72 Search Results

For a high-performance proton exchange membrane water electrolyzer (PEMWE), acidic oxygen evolution reaction (OER) electrocatalysts require highly dispersed iridium oxide (IrO_x) nanoparticles. Although carbon-based materials have been explored as promising supports for IrO_x nanoparticles, their limited stability under harsh oxidative and acidic PEMWE conditions remains a significant challenge. Here, in this study, we report the synthesis and in situ characterization of active and durable IrOx electrocatalysts supported on electrochemically stable and electrically conducting tantalum carbide (TaC). When applied in a PEMWE, the IrO_x/TaC electrocatalyst achieves a cell voltage of 1.71 V at 1.0 A cm^–2, outperforming the commercial IrO₂ catalystmore » (1.82 V at 1.0 A cm^–2). Furthermore, the IrO_x/TaC catalyst maintains a stable operation for 200 h at 0.5 A cm^–2 with a low degradation rate of 36 μV h^–1. Density functional theory calculations further confirm that Ir–O–Ta bond formation at the IrO_x/TaC interface reduces the overpotential of the OER compared to IrO₂. This study underscores the pivotal role of supporting IrO_x over stable and conducting metal carbides, providing guidance for the design of advanced acidic OER catalysts.« less

Resistive switching, a behavior found in many oxide materials, has the potential to enable emerging computer hardware technologies and architectures. We present resistive switching devices fabricated from epitaxial brownmillerite SrFeO2.5 films with two distinct film orientations, wherein facile oxygen ion diffusion planes are aligned parallel (in-plane) and perpendicular (out-of-plane) with the electrodes. SrFeO2.5 films were grown on (001) oriented Nb:SrTiO3 to enable high-quality interfaces and future integration with Si CMOS technologies. Post-growth vacuum annealing and growth pressure were used to control film orientations, as confirmed by transmission electron microscopy and x-ray diffraction measurements. Films grown with diffusion planes oriented in-planemore » had oxygen-rich, perovskite-like nanodomains spread throughout the film, and fabricated devices exhibited worse switching consistency and more stochasticity. In contrast, films grown with diffusion planes oriented out-of-plane had a more uniform oxygen-rich perovskite interfacial layer above the bottom electrode, and devices built from this film orientation showed significant statistical improvements in switching voltages and cycling consistency.« less

State-of-the-art OER (oxygen evolution reaction) catalyst syntheses require the use of expensive metals (i.e. Ir) with complex and time-consuming synthetic routes, difficulty in control, and impractical yields. Although some reported catalysts show improved performance (i.e. activity, stability, lowering Ir content with Ru), their synthesis is costly and not viable for scale-up. Here we demonstrate a practical, reliable, and scalable one-pot synthesis method for OER catalysts based on borohydride reduction to quickly yield >100 mg of Ir, Ru, and IrRu nanoparticles (1.6 ± 0.2 nm) with outstanding batch-to-batch consistency. Both mono- and bi-metallic compositions exhibit a metal-core/metal-oxide-shell nanoparticle structure. We furthermore » demonstrate the versatility of this method by incorporating earth-abundant yttrium, resulting in a catalyst with improved precious metal utilization for OER. This method serves as a robust platform for generating ultrasmall (<2 nm) multi-metal particles useful for electrocatalysis research.« less

The modification of surface oxide layers formed on niobium (Nb) thin films via chemical mechanical planarization (CMP) and accelerated neutral atom beam (ANAB) processing provides a promising route toward tailoring their emergent properties and performance when used as superconducting qubits. Here, in this study, we show that CMP- and ANAB-formed Nb oxides are significantly thinner and smoother than the native oxide, as revealed by transmission electron microscopy (TEM) and atomic force microscopy. Scanning TEM and energy-dispersive X-ray spectroscopy along with X-ray photoelectron spectroscopy identified an oxidation gradient within the native and surface-engineered oxides. The topside layer is dominated by Nb⁵⁺more » (Nb₂O₅), with various Nb suboxides present closer to the oxide/metal interface. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling confirmed the presence of an oxygen content gradient and demonstrated the enhanced resistance of the CMP- and ANAB-formed oxides to oxygen surface exchange and subsequent diffusion via ¹⁸O₂ isotopic labeling experiments. ToF-SIMS also identified an interfacial layer containing trapped hydrogen (H)-containing species at the Nb oxide/metal interface. In situ ToF-SIMS and TEM revealed migration of the H/OH interfacial layer coinciding with decomposition of the surface oxide. Furthermore, our density functional theory calculations indicated that both H from moisture present in ambient air and bulk H in Nb films tend to segregate at the interface. These findings underscore the importance of understanding surface oxidation mechanisms, hydrogen incorporation, and their impact on the designed functionalities of Nb-based devices.« less

Recent predictions suggest that oxides, such as MgO and CaO, could serve as hosts of spin defects with long coherence times and thus be promising materials for quantum applications. However, in most cases, specific defects have not yet been identified. Here, by using a high-throughput first-principles framework and advanced electronic structure methods, we identify a negatively charged complex between a nitrogen interstitial and a magnesium vacancy in MgO with favorable electronic and optical properties for hybrid quantum technologies. We show that this NV⁻ center has stable triplet ground and excited states, with singlet shelving states enabling optical initialization and spin-dependentmore » readout. We predict several properties, including absorption, emission, and zero-phonon line energies, as well as zero-field splitting tensor, and hyperfine interaction parameters, which can aid in the experimental identification of this defect. Our calculations show that due to a strong pseudo-Jahn Teller effect and low-frequency phonon modes, the NV⁻ center in MgO is subject to a substantial vibronic coupling. We discuss design strategies to reduce such coupling and increase the Debye-Waller factor, including the effect of strain and the localization of the defect states. We propose that the favorable properties of the NV⁻ defect, along with the technological maturity of MgO, could enable hybrid classical-quantum applications, such as spintronic quantum sensors and single qubit gates.« less

