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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Investigating the Surface Structure of γ-Al 2 O 3 Supported WO X Catalysts by High Field 27 Al MAS NMR and Electronic Structure Calculations

The metal-support interaction in γ-Al2O3 supported WOX catalysts is investigated by a combination of high field quantitative single pulse (SP) 27Al MAS NMR spectroscopy, 2D MQMAS, 1H-27Al CP/MAS, and electronic structure calculations. NMR allows the observation of at least seven different Al sites, including a pentahedral Al site, three different tetrahedral Al sites, and three octahedral Al sites. It is found that the penta-coordinated Al (AlP) site density decreases monotonically with an increased WOX loading while the octahedral Al (AlO) site density increases concurrently. This suggests that the Alp sites are the preferred surface anchoring positions for the WOX species. Importantly, the AlP site isotropic chemical shift observed for the unsupported γ-Al2O3 at about 38 ppm migrates into the octahedral region with a new isotropic chemical shift value appearing near 7 ppm when the Alp site is anchored by WOX species. Density functional theory (DFT) computational modeling of the NMR parameters on proposed cluster models is carried out to accurately interpret the dramatic chemical shift changes from which the detailed anchoring mechanisms are obtained. It is found that tungsten dimers and monomers are the preferred supported surface species on γ-Al2O3, wherein one monomeric and several dimeric structures are identified as the most likely surface anchoring structures.

27Al MAS NMR↗
DOE OSTI2022

Hierarchical triply periodic minimal surface structures as heat exchangers and reactors

The present disclosure relates to a transport mechanism apparatus for transporting at least one of a gas or a fluid. The transport mechanism may have an inlet, an outlet and a triply periodic minimal surface (TPMS) structure. The TPMS structure is formed in a layer-by-layer three dimensional (3D) printing operation to include cells propagating in three dimensions, where the cells include wall portions having openings, and where the cells form a plurality of flow paths throughout the transport mechanism from the inlet to the outlet, and where the cells form the inlet and the outlet.

Roy, Pratanu↗
DOE OSTI2017

Surface structures of L1 0 -MnGa (001) by scanning tunneling microscopy and first-principles theory

We report on the surface reconstructions of L10-ordered MnGa (001) thin films grown by molecular beam epitaxy on a 50 nm Mn 3 N 2 (001) layer freshly grown on a magnesium oxide (001) substrate. Scanning tunneling microscopy, Auger electron spectroscopy, and reflection high energy electron diffraction, are combined with first-principles density functional theory calculations to determine the reconstructions of the L1 0 -ordered MnGa (001) surface. We find two lowest energy reconstructions of the MnGa(001) face: a 1×1 Ga-terminated structure and a 1×2 structure with a Mn replacing a Ga in the 1×1 Ga-terminated surface. The 1×2 reconstruction forms a row structure along [100]. The manganese:gallium stoichiometry within the surface based on the oretical modeling is in good agreement with experiment. Magnetic moment calculations for the two lowest energy structures reveal important surface and bulk effects leading to oscillatory total magnetization for ultra-thin MnGa(001) films.

density functional theory↗
DOE OSTI2019

Boosting Cell Performance of LiNi 0.8 Co 0.15 A 0.05 O 2 via Surface Structure Design

Although the high energy density and environmental benignancy of LiNi 0.8 Co 0.15 A 0.05 O 2 (NCA) holds promise for use as cathode material in Li-ion batteries, present low rate capabilities, and fast capacity fade limit its broad commercial applications. Here, it is reported that surface modification of NCA cathode (R-3m) with 5 nm-thick nanopillar layers and Fm-3m structures significantly improves electrode structure, morphology, and electrochemical performance. The formation of nanopillar layers increases cycling and working voltage stability of NCA by shielding the host material from hydrofluoric acid and improves structural stability with the electrolyte. The modified NCA cathode exhibits an enhanced 89% capacity retention at a rate of 1 C over that of pristine NCA (75.2%) after 150 cycles and effectively suppresses working voltage fade (a drop of 0.025 V after 300 cycles) during repeated charge-discharge cycles. In addition, the diffusion barrier of Li ions in NCA crystals at 0.80 V is noticeably smaller than that of Li ions in pristine NCA (0.87 eV). These findings demonstrate that this unique surface structure design considerably enhances cycle and rate performance of NCA, which has potential applications in other Ni-rich layered cathode materials.

