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High quality, epitaxial thin-films are key components of almost all modern electronic devices. During deposition, lattice mismatch between the substrate and the film generates elastic strain. The strain energy grows with film thickness until a defect is generated that relieves the strain. The strain relaxation mechanism is critical in epitaxial electrodeposition. Here, we study how a metal (bismuth) film growing via electrodeposition on a semiconductor substrate [GaAs(110)] relaxes the lattice mismatch-induced strain. Using in situ synchrotron-based X-ray techniques, we monitor the crystallographic orientation and grain size of the growing film during electrochemical deposition. We (1) confirm that a single crystallographic orientation of the film, with ( 01 1 ¯ 8 ) as the oriented plane, can be selected by controlling the overpotential, η, and (2) find that, after a threshold thickness is reached, the tilt angle varies monotonically with film thickness. Our data are consistent with the film relaxing the strain by forming low-energy, asymmetric tilt boundaries. [ABSTRACT FROM AUTHOR]

The miniaturization and integration of frequency-agile microwave circuits--relevant to electronically tunable filters, antennas, resonators and phase shifters--with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field (1). Appropriate systems such as [Ba.sub.x][Sr.sub.1-x]Ti[O.sub.3] have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability (1-3). Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss--[Sr.sub.n+1][Ti.sub.n][O.sub.3n+1] phases (4,5)--in which [(SrO).sub.2] crystallographic shear (6,7) planes provide an alternative to the formation of point defects for accommodating non-stoichiometry (8,9). Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability (10) in bi-axially strained [Sr.sub.n+1][Ti.sub.n][O.sub.3n+1] phases with n [greater than or equal to] 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics--doping (1-3,11,12) or strain (13-16)--in this unique system an increase in the separation between the [(SrO).sub.2] planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics (3)., Ferroelectric thin films possessing a nonlinear dielectric response to a quasi-static electric field have been widely pursued for tunable dielectric devices (17-20) that work at gigahertz frequencies. [Ba.sub.x][Sr.sub.1-x]Ti[O.sub.3] is the [...]

We have developed an improved method of time-resolved x-ray reflectivity (XRR) using monochromatic synchrotron radiation. Our method utilizes a polycapillary x-ray optic to create a range of incident angles and an area detector to collect specular reflections. By rotating the sample normal out of the plane of the incident fan, we can separate the surface diffuse scatter from the reflectivity signal, greatly improving the quality of the XRR spectra compared to previous implementations. We demonstrate the time-resolved capabilities of this system, with the temporal resolution as low as 10 ms, by measuring XRR during the annealing of Al/Ni nano-scale multilayers, and use this information to extract the activation energy for interdiffusion in this system. [ABSTRACT FROM AUTHOR]

Ordered intermetallic nanoparticles are promising electrocatalysts with enhanced activity and durability for the oxygen-reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). The ordered phase is generally identified based on the existence of superlattice ordering peaks in powder X-ray diffraction (PXRD). However, after employing a widely used postsynthesis annealing treatment, we have found that claims of "ordered" catalysts were possibly/likely mixed phases of ordered intermetallics and disordered solid solutions. Here, we employed in situ heating, synchrotron-based, X-ray diffraction to quantitatively investigate the impact of a variety of annealing conditions on the degree of ordering of large ensembles of Pt3Co nanoparticles. Monte Carlo simulations suggest that Pt3Co nanoparticles have a lower order-disorder phase transition (ODPT) temperature relative to the bulk counterpart. Furthermore, we employed microscopic-level in situ heating electron microscopy to directly visualize the morphological changes and the formation of both fully and partially ordered nanoparticles at the atomic scale. In general, a higher degree of ordering leads to more active and durable electrocatalysts. The annealed Pt3Co/C with an optimal degree of ordering exhibited significantly enhanced durability, relative to the disordered counterpart, in practical membrane electrode assembly (MEA) measurements. The results highlight the importance of understanding the annealing process to maximize the degree of ordering in intermetallics to optimize electrocatalytic activity. [ABSTRACT FROM AUTHOR]

Metal overlayers can be electrodeposited onto dissimilar metal substrates at potentials less negative than the Nernst value, otherwise called underpotential deposition (UPD). The evolution of UPD processes was studied using time-resolved surface X-ray scattering method on Cu UPD and Pt(111) in the presence of Cl anion. Findings show that the development of long-range periodic order in the CuCl overlayer requires significantly more time than that required for electrodeposition. This suggest that the latter is not the rate-limiting process.

