**2. Synthesis process of NS TiO2**

Various synthesis methods such as sol-gel, hydrothermal and solvothermal methods, vapor deposition, electrochemical deposition, oxidation, and sonochemical and micro-waves methods are used to obtain high-quality TiO2 nanostructures [12, 15]. In this section, we will analyze the most used methods for the preparation of TiO2 nanostructures.

### **2.1 Chemical and physical methods**

### *2.1.1 Sol: gel process*

Sol-gel is a versatile method used for the synthesis of TiO2 nanostructures of different morphologies such as sheets, tubes, particles, wires, rods, mesoporous, and aerogels [30–32]. Mehrotra and Singh [33] suggested different steps and conditions that can control the morphology of the final products in the sol-gel process (**Figure 1**).

The sol-gel method can use two ways of synthesis: the inorganic or colloidal route in which the precursors used are metal salts such as chlorides, nitrates, and oxychlorides in an aqueous solution. The Metallo-organic or polymeric route: obtained from metal

*Nanostructured Titanium Dioxide (NS-TiO2) DOI: http://dx.doi.org/10.5772/intechopen.111648*

**Figure 1.** *Sol-gel process steps for the synthesis of TiO2 nanostructures [30, 32, 33].*

alkoxides in organic solutions. The polymerization reaction to obtain titanium dioxide takes place in two steps, namely hydrolysis, and condensation.
