**4. Surface modification by lanthanides**

The surface modification of semiconductors that occurs by the addition of tiny and controlled impurities is denominated doping, and this process is applied to achieve distinct electronic properties from those raw materials. Metal doping has a main feature, the bandgap reduction, which optimizes the ability of a semiconductor to absorb wavelengths with less energy, reduce recombination of the vacancy/electron pairs, and modify the adsorptive capacity of the surface of this photocatalyst. Bandgap reduction occurs possibly due to the formation of energy levels between the conduction and valence bands, such as when transition metals are used, because these incorporate levels 3d in the TiO2 lattice, which leads to the formation of electronic levels occupied near to the conduction band. However, visible light absorption by semiconductor doped with noble metals, such as gold and silver, may be associated with the surface resonance effect of Plasmon, which is described as an oscillation of the electrons on the surface of these metals, which cause a separation of charge carriers [13, 44, 56].

photocatalytic activity responses, such as studies performed by Liu et al. [66] which evaluated the cerium doping on titanium dioxide using chemical coprecipitation method, with molar ratio from 0.5 to 2.5% of cerium in relation to the semiconductor to evaluate the degradation of methylene blue 2,3-dichlorophenol and benzene, under visible and UV light. The best results

molar ratio applied was 0.1–1.0% of the lanthanide in relation to the semiconductor, with subsequent calcination of all samples at 673 K. The results obtained in the degradation of glyphosate under UV light showed that the molar ratio equal 0.15% optimizes the photocatalysis.

Cerium doping was also recently evaluated by Pei et al. [68] in the titanium dioxide nanorods formation. The inclusion of cerium in the semiconductor occurred by hydrothermal method. The inclusion of this element changed the semiconductor wavelength absorption to 480 nm, with bandgap energy equal to 2.65 eV, meaning that photocatalysis can occur in visible light. Neodymium (Nd) has attracted interest as a semiconductor dopant because of its ability to absorb and emit wavelengths in the range from UV to IR radiation, besides the usual features of RE, such as discussed above. Du et al. [69] used this element in the titanium dioxide film doping by sol–gel method. In this research, the photocatalytic effects of the mass ratios of neodymium (0.1 and 0.9%) after calcination at 823 K for 2 h were evaluated. The obtained films were used in the degradation of methylene blue under UV light, with efficiency upper of 92% in the removal of this dye when using a mass ratio of 0.10%. Possibly, this increment occurs due to a reduction in bandgap energy, since analysis showed an increase in the visible light wavelengths absorption by the doped semiconductor. Another important research was done by Thomas et al. [70] who applied molar ratios equal to 0.30, 0.50, and 0.70% of neodymium to doping titanium dioxide via sol-gel method. It evaluated the insertion of a third component, phosphotungstic acid, in the degradation of methylene blue and 4-chlorophenol under visible light. The results obtained showed an increase in the degradation in relation to the pure

semiconductor, with the best result when the molar ratio was equal to 0.50%.

of the reactive yellow dye 4 under UV and visible radiation using TiO2

rare earth with molar mass equal to 140.9 g·mol−<sup>1</sup>

Ln3+:TiO2

the crystalline phase.

Another element evaluated for titanium dioxide doping is praseodymium, classified as a light

ionic ratios of praseodymium (0.50–1.80%), which was included in the crystal lattice by sol–gel method, with calcination at 773 K for 2 h. The photodegradation results demonstrated that

in degrading the contaminant under UV and visible light, respectively. This increase in the catalytic activity of praseodymium doped semiconductors was also confirmed according to research published by Kralova et al. [72], which demonstrated that titanium dioxide doped with 0.3 mol% of praseodymium at different temperatures (723, 823 and 923 K) and calcination times (4, 8 and 12 h) promotes a reduction in bandgap energy (from 3.20 to 3.14 eV) when the photocatalyst was calcined at 723 K for 8 h. The efficiency increase in the degradation of an organochlorine pesticide under UV light (LED) possibly is related to milder temperatures joined with intermediate times, which ensure a diffusion of the lanthanide without changes in

equal to 1.0 and 1.5% optimizes the photocatalytic capacity of the semiconductor

). Xue et al. [67] evaluated the

Lanthanides Effects on TiO2 Photocatalysts http://dx.doi.org/10.5772/intechopen.80906 87

