Graphene: important results and applications 9781927885512, 9781927885529, 9781523124565, 1523124563, 1927885523

Table of contents :
Front Cover......Page 1
Graphene: Important Results and Applications......Page 2
Copyright Page......Page 3
Table of Contents......Page 4
Chapter 1. Introduction. Nobel Prize Laureates and Award Justification......Page 6
REFERENCES......Page 7
Chapter 2. Analysis of Publications......Page 10
REFERENCES......Page 13
3.1 CHEMICAL VAPOR DEPOSITION......Page 15
3.2 MOLECULAR BEAM EPITAXY......Page 32
3.3 ION IMPLANTATION......Page 37
3.4 DESORPTION OF SILICON FROM SILICON CARBIDE......Page 43
3.5 GRAPHITE OXIDATION......Page 44
3.6 REDUCTION OF GRAPHENE OXIDE......Page 53
3.7 ULTRASOUND-ASSISTED EXFOLIATION......Page 67
3.8 ELECTROCHEMICAL PROCESS......Page 72
3.9 DETONATION REACTION......Page 79
3.10 GRAPHITE INTERCALATION......Page 85
3.11 AGRICULTURAL WASTE PROCESSING......Page 89
Chapter 4. Manufacturers of Graphene, Its Grades, and the Production Output......Page 91
REFERENCES......Page 95
5.1 MORPHOLOGY AND THICKNESS......Page 97
5.2 CRYSTALLINITY......Page 116
5.3 MECHANICAL PROPERTIES......Page 118
5.4 TRIBOLOGICAL PROPERTIES......Page 123
5.5 ELECTRONIC PROPERTIES......Page 127
5.6 ELECTRICAL PROPERTIES......Page 129
5.7 MAGNETIC PROPERTIES......Page 132
5.8 THERMAL STABILITY......Page 135
5.9 THERMAL CONDUCTIVITY......Page 137
5.10 OPTICAL PROPERTIES......Page 141
5.11 BARRIER PROPERTIES......Page 143
5.12 SOUND AND MICROWAVE ABSORPTION......Page 146
5.13 RHEOLOGICAL PROPERTIES......Page 149
5.14 CHEMICAL RESISTANCE......Page 152
5.15 ANTIBACTERIAL PROPERTIES......Page 154
6.1 METHODS OF DISPERSION......Page 161
6.2 STABILITY OF DISPERSIONS......Page 167
6.3 DISPERSION MORPHOLOGY......Page 173
6.4 SPATIAL CONFIGURATIONS OF GRAPHENE SHEETS......Page 177
6.5 RIBBON SIZE......Page 180
6.6 RESULTS IN DIFFERENT MATRICES......Page 181
7.1 FUNCTIONAL GROUPS AND SIDE CHAINS......Page 189
7.2 DOPING......Page 195
7.3 EDGE FUNCTIONALIZATION......Page 198
8.1 AEROGELS......Page 201
8.2 ANTIBACTERIAL SURFACES......Page 206
8.3 BATTERIES......Page 209
8.4 BIOMEDICAL APPLICATIONS......Page 214
8.5 CATALYSIS......Page 217
8.6 COMPOSITES......Page 220
8.7 CONCRETE ADMIXTURES......Page 225
8.8 CORROSION PROTECTION......Page 229
8.9 DRUG DELIVERY SYSTEMS......Page 235
8.10 ENCAPSULATION......Page 240
8.11 ENERGY STORAGE......Page 242
8.12 INKS AND 3D PRINTS......Page 245
8.13 LUBRICATION......Page 247
8.14 ORGANIC LIGHT-EMITTING DIODES......Page 251
8.15 PACKAGING......Page 252
8.16 SELF-HEALING MATERIALS......Page 254
8.17 SEMICONDUCTORS......Page 259
8.18 SENSORS......Page 261
8.19 SPORTING EQUIPMENT......Page 266
8.20 TRANSPARENT FUNCTIONAL MATERIALS......Page 267
8.21 THERMAL MANAGEMENT SOLUTIONS......Page 270
8.22 WATER TREATMENT......Page 272
8.23 WEARABLE ELECTRONICS......Page 275
Chapter 9. Comparison of Justifications of Nobel Prize by the Selection Committee with Actual Results of Research Reported......Page 279
Index......Page 281

Citation preview

Impact of Award

Graphene Important Results and Applications George Wypych

Toronto 2019

Published by ChemTec Publishing 38 Earswick Drive, Toronto, Ontario M1E 1C6, Canada © ChemTec Publishing, 2019 ISBN 978-1-927885-51-2 (hard copy); 978-1-927885-52-9 (ebook) Cover design: Anita Wypych

All rights reserved. No part of this publication may be reproduced, stored or transmitted in any form or by any means without written permission of copyright owner. No responsibility is assumed by the Author and the Publisher for any injury or/and damage to persons or properties as a matter of products liability, negligence, use, or operation of any methods, product ideas, or instructions published or suggested in this book.

Library and Archives Canada Cataloguing in Publication Wypych, George, author Graphene: important results and applications / George Wypych. -1st edition. (Impact of award) Includes bibliographical references and index. Issued in print and electronic formats. ISBN 978-1-927885-51-2 (hardcover).--ISBN 978-1-927885-52-9 (PDF) 1. Graphene. I. Title. TA455.G65W97 2019

620.1'15

C2018-903999-X C2018-904000-9

Printed in United States, United Kingdom, France, and Australia

Table of Contents

iii

Table of Contents 1

Introduction. Nobel Prize Laureates and Award Justification

1

2

Analysis of Publications

4

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11

Production of Graphene and its Derivatives Chemical vapor deposition Molecular beam epitaxy Ion implantation Desorption of silicon from silicon carbide Graphite oxidation Reduction of graphene oxide Ultrasound-assisted exfoliation Electrochemical process Detonation reaction Graphite intercalation Agricultural waste processing

9 9 26 31 37 38 47 61 66 73 79 83

4

Manufacturers of Graphene, Its Grades, and the Production Output

85

5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15

Unique Nature of Graphene. Research Results Morphology and thickness Crystallinity Mechanical properties Tribological properties Electronic properties Electrical properties Magnetic properties Thermal stability Thermal conductivity Optical properties Barrier properties Sound and microwave absorption Rheological properties Chemical resistance Antibacterial properties

91 91 110 112 117 121 123 126 129 131 135 137 140 143 146 148

iv

Table of Contents

6 6.1 6.2 6.3 6.4 6.5 6.6

Dispersion of Graphene in the Polymer Matrix Methods of dispersion Stability of dispersions Dispersion morphology Spatial configurations of graphene sheets Ribbon size Results in different matrices

155 155 161 167 171 174 175

7 7.1 7.2 7.3

Chemical Modifications and Their Applications Functional groups and side chains Doping Edge functionalization

183 183 189 192

8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12 8.13 8.14 8.15 8.16 8.17 8.18 8.19 8.20 8.21 8.22 8.23 9

Current Developments in Some Applications of Graphene Aerogels Antibacterial surfaces Batteries Biomedical applications Catalysis Composites Concrete admixtures Corrosion protection Drug delivery systems Encapsulation Energy storage Inks and 3D prints Lubrication Organic light-emitting diodes Packaging Self-healing materials Semiconductors Sensors Sporting equipment Transparent functional materials Thermal management solutions Water treatment Wearable electronics Comparison of Justification of Nobel Prize by the Selection Committee with Actual Results of Research Reported

195 195 200 203 208 211 214 219 223 229 234 236 239 241 245 246 248 253 255 260 261 264 266 269

Index

275

273

1

Introduction. Nobel Prize Laureates and Award Justification According to the press release,1 on October 5, 2010, the Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2010 to Andre Geim, University of Manchester, UK and Konstantin Novoselov, University of Manchester, UK “for groundbreaking experiments regarding the two-dimensional material graphene.” The announcement further contained an introduction to the topic of Nobel Prize entitled Graphene − the perfect atomic lattice which may give us hints on the reasons behind the selection of the topic for the award. The pointers to the merits of the prize can be selected from the press release, as follows.1 • a thin flake of ordinary carbon, just one atom thick, lies behind this year’s Nobel Prize in Physics • carbon in such a flat form has exceptional properties that originate from the remarkable world of quantum physics • the material is the thinnest ever but also the strongest • a conductor of electricity that performs as well as copper • a conductor of heat which outperforms all other known materials • almost completely transparent • so dense that not even helium, the smallest gas atom, can pass through it • the graphene was extracted from a piece of graphite as found in ordinary pencils • the regular adhesive tape was used to obtain a flake of carbon with a thickness of just one atom • many believed (at the time of experiment) that it was impossible for such thin crystalline materials to be stable • opening for new research in physics (quantum Hall effect, Klein tunneling, electron properties and movement) • potential practical applications (transistors, touch screens, light panels, solar cells, conductive plastics, reinforced polymers, lightweight materials) • playfulness is one of their (Nobel Prize winners) hallmarks, one always learns something in the process and, who knows, you may even hit the jackpot Royal Swedish Academy of Sciences compiled Scientific Background on the Nobel Prize in Physics 2010 entitled Graphene which includes the above points with slightly enhanced and more structured information than contained in the press release. This information was

2

Introduction. Nobel Prize Laureates and Award Justification

supported by 44 references out of which 162-17 were authored and coauthored by the Nobel Prize laureates. It can be assumed that these were the papers considered by the Committee in evaluating the candidates’ achievements and the contribution which eventually resulted in awarding the price.

The next chapter of this book contains an analysis of publications on graphene prior and after awarding the Nobel Prize in Physics. The chapters which follow contain research data and their analysis based on the available literature. The following topics are included in the analysis of the research and technology related to graphene. • methods of graphene and its relevant derivatives production • manufacturers of graphene, available grades, and the production output • research results and data on graphene morphology, mechanical properties, 2D & 3D structures, electric conductivity, magnetic properties, thermal conductivity, optical properties, crystallinity, oxidative and thermal stability, and chemical resistance • dispersion of graphene in a polymer matrix • chemical modifications of graphene and their potential applications • current developments in 23 groups of product in which graphene made a significant impact All the above topics are discussed based on the large sample of publications with preference given to the most recently published data. The main direction of this book is to amplify potential applications of graphene and proven benefits of its use, provide a full account of its physical and chemical properties and fundamental principles which facilitate applications, the know-how of preparation of graphene and its derivatives and their most beneficial use in a large number of products. Three chapters (the first two and the last one) shade light on Nobel Prize selection and influence. In the last chapter, the currently known research findings are compared with included in this chapter list of reasons for awarding the Prize to review the wisdom of selection, impact on scientific developments, and (un)predictability of pathways of scientific developments based on a proven research track record.

REFERENCES 1 2 3 4 5 6 7 8 9 10

https://www.nobelprize.org/nobel_prizes/physics/laureates/2010/press.html K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science, 306, 666 (2004). K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, Nature, 438, 197 (2005). R. Nair, P. Blake, A. Grigorenko, K. Novoselov, T. Booth, T. Stauber, N. Peres, and A. Geim, Science, 320, 1308 (2008). M. I. Katsnelson, K. S. Novoselov, and A. K. Geim, Nature Physics, 2, 620 (2006). K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, and A. K. Geim, Proceedings of the National Academy of Sciences of the United States of America, 102, 10451 (2005). A. K. Geim and K. S. Novoselov, Nature Materials, 6, 183 (2007). K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Supporting material for Science, 306, 666 (2004). Barbolina, II, K. S. Novoselov, S. V. Morozov, S. V. Dubonos, M. Missous, A. O. Volkov, D. A. Christian, I. V. Grigorieva, and A. K. Geim, Applied Physics Letters, 88, 013901 (2006). E. W. Hill, A. K. Geim, K. Novoselov, F. Schedin, and P. Blake, IEEE Transactions on Magnetics, 42, 2694 (2006).

Introduction. Nobel Prize Laureates and Award Justification

11 12 13 14 15 16 17

3

E. V. Castro, K. S. Novoselov, S. V. Morozov, N. M. R. Peres, J. Dos Santos, J. Nilsson, F. Guinea, A. K. Geim, and A. H. C. Neto, Physical Review Letters, 99, 216802 (2007). A. B. Kuzmenko, E. van Heumen, D. van der Marel, P. Lerch, P. Blake, K. S. Novoselov, and A. K. Geim, Physical Review B, 79, 115441 (2009). S. V. Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin, D. C. Elias, J. A. Jaszczak, and A. K. Geim, Physical Review Letters, 100, 016602 (2008). K. S. Novoselov, E. McCann, S. V. Morozov, V. I. Fal'ko, M. I. Katsnelson, U. Zeitler, D. Jiang, F. Schedin, and A. K. Geim, Nature Physics, 2, 177 (2006). R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, Science, 320, 1308 (2008). Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson, and K. S. Novoselov, Nature Materials, 6, 652 (2007). K. Novoselov and A. Geim, Materials Technology, 22, 178 (2007).

4

Introduction. Nobel Prize Laureates and Award Justification

2

Analysis of Publications

Table 2.1. Percentage of publications on graphene by countries in 2008. % >30 10-30

Countries US (33) Japan (28), China (17), UK (11)

4-9

Italy (9), France, S. Korea (6), Germany, Netherlands, Russia (5), Brasil, Hungary, Iran, ROC (4)

2-3

Canada, Chile, India, Swiss (3), Australia, Belgium, Poland, Spain, Thailand, Ukraine (2)

1 or less Argentina, Finland, New Zealand, Norway, Peru, Vietnam Figure 2.1. Number of publications on graphene included in ScienceDirect vs. publication year.

Publications on graphene are analyzed regarding their yearly frequency, topics and geographic distribution before and after the Nobel Prize was awarded by the Nobel Prize Committee to Andre Geim and Konstantin Novoselov in 2010. Figure 2.1 shows that the number of publications on graphene dramatically increased after the Nobel Prize was awarded. It appears that further acceleration in research on this subjects occurs again in 2017-2018. In 2008, papers were published by authors from 31 countries as compared with 54 countries in 2018, meaning that in the countries with relatively smaller research budgets, the research on graphene was considered sufficiently promising as to increase the funding. Table 2.1 illustrates the frequency of publications on graphene in 2008 as per the country of the original authors. The field was dominated then by the USA research, followed by Japan and China, with UK and Netherlands (countries of Nobel Prize winners) participating in substantially less publishing activities. Table 2.2 shows a similar table characterizing the spread of publications on graphene in 2018. China becomes the clear leader in the publication on graphene with the USA, S. Korea, and Iran to be the distant followers. The industrialized European countries contributed to the world literature on graphene in a limited way as compared to the previously listed countries.

5

Analysis of Publications

Table 2.2. Percentage of publications on graphene by countries in 2018. %

Countries

>20

China (29.6)

5-10

US (9.2), S. Korea (6.8), Iran (5.2)

3-5

India (4.7), Australia (3.7), Italy (3.1)

2

Russia (2.6), Brazil, Malaysia, Spain (2.4)

1

France, Germany, Japan, ROC, UK (1.6), Egypt (1.3), Pakistan, Poland, Turkey, Vietnam (1)

50

USA (50.5)

>10

S.Korea (17.9)

9-5

Japan (7.4), Taiwan ROC (5.8), China (5.4)

4-1

All remaining countries not listed by name (3.5), Germany (2.3), Finland cations on graphene in 2008 and 2018. Asia has increased dominance in the published (1.0)

0

Figure 2.2 compares spread of publi-

papers on the subject. Contributions from African countries are as numerous as Australian and South American. North American and European contributions have substantially decreased. Brazil, Cyprus, Denmark, Malaysia, Figure 2.3 shows that the logically Poland, Russia expected changes occurred in the research Algeria, Chile, Hungary, Iran, New area on graphene. Initially, in 2008, the Zealand, Norway, Portugal, Uruguay, research was dominated by the evaluation

0.9-0.1 France (0.9), UK (0.7), Netherlands (0.6), Italy, Switzerland (0.5), Canada, India (0.4), Australia, Egypt (0.3), Austria, Israel, Spain, Sweden (0.2), Ireland, Luxembourg (0.1) 80 mesh) and larger amount of sulfuric acid (60 mL).16 The increased amounts of sulfuric acid were selected to ensure the sufficient mass and heat transfer in the reaction system.16 Figure 3.32 shows SEM micrographs and size distribution graphs of different graphene sizes.16 Hummers method uses cost-effective raw materials and has low energy demand, but purification, scale-up, and safety problems prompt numerous challenges for large-scale production.17 Conventional dead-end-filtration is difficult (the severe plugging of the filters by water-swollen graphene oxide and dialysis require massive amounts of water).17 Plugging problems are solved when using membrane separation with average membrane pores smaller than the lateral micrometer dimensions of graphene oxide nanoplatelets.17 A short dead-end-filtration prefiltration of acidified graphene oxide dispersions, combined with subsequent cross-flow filtration enables easy graphene oxide scale-up and production of high purity graphene oxide (Figure 3.33).17 Equipped with on-line monitoring of wastewater, the cross-flow filtration process is automated and enables concentration of graphene oxide dispersions in the final stage.17 The pH switching represents the key to removing Mn and S impurities.17 To minimize the amount of impurities, the acidification with HNO3 is preferred with respect to HCl because nitrates are readily removed.17 Graphene oxide was manufactured in a Couette-Taylor flow reactor for the oxidation of bulk graphite flakes (Figure 3.34).18 The turbulent Couette-Taylor flow in the reactor resulted in the efficient mixing and mass transfer of graphite and oxidizing agents (KMnO4 and H2SO4), thereby improving the efficiency of conversion graphite to

46

Production of Graphene and its Derivatives

graphene oxide.18 As compared to the standard Hummers method, a higher fraction of a single- and few-layer graphene oxide was produced in a dramatically shortened reaction time.18 By optimizing the processing parameters, the 93% yield of graphene oxide was achieved within 60 min of reaction time.18 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Jang, BZ; Zhamu, A, US8,501,318, Nanotek Instruments, Inc., Aug. 6, 2013. Zhamu, A; Jang, BZ, US8,753,539, Nanotek Instruments, Inc., Jun. 17, 2014. Kwon, YJ; Gu, JW; Park, WH; Shin, CM; Ji, BK; Kwon, DH, US8,968,695, IDT International Co., Ltd., Mar. 3, 2015. Blair, RG, US9,114,999, University of Central Florida Research Foundation, Inc., Aug. 25, 2015. Liu, Z; Zhou, X; Qin, Z; Tang, C, US9,162,894, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Oct. 20, 2015. Cruz-Silva, R; Morelos, A; Terrones, M; Elias, AL; Perea-Lopez, N; Endo, M, US9,284,193, The Penn State Research Foundation and Shinshu University, Mar. 15, 2016. Plomb, B; Geinoz, J; Marti, R; Vanoli, E; Bourgeois, J-P, US9,422,163, Belenos Clean Power Holding AG, Aug. 23, 2016. Li, CP; Shi, Y; Chen, X; He, D; Shen, L; Bao, N, Chem. Eng. Sci., 176, 319-28, 2018. Asgar, H; Deen KM; Riaz, U; Rahman, ZU; Shah, UH; Haider, W, Mater. Chem. Phys., 206, 7-11, 2018. Tran, M-H; Yang, C-S; Yang, S; Kim, I0J; Jeong, HK, Current Appl. Phys., 14, Suppl. 1, S74-9, 2014. Jankovsky, O; Marvan, P; Novacek, M; Luxa, J; Mazanek, V; Klimova, K; Sedmidubsky, D; Sofer, Z, Appl. Mater. Today, 4, 45-53, 2016. Ramesh, P; Bhagavathsingh, J, Mater. Lett., 193, 305-8, 2017. Zhao, J; Li, Y; Mao, J; He, Y; Luo, J, Trib. Int., 116, 303-9, 2017. Pendolino, F; Armata, N; Masullo, T; Cuttitta, A, Mater. Chem. Phys., 164, 771-7, 2015. Yuan, R; Yuan, J; Wu, Y; Chen, L; Zhou, H; Chen. J, Appl. Surf. Sci., 868-77, 2017. Chen, J; Chi, F; Huang, L; Zhang, M; Yao, B; Li, Y; Li, C; Shi, G, Carbon, 110, 34-40, 2016. Tölle, FJ; Gamp, K; Mülhaupt, R, Carbon, 75, 432-442, 2014. Park, WK; Kim, H; Kim, TY; Kim, Y; Yoo, S; Kin, S; Yoon, DH; Yang, WS, Carbon, 83, 217-23, 2015.

3.6 Reduction of graphene oxide

47

3.6 REDUCTION OF GRAPHENE OXIDE A transparent graphene film is prepared by maintaining the primary reduced state of a graphene oxide thin film via chemical reduction, reducing the graphene oxide thin film with chemical vapor deposition, and doping nitrogen which enhances its conductivity.1 The primary chemical reduction employs a reducing agent which is followed by the secondary thermal reduction, and nitrogen doping by injecting hydrogen and ammonia gas into a chemical vapor deposition equipment.1 The substrate is selected from the group consisting of glass, quartz, Si/SiO2, polyethylene terephthalate, polycarbonate, polyimide, polyethylene naphthalate, and polystyrene.1 A graphene oxide thin film is deposited on the substrate by spin or spray coating using ethanol and a graphene oxide aqueous solution at a mixing ratio of 1:1 by volume (or the graphene oxide aqueous solution has a concentration from 0.2 to 0.4 wt%).1 The reducing agent is selected from a group consisting of hydrazine, potassium hydroxide, sodium hydroxide, sodium bisulfate, sodium sulfite, thionyl chloride, and sulfur dioxide.1 The thermal reduction and nitrogen doping are performed by injecting hydrogen and ammonia gas (the mixing ratio of hydrogen and ammonia gas is 55% to 65%:45% to 35%) while heating from 500 to 1,200oC.1 The resultant product is used in solar cells and displays.1 A transformation of carbon nanotubes to nanoribbons composed of a few layers of graphene by an electrochemical process involving dispersing carbon nanotubes by sonication and depositing onto a conducting substrate, oxidizing carbon nanotubes at a controlled potential, followed by the reduction to form graphene nanoribbons having smooth edges and fewer defects.2 The dispersion liquid containing graphene oxide is applied to the conductive layer, and a solvent in the dispersion liquid is removed by drying.3 The graphene oxide formed on the conductive layer is electrochemically reduced by application of a sufficient reduction potential.3 A mild enzymatic oxidation of graphene oxide (using horseradish peroxidase or other enzymes) results in the formation of holey graphene oxide nanostructure.4 Enzymatic oxidation introduces defects in the basal plane of graphene oxide to enable alteration of the electronic transport properties of the reduced form (for example, via a change/ increase in an edge to plane ratio).4 Enzymatic oxidation can lead to the formation of holes in graphene oxide having an average diameter of 1 to 100 nm.4 After enzymatic oxidation of graphene oxide, reduction of the material may be accomplished via a chemical technique or via heating.4 For example, reduction with hydrazine hydrate results in the formation of holey reduced graphene oxide.4 The holey reduced graphene oxide demonstrated ptype semiconducting behavior, which makes this material desirable for a number of applications, including field-effect transistors.4 Graphene sheets having a surface area of at least about 100 m2/g with a carbon to oxygen molar ratio of between 3:2 to 1000:1 were produced by thermal exfoliation of graphite oxide to produce graphene sheets by heating the graphite oxide to a temperature of 600oC at a rate of 120oC/min, conveying the graphene sheets in a closed system to a compression apparatus, and compressing the graphene sheets with a pressure of 5 to 1000 psi.5 Deoxidizing the graphene oxide may be accomplished by dissolving the graphene oxide in an aqueous solvent that is a mixture of water and 1-butyl-2,3-dimethylimidaz-

48

Production of Graphene and its Derivatives

olium iodide and optionally iodine to form a solution and centrifuging the solution to obtain the graphene.6 The inventive concept relates to solar cells.6 The solution of graphene oxide is coated on a supporting fiber (polymer fibers of different chemical composition).7 The graphene oxide is then converted on the supporting fiber by thermal (100-200oC in the case of chitosan and polyamide fibers), optical (wavelength in the range of 200 to 1500 nm), or chemical method (hydrazine, dimethylhydrazine, sodium borohydride, sodium hydroxide, ascorbic acid, glucose, hydrogen sulfide, hydroquinone, and sulfuric acid).7 Preparation of a reduced graphene oxide film comprises: coating a graphene oxide solution on a substrate (polyethersulfone, polyimide, polycarbonate, poly(ethylene naphthalate), and poly(ethylene terephthalate)) to form a graphene oxide thin film and reducing the graphene oxide thin film using a chemical reduction method (hydrazine, sodium borohydride, and sulfuric acid) and a pressure-assisted thermal reduction method (70-200oC).8 Graphene fibers are produced using the following steps 1) oxidation of graphite to graphene oxide, 2) preparation of graphene oxide slurry with high solid contents and residues of sulfuric acid impurities. 3) preparation of large area films by bar-coating or dropcasting the graphene oxide dispersion and drying at low temperature. 4) spinning the graphene oxide film into a fiber, and 5) thermal or chemical reduction of the graphene oxide fiber into an electrically conductive graphene fiber.9 The graphene oxide fiber is reduced by chemical reduction using hydrazine, sodium borohydride, hydroiodic acid, ascorbic acid, or cysteamine or the thermal reduction occurring at temperatures between 1800 and 3000oC.9 The graphene oxide was first reduced with hydrazine or hydrazine hydrate and then reduced with hydrogen.10 The second reduction restored the sp2 structure of pristine graphene sheets over that obtained in the first reduction.10 Hydrogen may be more efficient than hydrazine in removing oxygen-containing functional groups from the graphene oxide since it removes even carboxylic acid groups in addition to the carbonyl and hydroxyl functionalities.10 Borane may also be used to reduce the graphene oxide.10 Borane is particularly effective at reducing carboxylic acids to alcohols, and the alcohols can be further removed with hydrogen and heat in a second reduction step.10 Graphene oxide reduction is frequently carried out in a neutral or alkaline environment, which causes the agglomeration of graphene oxide, and the reduction reaction would not continue until graphene oxide is dispersed.11 In an acidic environment, all these problems can be avoided, and the reduction reaction occurs homogeneously.11 The graphene oxide layer on the flat sponge of hollow tubular oil absorbing material is formed by immersion and coating under negative pressure (0.05-0.1 MPa).12 The reduction of graphene oxide is performed with hydrazine hydrate vapor (at 80-90oC for 12-24 h) followed by washing and drying.12 A storage battery electrode is manufactured using the following steps: a paste containing an active material, a binder, graphene oxide, and a solvent is formed; the paste is applied to a current collector and the solvent contained in the paste is evaporated to form an active material layer; the active material layer is immersed in a liquid containing alcohol; and the active material layer is taken out from the liquid and heated (80-150oC) so that the graphene oxide is reduced.13

3.6 Reduction of graphene oxide

49

Figure 3.35. SEM images of graphite (A, B), graphene oxide (C, D) and the reduced graphene oxide (E, F). [Adapted, by permission, from Kanishka, K; De Silva, H; Huang, H-H; Yoshimura, M, Appl. Surf. Sci., 447, 338-46, 2018.]

A material comprises a reduced graphene oxide having the degree of reduction of the graphene oxide with a spatial variation so that the material exhibits a gradient in the electrical conductivity and/or permittivity.14 The material can be obtained by means of applying a thermal gradient to a graphene oxide element, or by irradiation of a graphene oxide element.14 The ratio of the electrical conductivity of the high conductivity/permittivity surface of the material to the electrical conductivity of the low conductivity/permittivity surface of the material exceeds 100 and the material has a thickness of 0.1-10 mm.14 The material can be used in an electric device for purposes of field grading and/or dissipation of charges.14 Examples of electric devices wherein the material is beneficial includes cable accessories, bushings, power cables, microelectronics, switchgear, etc.14

50

Production of Graphene and its Derivatives

Figure 3.36. Schematic representation of the oxygen functionalities presents in graphene oxide and reduced graphene oxide. [Adapted, by permission, from Kanishka, K; De Silva, H; Huang, H-H; Yoshimura, M, Appl. Surf. Sci., 447, 338-46, 2018.]

The synthesis of zero dimension graphene quantum dots based on exfoliation/reduction of surface passivated functionalized graphite oxide is described.15 The synthesis procedures include exfoliation/reduction of functionalized graphite oxide in the presence of hydrogen gas, using focused solar radiation under vacuum.15 Graphene oxide prepared from anthracite is reduced either by heating the graphene oxide-dispersant solution in a microwave oven for 5-20 min or by reducing it by refluxing the mixture of reducing agent (e.g., hydrazine) to graphene oxide of 1:1-5 for 1 h.16 Graphene oxide synthesized by oxidizing natural graphite was reduced by ascorbic acid which is a green reducer.17 The oxidation of graphite produced a highly oxidized graphene oxide with a 9.30 Å interlayer space and about 33% of oxygen atomic percentage.17 Until 50 min of the reduction, both graphene oxide and reduced graphene oxide coexist.17 The reduction rate is fast within the first 30 min.17 Figure 3.35 shows the morphology of graphite, graphene oxide, and the reduced graphene oxide.17 Graphite (Figure 3.35A and B) has well developed a layered structure with sharp edges, and graphene oxide (Figure 3.35C and D) has re-stacked exfoliated sheets with wavy edges.17 The stacking nature of graphene oxide was observed after drying.17 The layered nature of graphene oxide disappeared with the increase in the reduction time (Figure 3.35E and F).17 Figure

Figure 3.37. Reduction of graphene oxide using artemisinin. [Adapted, by permission, from Hou, D; Liu, Q; Wang, X; Quan, Y; Qiao, Z; Yu, L; Ding, S, J. Materiomics, in press, 2018.]

3.6 Reduction of graphene oxide

51

Figure 3.38. TEM images of graphene oxide (a), reduced graphene oxide (b) and their corresponding selected area electron diffraction (SAED) patterns; High-resolution image of reduced graphene oxide (c) and the inset raw image of reduced graphene oxide after Fast Fourier Transform Algorithm which shows the lattice and atomic structure; the magnified lattice image of defects (d). [Adapted, by permission, from Hou, D; Liu, Q; Wang, X; Quan, Y; Qiao, Z; Yu, L; Ding, S, J. Materiomics, in press, 2018.]

3.36 shows the chemical structure of graphene oxide and its reduced form by ascorbic acid. With the reduction progressing, the intensities of the peaks responsible for oxygen functionalities in graphene oxide have been decreased (especially epoxy and hydroxyl groups).17 The intensity of the peak for C=C bond was increased.17 The interlayer space has declined due to the removal of oxygen functionalities.17 The preparation of reduced graphene oxide by chemical reduction of graphene oxide usually involves highly toxic reducing agents which are harmful to the environment and human health.18 A mediated facile and relative green approach for the preparation of reduced graphene oxide in ethanol using artemisinin as a reducing agent was reported (Figure 3.37).18 Figures 3.38a and b display typical transparent morphology and rippled surface of graphene oxide and reduced graphene oxide, respectively.18 The diffraction spots are ring-like partially destroyed during oxidation-reduction.18 The typical thickness of the reduced graphene oxide platelet is about 1.2 nm, corresponding to 3 layers and interlayer distance is 0.34 nm typical of the natural graphite.18 After reduction by artemis-

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Figure 3.39. Methods of reduction of the graphene oxide. [Adapted, by permission, from Lavin-Lopez, MP; Paton-Carrero, A; Sanchez-Silva, L; Valverde, JL; Romero, A, Adv. Powder Technol., 28, 12, 3195-203, 2017.]

Figure 3.40. Advantages and disadvantages of reduction strategies used in the production of reduced graphene oxide. [Adapted, by permission, from Lavin-Lopez, MP; Paton-Carrero, A; Sanchez-Silva, L; Valverde, JL; Romero, A, Adv. Powder Technol., 28, 12, 3195-203, 2017.]

inin, reduced graphene oxide layers are comprised of defect-free nanosize graphene-like domains surrounded by the topological defect areas dominated by pentagons-heptagons pairs or quasi-amorphous, as shown in Figure 3.38d.18 Three reduction routes including chemical, thermal, and multiphase methods were used to determine the most effective reduction strategy (Figure 3.39).19 Figure 3.40 compares the advantages and disadvantages of different methods of reduction of graphene oxide.19 A mild thermal treatment followed by a chemical reduction with ascorbic acid

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reduced 47% of oxygen functional groups.19 A simple method to produce reduced graphene oxide by refluxing graphene oxide in N,Ndimethylformamide without reducing agent has been discussed. The reduced graphene oxide having a high degree of reduction was prepared, and the level of reduction was controlled by reflux time.20 The C/O atomic ratio of reduced graphene oxide was 8.4 (compared to 1.4 for graphene oxide).20 It is also possible to reduce graphene oxide by refluxing its suspension in water but the process is time-consuming, and the degree of reduction achieved is mediocre.20 In order to improve the electrochemical properties of reduced graphene oxide, multiple-step reduction of graphene oxide was found to be more effective compared to single-step methods including thermal annealing (heatFigure 3.41. Schematic representation of the major oxidation meth- ing at 1000oC for 10 min under ods of graphite to graphene oxide and the chemical reduction of graphene oxide by some reductants. [Adapted, by permission, from nitrogen), microwave treatment (treatment under 700 W for 1 De Silva, KKH; Huang, H-H; Joshi, RK; Yoshimura, M, Carbon, 119, 190-9, 2017.] min), hydrazine reduction (134 μL N2H4 added to 100 mL of 1.0 mg mL-1 graphene oxide suspension at 90°C and held at this temperature for 1 h with constant stirring), and borohydride reduction (0.25 g of NaBH4 was dissolved in 10 mL H2O and mixed with a 25 mL GO suspension, (1.0 mg mL-1) at 90°C and held at this temperature for 2 h).21 The two-step borohydride and hydrazine reduction (reduced graphene oxideNaBH4-N2H4) with optimal sequence and conditions was able to completely recover the conjugated carbon structure with minimum defects.21 The two-step reduction produced the greatest capacitance (162 F g-1 in alkaline media and 164 F g-1 in acidic media) and highest oxygen reduction reaction activity when compared to other individual and twostep combined methods.21 Figure 3.41 illustrates developments in oxidation and reduction.22 The first known reducing agent, H2S was introduced in 1934.22 Hydrazine (N2H4·H2O) is the best-known reductant in terms of giving reduced graphene oxide with improved electrical and structural properties with the best resemblance of pristine graphene (but hydrazine is toxic).22

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Figure 3.42. Proposed mechanism of microbial reduction of graphene oxide. Orange dots, blue circles with white dots, and red dashed circle represent self-secreted electron mediators, multiheme-containing outer-membrane c-type cytochromes, and the molecular structure of the heme group respectively. [Adapted, by permission, from De Silva, KKH; Huang, H-H; Joshi, RK; Yoshimura, M, Carbon, 119, 190-9, 2017.]

