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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Time-resolved absorption spectroscopic characterization of ultrafast laser-produced plasmas under varying background pressures

Time-resolved tunable laser absorption spectroscopy is used to characterize the physical properties of ultrafast laser-produced plasmas. The plasmas were produced from an Inconel target, with = 0.4 wt. % Al, using ~ 35 fs, ~800 nm laser pulses at varying Ar background pressures from 1-100 Torr. The absorption spectrum of atomic Al is measured with high spectral and temporal resolution when the probe laser is stepped across the selected Al transition at 394.4 nm. Spectral fitting is used to infer linewidths, kinetic temperature, Al column density, and pressure broadening coefficient. The late time physical properties of plasmas are compared for various pressure levels. Our studies highlight that a significant lower state population exists even at early times of ultrafast laser-produced plasma evolution and lower state population persistence decreases with increasing ambient pressure. We also show that the fundamental optical properties, such as pressure-broadening, can be measured using ultrafast laser-produced plasmas combined with laser absorption spectroscopy.

laser absorption spectroscopy, laser produced plas↗
DOE OSTI2025

Selective reduction in epitaxial SrFe 0.5 Co 0.5 O 2.5 and its reversibility

Oxygen-vacancy engineering in transition metal oxides enables programmable functionalities by modulating the valence states and local coordination of constituents. Here, we report the selective reduction of cobalt ions in epitaxial SrFe 0.5 Co 0.5 O 2.5 thin films under reducing gas environments, while iron ions remain unchanged. X-ray absorption spectroscopy reveals an absorption edge shift of 1.65 eV in the Co L-edge upon reduction, and multiplet simulations estimate a decrease in the average Co valence from Co 2.91+ to Co 2.00+ . This site- and element-specific reduction leads to the formation of a structurally distinct oxygen-deficient phase stabilized by oxygen vacancies at tetrahedral sites, as confirmed by density functional theory. Optical spectroscopy reveals an increase in the bandgap from 2.47 eV to 3.04 eV, accompanied by enhanced transparency. Furthermore, simultaneous in situ diffraction and transport measurements confirm fully reversible redox-driven transitions among three phases: reduced defective perovskite, brownmillerite, and oxygen-rich perovskite phases. These findings demonstrate that selective redox control in multi-cation oxides enables the realization of chemically and functionally distinct oxygen-deficient phases.

Fuel cells↗
DOE OSTI2022

Optical and photophysical properties of platinum benzoporphyrins with C 2v and D 2h symmetry

Platinum(II) mono- and di-benzoporphyrins carrying either ester or imide peripheral groups have been successfully synthesized. These benzoporphyrins possessing C 2v and D 2h symmetry were investigated using a range of techniques including UV-Vis and phosphorescence spectroscopy, differential pulse voltammetry, and transient absorption spectroscopy covering wide spatial and temporal ranges. Up to three oxidation and three reductions within the accessible potential window of dichlorobenzene were observed. Facile reductions for both mono- and dibenzoporphyrin derivatives, more so with imide peripheral substituents were witnessed. Computational studies were supportive of push-pull type interactions within these molecular systems. Here, the formation of triplet excited states within 5 ps after laser excitation was confirmed by femtosecond transient absorption studies. Nanosecond transient absorption studies coupled with phosphorescence lifetime studies revealed relatively rapid relaxation of the triplet excited states as compared to a controlled platinum porphyrin suggesting push-pull type interactions on a slower time scale.

DFT calculations↗
DOE OSTI2021

Synthesis of Non-Aqueous Neptunium(III) Halide Solvates from NpO 2

We report two Np(III) halides, NpI 3 (THF) 4 and NpBr 3 (THF) 4 , have been prepared and isolated in high yields as described in this work. Starting with neptunia (NpO 2 ), NpCl 4 (DME) 2 was first generated in an updated, higher yielding synthesis than what was previously reported by using HCl/HF. This material was then reduced with KC 8 , followed by subsequent ligand exchange, to generate NpBr 3 (THF) 4 and NpI 3 -(THF) 4 . Full characterization by single-crystal X-ray crystallography, 1 H NMR spectroscopy and electronic absorption spectroscopy confirmed the molecular formulas and oxidation states. These trivalent materials are straightforward to synthesize and can be used as starting materials for non-aqueous Np(III) chemistry, obviating the need for rare and restricted Np metal and elemental halogens.

