Structural and optical properties of visible active photocatalytic Al doped ZnO nanostructured thin films prepared by dip coating

Transparent pure and Al-doped zinc oxide (ZnO, AZO) thin films have been deposited onto glass substrates by sol-gel dip-coating technique. The effect of Al doping and annealing temperature on structural, morphological, and optical properties as well as photocatalytic activity of the films have been investigated. XRD results confirm the formation of a hexagonal wurtzite phase with improved crystallinity at a annealing temperature of 550 °C, and preferential orientation along (100) plane. The crystallinity is found to deteriorate with doping, and large crystallite size is observed for pure ZnO film. All films exhibit a homogenous, dense, and granular nanostructure with a typical thickness of about 100 nm. The effective Al³⁺ incorporation into ZnO lattice and the existence of Al–O bonds are confirmed by XPS analysis. An excellent transmittance of 95% is recorded in the visible range. The bandgap E_g value is found to change slightly with doping and higher Urbach energy values are obtained for AZO films. Al doping is found to trigger the formation of visible-active deep-trap levels within the bandgap. Photocatalytic degradation of phenol under simulated sunlight significantly increases upon doping with 1%–2% Al. The degradation rate reaches 80% after 5 h of sunlight irradiation and the photocatalytic kinetics in AZO (2%) is found to be three times greater than that obtained in pure ZnO.