Band-gap engineering in AB(OxS1−x)3 perovskite oxysulfides: a route to strongly polar materials for photocatalytic water splitting

Abstract

Polarity in heterogeneous photocatalysts was shown to enhance charge-carrier separation, resulting in superior efficiency for example for photocatalytic water splitting. Oxynitrides are promising photocatalysts due to their small band gaps and can be rendered polar by epitaxial strain. However, while the low electronegativity of nitrogen results in small band gaps in non-polar oxynitrides, it is also responsible for a large increase in band gap with increasing polar distortion amplitude. This suggests a trade-off between small band gaps and polar distortions – both being crucial for the catalyst's performance. Due to the even lower electronegativity of sulfur, sulfides normally have band gaps that are too small for water splitting but polar distortions could shift their band gap to a suitable range. In this paper we investigate, using density functional theory calculations, the suitability of polar AB(O_xS_1−x)₃ perovskites for photocatalytic water splitting. We find that polar distortions – induced by epitaxial strain or substitution of sulfur by oxygen – can indeed increase the band gap to a suitable range. In particular, our calculations predict compressively strained BaZr_yTi_1−yO₂S compounds to be highly promising for photocatalytic water splitting.