Effect of Substrate and Film Thickness on Structural and Photocatalytic Properties of ZnO Thin Films

Zinc oxide (ZnO) thin films have attracted considerable attention due to their versatile applications in optoelectronic devices, transparent electrodes and surface acoustic wave devices. In particular, their photocatalytic properties make them interesting for wastewater treatment. In this study, we investigate the influence of substrate and film thickness on the structure and photocatalytic activity of ZnO thin films prepared by atomic layer deposition (ALD). The photocatalytic activity of ZnO films on Si, glass, Al, and porous Al substrates was investigated under UV irradiation, focusing on the decomposition of methylene blue (MB) as a model for an organic pollutant. To understand the mechanism of photodegradation, detailed information on the morphology of the nanostructured ZnO surface and the surface chemistry was obtained by scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). We have shown that the photocatalytic activity depends on ZnO film thickness and that it reaches saturation at a film thickness of about 20 nm, independent of the substrate. Furthermore, we compared the photocatalytic activity of ZnO films on flat substrates and porous aluminum (prepared by electrochemical anodization) at an optimal film thickness of 20 nm. Our results show that the ZnO thin film on porous aluminum has a significantly higher photocatalytic activity. After 300 minutes of UV lamp exposure, the ZnO thin film deposited on flat aluminum demonstrated the least photocatalytic activity, leading to a reduction of approximately 35% in the concentration of the MB solution. In contrast, the ZnO film coated on a porous anodic aluminum substrate exhibited the highest photocatalytic efficiency, with a reduction in the MB solution concentration by approximately 85%.