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Results for “Density Functional calculations”

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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

The Interaction of K and O 2 on Au(111): Multiple Growth Modes of Potassium Oxide and Their Catalytic Activity for CO Oxidation

Abstract In industrial catalysis, alkali cations are frequently used to promote activity or selectivity. Scanning tunneling microscopy, ambient‐pressure X‐ray photoelectron spectroscopy, and density‐functional calculations were used to study the structure and reactivity of potassium oxides in contact with the Au(111) surface. Three different types of oxides (K 2 O 2 , K 2 O and KO y with y <0.5) were observed on top of the gold substrate at 300–525 K. Initially, small aggregates of K 2 O 2 /K 2 O (1–2 nm in size) were seen at the elbows of the herringbone structure. After increasing the K coverage (>0.15 ML), large islands of the oxide (20–40 nm in size) appeared. These islands contained a mixture of K 2 O and KO y ( y <0.5). A key correlation was found involving the structure, oxidation state, and chemical activity of the alkali oxide. The small aggregates of potassium oxide had a very high catalytic activity for the oxidation of CO, being much more than plain promoters.

Shi, Rui↗
DOE OSTI2022

The Interaction of K and O 2 on Au(111): Multiple Growth Modes of Potassium Oxide and Their Catalytic Activity for CO Oxidation

Abstract In industrial catalysis, alkali cations are frequently used to promote activity or selectivity. Scanning tunneling microscopy, ambient‐pressure X‐ray photoelectron spectroscopy, and density‐functional calculations were used to study the structure and reactivity of potassium oxides in contact with the Au(111) surface. Three different types of oxides (K 2 O 2 , K 2 O and KO y with y <0.5) were observed on top of the gold substrate at 300–525 K. Initially, small aggregates of K 2 O 2 /K 2 O (1–2 nm in size) were seen at the elbows of the herringbone structure. After increasing the K coverage (>0.15 ML), large islands of the oxide (20–40 nm in size) appeared. These islands contained a mixture of K 2 O and KO y ( y <0.5). A key correlation was found involving the structure, oxidation state, and chemical activity of the alkali oxide. The small aggregates of potassium oxide had a very high catalytic activity for the oxidation of CO, being much more than plain promoters.

Shi, Rui↗
DOE OSTI2021

Backbonding in Thorium(IV) and Uranium(IV) Diarsenido Complexes with t BuNC and CO

The coordination of tBuNC and CO with the diarsenido complexes (C 5 Me 5 ) 2 An(η 2 -As 2 Mes 2 ), An=Th, U, has been investigated. For the first time, a comparison between isostructural complexes of Th IV and U IV has been possible with CO; density functional calculations indicated an appreciable amount of π backbonding that originates from charge transfer from an actinide-arsenic sigma bond. The calculated CO stretching frequencies in the Th IV and U IV diarsenido complexes are consistent with the experimental measurements, both show large shifts to lower frequency. We demonstrate that the π backbonding is crucial to explaining the red shifts of CO frequency upon An IV complex formation. Interestingly, this interaction essentially correlates to the parallel orientation of π*(C-O) orbitals relative to the An-As bond.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Theoretical Investigation of the Adsorbate and Potential–Induced Stability of Cu Facets During Electrochemical CO 2 and CO Reduction

The activity and product selectivity of electrocatalysts for reactions like the carbon dioxide reduction reaction (CO 2 RR) are intimately dependent on the catalyst's structure and composition. While engineering catalytic surfaces can improve performance, discovering the key sets of rational design principles remains challenging due to limitations in modeling catalyst stability under operating conditions. Herein, we perform first-principles density functional calculations adopting implicit solvation methods with potential control to study the influence of adsorbates and applied potential on the stability of different facets of model Cu electrocatalysts. Using coverage dependencies extracted from microkinetic models, we describe an approach for calculating potential and adsorbate-dependent contributions to surface energies under reaction conditions, where Wulff constructions are used to understand the morphological evolution of Cu electrocatalysts under CO 2 RR conditions. Here we identify that CO*, a key reaction intermediate, exhibits higher kinetically and thermodynamically accessible coverages on (100) relative to (111) facets, which can translate into an increased relative stabilization of the (100) facet during CO 2 RR. Our results support the known tendency for increased (111) faceting of Cu nanoparticles under more reducing conditions and that the relative increase in (100) faceting observed under CO 2 RR conditions is likely attributed to differences in CO* coverage between these facets.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2022

