Hydrophilicity of TiO2 (titanium oxide) thinfilm

 

The photochemical properties of titanium dioxide make it can be used in many fields, such as air, water, and fluid purification. Photocatalysts doped with carbon or other heteroatoms can also be used in sealed spaces or regions with scattering light sources. When used in coatings on buildings, pedestrian slabs, concrete walls, or roof tiles, they can significantly increase the decomposition of air pollutants such as nitrogen oxides, aromatic hydrocarbons, and aldehydes. Besides, it is widely used in the production of sunscreen, non-toxic, harmless to the human body. Ultrafine titanium dioxide has excellent UV shielding and transparency. It is widely used in cosmetics, wood protection, food packaging plastics, durable household film, man-made fiber, and natural fiber, transparent coating. Because of its special optical effect in the metal flash coating, it has been paid attention to and applied in high-grade car paint.

 

 

 

Principle of super hydrophilicity of TiO2 surface

In general, the contact angle between the TiO2 coating surface and water is large. However, after UV irradiation, the contact angle of water is reduced to 5 degrees or even 0 degrees (i.e. water droplets completely diffuse on the surface of TiO 2). It shows a very strong hydrophilicity. After the light was stopped, the super hydrophilicity of the surface could be maintained for several hours to about one week, and then slowly returned to the hydrophobic state before irradiation. The surface is super hydrophilic by ultraviolet irradiation. The surface can be kept super hydrophilic by intermittent ultraviolet irradiation.

 

At first, it was thought that the super hydrophilicity of the TiO2 surface is due to the photocatalytic decomposition reaction of adsorbing organic molecules on the surface. The chemically adsorbed water on the surface of TiO2 is hydrophilic. After adsorbing organic matters in the air, the surface becomes hydrophobic. Under UV irradiation, the surface forms strong oxidizing active hydroxyl groups, and the hydrophobic organic compounds are oxidized and decomposed by the active hydroxyl groups through the photocatalytic decomposition reaction The surface shows a hydrophilic state; when the light stops, the organics will slowly adsorb on the surface of TiO2 and return to the hydrophobic state. However, further studies show that the super hydrophilicity of the TiO2 surface is different from the photocatalytic oxidative decomposition characteristics of TiO2, but it is another reaction induced by the surface of TiO2 itself. The reasons are as follows: 1) the degree of super-hydrophilicity of the TiO2 surface has nothing to do with the photodegradation efficiency of organic compounds. Super-hydrophilic properties have been observed on some single crystal or polycrystalline surfaces of TiO2 with no or very low photocatalytic activity; 2) some metal ions (such as copper) doping can improve the photocatalytic oxidation reaction of TiO2, but decrease the super-hydrophilic property of TiO2 surface; ③ The results show that the surface of TiO2 is porous and the reaction area is as large as possible; ④ Under normal conditions, the contact angle between oily substances such as ethylene glycol hexadecane, glyceryl tri-acylate, and TiO2 surface is large. However, after UV irradiation, these liquids will completely infiltrate the glass coating surface. That is to say, after UV irradiation, the TiO2 surface has water oil amphiphilic activity.