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We develop a framework for controlling and investigating reversible ionic transfer between two solid metal oxides layers by examining field-driven changes in electrical properties of the thin film bilayer oxide system Pr0.1Ce0.9O2/La1.85Ce0.15CuO4(PCO/LCCO). We show that we can reversibly redistribute oxygen ions by applied voltage in a highly controlled and reversible fashion near ambient temperatures over large oxygen ion activity limits, which, for the first time, is directly interpretable by defect chemical models. This allowed us to determine how defect concentrations in each layer systematically varied with voltage and the subsequent impact on each film’s conductance. These results showcase the relevance and applicability of defect chemical models, traditionally considered only at elevated temperatures, to the development of bilayer devices of importance to neuromorphic memory applications. This allows for a more systematic approach for studying and understanding the solid–solid exchange process in electrochemically controlled microelectronic devices. Moreover, our work sets the foundation for the development of large-area field-driven defect-controlled bilayer switching devices with potential application to a broad array of functionally modulated devices.

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 Fe0exsolution 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 Fe0over 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.

The chemical expansion of Pr0.1Ce0.9O2–δ(PCO) and CeO2–δthin films is investigated in the temperature range between 600 °C and 800 °C by laser Doppler vibrometry (LDV). It enables non-contact determination of nanometer scale changes in film thickness at high temperatures. The present study is the first systematic and detailed investigation of chemical expansion of doped and undoped ceria thin films at temperatures above 650 °C. The thin films were deposited on yttria stabilized zirconia substrates (YSZ), operated as an electrochemical oxygen pump, to periodically adjust the oxygen activity in the films, leading to reversible expansion and contraction of the film. This further leads to stresses in the underlying YSZ substrates, accompanied by bending of the overall devices. Film thickness changes and sample bending are found to reach up to 10 and several hundred nanometers, respectively, at excitation frequencies from 0.1 to 10 Hz and applied voltages from 0–0.75 V for PCO and 0–1 V for ceria. At low frequencies, equilibrium conditions are approached. As a consequence maximum thin-film expansion of PCO is expected due to full reduction of the Pr ions. The lower detection limit for displacements is found to be in the subnanometer range. At 800 °C and an excitation frequency of 1 Hz, the LDV shows a remarkable resolution of 0.3 nm which allows, for example, the characterization of materials with small levels of expansion, such as undoped ceria at high oxygen partial pressure. As the correlation between film expansion and sample bending is obtained through this study, a dimensional change of a free body consisting of the same material can be calculated using the high resolution characteristics of this system. A minimum detectable dimensional change of 5 pm is estimated even under challenging high-temperature conditions at 800 °C opening up opportunities to investigate electro-chemo-mechanical phenomena heretofore impossible to investigate. The expansion data are correlated with previous results on the oxygen nonstoichiometry of PCO thin films, and a defect model for bulk ceria solid solutions is adopted to calculate the cation and anion radii changes in the constrained films during chemical expansion. The constrained films exhibit anisotropic volume expansion with displacements perpendicular to the substrate plane nearly double that of bulk samples. The PCO films used here generate high total displacements of several 100 nm’s with high reproducibility. Consequently, PCO films are identified to be a potential core component of high-temperature actuators. They benefit not only from high displacements at temperatures where most piezoelectric materials no longer operate while exhibiting, low voltage operation and low energy consumption.

Understanding the surface defect chemistry and its strain dependency is essential in developing next-generation electrochemical devices. However, due to their nanoscale dimensions, surface defects cannot be accessed by conventional techniques used in bulk defect studies. Here, we constructed the strain-dependent surface defect equilibria (i.e., the Brouwer diagram) of mixed ionic-electronic conducting perovskite oxides with near ambient pressure X-ray absorption spectroscopy. Using coherently strained thin-film La0.6Sr0.4FeO3(LSF) as model systems, we probed their surface defect equilibria at 400 °C in oxygen partial pressures between 1 and 10–5Torr. We found that the electron holes on the LSF surfaces have strong oxygen character, regardless of the strain states. Nevertheless, tensile strain makes the LSF surface more reducible than the compressed counterpart. These two observations were then validated using first-principles calculations. Finally, with the aid of thermodynamic analyses, we showed that the strain-dependent surface defect equilibria of LSF can be captured by bulk-like ideal solution defect models with shifted oxygen chemical potentials. The findings and methodology presented in this study enable quantitative determination of the surface defect chemistry, which is crucial to understanding and designing functional surfaces for efficient conversions of energy and fuels.

