3.1 Results of characterization of TiO2 doped with Ce3+, Pr3+, La3+, Nd3+ Sm3+, Eu3+, and Gd3+

The diffuse reflectance spectra in the materials describe a change in the absorption band toward wavelengths corresponding to the visible region in all materials calcined at 500 (Figure 1a and b) and 800°C (Figure 1c). Due to the elimination of impurities, organic material and hydroxylated groups from the precursors in the synthesis, which use NH4OH as hydrolysis catalyst and NO3 ˜ ions as precursors of the dopants, so it is expected that N atoms have been incorporated and eliminated by the effect of thermal treatment, causing a shift of the absorption bands toward longer [52], at the same time, this caused the production of oxygen vacancies, which also produced the same effect [53].The spectra of Figure 1a present a greater absorption toward the visible for the solids doped with Pr 0.3%, Nd 0.1%, and 0.3%, however, for Figure 1b, the sample with Sm 0.3% is the one that greater absorption shows and in Figure 1c, the materials doped with Sm 0.3% and Gd 0.3% have this same behavior. In Figure 1c, it is evident that materials calcined at 800°C have a better absorption toward the visible, compared with those calcined at 500°C, due to the presence of the rutile crystalline phase, due to the increase in temperature in the thermal treatment, which consequently produces a decrease in the value of the band gap energy, the same happens with the sample of pure TiO2 treated at 800°C. All doped materials already treated at 500 and 800°C show greater absorption than pure TiO2. The values of the Eg are shown in Table 1, here, it is observed that most of the samples treated at 500°C have a value ranging between 3.02 and 3.16 eV, including doped materials and pure TiO2, however, only the sample doped with Pr 0.3% decreased this value considerably (2.88 eV). With respect to the solids treated at 800°C, the photocatalyst doped with Eu 0.3% obtained the lowest value of Eg compared with all materials (2.5 eV).

Figure 2 shows the diffractograms corresponding to pure TiO2 and TiO2-doped with rare earth ions, all thermally treated at 500°C. Only the anatase crystalline phase was detected in these samples presenting a low crystallinity (slightly amorphous), which is observed in the morphology by SEM in Figure 6a.

The results in Table 1 show that an average crystal size is 8.14 nm for the TiO2 calcined at 500°C when introducing the dopants at the same treatment temperature the value decreases until 7.00 nm for the case of the catalyst with Sm 0.5%. In most of the samples doped to increase their content in the titania, the average crystal size decreases; this is due to the separation of the dopant in the limits of the crystal, which prevents its growth by not being able to be in contact with other crystals to grow by coalescence [54], except for the Nd and Pr, where their values are fixed

#### Figure 1.

UV-Vis diffuse reflectance spectra of TiO2 without treatment and TiO2 (a) doped to 0.1 and 0.3% with La, Ce, Nd, and Pr calcinated at 500°C; (b) doped to 0.3 and 0.5% with Sm, Eu, and Gd calcinated at 500°C; (c) doped to 0.3 and 0.5% with Sm, Eu, and Gd calcinated at 800°C.


Photocatalytic Treatment of Pesticides Using TiO2 Doped with Rare Earth DOI: http://dx.doi.org/10.5772/intechopen.84677

#### Table 1.

Results of characterization techniques applied to TiO2 and doped TiO2: N2 physisorption, XRD and UV-Vis spectroscopy with RD.

and possibly with these ions to increase their content will no longer reduce their average crystal size. As mentioned above, the presence of rare earth ions inhibits the complete transformation of phases due to temperature; for our case, it happens at 800°C where catalysts stabilized at this temperature are present and clearly show mixtures of crystalline phases (anatase-rutile), and this is observed in Figure 3. A commercial sample of titania (P25-TiO2) was compared with the materials calcined at the highest temperature with this technique. It was observed that the materials doped with rare earth ions showed a greater intensity in the peaks corresponding to the rutile phase, which describes a greater abundance of this phase and is less than 70% because this is the approximate percentage of rutile phase reported for this solid [55]. This can be corroborated with the results of Table 1, where the percentage of crystalline phase is described; here, the samples doped and calcined at 800°C indicate mixtures of phases with close proportions for Eu and Gd (40% A-60% R); however, for the Sm, the proportion percentage was almost equal (47% A-53% R). The above can also be confirmed according to the Raman spectra shown in

Figure 3. XRD patterns of P25-TiO2 and doped TiO2 with rare earth thermally treated to 800°C.

