Self cleaning coatings
The technology of self cleaning coatings has developed rapidly in recent years. The use of self cleaning coatings is attractive as these are labour saving and effectively improve the appearance. The field of self cleaning coating is divided into two categories: (a) hydrophobic and (b) hydrophilic. Both these types of coatings clean themselves through the action of water, the former by rolling droplets and the latter by sheeting water that carries away dirt. Hydrophilic coatings, however, have an additional property that they can chemically breakdown absorbed dirt in sunlight and the process is known as ‘photocatalysis’. Semiconductor nanoparticles as TiO2, ZnO α-Fe2O3, ZrO2, CdS, WO3 and SnO2 are successfully utilized in photocatalytic applications.
Why Titanium dioxide?
Titanium dioxide (TiO2) belongs to the family of transition metal oxides. In recent years, crystalline Titania or Titanium dioxide (TiO2) has received much attention due to their interesting properties as photocatalyst. Among many semiconductor photocatalysts, there is a general consensus among researchers that TiO2 is more superior because of its high activity, large stability to light illumination, low price, and nontoxicity.
Mechanism of Self cleaning coating
When a photocatalytic surface is illuminated by light with energy equal to or larger than the bandgap energy ΔEbg (bandgap energy of TiO2 is 3.03), it excites the electrons in the valance band to the conduction band, resulting in the formation of a positive hole (p+) in the valance band and an electron (e−) in the conduction band. The positive hole oxidizes either pollutants directly or water to produce HO˙ radicals, whereas the electron in the conduction band reduces oxygen adsorbed to photocatalyst (TiO2).