Metal Chalcogenides Janus Monolayers for Efficient Hydrogen Generation by Photocatalytic Water Splitting

We investigated the structural, electronic, and photocatalytic properties of the Janus monolayers (MLs) composed of the metal chalcogenides Ga2XY and In2XY (X = S, Se, Te; Y = S, Se, Te). The calculated phonon dispersion curves show that all Janus materials are mechanically stable, presenting the same structures as their pristine counterparts. Janus MLs are characterized by the presence of different chalcogen atoms lying on the opposite sides, giving rise to an electric dipole embedded perpendicularly to the ML, and thus the electronic band alignment becomes side-dependent, a property that can be exploited for different technological applications. Except for Ga2SSe, the other Janus crystals exhibit direct band gaps. On the basis of the GW Bethe–Salpeter approach, we found lower exciton binding energies (0.3–0.7 eV) in comparison with the pristine systems. Once the Janus MLs combine small values of the exciton binding energies and electron–hole separation, our results reveal that they fulfill all of the required conditions to be used in the hydrogen generation by photocatalytic water splitting.