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

The Role of Surface Hydroxyls in Dehydration and Dehydrogenation of Formic Acid on Fe 3 O 4 (001)

Understanding the role of surface structure and hydroxylation in catalytic reactions on metal oxide surfaces is important for developing a mechanistic insight into the complex interface processes. Here, we investigate the reactivity of formic acid on reconstructed Fe 3 O 4 (001) using a combination of X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, temperature-programmed reaction spectroscopy, low energy electron diffraction, and electronic structure calculations. We find that formic acid initially dissociates at low temperatures (< 80 K) into bidentate formate and a hydroxyl up to an initial dosed coverage of two HCOOH per Fe 3 O 4 (001) unit cell. At higher temperatures (> 450 K), formate largely decomposes along the dehydration pathway, producing CO and H 2 O, with dehydrogenation to CO 2 being a minority side reaction. As a first step, water formation leads to surface oxygen extraction via the Mars-van Krevelen mechanism. Computational studies reveal formate embedded in oxygen vacancies as a key intermediate in the CO formation mechanism. CO formation proceeds via two reaction pathways with desorption that peaks at 530 K on the hydroxyl-rich surface and 560 K on the hydroxyl-deficient surface. Atomic hydrogen coadsorption experiments and ab initio calculations reveal that the presence of surface hydroxyls reduces the CO formation barrier. Furthermore, these results highlight the complex interactions between substrate and intermediate species occurring during reactions on metal oxide surfaces.

Adsorption↗
DOE OSTI2023

Mediating Photochemical Reaction Rates at Lewis Acidic Rare Earths by Selective Energy Loss to 4f-Electron States

Manifesting chemical differences in individual rare earth (RE) element complexes is challenging due to the similar sizes of the tripositive cations and the corelike 4f shell. In this work, we disclose a new strategy for differentiating between similarly sized Dy 3+ and Y 3+ ions through a tailored photochemical reaction of their isostructural complexes in which the f-electron states of Dy 3+ act as an energy sink. Complexes RE(hfac) 3 (NMMO) 2 (RE = Dy (2-Dy) and Y (2-Y), hfac = hexafluoroacetylacetonate, and NMMO = N-methylmorpholine-N-oxide) showed variable rates of oxygen atom transfer (OAT) to triphenylphosphine under ultraviolet (UV) irradiation, as monitored by 1 H and 19 F NMR spectroscopies. Ultrafast transient absorption spectroscopy (TAS) identified the excited state(s) responsible for the photochemical OAT reaction or lack thereof. Competing sensitization pathways leading to excited-state deactivation in 2-Dy through energy transfer to the 4f electron manifold ultimately slows the OAT reaction at this metal cation. The measured rate differences between the open-shell Dy 3+ and closed-shell Y 3+ complexes demonstrate that using established principles of 4f ion sensitization may deliver new, selective modalities for differentiating the RE elements that do not depend on cation size.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Pushing the heterometal doping limit while preserving long-lived charge separation in a Ti-based MOF photocatalyst

Here, this study explores the nature, dynamics, and reactivity of the photo-induced charge separated excited state in a Fe 3+ -doped titanium-based metal organic framework (MOF), xFeMIL125-NH 2 , as a function of iron concentration. The MOF is synthesized with doping levels x = 0.5, 1 and 2 Fe node sites per octameric Ti-oxo cluster and characterized by powder x-ray diffraction, UV-vis diffuse reflectance, atomic absorption, and steady state Fe K-edge X-ray absorption spectroscopy. For each doping level, time-resolved X-ray transient absorption spectroscopy studies confirm the electron trap site role of the Fe sites in the excited state. Time scan data reveal multiexponential decay kinetics for the charge recombination processes which extend into the microsecond range for all three concentrations. A series of dye photodegradation studies, based on the oxidative decomposition of Rhodamine B, demonstrates the reactivity of the charge separated excited state and the photocatalytic capacity of these MOF materials compared to traditional heterometal-doped semiconductor photocatalysts.

