Crucial impact of reduction on the photocarrier dynamics of SrTiO₃ powders studied by transient absorption spectroscopy

Inducing oxygen vacancy defects in SrTiO₃ powders via high temperature treatment in the presence of a mixture of Ar, O₂, dry air, and H₂ ambient gases is a promising strategy to produce homogeneously defective SrTiO₃ photocatalysts with a remarkable 40-fold enhancement of H₂ evolution activity. Electron doping of SrTiO₃ due to oxygen vacancies triggers the development of a highly active SrTiO₃ photocatalyst; however, the photodynamic processes involved in these modifications of SrTiO₃ have not been fully elucidated yet. In this work, we investigated the impact of high temperature treatment based on the dynamics of photocarriers by transient absorption spectroscopy (TAS). TAS results revealed that upon band gap excitation of SrTiO₃, most of the photoexcited electrons in non-reduced SrTiO₃ are deeply trapped in the intrinsic defects as evident from the strong broad absorption signals peaking at 11 000 cm^-1 (909 nm, 1.36 eV) and 20 000 cm^-1 (500 nm, 2.48 eV), whereas the absorption intensities in this wavenumber region largely decreased in highly reduced SrTiO₃, suggesting a possible electron filling of deeply trapped states via reduction treatment (or electron doping). Interestingly, the photoexcited electrons in oxygen-deficient SrTiO₃ preferably occupy the shallower electron traps. The lowest energy limit of the electron trap filled by photoexcited electrons is estimated to be at the absorption edge located at 1000 cm^-1 (∼0.12 eV below the conduction band), which is much shallower than that of non-reduced SrTiO₃ (>0.7 eV). Furthermore, it was found that the electron population in the shallow traps in highly reduced SrTiO₃ is nearly 2 orders of magnitude higher compared to that in non-reduced SrTiO₃, indicating a large improvement in the electron lifetime. The findings herein offer significant insights into the crucial impact of the reduction of SrTiO₃via induced oxygen vacancy defects to provide available photoexcited electrons that can be readily utilized for the H₂ generation reaction.