High Atomic Density Research Articles

Quantifying Electrocatalytic Structure-Property Relationships with Surface Science, Electrochemistry, and Density Functional Theory

We have coupled electrochemistry, surface science, and density functional theory (DFT) calculations to quantify structure-property relationships in well-defined oxygen evolution reaction (OER) electrocatalysts. The OER is an essential component in electrochemical CO2 reduction, H2 evolution, and NH3 synthesis reactions. Mixtures of nickel, cobalt, and iron have shown remarkable alkaline OER activity, but atomic-level understanding of how OER activity evolves with the catalyst shape, size, composition, and support are still lacking in the literature. Our approach uses ultra-high vacuum techniques to grow well-defined transition metal electrocatalysts with precisely controlled morphology, composition, and loading. Scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy determine the catalyst’s size, aspect ratio, thickness mass loading, and oxidation state. Electrochemical OER testing then correlates mass activity, turnover frequency, and specific current density with the catalyst structure and composition. Our results with ultrathin, two-layer thick (2L) Fe2O3 nanocatalysts showed a linear relationship between the population of edge-site Fe atoms and overall OER activity, and edge-site Fe atoms are approximately 150 times more OER active than Fe atoms on the catalyst surface. In fact, 2L-Fe2O3 nanocatalysts with a high density of Fe edge-site atoms demonstrated better OER performance than an Ir-oxide catalyst. Post-reaction STM and XPS analysis confirmed that particles retained their structure and identified hydroxylated Fe edge-site atoms along the catalyst perimeter as key OER active sites. Complementary DFT modeling confirmed these experimental observations by predicting more thermodynamically favorable OER at Fe edge-site atoms due to beneficial modification of intermediate binding. Extension of this work to Ni/Fe and Co/Fe systems is revealing additional insight into the structure-dependent activity of bimetallic OER electrocatalysts, and our approach provides atomic-level structure-property relationships that would be extremely difficult, if not impossible, to quantify with traditional approaches.

Effect of the Random Defects Generated on the Surface of Pt(111) on the Electro-oxidation of Ethanol: An Electrochemical Study.

In the present work, the Pt(111) surface was disordered by controlling the density of {110}- and {100}-type defects. The cyclic voltammogram (CV) of a disordered surface in acid media consists of three contributions within the hydrogen adsorption/desorption region: one from the well-ordered Pt(111) symmetry and the other two transformed from the {111}-symmetry with contributions of {110}- and {100}-type surface defects. The ethanol oxidation reaction (EOR) was studied on these disordered surfaces. Electrochemical studies were performed in 0.1 M HClO4 +0.1 M ethanol using cyclic voltammetry and chronoamperometry. Changes in current densities associated to the specific potentials at which each oxidation peak appears suggest that different surface domains of disordered platinum oxidize ethanol independently. Additionally, as the surface-defect density increases, the EOR is catalysed better. This tendency is directly observed from the CV parameters because the onset and peak potentials are shifted to less positive values and accompanied by increases in the oxidation-peak current on disordered surfaces. Similarly, the CO oxidation striping confirmed this same tendency. Chronoamperometric experiments showed two opposite behaviors at short oxidation times (0.1 s). The EOR was quickly catalyzed on the most disordered surface, Pt(111)-16, and was then rapidly deactivated. These results provide fundamental information on the EOR, which contributes to the atomic-level understanding of real catalysts.

Excavated and dendritic Pt-Co nanocubes as efficient ethylene glycol and glycerol oxidation electrocatalysts

Noble metal electrocatalysts with excavated and dendritic architectures have recently attracted growing attention for their large accessible surface areas and high density of under-coordinated atoms. However, it is a tough challenge to avoid the atoms migrating to the corners and kinks and finally obtain the excavated structure, after the complicated interaction among multiple factors during the synthesis process, let alone the growth control of dendritic shape. Herein we demonstrate a facile strategy to synthesize Pt-Co nanocubes having excavated and dendritic structure (Pt-Co EDNCs). Specifically, the Pt-Co EDNC nanocrystal is composed of a deeply excavated nanocube and nanodendrites selectively protruding on the vertexes. Further electrocatalytic investigations showed that Pt-Co EDNCs exhibited enhanced catalytic properties towards the electro-oxidation of ethylene glycol and glycerol compared with commercial Pt/C, which could be attributed to both its unique structure and the synergism between Pt and Co.

