(Invited) Water Splitting and Carbon Dioxide Conversion By Mixed-Anion Materials

Abstract

Mixed-anion materials that consist more than one anionic species in a single-phase have attracted attention for various applications. Oxynitrides containing d0- or d10-metal cations are typical example of visible-light-driven photocatalysts for water splitting and CO2conversion. In this presentation, recent progress on the development of mixed-anion materials for water splitting and CO2 conversion made by our group will be given. While two-dimensional (2D) undoped layered oxynitrides are potential candidates as high performance visible-light photocatalysts, synthesis of 2D layered oxynitrides is generally difficult because most of them are metastable phases and are not chemically very stable. We found an exceptional example of a 2D layered oxynitride, K2LaTa2O6N (more specifically, K2LaTa2O6N·1.6H2O), which is a Ruddlesden-Popper phase two-layer perovskite analogous to Li2LaTa2O6N and has a band gap of 2.5 eV. This material undergoes in situH+/K+ exchange in aqueous solution while keeping its visible light absorption capability. The protonated, Ir-modified K2LaTa2O6N exhibited photocatalytic activity for H2 evolution from aqueous NaI solution under visible light, outperforming Pt/ZrO2/TaON and Pt/SrTiO3:Rh, which are one of the best-performing oxynitride and oxide photocatalysts for H2 evolution, respectively. The use of mixed-anion materials is of interest not only for photocatalysts but also for catalysts promoting water oxidation and CO2reduction. For example, we found that certain oxyfluorides work as electrocatalysts for water oxidation. A 2D perovskite oxyfluoride Pb3Fe2O5F2 exhibits activity for electrochemical water oxidation to O2, which is 8 times more active than a 3D bulk PbFeO2F.