Bioinspired mechanically interlocking holey graphene@SiO 2 anode

Mechanically interlocking structures that can enhance adhesion at the interface and regulate the stress distribution have been widely observed in biological systems. Inspired by the biological structures in the wings of beetles, we synthesized a holey graphene@SiO 2 anode with strong mechanical interlocking, characterized it electrochemically, and explained its performance by finite element analysis and density functional calculations. The mechanically interlocking structure enhances lithium-ion (Li + ) storage by transmitting the strain from SiO 2 to the holey graphene and by a mechano-electrochemical coupling effect. The interlocking fit hinders the abscission of SiO 2 and the distinctive structure reduces the stress and strain of SiO 2 during (de)lithiation. The positive mechano-electrochemical coupling effect preserves the amount of electrochemically active phase (Li x Si) during cycles and facilitates Li + diffusion. Therefore, the capacity shows only a slight attenuation after 8000 cycles (cycling stability), and the specific capacity is ~1200 mA h g –1 at 5 A/g (rate-performance). This study furnishes a novel way to design high-performance Li + /Na + /K + /Al 3+ anodes with large volume expansion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Monolayer TiS 2 Nanosheets on Au(111)–Structural Characterization and Effect of Edge Stability for Shape Control

Transition metal dichalcogenides are promising alternatives to noble metal catalysts, e.g., for (photo-)activation of greenhouse gases or hydrogenations. Herein, a direct synthetic route for 2D TiS 2 nanosheets on Au(111) by titanium deposition in the presence of a mild, organic, non-oxidizing sulfur source is presented. High-resolution scanning tunneling microscopy (STM) is used to gain atomic-level insights into the TiS 2 nanosheet morphology. In contrast to the literature, this protocol gains mostly hexagonal and truncated triangular nanosheets with an increased edge contrast in STM, analog to metallic edge states in MoS 2 . Synchrotron-based photoelectron spectroscopy allows insights into compositional details, specifically to distinguish different S sites on the TiS 2 sheets and other S species on the sample. Further, a minimum size is identified (9 S atoms side length), which underlines the importance of moiré reconstructions for stress relief. The TiS 2 sheets coexist with [Au]Ti 1 S 3 clusters, in which a single gold atom is alloyed into the surface and capped by three S atoms. Together with the finding of a critical sheet size, this points toward on-surface Ostwald ripening as a relevant process in the sheet formation. Ab-initio calculations (density functional theory) underscore that the chemical potential of S is an essential descriptor to maintain shape control.

2D nanomaterials↗
DOE OSTI2024

Stabilizing lithium superoxide formation in lithium-air batteries by Janus chalcogenide catalysts

Solid lithium peroxide (Li 2 O 2 ) is the major discharge product in Li-air batteries. However, the electronically insulating nature of Li 2 O 2 tends to affect the battery’s performance such as the polarization gap and cyclability. On the other hand, lithium superoxide (LiO 2 ), generated through a one-electron transfer process, offers greater electronic conductivity, lower charge transfer resistance, and thus reduced charge potential. Nevertheless, LiO 2 long-term stabilization as a final product remains a significant challenge. Here, in this study, we present the molybdenum (Mo)-based Janus chalcogenide family featuring asymmetric structures as a new generation of cathode catalysts for Li-air batteries. These catalysts demonstrate remarkable efficacy in stabilizing LiO 2 discharge products, even under high current densities of 5000 mA/g (corresponding to 0.5 mA/cm 2 ). Our density functional calculations provide an understanding of why the asymmetric Mo-Janus chalcogenides result in LiO 2 formation whereas the symmetric Mo-dichalcogenides produce Li 2 O 2 as the discharge product. These results pave the way to explore a new generation of advanced catalysts for superoxide-based Li-air batteries.