nanotubes by chemical impregnation and ultrasound. The

. Liang et al. [71] evaluated the degradation

doped with different

occur when the RE concentration was equal to 0.50% (mol∙mol−<sup>1</sup>

effects of cerium doping into TiO<sup>2</sup>

Another advantage of metal doping is the decrease in photogenerated charges recombination, which is due to the Schottky barrier formation in the interface of semiconductor and metal. The electrons formed migrate, and they are captured by the metal particles, which become active sites for oxygen reduction. This barrier is formed by the electronic binding between the lanthanides and titanium dioxide, allowing the mobility of electrons formed in the semiconductor valence band to the metal, until equilibrium occurs between the Fermi levels [57, 58].

The rare earths when applied in TiO2 doping have shown one or both features above discussed, mainly due to the presence of incomplete 4f levels, surface defects, and oxygen vacancies. The first ones reduce the TiO<sup>2</sup> bandgap due to the appearance of orbitals between the conduction and valence bands, which generates impure energy levels. This reduction provides an electron transfer from VB until the orbital created and/or from this to the CB requires less energy. In addition, these orbitals *f* have the ability to produce complexes with several Lewis bases, increasing the concentration of these species on the surface of the catalyst and, consequently, the photocatalytic activity. Moreover, these orbitals have the ability to trap the photogenerated electrons due to their incomplete levels, which avoids the recombination of the charge carriers [11–15].

Another advantage of lanthanides is the ability of some ions of these elements (Ln3+) to generate surface defects and titanium and oxygen vacancies. This occurs possibly because some lanthanides ions have an atomic radius higher than Ti4+ (0.68 Å), such as La3+ (1.15 Å) and Sm3+ (1.24 Å). In these cases, Ln3+ is not able to replace the titanium ions in the semiconductor lattice, and these ions only are adsorbed on the surface, forming Ti–O–Ln3+ bonds. This fact leads to an imbalance in the surface charges of the TiO2 crystal structure. Since titanium atom has a Pauling electronegativity value (1.54) higher than those presented by lanthanum (1,10) and samarium atoms (1,17), a transfer of electrons occur from these lanthanides to the titanium, which is converted from Ti4+ to Ti3+. The excess of negative charge disturbs the electronegativity, and it is necessary remove an ion of O2− for every two Ti3+ ions produced, which causes the formation of oxygen and titanium vacancies, generating a surface defect. Furthermore, the presence of lanthanides, dispersed as interstitial impurities on the TiO2 lattice causes surface defects. The presence of these vacancies and defects causes the capture of electrons produced on the surface of the photocatalyst and decreases the recombination of photogenerated charge carriers. However, an excess of lanthanides on the photocatalyst surface can lead to a high density of defects and vacancies, which creates recombination centers [30, 59–64].

Rare earth application as dopant also can increase the surface area of the catalyst because it reduces the growth of the crystal structure. Besides, these elements can increase the thermal stability, since they interfere in the conversion of the anatase phase to rutile during the calcination of the doped semiconductors [12, 25, 65].

Lanthanides have been applied as TiO2 dopants in many researches, highlighting yttrium, cerium, neodymium, praseodymium, europium, samarium, and lanthanum, with promising photocatalytic activity responses, such as studies performed by Liu et al. [66] which evaluated the cerium doping on titanium dioxide using chemical coprecipitation method, with molar ratio from 0.5 to 2.5% of cerium in relation to the semiconductor to evaluate the degradation of methylene blue 2,3-dichlorophenol and benzene, under visible and UV light. The best results occur when the RE concentration was equal to 0.50% (mol∙mol−<sup>1</sup> ). Xue et al. [67] evaluated the effects of cerium doping into TiO<sup>2</sup> nanotubes by chemical impregnation and ultrasound. The molar ratio applied was 0.1–1.0% of the lanthanide in relation to the semiconductor, with subsequent calcination of all samples at 673 K. The results obtained in the degradation of glyphosate under UV light showed that the molar ratio equal 0.15% optimizes the photocatalysis.

