Structure and Formation Mechanism of Black TiO₂ Nanoparticles

The remarkable properties of black TiO₂ are due to its disordered surface shell surrounding a crystalline core. However, the chemical composition and the atomic and electronic structure of the disordered shell and its relationship to the core remain poorly understood. Using advanced transmission electron microscopy methods, we show that the outermost layer of black TiO₂ nanoparticles consists of a disordered Ti₂O₃ shell. The measurements show a transition region that connects the disordered Ti₂O₃ shell to the perfect rutile core consisting first of four to five monolayers of defective rutile, containing clearly visible Ti interstitial atoms, followed by an ordered reconstruction layer of Ti interstitial atoms. Our data suggest that this reconstructed layer presents a template on which the disordered Ti₂O₃ layers form by interstitial diffusion of Ti ions. In contrast to recent reports that attribute TiO₂ band-gap narrowing to the synergistic action of oxygen vacancies and surface disorder of nonspecific origin, our results point to Ti₂O₃, which is a narrow-band-gap semiconductor. As a stoichiometric compound of the lower oxidation state Ti³⁺ it is expected to be a more robust atomic structure than oxygen-deficient TiO₂ for preserving and stabilizing Ti³⁺ surface species that are the key to the enhanced photocatalytic activity of black TiO₂.