25 ENERGY STORAGE↗
DOE OSTI2018

Enhanced Catalytic Properties of Molybdenum Promoted Mesoporous Cobalt Oxide: Structure-Surface-Dependent Activity for Selective Synthesis of 2-Substituted Benzimidazoles

High-valent molybdenum ions were substituted into the cobalt oxide lattice through a one step, sol-gel method and investigated for selective synthesis of 2-substituted benzimidazoles. Catalyst synthesis involves surfactant assisted soft templating inverse micelle method, which forms mesopores by interconnected intraparticle voids. Substitutional doping of Mo 6+ resulted in materials with modified structural, morphological, surface, and redox properties. The catalytic activity increased with Mo concentration until an optimum amount (3 % Mo incorporation). Modified material shows lattice expansion, increased surface oxygen vacancies, and high surface area, which are responsible for the higher catalytic activity in selective benzimidazole synthesis reaction. A strong correlation between surface properties of the catalyst and the product selectivity was observed and plausible mechanistic and kinetic data are proposed and collected, respectively.

2-substituted benzimidazoles↗
DOE OSTI2023

X-Ray Absorption Spectroscopy (XAS): Surface Structural Determination of Alloy Nanoparticles

In catalysis since reactions occur on the surface of nanoparticles (NP), it is essential to determine the composition of this structure, rather than that of the nanoparticles, since the two may be, and often are, different. Conventional techniques including X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) are powerful techniques, but these data reflect the average composition of the entire particle. In this chapter, we introduce the method of EXAFS analysis, which isolates the surface atoms of nanoparticles based on its sensitivity to chemical reactions, specifically surface oxidation. As shown in our case studies, if the surface of a Pt-based nanoparticle is contacted by air at room temperature, the surface will selectively oxidize, resulting in the loss of Pt-Pt and Pt-M bonds to the formation of Pt-O bonds. The difference between the completely reduced and surface-oxidized nanoparticle allows for the isolation of signal from the catalytic surface. Although these examples highlight Pt alloys, similar analysis is also possible for other group 8 and IB bimetallic catalysts.

Zhang, Guanghui↗
DOE OSTI2019

Influence of Atomic Surface Structure on the Activity of Ag for the Electrochemical Reduction of CO 2 to CO

This work was undertaken to elucidate the facet-dependent activity of Ag for the electrochemical reduction of CO 2 to CO. To this end, CO2 reduction was investigated over Ag thin films with (111), (100), and (110) orientations prepared via epitaxial growth on single-crystal Si wafers with the same crystallographic orientations. Thus, this preparation technique yielded larger area electrodes than can be achieved using single-crystals, which enabled the electrocatalytic activity of the corresponding Ag surfaces to be quantified in the Tafel regime. The Ag(110) thin films exhibited higher CO evolution activity compared to the Ag(111) and Ag(100) thin films, consistent with previous single-crystal studies. Density functional theory calculations suggest that CO 2 reduction to CO is strongly facet-dependent, and that steps are more active than highly coordinated terraces. This is the result of both a higher binding energy of the key intermediate COOH as well as an enhanced double-layer electric field stabilization over undercoordinated surface atoms located at step edge defects. As a consequence, step edge defects likely dominate the CO 2 reduction activity observed over the Ag(111) and Ag(100) thin films. The higher activity observed over the Ag(110) thin film is then related to the larger density of undercoordinated sites compared to the Ag(111) and Ag(100) thin films. Our conclusion that undercoordinated sites dominate the CO 2 reduction activity observed over close-packed surfaces highlights the need to consider the contribution of such defects in studies of single-crystal electrodes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Surface structure of mass-selected niobium oxide nanoclusters on Au(111)

The structures formed by the deposition of mass-selected niobium oxide clusters, Nb 3 O y ( y = 5, 6, 7), onto Au(111) were studied by scanning tunneling microscopy. The as-deposited Nb 3 O 7 clusters assemble into large dendritic structures that grow on the terraces as well as extend from the top and bottom of step edges. The Nb 3 O 6 cluster also forms dendritic assemblies but they are generally much smaller in size. The assemblies are composed of smaller discrete structures (<1 nm) which are likely to be single clusters. Furthermore, the dendritic assemblies for both the Nb 3 O 7 and Nb 3 O 6 clusters have fractal dimensions of about 1.7 which is very close to that expected for simple diffusion limited aggregation. Annealing the Nb 3 O 7,6 /Au(111) surfaces up to 550 K results in changes in assembly sizes and increases in heights, while heating to 700 results in the disruption of the assemblies into smaller structures. By contrast, the as-deposited Nb 3 O 5 /Au(111) surface at RT exhibits compact cluster structures which become 3D nanoparticles when annealed above 550 K. Differences in the observed surface structures and thermal stability are attributed to differences in metal-oxygen stoichiometry which can influence cluster binding energies, mobility and inter-cluster interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

