Nanoscale Iron Redistribution during Thermochemical Decomposition of CaTi1-x Fe x O3-δ Alters the Electrical Transport Pathway: Implications for Oxygen-Transport Membranes, Electrocatalysis, and Photocatalysis.

Potential applications of the earth-abundant, low-cost, and non-critical perovskite CaTi1-x Fe x O3-δ in electrocatalysis, photocatalysis, and oxygen-transport membranes have motivated research to tune its chemical composition and morphology. However, investigations on the decomposition mechanism(s) of CaTi1-x Fe x O3-δ under thermochemically reducing conditions are limited, and direct evidence of the nano- and atomic-level decomposition process is not available in the literature. In this work, the phase evolution of CaTi1-x Fe x O3-δ (x = 0-0.4) was investigated in a H2-containing atmosphere after heat treatments up to 600 °C. The results show that CaTi1-x Fe x O3-δ maintained a stable perovskite phase at low Fe contents while exhibiting a phase decomposition to Fe/Fe oxide nanoparticles as the Fe content increases. In CaTi0.7Fe0.3O3-δ and CaTi0.6Fe0.4O3-δ, the phase evolution to Fe/Fe oxide was greatly influenced by the temperature: Only temperatures of 300 °C and greater facilitated phase evolution. Fully coherent Fe-rich and Fe-depleted perovskite nanodomains were observed directly by atomic-resolution scanning transmission electron microscopy. Prior evidence for such nanodomain formation was not found, and it is thought to result from a near-surface Kirkendall-like phenomenon caused by Fe migration in the absence of Ca and Ti co-migration. Density functional theory simulations of Fe-doped bulk models reveal that Fe in an octahedral interstitial site is energetically more favorable than in a tetrahedral site. In addition to coherent nanodomains, agglomerated Fe/Fe oxide nanoparticles formed on the ceramic surface during decomposition, which altered the electrical transport mechanism. From temperature-dependent electrical conductivity measurements, it was found that heat treatment and phase decomposition change the transport mechanism from thermally activated p-type electronic conductivity through the perovskite to electronic conduction through the iron oxide formed by thermochemical decomposition. This understanding will be useful to those who are developing or employing this and similar earth-abundant functional perovskites for use under reducing conditions, at elevated temperatures, and when designing materials syntheses and processes.

Exsolution-Driven Surface Transformation in the Host Oxide.

Exsolution synthesizes self-assembled metal nanoparticle catalysts via phase precipitation. An overlooked aspect in this method thus far is how exsolution affects the host oxide surface chemistry and structure. Such information is critical as the oxide itself can also contribute to the overall catalytic activity. Combining X-ray and electron probes, we investigated the surface transformation of thin-film SrTi0.65Fe0.35O3 during Fe0 exsolution. We found that exsolution generates a highly Fe-deficient near-surface layer of about 2 nm thick. Moreover, the originally single-crystalline oxide near-surface region became partially polycrystalline after exsolution. Such drastic transformations at the surface of the oxide are important because the exsolution-induced nonstoichiometry and grain boundaries can alter the oxide ion transport and oxygen exchange kinetics and, hence, the catalytic activity toward water splitting or hydrogen oxidation reactions. These findings highlight the need to consider the exsolved oxide surface, in addition to the metal nanoparticles, in designing the exsolved nanocatalysts.

Bulk and surface exsolution produces a variety of Fe-rich and Fe-depleted ellipsoidal nanostructures in La0.6Sr0.4FeO3 thin films.

The past several years have seen a resurgence in the popularity of metal exsolution as an approach to synthesize advanced materials proposed for novel catalytic, magnetic, optical, and electrochemical properties. Whereas most studies to-date have focused on surface exsolution (motivated by catalysis), we instead report on the diversity of nanostructures formed in La0.6Sr0.4FeO3 thin films during sub-surface or so-called 'bulk' exsolution, in addition to surface exsolution. Bulk exsolution is a promising approach to tuning the functionality of materials, yet there is little understanding of the nanostructures exsolved within the bulk and how they compare to those exsolved at gas-solid interfaces. This work combines atomic- and nano-scale imaging and spectroscopy techniques applied using a state-of-the-art aberration-corrected scanning transmission electron microscope (STEM). In doing so, we present a detailed atomic-resolution study of a range of Fe-rich and Fe-depleted nanostructures possible via exsolution, along with qualitative and quantitative chemical analysis of the exsolved nanostructures and oxide phases formed throughout the film. Local structural changes in the perovskite matrix, coinciding with nanostructure exsolution, are also characterized with atomic-resolution STEM imaging. Fe exsolution is shown to create local A-site rich domains of Ruddlesden-Popper phase, and some stages of this phase formation have been demonstrated in this work. In particular, phase boundaries are found to be the primary nucleation sites for bulk and surface exsolution, and the exsolved particles observed here tend to be ellipsoidal with shape factor of 1.4. We report a range of nanostructure types (core-shell, bulk core-shell, adjacent, and independent particles), revealing several possible avenues of future exploration aimed to understand the formation mechanism of each exsolution type and to develop their functionality. This work is thus relevant to materials scientists and engineers motivated to understand and utilize exsolution to synthesize materials with predictable nanostructures.

