## 3 Results and Discussion
than those corresponding to ceria (2.98 eV)--Table 1. The band gap narrowing of CeO\({}_{2}\) by doping with RE ions can be explain by the formation of 4\(f\) for Gd and Eu or 4\(d\) for Y states below the conduction band edge, which extend the absorption of ceria into the visible region. Photoluminescence spectroscopy is considered an important technique for understanding the role of the recombination of electron pairs and defects in modifying the photocatalytic activity of a material. The registered spectra for the ceria-based samples are presented in Figure 4. The literature data reported that the energy of emitted photons due to the electron transitions from the 4\(f\) band to the valence band in CeO\({}_{2}\) lies in the range of 3.0-3.8 eV [21]. The band centred at 364 nm, presented in all spectra, should be attributed to this transition. Also, it is reported that the emission bands located from 400 to  
Figure 3: (**a**) Room temperature optical absorption spectra and (**b**) Tauc plots of undoped and RE doped ceria nanoparticles. Figure 2: TEM images of undoped ceria, CeO\({}_{2}\); Y-doped CeO\({}_{2}\), CeO\({}_{2}\);Y; Eu-doped ceria, CeO\({}_{2}\):Eu; and Gd-doped ceria, CeO\({}_{2}\):Gd. The scale bar is 50 nm for all images, except the inset of CeO\({}_{2}\):Y; here, the scale bar is 2 μm. 500 nm are attributed to some defects, such as vacancies or dislocations in CeO\({}_{2}\)[21; 55]. Therefore, the conclusive transitions from Ce 4\(f\) level to O 2\(p\) level due to abundant defects like O vacancies and dislocations are responsible for emission peaks from the 430 nm to 494 nm domains observed in all spectra. The replacement of Ce\({}^{4+}\) cations by RE\({}^{3+}\) cations in the lattice of CeO\({}_{2}\) generates the formation of defects and oxygen vacancies in order to preserve the charge balance. The photoluminescence spectrum of Eu-doped ceria is significantly different from the other registered spectra, showing well-defined emission bands in the 540-620 region. These bands result directly from the incorporation of the Eu\({}^{3+}\) ions into the CeO\({}_{2}\) lattice. Depending on the excitation wavelength, possible f-f transitions of Eu\({}^{3+}\) cations (\({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{1}\)) take place at 550 nm (J = 0), 593 nm (J = 1), 610-630 nm (J = 2), 650 nm (J = 3), and 710 nm (J = 4) [56]. It is already known that the Ce site in the fluorite CeO\({}_{2}\) lattice presents Oh symmetry with eightfold oxygen coordination. Therefore, the Ce site possesses inversion symmetry and the electric dipole emission \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{2}\) could be forbidden and could not be observed only if the Eu\({}^{3+}\) ion occupied the Ce site without changing the original inversion symmetry. When Eu\({}^{3+}\) replaces Ce\({}^{4+}\), it needs charge compensation in order to obtain a charge equilibrium. Two kinds of defects could be formed during charge compensation: * interstitial Eu\({}^{3+}\) (four Eu\({}^{3+}\) replacing three Ce\({}^{4+}\)) and **O\({}^{2-}\) vacancies (one Eu\({}_{2}\)O\({}_{3}\) replacing two CeO\({}_{2}\)). These defects induce the symmetry distortion of the local environment around the Eu\({}^{3+}\) ions. Moreover, due to the difference in ionic radius between Ce\({}^{4+}\) and Eu\({}^{3+}\), the replacement of Ce\({}^{4+}\) ions with Eu\({}^{3+}\) ions also generates the symmetry distortion of the local environment around the Eu\({}^{3+}\) ions [56]. The presence in the spectrum (\(\lambda_{\rm{exc}}\) = 325 nm) of \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{0}\) transition, around 549 nm, \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{1}\) transition at 592 nm and \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{2}\) at 606 and 612 nm, respectively, suggests that Eu\({}^{3+}\) cations replace Ce\({}^{4+}\) sites in the CeO\({}_{2}\) lattice and moreover is indicative of the formation of oxygen vacancies [57; 58; 59; 60]. To evidence the presence of paramagnetic species, the EPR technique was used.