Structure and chemistry of epitaxial ceria thin films on yttria-stabilized zirconia substrates, studied by high resolution electron microscopy

Electrochemical Impedance Spectroscopy Integrated with Environmental Transmission Electron Microscopy

The concept of combining electrical impedance spectroscopy (EIS) with environmental transmission electron microscopy (ETEM) is demonstrated by testing a specially designed micro gadolinia-doped ceria (CGO) sample in reactive gasses (O₂ and H₂ /H₂ O), at elevated temperatures (room temperature-800 °C) and with applied electrical potentials. The EIS-TEM method provides structural and compositional information with direct correlation to the electrochemical performance. It is demonstrated that reliable EIS measurements can be achieved in the TEM for a sample with nanoscale dimensions. Specifically, the ionic and electronic conductivity, the surface exchange resistivity, and the volume-specific chemical capacitance are in good agreement with results from more standardized electrochemical tests on macroscopic samples. CGO is chosen as a test material due to its relevance for solid oxide electrochemical reactions where its electrochemical performance depends on temperature and gas environment. As expected, the results show increased conductivity and lower surface exchange resistance in H₂ /H₂ O gas mixtures where the oxygen partial pressure is low compared to experiments in pure O₂ . The developed EIS-TEM platform is an important tool in promoting the understanding of nanoscale processes for green energy technologies, e.g., solid oxide electrolysis/fuel cells, batteries, thermoelectric devices, etc.

Exploring Blob Detection to Determine Atomic Column Positions and Intensities in Time-Resolved TEM Images with Ultra-Low Signal-to-Noise

Spatially resolved in situ transmission electron microscopy (TEM), equipped with direct electron detection systems, is a suitable technique to record information about the atom-scale dynamics with millisecond temporal resolution from materials. However, characterizing dynamics or fluxional behavior requires processing short time exposure images which usually have severely degraded signal-to-noise ratios. The poor signal-to-noise associated with high temporal resolution makes it challenging to determine the position and intensity of atomic columns in materials undergoing structural dynamics. To address this challenge, we propose a noise-robust, processing approach based on blob detection, which has been previously established for identifying objects in images in the community of computer vision. In particular, a blob detection algorithm has been tailored to deal with noisy TEM image series from nanoparticle systems. In the presence of high noise content, our blob detection approach is demonstrated to outperform the results of other algorithms, enabling the determination of atomic column position and its intensity with a higher degree of precision.

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

Reducible oxides are widely used catalyst supports that can increase oxidation reaction rates by transferring lattice oxygen at the metal-support interface. There are many outstanding questions regarding the atomic-scale dynamic meta-stability (i.e., fluxional behavior) of the interface during catalysis. Here, we employ aberration-corrected operando electron microscopy to visualize the structural dynamics occurring at and near Pt/CeO₂ interfaces during CO oxidation. We show that the catalytic turnover frequency correlates with fluxional behavior that (a) destabilizes the supported Pt particle, (b) marks an enhanced rate of oxygen vacancy creation and annihilation, and (c) leads to increased strain and reduction in the CeO₂ support surface. Overall, the results implicate the interfacial Pt-O-Ce bonds anchoring the Pt to the support as being involved also in the catalytically-driven oxygen transfer process, and they suggest that oxygen reduction takes place on the highly reduced CeO₂ surface before migrating to the interfacial perimeter for reaction with CO.

Cyclic oxygen exchange capacity of Ce-doped V2O5 materials for syngas production via high-temperature thermochemical-looping reforming of methane

Synthesis gas production via solar thermochemical reduction-oxidation reactions is a promising pathway towards sustainable carbon-neutral fuels. The redox capability of oxygen carriers with considerable thermal and chemical stability is highly desirable. In this study, we report Ce-doped V₂O₅ structures for high-temperature thermochemical-looping reforming of methane coupled to H₂O and CO₂ splitting reactions. Incorporation of fractional amounts of large cerium cations induces a V⁵⁺ to V³⁺ transition and partially forms a segregated CeVO₄ phase. More importantly, the effective combination of efficient ion mobility of cerium and high oxygen exchange capacity of vanadia achieves synergic and cyclable redox performance during the thermochemical reactions, whereas the pure vanadia powders undergo melting and show non-cyclic redox behaviour. These materials achieve noteworthy syngas production rates of up to 500 mmol mol_V⁻¹ min⁻¹ during the long-term stability test of 100 CO₂ splitting cycles. Interestingly, the cerium ions are mobile between the lattice and the surface of the Ce-doped vanadia powders during the repeated reduction and oxidation reactions and contribute towards the cyclic syngas production. However, this also causes the formation of the CeVO₄ phase in Ce-rich powders, which increases the H₂/CO ratios and lowers fuel selectivity, which can be controlled by optimizing the cerium concentration. These findings are noteworthy towards the experimental approach of evaluating the oxygen carriers with the help of advanced characterization techniques.

