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Oxygen non-stoichiometry profoundly impacts the electrical, magnetic, and catalytic properties of metal oxide. Limited by the low mass and volume of thin oxide films, conventional quantification methods, such as thermogravimetry, are not directly applicable. While chemical capacitance has been successfully applied to monitor oxygen non-stoichiometry in thin oxide films, detailed a-priori understanding of the defect chemistry is often very helpful in its interpretation. In this study, changes in non-stoichiometry in Pr doped CeO2 (PCO) thin films are measured by coulometric titration. I-V titration measurements are performed on electrochemical cells, over the temperature range from 550 to 700 ℃, oxygen partial pressure range from 10-4 to 0.21 atm, and bias range of -50 mV to 50 mV, to extract changes in stoichiometry. The results agree well with values obtained by chemical capacitance, demonstrating the utility in applying coulometric titration to investigate oxygen non-stoichiometry in oxide thin films.

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.

In this review, we consider the detrimental effects of Si-contamination on electrochemical applications, broadly conceived, in which both ions and electrons play key roles in device operation and where exchange of oxygen between the gas and solid phase is likewise essential for operation.

Copyright © 2020 American Chemical Society. Sr-doped LaMnO3 (LSM) is a promising oxygen reduction reaction electrocatalyst in solid oxide fuel cells and other electrochemical devices. The presence of CO2 and H2O has been reported to promote the oxygen dissociation reaction on LSM surfaces. Here, we investigate the coadsorption mechanism of O2 with H2O or CO2 by 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 CO2 do not compete with O2 for 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.

Despite improved sensitivity, simple downsizing of gas-sensing components to randomly arranged nanostructures often faces challenges associated with unpredictable electrical conduction pathways. In the present study, controlled fabrication of three-dimensional (3D) metal oxide nanowire networks is demonstrated that can greatly improve both signal stability and sensor response compared to random nanowire arrays. For example, the highest ever reported H2S gas response value, and a 5 times lower relative standard deviation of baseline resistance than that of random nanowires assemblies, are achieved with the ordered 3D nanowire network. Systematic engineering of 3D geometries and their modeling, utilizing equivalent circuit components, provide additional insights into the electrical conduction and gas-sensing response of 3D assemblies, revealing the critical importance of wire-to-wire junction points and their arrangement. These findings suggest new design rules for both enhanced performance and reliability of chemical sensors, which may also be extended to other devices based on nanoscale building blocks.

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.

Air Force Office of Scientific Research; National Research Foundation of Korea; U.S. Department of Energy

© 2019 American Physical Society. Magneto-ionic control of magnetism has garnered great interest in recent years due to the large magnetic changes that can be induced using a relatively small voltage. One model structure for this is Pt/Co/GdOx/Au, where Co is the magnetic layer and GdOx is the ionic conductor, with the magnetic properties dependent on the oxidation state of Co. While this structure is commonly used, there is limited understanding of the effect of GdOx properties on voltage-induced magnetic changes. In this work, we show that hydration of Gd2O3 to form Gd(OH)3 is crucial for voltage-induced Co oxidation in a Pt/Co/GdOx/Au device. By examining the rate of Co oxidation in nonhydrated and hydrated devices, we conclude that H2O in the GdOx layer acts as an oxidant during the voltage-induced Co oxidation process. Co oxidation through this interfacial reaction process is confirmed by in situ x-ray absorption spectroscopy.

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Distinct properties of multiple phases of vanadium oxide (VOx) render this material family attractive for advanced electronic devices, catalysis, and energy storage. In this work, phase boundaries of VOx are crossed and distinct electronic properties are obtained by electrochemically tuning the oxygen content of VOx thin films under a wide range of temperatures. Reversible phase transitions between two adjacent VOx phases, VO2 and V2O5, are obtained. Cathodic biases trigger the phase transition from V2O5 to VO2, accompanied by disappearance of the wide band gap. The transformed phase is stable upon removal of the bias while reversible upon reversal of the electrochemical bias. The kinetics of the phase transition is monitored by tracking the time-dependent response of the X-ray absorption peaks upon the application of a sinusoidal electrical bias. The electrochemically controllable phase transition between VO2 and V2O5 demonstrates the ability to induce major changes in the electronic properties of VOx by spanning multiple structural phases. This concept is transferable to other multiphase oxides for electronic, magnetic, or electrochemical applications.

