Development of Ultra-Small Rare-Earth-Doped Cerium Oxide Nanoparticles for Environmental Remediation

Abstract

Cerium oxide nanoparticles (CeO2) have earned a well-deserved scientific interest due to their exceptional redox flexibility, which arises from the reversible transition between the Ce3+ and Ce4+ oxidation states and the subsequent generation of oxygen vacancies. This work presents the development, characterization, and evaluation of a series of nanomaterials based on CeO2 structurally modified through doping with transition metals (Iron) and rare-earth elements (Erbium) in order to enhance their physicochemical properties. The synthesis of these ultra-small (2-4 nm) nanoparticles (NPs) was carried out using a citrate complexation method via wetchemistry routes. Transmission electron microscopy (TEM) revealed the high crystallinity and homogeneous composition of the samples across almost all doping levels. From the analysis of the high-resolution images, it could be determined that the lattice fringes corresponded to the d-spacing values of the CeO2 FCC (Face-Centred Cubic) structure. Dynamic Light Scattering results were in line with the results obtained from TEM and confirmed no aggregation phenomena as particles remained stable in water media thanks to citrate functionalization. The peaks gathered from X-Ray Diffraction confirmed the CeO2 structure, with variable shifts in position strongly related to the dopant percentage. Finally, to ensure that the dopant ions got inside the structure, Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) measurements were conducted after purification of the product. As for the photocatalysis studies, methylene blue (MB) was chosen as a model dye pollutant and a series of samples of different chemical composition were tested. The results exhibited that for the doped samples the catalytic performance was enhanced in comparison to pure CeO2. This was further enhanced when Fe3+ ions were present as a Fenton-like reaction is induced. The best decolouration came from the Ce0.8Er0.1Fe0.1O2 sample, reaching a degradation of 92% of MB in less than 120 minutes due to a synergistic effect between Er3+ and Fe3+ ions, thus proving their efficacy as potential dye degradation candidates. These results also suggest a strong dependence of the catalytic properties of the NPs in the doping agent and percentage, which should be further optimized.