Figure 4, in which the peaks corresponding to the anatase and rutile phases respectively are described. The phase mixtures at high temperatures can be explained due to the connection between the Ti4+ (octahedral) and RE3+ (tetrahedral) ions, where the Ti4+ ions replace the surface RE3 + ions in the network the rare earth oxide to form sites tetragonal of Ti, the interaction between atoms of Ti4+ octahedral and Ti4+ tetrahedral prevents the transformation of phases in the thermal treatment [56]. The average crystal size for these samples is also found in Table 1 and calculated individually for each crystalline phase. With respect to the anatase phase, the highest value was presented by the sample doped with Eu (34.43 nm), which is

Photocatalytic Treatment of Pesticides Using TiO2 Doped with Rare Earth DOI: http://dx.doi.org/10.5772/intechopen.84677

more than three times greater than the sample of pure titania at 500°C. However, the material doped with Sm obtained the highest average crystal size for the rutile phase (140.81 nm).

Figure 5 shows the adsorption-desorption isotherms of the materials calcined at 500°C. It is noteworthy that the incorporation of the dopant into TiO2 generates greater physical adsorption by increasing the relative pressure, explained by a possible uniform surface dispersion of the dopant, which demonstrates the increase in the specific area with respect to pure TiO2 and the capacity of the dopant. Rare

Figure 4. Raman spectra of TiO2 doped with Sm, Eu, and Gd, calcinated at 800°C.

Figure 5. Adsorption-desorption isotherms of TiO2 and doped TiO2 with Sm, Eu, and Gd calcinated al 500°C.

earth ions to form complexes with several Lewis bases. It is observed that all the isotherms have a type IV behavior according to the IUPAC classification, which is a characteristic of mesoporous solids and has multiple layer adsorption mechanism, with a hysteresis loop of type A according to the same organism, which indicates the description of mesoporous solids with capillaries in tubular form and ink cans; these samples have a desorption of similar geometric shape although their adsorption varied with respect to the metal and the amount of dopant.

The specific area values of the pure, doped, and calcined catalysts at 500 and 800°C are compiled in Table 1. As expected, the presence of the dopants in the titania increases the specific area for all materials treated at 500°C, which previously had already been described with other materials [51]. The value of this parameter was between 90.10 and 119.50 m<sup>2</sup> /g, the sample doped with Ce 0.3% presented the highest value, and this increase can be attributed to (1) the high dispersion that had the rare earth ions and this can be seen in Figure 6b with the image describing the elemental dispersion of Sm at 0.3% in titania, which also manifests with all doping ions, (2) to the impediment of rare earth ions to enter the lattice titania due to the large size of its ions, (3) the low amount of dopant that was used and (4) the reduction in the size of the crystal. This confirms that the rare earth ions inhibit the sintering of TiO2 [57].

The thermal transformation to rutile in the titania decreases considerably the specific area to 800°C and therefore, increases its crystallinity and the sintering process, but when inserting Sm and Eu to 0.3%, the area increases due to the formation of mixtures of crystalline phases, distorting the surface of the titania due to the presence of dopants. The titania catalyst doped with Eu at 0.3% and calcined at 800°C does not increase its specific area; it is the only one that shows a value below pure TiO2; this possibly at the low percentage ratio that it has anatase phase.

Figure 6. Images obtained by SEM to the Sm 0.3 TiO2 500°C: (a) morphology and (b) elemental mapping.

Photocatalytic Treatment of Pesticides Using TiO2 Doped with Rare Earth DOI: http://dx.doi.org/10.5772/intechopen.84677

The pore diameter showed an increase with respect to pure TiO2 with samples doped with La, Ce at different contents, with Eu at 0.1 and 0.3%, with Sm 0.3 and 0.5%, and with Gd 0.3 and 0.5% all calcined at 500°C. This increase is attributed to the fusion of small pores present in the anatase phase to form large pores or stacked cavities. However, the reduction in the value of this parameter is explained by the possible blocking of porous cavities by the dopants. Similarly, at 800°C, the materials showed similar behavior, but to these conditions, the increase in the pore diameter is due to pore coalescence during calcination [51].
