Synthesis and characterization of graphene oxide and reduced graphene oxide

Oxidation of graphite to graphite oxide (now commonly called graphene oxide [GO]) traces its origins to 1859, when Brodie used fuming nitric acid and potassium chlorate (KClO₃) to oxidize graphite.Reference Brodie⁵ Approximately a century later, Hummers and Offeman developed a procedure for oxidizing graphite that used potassium permanganate (KMnO₄) and sulfuric acid as oxidative agents.Reference Hummers and Offeman⁶ Following this pioneering work, the Hummers method and its variantsReference Kovtyukhova, Ollivier, Martin, Mallouk, Chizhik, Buzaneva and Gorchinskiy⁷ have been widely used for GO formation.Reference Kovtyukhova, Ollivier, Martin, Mallouk, Chizhik, Buzaneva and Gorchinskiy^7–Reference Sun, Liu, Welsher, Robinson, Goodwin, Zaric and Dai¹²

Nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies and x-ray diffraction (XRD) indicate that hydroxyl and carbonyl groups decorate the surface of GO formed by these oxidation methods (Figure 1a). These hydroxyl and carbonyl groups provide electrostatic repulsion between GO sheets, thus minimizing aggregation in solution.Reference Gao, Alemany, Ci and Ajayan^13, Reference Josepovits, Sanakis, Petridis and De^16–Reference Rodriguez and Jimenez¹⁸ Furthermore, these oxygen moieties render GO hydrophilic, which enables straightforward dispersion in aqueous solution through sonication.Reference Wang, Yang, Park, Gou, Wang, Liu and Yao^19, Reference Stankovich, Dikin, Piner, Kohlhaas, Kleinhammes, Jia, Wu, Nguyen and Ruoff²⁰ Once in solution, the dispersed graphene oxide sheets can then be converted to reduced graphene oxide (RGO) through chemical reduction, as schematically depicted in Figure 1b.Reference Li, Müller, Gilje, Kaner and Wallace¹⁴Figure 1c shows an atomic force microscopy image of GO sheets produced by a modified Hummers method. The GO sheets have an average lateral size of about 1 μm and a thickness of approximately 1 nm.Reference Gao, Alemany, Ci and Ajayan¹³

Figure 1.

(a) Structural model of graphene oxide (GO) showing hydroxyl and carbonyl groups. Adapted from Reference Reference Gao, Alemany, Ci and Ajayan13. (b) Schematic illustration of the synthesis of aqueous graphene dispersions: ((1)) oxidation of graphite (black blocks) to graphite oxide (light gray blocks) with greater interlayer distance, ((2)) exfoliation of GO in water by sonication to obtain GO colloids, and ((3)) chemical conversion of GO to RGO in solution. Reproduced with permission from Reference Reference Li, Müller, Gilje, Kaner and Wallace14. ©2008, Nature Publishing Group. (c) Atomic force microscopy image of GO sheets on a silicon substrate. Reproduced with permission from Reference Reference Gao, Alemany, Ci and Ajayan13. ©2009, Nature Publishing Group. (d) Optical absorbance spectra of GO and RGO. (e) Raman spectra of ((1)) GO; RGO obtained by hydrothermal treatment of GO at ((2)) 180°C, ((3)) 150°C, and ((4)) 120°C for 6 h; and ((5)) hydrazine-reduced GO, indicating the increased intensity of defect peaks (I _D) relative to graphene peaks (I _G) for chemically and hydrothermally reduced GO compared to GO. Parts (d) and (e) reproduced with permission from Reference Reference Zhou, Bao, Tang, Zhong and Loh15. ©2009, American Chemical Society.

Several applications have been demonstrated for GO prepared by the Hummers method. For example, as-produced GO has been used as a catalyst in reactions such as oxidation of alcohols and alkyls,Reference Pyun²¹ as a surfactant to disperse other nanomaterials such as carbon nanotubes (CNTs) in aqueous solution,Reference Cote, Kim, Tung, Luo, Kim and Huang²² and as a hole-transport and electron-blocking layer in organic photovoltaics.Reference Li, Tu, Lin, Chen and Chhowalla^23, Reference Murray, Lou, Cote, Loser, Kadleck, Xu, Szarko, Rolczynski, Johns, Huang, Yu, Chen, Marks and Hersam²⁴ Furthermore, GO exhibits intrinsic and tunable fluorescence, indicating potential application as an optical material.Reference Loh, Bao, Eda and Chhowalla²⁵

