Oxygen Vacancies and Ti3+ In-Gap Defects Dictate Photocatalytic H2 Generation in BaTiO3

Ubiquitous oxygen vacancies and mutually correlated Ti3+ defects in ABO3-type perovskite titanate, such as BaTiO3 (BTO), critically impact optoelectronic properties. However, rationally tuning such defects via synthesis routes and obtaining insights into their impact on photocatalytic H2 generation is limited. Herein, the effect of heating as-synthesized BTO in an H2 atmosphere at 400 °C for an hour on the photocatalytic activity is investigated. Such post-synthesis modification did not induce changes in the bulk properties of BTO, such as crystalline phase and optical properties. However, the photocatalytic H2 evolution activity under ultraviolet light decreased by ≈1.8 times after the H2 reduction treatment. Under visible light (λ > 400 nm) that majorly populates in-gap defects, virtually no photocatalytic activity was observed in BTO after being subjected to the H2 reduction process. This observation is attributed to an enhancement in the density of electron-trapping Ti3+ and oxygen vacancies, revealed via complementary microscopic and spectroscopic tools. Insights from nonlinear optical measurement revealed the location of such electron-trapping in-gap states to be ≈0.8 eV below the conduction band of BTO. Results show how vulnerable these defects can be toward reduction treatment with 5% H2 for an hour and its crucial impact on the photocatalytic H2 evolution efficiency. Hence, elucidating the inherent nature of defects and controlling them should be considered as a key parameter in photocatalyst design.