TiO₂ thin films were prepared on fused quartz by the liquid-phase deposition (LPD) method from a (NH₄)₂TiF₆ aqueous solution upon addition of boric acid (H₃BO₃) and calcined at various temperatures. The as-prepared films were characterized with thermogravimetry (TG), Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), UV−Visible spectrophotometry (UV−Vis), scanning electron microscopy (SEM), photoluminescence spectra (PL), and X-ray photoelectron spectroscopy (XPS), respectively. The photocatalytic activity of the samples was evaluated by photocatalytic decolorization of methyl orange aqueous solution. It was found that the as-prepared TiO₂ thin films contained not only Ti and O elements, but also a small amount of F, N, and Si elements. The F and N came from the precursor solution, and the amount of F decreased with increasing calcination temperature. Two sources of Si were identified. One was from the SiF₆²⁻ ions, which were formed by a reaction between the treatment solution and quartz substrate. The other was attributed to the diffusion of Si from the surface of quartz substrate into TiO₂ thin film at 700 °C or higher calcination temperatures. With increasing calcination temperature, the photocatalytic activity of the TiO₂ thin films gradually increased due to the improvement of crystallization of the anatase TiO₂ thin films. At 700 °C, the TiO₂ thin film showed the highest photocatalytic activity due to the increasing amount of SiO₂ as an adsorbent center and better crystallization of TiO₂ in the composite thin film. Moreover, the SiO₂/TiO₂ composite thin film showed the lowest PL intensity due to a decrease in the recombination rate of photogenerated electrons and holes under UV light irradiation, which further confirms the film with the highest photocatalytic activity at 700 °C. When the calcination temperature is higher than 700 °C, the decrease in photocatalytic activity is due to the formation of rutile and the sintering and growth of TiO₂ crystallites resulting in the decrease of surface area. Copyright © 2003 American Chemical Society.