More commonly, however, GO serves as a precursor to RGO. The reduction of GO has been achieved chemically with reducing agents such as hydrazine,Reference Li, Müller, Gilje, Kaner and Wallace^14, Reference Yan, Zhao and Pan^26–Reference Xu, Bai, Lu, Li and Shi²⁸ zinc powder,Reference Mei and Ouyang²⁹ iron powder,Reference Fan, Kai, Yan, Wei, Zhi, Feng, Ren, Song and Wei⁸ dimethylformamide (DMF),Reference Ai, Liu, Lu, Cheng and Huo³⁰ laser irradiation,Reference Huang, Liu, Ji, Xie, Wang and Shi³¹ poly(diallyldimethylammonium chloride) (PDDA),Reference Zhang, Shao, Liao, Engelhard, Yin and Lin³² and nontoxic reducing agents such as green teaReference Wang, Shi and Yin³³ and vitamin C.Reference Fernandez-Merino, Guardia, Paredes, Villar-Rodil, Solis-Fernandez, Martinez-Alonso and Tascon³⁴ Other GO reduction methods include simultaneous electrochemical reduction and deposition on surfacesReference Guo, Wu, Liu, Ma, Yang, Zheng, Yu and Liu³⁵ and high-temperature reduction of GO sheets in a reducing gas atmosphere such as hydrogen. The latter approach is often used for synthesizing RGO composites, papers, or thin films for electronics and energy applications.Reference Lee, Smith, Hayner and Kung^36, Reference Wang, Zhi and Müllen³⁷

As illustrated in Figure 1d, GO colloidal suspensions undergo a color change from yellow to black when RGO is formed.Reference Zhou, Bao, Tang, Zhong and Loh¹⁵ RGO has been employed in many applications, including transistors and transparent conductors,Reference Kim, Pham, Cuong, Kim, Chung, Hur and Cho^38–Reference Becerril, Mao, Liu, Stoltenberg, Bao and Chen⁴⁵ biological and chemical sensors,Reference Guo, Wu, Liu, Ma, Yang, Zheng, Yu and Liu^35, Reference Jiang^46–Reference Ji, Honma, Paek, Akada, Hill, Vinu and Ariga⁴⁸ and photovoltaics.Reference Xu, Long, Huang, Huang, Wan, Ma and Chen^49, Reference Liu, He, Wang, Wu and Wang⁵⁰ In comparison with mechanically exfoliated graphene, RGO exhibits more topological defects and a higher surface roughness.Reference Gómez-Navarro, Meyer, Sundaram, Chuvilin, Kurasch, Burghard, Kern and Kaiser⁵¹Figure 1e shows the defect (D) peaks in the Raman spectra of chemically and hydrothermally reduced GO.Reference Zhou, Bao, Tang, Zhong and Loh¹⁵ As a result of these defects and remnant oxygen functionalities, the electrical properties of RGO are compromised compared to those of pristine graphene.Reference Tung, Allen, Yang and Kaner⁵²

Exfoliation of graphite in aqueous solutions with amphiphilic surfactants

In an effort to avoid the defects introduced during the aforementioned oxidative and reductive chemical treatments, researchers have also developed methods for directly exfoliating graphene from graphite in aqueous solutions using ultrasonication in amphiphilic surfactants such as sodium dodecyl benzene sulfonate (SDBS),Reference Lotya, Hernandez, King, Smith, Nicolosi, Karlsson, Blighe, De, Wang, Mcgovern, Duesberg and Coleman⁵³ sodium cholate (SC), and sodium dodecyl sulfate (SDS).Reference Green and Hersam⁵⁴ Biocompatible, amphiphilic block copolymers (such as those sold under the trade names Pluronic and Tetronic) have also yielded effective dispersion of graphene in aqueous solutions.Reference Seo, Green, Antaris and Hersam⁵⁵ These block copolymers contain hydrophobic poly(propylene oxide) (PPO) and hydrophilic poly(ethylene oxide) (PEO) domains that preferentially interact with graphene sheets and water molecules, respectively, enabling the graphene sheets to disperse in water without a covalent modification step. The concentration of graphene in aqueous solution depends on the lengths of the PPO and PEO chains, as shown in Figure 2a.Reference Seo, Green, Antaris and Hersam⁵⁵

Figure 2.

