**2.1. Experimental**

TiO2 thin films were prepared by sol-gel process [2, 5] using titanium isopropoxide (Aldrich, 98%) as the precursor of titania mixed with isopropyl alcohol and hydrochloric acid in stoichiometric amounts. The precursor solution was kept under agitation at room temperature for 1 h and rested until the viscosity reaches the best value condition, between 2 and 5 cP. The films were prepared using solutions with 2 < pH < 4 and atmosphere relative humidity <40%, since they are opaque and not adherent for other pH and relative humidity values. The films were deposited onto clean substrates (borosilicate glass, steel, silicon, and magnets) at room conditions (25°C, relative air humidity lower than 30%), using a dip-coating equipment with withdrawal speed between 0.2 and 1.5 mm/s. The substrates were washed with standard cleaning method before dipping. After each dip-coating process, the wet films were dried in air for 30 min and thermally treated at temperatures between 100 and 500°C for a range of time (between 10 and 60 min) to convert them into porous or densified oxide films. Depending on the thermal treatment temperature, the films can be amorphous or nanocrystalline. Some samples were submitted at UV-C light (lamp Girardi RSE20B, 254 nm—15 W) to crystallize without increasing the temperature. Crystalline structures were investigated by an X-ray diffraction (incidence angle of 5°) using a diffractometer Rigaku (Geigerflex model 3034). The samples were analyzed by atomic force microscopy (AFM) in an Asylum Research, model MFP-3D-SA, to observe the topography and possible coating defects, such as cracks and peeling. Morphological characterization was evaluated by transmission electron microscopy (FEI TECNAI G2 20 at acceleration tension of 200 kV). Electron diffraction was also used to determine the structure of the crystalline phases. The films were pulled from glass substrates and mounted onto 200 mesh copper grids coated with holey carbon films for examination. The morphology and composition were evaluated by a scanning electron microscope (SEM) FEI Quanta 200 FEG with an energy-dispersive spectrometer (EDS). The transparency and thickness of the films deposited on glasses were verified by the optical transmission spectra measured with an ultraviolet and visible spectrometer (U3010, Hitachi).

from a glass substrate. The thickness of the films deposited in glass and dried in air can range from 40 to 800 nm for each coating, depending on the withdrawal speed and viscosity. After heating, the film thickness decreases due to the densification process, reaching values between 20 and 300 nm each coating. When the number of coating increased, the thickness can reach

thin films removed from a glass substrate and (b) thickness and refractive index of TiO<sup>2</sup>

Pure and Nanocomposite Thin Films Based on TiO2 Prepared by Sol-Gel Process…

http://dx.doi.org/10.5772/intechopen.74335

After drying, the films are porous when treated at low temperatures, and the density increases as a function of heating temperature and time. The porosity of the films leads to a variation in the refraction index that can change from 1.9 to 2.3 (λ = 550 nm) for porosities between 20 and 5%, respectively. **Figure 2b** shows an example of the variation of thickness and refractive

deposited in substrates that cannot be thermally treated, such as polymers and cotton, the densification and crystallization can be made by UV light treatment. **Figure 3** shows images

The film formed after drying at room temperature is amorphous and contains organic contaminants in the network. With increasing in temperature of thermal treatment, the film struc-

degrees also depending on the type and time of drying, used dopant, and particle size, among

in two temperatures, generating an amorphous material at 100°C and a nanocrystalline material at 400°C. **Figure 5** shows SEM and TEM images of the film and the respective electron

ture changes to anatase phase around ~300°C and to rutile phase above ~600°C.

According to the literature, the values of the phase transition of TiO<sup>2</sup>

film. When the TiO<sup>2</sup>

films are

thin films in

85

can change in some

films deposited in glass substrate

800 nm after calcination without cracks.

function of thermal treatment temperature.

of TiO<sup>2</sup>

**Figure 2.** (a) TiO2

index in the function of thermal treatment temperature of TiO<sup>2</sup>

films heated at 100 and 400°C for 10 min.

others factors. **Figure 4** shows typical diffractograms of TiO<sup>2</sup>

diffraction that confirm its anatase phase.
