**2.6 Mechanical alloying**

Mechanical alloying (MA) is a low cost and simple route for preparing nanostructured materials among them TiO2 NPs. The schematic illustration of the MA is presented in **Figure 10**.

**Figure 10.** *Schematic illustration of the MA process.*

Yao et al. [52] prepared nanostructured TiO2 coating by mechanical alloying process. The results showed that the obtained material exhibited an excellent photocatalytic activity. Vilchez et al. [53] synthesized TiO2 NPs by MA during 5 min. The TEM images of TiO2 NPs are presented in **Figure 11**. The obtained material, with size in the range of 2–4 nm and specific surface area of 298 m<sup>2</sup> g−1, exhibiting a good photocatalytic activity.

Kim et al. [54] prepared TiO2 NPs by MA and heat treatment. The mean crystallite size was less than 6 nm. The UV-Visible spectrum showed that the obtained TiO2 NPs had an elevated wavelength rage (in the range of 650 and 700 nm) compared to Ni doped TiO2 (480–500 nm) and rutile (380–400 nm). In addition, PL spectrum exhibited a new emission peak confirming the decrease in the band gap. Furthermore, Fe (III) doped TiO2 NPs have been synthesized via MA [55]. The final product showed excellent selectivity, stability, sensitivity, and fast response. Additionally, Eadi et al. [56] developed new Fe doped TiO2 NPs by MA from FeCl3 and TiO2 powder. The results showed that the mean particle size was about 28 nm and the prepared material could be applied for gas sensing and photocatalytic degradation. Carniero et al. [57] investigated the effect of process parameters on the structural, optical, magnetic,

**Figure 11.** *TEM images of TiO2 NPs prepared by MA [53].*

and photocatalytic properties of iron doped TiO2 NPs prepared by MA. The results showed that the incorporation of iron in the TiO2 NPs has improved their photocatalytic activity.