(a) Graphene concentration map for various Pluronic and Tetronic compositions (identified by trade name), showing the dependence of the concentration of graphene in aqueous solution on the lengths of the poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEO) blocks of the block copolymers. Reproduced with permission from Reference Reference Seo, Green, Antaris and Hersam55. ©2011, American Chemical Society. (b) Schematic illustration of a surfactant-based graphene exfoliation process in aqueous solution, along with a photograph of a 90 μg ml^–1 graphene dispersion in sodium cholate (SC). (c) Photograph of a centrifuge tube containing an aqueous solution of SC-coated graphene after the application of density gradient ultracentrifugation (DGU) and representative atomic force microscopy images of graphene from fractions designated ((1)) f4 and ((2)) f16 on a SiO₂substrate. The height profiles corresponding to the indicated line scans from the two fractions demonstrate the different thicknesses of graphene flakes, in this case about 1 nm and 1.5 nm, segregated using DGU. Parts (b) and (c) reproduced with permission from Reference Reference Green and Hersam54. ©2009, American Chemical Society.

Figure 2b schematically illustrates the ultrasonic exfoliation of graphene from graphite in aqueous solution containing SC.Reference Green and Hersam⁵⁴ As indicated in the schematic, solution-phase methods for directly exfoliating graphene from graphite typically offer minimal control over the thickness (i.e., number of layers) in the dispersed graphene sheets, which is suboptimal for many applications because many properties of graphene depend on the number of layers.Reference Green and Hersam⁵⁶ For example, single-layer graphene is a zero-bandgap semimetal,Reference Castro Neto, Guinea, Peres, Novoselov and Geim⁵⁷ whereas bilayer graphene has a tunable bandgap in the infrared regionReference Zhang, Tang, Girit, Hao, Martin, Zettl, Crommie, Shen and Wang⁵⁸ and trilayer graphene has a band overlap that can be controlled by applying an electric field.Reference Craciun, Russo, Yamamoto, Oostinga, Morpurgo and Tarucha⁵⁹

An important advantage of dispersing graphene in aqueous solutions is that these solutions are compatible with methods for separating and purifying nanomaterials such as density-gradient ultracentrifugation (DGU).Reference Arnold, Green, Hulvat, Stupp and Hersam^60, Reference Green and Hersam⁶¹ For example, SC has a planar structure and can encapsulate few-layer-graphene particles to form well-ordered graphene/SC assemblies (as illustrated in Figure 2b). With this geometry, the buoyant density of these graphene/SC particles varies as a function of their thickness, presenting an opportunity for thickness sorting by DGU. In the graphene DGU process, SC-coated graphene sheets are initially loaded into a fluid of linearly varying density. During subsequent ultracentrifugation, the graphene sheets sediment through the density gradient until they reach the point where their density matches the local density of the fluid, forming distinct bands in the centrifuge tube. These bands contain graphene sheets with specific numbers of layers, which are then harvested in a fractionation step. Figure 2c shows these bands in a centrifuge tube along with atomic force microscopy characterization of selected fractions, illustrating separation by graphene thickness and thus number of layers.Reference Green and Hersam⁵⁴ Single-layer graphene isolated by DGU was recently employed in high-performance flexible electronic devices that operate at frequencies in the gigahertz range.Reference Sire, Ardiaca, Lepilliet, Seo, Hersam, Dambrine, Happy and Derycke⁶²

Direct exfoliation of graphene in organic solvents

Another effective solution-phase method for exfoliating graphene from graphite is sonication of pristine graphite in organic solvents.Reference Hernandez, Nicolosi, Lotya, Blighe, Sun, De, McGovern, Holland, Byrne, Gun’Ko, Boland, Niraj, Duesberg, Krishnamurthy, Goodhue, Hutchison, Scardaci, Ferrari and Coleman^63–Reference O’Neill, Khan, Nirmalraj, Boland and Coleman⁶⁵ For example, Hernandez et al. used organic solvents such as N-methyl-2-pyrrolidone (NMP), γ-butyrolactone (GBL), N,N-dimethylacetamide (DMA), and N,N′-dimethylethyleneurea (DMEU) to obtain graphene dispersions with ~1 wt% yield (although the yield could be increased to ~12 wt% by recycling the graphite sediment). Because a mild-bath sonication was used for graphite exfoliation followed by a gentle centrifugation step for sedimentation and separation of the remaining graphite, the resulting graphene was reported to have a low defect density as determined by Raman spectroscopy.Reference Hernandez, Nicolosi, Lotya, Blighe, Sun, De, McGovern, Holland, Byrne, Gun’Ko, Boland, Niraj, Duesberg, Krishnamurthy, Goodhue, Hutchison, Scardaci, Ferrari and Coleman⁶³ With this method, graphene dispersions with concentrations of approximately 0.01 mg/ml were obtained in various solvents. Figure 3a–b show the optical absorbance data for such solutions. NMP was found to be the best choice among the four tested solvents for graphene exfoliation, yielding the highest graphene concentration, because its surface energy provides the best match with the surface energy of graphene.Reference Hernandez, Nicolosi, Lotya, Blighe, Sun, De, McGovern, Holland, Byrne, Gun’Ko, Boland, Niraj, Duesberg, Krishnamurthy, Goodhue, Hutchison, Scardaci, Ferrari and Coleman⁶³

Figure 3.

