Single-crystalline Ba_xSr_1-xTaO₂N solid-solution photocatalyst with low defect concentrations for solar-driven water splitting

Perovskite-type tantalum-based oxynitride photocatalysts are promising candidates for water splitting due to their suitable band positions and extended light absorption beyond 600 nm. However, their associated photocatalytic activities and quantum yields remain relatively low. Here, we show that a nano-sized single-crystalline Ba_xSr_1-xTaO₂N solid-solution perovskite photocatalyst exhibits state-of-the-art activity in separate oxygen and hydrogen evolution half-reactions. The improved performance is attributed to the nanoscale particle sizes, as well as the reduced defect densities achieved by using a mixed precursor comprising TaS₂ and Ta₃N₅. The half-reaction activities can be modulated by applying a post-synthetic high-temperature treatment. Assessments of charge carrier dynamics, in conjunction with a mechanistic kinetic model, reveal that exponential-tail trap states are formed during this post-treatment. Such trap states, present on the photocatalyst surface, facilitate participation of holes during the oxygen evolution reaction. The development of such solid-solution photocatalysts broadens the range of potential materials for solar-driven hydrogen production. In addition, the present findings are expected to enable the selective tuning of bifunctional photocatalysts for either the hydrogen or oxygen evolution reaction.