Influencing Factors for Superhydrophilicity of TiO2 Surfaces
1. UV light irradiation time and intensity
In general, the size of the hydrophilic zone on the TiO2 surface increases and the size of the lipophilic zone decreases as the light exposure time increases. The increase in te size of hydrophilic zone is conducive to improving the hydrophilic properties of the surface, and the decrease in the size of the lipophilic zone increases the capillary gravitational force, which is conducive to the expansion of oil on the TiO2 surface. When the hydrophilic and oleophilic zones reach the critical point that is, the two-philic phenomenon of water and oil can be realized on the same surface, the contact angle of both water and oil can reach 0°, and the super hydrophilic and super oleophilic characteristics appear; when the light exposure time continues to increase, the size of hydrophilic zone will further increase, and the surface can still remain super hydrophilic, but the contact angle with oil increases; when the UV light exposure time is too long, the contact angle between the surface and water will also increase, that is, the long time UV irradiation does not cause the surface to become more hydrophilic and lipophilic.
The UV intensity also affects the superhydrophilic properties of the TiO2 surface. If the UV intensity is lower than 20mW/c㎡, the long time UV irradiation can not make the TiO2 surface hydrophilic, which is mainly due to the low UV energy can not excite the TiO2 valence band electron leap, it is also difficult to form the surface lack of junction state.
2, TiO2 crystal surface
TiO2 different single crystal surface superhydrophilic research shows that TiO2 [ll0) surface and (100) surface than (0O1) surface is more susceptible to light excitation so that the surface has superhydrophilic properties. Such as in 40mW / cm2 light, l0mm within (1l0) surface and (001) surface and the contact angle of water up to 0 °. And (001) surface is required 40min, which is mainly due to TiO2 each crystal surface has a different titanium coordination structure. On the (110) crystal face, half of the titanium ions are in a five-ligand structure, with thorium a titanium connected to each other by bridging oxygen, and the other half is in a six-ligand structure. On the (100) crystal plane, all of the titanium ions are in a pentacoordinated lift-off oxygen structure, while the {001) crystal plane is in the same original arrangement as the TiO2 crystal interior, with the thorium free as a tetra-coordinated structure. Compared to other oxygen structures, the bridge oxygen has a higher position on the surface, is energetically more reactive and is more easily released by oxidation to form surface oxygen vacancies. The formation of other oxygen defects will cause greater lattice distortion, and therefore requires a larger external reaction energy
3. Ambient atmosphere
The TiO2(110) single crystals are placed in air and oxygen atmosphere respectively. Under UV irradiation, the contact angle between the single crystal surface and water in air is reduced quickly and a high hydrophilic state can be reached in a relatively short period of time. In the oxygen atmosphere, the contact angle decreases slowly to 35° when saturation is reached. Similarly, TiO2 crystalline surfaces that have been hydrophilized can maintain a hydrophilic state in air for several days, while in an oxygen atmosphere they quickly change to a hydrophobic state. This is mainly due to the presence of oxygen is not conducive to the generation of oxygen vacancies, and will not form more chemisorbed water in the surface, generated by the surface chemisorbed water will also be replaced by oxygen and restore the previous hydrophobic state, which is not conducive to the generation and maintenance of surface hydrophilicity.
4、Heat treatment
The surface hydrophobic TiO2 is heat treated at 150°C and the surface gradually becomes hydrophilic, with a contact angle of O° with water at around 400°C. This means that the TiO2 exhibits good hydrophilicity. That is, the performance of good hydrophilicity TiO2 surface before and after heat treatment of water wettability changes may be due to the formation of the same surface defect state with light irradiation conditions, that is, the formation of a non-chemical proportion of the TiO2 surface structure, which is conducive to the adsorption of water in, that is, the use of heat treatment can also stimulate or restore the surface superhydrophilic.
