Study of Fluorine-Doped $TiO2$ Sol-Gel Thin Coatings

$TiO2$ 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 $TiO2$ thin films is reported. Trifluoroacetic acid $(CF3COOH)$ was used as fluorine source with starting concentrations of $5at.%$⁠, $10at.%$⁠, and $20at.%$ F to Ti. Gel films were deposited on $SiO2$∕soda lime glass substrates by dip-coating technique. After thermal treatment at $450°C$ fluorine containing $TiO2$ 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 $TiO2−xFx$⁠. The linear part of the absorption edge of $TiO2$ films for mentioned fluorine concentrations gives slightly increased values of Tauc band gap $(∼3.7eV)$ than that for bulk $TiO2$$(∼3.2eV)$⁠. Also, its broadening at lower frequencies with a FB band gap $∼2.7eV$ is observed. The results show that, although the fluorine is incorporated in the $TiO2$ lattice, it does not affect the crystallinity and band gap of the formed $TiO2$ film for the $F∕Ti$ ratios used in the starting sols.