Synthesis and Photocatalytic Activity of Perovskite Niobium Oxynitrides with Wide Visible-Light Absorption Bands

Photocatalytic activities of perovskite-type niobium oxynitrides (CaNbO2N, SrNbO2N, BaNbO2N, and LaNbON2) were examined for hydrogen and oxygen evolution from water under visible-light irradiation. These niobium oxynitrides were prepared by heating the corresponding oxide precursors, which were synthesized using the polymerized complex method, for 15 h under a flow of ammonia. They possess visible-light absorption bands between 600-750 nm, depending on the A-site cations in the structures. The oxynitride CaNbO2N, was found to be active for hydrogen and oxygen evolution from methanol and aqueous AgNO3, respectively, even under irradiation by light at long wavelengths (lambda < 560 nm). The nitridation temperature dependence of CaNbO2N was investigated and 1023 K was found to be the optimal temperature. At lower temperatures, the oxynitride phase is not adequately produced, whereas higher temperatures produce more reduced niobium species (e. g., Nb3+ and Nb4+), which can act as electron-hole recombination centers, resulting in a decrease in activity.