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DOE OSTI2024

Superconductivity in CH 4 and BH – 4 containing compounds derived from the high-pressure superhydrides

Inspired by the synthesis of the high-pressure Fm3m LaH 10 superconducting superhydride, systematic density functional theory (DFT) calculations are performed to study ternaries that could be derived from it by replacing two of the hydrogen atoms with boron or carbon and varying the identity of the electropositive element. Though many of the resulting alkali-metal and alkaline-earth MC 2 H 8 phases are predicted to be dynamically stable at mild pressures, their superconducting critical temperatures (T c s) are low because their metallicity results from the filling of an electride-like band. Substitution with a trivalent element leads to phases with substantial metal d- character at the Fermi level whose T c s are typically above 40 K. Here, among the MB 2 H 8 phases examined, KB 2 H 8 , RbB 2 H 8 and CsB 2 H 8 are predicted to be dynamically stable at very mild pressures, and their stability is rationalized by a DFT-Chemical Pressure analysis that elucidates the role of the M atom size. Quantum anharmonic effects strongly affect the properties of KB 2 H 8 , the highest predicted T c compound, near 10 GPa, but molecular dynamics simulations reveal it would decompose below its T c at this pressure. Nonetheless, at ca. 50 GPa KB 2 H 8 is predicted to be thermally stable with a superconducting figure of merit surpassing that of the recently synthesized LaBeH 8 .

36 MATERIALS SCIENCE↗

DOE OSTI2021

Isopropyl 3-deoxy-α- D - ribo -hexopyranoside (isopropyl 3-deoxy-α-D-glucopyranoside): evaluating trends in structural parameters

Isopropyl 3-deoxy-α-D-ribo-hexopyranoside (isopropyl 3-deoxy-α-D-glucopyranoside), C 9 H 18 O 5 , (I), crystallizes from a methanol–ethyl acetate solvent mixture at room temperature in a 4C1 chair conformation that is slightly distorted towards the C5 S C1 twist-boat form. A comparison of the structural parameters in (I), methyl α-D-glucopyranoside, (II), α-D-glucopyranosyl-(1→4)-D-glucitol (maltitol), (III), and 3-deoxy-α-D-ribo-hexopyranose (3-deoxy-α-D-glucopyranose), (IV), shows that most endocyclic and exocyclic bond lengths, valence bond angles and torsion angles in the aldohexopyranosyl rings are more affected by anomeric configuration, aglycone structure and/or the conformation of exocyclic substituents, such as hydroxymethyl groups, than by monodeoxygenation at C3. The structural effects observed in the crystal structures of (I)–(IV) were confirmed though density functional theory (DFT) calculations in computed structures (I) c –(IV) c . Exocyclic hydroxymethyl groups adopt the gauche–gauche (gg) conformation (H5 anti to O6) in (I) and (III), and the gauche–trans (gt) conformation (C4 anti to O6) in (II) and (IV). The O-glycoside linkage conformations in (I) and (III) resemble those observed in disaccharides containing β-(1→4) linkages.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