In situ transmission electron microscopy observation of ZnO polar and non-polar surfaces structure evolution under electron beam irradiation

In this paper, using in situ transmission electron microscopy, we investigated the dynamic reconstruction and evolution of ZnO polar and non-polar surfaces under high-energy electron beam irradiation. Electron beam radiolysis creates oxygen vacancies and a Zn rich (0001) surface. Positive polar charges at the (0001) surface expel loosely bonded Zn ions to diffuse away from the (0001) polar surface. As a result, mass loss was observed around the (0001) surface. Dehydration by the electron beam breaks the charge balance on the ( 000 1 ¯ ) polar surface. The negative charges on the ( 000 1 ¯ ) surface suppress the radiolysis effect and further absorb Zn ions to the surface to neutral the polar charges. The ideal stacking sequences of Zn ions in hexagonal ZnO structure can be considered as ABAB… along its c axis, while the absorbed individual Zn ion on the ( 000 1 ¯ ) surface occupies the C site to form three bonds with surface O ions beneath, instead of one bond in the ideal structure. With more Zn ion absorption and surface oxidization, new nanocrystals grow up from the ( 000 1 ¯ ) polar surface. Finally, new nanocrystals nucleated at the ( 01 1 ¯ 0 ) non-polar surface are driven by the electric field of the polar charges as well, for the Zn ions were always observed to absorb on the negatively charged [ 000 1 ¯ ] end of the newly formed ( 01 1 ¯ 0 ) surface layer.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Fe Oxides on Ag Surfaces: Structure and Reactivity

One layer thick iron oxide films are attractive from both applied and fundamental science perspectives. The structural and chemical properties of these systems can be tuned by changing the substrate, making them promising materials for heterogeneous catalysis. In the present work, we investigate the structure of FeO(111) monolayer films grown on Ag(100) and Ag(111) substrates by means of microscopy and diffraction techniques and compare it with the structure of FeO(111) grown on other substrates reported in literature. We also study the NO adsorption properties of FeO(111)/Ag(100) and FeO(111)/Ag(111) systems utilizing different spectroscopic techniques. Finally, we discuss similarities and differences in the data obtained from adsorption experiments and compare it with previous results for FeO(111)/Pt(111).

36 MATERIALS SCIENCE↗
DOE OSTI2017

Submonolayer adsorption of Na onto the Cu(110) surface: Structure and vibrational properties

The submonolayer adsorption of Na onto the Cu(110) surface is studied. At small Na coverages (Θ = 0.16–0.25 ML), the substrate surface subjected to missing-row reconstruction (1 × 2) is shown to be most stable dynamically. When the coverage increases to Θ = 0.5 ML, the unreconstructed substrate surface with a c(2 × 2) sodium adlayer becomes dynamically stable. For an analysis, we used data on the equilibrium atomic configuration, the adsorption energy, the phonon spectra, the local density of phonon states, and the polarization of localized vibrational modes. All calculations were performed using the interatomic potentials obtained in terms of the embedded-atom method. The calculated frequencies of localized vibrational modes agree well with the existing experimental data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Evolution of surface structure in laser-preheated, perturbed materials

Here, we report an experimental and computational study investigating the effects of laser preheat on the hydrodynamic behavior of a material layer. In particular, we find that perturbation of the surface of the layer results in a complex interaction, in which the bulk of the layer develops density, pressure, and temperature structure and in which the surface experiences instability-like behavior, including mode coupling. A uniform one-temperature preheat model is used to reproduce the experimentally observed behavior, and we find that this model can be used to capture the evolution of the layer, while also providing evidence of complexities in the preheat behavior. Lastly, this result has important consequences for inertially confined fusion plasmas, which can be difficult to diagnose in detail, as well as for laser hydrodynamics experiments, which generally depend on assumptions about initial conditions in order to interpret their results.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Tungsten Incorporation into Gallium Oxide: Crystal Structure, Surface and Interface Chemistry, Thermal Stability and Interdiffusion

