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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 o rates in C/min: (a) 1, (b) 10, (c) 30, (d) 60, and (e) holey graphene. 9 The red circles in (d) reveal the presence of holes in sample heated graphene. 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 delamination where: a) For uninhibited coatings, cathodic O2 properties, but, in practice, the graphene 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 Figure 8.29 shows the method of coating preparation.11 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.

8.14 Organic light-emitting diodes

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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 (<4% strain) and bending (<34°) of the filaments (Figure 8.51).4 A flexible, lightweight, and conductive porous graphene network was used as the humidity sensor for respiration monitoring.5 Graphene oxide was modified with poly(3, 4ethylenedioxythiophene)-poly(styrenesulfonate) and Ag colloids to enhance its performance.5 The breathing patterns including mouse and nose respiration, normal and deep respiration were monitored as well as skin moisture, speaking and whistle rhythm.5 A quartz crystal microbalance humidity sensor was based on fullerene/graphene oxide nanocomposite.6 The fullerene molecules reduced the aggregation of the graphene oxide sheets and formed hydrophobic isolation layers between the graphene oxide sheets, which inhibited the water molecules permeation and maintained the mechanical stiffness of the fullerene/graphene oxide film.6 Elastic polyurethane core fiber and polyester fibers wound helically around the polyurethane core were used as an elastic scaffold which was air plasma-etched and dip-coated with graphene.7 The fiber sensor maintains stable and accurate performance in stretching, bending, and torsion testing.7 The wearable sensors were attached to the elbow and wrist to monitor the basic movements of the human body during the typical actions of soccer and basketball.7 Graphene-elastomer (polydimethylsiloxane) nanocomposites were used for flexible piezoresistive sensors for strain and pressure detection.8 The static strain up to 20% could

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be monitored due to the flexibility and stretchability of PDMS elastomer.8 A flexible sensor detecting the finger motions was fabricated.8 Polyurethane sponges decorated with reduced graphene oxide and silver nanowires were fabricated by dip coating to produce highly stretchable gas sensors.9 They detected an oxidizing gas, NO2, and reducing gases, acetone, and ethanol even under large strains of up to 60%.9 Graphene sensing properties depended on the target gases. The performance of graphene devices was affected by substrate.10 The ammonia sensing depended on the separation distance between graphene and substrate (the average distance between graphene and boron nitride crystals was smaller than for silica or talc).10 Graphene/boron nitride sensor had the fastest recovery times for NH3 exposure (slightly affected by wet or dry air environment).10 Surface plasmon resonance occurs when electron density excitations are controlled by the coupling of p-polarized electromagnetic radiation with the surface plasmons (plasmon is a quantum of plasma oscillation) propagating along a metal-dielectric interface.11 Sensor composed of the glass substrate, silver film, and insulating layer (graphene) was used in the NIR region.11 At λ = 1550 nm, maximum (and almost constant) sensing performance has been achieved for 0.9 < μ < 1 eV for graphene monolayer.11 Fluoride glass gave the best sensing performance.11 A high sensitivity optical fiber surface plasmon resonance sensor for liquid concentration detection was based on coreless optical fiber, silver film, and graphene.12 Graphene enhanced the evanescent field of traditional optical fiber increasing sensitivity.12 Figure 8.52 shows concentration measurement system.12 The graphene-covered Ag prevented silver film from oxidation.12 Graphene-based optical sensors were developed for measurement of the biological intercellular refractive index.13 They offered greater detection depth than the surface plasmon resonance sensors.13 The detection depths of 2.5 and 3 μm were achieved at wavelengths of 532 and 633 nm, respectively.13 The sensors are useful for refractive index tomography.13 Imidazole-functionalized graphene oxide was used as an artificial enzymatic active site for voltammetric determination of progesterone.14 The imidazole groups promoted a significant enhancement of the electrochemical reduction of progesterone.14 Phosphorus-doped reduced graphene oxide coated on glass was used in electrochemical sensor for paracetamol.15 It had electrocatalytic activity causing the oxidation of paracetamol due to highly enhanced electrochemical conductivity and accelerated electron transfer.15 A linear relationship between current intensity and concentration of paracetamol was obtained in the range of 1.5-120 μM with a detection limit of 0.36 μM.15 The sensor detected paracetamol in pharmaceutical tablets with recovery results of 99.8-104.9 and relative standard deviations of 0.27-1.86%.15 A lab-on-a-chip for healthcare, point-of-care tests may use a biosensor to detect a stress biomarker of cortisol using cortisol monoclonal antibody covalently immobilized on reduced graphene oxide channel as an electrical sensing element for salivary cortisol monitoring.16 Potentiometric and chemiresistive pH sensing of polyaniline functionalized electrochemically reduced graphene oxide was used to monitor continuous pH change in Lactococcus lactis fermentation in real time.17 Graphene and polyaniline were prepared by