High-entropy Li-rich layered oxide cathode for Li-ion batteries

High-entropy oxides (HEOs) are emerging as promising cathode materials for Li-ion batteries (LIBs) due to their stable solid-state phase and compositional flexibility. Herein, we investigate the structural and electrochemical properties of a novel non-equimolar high-entropy cathode material, termed high-entropy Li-rich layered oxide (HE-LLO, Li 1.15 Na 0.05 Ni 0.19 Mn 0.56 Fe 0.02 Mg 0.02 Al 0.02 O 1.97 F 0.03 ), in comparison to a pristine Li-rich layered oxide (PR-LLO, Li1.2Ni0.2Mn0.6O2). The incorporation of multiple cations (Na + , Al 3+ , Mg 2+ , Fe 3+ ) and anion (F - ) into HE-LLO introduces compositional diversity, enhancing structural stability through the entropy stabilization effect. Theoretical calculations confirm a significantly higher configurational entropy in HE-LLO compared to PRLLO, supporting its high-entropy nature. Electrochemical evaluations demonstrate that HE-LLO exhibits considerable capacity retention, preserving 76.8 % of its discharge capacity at 0.5C after 200 cycles, compared to only 36.2 % for PR-LLO. Even under high-temperature conditions, HE-LLO outperformed PR-LLO, maintaining 76.1 % of its discharge capacity after 100 cycles at 5C, while PR-LLO retained only 12.4 %. These enhancements are attributed to the improved phase reversibility and higher Li + ion diffusion coefficients of HE-LLO, validated by ex-situ characterizations using a synchrotron X-ray technique, along with density functional theory (DFT) calculations. In conclusion, these findings highlight the promise of non-equimolar HEOs as a novel design strategy for highperformance cathode materials.

25 ENERGY STORAGE↗

DOE OSTI2023

Effect of the Precursor Chemistry on the Crystallization of Triple Cation Mixed Halide Perovskites

Triple cation, mixed halide perovskite compositions have been reported to be more thermally stable, exhibit fewer phase impurities, and show higher power conversion efficiency and better reproducibility than single cation perovskites. In this work, we explain the formation of Cs0.05FA0.81MA0.14Pb(I0.85Br0.15)3 via a multimodal in situ study combining structural information from synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS) and optical properties from photoluminescence (PL) spectroscopy with density functional theory calculations (DFT). The focus here is on the effects of the solvent and antisolvent during crystallization. The predominantly used solvents N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and the antisolvent chlorobenzene (CB) as well as the solvent-antisolvent-precursor interactions are investigated. Given the high elemental complexity and mutual interdependencies between solvent, antisolvent, and perovskite precursors, we found significant differences in the crystallization pathways. DMF-pure precursors show the formation of the DMF-containing intermediate phase and the nucleation of compositionally distinct perovskite phases, while when DMSO is added, only crystalline α- and δ-phases were found. In addition, the presence of DMSO helps the formation of α-perovskite. Coordination energy and bond order (BO) calculations support our experimental findings. Dripping of CB induces nucleation at room temperature, slows the α-phase formation rate, and appears to reduce the nucleation radius. These findings provide novel insights into solvent, antisolvent, and perovskite precursor interactions and their formation pathways. The complexity of interactions between solvents and reagents highlights the importance of understanding these effects to further improve the reproducibility and optimize processing conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

Experimental and Theoretical Insights into the Potential of V 2 O 3 Surface Coatings for Hydrogen Permeable Vanadium Membranes

A grand challenge of vanadium-based H 2 permeable membranes is the development of effective, cheap, and stable catalysts to facilitate H 2 dissociation and recombination. This article investigates a facile air treatment to form catalytically active vanadium oxide on the surfaces of dense vanadium foils. The treatment consisted of short air exposure followed by H 2 reduction at 823 K, which produced a well-faceted and nanocrystalline V 2 O 3 layer on the foil surfaces. The resulting membranes display stable H 2 permeability of 2 ± 0.25 × 10 -8 mol·m -1 ·s -1 ·Pa -0.5 , but transient declines in permeation are observed when operated at both elevated and reduced temperatures. DFT calculations revealed that V 2 O 3 (0001) surfaces display barriers and adsorption energies for H 2 dissociation/recombination that are comparable to known H 2 activation catalysts. It was found that H 2 dissociation is expected to proceed spontaneously on metal-terminated V 2 O 3 , with recombinative-desorption anticipated as the rate limiting step.

