Oxidation-state sensitive imaging of cerium dioxide by atomic-resolution low-angle annular dark field scanning transmission electron microscopy

Abstract

Low-angle annular dark field (LAADF) scanning transmission electron microscopy (STEM) imaging is presented as a method that is sensitive to the oxidation state of cerium ions in CeO2 nanoparticles. This relationship was validated through electron energy loss spectroscopy (EELS), in situ measurements, as well as multislice image simulations. Static displacements caused by the increased ionic radius of Ce3+ influence the electron channeling process and increase electron scattering to low angles while reducing scatter to high angles. This process manifests itself by reducing the high-angle annular dark field (HAADF) signal intensity while increasing the LAADF signal intensity in close proximity to Ce3+ ions. This technique can supplement STEM-EELS and in so doing, relax the experimental challenges associated with acquiring oxidation state information at high spatial resolutions.

In situ oxidation and reduction of cerium dioxide nanoparticles studied by scanning transmission electron microscopy

“…In electron microscopy studies it is commonly reported that the surface of CeO 2 nanoparticles are reduced. This observation has been made with direct measurements using STEM-EELS and imaging techniques (Goris et al, 2014; Haigh et al, 2011; Johnston-Peck et al, 2016b; Lin et al, 2014; Spadaro et al, 2016; Turner et al, 2011; Wu et al, 2004), and can be inferred by observing the Ce 3+ concentration in ceria particles is inversely related to diameter (Sims et al, 2018; Spadaro et al, 2016; Wu et al, 2004). To determine if the microscope environment influences the oxidation state of CeO 2 particles the oxygen chemical potential of the ESTEM was changed.…”

“…This can be understood as atomic static displacements in proximity to oxygen vacancies affecting channeling of the incident electrons along the atomic columns while suppressing coherent scattering. The phenomenon of points defects influencing electron scattering has been covered in-depth elsewhere (Cowley, 1995; Grillo et al, 2008; Hall et al, 1966; Johnston-Peck et al, 2016b; Muller et al, 2004; Perovic et al, 1993; Rossouw et al, 1994). Practically this means under equivalent conditions a HAADF image of CeO 2-x (fluorite structure with oxygen vacancies or anion-deficient fluorite) will exhibit lower intensity (image grayscale values) than CeO 2 (fluorite structure without oxygen vacancies) thereby generating contrast.…”

“…The probe convergence angle was ≈ 13.5 mrad; HAADF-STEM images were acquired with collection angles of ≈ (88 to 175) mrad or ≈ (112 to 168) mrad. CeO 2 nanocubes were synthesized using wet chemical methods, details can be found in previous reports (Johnston-Peck et al, 2016a; Johnston-Peck et al, 2016b). CeO 2 truncated octahedron and nanoparticles were acquired from Sigma-Aldrich and Strem Chemicals, respectively.…”

“…To this point, when the sample is oriented to a high-symmetry-crystallographic axis, electron channeling becomes pronounced and local changes in the sample can manifest as contrast within the resulting ADF images. In CeO 2 specifically, atomic static displacements due to oxygen vacancies and the different ionic radius of Ce 3+ and Ce 4+ cations will generate contrast (Johnston-Peck et al, 2016b). The relative simplicity of a single-crystal nanomaterial, without additional features (e.g., line or planar defects) that can also be sources of contrast, presents a situation where the image contrast can be interpreted beyond the mass-thickness relationship of high-angle annular dark field STEM (HAADF-STEM) images to being qualitatively sensitive to relative differences in the extent of reduction.…”

Dose-rate-dependent damage of cerium dioxide in the scanning transmission electron microscope

“…This technique has been used to identify strain fields around interfaces and line defects [15, 16]. It is also sensitive to point defects [17, 18] and has been used to identify regions in CeO 2 nanoparticles that were reduced [19]. By using this imaging technique in combination with EELS we were able to quantitatively assess the effects of electron beam irradiation in CeO 2 nanocubes.…”