Study of fluorine-doped TiO2 sol-gel thin coatings

TiO(2) is an excellent material for degradation of many environmental contaminants. Its photocatalytic activity is restricted by UV spectral region that can be extended to visible region using different doping techniques. Effect of fluorine-doping on the optical properties of sol-gel prepared TiO(2) thin films is reported. Trifluoroacetic acid (CF(3)COOH) was used as fluorine source with starting concentrations of 5 at. %, 10 at. %, and 20 at. % F to Ti. Gel films were deposited on SiO(2)/soda lime glass substrates by dip-coating technique. After thermal treatment at 450 degrees C fluorine containing TiO(2) thin films were obtained. The crystallinity of the films was determined by X-ray diffraction (XRD). The X-ray photoemission spectrometry (XPS) was applied to determine valence state of the dopant and bonding element. The optical constants, namely, refractive index and the extinction coefficient, were obtained by fitting UV-visible theoretical transmittance curves to experimental ones. The simulated transmittance was calculated in the frames of the Forouhi-Bloomer (FB) dispersion model. The values of the FB energy band gap determined as onset of the absorption were compared with Tauc band gap values received by extrapolation of the linear part of the absorption edge. The XRD patterns reflect beginning of the anatase crystallization process. The XPS analysis reveals the presence of fluorine in the films in the form of titanium fluoride, titanium oxyfluorides, and nonstoichiometric TiO(2-x)F(x). The linear part of the absorption edge of TiO(2) films for mentioned fluorine concentrations gives slightly increased values of Tauc band gap (similar to 3.7 eV) than that for bulk TiO(2) (similar to 3.2 eV). Also, its broadening at lower frequencies with a FB band gap similar to 2.7 eV is observed. The results show that, although the fluorine is incorporated in the TiO(2) lattice, it does not affect the crystallinity and band gap of the formed TiO(2) film for the F/Ti ratios used in the starting sols.