Effects of MoS Layer Thickness on Its Photochemically Driven Oxidative Dissolution.

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