08 HYDROGEN↗

DOE OSTI2015

Electrospray production and collisional dissociation of lanthanide/methylsulfonyl anion complexes: Sulfur dioxide anion as a ligand

For this study, gas-phase lanthanide-SO 2 complexes, Ln(CH 3 SO 2 ) 3 (SO 2 ) - , were produced by collision induced dissociation (CID) of Ln(CH 3 SO 2 ) 4 - precursors prepared by electrospray ionization. For all lanthanides except Eu, CID of Ln(CH 3 SO 2 ) 4 - resulted in CH 3 loss to form Ln(CH 3 SO 2 ) 3 (SO 2 ) - , which spontaneously react with O 2 to form Ln(CH 3 SO 2 ) 3 (O 2 ) - . CID of Eu(CH 3 SO 2 ) 4 - produced only Eu(CH 3 SO 2 ) 3 - , with reduction from Eu(III) to Eu(II). For Ln = Yb and Sm, the Ln(CH 3 SO 2 ) 4 - underwent neutral ligand loss to form Ln(CH 3 SO 2 ) 3 - , which reacted with O 2 to yield Ln(CH 3 SO 2 ) 3 (O 2 ) - , recovering the Ln(III) oxidation state. The CID results show parallels to condensed-phase Ln 3+ /Ln 2+ redox chemistry. Density functional theory (DFT) calculations on Ln(CH 3 SO 2 ) 3 (SO 2 ) - for Ln = La, Yb and Lu reveal that SO 2 acts as a bidentate oxygen bound ligand for doublet ground-state La(CH 3 SO 2 ) 3 (SO 2 ) - and Lu(CH 3 SO 2 ) 3 (SO 2 ) - , while the ground state for Yb(CH 3 SO 2 ) 3 (SO 2 ) - is an open-shell singlet with a monodentate SO 2 ligand. Loss of CH 3 is computed to be much more favorable than neutral ligand loss for La(CH 3 SO 2 ) 4 - and Lu(CH 3 SO 2 ) 4 - , whereas both channels are comparable in energy for Yb(CH 3 SO 2 ) 4 - , in accord with the experiments. DFT results for fragmentation of Cu(CH 3 SO 2 ) 2 - reveal that formation of the organometallic complex, Cu(CH 3 SO 2 )(CH 3 ) - , is energetically most favorable, in agreement with contrasting fragmentation pathways of copper and lanthanide complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Leveraging Curvature on N–Doped Carbon Materials for Hydrogen Storage

Carbon sorbent materials have shown great promise for solid-state hydrogen (H 2 ) storage. Modification of these materials with nitrogen (N) dopants has been undertaken to develop materials that can store H 2 at ambient temperatures. In this work density functional theory (DFT) calculations are used to systematically probe the influence of curvature on the stability and activity of undoped and N-doped carbon materials toward H binding. Specifically, four models of carbon materials are used: graphene, [5,5] carbon nanotube, [5,5] D 5d -C 120 , and C 60 , to extract and correlate the thermodynamic properties of active sites with varying degrees of sp 2 hybridization (curvature). From the calculations and analysis, it is found that graphitic N-doping is thermodynamically favored on more pyramidal sites with increased curvature. In contrast, it is found that the hydrogen binding energy is weakly affected by curvature and is dominated by electronic effects induced by N-doping. These findings highlight the importance of modulating the heteroatom doping configuration and the lattice topology when developing materials for H 2 storage.