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Figure 8.52. Concentration measurement system (a) System structure of liquid concentration sensor, (b) physical map of sensing probe, (c) schematic diagram of sensing probe. [Adapted, by permission, from Zhou, X; Li, X; Cheng, TL; Li, S; An, G, Optical Fiber Technol., 43, 62-6, 2018.]

electrochemical reduction of graphene oxide and electropolymerization of aniline using cyclic voltammetry.17 Flexible, implantable glucose biosensors are required for continuous monitoring of blood-glucose of diabetes.18 The laser-scribed graphene was used as a flexible conductive substrate, and copper nanoparticles were electrodeposited as the catalyst.18 The glucose sensor had a wide linear glucose detection range from 1 μM to 4.54 mM and high sensitivity (1.518 mA mM-1 cm-2) and low limit of detection (0.35 μM).18 More than 12,000 papers were published on this important and fast developing subject of the application of graphene and its derivatives. There are available review papers to farther learn about the sensors, their use, and structure.19-22 REFERENCES 1 3 2 4

Luo, S; Yang, J; Song, X; Zhou, X; Wei, D, Solid-State Electronics, 145, 29-33, 2018. Kou, H; Zhang, L; Tan, Q; Liu, G; Lv, W; Lu, F; Dong, H; Xiong, J, Sensors Actuators A: Phys., 277, 150-6, 2018. Chun, S; Choi, Y; Park, W, Carbon, 116, 753-9, 2017. Jin, Y; Boon, EP; Le, LT; Lee, W, Sensors Actuators A: Phys., 280, 92-8, 2018.

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Pang, Y; Jian, J; Tu, T; Yang, Z; Ling, J; Li, Y; Wang, X; Qiao, Y; Tian, H; Yang, Y; Ren, T-L, Biosensors Bioelectronics, 116, 123-9, 2018. Zhang, J; Cao, Y; Qiao, M; Ai, L; Sun, K; Mi, Q; Zang, S; Zuo, Y; Yuan, X; Wang, Q, Sensors Actuators A: Phys., 274, 132-40, 2018. Ding, X; Chen, X; Chen, X; Zhao, X; Li, N, Sensors Actuators B: Chem., 266, 534-42, 2018. Niu, D; Jiang, W; Ye, G; Wang, K; Yin, L; Shi, Y; Chen, B; Luo, F; Liu, H; Mater. Res. Bull., 102, 92-9, 2018. Luan, Y; Zhang, S; Nguyen, TN; Yang, W; Noh, J-S, Sensors Actuators B: Chem., 265, 609-16, 2018. Cadorea, AR; Mania, E; Alencar, AB; Rezende, NP; de Oliveira, S; Watanabe, K; Taniguchi, T; Chacham, H; Campos, C; Lacerda, RG, Sensors Actuators B: Chem., 266, 438-46, 2018. Sharma, AK; Kaur, B, Solid State Commun., 275, 58-62, 2018. Zhou, X; Li, X; Cheng, TL; Li, S; An, G, Optical Fiber Technol., 43, 62-6, 2018. Yang, Y; Sun, J; Liu, Lu, Zhu, S; Yuan, X, Optics Commun., 411, 143-7, 2018. Gevaerd, A; Blaskievicz, SF; Zarbin, AJG; Orth, ES; Bergamini, MF; Marcolino-Junior, LH, Biosensors Bioelectronics, 112, 108-13, 2018. Zhang, X; Wang, K-P; Zhang, L-N; Zhang, Y-C; Shen, L, Anal. Chim. Acta, in press, 2018. Kim, Y-H; Lee, K; Jung, H; Kang, HK; Lee, HH, Biosensors Bioelectronics, 98, 473-7, 2017. Chinnathambi, S; Euverink, GJW, Sensors Actuators B: Chem., 264, 38-44, 2018. Lin, S; Feng, W; Miao, X; Zhang, X; Chen, S; Chen, Y; Wang, W; Zhang, Y, Biosensors Bioelectronics, 110, 89-96, 2018. Justino, CIL; Gomes, AR; Freitas, AC; Duarte, AC; Rocha-Santos, TAP, TrAC Anal. Chem., 91, 53-66, 2017. Nag, A; Mitra, A; Mukhopadhyay, SC, Sensors Actuator A: Phys., 270, 177-94, 2018. Yang, T; Zhao, X; He, Y; Zhu, H, Graphene. Graphene-Based Sensors, Elsevier, 2018, pp. 157-74. Bo, X; Zhou, M; Guo, L, Biosensors Bioelectronics, 89, 1, 167-86, 2017.

