WO2008097343A2 - Functionalized graphene materials and method of production thereof - Google Patents

Abstract

A method of functionalizing graphene sheets of graphite includes treating graphite with an alkali metal in substantially dry ammonia in an inert atmosphere. The resultant mixture is treated with an alkylating agent to form functionalized graphene sheets. A polymer composite may be formed that incorporates such derivatized graphene sheets. Additionally, water soluble and organic soluble graphite compounds may be formed by this method.

Description

FUNCTIONALIZED GRAPHENE MATERIALS AND METHOD OF PRODUCTION

THEREOF

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 60/836,341, filed August 8, 2006 and is incorporated herein by reference in its entirety.

STATEMENT OF FEDERALLY SPONSORED RESEARCH [0002] This work was sponsored in part by a grant from the National Science Foundation, Grant No. CHE-0450085.

FIELD OF THE INVENTION

[0003] This invention relates to chemical modification of graphene (individualized sheets of graphite), particularly wherein such modification renders them soluble or otherwise easy to process.

BACKGROUND

[0004] Individual sheets of graphite, called graphene, have interesting and useful properties such as high strength, stiffness and electrical conductivity. Indeed, recent experiments indicate that graphene exhibits important properties as an electronic material. Novoselov, K. S. et al. Electric field effect in atomically thin carbon films. Science 306, 666-669 (2004). Zhang, Y., Tan, Y. W., Stormer, H. L. & Kim, P. Experimental observation of the quantum hall effect and Berry's phase in graphene. Nature 438, 201-204 (2005). Novoselov, K. S. et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 438, 197-200 (2005). Dresselhaus, M. S., Dresselhaus, G. & Eklund, P. C. Science of Fullerenes and Carbon Nanotubes (Academic, San Diego, California, 1996). Berger,C. et al. Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics. J. Phys. Chem. B 108, 19912-19916 (2004). Rotkin, S. V. & Hess, K. Possibility of a metallic field-effect transistor. Appl. Phys. Lett. 84, 3139 (2004). [0005] Chemically, graphene exists as extended aromatic arrays of carbon atoms arranged in a planar hexagonal lattice. The parent material, graphite, is comprised of a thick stack of such sheets; these sheets are weakly bound together by Van der Waals forces, and generally only the edges of such stacks are accessible for covalent chemical modification. Even though the individual graphene planes are strong in planar tension, the macroscopic solid graphite is mechanically rather soft, since these planes can easily shear and slide past one another. In fact, bulk graphite is often used as a dry lubricant film, even though the individual sheets are theoretically quite strong.

[0006] The ability to process carbon nanomaterials is of fundamental importance if these materials are to be exploited in the fields of electronics, nanohealth, and composites. In order to take advantage of the unusual properties of a single graphene sheet, a means to separate individual graphene sheets may prove beneficial. Toward this end, interruption of the Van der Waals attractive forces between graphene sheets may be realized through graphene functionalization.

SUMMARY OF THE INVENTION

[0007] In some aspects, the present disclosure provides a method of functionalizing graphene sheets in graphite that includes treating graphite with an alkali metal in substantially dry ammonia in an inert atmosphere. To the resultant mixture is added an alkylating agent to yield a functionalized graphene sheets.

[0008] Another aspect of the present disclosure is the use of this method to provide a polymer composite. The polymer portion may include more than one monomer and may be attached to a derivatized graphene sheet. The derivatized graphene sheet may display functional groups for covalent attachment and integration into the polymer composite.

[0009] Finally, both water soluble and organic soluble graphite compounds are accessible by the method for functionalizing the graphene sheets. Functionalization of graphene sheets provides a means for separating individual graphene sheets.

[0010] The foregoing has outlined rather broadly the features of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the claims of the invention. DESCRIPTION OF THE DRAWINGS

[0011] The foregoing and other features and aspects of the present invention will be best understood with reference to the following detailed description of a specific embodiment of the invention, when read in conjunction with the accompanying drawings, wherein:

[0012] Figure 1 shows from lithium intercalation into the layers of graphene.

[0013] Figure 2 shows Raman spectra for graphene before, line a, and after functionalization with dodecyl radicals, line b.

[0014] Figure 3 a shows an atomistic model of graphene sheet using MM2 energy minimization calculation.

[0015] Figure 3b shows the irregular, ragged shapes of the graphene after functionalization found in an optical microscopy image.

[0016] Figures 3c and 3d show different shapes of graphene structure (1 μm x 1 μm) found by APM analyses.

[0017] Figure 3e shows a 1.1 μm x 1.1 μm AFM scan of an individual graphene sheet.

[0018] Figure 3f shows a pseudo-3D representation of a 1.1 μm x 1.1 μm AFM scan of an individual graphene sheet shown in figure 3e.