© 2017 Author(s). Metallic LaAlO3/SrTiO3 (LAO/STO) interfaces attract enormous attention, but the relationship between the electron mobility and the sheet electron density, ns, is poorly understood. Here, we derive a simple expression for the three-dimensional electron density near the interface, n3D, as a function of ns and find that the mobility for LAO/STO-based interfaces depends on n3D in the same way as it does for bulk doped STO. It is known that undoped bulk STO is strongly compensated with N5×1018 cm-3 background donors and acceptors. In intentionally doped bulk STO with a concentration of electrons n3DN, the mobility collapses because scattering happens on n3D intentionally introduced donors. For LAO/STO, the polar catastrophe which provides electrons is not supposed to provide an equal number of random donors and thus the mobility should be larger. The fact that the mobility is still the same implies that for the LAO/STO, the polar catastrophe model should be revisited.

© 2018, The Author(s), under exclusive licence to Springer Nature Limited. 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 O 2− 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/GdO x 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.

Copyright © 2020 American Chemical Society. Sr-doped LaMnO3 (LSM) is a promising oxygen reduction reaction electrocatalyst in solid oxide fuel cells and other electrochemical devices. The presence of CO2 and H2O has been reported to promote the oxygen dissociation reaction on LSM surfaces. Here, we investigate the coadsorption mechanism of O2 with H2O or CO2 by 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 CO2 do not compete with O2 for 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 effect of an applied overpotential on oxygen isotope incorporation and diffusion in oxide thin film electrodes is investigated by a novel experimental approach. A special electrode geometry leads to in-plane electron flow, perpendicular oxide ion flow and a well-defined laterally varying driving force. This design allows one to obtain a series of tracer depth profiles induced by a range of overpotentials on one and the same thin film. The approach was applied to La0.8Sr0.2MnO3(LSM) thin films deposited by pulsed laser deposition (PLD) on yttria stabilized zirconia (YSZ) single crystals. Tracer depth profiles were measured by secondary ion mass spectrometry (SIMS). These depth profiles include examples of pronounced apparent uphill diffusion that can be explained by considering an interplay of polarization-induced changes in stoichiometry within the LSM grains combined with fast oxygen transport along the grain boundaries., United States. Department of Defense. Basic Energy Sciences (grant No. DE-SC0002633)

Pr0.1Ce0.9O2 (PCO) films, with columnar structure containing voids, were fabricated by pulsed laser deposition (PLD) and integrated into YSZ-based potentiometric gas sensor as a sensing electrodes (SEs). Sensors with PCO film thicknesses of 550, 1100 and 2200 nm, fabricated by controlling the number of PLD shots of 750, 1500 and 3000 respectively, were examined over the temperature range of 450?600 °C. The sensor output voltage, E, of all sensors showed increasing negative change with increasing toluene concentration. The sensor with the thickest PCO SE film (thickness: 2200 nm) showed a much larger toluene response at 450 °C than the others (PCO-SE thickness: 550, 1100 nm). Because PCO is a mixed ionic and electronic conductor and the morphology of SE was porous, the electrochemical oxidation reaction is viewed as occurring at the double phase boundaries ? DPBs (interfaces between PCO/gas) as well as triple phase boundaries ? TPBs (interfaces of PCO/YSZ/gas). Because a fraction of the toluene is catalytically oxidized in part during gas diffusion within the SE films, especially at lower operating temperatures, the much larger toluene response can be explained by the electrochemical oxidation reaction of the partially oxidized products together with toluene at the TPBs and DPBs., Sensors and Actuators B: Chemical, 317, art.no.128037; 2020

The local electronic properties of tantalum oxide (TaO[subscript x], 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO[subscript 3]) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaO[subscript x] films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO[subscript 3], we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaO[subscript x] and SrRuO[subscript 3] differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides., National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant DMR-1419807)

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 SrTi[subscript 0.65]Fe[subscript 0.35]O[subscript 3-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 (~Fe[superscript 4+]) concentrations; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry. The k[subscript chem] values obtained by OTR were significantly lower than the AC-IS derived k[subscript chem] values and k[subscript q] 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 kchemand kqvalues 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 kchemvalues. 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. Keywords: Thin films; perovskite; mixed ionic and electronic conductor; oxygen surface exchange; optical absorption; impedance spectroscopy, United States. Department of Energy. Office of Basic Energy Sciences (Grant DE-SC0002633)

