No abstract

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+ x O2– (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.

Ti0 2 nanoparticles with tailored morphology have been synthesized under exceptionally soft conditions. The strategy is based on the use of a non-aqueous alcoholic reaction medium in which water traces, coming either from the air (atmospheric water) or from an ethanol-water azeotropic mixture (ethanol 96%), are incorporated in order to accelerate hydrolysis of the Ti-precursor. Moreover, organic surfactants have been used as capping agents so as to tailor crystal growth in certain preferential directions. Combinations of oleic acid and oleylamine, which lead to the formation of another surfactant, dioleamide, are employed instead of fluorine-based compounds, thus increasing the sustainability of the process. As a result, Ti0 2 nanostructured hierarchical microspheres and individual nanoparticles with exposed high-energy facets can be obtained at atmospheric pressure and temperatures as low as 78 °C.

Fluorine species adsorbed on the surface of TiO 2 anatase nanoparticles improve their photocatalytic performance by reducing the recombination rate of photogenerated electrons and holes. Trifluoroacetic acid (TFAA) has been recently proposed as a promising harmless substitute for hydrofluoric acid (HF) in preparing fluorine-modified anatase with good scavenger properties. However the photocatalytic performance of the TiO 2 nanoparticles also depends on their specific morphology, by which the number and type of crystal facets, which remain exposed, are determined. A comprehensive study is presented in this contribution which for the first time describes the role of TFAA in stabilizing the highly photoactive {001} TiO 2 facets. Furthermore, by reducing the amount of water incorporated into the reaction medium, a simple one-step synthesis method is also proposed that allows the preparation of TFAA-modified anatase nanoparticles with specific morphology and selected stabilized facets, eventually yielding a semiconductor material with excellent photocatalytic properties.