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8.19 SPORTING EQUIPMENT High-performance materials are often first employed in sporting goods because no lengthy period of testing is required (if a failure occurs it does not lead to expensive compensations or live-costing accidents) but there is a potential for the introduction of product having exceptional performance.1 Graphene-based nanoparticles were used to reinforce resin-rich regions in the shaft of the tennis racket at the discontinuity in the fiber tows, where the handle is joined to the racquet’s head.1 The nanoparticles employed in the tennis racket were most probably graphite nanoplatelets added to improve the mechanical properties of the resin-rich regions.1 The first tennis racket containing graphene was manufactured by Head in Austria.1 It was most likely based on the patents in which “at least one prepreg layer may also include graphene material.”2-4 Figure 8.53 shows the effect of a number of layers on the position of Raman bands which increases when the number of 1 Figure 8.53. Differences in the shape and position of the layers increases. The 2D peak characteris2D Raman band for different forms of graphene: mono- tic of graphene found in the tennis racket is layer, bilayer and XG graphite nanoplatelets. [Adapted, consistent with a peak for multilayer by permission, from Young, RJ; Liu, M, J. Mater. Sci., graphene.1 This implies that graphene 51, 8, 3861-7, 2016.] nanoplatelets were added to the epoxy resin in the resin-rich regions of the area where the head was joined onto the racket handle.1 This resin-rich region is a potential point of weakness of the racket and, therefore, the addition aimed at improving the mechanical performance.1 Other potential application of graphene in sporting equipment may include skis, golf shafts, and golf balls. REFERENCES 1 2 3 4

Young, RJ; Liu, M, J. Mater. Sci., 51, 8, 3861-7, 2016. Lammer, H, US Patent 8,342,989, Head Technology GmbH, Nov. 13, 2009. Lammer, H, US Patent 8,894,517, Head Technology GmbH, Nov. 12, 2012. Lammer, H, US Patent 20150051027, Head Technology GmbH, Oct. 22, 2014.

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8.54. Applications of transparent conductive electrodes. Also included are the current challenges of ITO and expected features in the ITO substitutes. [Adapted, by permission, from Zhang, CJ; Nicolosi, V, Energy Storage Mater., 16, 102-25, 2019.]

8.20 TRANSPARENT FUNCTIONAL MATERIALS Interactive devices such as smartphone, tablets, and other touchable devices require transparent conductive electrodes.1 Also, transparent supercapacitors are required for transparent electronics.1 Figure 8.54 shows some applications of transparent electrodes which emphasize the importance of transparency in these applications.1 Graphene oxide/Au/graphene and graphene/Au/graphene hybrids were evaluated for air-stable, graphene-based, transparent, and flexible electrodes.2 The optical transmittance and a sheet resistance of graphene oxide/Au/graphene were 94.9% and 198±29 Ohm/sq, respectively.2 The size and density (about 1 mg/ml optimal) of Au nanoparticles strongly influenced their reduction reaction.2 Transparent conducting films composed of Ag nanowires sandwiched between graphene oxide and poly(ethylene terephthalate) have been prepared by coating and drying.3 The sheet resistance was 10.4 Ω/sq, and the total transmittance was 82% (at 550 nm wavelength).3 The resistance values were reduced with a sandwich structure graphene oxide/Ag nanowires/graphene oxide.3 The transparent conductive layers were fabricated for planar perovskite solar cells.4 A large-area conductive graphene film was made by a solution printing method.4 The sheet resistance of films was 3.4-3.6 kΩ/sq at around 75% transparency at 550 nm wavelengths.4 The superhydrophilic property of the surface allows water to spread entirely across the surface rather than remain as droplets, thus making the surface anti-fogging.5 The

