Publication date: 10th April 2024
In the vast majority of technologically relevant electrochemical devices, the reactions of interest occur at interfaces, where defects often serve to mediate the reaction pathway. In oxides, point defects, and in particular surface oxygen vacancies are generally the relevant defects in mediating electrocatalysis. Furthermore, surface defect formation energies are often lower than those of the bulk, resulting in defect enrichment on surfaces. In the case of cerium dioxide (ceria), an exemplary (electro)catalyst material, computational prediction of enhanced oxygen vacancy concentrations on {011} surfaces has motivated the development of synthetic routes for preparing nanoparticles terminated with these surfaces. In many instances, such nanoparticles indeed display higher activity for reactions of interest than those with predominant {100} or {111} termination. Yet, studies quantifying the concentration and crystallographic environment of such defects, particularly under relevant reaction conditions, are scarce. Here we demonstrate glancing angle X-ray absorption spectroscopy about Ce L edges as a tool for studying the surface features of epitaxial ceria thin films. Analysis of the near-edge (XANES) portion of the data yields the Ce3+ concentration, whereas the thin-film geometry avoids ambiguities that otherwise arise from the corners and edges of nanoparticles and provides access to (001), (110) and (111) termination via selection of the substrate (yttria-stabilized zirconia) termination. The use of X-rays enables easy integration with environmental chambers for gas and temperature control, a stark contrast to electron-based methods. At selected conditions we find the surface Ce3+ concentration to be surprisingly insensitive to termination. Using a series of angle-resolved measurements, we extract the depth profile for the (111) termination at high temperature (800 °C) and reducing conditions (oxygen partial pressure of 10-19 atm). We find the surface to be essentially fully reduced, followed by a sharp drop in Ce3+ concentration to the bulk value within approximately 5 nm of the surface. The implications of these findings on (electro)catalytical reaction pathways and rates is discussed.
US National Science Foundation Award DMR-2130831
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