chemical methods for the production of graphenes

nature nanotechnology | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology 1 review article Published online: 29 march 2009 | doi: 10.1038/nnano.2009.58 The development of various methods for producing graphene — a single layer of carbon atoms bonded together in a hexagonal lattice — has stimulated a vast amount of research in recent years1. The remarkable properties of graphene reported so far include high values of its Young’s modulus (~1,100 GPa)2, fracture strength (125 GPa)2, thermal conductivity (~5,000 W m−1K−1)3, mobility of charge carriers (200,000 cm2 V−1 s−1)4 and specific sur- face area (calculated value, 2,630 m2 g−1)5, plus fascinating trans- port phenomena such as the quantum Hall effect6. Graphene and chemically modified graphene (CMG) are promising candidates as components in applications such as energy-storage materi- als5, ‘paper-like’ mater ials7,8, polymer composites9,10, liquid crystal devices11 and mechanical resonators12. Graphene has been made by four different methods. The first was chemical vapour deposition (CVD) and epitaxial growth, such as the decomposition of ethylene on nickel surfaces13. These early efforts (which started in 1970) were followed by a large body of work by the surface-science community on ‘monolayer graphite’14. The second was the micromechanical exfoliation of graphite15. This approach, which is also known as the ‘Scotch tape’ or peel-off method, followed on from earlier work on micromechanical exfo- liation from patterned graphite16. The third method was epitaxial growth on electrically insulating surfaces such as SiC (ref. 17) and the fourth was the creation of colloidal suspensions. Micromechanical exfoliation has yielded small samples of graphene that are useful for fundamental study. Although large- area graphene films (up to ~1 cm2) of single- to few-layer graphene have been generated by CVD growth on metal substrates18–20, and graphene-type carbon materials have been produced by substrate- free CVD21, radio-frequency plasma-enhanced CVD22, aerosol pyrolysis23 and solvothermal synthesis24, the uniform growth of single-layer graphene is still a challenge. In this review, we dis- cuss the production of graphene and CMG from colloidal suspen- sions made from graphite, derivatives of graphite (such as graphite oxide) and graphite intercalation compounds. This approach is both scalable, affording the possibility of high-volume produc- tion, and versatile in terms of being well-suited to chemical func- tionalization. These advantages mean that the colloidal suspension method for producing graphene and CMG could be used for a wide range of applications. graphenes from graphite oxide Since it was first prepared in the nineteenth century25,26, graphite oxide has been mainly produced by the Brodie25, Staudenmaier27 and Hummers28 methods. All three methods involve oxidation of chemical methods for the production of graphenes sungjin Park1 and rodney s. ruoff1* Interest in graphene centres on its excellent mechanical, electrical, thermal and optical properties, its very high specific surface area, and our ability to influence these properties through chemical functionalization. There are a number of methods for gen- erating graphene and chemically modified graphene from graphite and derivatives of graphite, each with different advantages and disadvantages. Here we review the use of colloidal suspensions to produce new materials composed of graphene and chemically modified graphene. This approach is both versatile and scalable, and is adaptable to a wide variety of applications. graphite in the presence of strong acids and oxidants. The level of the oxidation can be varied on the basis of the method, the reaction conditions and the precursor graphite used. Although extensive research has been done to reveal the chemical structure of graphite oxide, several models