Bi2Pt2O7 pyrochlore is thought to be one of the most promising oxide catalysts for application in fuel cell technology. Unfortunately, direct film growth of Bi2Pt2O7 has not yet been achieved, owing to the difficulty of oxidizing platinum metal in the precursor material to Pt4+. In this work, in order to induce oxidation of the platinum, we annealed pulsed laser deposited films consisting of epitaxial δ-Bi2O3 and co-deposited, comparatively disordered platinum. We present synchrotron x-ray diffraction results that show the nonuniform annealed films contain the first epitaxial crystals of Bi2Pt2O7. We also visualized the pyrochlore structure by scanning transmission electron microscopy, and observed ordered cation vacancies in the epitaxial crystals formed in a bismuth-rich film but not in those formed in a platinum-rich film. The similarity between the δ-Bi2O3 and Bi2Pt2O7 structures appears to facilitate the pyrochlore formation. These results provide the only route to date for the formation of epitaxial Bi2Pt2O7. [ABSTRACT FROM AUTHOR]

Recently, Freedman et al. [Phys. Rev. B 80, 064108 (2009)] calculated the elastic dipole tensor for several types of point defects in SrTiO3 and showed that it is nearly traceless for oxygen vacancies. Thus, mobile oxygen vacancies are predicted to screen elastic strain fields. Here, we report detailed diffuse x-ray scattering measurements of bulk SrTiO3 crystals prepared with controlled oxygen vacancy distributions. We verify the traceless nature of the elastic dipole tensor of an oxygen vacancy and demonstrate both correlations between oxygen vacancies and elastic strain screening by oxygen vacancies. [ABSTRACT FROM AUTHOR]

A major goal of energy research is to use visible light to cleave water directly, without an applied voltage, into hydrogen and oxygen. Although SrTiO3 requires ultraviolet light, after four decades, it is still the "gold standard" for the photo-catalytic splitting of water. It is chemically robust and can carry out both hydrogen and oxygen evolution reactions without an applied bias. While ultrahigh vacuum surface science techniques have provided useful insights, we still know relatively little about the structure of these electrodes in contact with electrolytes under operating conditions. Here, we report the surface structure evolution of a n-SrTiO3 electrode during water splitting, before and after "training" with an applied positive bias. Operando high-energy X-ray reflectivity measurements demonstrate that training the electrode irreversibly reorders the surface. Scanning electrochemical microscopy at open circuit correlates this training with a 3- fold increase of the activity toward the photo-induced water splitting. A novel first-principles joint density functional theory simulation, constrained to the X-ray data via a generalized penalty function, identifies an anatase-like structure as the more active, trained surface. [ABSTRACT FROM AUTHOR]

Recently, Freedman et al. [Phys. Rev. B 80, 064108 (2009)] calculated the elastic dipole tensor for several types of point defects in SrTiO3 and showed that it is nearly traceless for oxygen vacancies. Thus, mobile oxygen vacancies are predicted to screen elastic strain fields. Here, we report detailed diffuse x-ray scattering measurements of bulk SrTiO3 crystals prepared with controlled oxygen vacancy distributions. We verify the traceless nature of the elastic dipole tensor of an oxygen vacancy and demonstrate both correlations between oxygen vacancies and elastic strain screening by oxygen vacancies. [ABSTRACT FROM AUTHOR]

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

There is a great deal of interest in developing battery systems that can exhibit self-healing behavior, thus enhancing cyclability and stability. Given that gallium (Ga) is a metal that melts near room temperature, we wanted to test if it could be employed as a self-healing anode material for lithium-ion batteries (LIBs). However, Ga nanoparticles (NPs), when directly applied, tended to aggregate upon charge/discharge cycling. To address this issue, we employed carbon-coated Ga 2 O 3 NPs as an alternative. By controlling the pH of the precursor solution, highly dispersed and ultrafine Ga 2 O 3 NPs, embedded in carbon shells, could be synthesized through a hydrothermal carbonization method. The particle size of the Ga 2 O 3 NPs was 2.6 nm, with an extremely narrow size distribution, as determined by high-resolution transmission electron microscopy and Brunauer-Emmett-Teller measurements. A lithium-ion battery anode based on this material exhibited stable charging and discharging, with a capacity of 721 mAh/g after 200 cycles. The high cyclability is due to not only the protective effects of the carbon shell but also the formation of Ga 0 during the lithiation process, as indicated by operando X-ray absorption near-edge spectroscopy.

A major goal of energy research is to use visible light to cleave water directly, without an applied voltage, into hydrogen and oxygen. Although SrTiO3 requires ultraviolet light, after four decades, it is still the "gold standard" for the photo-catalytic splitting of water. It is chemically robust and can carry out both hydrogen and oxygen evolution reactions without an applied bias. While ultrahigh vacuum surface science techniques have provided useful insights, we still know relatively little about the structure of these electrodes in contact with electrolytes under operating conditions. Here, we report the surface structure evolution of a n-SrTiO3 electrode during water splitting, before and after "training" with an applied positive bias. Operando high-energy X-ray reflectivity measurements demonstrate that training the electrode irreversibly reorders the surface. Scanning electrochemical microscopy at open circuit correlates this training with a 3-fold increase of the activity toward the photo-induced water splitting. A novel first-principles joint density functional theory simulation, constrained to the X-ray data via a generalized penalty function, identifies an anatase-like structure as the more active, trained surface.