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Figure 8.55. FE-SEM images in (a) top view and (b) cross-sectional view of a graphene oxide coating, (c) threedimensional AFM image, (d) water contact angle of graphene oxide coating on the glass substrate. [Adapted, by permission, from Hu, X; Yu, Y; Wang, Y; Wang, Y; Zhou, J; Song, L, Mater. Lett., 182, 372-8, 2016.]

transparent functional graphene oxide coating had been fabricated on the glass substrate through a spin coating process (transmittance of 76% in the visible region).5 Figure 8.55 shows the morphology of coating and its effect on water contact angle.5 The direct-write graphene resistors were produced using a CO2 laser to irradiate a spin-coated aromatic polyimide thin film on the top of the glass substrate on which graphene resistors were printed.6 The heater can produce a temperature of 92°C on the glass substrate in one minute.6 The transparent hole transporting layer was manufactured from a composite of graphene and water-dispersible polyaniline-poly(2-acrylamido-2-methyl-1-propanesulfonic acid).7 The graphene nanostacks enhanced the performance of the solar cells by increasing the surface roughness of the hole transporting layers causing improvement of the short-circuit current.7 Transparent conducting thin film of reduced graphene oxide is normally obtained at temperatures of 1000°C, but the highest temperatures employed during the thermal treatment of 400°C are a mandatory condition for the development of organic electronic devices on glass substrates.8 A two-step oxidation process (modified Hummers and “hidden oxidation step”) was employed in order to allow the formation of carbonyl chemical groups rather than epoxy functionalization.8 The sheet resistance was 3.2×103 Ω/sq, the transmittance of 80% at 550 nm, and roughness less than 0.5 nm.8

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Many other solutions have been published in several thousand original papers, and numerous review papers are also available.9-13 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13

Zhang, CJ; Nicolosi, V, Energy Storage Mater., 16, 102-25, 2019. Lee, SJ; Lim, YR; Ji, S; Kim, SK; Yoon, Y; Song, W; Myung, S; Lim, J; An, K-S; Park, J-S; Lee, SS, Carbon, 126, 241-6, 2018. Naito, k; Inuzuka, R; Yoshinaga, N; Mei, W, Synthetic Metals, 237, 50-5, 2018. Liu, Z; Xie, Y; Zhao, J; Wu, S; Zhong, J, Thin Solid Films, 647, 24-31, 2018. Hu, X; Yu, Y; Wang, Y; Wang, Y; Zhou, J; Song, L, Mater. Lett., 182, 372-8, 2016. Wu, D; Deng, L; Mei, X; Teh, KS; Cai, W; Tan, Q; Zhao, Y; Wang, L; Zhao, G; Sun, D; Lin, L, Sensors Actuators A: Phys., 267, 327-33, 2017. Iakobson, OD; Gribkova, OL; Tameeva, AR; Nekrasov, AA; Saranin, DS; Di Carlo, A, J. Ind. Eng. Chem., in press, 2018. Lima, AH; Mendonça, JP; Duarte, M; Stavale, F; Legnani, C; De Carvalho, GSG; Maciel, IO; Sato, F; Fragneaud, B; Quirino, WG, Org. Electronics, 49, 165-73, 2017. Song, Y; Fang, W; Brenes, R; Kong, J, Nano Today, 10, 6, 681-700, 2015. Park, J; Cho, YS; Sung, SJ; Byeon, M; Park CR, Energy Storage Mater., 14, 8-21, 2018. Iqbal, MZ; Rehman, A-U, Solar Energy, 169, 634-47, 2018. Lu, Y; Liu, X; Kuzum, D, Current Opinion Biomed. Eng., 6, 138-47, 2018. Mahmoudi, T; Wang, Y; Hahn, Y-B, Nano Energy, 47, 51-65, 2018.

