Understanding the exfoliation and dispersion of MoS2 nanosheets in pure water.

Exfoliation and dispersion of two dimensional (2D) transition metal dichacogenides (TMDCs) such as MoS2 in water are highly demanded. In this paper, we exfoliate MoS2 nanosheets in pure water via ultrasonication and attempt to understand the exfoliation process and the dispersion behavior of hydrophobic MoS2. Cavitation-induced exfoliation leads to thinning and fragmentation of MoS2 flakes. Formation of mesoporous MoS2 sheets suggests that the exfoliation initiates from the basal planes. As for the dispersion of MoS2 nanosheets in water, we find that higher centrifugation rate and thus smaller lateral size, results in improved stability. This can be attributed to the enhanced edge effects. Fragmentation of MoS2 flakes generates many edges, to which hydrophilic and ionizable groups are attached. Herein, the edge effects include two aspects. On the one hand, edge-attached polar and hydrophilic groups promote interactions of MoS2 nanosheets with water molecules and result in better wettability, which can be explained by the hemi-wicking model. On the other hand, dissociation of these groups makes MoS2 sheets negatively charged. It is found that edge charge dominates. As the flake size reduces, attractive van der Waals force and hydrophobic force can be overcome by electrostatic repulsions, which prevent MoS2 nanosheets from aggregation. This study opens a new vista on exfoliating and dispersing hydrophobic 2D nanomaterials in water.