Unified defect mechanism for illumination effects on the rutile TiO2 (110) surface from first principles

Many phenomena observed on the TiO2 surface under illuminations have been attributed to the formation of light-induced surface oxygen vacancies (VO), which were believed to act as electron donors and hole traps. However, direct experimental evidence supporting this explanation has been elusive, leading to a long-standing debate on the underlying mechanisms. Here, we employ first-principles calculations and defect theory under illumination to elucidate the underlying mechanisms governing diverse illumination effects on the rutile TiO2 (110) surface. We find that the illumination cannot directly create new surface defects under experimental conditions, while illumination can promote the ionization rate of Ti interstitials (Tii) and disrupt their original equilibrium distribution, thus triggering the associated defect dynamics. Specifically, under illumination, an increased number of subsurface Tii become ionized, creating a concentration gradient relative to surface ionized Tii. Consequently, subsurface Tii would diffuse towards the surface sites which not only provides a plausible explanation for the increased concentration of surface electron donors and hole traps, but also explains the time-dependent nature of illumination effects. After the removal of illumination, surface Tii are temporarily retained, thereby extending the effects of illumination. Our proposed dynamics of Tii are consistent with experimental observations, providing a picture for understanding the complex interplay between illumination and surface properties in rutile TiO2. Our findings not only resolve the historical debate but also offer a foundation for understanding illumination effects in a broader range of materials and systems.