Effects of atomic layer deposition conditions on the formation of thin ZnO films and their photocatalytic characteristics

The use of photocatalysts in water treatment systems is regarded as an advanced technology. To ensure efficiency and stability, the optimization of photocatalyst immobilization is essential for application in water treatment processes. In this study, we investigated the effect of atomic layer deposition (ALD) conditions on the development of highly photocatalytically active thin ZnO films. Three different temperatures and three ALD cycles were employed to evaluate the photocatalytic activity of thin ZnO films (represented by the production rate of reactive oxygen species and the degradation rate of methylene blue). We found that the surface properties of the thin ZnO films, such as grain size and homogeneity, exerted a dominant influence on the photocatalytic activity. At a low temperature (50 °C), nanograins were not formed properly, while various nanograin shapes were obtained at a high temperature (250 °C). The optimized grain had a grain size of 20 nm and a (002)/(101) crystalline orientation ratio of 2.2. The UV light absorption increased in proportion to the film thickness, and a minimum film thickness (50 nm) was necessary to ensure high photocatalytic activity at the film surface. In addition, the increase in the photocatalytic activity was not significant as the thickness increased beyond the optimum thickness. These results will provide useful guidelines for the fabrication of thin ZnO films with excellent photocatalytic activity for water treatment.