**2.1 Polyol method**

 Recently, the polyol method has been found to be a very powerful route for the fabrication of nano-oxide and chalcogenide materials [ 17 , 18 ]. In addition, polyol method is a simple and low cost route for fabricating metal oxide NPs.

 Thus, a number of studies have been reported on the synthesis of TiO 2 NPs by polyol method. For example, Shah and Rather [ 19 ] prepared TiO 2 NPs by polyol method using titanium (IV) butoxide, ethylene glycol, and acetone. They concluded that the mean crystallite size increased from 9.3 to 66.9 nm when calcination temperature rises from 300 to 1000°C. In addition, the obtained products showed greater stability (zeta potential of −30.8 to −37.5 mV) in aqueous solutions. Also, Sasikala et al. [ 20 ] prepared a dispersed SnO 2 on TiO 2 NPs via polyol method at calcination temperature of 500°C. They concluded that the TiO 2 containing SnO 2 showed improved photocatalytic activity compared to pure TiO 2 because of improved charge separation. Ultrafine anatase TiO 2 nanocrystals, with size of 2–5 nm, have been prepared through polyol process [ 21 ]. **Figure 1** shows the TEM images of TiO 2 nanocrystals. The samples exhibited excellent photocatalytic activities.

 Furthermore, polyol method was used to synthesize TiO 2 NPs by using different mole ratios of titanium tetrachloride and polyvinylpyrrolidone [ 22 ]. The

 **Figure 1.**  *TEM images of TiO 2 NPs prepared by polyol method [ 21 ].* 

photocatalytic performance of the prepared TiO2 NPs attained 97.83% with a power conversion efficiency of 4.6%. Kang and co-workers [23] synthesized TiO2 NPs, with average size of 25 nm, via polyol from titanium isopropoxide by refluxing at 270°C during 12 h. After that, the sample was heated at 600°C for 3 h. The final product showed an excellent electrochemical performance.