Ascorbic acid is the focus of the review article to propose non-toxic alternative but several other green alternatives exist. For example, many plant extracts were used for the reduction of graphene oxide (e.g., green tea, carrot juice, orange peel, pomegranate juice, etc.).22 The problem arises from the fact that the plant extracts have large organic molecules as the reducing components that can interact with the reduced graphene oxide sheets forming stable dispersions. For some applications, such as materials for conductive applications, such kind of incorporation of molecules can be disadvantageous.22 The metabolically generated electrons from Shewanella are transferred to an external electron acceptor either directly from the cell surface or using self-secreted electron mediators (Figure 3.42) which can be used to reduce graphene oxide.22 This process results in a reduced graphene oxide having good qualities in applications in the biomedical field.22 The reduction may last for more than 3 days and care has to be taken in handling cultures of microbes.22 Glucose oxidized to aldonic acid in a basic medium and converted into lactone can form Hbonds with the residual oxygen functionalities present in the reduced graphene oxide.22 A high-density atomic hydrogen was generated on a hot tungsten surface through a catalytic cracking reaction and subsequently used to reduce graphene oxide by the atomic hydrogen annealing at low temperature.23 The tungsten mesh temperature was 1780°C, a sample temperature of 241°C and a treatment time was 3600 s.23 A reduced graphene oxide film had a low resistance of 272 Ω.23 The atomic hydrogen annealing apparatus used in this study is shown in Figure 3.43.23 The tungsten mesh was heated electrically to a setpoint. The flow rate of H2 was 400 sccm.23 The sample surface was modified by the combined effects of local heating and chemical reaction (hydrogenation and reduction) in atomic hydrogen annealing as shown in Figure 3.43b.23 High density of atomic hydrogen was achieved by an efficient H2 decomposition via a catalytic cracking reaction on the cat-

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Figure 3.43. (a) Schematic diagram of the atomic hydrogen annealing apparatus. H2 molecules are decomposed on a heated tungsten mesh. The graphene oxide film surface on a quartz substrate is subjected to atomic hydrogen. The distance between the tungsten mesh and sample, Dms, was changed from 164 to 249 mm by a transfer rod. (b) Concept of atomic hydrogen annealing. The decomposition and recombination reactions, and the effects of atomic hydrogen on a sample surface are illustrated. [Adapted, by permission, from Heya, A; Matsuo, N, Thin Solid Films, 625, 93-9, 2017.]

alyst surface.23 The generated atomic hydrogen atoms were recombined to H2 on the sample surface.23

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Figure 3.44. Laser induced graphene oxide reduction and patterning. [Adapted, by permission, from Kasischke, M; Maragkaki, S; Volz, S; Ostendorf, A; Gurevich, EL, Appl. Surf. Sci., 445, 197-203, 2018.]

The periodical nanopatterning and reduction of graphene oxide was achieved by femtosecond laser at fluencies slightly higher than the fluency needed for reduction of the graphene oxide.24 The periodic pattern is formed either simultaneously with or due Figure 3.45. SEM images of graphene oxide powder. (a) Before to the reduction of the graphene thermal reduction; (b) after thermal reduction upon heating to 400°C in flowing argon. [Adapted, by permission, from Ren, Y; oxide (Figure 3.44).24 First, the Zhou, T; Su, G; Ma, Y, Vibrational Spectr., 96, 32-45, 2018.] graphene oxide is reduced, and the low spatial frequency ripples are formed in the conductive reduced graphene oxide, or they are formed simultaneously.24 In the first case, the ripples are formed on a conductive reduced graphene oxide surface, on which surface electromagnetic waves can be excited. In the second case, the ripples are formed on a non-conductive surface; hence the low spatial frequency could not be explained only through the excitation of plasmons and their interference with the incident light.24 A variety of possible applications in printed and flexible electronics may use this method.24

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Figure 3.46. Illustration of the detailed steps of processes I (35-182°C) and II (182-385°C) inferred from the 2D correlation analysis. The carbon skeleton of graphene oxide is displayed in 3D ball-and-stick model. [Adapted, by permission, from Ren, Y; Zhou, T; Su, G; Ma, Y, Vibrational Spectr., 96, 32-45, 2018.]

The thermal reduction of graphene oxide (from 20 to 400°C in argon atmosphere) was monitored online and investigated through temperature-dependent FTIR spectroscopy combined with scaling-moving-window 2D correlation spectroscopy, MW2D, FTIR spec-

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troscopy and generalized 2D correlation analyses.25 The graphene oxide sheets before thermal reduction were large and had many wrinkles (Figure 3.45a).25 After reduction, the large sheets were broken into small pieces.25 A phenomenon is probably caused by the removal of a large number of carbon atoms producing CO/CO2 from the graphene oxideoxidized area during thermal reduction.25 This causes many vacancies in the lattice of the reduced graphene oxide, and therefore, after the vacancy aggregation, the sheets cannot maintain the original morphology (Figure 3.45b).25 Figure 3.46 shows the detailed steps of the process of reduction in two temperature ranges.25 For the process I (35-182°C), a sixstep mechanism was revealed according to the obtained sequential order:25 • Dissociation of the intercalated or adsorbed water (35-100°C). • Dissociation of hydroxyl groups attached to the interior aromatic domains of graphene oxide below 148°C. • Elimination of epoxy groups in the form of CO2 (148-158°C). • Generation of new species containing a carbonyl (ketones). • Restoration of the graphene structure (sp2 aromatic structure) because of the rapid removal of oxygen functional groups. • Generation of ether/lactone species. For the process II (182-385°C), the sequential order demonstrated a four-step mechanism:25 • Removal of carboxyl groups in graphene oxide. • Removal of hydroxyl groups on the edges of the graphene oxide aromatic domains. • Further restoration of the graphene structure after the removal of carboxyl and hydroxyl groups. • Removal of lactones/ethers generated in the process I ( G″) behavior at a low graphene oxide composition (ϕ ~ 0.08%) and solid-like (liquid crystalline) behavior at higher compositions (ϕ ~ 0.45%) are typical of graphene oxide dispersions.1 The nematic gel-like phase is present at a higher graphene oxide contents when ϕ > 0.83% with both G′ and G″ moduli nearly independent of frequency (ω).1 The yield stress and rigidity percolation transition are also observed above phase transition composition ϕc > 0.33%.1 Graphene oxide sheets self-organize above some critical concentration.2 Aggregates are formed or disassociate depending on the flow conditions.2 Aggregates are reversibly formed when the flow is arrested (thixotropy). Figure 5.52 illustrates results of shear rheological characterizations using measurements in oscillatory linear shear flow, steady shear flow, and transient shear flow.2 Graphene sheets with different oxygen contents were prepared for incorporation in polymethylmethacrylate.3 Due to the favorable interfacial interaction arising from polarity matching, the graphene with a C/O ratio of 13.2 had a better dispersion in PMMA than graphene oxides with lower C/O ratios.3 The PMMA composites exhibited lower rheological and electrical percolation thresholds.3 The crystallization behavior and non-isothermal crystallization kinetics of polypropylene/reduced graphene oxide nanocomposites were studied.4 The crystallization peak temperature, crystallization rate, and crystallization degree of polypropylene/reduced graphene oxide nanocomposites were enhanced by the increase in reduction extent of reduced graphene oxide.4 The non-Newtonian behavior of nanocomposites became stronger with the increase in reduction extent of reduced graphene oxide.4 Melt rheology reveals the presence of strong interaction between high-density polyethylene chains and reduced graphene oxide nanoplatelets.5 The restriction imposed by the filler on the chain mobility preserved oriented state that caused anisotropy in crystallization on cooling.5 The enhanced orientation with increasing filler content was attributed to the strong chain-filler interaction.5 The initial drop in the storage modulus (at lower concentrations of filler) was attributed to the adhesion of longer chains to the filler surface.5 This was followed by an increase in the modulus with the increase in the concentration of reduced graphene oxide nanoplatelets which was attributed to the clustering of the reduced graphene oxide nanoplatelets.5 The rheological threshold (~0.1 vol%) was distinctly lower than the conductivity threshold (~0.5 vol%) because of the formation of the polymer-nanofiller hybrid network.6 The 2D graphene-related network is more effective than the 1D carbon nanotube-related network in enhancing the melt elasticity.6 The graphene-oil-based fluid is a Bingham fluid but behaved similarly to a Newtonian fluid as it possessed zero shear stress.7 Bingham model data fitted closely the experimental data whereas power law deviated at higher shear rates7. The slight thickening of graphene-oil-based fluid was attributed to the agglomeration of graphene nanosheets.7

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Figure 5.52. Graphene oxide aqueous dispersions at different volume concentrations, ϕ, are prepared by adding graphene oxide to water. Oscillatory linear shear flow: When ϕ < ϕc (ϕc is the critical volume or mass concentration) viscous response due to the drag of the graphene oxide sheets prevails over the elastic response. When ϕ > ϕc, elastic response due to the graphene oxide self-aggregation prevails over the viscous response due to the · 2 · drag of the clusters. Steady shear flow: When ϕ < ϕc and at Peclet number ( Pe = γ a ⁄ D 0 < 1 ) where γ is the applied shear rate, a is the average sheet radius, and D0 is the sheet diffusivity, graphene oxide sheets are randomly oriented; while at Pe > 1 graphene oxide sheets are oriented along the flow direction. When ϕ > ϕc and at Peclet number Pe < 1, graphene oxide platelets are arranged in randomly oriented clusters; while at Pe > 1 clusters are broken down. Transient shear flow: Only the case ϕ > ϕc is considered. Initially, the dispersion is arranged in randomly oriented clusters. After applying a flow at Pe > 1, clusters are broken down and graphene oxide sheets are oriented along the flow direction. When the flow is arrested, Pe = 0, graphene oxide sheets start to self-arrange. After sufficient resting time, graphene oxide sheets recover the initial cluster configuration. [Adapted, by permission, from Del Guidice, F; Shen, AQ, Current Opinion Chem. Eng., 16, 23-30, 2017.]

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The addition of graphene oxide into the cement caused a reduction in fluidity and increased rheological parameters.8 A mechanism was proposed to explain changes in the rheological properties of cement pastes (Figure 5.53).8 The negatively charged Figure 5.53. Schematic illustration of (a) the formation of floccugraphene oxide on silica fume surlated structure and (b) electrostatic repulsion. [Adapted, by permission, from Shang, Y; Zhang, D; Yang, C; Liu, Y; Liu, Y, Con- face provides electrostatic repulstr. Build. Mater., 96, 20-28, 2015.] sion between particles which causes better dispersion and releases the entrapped water.8 REFERENCES 1 2 3 4 5 6 7 8

Kumar, P; Maiti, UN; Lee, KE; Kim, SO, Carbon, 80, 453-61, 2014. Del Guidice, F; Shen, AQ, Current Opinion Chem. Eng., 16, 23-30, 2017. Zhang, H-B; Zheng, W-G; Yan, Q; Jiang, Z-G; Yu, Z-Z, Carbon, 50, 14, 5117-25, 2012. Chen, Y; Yin, Q; Zhang, X; Xue, X; Jia, H, Thermochim. Acta, 661, 124-36, 2018. Liu, K; Andablo-Reyes, E; Patil, N; Merino, DH; Ronca, S; Rastogi, S, Polymer, 87, 8-16, 2016. Chiu, Y-C; Huang, C-L; Wang, C, Compos. Sci. Technol., 134, 153-60, 2016. Ho, CY; Yusup, S; Soon, CV; Arpin, MT, Procedia Eng., 148, 49-56, 2016. Shang, Y; Zhang, D; Yang, C; Liu, Y; Liu, Y, Constr. Build. Mater., 96, 20-28, 2015.

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5.14 CHEMICAL RESISTANCE The chemical vapor deposition graphene shows an excellent performance as a passivation layer below 200°C, but the protective action degenerates rapidly with increasing air temperature.1 The most adverse effect on the degeneration of oxidation resistance at a hightemperature air comes from wrinkles.1 Lack of uniformity and defects (grain boundaries and wrinkles) in graphene coatings synthesized by the chemical vapor deposition adversely affected the durability of these coatings.2 A slow cooling hindered the formation of graphene coating irrespective of the presence or absence of hydrogen flow.2 Under the rapid cooling conditions, the absence of

Figure 5.54. Preparation of modified graphene oxide. [Adapted, by permission, from Khaleghi, M; Didehban, K; Shabanian, M, Ultrasonics Sonochem., 43, 275-84, 2018.]

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hydrogen flow restricted wrinkle formation.2 The reduced wrinkle formation significantly improved the durability of the resultant coating.2 Graphene and its derivatives are inert under various atmospheres.3 In addition, they can act as barriers for molecules diffusion. The chemical resistance properties of PVC was improved by modified graphene oxide.3 The melamine terephthaldehyde resin was used for modification of graphene oxide (Figure 5.54) to improve PVC chemical resistance at very low loading levels.3 PVC containing modified graphene oxide had improved resistance to acetone and sodium hypochlorite in addition to the improved thermal stability.3 The addition of nanofiller reduced swelling and protected PVC chains from chemical attack.3 Hybrid sol-gel coatings containing graphene oxide have been deposited on the crosslinked polyethylene films.4 The presence of graphene oxide significantly increased the thermooxidative resistance of polymer.4 Its effect was attributed to an improved barrier against oxygen diffusion.4 A thin layer (two or more layers of graphene) protected the copper from oxidation, corrosion, and increased its resistance to wear.5 It can be used as a protective coating of components if exposed to an oxygen atmosphere.5 Because graphene coating does not impair the electrical resistance, it can be used in electrical contacts.5 At the optimal concentration of 0.5 wt%, graphene improved corrosion protection of epoxy coating.6 Graphene gave better corrosion resistance than fullerene which suggested that nanofiller shape had also an important influence.6 REFERENCES 1 2 3 4 5 6

Zhang, YH; Wang, B; Zhang, HR; Chen, ZY; Zhang, YQ; Wang, B; Sui, YP; Li, XL; Xie, XM; Yu, GH; Jin, Z; Liu, XY, Carbon, 70, 81-6, 2014. Anisur, MR; Banerjee, PC; Easton, CD; Raman, RKS, Carbon, 127, 131-40, 2018. Khaleghi, M; Didehban, K; Shabanian, M, Ultrasonics Sonochem., 43, 275-84, 2018. Toselli, M; Saccani, A; Pilati, F, Surf. Coat. Technol., 258, 503-8, 2014. Pietrzak, K; Strojny-Nędza, A; Olesińska, W; Bańkowska, A; Gładki, A, Appl. Surf. Sci., 421A, 228-33, 2017. Liu, D; Zhao, W; Liu, S; Cen, Q; Xue, Q, Surf. Coat. Technol., 286, 354-64, 2016.

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5.15 ANTIBACTERIAL PROPERTIES The graphene oxide coatings on silicone rubber were used to test the antibacterial activity against E. coli and S. aureus.1 The colony counting and fluorescent staining test validated that graphene oxide coatings confirmed the antibacterial activity.1 The cells lost the membrane integrity, and cytoplasm was leaked, leading to their death.1 The oxidative stress mechanism was considered to be the main reason for the antibacterial activity.1 The schematic diagram in Figure 5.35 illustrates the antibacterial activity of graphene oxide towards E. coli.2 The sharp graphene oxide sheets can penetrate the bacterial membranes through 'physical interaction'.2 The smaller sized graphene oxide sheets may stick onto the bacteria surface and undergo the 'chemical interaction' processes (electron exchange between the bacterial enzymes and functional oxygen groups of graphene oxide).2 This electron reshuffle may cause formation of the free radicals and reactive oxygen species, exerting oxidative stress, leading to the denaturation of bacterial proteins and cell entrapment.2 Antibacterial activities of graphene derivatives depend on the platelet size, a number of layers, oxygen-containing groups, etc.3 Also, the deposition method of graphene oxide on the substrate may affect the antibacterial activity.3 The graphene derivative was fixed on the titanium surface by drop with gravitational effects, electrostatic interaction, and electrophoretic deposition.3 The method of deposition affected the ability of graphene oxide to prevent Staphylococcus aureus from gathering, sharpness of wrinkles or edges, and levels of reactive oxygen species formation.3 Once S. aureus was not aggregated, graphene oxide could effectively interact with bacteria and kill them with sharp wrinkles or edges and highly reactive oxygen species levels.3 The graphene oxide-electrophoretic deposition can effectively prevent S. aureus from gathering with own sharp wrinkles or edges and can generate higher reactive oxygen species levels.3 As a result, the graphene oxide-electrophoretic, GO-EPD, deposition exhibited optimal antibacterial activity against

Figure 5.55. SEM observation of S. aureus on Ti, GO-D, GO-APS and GO-EPD at different magnifications. [Adapted, by permission, from Qiu, J; Liu, L; Zhu, H; Liu, X, Bioactive Mater., 3, 3, 341-6, 2018.]

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5.56. The mechanism of antibacterial activity. [Adapted, by permission, from Sandhya, PK; Jose, J; Sreekala, MS; Padmanabhan, M; Kalarikkal, N; Thomas, S, Ceramics Int., 44, 13, 15092-98, 2018.]

S. aureus, followed by the graphene oxide-electrostatic interaction, GO-APS, and the graphene oxide-gravitational, GO-D. S. aureus grew well on titanium, Ti, with the typical features of smooth, round and intact cell morphology (Figure 5.55).3 In the case of GO-D, many bacteria are gathered on the sample surface.3 Fewer bacteria adhered on the GOAPS sample surface and bacteria of small size were dead with the cell membrane badly damaged as pointed out by the yellow arrows, while bacteria in the form of aggregation (large size) had a very rough surface but the cell membrane was relatively intact.3 Bacteria on GO-EPD sample were separated and dead with the evident distortion of the cell membrane as shown by the yellow arrows.3 The nanocomposites of graphene oxide with 12-molybdophosphoric acid were analyzed for their antibacterial activity.4 Only small improvement of antibacterial properties compared with graphene oxide was detected.4 The graphene-based materials reduced the antibiotic resistance, improving antibacterial activity.5 The zinc oxide-decorated reduced graphene oxide was more efficient than reduced graphene oxide regarding antibacterial properties.5 This is attributable to the synergistic effect of ZnO and reduced graphene oxide towards the bacteria.5 The antibacterial effect of zinc oxide-decorated-reduced graphene oxide towards E. coli was due to the disruption of the bacterial cell which could be confirmed by AFM images.5 Figure 5.56 illustrates the mechanism of antibacterial activity.5 The Marigold flower extract was used for the reduction of graphene oxide to obtain material suitable for biomedical applications (does not contain toxic substances which are typically present in the reduced graphene oxide produced by other methods).6

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Figure 5.57. Schematic diagram of (a) preparation of poly(l-lactic acid), PLLA-poly(glycolic acid), PGA/ graphene oxide-silver nanocomposite powders; (b) fabrication of the scaffolds via selective laser sintering, SLS. [Adapted, by permission, from Shuai, C; Guo, W; Wu, P; Yang, W; Hu, S; Xia, Y; Feng, P, Chem. Eng. J., 347, 322-33, 2018.]

Fluorinated graphene was prepared from graphene oxide by a hydrothermal reaction to be used in dental glass ionomer cement.7 The amount used was in the range of 0.5-4 wt%.7 The fluoride ion release was measured by fluoride ion selective electrode and the antibacterial effect was tested using Staphylococcus aureus and Streptococcus mutans by pellicle sticking method.7 The fluoride ion release was not significantly different from control.7 The inhibitory rates of pure glass ionomer cement for S. mutans and S. aureus were 46.9% and 39% while that of glass ionomer cements/fluorinated graphene (4 wt%)

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Figure 5.58. Schematic of a possible synergistic antibacterial mechanism of graphene oxide-silver nanosystem: combination of the capturing effects of graphene oxide nanosheets and killing effects of silver nanoparticles. (1) Attacking cytomembrane and disturbing transmembrane transport, (2) attacking respiratory enzyme and inhibiting the synthesis of ATP, (3) attacking DNA and inhibiting the replication and transcription, (4) attacking mRNA and disturbing the transcription, (5) attacking proteins and damaging their physiological function. [Adapted, by permission, from Shuai, C; Guo, W; Wu, P; Yang, W; Hu, S; Xia, Y; Feng, P, Chem. Eng. J., 347, 322-33, 2018.]

were 85.3% and 88.1%, respectively.7 The phytochemically reduced graphene oxide had significant antibacterial activity.7 Bone scaffolds with antibacterial properties were developed using a graphene oxidesilver co-dispersing nanosystem with dual synergistic effect.8 The graphene oxide nanosheets were loaded with silver nanoparticles while silver nanoparticles were intercalated into the interlayers of graphene oxide nanosheets (Figure 5.57).8 The graphene oxide-silver nanosystem showed a synergistic effect via a combination of the graphene oxide nanosheets and the killing efficiency of silver (Figure 5.58).8 A scaffold containing

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Figure 5.59. Sample morphologies. (a) SEM image of spindle-shaped graphene oxide, GO, with a length of 1.0 μm, (b) TEM image of spindle-shaped GO, (c) SEM image of ZnO nanoparticles (diameter ≤ 50 nm), (d) TEM image of uniform ZnO nanoparticles, (e) SEM of spindle-shaped GO were fully covered by ZnO nanoparticles, and (f) TEM image of ZnO crystal structure on the GO surface. [Adapted, by permission, from Zhong, L; Liu, H; Samal, M; Yun, K, J. Photochem. Photobiol. B: Biol., 183, 293-301, 2018.]

1 wt% graphene oxide with 1 wt% silver showed a bacterial inhibition rate of more than 95%.8 Bioactive glasses have excellent bioactivity and bone bonding ability.9 By doping them with graphene oxide, their lack of intrinsic antibacterial effects is effectively eliminated.8

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The N-alkylated poly(4-vinylpyridine), a cationic polymer was used for the surface modification of Fe3O4 nanoparticles.10 The modified Fe3O4 nanoparticles were combined with graphene oxide through a simple electrostatic binding.10 Then, silver nanoparticles were deposited to form the multiple antibacterial nanocomposites.10 The presence of Fe3O4 permits recycling the antibacterial additive by magnetic separation.10 The nanocomposite exhibited excellent antibacterial properties and a low cytotoxicity.10 Figure 5.59 shows still another application of graphene oxide in the antibacterial applications.11 The composites containing spindle-shaped graphene oxide decorated with ZnO nanoparticles prevented bacterial proliferation and destroyed bacterial membrane by the release of Zn2+ and generation of abundant reactive oxygen species.11 The spindleshaped graphene oxide with a length of ~1.0 μm and diameter of 100 nm were elongated and sharp at both ends and were able to destroy the cell integrity by slicing the cell resulting in cell death.11 The rough surface of spindle-shaped graphene oxide enhanced the antimicrobial activity by damaging the outer membrane of cells through attrition and rubbing.11 The exposure of bacterial membranes to Zn2+, which was generated from ZnO, resulted in protein solidification and synthetase inactivation.11 Finally, the contact between the negatively charged bacterial membrane and Zn2+ caused a strong Coulombic force which caused the bacteria to lose the ability to divide and proliferate.11 Numerous review papers contain more information on the subject.12-14 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Liu, Y; Wen, J; Gao, Y; Li, T; Wang, H; Yan, H; Niu, B; Guo, R, Appl. Surf. Sci., 436, 624-30, 2018. Sharma, A; Varshney, M; Nanda, SS; Shin, HJ; Kim, N; Yi, DK; Chae, K-H; Won, SO, Chem. Phys. Lett., 698, 85-92, 2018. Qiu, J; Liu, L; Zhu, H; Liu, X, Bioactive Mater., 3, 3, 341-6, 2018. Jovanović, S; Holclajtner-Antunović, I; Uskoković-Marković, S; Bajuk-Bogdanović, D; Pavlov, V; Tošić, D; Milenković, M; Todorović Marković, B, Mater. Chem. Phys., 213, 157-67, 2018. Sandhya, PK; Jose, J; Sreekala, MS; Padmanabhan, M; Kalarikkal, N; Thomas, S, Ceramics Int., 44, 13, 15092-98, 2018. Rani, MN; Ananda, S; Rangeppa, D, Mater. Today, 4, 11, 3, 12300-5, 2017. Sun, L; Yan, Z; Duan, Y; Zhang, J; Liu, B, Dental Mater., 34, 6, 115-27, 2018. Shuai, C; Guo, W; Wu, P; Yang, W; Hu, S; Xia, Y; Feng, P, Chem. Eng. J., 347, 322-33, 2018. Shih, S-J; Chen, C-Y; Lin, Y-C; Chung, R-J, Adv. Powder Technol., 27, 3, 1013-20, 2016. Li, Q; Yong, C; Cao, W; Wang, X; Wang, L; Zhou, J; Xing, X, J. Colloid Interface Sci., 511, 285-95, 2018. Zhong, L; Liu, H; Samal, M; Yun, K, J. Photochem. Photobiol. B: Biol., 183, 293-301, 2018. Zheng, H; Ma, R; Gao, M; Tian, X; Li, Y-Q; Zeng, L; Li, R, Sci. Bull., 63, 2, 133-42, 2018. Hegab, HM; ElMekawy, A; Zou, L; Mulcahy, D; Saint, CP; Ginic-Markovic, M, Carbon, 105, 362-76, 2016. Singh, DP; Herrera, CE; Singh, B; Singh, S; Singh, RK; Kumar, R, Mater. Sci. Eng.: C, 86, 173-97, 2018.

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6

Dispersion of Graphene in the Polymer Matrix In this chapter, the following aspects of the graphene dispersion process are discussed in the separate sections • dispersion methods • dispersion stability • morphology • spatial distribution • ribbons • folding • effects on properties in selected matrices

6.1 METHODS OF DISPERSION The free energy of any colloidal system is determined by both the interfacial area and tension.1 The theoretical surface area of a monolayer graphene is ~2590 m2 g-1.1 There is a limited range of conditions under which graphene can be dispersed, typically involving sonication and polar aprotic solvents.1 When the proper dispersion is accomplished, its stability requires that an energy barrier to aggregation is maintained by either electrostatic or steric repulsion which can be influenced either by a solvent selection or the graphene modification.1 The dispersibility of graphene depends on its source, as follows graphene oxide > reduced graphene oxide > pristine graphene.1 The chemical functionalization of graphene usually improves its dispersibility.1 Several solvents were identified as particularly useful in graphene dispersion, including N-methyl-2-pyrrolidone, dimethylsulfoxide, dimethylformamide, and ionic liquids (NMP ~ DMSO > DMR > water).1 Graphene oxide is the most frequently dispersed in water.1 Dispersibility of the reduced graphene oxide in water is poor, but it was demonstrated that it can be dispersed in water.1 Sonication assists process of dispersion in solvents and water.1 The functionalization methods of graphene to aid its dispersion include π−π interaction (e.g., polyaromatic hydrocarbons, peptides, conducting polymers, etc. used with pristine graphene and reduced graphene oxide), cation π interactions (e.g., between reduced graphene oxide and monovalent inorganic cations; imidazolium cations), surfactants, nanoparticles (e.g., silica, clay, titanium dioxide, etc.), and functionalization of graphene oxide prior to its reduction.1 Functionalization frequently causes damage to the regular structure of graphene similar to its oxidation and reduction.1

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Sonochemistry is used in cleaning and extraction but its scope has been expanded to the synthesis of nanoparticles.2 Sonication typically uses ultrasound waves with frequencies of 20 kHz (20,000 cycles per second) or higher.2 The ultrasonication has been employed in the graphene synthesis and its dispersion in various solvents to shorten the time of synthesis which otherwise would take days and result in a poor yield.2 Graphene oxide and reduced graphene oxide can be produced in a form of single layer.2 Oxidation to graphene oxide can be carried out within minutes whereas reduction can be done without any reducing agents. Graphite can be directly exfoliated to graphene layers.2 Various geometries of graphene including scrolled graphene, sponge or foam graphene, smooth or having rough edges, etc. can be produced using ultrasonication.2 Reactions which take hours and days can be completed in minutes with very high yields.2 A review paper provides fundamental concepts of ultrasonochemistry for the synthesis of graphene, its dispersion, exfoliation, and functionalization.2 The parameters of sonication such as frequency, power input, sonication time, type of sonication, and temperature are discussed in relation to the formation of the resultant graphene products.2 Sonication, calendering, and high shear forces were used to compare induced defects and size changes.3 The high shear forces caused the extension of the graphene nanoplatelets whereas sonication induced sheet wrinkling.3 Calendering caused the separation of the stacked nanoplatelets their dispersion in the matrix and the extension of the sheets.3 Figure 6.1 shows the morphology of graphene processed by three methods. Residual stresses were induced in the nanoplatelets structure showing an increase in the Raman intensities ratios ID/IG.3 Sonication and three-roll milling were used to separate the graphene nanoplatelets and disperse them in an epoxy resin.4 Folding and wrinkling were the Figure 6.1. TEM micrographs of a graphene platelets in (a) sonimajor modes of deformation, cated and (b) calendering cured samples and (c) graphene platelets schematic diagram. [Adapted, by permission, from Moriche, R; while tearing and peeling were the Prolongo, SG; Sanchez, M; Jimenez-Suarez, A; Sayagues, MJ; dominant modes of fracture.4 FigUrena, A, Compos. Part B: Eng., 72, 199-205, 2015.] ure 6.2 shows examples of folding. Folding by sliding is illustrated in Figures 6.2a&b.4 Figures 6.2c&d illustrate buckling mode of folding.4 Under some conditions, the compressive stress leads to layer splitting and buckling.4 Once buckling occurs, the inner layers undergo kinking at the point of folding.4 The sliding mode is more likely to occur in smaller graphene platelets, and buckling

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mode is more likely in the case of folded graphene platelets.4 The “freeze-drying masterbatch” method was used to promote dispersion of graphene in poly(lactic acid).5 The graphene oxide sheets were first dispersed in poly(ethylene glycol) aqueous solution then hydrazine hydrate was added to transform the graphene oxide sheets into the reduced graphene sheets, and the resulting mixture was freeze-dried to obtain the masterbatch of poly(ethylene glycol) and Figure 6.2. Two modes of folding. (a) The graphene layers drape graphene.5 The masterbatch was smoothly around the point of folding. (b) Illustration of the sliding then melt blended with poly(lacmode. (c) Buckling of inner layers at the point of folding. tic acid).5 The excellent dispersion (d) Illustration of the buckling mode in reduced graphene oxide. [Adapted, by permission, from Kuo, W-S; Tai, N-H; Chang, T-W, improved nucleation and crystalliCompos. Part A: Sci. Manuf., 51, 56-61, 2013.] zation which, in turn, the improved mechanical performance of the composite.5 The graphene quantum dots generated by electrooxidation of graphene had a uniform size of 35 nm.6 Larger graphene quantum dots with an average diameter of 52 nm were obtained using a similar method of electrooxidation of graphene film.6 Figure 6.3 comFigure 6.3. TEM images of graphene (a) and graphene quantum pares the morphology of graphene dots (b). [Adapted, by permission, from Luo, P; Guan, X; Yu, Y; Li, and quantum dots. X, Chem. Phys. Lett., 690, 129-32, 2017.] The oxidation level of graphene has a significant impact on its dispersion in the epoxy matrix.7 Carbonyl, carboxylic, and phenolic groups are the primary functional groups at the beginning of the oxidation process.7 As the oxidation progresses, the ether, epoxy, and aliphatic hydroxyl groups gradually become dominant at the surfaces of graphene.7 Graphene oxide with the low and high oxidation levels tend to have low degree of dispersion and even forms aggregates.7 The oxygen-containing groups on graphene oxide, such as epoxy, carboxyl, and ketone groups readily react with amines, and, therefore, graphene oxide performs better when it is first dispersed in the curing agent (isophorone diamine).7 The addition of graphene oxide can accelerate the curing reaction and decrease the pot-life of the epoxy matrix due to the presence of reactive hydrogen on graphene oxide surfaces.7 Also, the incorporation of graphene oxide decreases the crosslinking density of epoxy matrix and

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results in the decreased Tg values.7 This shows that the filler dispersion is one of a few factors controlling performance of cured epoxy.7 Oxidized graphene bears hydroxyl, carboxyl and epoxy functionalities which may serve as “anchoring sites” in further chemical reactions.8 Cationic surfactants are capable of π-cation interactions between the π electron cloud of the sp2 hybridized C atoms of the graphitic layer of graphene.8 The π-cation interaction has the electrostatic origin.8 The π-π interactions, causing stacking, are weaker than the π-cation interactions and the last ones are stronger than hydrogen bonding.8 The cetyltrimethyl ammonium bromide, participating in π-cation interactions with oxidized graphene markedly influenced its dispersion and mechanical and viscoelastic properties of natural rubber nanocomposites.8 The cetyltrimethyl ammonium bromide strongly influenced the filler-filler and rubber-filler interactions.8 Surfactants are used for dispersing graphene and functionalized graphene sheets in colloidal suspensions.9 The dispersions of graphene in aqueous solutions will eventually form irreversible agglomerates by restacking to graphitic structures due to the effect of attractive van der Waals forces.9 To prevent agglomeration, the electrostatic, steric, and electrosteric stabilization mechanisms have been adopted.9 In suspensions, where the surfactant does not strongly bind to graphene, the surfactant may detach from the graphene surface during aging and cause graphene restacking and formation of the graphene aggregates.9 The surfactants must provide long-range electrostatic or steric repulsion.9 The ionic surfactants having strong interfacial binding and large molecular weight increase the dispersing power by over an order of magnitude.9 Sodium dodecyl sulfonate and carboxyl methyl cellulose were used as surfactants to prepare graphene/water nanofluids.10 The external electromagnetic field enhanced dispersion by driving graphenes to move towards the solidification interface.10 The magnetic field effect increased with the amount of surfactant adsorbed on the graphenes increasing.10 The aqueous graphene suspensions containing different surfactants were used to fabricate graphene films by two economic techniques: spray and drop coating.11 The performances of four types of surfactants, including a cationic type surfactant tetradecyltrimethylammonium bromide, non-ionic type nonylphenylether, an anionic type sodium dodecyl sulfate, and a polymeric type polycarboxylate were compared.11 The nonionic type surfactant, nonylphenylether, at a concentration of 200-300 ppm gave the best performance.11 The reduction of graphene oxide leads to the re-stacking/agglomeration of graphene layers followed by precipitation from an aqueous dispersion.12 A laponite colloid can be used to prevent re-stacking and for stabilization of an aqueous dispersion of reduced graphene oxide. This can be done by dispersing graphene oxide in the colloid, followed by reduction with hydrazine hydrate.12 Results indicate that re-stacking can be prevented. Electrostatic interaction between negatively-charged reduced graphene oxide layers and positively-charged laponite edges lead to an intercalated structure.12 Figure 6.4 shows a schematic explanation of the mechanism of intercalation by laponite.12 The surface area of the laponite-containing reduced graphene oxide (1:1) was significantly increased by 17.6%, compared to the reduced graphene oxide.12

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Figure 6.4. Schematics of the evolution and the structure of reduced graphene oxide and laponite-containing reduced graphene oxide. [Adapted, by permission, from Li, J; Cui, J-c; Yang, Z-z; Qiu, H-x; Tang, Z-h; Yang, J-h, New Carbon Mater., 33, 1, 19-25, 2018.]

