F-doped and N,F-codoped TiO2 materials : photocatalytic activity vs. time-resolved photoluminescence

A systematic investigation was performed on an extended series of singly doped or co-doped TiO2 photocatalysts prepared by the sol-gel method starting from different amounts of N and F dopants and calcined at different temperature in the 500-700 °C range. Their photocatalytic behavior was investigated, also as a function of the irradiation wavelength, in both thermodynamically down-hill reactions, such as formic and acetic acid degradation in aqueous suspension and gas phase mineralization of acetaldehyde, and up-hill reactions for solar fuels production, such as hydrogen production from methanol-water vapor mixtures. Inactive light absorption, i.e. not leading to any photocatalytic reaction, could be clearly distinguished from effective photoactivity by comparing the absorption spectra of the materials with their action spectra in acetic acid oxidation. This demonstrated that fluorine, and not nitrogen, is responsible for the photoactivity increase in the UVA region observed with N,F-codoped TiO2 calcined at high temperature. Besides by well established XRPD, BET, XPS and DR characterization analyses, the investigated photocatalyst materials were characterized by time resolved photoluminescence (PL) spectroscopy in the pico-second range, aiming at obtaining a better and direct insight on the effects that doping TiO2 with p-block elements has on the dynamics of the charge carriers generated by band gap excitation, in relation to the photocatalytic activity of these materials. Both the photoluminescence spectral shapes and lifetimes were found to be strongly affected by the presence of dopants, as well as by the calcination temperature. In particular a clear correlation can be outlined between the long-lasting PL decay component and the photocatalytic performances of the fluorine doped TiO2-based materials, providing direct evidence that photoactivity is higher in semiconducting materials with longer living electron-hole couples.