(a) Absorption spectra for few-layer graphene dispersed in N-methyl-2-pyrrolidone (NMP), γ-butyrolactone (GBL), N,N-dimethylacetamide (DMA), and N,N′-dimethylethyleneurea (DMEU) at concentrations from 2 to 8 μg ml^–1. (b) Optical absorbance, A (at λ = 660 nm), divided by cell length (l) for graphene in the four solvents NMP, GBL, DMA, and DMEU, showing Beer–Lambert behavior (absorbance proportional to concentration and path length) with an average absorption coefficient of 〈α₆₆₀〉 ≈ 2460 l g^–1 m^–1. The x-axis error bars indicate the uncertainty in measuring the mass of graphene/graphite in solution. Parts (a) and (b) reproduced with permission from Reference Reference Hernandez, Nicolosi, Lotya, Blighe, Sun, De, McGovern, Holland, Byrne, Gun’Ko, Boland, Niraj, Duesberg, Krishnamurthy, Goodhue, Hutchison, Scardaci, Ferrari and Coleman63. ©2008, Nature Publishing Group. (c) Vials of a 1:5 mixture of terpineol and ethyl cellulose stabilized graphene/ethanol solution before and after water addition. Upon the addition of water, the hydrophobic graphene flakes preferentially separate into the concentrated terpineol fraction, leaving behind a predominantly ethanol and water mixture. The concentration factor C/C ₀ of graphene (for an initial concentration of C ₀= 102.4 μg/ml) is plotted after each solvent-exchange concentration and graphene/ethanol addition step for three iterations. Reproduced with permission from Reference Reference Liang and Hersam66. ©2010, American Chemical Society.

For applications that require higher graphene concentrations, Liang et al. employed a stabilizer and solvent-exchange method to increase the concentration of graphene in organic solvents. They initially used ethanol as a solvent to exfoliate graphite and obtained a graphene dispersion with a concentration of 1.6 μg/ml. However, upon addition of ~1% w/v ethyl cellulose (EC) to ethanol, the graphene concentration was substantially enhanced to more than 120 μg/ml. Terpineol was then added to dilute this solution to about 100 μg/ml. Because the EC-stabilized graphene flakes are hydrophobic, subsequent addition of water to the graphene/terpineol/ethanol/EC solution induced the graphene to leave the EC phase and concentrate in the small-volume terpineol phase, resulting in a graphene ink with a concentration of about 1 mg/ml following three iterations of this solvent-exchange process (Figure 3c).Reference Liang and Hersam⁶⁶

Exfoliation of graphene using intercalation and electrochemical methods

Graphite intercalation compounds such as tetrabutylammonium hydroxide (TBA) have also been used for exfoliation of graphite or weakly oxidized graphite oxide. Although this method yields large graphene flakes, the resulting graphene contains significant quantities of defects.Reference Ang, Wang, Bao, Thong and Loh^67, Reference Li, Zhang, Bai, Sun, Wang, Wang and Dai⁶⁸ Ionic intercalation compounds can also be used for graphite exfoliation, but they either leave oxygen moieties on the graphene sheets or produce graphene flakes that are relatively small compared to those obtained using competing intercalation compounds (e.g., TBA).Reference Zheng, Di, Liu, Liu, Guo, Du, Wu, Yu and Zhu^69, Reference Behabtu, Lomeda, Green, Higginbotham, Sinitskii, Kosynkin, Tsentalovich, Parra-Vasquez, Schmidt, Kesselman, Cohen, Talmon, Tour and Pasquali⁷⁰

To address these issues, Shih et al. used halides (ICl and IBr) as intercalants for graphite.Reference Shih, Vijayaraghavan, Krishnan, Sharma, Han, Ham, Jin, Lin, Paulus, Reuel, Wang, Blankschtein and Strano⁷¹ In this method, a piece of highly oriented pyrolytic graphite (HOPG) was placed in a reactor along with the intercalation compounds and heated to 100–160°C for 48 h. The intercalated graphite was then immediately heated to 800°C in an argon atmosphere, leading to volumetric expansion. The degree of intercalation is controlled by the intercalation temperature, providing significant tunability. For example, every third layer of the graphite lattice can be intercalated (stage 3), or every second layer can be intercalated (stage 2), as shown in Figure 4a. These well-ordered crystallographic stages are demonstrated by the XRD patterns in Figure 4b.

