Surface, Subsurface, and Bulk Oxygen Vacancies Quantified by Decoupling and Deconvolution of the Defect Structure of Redox-Active Nanoceria

Abstract

Oxygen vacancy concentrations are critical to the redox/photocatalytic performance of nanoceria, but their direct analysis is problematic under controlled atmospheres but essentially impossible under aqueous conditions. The present work provides three novel approaches to analyze these data from XPS data for the three main morphologies of nanoceria synthesized under aqueous conditions and tested using in vacuo analytical conditions. First, the total oxygen vacancy concentrations are decoupled quantitatively into surface-filled, subsurface-unfilled, and bulk values. Second, the relative surface areas are calculated for all exposed crystallographic planes. Third, XPS and redox performance data are deconvoluted according to the relative surface areas of these planes. Correlations based on two independent empirical results from volumetric surface XPS, combined with sequential deep XPS and independent EELS data, confirm that these approaches provide quantitative determinations of the different oxygen vacancy concentrations. Critically, the redox/photocatalytic performance depends not on the total oxygen vacancy concentration but on the concentration of the active sites on each plane in the form of subsurface-unfilled oxygen vacancies. This is verified by the pH-dependent performance, which can be increased significantly by exposing these vacancies to the surroundings. These approaches have significance to the design and engineering of semiconducting materials exposed to the environment.

Deep Eutectic Solvent‐Mediated Construction of Oxygen Vacancy‐Rich Fe‐Based Electrocatalysts for Efficient Oxygen Evolution Reaction

“…[ 24 ] As shown in Figure 3c, the O 1s high‐resolution spectrum can be deconvoluted into three peaks: oxygen atoms bound to metals (O atom at 530.23 eV), oxygen vacancy (O vacancy at 531.62 eV), and surface‐absorbed oxygen (O absorb at 532.42 eV). [ 39–41 ] In S 2p region (Figure 3d), it is found that the SO peak was located at 168.39 eV. Meanwhile, the deconvoluted peaks at 163.40, 162.25, and 161.24 eV are assigned to S 2− .…”

Design strategies for ceria nanomaterials: untangling key mechanistic concepts

“…51 In contrast, most reports of the synthesis of CeO 2Àx typically detail procedures based on precipitation and hydrothermal synthesis, which have been done at considerably lower temperatures (r200 1C). However, in the case of CeO 2Àx , the ready Ce 4+ 2 Ce 3+ redox switching, even at room temperature under the influence of pH change, 6,14 demonstrates that at least some point defects in CeO 2Àx (viz., surface defects) are mobile at low temperatures.…”

“…Ceria (CeO 2 ) is a wide band gap semiconductor of considerable importance through its multifarious uses in catalysis, [1][2][3][4] photocatalysis, 5,6 energy storage, 7 energy conversion, 2,8,9 gas sensing, 3 and health. 10 Most of these applications depend on the oxygen storage capacity of what effectively is CeO 2Àx .…”

Redox Active Cerium Oxide Nanoparticles: Current Status and Burning Issues

“…In contrast, at pH 9 the reconstruction of the surface allows for the exposure of surface and sub-surface oxygen vacancies, underscoring the importance of vacancies in the redox activity of nanoceria. [125]…”

“…In this context, it was recently demonstrated that the redox activities of nanoceria are pH dependent and that at pH 6, the ceria surface undergoes dynamic reconstruction in the presence of UV irradiation with partial removal of Ce(III) and associated surface and subsurface vacancies. In contrast, at pH 9.0 the reconstruction of the surface allows the exposure of surface and sub-surface oxygen vacancies, underscoring the importance of vacancies in the redox activity of nanoceria [124] .…”