New Insights into the Bulk and Surface Defect Structures of Ceria Nanocrystals from Neutron Scattering Study

Cite this: Chem. Mater. 2021, 33, 11, 3959–3970
Publication Date (Web):May 27, 2021
https://doi.org/10.1021/acs.chemmater.1c00156
Copyright © 2021 American Chemical Society
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Abstract

Neutron diffraction and pair distribution function studies coupled with Raman spectroscopy have successfully unraveled the detailed oxygen defect structures of ceria nanocubes and nanorods. Two types of defect sites are revealed for the ceria nanocrystals: surface and bulk defects. It is proposed that the surface oxygen defects in both types of CeO2 nanocrystals are predominantly the partially reduced Ce3O5+x, with the bulk defect structures dominated by interstitial Frenkel-type oxygen vacancies. Ceria nanorods possess much higher concentration of surface oxygen defects relative to the nanocubes, albeit with only slightly higher concentration of bulk Frenkel-type oxygen vacancies. Upon annealing the nanorod sample at 600 °C under vacuum (∼10–4 to 10–5 mbar), a partially reduced ceria phase with long-range oxygen vacancy ordering (Ce3O5+x) has been observed experimentally for the first time. This intriguing observation that surface defect phases can take on ordered defect sublattices under certain conditions is of great value in understanding the temperature-dependent catalytic performance of ceria nanocrystals. Furthermore, a drastic decrease of the surface vacancies in the ceria nanocrystals is observed upon exposure to SO2, especially for the nanorods, a likely origin for the sulfur poisoning effect on ceria-based materials. This study suggests that tailoring surface morphology is a promising strategy to control defect properties of ceria nanomaterials. It also provides fundamental insights to stabilize surface oxygen defects in CeO2 nanocrystals to achieve high redox performance under corrosive environments such as under SO2/SOx exposure.

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.1c00156.

  • Structure of the fluorite CeO2; Rietveld refinements of nanorod/nanocube using neutron Bragg diffraction data; Fourier difference map for identifying residual scattering intensities in nanocube/nanorod; long-range oxygen vacancy-ordered structure (Ce3O5+x) based on {111}, {110}, and {100} facets; local and average structure refinements of nanorod/nanocube using neutron PDF data; PDF data of nanorod and nanocube samples at elevated temperature; UV Raman spectra of pre-reduced CeO2 nanocube and nanorod; structure refinement results (PDF); and DFT results for formation energies of different oxygen vacancy arrangments in the three plausible bulk Ce3O5 phases.

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