Determination of the Radiative Characteristics of Molecular Transitions in the band HgXe(A3O+) – HgXe(X1O+)

This study is aimed at the explanation of peculiarities in emission spectral profiles obtained by different authors for the excimer band HgXe(A30+) – HgXe(X10+). The calculations of the spectral profiles have been fulfilled for two limiting cases: for the high density and low density of buffer gas atoms. The results obtained lead to the conclusion that the discrepancies can be mainly caused by the recombination and relaxation processes determining the population ro-vibrational states of the HgXe(A30+) excimer. The inverse spectroscopic problem is considered.

Unprecedented Eighteen-Faceted BiOCl with a Ternary Facet Junction Boosting Cascade Charge Flow and Photo-redox.

Exposure of anisotropic crystal facets allows the directional transfer of photoexcited electrons (e- ) and holes (h+ ), for spatial charge separation. High-index facets with a high density of low-coordinated atoms always serve as reactive catalytic sites. However, preparation of multi-facets or high-index facets is highly challenging for layered bismuth-based photocatalysts. Herein, we report the preparation of unprecedented eighteen-faceted BiOCl with {001} top facets and {102} and {112} oblique facets via a hydrothermal process. Compared to the conventional BiOCl square plates with {001} top facets and {110} lateral facets, the eighteen-faceted BiOCl has highly enhanced photocatalytic activity for H2 evolution and hydroxyl radicals (. OH) production. Theoretical calculations and photodeposition results disclose that the of eighteen-faceted BiOCl has a well-matched {001}/{102}/{112} ternary facet junction, which provides a cascade path for more efficient charge flow than the binary facet junction in BiOCl square plates.

Synthesis, characterization, and X-ray attenuation properties of polyacrylic acid-coated ultrasmall heavy metal oxide (Bi2O3, Yb2O3, NaTaO3, Dy2O3, and Gd2O3) nanoparticles as potential CT contrast agents

Ultrasmall heavy metal oxide nanoparticles are potential candidate materials for X-ray computed tomography (CT) contrast agents because they possess high X-ray attenuation powers owing to high X-ray attenuation coefficients of heavy metal atoms and high density of heavy metal atoms per nanoparticle. In this study, five kinds of polyacrylic acid (PAA)-coated ultrasmall heavy metal oxide (Bi2O3, Yb2O3, NaTaO3, Dy2O3, and Gd2O3) nanoparticles were synthesized and their X-ray attenuation properties were investigated. The estimated average particle diameters were 2.3 ± 0.1, 1.7 ± 0.1, 1.5 ± 0.1, 1.8 ± 0.1, and 1.9 ± 0.1 nm for PAA-coated ultrasmall Bi2O3, Yb2O3, NaTaO3, Dy2O3, and Gd2O3 nanoparticles, respectively. All of the nanoparticle suspension samples exhibited a high colloidal stability, a high biocompatibility, and X-ray attenuation powers which were stronger than that of a commercial iodine contrast agent Ultravist® at the same atomic concentration and much stronger, at the same number density. The effectiveness of the nanoparticle suspension samples as CT contrast agents was demonstrated by acquiring in vivo CT images by using one of the samples (i.e., PAA-coated ultrasmall Bi2O3 nanoparticles). After intravenous injection into the mouse tail vein, positive contrast enhancements in various organs were observed.

