Sol–gel SiO2/TiO2 bilayer films with self-cleaning and antireflection properties

We report here that a facile sol–gel dip-coating technique can be used to fabricate a SiO2/TiO2 bilayer film with self-cleaning and antireflection properties. The bottom SiO2 layer acts as an antireflection coating due to its lower refractive index; the top TiO2 layer acts as a self-cleaning coating generated from its photocatalysis and photo-induced superhydrophilicity. The maximal transmittance of SiO2/TiO2 bilayer film at normally incident light can be reached 96.7%, independent of the high refractive index and coverage of TiO2 nanoparticles. However, the photocatalytic activity of the bilayer film shows a close dependence on coverage of TiO2 nanoparticles. After illuminated by ultraviolet light, the SiO2/TiO2 bilayer films are superhydrophilic with water contact angle less than 2°, which favors greatly the self-cleaning function of the films.