Redox transformations at metal oxide (MO_x)/solution interfaces are broadly important, and oxygen atom transfer (OAT) is one of the simplest and most fundamental examples of such reactivity. OAT is a two-electron transfer process, well-known in gas/solid reactions and catalysis. However, OAT is rarely directly observed at oxide/water interfaces, whose redox reactions are typically proposed to occur in one-electron steps. Reported here are stoichiometric OAT reactions of organic molecules with aqueous colloidal titanium dioxide and iridium oxide nanoparticles (TiO₂ and IrO_x NPs). Me₂SO (DMSO) oxidizes reduced TiO₂ NPs with the formation of Me₂S, and IrO_x NPs transfer O atoms to amore » water-soluble phosphine and a thioether. The reaction stoichiometries were established and the chemical mechanisms were probed using typical solution spectroscopic techniques, exploiting the high surface areas and transparency of the colloids. Furthermore, these OAT reactions, including a catalytic example, utilize the ability of the individual NPs to accumulate many electrons and/or holes. Observing OAT reactions of two different materials, in opposite directions, is a step toward harnessing oxide nanoparticles for valuable multi-electron and multi-hole transformations.« less

Oxygen electrochemistry goes beyond O₂, as the formation of reactive oxygen species (ROS) such as H₂O₂ and O₃ during water oxidation is key in the destruction of persistent pollutants in water remediation technologies as well as in the degradation of fuel cell and electrolyzer components. In this study, we developed an in situ method utilizing the rotating ring-disk electrode technique to quantify the formation of O₂, O₃, and H₂O₂ species across a broad pH range (1–8.3). Oxygen selectivity trends over Pt, IrO₂, and PbO₂ surfaces reveal that even O₂ evolution catalysts may produce small yet measurable amounts of ROS, furthermore » modulated by pH and electrode potential. In conclusion, these findings emphasize the need to probe the selectivity of oxygen electrochemistry for a more complete picture of advanced materials for electrochemical technologies.« less

Perovskite oxide heterostructures host a large number of interesting phenomena such as ferroelectricity, which are often driven by octahedral distortions in the crystal that may induce polarization. SrHfO₃ (SHO) is a perovskite oxide with a pseudocubic lattice parameter of 4.08 Å that previous density functional theory (DFT) calculations suggest can be stabilized in a ferroelectric P4mm phase when stabilized with sufficient compressive strain. Additionally, it is insulating and possesses a large band gap and a high dielectric constant, making it an ideal candidate for oxide electronic devices. Here, to test the viability of epitaxial strain as a driver of ferroicmore » phase transitions, SHO films were grown by hybrid molecular beam epitaxy (hMBE) with a tetrakis(ethylmethylamino)hafnium(IV) source on GdScO₃ and TbScO₃ substrates. Strained SHO phases were characterized using X-ray diffraction, X-ray absorption spectroscopy, and scanning transmission electron microscopy to determine the space group of the strained films, with the results compared to those of DFT-optimized models of phase stability versus strain. Contrary to past reports, we find that compressively strained SrHfO₃ undergoes octahedral tilt distortions without associated ferroelectric polarization and most likely takes on the I4/mcm phase with the a⁰a⁰c^– tilt pattern.« less

Photoelectrochemical cells (PECs) can directly utilize solar energy to drive chemical reactions to produce fuels and chemicals. Oxide-based photoelectrodes in general exhibit enhanced stability against photocorrosion, which is a critical advantage for building a sustainable PEC. However, most oxide-based semiconductors are n-type, and p-type oxides that can be used as photocathodes are limited. In this study, we report the synthesis, characterization, and application of p-type BiVO₄ with a monoclinic scheelite (ms) structure. ms-BiVO₄ is inherently n-type, and it has been investigated only as a photoanode to date. In this study, we prepared p-type ms-BiVO₄ (bandgap of 2.4 eV) via atomicmore » doping of Ca²⁺ at the Bi³⁺ site under an O₂-rich environment and examined its performance as a photocathode. We then demonstrated that the Ca-doped ms-BiVO₄ photocathode can be used for solar O₂ reduction to H₂O₂ when coupled with appropriate catalysts. Our computational investigation using hybrid density functional theory revealed that holes are stable as polarons in ms-BiVO₄ and have a low self-trapping energy, that may lead to free carriers in the valence band at finite temperature. Our calculations also show that Ca is an effective shallow acceptor dopant with low formation energy and thermal ionization energy leading to p-type conductivity. In conclusion, our joint experimental and computational results provide critical insights into the design of p-type ms-BiVO₄, enabling its use as a polaronic oxide photocathode.« less

Iron metal was investigated for use as a consumable anode for electrolytic reduction of solid metal oxides in molten LiCl-Li₂O (2.0 - 2.4 wt%). Tests were performed where the potential of Fe anodes was increased incrementally from 0.1 to 1.0 V (vs Ni/NiO). Oxide formation on the anode started at a potential of 0.4 V and was identified as FeO via X-ray diffraction. In the absence of a pre-formed oxide layer, severe attack of the anode started at a potential of 0.7 V and was accompanied by an increase in Fe concentration in the salt. When an oxide layer wasmore » allowed to form on the anode, the Fe concentration did not increase in the salt. O₂ was detected in the headspace gas at an anode potential of 1.0 V only when an oxide layer was present on the anode. Finally, the results of this study support the idea that an inexpensive sacrificial anode could be an ideal replacement for expensive Pt that is currently widely used for this process.« less