[0019] Figure 4a shows the SEM image of unfunctionalized graphene. [0020] Figure 4b shows the SEM image of dodecylated graphene.

[0021] Figure 4c shows the SEM image of dodecylated graphene after thermal analysis.

[0022] Figure 5a shows the Raman spectra of unfunctionalized graphite.

[0023] Figure 5b shows the Raman spectra of dodecylated graphite (1) made from C₈K.

[0024] Figure 6a shows the high-resolution transmission electron microscopy (HRTEM) study of PEGylated graphite 3. [0025] Figure 6b shows the Cryogenic TEM (Cryo-TEM) study of PEGylated graphite 3 in the aqueous phase.

[0026] Figure 7a shows AFM images of dodecyl functionalized graphite 1.

[0027] Figure 7b shows statistical distributions of average height, from 50 nanoplatelets of dodecyl functionalized graphite 1.

[0028] Figure 7c shows AFM images of PEGylated graphite 3.

[0029] Figure 7d shows statistical distributions of average height, from 50 nanoplatelets of PEGylated graphite 3.

[0030] Figure 8a shows scanning electron microscopy (SEM) images of polyacrylonitrile (PAN)-functionalized graphene at 100 microns.

[0031] Figure 8b shows scanning electron microscopy (SEM) images of polyacrylonitrile (PAN)-functionalized graphene at 50 microns.

[0032] Figure 8c shows scanning electron microscopy (SEM) images of polyacrylonitrile (PAN)-functionalized graphene at 10 microns.

[0033] Figures 8d and 8e show scanning electron microscopy (SEM) images of polyacrylonitrile (PAN)-functionalized graphene at 1 micron.

DESCRIPTION OF THE INVENTION

[0034] The present disclosure provides methods for covalently functionalizing the planes and edges of graphene using reductive functionalization and other functionalization techniques. Such methods are useful for (a) solubilizing graphene, (b) for rendering graphene dispersible in polymers, and (c) rendering the graphene suitable for direct chemical attachment in polymer matrices for producing composites. In addition to these methods, the present disclosure is also directed to compositions generated by such methods.

[0035] Previous work with graphite/graphene has been limited to either surfactant dispersion, or oxidation (acids, ozone, and the like) to make edge-pendant acids/carbonyls, or by simply exfoliating/grinding and blending into polymers. Treatment of graphene in a strongly oxidizing medium leads to an oxide that forms unstable dispersions in organic solvents. Recently, it was reported that exfoliation by strong acid, followed by functionalization with a long-chain alkylamine gives rise to materials that are soluble in organic solvents. Niyogi, S. et al. Solution properties of graphite and graphene. J. Am. Chem. Soc. 128, 7720-7721 (2006).

[0036] The present disclosure provides a method of functionalizing graphene sheets of graphite that addresses functionalization of basal plane carbons of the graphene sheet, in addition to functionalization at the pendant edges. Graphene may be converted to graphene salts and alkylated using methodology that Applicants have used previously to functionalize single-walled carbon nanotubes (PCT Publication No. WO 2005/090233). The present method includes treating graphite with an alkali metal in substantially dry ammonia in an inert atmosphere. To this mixture one adds an alkylating agent to complete the functionalization.

[0037] The alkali metal may include lithium, sodium, potassium, cesium, rubidium, and combinations thereof. In particular embodiments, lithium and potassium are particularly effective alkali metals for generating graphene salts.

[0038] Without being bound by the mechanism, as used herein the alkylating agent may include any organohalide such as those capable of undergoing a radical reaction to form an organoradical species capable of addition to a graphene salt. Alkylating agents may include alkyl groups, both straight chain and branched, alkenes, alkynes, di-, tri, and multi-functional molecules having other common organic functional groups, any of which may be optionally protected. Common organic functional groups may include carboxylic acids, esters, alcohols, ketones, aldehydes, thiols, ethers, nitriles, amines, and amides, for example.