Chalcogenide↗
DOE OSTI2020

Highly Active Rhenium-, Ruthenium-, and Iridium-Based Dichalcogenide Electrocatalysts for Oxygen Reduction and Oxygen Evolution Reactions in Aprotic Media

Transition metal dichalcogenides (TMDCs) have garnered much attention recently due to their remarkable performance for different electrochemical systems. In this study, we report on the synthesis and catalysis of less studied TMDC nanoflakes (NFs) with a design space comprised of three transition metals (rhenium, ruthenium, and iridium) and three chalcogens (sulfur, selenium, and tellurium) for the oxygen reduction and evolution reactions (ORR and OER) in an aprotic hybrid electrolyte containing 0.1 M lithium bis(trifluoromethanesulfonypimide salt in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid and dimethyl sulfoxide. Our results indicate that among the tested catalysts, ReS2 exhibits the highest current density for both ORR and OER, beyond those of the state-of-the-art catalysts used in aprotic media with Li salts. We performed density functional calculations to provide a mechanistic understanding of the reactions in the ReS2 NFs/ionic liquid system. These novel bifunctional catalyst results could open a way for exploiting the unique properties of these materials in Li-O-2 batteries as well as other important electrochemical systems.

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

Homoleptic Perchlorophenyl “Ate” Complexes of Thorium(IV) and Uranium(IV)

The reaction of AnCl 4 (DME) n (An = Th, n = 2; U, n = 0) with 5 equiv of LiC 6 Cl 5 in Et 2 O resulted in the formation of homoleptic actinide-aryl “ate” complexes [Li(DME) 2 (Et 2 O)] 2 [Li(DME) 2 ][Th(C 6 Cl 5 ) 5 ] 3 ([Li][1]) and [Li(Et 2 O) 4 ][U(C 6 Cl 5 ) 5 ] ([Li][2]). Similarly, the reaction of AnCl 4 (DME)n (An = Th, n = 2; U, n = 0) with 3 equiv of LiC 6 Cl 5 in Et 2 O resulted in the formation of heteroleptic actinide-aryl “ate” complexes [Li(DME) 2 (Et 2 O)][Li(Et 2 O) 2 ][ThCl 3 (C 6 Cl 5 ) 3 ] ([Li][3]) and [Li(Et 2 O) 3 ][UCl 2 (C 6 Cl 5 ) 3 ] ([Li][4]). Density functional calculations show that the An–C ipso σ-bonds are considerably more covalent for the uranium complexes vs the thorium analogues, in line with past results. Additionally, good agreement between experiment and calculations is obtained for the 13 C ipso NMR chemical shifts in [Li][1] and [Li][3]. Here, the calculations demonstrate a deshielding by ca. 29 ppm from spin–orbit coupling effects originating at Th, which is a direct consequence of 5f orbital participation in the Th–C bonds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Vacancy-Driven Disorder and Elevated Dielectric Response in the Pyrochlore Pb 1.5 Nb 2 O 6.5

Lone pair-driven distortions are a hallmark of many technologically important lead (Pb)-based materials. Here, the role of Pb 2+ in polar perovskites is well understood and easily manipulated for applications in piezo- and ferroelectricity, but the control of ordered lone pair behavior in Pb-based pyrochlores is less clear. Crystallographically and geometrically more complex than the perovskite structure, the pyrochlore structure is prone to geometric frustration of local dipoles due to a triangular arrangement of cations on a diamond lattice. The role of vacancies on the O' site of the pyrochlore network has been implicated as an important driver for the expression and correlation of stereochemically active lone pairs in pyrochlores such as Pb 2 Ru 2 O 6.5 and Pb 2 Sn 2 O 6 . In this work we report on the structural, dielectric, and heat capacity behavior of the cation- and anion-deficient pyrochlore Pb 1.5 Nb 2 O 6.5 upon cooling. We find that local distortions are present at all temperatures that can be described by cristobalite-type cation ordering, and this ordering persists to longer length scales upon cooling. From a crystallographic perspective, the material remains disordered and does not undergo an observable phase transition. In combination with density function calculations, we propose that the stereochemical activity of the Pb 2+ lone pairs is driven by proximity to O' vacancies, and the crystallographic site disorder of the O' vacancies prohibits long range correlation of lone pair-driven distortions. This in turn prevents a low-temperature phase transition and results in an elevated dielectric permittivity across a broad temperature range.

