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 similar to 18 nm or similar to 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 (O-2(center dot-)) 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.