[0039] The organohalide may be optionally substituted. The term "optionally substituted" means the proceeding group may be substituted or unsubstituted. When substituted, the substituents of an "optionally substituted" group may include, without limitation, one or more substituents independently selected from the following groups or designated subsets thereof: (Q-C^alkyl, (C₂-C₆)alkenyl, (C₂-C₆)alkynyl, (C₁- C₆)heteroalkyl, (d-C₆)haloalkyl, (C₂-C₆)haloalkenyl, (C₂-C₆)haloalkynyl, (C₃- C₆)cycloalkyl, phenyl, (C₁-C₆)alkoxy, phenoxy, (Q-C^haloalkoxy, amino, (C₁- C₆)alkylamino, (d-C^alkylthio, phenyl-S-, oxo, (Q-C^carboxyester, (C₁- C₆)carboxamido, (Q-C^acyloxy, H, halogen, CN, NO₂, NH₂, N₃, NHCH₃, N(CH₃)₂, SH, SCH₃, OH, OCH₃, OCF₃, CH₃, CF₃, C(O)CH₃, CO₂CH₃, CO₂H, C(O)NH₂, pyridinyl, thiophene, furanyl, (Cl-C6)carbamate, and (C₁-C₆)UrCa. Two substituents may be joined together to form a fused five-, six-, or seven-membered carbocyclic or heterocyclic ring consisting of zero to three heteroatoms. An optionally substituted group may be unsubstituted (e.g., -CH₂CH₃), fully substituted (e.g., -CF₂CF₃), monosubstituted (e.g., - CH₂CH₂F) or substituted at a level anywhere in-between fully substituted and monosubstituted (e.g., -CH₂CF₃).

[0040] Organohalides generally include iodides, bromides and chlorides. One skilled in the art will recognize that organoiodides and organobromides are particularly effective in the mechanistic mode described above. As an alternative to organohalides, graphene can be functionalized using diazonium species such as those described in Dyke et al., "Covalent Functionalization of Single-Walled Carbon Nanotubes for Materials Applications," J. Phys. Chem. A, vol. 108(51), pp. 11151-11159, 2004. Furthermore, it is likely that other functionalization routes exist, particularly wherein such functionalizations proceed by way of a free radical.

[0041] The method as disclosed herein may be useful for the preparation of soluble graphite compounds bearing lipophilic groups. Compounds bearing lipophilic groups may have appreciable solubility in organic solvents, which may include, without limitation, methylene chloride, carbon tetrachloride, chloroform, benzene, tetrahydrofuran, diethyl ether, and the like. Lipophilic groups include, without limitation, C₃ through C₃o alkyl groups in one embodiment. In another embodiment the lipophilic groups may be C₁₀ through C₁₅ alkyl groups. The lipophilic groups may be straight chain or branched alkyl groups.

Exemplary Embodiments

[0042] Method Using Graphite-Lithium: Commercially available graphene (SupraCarbonic, LLC, 1214 Keel Drive, Corona del Mar, CA) can be used to carry out the methods disclosed herein. This hydrophobic material is a dark-grey insoluble powder, with a high specific surface area (-2500 m²/g). Functionalization reactions may be carried out by adding lithium to a dispersion of the graphene in liquid ammonia. Liang, F. et al. A convenient route to functionalized carbon nanotubes. Nano Lett. 4, 1257-1260 (2004). Chattopadhyay, J. et al. Carbon nanotube salts. Arylation of single- wall carbon nanotubes. Org. Lett. 7, 4067-4069 (2005). Liang, F., Alemany, L. B., Beach, J. M. & Billups, W. E. Structure analyses of dodecylated single- walled carbon nanotubes. J. Am. Chem. Soc. 127, 13941-13948 (2005). The resulting graphene salt reacts with «-dodecyl iodide, for example, to give soluble sheets of functionalized graphene as shown in Scheme 1 below.

Scheme 1. Functionalization via Li-graphene salt.

Li/NH_{3 r} . . R_I _r ,

Graphene ► Graphene salt ¹^¹ ► Graphene — R

R = «-dodecyl

[0043] Without imposing limitations or being bound by the mechanism, the exfoliation of the graphene maybe explained in terms of negatively charged sheets of graphene that are formed by electron transfer from lithium as the metal intercalates into the layers of graphene as shown in Figure 1. This leads to lithium ions dispersed between the sheets of graphene. The intense blue color associated with solvated electrons disappears as the lithium is added to the suspension of graphene in liquid ammonia, suggesting that electron transfer to the graphene is a facile process. Addition of an organohalide can lead to the formation of a radical anion by electron transfer from the graphene salt to the alkyl iodide. This unstable species may dissociate readily to yield iodide and dodecyl radicals.

[0044] Covalent attachment of the dodecyl radicals to graphene can be demonstrated by Raman spectroscopy. Figure 2 shows Raman spectra of the graphene before, line a and after functionalization line b. The two prominent bands are the disorder mode (D band) at 1299 cm^"1 and the tangential mode (known as graphitic "G" band) at 1580 cm^"1. The enhanced D band exhibited by the functionalized material results from chemical disruption of the sp² hybridized carbon atoms in the hexagonal framework of the graphene and reflects a high level of functionalization. Dodecylated single- walled carbon nanotubes (SWNTs) exhibit these bands at 1290 cm^"1 and 1590 cm^"1, respectively. Liang, F. et al. A convenient route to functionalized carbon nanotubes. Nano Lett. 4, 1257-1260 (2004).