(Invited) Atomically Dispersed Eletrocatalysts for High-Efficiency Ambient N2 Fixation

NH3 synthesis by the electrocatalytic N2 reduction reaction (NRR) under ambient conditions is regarded as an appealing alternative to the industrial method that requires high temperature and pressure. Finding a material that can efficiently catalyze electrochemical N2 fixation at ambient condition is a subject of considerable current interest. Atomically dispersed catalysts with mononuclear metal complexes or single metal atoms anchored on supports would be a promising candidate, because of their maximum atom efficiency, unique catalytic performances, and the similarity of the metal coordination environment to the ligand fields in molecular catalysts. A series of cost-effective and optimized atomically-dispersed eletrocatalysts for NRR will be reported. Our results show that the catalysts are featured with high density of single metal atoms supported on hierarchically porous carbon frameworks. They exhibited remarkable selectivity of NH3 formation and high NH3 yield rate at low applied potentials at room temperature. Moreover, the catalysts show negligible activity decay in an NRR electrolysis as long as 50,000 s. On the basis of our results and the previously reported work, a possible mechanism for NRR on the catalyst will be proposed to provide a fundamental insight into the high-efficiency NRR.

The Ultrahigh Quantum Thermal Conductance of Hydrogenated Boron Nanotubes

Using first‐principles calculations and phonon dispersion analysis, the authors demonstrate theoretically that a hydrogenated monolayer borophene sheet can form a stable nanotube structure, denoted as H‐BNT. Combined with non‐equilibrium Green's function method, it is found that although thermal conductance of H‐BNT is lower than its carbon counterpart in the high‐temperature limit, at room temperature the phonon thermal conductance of H‐BNT is only slightly lower (2%) than carbon nanotube with the similar diameter. The high room temperature thermal conductance of boron nanotube is attributed to the light atomic mass of boron, strong interatomic force between boron atoms and high atomic density. Our studies shed light on the nature of quantum thermal transport in low‐dimensional materials, and can help in developing boron‐based nanoscale devices.

Ultrathin Co3O4 Nanosheets with Edge-Enriched {111} Planes as Efficient Catalysts for Lithium–Oxygen Batteries

Li-O2 batteries have aroused great interest as promising chemical power sources due to their high energy density. However, enhancement of the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the cathodes is still a great challenge. Herein, we design and prepare Co3O4 single-crystal nanosheets with edge-enriched {111} facets via a simple pyrolysis of α-Co(OH)2, which show a high activity as cathode catalysts for Li-O2. The catalytic activity and stability have been significantly improved in comparison with Co3O4 poor-edge {111} facets. This enhancement can be largely attributed to the edge-enriched {111} facets. Co3O4 nanosheets with edge-enriched {111} facets expose more metal atoms to boost the catalytic activity. Moreover, the edge-enriched {111} facets contain a high density of atomic steps and kink atoms which supply a wide space for breaking chemical bonds and increasing the reaction activity. This study also presents some insights into designing high-pe...

Study on Thallium Bromide Radioactive Detector

Thallium bromide (TlBr) is a compound semiconductor material, which can be used for X-ray and gamma-ray detectors and can be used at room temperature. It has excellent physical properties, high atomic number and density, wide bandgap (B = 2.68 eV), and low ionization energy. Compared with other X-ray and gamma-ray detection materials, TlBr devices have high detection efficiency and excellent energy resolution performance. So TlBr is suitable for housing in small tubes or shells, and it can be widely used in nuclear material measurement, safeguards verification, national security, space high-energy physics research, and other fields. Based on the fabrication of TlBr prototype detector, this paper focuses on the device fabrication and signal acquisition technology. Gamma-ray spectrum measurements and performance tests are carried out with AM-241 radioactive source. The results show that the special photoelectric peak of 59.5 keV is clearly visible, and the optimal resolution is 4.15 keV (7%).

Excitation of Rydberg Molecules in Ultracold Quantum Gases

Recent experiments in our group on the excitation of Rydberg molecules in ultracold quantum gases have been reviewed. Quantum gases are a well‐suited environment to excite Rydberg molecules, due to the high atomic density and the perfect control of the internal spin degrees of freedom and the center of mass motion of the atoms. In parallel, the coupling to Rydberg molecules can be used to probe and manipulate the interaction in ultracold quantum gases. In this Feature Article, a comprehensive introduction to the field of Rydberg molecules, including ultralong‐range molecules and butterfly molecules is given. The Rydberg molecules have been characterized spectroscopically and their internal spin structure is discussed. Then, it is shown how these Rydberg molecules work as a tool to monitor the double occupancy in optical lattice experiments and to change the inter‐particle interaction with the help of a so‐called optical Feshbach resonance.