08 HYDROGEN↗

DOE OSTI2018

Elucidating CO 2 Chemisorption in Diamine-Appended Metal–Organic Frameworks

The widespread deployment of carbon capture and sequestration as a climate change mitigation strategy could be facilitated by the development of more energy-efficient adsorbents. Diamine-appended metal-organic frameworks of the type diamine-M 2 (dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc 4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) have shown promise for carbon-capture applications, although questions remain regarding the molecular mechanisms of CO 2 uptake in these materials. Here we leverage the crystallinity and tunability of this class of frameworks to perform a comprehensive study of CO 2 chemisorption. Using multinuclear nuclear magnetic resonance (NMR) spectroscopy experiments and van-der-Waals-corrected density functional theory (DFT) calculations for 13 diamine-M 2 (dobpdc) variants, we demonstrate that the canonical CO 2 chemisorption products, ammonium carbamate chains and carbamic acid pairs, can be readily distinguished and that ammonium carbamate chain formation dominates for diamine-Mg 2 (dobpdc) materials. In addition, we elucidate a new chemisorption mechanism in the material dmpn-Mg 2 (dobpdc) (dmpn = 2,2-dimethyl-1,3-diaminopropane), which involves the formation of a 1:1 mixture of ammonium carbamate and carbamic acid and accounts for the unusual adsorption properties of this material. Finally, we show that the presence of water plays an important role in directing the mechanisms for CO 2 uptake in diamine-M 2 (dobpdc) materials. Overall, our combined NMR and DFT approach enables a thorough depiction and understanding of CO 2 adsorption within diamine-M 2 (dobpdc) compounds, which may aid similar studies in other amine-functionalized adsorbents in the future.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Reactive Separations of CO/CO 2 mixtures over Ru–Co Single Atom Alloys

Reactive separations of CO/CO 2 mixtures are a promising pathway to lower the energy requirement of CO 2 hydrogenation to chemicals and fuels, with applications in the U.S. Navy’s seawater-to-fuel process. With the CO/CO 2 feedstock, a challenge is activating CO to produce heavier hydrocarbons while preventing CO 2 methanation, requiring low-temperature Fischer-Tropsch synthesis (FTS) catalysts. In this work, we demonstrate that a Ru–Co single atom alloy (SAA) catalyst produces C 5+ hydrocarbons at a rate of 11.7 μmol/s/g-cobalt (hexane basis) in a 50/50 CO/CO 2 stream with ≤1% CO 2 conversion. The reaction operates at a relatively low temperature (200 °C) and high gas hourly space velocity (GHSV: 84,000 mL/g/h) that is compatible with the upstream reverse water-gas shift reaction. Detailed experiments, catalyst characterizations, and density functional theory (DFT) calculations have been conducted to understand the active phase, the role of the Ru dopant, and catalyst restructuring that occurs at elevated temperatures (>200 °C). Ru dopants are found to promote the reduction of Co species, enabling catalytic activity for CO hydrogenation without pre-reduction, but may not enhance the FTS activity or desired C 5+ hydrocarbon selectivity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Density functional theory calculations of defect and fission gas properties in U-Si fuels

Accident tolerant fuels (ATF) are being developed in response to the Fukushima Daiichi accident in Japan. One of the options being pursued is U-Si fuels, such as the U 3 Si 2 and U 3 Si 5 compounds, which benefit from high thermal conductivity (metallic) compared to the UO 2 fuel (insulator or semi-conductor) used in current Light Water Reactors (LWRs). The U-Si fuels also have higher fissile density. In order to perform meaningful engineering scale nuclear fuel performance simulations, the material properties of the fuel, including the response to irradiation environments, must be known. Unfortunately, the data available for U-Si fuels are rather limited, in particular for the temperature range where LWRs would operate. The ATF HIP is using multi-scale modeling and simulations to address this knowledge gap. The present study investigates point defect and fission gas properties in U 3 Si 2 , which is one of the main fuel candidates, using density functional theory (DFT) calculations. Based on a few assumption regarding entropy contributions, defect and fission diffusivities are predicted. Even though uranium silicides have been shown to amorphize easily at low temperature, we assume that U 3 Si 2 remains crystalline under the conditions expected in Light Water Reactors (LWRs). The temperature and dose where amorphization occurs has not yet been well established.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2023

In-situ Synthesis of PuCl 3 and Corresponding Raman and Density Functional Theory Vibrational Modes

