(Invited) Electronic Properties of ABO4 Photo-Catalysts Under Nb-Substitution and Phase/Pressure-Related Distortions

Abstract

Both SbTaO4 and SbNbO4 in the orthorhombic α-Sb2O4 phase have demonstrated pure water splitting as photo-catalysts. Previous experimental studies have shown a significant difference in reported bandgap values for SbTaO4 such as 3.71 eV and 2.28 eV. We have studied SbTa1-xNbxO4 (x = 0, 0.25, 0.5, 0.75, 1) in the tetragonal, orthorhombic, and monoclinic phases. The resulting Sb3+ M 5+O-2 4 material can be classified as a ns2 md0 ternary metal oxide. We have studied the effects of structural symmetry related distortions on the band gaps and overall energetics of SbMO4 compounds (M = Ta, Nb). Evolution of the electronic structures was studied as one transitions from a system of 4d bands to 5d bands, and whether this has any effect on the Sb oxidation state or its lone pair s electrons. From the atomic point of view, electronic correlation energy is higher in Ta 5d than in Nb 4d, and Nb 4d band is higher in energy than Ta 5d. These were examined in terms of oxidation states in their respective solids. We look at the effects of increasing Nb concentration on the electronic structure of SbTa1-xNbxO4 alloy with emphasis on polyhedral and overall cell distortions. We decouple the effects caused by the presence of different nd cations from those caused by distortions and reorientation of various polyhedra. BiVO4 Belongs to the same group of materials; we will also present the electronic structure changes caused by Nb incorporation in BiVO4 and the application of external pressure. Through the analyses of the chemical-potential landscape, we have determined the single-phase stability zone of BiVO4 with the Nb doping. Application of external pressure improved the single-phase stability zone. We have also focused on the local structural distortions near the Nb doping site, especially on the BiO8 octahedra. We have shown here that pressure-induced symmetrization of BiO8 dodecahedron could lower the electron’s effective mass and therefore can help to improve the photo-conduction property of BiVO4. This work is supported by NSF-award # 1609811. We also acknowledge the computational time from Texas Advanced Computing Center (TACC).