Novel two-dimensional beta-GeSe and beta-SnSe semiconductors: anisotropic high carrier mobility and excellent photocatalytic water splitting

Photocatalytic water splitting is a promising method which uses sunlight to directly generate hydrogen from water to meet the requirements of energy consumption. Inspired by the experimental realization of monolayer beta-phase GeSe which has a moderate bandgap and excellent transport properties, based on the first-principles calculations, we identify a new class of compelling two-dimensional (2D) materials, beta-MX (GeS, GeSe, SnS and SnSe), which not only exhibit good feasibility for experimental exfoliation and excellent chemical stability, but also satisfy the bandgap and band edge requirements of the redox potential of water splitting. Excitingly, monolayer beta-GeSe and beta-SnSe exhibit remarkable optical absorption ability in the visible region, strongly anisotropic transport properties and a relatively small exciton binding energy of 0.50 eV (beta-GeSe) and 0.55 eV (beta-SnSe), favorable for the separation of photogenerated carriers (electrons and holes). The low overpotential in the oxygen evolution reaction and high overpotential in the hydrogen evolution reaction for monolayer beta-SnSe are attributed to the relatively large exciton binding energies, indicating that it can make a directional adjustment to the effective separation of photoexcited carriers for monolayer beta-SnSe. Therefore, we can expect a high photocatalytic activity for solar-energy harvesting and water splitting for beta-GeSe and beta-SnSe monolayers.