###### Abstract
Additionally, it is nontoxic, abundant and absorbs light in the near UV and also marginally in the visible region, properties which can be used in heterogeneous photocatalysis [23]. Thus, the application of CeO2 is obstructed. Many strategies have been used to enhance the catalytic/photocatalytic activity by tuning the structural and morphological characteristics [24]. One strategy to extend the utilization of visible light over CeO2 consists of suitable doping with various metal ions: isovalent cations (Ti4+, Zr4+, Hf4+, Sn4+, etc.) or aliovalent cations (Mn2+, Zn2+, Ca2+, La3+, Eu3+, Sm3+, Y3+, Eu3+, Mn3+, Sc3+, Gd3+, Sm3+, etc.) [25]. It has been demonstrated theoretically and experimentally that the substitution of cerium ions with isovalent dopant cations induces a decrease in oxygen vacancy formation energy due to the structural distortion, whereas in the case of aliovalent dopants the generation of extra oxygen vacancies into the ceria lattice has been evidenced due to the synergism between the structural distortion and electron modifications [25; 26; 27]. On the other hand, the trivalent cations of rare earth metals drastically modify the reactivity structure and optical emission of ceria [21; 28; 29; 30]. The luminescent properties of undoped or rare earth (Eu3+, Sm3+, or Tb3+)-doped ceria have been reported and the emission mechanism proposed by considering the transfer of the charge carrier between the Ce 4f conduction band and the O 2p valence band. Also, the broad photoluminescence emission in the visible domain is attributed to defects, including oxygen vacancies in the ceria lattice with electronic energy levels below the 4f band [31; 32; 33]. To date, a lot of chemical strategies have been developed to synthesize undoped or doped ceria nanoparticles, such as solid-state reactions [34], co-precipitation [35], hydrothermal techniques [36], sol-gel [37], microwave-assisted synthesis [38], ultrasonic spray pyrolysis [39], sonochemical synthesis [40], combustion method [41] and so on, proving that the physical and chemical properties are strongly dependent on the synthetic route used. In this regard, [PERSON] et al. studied the correlation between a wide range of CeO2 morphologies, such as nanorods, nanocubes, nanobeans and nanohexagons and the photocatalytic activity [42]. A similar study has been published by [PERSON] et al. with emphasis on the Eu-doped ceria nanostructures [43]. It has been demonstrated that enhanced photocatalytic performance is obtained by increasing the charge carrier concentration and light absorption capacity by defects induced by the dopants into CeO\({}_{2}\) nanostructures. Nevertheless, studies are still ongoing for a deeper understanding of the correlations between the experimental parameters, structure, morphology, luminescence properties and photocatalytic activity. Considering the previous research works reported in the literature, this paper focuses on the effects induced by the incorporation into the ceria lattice of Eu\({}^{3+}\), Gd\({}^{3+}\) and Y\({}^{3+}\) aliovalent cations by studying the correlations between the structural and morphological characteristics and luminescence properties and photocatalytic activity. These materials have been synthesized by a green hybrid sol-gel combustion method and characterized by using various techniques, namely X-ray Diffraction, Transmission Electron Microscopy, UV-Vis spectroscopy, Photoluminescence (PL) and Electron Paramagnetic Resonance (EPR). Finally, we attempt to provide an overview of how the selected rare earth ions influence the ceria's structure, morphology and photocatalytic activity.