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8.21 THERMAL MANAGEMENT SOLUTIONS Paraffin/expanded graphite/graphene composite was used for the improvement of thermal management of chips.1 Graphene was used to improve compatibility between the other two components.1 The composite had a lower latent heat than the paraffin and paraffin/ expanded graphite combination.1 The apparent heat transfer coefficient of chips was improved by the composite.1 The incorporation of graphene heat spreaders lowered the maximum temperature of the chip.2 The maximum temperature within the chip was reduced with graphene lateral heat spreaders placed underneath the device and interconnect layers.2 A few layer graphene is suitable for this purpose.2 The thermal conductivity of the cellulose/graphene oxide aerogel composite was 0.67 W m-1 K-1 in the vertical direction (increase by 219%) and 0.72 W m-1 K-1 in the parallel direction (increase by 44%).3 It was possible to enhance thermal conductivity to 6.17 W m-1 K-1 in the aligned reduced graphene oxide nanosheets in the cellulose matrix, but the through-plane thermal conductivity did not change because of a large interfacial thermal resistance between the cellulose fibers surfaces.3 Li-ion batteries suffer from self-heating limiting their lifetime.4 Overheating leads to thermal runaway, cell rupture, or even explosion (batteries on-board of the Boeing 787 Dreamliner).4 Addition of graphene to the hydrocarbon-based phase change material increased its thermal conductivity by more than two orders of magnitude while preserving its latent heat storage ability.4 Lithium-ion battery thermal management system was composed of multi-walled carbon nanotubes and graphene in paraffin (phase change material).5 Multi-walled carbon nanotubes/graphene at a mass ratio of 3/7 (total carbon additive concentration was 1 wt%) gave the best synergistic enhancement heat transfer.5 The thermal conductivity increased by 31.8%, 55.4%, and 124% as compared to graphene-containing paraffin, multi-walled carbon nanotubes-containing paraffin, and pure paraffin, respectively.5 Thermally conductive materials were developed from polyamide/reduced graphene oxide nanocomposites by melt blending.6 A titanate coupling agent was used to enhance the chemical compatibility of reduced graphene oxide.6 Titanate coupling agent replaced water of hydration on the reduced graphene oxide surface, which eliminated the air voids within the composites and improved thermal conductivity.6 The LED lamps containing this composite as heat sink had better durability due to excellent thermal dissipation.6 Three–dimensional materials made out of graphene oxide and copper nanoparticles were used for light-emitting diode heat dissipation.7 The reduced thermal resistance of the mixed particles was attributed to their arrangement at the step edge of graphene oxide sheets which enhanced the out-of-plane heat transfer at wrinkles/folds of graphene oxide interlayers.7 Magnetically-functionalized, self-aligning graphene fillers were used in high-efficiency thermal management applications.8 Graphene and few-layer-graphene flakes were functionalized with Fe3O4 nanoparticles.8 Graphene alignment resulted in a sharp increase of the thermal conductivity.8 The temperature of chip was decreased by 10oC with 1 wt% oriented filler loading.8 Figure 8.56 gives morphological characteristics of the system.8

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Figure 8.56. Preparation and characterization of the graphene and few-layer-graphene fillers functionalized with magnetic nanoparticles. (a) Scanning electron microscopy of graphene and few-layer-graphene flakes synthesized by the liquid-phase exfoliation technique. (b) Transmission electron microscopy image of the graphene flake with attached Fe3O4 nanoparticles. Observed agglomeration of graphene flakes did not prevent alignment and thermal applications. (c) Photograph illustrating a reaction of the magnetically functionalized graphene fillers on a permanent magnet (B = 1.5 T). (d) Two copper foils with the functionalized graphene between them placed on a flat permanent magnet for alignment of the “magnetic graphene” fillers. (e) Optical microscopy image of epoxy with aligned graphene fillers. Higher loading of graphene and few-layer-graphene fillers were used to reveal alignment at a larger length scale. [Adapted, by permission, from Renteria, J; Legedza, S; Salgado, R; Balandin, MP; Ramirez, S; Saadah, M; Kargar, F; Balandin, AA, Mater. Design, 88, 214-21, 2015.]

REFERENCES 1 2 3 4 5 6 7 8

Xu, T; Li, Y; Chen, J; Wu, H; Zhou, X; Zhang, Z, Appl. Thermal Eng., 140, 13-22, 2018. Barua, A; Hossain, MS; Masood, KI; Subrina, S, Phys. Procedia, 25, 311-6, 2012. Chen, L; Hou, X; Song, N; Shi, L; Ding, P, Compos. Part A: Appl. Sci. Manuf., 107, 189-96, 2018. Goli, P; Legedza, S; Dhar, A; Salgado, R; Balandin, AA, J. Power Sources, 248, 37-43, 2014. Zou, D; Ma, X; Liu, X; Zheng, P; Hu, Y, Int. J. Heat Mass Transfer, 120, 33-41, 2018. Cho, E-C; Huang, J-H; Li, C-P; Jian, C-W; Huang, J-H, Carbon, 102, 66-73, 2016. Ryu, BD; Han, M; Ko, KB; Lee, K-H; Cuong, TV; Han, N; Kim, K; Ryu, N-J; Lim, Y, Thanh, DT; Jo, CH; Ju, K; Hong, C-H, Mater. Res. Bull., in press, 2018. Renteria, J; Legedza, S; Salgado, R; Balandin, MP; Ramirez, S; Saadah, M; Kargar, F; Balandin, AA, Mater. Design, 88, 214-21, 2015.