A Bismuth Metal-Organic Framework as a Contrast Agent for X-ray Computed Tomography.

A new bismuth metal-organic framework (MOF), bismuth-NU-901 (Bi-NU-901), featuring the scu topology and a pore with a diameter of ∼11 Å, was solvothermally synthesized, and its use as an X-ray computed tomography (CT) contrast agent was tested. X-ray CT is a common diagnostic method used in the medical field. Inside the body, contrast media enhance the distinction between tissues and organs of similar density. Bi-NU-901 consists of eight connected Bi6 nodes and tetratopic 1,3,5,8-(p-benzoate)pyrene linkers (TBAPy). Numerous material characterization studies including powder X-ray diffraction (PXRD), scanning transmission electron microscopy (STEM), and DFT pore size distribution support the scu structure. Additionally, given the framework's high density of nontoxic heavy atoms, Bi-NU-901 was evaluated as an X-ray computed tomography (CT) agent. Importantly, in vitro studies revealed this new bismuth MOF demonstrates ∼7 times better contrast intensity compared to a zirconium MOF featuring the same topology and ∼14 times better contrast than a commercially available CT contrast agent. These results suggest bismuth MOFs may be promising CT contrast agents.

Saturation and alternate pathways in four-wave mixing in rubidium

We have examined the frequency spectrum of the blue light generated via four-wave mixing in a rubidium vapor cell inside a ring cavity. At high atomic density and input laser power, two distinct frequency components separated by $116 \pm 4$ MHz are observed, indicating alternate four-wave mixing channels through the $6p_{3/2}$ hyperfine states. The dependence of the generated light on excitation intensity and atomic density are explored, and indicate the primary process has saturated. This saturation results when the excitation rate through the 6p state becomes equal to the rate through the 5p state, giving no further gain with atomic density while a quadratic intensity dependence remains.

Anisotropy control in magnetic nanostructures through field-assisted chemical vapor deposition.

Chemical vapor deposition of iron pentacarbonyl (Fe(CO)5) in an external magnetic field (B = 1.00 T) was found to significantly affect the microstructure and anisotropy of as-deposited iron crystallites that could be transformed into anisotropic hematite (α-Fe2O3) nanorods by aerobic oxidation. The deterministic influence of external magnetic fields on CVD deposits was found to be substrate-independent as demonstrated by the growth of anisotropic α-Fe columns on FTO (F:SnO2) and Si (100), whereas the films deposited in the absence of the magnetic field were constituted by isotropic grains. TEM images revealed gradual increase in average crystallite size in correlation to the increasing field strength and orientation, which indicates the potential of magnetic field-assisted chemical vapor deposition (mfCVD) in controlling the texture of the CVD grown thin films. Given the facet-dependent activity of hematite in forming surface-oxygenated intermediates, exposure of crystalline facets and planes with high atomic density and electron mobilities is crucial for oxygen evolution reactions. The field-induced anisotropy in iron nanocolumns acting as templates for growing textured hematite pillars resulted in two-fold higher photoelectrochemical efficiency for hematite films grown under external magnetic fields (J = 0.050 mA cm−2), when compared to films grown in zero field (J = 0.027 mA cm−2). The dark current measurements indicated faster surface kinetics as the origin of the increased catalytic activity.

Benzophenone assisted UV-activated synthesis of unique Pd-nanodendrite embedded reduced graphene oxide nanocomposite: a catalyst for C-C coupling reaction and fuel cell.

