Tailoring non-stoichiometry and mixed ionic-electronic conductivity in nanostructured Pr-substituted ceria

High concentrations of mobile oxygen vacancies are crucial for devices such as SOFCs, SOECs, gas permeation membranes, and sensors, while for other applications such as ferroelectrics and piezoelectrics, oxygen vacancies are detrimental. Hence there is great interest in tailoring the oxygen vacancy concentration and mobility for given materials. Changes in oxygen non-stoichiometry also result in dilation of the crystal lattice, known as chemical expansion, and therefore there is a coupling between the electrical, chemical, and mechanical properties known as electro-chemo-mechanical coupling. Confined systems, such as thin films, are being investigated as a way to tailor the non-stoichiometry and transport properties of materials, shifting the paradigm away from searching for new materials or compositions. Please click Additional Files below to see the full abstract

Active Tuning of Optical Constants in the Visible–UV: Praseodymium‐Doped Ceria—a Model Mixed Ionic–Electronic Conductor

Mixed ionic–electronic conductors offer chemical and electrical means for active tuning of their optical constants, e.g., with variations in oxygen non-stoichiometry in Pr0.1Ce0.9O2–δ, enabling implementation of adaptive thin film optical devices. In situ chemo-tuning of the extinction coefficient in Pr0.1Ce0.9O2–δ at elevated temperatures is demonstrated and a tuning model is provided that treats the interdependence of mobile oxygen vacancies and small polarons coupled to variations in optically active praseodymium ions. Furthermore, a new means for electro-tuning of the optical constants of mixed ionic–electronic conductors is demonstrated experimentally and modeled for Pr0.1Ce0.9O2–δ thin films deposited on grid-like electrode structures. Modeling of non-steady-state optical transmittance modulations in the latter allows for estimation of oxygen vacancy mobility that determines the switching speed of the device. Quenched-in values of nr and k to room temperature become nonvolatile, providing a modulation range in the extinction coefficient of Δk ≈ 0.1 (change of ≈800%) and in the refractive index of Δnr ≈ 0.1 (relative to initial nr of ≈2.35). Key figures of merit, including transmission optical modulation of ≈0.04 per 1 mV nm–1, switching energy per area of 1.9 nJ µm–2, and switching times of seconds, are demonstrated, with further improvements possible

Impact of Oxygen Non‐Stoichiometry on Near‐Ambient Temperature Ionic Mobility in Polaronic Mixed‐Ionic‐Electronic Conducting Thin Films

Enhanced ionic mobility in mixed ionic and electronic conducting solids contributes to improved performance of memristive memory, energy storage and conversion, and catalytic devices. Ionic mobility can be significantly depressed at reduced temperatures, for example, due to defect association and therefore needs to be monitored. Measurements of ionic transport in mixed conductors, however, proves to be difficult due to dominant electronic conductivity. This study examines the impact of different levels of quenched-in oxygen deficiency on the oxygen vacancy mobility near room temperature. A praseodymium doped ceria (Pr0.1Ce0.9O2–δ ) film is grown by pulsed laser deposition and annealed in various oxygen partial pressures to modify its oxygen vacancy concentration. Changes in film non-stoichiometry are monitored by tracking the optical absorption related to the oxidation state of Pr ions. A 13-fold increase in ionic mobility at 60 °C for increases in oxygen non-stoichiometry from 0.032 to 0.042 is detected with negligible changes in migration enthalpy and large changes in pre-factor. Several factors potentially contributing to the large pre-factor changes are examined and discussed. Insights into how ionic defect concentration can markedly impact ionic mobility should help in elucidating the origins of variations seen in nanoionic devices

Effect of structure on oxygen diffusivity in layered oxides: a combined theoretical and experimental study

Effect of structure on oxygen diffusion D0 in two different structures of La2CuO4. Higher density of mobile defects in T-structure leads to higher D0, but the lower activation energy for T′-structure is correlated with its lower migration energy.</jats:p

Fast Surface Oxygen Release Kinetics Accelerate Nanoparticle Exsolution in Perovskite Oxides

Exsolution is a recent advancement for fabricating oxide-supported metal nanoparticle catalysts via phase precipitation out of a host oxide. A fundamental understanding and control of the exsolution kinetics are needed to engineer exsolved nanoparticles to obtain higher catalytic activity toward clean energy and fuel conversion. Since oxygen release via oxygen vacancy formation in the host oxide is behind oxide reduction and metal exsolution, we hypothesize that the kinetics of metal exsolution should depend on the kinetics of oxygen release, in addition to the kinetics of metal cation diffusion. Here, we probe the surface exsolution kinetics both experimentally and theoretically using thin-film perovskite SrTi0.65Fe0.35O3 (STF) as a model system. We quantitatively demonstrated that in this system the surface oxygen release governs the metal nanoparticle exsolution kinetics. As a result, by increasing the oxygen release rate in STF, either by reducing the sample thickness or by increasing the surface reactivity, one can effectively accelerate the Fe0 exsolution kinetics. Fast oxygen release kinetics in STF not only shortened the prereduction time prior to the exsolution onset, but also increased the total quantity of exsolved Fe0 over time, which agrees well with the predictions from our analytical kinetic modeling. The consistency between the results obtained from in situ experiments and analytical modeling provides a predictive capability for tailoring exsolution, and highlights the importance of engineering host oxide surface oxygen release kinetics in designing exsolved nanocatalysts