Electronic structure, charge transfer, and intrinsic luminescence of gadolinium oxide nanoparticles: Experiment and theory

Abstract

The cubic (c) and monoclinic (m) polymorphs of Gd 2 O 3 were studied using the combined analysis of several materials science techniques – X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy. Density functional theory (DFT) based calculations for the samples under study were performed as well. The cubic phase of gadolinium oxide (c-Gd 2 O 3 ) synthesized using a precipitation method exhibits spheroidal-like nanoclusters with well-defined edges assembled from primary nanoparticles with an average size of 50 nm, whereas the monoclinic phase of gadolinium oxide (m-Gd 2 O 3 ) deposited using explosive pyrolysis has a denser structure compared with natural gadolinia. This phase also has a structure composed of three-dimensional complex agglomerates without clear-edged boundaries that are ∼21 nm in size plus a cubic phase admixture of only 2 at.% composed of primary edge-boundary nanoparticles ∼15 nm in size. These atomic features appear in the electronic structure as different defects ([Gd…O–OH] and [Gd…O–O]) and have dissimilar contributions to the charge-transfer processes among the appropriate electronic states with ambiguous contributions in the Gd 5р – O 2s core-like levels in the valence band structures. The origin of [Gd…O–OH] defects found by XPS was well-supported by PL analysis. The electronic and atomic structures of the synthesized gadolinias calculated using DFT were compared and discussed on the basis of the well-known joint OKT–van der Laan model, and good agreement was established. © 2017