Surface and Electronic Features of Fluorinated TiO2 and Their Influence on the Photocatalytic Degradation of 1-Methylnaphthalene

Surface fluorination improves the photocatalytic activity of TiO2, and the influences of various features of fluorinated TiO2 (TiO2-F) have often been discussed in the literature. The present paper addresses the changes induced by surface fluorination on the morphological, structural, surface, and electronic features of TiO2. In particular, X-ray diffraction, specific surface area analysis, and transmission and scanning electron microscopy give evidence that surface fluorination does not affect the structural properties and the morphology of TiO2 nanoparticles. In contrast, fluorination induces changes of surface and electronic properties. Chemical and thermogravimetric analyses show that surface fluorination can reach up to 50% of original TiOH surface sites. The surface charge of TiO2 turns more negative upon fluorination, as shown by a shift of the point of zero charge toward lower pH. Electronic properties are deeply characterized by combining diffuse reflectance, X-ray, and UV photoelectron spectroscopies, as well as time-resolved fluorescence spectroscopy, and photoelectrochemical measurements. Results show the presence of intra-band-gap energy states induced by the local interaction of chemisorbed fluorine atoms. Such energy levels are close to the valence band. The unique surface and electronic properties of TiO2-F make it a promising material for the photocatalytic degradation of poorly soluble emerging pollutants such as 1-methylnaphtalene. In particular, TiO2-F demonstrates faster degradation kinetics with respect to both the pristine material and TiO2 P25 used as a benchmark standard.