Tracking Oxygen Vacancies in Thin Film SOFC Cathodes

ACS Nano, 2013

Spatial localization of the oxygen reduction/evolution reactions on lanthanum strontium cobaltite (LSCO) surfaces with perovskite and layered perovskite structures is studied at the sub-10 nm level. Comparison between electrochemical strain microscopy (ESM) and structural imaging by scanning transmission electron microscopy (STEM) suggests that small-angle grain boundaries act as regions with enhanced electrochemical activity. The ESM activity is compared across a family of LSCO samples, demonstrating excellent agreement with macroscopic behaviors. This study potentially paves the way for deciphering the mechanisms of electrochemical activity of solids on the level of single extended structural defects such as grain boundaries and dislocations.

Nature Materials, 2012

Oxygen vacancy distributions and dynamics directly control the operation of solid-oxide fuel cells and are intrinsically coupled with magnetic, electronic and transport properties of oxides. For understanding the atomistic mechanisms involved during operation of the cell it is highly desirable to know the distribution of vacancies on the unit-cell scale. Here, we develop an approach for direct mapping of oxygen vacancy concentrations based on local lattice parameter measurements by scanning transmission electron microscopy. The concept of chemical expansivity is demonstrated to be applicable on the subunit-cell level: local stoichiometry variations produce local lattice expansion that can be quantified. This approach was successfully applied to lanthanum strontium cobaltite thin films epitaxially grown on substrates of different symmetry, where polarized neutron reflectometry revealed a strong difference in magnetic properties. The different vacancy content found in the two films suggests the change in oxygen chemical potential as a source of distinct magnetic properties, opening pathways for structural tuning of the vacancy concentrations and their gradients.

Journal of Applied Physics, 2011

Ferroelectric domain nucleation and growth in multiferroic BiFeO 3 films is observed directly by applying a local electric field with a conductive tip inside a scanning transmission electron microscope. The nucleation and growth of a ferroelastic domain and its interaction with pre-existing 71˚ domain walls are observed and compared with the results of phase-field modeling. In particular, a preferential nucleation site and direction-dependent pinning of domain walls is observed due to slow kinetics of metastable switching in the sample without a bottom electrode. These in-situ spatially-resolved observations of a first-order bias-induced phase transition reveal the mesoscopic mechanisms underpinning functionality of a wide range of multiferroic materials.

Journal of Chemical Theory and Computation, 2017

Strong electronic correlations, interfaces, defects and disorder each individually challenge theoretical methods for predictions of materials properties. These challenges are all simultaneously present in complex transition metal-oxide interfaces and superlattices, which are known to exhibit unique and unusual properties caused by multiple coupled degrees of freedom and strong electronic correlations. Here we show that ab initio quantum Monte-Carlo (QMC) solutions of the many-electron problem are now possible for the full complexity of these systems. Within a single non-empirical theoretical approach, we unambiguously establish the site-specific stability of oxygen vacancies in the (LaFeO 3) 2 /(SrFeO 3) superlattice, accounting for experimental data, and predict their migration pathways. QMC calculations are now capable of playing a major role in the elucidation of many-body phenomena in complex oxides previously out of reach of first-principles theories.

Applied Physics Letters, 2019

The presence and potential ordering of oxygen vacancies plays an important role in determining the electronic, ionic and thermal transport properties of many transition metal oxide materials. Controlling the concentration of oxygen vacancies, as well as the structures of ordered oxygen vacancy domains has been the subject of many experimental and theoretical studies. In epitaxial thin films, the concentration of oxygen vacancies and the type of ordering depends on the structure of the substrate as well as the lattice mismatch between the thin films and the substrate. However, the role of temperature or structural phase transitions in either the substrate or the epitaxial thin films on the oxygen vacancy ordering has remained largely unexplored. In particular, atomicresolution imaging and spectroscopy analysis of oxygen vacancy ordering in thin films at temperatures below 300 K has not yet been reported. Here, we use aberration-corrected scanning transmission electron microscopy (STEM) combined with an in-situ cooling experiments to characterize the atomic/electronic structures of oxygen-deficient La0.5Sr0.5CoO3-δ thin films grown on SrTiO3 across its anti-ferrodistortive phase transition at 105 K. We demonstrate that atomicresolution imaging and electron energy-loss spectroscopy (EELS) can be used to examine variations in the local density of states as a function of sample temperature.

nature materials | VOL 3 | MARCH 2004 | www.nature.com/naturematerials 143 T he design of catalysts with desired chemical and thermal properties is viewed as a grand challenge for scientists and engineers 1 . For operation at high temperatures, stability against structural transformations is a key requirement. Although doping has been found to impede degradation, the lack of atomistic understanding of the pertinent mechanism has hindered optimization. For example, porous γ-Al 2 O 3 , a widely used catalyst and catalytic support 2-6 , transforms to non-porous α-Al 2 O 3 at ~1,100°C (refs 7-10). Doping with La raises the transformation temperature 8-11 to ~1,250°C, but it has not been possible to establish if La atoms enter the bulk, adsorb on surfaces as single atoms or clusters, or form surface compounds 10-15 . Here, we use direct imaging by aberrationcorrected Z-contrast scanning transmission electron microscopy coupled with extended X-ray absorption fine structure and firstprinciples calculations to demonstrate that, contrary to expectations, stabilization is achieved by isolated La atoms adsorbed on the surface. Strong binding and mutual repulsion of La atoms effectively pin the surface and inhibit both sintering and the transformation to α-Al 2 O 3 .

