###### Abstract  
Our study is focused on the structural and morphological characteristics, optical behaviour and photocatalytic properties of undoped and 5 at% Eu\({}^{3+}\)-, Gd\({}^{3+}\)- and Y\({}^{3+}\)-doped CeO\({}_{2}\) nanoparticles prepared by a green hybrid sol-gel combustion method. Several techniques such as X-ray diffraction powder (XRD), Transmission Electron Microscopy (TEM), UV-Vis spectroscopy, Photoluminescence spectroscopy (PL) and Electron Paramagnetic Resonance (EPR) have been used to investigate the obtained samples. Moreover, the correlation between the characteristics and properties has been studied. The nanoparticles observed by TEM exhibit a pseudo-spherical shape, except for Y\({}^{3+}\)-doped CeO\({}_{2}\), which presents an acicular shape. The average size of undoped and rare-earth-doped CeO\({}_{2}\) nanoparticles is below 10 nm, in good agreement with the calculations performed based on XRD analyses. From UV-Vis analyses it has been deduced that with doping the band gap energy decreases, which shows that additional levels are introduced by doping into the CeO\({}_{2}\) band gap. The EPR spectra evidence similar behaviour for all doped samples. The photocatalytic activity was evaluated by the degradation of rhodamine B (RhB) under UV light irradiation. The photodegradation mechanism has been studied in depth based on the formation of electron-hole pairs, and to evidence the reactive oxygen species, ESR coupled with spin-trapping experiments was performed. In the case of Y-doped CeO\({}_{2}\) nanoparticles, the generation of both \(\bullet\)OOH and \(\bullet\)O\({}_{2}^{-}\) radicals involved in RhB photodegradation was highlighted. Acticle phenomena, electronic properties, electronic properties, electronic properties, redox properties,  
contaminants from wastewater is Advanced Oxidation Processes (AOPs). AOPs using semiconductor photocatalysts have been proven to be a promising technique for water disinfection due to its high efficiency, acceptable price, high stability and the capability to degrade organic pollutants and mineralization to CO2 and H2O as the final products [6]. The efficiency of the photocatalytic process depends on the operational parameters that dominate the adsorption and photodegradation of dye molecules. Among these parameters, the most important are the initial pH of the dye solution, the concentration of the photocatalyst and dye, the reaction temperature and the dopants content [6; 7]. [PERSON] et al. show that controlling the incident light intensity and wavelength is a mode for controlling and tuning the activity and selectivity of the catalyst [8]. Also, in a doped semiconductor photocatalyst, the positively or negatively charged microenvironment around trapped holes or electrons influences the ability of photocatalyst to generate reactive oxygen species (ROS) involved in reactions of photo-oxidation and photo-reduction [9; 10]. A great number of semiconductor metal oxides, such as TiO2 [11], ZnO [12], SnO2 [13], etc., have been used as photocatalytic materials. As one of the most popular metal oxide semiconductors, the rare earth oxide ceria (CeO2) is extensively used in many fields due to its ability to extract, store and release oxygen. Its applicative potential based on this special feature includes domains like catalysis, photocatalysis, solid oxide fuel cells, ceramics, sunscreens, solar cells, gas sensors and biomedical areas such as biosensing, bioimaging or targeted drug therapy, etc. [14; 15; 16; 17; 18; 19]. Moreover, ceria has proved to be an ideal candidate as an artificial oxygen modulator, and this is vital for many biological processes [20]. Also, nanoceria is well-known for its enhanced catalytic reactivity due to easy conversion between cerium's tetravalent and trivalent valence surface states [21]. In ceria, the tetravalent cerium ion, Ce4+, transforms into a trivalent state, Ce3+, and this transformation produces a good yield of oxygen vacancies without changes in the chemical environment [22].