Photocatalytic Activity vs Structural Features of Titanium Dioxide Materials Singly Doped or Codoped with Fluorine and Boron

TiO2 photocatalysts, either doped with different amounts of fluorine or boron, or codoped with boron and fluorine, were prepared by sol-gel synthesis, followed by calcination at different temperatures (500-700°C), and characterized by XRPD, BET, XPS, and UV-VIS absorption analyses. The oxidative decompositions of both formic and acetic acid were employed as photoactivity test reactions, also in comparison with previous results obtained with NF-codoped TiO2. A detailed XPS analysis revaled the presence of B2O3 on the surface of B- doped and BF-codoped TiO2, which could be removed by washing under acidic conditions and had no beneficial role in photocatalysis. A correlation was found between the photoactivity of full anatase TiO2 materials calcined at 500°C, singly doped or codoped with nitrogen, fluorine, and boron, and their specific surface area, pointing to a major role of the latter, not of the different dopants, on photoactivity. Only samples containing fluorine, as dopant or codopant, exhibit a photoactivity increase with increasing the calcination temperature to 700°C. This fact, together with an action spectra analysis on photocatalysts calcined at different temperatures, evidenced that only fluorine is responsible for the photoactivity increase in the UVA region observed with full-anatase, highly crystalline doped TiO2 calcined at high temperature.