NaNbO₃/NaTaO₃ Superlattices: Cation-Ordering Improved Band-Edge Alignment for Water Splitting and CO₂ Photocatalysis

Perovskite oxide heterostructures have been extensively investigated for their excellent photocatalytic properties. Here, through hybrid density functional theory calculations, we systematically investigate the formation of NaNbO₃-NaTaO₃ (NBO-NTO) heterostructures. The sequential cations replacement in the superlattices reveals the Nb-Ta ratio range that allows the effective formation of heterostructures, which occurs through a spontaneous polarization mechanism induced by the electrostatic potential discontinuity in the interface. The resulting cation ordering is responsible for the sawtooth-like potential distribution that spatially separates valence and conduction charges and reduces the heterostructure bandgap. The symmetric NBO₅/NTO₅ junction has the smallest bandgap (2.50 eV) whose transitions are associated with Nb 5d orbitals on the interfacial plane. Such a relaxation mechanism provides the heterostructure with anisotropic optical properties and interface absorption peaks closer to the visible light spectrum. The phenomena strongly suggest the use of these heterostructures in photocatalytic reactions, supported by their coherent band-edge alignment with both water splitting and CO₂ reforming potentials.