Effect of annealing temperature on the photocatalytic activity of sol-gel derived TiO2 thin films

Abstract

Transparent TiO2 thin films were deposited on glass substrates by a sol-gel dip coating technique, where crystallinity can be achieved at a relatively low annealing temperature. In this sol-gel preparation technique, acetic acid was used as a catalyst. Repetitive dip coating and drying, with a single annealing has made this technique easier to prepare TiO2 thin film with desired thickness within a short period of time. The crystallographic and optical properties of TiO2 films were investigated in detail. The photocatalytic activity was evaluated by the measurement of decomposition of methanol under UV-visible irradiation. The rate of decomposition of gaseous methyl alcohol depends on the structures and surface properties of the TiO2 films. The structure of the TiO2 films, prepared at various annealing temperatures, shows different crystallinities. Although crystallinity has been observed for 300°C annealing temperature, it improved with increasing annealing temperature. The average transmittances of the TiO2 films slightly decrease with the increase of the annealing temperature in the range of 900–380nm. The optical transmission edge of the TiO2 films, annealed at different annealing temperatures shifts toward higher wavelength with higher annealing temperature. All the as-deposited TiO2 films successfully photocatalyzed methanol to CO2 and H2O, measured by Fourier transform infrared spectrometer. The decomposition efficiency of TiO2 films increased with decrease of annealing temperature. The surface morphology of all the samples was investigated by field emission scanning electron microscopy and atomic force microscopy measurements. The higher photocatalytic activity of the sample prepared at lower annealing temperature was explained by the effect of more porous structure with smaller grains and open surface morphology of the sample.