Direct Observation of Oxygen Vacancy Self-Healing on TiO2 Photocatalysts for Solar Water Splitting.

Oxygen vacancy (Vo) on transition metal oxides plays a crucial role in determining their chemical/physical properties. Conversely, the capability to directly detect the changing process of oxygen vacancies (Vos) will be important to realize their full potentials in the related fields. Herein, with a novel synchronous illumination X-ray photoelectron spectroscopy (SI-XPS) technique, we found that the surface Vos (surf-Vos) exhibit a strong selectivity for binding with the water molecules, and sequentially capture an oxygen atom to achieve the anisotropic self-healing of surface lattice oxygen. After this self-healing process, the survived subsurface Vos (sub-Vos) promote the charge excitation from Ti to O atoms due to the enriched electron located on low-coordinated Ti sites. However, the excessive sub-Vos would block the charge separation and transfer to TiO2 surfaces resulted from the destroyed atomic structures. These findings open a new pathway to explore the dynamic changes of Vos and their roles on catalytic properties, not only in metal oxides, but in crystalline materials more generally.