Ionic Conduction-Based Polycrystalline Oxide Gamma Ray Detection - Radiation-Ionic Effects.

Newly discovered opto-ionic effects in metal oxides provide unique opportunities for functional ceramic applications. The authors generalize the recently demonstrated grain boundary opto-ionic effect observed in solid electrolyte thin films under ultraviolet (UV) irradiation to a radiation-ionic effect that can be applied to bulk materials and used for gamma-rays (γ-rays) detection. Near room temperature, lightly doped Gd-doped CeO2, a polycrystalline ion conducting ceramic, exhibits a resistance ratio change ≈103 and reversible response in ionic current when exposed to 60Co γ-ray (1.1 and 1.3 MeV). This is attributed to the steady state passivation of space charge barriers at grain boundaries, that act as virtual electrodes, capturing radiation-induced electrons, in turn lowering space charge barrier heights, and thereby exclusively modulating the ionic carrier flow within the ceramic electrolytes. Such behavior allows significant electrical response under low fields, that is, < 2 V cm-1, paving the way to inexpensive, sensitive, low-power, and miniaturizable solid-state devices, uniquely suited for operating in harsh (high temperature, pressure, and corrosive) environments. This discovery presents opportunities for portable and/or scalable radiation detectors benefiting geothermal drilling, small modular reactors, nuclear security, and waste management.

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.

Tuning Surface Acidity of Mixed Conducting Electrodes: Recovery of Si-Induced Degradation of Oxygen Exchange Rate and Area Specific Resistance.

Metal oxides are an important class of functional materials, and for many applications, ranging from solid oxide fuel/electrolysis cells, oxygen permeation membranes, and oxygen storage materials to gas sensors (semiconducting and electrolytic) and catalysts, the interaction between the surface and oxygen in the gas phase is central. Ubiquitous Si-impurities are known to impede this interaction, commonly attributed to the formation of glassy blocking layers on the surface. Here, the surface oxygen exchange coefficient (kchem ) is examined for Pr0.1 Ce0.9 O2-δ (PCO), a model mixed ionic electronic conductor, via electrical conductivity relaxation measurements, and the area-specific resistance (ASR) by electrochemical impedance spectroscopy. It is demonstrated that even low silica levels, introduced by infiltration, depress kchem by a factor 4000, while the ASR increases 40-fold and we attribute this to its acidity relative to that of PCO. The ability to fully regenerate the poisoned surface by the subsequent addition of basic Ca- or Li-species is further shown. This ability to not only recover Si-poisoned surfaces by tuning the relative surface acidity of an oxide surface, but subsequently outperform the pre-poisoned response, promises to extend the operating life of materials and devices for which the catalytic oxygen/solid interface reaction is central.

Photo-enhanced ionic conductivity across grain boundaries in polycrystalline ceramics.

Grain boundary conductivity limitations are ubiquitous in material science. We show that illumination with above-bandgap light can decrease the grain boundary resistance in solid ionic conductors. Specifically, we demonstrate the increase of the grain boundary conductance of a 3 mol% Gd-doped ceria thin film by a factor of approximately 3.5 at 250 °C and the reduction of its activation energy from 1.12 to 0.68 eV under illumination, while light-induced heating and electronic conductivity could be excluded as potential sources for the observed opto-ionic effect. The presented model predicts that photo-generated electrons decrease the potential barrier heights associated with space charge zones depleted in charge carriers between adjacent grains. The discovered opto-ionic effect could pave the way for the development of new electrochemical storage and conversion technologies operating at lower temperatures and/or higher efficiencies and could be further used for fast and contactless control or diagnosis of ionic conduction in polycrystalline solids.

Synergistic Integration of Chemo-Resistive and SERS Sensing for Label-Free Multiplex Gas Detection.

Practical sensing applications such as real-time safety alerts and clinical diagnoses require sensor devices to differentiate between various target molecules with high sensitivity and selectivity, yet conventional devices such as oxide-based chemo-resistive sensors and metal-based surface-enhanced Raman spectroscopy (SERS) sensors usually do not satisfy such requirements. Here, a label-free, chemo-resistive/SERS multimodal sensor based on a systematically assembled 3D cross-point multifunctional nanoarchitecture (3D-CMA), which has unusually strong enhancements in both "chemo-resistive" and "SERS" sensing characteristics is introduced. 3D-CMA combines several sensing mechanisms and sensing elements via 3D integration of semiconducting SnO2 nanowire frameworks and dual-functioning Au metallic nanoparticles. It is shown that the multimodal sensor can successfully estimate mixed-gas compositions selectively and quantitatively at the sub-100 ppm level, even for mixtures of gaseous aromatic compounds (nitrobenzene and toluene) with very similar molecular structures. This is enabled by combined chemo-resistive and SERS multimodal sensing providing complementary information.