A nearly complete exfoliation of graphene may be obtained at concentrations up to 4.9 wt% in water.13 Molecules and polymers exhibiting delocalized π-systems emanating from single to multiple ring structures have binding affinities for sp2 graphene surfaces. Imidazolium-based polymeric surfactants and nanolattices give the highest degree of graphene dispersion.13 Graphene oxide had excellent solubility in N-methyl-2-pyrrolidone and a slight precipitation in N,N-dimethylformamide, water, and ethylene glycol.14 The graphene oxide dispersions were stable in nonpolar solvents, such as toluene, chlorobenzene, and odichlorobenzene.14 Reduced graphene oxide formed very good dispersions in N-methyl-2pyrrolidone, water, and ethylene glycol.14 The relatively stable aqueous solutions of graphene oxide and reduced graphene oxide can be attributed to the electrostatic repulsion due to the negatively charged graphene oxide and reduced graphene oxide sheets when they are dispersed in water.14 The reduced graphene oxide presented greater interaction with nonpolar solvents (chloroform, toluene, chlorobenzene) than the graphene oxide.14 The mixing of colloidal polymer particles and graphene oxide sheets resulted in molecular level dispersion because colloidal polymers stabilized dispersion of graphene sheets.15 Surface polarity of the polymer was crucial for the successful stabilization of graphene oxide layers.15 The presence of colloidal particles at the surface of graphene prohibited restacking and agglomeration of nanolayers during reduction.15 The mixing sequence affected dispersion of graphene oxide in polycarbonate/polymethylmethacrylate blend nanocomposite obtained by melt compounding under the controlled temperature and pressure.16 The best performance was obtained when graphene oxide was first mixed with PMMA, and then with PC.16 The incorporation of 1 wt% of graphene oxide resulted in excellent interfacial adhesion and uniform load transfer at the

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interfaces of polymer blend nanocomposites.16 Figure 6.5 shows a proposed mechanism of blend compatibilization.16 Vacuum filtration was the most effective method of dispersant removal, regardless of the dispersant type, removing up to 95 wt% of the polymeric dispersant with only 7.4 wt% decrease in graphene content.17 Dialysis also removed a significant fraction (70 Figure 6.5. Possible interaction between graphene oxide and blend wt% of polymeric dispersant) of polymers. [Adapted, by permission, from Tiwari, ST; Hatui, G; Oraon, R; Adhikari, AD; Nayak, GC, Current Appl. Phys., 17, un-adsorbed dispersant without 1158-68, 2017.] disturbing the dispersion quality.17 Liquid-phase exfoliation methods are effective for the production of a large volume of graphene.18 Organic solvents and aqueous surfactant solutions are used successfully, but mechanisms of exfoliation and stabilization processes are still ambiguous.18 Molecular simulations which may offer comprehensive microscopic insight into processing mechanisms are the subject of a comprehensive review.18 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Johnson, DW; Dobson, BP; Coleman, KS, Current Opinion, Colloid Interface Sci., 20, 367-82, 2015. Muthoosamy, K; Manickam, S, Ultrasonics Sonochem., 39, 478-93, 2017. Moriche, R; Prolongo, SG; Sanchez, M; Jimenez-Suarez, A; Sayagues, MJ; Urena, A, Compos. Part B: Eng., 72, 199-205, 2015. Kuo, W-S; Tai, N-H; Chang, T-W, Compos. Part A: Sci. Manuf., 51, 56-61, 2013. Liu, C; Ye, S; Feng, J, Compos. Sci. Technol., 144, 215-22, 2017. Luo, P; Guan, X; Yu, Y; Li, X, Chem. Phys. Lett., 690, 129-32, 2017. Wei, Y; Hu, X; Jiang, Q; Sun, Z; Wang, P; Qiu, Y; Liu, W, Compos. Sci. Technol., 161, 74-84, 2018. Berki, P; Khang, DQ; Minh, DQ; Hai, LN; Tung, NT; Karger-Kocsis, J, Polym. Testing, 67, 46-54, 2018. Lee, YJ; Huang, L; Wang, H; Sushko, ML; Schwenzer, B; Aksay, IA; Liu, J, Colloids Interface Sci. Commun., 8, 1-5, 2015. Jia, L; Chen, Y; Lei, S; Mo, S; Liu, Z; Shao, Energ. Procedia, 61, 1348-51, 2014. Pu, N-W; Wang, C-A; Liu, Y-M; Sung, Y; Wang, D-S; Ger, M-D, J. Taiwan Inst. Chem. Eng., 43, 140-6, 2012. Li, J; Cui, J-c; Yang, Z-z; Qiu, H-x; Tang, Z-h; Yang, J-h, New Carbon Mater., 33, 1, 19-25, 2018. Texter, J, Current Opinion Colloid Interface Sci., 19, 163-74, 2014. Konios, D; Stylianakis, MM; Stratakis, E; Kymakis, E, J. Colloid Interface Sci., 430, 109-12, 2014. Gudarzi, MM; Sharif, F, J. Colloid Interface Sci., 366, 44-50, 2012. Tiwari, ST; Hatui, G; Oraon, R; Adhikari, AD; Nayak, GC, Current Appl. Phys., 17, 1158-68, 2017. Irin, F; Hansen, MJ; Bari, R; Parviz, D; Metzler, SD; Bhattacharia, SK; Green, MJ, J. Colloid Interface Sci., 446, 282-9, 2015. Yang, J; Yang, X; Li, Y, Current Opinion Colloid Interface Sci., 20, 339-45, 2015.

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Figure 6.6. Intercalation of graphene. PTCA − 3,4,9,10-perylenetetra-carboxylic acid, GE − graphene, PHP − perylene-polyglycidol, HP − hyperbranched polyglycidol. [Adapted, by permission, from Liu, Y; Zhu, E; Bian, L; Hai, J; Tang, J; Tang, W, Mater. Lett., 118, 188-91, 2014.]

6.2 STABILITY OF DISPERSIONS Figure 6.4 shows the mechanism of stabilization of graphene dispersion.1 After centrifugation, laponite-containing dispersion remained suspended whereas the reduced graphene oxide, which did not contain laponite, precipitated.1 Zeta potential of water suspension of graphene oxide was -43.1 mV.1 Particles were less charged (-21.1 mV) due to the elimination of oxygen-containing functional groups when graphene oxide was reduced and charge was increased on addition of laponite to -36.2 mV.1 The intercalation of perylene-polyglycidol into graphene multilayers was achieved due to π−π interaction between perylene core and the multilayer graphene structure (Figure 6.6).2 The composite exhibited stable dispersion in both water and dimethylformamide even after 2 months.2 Graphene oxide nanoplatelets were used to stabilize oil/water emulsion.3 The emulsions remained stable for 1 year.3 The droplet sizes were as small as 1 μm with a low nanoplatelet concentration of 0.2 wt% (even with as low concentration as 0.001 wt%).3 The stability of the emulsion even at high salinity have been attributed to the high anion density at the graphene oxide nanoplatelet edges which protruded into the water phase.3

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Figure 6.7. Schematic illustrations of the formation process of edge-oxidized graphite (A), edge-oxidized graphene nanosheets (B, C) and edge-oxidized graphene nanosheet film (D, E). [Adapted, by permission, from Bai, M; Chen, J; Wu, W; Zeng, X; Wang, J; Zou, H, Colloids Surf. A: Physicochem. Eng. Aspects, 490, 59-66, 2016.]

Graphene oxide dispersed nanofluids were used in the direct absorption solar collectors.4 The sedimentation experiment and zeta potential showed that the graphene oxide/ water nanofluids containing 0.001–0.10% graphene oxide had an excellent long-term stability and were stable at elevated temperatures (30-70oC).4 This excellent dispersion resulted in a remarkable enhancement of optical absorption (reduced transmittance in the wavelength range from 220 to 2000 nm).4 Nanofluids containing graphene oxide nanoplatelets/deionized water were used as the working fluid for low-temperature direct absorption solar collectors.5 Calculation of absorbed energy's fraction suggested that the nanofluid layer with a weight percentage of 0.045 graphene oxide and height of 3 cm had the ability to absorb 99.6% of solar energy.5 The increase in the weight percentage of nanofluid and increase in temperature improved nanofluid's thermal properties.5 A nanofluid with a zeta potential more than +30 and less than -30 had good stability.5 The 0.015 wt% graphene oxide nanofluid/deionized water maintained its stability for 340 days without stirring or shaking.5 The oxidation of graphite with low concentration KMnO4 led to the formation of edge-oxidized graphite which preserved high crystalline graphitic structure on the basal planes while the edges were functionalized by oxygen-containing groups.6 Sonication of such oxidized graphite in water in the presence of sodium cholate as a stabilizer yielded dispersion at a concentration of up to 0.59 mg/ml.6 The dispersion was stable at room temperature for more than 6 months.6 Figure 6.7 shows a schematic diagram of the process.6 The high stability of the dispersion resulted from the steric and electrostatic repulsion between edge-oxidized graphite provided by sodium cholate which prevented the sheets

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Figure 6.8. Effect of exfoliation method on dispersion stability. [Adapted, by permission, from Chen, L; Li, N; Zhang, M; Li, P; Lin, Z, J. Solid State Chem., 249, 9-14, 2017.]

Figure 6.9. Modification of graphene. [Adapted, by permission, from Liu, J; Xu, C; Chen, LL; Fang, X; Zhang, Z, Solar Energ. Mater. Solar Cells, 170, 219-32, 2017.]

from the formation of irreversible agglomerates or even restacking to form graphite by van der Waals interactions.6 Three Hummers methods were used for preparation of graphene oxide: Hummers method, its modification (double addition of KMnO4), and improved Hummers method (phosphoric acid used for dispersion, higher reaction temperature, and double amount of KMnO4 added in one shot).7 Graphene dispersion obtained by improved method was the most stable because exfoliation by this method was more complete (Figure 6.8).7 The dispersion stability of graphene in the ionic liquid used as working fluids in medium- and high-temperature direct absorption solar collectors has been studied.8 The modified graphene (grafted with chains similar to 1-hexyl-3-methylimidazolium tetrafluoroborate − Figure 6.9) was dispersed in 1-hexyl-3-methylimidazolium tetrafluoroborate.8 The nanofluid exhibited good dispersion stability even after heating.8

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Dispersion of Graphene in the Polymer Matrix

Figure 6.10. Dispersion of graphene in water and ethanol. [Adapted, by permission, from Perumal, S; Park, KT; Lee, HM; Cheong, IW, J. Colloid Interface Sci., 464, 25-35, 2016.]

Figure 6.11. Use of surfactant in dispersion of reduced graphene oxide. [Adapted, by permission, from Uddin, ME; Kuila, T; Nayak, GC; Kim, NH; Ku, B-C; Lee, JH, J. Alloys Comp., 562, 134-42, 2013.]

The dispersion of graphene in methanol which was previously exfoliated using supercritical carbon dioxide was stabilized using poly(2,2,2-trifluoroethyl methacrylate)block-poly(4-vinylpyridine).9 The presence of a small amount of copolymer retarded precipitation of graphene and resulted in the formation of a stable dispersion.9 The amphiphilic copolymers of poly(4-vinyl pyridine)-block-poly(ethylene oxide) can be used to disperse graphene via sonication and centrifugation.10 Ethanolic and aqueous highly-ordered pyrolytic graphite dispersions with block copolymers were prepared, and they were compared with the dispersions stabilized by Pluronic (P123) and poly(styrene)-block-poly(ethylene oxide).10 Figure 6.10 compares the dispersion of graphene in the presence of the copolymer in water and ethanol.10 The amphiphilic copolymers of poly(4-vinyl pyridine)-block-poly(ethylene oxide) were much better dispersants than other two products studied.10 The longer 4-vinyl pyridine block improves dispersion.10 Delocalization of the 4-vinyl pyridine electron lone pair with graphene led to an increase in the N1s binding energy and increased stability.10

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Figure 6.12. Schematic illustration of synthesis of two-dimensional zirconium phosphate nanoplatelets (a), debundling reduced graphene oxide in liquid phase (b), dispersing and assembling reduced graphene oxide in chiral nematic liquid crystals by zirconium phosphate nanoplatelets. [Adapted, by permission, from Lin, P; Yan, Q; Chen, Y; Chen, Z, Chem. Eng. J., 334, 1023-33, 2018.]

The graphene dispersions were obtained by liquid-phase exfoliation using amphiphilic diblock copolymers; poly(ethylene oxide)-block-poly(styrene), poly(ethylene oxide)-block-poly(4-vinylpyridine), and poly(ethylene oxide)-block-poly(pyrenemethyl methacrylate) having similar block lengths.11 The graphite platelets and reduced graphene oxide were used as graphene sources.11 The graphene dispersions with poly(ethylene oxide)-block-poly(4-vinylpyridine) were much more stable, and graphene had fewer defects than these with two other diblock copolymers, as confirmed by turbidity and Raman analyses.11 The graphene concentration of up to 1.7 mg/mL has been obtained.11 Sodium dodecyl benzene sulfonate (1), sodium dodecyl sulfate (2), and 4-(1,1,3,3tetramethylbutyl) phenyl-polyethylene glycol (3) were used as ionic and non-ionic surfactants in preparation of water-dispersible reduced graphene (Figure 6.11).12 The dispersion stability of the surface modified graphene at surfactant:graphene oxide ratios of 0.5:1 and 1:1 were found to be stable for more than 90 days.12 The average thicknesses of 1.23 nm, 1.28 nm and 1.62 nm were measured by AFM for surfactants 1, 2, and 3, respectively at ratio 0.5:1.12

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A two-dimensional charged zirconium phosphate nanoplatelet surfactant was used to construct reduced graphene oxide 3D chiral architecture.13 The reduced graphene oxide suspensions changed from inhomogeneous non-Newtonian nanofluids to homogeneous Newtonian nanofluids after exfoliation in the presence of surfactant nanoplatelets.13 The dispersion stability was improved with the increased content of surfactant (required mass ratio of surfactant to the reduced graphene oxide was eight).13 Figure 6.12 illustrates the synthesis of surfactant and stages of dispersion. 13 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13

Li, J; Cui, J-c; Yang, Z-z; Qiu, H-x; Tang, Z-h; Yang, J-h, New Carbon Mater., 33, 1, 19-25, 2018. Liu, Y; Zhu, E; Bian, L; Hai, J; Tang, J; Tang, W, Mater. Lett., 118, 188-91, 2014. Yoon, KY; An, SJ; Chen, Y; Lee, JH; Bryant, SL; Ruoff, RS; Huh, C; Johnston, KP, J. Colloid Interface Sci., 403, 1-6, 2013. Chen, L; Xu, C; Liu, J; Fang, X; Zhang, Z, Solar Energy, 148, 17-24, 2017. Khosrojerdi, S; Lavasani, AM; Vakili, M, Solar Ener. Mater. Solar Cells, 164, 32-39, 2017. Bai, M; Chen, J; Wu, W; Zeng, X; Wang, J; Zou, H, Colloids Surf. A: Physicochem. Eng. Aspects, 490, 59-66, 2016. Chen, L; Li, N; Zhang, M; Li, P; Lin, Z, J. Solid State Chem., 249, 9-14, 2017. Liu, J; Xu, C; Chen, LL; Fang, X; Zhang, Z, Solar Ener. Mater. Solar Cells, 170, 219-32, 2017. Kim, YH; Lee, HM; Choi, SW; Cheng, IW, J. Colloid Interface Sci., 510, 162-171, 2018. Perumal, S; Park, KT; Lee, HM; Cheong, IW, J. Colloid Interface Sci., 464, 25-35, 2016. Perumal, S; Lee, HM; Cheong, IW, J. Colloid Interface Sci., 497, 359-67, 2017. Uddin, ME; Kuila, T; Nayak, GC; Kim, NH; Ku, B-C; Lee, JH, J. Alloys Comp., 562, 134-42, 2013. Lin, P; Yan, Q; Chen, Y; Chen, Z, Chem. Eng. J., 334, 1023-33, 2018.

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6.3 DISPERSION MORPHOLOGY The morphology of the reinforcement plays a crucial role in the development of the mechanical properties of polymer-based nanocomposites.1 An aqueous environment is not suitable for graphene dispersion.2 The dispersed graphene collapsed rapidly over each other. The aggregate produced from collapsed graphene had an ill-defined structure with multiple joints/links of varying strengths.2 Agitation required to disperse them in a suitable solvent further increased the scatter in graphene layer numbers.2 The effect of dispersion medium on the morphological properties of graphene oxidebased thin films was examined using Hansen solubility parameters.3 Graphene oxide had the following values of Hansen solubility parameters: δD = 16.6, δP = 14.1, δH = 14.9.3 DMF, DMSO, ethanol, NMP, water, and acetone were used in the study of dispersion media.3 Water, a frequently used graphene oxide dispersion medium, did not produce the highest graphene oxide surface coverage.3 Both protic and aprotic solvents were among those which were equally effective at dispersing graphene oxide. Ethanol, DMF, DMSO were the bestdispersing media.3 All three solvents had RED less than 1.3 Polypropylene-graft-maleic anhydride was used as a compatibilizer to achieve uniform dispersion of graphene in polypropylene.4 Raman study showed the strong interfacial interaction between the surface of functionalized graphene sheets and the compatibilizer.4 The morphological study indicated the homogeneous dispersion of graphene in polypropylene.4 The orientation control of graphene significantly influenced the properties of the composite.5 The highly aligned, large graphene oxide platelets were incorporated in polydimethylsiloxane matrix exploiting liquid crystallinity.5 Graphene oxide treated with two types of surfactants, i.e., silane coupling agent (KH550) and 4,4’-diphenylmethane diisocyanate was incorporated Figure 6.13. TEM images of pristine graphene oxide (a,b), KH550 modified graphene oxide (c,d), and MDI modified graphene oxide into phenyl silicone rubber at a (e,f) at a nanofiller content of 0.06 wt% in the phenyl silicone rublow concentration (≤0.2 wt%).6 ber matrix. [Adapted, by permission, from Xu, Y; Gao, Q; Liang, H; The particle size changed after Zheng, K, Polym. Testing, 54, 168-75, 2016.]

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Figure 6.14. SEM images of graphene oxide (a,b) and TiO2–graphene oxide hybrids (c,d). TEM images of (e) graphene oxide and (f) TiO2–graphene oxide hybrids. [Adapted, by permission, from Yu, Z; Di, H; Ma, Y; He, Y; Liang, L; Lv, L; Ran, X; Pan, Y; Luo, Z, Surf. Coat. Technol., 276, 471-8, 2015.]

modification and the modified graphene oxide dispersed well in the phenyl silicone rubber.6 The untreated graphene oxide was stacked layer by layer in the composite.6 The dispersion size of pristine graphene oxide in the silicone rubber was approximately 30-80 nm (Figure 6.13a,b).6 The dispersion size of KH550 modified graphene oxide in the silicone rubber (Figure 6.13c,d) of approximately 20-40 nm was smaller than that of pristine graphene oxide, which implies that the modification was effective.6 Figure 6.13e,f shows that MDI modified graphene oxide arranged row by row in a regular manner in the com-

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posites which implied that the agglomeration structure of graphene oxide was detached and that MDI modified graphene oxide led to a better dispersion than KH550 modified graphene oxide.6 TiO2-graphene oxide sheet hybrids were obtained using titanium dioxide loading on graphene oxide sheets with the help of 3-aminopropyltriethoxysilane, and dispersing the sheets into epoxy resin at a low weight fraction of 2%.7 The corrosion performance of the coating was enhanced by the addition of the hybrid which plugged micropores of epoxy structure.7 Figure 6.14 shows SEM images of graphene oxide and its hybrids.7 Figure 6.14a&b shows the morphology of graphene oxide.7 With nano-TiO2 introduced to graphene oxide, the nanocomposite showed a dispersion of TiO2 particles on the surface of graphene oxide (Figure 6.14c&d).7 Its morphology indicated that the lamella of graphene oxide was not destroyed in the decorating process.7 Figure 6.14e&f depicted the HR-TEM images of graphene oxide and TiO2-graphene oxide, respectively.7 Polymethylmethacrylate/reduced graphene oxide composites were obtained by latex mixing of anionic PMMA latex particles and graphene oxide dispersion, followed by coagulation, and in situ hydrazine reduction.8 The morphological evaluation showed a uniform distribution of graphene in the polymer matrix.8 REFERENCES 1 2 3 4 5 6 7 8

Duan, K; Li, Y; Li, L; Hu, Y; Wang, X, Mater. Design, 147, 11-8, 2018. Mir, A; Shukla, A, Appl. Surf. Sci., 443, 157-66, 2018. Gallerneault, M; Truica-Marasescu, F; Docoslis, A, Surf. Coat., Technol., 334, 196-203, 2018. Li, C-Q; Zha, J-W; Long, H-Q; Wang, S-J; Zhang, D-L; Dang, Z-M, Compos. Sci. Technol., 153, 111-8, 2017. Lee, KE; Oh, JJ; Yun, T; Kim, SO, J. Solid State Chem., 224, 115-9, 2015. Xu, Y; Gao, Q; Liang, H; Zheng, K, Polym. Testing, 54, 168-75, 2016. Yu, Z; Di, H; Ma, Y; He, Y; Liang, L; Lv, L; Ran, X; Pan, Y; Luo, Z, Surf. Coat. Technol., 276, 471-8, 2015. Lin, Y; Liu, Y; Zhang, D; Chen, C; Wu, G, Chem. Eng. J., 315, 516-26, 2017.

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Figure 6.15. Schematic illustration of the preparation of hydrogel and aerogel. [Adapted, by permission, from Lu, K-Q; Yuan, L; Xin, X; Xu, Y-J, Appl. Catalysis B: Env., 226, 16-22, 2018.]

Figure 6.16. Synthesis of 3D composite. [Adapted, by permission, from Yang, G; Wang, Y; Xu, H; Zhou, S; Jia, S; Zang, J, Appl. Surf. Sci., 447, 837-44, 2018.]

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6.4 SPATIAL CONFIGURATIONS OF GRAPHENE SHEETS Metal-free 3D aerogel composites were obtained from highly conductive commercial Elicarb graphene with graphene oxide acting as a “macromolecular surfactant”.1 This was done to overcome restrictions imposed by the intrinsic population of defects and disruption of 2D π-conjugation in the domains of the reduced graphene oxide sheets.1 Figure 6.15 illustrates the method of preparation of 3D hydrogel and aerogel.1 Graphene was dispersed using ultrasound, and graphene oxide played a role of surfactant.1 Subsequently, graphene oxide was converted to reduced graphene oxide resulting in a product without any unwanted impurity and generating a clean, electrically addressable carbon-carbon interfaces.1 The three-dimensional graphene-based structure has been built for the lightweight application using direct ink-writing technology.2 A homogeneous graphene dispersion obtained by ultrasonic dispersion of graphene in ethanol was used as an ink.2 A 3D printing technology has been used to manufacture of lightweight objects.2 Three-dimensional graphene/phenolic resin composites were synthesized via in-situ polymerization in graphene hydrogels.3 The water in the graphene hydrogel was replaced by infiltration of resole resin and the evaporation of water from the graphene hydrogel by heating.3 Figure 6.16 illustrates the process.3 The graphene oxide nanocomposites in PVA were prepared by a simple mixing and casting from aqueous solution.4 The intensity of scattering of the Raman band depended on the axis of laser polarization when the laser beam is parallel to the surface of the graphene plane.4 This effect was used to quantify the spatial orientation of the graphene.4 The typical thickness of 1 nm of the exfoliated graphene oxide monolayer was determined using atomic force microscopy.4 The diffraction peak of pure Figure 6.17. SEM image of the cross section of the 2 graphene oxide at 2θ=10.5° indicated dwt% graphene oxide/PVA. [Adapted, by permission, from Li, Z; Young, RJ; Wilson, NR; Kinloch, IA; spacing of 0.84 nm.4 SEM indicated that the Valles, C; Li, Z, Compos. Sci. Technol., 123, 125-33, graphene oxide flakes were aligned parallel 2016.]

Figure 6.18. Schematic illustrating the synthesis process of spongy bone-like graphene@SiC aerogels. [Adapted, by permission, from Jiang, Y; Chen, Y; Liu, Y-J; Sui, G-X, Chem. Eng. J., 337, 522-31, 2018.]

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the top surface of the nanocomposites film (Figure 6.17).4 The graphene oxide flakes have been aligned by a settlement after the water evaporated.4 At a higher concentration, a poorer orientation of the graphene oxide was observed.4 The spongy bone-like graphene oxide@SiC aerogels were fabricated by the directional freeze-casting of graphene oxide coated SiC whiskers slurry and thermal reduction of graphene oxide@SiC aerogels.5 The structure had two advantages such as low density (72 mg/cm3) and high microwave absorption.5 Figure 6.18 shows the method of structure preparation and Figure 6.19 characterizes the morphological features of its components and the full assembly.5 To obtain the spongy bone-like structure, the Figure 6.19. (a) Digital photographs of graphene oxide, GO@SiC suspension of SiC whiskers have and thermally annealed, TGO@SiC aerogels; (b) SEM image of SiC been maintained homogeneous whiskers; (c) SEM image showing the surface of GO@SiC aerogel; (d) SEM image and the element mapping of TGO@SiC aerogel; (e) during the freeze-casting. In this TEM image of TGO@SiC; (f) SEM image showing the typical hier- process, graphene oxide acted as a archically ordered structure of spongy bone-like graphene@SiC stabilizer which was well disaerogel. [Adapted, by permission, from Jiang, Y; Chen, Y; Liu, Y-J; persed in water due to a large Sui, G-X, Chem. Eng. J., 337, 522-31, 2018.] number of hydrophilic hydroxyl groups and negatively-charged oxygen-containing functional groups existing on the surface.5 The SiC has been absorbed onto the graphene oxide sheets because of the hydrogen bonding between the abundant surface hydroxyl groups and oxygen-containing groups. During freezing of the graphene oxide-coated SiC slurry, SiC whiskers and graphene oxide were ejected from the moving solidification front of growing ice, then stack up between the lamellar ice and a microporous network. After freeze-drying, the ice crystals were removed, and graphene oxide@SiC aerogel with the spongy bone-like structure remained.5 The SiC whiskers had an average length of 18 μm and diameter of 1 μm (Figure 6.19b). The stacked SiC whiskers were tightly wrapped with the corrugated graphene oxide film, forming the inorganic scaffolds (Figure 6.19c).5 The TEM image shows the rough and wrinkled surface of SiC whiskers indicating the TGO@SiC composite retains a well-wrapped architecture (Figure 6.19e).5 The aerogels present a highly interconnected hierarchical architecture with a large number of pores which have an average diameter of 80 μm (Figure 6.19f).5

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REFERENCES 1 2 3 4 5

Lu, K-Q; Yuan, L; Xin, X; Xu, Y-J, Appl. Catalysis B: Env., 226, 16-22, 2018. You, X; Yang, J; Feng, Q; Huang, K; Zhou, H; Hu, J; Dong, S, Int. J. Lightweight Mater. Manuf., in press, 2018. Yang, G; Wang, Y; Xu, H; Zhou, S; Jia, S; Zang, J, Appl. Surf. Sci., 447, 837-44, 2018. Li, Z; Young, RJ; Wilson, NR; Kinloch, IA; Valles, C; Li, Z, Compos. Sci. Technol., 123, 125-33, 2016. Jiang, Y; Chen, Y; Liu, Y-J; Sui, G-X, Chem. Eng. J., 337, 522-31, 2018.

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6.5 RIBBON SIZE The thermal conductivity of hybrid nanoribbon depends on its length.1 The anisotropy of thermal conductivity in graphene nanoribbons is caused by the edge effect.1 It becomes negligible when the graphene nanoribbon has an infinite size.1 Graphene can be effectively modified via surface functionalization, in which the carbon atoms are converted from sp2 to sp3 hybrids to bond with the adatoms.1 The tunable properties can be obtained by changing the hydrogen coverage and distribution.1 The thermal conductivity of graphene deteriorates with the increase of random hydrogen coverage.1 Due to interconnected structure and the synergistic effect of graphene ribbons and sheets, the graphene hybrid fiber shows the strength of 223 MPa, higher than any previously reported.2 The graphene hybrid fiber also exhibits an ultra-high toughness of 30 MJ m-3 which is again much higher than ever previously reported.2 Graphene ribbon can enhance transmission of light through a metallic grating.3 The enhancement factor can be adjusted by the Fermi energy level of graphene and change in geometrical parameters, including the width of graphene ribbon.3 A 154 fold enhancement was achieved.3 The resistivity of a ribbon increased as its width decreased, indicating the impact of edge states.4 Graphene nanoribbons narrower than 40 nm showed a distinct change of their electrical characteristics with temperature while wider ribbons did not.4 Graphene nanoribbons as narrow as 20 nm can be fabricated by e-beam lithography and etching techniques.4 Both boundary scattering and trapped charges in the substrate strongly affected the transport properties and minimum conductivity of the graphene nanoribbons.4 A confinement-induced gap of the order of 30 meV was inferred in the narrowest 20 nm ribbon.4 The electronic structure of finite-size graphenes depends on their shape and size.5 The electronic energy gap between graphene low spin ground state and the other high spin excited states decreased when its size increased.5 When graphene reached a certain critical size, the energy gap became sufficiently small for excited states to be thermally accessible.5 The energy gap between the electronic low spin ground state and its high spin excited state at stationary points along the CO desorption reaction was decreased to the thermally accessible range when the ribbon size increased.5 REFERENCES 1 2 3 4 5

Li, Y; Wei, A; Datta, D, Carbon, 113, 274-82, 2017. Sheng, L; Wei, T; Liang, Y; Jiang, L; Qu, L; Fan, Z, Carbon, 120, 17-22, 2017. Peng, Y-X; He, M-D; Li, Z-J; Wang, K-J; Li, S; Li, J-B; Liu, J-Q; Long, M; Hu, W-D; Chen, X, Optics Commun., 382, 86-92, 2017. Chen, Z; Lin, Y-M; Rooks, MJ; Avouris, P, Physica E: Low-dimensional Systems Nanostructures, 40, 2, 228-32, 2007. Pham, BQ; Nguyen, VH; Truong, TN, Carbon, 101, 16-21, 2016.

