The role of Al concentration on improving the photocatalytic performance of nanostructured ZnO/ZnO:Al/ZnO multilayer thin films

Abstract

Nanostructured ZnO, ZnO/AZO, and ZnO/AZO/ZnO multilayer thin films were deposited on glass substrates by the sol-gel spin coating method. Structural, optical and photocatalytic properties of multilayer thin films as a function of Al doping concentration were studied. Field emission scanning electron microscopy (FESEM) images showed that surfaces of ZnO multilayer thin films have granular character. X-ray diffraction (XRD) analysis indicated that all multilayer thin films are polycrystalline with a hexagonal Wurtzite structure, and the crystallinity of multilayer thin films deteriorate with an increase in the Al doping concentration. X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy showed that oxygen vacancies in ZnO/AZO/ZnO thin films are higher than in ZnO/AZO thin films. The DRS result showed that the band-gap energy of ZnO multilayer thin films increases with increasing Al doping concentration. The prepared samples were examined by the photodegradation reaction of methylene blue (MB) under visible light irradiation. The AZO as the middle layer in the ZnO/AZO/ZnO thin films was found to improve the photocatalytic activity of the samples. The maximum photocatalytic activity (95.2%) was obtained at 20% Al concentration, and the photocatalytic activity of this sample stayed nearly unchanged after three cycles.