Temporal and spatial tuning of the refractive index of optical thin films is desired for flat optics applications. The redistribution of mobile ions in mixed ionic-electronic conductors (MIEC) has been demonstrated to serve as a viable means for achieving optical tuning down to the nanoscale. Here we studied the dynamic range of the optical tuning achievable in the refractive index, in the MIEC oxide – PrxCe1−xO2−δ(PCO), for x= 0.1, 0.2 and 0.4, at 500 °C, by in-situspectrophotometry. Significant increases in the modulation of both the imaginary and real optical constants in the visible and the adjacent spectra were obtained for increased doping levels. Device employing an electrochemical titration method was implemented to modulate the oxygen concentration, and thereby the optical transmission of PCO. Incorporation of a patterned top electrode allowed for the demonstration of spatial control of PCO thin film properties by in-situvideo imaging of the optical switching process. The electrochemically induced optical state is shown to remain non-volatile upon quenching the device to room temperature under applied bias.

The oxygen exchange reaction kinetics of a large number of ternary mixed ionic and electronic conducting oxides were analyzed with respect to their calculated surface acidity factors derived from the Smith acidity factors of the corresponding binary oxides from which the ternary oxides are composed. The analysis suggests that the surface exchange kinetics have a direct link to the acidity of the mixed conducting oxide. The possible interplay between acidity and transport properties is discussed. The analysis is also applied to oxygen evolution reaction (OER) electrocatalysts but does not show a link between OER overpotentials and acidity, suggesting that the link is more relevant for gas/oxide interactions at high temperatures. The observations suggest that acidity impacts most directly the oxygen adsorption process while impacting the electron charge transfer only indirectly.

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.

Sr-doped LaMnO3(LSM) is a promising oxygen reduction reaction electrocatalyst in solid oxide fuel cells and other electrochemical devices. The presence of CO2and H2O has been reported to promote the oxygen dissociation reaction on LSM surfaces. Here, we investigate the coadsorption mechanism of O2with H2O or CO2by combining first-principles calculations of the (0 0 1) surface containing 25–100% Sr with thermodynamic adsorption models. The molecules were found to chemisorb by formation of charged oxygen, hydroxide, and carbonate species, and the adsorption energies were exothermic up to monolayer coverage. Low concentrations of H2O or CO2do not compete with O2for adsorption sites under relevant conditions. However, their presence contributes to the total amount of oxygen-containing species. The increased coverage of oxygen species provides a quantitative explanation for the reported enhancement in oxygen dissociation kinetics in the presence of H2O/CO2. This study thereby provides insights into oxygen exchange mechanisms on LSM surfaces.

The adsorption of CO2and CO on BaZrO3(0 0 1) was investigated by first-principles calculations with a focus on the BaO termination. CO2was found to strongly chemisorb on the surface by formation of carbonate species with an adsorption enthalpy of up to −2.25 eV at low coverage and −1.05 eV for a full monolayer. An adsorption entropy of −8.8 × 10–4eV K–1was obtained from the vibrational properties of the adsorbates. Surface coverages were evaluated as a function of temperature and CO2partial pressure, and the obtained coverage under 1 bar CO2was more than 0.8 at 1000 K (conditions relevant for steam methane reforming). The fully saturated surface was stable up to about 400 K under ambient atmosphere, i.e., 400 ppm of CO2. The initial stage of BaCO3formation was evaluated according to migration of barium to the carbonate overlayer, which was found to result in a significant stabilization of the system. The barium migration was found to be essentially unobstructed with a barrier of only ∼5 meV. In light of the stability of carbonate adsorbates at the surface, the prospect of bulk dissolution of carbonate species was evaluated but ultimately found to be negligible in acceptor-doped BaZrO3.

