**1. Introduction**

Titanium dioxide (TiO2 ) is a multifunctional, semiconductor and polymorphic material, which is commercialized in rutile or anatase phases, both in tetragonal crystal structures. TiO<sup>2</sup> is used in industry since 1918 as pigment in paints, paper, plastic, drugs, cosmetics, etc. In the last years, with the beginning of nanotechnology, powder and films of titanium dioxide have been widely studied due to its new properties obtained by decreasing the particles size. The wide range of application is due to its electronic and structural properties, such as high transmittance in the visible, high refractive index (n = 2.6), high photocatalytic activity, and chemical stability. These properties make TiO<sup>2</sup> an excellent material for use in photocatalysis, antimicrobial surfaces, selfcleaning and hydrophobic surfaces, photovoltaic cells, gas sensor, photochromic devices, etc. [1].

titanium ethoxide (Ti4

the equation below:

duce Ti─O─Ti and by-products (H<sup>2</sup>

Ti (OR)

Ti (OH)

to the introduction of other metals in the gel network.

(OEt)16), among others, that need to be used preferentially with their

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

O and ROH), leading to formation of the gel, according to

<sup>n</sup> → Ti(On/2) + n/2 H2 O (2)

<sup>n</sup> + nROH (1)

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

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correspondent alcohol. The precursor solution, also called sol, is a colloidal suspension of Ti surrounded by ligands, with physical-chemical properties adequate to the formation of a film. After a deposition, which can be by dip-coating, spin-coating, and spray-coating processes, the film is formed by a wet gel that became a dry gel after drying process. The hydrolysis of the alkoxide group to form Ti─OH occurs due to nucleophilic substitution of O─R groups (alkyl group) by hydroxyl groups (─OH) and the condensation of the group Ti─OH, to pro-

<sup>n</sup> + nH2 O → Ti (OH)

TiOR + TiOH → TiO2 + ROH (3)

This mechanism is relatively complex because the reactions occur simultaneously during the process of deposition. In this proposed mechanism, the alkoxide precursor passes by the sequences, oligomer, polymer, and colloid, and it finishes as an amorphous porous solid structure. Thermal treatments are used for the preparation of nanocrystalline thin films. With the use of doping salts in the precursor solutions, the mechanism becomes more complex due

The dip-coating technique [4] consists into dip a substrate in the sol and removes it at constant speed (**Figure 1**), resulting in an M─O─M oxide network that forms a wet gel film. The network structure, the morphology, and the thickness of the film depend on the contributions of the reactions of hydrolysis and condensation that must occur in approximately the same velocity of substrate withdrawal. Otherwise, the solution may run down the substrate. These properties may be controlled varying the experimental conditions: type of organic binder, the molecular structure of the precursor, water/alkoxide ratio, type of catalyst and solvent,

**Figure 1.** (a) Dip-coating equipment and (b) substrate withdrawal of the solution for film formation.

Titanium is the second transition metal on the periodic table and has Ar-3d<sup>2</sup> 4s<sup>2</sup> distribution. It was discovered in 1791 by the mineralogist William Gregor, in the region of Cornwall, United Kingdom, in the mineral ilmenite (FeTiO3 ). In 1795, it was isolated by the German chemist Heinrich Klaproth in the form of TiO<sup>2</sup> rutile phase. Titanium dioxide can be found in three different crystalline phases: anatase, brookite, and rutile. By thermal treatment, it is possible to convert the anatase and brookite phases in rutile, which is thermodynamically stable at high temperatures. The anatase phase is more reactive, mainly in nanometric dimension, and is frequently used in photocatalytic applications.

As semiconductor, TiO<sup>2</sup> can be studied in terms of the energy band theory, whose bandgap energy (3.2–3.6 eV) can be supplied by photons with energy in the near ultraviolet range and whose separation between valence and conduction bands is intrinsically linked with its optical and electronic properties. These bandgap values depend on the particle size, phase, and used dopant, making possible the modulation of these values. In the case of thin films, which traditionally are formed by TiO<sup>2</sup> nanoparticles, the thickness also contributes to the modulation of the bandgap values. Several studies are made aiming the best quality of the films and the decrease in the bandgap energy by introduction of dopants in the TiO<sup>2</sup> structures to improve the photocatalytic propriety in the visible region of the light [1, 2].

The introduction of dopants in the TiO<sup>2</sup> thin film structure such as SiO<sup>2</sup> , Ag, and Nb, among others, changes its properties expanding the range of possible applications. The methods of preparation also influence significantly its morphology, structure, and texture, modifying its properties. Several methods can be used to obtain thin films such as chemical vapor deposition, sputtering, spray pyrolysis, and sol-gel process. The sol-gel process [3] allows the preparation of thin films with high purity, thermal and mechanical resistance, chemical durability and the control of morphology, composition, thickness, and porosity. Thin film depositions using the sol-gel process can be realized by dip-coating, spin-coating, or spray-coating techniques. These techniques are economically feasible and can be applied to substrates with large surfaces and different forms.