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Figure 8.57. Schematic diagram illustrating the enhanced water flux and ions rejection. [Adapted, by permission, from Zhang, Q; Chen, S; Fan, X; Zhang, H; Yu, H; Quan, X, Appl. Catalysis B: Environ., 224, 204-13, 2018.]

8.22 WATER TREATMENT Graphene was prepared by combustion synthesis from Mg powders and carbon dioxide.1 The MgO diluent has an important influence on morphology, crystallinity, surface properties, and thus adsorption of graphene.1 Graphene exhibited a macroporous microstructure with abundant wrinkles and ripples.1 The adsorption kinetics followed the pseudo-secondorder reaction rate.1 The monolayer adsorption capacity was 125.2 mg/g.1 The optimum specific surface area and pore volume were 393.1 m2/g and 1.042 cm3/g, respectively.1 Graphene oxide membranes were used for the removal of natural organic matter from raw water sources used for drinking water supply.2 Trivalent cations, such as Al3+ and Fe3+, acted as crosslinking agents to stack graphene nanosheets layer-by-layer on a PVDF membrane support.2 The initial interlayer spacing of graphene nanosheets (0.80 nm) was increased to 0.86-0.95 nm by changing the Al3+ or Fe3+ ion concentrations.2 A multifunctional graphene-based nanofiltration membrane was used with photoassistance for enhanced water treatment based on layer-by-layer sieving.3 The membrane coupled with photocatalysis exhibited an efficient removal of ammonia (50%), antibiotic (80%) and bisphenol A (82%).3 Figure 8.57 illustrates features of the membrane.3 The graphene-based nanocomposite membranes were used for the treatment of water from oil sands.4 The graphene oxide derivatives were processed with a polyethersulfone matrix via a non-solvent induced phase separation.4 The graphene oxide nanoribbons at an optimum loading of 0.1 wt% provided the maximum water flux of 70 L/m2/h at 60 psi, organic matter rejection (59%), and antifouling properties (30% improvement compared to polyethersulfone membrane).4 Figure 8.58 shows the effect of different graphene derivatives on the membrane morphology.4 Magnetite derivative of graphene was used for removal of emulsified oil from water.5 The column was regenerated using n-hexane and reused several times with negligible decrease in its initial capacity.5 The graphene nanoplatelets and graphene magnetite showed 90 and 72.2% oil removal efficiency, respectively.5

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Figure 8.58. Cross-sectional FESEM images of unmodified PES (a-c) and nanocomposite membranes loaded with 0.1 wt% of (d–f) graphene nanoplatelets, (g-i) graphene oxide, (j-l) longitudinally unzipped nanoribbons, and (m-o) helically unzipped nanoribbons at different magnification. [Adapted, by permission, from Karkooti, A; Yazdi, AZ; Chen, P; McGregor, M; Sadrzadeh, M, J. Membrane Sci., 560, 97-107, 2018.]

The removal of organic dyes and heavy metal ions was achieved using the graphene oxide reduction-based method.6 Graphene, Zn powder, and NH4Cl were added to wastewater stirring for < 10 min.6 The NH4Cl catalyzed the reduction of graphene oxide by Zn powder which caused the rapid in situ adsorption and precipitation of pollutants improv-

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ing adsorption capacity and reducing adsorption time for water treatment.6 The removal efficiencies were all above 98.46%, and the maximum removal efficiency of 99.99% was obtained for Pb2+.6 Poly(vinyl alcohol) stabilized the 3D porous structure of graphene oxide in aerogel used in oil/water separation.7 Oil and water were separated in 5 s.7 The aerogels had a high absorption selectivity for dyes with positive charges (the adsorption efficiency over 96%).7 REFERENCES 1 2 3 4 5 6 7