Tailoring Nonstoichiometry and Mixed Ionic Electronic Conductivity in Pr0.1Ce0.9O2-δ/SrTiO3 Heterostructures.

The oxygen deficiency or excess, as reflected in the nonstoichiometry of oxide films, plays a crucial role in their functional properties for applications such as micro solid oxide fuel cells, catalysis, sensors, ferroelectrics, and memristors. High concentrations of oxygen vacancies may be beneficial or detrimental according to the application, and hence there is interest in controlling the oxygen content of films without resorting to compositional changes. Here, we demonstrate that substantial changes in the nonstoichiometry of Pr0.1Ce0.9O2-δ (PCO), a model mixed ionic electronic conductor, can be achieved by fabricating multilayers with an inert material, SrTiO3 (STO). We fabricated heterostructures using pulsed laser deposition, keeping the total thickness of PCO and STO constant while varying the number of layers and thickness of each individual layer, to probe the effects of the PCO/STO interfaces. Conductivity measurements as a function of oxygen partial pressure (PO2) and temperature showed a significant weakening of the PO2 dependence compared to bulk PCO, which scaled with the density of interfaces. We confirmed that this change was due to variations in nonstoichiometry, by optical transmission measurements, and show that the lower oxygen content is consistent with a decrease in the effective oxygen reduction enthalpy of PCO. These results exemplify the dramatic differences in properties between films and their bulk counterparts, achievable by interface engineering, and provide generalized insight into tailoring the properties of mixed ionic electronic conductors at the nanoscale.

In Situ Method Correlating Raman Vibrational Characteristics to Chemical Expansion via Oxygen Nonstoichiometry of Perovskite Thin Films.

Effective integration of perovskite films into devices requires knowledge of their electro-chemomechanical properties. Raman spectroscopy is an excellent tool for probing such properties as the films' vibrational characteristics couple to the lattice volumetric changes during chemical expansion. While lattice volumetric changes are typically accessed by analyzing Raman shifts as a function of pressure, stress, or temperature, such methods can be impractical for thin films and do not capture information on chemical expansion. An in situ Raman spectroscopy technique using an electrochemical titration cell to change the oxygen nonstoichiometry of a model perovskite film, Sr(Ti,Fe)O3- y  , is reported and the lattice vibrational properties are correlated to the material's chemical expansion. How to select an appropriate Raman vibrational mode to track the evolution in oxygen nonstoichiometry is discussed. Subsequently, the frequency of the oxygen stretching mode around Fe4+ is tracked, as it decreases during reduction as the material expands and increases during reoxidation as the material shrinks. This methodology of oxygen pumping and in situ Raman spectroscopy of oxide films enables future in operando measurements even for small material volumes, as is typical for applications of films as electrodes or electrolytes utilized in electrochemical energy conversion or memory devices.

Magneto-ionic control of magnetism using a solid-state proton pump.

Voltage-gated ion transport as a means of manipulating magnetism electrically could enable ultralow-power memory, logic and sensor technologies. Earlier work made use of electric-field-driven O2- displacement to modulate magnetism in thin films by controlling interfacial or bulk oxidation states. However, elevated temperatures are required and chemical and structural changes lead to irreversibility and device degradation. Here we show reversible and non-destructive toggling of magnetic anisotropy at room temperature using a small gate voltage through H+ pumping in all-solid-state heterostructures. We achieve 90° magnetization switching by H+ insertion at a Co/GdOx interface, with no degradation in magnetic properties after >2,000 cycles. We then demonstrate reversible anisotropy gating by hydrogen loading in Pd/Co/Pd heterostructures, making metal-metal interfaces susceptible to voltage control. The hydrogen storage metals Pd and Pt are high spin-orbit coupling materials commonly used to generate perpendicular magnetic anisotropy, Dzyaloshinskii-Moriya interaction, and spin-orbit torques in ferromagnet/heavy-metal heterostructures. Thus, our work provides a platform for voltage-controlled spin-orbitronics.

