Fluorine- and Niobium-Doped TiO2: Chemical and Spectroscopic Properties of Polycrystalline n-Type-Doped Anatase

Doping titanium dioxide (anatase) with elements carrying an extra electron, such as Nb and F, or with their mixtures leads to n-type materials showing peculiar properties with respect to the pristine oxide. Niobium and fluorine are present in the lattice in the form of Nb5+ and F- ions (detected by XPS), and the extra electrons carried by the dopants are stabilized on titanium ions, which become EPR-visible as Ti3+ ions homogeneously dispersed in the bulk of the crystals. Under such conditions, the optical band gap transition is slightly red-shifted (by a few tenths of an electronvolt) for all samples containing fluorine, and the Fermi level lies, depending on the material, at the boundary or even in the lower region of the conduction band. The typical Ti3+(1) centers generated by valence induction are responsible for the already reported conductivity properties of the system. The presence of these centers also influences the process of electron injection in the solid, favoring the dilution of additional reduced centers in the bulk, thereby leading to a homogeneously reduced material with optoelectronic properties differing from those of reduced anatase.