are still being debated in the literature. Solid-state 13C NMR spectroscopy of graphite oxide and recently of 13C-labelled graphite oxide favours the model shown in Fig. 1a; the sp2-bonded carbon network of graphite is strongly disrupted and a sig- nificant fraction of this carbon network is bonded to hydroxyl groups or participates in epoxide groups29–32. Minor components of carboxylic or carbonyl groups are thought to populate the edges of the layers in graphite oxide. This indicates that further work with solid-state NMR on 13C-labelled graphite oxide is necessary, along with (for example) titration with fluorescent tags of carboxylic and other groups to iden- tify their spatial distribution on individual graphene oxide platelets derived from graphite oxide as discussed further below. Graphite oxide thus consists of a layered structure of ‘graphene oxide’ sheets that are strongly hydrophilic such that intercalation of water molecules between the layers readily occurs33. The inter- layer distance between the graphene oxide sheets increases revers- ibly from 6 to 12 Å with increasing relative humidity33. Notably, graphite oxide can be completely exfoliated to produce aqueous colloidal suspensions of graphene oxide sheets by simple sonication (Fig. 1b)34 and by stirring the water/graphite oxide mixture for a long enough time35. The measurement of the surface charge (zeta potential) of graphene oxide sheets36 shows that they have negative charges when dispersed in water. This suggests that electrostatic repulsion between negatively charged graphene oxide sheets could generate a stable aqueous suspension of them. A considerable body of work37,38 on such aqueous colloidal suspensions was carried out in the 1950s and 1960s. Such graphene oxide sheets probably have a similar chemical structure to the layers in graphite oxide and are a promising starting material in the generation of colloidal suspen- sions of other CMGs through chemical tuning. Filtration of CMG suspensions has produced free standing paper-like materials7,36,39–41 that have a layered structure (Fig. 1c, d). Significant advances have also been made in using homogeneous suspensions of CMG sheets to produce thin films, which can be relevant to transparent and elec- trically conductive thin-film applications, among others36,39–44 unreduced graphene oxide sheets Several authors have stated that homogeneous colloidal suspen- sions of graphene oxide in aqueous and various organic solvents can be achieved by simple sonication of graphite oxide8,34,45–47. The hydrophilic graphene oxide can be easily dispersed in water 1Department of Mechanical Engineering and the Texas Materials Institute, University of Texas at Austin, One University Station C2200, Austin, Texas 78712-0292, USA. *e-mail: r.ruoff@mail.utexas.edu © 2009 Macmillan Publishers Limited. All rights reserved. 2 nature nanotechnology | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology review article NaTure NaNoTecHNology doi: 10.1038/nnano.2009.58 (at concentrations up to 3 mg ml−1)8,34,45,47, affording brown/dark- brown suspensions. (See Table 1 for a list of solvents used, the concentrations of colloidal suspensions, the lateral dimensions and heights of graphene oxide sheets, and the type of precur- sor material used, be it graphite oxide or graphite or expandable graphite.) The exfoliation to achieve graphene oxide sheets has been most typically confirmed by thickness measurements of the single graphene sheet (~1-nm height on substrates such as mica) using atomic force microscopy (AFM). Graphite oxide can be dispersed directly in several polar solvents such as ethylene glycol, DMF, NMP and THF at about 0.5 mg ml−1 (ref. 46). It has also been shown that the chemical modification of graphene oxide sheets by organic molecules yields homogeneous suspensions in organic solvents45; reaction of graphite oxide with iso- cyanate groups produced