The nature of defects in fluorine-doped TiO2

Fluorine-doped titanium dioxide was prepared via sol-gel synthesis and subsequent calcination in air. The presence of fluorine in the lattice induces the formation of reduced Ti3+ centers that localize the extra electron needed for charge compensation and are observed by electron paramagnetic resonance. Density functional theory calculations using hybrid functionals are in full agreement with such description. The extra electron is highly localized in a 3d orbital of titanium and lies a few tenths of an electron volt below the bottom of the conduction band. The preparation via sol-gel synthesis using aqueous solutions of fluorides also causes the formation of surface F- ions that substitute surface hydroxyl groups (OH-) without generating reduced centers.