**1.1. Sol-gel process**

The sol-gel process [3] that leads to the formation of TiO<sup>2</sup> films is based on mechanisms of hydrolysis and polycondensation of titanium alkoxides mixed with alcohol and catalytic agents. There are various kinds of Ti alkoxides such as titanium isopropoxide (Ti(O*<sup>i</sup>* Pr)<sup>4</sup> ) and titanium ethoxide (Ti4 (OEt)16), among others, that need to be used preferentially with their 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 produce Ti─O─Ti and by-products (H<sup>2</sup> O and ROH), leading to formation of the gel, according to the equation below:

**1. Introduction**

Titanium dioxide (TiO2

properties make TiO<sup>2</sup>

As semiconductor, TiO<sup>2</sup>

**1.1. Sol-gel process**

traditionally are formed by TiO<sup>2</sup>

The introduction of dopants in the TiO<sup>2</sup>

Kingdom, in the mineral ilmenite (FeTiO3

82 Titanium Dioxide - Material for a Sustainable Environment

is frequently used in photocatalytic applications.

Heinrich Klaproth in the form of TiO<sup>2</sup>

) is a multifunctional, semiconductor and polymorphic material, which

an excellent material for use in photocatalysis, antimicrobial surfaces, self-

can be studied in terms of the energy band theory, whose bandgap

nanoparticles, the thickness also contributes to the modu-

is used

distribution. It

structures to

Pr)<sup>4</sup>

) and

, Ag, and Nb, among oth-

films is based on mechanisms of

4s<sup>2</sup>

). In 1795, it was isolated by the German chemist

rutile phase. Titanium dioxide can be found in three

is commercialized in rutile or anatase phases, both in tetragonal crystal structures. TiO<sup>2</sup>

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

cleaning and hydrophobic surfaces, photovoltaic cells, gas sensor, photochromic devices, etc. [1].

was discovered in 1791 by the mineralogist William Gregor, in the region of Cornwall, United

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

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

lation of the bandgap values. Several studies are made aiming the best quality of the films

ers, 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.

hydrolysis and polycondensation of titanium alkoxides mixed with alcohol and catalytic

agents. There are various kinds of Ti alkoxides such as titanium isopropoxide (Ti(O*<sup>i</sup>*

thin film structure such as SiO<sup>2</sup>

and the decrease in the bandgap energy by introduction of dopants in the TiO<sup>2</sup>

improve the photocatalytic propriety in the visible region of the light [1, 2].

The sol-gel process [3] that leads to the formation of TiO<sup>2</sup>

Titanium is the second transition metal on the periodic table and has Ar-3d<sup>2</sup>

$$\text{Ti(OR)}\_{\text{n}} \text{+ nH}\_{\text{2}}\text{O} \rightarrow \text{Ti(OH)}\_{\text{n}} \text{+ nROH} \tag{1}$$

$$\text{Ti} \,\text{(OH)}\_{\text{n}} \rightarrow \text{Ti} \,\text{(O}\_{\text{n/2}}\text{)} + \text{n/2} \,\text{H}\_{2}\text{O} \tag{2}$$

$$\text{TiOR} + \text{TiOH} \rightarrow \text{TiO}\_2 + \text{ROH} \tag{3}$$

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 to the introduction of other metals in the gel network.

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.

withdrawal speed, and solution viscosity. After the deposition, the gel film is formed by a solid structure impregnated with the solvent, and a drying process can be used to convert the wet gel in a dry porous film. Denser film can be tailored by different temperatures of thermal treatment, leading to films with different specific surface areas and porosities.

The advantage of the dip-coating process is the ease of deposition in substrates of any size and shape, facilitating the industrial process.

#### **2. TiO2 thin films**
