Controlled Edge-Mediated Photoelectrochemical Thinning and Patterning of Transition Metal Dichalcogenides.

Transition metal dichalcogenides (TMDs) are widely investigated for electrochemical and photoelectrochemical energy conversion applications, yet the oxidation processes that govern their stability in reactive electrolytes remain poorly understood. Here, we systematically compare dark electrochemical and illuminated photoelectrochemical anodic etching and thinning of mechanically exfoliated MoS2, WS2, MoSe2, and WSe2 flakes as a function of applied potential, electrolyte pH, irradiance, and oxidation time. In the dark, oxidation initiates at defect-rich flake edges and produces lateral etching without appreciable basal-plane thinning. The oxidation peak potentials exhibit significantly different pH dependences for sulfide- and selenide-based TMDs, revealing chalcogen-dependent oxidation chemistry and kinetics. Under illumination, an additional oxidation regime emerges at lower anodic potentials, in which multilayer n-type MoS2, WS2, and MoSe2 undergo vertical thinning with minimal lateral etching, whereas no thinning is observed for p-type WSe2. This observation, together with strong suppression of thinning by sacrificial hole acceptors, indicates that photogenerated holes accumulated in the upper layers of n-type TMDs drive a photoelectrochemical self-oxidation process. Tuning the bias, irradiance, and oxidation time enables controlled conversion of initially nonuniform multilayer flakes into homogeneous few-layer structures, while focused laser excitation yields spatially localized thinning and patterning. In addition to elucidating stability windows and oxidation mechanisms of TMD photoelectrodes, these results provide a versatile strategy for controllably tuning the thickness and morphology of layered semiconductors.