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Effects of oxide surface chemistry on diffusioosmosis
Diffusioosmosis is the movement of fluid induced by gradients in solute concentration. Recent studies suggest that in low permeability rocks, it may be the dominant mode of reactant transport and thus control rates of diagenesis, which cannot be adequately explained by pressure-driven flow alone. In this paper we investigate how the equilibrium between an oxide mineral and the surrounding fluid phase influences the diffusioosmotic velocity. We have developed a theory for the case of a thin double layer, where the Debye length is smaller than the characteristic pore size. Several factors contribute to the total fluid velocity: the chemical structure of the mineral surface, the electrolyte type and concentration gradient, and the solution pH. Individual factors can act in concert or in opposition, leading to widely varying magnitudes and directions of the velocity. The numerical results are within the range of the limited experimental data. Our results highlight how surface charging and surface complexation impact the flow, and how they depend on pH.
The effect of the surface composition of Ru-Pt bimetallic catalysts for methanol oxidation
Here, a series of Ru-Pt bimetallic catalysts prepared by the electroless deposition of controlled and variable amounts of Ru on the Pt surface of a commercially-available 20 wt% Pt/C catalyst has been characterized and evaluated for the oxidation of methanol. The activity of each Ru-Pt catalyst was determined as a function of surface composition via cyclic voltammetry. For the Ru-Pt bimetallic catalysts, activity passed through a maximum at approximately 50% monodisperse Ru surface coverage. However, due to the monolayer coverage of Ru on Pt, the amount of metal in the catalyst is minimized compared to a bulk 1:1 atomic ratio of Ru:Pt seen in commercial bimetallic catalysts. Chemisorption and temperature programmed reduction experiments confirmed that the surface had characteristics of a true bimetallic catalyst. On a mass of Pt basis, the activity of this composition for methanol oxidation was 7 times higher than pure Pt and 3.5 times higher than a commercial catalyst with a 1:1 Pt:Ru bulk atomic ratio.
Understanding photocatalytic overall water splitting on CoO nanoparticles: Effects of facets, surface stoichiometry, and the CoO/water interface
Unlike CoO micropowder, which is unable to split water, CoO nanoparticles have been observed to photocatalytically split water into H 2 and O 2 at room temperature without an externally applied potential or co-catalyst. The photocatalytic activity of CoO nanoparticles has been suggested to stem from an upwards shift in the band edges relative to bulk CoO such that the conduction and valence band edges span the water redox potentials. However, the origin of this shift in the band edges is unknown. In this study, we use first-principles density functional theory (DFT) calculations to explore the thermodynamically stable surface configurations of CoO as a function of oxygen chemical potential. Here we show that the band edge positions are sensitive to surface chemistry which is determined by surface orientation, adsorbates, and stoichiometry, and thus growth conditions and operating environment. In particular, we predict that CoO nanoparticles have fully hydroxylated CoO(1 1 1) facets, with band edges spanning the water redox potentials, while larger CoO particles (such as CoO micropowders) have a full monolayer of hydrogen on the CoO(1 1 1) facets, with a band alignment that favors water oxidation but not water reduction. Furthermore, we demonstrate that explicit inclusion of liquid water is crucial for accurately predicting the band edge positions, and thus photocatalytic behavior, of CoO in an aqueous solution. Our work explains why photocatalytic overall water splitting has only been observed on CoO nanoparticles, and provides new insights into the relationships between environmental conditions, surface structure, and band alignment, which may lead to new approaches for optimizing activity in CoO and other oxide photocatalysts.
Effect of free surfaces on localized plastic deformation in single-crystal nickel containing helium bubbles and radiation-induced self-interstitial atom clusters
This paper presents molecular dynamics (MD) simulations of plastic deformation in single-crystal Ni models containing internal distributions of helium (He) bubbles and self-interstitial atom (SIA) clusters corresponding to experimental characterization of ex-service Inconel X-750 components. Here, we simulate single crystals with and without free surfaces at room temperature and 588K, the latter temperature corresponding to the Inconel X-750 service conditions. Both models are deformed under multiple slip to a maximum deviatoric strain of ~10 %. A dislocation slip band forms in the model without free surfaces, but not in the model with free surfaces. However, the latter exhibits nano-twin nucleation from surfaces. Helium (He) bubbles that overlap with a slip band or a nano-twinned region deform more severely than other bubbles in the model. Nevertheless, the bubbles do not link up and do not form configurations that resemble incipient cracks.
