Non-Janus WSSe/MoSSe Heterobilayer and Its Photocatalytic Band Offset

We propose two non-Janus configurations, i.e., S₂Se₁ and S₂Se₂, of MoSSe and WSSe monolayers, which are appreciably more stable than their parent Janus configuration by proper exchanges of equivalent S and Se atoms. Specifically, S₂Se₁ is not only the most stable in the energetic sense but also dynamically stable. Our HSE06+SOC band structure calculation shows that the monolayers exhibit band gaps in the visible region comparable to those of the Janus configuration. S₂Se₁ WSSe exhibits a direct gap for the spin-up component, while it displays an indirect gap for the spin-down component due to the strong spin-orbit interaction. Our calculation of the activation barrier indicates that the Janus configuration can be converted to S₂Se₁ by a 15% uniaxial strain. Due to the absence of vertical polarization, we finally show that a S₂Se₁ WSSe/MoSSe heterobilayer can be much more advantageous than a Janus heterobilayer for the photocatalytic reduction of CO₂ to CO or HCOOH in almost the entire pH range.