In this work we report the use of benzophenone (BP) for the synthesis of a palladium (Pd) embedded on reduced graphene oxide (rGO) nanocomposite (Pd/rGO) using a simple aqueous solution and UV irradiation. The simple and facile evolution of thermodynamically unstable branched Pd(0) nanodendrites was achieved by BP photoactivation, circumventing the growth of more stable nanomorphologies. The synthesis of Pd(0)-embedded rGO nanosheets (PRGO-nd) was made possible by the simultaneous reduction of both the GO scaffold and PdCl2 by introducing BP into the photoactivation reaction. The nanocomposites obtained in the absence of BP were common triangular and twinned Pd(0) structures which were also implanted on the rGO scaffold (PRGO-nt). The disparity in morphologies presumably occurs due to the difference in the kinetics of the reduction of Pd2+ to Pd0 in the presence and absence of the BP photoinitiator. It was observed that the PRGO-nd was composed of dense arrays of multiple Pd branches around nucleation site which exhibited (111) facet, whereas PRGO-nt showed a mixture of (100) and (111) facets. On comparing the catalytic efficiencies of the as-synthesized nanocatalysts, we observed a superiority in efficiency of the thermodynamically unstable PRGO-nd nanocomposite. This is due to the evolved active facets of the dendritic Pd(0) morphology with its higher surface area, as testified by Brunauer–Emmett–Teller (BET) analysis. Since both PRGO-nd and PRGO-nt contain particles of similar size, the dents and grooves in the structure are the cause of the increase in the effective surface area in the case of nanodendrites. The unique dendritic morphology of the PRGO-nd nanostructures makes them a promising material for superior catalysis, due to their high surface area, and the high density of surface atoms at their edges, corners, and stepped regions. We investigated the efficiency of the as-prepared PRGO-nd catalyst in the Suzuki–Miyaura coupling reaction and showed its proficiency in a 2 h reaction at 60 °C using 2 mol% catalyst containing 0.06 mol% active Pd. Moreover, the electrochemical efficiency for the catalytic hydrogen evolution reaction (HER) was demonstrated, in which PRGO-nd provided a decreased overpotential of 68 mV for a current density of 10 mA cm−2, a small Tafel slope of 57 mV dec−1 and commendable stability during chronoamperometric testing for 5 h.

Prediction on the high-pressure physical properties of a new metastable structure for BC<sub>2</sub>N compound

Light elements boron (B), carbon (C) and nitrogen (N) have small atomic radius and strong bonding ability, and the formed compounds are easy to form strong covalent bonds and high atomic density three-dimensional networks structures, makes it an alternative system for the preparation of superhard materials. Based on the newly studied body-centered tetragonal carbon structure model with excellent mechanical properties, a new structure of BC2N potential superhard compound with tetragonal symmetry structure and space group of I 4/ mmm was constructed in this paper. Using the first-principles calculation method based on density functional theory, the thermodynamic, mechanical and dynamic stability of the new tetragonal phase structure of the compound was systematically studied. It is shown that the tetragonal phase BC2N is atable in mechanics and dynamics at least in the pressure range of 0– 60 GPa. The thermodynamic calculation results show that the structural formation energy is slightly higher than the most stable wurtzite structure in the BC2N compound, indicating that the newly constructed tetragonal structure is a metastable structure of the BC2N compound, and its incompressibility is greater than other boron-carbon-nitrogen materials such as B2CN and BC4N. Based on the structural stability study, the electronic structure of the tetragonal BC2N in the pressure range of 0– 60 GPa is calculated. It is found to have a bandwidth of 2.16 eV at zero pressure, which is a semiconductor, and with the increase of pressure, The band gap is gradually widened; the high-pressure elastic properties show that the bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, Debye temperature, minimum thermal conductivity and elastic wave velocity of the tetragonal structure of BC2N compound are all increased in different increasing tendency with pressure, it shows brittleness and its elastic anisotropy becomes more obvious under high pressure. At the same time, the large bulk modulus, large shear modulus and high Vickers hardness indicate that the constructed tetragonal structure with I 4/ mmm space group is a potential superhard new structure of BC2N compounds.

