A catalyst functions by stabilizing reaction intermediates, usually through surface adsorption. In the oxygen evolution reaction (OER), surface oxygen adsorption plays an indispensable role in the electrocatalysis. The relationship between the adsorption energetics and OER kinetics, however, has not yet been experimentally measured. Herein we report an experimental relationship between the adsorption of surface oxygen and the kinetics of the OER on IrO(110) epitaxially grown on a TiO(110) single crystal. The high quality of the IrO film grown using molecular-beam epitaxy affords the ability to extract the surface oxygen adsorption and its impact on the OER. By examining a series of electrolytes, we find that the adsorption energy changes linearly with pH, which we attribute to the electrified interfacial water. We support this hypothesis by showing that an electrolyte salt modification can lead to an adsorption energy shift. The dependence of the adsorption energy on pH has implications for the OER kinetics, but it is not the only factor; the dependence of the OER electrocatalysis on pH stipulates two OER mechanisms, one operating in acidic solution and another operating in alkaline solution. Our work points to the subtle adsorption-kinetics relationship in the OER and highlights the importance of the interfacial electrified interaction in electrocatalyst design.
Bismuth vanadate (BiVO4) is a widely studied oxide in solar water splitting, known for its ease of synthesis, high charge extraction yields, and advantageous band alignment with water. We present a combined first-principles and experimental study of the electronic structure of the (010) surface of BiVO4 aimed at disentangling the impact of the surface and bulk oxygen vacancies on the electronic structure and transport properties. We found that oxygen vacancies are deep donors at the surface as they are in the bulk; our calculations on defect and polaron formation energies suggest that, while polarons formed from oxygen vacancies in the bulk can contribute to conductivity, those at the surface likely do not. Our results also show that out-of-plane structural relaxations at the surface contribute to the relatively immobile nature of electron polarons derived from surface oxygen vacancies. The structural model derived from first-principles calculation was validated by comparing computed results with experimental measurements of single-crystal and epitaxially grown single-crystalline BiVO4 samples. We also found a reasonably good agreement between our calculated and measured work functions for BiVO4 samples with and without oxygen vacancies.
Directed electrochemical nanowire assembly is a promising high growth rate technique for synthesizing electrically connected nanowires and dendrites at desired locations. Here we demonstrate the directed growth and morphological control of edge-supported platinum nanostructures by applying an alternating electric field across a chloroplatinic acid solution. The dendrite structure is characterized with respect to the driving frequency, amplitude, offset, and salt concentration and is well-explained by classical models. Control over the tip diameter, side branch spacing, and amplitude is demonstrated, opening the door to novel device architectures for sensing and catalytic applications.
Electrochemical generation of oxygen via the oxygen evolution reaction (OER) is a key enabling step for many air-breathing electrochemical energy storage devices.
Remote epitaxy is a promising approach for synthesizing exfoliatable crystalline membranes and enabling epitaxy of materials with large lattice mismatch. However, the atomic scale mechanisms for remote epitaxy remain unclear. Here we experimentally demonstrate that GaSb films grow on graphene-terminated GaSb (001) via a seeded lateral epitaxy mechanism, in which pinhole defects in the graphene serve as selective nucleation sites, followed by lateral epitaxy and coalescence into a continuous film. Remote interactions are not necessary in order to explain the growth. Importantly, the small size of the pinholes permits exfoliation of continuous, free-standing GaSb membranes. Due to the chemical similarity between GaSb and other III-V materials, we anticipate this mechanism to apply more generally to other materials. By combining molecular beam epitaxy with in-situ electron diffraction and photoemission, plus ex-situ atomic force microscopy and Raman spectroscopy, we track the graphene defect generation and GaSb growth evolution a few monolayers at a time. Our results show that the controlled introduction of nanoscale openings in graphene provides an alternative route towards tuning the growth and properties of 3D epitaxial films and membranes on 2D material masks.
Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO2 thin films on (110)-oriented TiO2 substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of d orbitals.
Understanding how physicochemical properties of materials affect the oxygen evolution reaction (OER) has enormous scientific and technological implications for the OER electrocatalyst design. We present our investigation on the role of strain on the surface–oxygen interaction and the OER on well-defined single-termination SrIrO3 films. Our approach employs a combination of molecular-beam epitaxy, electrochemical characterizations, ambient-pressure X-ray photoelectron spectroscopy, and density functional theory (DFT). We find that inplane compressive strain weakens the surface oxygen binding strength on SrIrO3; however, it has a negligible effect on the surface oxygen electroadsorption and the OER. We explain this observation, which goes against a commonly held intuition that a change in the surface oxygen binding strength should influence surface oxygen electroadsorption and OER by recognizing that the trend in surface oxygen adsorption measured in the gas phase does not account for the presence of water in the surface oxygen electroadsorption. Inclusions of surface water molecules allow DFT to qualitatively reproduce the electroadsorption trend, highlighting the importance of surface water in the surface–oxygen interaction. Our finding suggests that a commonly held assumption between surface oxygen binding strength (in vacuum, no water) and electroadsorption (requiring water) is not always a simple one-to-one description and calls for a more in-depth investigation on the structure of water at electrochemical interfaces.
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