On the Theoretical and Experimental Control of Defect Chemistry and Electrical and Photoelectrochemical Properties of Hematite Nanostructures.

Hematite (α-Fe2O3) is regarded as one of the most promising cost-effective and stable anode materials in photoelectrochemical applications, and its performance, like other transition-metal oxides, depends strongly on its electrical and defect properties. In this work, the electrical and thermomechanical properties of undoped and Sn-doped α-Fe2O3 nanoscale powders were characterized in situ at controlled temperatures ( T = 250 to 400 °C) and atmospheres ( pO2 = 10-4 to 1 atm O2) to investigate their transport and defect properties. Frequency-dependent complex impedance spectra show that interfacial resistance between particles is negligible in comparison with particle resistance. Detailed defect models predicting the dependence of electron, hole, and iron and oxygen vacancy concentrations on temperature and oxygen partial pressures for undoped and doped α-Fe2O3 were derived. Using these defect equilibria models, the operative defect regimes were established, and the bandgap energy of undoped α-Fe2O3 and oxidation enthalpy of Sn-doped α-Fe2O3 were obtained from the analysis of the temperature and pO2 dependence of the electrical conductivity. On the basis of these results, we are able to explain the surprisingly weak impact of donor doping on the electrical conductivity of α-Fe2O3. Furthermore, experimental means based on the results of this study are given for successfully tuning hematite to enhance its photocatalytic activity for the water oxidation reaction.

Origin of fast oxide ion diffusion along grain boundaries in Sr-doped LaMnO3.

The prospect of significantly enhanced oxide ion diffusion along grain boundaries in Sr-doped LaMnO3 (LSM) was investigated by means of density functional theory calculations applied to a Σ5 (3 1 0)[0 0 1] grain boundary. The structure of the grain boundary was optimized by rigid body translation, and segregation energies were calculated for oxygen vacancies and Sr-acceptors. Two potentially fast diffusion paths were identified along the grain boundary core based on the interconnectivity between neighbouring sites with a strong tendency for segregation of oxygen vacancies. The migration barriers for these paths, obtained with the nudged elastic band method, amounted to about 0.6 eV. Based on the obtained migration barriers and concentrations of oxygen vacancies for the relevant core sites, the grain boundary diffusion coefficient was estimated to be enhanced by 3 to 5 orders of magnitude relative to the bulk in the temperature range 500-900 °C. Space-charge effects were determined to be quite insignificant for the transport properties of LSM grain boundaries.

Oxygen surface exchange kinetics measurement by simultaneous optical transmission relaxation and impedance spectroscopy: Sr(Ti,Fe)O3-x thin film case study.

We compare approaches to measure oxygen surface exchange kinetics, by simultaneous optical transmission relaxation (OTR) and AC-impedance spectroscopy (AC-IS), on the same mixed conducting SrTi0.65Fe0.35O3-x film. Surface exchange coefficients were evaluated as a function of oxygen activity in the film, controlled by gas partial pressure and/or DC bias applied across the ionically conducting yttria-stabilized zirconia substrate. Changes in measured light transmission through the film over time (relaxations) resulted from optical absorption changes in the film corresponding to changes in its oxygen and oxidized Fe (~Fe4+) concentrations; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry. The kchem values obtained by OTR were significantly lower than the AC-IS derived kchem values and kq values multiplied by the thermodynamic factor (bulk or thin film), suggesting a possible enhancement in k by the metal current collectors (Pt, Au). Long-term degradation in kchem and kq values obtained by AC-IS was also attributed to deterioration of the porous Pt current collector, while no significant degradation was observed in the optically derived kchem values. The results suggest that, while the current collector might influence measurements by AC-IS, the OTR method offers a continuous, in situ, and contact-free method to measure oxygen exchange kinetics at the native surfaces of thin films.

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 PrxCe1-xO2-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.

Exceptional High-Performance of Pt-Based Bimetallic Catalysts for Exclusive Detection of Exhaled Biomarkers.

