Optical and structural properties of the sol–gel-prepared ZnO thin films and their effect on the photocatalytic activity

ZnO thin films were obtained by the sol–gel method, using the dip-coating procedure. Glass slides were used as substrates. The sintering temperature (Ts) was varied in the range of 200–600 °C in intervals of 50 °C, in an open atmosphere. Films with 1 and 5 coatings were prepared for each Ts. An increase of the grain size from 10 to 34 nm as the Ts increased was observed from X-ray diffraction measurements. The thickness of the films prepared starting from five coatings, decreased by 36% when Ts increased, and denser films were obtained. This result was corroborated with the refractive index values, calculated from the UV–Vis transmission spectra. The films were tested as a photocatalyst by the photobleaching of methylene blue in an aqueous solution under UV light exposure during 5 h. The photocatalytic activity (PA) increased with Ts, around 72% for the films with one coating and 66% for those with five coatings. The samples with one coating and a Ts=500 °C showed the best PA. However, the glass substrate had a negative effect on the PA for Ts>500 °C, even when the surface morphology of the samples showed an increase in roughness when Ts increased. The observed negative effect can be due to the presence of an amorphous compound formed by Si, Zn and O at the glass–ZnO interface.