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

Abstract

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.