**4.6 Hydrothermal method**

The hydrothermal technique (**Figure 11**) is a convenient wet way to produce well crystalline materials, with suitably tuned size and shape of particles at temperatures between 180 and 300°C. J. Ben Naceur et al. [13] have prepared TiO2 nanorods (NRAs) thin film by mixing of hydrochloric acid (HCl), distilled water and Titanium-isopropoxide [Ti(OCH(CH3)2)]. The mixture was magnetically agitated then transferred to Teflon-coated stainless-steel autoclave. The substrate was placed inside a furnace and kept at 180°C for 5 hours. The hydrothermal method has same advantages such as easy to obtain nanotube morphology, variation in the synthesis method can be implemented to enhance the properties of TiO2 nanotubes, and it is a feasible method for different applications.

### **4.7 Spin-spraying method**

The spin-spraying technique (**Figure 12**) consists of pulverization of an oxidizing and treatment solutions respectively, onto substrates mounted on a rotating table at a constant temperature. M. O. Abou-Helal et al. [14] have deposited TiO2 on glass substrates. The solution was prepared by dissolve Titanium (IV) isobutoxide [Ti((CH3)2CHCH2O)4], in a mixture of HNO3 and methanol solution. The deposition parameters used are 0.1 M of prepared solution, with rate of 1.0 ml/min, at 450–600°C for 5–30 min spray time.

### **4.8 Spin-coating method**

The spin-coating technique (**Figure 13**) is used to deposit uniform thin films onto flat substrates. Usually a small amount of coating material is applied on the

**Figure 11.** *Schematic of the hydrothermal method.*

**Figure 12.** *Schematic of the spin-spraying method.*

**Figure 13.** *Schematic of the spin-coating method.*

center of the substrate, which is either spinning at low speed or not spinning at all. The substrate is then rotated at speed up to 10,000 rpm to spread the coating material by centrifugal force. F. Joudi et al. [15] have prepared TiO2 thin film by mixing 2 g of P25 in 10 ml of solution of ethanol and acetylacetone under stiring for 10 min. Then the solution was deposited on substrates by spin coating technique. To remove organic solvents from the samples, the films were dried at 150°C and annealed at 450°C for 2 h.
