The chemistry of graphene oxide is discussed in this critical review Particular emphasis is directed toward the synthesis of graphene oxide, as well as its structure Graphene oxide as a substrate for a variety of chemical transformations, including its reduction to graphene-like materials, is also discussed This review will be of value to synthetic chemists interested in this emerging field of materials science, as well as those investigating applications of graphene who would find a more thorough treatment of the chemistry of graphene oxide useful in understanding the scope and limitations of current approaches which utilize this material (91 references)

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The chemistry of graphene oxide

Graphene sheets offer extraordinary electronic, thermal and mechanical properties and are expected to find a variety of applications. A prerequisite for exploiting most proposed applications for graphene is the availability of processable graphene sheets in large quantities. The direct dispersion of hydrophobic graphite or graphene sheets in water without the assistance of dispersing agents has generally been considered to be an insurmountable challenge. Here we report that chemically converted graphene sheets obtained from graphite can readily form stable aqueous colloids through electrostatic stabilization. This discovery has enabled us to develop a facile approach to large-scale production of aqueous graphene dispersions without the need for polymeric or surfactant stabilizers. Our findings make it possible to process graphene materials using low-cost solution processing techniques, opening up enormous opportunities to use this unique carbon nanostructure for many technological applications.

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Processable aqueous dispersions of graphene nanosheets

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

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 generating 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.

Chemical methods for the production of graphenes.

Fully exploiting the properties of graphene will require a method for the mass production of this remarkable material. Two main routes are possible: large-scale growth or large-scale exfoliation. Here, we demonstrate graphene dispersions with concentrations up to approximately 0.01 mg ml(-1), produced by dispersion and exfoliation of graphite in organic solvents such as N-methyl-pyrrolidone. This is possible because the energy required to exfoliate graphene is balanced by the solvent-graphene interaction for solvents whose surface energies match that of graphene. We confirm the presence of individual graphene sheets by Raman spectroscopy, transmission electron microscopy and electron diffraction. Our method results in a monolayer yield of approximately 1 wt%, which could potentially be improved to 7-12 wt% with further processing. The absence of defects or oxides is confirmed by X-ray photoelectron, infrared and Raman spectroscopies. We are able to produce semi-transparent conducting films and conducting composites. Solution processing of graphene opens up a range of potential large-area applications, from device and sensor fabrication to liquid-phase chemistry.

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High-yield production of graphene by liquid-phase exfoliation of graphite

Naked metallic and semiconducting single-walled carbon nanotubes (SWNTs) were dissolved in organic solutions by derivatization with thionychloride and octadecylamine. Both ionic (charge transfer) and covalent solution-phase chemistry with concomitant modulation of the SWNT band structure were demonstrated. Solution-phase near-infrared spectroscopy was used to study the effects of chemical modifications on the band gaps of the SWNTs. Reaction of soluble SWNTs with dichlorocarbene led to functionalization of the nanotube walls.

http://science.sciencemag.org/content/sci/282/5386/95.full.pdf

Solution Properties of Single-Walled Carbon Nanotubes

In this Account we highlight the experimental evidence in favor of our view that carbon nanotubes should be considered as a new macromolecular form of carbon with unique properties and with great potential for practical applications. We show that carbon nanotubes may take on properties that are normally associated with molecular species, such as solubility in organic solvents, solution-based chemical transformations, chromatography, and spectroscopy. It is already clear that the nascent field of nanotube chemistry will rival that of the fullerenes.

Chemistry of single-walled carbon nanotubes.

Covalent derivatization of the acidic functional groups in oxidized graphite with octadecylamine renders graphite soluble in common organic solvents. Atomic force microscopic characterization of the soluble species supports the idea that the solutions consist of single and few layer graphene sheets, and we report the first solution properties of graphite.

Solution Properties of Graphite and Graphene

834 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/1007-0834 $ 17.50+.50/0 Adv. Mater. 1999, 11, No. 10 be detected. Furthermore, the mechanical properties of the complex are solid-like at temperatures below the observable glass transition, which would not be the case for a phase separated material containing rubbery microdomains. Instead, we advance an explanation for the extraordinary properties of this liquid crystal complex by invoking a high degree of coupling between mesogenic side groups and the ethylene oxide backbone which thereby inhibits the formation of helical conformations favored by afreeo poly(ethylene oxide) chains. Within such helical arrangements the lithium ions would be tightly coordinated below Tg and effectively trapped. By suppressing the formation of such helical structures, an open ethylene oxide structure is obtained within which lithium ions are free to move and where empty sites exist for the ions to occupy. It is remarkable that a similar (but weaker) effect is observed in the amorphous material. It seems that insertion of rigid isophthalate units also inhibits helical formation, sufficient to provide a measure of ionic decoupling, but for the liquid crystal complex the open structure is further stabilized via the interactions between the liquid crystal side groups. It is worth noting that this view is supported by the continuity of behavior from the melt into the glassy state in both the heat capacity and electrical conductivity, indicating that the structure of the melt is not strongly influenced by temperature. In addition, the dissolution of the ions in the backbone does not swell the smectic layer and thus, the open network must be relatively unchanged at least in the direction normal to the smectic layers. This suggests that by careful engineering of the types of liquid crystal phase present, it should be possible to tailor the conductivity mechanism to particular applications. It is also remarkable that the conductivity in the MeOC6G6 complex increases strongly (by several orders of magnitude) as the AO:Li ratio is decreased from 10:1 to 3:1. We should now be able to employ more concentrated polymer electrolytes than is presently possible with conventional materials. Transport number data for these electrolytes are not yet available, but it is tempting to speculate that the elimination of acation trappingo within the ethylene oxide helix will lead to substantial increases in cation mobility. For many years this has been one of the principal goals of polymer electrolyte research.

Dissolution of Single-Walled Carbon Nanotubes

Full-length single-walled carbon nanotubes (SWNTs) were rendered soluble in common organic solvents by noncovalent (ionic) functionalization of the carboxylic acid groups present in the purified SWNTs. Atomic force microscopy (AFM) showed that the majority of the SWNTs ropes were exfoliated into small ropes (2−5 nm in diameter) and individual nanotubes with lengths of several micrometers during the dissolution process. The combination of multiwavelength laser excitation Raman scattering spectroscopy and solution-phase visible and near-infrared spectroscopies was used to characterize the library of SWNTs that is produced in current preparations. The average diameter of metallic nanotubes was found by Raman spectroscopy to be smaller than that of semiconducting nanotubes in the various types of full-length SWNT preparations. This observation sheds new light on the mechanism of SWNT formation.

M. A. Hamon

Papers

Solution Properties of Single-Walled Carbon Nanotubes

Chemistry of single-walled carbon nanotubes.

Solution Properties of Graphite and Graphene

Dissolution of Single-Walled Carbon Nanotubes

Dissolution of Full-Length Single-Walled Carbon Nanotubes