Synthesis and characterization of blue faceted anatase nanoparticles through extensive fluorine lattice doping

An effective synthesis strategy for the extensive fluorination of the TiO₂ anatase lattice has been developed which provides a highly stable blue-colored titania powder. The process also produces a convenient faceted morphology of the doped nanoparticles. Both theoretical and experimental data indicate an ordered atomic structure, in which an exceptionally high amount of fluorine ions substitute oxygen ions in the TiO₂ lattice. The extra-electrons borne by fluorine are stabilized by lattice Ti cations via a mechanism of valence induction, eventually leading to a consequent high amount of reduced Ti³⁺ centers. Such structure, whose general formula can be expressed as Ti⁴⁺_(1-x)Ti³⁺_xO^2–_(2-x)F^–_x, confers an excellent stability to the as-synthesized nanoparticles (in spite of the excess electrons), explaining for example why the blue color is retained even upon storage in ambient atmosphere. But moreover, the high concentration of the relatively shallow Ti³⁺ generated states also form a sort of sub-band close to the bottom of the conduction band itself. The whole effect results in a tangible decrease (0.2 eV) of the anatase band gap which allows an inspiring upgrading of its UV photocatalytic activity. Our results also suggest that narrowing the band gap is insufficient for a substantial improvement in the visible light harvesting.