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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 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Thermal Stability of Metal Nanocrystals: An Investigation of the Surface and Bulk Reconstructions of Pd Concave Icosahedra [On the Thermal Stability of Metal Nanocrystals: An Investigation of the Surface and Bulk Reconstructions of Pd Concave Icosahedra]

Despite the remarkable success in controlling the synthesis of metal nanocrystals, it still remains a grand challenge to stabilize and preserve the shapes or internal structures of metastable kinetic products. In this work, we address this issue by systematically investigating the surface and bulk reconstructions experienced by a Pd concave icosahedron when subjected to heating up to 600 °C in vacuum. We used in situ high-resolution transmission electron microscopy to identify the equilibration pathways of this far-from-equilibrium structure. We were able to capture key structural transformations occurring during the thermal annealing process, which were mechanistically rationalized by implementing self-consistent plane-wave density functional theory (DFT) calculations. Specifically, the concave icosahedron was found to evolve into a regular icosahedron via surface reconstruction in the range of 200–400 °C, and then transform into a pseudospherical crystalline structure through bulk reconstruction when further heated to 600 °C. As a result, the mechanistic understanding may lead to the development of strategies for enhancing the thermal stability of metal nanocrystals.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Tailoring the Surface of Metal Halide Perovskites to Enable the Atomic Layer Deposition of Metal Oxide Contacts

Replacing organic contact layers with inorganic counterparts, such as metal oxides, is one strategy for improving long-term device stability in metal halide perovskite solar cells. Often, the methods used to deposit metal oxide thin films are incompatible with metal halide perovskites, creating challenges for the fabrication of contacts above the perovskite absorber layer. In this study, we utilize a one-step, solution treatment of the top surface of Cs 0.25 FA 0.75 Pb(Br 0.20 I 0.80 ) 3 to create a thin (~1 nm) overlayer of lead sulfide (PbS) to protect the underlying perovskite during subsequent deposition. X-ray characterization of the surface region shows that the PbS overlayer limits undesirable changes to the perovskite structure and stoichiometry during atomic layer deposition (ALD) of SnO 2 . This protection enables ALD growth of SnO 2 electron contacts on top of the perovskite without an organic transport layer (e.g., C 60 ), resulting in a solar cell with a power conversion efficiency of 5.8%. This result is a marked improvement over devices with ALD SnO 2 grown directly on the perovskite without a PbS overlayer, which produce no power output. Furthermore, the interface characterization and device results in this study highlight some of the key challenges associated with ALD metal oxide growth on perovskite materials and can help inform the future design of inorganic contact layer deposition in solar photovoltaic technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Sequential Infiltration Synthesis for the Design of Low Refractive Index Surface Coatings with Controllable Thickness

Control over refractive index and thickness of surface coatings is central to the design of low refraction films used in applications ranging from optical computing to antireflective coatings. Here, we introduce gas-phase sequential infiltration synthesis (SIS) as a robust, powerful, and efficient approach to deposit conformal coatings with very low refractive indices. We demonstrate that the refractive indices of inorganic coatings can be efficiently tuned by the number of cycles used in the SIS process, composition, and selective swelling of the of the polymer template. We show that the refractive index of Al2O3 can be lowered from 1.76 down to 1.1 using this method. The thickness of the Al2O3 coating can be efficiently controlled by the swelling of the block copolymer template in ethanol at elevated temperature, thereby enabling deposition of both single-layer and graded-index broadband antireflective coatings. Using this technique, Fresnel reflections of glass can be reduced to as low as 0.1% under normal illumination over a broad spectral range.

Berman, Diana↗
DOE OSTI2019

1,2-Propanediol as a Surrogate Molecule of Glycerol for Mechanistic Studies of Selective Hydrodeoxygenation Reactions over Mo 2 C and Cu/Mo 2 C Surfaces

The upgrading of glycerol, a byproduct from biodiesel production, is important for developing more sustainable biomass conversion technologies. Nonetheless, the low vapor pressure and structural complexity of glycerol limit mechanistic studies of gas-phase reactions on model surfaces and powder catalysts. In this work, we demonstrate the feasibility of using 1,2-propanediol (PDO) as a surrogate compound for the mechanistic study of glycerol over Mo 2 C and Cu/Mo 2 C surfaces. Here, the results show that glycerol and 1,2-PDO undergo similar reaction pathways on both the Mo and Cu sites, suggesting that 1,2-PDO could be used as a higher vapor pressure surrogate to glycerol for gas-phase mechanistic studies.