isocyanate-modified graphene oxide sheets that are well dispersed in polar aprotic solvents. It was proposed that carbamate and amide functional groups are generated by the reac- tion of isocyanate with hydroxyl and carboxyl groups (Fig. 2a)45. The amide-coupling reaction48 between the carboxyl acid groups of graphene oxides and octadecylamine (after SOCl2 activation of the COOH groups) was used in ref. 49 to mod- ify graphene oxides by long alkyl chains with 20 wt% yield. Interestingly, chemical modification of an alternative starting material, graphite fluoride, with alkyl lithium reagents produced alkyl-chain-modified graphene sheets that could be dispersed in organic solvents after sonication50. reduced graphene oxides Although the chemical modification of graphene/graphite oxide or graphite fluoride can generate homogeneous colloidal suspen- sions, the resulting CMGs are electrically insulating owing to disruption of the ‘graphitic’ networks. On the other hand, the reduction of the graphene oxide by chemical methods (using reductants such as hydrazine47,51,52, dimethylhydrazine9, hydro- quinone53 and NaBH4 (refs 42 and 54), thermal methods55,56 and ultraviolet-assisted methods57 has produced electrically conduct- ing CMGs. (See Table 2 for a list of electrical properties of graph- ene-based materials generated using their suspensions.) The reduction of aqueous graphene oxide suspension by hydra- zine at the pH of the suspension when used as made results in 0.0 0.5 1.8 1.5 2.0 Distance (μm) 4 3 2 1 0 H ei gh t ( nm ) 0.0 0.5 1.8 1.5 2.0 Distance (μm) 4 3 2 1 0 H ei gh t ( nm ) 0.0 0.5 1.8 1.5 2.0 Distance (μm) 4 3 2 1 0 H ei gh t ( nm ) 2+2+ 2+2+22 2+2+ 22+ 2+ 2+ 2+ 2+ 2+22222+2+22 2+2+2+ 2+2+ 2+ 2+ 2 2 2+ 2+ 2 2222+ 2+2 2+ 2 2 a b c d 250 nm Figure 1 | graphite oxide and graphene oxide. a, Chemical structure of graphite oxide30. For clarity, minor functional groups, carboxylic groups and carbonyl groups have been omitted at the edges. Reproduced with permission from ref. 30. © 1998 Elsevier. b, An AFM image of exfoliated graphene oxide sheets47; the sheets are ~1 nm thick. The horizontal lines indicate the sections corresponding (in order from top to bottom) to the traces shown on the right. Reproduced with permission from ref. 47. © 2007 Elsevier. c, Photograph of folded graphene oxide paper7 (© 2007 NPG). d, A scanning electron microscope image of the cross-section of the graphene oxide paper, showing layered structure7 (© 2007 NPG). © 2009 Macmillan Publishers Limited. All rights reserved. nature nanotechnology | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology 3 review articleNaTure NaNoTecHNology doi: 10.1038/nnano.2009.58 agglomerated graphene-based nanosheets, and, when dried, a black powder (Fig. 2b)47 that is electrically conductive (powder conductivity, ~2 × 102 S m−1). Elemental analysis (atomic C/O ratio, ~10) of the reduced graphene oxides measured by com- bustion revealed the existence of a significant amount of oxygen, indicating that reduced graphene oxide is not the same as pris- tine graphene. Theoretical calculations of the reduction of graph- ene oxide (the model used for graphene oxide had the graphene decorated with hydroxyl and epoxide groups) suggest that reduc- tion below 6.25% of the area of the graphene oxide (C/O = 16 in atomic ratio) may be difficult in terms of removing the remaining hydroxyl groups58. Homogeneous colloidal suspensions of electrically conducting CMGs have been produced by chemical reduction with dimethyl- hydrazine or hydrazine in the presence of either polymer or surfactant9,34. The reduction of an aqueous suspension contain- ing a mixture of graphene oxide sheets and poly(sodium 4-sty- renesulphonate) afforded an aqueous black suspension of reduced graphene oxide sheets coated in the polymer34. The reduction of isocyanate-modified graphene oxide in the presence of polystyrene generated a suspension of