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 PO2dependence 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.

Surface oxygen reduction reaction (ORR) rates at n-type oxide-based mixed ionic–electronic conducting (MIEC) solid oxide fuel cell (SOFC) cathodes can be expected to be enhanced relative to that at p-type MIEC cathodes due to the greater availability of electrons at higher energies in the band structure needed for the charge transfer reaction. However, given the difficulty of achieving coexisting oxygen vacancies and electrons in the conduction band under oxidizing cathode conditions, no stable n-type MIEC cathodes have been reported to date. In this study, a predominantly n-type MIEC conductivity is confirmed in a Ba–In-based oxide (BNIM) co-doped with Nd and Mn at high temperature and high PO2confirmed by the PO2dependence of the electrical conductivity and negative Seebeck coefficients, combined with readily measurable oxide ion transference numbers. This coexistence of n-type electronic and oxide ion conductivities is discussed based on the electrical behavior of BNIM with different Mn levels and is attributed to the significant change in the degree of anion Frenkel ordering and the band structure associated with heavy donor doping of Ba2In2O5. This novel n-type MIEC has the potential for enhancing the ORR at SOFC cathodes at reduced temperatures and thereby identifying new potential candidate cathode materials for next-generation SOFCs.

Understanding and rationally tailoring defect-mediated optical absorption of nondilute oxide solid solutions represents a complex challenge. In this work, we investigate compositions in the SrTiO3–SrFeO2.5solid solution, departing from the simpler dilute Fe-substituted SrTiO3case. Through ex situand in situoptical absorption measurements of mixed conducting thin films prepared by pulsed laser deposition and through density functional theory simulations, we demonstrate understanding and rational tailoring of the optical absorption behavior. Experimentally, broad subgap absorption peaks, centered around 2.1 and 2.8–3 eV, increase in intensity with increasing Fe and/or O concentrations and decrease with increasing La donor dopant concentration. Consistent with these observations, the absorption is found to be proportional to the hole concentration and to the Fe concentration under fully oxidized conditions. This behavior is similar to the dilute case; however, the solid solution electronic structure and optical absorption behavior cannot be represented simply by Fermi level shifts in a rigid-band model. Simulations confirm these trends and identify transitions responsible for absorption as occurring from within the valence band to empty states at the hybrid (O 2p/Fe 3d) valence band maximum and to empty states at the hybrid (Ti/Fe 3d) conduction band minimum. Adding oxygen or iron, or removing La, increases the density of empty states at the top of the valence band and bottom of the conduction band, increasing the intensity of these transitions. This approach for studying solid solution behavior can be extended to other systems in the future, and the fundamental understanding of the origins of absorption also enables its in situuse as a quantitative probe of thin film point defect concentrations and kinetics.

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/GdOxinterface, 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.

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-Fe2O3nanoscale powders were characterized in situ at controlled temperatures (T= 250 to 400 °C) and atmospheres (pO2= 10–4to 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-Fe2O3were derived. Using these defect equilibria models, the operative defect regimes were established, and the bandgap energy of undoped α-Fe2O3and oxidation enthalpy of Sn-doped α-Fe2O3were obtained from the analysis of the temperature and pO2dependence 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.

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 operandodefect concentrations, transport properties, and mechanical properties. Here, we present an in situinvestigation 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 situheating, 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 operandochemical expansion or “breathing” of such oxide films.

The oxygen exchange kinetics in mixed conducting oxide surfaces are critical to a variety of energy-related technologies, including solid oxide fuel/electrolyzer cells (SOFCs/SOECs), permeation membranes, batteries, and sensors. Such devices, however, commonly suffer from performance degradation from impurity-poisoning during long-term operation. In our previous findings, it was found that while acidic additives can depress the surface exchange kinetics, basic additives can enhance these kinetics. Inspired by these findings, we demonstrated the ability to regenerate acidic (Cr and Si)-poisoned surfaces by compensating with basic additive infiltrants (Li and Ca). Electrical conductivity relaxation measurements on porous mixed conducting oxide specimens were used to examine the impact of infiltrants on the oxygen exchange rate coefficient (kchem) and revealed that the recovery of poisoned surface kinetics can be attributed to the compensation of depleted surface electrons by basic additives.