Optical properties of bimetallic Au-Cu nanocrystals embedded in glass
Highlights: • Bimetallic Au-Cu/Au-Cu sosoloid nanocrystals embedded in glass have been synthesized. • We achieved good size control of the nanocrystals. • The glasses with bimetallic nanoparticles exhibit enhanced for NLO. • The mechanisms on the nonlinear effects of the obtained glasses are discussed. - Abstract: Bimetallic gold-copper and gold-copper solid solutions nanocrystals (NCs) with orthorhombic morphologies were successfully embedded in sodium borosilicate glass; these materials were fabricated via a facile and synthetic sol-gel technique. Microstructural investigations demonstrate that the Au-Cu intermetallic nanostructure is generated in the glass host. Z-scan technique was applied to reveal the third-order nonlinear optical properties of the as-obtained glasses at the wavelength of 800 nm. The as-synthesized glasses exhibit similar nonlinear behaviors with reverse saturation absorption effects and self-focusing performances. The third-order nonlinear susceptibility χ{sup (3)} values were optimized by tuning the sizes of the Au-Cu NCs. The χ{sup (3)} of Au-Cu(1), Au-Cu(2) and Au-Cu(3) measured to be 3.1 × 10{sup −12}, 5.4 × 10{sup −12}, and 4.4 × 10{sup −12} esu for the single-pulse energy of 1 μJ. We have discussed the nonlinear optical mechanism of Au-Cu sodium borosilicate glass that may be due to the quantum confinement effects and surface plasmon resonance effects.
Effect of Fermi surface topology change on the Kagome superconductor CeRu 2 under pressure
The cubic Laves phase compound CeRu 2 with a Kagome substructure of Ru has been investigated to understand myriad fascinating phenomena resulting from competition among its various physical and geometric features. Such phenomena include flat bands, van Hove singularities, Dirac cones, reentrant superconductivity, magnetism, the Fulde–Ferrell–Larkin–Ovchinnikov state, valence fluctuations, time-irreversible anisotropic s-state superconductivity, etc. Extensive studies have thus been carried out since 1958 when the highly unusual coexistence of superconductivity and ferromagnetism was proposed for the mixed compounds (Ce,Gd)Ru 2 . Activity has accelerated in recent years due to increasing interest in topological states in superconductors. However, there has been little investigation of the mutual influence of these fascinating states. Therefore, we systematically investigated the superconductivity and possible Fermi surface topological change in CeRu 2 via magnetic, resistivity, and structural measurements under pressure up to ~168 GPa. An unusual phase diagram that suggests an intriguing interplay between the compound’s superconducting order and Fermi surface topological order has been constructed. A resurgence in its superconducting transition temperature was observed above 28 GPa. Our experiments have identified a structural transition above 76 GPa and a few tantalizing phase transitions driven by high pressure. Our high-pressure results further suggest that superconductivity and Fermi surface topology in CeRu 2 are strongly intertwined.
Nuclear reactor radiation and temperature effects on piezoelectric surface acoustic wave devices
Surface acoustic wave (SAW) resonators were characterized in-situ in a nuclear reactor environment at high temperature. Devices based on lithium niobate (LiNbO 3 ), aluminum nitride (sc-AlN), and thin-film aluminum nitride on sapphire substrate (AlN/sapphire) were tested up to 400 °C temperature and 1.9 × 10 12 n/cm 2 s neutron flux. Shifts in device resonant frequency were detected in response to temperature and neutron flux. Devices undergo a frequency change when exposed to neutron flux. At 300 °C, AlN/sapphire produced the strongest neutron flux response about 1.02 ppm at 1.27 × 10 12 n/cm 2 s neutron flux (5.7 × 10 4 rad-Si/hr neutron dose rate), compared to 0.30 ppm for LiNbO 3 and 0.17 ppm for sc-AlN. While the transient kinetics in response to step change in neutron flux support the defect-accumulation mechanism, the concurrent measurement of device temperature using resistive temperature sensor suggests additional heating caused by absorption of gamma rays can also play a role. These results make SAW devices attractive candidates for sensor applications in extreme environments.
