**3. Lanthanides**

However, the rutile phase has a bandgap equal to 3.0 eV, having the ability to absorb radiation in the visible light spectrum, while the anatase has a bandgap of approximately 3.2 eV. Therefore, the interaction of these two crystalline structures often has better photocatalytic results than when they are purely applied, which is possibly due to the visible light absorption by the rutile, which serves as a photosensitizer for the anatase phase. An example of this phase interaction is the commercial P25® titanium dioxide from Degussa Evonik, which has a composition of 80% anatase and 20% rutile, exhibiting high photocatalytic activ-

Regarding to the electronic structure, this catalyst can be considered a semiconductor with indirect bandgap for anatase phase, meaning that the valence and conduction bands of this lattice do not have the same maximum and minimum momentum, respectively. This condition leads to a recombination of the electron/vacancy pair that only occurs if assisted by a pho-

semiconductor; in other words it has a greater density of electrons than vacancies produced by oxygen, because they are compensated by the presence of Ti3+, which brings the Fermi level

Bandgap energy for the structure with the highest photocatalytic activity is approximately 3.2 eV, and the valence band, formed mainly by the 2p orbital of the oxygen, has an energy level of approximately 2.60 V, which is more positive than the oxidation potential of water to hydroxyl radicals (*E0* = 2.27 V). In the case of the conduction band, which is constituted mainly by titanium 3d orbitals, the energy level is equal to −0.51 V, with a value that is negatively greater than the oxygen reduction potential of the superoxide radical (*E0* = −0.33 V). As depicted in **Figure 1**, with these conditions titanium dioxide can degrade organic compounds through activation by UV light, which makes it a prominent catalyst

is an n-type

non, resulting in a longer lifetime of this electron generated. Furthermore, TiO2

**Figure 1.** Redox potential of conduction and valence bands and the radical formation.

ity [32, 38, 39].

84 Photocatalysts - Applications and Attributes

[13, 32, 43–45].

closer to the conduction band [40–42].

The rare earths (RE) are a group of elements with physicochemical properties that are very similar, which is constituted by lanthanides (lanthanum to lutetium), scandium, and yttrium, totalizing 17 elements of the periodic table. This series can be separated into light rare earths that comprise low atomic mass elements (lanthanum to europium) and heavy rare earths, which are constituted from lutetium to gadolinium, besides yttrium [19, 46].

These elements have shown great potential for industrial applications because of their unique magnetic, optical, and/or redox properties, with important uses in the field of catalysis, high temperature superconductors, hybrid cars, permanent magnets, nuclear magnetic resonance, rechargeable batteries, manufacture of glass and ceramic materials, shift reagents, etc. The electronic configuration of these chemical elements assure these properties due to the distribution of the electrons with the complete levels until 5, such as xenon, but with the level *f*, it was incomplete and protected by orbitals 6 *s* and/or 5*d*, except for of the scandium and yttrium. Thus, the representation of the electronic configuration is described as [Xe] 4fn6s2 or [Xe] 4fn5d1 6s2 for lanthanides (Ln), which usually have an oxidation state equal to 3+ (Ln3+) [47–51].

The orbitals 4*f* that were partially completed are responsible for the optical properties of rare earths, since different arrangements of these orbitals generate different levels of energy, allowing the absorption in a broad spectrum of light radiation, which can vary from ultraviolet to visible. Moreover, the luminescence of some rare earth ions arises from the *f-f* electronic transitions within their partially filled 4*f* orbitals that can occur by electric or magnetic dipole [52, 53].

Another advantage of this configuration is that these orbitals are sterically shielded from the surrounding microenvironment by the filled 5s and 5p orbitals, meaning that there are almost no perturbations of these transitions by other bonding elements, which assure that the optical properties do not undergo sudden changes [48, 54, 55].
