**1. Introduction**

Titanium dioxide, commonly known as titanium white, is in normal conditions a colourless crystalline solid. It is non-toxic, non-hygroscopic, inflammable and non-volatile. It shows high chemical stability: it does not dissolve in water, organic solvents, acids (except for concentrated hydrofluoric acid and sulphuric acid) or alkalis. It is amphoteric, but more acidic

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

than basic. Titanium dioxide is thermally stable: it loses oxygen only at a temperature of a few hundred degrees Celsius and under the influence of reducing agents (carbon, magnesium, hydrogen and halogens). Its melting point is 1825°C, while its boiling point is close to 2500°C. Above 400°C, a reversible change in colour to yellow takes place as a result of thermal expansion of the crystalline lattice. Above 1000°C, the oxide forms of titanium are formed, characterised by a lower content of oxygen than in TiO<sup>2</sup> , an undesirable colour change takes place and the electrical conductivity changes. Titanium dioxide does not show activity towards living organisms [1–4].

Titanium dioxide occurs in nature in three polymorphous varieties: tetragonal rutile, anatase and rhombic brookite. Anatase and rutile are of practical importance and are commonly used in many applications, while brookite is not used because of the instability of its structure [4–6].

The rapidly developing technologies for obtaining new functional materials based on titania are an especially important topic for both theoretical study and practical application. The continually growing requirements of different technologies require new directions to be sought in order to obtain materials with precisely designed physicochemical and structural properties. Hybrid systems based on TiO<sup>2</sup> constitute a new group of compounds exhibiting strictly designed physicochemical properties resulting from the effects of combining the characteristic behaviours of the individual compounds from which they are made. The presence of a foreign element in the matrix of pure titania can greatly affect the structural, textural, acid/base and catalytic properties [7]. The textural properties of the hybrid materials, such as pore size distribution, surface area, etc., are strongly dependent upon the conditions of synthesis, including the nature and composition of the precursors, solvent, complexing/templating agent, hydrolysis and calcination conditions [8]. Research into the production and potential applications of new functional materials based on titanium dioxide is only possible when the final materials have a strictly defined dispersive character, crystalline structure, morphology and porous structure [9–13].
