First-Principles Modeling of Oxygen-Deficient Anatase TiO₂ Nanoparticles

First-principles calculations of the electronic structure of reduced anatase TiO2 nanoparticles are performed using a hybrid density functional theory approach for an accurate description of charge trapping. It is found that, in the bulk and at extended surfaces, electrons introduced by oxygen vacancies delocalize but, in reduced nanoparticles, electrons preferentially localize (forming Ti3+ species) at low-coordinated sites on the surface of the particle. It is favorable for nanoparticles to be oxygen-deficient in oxygen-poor conditions with the Ti33O66 nanoparticle being significantly easier to reduce than the larger Ti151O302 nanoparticle. Since low-coordinated sites are more prevalent in smaller nanoparticles, this suggests that there is a delicate balance between the number of carriers introduced by vacancies and the number of trapped electrons.