Synthesis, characterization and computational simulation of visible-light irradiated fluorine-doped titanium oxide thin films

5% F doped TiO2 (FT) thin films deposited on glass substrates have been synthesized by the sol-gel dipcoating method. The as-synthesized samples are characterized using XRD, AFM, SEM, XPS, UV-vis and PL analysis technologies. The electron structures of anatase TiO2 and F doped TiO2 have been calculated using first principles based on density functional theory (DFT). The visible light induced hydrophilic properties of FT thin films are primarily investigated and compared with those of TiO2 thin films. The results demonstrate that the surface of FT thin films is smoother with a root mean square roughness of 2.5 nm. The absorption edge of the FT thin films does not display an appreciable shift to the visible region, however, the surface chemical states have changed compared with those of TiO2 thin films. The water contact angle is about 5 degrees after daylight lamp irradiation for 10 min. The theoretical band calculations indicate that the band gap of F doped TiO2 is not narrowed, but the conduction band (CB) of F doped TiO2 would be modified compared with that of TiO2 and the calculated imaginary parts of the dielectric functions of FT show that the peak near the absorption edge shifts to lower energy upon F doping, so the electron transition from the localized states to the CB would lead to a decrease of absorption energy and induce visible-light irradiation, which gives a good explanation for the experimental visible light induced superhydrophilic properties of FT thin films.