**5. Noble metal deposited TiO2 photocatalyst**

Apart from these metals, non-metals and graphene oxide as dopant/hybrid, few reseachers have also utilized the noble metals like platinum (Pt), gold (Au) and silver (Ag) as a potential source of dopant. The expensive Pt deposition on both rutile and anatase TiO2 was performed by Scalfani et al., 1998, Kim et al., 2002, Sun et al., 2003, Bosc and coworkers, 2006 and Hidalgo and coworkers, 2007. All their study showed that Pt had beneficial effects for the photocatalytic oxidation. The Pt as a dopant/hybrid has increased the electron mobility rate of TiO2. This mobility has contributed for the electron transfer to the adsorbed oxygen (O<sup>−</sup> ads) especially in rutile TiO2. A negligible such effect was observed in the anatase phase [68–72].

Owing to the versatile potential of the Au, it was also chosen as a dopant by the reseachers [73, 74]. The study by reseachers showed that Au can behaves like a semiconductor rather than metallic. The size of Au plays a major role in the interfacial electron transfer to adsorbed oxygen (O<sup>−</sup> ads). The Fermi level of the Au particle will

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

be lowered than that of adsorbed oxygen (O<sup>−</sup> ads) in TiO2 and this depends on the particle size of Au. The Au also contributed for the hindering effect of the surface recombination. Thus, they exhibited an improvised visible light photoactivity.

Finally, the influence of Ag as a dopant/hybrid was studied by Szabo et al., 2003 and Christopher et al., 2010. Their study showed that Ag as a dopant contributed for the uniform morphology of the TiO2 and increased the probability function of excited oxygen atoms via electron transfer from adsorbed oxygen (O− ads) produced from O2 2− to the hole. The Ag also further exhibited a unique Surface Plasmon Resonance (SPR) property. Thus, exhibited SPR lowered the electron–hole recombination rate. All these potentially improvised the photoactivity of TiO2 with a better degradation of the pollutants either under natural or artificial visible light illumination [30, 75].