Different fuel electrodes for reversible solid oxide cells (r-SOC) are investigated with the aim of improving performance in both solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) modes, and durability in reversible operation mode. Electrodes based on gadolinium-doped ceria (GDC) as a mixed ionic electronic conductor, and lanthanum-doped strontium titanate (LST) as an electronic conductor are selected. The current-voltage characteristics of r-SOC single cells, and their cycling durability up to 1000 cycles are evaluated. LST-GDC co-impregnated with Ni and GDC prove to be highly durable in reversible operation, as a suitable fuel electrode material for r-SOCs.

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.

Highly textured thin films of undoped, Ce-doped, and Sr-doped Pr2CuO4were 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. Pr1.6Sr0.4CuO4is 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-d. 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.

Mixed ionic and electronic conducting perovskites that can readily exchange oxygen with the atmosphere exhibit a chemo-mechanical coupling between their oxygen content and their lattice dimensions. The lattice dilation accompanying oxygen loss, termed "chemical expansion," causes large chemical stresses in devices during operation that can lead to mechanical failure. This paper describes our work aimed at understanding, across multiple length scales, which factors impact chemical expansion coefficients in perovskites. Polycrystalline gallate and titanate perovskites containing multivalent Ni, Fe, and Co have been studied using in situ thermogravimetry, dilatometry, and diffraction to probe the chemical expansion process at macroscopic and crystal structure levels. Density functional theory, molecular dynamics, and empirical simulations have provided atomistic insight into changes taking place on the anion and cation sublattices during oxygen loss. Factors impacting the magnitude of the chemo-mechanical coupling, including oxygen vacancy radii, charge localization on cations, temperature, and crystal symmetry have been identified.

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 WO3NFs, 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 WO3NFs with nanoscale catalysts are demonstrated to show great promise for the reliable diagnosis of diseases.

Experimental results obtained on electrochemical cells of the form PCO/YSZ/Ag by a novel experimental technique capable of simultaneously performing in situ optical absorption and electrochemical impedance spectroscopy (EIS) measurements under bias were used to demonstrate the ability to extract and control nonstoichiometry in thin film oxides. We showed, that when applied to model materials systems such as PrxCe1-xO2-d (PCO), these techniques are capable of deriving a detailed picture of the defect equilibria and transport properties of such materials and determining whether, and to what degree, they differ from bulk properties. Key studies of chemical capacitance, optical absorption and color front motion kinetics are used to extract key thermodynamic and kinetic (kex, DO) parameters in PCO.

We present a route toward a radical improvement in solar cell efficiency using resonant energy transfer and sensitization of semiconductor metal oxides with a light-harvesting quantum dot (QD)/bacteriorhodopsin (bR) layer designed by protein engineering. The specific aims of our approach are (1) controlled engineering of highly ordered bR/QD complexes; (2) replacement of the liquid electrolyte by a thin layer of gold; (3) highly oriented deposition of bR/QD complexes on a gold layer; and (4) use of the Forster resonance energy transfer coupling between bR and QDs to achieve an efficient absorbing layer for dye-sensitized solar cells. This proposed approach is based on the unique optical characteristics of QDs, on the photovoltaic properties of bR, and on state-of-the-art nanobioengineering technologies. It permits spatial and optical coupling together with control of hybrid material components on the bionanoscale. This method paves the way to the development of the solid-state photovoltaic device with the efficiency increased to practical levels.

