Effect of annealing on the photocatalytic activity of chemically prepared TiO2 thin films under visible light

Porous TiO2 thin film was fabricated onto glass substrate using a simplified chemical bath deposition method at moderately low temperature and the effects of thermal annealing on the structural, morphological, optical and photocatalytic properties were investigated. We report here that the as-prepared TiO2 thin films showed the amorphous phase while the little improvement in the crystallinity was observed after annealing. FESEM image of the as-deposited TiO2 thin film shows that the total surface of the substrate uniformly acquired by nearly spherical TiO2 clusters and a significant grain growth takes place and porous structure generates after the thermal annealing at 773 K. The average transmittance of the TiO2 films slightly decreases after annealing thus absorption band edge shifted towards higher wavelength. The decrease in bandgap value from 3.32 eV to 3.08 eV was observed after thermal annealing. The photocatalytic activity of the as-prepared and annealed TiO2 films was evaluated by photodegradation of congo red dye solution. After annealing, the photocatalytic activity of the TiO2 films was found to be increased because of the improvement of crystallization of anatase TiO2 thin films as evident from XRD and FESEM studies. The photocatalytic reaction for both as-deposited and annealed TiO2 thin film follows pseudo-first order kinetics.