Cerium doping into the V₂O₅ lattice forms a reversible V₂O₃/VO redox pair after sequential methane partial oxidation and CO₂/H₂O splitting reactions and produces syngas (H₂, CO) with fast rates and high oxygen exchange capacity.

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

Oxygen vacancy creation and annihilation are key processes in nonstoichiometric oxides such as CeO₂. 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.

Synergistic effects between polyvinylpyrrolidone and oxygen vacancies on improving the oxidase-mimetic activity of flower-like CeO2 nanozymes

Both oxygen vacancies and surface chemistry can affect the enzyme-like catalytic activities of CeO2-based nanozymes. However, the mechanism of the enzyme-mimetic process is not yet clearly elucidated, which is of great importance to guide the synthesis of high-performance nanozymes with desirable properties. Herein, we report a facile one-pot solvothermal method for the preparation of polyvinylpyrrolidone (PVP)-capped CeO2 nanoflowers with adjustable oxygen vacancies by changing appropriate solvothermal reaction parameters. Oxygen vacancies effectively increase under a higher precursor concentration, extended solvothermal time, and proper reaction temperature. The maximum content of surface Ce(iii) cations is up to 50% for 31.1 nm CeO2 nanoflowers, which exhibit 0.07 mM apparent Michaelis constant towards 3,3',5,5'-tetramethylbenanozymeidine and show a higher binding affinity than the other CeO2-based catalysts. Theoretical results indicate that the synergy between PVP and oxygen vacancies can significantly promote the adsorption of O2 and TMB on CeO2, which directly enhances the oxidase-mimetic activity of flower-like CeO2 nanozymes. This work can shed light on a new perspective on the enzyme-like performance promotion of CeO2-based catalysts and surface engineering of nanozymes.

Approaches to Exploring Spatio-Temporal Surface Dynamics in Nanoparticles with In Situ Transmission Electron Microscopy

Many nanoparticles in fields such as heterogeneous catalysis undergo surface structural fluctuations during chemical reactions, which may control functionality. These dynamic structural changes may be ideally investigated with time-resolved in situ electron microscopy. We have explored approaches for extracting quantitative information from large time-resolved image data sets with a low signal to noise recorded with a direct electron detector on an aberration-corrected transmission electron microscope. We focus on quantitatively characterizing beam-induced dynamic structural rearrangements taking place on the surface of CeO2 (ceria). A 2D Gaussian fitting procedure is employed to determine the position and occupancy of each atomic column in the nanoparticle with a temporal resolution of 2.5 ms and a spatial precision of 0.25 Å. Local rapid lattice expansions/contractions and atomic migration were revealed to occur on the (100) surface, whereas (111) surfaces were relatively stable throughout the experiment. The application of this methodology to other materials will provide new insights into the behavior of nanoparticle surface reconstructions that were previously inaccessible using other methods, which will have important consequences for the understanding of dynamic structure-property relationships.

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

Cerium dioxide nanocubes and truncated octahedra were reduced and oxidized in the scanning transmission electron microscope. The reduction process was stimulated by the electron beam and oxidation was supported by background gases in the microscope environment. High-angle annular dark field imaging is sensitive to local lattice distortions that arise as oxygen vacancies are created and cerium cations reduce enabling high spatial resolution characterization of this process with temporal resolution on the order of seconds. Such measurements enable us to differentiate and infer that the observed behavior between the nanocubes and truncated octahedra may be due to the difference in crystallographic termination of surfaces. In situ measurements taken with different partial pressures of oxygen reveal the cerium oxidation state and the dose rate threshold for the onset of beam reduction are influenced by the environment. Increasing oxygen partial pressure reduces the Ce3+ content and decreases susceptibility to electron beam driven reduction.