The experimental and calculated Raman spectrum of PuCl 3 has been reported for the first time. PuCl 3 is a primary species found in plutonium metal refinement, specifically in pyrochemical salt processes including multicycle direct oxide reduction, metal chlorination, and electrorefining. As such, Raman signatures of PuCl 3 could serve as potential forensic indicators of material process history. A novel technique for synthesizing PuCl 3 from the in situ chlorination of plutonium metal with HCl was developed to establish these signatures. Cerium metal surrogates were utilized to ensure optimization of the plutonium experiments and to minimize personnel exposure, and all experiments were carried out in a Raman reaction chamber designed for air-tight, high vacuum environments. In situ Raman spectroscopy was employed in conjunction with density functional theory (DFT) to investigate the vibrational modes of PuCl 3 . Associated mixed oxy and hydroxyl phases are also reported. The combined Raman and DFT results have eliminated inconsistencies in Raman mode assignments for the MX 3 family of metal chlorides having P6 3 /m symmetry, and IR modes derived from the DFT calculations are additionally presented. The data observed in this study are of potential interest to nuclear forensic analyses, nuclear sample aging, nuclear energy, plutonium processing, and stockpile stewardship.

36 MATERIALS SCIENCE↗

DOE OSTI2023

In-situ Synthesis of PuCl 3 and Corresponding Raman and Density Functional Theory Vibrational Modes

The experimental and calculated Raman spectrum of PuCl 3 has been reported for the first time. PuCl 3 is a primary species found in plutonium metal refinement, specifically in pyrochemical salt processes including multicycle direct oxide reduction, metal chlorination, and electrorefining. As such, Raman signatures of PuCl 3 could serve as potential forensic indicators of material process history. A novel technique for synthesizing PuCl 3 from the in-situ chlorination of plutonium metal with HCl was developed to establish these signatures. Cerium metal surrogates were utilized to ensure optimization of the plutonium experiments and to minimize personnel exposure, and all experiments were carried out in a Raman reaction chamber designed for air-tight, high vacuum environments. In-situ Raman spectroscopy was employed in conjunction with Density Functional Theory (DFT) to investigate the vibrational modes of PuCl 3 . Associated mixed oxy and hydroxyl phases are also reported. The combined Raman and DFT results have eliminated inconsistencies in Raman mode assignments for the MX 3 family of metal chlorides having P6 3 /m symmetry, and IR modes derived from the DFT calculations are additionally presented. The data observed in this study is of potential interest to nuclear forensic analyses, nuclear sample aging, nuclear energy, plutonium processing, and stockpile stewardship.

36 MATERIALS SCIENCE↗

DOE OSTI2023

DFT-guided flux synthesis of a family of layered titanates crystallizing in the lepidocrocite structure type

K 0.72 Fe 0.72 Ti 1.28 O 4 , a layered mixed-metal oxide belonging to the lepidocrocite structure type, was grown out of a molten KCl-KF mixture at 800 °C. The compound crystallizes in orthorhombic space group Cmcm and exhibits two-dimensional layers, composed of mixed Fe/Ti octahedra, separated by K ions that occupy the interlayer layer space and maintain charge balance. The targeted flux synthesis of the Ni, Cu, and Zn analogs was carried out after density functional theory (DFT) calculations predicted their formation to be energetically favorable, yielding K 0.77 Ni 0.38 Ti 1.62 O 4 , K 0.77 Cu 0.38 Ti 1.62 O 4 , and K 0.73 Zn 0.36 Ti 1.64 O 4 . Finally, all four compositions can be prepared via conventional solid-state technique by mixing and heating stoichiometric amounts of KNO 3 , appropriate transition metal oxide precursors, and TiO 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

SERS Theory: The Chemical Effect of Rhodamine 6G Adsorption on Silver Surfaces on Its Raman Spectrum

