Understanding the light-induced hydrophilicity of metal-oxide thin films involves studying how ultraviolet (UV) light affects the surface properties of metal oxides like TiO₂ and ZnO. These materials, which are n-type doped wide bandgap semiconductors, exhibit photocatalytic effects such as water splitting, carbon dioxide reduction, and organic compound decomposition. The study proposes a model to explain the photo-induced hydrophilicity observed on these surfaces, testing it on TiO₂/Si and ZnO/Si heterojunctions. The model uses a wet exposure technique where a water droplet is placed on the surface while it is exposed to UV light, allowing for continuous contact angle measurements. This method helps determine minority carrier diffusion lengths and provides design rules for materials with photocatalytic hydrophilicity. The study also explores the role of optical constants and layer thickness in UV light-induced hydrophilicity. It shows that the absorption of UV light by the metal oxide layers leads to oscillatory behavior in the absorbed optical power, influenced by internal reflections and electron-hole pair diffusion. Theoretical models and experimental techniques are used to analyze the effects of different wavelengths and layer thicknesses on the photo-induced wetting behavior. The results indicate that the switching rate of contact angles depends on the illumination intensity, with faster switching observed for shorter wavelengths in TiO₂/Si heterojunctions and longer wavelengths in ZnO/Si heterojunctions. The study compares dry and wet exposure methods, finding that wet exposure, where the surface is in contact with water, leads to faster activation of surface groups due to the oxidation of water molecules. The significance of bandgaps and illumination wavelengths is highlighted, showing that photon energies above the bandgap of the metal oxides are necessary for photo-induced hydrophilicity. The results demonstrate that the optical and electronic bandgaps of TiO₂ and ZnO are crucial for their photocatalytic properties, with TiO₂ having a slightly higher bandgap than commonly reported values. The findings provide insights into the design of materials for efficient photocatalytic surface effects and highlight the importance of optical constants and layer thickness in determining the performance of these materials.Understanding the light-induced hydrophilicity of metal-oxide thin films involves studying how ultraviolet (UV) light affects the surface properties of metal oxides like TiO₂ and ZnO. These materials, which are n-type doped wide bandgap semiconductors, exhibit photocatalytic effects such as water splitting, carbon dioxide reduction, and organic compound decomposition. The study proposes a model to explain the photo-induced hydrophilicity observed on these surfaces, testing it on TiO₂/Si and ZnO/Si heterojunctions. The model uses a wet exposure technique where a water droplet is placed on the surface while it is exposed to UV light, allowing for continuous contact angle measurements. This method helps determine minority carrier diffusion lengths and provides design rules for materials with photocatalytic hydrophilicity. The study also explores the role of optical constants and layer thickness in UV light-induced hydrophilicity. It shows that the absorption of UV light by the metal oxide layers leads to oscillatory behavior in the absorbed optical power, influenced by internal reflections and electron-hole pair diffusion. Theoretical models and experimental techniques are used to analyze the effects of different wavelengths and layer thicknesses on the photo-induced wetting behavior. The results indicate that the switching rate of contact angles depends on the illumination intensity, with faster switching observed for shorter wavelengths in TiO₂/Si heterojunctions and longer wavelengths in ZnO/Si heterojunctions. The study compares dry and wet exposure methods, finding that wet exposure, where the surface is in contact with water, leads to faster activation of surface groups due to the oxidation of water molecules. The significance of bandgaps and illumination wavelengths is highlighted, showing that photon energies above the bandgap of the metal oxides are necessary for photo-induced hydrophilicity. The results demonstrate that the optical and electronic bandgaps of TiO₂ and ZnO are crucial for their photocatalytic properties, with TiO₂ having a slightly higher bandgap than commonly reported values. The findings provide insights into the design of materials for efficient photocatalytic surface effects and highlight the importance of optical constants and layer thickness in determining the performance of these materials.