Figure 4.

(a) Schematic illustration of highly oriented pyrolytic graphite (HOPG) (top), IBr stage-3 graphite intercalation compound (GIC) (middle), and ICl stage-2 GIC (bottom) (carbon, gray; iodine, red; bromine, yellow; chlorine, cyan). (b) X-ray diffraction patterns of HOPG, IBr stage-3 GIC, ICl stage-2 GIC, IBr stage-3 expanded graphite (ExG), and ICl stage-2 ExG. For IBr stage-3 GIC, three additional peaks appear between two of the three main peaks corresponding to the (002), (004), and (006) planes in HOPG, which is a clear signature of stage-3 GIC (the intercalant layer inserts between every three graphene layers). (c) Photographs of HOPG (top), stage-3 GIC (middle), and stage-3 ExG formed by lithium intercalation (bottom). (The HOPG and GIC materials are held up with tweezers with the edges toward the viewer). (d) Photographs of HOPG (top), stage-2 GIC (middle), and stage-2 ExG (bottom). Parts (a)–(d) reproduced with permission from Reference Reference Shih, Vijayaraghavan, Krishnan, Sharma, Han, Ham, Jin, Lin, Paulus, Reuel, Wang, Blankschtein and Strano71. ©2011, Nature Publishing Group. (e) Schematic illustration of electrochemically charged graphite in Li⁺/propylene carbonate (PC). The expanded graphite is then exfoliated by power sonication in LiCl in a dimethylformamide/PC mixed solvent. Part (e) reproduced with permission from Reference Reference Wang, Manga, Bao and Loh72. ©2011, American Chemical Society.

Stage-2 and stage-3 expanded samples exhibit a substantial volume expansion, as shown in Figure 4c–d. The two bottom profiles in Figure 4b show the XRD patterns of stage-2 and stage-3 expanded graphite. The corresponding (002) and (004) peak positions are almost identical to those of HOPG, indicating that the spacing of the graphene planes in graphite is restored. Furthermore, the decrease in (004) peak intensity and the absence of the (006) peak show that the two materials are made of fewer than four graphite layers.Reference Shih, Vijayaraghavan, Krishnan, Sharma, Han, Ham, Jin, Lin, Paulus, Reuel, Wang, Blankschtein and Strano⁷¹

Inspired by electrochemical reactions in lithium-ion batteries, Wang et al. used intercalation of lithium ions in graphite to exfoliate graphite. The primary advantage of this method is the high yield of single- and few-layer graphene (more than 70%), whereas the yield is often less than 1% in competing exfoliation methods. In the lithium intercalation process, graphite is used as the negative electrode in a lithium-ion battery cell.Reference Wang, Manga, Bao and Loh⁷² In many commercial lithium-ion batteries, electrolytes are selected so that they form a protective layer on the graphite anode that is referred to as the solid electrolyte interphase (SEI). SEI formation prevents graphite exfoliation and promotes a longer battery life.Reference Shu, Mcmillan, Murray and Davidson⁷³ Therefore, for effective graphite exfoliation with the highest yield, electrolytes are needed that do not form an SEI layer, such as propylene carbonate (PC).Reference Rauh and Brummer⁷⁴ To further promote exfoliation, Wang et al. used a high applied potential (–15 ± 5 V) to activate the intercalation of PC molecules in addition to Li⁺ ions into the graphite.Reference Wang, Manga, Bao and Loh⁷² The resulting expanded graphite was then exfoliated to graphene by sonication in LiCl in a DMF/PC mixed solvent. Figure 4e schematically illustrates this process.Reference Wang, Manga, Bao and Loh⁷²

Amphiphilic surfactants such as SDS have also been used as intercalating compounds. In addition to serving as intercalants, the surfactants that adsorb on the surface of the exfoliated graphene sheets prevent the sheets from restacking in solution following exfoliation. In this method, a graphite rod is used as a working electrode with a platinum counter electrode in a 1% SDS aqueous solution. The graphite rod is initially treated as an anode in an effort to drive SDS intercalation. Then, the intercalated graphite is treated as a cathode, leading to the exfoliation and dispersion of SDS-functionalized graphene sheets. The size and concentration of the graphene sheets can be controlled by the intercalation potential, thus providing a degree of dispersion tunability.Reference Alanyalıoğlu, Segura, Oró-Solè and Casañ-Pastor⁷⁵