Tungsten (W) incorporated gallium oxide (Ga2O3) (GWO) thin films were deposited by radio-frequency magnetron co-sputtering of W-metal and Ga2O3-ceramic targets. Films were produced by varying sputtering power applied to the W-target in order to achieve variable W-content (0-12 at%) into Ga2O3 while substrate temperature was kept constant at 500 °C. Chemical composition, chemical valence states, microstructure and crystal structure of as-deposited and annealed GWO films were evaluated as a function of W-content. The structural and chemical analyses indicate that the samples deposited without any W-incorporation are stoichiometric, nanocrystalline Ga2O3 films, which crystallize in β-phase monoclinic structure. While GWO films also crystallize in monoclinic β-Ga2O3 phase, W-incorporation induces surface amorphization as revealed by structural studies. The chemical valence state of Ga ions probed by X-ray photoelectron spectroscopic (XPS) analyses is characterized by the highest oxidation state i.e., Ga3+. No changes in Ga chemical state are noted for variable W-incorporation in the range of 0-12 at%. Rutherford backscattering spectrometry (RBS) analyses indicate the uniform distribution of W-content in the GWO films. However, XPS analyses indicate the formation of mixed valence states for W ions, which may be responsible for surface amorphization in GWO films. GWO films were stable up to 900 oC, at which point thermally induced secondary phase (W-oxide) formation was observed. A transition to mesoporous structure coupled with W interdiffusion occurs due to thermal annealing as derived from the chemical analyses at the GWO films’ surface as well as depth-profiling towards the GWO-Si interface. A model has been formulated to account for the mechanism of W-incorporation, thermal stability and interdiffusion via pore formation in GWO films.

Chemical composition↗
DOE OSTI2025

Effect of Thermal Oxidation on the Structure, Surface Texturing, and Microstructure Evolution in Nanocrystalline Ga-O-N Films

An extensive examination of the nanoscale, crystallographic growth dynamics of the system, which is impacted by the thermal energy given to the GaN, is carried out to derive a deeper understanding of the growth kinetics, morphology and microstructure evolution, chemical bonding, and optical properties of Ga-O-N films. Thermal annealing of GaN films is performed in the temperature range of 900–1200 °C. Crystal structure, phase formation, chemical composition, surface morphology, and microstructure evolution of Ga-O-N films are investigated as a function of temperature. Increasing temperature induces surface oxidation, which results in the formation of stable β-Ga2O3 phase in the GaN matrix, where the overall film composition evolves from nitride (GaN) to oxynitride (Ga-O-N). While GaN surfaces are smooth, planar, and featureless, oxidation induced granular-to-rod shaped morphology evolution is seen with increasing temperature to 1200 °C. The considerable texturing and stability of the nanocrystalline Ga-O-N on Si substrates can be attributed to the surface and interface driven modification because of thermal treatment. Corroborating with structure and chemical changes, Raman spectroscopic analyses also indicate that the chemical bonding evolution progresses from fully Ga-N bonds to Ga-O-N. While the GaN oxidation process starts with the formation of β-Ga 2 O 3 at an annealing temperature of 1000 °C, higher annealing temperatures induce structural distortion with the potential formation of Ga-O-N bonds. The structure-phase-chemical composition correlation, which will be useful for nanocrystalline materials for selective optoelectronic applications, is established in Ga-O-N films made by thermal treatment of GaN.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Be Surface Structures on W(110) and W(211): A DFT Study

Beryllium and Tungsten are promising candidates for use as plasma facing materials (PFMs) in upcoming fusion reactors. Many complex and competing phenomena however complicate the understanding and development of materials that operate in the harsh conditions of the reactor. In particular, redeposition of Be onto the W divertor must be considered due to the expected erosion of the Be first wall under exposure to the plasma. It is known that a build up of Be on W allows for the formation of BeW alloys which can harm the longevity and performance of the fusion divertor. In an effort to understand the interaction of Be with W surfaces, a study of Be structures on W(110) and W(211) as a function of Be coverage has been carried out using Density Functional Theory. We have found that both W surfaces develop monolayers of Be characterized by a densely packed hexagonal structure. Below this monolayer coverage, Be structures are found to have two motifs of coverage, a densely packed hexagonal pattern punctuated by areas of low-density coverage. The structures found here produce work function values and trends in good agreement with experimental measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electronic structure, surface doping, and optical response in epitaxial WSe 2 thin films

High quality WSe 2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe 2 /BLG. We observe that a bilayer of WSe 2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct–indirect band gap crossover to trilayer WSe 2 . In the monolayer limit, WSe 2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX 2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe 2 /BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Lastly, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.

36 MATERIALS SCIENCE↗