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Figure 6.20. SEM images of the one day-cured graphene/cement composite containing different silica fume content: (a) the composite without silica fume, (b-e) the composite with 15% of silica fume. [Adapted, by permission, from Bai, S; Jiang, L; Xu, N; Jin, M; Jiang, S, Constr. Build. Mater., 164, 433-41, 2018.]

6.6 RESULTS IN DIFFERENT MATRICES In this section, the effect of dispersion on graphene performance in various systems will be analyzed. The selected applications include cementitious products, polymers, and rubbers as well as the impact of dispersion on different properties such as mechanical performance, rheology, and tribological properties. This review is included to stress importance and benefits of a proper dispersion by the discussion of selected examples rather than to provide a representative account of publications in the field.

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Figure 6.21. Illustration of (a) Polycarboxylate superplasticizer molecule, (b) Agglomeration of graphene platelets, and (c) Dispersion of graphene platelets. [Adapted, by permission, from Du, H; Pang, SD, Constr. Build. Mater., 167, 403-13, 2018.]

Effective dispersion of graphene improves the mechanical performance of cement composites.1 Silica fume improved the dispersion of graphene and increased the interfacial strength between graphene and cement matrix.1 Figure 6.20 shows dispersion of graphene without and with fumed silica.1 Without silica fume, the length of the graphene agglomeration reached up to 100 μm (Figure 6.20a).1 With 15% silica fume in the formulation, the graphene clumps had a maximum size of less than 20 μm (Figure 6.20b).1 Fumed silica particles were inserted between the graphene layers and disrupted the interaction of graphene layers (Figure 6.20c-e).1 One hour of sonication and 15% of superplasticizer (polycarboxylate) was sufficient to produce a 1% graphene platelets suspension that was stable for 6 h.2 The addition of 1% graphene caused reduction of 37% and 30% of the effective porosity and critical pore diameter, respectively.2 Figure 6.21 illustrates the effect of addition of superplasticizer.2

Figure 6.22. Schematic illustration of preparation of a graphene/bacterial cellulose nanocomposite via layer-bylayer interface culture approach. [Adapted, by permission, from Luo, H; Dong, J; Xu, X; Wang, J; Yang, Z; Wan, Y, Composites Part A, 109, 290-7, 2018.]

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Figure 6.23. SEM images (the insets are the macrophotographs) and diameter distributions of conventional bacterial cellulose (a and b) and bacterial cellulose from this study (c and d). Photos of graphene/bacterial cellulose-2 samples with varying thickness (e). SEM images of surface (f), core (g), and cross-section (h) of graphene/bacterial cellulose-2. Magnified SEM image of graphene/bacterial cellulose-2 (i), showing the close bundling of graphene nanosheets by bacterial cellulose nanofibers. Proposed mechanism of formation of bundling of graphene nanosheets by bacterial cellulose nanofibers (j). SEM images of graphene/bacterial cellulose-1 (k) and graphene/bacterial cellulose-3 (l). [Adapted, by permission, from Luo, H; Dong, J; Xu, X; Wang, J; Yang, Z; Wan, Y, Composites Part A, 109, 290-7, 2018.]

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The dispersion of graphene oxide in water is independent of the sonication degree.3 Only a small amount of graphene oxide reagglomerates. Graphene oxide has excellent dis-

Figure 6.24. Schematic illustration of the green synthesis of reduced graphene oxide and fabrication of chitosan based hydrogels. [Adapted, by permission, from Kosowska, K; Domalik-Pyzik, P; Nocun, M; Chlopek, J, Mater. Chem. Phys., 216, 28-36, 2018.]

persibility in water.3 High alkalinity and calcium ions are key factors causing agglomeration of graphene oxide in cement systems.3 Polycarboxylate-based superplasticizers were the most suitable aids of graphene oxide dispersion.3 They ensured dispersion stability.3 The flexural strength of cement composite was increased by 67% with the addition of 0.03 wt% graphene oxide.3 The method of distribution of graphene nanosheets in three-dimensional bacterial cellulose matrix has been presented.4 The bacterial cellulose is a natural nanofibrous cellulosic material produced by the bacteria, Komagataeibacter xylinus (former name Acetobacter xylinum).4 Figure 6.22 illustrates the method of dispersion of graphene in bacterial cellulose matrix.4 Figure 6.23 illustrates the development of properties of the composite using this less commonly-known method of dispersion.4 The graphene/bacterial cellulose2 and graphene/bacterial cellulose-3 were obtained when the volume ratio of graphene suspension to culture medium was 1:3 and 1:1, respectively, as compared with graphene/ bacterial cellulose having ratio of 1:5.4 These micrographs demonstrate that a good dispersion can be maintained throughout all range of concentrations.4 A 91% improvement in tensile strength and a 279% improvement in tensile modulus over bare bacterial cellulose were observed for the graphene/bacterial cellulose hydrogel with 0.3 wt% graphene.4

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Figure 6.25. (a-c) TEM images of epoxy/graphene nanocomposites (0.8 wt% diazonium-functionalized graphene) at low magnification (a-b) and high magnification (c), and (d) XRD spectra of neat epoxy and epoxy/ graphene nanocomposites. [Adapted, by permission, from Yao, H; Hawkins, SA; Sue, H-J, Compos. Sci. Technol., 146, 161-8, 2017.]

Figure 6.24 illustrates three methods of graphene oxide transformation to the reduced graphene oxide using green reducing agents and hydrogel formation.5 Alkaline conditions were used to prevent agglomeration after dispersions of reduced graphene oxide in chitosan were prepared.5 Chitosan dispersion of reduced graphene oxide obtained by reaction with ascorbic acid was stable after one month of storage unlike the other two grades of reduced graphene oxide.5 A slurry compounding method was used for the preparation of chlorosulfonated polyethylene/graphene composites with well-dispersed graphene.6 The tea polyphenol reduced graphene/ethanol slurry was first produced by the solvent exchange which was melt-compounded with the polymer.6 The reduced graphene oxide was well dispersed in the matrix, and it had strong interfacial interaction with polymer.6 The composites containing 7 phr of reduced graphene oxide had a 160% and 400% increase in tensile strength and modulus, respectively, and 76% improvement in Akron abrasion resistance as compared with those of neat chlorosulfonated polyethylene.6 As in any other case, a good exfoliation, a homogeneous dispersion, and a strong affinity of graphene nanosheets to a polymer matrix are needed to produce well-performing composite from epoxy resin.7 The chemical functionalization of graphene oxide with 4-nitrobenzenediazonium salt was used to achieve a homogeneous dispersion of graphene nanosheets in epoxy resin.7 Figure 6.25 shows TEM images characterizing dispersion and XRD spectra.7 The nanosheets are individually and randomly dispersed in the epoxy matrix without detectable aggregation.7 The wide diffraction hump in the neat epoxy between 10 and 30o is caused by the scattering of the amorphous epoxy molecules.7

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Figure 6.26. TEM micrographs of composites obtained without (a) and with (b) injection of water. [Adapted, by permission, from Tong, J; Huang, H-X; Wu, M, Chem. Eng. J., 348, 693-703, 2018.]

Epoxy/diazonium-functionalized graphene nanocomposites exhibit similar diffraction patterns as the neat epoxy without showing other peaks corresponding to the graphene restacking which indicates high exfoliation and excellent dispersion.7 Dispersion of thermally reduced graphene in polyethylene was improved by blending with oxidized polyethylene which is well-known process aid.8 Improved dispersion of graphene in polyethylene/oxidized polyethylene blends substantially decreased percolation from both rheological (0.3 vol%) and electrical (0.13 vol%) measurements compared to a neat polyethylene nanocomposites (1 and 0.29 vol%), respectively.8 A ~3-fold increase in elastic properties of thermally reduced graphene-filled blends with a ~7-fold lower melt viscosity indicates the formation of processable blend nanocomposite with improved filler/matrix interactions.8 Dispersion and thermal reduction of graphene oxide to enhance the thermal conductivity of poly(vinylidene fluoride)/graphene nanocomposites was facilitated by water in a melt mixing extrusion.9 The injected water facilitated the dispersion and in situ thermal reduction of the unfunctionalized graphene oxide in the PVDF matrix.9 The injected water accelerated the removal of oxygen-containing groups especially C=O groups and the transformation of carbon sp3 into sp2 bonds.9 Figure 6.26 illustrates the influence of water injection on dispersion of graphene in PVDF matrix.9 Agglomerates were present when water was not used and dispersion was significantly improved when water was injected.9 A small number of graphene platelets was introduced into polystyrene through the one-step process (solution compounding) and two-step process (solution compounding and subsequent melt compounding).10 The first method produced poor dispersion which was inferior compared with the second method, but the morphological and microstructural characterization demonstrated that although solution compounding was not favorable for the homogeneous dispersion of graphene particles, it was supporting the construction of highly efficient conductive pathways.10 Consequently, the first method enhanced the thermal conductivity of the composite.10 Graphene/hydrogenated acrylonitrile-butadiene rubber nanocomposites were prepared via master batch by ultrasonically-assisted solution-mixing and subsequent conven-

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tional two-roll milling using 3 phr of multilayer graphene nanoparticles having 10 sheets.11 The dispersed multilayer graphene particles had a diameter of the plate area of 200±80 nm and thickness of 3-5 nm.11 The multilayer graphene had an average aspect ratio of 40.11 Curing, rheological and mechanical properties, and flame retardancy were improved.11 The natural rubber latex was modified with 5 phr graphene or graphene oxide in the presence and absence of cetyltrimethylammonium bromide to trigger π-cation interactions.12 Cetyltrimethylammonium bromide affected the reinforcing efficiency and mechanical loss of the natural rubber nanocomposites affecting changes in the natural rubber fraction immobilized on the filler's surfaces.12 Graphene nanoplatelet incorporation and dispersion state affected thermal, mechanical, and electrical properties of biodegradable matrices (polylactide and poly(butylene adipate-co-terephthalate)).13 The crystallinity of polylactide increased markedly on the addition of graphene nanoplatelets but decreased in poly(butylene adipate-co-terephthalate).13 Dispersion of graphene in poly(butylene adipate-co-terephthalate) was better than in polylactide.13 The relatively poorer dispersion appears to have a significant positive effect in enhancing the electrical conductivity of polylactide/graphene nanoplatelet nanocomposite.13 Figure 5.52 illustrates a complex relationships between flow characteristics, graphene oxide concentration in water dispersions, Peclet number, and distribution and dispersion of particles.14 Below critical concentration graphene oxide sheets are dispersed, and above they self-organize.14 At low Pe number, graphene oxide sheets self-aggregate, whereas at high Pe aggregates disassociate.14 Graphene oxide aggregates are reversibly formed once the flow is arrested; indicating that graphene oxide dispersions act as thixotropic fluids.14 The rheology of graphene oxide dispersion can be modified by the addition of salts or poly(ethylene glycol) to influence interactions between graphene oxide sheets.14 Graphene oxide can also be used as the rheological additive.15 Two lubricant additives were synthesized and characterized by the reaction between the carboxyl of graphene oxide and 1-dodecanethiol and tert-dodecyl mercaptan, respectively.15 Their dispersion stabilities and tribological properties in rapeseed oil were investigated.15 The dispersion stability and tribological properties of 1-dodecanethiol-reacted graphene oxide in rapeseed oil were superior to that of tert-dodecyl mercaptan-reacted graphene oxide.15 Both rheological additives formed the tribochemical film on the rubbing surface.15 Also, both additives decreased coefficient of friction (1-dodecanethiol-reacted graphene oxide by 44.5% and tert-dodecyl mercaptan-reacted graphene oxide by 40.1% at a concentration of 0.2 wt%).15 The applications of graphene oxide and graphene as Pickering stabilizers in emulsification, emulsion polymerization, and suspension polymerization are discussed in a review paper.16 These Pickering stabilizers are presented for advanced applications, including electro-rheological fluids, opto-rheological fluids, particles for supercapacitors, phase change materials, catalysis, and stabilizers.16

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

Bai, S; Jiang, L; Xu, N; Jin, M; Jiang, S, Constr. Build. Mater., 164, 433-41, 2018. Du, H; Pang, SD, Constr. Build. Mater., 167, 403-13, 2018. Chuah, SC; Li, W; Chen, SJ; Sanjayan, JG; Duan, WH, Constr. Build. Mater., 161, 519-27, 2018. Luo, H; Dong, J; Xu, X; Wang, J; Yang, Z; Wan, Y, Composites Part A, 109, 290-7, 2018. Kosowska, K; Domalik-Pyzik, P; Nocun, M; Chlopek, J, Mater. Chem. Phys., 216, 28-36, 2018. Yang, Z; Xu, Z; Zhang, L; Guo, B, Compos. Sci. Technol., 162, 156-62, 2018. Yao, H; Hawkins, SA; Sue, H-J, Compos. Sci. Technol., 146, 161-8, 2017. Igbal MZ; Abdal, AA; Mittal, V; Seifert, S; Herring, AM; Liberatore, MW, Polymer, 98, 143-55, 2016. Tong, J; Huang, H-X; Wu, M, Chem. Eng. J., 348, 693-703, 2018. Bai, Q-q; Wei, X; Yang, J-h; Zhang, N; Huang, T; Wang, Y; Zhou, Z-w, Composites Part A: 96, 89-98, 2017. Zirnstein, B; Tabaka, W; Frasca, D; Schulze, D; Schartel.B, Polym. Testing, 66, 268-79, 2018. Berki, P; Khang, DQ; Minh, DQ; Hai, LN; Tung, NT; Karger-Kocsis, J, Polym. Testing, 67, 46-54, 2018. Kashi, S; Gupta, RK; Kao, N; Hadigheh, A; Bhattacharya, SN, J. Mater. Sci. Technol., 34, 1026-34, 2018. Del Giudice, F; Shen, AQ, Current Opinion Chem. Eng., 16, 23-30, 2017. Zhang, G; Xu, Y; Xiang, X; Zheng, G; Zeng, X; Li, Z; Ren, T; Zhang, Y, Tribol. Int., 126, 39-48, 2018. Texter, J, Current Opinion Colloid Interface Sci., 20, 454-64, 2015.

7

Chemical Modifications and Their Applications 7.1 FUNCTIONAL GROUPS AND SIDE CHAINS The quantum chemical calculations based on the density functional theory were used to reveal the elementary oxygen-transfer processes that was consistent with the oxidation of sp2-hybridized carbon materials.1 The study aimed at understanding the mechanism of carboxyl group formation during functionalization of graphene.1 Carboxyl groups were formed by sequential hydroxyl attack at the carbon active sites which weakened and cleaved the Figure 7.1. Mechanisms of formation of carboxyl group. [Adapted, contiguous aromatic C−C bonds by permission, from Tajima, K; Isaka, T; Yamashima, T; Ohta, Y; (Figure 7.1).1 The carboxyl groups Matsuo, Y; Takai, K, Polyhedron, 136, 155-8, 2017.] are more likely to form at graphene edges than at defect sites within the basal plane, except of monolayer graphene.1 Direct titration with NaOH was used to determine the acidic functional group content in graphene oxide.2 The carboxylic acid group contents ranged from 0.80 to 1.70 mmol/g.2 The carboxylated graphene oxide prepared by the reaction with chloroacetic acid contained significantly more carboxylic acid groups (up to 4.65 mmol/g).2 The acidic groups on carbonaceous materials can be divided into two classes: stronger acids with pKa ˂ 7.0 (including carboxylic acids) and weaker acids with pKa ˃ 7.0 (including hydroxyl groups).2 The structure, dispersion, and surface properties of reduced graphene oxide sheets in epoxy matrix are controlled by the content of oxygen-containing functional groups.3 The epoxy-based composite coating that contains 0.25 wt% graphene sheets (reduced by hydrazine hydrate for one hour) exhibited excellent corrosion protection behavior owing to the synergism of the barrier effect, impenetrability, and hydrophobicity.3

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The graphene oxide was synthesized by Brodie and Hummers methods.4 The spin concentration of graphene oxide produced by Hummers method 19 Figure 7.2. Effect of functional group on spin magnetism. [Adapted, (Ns = 2×10 ) was larger than that by permission, from [Adapted, by permission, from Radovic, LR; of produced by Brodie method (Ns Mora-Vilches, CV; Salgado-Casanova, AJA; Buljan, A, Carbon, = 2×1018).4 Although similar 130, 340-9, 2018.] groups were detected by FTIR (hydroxyl, carboxyl, epoxy, and carbonyl) but the peak for C−OH stretching for Hummers graphene oxide was larger, and peak for C−O−C stretching was smaller than in the case of Brodie produced graphene oxide which explained the differences in spin concentration (Figure 7.2).4 Acryloyl-group on functionalized graphene led to a higher degree of dispersion in acrylate epoxidized soybean oil matrix, and it permitted generation of crosslinks during UV curing (Figure 7.3).5 Thermal and mechanical properties were improved due to the better dispersion and improved interaction with the matrix.5

Figure 7.3. Functionalized reduced graphene synthesis and its composite. [Adapted, by permission, from Dong, B; Yuan, Y; Luo, J; Dong, L; Liu, R; Liu, X, Prog. Org. Coat., 118, 57-65, 2018.]

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An electrochemical route is used to functionalize graphene nanosheets.6 The amidation level was affected by the applied potential and electrolyte.6 The N/C ratio can reach 8.4 at%.6 The scanning probe lithography and pulsed laser two-photon oxidation were used to oxidize the chemical vapor deposition grown graphene resulting in the formation of epoxy, ether, hydroxyl, carbonyl, and carboxyl functional groups.7 Three-dimensional graphene functionalized with cysteine was produced using L-cysteine as grafting modification agent.8 Three reactions were taking place:8 • The redox reaction −SH in cysteine and C−O−C in graphene oxide. The −OH and C−O−C were reduced, while the cysteine was oxidized to cystine which served as the template of three-dimensional graphene to inhibit the aggregation of graphene layers • Dehydration reactions occurred between −SH in cysteine and −COOH in cysteine, forming the −C(O)−S−C− bond to realize the graft modification • The amidation reaction occurred between −NH2 in cysteine and −COOH in graphene oxide, the formation of a peptide bond (−C(O)−N(H)−) to connect cysteine and graphene oxide. Through all these reactions, the monolithic three-dimensional functionalized graphene oxide was fabricated with cysteine and cysteine.8 The reactions promoted the crosslinking of graphene oxide nanosheets and induced the graphene oxide nanosheets to assemble into a 3D porous framework.8 With the increase of the mass ratio of cysteine to graphene oxide, more pores were formed, and the pore size became smaller.8 The oxygen-containing functional groups of graphene oxide assisted in hydrogen storage.9 The specific surface area, porosity, and the interactions of the functional groups with hydrogen molecules are important for hydrogen storage.9 Graphene oxide had better H2 uptake capacity (0.74 wt%) as compared to the reduced graphene oxide (0.47 wt%).9 The adsorption was promoted by surface functional groups, such as epoxy, hydroxyl, car-

Figure 7.4. Functionalization of graphene oxide with TDI. [Adapted, by permission, from Lin, P; Meng, L; Huang, Y; Liu, L; Fan, D, Appl. Surf. Sci., 324, 784-90, 2015.]

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Figure 7.5. Schematic illustration of graphene oxide functionalization with hindered phenol and its use in rubber composite. [Adapted, by permission, from Zhang, L; Li, H; Lai, X; Wu, W; Zeng, X, Mater. Lett., 210, 239-42, 2018.]

boxyl and carbonyl.9 The enhancement of hydroxyl was more significant than in the case of other species on the surface. Carbonyl group had a small contribution to adsorption.9 With the increase of functional groups in the reduced graphene oxide, the thermal conductivity increased until the concentration of the groups attained 2 wt% for monolayer graphene (1 wt% for bilayer graphene) then it declined.10 The functional groups and oxygen density influenced the electronic and vibrational properties of graphene oxide.11 The high oxygen content in graphene leads to the gap opening resulting in a transition from semimetal to semiconductor.11 The group chemical structure and the oxygen density influenced characteristics of phonon modes.11 The lattice constant and C−O bond length increased with oxygen density while C−C bond length decreased.11 Graphene oxide was simultaneously functionalized and reduced with toluene-2,4diisocyanate.12 The isocyanate groups of TDI reacted with oxygen-containing functional groups of graphene oxide (Figure 7.4).12 Hindered phenol functionalized graphene oxide was prepared by grafting 2,6-di-tertbutyl-4-hydroxymethyl phenol onto graphene oxide using isophorone diisocyanate as a bridging agent (Figure 7.5).13 The modified graphene oxide was used as an additive in rubber to improve its thermooxidative aging resistance by the antioxidative effect of hindered phenol and reduction of oxygen diffusion.13 The graphene oxide was made from graphite powder via a modified Hummers method, then carboxylated to graphene oxide-COOH in a mixture having a strong basic pH of chloroacetic acid with NaOH.14 The Cu(II) was adsorbed onto the graphene oxide and graphene oxide-COOH in aqueous solutions, the highest removal efficiencies of Cu2+ were 97 and 99.4%, respectively.14

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Figure 7.6. Effect of functional groups on copper adsorption and system toxicity. [Adapted, by permission, from Liu, Y; Fan, W; Xu, Z; Peng, W; Luo, S, Environ. Pollution, 236, 962-70, 2018.]

Graphene and graphene oxide had different effects on the toxicity of copper towards Daphnia magna.15 Graphene oxide had high adsorption of copper ions by hydroxyl and epoxy groups which, thus, reduced availability of copper ions in solution resulting in reduced toxicity of the system (with graphene situation was just opposite as Figure 7.6 shows).15 Graphene was functionalized with furfuryl alcohol using the Diels-Alder reaction to improve its sorption of radionuclides.16 The adsorption kinetics followed the pseudo-second-order model.16 The thermodynamic parameters indicated an endothermic and spontaneous adsorption process.16 The relative binding affinity of radionuclides was ranked in the order Eu(III) > Co(II) > Sr(II) > U(VI).16 Dithiocarbamate group functionalized graphene oxide was prepared to promote the adsorption efficiency for Pb(II) ions in aqueous solution.17 Due to the strong affinity of dithiocarbamate group containing sulfur towards Pb(II) ions, the maximum adsorption capacity of modified graphene oxide was increased to 132.10 mg/g at 25℃, and adsorption was exothermic and spontaneous.17 Graphene oxide was fluorinated, and the concentrations of oxygen functional groups on graphene oxide were decreased by fluorination.18 The crystallite size and interplanar distance of fluorinated-graphene oxide changed depending on fluorine concentration.18 The d-spacing and crystallite size of fluorinated graphene oxide were decreased to 0.771 (from 0.853) and 2.125 nm (from 40.07), respectively.18 Alkyl chains and benzyl moieties were covalently attached to graphene oxide.19 The hydroxyl and epoxide groups on graphene oxide were used as the sites for grafting alkanes.19 The resultant products had better dispersion and significantly improved the mechanical performance of polyethylene.19

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REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Radovic, LR; Mora-Vilches, CV; Salgado-Casanova, AJA; Buljan, A, Carbon, 130, 340-9, 2018. Edener, J; JAnos, P; Ecorchard, P; Stengl, V; Belcicka, Z; Stasny, M; Pop-Georgievski, O; Dohnal, V, React. Funct. Polym., 103, 44-53, 2016. Wang, M-H; Li, Q; Li, X; Liu, Y; Fan, L-Z, Appl. Surf. Sci., 448, 351-61, 2018. Tajima, K; Isaka, T; Yamashima, T; Ohta, Y; Matsuo, Y; Takai, K, Polyhedron, 136, 155-8, 2017. Dong, B; Yuan, Y; Luo, J; Dong, L; Liu, R; Liu, X, Prog. Org. Coat., 118, 57-65, 2018. Gu, S-Y; Hsieh, C-T; Yuan, J-Y; Hsueh, J-H; Gandomi, JA, Diamond Related Mater, 87, 99-106, 2018. Hong, Y-Z; Tsai, H-C; Wang, Y-H; Aumanen, J; Myllyperkio, P; Johansson, A; Kuo, Y-C; Chang, L-Y; Yue, H; Sun, H; Peng, T; Liu, B; Xie, Y, J. Molec. Struct., 1163, 449-54, 2018. Luo, D; Zhang, X, Int. J. Hydrogen Energy, 43, 11, 5668-79, 2018. Sun, Y; Chen, L; Cui, L; Zhang, Y; Du, X, Comput. Mater. Sci., 148, 176-83, 2018. Dabhi, SD; Jha, PK, Phys. E: Low-dimensional Syst. Nanostruct., 93, 332-338, 2017. Lin, P; Meng, L; Huang, Y; Liu, L; Fan, D, Appl. Surf. Sci., 324, 784-90, 2015. Zhang, L; Li, H; Lai, X; Wu, W; Zeng, X, Mater. Lett., 210, 239-42, 2018. White, RL; White, CM; Turgut, H; Massoud, A; Tian, ZR, J. Taiwan Inst. Chem. Eng., 85, 18-28, 2018. Liu, Y; Fan, W; Xu, Z; Peng, W; Luo, S, Environ. Pollution, 236, 962-70, 2018. Chen, X; Wang, X; Wang, S; Qi, J; Xie, K; Liu, X; Li, J, Arabian J. Chem., 10, 6, 837-44, 2017. Gao, T; Yu, J; Zhou, Y; Jiang, X, J. Taiwan Inst. Chem. Eng., 71, 426-32, 2017. Park, M-S; Lee, Y-S, J. Fluorine Chem., 182, 91-7, 2016. Park, S; He, S; Wang, J; Stein, A; Macosko, CW, Polymer, 104, 1-9, 2016.

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Figure 7.7. Variation of the structures of graphene with different types of defects before and after structure optimization. [Adapted, by permission, from Wu, X; Zhao, H; Yan, D; Pei, J, Comput. Mater. Sci., 129, 184-93, 2017.]

7.2 DOPING Graphene doped with nitrogen exhibits unique properties different than perfect graphene.1 The temperature distribution in nitrogen-doped graphene nanoribbon, containing two types of grain boundaries, was found to be sensitive to the number of dopants and grain boundary.1 The nitrogen atoms enhance the roughness of N-graphene and decrease thermal conductivity.1 The graphene is mechanically weaker when doped with nitrogen hav-

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ing decreased Young's modulus and breaking stress.1 The formation energies for N-doped AB-stacked bilayers varied depending on dopant sites, but B-doped ones were almost siteindependent.2 The B-doped and N-doped bilayer graphenes exhibit p-type and n-type electronic properties, respectively.2 Be-N is more stable than Bedoped graphene.3 The transition from semi-metallicity to semiconducting of graphene doped Figure 7.8. Defective graphene supercells. [Adapted, by with Be, N, and pairs of Be-N has permission, from del Castillo, RM; Calles, AG; Espejel-Morales, R; Hernández-Coronado, H; Comput. been observed. Both Be and Be-N Condensed Matter., 16, e00315, 2018.] co-doped graphene exhibit p-type character.3 Graphene was doped using N ion beam irradiation.4 Under the irradiation of low energy ion beams, the adsorbed ions and vacancy defects were generated, and, during the equilibrium process of the structures, the adsorbed ions combined with the vacancy defects, which led to the substitution doping results.4 With complicated defects (multivacancy defects), or mismatch between the adsorbed ions and sputtered atoms, the vacancies cannot be wholly repaired by adsorbed ions; thus the doping results of substitution with polygon defects have been generated (Figure 7.7).4 Graphene doped with nitrogen acts as a metal-free electrode.5 It has three times higher catalytic activity than graphene and achieves the catalytic activity of the platinum.5 The graphene had the following defects: graphitic-N type defect, pyridinic-N type defect, and a vacancy in graphene layer (Figure 7.8).5 Graphene with a vacancy and pyridinic-N systems are p-type doping.5 The graphitic-N, which present n-type doping, is the most stable system.5 Silica-doped graphene had the lowest formation energy although it was semimetallic.6 Phosphorus-doped graphene had a magnetic moment of 1 μB and for 3 at% of doping the band gap was 0.67 eV.6 Phosphorus can open the largest band gap in graphene. It is possible to tune the band gap of graphene from 0.66 to 0.1 eV just by varying the amount of phosphorus introduced.6 Aluminum-doped graphene is very unstable, but it is an attractive material because it is metallic.6 Graphene p-doping was obtained through the cell exposure to the vapor of HNO3 or SOCl2 and was used for a double antireflection coating of the solar cell.7 The solar cells had larger open circuit voltage and higher efficiency.7 Tetrachloroauric acid was used for graphene doping resulting in a direct correlation between its concentration and modification of graphene's electrical properties.8 The doping mechanism entails the charge transfer (CT) between AuIII ions and graphene, and the formation of Au nanoparticles (AuNPs) on the surface (Figure 7.9).8

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Figure 7.9. Illustration of the induced Fermi level shifts with HAuCl4 solution concentrations (a) and proposed reduction mechanism of AuIII ions to AuNPs after doping CVD graphene layers with HAuCl4 solution (b). [Adapted, by permission, from Krajewska, A; Oberda, K; Azpeitia, J; Gutierrez, A; Pasternak, I; Lopez, MF; Mierczyk, Z; Munuera, C; Strupinski, W, Carbon, 100, 625-31, 2016.]

REFERENCES 1 2 3 4 5 6 7 8

Lofti, E; Neek-Amal, M, J. Molec. Graphics Modelling, 74, 100-4, 2017. Fujimoto, Y; Saito, S, Surf. Sci., 634, 57-61, 2015. Olaniyan, O; Maphasha, RE; Madito, MJ; Khaleed, AA; Manyala, N, Carbon, 129, 207-227, 2018. Wu, X; Zhao, H; Yan, D; Pei, J, Comput. Mater. Sci., 129, 184-93, 2017. del Castillo, RM; Calles, AG; Espejel-Morales, R; Hernández-Coronado, H; Comput. Condensed Matter., 16, e00315, 2018. Denis, PA, Chem. Phys. Lett., 492, 4-6, 251-7, 2010. Lancellotti, L; Bobeico, E; Castaldo, A; Delli Veneri, P; Lago, E; Lisi, N, Thin Solid Films, 646, 21-7, 2018. Krajewska, A; Oberda, K; Azpeitia, J; Gutierrez, A; Pasternak, I; Lopez, MF; Mierczyk, Z; Munuera, C; Strupinski, W, Carbon, 100, 625-31, 2016.