Macroporous WO₃ nanotubes (NTs) functionalized with nanoscale catalysts were fabricated using coaxial electrospinning combined with sacrificial templating and protein-encapsulated catalysts. The macroporous thin-walled nanotubular structures were obtained by introducing colloidal polystyrene (PS) particles to a shell solution of W precursor and poly(vinylpyrrolidone). After coaxial electrospinning with a core liquid of mineral oil and subsequent calcination, open pores with an average diameter of 173 nm were formed on the surface of WO₃ NTs due to decomposition of the PS colloids. In addition, catalytic Pd nanoparticles (NPs) were synthesized using bio-inspired protein cages, i.e., apoferritin, and uniformly dispersed within the shell solution and subsequently on the WO₃ NTs. The resulting Pd functionalized macroporous WO₃ NTs were demonstrated to be high performance hydrogen (H₂) sensors. In particular, Pd-functionalized macroporous WO₃ NTs exhibited a very high H₂ response (R[subscript air]/R[subscript gas]) of 17.6 at 500 ppm with a short response time. Furthermore, the NTs were shown to be highly selective for H₂ compared to other gases such as carbon monoxide (CO), ammonia (NH₃), and methane (CH₄). The results demonstrate a new synthetic method to prepare highly porous nanotubular structures with well-dispersed nanoscale catalysts, which can provide improved microstructures for chemical sensing., Intel Corporation

In this study, YSZ-based potentiometric gas sensors of the type Au*/YSZ/Pt, where Au* represents either dense (d-Au), porous Au (p-Au) or porous Au/4MOx (4 wt% CeO2, Gd2O3, La2O3, Pr6O11, Sm2O3, Pr0.1Ce0.9O2 or Ce0.8Zr0.2O2) composite sensor electrodes (SEs), were fabricated, and their toluene-sensing properties examined over the temperature range of 400?500 °C. The roles of microstructure of the SEs, and improvements in sensing properties due to MO additions were examined with respect to their impact on catalytically enhanced toluene oxidation as well as electrochemical reactions at the SE/electrolyte interface. Au(4 MO)/Pt sensors showed larger response to toluene than d-Au/Pt, with the Au(4CeO2)/Pt sensor showing the largest response among all of the Au(4 MO)/Pt sensors. While the response of Au(4CeO2)/Pt was smaller than that of p-Au/Pt, its recovery time was much shorter. These observations were shown to result from Au(4CeO2) exhibiting the largest catalytic activity of toluene oxidation among all of the oxides tested, leading to enhanced toluene oxidization during its diffusion through the SE, as well as the removal of residual toluene at TPBs, during the recovery process. This resulted in decreased response, but at the same time, faster recovery. The results demonstrated that both high electrochemical activity of toluene oxidation and low electrochemical activity of oxygen reduction are indispensable for achieving large sensor response. The Au(4CeO2)/Pt sensor also showed a nearly linear response vs logarithm of toluene concentration (0.5?50 ppm), with strong response down even to 0.5 ppm toluene (ca. +55 mV) in dry air., Sensors and Actuators B: Chemical, 252, pp.268-276; 2017

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 CO[subscript 2] and H[subscript 2]O 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 H[subscript 2]O and CO[subscript 2] to H[subscript 2] 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., United States. Dept. of Energy. Office of Basic Energy Sciences (Award# DE SC0002633), National Science Foundation (U.S.) (Award# DMR-1507047), MIT Skoltech Initiative

Metallic LaAlO3/SrTiO3 (LAO/STO) interfaces attract enormous attention, but the relationship between the electron mobility and the sheet electron density, ns, is poorly understood. Here, we derive a simple expression for the three-dimensional electron density near the interface, n3D, as a function of ns and find that the mobility for LAO/STO-based interfaces depends on n3D in the same way as it does for bulk doped STO. It is known that undoped bulk STO is strongly compensated with N similar or equal to 5 x 1018 cm-3 background donors and acceptors. In intentionally doped bulk STO with a concentration of electrons n3D < N, background impurities determine the electron scattering. Thus, when n3D < N, it is natural to see in LAO/STO the same mobility as in the bulk. On the other hand, in the bulk samples with n3D > N, the mobility collapses because scattering happens on n3D intentionally introduced donors. For LAO/STO, the polar catastrophe which provides electrons is not supposed to provide an equal number of random donors and thus the mobility should be larger. The fact that the mobility is still the same implies that for the LAO/STO, the polar catastrophe model should be revisited.