Critical Parameters in Exfoliating Graphite into Graphene

Graphite exfoliation into few-layers graphene sheets (GS) is an appealing source of graphene: environmental-friendly, low-cost and low-energy. We obtained relatively high GS concentrations in water (0.7mg·mL^-1) by optimizing the nature of dispersant and the ultra-sonic generator intensity.  We employed a wide range of dispersant types: anionic, cationic, and nonionic surfactants, and found that neither the polarity of the dispersant nor its surface tension have a clear effect on the GS concentration. In contrast, the nature of the hydrophobic part of the dispersant strongly affects the dispersion quality. The dispersant Triton X-100 gives the highest GS concentration since it includes a benzene ring with strong π-π interactions with the aromatic structure of the GS. We found that a multi-step sonication procedure involving both tip and bath sources considerably enhances the yield of exfoliated GS. Tip sonication (TS) and bath sonication (BS) differ considerably in the power they supply. It is therefore expected that the substantially higher power of TS would be more efficient than the weak BS. However, when comparing the efficiency of either TS or BS alone for the same sonication energy, TS is the least efficient option, while the weaker BS leads to a much higher GS concentration. Still, combining the two sources gives the highest GS concentration. This marked difference is ocularly visible: a much darker supernatant (higher GS concentration) is obtained by TBT than by BS alone. In order to characterize the GS, we used Raman spectroscopy and transmission electron microscopy, which indicates few-layers graphene patches of ~μm lateral dimension.