Titanium Dioxide Surface Superhydrophilic Principle
Normally, the TiO2 coated surface has a large contact angle with water. However, after UV irradiation, the contact angle of water is reduced to less than 5 degrees, or even to 0 degrees (i.e. water droplets are completely diffused on the Tio2 surface). It shows very strong hydrophilicity. After the light is stopped. The superhydrophilicity of the surface can be maintained for several hours to about 1 week. Then it slowly returns to the hydrophobic state before irradiation. Re-irradiation with UV light. The fork can be superhydrophilic, and intermittent UV irradiation can maintain the superhydrophilic state of the surface.
It is initially believed that the superhydrophilicity of TiO2 surface starts from the photocatalytic decomposition reaction of organic molecules adsorbed on the surface: the chemisorbed water on the surface of TlO2 itself is hydrophilic. And the adsorption of organic matter in the air makes the surface hydrophobic. Under UV light irradiation. The surface generates strong oxidizing active hydroxyl groups, and the hydrophobic organic matter is oxidized and decomposed by the active hydroxyl groups through the photocatalytic decomposition reaction. Thus, the surface becomes hydrophilic; when the light is stopped, the organic matter will be slowly adsorbed on the TiO2 surface and return to the hydrophobic state.
However, further studies show that the superhydrophilicity of TiO2 surface is different from the photocatalytic oxidative decomposition property of TiO2. Rather, it is another reaction induced by light on the TiO2 surface itself. The reasons are as follows: ① The degree of superhydrophilicity of TiO2 surface is not related to the photodecomposition efficiency of organic matter. In some TiO2 single crystals or polycrystals with no photocatalytic activity or very low photocatalytic activity are observed superhydrophilic properties; ② some metal ions (such as copper) doping can improve the photocatalytic oxidation reaction of TiO2. However, it reduces the superhydrophilic property of TiO2 surface; (3) Unlike the porous TiO2 surface and the largest possible reaction area required for photocatalytic oxidation reaction. The smooth and dense surface is more conducive to its superhydrophilic property; ④ The surface of TiO2 also has great affinity for oil after UV irradiation, and under normal conditions, oily marketers such as ethylene glycol hexadecane and glyceryl trioleate have large contact angles with TiO2 surface. But 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 dual affinity.
The current study concluded that. The superhydrophilicity of TiO2 surface under light conditions is solidified by the change of its surface hollowing: the electrons in the valence band of TiO2 are excited to the conduction band under UV irradiation conditions. Electrons and holes migrate to the TiO2 surface and generate electron-hole pairs on the surface. The electrons react with Ti4+. The holes react with the surface bridge oxygen ions to form positive trivalent titanium ions and oxygen vacancies, respectively. At this time. The water in the air is dissociated and adsorbed in the oxygen vacancies. It becomes chemisorbed water (surface hydroxyl facsimile). The chemisorbed water can further adsorb the water in the air and become a physical adsorption layer. That is, a highly hydrophilic emblem zone is formed around the Ti3+ defect. The remaining area of the surface remains hydrophobic, so that the TiO2 surface constitutes a uniformly separated nano-sized hydrophilic and oleophilic emblem zone, similar to the two-dimensional capillary phenomenon. Since the size of water or oily droplets is much larger than the area of hydrophilic or oleophilic zone, the macroscopic TiO2 surface exhibits hydrophilic and oleophilic properties. The water or oil drops are adsorbed by the hydrophilic micro-zone or the lipophilic emblem zone, respectively. Thus, the surface is infiltrated. After stopping UV irradiation, the chemisorbed hydroxyl groups are replaced by oxygen in the air and return to the hydrophobic state again.
