Synthesis and characterization of ZnO and ZnO:Al by spray pyrolysis with high photocatalytic properties

The pneumatic spray pyrolysis technique was used to deposit zinc oxide and aluminum doped zinc oxide films. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical transmission were used to characterize the films and their photocatalytic activities were measured by the degradation of an organic dye under ultraviolet irradiation. A thickness-based rate constant was calculated for both, undoped and Al-doped thin films, obtaining values of k_t(ZnO) = 0.25 and k_t(ZnO:Al) = 0.32 h^-1 μm^-1, respectively, which resulted higher than that of a TiO₂ film (k_t = 0.21 h^-1 μm^-1). The stability of the films was also evaluated by the repetition of the degradation test three times. A slight decrease in the activity was observed after the third cycle particularly for the doped film. These results along with the characterization results show that the incorporation of aluminum to the zinc oxide matrix produce a slight increase in the optical band gap of ZnO:Al (3.26 eV) compared to the pure ZnO film (3.23 eV), and favors the formation of surface structures that contributed to enhance the photocatalytic activity.