Oxygen-Vacancy-Driven Orbital Reconstruction at the Surface of TiO₂ Core-Shell Nanostructures

Oxygen vacancies and their correlation with the electronic structure are crucial to understanding the functionality of TiO₂ nanocrystals in material design applications. Here, we report spectroscopic investigations of the electronic structure of anatase TiO₂ nanocrystals by employing hard and soft X-ray absorption spectroscopy measurements along with the corresponding model calculations. We show that the oxygen vacancies significantly transform the Ti local symmetry by modulating the covalency of titanium-oxygen bonds. Our results suggest that the altered Ti local symmetry is similar to the C₃, which implies that the Ti exists in two local symmetries (D₂ and C₃) at the surface. The findings also indicate that the Ti distortion is a short-range order effect and presumably confined up to the second nearest neighbors. Such distortions modulate the electronic structure and provide a promising approach to structural design of the TiO₂ nanocrystals.