Copper adparticle enabled selective electrosynthesis of n-propanol

The electrochemical reduction of carbon monoxide is a promising approach for the renewable production of carbon-based fuels and chemicals. Copper shows activity toward multi-carbon products from CO reduction, with reaction selectivity favoring two-carbon products; however, efficient conversion of CO to higher carbon products such as n-propanol, a liquid fuel, has yet to be achieved. We hypothesize that copper adparticles, possessing a high density of under-coordinated atoms, could serve as preferential sites for n-propanol formation. Density functional theory calculations suggest that copper adparticles increase CO binding energy and stabilize two-carbon intermediates, facilitating coupling between adsorbed *CO and two-carbon intermediates to form three-carbon products. We form adparticle-covered catalysts in-situ by mediating catalyst growth with strong CO chemisorption. The new catalysts exhibit an n-propanol Faradaic efficiency of 23% from CO reduction at an n-propanol partial current density of 11 mA cm−2.

Advances in the accuracy, stability, and reliability of the PTB primary fountain clocks

Improvements of the systematic uncertainty, frequency instability, and long-term reliability of the two caesium fountain primary frequency standards CSF1 and CSF2 at PTB (Physikalisch-Technische Bundesanstalt) are described. We have further investigated many of the systematic effects and made a number of modifications of the fountains. With an optically stabilized microwave oscillator, the quantum projection noise limited frequency instabilities are improved to for CSF1 and for CSF2 at high atom density. The systematic uncertainties of CSF1 and CSF2 are reduced to and , respectively. Both fountain clocks regularly calibrate the scale unit of International Atomic Time (TAI) and the local realization of Coordinated Universal Time, UTC(PTB), and serve as references to measure the frequencies of local and remote optical frequency standards.

Highly efficient iodine capture by task-specific polyethylenimine impregnated hypercrosslinked polymers

Abatract The primary challenge in environmental remediation is keeping the balance between performance and economy for practical application. The usage of hypercrosslinked polymers as iodine capture adsorbents overcomes the shortcomings of commonly used porous organic polymers adsorbents including the usage of expensive catalysts or monomers and complicated synthesis procedures. This issue is the first attempt to prepare polyethylenimine impregnated hypercrosslinked polymers with a high density of nitrogen atoms designed for iodine capture. The resulting polymers exhibited ultrahigh iodine uptake capacities up to 607 wt% and the capture process fitted the pseudo-second-order model well. Porous characteristics, electron-rich aromatic networks, and abundance nitrogen atoms in polyethylenimine contributed to the high capture capacity. Results indicated that the interactions between iodine and adsorbents were charge transfer and the form of captured iodine was polyiodide. Furthermore, the polymers could be recycled with 93.4% iodine uptake capacity maintained after four cycles. The work presented a new benchmark for using hypercrosslinked polymers as economical materials for iodine capture.

PRISM reference fuel design

The basis for GE-Hitachi Nuclear Energy America’s, LLC (GEH’s) decision to select metallic fuel as the reference core for its sodium-cooled fast reactors (SFRs), such as PRISM (Power Reactor Inherently Safe Module), is based on the fuel’s physical properties, manufacturability, and accident response. The ideal fuel for a fast reactor has high thermal conductivity, a high melting temperature, chemical compatibility with the coolant, dimensional stability during irradiation, high fissile atom density, and few moderating atoms. Though each material has unique benefits, metallic fuel was chosen because 1) its high heat transport capability throughout its irradiation life, while still maintaining similar margin to melt as oxide fuel, 2) it is completely compatible with the coolant, making run beyond cladding breach a feasible operational strategy, and 3) it avoids the potential for core-wide boiling by providing a lower accident condition fuel centerline temperature and low Doppler reactivity feedback in stabilization of the core power during transient events. Metallic fuel is judged to have a more robust implementation for multiple SFR missions due to the relatively limited impact on overall thermo-mechanical properties with varying isotopic content, and therefore aligns PRISM better with the GEN-IV and PRISM’s original design goals of economic power generation, transuranic recycle and plutonium disposition, which in turn makes the commercialization of an advanced SFR possible in the near-term.