**1. Introduction**

Photocatalysis applied in gas or aqueous media involves the reactive oxygen species (ROS) formation, mainly hydroxyl radical (•OH), superoxide (•O2 <sup>−</sup>), and singlet oxygen (1 O2 ), which are generated by the oxidation of water molecules or capture of electrons by oxygen.

© 2016 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. © 2018 The Author(s). Licensee IntechOpen. 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.

These species are very effective oxidant agents, and they are capable of degrading recalcitrant compounds due to the high potential of oxidation. These ROS are produced on the semiconductors' surface, such as titanium dioxide (TiO2 ), which is considered the most promising material because it is inexpensive, nontoxic, chemically and thermally stable, abundant, and environmentally friendly. However, this semiconductor only will produce these species when it receives a minimum energy amount, called the bandgap energy (*Eg* ), which is capable to remove an electron from the valence band (VB) and transfer it to the conduction band (CB), thus creating the electron-hole pair. For TiO2 , this minimum energy is supplied by photons with frequency in the ultraviolet light region, which possess wavelengths below 400 nm [1–6].

other hand, some doping methods do not supply enough energy to promote the ions substi-

Different methods are currently used for doping, like solvothermal, microemulsion, electrospinning, cathodic sputtering, hydrothermal, microwave, sonochemical, and sol–gel, which is the most usual process. However, these methods usually employ reactants such as tetrabutoxide and titanium isopropoxide, which are aggressive to the environment, besides using severe conditions of temperature and pressure. Therefore, the methodology using ultrasound appears as an alternative to avoid the application of secondary reactants and extreme condi-

Lanthanides have particular features when compared with other metal oxides dopants, because they inhibit the transition from the anatase phase to the rutile phase, reduce the crystalline size, can easily react with the functional groups of Lewis bases to mineralize organic

Thus, titanium dioxide doped with lanthanides, which avoid the recombination mechanisms, has been studied in the sense of optimizing the capacity of radical formation due to the increased numbers of defects and oxygen vacancies on the surface of the lattice to trap e−/h+ pairs and reduce the recombination, besides that increase the density of –OH groups or water molecules attached to the surface of the catalyst. However, the excess of these dopants reduces the photocatalytic capacity of the semiconductor, possibly due to the excess of vacancies generated, which act as recombination centers instead of electrons

The crystalline structures of titanium dioxide are anatase, rutile, and brookite, which is difficult to be synthesized in laboratory. Therefore, the first two lattices are the most prominent in

between these structures, since anatase has four connections between the edges, brookite has three connections, and rutile has only two. This structure confers to rutile a greater thermodynamic stability among the other polymorphs, as established by the third Pauling rule [31–33]. There is a great interest in the application of titanium dioxide as a photocatalyst because this compound is nontoxic, economical, chemically inert, and photostable to corrosion, besides high thermal stability, intense photocatalytic activity, and strong oxidant power. However, not all crystalline structures have the same efficiency in the absorption of light for catalysis; for example, although rutile is the polymorph thermodynamically more stable, its photocatalytic activity is lower than anatase, possibly due to the high temperature required for its preparation, which causes an increase in particle size, a high rate of electron/vacancy recombination that reduces the number of hydroxyl groups on the surface, and lower electron mobility in

compounds, and bring luminescent properties to the titanium dioxide [19, 26, 27].

surface or boundaries of the

Lanthanides Effects on TiO2 Photocatalysts http://dx.doi.org/10.5772/intechopen.80906 83

octahedra, but with distinct connections

tution in the lattice, and the lanthanides are located in the TiO<sup>2</sup>

semiconductor particles, creating Ti–O–Ln bonds [16–20].

researches. All crystal lattices are composed of TiO<sup>6</sup>

tions [21–25].

scavenger [28–30].

relation to anatase [34–37].

**2. TiO2**

Despite the ability of photocatalytic processes to degrade several compounds by the use of hydroxyl radicals, their use is still not widespread, with scarce industrial applications. This is due to some inherent features of the catalysts employed, such as the recombination of photogenerated charges, which reduces the formation of radicals and, consequently, the efficiency of photocatalytic degradation. Therefore, plenty of studies have been performed to overcome this drawback, like surface modifications that allow the capture of the generated electrons and avoid the recombination. Furthermore, the bandgap energy required for the formation of the electron/vacancy pair, which for TiO2 is equal to 3.2 eV, restricts this catalyst to the use of a light source that has wavelengths in the ultraviolet region (≈390 nm). Therefore, only 5% of sunlight can be used in photocatalysis that applies this semiconductor, which makes the process more expensive. In this way, superficial modification through metal and nonmetal doping is fundamental to overcome these drawbacks [7–10].

Among the possible dopants, rare earths have been investigated for their ability to increase photocatalytic activity, possibly by reducing bandgap energy due to the introduction of orbitals between the conduction and valence bands, generating impurity energy levels in the semiconductor elements. These states are generated from the 4f level, which are electron deficient. Another hypothesis for this increase in contaminant degradability is that the adsorption of these lanthanides on the surface of the semiconductors generates an imbalance of surface charges, which can produce surface defects and vacancies of oxygen and titanium. These two propositions lead to states that serve as electron scavengers and reduce the recombination of photogenerated charges, increasing the probability of •OH formation. Moreover, the lanthanides adsorbed on the surface may act as traps to water molecule and –OH (hydroxyl anion) groups, which increase their density on the photocatalyst surface and can promote more intense formation of hydroxyl radicals. Another important feature related to the use of these compounds is that they serve as a Lewis base, which could concentrate the contaminants dispersed in aqueous medium on the semiconductor surface enhancing the electron transfer for the direct degradation of the contaminant or increasing the probability of interaction between the molecules and the radicals formed [11–15].

The mechanisms of lanthanides doping on TiO2 proposed by different researchers have not been the same, with two ways most discussed. Rare earths can be included in the TiO2 lattice by direct linking or substitution producing a≡Ti–O–Ln–O–Ti≡arrangement, which cause distortions/defects in the lattice due to the mismatch of ionic radius of Ln3+ and Ti4+. On the other hand, some doping methods do not supply enough energy to promote the ions substitution in the lattice, and the lanthanides are located in the TiO<sup>2</sup> surface or boundaries of the semiconductor particles, creating Ti–O–Ln bonds [16–20].

Different methods are currently used for doping, like solvothermal, microemulsion, electrospinning, cathodic sputtering, hydrothermal, microwave, sonochemical, and sol–gel, which is the most usual process. However, these methods usually employ reactants such as tetrabutoxide and titanium isopropoxide, which are aggressive to the environment, besides using severe conditions of temperature and pressure. Therefore, the methodology using ultrasound appears as an alternative to avoid the application of secondary reactants and extreme conditions [21–25].

Lanthanides have particular features when compared with other metal oxides dopants, because they inhibit the transition from the anatase phase to the rutile phase, reduce the crystalline size, can easily react with the functional groups of Lewis bases to mineralize organic compounds, and bring luminescent properties to the titanium dioxide [19, 26, 27].

Thus, titanium dioxide doped with lanthanides, which avoid the recombination mechanisms, has been studied in the sense of optimizing the capacity of radical formation due to the increased numbers of defects and oxygen vacancies on the surface of the lattice to trap e−/h+ pairs and reduce the recombination, besides that increase the density of –OH groups or water molecules attached to the surface of the catalyst. However, the excess of these dopants reduces the photocatalytic capacity of the semiconductor, possibly due to the excess of vacancies generated, which act as recombination centers instead of electrons scavenger [28–30].