Nature Materials, 2004

nature materials | VOL 3 | MARCH 2004 | www.nature.com/naturematerials 143 T he design of catalysts with desired chemical and thermal properties is viewed as a grand challenge for scientists and engineers 1 . For operation at high temperatures, stability against structural transformations is a key requirement. Although doping has been found to impede degradation, the lack of atomistic understanding of the pertinent mechanism has hindered optimization. For example, porous γ-Al 2 O 3 , a widely used catalyst and catalytic support 2-6 , transforms to non-porous α-Al 2 O 3 at ~1,100°C (refs 7-10). Doping with La raises the transformation temperature 8-11 to ~1,250°C, but it has not been possible to establish if La atoms enter the bulk, adsorb on surfaces as single atoms or clusters, or form surface compounds 10-15 . Here, we use direct imaging by aberrationcorrected Z-contrast scanning transmission electron microscopy coupled with extended X-ray absorption fine structure and firstprinciples calculations to demonstrate that, contrary to expectations, stabilization is achieved by isolated La atoms adsorbed on the surface. Strong binding and mutual repulsion of La atoms effectively pin the surface and inhibit both sintering and the transformation to α-Al 2 O 3 .

The Journal of Physical Chemistry Letters, 2014

Heterostructured oxides have shown unusual electrochemical properties including enhanced catalytic activity, ion transport, and stability. In particular, it has been shown recently that the activity of oxygen electrocatalysis on the Ruddlesden−Popper/ perovskite (La 1-y Sr y ) 2 CoO 4±δ /La 1−x Sr x CoO 3−δ heterostructure is remarkably enhanced relative to the Ruddlesden−Popper and perovskite constituents. Here we report the first atomic-scale structure and composition of (La 1−y Sr y ) 2 CoO 4±δ /La 1−x Sr x CoO 3−δ grown on SrTiO 3 . We observe anomalous strontium segregation from the perovskite to the interface and the Ruddlesden−Popper phase using direct X-ray methods as well as with ab initio calculations. Such Sr segregation occurred during the film growth, and no significant changes were found upon subsequent annealing in O 2 . Our findings provide insights into the design of highly active catalysts for oxygen electrocatalysis.

Nat Mater, 2004

nature materials | VOL 3 | MARCH 2004 | www.nature.com/naturematerials 143 T he design of catalysts with desired chemical and thermal properties is viewed as a grand challenge for scientists and engineers 1 . For operation at high temperatures, stability against structural transformations is a key requirement. Although doping has been found to impede degradation, the lack of atomistic understanding of the pertinent mechanism has hindered optimization. For example, porous γ-Al 2 O 3 , a widely used catalyst and catalytic support 2-6 , transforms to non-porous α-Al 2 O 3 at ~1,100°C (refs 7-10). Doping with La raises the transformation temperature 8-11 to ~1,250°C, but it has not been possible to establish if La atoms enter the bulk, adsorb on surfaces as single atoms or clusters, or form surface compounds 10-15 . Here, we use direct imaging by aberrationcorrected Z-contrast scanning transmission electron microscopy coupled with extended X-ray absorption fine structure and firstprinciples calculations to demonstrate that, contrary to expectations, stabilization is achieved by isolated La atoms adsorbed on the surface. Strong binding and mutual repulsion of La atoms effectively pin the surface and inhibit both sintering and the transformation to α-Al 2 O 3 .

ACS Nano

The robust approach for real-time analysis of the scanning transmission electron microscopy (STEM) data streams, based on the ensemble learning and iterative training (ELIT) of deep convolutional neural networks, is implemented on an operational microscope, enabling the exploration of the dynamics of specific atomic configurations under electron beam irradiation via an automated experiment in STEM. Combined with beam control, this approach allows studying beam effects on selected atomic groups and chemical bonds in a fully automated mode. Here, we demonstrate atomically precise engineering of single vacancy lines in transition metal dichalcogenides and the creation and identification of topological defects graphene. The ELITbased approach opens the pathway toward the direct on-the-fly analysis of the STEM data and engendering real-time feedback schemes for probing electron beam chemistry, atomic manipulation, and atom by atom assembly.