semiconductor doped with noble metals, such as gold and silver, may be associated with the surface resonance effect of Plasmon, which is described as an oscillation of the electrons on the

Another advantage of metal doping is the decrease in photogenerated charges recombination, which is due to the Schottky barrier formation in the interface of semiconductor and metal. The electrons formed migrate, and they are captured by the metal particles, which become active sites for oxygen reduction. This barrier is formed by the electronic binding between the lanthanides and titanium dioxide, allowing the mobility of electrons formed in the semiconductor valence band to the metal, until equilibrium occurs between the Fermi levels [57, 58].

cussed, mainly due to the presence of incomplete 4f levels, surface defects, and oxygen vacan-

conduction and valence bands, which generates impure energy levels. This reduction provides an electron transfer from VB until the orbital created and/or from this to the CB requires less energy. In addition, these orbitals *f* have the ability to produce complexes with several Lewis bases, increasing the concentration of these species on the surface of the catalyst and, consequently, the photocatalytic activity. Moreover, these orbitals have the ability to trap the photogenerated electrons due to their incomplete levels, which avoids the recombination of

Another advantage of lanthanides is the ability of some ions of these elements (Ln3+) to generate surface defects and titanium and oxygen vacancies. This occurs possibly because some lanthanides ions have an atomic radius higher than Ti4+ (0.68 Å), such as La3+ (1.15 Å) and Sm3+ (1.24 Å). In these cases, Ln3+ is not able to replace the titanium ions in the semiconductor lattice, and these ions only are adsorbed on the surface, forming Ti–O–Ln3+ bonds. This fact leads

a Pauling electronegativity value (1.54) higher than those presented by lanthanum (1,10) and samarium atoms (1,17), a transfer of electrons occur from these lanthanides to the titanium, which is converted from Ti4+ to Ti3+. The excess of negative charge disturbs the electronegativity, and it is necessary remove an ion of O2− for every two Ti3+ ions produced, which causes the formation of oxygen and titanium vacancies, generating a surface defect. Furthermore, the

defects. The presence of these vacancies and defects causes the capture of electrons produced on the surface of the photocatalyst and decreases the recombination of photogenerated charge carriers. However, an excess of lanthanides on the photocatalyst surface can lead to a high

Rare earth application as dopant also can increase the surface area of the catalyst because it reduces the growth of the crystal structure. Besides, these elements can increase the thermal stability, since they interfere in the conversion of the anatase phase to rutile during the calci-

cerium, neodymium, praseodymium, europium, samarium, and lanthanum, with promising

presence of lanthanides, dispersed as interstitial impurities on the TiO2

density of defects and vacancies, which creates recombination centers [30, 59–64].

doping have shown one or both features above dis-

bandgap due to the appearance of orbitals between the

crystal structure. Since titanium atom has

dopants in many researches, highlighting yttrium,

lattice causes surface

surface of these metals, which cause a separation of charge carriers [13, 44, 56].

The rare earths when applied in TiO2

to an imbalance in the surface charges of the TiO2

nation of the doped semiconductors [12, 25, 65].

Lanthanides have been applied as TiO2

cies. The first ones reduce the TiO<sup>2</sup>

86 Photocatalysts - Applications and Attributes

the charge carriers [11–15].

Cerium doping was also recently evaluated by Pei et al. [68] in the titanium dioxide nanorods formation. The inclusion of cerium in the semiconductor occurred by hydrothermal method. The inclusion of this element changed the semiconductor wavelength absorption to 480 nm, with bandgap energy equal to 2.65 eV, meaning that photocatalysis can occur in visible light.

Neodymium (Nd) has attracted interest as a semiconductor dopant because of its ability to absorb and emit wavelengths in the range from UV to IR radiation, besides the usual features of RE, such as discussed above. Du et al. [69] used this element in the titanium dioxide film doping by sol–gel method. In this research, the photocatalytic effects of the mass ratios of neodymium (0.1 and 0.9%) after calcination at 823 K for 2 h were evaluated. The obtained films were used in the degradation of methylene blue under UV light, with efficiency upper of 92% in the removal of this dye when using a mass ratio of 0.10%. Possibly, this increment occurs due to a reduction in bandgap energy, since analysis showed an increase in the visible light wavelengths absorption by the doped semiconductor. Another important research was done by Thomas et al. [70] who applied molar ratios equal to 0.30, 0.50, and 0.70% of neodymium to doping titanium dioxide via sol-gel method. It evaluated the insertion of a third component, phosphotungstic acid, in the degradation of methylene blue and 4-chlorophenol under visible light. The results obtained showed an increase in the degradation in relation to the pure semiconductor, with the best result when the molar ratio was equal to 0.50%.