1,2-Propanediol↗
DOE OSTI2015

Water Oxidation by Cobalt Centers on Various Oxide Surfaces: The Effects of Oxide Surface Acidity and Oxygen Atom Affinity on Catalysis

© 2015 American Chemical Society. Single-atom cobalt centers on various oxide surfaces (TiO2, MgO, SBA-15, AlPO, and Y-Zeolite) were prepared and evaluated as water oxidation catalysts by photochemical water oxidation experiments. Superior catalytic rates were observed for cobalt sites on basic supporting oxides (TiO2 and MgO) relative to those on acidic oxides (Y-Zeolite, AlPO, and SiO2). Per-atom turnover frequencies of ca. 0.04 s-1 were achieved, giving initial rates 100 times greater than that of a surface atom of a Co3O4 nanoparticle. Contrary to expectations based on theoretical work, no apparent correlation was observed between the catalytic rates and the oxygen atom affinities of the supporting oxides. (Graph Presented).

Ahn, Hyun S↗
DOE OSTI2014

Spatial Arrangment of Organic Compounds on a Model Mineral Surface: Implications for Soil Organic Matter Stabilization

The complexity of the mineral organic carbon interface may influence the extent of stabilization of organic carbon compounds in soils, which is important for global climate futures. The nanoscale structure of a model interface was examined here by depositing films of organic carbon compounds of contrasting chemical character, hydrophilic glucose and amphiphilic stearic acid, onto a soil mineral analogue (Al2O3). Neutron reflectometry, a technique which provides depth-sensitive insight into the organization of the thin films, indicates that glucose molecules reside in a layer between Al2O3 and stearic acid, a result that was verified by water contact angle measurements. Molecular dynamics simulations reveal the thermodynamic driving force behind glucose partitioning on the mineral interface: The entropic penalty of confining the less mobile glucose on the mineral surface is lower than for stearic acid. The fundamental information obtained here helps rationalize how complex arrangements of organic carbon on soil mineral surfaces may arise

Petridis, Loukas↗
DOE OSTI2017

Water Dynamics from the Surface to the Interior of a Supramolecular Nanostructure

Water within and surrounding the structure of a biological system adopts context-specific dynamics that mediate virtually all of the events involved in the inner workings of a cell. These events range from protein folding and molecular recognition to the formation of hierarchical structures. Water dynamics are mediated by the chemistry and geometry of interfaces where water and biomolecules meet. Here we investigate experimentally and computationally the translational dynamics of vicinal water molecules within the volume of a supramolecular peptide nanofiber measuring 6.7 nm in diameter. Using Overhauser dynamic nuclear polarization relaxometry, we show that drastic differences exist in water motion within a distance of about one nanometer from the surface, with rapid diffusion in the hydrophobic interior and immobilized water on the nanofiber surface. Furthermore, these results demonstrate that water associated with materials designed at the nanoscale is not simply a solvent, but rather an integral part of their structure and potential functions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Dynamics of Back Electron Transfer in Dye-Sensitized Solar Cells Featuring 4-tert-Butyl-Pyridine and Atomic-Layer-Deposition Alumina as Surface Modifiers.

A series of dye-sensitized solar cells (DSCs) was constructed with TiO2 nanoparticles and N719 dye. The standard I-3(-)/I- redox shuttle and the Co(1,10-phenanthroline)(3)(3+/2+) shuttle were employed. DSCs were modified with atomic-layered-deposited (ALD) coatings of Al2O3 and/or with the surface-adsorbing additive 4-tert-butyl-pyridine. Current-voltage data were collected to ascertain the influence of each modification upon the back electron transfer (ET) dynamics of the DSCs. The primary effect of the additives alone or in tandem is to increase the open-circuit voltage. A second is to alter the short-circuit current density, J(SC). With dependence on the specifics of the system examined, any of a myriad of dynamics-related effects were observed to come into play, in both favorable (efficiency boosting) and unfavorable (efficiency damaging) ways. These effects include modulation of (a) charge-injection yields, (b) rates of interception of injected electrons by redox shuttles, and (c) rates of recombination of injected electrons with holes on surface-bound dyes. In turn, these influence charge-collection lengths, charge-collection yields, and onset potentials for undesired dark current. The microscopic origins of the effects appear to be related mainly to changes in driving force and/or electronic coupling for underlying component redox reactions. Perhaps surprisingly, only a minor role for modifier-induced shifts in conduction-band-edge energy was found. The combination of DSC-efficiency-relevant effects engendered by the modifiers was found to vary substantially as a function of the chemical identity of the redox shuttle employed. While types of modifiers are effective, a challenge going forward will be to construct systems in ways in which the benefits of organic and inorganic modifiers can be exploited in fully additive, or even synergistic, fashion.