reduced graphene oxide sheets in DMF that could then be ‘crashed out’ with methanol to yield a compos- ite with well-dispersed and electrically conductive CMG sheets9. Reduction of sodium dodecylbenzenesulfonate (SDBS)-wrapped graphene oxide with hydrazine and then its chemical modifica- tion (Fig. 2c) with aryl diazonium salt produced SDBS-wrapped CMG that was dispersible in DMF, N,N′-dimethylacetamide, and NMP at concentrations up to 1 mg ml−1 (ref. 51). Colloidal suspensions of modified graphenes decorated with small organic molecules or nanoparticles have also been reported. In ref. 39, the reduced graphene oxide sheets were functionalized using pyrenebutyric acid (a well-known organic molecule48 with a strong adsorption affinity for the graphitic plane via π stack- ing). The aqueous graphene oxide suspension was reduced using hydrazine in the presence of pyrenebutyric acid, yielding a black aqueous colloidal suspension (0.1 mg ml−1) of CMG adsorbed by pyrenebutyric acid. Its paper-like material, prepared by filtration, showed moderate electrical conductivity (2 × 102 S m−1). The suspension (<0.48 mg ml−1) of gold-nanoparticle-modified graphene sheets in THF was generated by the reaction of NaBH4 and octadecylamine-modified graphene oxide49 and then the addi- tion of AuCl4 − to the suspension54. The gold nanoparticles (diam- eter, ~5–11 nm) were anchored to the modified graphene sheets. Graphene modified with titanium dioxide nanoparticles has also been studied57; ultraviolet irradiation of this TiO2/graphene oxide hybrid in ethanol was used to reduce the graphene oxide sheets (Fig. 2d), producing a black suspension of TiO2-attached CMG sheets in ethanol. The TiO2 was suggested to act as a photocata- lyst, transferring photoelectrons from the TiO2 to the graphene oxide sheets57. A few methods for creating colloidal suspensions of graphene sheets without the help of stabilizers or surfactants have been Table 1 | comparison of a set of chemical approaches to produce colloidal suspensions of cMg sheets ref. starting materials dispersible solvents concentration (mg ml−1) lateral size thickness (nm) 34 GO/MH Water 1 — — 36 GO/MH Water 0.5 Several hundred nm ~1 39 GO/MH Water 0.1 — ~1.7 40 GO/MH Water 7 Several hundred nm ~1 42 GO/H Water/methanol, acetone, acetonitrile mixed solvents 3–4 Several hundred nm ~1.2 45 GO/MH DMF, NMP, DMSO, HMPA 1 ~560 nm ~1 46 GO/H Water, acetone, ethanol, 1-propanol, ethylene glycol, DMSO, DMF, NMP, pyridine, THF 0.5 100–1,000 nm 1.0–1.4 49 GO/O DMF, THF, CCl4, DCE 0.5 — 0.5–2.5 50 Graphite fluoride DCB, MC, THF 0.002–0.54 1,600 nm ~0.95 51 GO/S DMF, DMAc, NMP 1 Several hundred nm 1.8–2.2 52 GO/MH Hydrazine 1.5 Up to 20 μm × 40 μm ~0.6 54 GO/S THF <0.48 — 1–2 55, 56, 10 GO/S NMP, DMF, DCB, THF, nitromethane 0.1 100–2,500 nm 1.1–3.5 (ave. 1.75) 57 GO/H Ethanol 1 Several hundred nm ~2 59 Graphite powder NMP, DMAc, GBL, DMEU 0.01 Several μm 1–5 60 GIC NMP 0.15 Several hundred nm ~0.35 61 EG DCE 0.0005 Nanoribbon (width <10 nm) 1–1.8 62 EG DMF — ~250 nm ~1 63 EG Water, DMF, DMSO 0.015–0.020 Several hundred nm to a few μm 2–3 (2–3 layers of graphene) 64 Graphite rod DMF, DMSO, NMP 1 500–700 nm ~1.1 GO, graphite oxide; MH, modified Hummers method; H, Hummers method; O, their own method; S, Staudenmaier method; EG, expandable graphite; GIC, graphite intercalation compound; DMF, dimethylformamide; DMAc, N,N′-dimethylacetamide; DMSO, dimethylsulphoxide; NMP, N-methylpyrrolidone; THF, tetrahydrofuran; MC, dichloromethane; DCE, 1,2-dichloroethane; DCB, 1,2-dichlorobenzene; HMPA, hexamethylphosphoramide; GBL, γ-butyrolactone; DMEU, 1,3-dimethyl-2-imidazolidinone. © 2009 Macmillan Publishers Limited. All rights reserved. 