Diagnostic sensing device using exhaled breath of human have critical advantages due to the noninvasive diagnosis and high potential for portable device with simple analysis process. Here, we report ultrafast as well as highly sensitive bumpy WO3hemitube nanostructure assisted by O2plasma surface modification with functionalization of graphene-based material for the detection of acetone (CH3COCH3) and hydrogen sulfide (H2S) which are biomarkers for the diagnosis of diabetes and halitosis, respectively. 0.1 wt % graphene oxide (GO)- and 0.1 wt % thin layered graphite (GR)- WO3hemitube composites showed response times of 11.5 ± 2.5 s and 13.5 ± 3.4 s to 1 ppm acetone as well as 12.5 ± 1.9 s and 10.0 ± 1.6 s to 1 ppm of H2S, respectively. In addition, low limits of detection (LOD) of 100 ppb (Rair/Rgas= 1.7 for acetone and Rair/Rgas= 3.3 for H2S at 300 °C) were achieved. The superior sensing properties were ascribed to the electronic sensitization of graphene based materials by modulating space charged layers at the interfaces between n-type WO3hemitubes and p-type graphene based materials, as identified by Kelvin Probe Force Microscopy (KPFM). Rapid response and superior sensitivity of the proposed sensing materials following cyclic thermal aging demonstrates good potential for real-time exhaled breath diagnosis of diseases.

Field effect transistors (FETs), incorporating metal-oxide nanofibers as the active conductive channel, have the potential for driving the widespread application of nanowire or nanofiber FETs-based electronics. Here we report on low voltage FETs with integrated electrospun In2O3–ZnO–ZnGa2O4composite fiber channel layers and high-K dielectric (MgO)0.3-(Bi1.5Zn1.0Nb1.5O7)0.7gate insulator and compare their performance against FETs utilizing conductive single phase, polycrystalline ZnO or In2O3channel layers. The polycrystalline In2O3–ZnO–ZnGa2O4composite fibers provide superior performance with high field effect mobility (∼7.04 cm2V–1s–1), low subthreshold swing (390 mV/dec), and low threshold voltage (1.0 V) combined with excellent saturation, likely resulting from the effective blocking of high current-flow through the In2O3and ZnO nanocrystallites by the insulating spinel ZnGa2O4phase. The microstructural evolution of the individual In2O3, ZnO, and ZnGa2O4phases in composite fibers is clearly observed by high resolution TEM. A systematic examination of channel area coverage, ranging from single fiber to over 90% coverage, demonstrates that low coverage results in relatively low current outputs and reduced reproducibility which we attribute to the difficulty in positioning fibers and fiber length control. On the other hand, those with ∼80% coverage exhibited high field effect mobility, high on/off current ratios (>105), and negligible hysteresis following 15 sweep voltage cycles. A special feature of this work is the application of the FETs to modulate the properties of complex polycrystalline nanocomposite channels.

The point defect induced strain and stress in thin films was analyzed using in situ wafer curvature and in-situ XRD measurements. The strains and stresses were modeled using a previously well-developed and experimentally validated point defect equilibria model. Values for the chemical expansion coefficient and Young's modulus for bulk ceramics were successfully used to predict stress and strain behavior of the corresponding thin films.

While Cr poisoning of the oxygen reduction reaction (ORR) at SOFC cathodes is widely agreed to involve deactivation of oxygen exchange sites, the degradation mechanism remains ambiguous. Here, we selected an alternative cathode material Pr0.1Ce0.9O2−δ,free of Sr segregation, to systematically investigate the effect of Cr-induced degradation in ORR. We expand on our previous studies in which the acidity/basicity of binary additives was found to be a strong indicator of the rate of oxygen surface exchange, by electrochemically investigating the ORR activity of the PCO cathode by impedance spectroscopy. Serial infiltration with acidic Cr-based oxides was found to degrade ORR activity as reflected in a 20-fold increase in area specific resistance (ASR) without corresponding changes in activation energy, with the opposite trend obtained with Ca-based oxides. Detected changes in total capacitance, attributed to changes in surface capacitance, also suggest depressed/enhanced PCO surface redox behavior with Cr/Ca-based infiltrants. Taken together, these results point to a more universal source of ORR poisoning/activation, based on acidity/basicity, rather than physical blocking of active sites. With this improved understanding, one can expect progress to be made in the coming years in optimizing means for protecting catalytic surfaces from degradation and/or improving their performance.

