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

Abstract

Ce_1−xEu_xO₂ (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_5,4-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, Ce_1−xEu_xO₂ (x = 0.1–0.3) NPs showed significant improvements in the 4f–4f, ⁵D₀–⁷F₂ and ⁵D₀–⁷F₁ 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 ⁵D₀–⁷F₂ 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 ⁵D₀–⁷F₁ induced PL properties. As an application of CeO₂ NPs towards the degradation of water pollutants, we demonstrated that the Ce_1−xEu_xO₂ (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.