36 MATERIALS SCIENCE↗
DOE OSTI2024

High-Throughput Screening for Boride Superconductors

A high-throughput screening using density functional calculations is performed to search for stable boride superconductors from the existing materials database. The workflow employs the fast frozen-phonon method as the descriptor to evaluate the superconducting properties quickly. Twenty-three stable candidates were identified during the screening. The superconductivity was obtained earlier experimentally or computationally for almost all found binary compounds. Previous studies on ternary borides are very limited. Here our extensive search among ternary systems confirmed superconductivity in known systems and found several new compounds. Among these discovered superconducting ternary borides, TaMo 2 B 2 shows the highest superconducting temperature of ∼12 K. Most predicted compounds were synthesized previously; therefore, our predictions can be examined experimentally. Our work also demonstrates that the boride systems can have diverse structural motifs that lead to superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Magic Numbers and Stabilities of Photoionized Water Clusters: Computational and Experimental Characterization of the Nanosolvated Hydronium Ion

The stability and distributions of small water clusters generated in a supersonic beam expansion are interrogated by tunable vacuum ultraviolet (VUV) radiation generated at a synchrotron. Time-of-flight mass spectrometry reveals enhanced population of various protonated water clusters (H + (H 2 O) $n$ ) based upon ionization energy and photoionization distance from source, suggesting there are "magic" numbers below the traditional $n$ = 21 that predominates in the literature. These intensity distributions suggest that VUV threshold photoionization (11.0-11.5 eV) of neutral water clusters close to the nozzle exit leads to a different nonequilibrium state compared to a skimmed molecular beam. This results in the appearance of a new magic number at 14. Metadynamics conformer searches coupled with modern density functional calculations are used to identify the global minimum energy structures of protonated water clusters between $n$ = 2 and 21, as well as the manifold of low-lying metastable minima. New lowest energy structures are reported for the cases of $n$ = 5, 6, 11, 12, 16, and 18, and special stability is identified by several measures. These theoretical results are in agreement with the experiments performed in this work in that $n$ = 14 is shown to exhibit additional stability, based on the computed second-order stabilization energy relative to most cluster sizes, though not to the extent of the well-known $n$ = 21 cluster. Other cluster sizes that show some additional energetic stability are $n$ = 7, 9, 12, 17, and 19. To gain insight into the balance between ion-water and water-water interactions as a function of the cluster size, an analysis of the effective two-body interactions (which sum exactly to the total interaction energy) was performed. In conclusion, this analysis reveals a crossover as a function of cluster size between a water-hydronium-dominated regime for small clusters and a water-water-dominated regime for larger clusters around $n$ = 17.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Sulfur-Donor Solvents Strongly Coordinate Pb 2+ in Hybrid Organic–Inorganic Perovskite Precursor Solutions

Strong coordination between Lewis-basic processing additives and the Lewis-acidic lead halide in hybrid organic–inorganic perovskite (HOIP) precursor solutions is required to solubilize the lead halide, and subsequently access the appropriate crystallization kinetics and attain the desired morphology of perovskite active layers. While oxygen-donor solvents and additives, such as dimethylformamide and dimethyl sulfoxide, are widely used for perovskite processing, we demonstrate here that “soft” sulfur-donor solvents exhibit stronger coordination to the “borderline soft” Lewis acid Pb 2+ center of PbI 2 relative to “hard” O-donor solvents in the precursor solution. The stronger coordination of S-donor solvents compared to O-donor solvents to Pb 2+ implies that such compounds can be useful additives to HOIP precursor solutions. Density-functional calculations of the enthalpy change resulting from the coordination of solvents to Pb 2+ provide direct numerical comparison of the strength of O-donor and S-donor coordination with Pb 2+ and expands the library of candidate S-donor compounds. Our results provide a roadmap for processing additive selection and expand the previously limited choice of perovskite processing additives to include strongly coordinating S-donor compounds.

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

Plasmon-Induced CO 2 Conversion on Al@Cu 2 O: A DFT Study

In plasmonic catalysis, localized surface plasmon was leveraged to drive chemical reactions. This approach is promising for catalyzing many challenging reactions at relatively low temperatures and pressures. We apply density functional calculations to provide the insight for the mechanism of plasmon-induced direct CO 2 dissociation on an Al@Cu 2 O core-shell structure, which was observed in a previous experimental study. We show that the interaction of Cu 2 O with CO 2 results in the accessible antibonding states of the adsorbed CO 2 in the range of visible light. Though the intrinsic activation barrier for direct CO 2 dissociation is as high as 3.6 eV, we find that an effective reaction barrier of CO 2 dissociation under photon excitation could be reduced by 2 eV. We also show that the charge transfer is more pronounced in the final state of the dissociation due to the strong hybridization of the dissociated species with Cu 2 O. These results thus provide an explanation for the visible-light-induced direct CO 2 dissociation at relatively low temperature and under ambient pressure. This finding also shows the promising impact of plasmonic catalysis in mitigating carbon emission.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Defect Stability in CdTe Based on Formation Energies and Migration Barriers