Fast oxygen incorporation, essential for low temperature SOFCs, is enhanced in mixed ionic electronic conducting electrodes. In this study, simultaneous in situ measurement of transient optical absorption changes and electrode impedance is demonstrated as a powerful and convenient means for examining oxygen exchange kinetics in oxide thin films, utilizing Pr0.1Ce0.9O2-d as a model system. Absorption in the visible spectrum is tied to the fraction of Pr in its oxidized state (4+), which in turn is related to the material's oxygen deficiency. The origin of discrepancies between kchem derived from optical relaxation measurements and kelec from electrochemical measurements is discussed in terms of surface contamination and the potential catalytic effect of metal contacts.

Non-stoichiometric oxides are frequently used in SOFC electrodes due to the ease with which they exchange oxygen with the atmosphere over their entire surface, as compared to composites limited to three phase boundaries (i.e. LSM/YSZ). The large fluctuations in oxygen content these materials exhibit during operation lead, not only to significant changes in transport properties, but also to a defect induced expansion, known as chemical expansion, potentially contributing to mechanical failure. In this presentation, new insights into the origins of chemical expansion, with consequent development of methods for reducing its impact in non-stoichiometric oxides, facilitated by collaborations of the authors, will be discussed.

PrxCe1-xO2-d can potentially serve as both cathode and anode in solid oxide fuel cells given that it exhibits significant mixed ionic-electronic conductivity (MIEC) under both cathodic and anodic conditions. While MIEC depends strongly on oxygen nonstoichiometry d, there have been few ways to extract this information reliably and in situ, particularly for thin films. In this work, this is achieved by analysis of chemical capacitance values extracted from impedance spectroscopy data obtained on electrochemical cells of the form Pr0.1Ce0.9O2[?]d/ Y0.16Zr0.84O1.92 / Pr0.1Ce0.9O2[?]d operating at temperatures between 500 to 700 oC and over the pO2 range of 10-31-10-16 atm. The enthalpy of reduction for the Pr0.1Ce0.9O2[?]d thin film is derived.

To understand ultra-fast oxygen reduction reaction kinetics near La0.8Sr0.2CoO3/(La0.5Sr0.5)2CoO4 (LSC113/214) interface, a combination of in-situ scanning tunneling spectroscopy and focused ion beam milling was used to study the local electronic structure near LSC113/214 interface at elevated temperature and in oxygen. The electronic activation of LSC214 by the coupling with LSC113, concurrent with the fast oxygen incorporation on LSC214, was identified to be a key reason. These results put forward the electronically coupled oxide structures as novel cathodes. We generalize and further test this idea on a La0.8Sr0.2CoO3/La2NiO4 system, which shows a similar electronic activation of La2NiO4 at elevated temperatures.

The influence of electron localization on the chemical expansion coefficient was examined experimentally, using a model electrode material, (La,Sr)(Ga,Ni)O3-d. Varying the Ni content yielded differing degrees of electron localization with resultant chemical expansion coefficients evaluated by dilatometry and thermogravimetric analysis in N2/O2 mixtures at 800 degC. In agreement with prior computational predictions, a decrease in chemical expansion coefficient was observed with electron delocalization.

Donor doping of the p-type solid oxide fuel cell cathode, Sr(Ti,Fe)O3-d, with La was pursued to examine the role of minority charge carriers (electrons) on the oxygen exchange rate. Model dense thin film cathodes of (La,Sr)(Ti,Fe)O3-d were fabricated by pulsed laser deposition. Optical transmission spectroscopy measurements demonstrated that the addition of the donor La resulted in a change in valence state of Fe from 4+ to 3+, suggesting a rise in Fermi level. The activation energy of the cathode's area specific resistance, studied by impedance spectroscopy, was found to be lower with La addition; however, the magnitude of the area specific resistance was not lowered.

The authors use SIMS measurements to examine the composition of a Pr0.1Ce0.9O2[?]d thin film following an oxygen surface exchange measurement via optical absorption relaxation. A significant concentration of impurities (Si, K, Na, and Ca) were observed to accumulate on the film surface, with a concentration profile extending approximately 60 nm into the film, following the high temperature measurements. This resulted in a significant decrease in the oxygen exchange coefficient, kex. Potential sources for the impurities are discussed.

