Effect of cobalt doping on the photocatalytic activity of zinc oxide thin films under uv light irradiation

The purpose of this research was to investigate the effect of cobalt dopant and their concentrations on the structure, functional group, surface morphology, optical properties and photocataltytic activity of ZnO films deposited on slide glass plates by dip-coating technique. Dipping solutions were prepared by 0.7 M Zn(CH3COO)2.2H2O in the mixed solvent of C2H5OH9 DEA and DI-water. The molar ration of DEA/zinc acetate and DI-water/zinc acetate were 1:1 and 2:1, respectively. In this work, Co(CH3COO)2.4H2O was selected as doping additive, which was varied from 0 to 10 mol%. The obtained thin films were annealed at 500 C for 1 hr.
The obtained thin films were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Atomic force microscopy, Photoluminescence spectroscopy, and Ultraviolet-Visible spectroscopy. The photocatalytic activity was investigated by the decomposition of Malachite green (MG) dye over the film both under UV-light and darkness conditions. The X-ray diffraction results indicated that the samples have polycrystallite nature with hexagonal wurtzite structure. Both crystallinity and crystallite sizes of film decreased with increasing of doping concentrations. From fourier transform infrared results, zinc ions were replaced by cobalt ions in ZnO lattice, resulting to shifts, the grain sizes and surface roughness of prepared thin films tended to decrease with increasing cobalt content compared with ZnO. However, the slight increase in both crystallite size and surface roughness were found at 7 and 10 mol% doping level as compared with cobalt-doped ZnO thin films (5 mol%). The photoluminescence analysis indicated that the photoluminescence intensity of ZnO films with cobalt-doping was lower than that of ZnO. Moreover, the band gap energy of the thin films slightly increased, whereas the transparency of films decreased with high cobalt-doping level.
The photocatalytic activities of cobalt-doped ZnO films were evaluated using MG dye irradiated with UV light. It was found that the rate constant of cobalt-doped ZnO thin films (0, 3, 5, 7 and 10 mol%) and self photolysis of MG dye were 0.0150, 0.0078, 0.0069, 0.0071, 0.0109 and 0.0006 min-1, respectively. The experiment demonstrated that the photo-degradation efficiency of undoped ZnO films was significantly higher than that cobalt-doped ZnO films.