Constructing junction architectures is one of the most promising strategies to improve the photocatalytic activity of two-dimensional semiconductors for the splitting of water. Using first-principles calculations, we demonstrate that the van der Waals heterojunction consisting of PtS2 and GaSe monolayers is a potential step-scheme photocatalyst with high solar-to-hydrogen (STH) efficiency. The stability of the heterojunction is confirmed by phonon dispersion spectrum calculation and ab initio molecular-dynamics simulation. In such a step-scheme heterojunction, GaSe serves as a reduction photocatalyst and PtS2 acts as an oxidation photocatalyst. The built-in electric field and band bending are formed since the work function difference and electrostatic potential difference promote the photo-generated electron (hole) to the conductance band minimum (valence band maximum) of GaSe (PtS2), inducing a step-scheme migrating route and guaranteeing strong redox ability of photo-generated carriers. The hydrogen evolution reduction can proceed driven solely by the photogenerated electrons, while the barrier of the oxygen evolution reaction is only 0.89 eV. More intriguingly, the STH efficiency is predicted up to 36.9% along with the improvement of visible light absorption. The STH efficiency can be enhanced effectively by both in-plane strain and compressive vertical strain. Our findings provide valuable guidance for the potential applications of PtS2/GaSe heterojunction as a photocatalyst for the photocatalytic splitting of water.

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