Surface electronic structure and photo activity of double perovskite Ba[formula omitted]PrBiO[formula omitted]: First-principles investigations

Abstract

Double perovskite Ba2PrBiO6 exhibits photo catalytic activity in the visible light range with the observed optical band gap of ∼1.0 eV. On the other hand, density functional theory (DFT) calculations predict a larger bulk band gap of 2.0 eV even with the standard Perdew–Burke–Ernzerhof (PBE) functional, which is known to underestimate the band gap of semiconductors. Much larger bulk band gap of 3.13 eV is predicted with a hybrid functional treatment for the exchange–correlation, which generally gives better descriptions for them. This discrepancy between experiment and theory implies the presence of unknown mechanism which effectively reduces the band gap of Ba2PrBiO6 and thereby gains the sensitivity to the visible light. The first finding is that the surface band gaps of Ba2PrBiO6 calculated using an appropriate hybrid functional are smaller than the bulk band gaps and are 2.49 eV and 2.85 eV for the Pr–Bi and Ba–O polar surfaces, respectively. We also find that work function is quite sensitive to the surface structures and its accurate calculations are essential to show that the reduction and oxidation levels are fit to the gap region. When a substitutional defect PrBi, in which Bi is substituted by Pr, is introduced into the Pr–Bi polar surface, surface band gap is further reduced to 0.93 eV, being very close to the experimental value. For the Ba–O polar surface, similar reduction to 1.59 eV is found but not substantial as compared to the Pr–Bi polar surface. These narrow surface band gaps may be the origin of the photo activity of Ba2PrBiO6 in the visible light region. We also performed Born–Oppenheimer molecular dynamics simulations to clarify the reactivity of the polar surfaces to water. We find that two hydroxyl groups were created in the adsorption of a single H2O molecule. The decomposition of water molecule in the adsorption indicates that the polar surfaces provide a promising stage for the photo catalytic activity. Further adsorption of H2O molecules leads to their diffusion on the surface.