Interface Effects on the Ionic Conductivity of Doped Ceria-Yttria-Stabilized Zirconia Heterostructures

Multilayered heterostructures of Ce_0.85Sm_0.15O_2-δ and Y_0.16Zr_0.92O_2-δ of a high crystallographic quality were fabricated on (001)-oriented MgO single crystal substrates. Keeping the total thickness of the heterostructures constant, the number of ceria-zirconia bilayers was increased while reducing the thickness of each layer. At each interface Ce was found primarily in the reduced, 3+ oxidation state in a layer extending about 2 nm from the interface. Concurrently, the conductivity decreased as the thickness of the layers was reduced, suggesting a progressive confinement of the charge transport along the YSZ layers. The comparative analysis of the in-plane electrical characterization suggests that the contribution to the total electrical conductivity of these interfacial regions is negligible. For the smallest layer thickness of 2 nm the doped ceria layers are electrically insulating and the ionic transport only occurs through the zirconia layers. This is explained in terms of a reduced mobility of the oxygen vacancies in the highly reduced ceria.

Structure and chemistry of interfaces between ceria and yttria-stabilized zirconia studied by analytical STEM

Epitaxial undoped and Gd₂O₃-doped ceria films were grown by pulsed laser deposition on (1 1 1) faced Y₂O₃-stabilized zirconia (YSZ). Highly localized cerium reduction at the film-substrate interfaces is revealed by atomically resolved valence EELS mapping using C_s aberration-corrected scanning transmission electron microscopy. The chemical profiles reveal interdiffusion of Ce, (Gd), Y, Zr, forming an intermixing zone at the interface 7-9 (1 1 1) lattice planes wide. In its vicinity, the fraction of Ce³⁺ raises gradually over 6-8 lattice planes from zero in the bulk ceria to ≈100% in one single plane at the interface. Beyond this plane the Ce³⁺ fraction drops sharply within the YSZ substrate. In the vicinity of the interface systematic scan deflections are observed during EELS line scans. The advancing electron probe experiences a retarding force at the ceria side, and an accelerating force at the YSZ side, irrespective of the scan direction. This behavior is suggestive of coulombic interactions between the electron probe and a charged interface. This is interpreted as an indication of the presence of a space-charge situation at the YSZ/ceria interface, resulting from an excess negative charge at the ceria side (due to Ce³⁺cations) and an excess positive charge at the YSZ side (due to oxygen vacancies).

Atomic Resolution Imaging of Nanoscale Chemical Expansion in PrxCe1-xO2-δ during In Situ Heating

Thin film nonstoichiometric oxides enable many high-temperature applications including solid oxide fuel cells, actuators, and catalysis. Large concentrations of point defects (particularly, oxygen vacancies) enable fast ionic conductivity or gas exchange kinetics in these materials but also manifest as coupling between lattice volume and chemical composition. This chemical expansion may be either detrimental or useful, especially in thin film devices that may exhibit enhanced performance through strain engineering or decreased operating temperatures. However, thin film nonstoichiometric oxides can differ from bulk counterparts in terms of operando defect concentrations, transport properties, and mechanical properties. Here, we present an in situ investigation of atomic-scale chemical expansion in Pr_xCe_1-xO_2-δ (PCO), a mixed ionic-electronic conducting oxide relevant to electrochemical energy conversion and high-temperature actuation. Through a combination of electron energy loss spectroscopy and transmission electron microscopy with in situ heating, we characterized chemical strains and changes in oxidation state in cross sections of PCO films grown on yttria-stabilized zirconia (YSZ) at temperatures reaching 650 °C. We quantified, both statically and dynamically, the nanoscale chemical expansion induced by changes in PCO redox state as a function of position and direction relative to the film-substrate interface. Additionally, we observed dislocations at the film-substrate interface, as well as reduced cation localization to threading defects within PCO films. These results illustrate several key aspects of atomic-scale structure and mechanical deformation in nonstoichiometric oxide films that clarify distinctions between films and bulk counterparts and that hold several implications for operando chemical expansion or "breathing" of such oxide films.

Direct Visualization and Control of Atomic Mobility at {100} Surfaces of Ceria in the Environmental Transmission Electron Microscope

Ceria is one of the world's most prominent material for applications in heterogeneous catalysis, as catalyst support or catalyst itself. Despite an exhaustive literature on the structure of reactive facets of CeO₂ in line with its catalytic mechanisms, the temporal evolution of the atomic surface structure exposed to realistic redox conditions remains elusive. Here, we provide a direct visualization of the atomic mobility of cerium atoms on {100} surfaces of CeO₂ nanocubes at room temperature in high vacuum, O₂, and CO₂ atmospheres in an environmental transmission electron microscope. Through quantification of the cationic mobility, we demonstrate the control of the surface dynamics under exposure to O₂ and CO₂ atmospheres, providing opportunities for a better understanding of the intimate catalytic mechanisms.