[0045] An FT-IR spectrum with ATR accessory of dodecylated graphene exhibited C-H stretching bands associated with the dodecyl groups at 2917 and 2840 cm^"1. A strong broad absorption at 1556 cm^"1 is assigned to the activated C=C double bonds in the dodecylated graphene structure.

[0046] Thermogravimetric analysis shows that the functionalized graphene exhibits substantial weight loss at 100-550 °C due to detachment of the dodecyl radicals. The weight loss (24%) is in agreement with the large D/G ratio observed in the Raman spectrum of the functionalized graphene.

[0047] Figure 3 a shows an atomistic model of graphene sheet using MM2 energy minimization calculation. Polarized light microscopy was used to image the dispersions of dodecylated graphene. As shown in Figure 3b the irregular, ragged shapes of the graphene after functionalization may reflect the high temperature during preparation of the graphene. Figure 3b shows a schematic representation of different shapes found in optical microscopy image. Figures 3c and 3d show different shapes of graphene structure (1 μm x 1 μm) found by AFM analyses. Figure 3e shows a 1.1 μm x 1.1 μm AFM scan of an individual graphene sheet. Figure 3f shows apseudo-3D representation of a 1.1 μm x 1.1 μm AFM scan of an individual graphene sheet (that shown in figure 3e) representing the wrinkled and rough structure of the surface.

[0048] The functionalized material is soluble in chloroform, benzene, and 1,2,4- trichlorobenzene. To characterize the soluble species by atomic force microscopy (AFM), solutions were spin coated onto mica from chloroform. The solutions consisted of irregular graphene sheets with a height of -0.3-0.8 nm. Schniepp, H. C. et al. Functionalized single graphene sheets derived from splitting graphite oxide. J. Phys. Chem. B 110, 8535-8539 (2006). The horizontal distance of these graphene sheets varies from 0.8-1.3 μm. Images with large holes and folded edges (Figures 3c and 3d) were observed in several instances. [0049] Figures 4a-c show scanning electron microscopy (SEM) images that reveal the layered carbon structure of graphene as it is broken down into separate sheets. Figure 4a is the SEM image of unfunctionalized graphene. Figure 4b is the SEM image of dodecylated graphene and Figure 4c is the SEM image of dodecylated graphene after thermal analysis. The functionalized graphene shows (Figure 4b) an increase of dead space. In Figure 4c separate sheets are observed after removal of the functional groups.

[0050] Applicant has found that soluble graphene can also be formed from graphite flake (Aldrich) by reductive alkylation using lithium in liquid ammonia. Greenish blue solutions are observed in tetrahydrofuran. Atomic force microscopy (AFM) images reveal irregular graphite crystallites with measured heights of -4-7 nm (from section analysis of height data). From the literature, it is known that graphene layers have a thickness of 3.4 A to 4 A, with a dead space of 5 A between graphene and the substrate occupied by trapped solvent. Novoselov, K. S. et al. Electric field effect in atomically thin carbon films. Science 306, 666-669 (2004). Niyogi, S. et al. Solution properties of graphite and graphene. J. Am. Chem. Soc. 128, 7720-7721 (2006). Kotov, N. A., Dekany, I. & Fendler, J. H. Ultrathin graphite oxide-polyelectrolyte composites prepared by self- assembly: Transition between conductive and non-conductive states. Adv. Mater. 8, 637- 641 (1996). Cassagneau, T. & Fendler, J. H. High density rechargeable lithium-ion batteries self-assembled from graphite oxide nanoplatelets and polyelectrolytes. Adv. Mater. 10, 877-881 (1998). Kovtyukhova, N. I. et al. Layer-by-layer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations. Chem. Mater. 11, 771-778 (1999). It is expected that extensive functionalization may increase the dead space resulting in an increase in the height.

