Oxygen\nVacancies and Ti<sup>3+</sup> In-Gap Defects\nDictate Photocatalytic H<sub>2</sub> Generation in BaTiO<sub>3</sub>

Ubiquitous\noxygen vacancies and mutually correlated Ti<sup>3+</sup> defects in\nABO<sub>3</sub>-type perovskite titanate, such as BaTiO<sub>3</sub> (BTO), critically impact optoelectronic properties. However,\nrationally tuning such defects via synthesis routes and obtaining\ninsights into their impact on photocatalytic H<sub>2</sub> generation\nis limited. Herein, the effect of heating as-synthesized BTO in an\nH<sub>2</sub> atmosphere at 400 °C for an hour on the photocatalytic\nactivity is investigated. Such post-synthesis modification did not\ninduce changes in the bulk properties of BTO, such as crystalline\nphase and optical properties. However, the photocatalytic H<sub>2</sub> evolution activity under ultraviolet light decreased by ≈1.8\ntimes after the H<sub>2</sub> reduction treatment. Under visible light\n(λ > 400 nm) that majorly populates in-gap defects, virtually\nno photocatalytic activity was observed in BTO after being subjected\nto the H<sub>2</sub> reduction process. This observation is attributed\nto an enhancement in the density of electron-trapping Ti<sup>3+</sup> and oxygen vacancies, revealed via complementary microscopic and\nspectroscopic tools. Insights from nonlinear optical measurement revealed\nthe location of such electron-trapping in-gap states to be ≈0.8\neV below the conduction band of BTO. Results show how vulnerable these\ndefects can be toward reduction treatment with 5% H<sub>2</sub> for\nan hour and its crucial impact on the photocatalytic H<sub>2</sub> evolution efficiency. Hence, elucidating the inherent nature of\ndefects and controlling them should be considered as a key parameter\nin photocatalyst design.