The SERS spectra of small organic molecules can be very sensitive to the physisorption or chemisorption of the molecules onto metallic surfaces. However it is challenging to model the results, as typical SERS substrates have a broad distribution of possible surface sites, and the populations of these sites and the SERS enhancement factors associated with them are very difficult to determine, especially when electromagnetic hot spots dominate the observed spectra. However it is important to account for these effects if one is to make a quantitative interpretation of measured SERS spectra. Indeed, the influence of different sites is important to both the electromagnetic and chemical mechanisms in SERS, where the former mechanism is concerned with local field enhancement effects arising from plasmon excitation, while the latter arises from the effect of chemisorption induced charged transfer on the Raman intensities, as can be modeled using static limit Raman intensity calculations[1]. Extensive studies have been performed on simple organics like pyridine bound to gold and silver clusters, and it was found that both the chemical and electrodynamic enhancement factors associated with binding pyridine in various orientations result in noticeable changes in the SERS spectra compared to the gas phase spectrum of pyridine.[2] One of the most important benchmark molecules for SERS and TERS studies is Rhodamine 6G (R6G). This is a strongly fluorescent dye molecule in solution, but when adsorbed onto SERS substrates, its fluorescence is quenched, and the resulting SERS intensities are among the largest observed for any molecule. As a result R6G has played a crucial role in the development of single molecule SERS, and in the development of SERS substrates. [3, 4] Previously, the SERS spectrum of R6G has been modeled as if it were a gas phase molecule. Jensen and Schatz[5] incorporated resonant effects as well as vibronic effects and found that resonance excitation to the S1 excited state of R6G resulted in good agreement with spectra obtained from SERS experiments. In another study, intermolecular charge transfer was studied with R6G bound to a Ag2 cluster.[6] It is unclear if a cluster model as small as two silver atoms can accurately capture the surface effects of a nanoparticle with thousands of atoms. Recent single molecule, ultra high vacuum, tip enhanced Raman spectroscopy (SM-UHV-TERS) measurements for R6G[3] were moderately reproduced by gas phase resonance Raman calculations. Of particular importance in this work was the comparison of TERS spectra at room temperature with those at a low temperature of 10K. It was found that lowering the temperature led to both sharpening of the vibrational spectra, and shifts in the spectra (up to 20 cm-1 with some lines shifting red, some blue and some not at all), suggesting that the molecule transitions from a largely gas phase molecule at room temperature to being strongly chemisorbed at 10K. A potential energy distribution analysis performed by Chulhai and Jensen[3] showed that the modes that were shifted from the gas phase had vibrational activity associated with the ethylamine component of R6G, which is the likely moiety associated with Raman calculations. What remains unresolved concerning the SM-UHV-TERS studies of R6G is direct evidence that the chemical binding of R6G to a silver nanoparticle surfaces causes frequency shifts and/or intensity changes in the Raman scattering intensities of specific modes, especially those involving vibrations of the ethylamine moiety. The theoretical work described in this chapter directly addresses this issue by calculating Raman spectra for R6G interacting with a 20-atom silver cluster, and with a flat Ag(111) surface. Tetrahedral Ag20 is chosen for part of these studies as it provides a model nanoparticle structure that has a well-defined fragment of a (111) surface, as well as having edges and vertices that can be used to model coordinately unsaturated sites on a particle. In addition, Ag20 has been found to provide a reasonable reference structure for determining the chemical contribution to SERS enhancements.[1] Two different geometries of the Ag20/R6G system are considered in the present study, corresponding to two different binding configurations of R6G to a pristine Ag(111) surface. In addition, we study adsorption of R6G on a flat Ag(111) surface of using plane wave DFT calculations to determine optimized geometries and normal mode properties.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2022

Catalytic N–H Bond Formation Promoted by a Ruthenium Hydride Complex Bearing a Redox-Active Pyrimidine-Imine Ligand