Graphene/polymer composites

Graphene, GO, and RGO composites with insulating and conducting polymers have been widely reported in the literature.Reference Alzari, Nuvoli, Scognamillo, Piccinini, Gioffredi, Malucelli, Marceddu, Sechi, Sanna and Mariani^78–Reference Santos, Tria, Vergara, Cui, Pernites, Advincula, Aileen and Vergara⁹¹ In the solution-mixing method for synthesizing graphene/polymer composites, a graphene, GO, or RGO colloidal dispersion is combined with the polymer in solution. This approach has been employed with a variety of both nonconducting polymers such as poly(vinyl alcohol),Reference Zhao, Zhang, Chen and Lu^83, Reference Liang, Huang, Zhang, Wang, Ma, Guo and Chen^88, Reference Das, Eswar Prasad, Ramamurty and Rao⁹² poly(methyl methacrylate),Reference Das, Eswar Prasad, Ramamurty and Rao⁹² poly(vinylidene fluoride),Reference Mohamadi and Sharifi-Sanjani⁹³ polystyrene,Reference Stankovich, Dikin, Dommett, Kohlhaas, Zimney, Stach, Piner, Nguyen and Ruoff^94, Reference Fang, Wang, Lu, Yang and Nutt⁹⁵ and polycarbonateReference Higginbotham, Lomeda, Morgan and Tour⁹⁶ and conducting polymers such as sulfonated poly(phenylene ethylene),Reference Yang, Zhang, Shan, Li, Han and Niu⁸⁴ poly(3-hexylthiophene-2,5-diyl),Reference Wang, Kurunthu, Scott and Bardeen⁹⁷ and poly(3,4-ethylenedioxythiophene).Reference Hong, Xu, Lu, Li and Shi⁹⁸Figure 5 provides an example of the processing and resulting structure of a representative graphene/polymer composite.

Figure 5.

An example of a fabrication process for a graphene/polymer composite: (a) Suspension of phenyl isocyanate-treated GO (1 mg/ml) and dissolved polystyrene in dimethylformamide before (left) and after (right) reduction of the GO by N,N-dimethylhydrazine. (b) Composite powder as obtained after coagulation in methanol. (c) Hot-pressed composite (opaque; 0.12 vol% graphene) and pure polystyrene (clear) of the same 0.4-mm thickness following identical processing. (d) Scanning electron microscopy (SEM) images obtained from a fracture surface of composite samples of 0.48 vol% (left) and 2.4 vol% (right) graphene in polystyrene. Reproduced with permission from Reference Reference Stankovich, Dikin, Dommett, Kohlhaas, Zimney, Stach, Piner, Nguyen and Ruoff94. ©2006, Nature Publishing Group.

In polymer composites that contain RGO, reduction of graphene oxide to RGO can be performed either before or after solution mixing. In the pregraphitization strategy, RGO is directly mixed with the polymer, thus avoiding exposure of the polymer to the graphene oxide reduction process that involves chemical treatments or high temperatures. On the other hand, the advantage of the postgraphitization strategy is that GO is easily dispersed in aqueous solutions because of its hydrophilicity, thus enabling the integration of GO with water-soluble polymers using a simple sonication step. Unfunctionalized graphene sheets that do not disperse well in water can be dispersed in solvents such as DMFReference Yu, Jiang, Wu, Wang and Wu⁹⁹ and then mixed with the polymer solution. After the solutions are mixed, the composite can be precipitated by water/solvent evaporation,Reference Li, Liu, Li, Wan, Neoh and Kang¹⁰⁰ filtration,Reference Li, Xie, Li, Liu and Li¹⁰¹ centrifugation,Reference Liu, Tao, Yang, Li, Boyer, Wuhrer, Braet and Davis¹⁰² or flocculation (for example, adding alcohol to a water-dispersed composite causes composite precipitation).Reference Salavagione, Martínez and Gómez¹⁰³

