**7. Doped/hybrid TiO2 photocatalytic mechanism**

The basic mechanism of TiO2 photocatalyst is described based on **Figure 1**, which inititated by the absorption of the photon *hv*1 with energy equal to the band gap of anatase TiO2 (3.20 eV). Electron–hole pair is produced on the surface of titania as schematized. As known, the electron from the CB is promoted and produced positive hole in VB. The excited state electrons and holes get trap in the metastable states as well as dissipate energy as heat. Besides, they also can react with the electron donor or acceptor adsorbed on its surface. Therefore, the • OH is produced with high oxidation potential which plays an important role in photocatalysis [1, 17].

The interstitial metal doped TiO2 on the other hand beneficially produced the new energy level in the band gap of TiO2 by the dispersion of metal nanoparticles in titania matrix. As shown in **Figure 1**, electron from the CB can be excited from the defect state by photon energy equals to *hv*2. The improvement of the electrons trapping to inhibit the electron–hole recombination during its photoactivation as well described the additional advantageous of the metal ion doping. Thus, decreasing in the charge-carrier recombination resulted in enhanced photoactivity of the photocatalyst.

*TiO2: A Semiconductor Photocatalyst DOI: http://dx.doi.org/10.5772/intechopen.99256*

#### **Figure 1.**

*Mechanism of TiO2 photocatalysts: hv1: Pure TiO2; hv2: Metal doped TiO2 and hv3: Non-metal doped TiO2 [7].*

The modification of TiO2 with the non-metal ion doping provides four main opinions regarding the changes in the nature mechanism of photocatalyst as described below [7]:


All these obtained modification in the mechanism will contribute for the better and sustainable treatment of the organic pollutants.

#### **8. Conclusions**

Though various studies have been carried out to find an ideal semiconductor photocatalyst, TiO2 however remains as a benchmark and active photocatalyst

among them and was proved in both laboratory and pilot studies. Other oxides such as ZrO2, SnO2, WO2 and MoO3 do not have the similar application prospects as TiO2 due to the fact that these oxides are much less active, chemically and biologically instable Several commercial TiO2 photocatalysts are produced worldwide. Among them Degussa P25®, an amorphous TiO2, emerged as the best photocatalyst due to its better utilization of the UV light. It has a phase ratio of 25:75 (rutile:anatase). It is also considered as standard photocatalyst for environmental applications. The wider band gap, greater recombination of electron–hole pair and low interfacial charger carrier transfer, limit the visible light or sustainable solar energy utilization of TiO2 photocatalyst. The limitations were successfully achieved by synthesizing a new and modified TiO2-based composite nanophotocatalysts through a series of simple preparation processes. The nano-size morphology of the composite photocatalysts well created the quantum effect that improved the photocatalytic properties.