The synthesis of a piano-stool ruthenium hydride, [(η 5 -C 5 Me 5 )Ru(PmIm)H] (PmIm = (N-(1,3,5-trimethylphenyl)-1-(pyrimidin-2-yl)ethan-1-imine) for the dual purpose of catalytic dihydrogen activation and subsequent hydrogen atom transfer for the formation of weak chemical bonds is described. Introduction of a neutral, potentially redox-active PmIm supporting ligand was designed to eliminate the possibility of deleterious C(sp 2 )–H reductive coupling and elimination that has been identified as a deactivation pathway with related rhodium and iridium catalysts. Treatment of [(η 5 -C 5 Me 5 )RuCl 2 ] n with one equivalent PmIm ligand in the presence of zinc and sodium methoxide resulted in isolation of the diruthenium complex, [(η 5 -C 5 Me 5 )Ru(PmIm)] 2 , arising from C–C bond formation between two PmIm chelates. Addition of H 2 to the ruthenium dimer under both thermal and blue light irradiation conditions furnished the targeted hydride, [(η 5 -C 5 Me 5 )Ru(PmIm)H] which has a relatively weak DFT-calculated Ru–H bond dissociation free energy (BDFE) of 47.9 kcal/mol. Addition of TEMPO to [(η 5 -C 5 Me 5 )Ru(PmIm)H] generated the 17-electron metalloradical, [(η 5 -C 5 Me 5 )Ru(PmIm)], which was characterized by EPR spectroscopy. The C–C bond forming process was reversible as irradiation of [(η 5 -C 5 Me 5 )Ru(PmIm)] 2 generated [(η 5 -C 5 Me 5 )Ru(PmIm)H] and a piano-stool ruthenium complex containing an enamide ligand derived from H-atom ab-straction from the PmIm chelate. Equilibration studies were used to establish an experimental estimate of the effective Ru–H BDFE and a value of 50.8 kcal/mol was obtained, in agreement with the observed loss of H 2 and the DFT-computed value. Here, the ruthenium hydride was an effective catalyst for the thermal catalytic hydrogenation of TEMPO, acridine and a cobalt-imido complex and for the selective reduction of azobenzene to diphenylhydrazine using H 2 as the terminal reductant, highlighting the role of this complex is catalytic weak bond formation using H 2 as the stoichiometric reductant.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Degradation and fate of 2,4-dinitroanisole (DNAN) and its intermediates treated with Mg/Cu bimetal: Surface examination with XAS, DFT, and LDI-MS

In this article, a novel Mg-based bimetal reagent (Mg/Cu) was used as an enhanced reductive system to degrade insensitive munition 2,4-dinitroanisole (DNAN), a contaminant found in energetic-laden waste. Degradation of DNAN was significantly impacted by dissolved oxygen and studied in anoxic and oxic bimetal systems (i.e., purging with N 2 , air, or O 2 gas). Degradation occurred through sequential nitroreduction: first one nitro group was reduced (ortho or para) to form short-lived intermediates 2-amino-4-nitroanisole or 4-amino-2-nitroanisole (2-ANAN or 4-ANAN), and then subsequent reduction of the other nitro group formed 2,4-diaminoanisole (DAAN). The nitro-amino intermediates demonstrated regioselective reduction in the ortho position to 2-ANAN; Regioselectivity was also impacted by the anoxic/oxic environment. Under O 2 -purging DNAN degradation rate was slightly enhanced, but most notably O 2 significantly accelerated DAAN generation. DAAN also further degraded only in the oxygenated Mg/Cu system. Adsorption of DNAN byproducts to the reagent occurred regardless of anoxic/oxic condition, resulting in a partition of carbon mass between the adsorbed phase (27%-35%) and dissolved phase (59%-72%). Additional surface techniques were applied to investigate contaminant interaction with Cu. Density functional theory (DFT) calculations identified preferential adsorption structures for DNAN on Cu with binding through two O atoms of one or both nitro groups. X-ray absorption spectroscopy (XAS) measurements determined the oxidation state of catalytic metal Cu and formation of a Cu-O-N bond during treatment. Laser desorption ionization mass spectrometry (LDI-MS) measurements also identified intermediate 2-ANAN adsorbed to the bimetal surface.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2017

