Structure tailoring of fluorine-doped TiO2 nanostructured powders

Fluorine-doped nanocrystalline TiO2 powders with controlled anatase–rutile phase content are synthesized by a sol–gel route using NH4F and CF3COOH as fluorine sources. The X-ray diffraction (XRD) study reveals that fluorine-doping through NH4F leads to the formation of anatase crystalline phase, whereas F-doping through CF3COOH favors the formation of rutile along with anatase phase. These results are connected to the influence of the fluorine precursor on the pH of the starting solutions. The presence of the fluorine in the powders is confirmed using X-ray photoelectron spectroscopy (XPS). The dopant in concentrations between 11 and 16 at.% is found mainly in the form of metal fluoride. The measured UV–vis diffuse reflectance spectra and the calculated band gap widths using Kubelka–Munk phenomenological theory are in good agreement with the crystallographic results. The calculations reveal no change in the band gap values due to fluorine doping. The enhanced absorbance in the vis region and the red shift of the absorption edge are attributed to the presence of fluorine and the anatase/rutile phase composition of the TiO2 powders.