Wafer-scale synthesis of monolayer WSe₂: A multi-functional photocatalyst for efficient overall pure water splitting

A multi-functional photocatalyst, that can combine the catalytic functions of water oxidation and proton reduction together with light harvesting capacity, is highly desired for low cost, high efficiency, and highly stable solar fuel production. Monolayer WSe₂, with a direct energy gap of ~ 1.65 eV is a nearly ideal light absorber to convert sunlight to hydrogen fuels through solar water splitting. To date, however, the controlled synthesis of monolayer WSe₂ on a wafer scale and the realization of overall water splitting on WSe₂ have remained elusive. Here, we report the van de Waals epitaxy of crystalline monolayer WSe₂ on large area amorphous SiO_x substrates. We have demonstrated, for the first time, the multi-functionality of monolayer WSe₂ in solar water splitting, including extraordinary capacities for efficient light harvesting, water oxidation, and proton reduction. The absorbed photon conversion efficiency exceeds 12% for a single monolayer WSe₂. This work provides a viable strategy for wafer-scale synthesis of multi-functional photocatalysts for the development of efficient, low cost, and scalable solar fuel devices and systems.