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7.3 EDGE FUNCTIONALIZATION The structural and electronic properties of edge-functionalized (by alkyl, alkene, and seven different polyacetylene side chains) as an alternative for a hydrogen atom in eight kinds of graphene sheets were investigated.1 The CH3 and longer side chains had high stability and remained unaffected by increasing the size of the graphene sheet.1 The weak binding energy and large formation energy were observed for CH2, C3H4, and C5H6 in all eight kinds of the graphene sheets.1 The central region remained unaffected by the side chain substitution in all kinds of graphene.1 The average thickness of graphene oxide sheets is ~1.4 nm, which is larger than the thickness of ~0.8 nm of single-layer graphene oxides, but it is still smaller than the typical thickness of a bilayer graphene oxide.2 The interlayer spacing along the c-axis changes during oxidation because hydroxyl, carbonyl, and epoxy groups are bonded to the edges of basal planes of the graphite structure.2 Also, carbon hydroxylation occurs, and the sp2 bonds change to sp3 bonds.2 The presence of dead space can explain the bumpy texture of the flat regions by the extensive edge functionalization changing the dimensional distance between the edges as compared to the flat regions.2 Two green emission bands in the range of 497-502 nm and 534-551 nm were assigned to the COOH and CO sub-band energy states belonging to the edge sites, while the blue emission was attributed to the localized states of sp2/sp3 domains and epoxy related in-plane functional groups in the chemically-derived graphene materials.3 The electronic structure of graphene can be altered by the size distribution of sp2 and sp3 domains and by tuning the selective oxygenated surface along with the edge functionalities on pristine graphene layer.3 The distribution and nature of the oxygenated functionalities among the sp2 and sp3 carbon domains at the basal plane and edges play a vital role in the band gap opening.3 Graphene oxide contains hydrophobic graphenic domains and hydrophilic edges anchored with carboxyl groups, which give a wide range of chemical functionalization opportunities and good water dispersibility.4 The flat, hydrophobic section can be used to combine with hydrophobic drugs if edge-functionalized graphene oxide is used as a drug carrier.4 In the case of dextrin-conjugated graphene oxide used as nanocarrier, the edge functionalization was barely visible on TEM images, but measurements by AFM showed that the thickness of nanocarrier increased from 1.5 nm to approximately 6 nm after the drug was inserted.4 The drug loading was mediated by π-π stacking and hydrogen bonding.4 The edge-functionalized graphene nanoribbons were incorporated into the brittle epoxy polymer matrix.5 The functionalization process by polyvinylamine chains occurred only at the edges, preserving the in-plane sp2 of the graphene.5 The edge functionalization improved compatibility with the epoxy resin and prevented agglomeration resulting in improvement of the fracture toughness, flexural strength, and shear strength at low loading (0.15 wt%).5 By improving the interfacial adhesion, the edge-functionalization of graphene nanoribbons altered the failure mechanism from pullout to fracture.5 Edge-terminating oligomers are frequently used to aid in the dispersion of solutionprocessed, “bottom-up” graphene nanoribbons.6 Polyethylene glycol edge-termination increased the barrier to aggregation (by 6.4 kcal/mol), whereas n-alkoxy chains decreased the energy barrier significantly and increased the range at which graphene nanoribbons

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Figure 7.10. (a) Schematic illustration of the edge-functionalization process. (b) covalent network between graphene and epoxy. (c) 3D structure of the conductive composite. [Adapted, by permission, from Liu, K; Chen, S; Luo, Y; Jia, D; Gao, H; Hu, G; Liu, L, Compos. Sci. Technol., 88, 84-91, 2013.]

begin to attract each other (the range of attraction between neighboring graphene nanoribbons increased by about 10 Å).6 Functionalized graphite was produced by Friedel-Craft acylation reaction with 4aminobenzoic acid.7 The functionalization predominantly occurred at the edge region of graphite with the initial structure of pristine graphene in the interior basal planes retained.7 The electrical conductivity of composites containing 0.6 wt% of functionalized graphene increased by 31.3%, and the shear strength improved from 8.7 MPa to 15.2 MPa because covalent interactions between graphene and epoxy matrix of terminal amino groups maintained excellent dispersion leading to a 3D network (Figure 7.10).7 A bilayer graphene nanoribbon has been edge-functionalized with OH, NH2, NO2, CH3 and COOH.8 Functionalization always leads to increased adsorption of both CO2 and N2 with the largest increase in the case of COOH but the selectivity of CO2 adsorption is only increased in the presence of COOH functionalized nanoribbons.8 The stability of edge magnetism in zigzag graphene nanoribbons with indene-type functionalized edges has been evaluated.9 The functionalized nanoribbons preserved most electronic properties of the pristine ribbon.9 The band modification made edge magnetism sensitive or unstable to environmental influences.9 It is possible to obtain atomically precise nanoribbons having the edge electronic and magnetic properties affected in a predictive manner for potential applications in nanoelectronics and spintronics.9

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The edge-sulfurized graphene nanoplatelets with 3D porous foam structure were prepared for cathode materials in lithium-sulfur batteries.10 The sulfur content around the edge was much higher than that on the inside of mono- or multi-layered graphene nanoplatelets.10 The material properties effectively suppressed Figure 7.11. The o-carborane edge-functionalized graphene oxide the dissolution and diffusion of layer. [Adapted, by permission, from Štengl, V; Bakardjieva, S; polysulfides into the electrolyte Bakardjiev, M; Štíbr, B; Kormunda, M, Carbon, 67, 336-43, 2014.] and significantly improved the electrical conductivity of sulfur.10 Half-metals are the primary ingredients for the realization of novel spintronic devices, and such material can be obtained by fluorine passivation of zigzag graphene nanoribbons.11 Fluorine passivation is analogous to H passivation (pristine) of the ribbons with zigzag edges, but fluorine passivated graphene nanoribbons are energetically more stable.11 Highly graphitic graphene quantum dots with abundant edge functional groups produced from pyrene exhibited high antioxidant activity, with inhibition effective concentrations much lower than those of ascorbic acid.12 A graphene-based material was covalently functionalized on the edges by stable ocarboranoyl groups.13 Figure 7.11 shows the chemical structure of functionalized graphene sheet.13 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13

Anithaa, VS; Vijayakumar, S, Comput. Theor. Chem., 1135, 34-47, 2018. Shen, J; Li, N; Shi, M; Hu, Y; Ye, M, J. Colloid Interface Sci., 348, 2, 377-83, 2010. Biroju, RK; Rajender, G; Giri, PK, Carbon, 95, 228-38, 2015. Kiew, S; Ho, YT; Kiew, LV; Kah, JCY; Lee, HB; Imae, T; Chung, LY, Int. J. Pharm., 534, 1-2, 297-307, 2017. Nadiv, R; Shtein, M; Buzaglo, M; Peretz-Damari, S; Kovalchuk, A; Wang, T; Tour, JM; Regev, O, Carbon, 99, 444-50, 2016. Saathoff, JD; Clancy, P, Carbon, 115, 154-61, 2017. Liu, K; Chen, S; Luo, Y; Jia, D; Gao, H; Hu, G; Liu, L, Compos. Sci. Technol., 88, 84-91, 2013. Dasgupta, T; Punnathanam, SN; Ayappa, KG, Chem. Eng. Sci., 121, 279-91, 2015. Shinde, PP; Gröning, O; Wang, S;Ruffieux, P; Pignedoli, CA;Fasel, R; Passerone, D, Carbon, 124, 123-32, 2017. Yan, L; Xiao, M; Wang, S; Han, D; Meng, Y, J. Ener. Chem., 26, 3, 522-29, 2017. Jaiswal, NK; Tyagi, N; Kumar, A; Srivastava, P, Appl. Surf. Sci., 396, 471-9, 2017. Ruiz, V; Yate, L; García, I; Cabanero, G; Grande, H-J, Carbon, 116, 366-74, 2017. Štengl, V; Bakardjieva, S; Bakardjiev, M; Štíbr, B; Kormunda, M, Carbon, 67, 336-43, 2014.

8

Current Developments in Some Applications of Graphene 8.1 AEROGELS The three-dimensional highly compressible, elastic, anisotropic, cellulose/graphene aerogels were manufactured by bidirectional freeze-drying.1 Grafting long carbon chains using chemical vapor deposition improved superhydrophobicity of cellulose/graphene aerogels.1 Flexible cellulose, stiff graphene, and the special bidirectionally aligned porous structure resulted in outstanding recoverability (99.8% and 96.3% when compressed to 60% and 90% strain, respectively).1 The aerogel absorption capacity was 80-197 times of its weight.1 Figure 8.1 shows the details of aerogel morphology.1 Three-dimensional graphene foam produced by template-directed chemical vapor deposition had enhanced thermal conductivity, improved the shape-stability, increased thermal energy storage density, thermal reliability, and chemical stability.2 The self-polymerized polydopamine can be homogeneously coated on graphene oxide sheets.3 Good adsorbing selectivity and excellent adsorption ability of organic solvents are typical of these aerogels.3 The electrostatic attraction, π-π interaction, and Eschenmoser structure assisted chemical interaction.3

Figure 8.1. (a) SEM images show the morphology of aerogel in different directions. Digital images show that cellulose/graphene aerogel turned from brown to black after chemical vapor deposition modification. Both aerogels can rest atop a dandelion, attesting to their ultra lightweight properties. Scale bar = 200 μm. (b) Pore size distribution histogram of aerogel in the z direction. (c) Pore aspect ratio distribution histogram of aerogel in the z direction. (d) graphene aerogel with 0.2 vol% MPS. [Adapted, by permission, from Mi, H-Y; Jing, X; Politowicz, AL; Chen, E; Huang, H-X; Turng, L-S, Carbon, 132, 199-209, 2018.]

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The biobased aerogel was obtained from carboxymethyl cellulose, and 2D graphene oxide using boric acid as crosslinker.4 When graphene oxide content was 5 wt%, the compressive strength and Young’s modulus of composite aerogels reached 349 kPa and 1029 kPa, which were 1.6 and 4.5 times higher than that of carboxymethyl cellulose aerogels, respectively.4 A high decontamination efficiency, capability of withstanding mechanical deformation without secondary pollution and degradation of performance are primary requirements for materials used in environmental purification.5 A flexible graphene aerogel was obtained by vacuum freeze-drying of hydrogel precursor obtained by heating the aqueous mixture of graphene oxide and ascorbic acid.5 The aerogel was used in water purification including enrichment of organic liquid solvents (alcohols, oil, and alkanes), removal of hexavalent chromium Cr(VI), and purification of industrial wastewater.5 The electrical conductivity and charge carrier mobility of reduced graphene oxidebased 3D aerogel is often restricted by defects causing disruption of 2D π-conjugation in reduced graphene oxide sheets.6 The improved photocatalytic activity results from the application of graphene oxide used to disperse commercial Elicarb graphene.6 The 3D porous structure of aerogel substantially inhibited the aggregation exposing more active sites for catalytic surface reaction.6 Cellulose acetate nanofibers were used in graphene aerogels to prevent graphene sheets from over-stacking and to enhance connectivity of cell walls.7 The co-deposition of polydopamine and polyethyleneimine made aerogel a superhydrophilic/underwater superoleophobic and shape-stable material able to separate oil-in-water emulsions with an extremely high flux (adsorption capacity of 230-734 g/g).7 Figure 8.2 illustrates a method of production and morphology of aerogel.7 The shape-stable composite for phase change materials was assembled based on lauric acid and graphene/graphene oxide complex aerogels to be used for enhancement of the thermal energy storage and electrical conduction.8 The reduction reaction and freeze-drying technology were used to produce the graphene/graphene oxide complex aerogels fol-

Figure 8.2. a) Preparation of graphene oxide/nanofiber aerogel via a direct freeze-drying procedure. b) Photographs of graphene oxide/nanofiber aerogel obtained from different molds. c) TEM image of graphene oxide/ nanofiber aerogel deposited on a copper mesh. d-e) SEM images of graphene oxide/nanofiber aerogel showing porous structure. f-i) SEM images of graphene oxide/nanofiber aerogel showing different sheets/nanofibers combinations (upper) and the corresponding cartoons (lower). The nanofiber fraction (f) in all graphene oxide/ nanofiber aerogel was 0.5. [Adapted, by permission, from Xiao, J; Lv, W; Song, Y; Zheng, Q, Chem. Eng. J., 338, 202-210, 2018.]

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Figure 8.3. Structure and morphology of graphene oxide/ methylmethacrylate, GO/MMT aerogels. (a) Photographs of GO/MMT gel and aerogels. (b) SEM images of the skin and core of a GO/MMT aerogel. (c) Zoom in images of the yellow dash area in b). (d) SEM images of the cross-section of a GO/MMT square rod aerogel. (e) SEM image and energy dispersive X-ray elemental mapping images of the yellow dash area in d). [Adapted, by permission, from Zhang, S; Zhao, K; Zhao, J; Liu, H; Chen, X; Yang, J; Bao, C, Carbon, 136, 196-203, 2018.]

lowed by lauric acid incorporation into the complex aerogels via vacuum-assisted impregnation.8 A high phase-change enthalpy of 198 J/g, high heat-charging and discharging efficiency of 90%, excellent cyclic stability, good phase-change reversibility, and good shape stability were achieved.8 Large-size graphene oxide sheets cover, wrap, and interact with nanoparticles, promoting their assembly.9 Extrusion devices were employed to control the shape and size of aerogels.9 This strategy was effective in the case of nanoparticles whose surface did not have polar functional groups.9 Figure 8.3 shows some examples of application.9 High-performance electrocatalysts for hydrogen evolution reaction have been constructed based on the graphene/graphene nanoribbon aerogels.10 The graphene nanosheets acted as the main building blocks of the monolithic aerogels.10 The graphene nanoribbons bridge graphene nanosheets, fostering good electric contact with electroactive materials.10 The density, hydrophobicity, and oil-uptake capability of graphene aerogels were influenced by reacting (3-mercaptopropyl)trimethoxysilane with graphene oxide.11 Functionalized graphene aerogels (density of 3.5 mg cm-3) had high oil absorption capacity (182 times for lubricating oil and 143 times for n-hexane of its weight).11 Figure 8.4 shows the morphology of aerogels.11 Compressible graphene-based aerogel has been developed by using a molecular glue strategy using γ-oxo-1-pyrenebutyric acid which can link the graphene skeleton sheets and dip-coated polymer layers.12 The aerogel can be used as hydrophilic and oleophilic intelligence and compressible electrical sensor.12

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Figure 8.4. SEM images for (a) graphene aerogel with 0.01 vol% MPS, (b) graphene aerogel with 0.1 vol% MPS, (c) graphene aerogel with 0.1 vol% MPS at high magnification showing spherical silicone polymer and (d) graphene aerogel with 0.2 vol% MPS. [Adapted, by permission, from Zhou, S; Zhou, X; Hao, G; Jiang, W; Wang, T, Appl. Surf. Sci., 439, 946-53, 2018.]

High-performance graphene oxide/carbon nanotube aerogel-polystyrene composites were prepared.13 Three-dimensional aerogel was prepared by self-assembly and a freezedrying method.13 It had highly a porous structure, low density, and good mechanical properties with only 0.41 wt% graphene oxide and 0.16 wt% carbon nanotubes.13 Ultralight (5.5 mg/cm3) graphene aerogels were fabricated by assembling graphene oxide using freeze-drying followed by a chemical reduction method.14 The EMI shielding effectiveness was increased from 20.4 to 27.6 dB when the graphene oxide was reduced by a high concentration of hydrazine vapor.14 The presence of sp2 graphitic lattice and free electrons from nitrogen atoms enhanced EMI shielding effectiveness.14 Poly(vinyl alcohol) was employed as an organic binder of three-dimensional graphene/carbon nanotube aerogels designed for the electrochemical energy storage.15 A high specific capacitance of 375 F/g with capacity retention of 88-94.8% after 5000 cycles was achieved.15 The exceptional electrocatalytic properties including high activity, good antipoisoning capacity, and outstanding durability resulted from decorating aerogel with ultrafine Pt nanoparticles.15 Hybrid aerogels exhibited a typical three-dimensional porous structure with rich graphene/PANI heterostructure and high specific surface area of up to 337 m2/g.16 As electrodes for symmetric and asymmetric all-solid-state supercapacitors, the aerogels delivered areal capacitances of up to 453 and 679 mF/cm2, respectively, due to the synergistic

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contribution of the local conductivity of graphene layers sandwiched between PANI layers and long-distance conductivity of 3D graphene frameworks.16 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Mi, H-Y; Jing, X; Politowicz, AL; Chen, E; Huang, H-X; Turng, L-S, Carbon, 132, 199-209, 2018. Yang, J; Qi, G-Q; Bao, R-Y; Yi, K; Li, M; Peng, L; Cai, Z; Yang, M-B; Wei, D; Yang, W, Ener. Storage Mater., 13, 88-95, 2018. Huang, T; Dai, J; Yang, J-h; Zhang, N; Wang, Y; Zhou, Z-w, Diamond Related Mater., 86, 117-27, 2018. Ge, X; Shan, Y; Wu, L; Mu, X; Peng, H; Jiang, Y, Carbohydrate Polym., 197, 277-83, 2018. Dong, S; Xia, L; Guo, T; Zhang, F; Cui, L; Su, X; Wang, D; Guo, W; Sun, J, Appl. Surf. Sci., 445, 30-8, 2018. Lu, K-Q; Yuan, in, X; Xu, Y-J, Appl. Catalysis B: Environ., 226, 16-22, 2018. Xiao, J; Lv, W; Song, Y; Zheng, Q, Chem. Eng. J., 338, 202-210, 2018. Liang, K; Shi, L; Zhang, J; Cheng, J; Wang, X, Thermochim. Acta, 664, 1-15, 2018. Zhang, S; Zhao, K; Zhao, J; Liu, H; Chen, X; Yang, J; Bao, C, Carbon, 136, 196-203, 2018. Sun, Z; Fan, W; Liu, T, Electrochim Acta, 250, 91-8, 2017. Zhou, S; Zhou, X; Hao, G; Jiang, W; Wang, T, Appl. Surf. Sci., 439, 946-53, 2018. Xiang, Y; Liu, L; Li, T; Dang, Z, Mater. Design, 110, 839-48, 2016. Cong, L; Li, X; Ma, L; Peng, Z; Yang, C; Han, P; Wang, G; Li, H; Song, W; Song, G, Mater. Lett., 214, 190-3, 2018. Bi, S; Zhang, L; Mu, C; Liu, M; Hu, X, Appl. Surf. Sci., 412, 529-36, 2017. Zhou, Y; Hu, XC; Guo, S; Yu, C; Zhiong, S; Li, X, Electrochim. Acta, 264, 12-9, 2018. Qu, Y; Lu, C; Su, Y; Cui, D; He, Y; Zhang, C; Cai, M; Zhang, F; Feng, X; Zhuang, X, Carbon, 127, 77-84, 2018.

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8.2 ANTIBACTERIAL SURFACES The effect of graphene oxide on bacterial growth is still a matter of controversy according to a paper which has reviewed 36 publications.1 In 2/3 of cases, the growth inhibition was confirmed, but the remaining papers did not find an antibacterial effect, and a couple of them even noted growth increase in the presence of graphene oxide.1 The antimicrobial mechanism of graphene oxide was studied by following the structural/electronic properties on its bioactivity with focus on the effect of the local hybridization of sp2/sp3 orbitals.2 The antibacterial activities of graphene derivatives are thought to depend on size, a number of layers, oxygen-containing groups, and the polymer matrix.2 Graphene oxide prevented Staphylococcus aureus from gathering because of sharpness of wrinkles or edges and presence of reactive oxygen species.2 Graphene oxide was able to kill bacteria with sharp wrinkles or edges.2 Figure 5.35 shows the proposed mechanism.2 A cellular oxidative stress-based molecular level mechanism is anticipated to cause the bacterial entrapment.2 The surface oxygen species can be reduced by the electron transfer from the bacterial enzymes to form reactive oxygen species such as H2O2 or O2-.2 The cellular oxidative stress is likely to cause molecular denaturation of bacterial protein on the surface of graphene oxide nanosheets.2 Graphene quantum dots generated reactive oxygen species when photoexcited (470 nm, 1 W) and killed two strains of pathogenic bacteria, methicillin-resistant Staphylococcus aureus and Escherichia coli.3 Antibacterial activity was demonstrated by the reduction in number of bacterial colonies, the increase in propidium iodide uptake confirming the cell membrane damage, and morphological defects visualized by atomic force microscopy.3 Neither graphene quantum dots nor light exposure alone was able to cause oxidative stress and reduce the viability of bacteria.3 E. coli and S. aureus were used as the test microorganisms for graphene containing a coating of silicone rubber.4 The smooth, sharp edges-free morphology coatings were deposited.4 The oxidative stress mechanism was suggested as the primary factor of antibacterial activity.4 The antibacterial effect was determined by the colony counting and fluorescent staining test.4 The SEM observations showed that cells lost the membrane integrity and cytoplasm was leaked, leading to the final cell death.4 Marigold flower extract was used for reduction of graphene oxide to avoid a presence of residual toxic reducing agent molecules used in chemical reduction processes.5 The phytochemically reduced graphene oxide had significant antibacterial activity in the case of Gram-positive and Gram-negative bacteria.5 Higher levels of inhibition were seen in the case of E.coli as compared to Bacillus subtilis.5 Vancomycin is a glycopeptide antibiotic used to treat a variety of Gram-positive bacterial infections.6 It was used in the chemical reduction of graphene oxide at a weak alkaline pH.6 The vancomycin-decorated reduced graphene oxide was assessed by the inhibition zone test and the bacterial adhesion assay.6 Its film exhibited antibacterial property against S. aureus and S. epidermidis.6 It provided better and faster wound healing efficiency than the graphene oxide film.6 Graphene oxide and reduced graphene oxide containing ornidazole were used as carriers in antibacterial materials.7 The hydrophilic graphene oxide paper showed a direct antibacterial effect due to the excellent antibacterial activities of graphene oxide and orni-

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dazole, while the hydrophobic reduced graphene oxide/ornidazole paper resisted the bacterial adhesion.7 The spindle-shaped graphene oxide was obtained by the self-assembly of graphene oxide decorated by ZnO nanoparticles.8 The composites prevented bacterial proliferation and destroyed bacterial integrated membranes by the release of Zn2+ and generation of abundant reactive oxygen species.8 Chitosan/poly(vinyl alcohol)/ graphene oxide composite nanofibrous membranes were prepared via electrospinning.9 A good antibacterial activity of the Figure 8.5. Activity changes of lysozyme upon the incuprepared nanofibrous membranes was bation with graphene oxide and reduced graphene oxide. [Adapted, by permission, from Bai, Y; Ming, Z; exhibited against Gram-negative EscheCao, Y; Feng, S; Yang, H; Chen, L; Yang, S-T, Colloids richia coli and Gram-positive StaphylococSurf. B: Biointerfaces, 154, 96-103, 2017.] cus aureus.9 The antibacterial effects of quaternary ammonium salts cannot be effectively utilized due to the uncontrolled release.10 Dodecyl dimethyl benzyl ammonium chloride and bromohexadecyl pyridine were assembled on surfaces of graphene oxide through π–π interactions.10 Graphene oxide increased the antibacterial effect when the weak antibacterial agent was adopted.10 According to the mechanism presented in Figure 7.5, the electrons (e−) in the valence band were excited to the conduction band of Ag3PO4 as a result of visible light irradiation, causing the generation of holes (h+) in the valence band.11 The photogenerated electrons have been trapped by O2 molecules absorbed on the surface of graphene oxide, GO, producing .O2− radicals and the photogenerated holes, which have been reacted with water, formed .OH radical.11 These two reactive oxygen species attacked bacteria cells.11 The high surface area of graphene oxide provided active adsorption sites for bacteria and excellent conductivity which facilitated the transfer of electrons to its sheets due to its πconjugated structure.11 The enzyme-graphene interaction helped to compare the effects of graphene oxide and reduced graphene oxide.12 Both graphene oxide and reduced graphene oxide adsorbed large quantities of lysozyme, but graphene oxide inhibited lysozyme active whereas reduced graphene oxide nearly did not influence on the enzyme activity (Figure 8.5).12 The differences in behavior were caused by the differences in affecting the lysozyme conformational changes.12 The graphene oxide induced substantial changes to the enzyme protein conformation exposing the active sites of lysozyme to the aqueous environment making it more susceptible to oxidation.12 Many attempts report the use of combinations with some known biocidal substances such as silver, copper, zinc oxide, titanium dioxide, fluorine, chitosan, etc.

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

Palmieri, V; Lauriola, MC; Ciassa, G; Conti, C; De Spirito, M; Papi, M, Nanotechnology, 28, 152001-18, 2017. Sharma, A; Varshney, M; Nanda, SS; Shin, HJ; Kim, N; Yi, DK; Chae, K-H; Won, SO, Chem. Phys. Lett., 698, 85-92, 2018. Ristic, BZ; Milenkovic, MM; Dakic, IR; Todorovic-Markovic, BM; Milosavljevic, MS; Budimir, MD; Paunovic, VG; Dramicanin, MD; Markovic, ZM; Trajkovic, VS, Biomaterials, 35, 15, 4428-35, 2014. Liu, Y; Wen, J; Gao, Y; Li, T; Wang, H; Yan, H; Niu, B; Guo, R, Appl. Surf. Sci., 436, 624-30, 2018. Rani, MN; Ananda, S; Rangappa, D, Mater. Today: Proc., 4, 11, 3, 12300-5, 2017. Xu, LQ; Liao, YB; Li, NN; Li, YJ; Zhang, JY; Wang, YB; Hu, XF; Li, CM, J. Colloid Interface Sci., 514, 733-9, 2018. Qian, W; Wang, Z; He, D; Huang, X; Su, J, J. Saudi Chem. Soc., 22, 5, 581-7, 2018. Zhong, L; Liu, H; Samal, M; Yun, K, J. Photochem. Photobiol. B: Biology, 183, 293-301, 2018. Yang, S; Lei, P; Shan, Y; Zhang, D, Appl. Surf. Sci., 435, 832-40, 2018. Ye, X; Qin, X; Yan, X; Guo, J; Huang, L; Chen, D; Wu, T; Shi, Q; Tan, S; Cai, X, Chem. Eng. J., 304, 873-81, 2016. Deng, C-H; Gong, J-L; Ma, L-L; Zend, G-M; Song, B; Zhang, P; Huan, S-Y, Process Safety Environ. Protection, 106, 246-55, 2017. Bai, Y; Ming, Z; Cao, Y; Feng, S; Yang, H; Chen, L; Yang, S-T, Colloids Surf. B: Biointerfaces, 154, 96-103, 2017.

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8.3 BATTERIES Lithium-sulfur batteries suffer from their quick capacity decay and short lifespan due to the insulating nature of sulfur/Li2S and high solubility of lithium polysulfides.1 Graphene oxide can be functionalized with groups that will provide graphene with effective polysulfide encapsulation.1 This and other methods of improvement of lithium-sulfur batteries are discussed in a review paper.1 The nitrogen-doped porous carbon and graphene composite was used as the sulfur scaffold for lithium-sulfur batteries.2 3D heteroatom-doped carbon framework from sp2hybridized nanocarbon (graphene) and sp3-hybridized porous carbon (activated carbon) provided excellent performance in the energy conversion and storage due to the synergistic effect between building blocks (capacity of 1372 mAh g-1 and cycling stability of 579 mAh g-1 after 500 cycles).2 The cabbage-like nitrogen-doped graphene/sulfur composite cathode gave a discharge specific capacity of 1309 mAh g-1 and stable reversible capacity (663 mAh g-1 after 300 cycles).3 A broad overview of options in the selection of the graphene-based nanomaterials for energy storage devices is included in a review paper.4 Metal/graphene oxide batteries have been developed to convert chemical energy into electricity.5 In these batteries, the metal plays the role of the anode, and graphene oxide acts as both cathode and separator.5 Lithium/graphene oxide battery generates the highest specific capacity of 1572 mAh cm-3 (1604 mAh g-1).5 The three-dimensional batteries deliver higher energy than 2D planar batteries.5 Graphene oxide was uniformly coated onto the zinc anode to suppresses the dissolution of zinc anodes.6 The graphene oxide layers on the zinc surface conduct electrons across insulating zinc oxide, but they also slow down zinc intermediates from dissolving into the electrolyte.6 A small amount of graphene oxide (1.92 wt%) on the Zn anode surface significantly reduced the electrochemical impedance and improved its lifetime capacity by 28%.6 Graphene having a low surface area, proper sheet size, a moderate oxygen functional groups, and conductivity showed a great improvement of the cathode performance in lithium-ion batteries.7 Conductivity, surface chemistry, and content of graphene significantly influenced the electrochemical performance of electrode.7 One percent graphene was sufficient to provide a conductive network in the electrode.7

Figure 8.6. Improved Li-ion diffusion in a holey graphene anode (right) in contrast to a non-holey one (left). [Adapted, by permission, from Wu, C-H; Pu, N-W; Liu, Y-M; Chen, C-Y; Peng, Y-Y; Cheng, T-Y; Lin, M-H; Ger, M-D, J. Taiwan Inst. Chem. Eng., 80, 511-7, 2017.]

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Multilayered graphene grown by chemical vapor deposition was used as the negative electrode for the lithium-ion batteries.8 A thin film obtained on nickel substrate used without conductive additive and binding agent produced a capacity of ~250 mAh g-1.8 Holey graphene improved the rate capability of lithium-ion batteries by providing shortcuts for Li-ion diffusion through the holes in the fast charge/discharge processes.9 The holey graphene had a high reversible capacity of 742 mAh g-1 after 80 cycles, which was 2.3 times larger than the non-holey graphene.9 Figure 8.6 shows the mechanism of improved ion diffusion.9 Figures 8.7a-d show that the heating rate affected the formation of holes with high heating rates required to Figure 8.7. SEM images of graphene obtained at different heating accomplish the formation of holey rates in oC/min: (a) 1, (b) 10, (c) 30, (d) 60, and (e) holey graphene. 9 graphene. The red circles in (d) reveal the presence of holes in sample heated at 60oC/min. (f) High-magnification SEM image of holey graphene. Graphene-wrapped silicon The blue circle indicates a region containing holes as small as nanoparticles were prepared for ~10 nm; the purple squares show the outward-opening morphology self-support and binder-free of the hole edges. [Adapted, by permission, from Wu, C-H; Pu, N-W; Liu, Y-M; Chen, C-Y; Peng, Y-Y; Cheng, T-Y; Lin, M-H; anodes of lithium-ion batteries.10 Ger, M-D, J. Taiwan Inst. Chem. Eng., 80, 511-7, 2017.] Liquid nitrogen fast freezing was followed by a freeze-drying and a thermal reduction of graphene oxide (200oC under argon).10 The composite of reduced graphene oxide and silicon nanoparticles (80-100 nm) significantly improved lithium storage performance (capacity of 1482 mAh g-1 at a current density of 210 mA g-1 after 300 cycles).10 An additive-free electrode fabrication process, in which reduced graphene oxide was coated onto a collector using a supersonic kinetic spray technique, was used in the development of flexible lithium-ion batteries.11 Figure 8.8 shows the process of preparation, intermediates, properties, morphology, and structure of the produced electrode.11 The intensity ratio ID/IG (a criterion for evaluating the defect level) was 0.166 for the pristine rGO particles and decreased to 0.112 after the spraying process (Figure 8.8b) meaning that the number of defects was decreased resulting in better flexibility.11 Figure 8.8c shows that the spray-rGO electrode formed had a large contact area at the interface whereas in a

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Figure 8.8. (a) Overall schematic of additive-free coating of an reduced graphene oxide, rGO, electrode using supersonic kinetic spraying. The inset shows an optical image of spray-rGO. (b) Raman spectra of spray-rGO and slurry-rGO in the frequency range of 1000-3000 cm-1. (c) Schematic structure of spray-rGO. (d-e) Cross-sectional SEM images of spray-rGO with different magnifications. (f) Schematic structure of slurry-rGO. (g-h) Cross-sectional SEM images of slurry-rGO with different magnifications. [Adapted, by permission, from Kim, SD; Lee, J-G; Kim, T-G; Rana, K; Jeong, JY; Park, JH; Yoon, SS; Ahn, J-H, Carbon, 139, 195-204, 2018.]

slurry processed electrode only binder particles held the reduced graphene oxide particles and formed an adhesive surface at the interface.11 Graphene microsheets prepared from microcrystalline graphite minerals by an electrochemical/mechanical process were used as conductive support to load sulfur as the cathode of the lithium-sulfur battery.12 The cathode had long-term cyclability and high coulombic efficiency (99.7% after 2000 cycles).12 Flower-like TiO2/graphene composites have been fabricated via a simple hydrothermal method for use as an anode material for lithium-ion batteries.13 During the hydrothermal treatment, most of the graphene oxide has been converted to reduced graphene oxide.13 The composite containing 18.8 wt% graphene gave an anode having the highest specific capacity and good capacity retention.13 Figure 8.9 shows the morphology of titanium oxide decorated graphene.13

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Figure 8.9. SEM images with different concentration of graphene in %: a 0; b 10, c 20, and d 30%. [Adapted, by permission, from Wang, J-F; Zhang, J-J; He, D-N, Nano-Structures Nano-Objects, 15, 216-23, 2018.]

The hexagonal nickel hydroxide nanoplates were grown on graphene as a binder-free anode for the lithium-ion battery of high capacity.14 When used as an anode for a lithiumion battery, the oriented hexagonal hydroxide nanoplates on graphene exhibited high initial discharge capacity of 1318 mAh/g at the current density of 50 mA/g.14 After 80 cycles, the capacity was maintained at 834 mAh/g.14 Vanadium redox flow batteries are suitable for large-scale energy storage due to long cycle life, flexible design, and high safety.15 The poor electrocatalytic activity of carbonbased materials results in a large polarization resistance and energy loss during charge/discharge and hampers their broader application.15 A combination of graphene oxide, reduced graphene oxide, and graphene foam gives material with a high electrocatalytic activity and a high electrical conductivity giving electrode with a low polarization, a high discharge capacity, a high energy density, and a high energy efficiency.15 Vanadium redox flow battery performance was enhanced by using graphene nanoplatelets to decorate carbon electrodes.16 Covalent functionalization of graphene oxide with phosphonic acid was carried out to enhance the electrode wettability for use in vanadium redox flow battery system.17 The defects in the modified graphene oxide structure increased the negative charge density on the surface resulting in higher vanadium ions tendency for electrooxidation/electroreduction reactions which improved battery performance.17

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REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Zhang, Y; Gao, Z; Song, N; He, J; Li, X, Mater. Today Energy, 9, 319-35, 2018. Wu, H; Xia, L; Ren, J; Zheng, Q; Xie, F; Jie, W; Xu, C; Lin, D, Electrochim. Acta, 278, 83-92, 2018. Cui, Z; Mei, T; Yao, J; Hou, B; Zhu, X; Liu, X; Wang, X, J. Alloys Compounds, 753, 622-9, 2018. Wu, S; Ge, R; Lu, M; Xu, R; Zhang, Z, Nano Energy, 15, 379-405, 2015. Ye, M; Gao, J; Xiao, Y; Xu, T; Zhao, Y; Qu, L, Carbon, 125, 299-307, 2017. Zhou, Z; Zhang, Y; Chen, P; Wu, Y; Yang, H; Ding, H; Zhang, Y; Wang, Z; Du, X; Liu, N, Chem. Eng. Sci., in press, 2018. Shi, Y; Wen, L; Pei, S; Wu, M; Li, F, J. Energ. Chem., in press, 2018. Saulnier, M; Trudeau, C; Cloutier, SG; Schougaard, SB, Electrochim. Acta, 244, 54-60, 2017. Wu, C-H; Pu, N-W; Liu, Y-M; Chen, C-Y; Peng, Y-Y; Cheng, T-Y; Lin, M-H; Ger, M-D, J. Taiwan Inst. Chem. Eng., 80, 511-7, 2017. Yue, H; Li, Q; Liu, D; Hou, X; Bai, S; Lin, S; He, D, J. Alloys Compounds, 744, 243-51, 2018. Kim, SD; Lee, J-G; Kim, T-G; Rana, K; Jeong, JY; Park, JH; Yoon, SS; Ahn, J-H, Carbon, 139, 195-204, 2018. Zhang, Y; Duan, X; Wang, J; Wang, C; Wang, J, J. Power Sources, 376, 131-7, 2018. Wang, J-F; Zhang, J-J; He, D-N, Nano-Structures Nano-Objects, 15, 216-23, 2018. Du, Y; Ma, H; Guo, M; Gao, T; Li, H, Chem. Phys. Lett., 699, 167-70, 2018. Hu, G; Jing, M; Wang, D-W; Sun, Z; Li, F, Energ. Storage Mater., 13, 66-71, 2018. Sankar, A; Michos, I; Dutta, I; Dong, J; Angelopoulos, AP, J. Power Sources, 387, 91-100, 2018. Etesami, M; Abouzari-Lotf, E; Ripin, A; Nasef, MM; Ting, TM; Saharkhiz, A; Ahmad, Int. J. Hydrogen Energy, 43, 1, 189-97, 2018.

