The Principle of Superhydrophilic TiO2 Surfaces
Normally, the TiO2 coated surface has a wide contact angle with water, but the contact angle of water is reduced to less than 5 degrees after UV irradiation, and can even reach 0 degrees (i.e. water droplets are completely diffused on the Tio2 surface), showing a very powerful hydrophilicity. After cessation of light exposure, the superhydrophilicity of the surface is maintained for a few hours to a week, and then slowly returns to the hydrophobic state before irradiation. Further irradiation with ultraviolet light can be superhydrophilic, and intermittent ultraviolet light irradiation can make the surface always maintain the superhydrophilic state.
It is originally thought that the superhydrophilicity of the TiO2 surface originates from the photocatalytic decomposition of organic molecules adsorbed on the surface: while the surface of TiO2 itself is hydrophilic due to the chemisorption of water, the adsorption of organic matter in the air makes the surface hydrophobic. When irradiated by UV light, the surface generates strong oxidizing active hydroxyl groups, and the hydrophobic organic matter is oxidized and decomposed by the positive hydroxyl groups through the photocatalytic decomposition reaction, thus making the surface behave in a hydrophilic state; when the light is stopped, the organic matter will be gradually adsorbed on the TiO2 surface and return to a hydrophobic state. Nevertheless, it has been shown that the superhydrophilicity of the TiO2 surface is different from the photocatalytic oxidation and decomposition of TiO2, rather it is another kind of reaction induced by light on the TiO2 surface itself. For the following reasons: ① The superhydrophilicity of the TiO2 surface is not related to the photodecomposition efficiency of organic matter. It has been observed in some TiO2 single crystals or polycrystals with no photocatalytic activity or very low photocatalytic activity; ② Some metal ions (e.g. copper) doping can improve the photocatalytic oxidation reaction of TiO2, while reducing the superhydrophilic property of TiO2 surface; ③ Different from the porous TiO2 surface and the largest possible reaction area required for photocatalytic oxidation reaction, a smooth and dense surface is more conducive to its superhydrophilic property. Under normal conditions, oily bodies such as ethylene glycol hexadecane and glyceryl trioleate have a large contact angle with the TiO2 surface. However, after UV irradiation, these liquids will also be completely infiltrated in the glass coating surface, that is, after UV irradiation TiO2 surface with water-oil amphiphilic activity.
It is believed that the super-hydrophilicity of TiO2 surface under light conditions is solidified by its changes in surface structure: under UV irradiation conditions, TiO2 valence band electrons are excited to the conduction band, electrons and holes migrate to the TiO2 surface, generating electron-hole pairs on the surface, electrons react with Ti4+, holes react with surface oxygen ions, which form positive trivalent titanium ions and oxygen vacancies respectively. In this case, the water in the air is dissociated and adsorbed in the oxygen vacancy, forming a chemisorbed surface (surface hydroxyl group). This chemisorption surface can further adsorb water in the air and form a physical adsorption layer, i.e. a highly hydrophilic emblem zone is formed around the Ti3+ defect, whereas the remaining area of the surface remains hydrophobic, thus constituting a uniformly zoned nano-size separated hydrophilic and oleophilic emblem zone on the TiO2 surface, similar to a two-dimensional capillary phenomenon. Since the size of water or oil droplets is much larger than the hydrophilic or oleophilic area, the macroscopic TiO2 surface exhibits hydrophilic and oleophilic properties. The droplets of water or oil are adsorbed by the hydrophilic micro-region or the lipophilic emblematic region respectively, thus infiltrating the surface. When the UV light irradiation is stopped, the chemisorbed hydroxyl groups are replaced by oxygen in the air and return to the hydrophobic state.