Achieving an improved understanding of catalyst properties, with ability to predict new catalytic materials, is key to overcoming the inherent limitations of metal oxide based gas sensors associated with rather low sensitivity and selectivity, particularly under highly humid conditions. This study introduces newly designed bimetallic nanoparticles (NPs) employing bimetallic Pt-based NPs (PtM, where M = Pd, Rh, and Ni) via a protein encapsulating route supported on mesoporous WO3 nanofibers. These structures demonstrate unprecedented sensing performance for detecting target biomarkers (even at p.p.b. levels) in highly humid exhaled breath. Sensor arrays are further employed to enable pattern recognition capable of discriminating between simulated biomarkers and controlled breath. The results provide a new class of multicomponent catalytic materials, demonstrating potential for achieving reliable breath analysis sensing.

Dynamic chemical expansion of thin-film non-stoichiometric oxides at extreme temperatures.

Actuator operation in increasingly extreme and remote conditions requires materials that reliably sense and actuate at elevated temperatures, and over a range of gas environments. Design of such materials will rely on high-temperature, high-resolution approaches for characterizing material actuation in situ. Here, we demonstrate a novel type of high-temperature, low-voltage electromechanical oxide actuator based on the model material PrxCe1-xO2-δ (PCO). Chemical strain and interfacial stress resulted from electrochemically pumping oxygen into or out of PCO films, leading to measurable film volume changes due to chemical expansion. At 650 °C, nanometre-scale displacement and strain of >0.1% were achieved with electrical bias values <0.1 V, low compared to piezoelectrically driven actuators, with strain amplified fivefold by stress-induced structural deflection. This operando measurement of films 'breathing' at second-scale temporal resolution also enabled detailed identification of the controlling kinetics of this response, and can be extended to other electrochemomechanically coupled oxide films at extreme temperatures.

Role of grain size on redox induced compositional stresses in Pr doped ceria thin films.

In constrained geometries and in varying oxygen partial pressures and operating temperatures, exchange of oxygen ions between non-stoichiometric oxide thin films (for example, doped and undoped ceria systems) and the gas phase can lead to stresses. In this study, these compositional stresses were investigated in thin films of nanocrystalline 10% praseodymium doped ceria (PCO), as a function of average grain size. In situ wafer curvature measurements, along with High Temperature X-Ray Diffraction (HTXRD), were employed to measure stresses and strains, respectively on the PCO films during oxidation-reduction cycling, over the pO2 range of 10-1-10-5 atm at 750 °C. For relatively large grain sizes, the stress values agree well with the amount of expansion induced by oxygen non-stoichiometry (chemical expansion) predicted by a thin film defect equilibria model that was developed previously. The compositional stresses were found to increase with decreasing grain size. The origin of this effect, including the role of space charge effects near surfaces and interfaces are discussed in this paper. To our knowledge, this is the first time that such comparisons are reported by simultaneously employing high temperature in situ wafer curvature and HTXRD measurements on doped ceria systems.

Solar to fuels conversion technologies: a perspective.

To meet increasing energy needs, while limiting greenhouse gas emissions over the coming decades, power capacity on a large scale will need to be provided from renewable sources, with solar expected to play a central role. While the focus to date has been on electricity generation via photovoltaic (PV) cells, electricity production currently accounts for only about one-third of total primary energy consumption. As a consequence, solar-to-fuel conversion will need to play an increasingly important role and, thereby, satisfy the need to replace high energy density fossil fuels with cleaner alternatives that remain easy to transport and store. The solar refinery concept (Herron et al. in Energy Environ Sci 8:126-157, 2015), in which captured solar radiation provides energy in the form of heat, electricity or photons, used to convert the basic chemical feedstocks CO2 and H2O into fuels, is reviewed as are the key conversion processes based on (1) combined PV and electrolysis, (2) photoelectrochemically driven electrolysis and (3) thermochemical processes, all focused on initially converting H2O and CO2 to H2 and CO. Recent advances, as well as remaining challenges, associated with solar-to-fuel conversion are discussed, as is the need for an intensive research and development effort to bring such processes to scale.

Praseodymium Cuprate Thin Film Cathodes for Intermediate Temperature Solid Oxide Fuel Cells: Roles of Doping, Orientation, and Crystal Structure.

