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.