Insights into the genesis of oxygen vacancies and atomically dispersed Ni^δ− species in anatase TiO₂: A spectroscopic study

Oxygen vacancies are commonly observed defects in metal oxides that contribute to the unique physiochemical properties of these materials. Despite the abundant evidence of oxygen vacancies in transition metal oxides and their intriguing properties in catalysis, there remain questions in understanding their formation, structure, and properties. In this study, we employ in situ diffuse reflectance infrared Fourier transform spectroscopy and electron paramagnetic resonance spectroscopy to investigate the genesis of oxygen vacancies and Ni species, focusing on the significance of Ni species relating to the reduction temperature. Our findings reveal a reduction temperature dependence on the formation of oxygen vacancies and atomically dispersed Ni^δ− species in anatase TiO₂ (TiO₂-A). In pristine TiO₂-A, the reduction temperature between 300 and 400 °C led to the creation of a substantial number of oxygen vacancies. With supported Ni on TiO₂-A, oxygen vacancies are favorably formed at 200 °C. As the reduction temperature is increased to 300 °C, Ni species migrate to the oxygen vacancies and become stabilized by forming Ni^δ− species while reducing the number of oxygen vacancies. Subsequent oxidation at 300 °C led to the oxidation of some Ni^δ− species alongside the liberation of the oxygen vacancies they previously occupied. These findings shed light on the mechanisms in forming oxygen vacancies and Ni^δ− species.