Optimization of the structural and optical properties of ALD grown ZnO thin films for photocatalytic applications: thickness dependence

Thin films of ZnO with different thicknesses (ranging from 8 to 40 nm) have been prepared by plasma-enhanced atomic layer deposition. Grazing incidence x-ray diffraction shows the nano-crystalline structure of the films with high degree of disorder. The films have also lattice oxygen and non-lattice oxygen where the film with 20 nm thickness has the highest percentage of the non-lattice oxygen. These films have indirect optical transitions. The energy gap increases slightly with decreasing the film thickness (2.96, 3.03 and 3.16 eV for the thicknesses 40, 20 and 8 nm, respectively). These films have strong photocatalytic activity to treat the water from the organic dyes such as Levafix Brilliant Red. The film with thickness 20 nm has the optimum photocatalytic activity and the lowest contact angle with water. The photoinduced super-hydrophilic nature of ZnO film (20 nm) renders this film suitable for antifogging application. The high photocatalytic activity and super-hydrophilicity are due to the low recombination rate of charge carriers accompanied to the excess of oxygen vacancies and the high degree of structural disorder.