Native point defects are thought to play a key role in CdTe, either as compensation centers in intentionally doped material, as a source of conductivity in nominally undoped material, or as electron–hole recombination centers. Here, the discussion of their concentration and impact has often centered only on formation energies and transition levels. Using hybrid density functional calculations, including the effects of spin–orbit coupling (SOC), we discuss the stability of native point defects in CdTe based on their formation energies and migration barriers. We show that although Cd interstitials are the lowest energy donor defects, they are unstable at room temperature due to a low migration barrier. They are important for maintaining charge neutrality during growth or annealing at high temperatures, but once the material is brought to room temperature, they are not frozen in as often assumed and are expected to anneal out, leaving the other more stable defects to determine the conductivity. Taking this into account in the solution of the charge neutrality equation, we are able to predict the conductivity type and carrier concentrations that are in good agreement with experimental observations.

14 SOLAR ENERGY↗
DOE OSTI2025

Selective Adsorption of Thiol-Containing Molecules on Copper Sulfide Surfaces via Molecule–Surface Disulfide Bridges

Recent results in the fields of nanoenhanced agriculture and expanding interest in prebiotic chemistry have placed increased emphasis on understanding the chemically selective interaction of small molecules with the surfaces of metal sulfides. Here, we present an integrated experimental and computational study of the interaction of thiol-containing molecules with copper sulfide (covellite) surfaces in aqueous media. In situ Fourier-transform infrared (FTIR) measurements and ex situ X-ray photoelectron spectroscopy (XPS) measurements show that molecules bearing free thiol groups, including glutathione and cysteine, bind strongly to CuS (covellite) nanoparticles and to CuS (001) single crystals, while control studies show that similar molecules lacking the free thiol group exhibit much less binding. Additional experiments show that these thiol-containing molecules interact transiently with CuO nanoparticle surfaces but are readily removed by rinsing. The FTIR and XPS experiments demonstrate that adsorption of molecular thiols to CuS surfaces occurs in a chemically selective manner. Further experimental studies and density functional calculations show that the preferred mode of binding is through the surface S atoms, forming a Solid–S–S–Molecule disulfide linkage. While the role of disulfide linkages in controlling structure and function of proteins and other biomolecules is widely known, the formation of surface disulfide linkages as a motif for covalent molecular binding at surfaces has not been established previously.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Probing Self-Assembly in Arginine–Oleic Acid Solutions with Terahertz Spectroscopy and X-ray Scattering

A study of the formation of microstructures in the reaction of oleic acid with arginine elucidates dynamical self-assembly processes at the molecular level. Terahertz spectroscopy combined with density functional calculations reveals the initial hydrogen-bonding motifs in the assembly process, leading to the formation of micelles and vesicles. Small-angle X-ray scattering measurements allow for kinetic analysis of the growth processes of these nanostructures, revealing a prenucleation pathway of vesicles and micelles which lead to spongelike structures. This final stage of the assembly into spongelike aggregates is investigated with optical microscopy. The formed structures only occur at pH > 8 and are resistant to extreme acidic and basic conditions. Here, a mechanistic pathway to the formation of the spongelike aggregates is described.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

X-ray Emission Spectroscopy of Single Protein Crystals Yields Insights into Heme Enzyme Intermediates

Enzyme reactivity is often enhanced by changes in oxidation state, spin state, and metal-ligand covalency of associated metallocofactors. The development of spectroscopic methods for studying these processes coincidentally with structural rearrangements is essential for elucidating metalloenzyme mechanisms. Herein, we demonstrate the feasibility of collecting X-ray emission spectra of metalloenzyme crystals at a third-generation synchrotron source. In particular, we report the development of a von Hamos spectrometer for the collection of Fe Kβ emission optimized for analysis of dilute biological samples. Here, we further showcase its application in crystals of the immunosuppressive heme-dependent enzyme indoleamine 2,3-dioxygenase. Spectra from protein crystals in different states were compared with relevant reference compounds. Complementary density functional calculations assessing covalency support our spectroscopic analysis and identify active site conformations that correlate to high- and low-spin states. These experiments validate the suitability of an X-ray emission approach for determining spin states of previously uncharacterized metalloenzyme reaction intermediates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