Interplay between Facets and Defects during the Dissociative and Molecular Adsorption of Water on Metal Oxide Surfaces.

Surface terminations and defects play a central role in determining how water interacts with metal oxides, thereby setting important properties of the interface that govern reactivity such as the type and distribution of hydroxyl groups. However, the interconnections between facets and defects remain poorly understood. This limits the usefulness of conventional notions such as that hydroxylation is controlled by metal cation exposure at the surface. Here, using hematite (α-Fe2O3) as a model system, we show how oxygen vacancies overwhelm surface cation-dependent hydroxylation behavior. Synchrotron-based ambient-pressure X-ray photoelectron spectroscopy was used to monitor the adsorption of molecular water and its dissociation to form hydroxyl groups in situ on (001), (012), or (104) facet-engineered hematite nanoparticles. Supported by density functional theory calculations of the respective surface energies and oxygen vacancy formation energies, the findings show how oxygen vacancies are more prone to form on higher energy facets and induce surface hydroxylation at extremely low relative humidity values of 5 × 10-5%. When these vacancies are eliminated, the extent of surface hydroxylation across the facets is as expected from the areal density of exposed iron cations at the surface. These findings help answer fundamental questions about the nature of reducible metal oxide-water interfaces in natural and technological settings and lay the groundwork for rational design of improved oxide-based catalysts.

Understanding the Electronic Structure Evolution of Epitaxial LaNi1-xFexO3 Thin Films for Water Oxidation.

Rare earth nickelates including LaNiO3 are promising catalysts for water electrolysis to produce oxygen gas. Recent studies report that Fe substitution for Ni can significantly enhance the oxygen evolution reaction (OER) activity of LaNiO3. However, the role of Fe in increasing the activity remains ambiguous, with potential origins that are both structural and electronic in nature. On the basis of a series of epitaxial LaNi1-xFexO3 thin films synthesized by molecular beam epitaxy, we report that Fe substitution tunes the Ni oxidation state in LaNi1-xFexO3 and a volcano-like OER trend is observed, with x = 0.375 being the most active. Spectroscopy and ab initio modeling reveal that high-valent Fe3+δ cationic species strongly increase the transition-metal (TM) 3d bandwidth via Ni-O-Fe bridges and enhance TM 3d-O 2p hybridization, boosting the OER activity. These studies deepen our understanding of structural and electronic contributions that give rise to enhanced OER activity in perovskite oxides.

Contribution of the Sub-Surface to Electrocatalytic Activity in Atomically Precise La0.7 Sr0.3 MnO3 Heterostructures.

Electrocatalytic reactions are known to take place at the catalyst/electrolyte interface. Whereas recent studies of size-dependent activity in nanoparticles and thickness-dependent activity of thin films imply that the sub-surface layers of a catalyst can contribute to the catalytic activity as well, most of these studies consider actual modification of the surfaces. In this study, the role of catalytically active sub-surface layers was investigated by employing atomic-scale thickness control of the La0.7 Sr0.3 MnO3 (LSMO) films and heterostructures, without altering the catalyst/electrolyte interface. The activity toward the oxygen evolution reaction (OER) shows a non-monotonic thickness dependence in the LSMO films and a continuous screening effect in LSMO/SrRuO3 heterostructures. The observation leads to the definition of an "electrochemically-relevant depth" on the order of 10 unit cells. This study on the electrocatalytic activity of epitaxial heterostructures provides new insight in designing efficient electrocatalytic nanomaterials and core-shell architectures.

Correlation between oxygen evolution reaction activity and surface compositional evolution in epitaxial La0.5Sr0.5Ni1-xFexO3-δ thin films.

Water electrolysis can use renewable electricity to produce green hydrogen, a portable fuel and sustainable chemical precursor. Improving electrolyzer efficiency hinges on the activity of the oxygen evolution reaction (OER) catalyst. Earth-abundant, ABO3-type perovskite oxides offer great compositional, structural, and electronic tunability, with previous studies showing compositional substitution can increase the OER activity drastically. However, the relationship between the tailored bulk composition and that of the surface, where OER occurs, remains unclear. Here, we study the effects of electrochemical cycling on the OER activity of La0.5Sr0.5Ni1-xFexO3-δ (x = 0-0.5) epitaxial films grown by oxide molecular beam epitaxy as a model Sr-containing perovskite oxide. Electrochemical testing and surface-sensitive spectroscopic analyses show Ni segregation, which is affected by electrochemical history, along with surface amorphization, coupled with changes in OER activity. Our findings highlight the importance of surface composition and electrochemical cycling conditions in understanding OER performance, suggesting common motifs of the active surface with high surface area systems.

Understanding the Role of Surface Heterogeneities in Electrosynthesis Reactions.

In this perspective, we highlight the role of surface heterogeneity in electrosynthesis reactions. Heterogeneities may come in the form of distinct crystallographic facets, boundaries between facets or grains, or point defects. We approach this topic from a foundation of surface science, where signatures from model systems provide understanding of observations on more complex and higher-surface-area materials. In parallel, probe-based techniques can inform directly on spatial variation across electrode surfaces. We call attention to the role spectroscopy can play in understanding the impact of these heterogeneities in electrocatalyst activity and selectivity, particularly where these surface features have effects extending into the electrolyte double layer.

In vitro effect of 20% N-acetylcysteine on the viscosity of normal canine bile.

OBJECTIVE To evaluate the in vitro effect of 20% N-acetylcysteine (NAC) on the viscosity of normal canine bile. ANIMALS Bile samples obtained from 10 adult dogs euthanized for reasons unrelated to biliary disease. PROCEDURES Each sample was centrifuged to remove particulates, then divided into 3 aliquots. One aliquot remained untreated (control). Each of the other aliquots was diluted 1:4 with 20% NAC or sterile water. The viscosity of all samples was measured with a rotational viscometer at 25°C. Viscosity of control samples was measured immediately after centrifugation and at 1 and 24 hours after treatment application to the diluted samples. Viscosity of diluted samples was measured at 1 and 24 hours after treatment application. RESULTS Mean viscosity differed significantly among the 3 groups at both 1 and 24 hours after treatment application. Relative to control samples, the addition of NAC and sterile water decreased the viscosity by approximately 3.35 mPa·s (95% confidence interval [CI], 1.58 to 5.12 mPa·s) and 2.74 mPa·s (95% CI, 1.33 to 4.14 mPa·s), respectively. Mean viscosity of the NAC-treated samples was approximately 0.61 mPa·s (95% CI, 0.21 to 1.01 mPa·s) less than that for the sterile water-treated samples. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that in vitro dilution of canine bile 1:4 with 20% NAC significantly decreased the viscosity of the resulting mixture. Further research is necessary to determine whether NAC is a safe and effective noninvasive treatment for dogs with persistent biliary sludge or gallbladder mucoceles.