Highly textured thin films of undoped, Ce-doped, and Sr-doped Pr2CuO4 were synthesized on single crystal YSZ substrates using pulsed laser deposition to investigate their area-specific resistance (ASR) as cathodes in solid-oxide fuel cells (SOFCs). The effects of T' and T* crystal structures, donor and acceptor doping, and a-axis and c-axis orientation on ASR were systematically studied using electrochemical impedance spectroscopy on half cells. The addition of both Ce and Sr dopants resulted in improvements in ASR in c-axis oriented films, as did the T* crystal structure with the a-axis orientation. Pr1.6Sr0.4CuO4 is identified as a potential cathode material with nearly an order of magnitude faster oxygen reduction reaction kinetics at 600 °C compared to thin films of the commonly studied cathode material La0.6Sr0.4Co0.8Fe0.2O3-δ. Orientation control of the cuprate films on YSZ was achieved using seed layers, and the anisotropy in the ASR was found to be less than an order of magnitude. The rare-earth doped cuprate was found to be a versatile system for study of relationships between bulk properties and the oxygen reduction reaction, critical for improving SOFC performance.

Oxygen diffusion and surface exchange in the mixed conducting oxides SrTi1-yFeyO3-δ.

Oxygen transport in the mixed ionic-electronic conducting perovskite-oxides SrTi1-yFeyO3-δ (with y = 0.5 and y = 1.0) was studied by oxygen isotope exchange measurements. Experiments were performed on thin-film samples that were grown by Pulsed Laser Deposition (PLD) on MgO substrates. Isotope penetration profiles were introduced by 18O2/16O2 exchanges into the plane of the films at various temperatures in the range 773 < T/K < 973 at an oxygen activity aO2 = 0.5. Isotope profiles were determined subsequently by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and their analysis yielded tracer diffusion coefficients D* and oxygen surface exchange coefficients k*. Activation energies for oxygen diffusion ΔHD* and surface exchange ΔHk* were obtained. Isothermal values of D* and values of ΔHD* are compared with literature data as a function of Fe content. D* is seen to increase monotonically with Fe content; ΔHD* shows more complex behaviour. D* and ΔHD* are also compared with the predictions of defect-chemical models. Analogous comparisons with literature data for k* and ΔHk* indicate, in contrast to prior studies, no mechanistic difference between electron-poor and electron-rich materials. It is concluded that the single operative mechanism of surface exchange for the entire series of STF compositions requires conduction-band electrons (minority electronic charge-carriers).

Heterogeneous Sensitization of Metal-Organic Framework Driven Metal@Metal Oxide Complex Catalysts on an Oxide Nanofiber Scaffold Toward Superior Gas Sensors.

We report on the heterogeneous sensitization of metal-organic framework (MOF)-driven metal-embedded metal oxide (M@MO) complex catalysts onto semiconductor metal oxide (SMO) nanofibers (NFs) via electrospinning for markedly enhanced chemical gas sensing. ZIF-8-derived Pd-loaded ZnO nanocubes (Pd@ZnO) were sensitized on both the interior and the exterior of WO3 NFs, resulting in the formation of multiheterojunction Pd-ZnO and ZnO-WO3 interfaces. The Pd@ZnO loaded WO3 NFs were found to exhibit unparalleled toluene sensitivity (Rair/Rgas = 4.37 to 100 ppb), fast gas response speed (∼20 s) and superior cross-selectivity against other interfering gases. These results demonstrate that MOF-derived M@MO complex catalysts can be functionalized within an electrospun nanofiber scaffold, thereby creating multiheterojunctions, essential for improving catalytic sensor sensitization.

Mesoporous WO3 Nanofibers with Protein-Templated Nanoscale Catalysts for Detection of Trace Biomarkers in Exhaled Breath.

Highly selective detection, rapid response (<20 s), and superior sensitivity (Rair/Rgas> 50) against specific target gases, particularly at the 1 ppm level, still remain considerable challenges in gas sensor applications. We propose a rational design and facile synthesis concept for achieving exceptionally sensitive and selective detection of trace target biomarkers in exhaled human breath using a protein nanocage templating route for sensitizing electrospun nanofibers (NFs). The mesoporous WO3 NFs, functionalized with well-dispersed nanoscale Pt, Pd, and Rh catalytic nanoparticles (NPs), exhibit excellent sensing performance, even at parts per billion level concentrations of gases in a humid atmosphere. Functionalized WO3 NFs with nanoscale catalysts are demonstrated to show great promise for the reliable diagnosis of diseases.