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8.4 BIOMEDICAL APPLICATIONS Figure 8.10 illustrates the range of graphene applications in biomedical field.1 Applications include therapy, imaging, biosensor, delivery, biological carrier, and tissue engineering.1 The review paper discussed these various applications in detail.1 Also, a book chapter includes a discussion of graphene in the biomedical field.2 Selected applications are given below based on their original papers. The biocompatibility of graphene and its derivatives is essential for biomedical applications such as drug and gene delivery, tissue engineering, biosensing, and imaging.3 They may activate some immune cells or cause suppression of maturation of Figure 8.10. Biomedical applications of graphene. others.3 The interaction of graphene prod[Adapted, by permission, from Foo, ME; Gopinath, ucts depends on lateral dimensions, oxidaSCB, Biomed. Pharmacotherapy, 94, 354-61, 2017.] tion, functionalization, layer number, and purity.3 These variables may decide whether the immune interaction will favor immune-suppression or immune-stimulation.3 The reduction of graphene oxide by eco-friendly reducing agents is of great interest for its applications in the medical field, such as, for example, the applications aiming at enhancement of antibiotic activity.4 Zinc oxide decorated reduced graphene oxide was found to have antibacterial properties attributable to the synergisFigure 8.11. Mechanism of antibacterial effect of zinc tic effect of zinc oxide and graphene oxide-decorated reduced graphene oxide. [Adapted, by towards the bacteria. The antibacterial permission, from Sandhya, PK; Jose, J; Sreekala, MS; Padmanabhan, M; Kalarikkal, N; Thomas, S, Ceramics effect is caused by the disruption of the Int., 44, 33, 15092-98, 2018.] bacterial cell.4 At the same time, both zinc oxide and reduced graphene oxide have little toxicity to the mammalian cells.4 Figure 8.11 shows the mechanism of cell wall damage.4 The zinc oxide-decorated reduced graphene sheets are capable of destroying the rigidity of cell walls of the bacteria by physical wrapping which then induces significant membrane stress on the surface of the cell walls leading to the death of the bacteria.4 Graphene was directly grown on the surface of biomedical 316 type stainless steel.5 The deposited graphene layer promoted the adhesion and collagen secretion of bone marrow mesenchymal stem cells.5 The orthopedic, dental, and surgical implants and intravascular stents require improved biological activity on the steel surface.5

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Figure 8.12. Schematic diagram of graphene grafting with short peptides via click chemistry. AMP − antimicrobial peptide, RGD − arginine-glycine-aspartic acid. [Adapted, by permission, from Shi, L; Wang, L; Chen, J; Chen, J; Ren, L; Shi, X; Wang, Y, Appl. Mater. Today, 5, 111-7, 2016.]

The cytocompatibility and antibacterial properties of graphene oxide were improved by modification with short peptide using click chemistry via Cu(I)-catalyzed azide-alkyne cycloaddition click reaction (Figure 8.12).6 Click reaction does not damage the bioactivity of grafted short peptide.6 The cytotoxicity of graphene oxide modified with RGD peptide was significantly reduced.6 The AMP grafted graphene oxide nanosheets had excellent antibacterial properties towards both Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria at a low concentration.6 The graphene oxide was functionalized with tryptamine without using any hazardous acylating and coupling reagents.7 The hybrid had excellent antibacterial activity and high cytocompatibility.7 The hydroxyapatite nanorods were grown on graphene oxide sheets using hydrothermal process.8 The nanorods had the diameter and length in the range of ~32 and 60-85 nm, and they were uniformly distributed on graphene oxide sheets.8 Due to their excellent biocompatibility, they can be used in orthopedic, drug delivery, and dentistry applications.8 Graphene-silver hybrid particles were dispersed in poly(ε-caprolactone) matrix to obtain biodegradable composite which was both cytocompatible and antibacterial.9 The synergistic effect of silver and reduced graphene oxide makes material potentially suitable for fracture fixation devices and tissue engineering.9 Figure 8.13 illustrates production method and potential applications of the composite.9 Various aspects of biomedical applications, such as general applications,10-l2 sensing devices,13-14 neural regeneration,15 orthopedic,16 bioimaging,17 and tissue engineering,18 are discussed in numerous review papers − some of which were brought into attention.

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Figure 8.13. Schematic diagram showing the production of composite and its potential applications. [Adapted, by permission, from Kumar, S; Raj, S; Jain, S; Chatterjee, K, Mater. Design, 108, 319-32, 2016.]

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Foo, ME; Gopinath, SCB, Biomed. Pharmacotherapy, 94, 354-61, 2017. Shi, J; Fang, Y, Biomedical Applications of Graphene. Graphene, Tsinghua University Press Limited, 2018. Saleem, J; Wang, L; Chen, C, Nanoimpact, 5, 109-18, 2017. Sandhya, PK; Jose, J; Sreekala, MS; Padmanabhan, M; Kalarikkal, N; Thomas, S, Ceramics Int., 44, 33, 15092-98, 2018. Zhou, H; Jiang, M; Xin, Y; Sun, G; Long, S; Bao, S; Cao, X; Ji, S; Jin, P, Mater. Lett., 192, 123-7, 2017. Shi, L; Wang, L; Chen, J; Chen, J; Ren, L; Shi, X; Wang, Y, Appl. Mater. Today, 5, 111-7, 2016. Maktedar, SS; Mehetre, SS; Avashthi, G; Singh, M, Ultrasonics Sonochem., 34, 67-77, 2017. Ramadas, M; Bharath, G; Ponpandian, N; Ballamurugan, AM, Mater. Chem. Phys., 199, 179-84, 2017. Kumar, S; Raj, S; Jain, S; Chatterjee, K, Mater. Design, 108, 319-32, 2016. Olszowska, K; Pang, J; Wrobel, PS; Zhao, L; Rummeli, MH, Synth. Metals, 234, 53-85, 2017. Singh, DP; Herrera, CE; Singh, B; Singh, S; Kumar, R, Mater. Sci. Eng. C, 86, 173-97, 2018. Rifai, A; Pirogova, E; Fox, K, Diamond, Carbon Nanotubes and Graphene for Biomedical Applications. Encyclopedia, Elsevier, 2018. Kumar, R; Singh. R, Prospect of Graphene for Use as Sensors in Miniaturized and Biomedical Sensing Devices. Encyclopedia, Elsevier, 2018. Kumar, R; Singh, R; Hui, D; Feo, L; Fraternali, F, Compos. Part A: Eng., 134, 193-206, 2018. Reddy, S; Xu, X; Guo, T; Zhu, R; Ramakrishna, S, Current Opinion Biomed. Eng., 6, 120-9, 2018. Li, M; Xiong, P; Yan, F; Li, S; Cheng, C, Bioactive Mater., 3, 1, 1-18, 2018. Lin, J; Chen, X; Huang, P, Adv. Drug Delivery Rev., 105B, 242-54, 2016. Shin, SR; Li, Y-C; Jang, HL; Khoshakhlagh, P; Khademhosseini, A, Adv. Drug Delivery Rev., 105B, 255-74, 2016.

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8.5 CATALYSIS The presence of a low concentration of heteroatoms on the graphene sheet (“doping”) modifies the electron density, electrical conductivity, and other properties of graphenes which influence the application of these materials in catalysis.1 Graphene and related materials can be considered as composed of a surface on which catalysis takes place.1 In classical solid catalysts, only atoms located on the external surface contribute to the catalytic activity.1 The atoms residing in the interior of the particle do not contribute to the catalytic activity.1 Also, strong adsorption of substrates or reagents on graphene as a consequence of the sterically favorable interaction with the extended π orbital is a contributing factor.1 The adsorption is favored by the geometry and the electronic configuration on the surface.1 The presence of heteroatoms may generate a distortion of the π cloud; therefore, doping may increase adsorption of certain substrates either by donating or by accepting electrons.1 Finally, various functional groups which can be introduced may act as the active centers of catalytic activity.1 Defect-free flat graphene (perfectly ordered hexagonal structure) is chemically less active, and, therefore, is not of practical interest in catalysis.2 The reactivity of intrinsic defects and dangling bonds is important for its applications involving chemical reactions. In these applications, the focus should be on the chemical reactivity of graphene rather than the ideal structure.2 The role of support (anchoring metal particles to prevent sintering) is frequently assigned to graphene in catalytic applications, but graphene also activates oxidative and reductive reaction steps in the catalytic cycle of palladium-catalyzed cross-coupling reactions.3 The high catalytic activity is linked to the ability of graphene support to act as both

Figure 8.14. The degradation mechanism of sulfamethoxazole by catalytic ozonation. [Adapted, by permission, from Yin, R; Guo, W; Du, J; Zhou, X; Zheng, H; Wu, Q; Chang, J; Ren, N, Chem. Eng. J., 317, 632-9, 2017.]

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Figure 8.15. Schematic view of aniline oligomer attached to reduced graphene oxide preparation and the charge transfer between graphene and tetramer aniline by means of π‒π interactions. [Adapted, by permission, from Wang, S; Li, L; Wang, Q; Fan, Y; Shen, J; Zhang, K; Yang, Zhang, W, Synthetic Metals, 243, 107-14, 2018.]

an efficient charge donor and acceptor.3 Defective graphene can catalyze reaction steps that require both charge donation and charge acceptance.3 The oxygen reduction reaction was catalyzed by the trace manganese content, and it reached its highest performance at an onset potential of 0.94 V when manganese exists as a mononuclear-centered structure within defective graphene.4 The trace metal acted when present below the detection limit of XPS and TEM analysis. For this reason, some socalled “metal-free” catalysts may have benefited from the presence of a trace metal.4 Reduced graphene oxide was doped with heteroatoms (N and P) and applied in catalytic ozonation of sulfamethoxazole (commonly used antibiotic found in wastewater effluents).5 Figure 8.14 shows a degradation mechanism by catalytic ozonation.5 The doping with heteroatoms modulated the configuration and properties of graphene oxide creating new active sites enhancing the catalytic performance.5 The catalyst can find application in the effective remediation technologies for hazardous pollutants removal.5 The graphene oxidation degree affects its catalytic activity in ozone catalysis.6 With the increase in the oxidation degree, the formation of hydroxyl and carboxyl groups increased.6 These groups then partially converted into epoxy groups.6 The catalytic efficiency of graphene oxide increased with an increase in the oxidation level.6 The nitrogen-doped graphene and aminated graphene effectively activated persulfate and removed sulfamethoxazole which was studied as a model contaminant.7 The study shows that the functionalization of graphene with heteroatom doping can be used in water treatment of organic pollutants including emerging contaminants.7

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The noble metal-free catalyst (N-doped graphene) is a promising replacement for platinum in low-temperature fuel cells considered as one of the most attractive alternatives to the combustion engines in automotive applications.8 An aniline oligomer was covalently attached to the surface of graphene after reaction of graphene oxide with aniline tetramer followed by the reduction process.9 The conductivity of the graphene was increased from 5.8 to 41.7 S cm-1 caused by the π‒π interactions (see the charge transfer mechanism in Figure 8.15).9 With the interfacial contact enhanced and the aggregation suppressed, the aniline oligomer attached to reduced graphene oxide became capable of fast charge transfer and enhanced electronic communication, leading to large specific capacitance, excellent cycling stability, high electrical conductivity, and better electrocatalytic behavior.9 Due to the synergistic effect, the capacitance of the oligomer attached to graphene was 370 F g-1 as compared with 114 and 129 F g-1 for graphene oxide and tetramer aniline, respectively.9 The oligomer attached to graphene had a significant electrochemical catalytic effect towards nifedipine.9 Graphene is a promising carbon material for hydrogenation catalysis.10 Thermal stability, modifiable surface, high electron mobility, conjugated π-bond system on the surface, potentially high specific surface area are the main reasons for the excellent catalytic hydrogenation activity of graphene.10 The above-discussed examples of the catalytic effects of graphene and its derivatives cannot be considered exhaustive in the field in which 7-8 thousand papers have been published. Each field of catalysis requires special additives and many of them may include or will include graphene for two main reasons: graphene support goes beyond supporting. It enhances activity by synergistic effects. Also, expensive noble metals can be replaced by cheaper systems. REFERENCES 1 2 3 4 5 6 7 8 9 10

Albero, J; Garcia, H, J. Molec. Catalysis A: Chem., 408, 296-309, 2015. Eftekhari, A; Garcia, H, Mater. Today Chem., 4, 1-16, 2017. Yang, Y; Reber, AC; Lilliland, SE; Castano, CE; Gupton, BF; Khanna, SN, J. Catalysis, 360, 20-6, 2018. Ye, R; Dong, J; Wang, L; Mendoza-Cruz, R; Li, Y; An, P-F; Yacaman, MJ; Yakobson, BI; Chen, D; Tour, JM, Carbon, 132, 623-31, 2018. Yin, R; Guo, W; Du, J; Zhou, X; Zheng, H; Wu, Q; Chang, J; Ren, N, Chem. Eng. J., 317, 632-9, 2017. Ahn, Y; Oh, H; Yoon, Y; Park, WK; Yang, WS; Kang, J-W, J. Environ. Chem. Eng., 5, 4, 3882-94, 2017. Chen, H; Carroll, KC, Environ. Pollution, 215, 96-102, 2016. Reda, M; Hansen, HA; Vegge, T, Catalysis Today, 312, 118-25, 2018. Wang, S; Li, L; Wang, Q; Fan, Y; Shen, J; Zhang, K; Yang, Zhang, W, Synthetic Metals, 243, 107-14, 2018. Wei, Z; Guo, D; Hou, Y; Xu, H; Liu, Y, J. Taiwan, Inst. Chem. Eng., 67, 126-39, 2016.

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8.6 COMPOSITES The thermal conductivity of graphene-based polymer composites largely depends on the intrinsic microstructure of graphene (the in-plane sp2 structure of carbon favors phonon transfer via lattice vibrations).1 The reduction and post-thermal treatment repaired the defective sp2 carbon structure enhancing thermal conductivity. Small inter-particle distance and better integrity increase thermal conductivity.1 High orientation enhances the thermal conductivity of composite in a specific direction but anisotropic thermal property limits applications.1 If graphene loading is below 20 wt%, the thermal conductivity of composite does not exceed 10 Wm-1 K-1 in the isotropic system (insufficient for commercial applications).1 However, when adopting the orientation strategy, the thermal conductivity of oriented composites can easily exceed 10 Wm-1 K-1 even at low loading such as 1 wt%.1 Graphene oxide and reduced graphene oxide have reduced peak heat release rate of epoxy (by 47% at 3 wt% loading) at a low loading of even 1 wt%.2 The drastic reduction in peak heat release rate was attributed to reduced permeability of volatiles (reduces risk of combustion) and reduced radiant conductivity (formation of a continuous and compact char layer decreasing temperatures and slowing down chemical reactions).2 In situ polymerization of poly(vinyl alcohol), formaldehyde and graphene sheets was used to design foam which had to fulfill a combination of properties (elastomeric, mechanically robust, and flame retardant).3 The graphene sheets ~5 nm thick had a carbon to oxygen atomic ratio of 9.8 and a Raman ID/IG of 0.03.3 The composite had a limiting oxygen index of 59.4.3 Poly(ethylene glycol) grafted graphene was obtained by amidation of graphene oxide using methoxypolyethylene glycol amine and NaHB4 reduction.4 The presence of grafted graphene increased the electroactive crystalline content in poly(vinylidene fluoride) from 24.6% for the pure poly(vinylidene fluoride) to 90.5% for the grafted graphene (15 wt%)/ poly(vinylidene fluoride) composite because of the interfacial interaction.4 The grafted graphene (10 wt.%)/poly(vinylidene fluoride) composite near its percolation threshold had a dielectric constant of 53.3 compared to 8.2 for the pure poly(vinylidene fluoride).4 The introduced bubbles formed by foaming with supercritical carbon dioxide enhanced electrical conductivity and decreased percolation threshold from 0.34 to 0.16 vol%.5 Smaller bubbles and higher cell density increased electrical conductivity.5 Graphene, graphene oxide, and reduced graphene oxide were used for fabrication of multifunctional micro-nanofibrillated cellulose nanocomposites using a simple aqueous dispersion based mixing method.6 Graphene and reduced graphene oxide composites had a high electrical conductivity of 1.7 S and 0.5 S m-1, respectively, and graphene oxide reinforced composite was insulating.6 Poly(vinyl alcohol) was combined with graphene oxide and thermally reduced graphene oxide to prepare composites for supercapacitor applications.7 Reduced graphene oxide composite had capacitance of 190 Fg-1 as compared with 13 Fg-1 for graphene oxide composite.7 Electrochemical impedance of reduced graphene oxide composite was more than ten times smaller than that of graphene oxide composite.7 Reduced graphene oxide composite is a material of choice for supercapacitor applications as compared to polymer and graphene oxide composite.7

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Figure 8.16. Conductive composite formation, activation energy, and morphology of conductive network. [Adapted, by permission, from Xiang, M; Li, C; Ye, L, J. Ind. Eng. Chem., 62, 84-95, 2018.]

Figure 8.17. Schematic illustration of working mechanism of PANI or PANI-graphene films used as rain-enabled converters in electricity generation. [Adapted, by permission, from Wang, Y; Duan, J; Zhao, Y; He, B; Tang, Q, Renewable Energy, 125, 995, 1002, 2018.]

The graphene precursor with high grafting ratio having TDI-reactive sites of isocyanate groups was prepared and compounded with polyamide-6 via reactive melt processing.8 Polymer molecules reactively intercalated into reduced graphene oxide resulting with almost monolayer dispersion. At low percolation threshold, a conductive composite was formed (Figure 8.16).8

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Polyaniline and polyaniline/graphene composite films can convert rain energy into electricity (Figure 8.17).9 The converter is made of a conductive substrate and two collectors.9 A film-type rain-enabled converter is a Lewis acid due to the enriched π-electrons, whereas rainwater containing various of ions is regarded as a Lewis base.9 When rainwater is dropping onto the surface of film converter, cations from raindrops absorb π-electrons from converter to yield electrical double layer.9 Three-dimensional graphene/phenolic resin composite was synthesized via in-situ polymerization in graphene hydrogel.10 The water was replaced with resole resin by infiltration and the evaporation of water by simple heating.10 The compressive strength of composite (441.11 MPa) increased by 14.24% on addition of 2.15% graphene.10 Polypropylene composites filled with multilayer graphene sheets and graphite platelets were fabricated via a facile melt-mixing procedure.11 Young’s modulus and tensile strength of composites were improved simultaneously.11 A self-sensing composite coating, providing an early-warning signal of corrosion, was prepared from graphene oxide which was chemically modified with 1,10-phenanthroline-5-amine, which can form a red complex with Fe2+ signalizing corrosion at very early stage.12 So prepared graphene was dispersed in the waterborne polyurethane coating.12 Graphene oxide–poly(urea-formaldehyde) composite with 8.6 wt% graphene oxide sheets exhibited the optimal corrosion protection of mild steel.13 Figure 8.18 demonstrates the effect of loading on the corrosion protection.13 At low concentration of graphene oxide (Figure 8.18b), uniform urea-formaldehyde microspheres of 7.1 μm size were present, and only a few graphene oxide sheets with wrinkles were attached to the surface of the resin microspheres.13 With increased concentration of graphene oxide (Figure 8.18c) and (Fig-

Figure 8.18. SEM high magnification images of (a) graphene oxide sheets, and graphene oxide/poly(urea-formaldehyde) composites containing (b) 2.56, (c) 4.28, (d) 8.62, (e) 20.79 and (f) 34.83 wt% graphene oxide. [Adapted, by permission, from Zheng, H; Guo, M; Shao, Y; Wang, Y; Liu, B; Meng, G, Corrosion Sci., 139, 1-12, 2018.]

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Figure 8.19. Optical micrographs of typical electric-field induced the alignment of graphene in the epoxy matrix: (a) Randomly-oriented graphene in original sample; (b), (c), and (d) after the field was serviced for 4 min, 10  min, and 20 min, respectively. (“ +” and “-” represent the positive and negative electrodes.) [Adapted, by permission, from Zhang, Z; Qu, J; Febg, Y; Feng, W, Compos. Commun., 9, 33-41, 2018.]

ure 8.18d), microspheres had sizes of 5.1 and 3.4 μm, respectively.13 These microspheres were grafted on large areas of graphene oxide sheets with wrinkles and wavy features.13 The lamellar structure of graphene oxide was not destroyed by the process.13 The size of the urea-formaldehyde microspheres decreased to 0.6 μm when the concentration of graphene oxide was further increased (Figures 8.18e&f), and agglomeration occurred.13 At the highest concentration of graphene oxide, the resin microspheres completely disappeared. The high concentrations of graphene oxide caused the formation of non-uniform stress, which led to cracks in the coatings and deteriorated the corrosion protection.13 Spherical polystyrene particles (340–370 nm) adsorbed on the graphene surface by π-π stacking prevented the graphene re-agglomeration.14 The graphene/polystyrene composite containing 0.0038 wt% graphene-enhanced laser patterning of composite.14 Being an efficient absorber of 1064 nm NIR laser, only 0.005 wt% graphene endows most polymer materials with an excellent laser patterning performance.14 The bulk-modified ultrahigh molecular weight polyethylene is used for joint replacement.15 Composite coatings were prepared with 0-4.6 wt% of graphene nanoplatelets and 1-2 layered graphene.15 The spray-coated polymer had 40% lower friction than UHM-

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WPE.15 The elastic modulus was increased by 10% and hardness by 30% in composite coatings containing 2-5 wt% graphene.15 The microwave absorption properties of silicone rubber were improved by small additions of holey graphene.16 With 1 wt% loading, the return loss of 2 mm thick composite reached -32.1 dB at 13.2 GHz.16 The defect-induced losses, interfacial polarization, and multiple reflection/scattering at the interfaces were the major loss mechanisms (Figure 5.50). Further details on microwave absorption can be found in a review paper.17 Composites containing aligned graphene nanoparticles have improved thermal conductivity. Figure 8.19 shows the effect of electric field on graphene alignment in an epoxy matrix.18 With an increase of time, graphene nanoparticles rotated from the original random dispersion to the oriented state and formed chain-like structures aligned in the direction of the external electric field.18 The composites show containing the aligned graphene had excellent thermal conductivities and anisotropic thermal conduction.18 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Fang, H; Bai, S-L; Wong, CP, Compos. Part A: Appl. Sci. Manuf., 112, 216-38, 2018. Zhang, Q; Wang, YC; Bailey, CG; Yuen, RKK; Parkin, J; Yang, W; Valles, C, Compos. Part B: Eng., 146, 76-87, 2018. Araby, S; Li, J; Shi, G; Ma, Z; Ma, J, Compos. Part A: Appl. Sci. Manuf., 101, 254-64, 2017. Chen, J-J; Li, Y; Zheng, X-M; He, F-A; Lam, K-H, Appl. Surf. Sci., 448, 320-30, 2018. Xiao, W; Liao, X; Jiang, Q; Zhang, Y; Yang, Q; Li, G, J. Supercritical Fluids, in press, 2018. Phiri, J; Johansson, L-S; Gane, P; Maloney, T, Compos. Part B: Eng., 147, 104-13, 2018. Theophile, N; Jeong, NK, Chem. Phys. Lett., 669, 125-9, 2017. Xiang, M; Li, C; Ye, L, J. Ind. Eng. Chem., 62, 84-95, 2018. Wang, Y; Duan, J; Zhao, Y; He, B; Tang, Q, Renewable Energy, 125, 995, 1002, 2018. Yang, G; Wang, Y; Xu, H; Zhou, S; Jia, S; Zang, J, Appl. Surf. Sci., 447, 837-44, 2018. Ren, Y; Zhang, Y; Fang, H; Ding, T; Bai, S-L, Compos. Part A: Appl. Sci. Manuf., 112, 57-63, 2018. Li, J; Jiang, Z; Gan, L; Qiu, H; Yang, G; Yang, J, Compos. Commun., 9, 6-10, 2018. Zheng, H; Guo, M; Shao, Y; Wang, Y; Liu, B; Meng, G, Corrosion Sci., 139, 1-12, 2018. Xie, Y; Wen, L; Zhang, J; Zhou, T, Mater. Design, 141, 159-69, 2018. Chih, A; Anson-Casaos, Pertolas, JA, Trib. Int., 116, 295-302, 2017. Chen, C-Y; Pu, N-W; Liu, Y-M; Chen, L-H; Wu, C-H; Cheng, T-Y; Lin, M-H; Ger, M-D; Gong, Y-J; Peng, Y-Y; Grubb, PM; Chen, RT, Compos. Part B: Eng., 135, 119-28, 2018. Meng, F; Wang, H; Huang, F; Guo, Y; Zhou, Z, Compos. Part B: Eng., 137, 260-77, 2018. Zhang, Z; Qu, J; Febg, Y; Feng, W, Compos. Commun., 9, 33-41, 2018.

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8.7 CONCRETE ADMIXTURES Concrete having 1.5% of graphene nanoplatelet showed the greatest reduction in transport; water penetration depth, chloride diffusion, and migration coefficients which were reduced by 80, 80, and 37%, respectively.1 The barrier effect was caused by tortuosity and pore refinement.1 Electromagnetic shielding and propagation in concrete structures are important for protection against radiation hazard and wireless communication protection.2 Use of conductive concrete composites in place of metallic shielded rooms can be effective due to the ease of plastering the existing walls.2 The addition of graphene oxide microparticles and steel fibers is a promising class of EM shielding materials.2 They provide strong efficacy with time.2

Figure 8.20. SEM images for different cement mortar components with MLG. a) Accumulation of several etringite crystals, which have been shaped in the presence of humidity from the hardened cement paste; b) Empty air space in which secondary etringite crystals seem to have been assembled, along with prismatic calcium hydroxides (CH’s); c) prismatic calcium hydroxides (CH’s), hexagonal crystals that are assembled in a rose-shaped arrangement and ettringite crystals; d) Graphene sheet immersed in the cement paste and non-hydrated area. [Adapted, by permission, from Alves e Silva, R; de Castro Guetti, P; Sérgio da Luz, M; Rouxinol, F; Valentim Gelamo, Constr. Build. Mater., 149, 378-85, 2017.]

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The graphene nanoplatelets improved the fracture energy of the mortar by 1700% with graphene contents of 0.4 wt%.3 The improvement was validated by the acoustic emission data which highlighted high energy dissipation potential of graphene nanoplatelets.3 The fracture mode of the mortars was changed from shear to tensile because of the presence of graphene platelets.3 The possible mechanisms include crack splitting and deflection at nanoplatelet locations, load-sharing by stretching of nanoplatelets, crack bridging by intact graphene, and pull-out of failed graphene platelets from the cement bulk.3 The shear stress-displacement curve, which represented the bond-slip relation, has been calculated for a graphene oxide/cement nanocomposite at a molecular level.4 The shear strength was 647.58±91.18 MPa, which indicated strong interfacial bonding strength in graphene oxide cement.4 Graphene oxide was used as a surface sealer for cementitious mortars.5 Graphene oxide effectively mitigated moisture loss and facilitated the hydration process, causing densification of the microstructure of cementitious mortars.5 The presence of graphene oxide reduced drying shrinkage of mortar due to better water retention.5 The addition of graphene promoted the cement hydration process and interfacial bond formation through the bridging and nucleation effects.6 Concrete with smaller pore size and homogeneous pore distribution was obtained which benefited its durability.6 Incorporation of multilayer graphene into mortars increased their strength, thermal stresses were reduced, and hydration increased.7 The optimal tensile strength was achieved with 0.033% addition of multilayer graphene.7 The addition of graphene accelerated cement hydration reactions, reduced the pore volume, and hardened cement properties.7 Figure 8.20 shows some morphological features of the mortars.7 Graphene acted as a structural binder, reducing microcrack occurrences.7 The compressive strengths after 3 and 7 days of composites containing 0.2 wt% graphene oxide were increased by 35.7 and 42.3%, respectively, as compared with control.8 The 29Si NMR measurements showed that the addition of graphene oxide improved hydration degree.8 During the initial stage of hydration, graphene oxide absorbed water, Ca2+ ions, and oxygen molecules which improved the hydration degree.8 The –COOH groups of graphene oxide reacted with the hydration product of cement (Ca(OH)2).9 The chemical reactions connected the graphene oxide nanosheets to each other, producing a 3D network in modified cement.9 The hydration products were inserted into this 3D network structure.9 Graphene oxide accelerated the hydration rate of cement due to its catalytic behavior in which the oxygen-containing functional groups of graphene oxide provided adsorption sites for both water molecules and cement components.10 Introduction of 0.02% graphene decreased the chloride penetration depth and coefficient by 37 and 42%, respectively, which can be attributed to the enhanced degree of cement hydration, filling effect, barrier effect, and crack-arresting effect.11 Figure 8.21 illustrates these effects.11 The addition of 2.5% graphene caused a decrease of 64, 70, and 31% of water penetration depth, chloride diffusion coefficient, and chloride migration coefficients, respectively.12 The reduced water and ions ingress were attributed to the reduction in the critical pore diameter by 30% and increased tortuosity of the pathway.12

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Figure 8.21. SEM images of graphene in cement matrix: (a) a typically pull-out graphene (b) a filling effect; (c) and (d) barrier effect; (e) and (f) crack-arresting effect. [Adapted, by permission, from Wang, B; Zhao, R, Constr. Build. Mater., 161, 715-22, 2018.]

Sacrificial concrete was designed to reduce the leakage potential of radioactive materials in nuclear accidents through its encasing function.13 The compressive strength, splitting tensile strength, thermal diffusivity, and decomposition enthalpy of sacrificial concrete were increased by 12.98-25.36%, 8.66-34.38%, 25.00-103.23% and 4.23%, respectively, with addition of 0.1 wt% graphene sulfonate nanosheets, whereas the porosity and ablation velocity of sacrificial concrete were reduced by 3.01-6.99% and 4.14%, respectively.13 The rheological properties and morphology of fresh cement pastes containing graphene oxide were investigated.14 The cement particles were re-agglomerated, and new flocculated structures were generated due to the addition of graphene oxide, which significantly altered the rheological properties of the pastes.14 The yield stress, plastic viscosity, and the area of the hysteresis loop were increased by the increase in the content of graphene oxide.14 REFERENCES 1 2 3 4 5 6 7 8

Du, H; Gao, HJ; Pang, SD, Cement Concrete Res., 83, 114-23, 2016. Mazzoli, A; Corinaldesi, V; Donnini, J; Di Perna, D; Micheli, D; Vricella, A; Pastore, R; Bastianelli, L; Moglie, F; Mariani Primiani, V, J. Build. Eng., 18, 33-9, 2018. Tragazikis, IK; Dassios, KG; Dalla, PT; Exarchos, DA; Matikas, TE, Eng. Fracture Mech., in press, 2018. Fan, D; Lue, L; Yang, S, Comput. Mater. Sci., 139, 56-64, 2017. He, J; Du, S; Yang, Z; Shi, X, Constr. Build. Mater., 162, 64-79, 2018. Yang, H; Cui, H; Tang, W; Li, Z; Han, N; Xing, F, Compos. Part A: Appl. Sci. Manuf., 102, 273-96, 2017. Alves e Silva, R; de Castro Guetti, P; Sérgio da Luz, M; Rouxinol, F; Valentim Gelamo, Constr. Build. Mater., 149, 378-85, 2017. Yang, H; Monasterio, M; Cui, H; Han, N, Compos. Part A: Appl. Sci. Manuf., 102, 263-72, 2017.

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Wang, M; Wang, R; Yao, H; Farhan, S; Zheng, S; Du, C, Constr. Build. Mater., 126, 730-9, 2016. Lin, C; Wei, W; Hu, YH, J. Phys. Chem. Solids, 89, 128-33, 2016. Wang, B; Zhao, R, Constr. Build. Mater., 161, 715-22, 2018. Du, H; Pang, SD, Cement Concrete Res., 76, 10-9, 2015. Chu, H-y; Jiang, J-y; Sun, W; Zhang, M, Constr. Build. Mater., 153, 682-94, 2017. Wang, Q; Wang, J; Lv, C-x; Cui, X-y; Li, S-y; Wang, X, New Carbon Mater., 31, 6, 574-84, 2016.

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Figure 8.22. The size of the remaining zinc particles at the failure stage. [Adapted, by permission, from Ding, R; Zheng, Y; Yu, H; Li, W; Wang, X; Gui, T, J. Alloys Compounds, 748, 481-95, 2018.]

