Electronic Structure of Orthorhombic Bi₄V₂O₁₁ and Role of Gap States in Photoelectrochemical Water Splitting

A group of bismuth vanadium oxidesshows promise as an excellentmaterial for photoanodes in water splitting. However, the efficiencyof Bi4V2O11 in water splitting isnot satisfactory compared to BiVO4. In this study, we investigatedthe electronic structure of orthorhombic Bi4V2O11 using density functional theory (DFT) and synchrotronsoft X-ray spectroscopy techniques to clarify the causes of inefficiency.Our DFT calculations of the two-fold superlattice, as confirmed bynear-edge X-ray absorption fine structure measurements, revealed itsindirect-band-gap nature. An angle-resolved photoelectron spectroscopystudy identified the states within the band gap, and the resonantphotoemission technique and DFT simulation clarified that the originof the gap states was anisotropic localization of electrons aroundV-O tetrahedra. Water absorption enhances the gap states, whichdramatically impedes the kinetics of water oxidation reactions. Ourresults reveal the role of gap states in water oxidation reactionsand offer new insights into manipulating gap states in 3d transitionmetal oxides.