Keeping glass clean from fogging up is one of growing interests among glass manufacturers and suppliers. Anti-fog glass is becoming more and more prevalent in day to day products such as bathroom mirrors, car windows, eye glasses, etc. Having anti-fog glass may increase safety in many instances such as in cars. When there is humidity or a rapid temperature change, fogging glass may disrupt the driver's ability to see through their windscreen or to see views in side view mirrors. In other applications, anti-fog glass may eliminate the inconvenience of fogging in kitchen and bathroom glass and mirrors caused by hot showers and boiling water. There are many other places and circumstances where anti-fog glass may help safety and convenience, such as on facade glass in the presence of a significant temperature gradient and high environmental humidity.
Because of the synergetic effect of photocatalysis and photo-induced hydrophilicity of titanium oxide (TiO2), TiO2 has been considered as a good candidate for a large-scale and relatively inexpensive applications in the fields of antifogging and self-cleaning coatings. It is known that TiO2 hydrophilic surfaces can be obtained by UV activation through a redox mechanism that results in trapping of the photo-generated holes at lattice sites and subsequent Ti—O bond rupture by adsorbed water molecules, and forming new hydroxyl groups. The rapid advance in surface science and the increasing industrial demand has significantly facilitated the development of anti-fogging and self-cleaning engineered surfaces showing a superhydrophilic character without any external stimuli, in which titanic is combined with other materials such as in the case of multilayer assemblies constituted by TiO2 nanoparticles and polyethylene glycol or yet porous ZnO/TiO2 composite films.
Few studies deal with the fabrication of coatings made exclusively by TiO2 and superhydrophilic without radiation. Existing methods include the preparation of porous TiO2 nanostructures by a sol-gel method, exhibiting stable super-wetting properties without the need of light activation. Other methods include fabricating perpendicular TiO2 nanosheet films by a hydrothermal treatment of a titanium metal sheet with aqueous urea, resulting in superhydrophilicity without UV irradiation due to the enhanced density of oxygen defects or dangling bonds present in these structures.
Many attempts have been made to control the wettability by tuning coating porosity and roughness, which may allow water to rapidly permeate the three-dimensional porous network that induces the complete wetting of the surface. Porous films, containing a mixture of all the main three polymorphs of TiO2, have previously been synthetized by supersonic aerosol deposition. These films became superhydrophilic without UV illumination after high-temperature annealing. Porous structures may not meet the requirements of high transparency in the visible region due to the high surface roughness dramatically depleting the transmittance of the coatings seriously affected by an enhanced diffuse scattering. In addition, their mechanical properties may be poorer than those of compact films and thus those may not be used as long-lasting building materials.