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

Spin-lattice entanglement in CoPS 3

Complex chalcogenides in the 𝑀⁢PS 3 family of materials (𝑀 =Mn, Fe, Co, and Ni) display remarkably different phase progressions depending upon the metal center orbital filling, character of the P–P linkage, and size of the van der Waals gap. There is also a stacking pattern and spin-state difference between the “lighter” and “heavier” transition-metal-containing systems that places CoPS 3 at the nexus of these activities. Despite these unique properties, this compound is underexplored. Here, we bring together Raman scattering spectroscopy and infrared absorption spectroscopy with x-ray techniques to identify a structural component to the 119 K magnetic ordering transition. With temperature-dependent Raman scattering, we discover a set of magnon-phonon pairs that engages in avoided crossings below 𝑇 Néel . Lastly, these findings point to strong spin-phonon entanglement as well as opportunities to control these effects under external stimuli.

Magnetism↗
NASA NTRS1981

To Produce Nutrients for Plant Growth

On July 1, 1979 a new research program entitled "Development of a Prototype Experiment for Treating CELSS and PCELSS Wastes to Produce Nutrients for Plant Growth" was undertaken by the Massachusetts Institute of Technology and the Georgia Institute of Technology under grant from the National Aeronautics and Space Administration's Ames Research Center. The program was designed to be completed over a five-year period, with the work divided into two phases. Phase I covers approximately two years and involves the development of necessary facilities, chemical methodologies and models to allow meaningful experiments during Phase II, which will involve interfacing waste oxidation with plant growth. Homogeneous samples of freeze-dried human feces and urine have been prepared to assure comparability of results among various CELSS waste treatment research groups. A model of food processing wastes within a PCELSS was developed, and an initial batch of waste was prepared. An automated gas chromatographic system to analyze oxidizer effluents was designed and is operational. A state-of-the-art quantitative elemental analysis capability, built around inductively coupled plasma emission spectroscopy and atomic absorption spectroscopy using electrothermal atomization, has been demonstrated. A continuous flow wet oxidation system was constructed and is now operational.

Michael Modell↗
NASA NTRS2005

Instrumentation for Sensitive Gas Measurements

An improved instrument for optical absorption spectroscopy utilizes off-axis paths in an optical cavity in order to increase detection sensitivity while suppressing resonance effects. The instrument is well suited for use in either cavity ring-down spectroscopy (CRDS) [in which one pulses an incident light beam and measures the rate of decay of light in the cavity] or integrated cavity output spectroscopy (ICOS) [in which one uses a continuous-wave incident light beam and measures the power of light in the cavity as a function of wavelength]. Typically, in optical absorption spectroscopy, one seeks to measure absorption of a beam of light in a substance (usually a gas or liquid) in a sample cell. In CRDS or ICOS, the sample cell is placed in (or consists of) an optical cavity, so that one can utilize multiple reflections of the beam to increase the effective optical path length through the absorbing substance and thereby increase the sensitivity for measuring absorption. If an absorbing substance is not present in the optical cavity, one can utilize the multiple passes of the light beam to increase the sensitivity for measuring absorption and scattering by components of the optical cavity itself. It is desirable to suppress the effects of resonances in the cavity in order to make the spectral response of the cavity itself as nearly constant as possible over the entire wavelength range of interest. In the present instrument, the desired flattening of the spectral response is accomplished by utilizing an off-axis beam geometry to effectively decrease the frequency interval between longitudinal electromagnetic modes of the cavity, such that the resulting transmission spectrum of the cavity is nearly continuous: in other words, the cavity becomes a broad-band optical device.

