Structural evolution and photocatalytic performance of ZnS-ZnO thin films synthesized by spray pyrolysis

ZnS thin films were synthesized using the spray pyrolysis technique at 350 degrees C. Following the annealing process, X-ray diffraction (XRD) analysis was performed to examine the structural characteristics of the films. This analysis revealed a gradual substitution of sulfur by oxygen during an extended annealing period, leading to the transformation of the samples into a mixed phase consisting of ZnS, ZnO, and a substitutional solid solution Zn(S, O). The surface morphology and elemental composition of the treated films were investigated using Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDX) and profilometry. These crystalline particles of growing ZnO phase adopt a hexagonal shape, displaying well defined edges and smoother surfaces, with dimensions of approximately 600 nm. Moreover, a change in surface properties, marked by a transition from hydrophilic to hydrophobic behavior, was observed. The optical characteristics were examined through transmittance, reflectance and absorbance spectra, recorded in the range of 200-2500 nm. Films with a high percentage of ZnO are more absorbent in the UV range and have a narrower bandgap energy. Additionally, the photocatalytic activity was evaluated by monitoring the degradation of an aqueous solution of methylene blue (MB) when exposed to sunlight. Enhanced degradation of MB dye was observed as the amount of zinc oxide in the annealed samples increased.