Photocatalytic water oxidation under visible light by valence band controlled oxynitride solid solutions LaTaON2–SrTiO3

Abstract

Solid solutions between LaTaON₂ and SrTiO₃ were successfully synthesized. In this solid solution, the valence band maxima (VBM) shifted to positive sides as the fraction of SrTiO₃ increased, resulting in the appearance of activity for O₂ evolution even without any cocatalysts. Despite widening of the band gap (E_g) by only 0.15 eV, La_0.5Sr_0.5Ta_0.5Ti_0.5O₂N gained 0.5 V larger driving force for water oxidation, which corresponds to the potential difference between VBM and potential for water oxidation, than LaTaON₂. It suggested that the contribution of Ti 3d to the conduction band suppressed the widening of E_g. This is an interesting feature in the present LaTaON₂–SrTiO₃ solid solution. The bare La_0.5Sr_0.5Ta_0.5Ti_0.5O₂N exhibited the activity for O₂ evolution 9 times higher than LaTaON₂ modified with a CoO_x cocatalyst. It has been found that the driving force for water oxidation of La_0.5Sr_0.5Ta_0.5Ti_0.5O₂N was 0.8 V, and was larger than those in other perovskite-type oxynitrides.