OKeefe, Anthony↗
DOE OSTI2023

Ball Milling-Enabled Fe 2.4+ to Fe 3+ Redox Reaction in Prussian Blue Materials for Long-Life Aqueous Sodium-Ion Batteries

Aqueous Na-ion batteries using Prussian blue materials have inherent advantages on safety, material sustainability, and economic cost. However, it is challenging to obtain long term cycling stability because many redox reactions have poor intrinsic stability in water. In this report we demonstrate reversible Fe 2.4+ to Fe 3+ redox reaction of Prussian blue electrodes cycled in 17 m NaClO 4 water-in-salt electrolyte. The cubic phase c-Na 1.17 Fe[Fe(CN) 6 ]·0.35H 2 O) derived from monoclinic Prussian blue (m-Na 1.88 Fe[Fe(CN) 6 ]·0.7H 2 O) through ball milling delivers excellent cycling stability of >18,000 cycles with >90% capacity retention at 10C rate. The specific capacity is ~75 mAh/g and ~67 mAh/g at 1C and 10C rate, respectively. Systematic characterizations including electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy have verified the phase transition and iron oxidation state evolution, revealing the mechanism that enables the material’s high rate and long durability as the battery cathode.

25 ENERGY STORAGE↗
DOE OSTI2023

Understanding non-stochiometric deposition of multi-principal elemental NiCoCr thin films

Multi-principal elemental NiCoCr thin films with compositional flexibility have potential applications as a thermal barrier, corrosion-resistant coatings, and functional energy materials. In this study, the non-stochiometric multi-principal element alloy thin film formation behavior from an equiatomic ablation target of NiCoCr using pulsed laser deposition has been reported. Here, the effect of varying laser fluence on compositions of constituent elements in the NiCoCr thin films is systematically investigated. The increase in laser fluence typically increases the likelihood of acquiring undesirable particulates in thin films. Therefore, to maintain the film quality the laser fluences are adjusted within the range of 1.3–3.3 J·cm -2 , while the film thicknesses are kept constant at ~30 nm. Results show that the constituent elements are well-distributed in the formed NiCoCr thin films, however, having average atomic percentages of Ni and Cr twice as high as Co irrespective of the equiatomic composition of individual elements in the ablation target. This stochiometric alteration in the as-deposited thin films is governed by the rate of ablation and subsequent evaporation of each element, which is facilitated by the optical properties and vapor pressure of individual constituent elements. Additionally, electron energy-loss spectroscopy and x-ray absorption spectroscopy analysis reveal that the thin films have partially oxidized, with the Cr oxidation contributing the most to the O-K edge in comparison to Ni and Co. Finally, this study presents a laser-assisted pathway for fabricating compositionally flexible NiCoCr thin films and develops an understanding of the non-equilibrium laser-material interactions and how the intrinsic properties of constituent elements affect the stoichiometric flow of materials from the target to the final film deposition.

36 MATERIALS SCIENCE↗
NASA NTRS2005

High-Sensitivity Optical Techniques for Atmospheric Spectroscopy, Kinetics and Remote Sensing

A principal objective of the work supported by this Grant has been to use IntraCavity Laser Absorption Spectroscopy (ICLAS) to acquire data on weakly absorbing species of atmospheric interest that are not accessible, or cannot be determined with sufficient precision, using conventional spectroscopic instrumentation. The principal focus has been to adapt the existing instrument to carry out Kinetic studies using IntraCavity Absorption Spectroscopy (KICAS) in order to measure rate parameters for weakly absorbing, environmentally significant species. Additional related work has been carried out in collaboration with Prof. M.J. Molina's program on air pollution in the Mexico City Metropolitan Area on modeling the role of these species in atmospheric chemistry.

