*2.2.2. Synthesis of metal doped TiO2*

What is the motivation for metal doping in TiO<sup>2</sup> ? As explained earlier, TiO<sup>2</sup> is wide band gap semiconductor and requires UV irradiation for its operation as a photo catalyst. The contribution of UV light in the solar radiation is less than 5%. Therefore, it is required to tune the band gap of TiO<sup>2</sup> to visible range so that no extra source of radiation energy (other than solar light) is required. Also, pristine TiO<sup>2</sup> suffers from higher recombination rate of charge carriers (electron and hole) resulting in less photocatalytic efficiency. Metals [10, 47–52] and nonmetals [53–58] are well known for their ability to reduce the band gap of TiO<sup>2</sup> by generating energy states between valence band and conduction band. These energy states serve as charge carrier trapping center and therefore reduce the electron hole recombination rate. The reduction in electron hole recombination rate results in remarkable improvement in photocatalytic performance. In addition, noble metals (Ag, Au and Pt), transition metals [59–61] and nonmetals doping in TiO<sup>2</sup> reduces the band gap of TiO<sup>2</sup> to visible range. Moreover, many doped TiO<sup>2</sup> nanoparticles are found to have small size as compared to pristine TiO<sup>2</sup> , which improves the surface area and consequently boosts up the photocatalytic performance. However, favorable change in properties of TiO<sup>2</sup> by doping is largely affected by synthesis methods.

In current chapter, synthesis and photocatalytic properties of TiO<sup>2</sup> nanoparticles doped with one transition metal, Zirconium (Zr) and one other alkali metal, sodium (Na) are discussed. Both these metals have higher ionic radii (~0.79 Å for Zr and ~1.02 Å for Na) as compared to titanium (~0.68 Å for Ti). Large ionic radii and low valence ionic metallic dopant in host Ti4+ always results in strain in the crystal structure that favors the formation of oxygen vacancies [62]. These oxygen vacancies are prone to trap electrons and suppress grain growth resulting in reduced charge recombination rate and small crystallite size respectively. There are many reports which claimed contradictory reports on photocatalytic activity of TiO<sup>2</sup> after doping of same dopant under different synthesis routes. Bessekhouad et al. [63] compared the photo catalytic efficiency of Na doped TiO<sup>2</sup> nanopowder prepared via two methods*:* sol-gel route and impregnation technology. They found that photo catalytic efficiency of nanopowder prepared by impregnation technology is higher than nanopowder prepared by aqueous sol-gel route. In addition, Na doping in TiO<sup>2</sup> matrix via aqueous sol-gel route decreases the photocatalytic efficiency due to migration of Na at TiO<sup>2</sup> surface instead of entering in the lattice. On the other hand, Yang et al. [64] showed higher photocatalytic activity of Na doped TiO<sup>2</sup> nanopowder prepared via solvothermal method. XRD analysis by Yang et al. [64], Xie et al. [65] as well as Bessekhouad et al. [63] could not confirm the doping of large sized Na into TiO<sup>2</sup> lattice and therefore hinted at the tendency of large sized Na + ions to migrate to TiO<sup>2</sup> surface. Therefore, it is required to study the effect of synthesis method on doping mechanism and photocatalytic activity of large sized metal dopants in TiO<sup>2</sup> matrix.

In [2, 10] Zr and Na has been doped in TiO<sup>2</sup> matrix individually by solvent controlled nonaqueous sol-gel route. Easily dissolvable zirconium oxy-nitrate and sodium nitrate are used as precursor of Zr and Na respectively. To achieve nominal dopants concentration, calculated amount of precursor is added to solvent prior to addition of Ti precursor. After complete dissolution of dopant precursor, Ti precursor is added to reaction solution and similar steps are followed as in preparation of doped TiO<sup>2</sup> nanopowder.
