Bifunctional Ce1-xEuxO2 (0 ≤ x ≤ 0.3) nanoparticles for photoluminescence and photocatalyst applications: an X-ray absorption spectroscopy study

Ce1-xEuxO2 (0 <= x <= 0.3) nanoparticles (NPs) were synthesized by the chemical precipitation method. The microstructures and morphology were characterized by synchrotron X-ray diffraction and high resolution transmission electron microscopy. X-ray absorption near edge structure (XANES) spectra at the Eu M-5,(4)-edge and atomic-multiplet calculations revealed that Eu3+ was predominantly present in the CeO2 lattice and Eu2+ was negligibly present within the entire doping range. The detailed analysis of the Ce M-5,M-4-edge and the O K-edge has shown strong dependence of the Ce3+/Ce4+ ratio and oxygen vacancy with Eu content. Extended X-ray absorption fine structure (EXAFS) spectra at the Ce K-edge, along with theoretical fitting, have shown systematic variation in the coordination number, bond length and Debye-Waller factor with Eu doping. A blue shift in the absorption edge was observed which implies a net increase in the charge transfer gap between the O 2p and Ce 4f bands due to the increased number of Ce3+ ions in the Eu doped samples. The excitation and emission spectra of pure CeO2 NPs did not show any photoluminescence (PL) characteristic; however, Ce1-xEuxO2 (x = 0.1-0.3) NPs showed significant improvements in the 4f-4f, D-5(0)-F-7(2) and D-5(0)-F-7(1) transitions induced luminescence properties. Eu doping has two major effects on the electronic structure and optical properties of CeO2 NPs: the first, at an Eu content of 10 mol%, is the formation of Ce4+-O-Eu3+ networks, i.e., Eu3+ ions substitute the Ce4+ ions and introduce oxygen vacancies and Ce3+ ions in the host lattice, which favors the D-5(0)-F-7(2) induced PL properties. The other, at an Eu doping over 10 mol%, is the formation of both Ce4+-O-Eu3+ and Ce3+-O-Eu3+, i.e., Eu3+ ions not only take substitutional sites of Ce4+ ions but also replace a fraction of Ce3+ ions in the CeO2 lattice which favors D-5(0)-F-7(1) induced PL properties. As an application of CeO2 NPs towards the degradation of water pollutants, we demonstrated that the Ce1-xEuxO2 (0 <= x <= 0.3) NPs can serve as effective photocatalyst materials towards the degradation of the methyl-orange aqueous pollutant dye under UV light irradiation.