[0051] Method Using Graphite-Potassium: Graphite also reacts with potassium to form lamellar compounds by intercalation of potassium atoms between the graphene sheets. L. Reghai, J. Conard, H. Fuzellier, M. Lelaurain, E. McRae, J. Phys. Chem. Solids 2001, 62, 2083-2090. M.-H. Whangbo, W. Liang, J. Ren, S. N. Magonov, A. Wawkuschewski, J. Phys. Chem. 1994, 98, 7602-7607. H. Estrade-Szwarckopf, B. Rousseau, Synth. Met. 1988, 23, 191-198. K. Okabe, S. Tanuma, Synth. Met. 1988, 23, 61 -66. D. Braga, A. Ripamonti, D. Savoia, C. Trombini, A. Umani-Ronchi, J. Chem. Soc. Chem. Commun. 1978, 927-928. D. T. Haworth, C. A. Wilkie, J. Solid State Chem. 1980, 31, 343 -345. Although, initially, C₈K was used as a catalyst in polymerization reactions, and in the nuclear and side-chain alkylation of aromatic compounds by ethylene, the use OfC₈K as a reducing agent has also been investigated. M. A. M. Boersma, Catal. Rev. 1974, 10, 243-280. H. Podall, W. E. Foster, J. Org. Chem. 1958, 23, 401-403. M. A. Sierra, P. Ramirez-Lόpez, M. Gόmez-Gallego, T. Lejon, M. J. Mancheήo, Angew. Chem. 2002, 114, 3592-3595; Angew. Chem. Int. Ed. 2002, 41, 3442- 3445. I. S. Weitz, M. Rabinovitz, J. Chem. Soc. Perkin Trans. 1 1993, 117- 120. A. Fϋrstner, H. Weidmann, J. Organomet. Chem. 1988, 354, 15-21. A. Fϋrstner, H. Weidmann, J. Org. Chem. 1989, 54, 2307- 2311. D. J. Mindiola, G. L. Hillhouse, J. Am. Chem. Soc. 2001, 123, 4623-4624. M. A. Schwindt, T. Lejon, L. S. Hegedus, Organometallics 1990, 9, 2814-2819. A. Fϋrstner, Tetrahedron Lett. 1990, 31, 3735- 3738. A. Fϋrstner, H. Weidmann, J. Carbohydr. Chem. 1988, 7, 773-783. M. Contento, D. Savoia, C. Trombini, A. Umani-Ronchi, Synthesis 1979, 30-32. C. Ungurenasu, M. Palie, J. Chem. Soc. Chem. Commun. 1975, 388. K. A. Jensen, B. Nygaard, G. Clisson, P. H. Nielson, Acta Chem. Scand. 1965, 19, 768-770. The use of C₈K as a metallation agent in the alkylation of nitriles, esters, and oxazines, and in the reductive cleavage of carbon-sulfur bonds in vinylic and allylic sulfones has been reported. D. Savoia, C. Trombini, A. Umani-Ronchi, Tetrahedron Lett. 1977, 18, 653 -656. D. Savoia, C. Trombini, A. Umani-Ronchi, J. Org. Chem. 1978, 43, 2907-2910. D. Savoia, C. Trombini, A. Umani-Ronchi, J. Chem. Soc. Perkin Trans. 1 1977, 123 -125. P. O. Ellingsen, K. Undeheim, Acta. Chem. Scand. B 1979, 33, 528 -530. Bergbreiter and Killough studied the Lewis basicity and the electron-transfer properties of C₈K D. E. Bergbreiter, J. M. Killough, J. Chem. Soc. Chem. Commun. 1976, 913 -914. D. E. Bergbreiter, J. M. Killough, J. Am. Chem. Soc. 1978, 100, 2126 -2134.

[0052] Biphenyl is formed in high yield when phenyl halides are treated with C₈K, whereas reactions of C₈K with alkyl halides lead to products ranging from alkanes to typical Wurtz-type coupling products. D. E. Bergbreiter, J. M. Killough, J. Am. Chem. Soc. 1978, 100, 2126 -2134. M. Rabinovitz, D. Tamarkin, Synth. Commun. 1984, 14, 377-379. F. Glockling, D. Kingston, Chem. Ind. 1961, 8, 1037. Novel ring-closure reactions leading to the coupling of α-diketones and nitrogen-containing heterocyclic compounds have also been reported. M. Rabinovitz, D. Tamarkin, Synth. Met. 1988, 23, 487-491. R. Setton, F. Beguin, S. Piroelle, Synth. Met. 1982, 4, 299 -318. Ebert studied reductive alkylations, as well as potassium intercalaction, with soot. L. B. Ebert, Science 1990, 247, 1468-1471.

[0053] The synthesis of C₈K, a bronze powder, is achieved readily by melting potassium over graphite (synthetic graphite powder, < 20 μm, Aldrich) under an atmosphere of argon. I. S. Weitz, M. Rabinovitz, J. Chem. Soc. Perkin Trans. 1 1993, 117-120. Freshly prepared C₈K was treated with 1-iodododecane, as illustrated in Scheme 2, to produce dodecylated graphite (1), which is soluble in chloroform, benzene, and 1,2,4-trichlorobenzene.

