Characteristics and photocatalytic activity of highly c-axis-oriented ZnO thin films

ZnO thin films are deposited on glass substrate using sol–gel spin coating methods with various annealing temperatures. The structural, morphological, and optical properties are studied by XRD, FE-SEM, UV–Vis, and photoluminescence spectrophotometer. The photocatalytic activity was assessed by examining the decomposition of Rhodamine-B (Rh-B) dye under UV illumination. The result shows that the ZnO films have a hexagonal wurtzite structure with highly preferred c-axis orientation. The (101) crystallographic plane appears at higher annealing temperatures. The transmittance of ZnO films is more than 80%, with a slight red shift in absorption and decrease in the optical band gaps as the annealing temperature increases to some extent. The photoluminescence spectra of the ZnO films at room temperature demonstrated the ultra-violet (UV) emission with a peak emission at 410 nm (3.03 eV). According to the photocatalytic activity examination, the presence of (101) plane in the c-axis oriented ZnO films enhances the photocatalytic performance by a factor of 5. The result revealed that ZnO films with multi-orientation structure possess better photocatalytic performance than that of the single-orientation ones. In particular, the photocatalytic performance is highly contributed by the polar crystal plane and slightly improved by the crystallinity and surface morphology.