Optical, structural, and morphological properties of photocatalytic TiO₂-ZnO thin films synthesized by the sol-gel process

(TiO₂)_{1 - x}-(ZnO)_x thin films, with x = 0.00, 0.25, 0.50, 0.75, and 1.00, were deposited on glass substrates by the sol-gel process. Pure TiO₂ and ZnO thin films were synthesized from titanium isopropoxide-based and zinc acetate-based precursor solutions, respectively, whereas the composite films were obtained from the mixture of these solutions at specific vol.% ratios. Using the Tauc model, it was determined that the band-gap energy increased from 3.43 eV for the TiO₂ thin films up to 3.60 eV for the sample with x = 0.50. With further increase in x, the absorption edges of the films were red-shifted and the band-gap energy decreased down to 3.29 eV for the ZnO thin films. Additionally, the films were evaluated for their ability to degrade methylene blue. The photocatalytic activity of pure ZnO thin films was enhanced by aging the ZnO sol for 48 h. Moreover, it was found that the photocatalytic performance improved by decreasing the x value, with the TiO₂ thin films displaying the highest response. These results are explained in terms of the crystallinity and the surface morphology of the (TiO₂)_{1 - x}-(ZnO)_x thin films.