**5. Water purification by TiO2 thin films**

The conduction band potentials of TiO2 is near 0 V, which is the reduction potential of H<sup>+</sup> to H2 and the large bandgap energy of the semiconductors makes them highly promising for photocatalytic and photoelectrochemical water splitting.

*Titanium Dioxide Thin Films for Environmental Applications DOI: http://dx.doi.org/10.5772/intechopen.99726*

**Figure 17.** *Schematic of TiO2 photocatalytic mechanism.*

The heterogeneous photocatalysis is widely used for the degradation of water pollutants. The process of photocatalysis can be described as the interaction of incident light with the photocatalyst, causing the formation of electrons and holes (**Figure 17**). The electrons and holes migrate to the surface of the semiconductor without recombining involved the oxydo-reduction reactions. In addition, the holes can also form hydroxyl radicals (·OH) with strong oxidizing properties and the photoexcited electrons can produce superoxide radicals (·O2−) and (·OH). These free radicals and e<sup>−</sup> /h+ pairs are highly reactive to decomposed organic substance, environmental pollutants or harmful microorganisms.

The mentioned reactions can be expressed as follow:

$$\cdot \text{H}^\* + \text{H}\_2\text{O} \rightarrow \cdot \text{OH} + \text{H}^\* \tag{1}$$

$$\text{e}^- + \text{O}\_2 \rightarrow \text{\textquotedblleft O}^{2-} \tag{2}$$

$$\cdot \text{h}^{+} + \text{OH}^{-} \rightarrow \cdot \text{OH} \tag{3}$$

$$\cdot \text{ Dye} + \cdot \text{OH} + \cdot \text{O}^{2-} \rightarrow \text{CO}\_2 + \text{H}\_2\text{O} \tag{4}$$

During the last two decades, researchers have widely studied the purification of water by TiO2 thin films for their excellent optical and catalytic properties. TiO2 thin films can act as oxidation and reduction catalysts for organic and inorganic contaminants.

Q. Zhu et al. [19] reported that the synthesis P/Ag/Ag2O/Ag3PO4/TiO2 composite films have degraded 99.9% of rhodamine B (Rh B) after 60 min under simulated solar. J. Singh et al. [20] demonstrated that the synthesized nanocomposite thin films Ag-TiO2 exhibit a higher photocatalytic activity for the degradation of methylene blue (MB) in water under sun light. S. Yan et al. [21] have determined the photocatalytic performance of MS@TiO2@PPy through the degradation of soluble organic dye rhodamine B (Rh B) in water under simulated solar irradiation illumination. After being irradiated by simulated sunlight, the degradation efficiency of RhB solution reached up to 90% after 90 min. D. Tekin et al. [22] have reported the efficient application of the ZnO/TiO2 thin film prepared by the sol–gel method to degrade Orange G dye which show a good performance. K.B. Chaudhari et al. [23] have fabricated TiO2 film onto glass substrate using a simplified chemical bath

deposition method at moderately low temperature. The photocatalytic activity of the TiO2 thin film was tested by photodegradation of congo red dye solution which proved enhanced photocatalytic activity under visible light.
