Long-Term Stable 2H-MoS₂ Dispersion: Critical Role of Solvent for Simultaneous Phase Restoration and Surface Functionalization of Liquid-Exfoliated MoS₂

Chemical exfoliation approaches such as Li-intercalation for the production of two-dimensional MoS₂ are highly attractive due to their high yield of monolayer forms, cost-effectiveness, and mass-scalability. However, the loss of the semiconducting property and poor dispersion stability in solvent have limited the extent of their potential applications. Here, we report simultaneous phase recovery and surface functionalization for the preparation of a highly stable 2H-MoS₂ dispersion in water. This study shows that high-yield restoration of the semiconducting 2H phase from a chemically exfoliated MoS₂ (ce-MoS₂) can be induced by a mild-temperature (180 °C) solvent thermal treatment in N-methyl-2-pyrrolidone (NMP). In addition to a phase transition, this solvent thermal treatment in NMP realizes concurrent surface functionalization of the 2H-MoS₂ surface, which provides an outstanding dispersion stability to 2H-MoS₂ in water for more than 10 months. Finally, we report the humidity sensor based on the functionalized 2H-MoS₂, which shows a substantial response enhancement compared with a nonfunctionalized 2H-MoS₂ or ce-MoS₂.