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Chapter 5 β Production of Few Layer Graphene in Organic Solvents flakes might escape through the holes in the TEM grid. This might result in over estimating the flake size.59, 61, 77 As observed in this study, even at low sonication times (24 hr), flakes as small as <100 nm and as large as >1300 nm are present and contribute to the calculated mean size (Figure 5.5).77 In addition to this, the present study is not a size selection study53 by using controlled centrifugation steps where relatively similar size of flakes can be extracted at a particular centrifugation speed. So, the presence of a large distribution of flake lengths in the samples is not surprising, thus justifying the standard deviation. With regards to π, samples with lower sonication time (24 hr) and a centrifugation speed (2000 rpm) had (γπγ - 8 Β± 4 (Figure 5.8b). As the sonication time increased up to 60 hr, even when using a low centrifuge speed of 2000 rpm, the γπγ decreased considerably to 4.2 Β± 1.6, suggesting that exfoliation efficiency increases with sonication time. At higher sonication times (48 and 60 hr), as centrifugation speed increased, γπγ remained between 3-5 suggesting that higher centrifugation speed remove heavier (γπγ > 7) and larger flakes (γπΏγ > 500 nm), resulting in a stable dispersion with less sedimentation. Derived π1βππ % and π1β5βππ % from the histogram data were and plotted in Figure 5.8c,d. As the sonication time increases π1βππ % increases steadily (e.g., at 6000 rpm, ~4 % at 24 hr to ~8 % at 60 hr). Most striking observation is that, π1βππ % increases drastically as centrifugation speed increases (e.g., at 60 hr, ~5 % at 2000 rpm to ~16 % at 10000 rpm). This suggests that the smaller sized flakes those remain at dispersed at higher centrifuge speeds must constitute to the higher π1βππ % than the large sized flakes. With regards to π1β5βππ, significant increase was observed as the sonication time increased (e.g., at 2000 rpm, ~50 % at 24 hr to ~80 % at 60 hr). At higher centrifugation speeds, π1β5βππ % was roughly constant between 80 % and 90 %. These observations were similar to literature.59, 350 Similar trend in γπΏπ΄πΉπγ and γπγ was observed for FLG/DMF sample. As mentioned in 5.2.1, only higher concentration samples were analysed for DMF based dispersions. At 60 hr sonication time, the γπΏπ΄πΉπγ and γπγ changed from ~170 nm and 5.8 at 2000 rpm to ~100 nm and 3.3 at 10000 rpm, respectively (see Appendix 9.1). Noteworthy, for DMF- based dispersions, the γπγ are slightly higher and π1βππ % and π1β5βππ % are slightly 142PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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