Another element evaluated for titanium dioxide doping is praseodymium, classified as a light rare earth with molar mass equal to 140.9 g·mol−<sup>1</sup> . Liang et al. [71] evaluated the degradation of the reactive yellow dye 4 under UV and visible radiation using TiO2 doped with different ionic ratios of praseodymium (0.50–1.80%), which was included in the crystal lattice by sol–gel method, with calcination at 773 K for 2 h. The photodegradation results demonstrated that Ln3+:TiO2 equal to 1.0 and 1.5% optimizes the photocatalytic capacity of the semiconductor in degrading the contaminant under UV and visible light, respectively. This increase in the catalytic activity of praseodymium doped semiconductors was also confirmed according to research published by Kralova et al. [72], which demonstrated that titanium dioxide doped with 0.3 mol% of praseodymium at different temperatures (723, 823 and 923 K) and calcination times (4, 8 and 12 h) promotes a reduction in bandgap energy (from 3.20 to 3.14 eV) when the photocatalyst was calcined at 723 K for 8 h. The efficiency increase in the degradation of an organochlorine pesticide under UV light (LED) possibly is related to milder temperatures joined with intermediate times, which ensure a diffusion of the lanthanide without changes in the crystalline phase.

The studies performed by Leal et al. [73] are prominent due to the extensive evaluation about lanthanides doping of TiO2 lattices. The light lanthanide series (La-Eu), besides gadolinium, were evaluated for the degradation of orange and violet methyl by UV light. The pH values (3.1 and 5.6) and the mass fraction of Ln3+:TiO2 (0.1 and 0.3%) were used as parameters for the experiments, with semiconductor doping by the sol–gel method and calcination at 773 K for 2 h. The results showed an increase (approximately 37%) in the degradation of the compounds when the doped semiconductors were applied instead of pure TiO2 . Therefore, this research provides an overview of the potential application of these lanthanides to degrade different compounds under different process conditions.

with best results when the lowest concentrations of samarium (0.30 and 0.43%) were applied. The photocatalytic degradations achieved results of upper 90% when applied by UV light, which demonstrates that an increase in the amount of rare earths can lead these photocata-

Xiao et al. [78] demonstrated that methylene blue degradation under UV light showed best responses when the lowest samarium molar ratio was applied (0.50%). The sol-gel method

Calcination is a vital step for doping, since it allows the activation and/or fixation of the dopant in the semiconductor crystal structure, besides the removal of impurities and the increase in the density of vacancies due to the removal of oxygen from the photocatalyst lattice, in addition to promoting an increase in crystallization. However, an excessive increase in the calcination temperature can lead to a particle aggregation and, consequently, reduction of the surface area, besides the conversion of the anatase phase to rutile, which can affect the photocatalytic activity. Therefore, the calcination temperature control is essential to assure

Usually anatase to rutile phase transformation occurs at temperatures between 500 and 750°C, as soon as there is an increase in the crystalline size of anatase when the calcination temperature enhances. However, the lanthanides doping shift the phase transformation to higher temperatures (above 700°C) and suppress the anatase crystalline growth between 500

Chen et al. [87] evaluated the effect of rare earth doping (0.20, 0.50, 1.0, and 2.0% mol) by hydrothermal method and the calcination temperature (673, 773, 873 and 1073 K). The results obtained showed that the best photocatalytic activity was achieved when the temperature of calcination was 773 K for 2 h, with a crystallite size equal to 15 nm. It was possible to verify that an increase of the temperature reduces the surface area of the crystal structures formed. Cruz et al. [88] published a research about titanium dioxide doped with samarium via sol–gel method, in which the photocatalytic degradation of a herbicide under UV light was investigated. The parameters evaluated were the calcination temperature and the samarium concentration, with the best efficiency in herbicide degradation when the samples were calcined at 773 K for 4 h. Similar results were obtained by Yang et al. [76], which showed that the

method was optimized when the temperature and time of calcination were equal to 773 K and