Scheme 2. Functionalization via K-graphene salt.

powder* *^* potassium graphite (C₈K) — →- [graphite |- docleey⁾

1 a) potassium. 200 ⁰C; b) ammonia; c) 1-iododecane

[0054] The Raman spectra of unfunctionalized and dodecylated graphite (1) made from C₈K are presented in Figure 5 a and 5b, respectively. The appearance of the prominent disorder mode (D band) at 1299 cm^"1 (Figure 5b) is indicative of the disruption of the sp²-hybridized carbon atoms in the hexagonal framework of graphite. Thermogravimetric analyses (TGA) of 1 (argon, 10 ⁰C min^"1 to 800 ⁰C) indicated a weight loss of 15%, which corresponds to approximately one dodecyl group per 78 graphite carbon atoms. Although protonation has been observed with other systems, a control experiment carried out with C₈K and ammonia led to a negligible increase in the intensity of the D band, indicating that the addition of hydrogen to the graphite is not a significant event. D. E. Bergbreiter, J. M. Killough, J. Chem. Soc. Chem. Commun. 1976, 913 -914. S. Pekker, J.-P. Salvetat, E. Jakab, J.-M. Bonard, L. Forrό, J. Phys. Chem. B 2001, 105, 7938 -7943. The FT-IR spectrum of 1 exhibits C-H stretching bands associated with the dodecyl groups at 2800-3000 cm^"1.

[0055] Water soluble graphite: The reductive alkylation methods described above can also be used to prepare a water soluble graphite compound. One strategy to accomplish this is to form derivatized graphene sheets having a functional group handle for covalent attachment of a water solubilizing polymer. In one embodiment the functional group handle may be carboxylic acid groups. PEG polymers may be especially well-suited for generating water soluble graphene sheets, especially where the applications my require biocompatibility. Other biological polymers and oligomeric entities may be attached to the graphene sheet to confer water solubility, including peptides, proteins, oligosaccharides, for example.

[0056] Although water-soluble platlets of graphene can be prepared by oxidation of graphite, hydrophobic interactions leads to re-aggregation of the graphene sheets as the oxygen functionality is removed by reduction. Amphiphilic polymers have been used to overcome some of these difficulties. Stankovich, S. et al. Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphitic oxide in the presence of poly(sodium 4-styrenesulfonate). J. Mater. Chem. 16, 155-158 (2006). Bourlinos, A. B. et al Graphite oxide: Chemical reduction to graphite and surface modification with primary aliphatic amines and amino acids. Langmuir 19, 6050-6055 (2003). Hahn, J. R., Kang, H., Lee, S. M. & Lee, Y. H. Mechanistic study of defect-induced oxidation of graphite. J. Phys. Chem. B 103, 9944-9951 (1999). Grant, L. M., Tiberg, F. & Ducker, W. A. Nanometer-scale organization of ethylene oxide surfactants on graphite, hydrophilic silica, and hydrophobic silica. J. Phys. Chem. B 102, 4288-4294 (1998).

[0057] In an exemplary embodiment, solubility in water was achieved by initial functionalization of the graphite surface using 5-bromovaleric acid to provide carboxylic acid decorated graphite intermediate 2, and subsequent reaction with amine-terrninated poly(ethylene glycol) (PEG) chains to yield PEGylated graphite 3, as illustrated in Scheme 3 below.

Scheme 3. Synthesis of PEGylated graphite potassium graphite (C₈K)

O [graphite |- (CH₂J₁C

NH-PEG-OMe 3 a) ammonia; b) 5-bromovaleric acid; c) NH2-PEG-0Me (Mr = 5000 Da), DCC, DMAP,

DMSO/DMF

[0058] As expected, the Raman spectrum of the acid-functionalized graphite (2) exhibits a strong D band, and the TGA trace of 2 reveals a weight loss of 12%, corresponding to one C₄H₈CO₂H group per 61 graphite carbon atoms. The FT-IR spectrum of the carboxylic acid functionalized material shows a broad hydroxy absorption at 3400 cm^"1 and a sharp carbonyl absorption at 1678 cm^"1. The carbonyl absorption in the spectrum of the PEGylated graphite (3) is shifted to 1624 cm^"1, and the N-H stretch is found at 3738 cm^"1, in accordance with amide-bond formation.

[0059] X-ray photoelectron spectroscopy (XPS) provides direct evidence for the linkage of nitrogen to the carboxylate group during the PEGylation reaction. XPS spectra of the region between 0-1100 eV indicate the presence of carbon, nitrogen, and oxygen in the PEGylated graphite. The C Is, O Is, and N Is XPS spectra show distinct peaks at 284.6, 533, and 400.2 eV, respectively. The presence of the distinct N Is peak is indicative of the amide bond in 3.

