Theoretically designing polar van der Waals heterostructures for photocatalytic overall water splitting

Conventional photocatalysts for overall water splitting face significant material selection constraints due to stringent bandgap requirements. To address this limitation, we propose an innovative strategy utilizing the intrinsic built-in electric field of two-dimensional (2D) polar materials to induce band bending. In this work, we constructed ten two-dimensional polar van der Waals (vdW) heterostructures by integrating five nonpolar materials (MoO₂, WO₂, PdO₂, Ti₂CO₂, and TiF₂) and Al₂O₃ monolayers with different polarization directions. These five nonpolar materials were identified from 4254 candidates through first-principles calculations combined with a material-screening strategy. The incorporation of the polar Al₂O₃ layer disrupts the structural symmetry of the heterostructures, leading to an electrostatic potential difference between two surfaces, which, in turn, gives rise to an internal built-in electric field. It is interesting that Ti₂CO₂/Al₂O₃-P↓ and WO₂/Al₂O₃-P↓ (symbol ↓ denotes the polarization direction of the Al₂O₃ layer) heterostructures are direct band gap semiconductors with strong optical absorption and suitable band edge positions with respect to the redox potentials of water. More importantly, the Ti₂CO₂/Al₂O₃-P↓ and WO₂/Al₂O₃-P↓ heterostructures exhibit impressive solar-to-hydrogen (STH) conversion efficiencies of 23.35% and 21.96%, respectively, highlighting their great potential for photocatalytic applications. These results provide a new strategy for designing polar vdW heterostructures with tunable built-in electric fields to achieve efficient charge separation and high photocatalytic water splitting performance.