Characterization of Silicon Nanocrystal Surfaces by Multidimensional Solid-State NMR Spectroscopy

The chemical and photophysical properties of silicon nanocrystals (Si NCs) are strongly dependent on the chemical composition and structure of their surfaces. Here we use fast magic angle spinning (MAS) and proton detection to enable the rapid acquisition of dipolar and scalar 2D 1 H– 29 Si heteronuclear correlation (HETCOR) solid-state NMR spectra and reveal a molecular picture of hydride-terminated and alkyl-functionalized surfaces of Si NCs produced in a nonthermal plasma. 2D 1 H– 29 Si HETCOR and dipolar 2D 1 H– 1 H multiple-quantum correlation spectra illustrate that resonances from surface mono-, di-, and trihydride groups cannot be resolved, contrary to previous literature assignments. Instead the 2D NMR spectra illustrate that there is large distribution of 1 H and 29 Si chemical shifts for the surface hydride species in both the as-synthesized and functionalized Si NCs. However, proton-detected 1 H– 29 Si refocused INEPT experiments can be used to unambiguously differentiate NMR signals from the different surface hydrides. Varying the 29 Si evolution time in refocused INEPT experiments and fitting the oscillation of the NMR signals allows for the relative populations of the different surface hydrides to be estimated. This analysis confirms that monohydride species are the predominant surface species on the as-synthesized Si NCs. A reduction in the populations of the di- and trihydrides is observed upon functionalization with alkyl groups, consistent with our previous hypothesis that the trihydride, or silyl (*SiH 3 ), group is primarily responsible for initiating surface functionalization reactions. Density functional theory (DFT) calculations were used to obtain quantum chemical structural models of the Si NC surface and reproduce the observed 1 H and 29 Si chemical shifts. Furthermore, the approaches outlined here will be useful to obtain a more detailed picture of surface structures for Si NCs and other hydride-passivated nanomaterials.

29Si NMR↗

DOE OSTI2024

Accelerating defect predictions in semiconductors using graph neural networks

First-principles computations reliably predict the energetics of point defects in semiconductors but are constrained by the expense of using large supercells and advanced levels of theory. Machine learning models trained on computational data, especially ones that sufficiently encode defect coordination environments, can be used to accelerate defect predictions. Here, we develop a framework for the prediction and screening of native defects and functional impurities in a chemical space of group IV, III–V, and II–VI zinc blende semiconductors, powered by crystal Graph-based Neural Networks (GNNs) trained on high-throughput density functional theory (DFT) data. Using an innovative approach of sampling partially optimized defect configurations from DFT calculations, we generate one of the largest computational defect datasets to date, containing many types of vacancies, self-interstitials, anti-site substitutions, impurity interstitials and substitutions, as well as some defect complexes. We applied three types of established GNN techniques, namely crystal graph convolutional neural network, materials graph network, and Atomistic Line Graph Neural Network (ALIGNN), to rigorously train models for predicting defect formation energy (DFE) in multiple charge states and chemical potential conditions. We find that ALIGNN yields the best DFE predictions with root mean square errors around 0.3 eV, which represents a prediction accuracy of 98% given the range of values within the dataset, improving significantly on the state-of-the-art. We further show that GNN-based defective structure optimization can take us close to DFT-optimized geometries at a fraction of the cost of full DFT. The current models are based on the semi-local generalized gradient approximation-Perdew–Burke–Ernzerhof (PBE) functional but are highly promising because of the correlation of computed energetics and defect levels with higher levels of theory and experimental data, the accuracy and necessity of discovering novel metastable and low energy defect structures at the PBE level of theory before advanced methods could be applied, and the ability to train multi-fidelity models in the future with new data from non-local functionals. The DFT-GNN models enable prediction and screening across thousands of hypothetical defects based on both unoptimized and partially optimized defective structures, helping identify electronically active defects in technologically important semiconductors.

Rahman, Md Habibur (ORCID:000000027705984X)↗