[0060] Additional evidence for functionalization is provided by the high-resolution transmission electron microscopy (HRTEM) study of 3 (Figure 6a). The PEGylated graphite shows a morphology expected for functionalized graphite. TEM images of unfunctionalized graphite are generally known to have smooth sidewalls. P. R. Buseck, H. Bo- Jun, L. P. Keller, Energy Fuels 1987, 1, 105 - 110. Analogous to SWNTs, the "bumps" along the sidewalls of the graphite structure are indicative of surface functionalization. F. Liang, L. B. Alemany, J. M. Beach, W. E. Billups, J. Am. Chem. Soc. 2005, 127, 13941 - 13948. B. K. Price, J. M. Tour, J. Am. Chem. Soc. 2006, 128, 12899 - 12904. The HRTEM image of 3 shows that the fringes are long and that 6-20 fringes are formed in tangled ribbons in a network-like structure. Cryogenic TEM (Cryo- TEM) study of 3 (Figure 6b) shows that, in the aqueous phase, the PEGylated-graphite particles have an average size of the order of 0.1 μm.

[0061] The average height distribution of the functionalized material determined by atomic force microscopy (AFM) is in good agreement with that detected in TEM images of 3. AFM images of two types of functionalized graphite (1 and 3) reveal irregular graphite nanoplatelets (Figure 7a and 7c). The statistical distributions, from 50 nanoplatelets each of 1 and 3, show that 70% of each of the functionalized materials has an average height of 7-9 nm, whereas 30% has an average height of 2-4 nm (Figure 7b and 7d. The horizontal distances across the nanoplatelets of these functionalized materials vary between 0.1-1.4 μm.

[0062] Using the methods described hereinabove, a polymer composite may be generated that incorporates functionalized graphene sheets. The polymer may be made from at least one monomer and a plurality of derivatized graphene sheets; wherein the derivatized graphene sheets are formed by reductive alkylation. The polymer may also include other monomer units and the functionalized graphene sheets incorporated in the resultant copolymer. In many respects, the potential technological benefits of graphene are similar to those described for single-wall carbon nanotubes (SWNTs). And like SWNTs, optimum benefit in composites will occur when the basic components, graphene sheets, are individually separated from the bulk parent material, and when these sheets are covalently functionalized to allow direct covalent linkage into the matrix of a composite. Derivatized graphene sheets may display one or more functional moieties for covalent attachment to a polymer matrix including, but not limited to a carboxylic acid, an alcohol, an^ amine, a thiol, an epoxide, and an alkene. Matrices of technological interest span epoxies, polyolefins, polycarbonates, nylons, polyimides, aramids, polyesters, and the like. In an exemplary embodiment, Figures 8a-e shows scanning electron microscopy (SEM) images of polyacrylonitrile (PAN)-functionalized graphene, where the graphene is high reaction (ability) carbon mixture (HRCM) graphene.

Experimental Examples

[0063] The following experimental examples are included to demonstrate particular embodiments of the present invention. It should be appreciated by those of skill in the art that the methods and compositions disclosed in the example that follows merely represent exemplary embodiments of the present invention. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments described and still obtain a like or similar result without departing from the spirit and scope of the present invention. [0064] Functionalized graphenes were characterized by Raman spectroscopy, thermogravimetric analyses (TGA), FT-IR spectroscopy with ATR accessory, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Raman spectra of solid samples were recorded using a Renishaw 1000 microraman system equipped with 780 nm excitation laser source. Multiple spectra (3-5) were obtained, normalized to the G band, and averaged to give a comprehensive snapshot of the material. Thermogravimetric (TGA) experiments were carried out under argon using a SDT 2960 TA instrument. Samples were degassed at 80 ⁰C, heated for 10 °C/min to 800 °C and held there for 30 min. FTIR spectra were obtained using a Nicolet spectrometer with the ATR accessory. Optical microscopy pictures were taken using a Digipro 4.0 instrument and were viewed by placing a TEM grid onto a glass. AFM images were taken using a Digital Instrument Nanoscope Ilia in tapping mode with a 3045 JVW piezo tube scanner. Samples were prepared by spin coating a chloroform solution onto mica. The tapping frequency was between 270 and 310 kHz. SEM images were obtained using an FEI Phillips Electroscan XL30 ESEM-FEG scanning electron microscope.

EXAMPLE 1

[0065] Organic soluble graphite compound-lithium: The functionalization reactions with lithium were carried out by adding the graphene (1.7 mmol) under an atmosphere of argon to a dry 100 mL three neck round-bottomed flask fitted with a dry ice condenser. Ammonia (60 mL) was then condensed into the flask followed by the addition of 17 mmol of lithium metal. The n-dodecyl iodide (6.8 mmol) was then added and the mixture stirred at -33 °C with slow evaporation of ammonia. The flask was then cooled in an ice bath and the reaction mixture quenched by the slow addition of ethanol followed by water. The mixture was acidified with 10% HCl and the graphenes were extracted into hexane and washed several times with water. The hexane layer was then filtered through a 0.2 μm PTFE membrane and washed successively with ethanol and chloroform. The functionalized graphenes were dried overnight in vacuo at 80 °C.

