Fluorescence Line-Narrowing Spectroscopy as a Tool to Monitor Phase Transitions and Phase Separation in Efficient Nanocrystalline Ce_xZr_1–xO₂:Eu³⁺Catalyst Materials

Abstract

Despite the wide range of industrial applications for ceria-zirconia mixed oxides (Ce x Zr 1−x O 2 ), the complex correlation between their atomic structure and catalytic performance is still under debate. Catalytically interesting Ce x Zr 1−x O 2 nanomaterials can form homogeneous solid solutions and, depending on the composition, show phase separation under the formation of small domains. The characterization of homogeneity and atomic structure of these materials remains a major challenge. High-resolution emission spectroscopy recorded under cryogenic conditions using Eu 3+ as a structural probe in doped CeZrO 2 nanoparticles offers an effective way to identify the different atomic environments of the Eu 3+ dopants and, subsequently, to monitor structural parameters of the ceria-zirconia mixed oxides. It is found that, in stoichiometric CeZrO 2 :Eu 3+ , phase separation occurs at elevated temperatures beginning with the gradual formation of (pseudo)cubic crystallites in the amorphous materials at 500 °C and a sudden phase separation into tetragonal, zirconia-rich and cubic, ceria-rich domains over 900 °C. The presented technique allows us to easily monitor subtle changes even in amorphous, high surface area samples, yielding structural information not accessible by conventional techniques such as X-ray diffraction (XRD) and Raman. Moreover, in reference experiments investigating the reducibility of largely unordered Ce 0.2 Zr 0.8 O 2 :Eu 3+ , the main reduction peak in temperature-programmed reduction measurements appeared at exceptionally low temperatures below 200 °C, thus suggesting the outstanding potential of this oxide to activate catalytic oxidation reactions. This effect was found to be dependent on the amount of Eu 3+ dopant introduced into the CeZrO 2 matrix as well as to be connected to the atomic structure of the catalyst material.

Toward a Unified Description of Luminescence–Local Structure Correlation in Ln Doped CeO2 Nanoparticles: Roles of Ln Ionic Radius, Ln Concentration, and Oxygen Vacancies

“…For the opposite case of a phase mixture, the optical signatures of Eu in cubic ceria and tetragonal zirconia are likely to coexist, and the relative contribution of these to the overall emission depended on both the excitation wavelength and delay. Our approach based on multiwavelength time-resolved excitation is different from those used in refs and , which are based on two-wavelength steady-state excitation (at 488 or 532 nm) or time-resolved selective excitation into 5 D 0 – 7 F 0 absorption (around 579 nm) of Eu. We only note that in these reports, since all of these f–f absorptions are forbidden in cubic symmetry, the existence of Eu in a ceria-like phase may be largely ignored.…”

Tracking the Motion of Lanthanide Ions within Core–Shell–Shell NaYF4 Nanocrystals via Resonance Energy Transfer

“…High-resolution luminescence experiments were performed using Eu 3+ as an optical probe in order to complement the XRD data. , Especially for the low dopant concentrations of Eu 3+ , the detection of slight differences in the lattice (or the formation of a separate phase) can be demanding and the use of high-resolution luminescence data at ultralow temperature yields additional structural information related to the location of the Eu 3+ ions in the lattice. ,, A typical total luminescence spectrum recorded at T = 4 K is shown in Figure together with selected spectral traces for emission (top) and excitation spectra (right), respectively. The respective luminescence bands resulting from the 5 D 0 → 7 F j transition ( j = 0, 1, 2) and the 5 D 1 → 7 F 3 are shown.…”

Structural Changes from Conventional SrSnO3 to Ruddlesden–Popper Sr2SnO4 Perovskites and Its Implication on Photoluminescence and Optoelectronic Properties

“…There have been reports wherein europium luminescence was successfully explored to decipher the structural information related to symmetry, defects, distortions, local site, a in NaYF 4 (both α and β phase), KGdF 4 , CeZrO 2 , thorium oxalate, SnO 2 , ABO 3 perovskite, Zn 2 P 2 O 7 , A 2 B 2 O 7 pyrochlore, and many more systems. ,,− …”