The effect of calcination temperature on the surface microstructure and photocatalytic activity of TiO2 thin films prepared by liquid phase deposition

TiO2 thin films were prepared on fused quartz by the liquid-phase deposition (LPD) method from a (NH4)(2)TiF6 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 degreesC 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 degreesC, 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 degreesC. When the calcination temperature is higher than 700 degreesC, 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.