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Chapter 5 – Production of Few Layer Graphene in Organic Solvents Figure 5.1: a) Photograph of graphene dispersions in NMP obtained after different sonication times and centrifugation speeds. b) Example UV-Vis absorption spectra obtained from graphene produced in NMP and DMF, with different concentrations, appearing flat and featureless. The calculated concentrations after centrifugation are shown in Figure 5.2 for both FLG/NMP and FLG/DMF. It can be seen clearly from Figure 5.2a, that the concentration of FLG/NMP increases steadily with the sonication time and reaches maximum value at 60 hr for all the centrifugation speeds. The maximum concentrations obtained at 60 hr for FLG/NMP were 0.28 ± 0.04 mg/ml, 0.22 ± 0.03 mg/ml, and 0.12 ± 0.01 mg/ml at 2000 rpm, 6000 rpm, and 10000 rpm, respectively. It has been shown direct exfoliation in NMP for longer sonication times of about 270 hr results in concentrations up to 1.2 mg/ml at 500 rpm.59 However, a controlled centrifugation study on graphene dispersions in NMP have shown that the mean number of graphene layers per flake (〈𝑁〉) increased as the speed of centrifugation decreased (〈𝑁〉 - 7.3, 3, and 3.2 for 500, 1000 and 3000 rpm, respectively).62 Hence in this study, relatively higher centrifugation speeds were chosen to remove the thicker flakes. It noteworthy that the larger size flakes are removed at higher centrifugation speeds leaving only smaller size flakes dispersed.59 This case was also observed in this study and is reported in the following section. 134PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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