Lu, N; He, G; Liu, G; Li, J, Ceramics Int., 44, 2, 2463-9, 2018. Liu, T; Yang, B; Graham, N; Yu, W; Sun, K, J. Membrane Sci., 542, 31-40, 2017. Zhang, Q; Chen, S; Fan, X; Zhang, H; Yu, H; Quan, X, Appl. Catalysis B: Environ., 224, 204-13, 2018. Karkooti, A; Yazdi, AZ; Chen, P; McGregor, M; Sadrzadeh, M, J. Membrane Sci., 560, 97-107, 2018. Chacra, LA; Sabri, MA; Ibrahim, TH; Khamis, MI; Fernandez, C, J. Environ. Chem. Eng., 6, 2, 3018-33, 2018. Hao, J; Ji, L; Li, C; Hu, C; Wu, K, J. Taiwan Inst. Chem. Eng., 88, 137-45, 2018. Dai, J; Huang, T; Tian, S-q; Xiao, Y-j; Yang, J-h; Zhang, N; Wang, Y; Zhou, Z-W, Mater. Design, 107, 187-97, 2016.

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8.23 WEARABLE ELECTRONICS Triboelectric nanogenerators formed directly on human skin and operating wearable devices without recharging process were fabricated with atomically thin graphene (<1 nm), polydimethylsiloxane (<1.5 µm) and poly(ethylene terephthalate) (<0.9 µm) as the electrode, electrification layer, and substrate, respectively.1 The triboelectric nanogenerators were adhered to a human body.1 They generated electricity by contact with various fabrics whose triboelectric effects depended on both the effective contact area and their differences in the electron affinity, ΔEA (polyamide=48.2, silk=66, cotton=72.5, and latex=83.3 nC/J).1 The e-skin has to mimic human skin, and self-generate feedback electric signals following the changes in the external mechanical stimulations.2 Triboelectric electronic-skin was developed based on graphene quantum dots-coated Ag nanowires for application in the self-powered, smart, artificial fingers.2 Micro-gaps were introduced to make the e-skin to respond sensitively to pressing, stretching, folding, and twisting.2 Ag nanowires

Figure 8.59. Basic structure and design of the e-skin. (a) Schematic of the e-skin and the graphene quantum dotcoated Ag NW (G-Ag NW) network. (b) 3D optical image of the patterned PDMS layer. The bottom panel presents the schematic cross-section of the device. (c) SEM image of the Ag NWs. The inset is a high-magnification SEM image. (d) Low-magnification TEM image of G-Ag NWs. (e) High-magnification TEM image of G-Ag NWs. (f) Photograph of the transparent and lightweight e-skin. (g) Photographs of the e-skin with demonstrations of it undergoing different mechanical deformations, including twisting and stretching. (h) Photographs demonstrating that the as-fabricated e-skin is cuttable and that a big e-skin can be cut into smaller pieces. [Adapted, by permission, from Xu, Z; Wu, C; Li, F; Chen, W; Guo, T; Kim, TW, Nano Energy, 49, 274-82, 2018.]

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Figure 8.60. Paper materials as promising substrates for burgeoning wearable electronics applications. Various paper materials with different compositions (e.g., cellulose, graphene, carbon microfiber, carbon nanotube, and composite materials) present a significant potential for applications in the rapidly growing wearable electronics industry for uses as healthcare sensors, memory devices, muscular actuators, and energy supply. [Adapted, by permission, from Liu, H; Qing, H; Li, Z; Han, YL; Lin, M; Yang, H; Li, A; Lu, TJ; Li, F; Xu, F, Mater. Sci. Eng.: R Reports, 112, 1-22, 2017.]

graphene quantum dots-coated acted as the electrode and the friction layer, increasing the sensitivity to the external movements.2 Figure 8.59 illustrates structure, elements and design of e-skin.2 Paper materials comprised of bio-origin ingredients (cellulose and carbon derivatives) were used for prototyping wearable electronics, such as in body-worn healthcare sensing systems, electro-stimulated artificial muscles, on-site memory storage, and wearable power supply on a paper substrate.3 Figure 8.60 illustrates a variety of materials tested.3 A review paper offers a broad discussion of stretchable electronic devices.4 Flexible supercapacitors were built from graphene and polyaniline used as an electrode on stainless steel fabric.5 They had a maximum specific capacitance of 1506.6 mF/ cm2 and capacitance retention of 92% after 5000 charge-discharge cycles − all essential in wearable electronics.5 A review paper provides more information on the flexible energy storage devices.6