8.8 CORROSION PROTECTION Graphene/zinc-containing coatings were used for corrosion protection of Q235 steel.1 The analysis of coating deterioration process was divided into five stages including initial shielding, fluctuation, cathodic protection, shielding, and failure.1 In the beginning, the initial infiltration of corrosive media and activation of zinc particles occurred, followed (cathodic protection stage) by the anode sacrificial reaction of zinc powder.1 Then, the steel substrate began to corrode (shielding and failure stage).1 Graphene was instrumental in the improvement of electrical contact between zinc particles and their interaction with steel which improved the utilization of zinc, increased cathodic protection current, and enhanced the protective functions of the coating.1 Figure 8.22 shows that at the failure state, the size of the remaining zinc particles in the graphene-containing coatings was sig-

Figure 8.23. Barrier effect of graphene on the diffusion of corrosive media in coatings. [Adapted, by permission, from Ding, R; Zheng, Y; Yu, H; Li, W; Wang, X; Gui, T, J. Alloys Compounds, 748, 481-95, 2018.]

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Figure 8.24. The maximum depth at which the corrosion products of zinc were detected by the microscopic Raman spectroscopy. [Adapted, by permission, from Ding, R; Zheng, Y; Yu, H; Li, W; Wang, X; Gui, T, J. Alloys Compounds, 748, 481-95, 2018.]

nificantly smaller than in the non-graphene coatings meaning that graphene improved the utilization of zinc.1 Graphene sheets improved the shielding performance of the coating and reduced water diffusion enhancing barrier properties of coating (Figure 8.23).1 The best performance of the coating was when a graphene content was 0.3 wt% which was sufficient to reduce the diffusion of water and increase the diffusion pathway.1 Also, graphene significantly delayed the formation of products of zinc corrosion within the coating by slowing down the penetration of corrosive media.1 Figure 8.24 shows the maximum depth at which the corrosion products of zinc, zinc oxide, and basic zinc chloride, were detected after immersion for different times.1 The orientation of the graphene sheets in the coatings parallel to the surface of the protected metal helps in maximizing its barrier Figure 8.25. Three scenarios of cathodic delaminaproperties, but, in practice, the graphene tion where: a) For uninhibited coatings, cathodic O2 reduction occurs at the coating/substrate interface nanosheets in the coatings are disordered, and b) in-coating graphene may increase the tortuosity there is no clear guidance on how to induce of O2 diffusion pathway c) in-coating graphene may 2 displace O2 reduction away from coating/substrate alignment. The graphene coatings give only interface. [Adapted, by permission, from Glover, passive protection (when mechanically damCF; Richards, CAJ; Williams, G; McMurray, HN, aged they lack a self-repair mechanism).2 Corrosion, Sci., 136, 304-10, 2018.]

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Figure 8.26. The corrosion mechanisms of pure epoxy (a), epoxy coating containing unmodified graphene (b) and epoxy coating containing modified graphene (c) immersed in salt spray chamber for 1000 h. [Adapted, by permission, from Ding, J-H; Zhao, H-R; Zheng, Y; Zhao, X; Yu, H-B, Carbon, 138, 197-206, 2018.]

The corrosion-driven cathodic delamination kinetics of coating comprising graphene nanoplatelets dispersed in polyvinylbutyral from iron and zinc (galvanized steel) was studied by in situ scanning Kelvin probe measurements.3 For iron surfaces, a vertical diffusion of oxygen through the coating was the rate-limiting process.3 A graphene volume fraction of 0.056 was required on iron, but only 0.028 on zinc to reduce delamination rates by >90%.3 Graphene slowed through-coating oxygen transport on iron; whereas on zinc, a galvanic couple formed between zinc and graphene, displacing cathodic oxygen reduction (Figure 8.25).3 The oxygen permeation rates through a polyvinylbutyral/graphene composite coating decreased by over an order of magnitude when graphene volume fraction increased to 0.056.4 Uniform graphene coating on a metal surface can inhibit corrosion-initiated degradation of copper and nickel.5 A non-uniform graphene coverage has an enormous effect on corrosion protection and may even lead to the acceleration of corrosion.5 When immersed

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in NaCl solution, the exposed edges of graphene become the centers for iron oxidation, and propagation.5 Chlorine increases metal dissolution and results in significant oxidation sites along the graphene edge line.5 It is thus concluded that graphene impermeability to liquids and gases can make it the thinnest anticorrosion coating possible, but the existence of defects may ruin all its advantages.5 When the concentration of the graphene edges is high, the corrosion is accelerated due to the trapping of chlorine ions near the iron surface.5 Graphene is impermeable to all molecules and has excellent chemical stability, but it is conductive, and it is cathodic to most metals which may aggravate metal corrosion at exposed metal-coatings interfaces.6 This effect caused rapid localized corrosion.6 It is possible to synthesize nonconductive graphene using the Diels-Alder reaction between exfoliated graphene and a biobased epoxy monomer for the benefit of epoxy anticorrosive coatings.6 Addition of 0.5 wt% of modified graphene to epoxy coating improved its barrier properties and gave superior corrosion resistance in comparison to pure epoxy.6 Figure 8.26 compares the performance of three coatings differing in composition.6 Graphene oxide/poly(urea-formaldehyde) composites containing 8.6 wt% graphene oxide sheets exhibited the optimal corrosion protection of mild steel.7 They were prepared by anchoring urea-formaldehyde resin prepolymer onto graphene oxide sheets through in situ polycondensation and addition to epoxy resin.7 Figure 8.18 and associated text provide a discussion of graphene dispersion and effect of its concentration on the composite properties.7 Graphene oxide was functionalized with 4-nitroaniline, added to the epoxy resin, and coated on mild steel.8 Incorporation of 0.5 wt% functionalized graphene oxide enhanced the corrosion resistance.8 The interlayer distance of the graphene oxide sheets has been significantly decreased by functionalization and dispersion was improved leading to an increased ionic resistance of the coating.8 3-(Aminopropyl)triethoxysilane was hydrolyzed with acidified demineralized water (pH=5) and used to chemically modify graphene.9 The functionalized graphene was applied on ultrasonically cleaned mild steel using a bar applicator (20-micron wet film thickness).9 Figure 8.27 illustrates Figure 8.27. Schematic diagram of functionalized functionalized graphene coating.9 A subgraphene coating. [Adapted, by permission, from stantial decrease in the water uptake and Aneja, KS; Mallika-Böhm, HL; Khanna, AS; diffusion coefficient, as a function of Böhm, S, FlatChem, 1, 11-9, 2017.] graphene concentration, provided improved barrier protection.9 Graphene increased the activation energy peak for the water diffusion process, making it difficult for ions to permeate through the coating.9 A trilaminar structure, composed of Fe-W amorphous alloy layer, silane crosslinked graphene oxide, and a hydrophobic organosilane, was formed layer by layer via electroplating, electrophoresis, and vapor deposition, respectively, (Figure 8.28) to hinder diffu-

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sion of water molecules and corrosive ions to the metal bases and to retard the corrosion reactions.10 The formation of Si−O− C− covalent bonds improved binding force between silane and the graphene oxide sheets.10 Polymer-graphene hybrid coating, comprising two single layers of chemical vapor deposited graphene sandwiched between three layers of polyvinylbutyral provided complete corrosion proFigure 8.28. Cross-section view and sketch map of composite coat- tection of commercial aluminum ing. [Adapted, by permission, from Liang, J; Wu, X-W; Ling, Y; alloys even after 120 days of Yu, S; Zhang, Z, Surf. Coat. Technol., 339, 65-77, 2018.] exposure to simulated sea water.11 11 Figure 8.29 shows the method of coating preparation. A hybrid silane-graphene film (heptadecafluorodecyl trimethoxysilane and γ-(2,3epoxypropoxy)propyltrimethoxysilane) was prepared on 2024 aluminum alloy surface.12 The high crosslink density promoted the barrier property of functionalized graphene film against aggressive ions and prolonged the performance time in NaCl solution.12 The 3,4,9,10-perylene tetracarboxylic acid functionalized graphene was used for the corrosion protection of epoxy-coated Q235 steel.13 The improved corrosion resistance of epoxy coating might be attributed to good dispersion of functionalized graphene and its barrier properties.13 A polyaniline-graphene oxide composite coating was deposited on the 316 stainless steel by a pulse current codeposition method, forming a compact coating on the stainless steel surface.14 The corrosion inhibition efficiency and protection efficiency of the com-

Figure 8.29. Schematic illustration of preparation steps of polymer-graphene hybrid coatings on aluminum, AA. [Adapted, by permission, from Yu, F; Camilli, L; Wang, T; MacKenzie, DMA; Curioni, M; Akid, R; Boggild, P, Carbon, 132, 78-84, 2018.]

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posite coating reached 98.4% and 99.3%, respectively.14 A composite coating having higher hydrophobicity, and lower porosity inhibited the adsorption and transfer of corrosive media (H2O, O2, Cl−, etc.).14 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Ding, R; Zheng, Y; Yu, H; Li, W; Wang, X; Gui, T, J. Alloys Compounds, 748, 481-95, 2018. Ding, R; Li, W; Wang, X; Gui, T; Li, B; Han, P; Tian, H; Liu, A; Wang, X; Gao, X; Wang, W; Song, L, J. Alloys Compounds, 764, 1039-55, 2018. Glover, CF; Richards, CAJ; Williams, G; McMurray, HN, Corrosion, Sci., 136, 304-10, 2018. Richards, CAJ; Glover, CF; Williams, G; McMurray, HN; Baker, J, Corrosion Sci., 136, 285-91, 2018. Lee, J; Berman, D, Carbon, 126, 225-31, 2018. Ding, J-H; Zhao, H-R; Zheng, Y; Zhao, X; Yu, H-B, Carbon, 138, 197-206, 2018. Zheng, H; Guo, M; Shao, Y; Wang, Y; Liu, B; Meng, G, Corrosion Sci., 139, 1-12, 2018. Nayak, SR; Mohana, KNS, Surf. Interfaces, 11, 63-73, 2018. Aneja, KS; Mallika-Böhm, HL; Khanna, AS; Böhm, S, FlatChem, 1, 11-9, 2017. Liang, J; Wu, X-W; Ling, Y; Yu, S; Zhang, Z, Surf. Coat. Technol., 339, 65-77, 2018. Yu, F; Camilli, L; Wang, T; MacKenzie, DMA; Curioni, M; Akid, R; Boggild, P, Carbon, 132, 78-84, 2018. Dun, Y; Zhao, X; Tang, Y; Dino, S; Zuo, Y, Appl. Surf. Sci., 437, 152-60, 2018. Yang, T; Cui, Y; Li, Z; Zeng, H; Luo, S; Li, W, J. Hazardous Mater., 357, 475-82, 2018. Qiu, C; Liu, D; Jin, K; Fang, L; Xie, G; Robertson, J, Mater. Chem. Phys., 198, 90-8, 2017.

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Figure 8.30. Schematic illustration of the preparation of MGO, loading PAC, linkage with APT and specific targeting to MCF-7 cancer cell. (Symbols explained in the text). [Adapted, by permission, from Hussien, NA; Isiklan, N; Turk, M, Mater. Chem. Phys., 211, 479-88, 2018.]

8.9 DRUG DELIVERY SYSTEMS Aptamer-conjugated magnetic graphene oxide (MGO) nanocarrier was developed to target tumor cells.1 The Fe3O4 was attached to the layer of graphene oxide (GO) followed by linking aptamer (APT) to a targeting moiety. An anti-cancer drug (Paclitaxel, (PAC)) was loaded onto the nanocarrier (entrapment efficiency 95.75% and high pH-responsive release).1 The π−π stacking and hydrophobic interactions facilitated drug loading onto nanocarrier.1 Figure 8.30 illustrates preparation and targeting.1 The nanocarrier was biocompatible (cell viability greater than 80% for L-929 fibroblast cell line).1 Amino groups were introduced into graphene oxide to form aminated fumed graphene which was then combined with carboxymethylcellulose to produce a drug carrier matrix.2 The anti-cancer drug, small molecule doxorubicin hydrochloride was bound to the carrier by π-π bond interaction and hydrogen bonding to form a drug loading system.2 The drug delivery system had a great anti-tumor activity and was safer than the simple doxorubicin administration.2 Graphene oxide synthesized by Hummer's method was loaded into polyethylene glycol, decorated with folic acid, and combined with anti-cancer drug camptothecin.3 The drug delivery system showed a pH-dependent drug release.3 Enhanced anti-cancer activity was found with this delivery system.3 Fluorinated graphene exhibits numerous excellent properties, but its chemical inertness and hydrophobicity limit its further application.4 The chemical introduction of oxygen facilitated the modification of fluorinated graphene oxide with folic acid and induced targeted endocytose into cancer cells.4 The combination had bright fluorescence (robust under both acidic and alkaline conditions), high near-infrared absorption, and pH-responsive drug delivery.4 The adjustment of size into nanoscale (~50 nm) provided fluorinated graphene oxide with superior photothermal performance. Enhanced hyperthermia, targeting specificity, and intracellular acid condition-triggered drug release guarantees the excellent therapeutic effects.4 It also allows switchable luminescence for monitoring the

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Figure 8.31. Schematic illustration for preparation of targeted fluorinated graphene oxide drug delivery system by controlling the structure and surface chemistry, switchable fluorescence, and synergistic therapy. (FGI − fluorinated graphite; FGO − fluorinated graphene oxide; FG − fluorinated graphene; FA − folic acid, DOX − doxorubicin). [Adapted, by permission, from Gong, P; Ji, S; Wang, J; Dai, D; Wang, F; Tian, M; Zhang, L; Guo, F; Liu, Z, Chem. Eng. J., 348, 438-46, 2018.]

drug loading and release in addition to better cancer therapeutic effects obtained by synergistic chemo-photothermal therapy.4 Figure 8.31 illustrates the formation and synergistic action of a drug delivery system.4 The nanosized fluorinated graphene oxide also exhibits bright photoluminescence.4 The fluorescence signal can be turned off or on depending on the loading or release of doxorubicin.4 Arginine-glycine-aspartic acid-conjugated graphene quantum dots were synthesized and utilized to load the anti-tumor drug doxorubicin for the targeted cancer fluorescence imaging as well as tracking and monitoring drug delivery without the need for external dyes.5 The release of doxorubicin demonstrated strong pH-dependence implying hydrogen-bonding interaction between graphene quantum dots and doxorubicin.5 Not only doxorubicin but also some graphene quantum dots penetrated into cell nuclei after 16 h of incubation which dramatically improved the cytotoxicity of doxorubicin.5 A graphene oxide-based, sodium alginate functionalized colon-targeting drug delivery system was loaded with 5-fluorouracil (5-FU) as the anti-cancer drug.6 The drug delivery system possessed much lower toxicity and better colon-targeting controlled-release behavior. It inhibited tumor growth and liver metastasis and prolonged the survival time of mice.6 Graphene oxide was incorporated into 3D porous bacterial cellulose which greatly increased the drug loading capacity of porous bacterial cellulose.7 Graphene-based nanocarriers prevented drugs from premature release outside the target cells.7 Ibuprofen was loaded onto the nanocomposite.7 The drug release followed a non-Fickian diffusion mech-

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Figure 8.32. SEM images of bacterial cellulose (a), ibuprofen in bacterial cellulose (b), ibuprofen in 0.19 wt% graphene-modified system (c and d), and ibuprofen in 0.48 wt% graphene-modified system (e and f) (red arrows indicate ibuprofen and yellow arrows indicate graphene oxide). [Adapted, by permission, from Luo, H; Ao, H; Li, G; Li, W; Xiong, G; Zhu, Y; Wan, Y, Current Appl. Phys., 17, 2, 249-54, 2017.]

anism.7 Figure 8.32 illustrates morphology of the drug release system as well as shows distribution of drug and graphene oxide.7 The ibuprofen loading capacity increased with the content of graphene oxide. The drug was simultaneously carried by both bacterial cellulose and graphene oxide.7 The targeted delivery and controlled release of cisplatin were accomplished using doubly decorated mesoporous silica nanoparticles, internally grafted with fluorescent conjugates and externally coated with polydopamine and graphene oxide layers.8 The brushlike internal conjugates granted fluorescent functionality and high capacity for cisplatin loading and also contributed to a sustained release of the cisplatin through a porous chan-

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Figure 8.33. Illustration of multifunctional mesoporous silica nanoparticles, MSNs, and their release and cytotoxic operations without (a) and with (b) graphene oxide. (A-F − fluorescent conjugates; PDA − polydopamine; GO − graphene oxide). [Adapted, by permission, from Tran, A-V; Shim, KH; Thi, T-TV; Kook, J-K; An, SSA; Lee, S-W, Acta Biomaterialia, 74, 397-413, 2018.]

nel with the assistance of external polydopamine layer.8 A double-layer formed by electrostatic interaction between the graphene oxide nanosheets and the polydopamine layer induced controlled release kinetics.8 The release was controlled by pH and NIR radiation, making it chemo-photothermal agent against cancer cells having a high cytotoxicity against human epithelial neuroblastoma cells.8 Figure 8.33 illustrates the structure and performance of drug release system.8 Graphene oxide wrapping improved the dispensability and cellular uptake of the mesoporous silica nanoparticles, as well as provided wellcontrolled drug release.8 The reduction of graphene oxide at body temperature by using a biopolymer (chitosan) provided a versatile platform for applying graphene in biomedical fields including tissue engineering and therapeutic delivery.9 Graphene oxide was reduced at 37°C.9 The 73% of oxygenated functional groups were removed from graphene oxide which is comparable to reduction effectiveness at higher temperature (90°C).9 The reduced graphene oxide was stable in water, phosphate buffered saline, and cell culturing media, and demonstrated pH-sensitive drug release profile.9 Chitosan derivatives/reduced graphene oxide blending with alginate was suitable for preparation of hydrogel beads for small-molecule drug delivery.10 A high drug-loading efficiency of 82.8% was obtained with small-molecule fluorescein sodium and outstanding release of 71.6% (150 h at a physiological pH) as well as a quick-release of 82.4% drug content (20 h in an acidic medium).10 Figure 8.34 illustrates assembly process.10 The literature contains thousands of variations of these important for the development of medicine drug carriers which may revolutionize the ways we treat cancer.

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Figure 8.34. Schematic illustration of assembly process of sodium fluorescein loaded chitosan derivative, CSD/ reduced graphene oxide, rGO/alginate beads. [Adapted, by permission, from Chen, K; Ling, Y; Cao, C; Li, X; Chen, X; Wang, X, Mater. Sci. Eng.: C, 69, 1222-8, 2016.]

REFERENCES 1 2 3 4 5 6 7 8 9 10

Hussien, NA; Isiklan, N; Turk, M, Mater. Chem. Phys., 211, 479-88, 2018. Rao, Z; Ge, H; Liu, L; Zhu, C; Min, L; Liu, M; Fan, L; Li, D, Int. J. Biol. Macromol., 107A, 1184-92, 2018. Deb, A; Vimal, R, J. Drug Delivery Sci. Technol., 43, 333-42, 2018. Gong, P; Ji, S; Wang, J; Dai, D; Wang, F; Tian, M; Zhang, L; Guo, F; Liu, Z, Chem. Eng. J., 348, 438-46, 2018. Dong, J; Wang, K; Sun, L; Sun, B; Dong, L, Sensors Actuators B: Chem., 256, 613-23, 2018. Zhang, B; Yan, Y; Shen, Q; Ma, D; Huang, L; Cai, X; Tan, S, Mater. Sci. Eng.: C, 79, 185-90, 2017. Luo, H; Ao, H; Li, G; Li, W; Xiong, G; Zhu, Y; Wan, Y, Current Appl. Phys., 17, 2, 249-54, 2017. Tran, A-V; Shim, KH; Thi, T-TV; Kook, J-K; An, SSA; Lee, S-W, Acta Biomaterialia, 74, 397-413, 2018. Justin, R; Chen, B, Mater. Sci. Eng.: C, 34, 50-3, 2014. Chen, K; Ling, Y; Cao, C; Li, X; Chen, X; Wang, X, Mater. Sci. Eng.: C, 69, 1222-8, 2016.

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Figure 8.35. TEM images of silicon nanoball encapsulated with graphene at different resolutions (a = 50 nm, b = 20 nm, c = 10 nm, d = 5 nm). [Adapted, by permission, from Kim, H; Hwang, T; Kang, K; Pichler-Nagi, J; So, D-S; Park, S; Huh, H, J. Ind. Eng. Chem., 50, 115-22, 2017.]

8.10 ENCAPSULATION Composites for encapsulation of solar cells and cooling of electronic devices were fabricated from poly(vinyl butyral) containing graphene as thermal conductivity enhancement filler using solution blending.1 The thermal conductivity of encapsulant containing 30 wt% graphene was 4.521 W/mK (20.55 times higher than that of pure PVB).1 The heating and cooling rates of solar cells were increased by 28 and 37%, respectively.1 The composite had ionic conductivity lower than 10-5 S/m.1 In situ grown (N-doped) graphene-encapsulated Ni nanoparticles were obtained using an arc-discharge method for syngas conversion.2 It is composed of a graphene sheath and a metallic nickel core.2 The carbon layer is to prevent the inner nickel nanoparticles from etching on exposure to air, H2O2, or acid.2 The encapsulated catalyst exhibited excellent activity, methane selectivity, and high stability in the methanation reaction.2 Its performance can be further improved by nitrogen doping into the graphene shell.2 Carbon materials are widely used as the anode materials, but, they are limited by a low capacity.3 Silicon has a four times higher capacity than carbon materials, but it has low stability due to the volume expansion.3 A silicon nanoball encapsulated with graphene is one of the potential solutions to overcome the problems.3 The nanoball has a core-shell structure. Figure 8.35 shows the morphology of graphene-encapsulated silica.3 There is a gap between the inner core (silicon) and the outer graphene shell (dark gray).3 The capacity of the nanoball anode was 611.26 mAh/g (10 times higher than that of the pure-Si anode − 56.297 mAh/g).3 The capacity retention after 10 cycles was 67% (the pure-Si

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anode 38%).3 The performance was improved because of the high electron carrier mobility of graphene supported the lithiation reaction of silicon and the protective action of encapsulation.3 The graphene-encapsulated copper nanoparticles were obtained by carbonizing a mixture of kraft lignin and copper sulfate pentahydrate at the temperature of 500°C.4 The copper ions were converted into its atoms at 300°C.4 The graphene layers surrounding copper nanoparticles began to form at 400°C.4 Most copper nanoparticles were encapsulated with less than five graphene layers when the temperature reached 500°C.4 The average diameter of graphene-encapsulated copper nanoparticle was 12.75 and 11.62 nm at 400 and 500°C, respectively.4 The formation of the graphene-layered shell surrounding copper nanoparticles was based on the mechanism of self-limiting theory.4 A slow-release fertilizer was developed by encapsulating KNO3 pellets with graphene oxide.5 When subjected to heat treatment, the graphene oxide sheets were soldered and reduced by potassium, dramatically improving fertilizer release characteristics.5 The release of fertilizer takes up to 8 h in water.5 REFERENCES 1 2 3 4 5

Huang, X; Lin, Y; Fang, G, Solar Energy, 161, 187-93, 2018. Wang, C; Zhai, P; Zhang, Z; Ma, D, J. Catalysis, 334,42-51, 2016. Kim, H; Hwang, T; Kang, K; Pichler-Nagi, J; So, D-S; Park, S; Huh, H, J. Ind. Eng. Chem., 50, 115-22, 2017. Leng, W; Barnes, M; Yan, C; Cai, Z; Zhang, J, Mater. Lett., 185, 131-4, 2016. Zhang, M; Gao, B; Chen, J; Li, Y; Creamer, AR; Chen, H, Chem. Eng. J., 255, 107-13, 2014.

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8.11 ENERGY STORAGE Pyrene (26.84%) decorated graphene composites synthesized via a facile solvothermal and subsequently activated route were used for supercapacitor electrodes.1 The electrode had a specific capacitance of 310.7 F g-1 at the current density of 1.5 A g-1 and excellent cycle stability with capacitance retention of 99% after 15,000 cycles.1 It also had an energy density of 64.5 W h kg-1 at a power density of 3.3 kW kg-1.1 The performance was attributed to the electrochemical activity of pyrene, conductive, porous structure, and improved wettability between electrode and electrolyte.1 A high specific capacitance of 335 F g-1 at 0.5 A g-1 in 6 M KOH was achieved in the case of honeycomb-like restacking-inhibited graphene architecture with open pores.2 Graphene oxide decorated ZnO prepared by simple, aqueous precipitation was used for supercapacitor electrode.3 The specific capacitance increased by 83% as compared to the graphene oxide electrode, and the electrode had 90.8% retention of the specific capacitance after 5000 cycles.3 The maximum specific capacitance of 97 F g-1 at a current density of 0.5 A g-1 by galvanostatic charging-discharging was achieved in 1M Na2SO4 electrolyte.3 The intercalation-type pseudocapacitors have high charge-storage capacity and fast charge/discharge rates derived from their unique charge storage mechanism of fast kinetics without the limitation of diffusion.4 Amorphous titanium dioxide, grown on highly conductive nanoporous graphene frameworks by atomic layer deposition, was capable of storing a large capacity at high rates by pseudocapacitive and bulk-form Li+ intercalation/ de-intercalation reactions.4

Figure 8.36. Structural characterization. (a) SEM and (b) low-magnification TEM images of 3D nanoparticle nitrogen-doped graphene, NP NDG/FeOx hybrid materials. (c) STEM-EDS mapping images of C, N, Fe and O elements in a marked square region in (b). (d) HRTEM image of iron oxides embedded in NP N-doped graphene. (e) XRD pattern of NP NDG/FeOx hybrid electrode. The line patterns correspond to Fe3O4, FeO, and Fe, respectively. (f) Raman spectrum of NP NDG/FeOx hybrid electrode. [Adapted, by permission, from Liu, B-T; Zhao, M; Han, L-P; Lang, X-Y; Wen, Z; Jiang, Q, Chem. Eng. J., 335, 467-74, 2018.]

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The 3D nanoporous electrodes were prepared by a sacrificial template route.5 Nitrogen-doped graphene/iron oxide hybrid electrode with three-dimensional nanoporous architecture was designed as an anode material for asymmetric supercapacitors.5 The hybrid electrode had low internal resistance (5.4 ohms) and high specific capacitance (409 mAh g-1).5 The FeOx served as electroactive material improving the charge-storage density whereas the in situ grown nitrogen-doped graphene facilitated the electron transport.5 The energy storage (maximum energy density of 142 Wh kg-1) was close to the values in lithium-ion batteries and much higher than that of lead-acid or Ni-MH batteries.5 Figure 8.36 shows morphology, structure, and composition characteristics of electrode material.5 The interfacial interaction between Fe atoms and pyrolytic carbon via coordination bonding with the doped N atoms facilitated the electron transport between the electroactive FeOx and the conductive graphene during the charge/discharge processes.5 The n-dodecanol/melamine resin composite microcapsules modified by graphene oxide with different oxidation degrees were evaluated for use in solar energy storage.6 The microcapsules were spherical with the latent heat of 170 J/g.6 Graphene was mixed with polypyrrole or magnetic polypyrrole to obtain the conductive ink which was then used for the supercapacitor electrodes.7 The supercapacitor cells were composed of the separator (PTFE) and electrolyte (ionic liquid materials or acids). The specific capacitance of 255 F g-1 was achieved.7

Figure 8.37. (a) SEM image of freeze-dried graphene oxide, SEM image of microwave reduced graphene oxide at low (b) and high magnification (c), (d) TEM images of microwave reduced graphene oxide. [Adapted, by permission, from Li, Z; Zhang, W; Guo, J; Yang, B; Yuan, J, Vacuum, 117, 35-9, 2015.]

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The size of graphene oxide from micrometers to tens of nanometers was changed using the sonochemical method.8 After conversion to graphene by thermal annealing, the materials were used for the construction of supercapacitor with ionic liquid electrolyte.8 The reduction of the graphene size results in changes in edge activity because the aromatic structure is disrupted on edges by the increased occurrence of sp3-carbon defects, and the electrical charge becomes hard to transfer; therefore, the increase of edges of small size graphene have no positive impact on its capacitance enhancement.8 Highly fluffy and wrinkled reduced graphene oxide produced by simple freeze-drying and microwave-expanding method in vacuum was found to have an improved electrochemical performance with the high specific capacitance of 246 F g-1 at scan rate 5 mV s-1 and excellent cycle stability for application in supercapacitors.9 The improvements in a supercapacitor were attributed to the highly fluffy and wrinkled graphene which created a three-dimensional network permitting fast electron, and ion transports.9 Figure 8.37 illustrates morphological changes of graphene oxide during processing.9 Combination of graphene and various conducting polymers such as polyaniline, polythiophene, polypyrrole, and their derivatives have potential use in electrochemical capacitive energy storage.10 This and some other review papers11-15 discuss a wide variety of possible solutions to energy storage. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Li, Z; Zhang, W; Li, Y; Wang, H; Qin, Z, Chem. Eng. J., 334, 845-54, 2018. Xie, Q; Zhang, Y; Zhao, P, Mater. Lett., 225, 93-6, 2018. Lee, KS; Park, CW; Lee, SJ; Kim, J-D, J. Alloys Compounds, 739, 522-8, 2018. Han, J; Hirata, A; Du, J; Ito, Y; Fujita, T; Kohara, S; Ina, T; Chen, M, Nano Energy, 49, 354-62, 2018. Liu, B-T; Zhao, M; Han, L-P; Lang, X-Y; Wen, Z; Jiang, Q, Chem. Eng. J., 335, 467-74, 2018. Liu, Z; Chen, Z; Yu, F, Solar Energy Mater. Solar Cells, 174, 453-9, 2018. Yanik, MO; Yigit, EA; Akansu, YE; Sahmetlioglu, E, Energy, 138, 883-9, 2017. Lu, L; Li, W; Zhou, L; Zhang, Y; Zhang, Y, Electrochim. Acta, 219, 463-9, 2016. Li, Z; Zhang, W; Guo, J; Yang, B; Yuan, J, Vacuum, 117, 35-9, 2015. Shen, F; Pankratov, D; Chi, Q, Current Opinion Electochem., 4, 1, 133-44, 2017. Chen, K; Wang, Q; Niu, Z; Chen, J, J. Energy Chem., 27, 1, 12-24, 2018. Guo, X; Zheng, S; Zhang, G; Xiao, X; Pang, H, Energy Storage Mater., 9, 150-69, 2017. Eftekhari, A; Shulga, YM; Baskakov, SA; Gutsev, GL, Int. J. Hydrogen Energy, 43, 4, 2307-26, 2018. Zang, X, Graphene-Based Flexible Energy Storage Devices, Tsinghua University Press Limited, 2018. Fan, X; Chen, X; Dai, L, Current Opinion Colloid Interface Sci., 20, 5-6, 429-38, 2015.

