ABSTRACT
Figure 8.1 Graphene: The mother of all graphitic carbon materials. Graphene (bottom from left to right) Graphite, Nanotube, and Fullerenes. Reprinted by permission from Macmillan Publishers Ltd: Geim, A. K., and Novoselov, K. S. (2007). The rise of graphene, Nat Mater, 6, pp.183-191], copyright @ 2010. The pristine graphene generally consists of single-and few-(three to nine) layer graphene sheets. Over the past few years, a number of methods have been reported to synthesize graphene sheets, which include mechanical or chemical exfoliation of graphite, epitaxial growth on SiC surface, chemical vapor deposition on various metal surfaces, solvothermal synthesis, total organic synthesis, unzipping carbon nanotubes, etc. [8−11]. However, the uniform growth of single-layer graphene in a large-scale is still a challenge. A family of chemically modified graphene (CMG) consisting of structural and chemical derivates of graphene has been prepared by using chemical methods from various precursors.[12, 13]. Reduced graphene oxide (RGO) sheet is one of the important CMGs for the promising energy
storage applications. The most common route to prepare RGO sheets begins with the oxidation of graphite to graphite oxide which consists of a layered structure of graphene oxide [12]. Due to the strong hydrophilicity of graphite oxide, the intercalation of water molecules between the layers occurs easily. The complete exfoliation of graphite oxide produces an aqueous colloidal suspension of graphene oxides (GO), which is readily carried out by sonication. Notably, the resulting GO sheets are electrically insulating owing to the disruption of graphene networks with oxygenate groups. The significant amount of oxygenate groups on the surface of GO sheets suggests that the electrostatic repulsion between the negatively charged GO sheets leads to the stable dispersion of GO sheets in water. The oxidation degree of graphite oxide is varied with the reaction conditions and the graphite precursors [14]. As a result, GO sheets with various levels of oxidation would be produced. The reduction of GO sheets using a reducing regent or thermal treatment result in the formation of RGO sheets which are electrically conductive. Nonetheless, element analysis revealed the existence of a large amount of residual oxygen (atomic ratio of C/O, ~10) on RGO sheets [15], suggesting that RGO is not the same as pristine graphene. To exploit the potential of graphene-based materials for wide applications, many efforts have been focused on CMGs to improve the diversity of graphene. CMGs have been prepared by structural and chemical modification and functionalization of graphene or RGO sheets. Blending CMG with a second component was also used to form graphene-based composite materials. Graphene-based materials are of special importance because of the intriguing properties from the synergistic effects of their counterparts except for the intrinsic properties from each component. However, information as to how graphene and CMGs are prepared is crucial because the properties of graphene strongly depend on the methods of fabrication. For example, mechanical exfoliation produces few-layered graphene of highest quality, while chemical method is demonstrated to give high throughput and at relatively low costs, which enables technical applications in a variety of fields such as energy conversion and storage materials, catalysis and sensors. Therefore, the effective synthesis strategies of graphene-based materials on a large scale with controlled sizes and layers are of great importance for the applications in different fields.