In situ polymerization is another common solution method for synthesizing graphene/polymer composites. In this method, the monomer is mixed with the graphene, GO, or RGO colloidal suspension, and then polymerization is induced. Both pre- and postgraphitization processes are compatible with this strategy.Reference Xu and Gao^104, Reference Hu, Wang, Wang, Wan, Liu, Zheng, Chen and Xu¹⁰⁵ As an example, Hu et al. prepared graphene/polystyrene nanocomposites by mixing GO with styrene monomers and then inducing polymerization by adding potassium persulfate and heating in an inert atmosphere. In this case, hydrazine was then added to induce GO reduction to RGO.Reference Hu, Wang, Wang, Wan, Liu, Zheng, Chen and Xu¹⁰⁵ This method tends to promote uniform coating of the graphene within the polymer matrix. However, the disadvantage is that interchain hydrogen bonding can be inhibited, resulting in a slight degradation in mechanical properties.Reference Kim, Miura and Macosko¹⁰⁶

Applications for graphene-based polymer composites attempt to exploit the unique electrical, physical, and chemical properties of graphene and graphene oxide. For example, because of their high conductivities, polymer composites of graphene and RGO have been used in field-effect transistors,Reference Eda and Chhowalla¹⁰⁷ supercapacitors,Reference Li, Xie, Li, Liu and Li¹⁰¹ and photovoltaics.Reference Liu, He, Wang, Wu and Wang^50, Reference Spitsina, Lobach and Kaplunov¹⁰⁸ Other applications such as biosensorsReference Lu, Do, Drzal, Worden and Lee^109, Reference Zhou, Zhu, Yang, Luo, Li and Luan¹¹⁰ and biomedicalReference Fan, Wang, Zhao, Li, Shi, Ge and Jin¹¹¹ applications rely on the high mechanical strength of graphene and GOReference Yang, Tu, Li, Shang and Tao^112, Reference Kulkarni, Choi, Singamaneni and Tsukruk¹¹³ and the chemical stability of graphene. Figure 6a–b show scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of a composite of graphene nanosheets (GNSs) and polyaniline (PANI). PANI is generally regarded as a promising electrode material for supercapacitors because of its high energy-storage capacity. Figure 6c shows that addition of graphene nanosheets to PANI further improves this capacity by increasing the available surface area and conductivity of the PANI electrode.Reference Li, Xie, Li, Liu and Li¹⁰¹

Figure 6.

(a) SEM and (b) transmission electron microscopy (TEM) images of a polyaniline (PANI)/graphene composite. (c) Variation of the specific capacitance of pure PANI, graphene nanosheets (GNSs), and graphene/polymer (GP) (6:94) composite electrodes as a function of cycle number measured at a rate of 50 mV s⁻¹ in 1 M H₂SO₄aqueous solution. The inset shows the ((1)) first and ((2)) 1000th cyclic voltammograms of the GP6:94 composite. Reproduced with permission from Reference Reference Li, Xie, Li, Liu and Li101. ©2011, Elsevier.

Graphene/nanoparticle composites

Graphene and GO composites with other carbonaceous nanomaterials such as fullerenes and CNTs have also been reported.Reference Cai, Song and Xu^114–Reference Tung, Chen, Allen, Wassei, Nelson, Kaner and Yang¹¹⁷ Typically, a colloidal dispersion of graphene or GO is directly mixed with another carbon nanomaterial dispersion and sonicated to form a uniform dispersion. The composite is then formed by vacuum filtration, drop casting, or blade coating of the mixed solution. In some cases, GO acts as a surfactant for the dispersion of CNTs in aqueous solutions because of its amphiphilic nature (GO has hydrophilic edges and hydrophobic regions on its basal plane).Reference Cote, Kim, Tung, Luo, Kim and Huang²² The addition of CNTs and fullerenes can increase the conductivity of graphene-based materials, which has utility for applications in electronics and transparent conductors.Reference Tung, Chen, Allen, Wassei, Nelson, Kaner and Yang¹¹⁷

Metal and metal-compound nanoparticles have also been explored as additives in graphene-based composites.Reference Liu, Pan, Lv, Zhu, Sun and Sun^118–Reference Chang, Sun, Ho, Tao, Yan, Kwok and Zheng¹³⁰ As for carbon-based nanomaterials, the most common method for preparing these composites is solution mixing of dispersions of graphene and a metal or metal compound. In addition, in situ methods for directly depositing nanoparticles on graphene sheets in solution have been reported.Reference Yang, Xu, Qiu, Chen, Deng, Zhang, Iwai, Watanabe and Chen^131–Reference HongKun and Chao¹³⁴ For example, HongKun and Chao demonstrated that gold, platinum, palladium, and silver nanoparticles can be deposited on RGO from their precursors (AuCl₃·HCl·H₂O, K₂PtCl₄, PdCl₂, and AgNO₃, respectively). These composites exhibited enhanced catalytic activity for the reduction reaction of potassium hexacyanoferrate(III).Reference HongKun and Chao¹³⁴ Yang et al. also reported enhancement in electrocatalytic activity of graphene following decoration with gold nanoparticles, with potential applications in biomedical and energy technologies.Reference Yang, Xu, Qiu, Chen, Deng, Zhang, Iwai, Watanabe and Chen¹³¹ Similarly, RGO/silver nanoparticle composites showed excellent antibacterial capability comparable to that of the antibacterial drug ampicillin,Reference Xu, Zhang, Li, Lu, Li, Ma, Wang and Yu¹²⁸ whereas graphene/iron nanoparticleReference Guo, Zhu, Song, Chen and Zhang¹³⁵ and graphene/tin nanoparticleReference Wang, Wang, Wang, Park, Dou, Ahn and Kim¹³⁶ composites were reported to have high lithium-ion capacities for use in rechargeable lithium-ion batteries.

