**5. Conclusion**

**4.2. Degradation results and mechanism**

108 Titanium Dioxide - Material for a Sustainable Environment

MB dye by WC, P25, pristine TiO<sup>2</sup>

photocatalytic activity of TiO<sup>2</sup>

The photocatalytic activity of prepared Na doped TiO<sup>2</sup>

reduced rate of electron-hole recombination. Similarly, Ti0.9Zr0.1O<sup>2</sup>

ature [56, 72]. The reactions occurring at the surface of semiconductor TiO<sup>2</sup>

recombination, band gap was also reduced due to Zr doping.

The photocatalytic degradation of dye at the surface of TiO<sup>2</sup>

gap of semiconductor. In case of pristine TiO<sup>2</sup>

e<sup>−</sup> + O<sup>2</sup> = O<sup>2</sup>

O<sup>2</sup>

**Figure 5**. Clearly, the degradation percentage of MB dye is highest for Ti0.92Na0.08O<sup>2</sup>

and Ti0.92Na0.08O<sup>2</sup>

superior to commercially available Degussa P25 catalyst. The k values for degradation of

43.24 × 10−3 min−1*,* respectively. TOC results are in accordance as well. This enhancement in

catalytic activity [10] for which, in addition to reduction in crystallite size and electron-hole

are depicted in the Eqs. 4–8. The electrons are excited to conduction band while holes are formed in conduction band after absorption of photons having energy greater than or equal to energy

holes in the valence band reacts with water molecules and form ROS OH\* radicals. These ROS

TiO<sup>2</sup> + hυ = e<sup>−</sup> + h+ (4)

H2 O + h+ = OH<sup>∗</sup> + H+ (6)

MB + OH<sup>∗</sup> = Degradation products (8)

**Figure 5.** Photocatalytic degradation (A) and mineralization (B) of MB dye under 60 minute of UV irradiation.

of them react with adsorbed oxygen forming reactive oxygen active specie (ROS) O<sup>2</sup>

actually reacts with dye molecules and degrade them into simple hydrocarbons H2

̅

photo catalyst can be compared from

is well explained in the existing liter-

, most of these electrons recombine with holes; few

̅ (5)

+ 2H+ + e<sup>−</sup> = 2 OH<sup>∗</sup> (7)

shows enhanced photo-

under UV irradiation

¯. Similarly

.

O and CO<sup>2</sup>

are 0.86 × 10−3, 30.52 × 10−3, 3.02 × 10−3 and

with Na doping is attributed to its smaller crystallite size and

, even

In this chapter the importance of recently reported nonaqueous solvent controlled sol-gel route for the synthesis of metal doped TiO<sup>2</sup> with improved photocatalytic properties discussed. Pristine as well as Zr and Na doped TiO<sup>2</sup> nanoparticles have been prepared by this modified sol-gel route and their photocatalytic activity evaluated. Successful doping of these large metal ions in TiO<sup>2</sup> lattice using this synthesis route was confirmed by shifts in XRD peak positions and increase in d spacing observed from HRTEM images.

The photocatalytic activity of metal doped TiO<sup>2</sup> nanopowder is found to be much higher than pristine TiO<sup>2</sup> , and even superior to commercially available Degussa P25 TiO<sup>2</sup> photo catalyst. This is attributed to large surface area due to small grain size and reduced electron hole recombination due to formation of oxygen vacancies in metal doped TiO<sup>2</sup> . The reduction in electron-hole recombination increases the availability of electrons and holes which reacts with adsorbed oxygen and water molecules forming large number of reactive oxygen active species leading to enhanced photocatalytic activity.

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In past years, only few MONPs have been prepared by this nonaqueous, solvent controlled, sol-gel route. This method has great potential to synthesize functional nanoparticles of desired composition, size and surface properties essential for different applications.