Probing Multiscale Disorder in Pyrochlore and Related Complex Oxides in the Transmission Electron Microscope: A Review.

Transmission electron microscopy (TEM), and its counterpart, scanning TEM (STEM), are powerful materials characterization tools capable of probing crystal structure, composition, charge distribution, electronic structure, and bonding down to the atomic scale. Recent (S)TEM instrumentation developments such as electron beam aberration-correction as well as faster and more efficient signal detection systems have given rise to new and more powerful experimental methods, some of which (e.g., 4D-STEM, spectrum-imaging, in situ/operando (S)TEM)) facilitate the capture of high-dimensional datasets that contain spatially-resolved structural, spectroscopic, time- and/or stimulus-dependent information across the sub-angstrom to several micrometer length scale. Thus, through the variety of analysis methods available in the modern (S)TEM and its continual development towards high-dimensional data capture, it is well-suited to the challenge of characterizing isometric mixed-metal oxides such as pyrochlores, fluorites, and other complex oxides that reside on a continuum of chemical and spatial ordering. In this review, we present a suite of imaging and diffraction (S)TEM techniques that are uniquely suited to probe the many types, length-scales, and degrees of disorder in complex oxides, with a focus on disorder common to pyrochlores, fluorites and the expansive library of intermediate structures they may adopt. The application of these techniques to various complex oxides will be reviewed to demonstrate their capabilities and limitations in resolving the continuum of structural and chemical ordering in these systems.

Linking Macroscopic and Nanoscopic Ionic Conductivity: A Semiempirical Framework for Characterizing Grain Boundary Conductivity in Polycrystalline Ceramics.

Understanding the chemical and charge transport properties of grain boundaries (GBs) with high point defect concentrations (beyond the dilute solution limit) in polycrystalline materials is critical for developing ion-conducting solids for electrochemical energy conversion and storage. Elucidation and optimization of GBs are hindered by large variations in atomic structure, composition, and chemistry within nanometers or Ångstroms of the GB interface, which limits a fundamental understanding of electrical transport across and along GBs. Here we employ a novel correlated approach that is generally applicable to polycrystalline materials whose properties are affected by GB composition or chemistry. We demonstrate the connection between the nanoscopic chemical and transport properties of individual boundaries and the macroscopic ionic conductivity in oxygen-conducting Pr0.04Gd0.11Ce0.85O2-δ. The key finding is that GBs with higher solute concentration have lower activation energy for cross-GB ion conduction through a polycrystalline conductor. The resultant semiempirical framework presented here provides a tool for understanding, designing and optimizing polycrystalline ionic conductors.

Enhanced ionic conductivity in electroceramics by nanoscale enrichment of grain boundaries with high solute concentration.

The enhancement of oxygen ionic conductivity by over two orders of magnitude in an electroceramic oxide is explicitly shown to result from nanoscale enrichment of a grain boundary layer or complexion with high solute concentration. A series of CaxCe1-xO2-δ polycrystalline oxides with fluorite structure and varying nominal Ca2+ solute concentration elucidates how local grain boundary composition, rather than structural grain boundary character, primarily regulates ionic conductivity. A correlation between high grain boundary solute concentration above ∼40 mol%, and four orders of magnitude increase in grain boundary conductivity is explicitly shown. A correlated experimental approach provides unique insights into fundamental grain boundary science, and highlights how novel aspects of nanoscale grain boundary design may be employed to control ion transport properties in electroceramics.

Designing Strained Interface Heterostructures for Memristive Devices.

Ionic heterostructures are used as a strain-modulated memristive device based on the model system Gd0.1 Ce0.9 O2-δ /Er2 O3 to set and tune the property of "memristance." The modulation of interfacial strain and the interface count is used to engineer the Roff /Ron ratio and the persistence of the system. A model describing the variation of mixed ionic-electronic mobilities and defect concentrations is presented.