A variety of metal compounds have also been employed in graphene/nanoparticle composites. For instance, graphene and RGO have been incorporated into composites with Fe₃O₄,Reference Ji, Tan, Kuykendall, Aloni, Xun, Lin, Battaglia and Zhang¹²⁴ vanadium nitride,Reference Zhang, Wang, He, Liu, Wang, Gu, Xu, Han, Dong, Zhang, Yao, Cui and Chen¹³⁷ MnO₂,Reference Deng, Zhu, Wang, Kang, Liu, Yang and Wang^138, Reference Sun, Hu, Luo and Huang¹³⁹ Mn₃O₄,Reference Wang, Cui, Yang, Casalongue, Robinson, Liang, Cui and Dai¹⁴⁰ SnO₂,Reference Zhao, Zhang, Song, Jiang, Zhuang, Liu and Fang^125, Reference Paek, Yoo and Honma¹⁴¹ MoO₂,Reference Sun, Hu, Luo and Huang¹³⁹ TiO₂,Reference Qiu, Yan, Yang, Jin, Deng and Li¹⁴² and CuOReference Wang, Wu, Shu, Guo and Wang¹⁴³ for application in high-performance lithium-ion battery electrodes. In addition, RGO composites with CdSReference Liu, Pan, Lv, Zhu, Sun and Sun¹¹⁸ and ZnOReference Liu, Pan, Lv, Lu, Zhu, Sun and Sun¹⁴⁴ were used for photocatalytic reduction of Cr(VI), and RGO composites with MnO₂ were utilized in supercapacitors.Reference Deng, Zhu, Wang, Kang, Liu, Yang and Wang¹³⁸Figure 7a–b show TEM and high-resolution TEM images, respectively, of an RGO/Mn₃O₄ composite. Figure 7c–d compare the specific lithium capacity and battery cycling performance of pure Mn₃O₄ and RGO/Mn₃O₄ composite, respectively, revealing improvement in the reversible capacity following the addition of RGO.Reference Wang, Cui, Yang, Casalongue, Robinson, Liang, Cui and Dai¹⁴⁰Figure 7e–f show SEM images of solvent-exfoliated graphene (SEG) and solvent-reduced graphene oxide (SRGO) composites with mixed-phase titania (TiO₂ P25) particles. These composites act as effective catalysts for CH₃CHO photo-oxidation and CO₂photoreduction reactions, respectively. In particular, SEG-based composites were found to exhibit higher catalytic activity than SRGO-based composites in both oxidation and reduction reactions. This observation was attributed to the reduced defect density and thus higher electron mean free path for SEG compared to SRGO.Reference Liang, Vijayan, Gray and Hersam¹⁴⁵

Figure 7.

(a) TEM and (b) high-resolution TEM images of reduced graphene oxide (RGO)/Mn₃O₄ composites. (c) Capacity retention of free Mn₃O₄ nanoparticles without graphene degrades after even one cycle at a current density of 40 mA/g. (d) Improved cycling performance of an RGO/Mn₃O₄ composite in a lithium-ion battery. Parts (a)–(d) reproduced with permission from Reference Reference Wang, Cui, Yang, Casalongue, Robinson, Liang, Cui and Dai140. ©2010, American Chemical Society. (e–f) SEM images of (e) an annealed solvent-exfoliated graphene (SEG)/P25 TiO₂ nanocomposite and (f) an annealed solvent-reduced graphene oxide (SRGO)/P25 TiO₂nanocomposite. Parts (e) and (f) reproduced with permission from Reference Reference Liang, Vijayan, Gray and Hersam145. ©2011, American Chemical Society.