Titanium, as a metal, is the ninth most abundant metal in Earth crust, which correspond to a 0,6% approximately. It is found in nature in the mineral form of ilmenite (FeTiO3), and the three principal forms of titanium dioxide (TiO2) (anatase, rutile and brookite) and perovskite (CaTiO3) . According to the US Geological Survey, only about the 5% of world’s annual production goes to the manufacture of titanium metal; the other 95% correspond to the production of TiO2.
The annual world consumption of TiO2 is up to 3 million tons, where the major use is as a pigment, since it has a high refractive index, an easiness to prepare particles of small sizes with a great superficial area resulting light-scattering ability, and also its whiteness [2-3]. Other uses include as sunscreen , skincare products , toothpaste , food additive , semiconductor , and more recent as a photocatalyst for the water oxidation reaction [9-15].
As mentioned before, TiO2 has three polymorphic forms, rutile, anatase and brookite. Rutile is the only stable phase, while anatase and brookite are metastable phases that can be converted to rutile irreversibly by heating. This forms has different crystal structures, despite all of them are distorted octahedra, since the brookite crystallized as an orthorhombic crystal system, rutile forms a distorted tetragonal system, and anatase a tetragonal crystal system. [16-17] (Fig. 1).
FIGURE 1. Schematic representation of the unit cell for the polymorphs of TiO2, (a) rutile a = b = 4.49 Å, c = 3.01 Å; and (b) anatase a = b = 3.77 Å, c = 9.56 Å. Ti4+ ions are in blue and O2− ions are in red. From .
In the last few years, researchers found different new physical and chemical properties, among others, the size of the material. The synthesis of this new nanomaterials include nanoparticles, nanotubes, nanowires, etc.; and are classified depending on their preparation method. Some of these methods include the sol-gel, micelle and inverse micelle, hydrothermal, solvothermal, direct oxidation, chemical and physical vapor deposition, electrodeposition, sonochemical, microwaves, etc, reviewed elsewhere. [19-20]
The applications of the TiO2 nanomaterials depend heavily on their optical properties, so currently many researchers are seeking the modification of this oxide with organic and inorganic molecules to improve the optical sensitivity and activity in the ultraviolet and visible region of the electromagnetic spectrum. In this review, the structural, thermodynamic, electronic and optical properties of TiO2 are discussed, and also the modification that can be made to improve their efficiency, either for use in solar cells or photocatalysis, among others.
II. Properties of titanium dioxide.
1. Structural properties.
Rutile and anatase are the two most important crystalline structures of TiO2. (Fig. 1) Both can be described as octahedral chains of TiO6, where each Ti4+ ion is surrounded by six O2− ions, in...