Processing of hard super-hydrophobic film with TiO_2 photocatalyst, and mechanisms responsible for self-cleaning performance

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Super-hydrophobic surfaces require appropriate surface roughness with surfaces having low surface energy. However, surfaces with a high roughness commonly show lower hardness than flat surfaces, and this is a crucial problem for the application of super-hydrophobic surfaces. We have prepared a hard super-hydrphobic silica film with visible light transmission by combining two different roughness dimensions. We combined a crater-like roughness (〜800nm) prepared by a phase separation of sol-gel method, and a fine roughness (〜20nm) provided by colloidal silica. Photocatalyst can be combined to this film by utilizing TiO_2 sol. Processing temperature has been lowered to 250℃ by utilizing the coagulation of colloidal silica. The concept of roughness combination is important for the design of super-hydrophobic films with high durability.It has been known that TiO_2 photocatalyst effectively provides a self-cleaning property to super-hydrophobic films and maintains high contact angles during long periods of outdoor exposure. We have investigated the mechanism that might be responsible for the overall self-cleaning performance. Based on the experimental results, it was confirmed that at least following two mechanisms are responsible : 1) a long diffusion distance of the radical species on the fluorine-coated surface ; and 2) reduction of static electricity by the photo-induced hydrophilicity of TiO_2.