**4. Application of TiO2 in environment protection**

### **4.1 Fundamental**

Excitation of TiO2 with UV light with energy greater than the band gap (>3.2 eV) promotes electrons from valence band into the conduction band and generates electron/hole pairs [68, 69]. **Figure 3** illustrates the mechanism of generating reactive radicals from TiO2 under irradiation of UV light. The conduction band electrons e can reduce molecular oxygen to generate (O2 •) superoxide radicals, and valance band holes h<sup>+</sup> is positive enough to generate (OH• ) radicals from H2O or OH on TiO2 surface. OH• radicals have the strongest oxidation potential. Superoxide radicals (O2 •) have moderate oxidation potentials, but their diffusion distances can reach up to hundreds of micrometers [70]. Both radicals are very reactive, and they attack the organic matter present or near the surface of TiO2 to degrade toxic and bio-resistant compounds or species into CO2, H2O, etc. [69, 71].

#### **Figure 3.**

*Mechanism of generating reactive radicals (OH• ) and superoxide (O2 •) from TiO2 under irradiation of UV light.*

The generation of reactive radicals (OH• ) and (O2 •) is affected by the crystalline state, and properties such as surface area and particle size. Although anatase and rutile have the similar band gaps, anatase has shown to have more rapid rate in photo-degradation of organic or bio-resistant compound than rutile [72, 73]. Therefore, nano-structured anatase TiO2 is often used as a catalyst in photo-degradation applications.

## **4.2 Self-clean and antibacterial uses of TiO2**

One application, which is commercially successful, is the nano-structured TiO2 material for self-clean and antibacterial uses [68, 74, 75]. Many nano-structured TiO2 material based products have been used as construction materials [76–82]. Self-clean application is based on the actions of sunlight, rainwater, and photocatalytic properties of TiO2. Under the irradiation of sunlight, adsorbed organic materials like oil can be decomposed by hydroxyl radicals on the surface of TiO2. Because of the hydrophilic property of TiO2 surface, contaminates and dust can be washed away off by rainwater. Tiles containing nano-structured TiO2 have been used to construct photocatalytic surface to decompose bacteria and viruses on the surface or bacteria floating in the air as they come in contact with surface.

Studies have shown that the photocatalytic properties of TiO2 can sometimes be enhanced by doping TiO2 with different elements. For example, TiO2 nano-particles containing Ag+ have been widely used in antibacterial plastics and coatings [83–85]. Fe or Sb-doped TiO2 have been used to make coatings with high antibacterial property [79, 80].

Nano-structured self-clean glass is now an important commercial product. Pilkington Glass has developed the first self-cleaning windows. The window glass is coated with a very thin and transparent TiO2 layer to have the properties of photocatalysis and hydrophilicity on the glass surface. Photocatalysis of TiO2 break down the organic dirt adsorbed onto the window in sunlight, and the decomposed organic species is washed away efficiently by rain or other water in the form of thin layer instead of droplets [86].

### **4.3 Application of TiO2 in water-treatment**

Another important application of nano-structured TiO2 is in the watertreatment, utilizing its photocatalytic properties [87–92]. Research of using nanostructured TiO2 for water-treatment has been very active in recent years. TiO2 has been used in the photocatalytic decomposition of organic dyes in waste water, and organic pollutants such as pesticides, dyes and pharmaceuticals in other contaminated water [93, 94]. The photocatalytic decomposition of organic matters in water are all based on the mechanism of the generation of highly reactive radicals (OH• ) and superoxide ions (O2 •) in TiO2 under UV irradiation, as illustrated in **Figure 3**. TiO2 has been considered to be the best choice to be used as photo-catalysts, as TiO2 is chemically inert, and cheap to manufacture and to apply.

The complete separation and recycling of TiO2 fine particles is important for the practical applications. A number of innovative methods have been developed for this purpose. For example, fixing TiO2 nano-particles on supports such as glass plates, aluminum sheets, and activated carbon are investigated to recycle the catalyst [95], or developing TiO2 catalyst system which can be separated from reaction liquid by applying external magnetic field [96, 97].

Because TiO2 and many other semiconductors have the large band gaps, the application of photocatalytic water treatment using TiO2 is limited by its relatively low efficiency. To improve photocatalytic efficiency of TiO2 for water treatment, as well as other photocatalytic applications, Enormous research has been carried out to extend the photocatalytic response of TiO2 into the visible range [98]. One of the strategies for improving photocatalytic efficiency for water treatment is to modify the band gap of TiO2 by incorporation of other ions into TiO2 structure, through metal and non-metal doping, metal implantation, noble metal loading, and others [99–104].
