Preparation and characterization of super-water-repellent Al2O3 coating films with high transparency

Alumina thin films with a roughness of 20 to 50 nm were formed by immersing the porous alumina gel films prepared by the sol-gel method in boiling water. When hydrolyzed fluoroalkyltrimethoxysilane was coated on the alumina films, the films showed super-water-repellency and high transparency; the contact angle for water of the film was 165 degrees and the transmittance for visible light was higher tl;an 92%. When the fluoroalkyltrimethoxysilane-coated thin films were heat-treated at temperatures higher than 500 degrees C, the films became super-hydrophilic; the contact angle for water on the films was less than 5 degrees. It was shown the existence of air in the pores on the surface caused the super-water-repellency and that of water in the pores caused the super-hydrophilic property. The transparent, super water-repellent and super-hydrophilic coating films formed on glasses, metals, and ceramics have practical applications such as optical lenses, eyeglasses, cover glasses for solar cells, windshields of automobiles, and so on.