Li et al. [89] also evaluate the effect of temperature on the photocatalytic activity of titanium dioxide doped with europium via hydrothermal method. It used calcination temperatures between 573 and 1173 K. The temperature at 773 K, with a calcination time of 4 h, improves the degradability of the contaminant, probably because of the increase in crystallization and

reduction of defects, which improve the ability to absorb visible light wavelengths.

lattice, and the samples were calcined for 2 h

Lanthanides Effects on TiO2 Photocatalysts http://dx.doi.org/10.5772/intechopen.80906 89

doped with neodymium and fluorine via sol-gel

lysts from electron traps to recombination centers.

was used for the inclusion of Sm in the TiO2

**4.1. Calcination temperature effects**

high photocatalytic activity [75, 79–81].

degradation of the methylene blue by TiO2

and 700°C [82–86].

3 h, respectively.

at 873 K.

Reszczyńska et al. [74] also evaluated the effects of the application of yttrium, praseodymium, erbium, and europium in the photodegradation efficiency of phenol, under visible and UV light. Furthermore, hydrothermal and sol-gel methods for semiconductor doping were compared in relation to photocatalytic activity response. It used the molar ratios of 0.25 and 0.50% of Ln3+ in relation to titanium dioxide. The results showed an increase in phenol degradation under visible light for doped photocatalysts when compared to pure TiO2 . When UV light was applied as radiation source only, the hydrothermal process showed better results than the pure semiconductor. However, if the decomposition efficiencies of the contaminant are compared, it is remarkable that the hydrothermal process overcomes the sol-gel method in most cases. This possibly is related to the high surface area, low crystallite dimensions, and higher density of –OH obtained in the first method employed. In addition, the lower concentration of rare earths in doping showed the best degradation results of the molecule evaluated.

Lanthanum (La) doping has an extensive discussion about the mechanisms, which are wellknown. However, different methods, as well as the optimized rare earth amount for each of these methods, are still evaluated, without being exhausted. Among the researches carried out, Li et al. [75] evaluated different amounts (0–0.50%) of lanthanum in the TiO<sup>2</sup> doping, which are impregnated in the semiconductor by sol-gel method, with subsequent calcination (973–1173 K). The best response in methyl orange photodegradation occurred when the concentration of lanthanum was 0.05% with a calcination temperature at 973 K.

Jun et al. [76] analyzed the photocatalytic activity of TiO2 doped with different concentrations of lanthanum (from 0 to 0.90% w∙w−<sup>1</sup> ) by sol–gel method followed by calcination at 823 K for 2 h. The results showed that mass ratio 0.30% (0.17% mol∙mol−<sup>1</sup> ) demonstrated the best results in methylene blue removal. It was proposed that the increase in photocatalytic efficiency is due to the formation of vacancies and defects from the presence of lanthanide on the TiO2 surface. It was also possible to verify that an increase in lanthanum concentration causes a reduction in photocatalytic efficiency, possibly because La serves as a mediator in the interfacial charge transfer or as a recombination center.

Samarium (Sm) has been used to doping semiconductors because its presence causes significant improvements in the degradation of compounds, which leads the semiconductors to absorb wavelengths in the visible light spectrum, besides low cost of the compound when compared with others lanthanides. Tang et al. [77] doped titanium dioxide with different concentrations of samarium (0–2.16% mol∙mol−<sup>1</sup> ) by the sol-gel method followed by different calcination temperatures (623–1123 K). The pollutants evaluated were methanol and acetone, with best results when the lowest concentrations of samarium (0.30 and 0.43%) were applied. The photocatalytic degradations achieved results of upper 90% when applied by UV light, which demonstrates that an increase in the amount of rare earths can lead these photocatalysts from electron traps to recombination centers.

Xiao et al. [78] demonstrated that methylene blue degradation under UV light showed best responses when the lowest samarium molar ratio was applied (0.50%). The sol-gel method was used for the inclusion of Sm in the TiO2 lattice, and the samples were calcined for 2 h at 873 K.
