Paper Title
Effect Ofnb/Ta Ratio In (111)-Layered Perovskite Ba5ta4o15crystals On Photocatalytic Water Oxidation Of Their Oxynitride Derivatives

Abstract
Due to their smaller band gaps, suitable band structures for water reduction/oxidation, stability under the reaction conditions, and nontoxicity, oxynitrides are considered as an emerging class of materials with potential applications in clean hydrogen energy production and environmental remediation by efficiently utilizing solar energy. BaTaO2N is one of the promising oxynitridecandidates as a photoanode for oxygen evolution from water. In this study, we have investigated the effect of the Nb/Ta ratio in (111)-layered perovskite Ba5Ta4O15 crystals grown by a flux method on photocatalytic water oxidation of their oxynitride derivatives converted by nitridationat 950C for 20 h under an NH3 flow. By increasing the Nb/Ta ratio in Ba5Ta4O15, it was found that the diffraction peaks of Nb-doped BaTaO2N crystals became slightly narrower with higher intensity and their positions shifted slightly to lower angle than that of pure BaTaO2N crystals, indicating the increase in crystallinity and an overall expansion of the lattice parameters due to successful incorporation of niobium into the BaTaO2N lattice. Porous structures were formed after nitridationpossibly due to the transformation from a layered perovskite oxide into a simple perovskite oxynitride and a lattice condensation process because of the partial replacement of O2 with N3. An increase in the Nb/Ta ratio also shifted an absorption edge toward longer wavelength (λ ≈ 730 nm). However, the intensity of a background absorption monotonically increased because of the presence of considerable amount of reduced niobium species. Higher photocatalytic activity for O2 evolution was observed with increasing the Nb/Ta ratio due to the decrease in band gap. Keywords- Perovskite, Ba5Ta4O15, BaTaO2N, Niobium, Flux method, Crystal growth, Water splitting.