Katz, Michael J.↗
DOE OSTI2025

Investigation of Arctic Cloud Properties and Surface Radiation Based on MOSAiC Shipborne Observations

The Arctic is rapidly changing due to changes of the Earth system. This study investigates cloud fraction, phase partition, cloud type, and their relationships with surface radiation based on yearlong shipborne observations in the Arctic regions. The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) campaign provided lidar and radar observations of cloud microphysical properties and surface shortwave (SW) and longwave (LW) radiation. Cloud and radiative properties were examined at daily and monthly resolutions in four seasons. Low clouds were found to be most prevalent throughout the year, followed by deep clouds. The ice phase is the dominant phase except for summer (June–August). Liquid and mixed phases show more significant monthly and annual mean radiative effects in SW and LW than the ice phase. The clouds show net warming effects due to LW heating in most months, while the SW cooling effects of clouds become more dominant for July and August. The cloud and radiation observations from MOSAiC were used to evaluate simulations of the atmospheric component of the Energy Exascale Earth System Model version 2. The simulations show large overestimations of the liquid and mixed phases in the Arctic regions from February to September. The simulations also underestimate the percentages of low clouds and overestimate the percentages of deep clouds throughout the year. Altogether, this work provides a unique analysis of cloud and radiation properties based on high-resolution shipborne observations, which can be used to assist future model evaluation and development.

58 GEOSCIENCES↗
DOE OSTI2021

Calculation and interpretation of classical turning surfaces in solids

Abstract Classical turning surfaces of Kohn–Sham potentials separate classically allowed regions (CARs) from classically forbidden regions (CFRs). They are useful for understanding many chemical properties of molecules but need not exist in solids, where the density never decays to zero. At equilibrium geometries, we find that CFRs are absent in perfect metals, rare in covalent semiconductors at equilibrium, but common in ionic and molecular crystals. In all materials, CFRs appear or grow as the internuclear distances are uniformly expanded. They can also appear at a monovacancy in a metal. Calculations with several approximate density functionals and codes confirm these behaviors. A classical picture of conduction suggests that CARs should be connected in metals, and disconnected in wide-gap insulators, and is confirmed in the limits of extreme compression and expansion. Surprisingly, many semiconductors have no CFR at equilibrium, a key finding for density functional construction. Nonetheless, a strong correlation with insulating behavior can still be inferred. Moreover, equilibrium bond lengths for all cases can be estimated from the bond type and the sum of the classical turning radii of the free atoms or ions.

Chemistry↗
DOE OSTI2022

All-perovskite tandem solar cells with improved grain surface passivation

All-perovskite tandem solar cells hold the promise of surpassing the efficiency limits of single-junction solar cells; however, until now, the best-performing all-perovskite tandem solar cells have exhibited lower certified efficiency than have single-junction perovskite solar cells. A thick mixed Pb–Sn narrow-bandgap subcell is needed to achieve high photocurrent density in tandem solar cells, yet this is challenging owing to the short carrier diffusion length within Pb–Sn perovskites. Here we develop ammonium-cation-passivated Pb–Sn perovskites with long diffusion lengths, enabling subcells that have an absorber thickness of approximately 1.2 μm. Molecular dynamics simulations indicate that widely used phenethylammonium cations are only partially adsorbed on the surface defective sites at perovskite crystallization temperatures. The passivator adsorption is predicted to be enhanced using 4-trifluoromethyl-phenylammonium (CF3-PA), which exhibits a stronger perovskite surface-passivator interaction than does phenethylammonium. By adding a small amount of CF3-PA into the precursor solution, we increase the carrier diffusion length within Pb–Sn perovskites twofold, to over 5 μm, and increase the efficiency of Pb–Sn perovskite solar cells to over 22%. Here, we report a certified efficiency of 26.4% in all-perovskite tandem solar cells, which exceeds that of the best-performing single-junction perovskite solar cells. Encapsulated tandem devices retain more than 90% of their initial performance after 600 h of operation at the maximum power point under 1 Sun illumination in ambient conditions.

14 SOLAR ENERGY↗
DOE OSTI2013

A Density Functional Theory Analysis of Trends in Glycerol Decomposition on Close-Packed Transition Metal Surfaces

We describe an accelerated density functional theory (DFT)-based computational strategy to determine trends in the decomposition of glycerol via elementary dehydrogenation, C–C, and C–O bond scission reactions on close-packed transition metal surfaces. Beginning with periodic DFT calculations on Pt(111), the thermochemistry of glycerol dehydrogenation on Pd(111), Rh(111), Cu(111) and Ni(111) is determined using a parameter-free, bond order-based scaling relationship. By combining the results with Brønsted–Evans–Polanyi (BEP) relationships to estimate elementary reaction barriers, free energy diagrams are developed on the respective metal surfaces, and trends concerning the relative selectivity and activity for C–C and C–O bond scission in glycerol on the various metals are obtained. The results are consistent with available theoretical and experimental literature and demonstrate that scaling relationships are capable of providing powerful insights into the catalytic chemistry of complex biomolecules.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2017

Isoreticular expansion of polyMOFs achieves high surface area materials

The concept of isoreticular chemistry has become a core principle in metal–organic framework (MOF) materials. Isoreticular chemistry has shown that organic ligands of different sizes, but with a common geometry/symmetry can be used to generate MOFs of related topologies, but with expanded pore sizes and volumes. In this study, polymer–MOF hybrid materials (polyMOFs) with a UiO (UiO = University of Oslo) architecture are shown to adhere to the principle of isoreticular expansion, generating polyMOFs with large surface areas and enhanced stability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