Entrapped Molecule‐Like Europium‐Oxide Clusters in Zinc Oxide with Nearly Unaffected Host Structure

Abstract

Nanocrystalline ZnO sponges doped with 5 mol% EuO1.5 are obtained by heating metal–salt complex based precursor pastes at 200–900 °C for 3 min. X‐ray diffraction, transmission electron microscopy, and extended X‐ray absorption fine structure (EXAFS) show that phase separation into ZnO:Eu and c‐Eu2O3 takes place upon heating at 700 °C or higher. The unit cell of the clean oxide made at 600 °C shows only ≈0.4% volume increase versus undoped ZnO, and EXAFS shows a ZnO local structure that is little affected by the Eu‐doping and an average Eu3+ ion coordination number of ≈5.2. Comparisons of 23 density functional theory‐generated structures having differently sized Eu‐oxide clusters embedded in ZnO identify three structures with four or eight Eu atoms as the most energetically favorable. These clusters exhibit the smallest volume increase compared to undoped ZnO and Eu coordination numbers of 5.2–5.5, all in excellent agreement with experimental data. ZnO defect states are crucial for efficient Eu3+ excitation, while c‐Eu2O3 phase separation results in loss of the characteristic Eu3+ photoluminescence. The formation of molecule‐like Eu‐oxide clusters, entrapped in ZnO, proposed here, may help in understanding the nature of the unexpected high doping levels of lanthanide ions in ZnO that occur virtually without significant change in ZnO unit cell dimensions.

Interplay between Growth Mechanism, Materials Chemistry, and Band Gap Characteristics in Sputtered Thin Films of Chalcogenide Perovskite BaZrS3

“…10−20 nm) precipitate out of the ZnO lattice when annealed. 51 Our description of annealing-induced phase distribution in Ba−Zr−S thin films closely resembles the chemical inhomogeneity captured by STEM-EDS spectral imaging of the BZS-900 film, which clearly revealed areas of the film containing Zrand O-rich zones, which are Ba and S-deficient (ref 37, Figure 7). When BaZrS 3 crystallites grow in size with increasing annealing temperature, one expects ZrO 2 and other secondary oxide and sulfide phases to be redistributed more toward crystallite boundaries, which eventually emerge out of the film upon further annealing (1000 °C).…”

“…Similar observations were made in Eu doping in ZnO nanosponges, where molecular Eu–O clusters embedded in hexagonal ZnO crystallites of similar dimensions (ca. 10–20 nm) precipitate out of the ZnO lattice when annealed …”

Atomistic structure investigation of Eu–doped ZnO nano-sponges

“…EXAFS and DFT modelling suggested that (EuO1.5)4-8 clusters were present inside the ZnO crystals, in excellent agreement with the experimental data. 22 This surprising finding explained how large amount of Eu-ions can be incorporated into ZnO without much change of XRD unit cell-parameters. With such high Eu-loading there should only be less than ca.…”

“…These sponges showed XRD patterns completely assignable to h-ZnO with very minor changes in unit cell-parameters. 22 However, TEM, XPS, TGA and IR spectroscopy showed the presence of minor amounts of organic residues which were replaced by Eu-oxo-carbonate when heating at 300 -500 o C for 3 minutes. Heating at 600 o C for 3 min.…”

“…Herein, XRD and IR spectroscopy were used to find out the temperature limits for the presence of carbonate (lowest T) and phase separation (highest T) versus Eu-oxide concentration. Considering the doping model with 4 to 8 Eu membered non-periodic oxide clusters in crystalline h-ZnO given in literature, 22 it should only be possible to reach 10-15% EuO1.5 in the ZnO before there is not enough ZnO to cover each cluster with one layer of ZnO. However, XRD and IR spectroscopy describe the materials as an average over the volume of the material which does not allow for obtaining important details on where the Eu resides, its oxidation state, the Zn-and Eu-to-oxygen coordination spheres, and the presence of more than one phase.…”