EXAMPLE 2

[0066] Organic soluble graphite compound 1-potassium: In a typical reaction, graphite powder (2.5 mmol) and a stir bar were added to a three-necked flask that was previously flushed with argon. The graphite powder was heated to 200⁰C, and then small pieces of potassium (0.32 mmol) were added. The mixture was stirred and heated at 200⁰C for 30 min. The resulting bronze-colored C₈K mixture was then cooled to room temperature. Dry ammonia (60 mL) was then condensed into the reaction vessel, and the mixture was stirred for 30 min in a dry-ice-acetone bath. 1-Iodododecane (10 mmol) was then added slowly, and the suspension was stirred overnight at room temperature, leading to slow evaporation of ammonia. The flask was then cooled in an ice bath, and the reaction mixture was quenched by slow addition of ethanol and water. The mixture was acidified with HCl (10%), and the product was extracted into hexanes and washed several times with water. The hexane layer was then filtered through a 0.2-μm polytetrafluoroethylene (PTFE) membrane. The precipitate was washed with ethanol, as well as chloroform, and dried overnight in vacuo at 80⁰C.

EXAMPLE 3

[0067] Water soluble graphite compound: Graphite was also functionalized using 5- bromovaleric acid to generate 2, in a manner similar to that mentioned above. The acid- functionalized graphite (2) (2.5 mmol) was then taken in dimethylformamide (DMF; 14 mL) and sonicated for approximately 15 min to achieve a homogeneous dispersion. 4- Dimethylaminopyridine (DMAP; 2.5 mmol) in DMF (3.5 mL) and H₂N-PEG-OMe (4 x 10^"5 mmol) in DMF (7.5 mL) were added slowly to this dispersion as the mixture was stirred. N,N'-Dicyclohexylcarbodiimide (DCC; 2.7 mmol) dissolved in a mixture of DMF (7.5 mL) and dimethyl sulfoxide (DMSO; 10 mL) was added dropwise over 1 h, and the resulting mixture was stirred at room temperature for 72 h. The solution was filtered through a 0.2-μm PTFE membrane and washed several times with DMF followed by chloroform. The product was then dried overnight in vacuo at 50⁰C. An aqueous solution of the PEGylated graphite (3) was dialyzed (SnakeSkinT Dialysis Tubing, 10000-Da molecular- weight cut-off) at room temperature in Νanopure water (Barnstead International). The dialyzed solution was used for further studies.

Claims

WHAT IS CLAIMED:

1. A method of functionalizing graphene sheets of graphite comprising: treating graphite with an alkali metal in substantially dry ammonia in an inert atmosphere to form a mixture; and adding an alkylating agent to the mixture.

2. The method of claim 1, wherein the alkali metal is chosen from lithium, sodium, potassium, cesium, and rubidium.

3. The method of claim 2, wherein the alkali metal is lithium.

4. The method of claim 2, wherein the alkali metal is potassium.

5. The method of claim 1 , wherein the alkylating agent is an organohalide.

6. The method of claim 5, wherein the alkylating agent is an organoiodide.

7. The method of claim 5, wherein the alkylating agent is an organobromide.

8. A polymer composite comprising: a polymer; wherein the polymer comprises at least one monomer; and a plurality of derivatized graphene sheets; wherein the derivatized graphene sheets are formed by the method of claim 1.

9. The composite of claim 8, wherein the polymer is chosen from epoxies, polyolefins, polycarbonates, nylons, polyimides, aramids, and polyesters.

10. The composite of claim 8, wherein the derivatized graphene sheets have at least one functional moiety chosen from a carboxylic acid, an alcohol, an amine, a thiol, an epoxide, and an alkene.

11. The composite of claim 10, wherein the functional moiety is capable of forming covalent attachments with the at least one monomer resulting in a structure in which the derivatized graphene sheets are integrated within the polymer.

12. A water soluble graphite compound comprising: derivatized graphene sheets having carboxylic acid groups; wherein the derivatized graphene sheets were formed by the method of claim 1 ; and a water soluble polymer covalently coupled to the carboxylic acid groups.

13. The water soluble graphite compound of claim 12, wherein the water soluble polymer is a PEG polymer.

14. A soluble graphite compound comprising graphene sheets having covalently bound lipophilic groups, wherein the lipophilic groups were introduced by the method of claim 1.

15. The soluble graphite compound of claim 14, wherein the lipophilic groups are C₃ through C₃₀ alkyl groups.

16. The soluble graphite compound of claim 15, wherein the lipophilic groups are C₁₀ through C₁₅ alkyl groups.