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106 A. Petrovski et al. Fig. 11.2 SEM images of the studied samples: (a) M1, H2SO4; (b) M1, H2SO4 + KOH; (c) M1, H2SO4 + NaOH; (d) M2, H2SO4; (e) M2, H2SO4 + KOH; (f) M2, H2SO4 + NaOH In Fig. 11.3 TEM images of some samples are shown. Generally, one can say that the morphology of the graphene produced by reverse electrolysis in aqueous electrolytes is similar to graphene published elsewhere. The graphene sheets are broken and intertwined. There is a low amount of impurities, fibers, non-exfoliated graphite and amorphous carbon. The thickness of the samples varies from a few nanometers to 20 nm. Also, according to the TEM analysis, the graphene produced from graphite M2 in sulfuric acid with NaOH shows the best characteristics: the lowest thickness and the least number of layers (4–6). According to the TEM images, the graphene sheets are monolayered, few-layered and multilayered in different parts of the sample. This is obvious from the graphene transparency in some parts and is in accordance with Raman analysis for the determination of the number of layers. Raman spectroscopy is an appropriate technique for a structural study of graphene which provides a lot of information such as a qualitative identification of graphene, the level of ordered and disordered structure, the presence of defects, the number of layers, the size of crystallites etc. Raman spectra of the studied graphene samples are shown in Fig. 11.4. The main peaks characteristic for graphene are the D, G and 2D bands. The G band (near 1,580 cm1) is the primary mode in graphene and graphite and attributed to the ordered crystalline structure [14]. Samples 4 and 6 produced from the second type of graphite M2 show a higher degree of crystallinity. The D band (near 1,350 cm1) is attributed to disorder in the graphene structure, i.e. to the defects present in the graphene plane. It is result of one-phonon vibrational lattice processes [14]. This band is typically less pro- nounced in graphite and graphene. If it is pronounced one can conclude that the tested material contains a lot of defects. The studied samples, especially those originating from graphite M1 show D bands higher than the G band, which is pericap@tmf.ukim.edu.mkPDF Image | Graphene Produced by Electrolysis
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