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8.12 INKS AND 3D PRINTS Direct ink writing technology was used to construct three-dimensional structure by layer stacking.1 Graphene was dispersed by ultrasonication in ethanol.1 The three-dimensional materials were obtained by 3D printing used for the lightweight applications of graphenebased structures.1 A surfactant-free graphene ink was prepared in terpineol and cyclohexanone mixture.2 It was used for ink-jet printing on rigid SiO2/Si and flexible polyimide substrates.2 The ink had desirable properties for flexible electronics including enhanced electronic transport, good mechanical robustness, and resistivity which only slightly varied with temperature.2 Stretchable strain sensor based on graphene flakes/ZnO composite was deposited on micro-random ridged type PDMS substrate.3 Its stretchability was 30%.3 The strain sensor has potential application in wearable electronics and human motion sensors.3 The properties of water/ethanol mixtures were adjusted to effectively exfoliate graphite and then disperse graphene flakes to formulate graphene-based inks.4 The inks can be printed in the form of conductive stripes (sheet resistance of ~13 kΩ/sq) on flexible substrates (poly(ethylene terephthalate)).4 Water-based graphene ink was used for inkjet printing.5 The ink was obtained by shear exfoliation process with the aid of bromine intercalation.5 After drying at 100°C in a vacuum oven, the printed films exhibited a conductivity of 1400 S/m.5 The combination of aqueous iodine doping and thermal annealing permitted to achieve a conductivity of 105 S/ m.5 Graphene/polyaniline inks were used in inkjet printing technology to produce thinfilm electrodes for supercapacitors.6 The inkjet printing gave good control over a pattern geometry, pattern location, film thickness, and electrical conductivity.6 Stable ink contained highly dispersed graphene nanosheets, sodium n-dodecyl sulfate, and pH adjusted to 10 by ammonia for inkjet printing on polyimide film.7 The conductivity of the resultant film was 121.95 S m-1.7 Self-supported, highly porous three-dimensional graphene oxide structures were fabricated by direct ink writing.8 They were infiltrated with a liquid organic-polysilazane and subsequently pyrolyzed at a temperature in the range of 800-1000ºC to cause the ceramic conversion.8 The graphene network provided the conductive path (electrical conductivity in the range 0.2-4 S cm-1), and the ceramic wrapping served as a protective barrier against the atmosphere, temperature (up to 900 °C in air), and direct flame.8 Graphene scaffolds having anisotropic properties were fabricated by three-dimensional printing.9 They contained 50 wt% aligned graphene exhibiting good bonding between layers. Figure 8.38 illustrates the process of fabrication.9 Reduced graphene oxide and Al2O3 with complex three-dimensional mesoscale architecture were prepared by a combination of 3D printing technique and thermal reduction process.10 The inks had shear-thinning behavior with graphene oxide having a strong effect on the fluidity and viscoelasticity of the inks.10 The increase of graphene oxide content caused the increase in viscosity, yield stress and elastic modulus of the ink.10 Highly conductive graphene nanoplatelet inks for the rapid surface coating of diverse substrates were prepared by a simple ball-milling.11 Direct yellow 50 dye was used as a modifier.11 The rigid, planar, and conjugated diazo dye bearing four sulfonate groups, and

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Figure 8.38. Schematic of the fabrication process. Following the arrows: graphene suspension was achieved when graphene nanoplatelets were dispersed in the mixture of graded solvents composing of ethylene glycol butyl-ether, EGB, dibutyl phthalate, DBP, and polyvinylbutyral, PVB, by ultrasonic cell disruptor. After evaporation of ethanol, ET, until the as-prepared graphene suspension became toothpaste-like, the graphene ink was obtained. Then the graphene ink was extruded through a nozzle with a diameter of 300–500 μm to form threedimensional graphene scaffolds. [Adapted, by permission, from Huang, K; Yang, J; Dong, S; Feng, Q; Zhang, X; Ding, Y; Hu, J, Carbon, 130, 1-10, 2018.]

its strong π-π and charge transfer interactions with exfoliated graphene were instrumental for the efficient formation of graphene nanoplatelets.11 The graphene nanoplatelets were dispersed in isopropanol forming stable, thick ink suitable for coating on a variety of substrates, e.g., glass beads, copper wires, plastic films, sponges, and plant leaves.11 Further information can be found in the review papers,12-16 and several thousand papers (7-8k) published on this subject. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

You, X; Yang, J; Feng, Q; Huang, K; Dong, S, Int. J. Lightweight Mater. Manuf., in press, 2018. Michel, M; Biswas, C; Kaul, AB, Appl. Mater. Today, 6, 16-21, 2017. Hassan, G; Bae, J; Hassan, A; Ali, S; Choi, Y, Composites Part A: Appl. Sci. Manuf., 107, 519-28, 2018. Capasso, A Del Rio Castillo, AE; Sun, H; Ansaldo, A; Bonaccorso, F, Solid State Commun., 224, 53-63, 2015. Majee, S; Liu, C; Wu, O; Zhang, S-L; Zhang, Z-B, Carbon, 114, 77-83, 2017. Xu, Y; Henning, I; Freyberg, D; Strudwick, AJ; Schwab, MG; Weitz, T; Cha, KC-P, J. Power Sources, 248, 483-8, 2014. Lee, C-L; Chen, C-H; Chen, C-W, Chem. Eng. J., 230, 296-302, 2013. Román-Manso, B; Moyano, JJ;Pérez-Coll, D; Belmonte, M; Miranzo, P; Osendi, MI, J. Eur. Ceramic Soc., 38, 5, 2265-71, 2018. Huang, K; Yang, J; Dong, S; Feng, Q; Zhang, X; Ding, Y; Hu, J, Carbon, 130, 1-10, 2018. Tubio, CR; Rama, A; Gomez, M; del Rio, F; Guitlan, F; Gil, A, Ceramics Int., 44, 5, 5760-7, 2018. Zhang, Z; Sun, J; Lai, C; Wang, Q; Hu, C, Carbon, 120, 411-8, 2017. Wisitsoraat, A; Mensing, JP; Karuwan, C; Sriprachuabwong, C; Tuantranont, A, Biosensors Bioelectr., 87, 7-17, 2017. Zhang, Y; Gao, Z; Song, N; He, J; Li, X, Mater. Today Energy, 9, 319-35, 2018. Ren, S; Rong, P; Yu, Q, Ceramics Int., 44, 11, 11940-55, 2018. Ye, M; Zhang, Z; Zhao, Y; Qu, L, Joule, 2, 2, 245-68, 2018. Grande, L; Chundi, WT; Wei, D; Bower, C; Ryhänen, T, Particuology, 10, 1, 1-8, 2012.

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Figure 8.39. (a)–(c) SEM; (d)–(f) TEM micromorphological images of the three types of graphene sheets: (a) and (d) for regular edge; (b) and (e) for irregular edge; (c) and (f) for irregular edge and wrinkled graphene. [Adapted, by permission, from Mao, J; Zhao, J; Wang, W; He, Y; Luo, J, Tribology Int., 119, 614-21, 2018.]

8.13 LUBRICATION The effect of micromorphology of graphene sheets on lubrication properties has been studied.1 Three types of reduced graphene oxide sheets were studied, including graphene with regular edges, irregular edges, and both irregular edges and wrinkles (Figure 8.39).1 Graphene with regular edges had the best lubrication properties (friction coefficient and wear-scar depth decreased to 27.9 and 14.1% of that of base oil, respectively).1 The morphological regularity of the graphene sheets improved their lubricating properties.1 Graphene sheets having the regular edges formed a thick, firm, and continuous tribofilm completely separating asperities in the rubbing surfaces, leading to the exceptional lubrication properties.1 The lubrication properties of graphene additives with different layer numbers and interlayer spacing have been investigated.2 The additives having a higher degree of exfoliation gave better lubrication properties.2 The additives with a lower degree of exfoliation had structural defects increasing friction.2 The ordered tribofilm deposited on the frictional interfaces had parallel orientation to the sliding direction, therefore, a slippage between its layers was instrumental in reducing friction.2 Figure 8.40 shows the morphology of 3 graphene grades.2 Graphene (a) is a few-layer graphene with larger interlayer spacing (FLG-Ls) which was thermally reduced by chemical activation of potassium hydroxide that can etch carbon atoms to highly exfoliated level.2 Graphene (b) was directly thermally reduced with moderate interlayer spacing (FLG-Ms), and graphene (c) was highly oriented multilayer graphene with the smallest interlayer spacing (MLG-Ss).2 The FLG-Ls had a broader peak at 2θ = 23.08° and shift the peak toward a lower angle than FLG-Ms (2θ = 25.82°) and MLG-Ss (2θ = 26.64°).2 This means a larger interlayer spacing for FLG-Ls (3.85 Å) as compared with FLG-Ms (3.45 Å) and MLG-Ss (3.35 Å).2

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Figure 8.40. Structure characteristic of graphene additives. SEM (a-c) and HRTEM (d-f) images: (a) and (d) for FLG-Ls, (b) and (e) for FLG-Ms, (c) and (f) for MLG-Ss, respectively. Comparison of specific surface area, SSA (g) and XRD pattern (h) for the additives. [Adapted, by permission, from Zhao, J; Mao, J; Li, Y; He, Y; Luo, J, Appl. Surf. Sci., 434, 21-7, 2018.]

A high fraction of oxygen functional groups was typical of graphene oxide. Their presence increased the interlayer spacing of graphene sheets.3 The chemical structure and crystallographic symmetry of reduced graphene oxide were preserved after controlled desorption process of oxygen functional groups.3 The graphene oxide caused higher friction when used in lubricating oil than reduced graphene oxide which was explained by the rigidity of interplanar graphene sheets caused by the oxygen functionalization which restricted shear between graphene sheets.3 The ultralow friction coefficient in reduced graphene oxide nanofluid was observed at high-pressure lubrication conditions.3 Dispersions of few-layers graphene in 1-ethyl-3-methylimidazolium ionic liquids with dicyanamide or bis(trifluoromethylsulfonyl)imide anions have been obtained by mechanical mixing and sonication.4 Graphene increased the load-carrying ability of ionic liquids, formed a surface layer on the sliding path, and retained wear debris, preventing the formation of large abrasive particles.4 Nanofluids containing higher graphene concentrations (0.75 and 1 wt%) showed anomalous viscosity-temperature behavior (a linear viscosity increased with increasing temperature) which was explained by the formation of

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Figure 8.41. The SEM image of (a) PVDF, (b) reduced graphene oxide, RGO, (c) RGO/PVDF composite, (d) RGO/PVDF hybrid and (e) acetanilide, AA/PVDF hybrid. [Adapted, by permission, from Li, X; Lu, H; Li, J; Dong, G, Tribology Int., 127, 351-60, 2018.]

stronger interactions between ionic liquid molecules and graphene sheets, which reverted when temperature decreased.4 Poly(vinylidene fluoride) particles wrapped by reduced graphene oxide (graphene concentration was 3.25 mg mL-1) in a tetrahydrofuran solution.5 The average friction coefficient and wear rate decreased by 44.4% and 98.7%, respectively, as compared to paraffin oil.5 Trace amounts of acetanilide were utilized as a special adhesive between reduced graphene oxide and PVDF.5 Figure 8.41 shows morphological features of components and their combinations. Some corrugations appear on extending reduced graphene oxide surfaces when reduced graphene oxide sheets wrap around VDF particles.5 The lubrication mechanisms of the composite have been ascribed to the protective tribofilm of extending reduced graphene oxide and mending effect of globular nanocomposite between frictional pairs.5 By the analysis of tribochemical action of graphene using first-principles calculations, it was suggested that graphene bound strongly to the native iron surfaces reducing their surface energy and caused a passivating effect on metal surfaces coated by graphene which became almost inert causing a very low adhesion and shear strength when mated in sliding contact.6 During the low-friction regime, graphene covered the wear track uniformly, but it was removed from the track at the high-friction regime.6 A high load caused peeling off graphene from the surface.6 Raman spectroscopy showed that graphene flakes tended to passivate the native iron surfaces that were exposed during sliding as a consequence of wear.7 In the steel-iron sliding contact, graphene was found on the iron surface while in the steel-bronze system graphene was bound to the active region of native iron.7

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Figure 8.42. Diagram of abrasion resistance testing of polyacrylate, polyacrylate/graphene oxide and polyacrylate/linear alkylbenzene sulfonate-modified reduced graphene oxide. [Adapted, by permission, from Wei, L; Ma, J; Zhang, W; Liu, C; Bao, Y, Prog. Org. Coat., 122, 64-71, 2018.]

Linear alkylbenzene sulfonate (anionic surfactant) was used to modify reduced graphene oxide by in situ reduction process to enhance its compatibility with polyacrylate latex.8 The surfactant-modified reduced graphene oxide formed self-lubrication and barrier layer on leather because of its amphiphilicity and π-π stacking.8 Figure 8.42 illustrates the effect of lubrication and abrasion resistance.8 The 3,5-di-tert-butyl-4-hydroxybenzaldehyde-grafted graphene in mineral lube base oil was used to lubricate steel balls.9 The van der Waals interaction between the tertiarybutyl group of modified graphene and hydrocarbon chains of mineral lube base oil facilitated dispersion.9 A small addition (0.2-0.8 mg mL-1) of the modified graphene showed the significant reduction in the coefficient of friction (40%) and wear scar diameter (17%) under the rolling contact between steel balls.9 Raman study of the worn area of steel ball revealed the deposition of a graphene-based tribo thin film in the form of irregular patches which reduced the friction and protected the tribo-surfaces against the wear.9 REFERENCES 1 2 3 4 5 6 7 8 9

Mao, J; Zhao, J; Wang, W; He, Y; Luo, J, Tribology Int., 119, 614-21, 2018. Zhao, J; Mao, J; Li, Y; He, Y; Luo, J, Appl. Surf. Sci., 434, 21-7, 2018. Mishra, KK; Panda, K; Kumar, N; Malpani, D; Ravindrana, TR; Khatrie, OP, J. Ind. Eng. Chem., 61, 97-105, 2018. Pamies, R; Avilés, MD; Arias-Pardilla, J; Espinosa, T; Carrión, FJ; Sanes, J; Bermúdez, MD, Tribology Int., 122, 200-9, 2018. Li, X; Lu, H; Li, J; Dong, G, Tribology Int., 127, 351-60, 2018. Restuccia, P; Righi, MC, Carbon, 106, 118-24, 2016. Marchetto, D; Restuccia, P; Ballestrazzi, A; Righi, MC; Valeri, S, Carbon, 116, 375-80, 2017. Wei, L; Ma, J; Zhang, W; Liu, C; Bao, Y, Prog. Org. Coat., 122, 64-71, 2018. Chouhan, A; Mungse, HP; Sharma, OP; Singh, RK; Khatri, OP, J. Colloid Interface Sci., 513, 666-76, 2018.

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8.14 ORGANIC LIGHT-EMITTING DIODES Graphene-based transparent electrodes are promising candidates for the photonic and optoelectronic applications, including organic light-emitting diodes.1 The ultraviolet ozone-assisted patterning and work function engineering were used to enhance the performance of polymer light-emitting diodes.1 Polymethylmethacrylate and fluoropolymer (CYTOP) were used as the supporting polymers for fabrication of organic light-emitting diodes.2 The doped graphene electrodes exhibit device efficiency comparable to that shown by indium tin oxide-based electrodes currently in use.2 CYTOP was more tolerant of UV-O3 treatment and thermal annealing than PMMA.2 Incorporation of graphene nanosheets improved the efficiency of poly[2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]-based light emitting diodes by enhancing photoluminescence emission by factor 6 using only 0.005 wt% graphene.3 The high charge carrier mobility in graphene nanostructure balanced the charge carrier concentration in the emissive layer.3 Graphene also improved electron injection from the cathode.3 The graphene concentration had to be kept below the percolation threshold level (higher concentration of graphene leads to short-circuiting of the device).3 Non-oxidized graphene nanoplatelets have been used as an efficient hole transport layer.4 Py+ ions from the pyridinium tribromide salt (Py+Br3-) assisted in exfoliation of graphene nanoplatelets.4 Low turn-on voltage of 4.1 V (at 100 mA/cm2), and high luminance of 36,000 cd/m2 (at 8.4 V) were obtained.4 A simple, cost-effective, and precisely controllable method was used to fabricate high-quality reduced films as a hole injection layer for high-efficiency polymer light-emitting diodes.5 The deposition and reduction of graphene films were done by electrical methods which permit strict control of thickness (80 Å) and the degree of reduction of graphene oxide films (10 s).5 The performance of organic light-emitting diodes was improved by using hybrid anodes composed of graphene and conducting polymer (poly(3,4-ethylenedioxythiophene) with poly(styrenesulfonic)) which helped to overcome low work function and high sheet resistance improving conductivity and forming a work function stairs for smooth hole injection property.6 REFERENCES 1 2 3 4 5 6

Ha, J; Park, S; Kim, D; Ryu, J; Lee, C; Hong, BH; Hong, Y, Organic Electronics, 14, 9, 2324-30, 2013. Kwon, KC; Kim, S; Kim,C; Lee, J-L; Kim, SY, Organic Electronics, 15, 11, 3154-61, 2014. Prasad, N; Singh, I; Kumari, A; Madhwal, D; Madan, S; Dixit, SK; Bhatnagar, PK; Mathur, PC, J. Luminescence, 159, 166-70, 2015. Vu, H-T; Yu, H-C; Chen, Y-C; Chen, I-WP; Huang, C-Y; Juang, F-S; Su, Y-K, Organic Electronics, 15, 3, 792-7, 2014. Kim, J; Ganorkar, S; Kim, Y-H; Kim, S-I, Carbon, 94, 633-40, 2015. Shin, S; Kim, J; Kim, Y-H; Kim, S-I, Current Appl. Phys., 13, 2, S144-47, 2013.

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8.15 PACKAGING The synergistic effect of graphene nanoplatelets and carbon black combination of conductive fillers was used for polymer film (poly(vinyl alcohol)) for electrostatic discharge packaging materials.1 The composite graphene/carbon black in the range of ratios from 10:90 to 30:70 gave a sharp drop in surface resistivity by 5-8 orders of magnitude at the filler loading of 8-10 wt%.1 The volume resistivity of the film was 108-1012 Ω-cm.1 Clove essential oil (plasticizer and biocide, 15-30 wt%) and graphene oxide nanosheets (1 wt%) were compounded with polylactide to produce an antimicrobial film suitable for food packaging.2 The composite film had antibacterial activity against Staphylococcus aureus and Escherichia coli.2 Reduced graphene oxide-zinc oxide in glycerol-plasticized poly(3-hydroxybutyrateco-3-hydroxyvalerate) (PHBV) film was prepared by melt extrusion.3 Bactericidal activity against Escherichia coli was a result of direct contact between bacteria cells and the hybrids surface.3 Chitosan functionalization with cinnamaldehyde and reinforcement with graphene resulted in a composite film suitable for food packaging.4 The fungicidal effect evaluated using R. stolonifer showed an increased inhibition effect with an increase in cinnamaldehyde concentration.4 The presence of graphite nanostacks increased the mechanical properties of the composite material.4 The barrier properties of polymers are significantly improved by lamellar fillers, increasing the diffusion path of gas and water vapor molecules.5 The optimal barrier and mechanical property enhancement occurred at low graphene loading of 2 and 1 wt%, respectively.5 The larger particle size of graphene (25 μm) exhibited an optimal barrier enhancement (50% reduction).5 Figure 8.43 shows the effect of particle size and loading of graphene on water vapor permeability.5

Figure 8.43. (a) Normalized water vapor permeability (Pcomposite/P0(Neat Polymer)) (left ordinate) and the composite viscosity (right ordinate) for polyurethane loaded with various graphene concentrations (the lines are guidance to the eye). Optimal nanoplatelet concentration for water vapor permeability (optimal concentration) values are indicated by the colored arrows at the abscissa (according to tangential interception). (b) SEM micrograph of fractured polyurethane surface loaded with 3 wt% 25 μm graphene. Air bubbles are indicated by arrows. [Adapted, by permission, from Damari, SP; Cullari, L; Nadiv, R; Nir, Y; Laredo, D; Grunlan, J; Regev, O, Composites Part B: Eng., 134, 218-24, 2018.]

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REFERENCES 1 2 3 4 5

Ge, D; Devar, G, J. Electrostatics, 89, 52-7, 2017. Arfat, YA; Ahmed, J; Ejaz, M; Mullah, M, Int. J. Biol. Macromol., 107A, 194-203, 2018. Gouvêa, RF; Del Aguila, EM; Paschoalin, VMF; Andrade, CT, Food Packaging Shelf Life, 16, 77-85, 2018. Demitri, C; De Benedictis, VM; Madaghiele, M; Corcione, CE; Maffezzoli, A, Measurement, 90, 418-23, 2016. Damari, SP; Cullari, L; Nadiv, R; Nir, Y; Laredo, D; Grunlan, J; Regev, O, Composites Part B: Eng., 134, 218-24, 2018.

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8.16 SELF-HEALING MATERIALS The cracks formed in the graphene sheet healed without any external aid within 0.4 ps.1 The self-healing was found to be independent of the length of the crack.1 The maximum crack opening distance for which healing took place was ≤5 Å for AA (armchair-zigzag) stacked pristine sheet and ≤13 Å for AB (armchair-armchair) stacked bilayer graphene sheet.1 The critical crack opening distance was around ten times the equilibrium C−C bond length (1.42 Å).1 The self-healing occurred by spontaneous recombination of the dangling bonds.1 Based on the molecular dynamics simulations, it was proposed that the self-healing of damaged graphene might occur under heat treatment.2 The self-healing mechanism in damaged graphene projected that the local curvature introduced by defects around the damage and curved surface was smoothed out via defect reconstruction which caused the damage shrinking.2 The thermal fluctuation and the size of damage determined the self-healing capability of graphene.2 Functional graphene nanosheets with Diels-Alder groups have been employed in the self-healing polyurethane system.3 Terminal maleimide groups were used in Diels-Alder reaction with the pendant furan groups of polyurethane.3 Composites were self-healed in the presence of NIR radiation which induced the photo-thermal effect.3 The conductivity was recovered by a 808 nm NIR irradiation.3 The graphene oxide was functionalized with maleimide groups and acted as a crosslinking point to fabricate dynamic dual-crosslinked polyurethane by Diels-Alder reaction.4 Polyurethane had both pendant furan and maleimide groups.4 The maleimide functionalization improved the compatibility between functionalized graphene oxide and polymer matrix, increasing mechanical performance and healing efficiency of polyurethane composites.4 The healing efficiency reached 99%.4 Self-healing multilayer polyelectrolyte film was prepared by layer-by-layer selfassembly technique from poly(acrylic acid), graphene, and branched poly(ethyleneimine).5 The excellent self-healing ability at high humidity and good electrical conductivity are required for potential applications in batteries, supercapacitors, and/or hydrogen fuel cells to improve their lifetime.5 The graphene oxide microcapsules containing linseed oil as the healing agent were prepared by a self-assembly process.6 The nanometer-thick shells of microcapsules were

Figure 8.44. Schematics of graphene oxide microcapsules formation in Pickering emulsions and the preparation of microcapsule/polyurethane coatings. [Adapted, by permission, from Li, J; Feng, Q; Cui, J; Yuan, Q; Qiu, H; Gao, S; Yang, J, Compos. Sci. Technol., 151, 282-90, 2017.]

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Figure 8.45. 3D digital microscope images of the graphene oxide microcapsules/polyurethane coatings (a) before and (b) after 15 days of healing. [Adapted, by permission, from Li, J; Feng, Q; Cui, J; Yuan, Q; Qiu, H; Gao, S; Yang, J, Compos. Sci. Technol., 151, 282-90, 2017.]

built by the liquid crystalline assembling of graphene oxide sheets at the liquid-liquid interface in Pickering emulsions (emulsion stabilized by solid microparticles adsorbed on the interface) (Figure 8.44).6 The shells were embedded in waterborne polyurethanes producing self-healing composite coatings.6 Figure 8.45 shows the results of the self-healing process.6 The scratched coating had a crack of 25 μm deep (equal to the thickness of the coating) which after 15 days healing completely disappeared and the integrity of the coating was recovered.6 Self-healable polyurethane/modified graphene nanocomposites were synthesized from poly(tetramethylene glycol) and 4,4′-methylene diphenyl diisocyanate with small addition (up to 1 wt%) of graphene oxide which was chemically modified with phenyl iso-

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Figure 8.46. Schematic of the production of a chitosan, CS-dopamine, DA-graphene oxide, GO composite hydrogel. Step 1: Electric interactions between CS and GO. Step 2: Introduction of DA and ammonium persulfate, APS into the CS/GO solution. Step 3: Covalent bonding between chitosan and DA and self-polymerization of DA to form CS-DA-GO composite hydrogel. (A) The self-healing mechanism of the hydrogel caused by the non-covalent bonds formed between catechol groups and the electric interactions between CS and GO. (B) The self-adhesiveness of the hydrogel imparted by the catechol groups. (C) The reduced GO formed electric pathways and endowed the hydrogel with good conductivity. [Adapted, by permission, from Jing, X; Mi, H-Y; Napiwocki, BN; Peng, X-F; Turng, L-S, Carbon, 125, 557-70, 2017.]

cyanate and reduced in the presence of phenylhydrazine.7 The self-healing effect was the most pronounced with 0.75 wt% modified graphene.7 Waterborne polyurethane/graphene oxide nanocomposites are eco-friendly and selfhealing polymeric materials with good thermal stability and mechanical properties.8 The materials containing 0.5 wt% graphene oxide had the best self-healing properties.8 Chitosan/graphene oxide hydrogel having self-adhesive and self-healing properties and electrical conductivity was prepared using the mussel-inspired protein polydopamine.9 During the oxidizing process of dopamine, graphene oxide was reduced and dispersed into the hydrogel network to form electrical pathways.9 The covalent bonds, supramolecular interactions, hydrogen bonding, and π−π stacking resulted in hydrogel having high stability, good adhesiveness, self-healing properties, and a fast recovery.9 The conductive hydrogel enhanced the cell viability and proliferation of human embryonic stem cell-derived fibroblasts and cardiomyocytes.9 Figure 8.46 illustrates formation of hydrogel and reasons for their improved properties.9 Graphene/rubber composites with the segregated network were prepared by latex mixing.10 The destroyed graphene networks were self-healed by the thermal treatment using electric heating.10 The composite containing 10 phr graphene had an electrical conductivity of 2.7 S/m which after stretching increased to 4.4 S/m indicating that the network was healed by post-thermal treatment by an applied voltage of 10-20 V which increased temperature from 57 to 152oC.10 The graphene@SiO2 were used as core and a nonionic copolymer as a shell in aqueous solution. The hybrid was able to flow above 45°C and had a particular thermal invertibility.11 The well-dispersed SiO2 nanoparticles were anchored onto the surface of graphene sheet via hydrogen bonding interaction under the synergistic effect of 3-(trime-

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Figure 8.47. Graphene/SiO2 hybrids. [Adapted, by permission, from Yang, S; Liu, J; Pan, F; Yin, X; Wang, L; Chen, D; Zhou, Y; Xiong, C; Wang, H, Compos. Sci. Technol., 136, 133-44, 2016.]

8.48. Coating formation and self-healing. [Adapted, by permission, from Yang, S; Liu, J; Pan, F; Yin, X; Wang, L; Chen, D; Zhou, Y; Xiong, C; Wang, H, Compos. Sci. Technol., 136, 133-44, 2016.]

thoxysilyl)-1-propanethiol and copolymer which prevented the aggregation of graphene sheet (Figure 8.47).11 The damaged coating can be self-healed either by immersion in water or by heating (Figure 8.48).11

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

Debroy, S; Miriyala, VPK; Sekhar, KV; Acharyya, SG; Acharyya, A, Superlattices Microstructures, 96, 26-35, 2016. Zhu, J; Shi, D, Computational Mater. Sci., 68, 391-5, 2013. Lin, C; Sheng, D; Liu, X; Xu, S; Yang, Y, Polymer, 140, 150-7, 2018. Lin, C; Sheng, D; Liu, X; Xu, S; Yang, Y, Polymer, 127, 241-50, 2017. Zhu, Y; Yao, C; Ren, J; Liu, C; Ge, L, Colloids Surf. A: Physicochem. Eng. Aspects, 465, 26-31, 2015. Li, J; Feng, Q; Cui, J; Yuan, Q; Qiu, H; Gao, S; Yang, J, Compos. Sci. Technol., 151, 282-90, 2017. Kim, JT; Kim, BK; Kim, EY; Kwon, SH; Jeong, HM, Eur. Polym. J., 49, 12, 3889-96, 2013. Wan, T; Chen, D, Prog. Org. Coat., 121, 73-9, 2018. Jing, X; Mi, H-Y; Napiwocki, BN; Peng, X-F; Turng, L-S, Carbon, 125, 557-70, 2017. Zhan, Y; Meng, Y; Li, Y, Mater. Lett., 192, 115-8, 2017. Yang, S; Liu, J; Pan, F; Yin, X; Wang, L; Chen, D; Zhou, Y; Xiong, C; Wang, H, Compos. Sci. Technol., 136, 133-44, 2016.

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8.17 SEMICONDUCTORS Graphene as a channel-material lacks the energy band-gap.1 But, it is possible to form an assembly of graphene and organic semiconductor molecules.1 The charge transport properties of graphene can, thus, be coupled to semiconducting properties of organic molecules. The nanosize graphene flakes can be blended in a solution with organic semiconductor molecules (polymer or small-molecule).1 Graphene in such assembly represents regions of high charge carrier mobility, while organic semiconductor provides energy gap required for an efficient transistor switching operation.1 Also, graphene flakes can be transferred onto the dielectric prior to the deposition of an organic semiconductor layer where graphene will cause disruptions in substrate morphology.1 A graphene phototransistor functionalized with poly(3-hexylthiophene)/graphene bulk heterojunction was fabricated by solution processing.2 It combined the high carrier mobility of graphene and the high visible light absorption of polymer which also had excellent photoresponse and air stability.2 Ferroelectric memories were fabricated based on electrochemically exfoliated graphene.3 A layer of graphene flakes bridging the gap between the source and drain electrodes has been obtained using Langmuir-Blodgett thin-film deposition. A random ferro-

Figure 8.49. Schematic description of the dual-gate ferroelectric transistor with electrochemically exfoliated graphene flakes in the channel. (b) Schematic representation of the chemical structure of poly(vinylidenefluorideco-trifluoroethylene). (c) Optical image of a transistor with interdigitated finger electrode geometry. The FET surface covered with EC-exfoliated graphene flakes appeared as orange spots on the surface. The insets show an SEM image of dark gray graphene flakes on the light gray SiO2 surface (left), the corresponding Raman spectra (middle) and on the right, a typical SEM image of the EC-graphene flakes between two gold electrodes (the horizontal strips) on the transistor substrate. [Adapted, by permission, from Heidler, J; Yang, S; Feng, X; Müllen, K; Asadi, K, Solid-State Electronics, 144, 90-4, 2018.]

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electric copolymer poly(vinylidenefluoride-co-trifluoroethylene) was used as the ferroelectric gate dielectric.3 Figure 8.49 shows a structure of a transistor, the chemical structure of the polymer, and distribution of graphene.3 The semiconductor/electrode interface affected performance of pentacene thin film transistors containing graphene as electrode.4 Increase in the stacking layers of graphene films (increased conductivity and energy level match between electrode and pentacene semiconductor) resulted in an increased surface roughness (breaks the connectivity of a single-phase domain in the active film) which decreased sheet resistance and increased work function.4 This specialized field is broadly covered in many review chapters,5-9 and about 10,000 contributed papers. REFERENCES 1 2 3 4 5 6 7 8 9

Mathew, J; Emin, S; Pavlica, E; Valant, M; Bratina, G, Surf. Sci., 664, 16-20, 2017. Che, Y; Zhang, G; Zhang, Y; Cao, X; Yao, J, Optics Commun., 425, 161-5, 2018. Heidler, J; Yang, S; Feng, X; Müllen, K; Asadi, K, Solid-State Electronics, 144, 90-4, 2018. Li, P; Wang, Q; Wang, X; Lu, H; Qiu, L, Synthetic Metals, 202, 103-9, 2015. Ho, K-I; Lai, C-S; Su, C-Y, Nanoelectronics Based on Fluorinated Graphene in New Fluorinated Carbons: Fundamentals and Applications, Elsevier, 2017. Justino, CIL; Gomes, AR; Freitas, AC; Duarte, AC; Rocha-Santos, TAP, TrAC Trends Anal. Chem., 91, 53-66, 2017. Giubileo, F; Di Bartolomeo, A, Prog. Surf. Sci., 92, 3, 143-75, 2017. Jin, Z; Owour, P; Lei, S; Ge, L, Current Opinion Colloid Interface Sci., 20, 5-6, 439-53, 2015. Gablech, I; Pekárek, J; Klempa, J; Svatoš, V; Pumera, TrAC Trends Anal. Chem., 105, 251-62, 2018.

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8.18 SENSORS Tunable-sensitivity and flexibility are considered as two crucial characteristics of pressure sensors and electronic skins.1 A graphene electrode exhibited flexibility and reliability, (stable when bent 50 times whereas ITO electrode was destroyed by bending).1 The graphene pressure sensors are suitable for wearable products for monitoring breath, pulse, and other physiological signals.1 The flexible sensor was able to respond to pressure in 50 ms.1 A transparent and stretchable strain sensor that can detect various types of strain induced via stretching, bending, and torsion has been developed from graphene.2 The sensor was fabricated using single-layer graphene as a force sensing material combined with a conductive film composed of graphene flake.2 Due to the selection of materials, the strain sensor is flexible and capable of stretching up to 20%.2 It also provides functional extension to bi-directional responses.2 The sensor can detect strain as low as 0.1%.2 Figure 8.50 illustrates the method of sensor fabrication.2 A flexible pressure sensor was fabricated using a micro-patterned graphene/ polydimethylsiloxane composite as the dielectric layer, which was sandwiched between the polydimethylsiloxane substrate and the wrinkled continuous gold pattern as the antenna and electrode.3 The composite with a thickness of 200 μm and a concentration of 2% graphene as the dielectric layer exhibited the highest sensitivity, stability, and durability.3 The sensors can be sensitive to hand bending and facial muscle movements.3 The textile-infused sensor array for spatiotemporal mapping of skin temperatures included reduced graphene oxide-coated nylon filaments which were stitched along with

Figure 8.50. Fabrication processes of the all-graphene strain sensor. (a) SLG on Cu foil after growth. (b) singlelayer graphene, SLG, channel lithography with stencil mask. (c) SLG channel patterning with serpentine shape. (d) O2 plasma etching of SLG. (e) PMMA coating the on etched SLG. (f) Attachment of PDMS on PMMAcoated SLG. (g) Cu etching with FeCl3 solution. (h) Cleaning with HCl for the removal of Cu residue and DI water rinsing. (i) Attachment of stencil mask for spray-coating of graphene flakes. (j) Spray coating of graphene flakes on both sides of the SLG channel. (k) Wiring them for two-channel electrical measurements. (l) Assembling a thin PDMS protecting layer and casting of liquid PDMS. [Adapted, by permission, from Chun, S; Choi, Y; Park, W, Carbon, 116, 753-9, 2017.]

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Figure 8.51. Resistance change upon stretching and bending: (a) irreversible increases in resistance, R, due to the cracking and buckling upon stretching up to 12% as shown by the inset SEM images before and after stretching; (b) resistance change of reduced graphene oxide, rGO, with moderate stretching (4% strain) up to 100 stretching cycles; (c) small fluctuations in resistance of rGO with bending to 34° and (d) resistance change of rGO up to 100 34°-bending cycles. (f) SEM images of rGO-coated nylon filament after severe bending of 117° with (e) cracking at the stretched area and (g) buckling observed at the compressed region. [Adapted, by permission, from Jin, Y; Boon, EP; Le, LT; Lee, W, Sensors Actuators A: Phys., 280, 92-8, 2018.]

silver conductive threads into polyester fabric.4 They created an array of individually addressable negative temperature coefficient sensing elements.4 The accuracy of the sensor array was comparable to infrared camera.4 The reduced graphene oxide film was mechanically and electrically stable upon stretching (