Novel Janus XGa-PbP (X = S, Se) monolayers: excellent photocatalysts for overall water splitting.

The search for efficient and sustainable materials for solar-driven water splitting has intensified with the emergence of two-dimensional (2D) Janus structures. In this work, we theoretically design and explore two novel Janus monolayers, SGa-PbP and SeGa-PbP, using first-principles calculations. Our results reveal that both monolayers are dynamically, thermally, and mechanically stable, and exhibit direct band gaps (0.93 eV for SGa-PbP and 1.24 eV for SeGa-PbP) ideally suited for visible-light absorption. Impressively, these systems display strong optical absorption in the visible and ultraviolet regions, with absorption coefficients reaching 10 cm. The asymmetric structure induces built-in electric fields that enhance charge separation, while the PbP surface provides active hydrogen adsorption sites with nearly optimal free energy |Δ | values for hydrogen evolution. Most notably, the corrected solar-to-hydrogen conversion efficiencies reach 40.69% and 31.75% for SGa-PbP and SeGa-PbP, respectively-significantly surpassing many state-of-the-art 2D photocatalysts. These findings position XGa-PbP Janus monolayers as highly promising candidates for next-generation solar hydrogen production technologies.