Effects of MoS₂ Layer Thickness on Its Photochemically Driven Oxidative Dissolution

The distinctive optical and electronic properties of two-dimensional (2D) molybdenum disulfide (MoS₂) make it a promising photocatalyst and photothermal agent in aqueous applications. In terms of environmental stability, MoS₂ has been considered insoluble, but 2D MoS₂ nanosheets can be susceptible to dissolution, owing to their large surface areas and highly accessible reactive sites, including defects at the basal plane and edge sites. Under light illumination, the dissolution of 2D MoS₂ nanosheets can be further accelerated by their photochemical reactivity. To elucidate MoS₂ reactivity in the environment, here we investigated the thickness-dependent dissolution of MoS₂ under illumination. To synthesize nanoscale thicknesses of MoS₂, we exfoliated bulk MoS₂ by ultrasonication and controlled the layer thickness by iterative cascade centrifugation, producing MoS₂ nanosheets averaging either ∼18 nm or ∼46 nm thick, depending on the centrifugation rate. Under simulated sunlight, MoS₂ dissolution was accelerated, the Mo⁶⁺ composition increased, and the solution pH decreased compared to those in the dark. These results suggest that light exposure promotes the oxidation of MoS₂, causing faster dissolution. Importantly, 18 nm thick MoS₂ exhibited faster dissolution than either 46 nm or bulk MoS₂, driven by the superoxide radical (O₂^•-) generation promoted by its relative thinness. These findings highlight the important role of the thickness-dependent photochemistry of MoS₂ nanosheets in their dissolution, which is directly linked to their environmental behavior and stability.