The predicted dependence of TlBr room temperature ionic conductivity on divalent donor and acceptor dopant concentration is presented, taking into account acceptor dopant exsolution and dopant-defect association. Donor-acceptor compensation is found to play a key role in determining the ultimate room temperature conductivity with donors increasing the solubility of acceptors from 0.04 ppm (0 donor) to 0.84 ppm (1 ppm donor) to 8.4 ppm (10 ppm donor).

Abstract

Atomic scale computer simulations, validated with experimental data, are used to uncover the factors responsible for defect‐induced chemical expansion observed in non‐stoichiometric oxides, exemplified by CeO2and ZrO2. It is found that chemical expansion is the result of two competing processes: the formation of a vacancy (leading to a lattice contraction primarily due to electrostatic interactions) and the cation radius change (leading to a lattice expansion primarily due to steric effects). The chemical expansion coefficient is modeled as the summation of two terms that are proportional to the cation and oxygen radius change. This model introduces an empirical parameter, the vacancy radius, which can be reliably predicted from computer simulations, as well as from experimental data. This model is used to predict material compositions that minimize chemical expansion in fluorite structured solid oxide fuel cell electrolyte materials under typical operating conditions.

The non-stoichiometry induced stress developed in the potential cathode material, Pr0.1Ce0.9O2-d (PCO), upon cooling from high temperature is modeled. Stresses can develop in such mixed ionic electronic materials due to changes in oxygen stoichiometry, resulting in defect induced dilation, or chemical expansion. In this example, a simulated bulk sample of PCO is cooled from 1400 C in air and O2 at different rates and the resulting diffusion limited concentration gradient is predicted and converted into a stress distribution using the chemical coefficient of expansion. Maximum tensile stresses of ~500 MPa are predicted at the sample surface, which can be reduced by cooling in more oxidizing atmospheres and/or at slower rates.

In-situ optical absorption measurements are used to derive the kinetics of oxidation and reduction of Pr0.1Ce0.9O2-d in an oxygen partial pressure range from 4 - 50% O2 at 600 oC without potential interference from catalytic metal electrodes. The chemical oxygen surface exchange coefficient is found to obey a power law dependence on pO2 suggesting an adsorption limiting process. The sources of discrepancies with values obtained in previous studies are discussed in terms of the catalytic activity of metal electrodes or changes in film surface chemistry.

Optimization of electrical, optical, mechanical, and other properties of many advanced, functional materials today relies on precise control of point defects. This article illustrates the progress that has been made in elucidating the often complex equilibria exhibited by many materials by examining two recently well-characterized model systems, TlBr for radiation detection and PrxCe1−xO2−δ, of potential interest in solid-oxide fuel cells. The interplay between material composition, electrical conductivity, and mechanical properties (electrochemomechanics) is discussed, and implications in these relations, for example, enhancing electrical properties through large mechanical strains, are described. The impact of space charge and strain fields at interfaces, particularly important in nanostructure materials, is also emphasized. Key experimental techniques useful in characterizing bulk and surface defects are summarized and reviewed.

We report on the facile synthesis of highly conductive RuO2/Mn3O4 composite nanofiber mats by electrospinning of mixed ruthenium and manganese precurosr solution and their potential suitability as an active electrode in electrochemical capacitors. Electrospun RuO2/Mn3O4 fiber mats calcined at 300degC exhibit multiple fiber networks composed of nanoparticles in a size range of 5-10 nm (average [?] 7.5 nm). Partially crystalline Mn3O4 phases are well mixed with the amorphous RuOx (a-RuO2) phases while pure Mn3O4 fibers exhibit only crystalline phase. Electrochemical capacitors utilizing a-RuO2/Mn3O4 fiber mats show a high specific capacitance of 293 Fg[?]1 at 10 mVs[?]1, high rate capability with a capacity loss of only 45% from 10 to 2000 mVs[?]1, and superior cycle performance of over 30,000 cycles as compared to those characteristics of Mn3O4 fiber mats (216 Fg[?]1 at 10 mVs[?]1, capacity loss of 94% at 2000 mVs[?]1, poor cycle performance of 2000 cycles). These results are correlated with the improved mixed ionic/electronic conductivity of the a-RuO2/Mn3O4 fiber mats as well as to its morphological evolution, i.e., highly porous flower-like-fiber networks, during cycling tests.