4 nature nanotechnology | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology review article NaTure NaNoTecHNology doi: 10.1038/nnano.2009.58 reported. An aqueous suspension (0.5 mg ml−1) of reduced graph- ene oxide sheets under basic conditions (pH 10) was described in ref. 36. The graphene oxide was reduced by hydrazine, and excess hydrazine was removed by dialysis. It was suggested that shifting to pH 10 converts neutral carboxylic groups to negatively charged carboxylate groups, so that when the interior of the graphene oxide sheets are reduced by hydrazine, the negatively charged particles do not agglomerate36; see Fig. 2e. Paper-like materials (1) (2) (3) 2+ 2+22 2 222+ 2+2 2+2+2 2 222 2 222 2 22 +12+1 2+ &2�51&2 51&25 +15 21+5 52 +1 5 2 1+ 52 +1 5 H H RR R RBF4N2 H O O OH OHO OH COOH COOH H O N2H4 H2O pH 10 80 °C, 24 h 1a, b R = CI 2a, b R = NO2 3a, b R = OCH3 4a, b R = Br 1a–4a 1b–4b RT, 1 h SDBS-wrapped GO a b d e c e h h– TiO2 250 nm – + Figure 2 | cMg oxide sheets. a, Proposed reaction of graphene oxide sheets with isocyanates forming carbamate (left oval) and amide (right oval) functionalities45. b, A scanning electron microscope image of aggregated graphene oxide sheets chemically reduced with hydrazine monohydrate47. Parts a and b reproduced with permission from ref. 47. © 2007 Elsevier. c, Starting with SDBS-wrapped graphene oxide, reduction and functionalization of intermediate SDBS-wrapped CMG with diazonium salts51. RT, room temperature. Reproduced with permission from ref. 51. © 2008 ACS. d, TiO2–graphene hybrid and its proposed response under UV excitation57. Reproduced with permission from ref. 57. © 2008 ACS. e, Chemical route to produce aqueous suspension of reduced graphene oxide36. (1) Oxidation of graphite to synthesize graphite oxide. (2) Exfoliation of graphene oxide in water by sonication of graphite oxide. (3) Controlled reduction of graphene oxide sheets by hydrazine yielding a colloidal suspension of conductive CMG sheets, which are stabilized by electrostatic repulsion (© 2008 NPG). © 2009 Macmillan Publishers Limited. All rights reserved. nature nanotechnology | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology 5 review articleNaTure NaNoTecHNology doi: 10.1038/nnano.2009.58 made by filtration and then dried in air showed good electrical conductivity (~7,200 S m−1)36. In other work, an aqueous suspension (7 mg ml−1) of partially reduced graphene oxides was produced, under basic conditions, by hydrazine reduction of KOH-modified graphene oxides40. It was suggested that the K+ ions and the carboxylate anions at the edges of the CMG sheets formed ion pairs. The electrical conductivity of air-dried ‘paper’ made from this material was ~690 S m−1. In a third approach42, a graphene-based suspension was pro- duced in three reaction steps: (1) ‘pre-reduction’ of graphene oxide by NaBH4; (2) sulphonation by aryl diazonium salt of sulphanilic acid; and (3) ‘post-reduction’ of aqueous sulphonated graphene (2 mg ml−1, pH 3–10) with hydrazine. The sulphonated graphene sheets of step 2 could be dispersed in water (2 mg ml−1, pH 3–10), and after the post-reduction step the CMG sheets could be dis- persed in mixed solvents of water with certain organic solvents. It was suggested that covalent functionalization in the CMG sheets by sulphonyl groups was occurring. The electrical conductivity of the thin evaporated film on a glass slide after drying at 120 °C was reported to be 1,250 S m−1 (ref. 42). A suspension of reduced graphene oxide sheets (some with lat- eral dimensions of up to ~20 mm × 40 mm) in anhydrous hydra- zine was produced by stirring graphene oxide paper in anhydrous hydrazine (10 ml for 15 mg of graphite oxide) for one week in a nitrogen-filled dry box52. The resulting CMG on SiO2 was reported to be ~0.6 nm in height, measured by AFM. The anhydrous hydrazine thus acts both as a reduction agent of graphene oxide sheets and as a dispersing solvent in this approach. The individual CMG sheets were tested as a field-effect transistor and showed p-type behaviour52. Thermal treatment of graphite oxide is another route that has been used to obtain redu