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Figure 8.61. (a) Illustration of the atmospheric pressure plasma jet (APPJ) method showing plasma jet surface irradiation and reduction of graphene oxide, GO, films; (b) APPJs experiment setup for reduction of GO films, in which GO film sandwiched between stainless steel dies is exposed to the plasma beam; (c) photo of circular film of GO to reduced graphene oxide, rGO, conversion showing that a GO film (yellowish) is effectively reduced for < 1 min achieving highly conductive P-rGO film (dark color); Photo of series of examples of prepared thick graphene film and patterns including: (d) thick and (e) thin graphene film on the flexible substrate fabricated by scanning plasma treatment of GO coated PET film, (f) square patterns of rGO after the plasma treatment, in which bright pitches are GO and dark color represents P-rGO, (g) graphene film with micro-patterns and array of graphene spots aligned over GO film matrix. [Adapted, by permission, from Alotaibi, F; Tung, TT; Nine, MJ; Kabiri, S; Moussa, M; Tran, DNH; Losic, D, Carbon, 127, 113-21, 2018.]

Spray coating was used to fabricate flexible polymer solar cells.7 Foldable conductive cellulose containing graphene nanoplatelets was used as a top electrode.7 The cellulose-graphene electrode has been applied to devices with the larger active area (0.75 cm2).7

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The scanning atmospheric plasma was used for an ultrafast reduction of graphene oxide (~60 s at room temperature) and preparation of highly conductive graphene films and patterns.8 Graphene films can be fabricated on different substrates including glass, plastic, ceramics, and metals with complex shapes required for flexible and wearable electronics and devices (Figure 8.61).8 The high performance of the plasma process results from plasma beam providing a unique combination of chemical reaction (charged oxygen ions) and mechanical bombardment which leads to the complete reduction of graphene oxide.8 Such improvements in the graphene structure and expanded sp2 cluster size cannot be obtained using conventional reduction methods.8 A thin transparent graphene film was produced with a surface sheets resistance of 22 kΩ/sq at the transparency of 88%, and a thick film (~25 μm) with a sheet resistance of 186 Ω/sq.8 Electrostatic powder coating of pristine graphene helped to avoid solution processed coating technologies which were limited by poor dispersibility of graphene in water and organic solvents.9 The method enabled to minimize moisture induced caking tendency of commercial urea prills at a relative humidity of 85%.9 Electrostatic powder coating applied to nonconductive acrylic fibers formed a stable conductive layer (~0.8±0.1 kΩ/sq) which made fibers suitable for wearable electronics, sensors, and electromagnetic interference shielding.9 Numerous review papers further elaborate on this very interesting topic.10-13 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13

Chu, H; Jang, H; Lee, Y; Chae, Y; Ahn, J-H, Nano Energy, 27, 298-305, 2016. Xu, Z; Wu, C; Li, F; Chen, W; Guo, T; Kim, TW, Nano Energy, 49, 274-82, 2018. Liu, H; Qing, H; Li, Z; Han, YL; Lin, M; Yang, H; Li, A; Lu, TJ; Li, F; Xu, F, Mater. Sci. Eng.: R Reports, 112, 1-22, 2017. Paek, J; Kim, J; An, BW; Park, J; Ji, S; Kim, S-Y; Jang, J; Lee, Y; Park, Y-G; Cho, E; Jo, S; Ju, S; Cheong, WH; Park, J-U, FlatChem, 3, 71-91, 2017. Yu, J; Xie, F; Wu, Z; Huang, T; Wu, J; Yan, D; Huang, C; Li, L, Electrochim. Acta, 259, 968-74, 2018. Chen. K; Wang, Q; Niu, Z; Chen, J, J. Energy Chem., 27, 1, 12-24, 2018. La Notte, L; Cataldi, P; Ceseracciu, L; Bayer, IS; Athanassiou, A; Marras, S; Villari, E; Brunetti, F; Reale, A, Mater. Today, Energy, 7, 105-12, 218. Alotaibi, F; Tung, TT; Nine, MJ; Kabiri, S; Moussa, M; Tran, DNH; Losic, D, Carbon, 127, 113-21, 2018. Nine, MJ; Kabiri, S; Tung, TT; Tran, DNH; Losic, D, Appl. Surf. Sci., 441, 187-93, 2018. Zhang, R; Li, X, Multidimensional Assemblies of Graphene. Graphene, Elsevier, 2018, pp. 27-72. Wu, H; Zhang, Y; Cheng, L; Zheng, L; Yuan, X, Energy Storage Mater., 5, 8-32, 2016. Chen, K; Wang, Q; Niu, Z; Chen, J, J. Energy Chem., 27, 1, 12-24, 2018. Ren, S; Rong, P; Yu, Q, Ceramics Int., 44, 11, 11940-55, 2018.