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12 1 An Introduction to Graphene 1000 (A) 1500 2000 Raman shift (cm−1) Figure 1.7 (A) Raman spectrum of graphene as produced and after anneal- ing at 500◦C in comparison with that of the starting graphite. (Reproduced with 3000 (a) (b) (c) 0 (B) 2500 500 1000 1500 2000 Raman shift (cm−1) a 1∕2 ratio (see Figure 1.6c). For a nanosheet with more than five layers the G′ band is similar to that of a sample with five layers. Raman spectra without D band are rarely observed for large pristine single graphene sheets prepared by micromechanical cleaving without defects. In most cases pristine graphene sheets have a sufficient number of defects to result in some D band intensity. The height of the D band directly depends on the number of the sp3 carbon atoms of graphene surface and thus, on the number of defects of the graphene nanosheets. As regards the quality of graphene, D band is an indication for the aromatic character and the ‘‘quality’’ of the graphene nanosheet and is related to the production method and the starting material. As an example, Figure 1.7A reports spectra relative to graphene sheets produced by the exfoliation of graphite in water and stabilized with a surfactant which show an intense D band that remains even after annealing at 500◦C [74]. A similarly intense D band is discerned in the Raman spectra of GO sheets. Here the D band is a common characteristic of the Raman spectra since the existence of sp3 carbon atoms in the graphitic surface goes along with the formation of oxygen groups (see Figure 1.7B) [20]. 1.3.3 Thermogravimetric Analysis Articles related to the characterization of graphene nanosheets and its derivatives typically include TGA since the structural changes of graphitic materials before permission from [74].) (B) The Raman spec- tra of graphite (a), GO (b), and the reduced GO (c). (Reproduced with permission from [20].) DG Graphite powder As-produced 500 C anneal 2D Intensity (a.u.)PDF Image | An Introduction to Graphene
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