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,...
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