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

•TiO2–ZnO thin films were synthesized on glass substrates by the sol–gel process.•The photoactivity of samples was evaluated for their degradation of methylene blue.•The effect of the ZnO–sol aging time on the photocatalytic activity was studied.•The TiO2 thin films showed the maximum photocatalytic response.•The film photoactivity mainly depends on its crystallinity and surface morphology.

(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.