The surface structure, chemical composition and electronic structure of Sr(Ti1-xFex)O3 under different temperatures and oxygen pressures were studied by Scanning Tunneling Microscopy / Spectroscopy (STM/S) and X-ray Photoelectron Spectroscopy (XPS). The surface structure remained stable to 430 oC at 10-3 mbar oxygen pressure. When annealing at low oxygen pressure, the A to B site ratio, Sr/(Fe+Ti) decreased, Fe content at the B site increased, and particles that are likely to be Fe enriched, precipitated to the grain boundaries. Moreover, Fe changed its valance state, in part, from Fe3+ to Fe2+, consistent with oxygen vacancy formation. The surface of STF was electrically insulating at room temperature, and exhibited by STS a well defined gap in the electronic density of states at 380 oC. With further increase to 430 oC, the energy gap disappeared. The variation in surface defects during thermal treatment is discussed as a likely reason for electronic structure changes at elevated temperatures.

This work presents a new route for enhancing the sensor response kinetics of macroporous films. Well-ordered arrays of hollow hemispheres, functionalized with catalytic Pt, were synthesized using colloidal templating and cosputtering, followed by subsequent heat-treatment. The diameter and shell thickness of the Pt-functionalized hollow hemispheres were 800 and 25 nm, respectively. High resolution transmission electron microscopy analysis indicates that catalytic Pt nanoparticles, with size of a few nanometers, coat the top surface of hemispheres, enabling chemical reactions to proceed at lower temperatures. Sensors composed of Pt-functionalized hollow hemispheres show attractive gas sensing characteristics, such as reduced operation temperature combined with improved sensitivity ( to 250 ppm ), reduced baseline resistance, and enhanced response/recovery properties compared to sensors using pristine hollow hemispheres at an operating temperature of .

Many advanced materials today derive their function from precise control of point defects. Through manipulation of point defects, the electrical, optical, mechanical, and other properties of materials can be systematically modified and optimized. This feature paper illustrates the type of progress that has been made in understanding the often complex equilibria exhibited by many materials and shows, by use of examples, how this knowledge can be applied to tailor materials’ properties through defect engineering. Examples which highlight key points of defect engineering include: dark conductivity of radiation detectors, chemomechanics and nanoionics of solid oxide fuel cell materials, sensitivity of high frequency resonators for high-temperature mass measurements, and emission lifetime and luminescence intensity of phosphor materials.

Over the past decade, the most commonly studied nanocrystalline electroceramic system has been cerium oxide. Ionic conduction is traditionally introduced into this system via homogeneous doping, though the maximum conductivity is limited by carrier mobility at high doping levels. In this work, heterogeneous doping is explored as a potential means for enhanced ionic conductivity. Nanocrystalline CeO2 thin films were deposited by pulsed laser deposition and modified by in-diffusion of Ni and Gd along columnar grain boundaries. To separate the ionic and electronic contributions to the conductivity, electron-blocking microelectrodes, consisting of dense yttria-stabilized zirconia and porous Pt layers, were fabricated on the microscale. After in-diffusion, both the ionic and electronic partial conductivities decreased relative to the as-deposited state. This result is analyzed in terms of the grain boundary chemistry and space charge potential, as well as segregation of acceptor impurities within the ceria thin films.

Abstract The gas sensing behavior of praseodymium doped cerium oxide (PrxCe1 - xO2 or PCO) has been examined for 0–1000 ppm CO or H2 in a 10% O2 atmosphere at temperatures ranging from 250–350_°C. Total conductivity as a function of temperature suggests that oxygen diffusion kinetics are slow below approximately 350_°C. Devices with x = 0.05 and 0.10 show stable, n-type gas sensing response, while those with x = 0.20 exhibit significant drift in sensor output, presumably due to bulk oxygen migration. The response to CO is significantly stronger than that to H2 at 300_°C, and at 350_°C the response to H2 is nearly zero, resulting in a CO-selective gas sensing element. Suggestions for the source of selectivity in PCO are presented.