Effects of substrate surface roughness and nano/micro particle additive size on friction and wear in lubricated sliding
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Synergistic effect on silicon surface under low energy he ion irradiation at elevated temperatures
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Effect of Protein Surface Charge Distribution on Protein–Polyelectrolyte Complexation
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Production of C 2 /C 3 Oxygenates from Planar Copper Nitride-Derived Mesoporous Copper via Electrochemical Reduction of CO 2
Electrochemical reduction of CO 2 provides an opportunity to produce fuels and chemicals in a carbon-neutral manner, assuming that CO 2 can be captured from the atmosphere. To do so requires efficient, selective, and stable catalysts. In this study, we report a highly mesoporous metallic Cu catalyst prepared by electrochemical reduction of thermally nitrided Cu foil. Under aqueous saturated CO 2 reduction conditions, the Cu 3 N-derived Cu electrocatalyst produces virtually no CH 4 , very little CO, and exhibits a faradaic efficiency of 68% in C 2+ products (C 2 H 4 , C 2 H 5 OH, and C 3 H 7 OH) at a current density of ~18.5 mA cm –2 and a cathode potential of -1.0 V versus the reversible hydrogen electrode. Under these conditions, the catalyst produces more oxygenated products than hydrocarbons. We show that surface roughness is a good descriptor of catalytic performance. The roughest surface reached 98% CO utilization efficiency for C 2+ product formation from CO 2 reduction and the ratio of oxygenated to hydrocarbon products correlates with the degree of surface roughness. These effects of surface roughness are attributed to the high population of undercoordinated sites as well as a high pH environment within the mesopores and adjacent to the surface of the catalyst.
Shape Effect Undermined by Surface Reconstruction: Ethanol Dehydrogenation over Shape-Controlled SrTiO 3 Nanocrystals
For this research, to gain an in-depth understanding of the surface properties relevant for catalysis using ternary oxides, we report the acid–base pair reactivity of shape-controlled SrTiO 3 (STO) nanocrystals for the dehydrogenation of ethanol. Cubes, truncated cubes, dodecahedra, and etched cubes of STO with varying ratios of (001) and (110) crystal facets were synthesized using a hydrothermal method. Low-energy ion scattering (LEIS) analysis revealed that the (001) surface on cubes of STO is enriched with SrO due to surface reconstruction, resulting in a high ratio of strong base sites. Chemical treatment with dilute nitric acid to form etched cubes of STO resulted in a surface enriched with Ti cations and strong acidity. Furthermore, the strength and distribution of surface acidic sites increase with the ratio of (110) facet from cubes to truncated cubes to dodecahedra for STO. Kinetic, isotopic, and spectroscopy methods show that the dehydrogenation of ethanol proceeds through the facile dissociation of the alcohol group, followed by the cleavage of the C α –H bond, which is the rate-determining step. Co-feeding of various probe molecules during catalysis, such as NH 3 , 2,6-di-tert-butylpyridine, CO 2 , and SO 2 , reveals that a pair of Lewis acid site and basic surface oxygen atom is involved in the dehydrogenation reaction. The surface density of acid–base site pairs was measured using acetic acid as a probe molecule, allowing initial acetaldehyde formation turnover rates to be obtained. Comparison among various catalysts reveals no simple correlation between ethanol turnover rate and the percentage of either surface facet ((001) or (110)) of the STO nanocrystals. Instead, the reaction rate is found to increase with the strength of acid sites but reversely with the strength of base sites. The acid–base property is directly related to the surface composition as a result from different surface reconstruction behaviors of the shaped STO nanocrystals. Lastly, the finding in this work underscores the importance of characterizing the top surface compositions and sites properties when assessing the catalytic performance of shape-controlled complex oxides such as perovskites.