Layer dependence of the photoelectrochemical performance of a WSe(2) photocathode characterized using in situ microscale measurements

Transition-metal dichalcogenide (TMD) materials are good candidates for photoelectrochemical (PEC) electrode materials because of their distinctive optoelectronic properties and catalytic activities. Monolayer WSe(2) is a p-type semiconductor with a direct bandgap that makes it a suitable PEC cathode material. In the present work, in situ PEC characterization of a single sheet device was carried out at the microscale to explore its performance. The PEC characteristics were found to be strongly related to the number of WSe(2) layers. Monolayer WSe(2) exhibited a dominant large current density and incident photo-to-current efficiency (IPCE) compared with those of multilayer WSe(2). Its PEC performance decreased with increasing number of layers. The photocurrent mapping results also revealed that the basal-plane sites and the edge sites on a monolayer WSe(2) sheet contributed equally to its catalytic activity, which is not consistent with traditional catalyst theory. The underlying mechanism is discussed.