**4.1. Calcination temperature effects**

The studies performed by Leal et al. [73] are prominent due to the extensive evaluation about

were evaluated for the degradation of orange and violet methyl by UV light. The pH values

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 com-

research provides an overview of the potential application of these lanthanides to degrade

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

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

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

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

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 interfa-

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

calcination temperatures (623–1123 K). The pollutants evaluated were methanol and acetone,

rare earths in doping showed the best degradation results of the molecule evaluated.

out, Li et al. [75] evaluated different amounts (0–0.50%) of lanthanum in the TiO<sup>2</sup>

concentration of lanthanum was 0.05% with a calcination temperature at 973 K.

Jun et al. [76] analyzed the photocatalytic activity of TiO2

cial charge transfer or as a recombination center.

concentrations of samarium (0–2.16% mol∙mol−<sup>1</sup>

2 h. The results showed that mass ratio 0.30% (0.17% mol∙mol−<sup>1</sup>

of lanthanum (from 0 to 0.90% w∙w−<sup>1</sup>

pounds when the doped semiconductors were applied instead of pure TiO2

under visible light for doped photocatalysts when compared to pure TiO2

lattices. The light lanthanide series (La-Eu), besides gadolinium,

(0.1 and 0.3%) were used as parameters for

. Therefore, this

. When UV light

doping,

doped with different concentrations

) by the sol-gel method followed by different

) demonstrated the best results

) by sol–gel method followed by calcination at 823 K for

lanthanides doping of TiO2

88 Photocatalysts - Applications and Attributes

(3.1 and 5.6) and the mass fraction of Ln3+:TiO2

different compounds under different process conditions.

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 high photocatalytic activity [75, 79–81].

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 and 700°C [82–86].

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 degradation of the methylene blue by TiO2 doped with neodymium and fluorine via sol-gel method was optimized when the temperature and time of calcination were equal to 773 K and 3 h, respectively.

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.
