  Owing to the versatility in their chemical and physical properties,
transition metal perovskite oxides have emerged as a new category of highly
efficient photocatalysts for photoelectrochemical water splitting. Here, to
understand the underlying mechanism for the enhanced photoelectrochemical water
splitting in mixed perovskites, we explore ideal epitaxial thin films of the
BiFeO3-SrTiO3 system. The electronic struture and carrier dynamics are
determined from both experiment and density-functional theory calculations. The
intrinsic phenomena are measured in this ideal sytem, contrasting to commonly
studied polycrstalline solid solutions where extrinsic structural features
obscure the intrinsic phenomena. We determined that when SrTiO3 is added to
BiFeO3 the conduction band minimum position is raised and an exponential tail
of trap states from hybridized Ti 3d and Fe 3d orbitals emerges near the
conduction band edge. The presence of these trap states strongly suppresses the
fast electron-hole recombination and improves the photocurrent density in the
visible-light region, up to 16 times at 0 VRHE compared to the pure end member
compositions. Our work provides a new design approach for optimising the
photoelectrochemical performance in mixed perovksite oxides.
