Reactivity of TiO₂ with water and oxygen: Surface science and perspective

This work reports the reactivity of oxygen and water with the surface of TiO₂ from the perspective of surface science. The reactivity is considered in terms of surface properties studied using surface sensitive tools and their effect on adsorption of oxygen and water, and the related charge transfer. There is a general agreement that defected surfaces exhibit substantially enhanced reactivity over perfect surfaces. In other words, surface defects, such as oxygen vacancies and Ti interstitials are the local surface sites, which are responsible for reactivity with water and oxygen. However, the reported reactivity mechanisms on specific surface sites and the related charge transfer are conflicting. The proposed reactivity models are also not consistent with the present state of understanding of TiO₂ reactivity with water and the related charge transfer. The apparent discrepancy is due to the difference between the specific environment applied in surface studies, which often requires the imposition of high vacuum, and that related to their performance in electrochemical cells.