Nanoscale oxygen defect gradients in UO _{2+ x} surfaces

Abstract

Oxygen defects govern the behavior of a range of materials spanning catalysis, quantum computing, and nuclear energy. Understanding and controlling these defects is particularly important for the safe use, storage, and disposal of actinide oxides in the nuclear fuel cycle, since their oxidation state influences fuel lifetimes, stability, and the contamination of groundwater. However, poorly understood nanoscale fluctuations in these systems can lead to significant deviations from bulk oxidation behavior. Here we describe the use of aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy to resolve changes in the local oxygen defect environment in UO2+x surfaces. We observe large image contrast and spectral changes that reflect the presence of sizable gradients in interstitial oxygen content at the nanoscale, which we quantify through first-principles calculations and image simulations. These findings reveal an unprecedented level of excess oxygen incorporated in a complex near-surface spatial distribution, offering additional insight into defect formation pathways and kinetics during UO2 surface oxidation.

Insight into the structure–property relationship of UO2nanoparticles

“…2 Nanoscale UO 2 is readily oxidized with the formation of UO 2+x , while the crystal structure does not significantly alter. [35][36][37] Similar AnO 2+x NPs with a structure close to bulk AnO 2 were also observed for plutonium, 38 which is not surprising as both UO 2 and PuO 2 are isostructural to the fluorite-type fcc structure with a very similar lattice parameter. However, recent publications show that PuO 2 NPs do not contain other oxidation states except for Pu(IV) 39,40 and their structural properties are close to the bulk.…”

“…Please do not adjust margins Please do not adjust margins therefore, to exist as UO 2+x , with some of the U oxidized at the surface. [35][36][37] However, it was found that U(IV) is the dominant oxidation state of the UO 2 NPs, synthesized by fast chemical deposition method at pH 8-11, but their stability is significantly lower then bulk-UO 2 in terms of time and oxidation sensitivity. They are easily oxidized not only in the air but also slowly under inert conditions or during X-ray exposure.…”

Probing electronic structure in berkelium and californium via an electron microscopy nanosampling approach

“…In light of these challenges, the use of transmission electron microscopy (TEM), a technique allowing detection and investigation of even single atoms, seems like a promising approach [15][16][17][18][19][20] . Yet, we are only aware of two research groups that have used TEM to study transplutonium elements.…”

Probing Multiscale Disorder in Pyrochlore and Related Complex Oxides in the Transmission Electron Microscope: A Review

“…STEM techniques have been used to explore the structure of various pyrochlore and fluorite systems ( Gallagher et al, 2016 ; Sachan et al, 2017b ; Sachan et al, 2018 ; Sachan et al, 2019 ; Spurgeon et al, 2019 ; Lee et al, 2020 ; Spurgeon, 2020 ). See reference ( MacLaren and Ramasse, 2014 ) for a review of STEM techniques applied to functional oxides beyond pyrochlore and fluorite systems.…”