Atomic Scale Characterization of Fluxional Cation Behavior on Nanoparticle Surfaces: Probing Oxygen Vacancy Creation/Annihilation at Surface Sites

Abstract

Oxygen vacancy creation and annihilation are key processes in nonstoichiometric oxides such as CeO 2 . The oxygen vacancy creation and annihilation rates on an oxide's surface partly govern its ability to exchange oxygen with the ambient environment, which is critical for a number of applications including energy technologies, environmental pollutant remediation, and chemical synthesis. Experimental methods to probe and correlate local oxygen vacancy reaction rates with atomic-level structural heterogeneities would provide significant information for the rational design and control of surface functionality; however, such methods have been unavailable to date. Here, we characterize picoscale fluxional behavior in cations using time-resolved in situ aberration-corrected transmission electron microscopy to locate atomic-level variations in oxygen vacancy creation and annihilation rates on oxide nanoparticle surfaces. Low coordination number sites such as steps and edges, as well as locally strained sites, exhibited the greatest number of cation displacements, implying enhanced surface oxygen vacancy activity at these sites. The approach has potential applications to a much wider class of materials and catalysis problems involving surface and interfacial transport functionalities.

Atomic level fluxional behavior and activity of CeO2-supported Pt catalysts for CO oxidation

“…In comparison, the subsurface and bulk Ce sites show sharp and clearly resolved atomic columns. The diffuse contrast or local streaking/blurring observed at the CeO 2 surface and Pt/CeO 2 interface is therefore not due to drift in the hot stage or the electron optics, but instead arises from dynamic structural reconfigurations occurring at these specific sites 25 .…”

Probing atom dynamics of excited Co-Mo-S nanocrystals in 3D

“…In fact, nano-scale objects have functional properties that strongly depend on the surface atoms. The surface atoms have reduced coordination, which consequently makes them more prone—compared to bulk atoms—to alterations induced by any stimuli, including gas/liquid environments, heat, electromagnetic fields as well as electron beams 11 – 16 . 3D atomic-resolution images have uncovered atom dynamics under an assumed uniform excitation 11 , 12 and 2D projected atomic-resolution images have revealed atom displacements at nanoparticle surfaces 15 , even with a site-dependency 16 .…”

“…The surface atoms have reduced coordination, which consequently makes them more prone—compared to bulk atoms—to alterations induced by any stimuli, including gas/liquid environments, heat, electromagnetic fields as well as electron beams 11 – 16 . 3D atomic-resolution images have uncovered atom dynamics under an assumed uniform excitation 11 , 12 and 2D projected atomic-resolution images have revealed atom displacements at nanoparticle surfaces 15 , even with a site-dependency 16 . Thus, the dynamic behavior of nano-scale objects is generally modulated in space and time and expected to heterogenize the electron microscopy image intensities and contrast blurring.…”

Describing Atomic-Level Fluxional Behavior in Nanoparticles

“…In situ electron microscopy in gas and liquid environments provides critical information on a wide range of materials phenomena such as nucleation and growth, phase transformations, and diffusion processes. Timeresolved in situ TEM observations can provide detailed atomic-level information on kinetic pathways that operate when the material is in its working state, offering the ability to directly observe dynamic changes in the material in real time [1][2][3][4]. For many processes, such as catalytic reactions or phase changes, materials may show significant atomic level structural dynamics or so-called fluxional behavior.…”