Source record↗
DOE OSTI2026

Metal oxide-promoted calcium cuprate catalysts for diol oxidative dehydrocyclization to lactones

Here, this work investigates structure-function relationships in electronically tunable, redox-active, basic Cu-Ca mixed metal oxide catalysts for oxidative dehydrocyclization of liquid diols to lactones. Compositional screening identified Ni 2+ and Zn 2+ as effective promoters that increase the surface Cu 2+ population by ∼1.7× and Cu-normalized activity for liquid 1,4-butanediol conversion to γ-butyrolactone by ∼3–4×. In situ Raman spectroscopy, in situ X-ray absorption spectroscopy (XAS), in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS), ex situ X-ray diffraction (XRD), and H 2 -temperature-programmed reduction (H 2 -TPR) show that Ni 2+ or Zn 2+ incorporation promotes the formation of Ca 0.82 Cu 1.00 O 2 nanoparticles under mild calcination conditions. This cuprate phase features stronger and shorter Cu–O bonds (1.90 Å) than inactive bulk CuO (1.95 Å) and square-planar Cu 2+ O 4 sites with enhanced d z2 electrophilicity, strengthening alkoxy adsorption. Pyridine-DRIFTS confirms the purely basic nature of the catalyst surface, while methanol-DRIFTS indicates Cu 2+ surface enrichment with Ni or Zn promotion, where Cu–O(Ca)–Cu sites can exist as amorphous domains or a truncation layer on crystalline nanoparticles.

09 BIOMASS FUELS↗
DOE OSTI2022

Role of Zeolite Structural Properties toward Iodine Capture: A Head-to-head Evaluation of Framework Type and Chemical Composition

This study evaluated zeolite-based sorbents for iodine gas [I2(g)] capture. Based on the framework structures and porosities, five zeolites, including two faujasite (FAU), one ZSM-5 (MFI), one mesoMFI, one ZSM-22 (TON), as well as two mesoporous materials, were evaluated for I2(g) capture at room temperature and 150 °C in an iodine-saturated environment. From these preliminary studies, the three best-performing zeolites were ion-exchanged with Ag+ and evaluated for I2(g) capture under similar conditions. Energy-dispersive X-ray spectroscopy data suggest that Ag-FAU frameworks were the materials with the highest capacity for I2(g) in this study, showing ~3× higher adsorption compared to Ag-mordenite (Ag-MOR) at room temperature, but X-ray diffraction measurements show that the faujasite structure collapsed during the adsorption studies because of dealumination. The Ag-MFI zeolites are decent sorbents in real-life applications, showing both good sorption capacities and higher stability. In-depth analyses and characterizations, including synchrotron X-ray absorption spectroscopy, revealed the influence of structural and chemical properties of zeolites on the performance for iodine adsorption from the gas phase.

aluminosilicate zeolites, iodine, porosity, acidit↗
DOE OSTI2026

Revealing key structures for reversible sulfur redox in amorphous polymeric sulfur

Amorphous polymeric sulfur cathodes, such as sulfurized polyacrylonitrile (SPAN), enable high-energy lithium-sulfur batteries without cobalt or nickel, leveraging abundant sulfur. However, the limited in situ understanding of their synthesis and electrochemistry has impeded targeted optimization. Here, in this study, we integrate operando high-energy total scattering with sulfur K-edge X-ray absorption spectroscopy to monitor SPAN's formation and cycling in real time. Our results show that S-C bond formation halts further fusion of cyclized polyacrylonitrile, fostering π-π stacking and a transition from long-chain to short-chain sulfur-critical for reversible sulfur redox. These features synergistically minimize polysulfide dissolution and charge-transfer resistance, enabling optimized SPAN to achieve high capacity retention over 1,000 cycles. Operando X-ray absorption spectroscopy reveals that residual protons drive thiol-thione tautomerism, with lithium replacement during the first discharge causing ~20% irreversible capacity loss. To enhance performance, minimizing -NH groups and expanding pyridine networks are key. These findings transform SPAN optimization from empirical tuning to mechanism‑guided engineering and point the way towards sulfur loadings and energy densities competitive with state‑of‑the‑art Li‑ion cathodes.

25 ENERGY STORAGE↗
DOE OSTI2025

Comparison of atomized mass and crater volume in laser ablation

The extent and dynamics of laser ablation are typically studied using crater imaging or by invoking the relationship between atomic emission and the mass removed. The former is a static view of a dynamic process and requires the accumulation of multiple shots in one location. The latter is complex and not absolute without calibration of the optical system with a standard of spectral radiance. We measure the mass of the atomized plume by laser atomic-absorption spectroscopy at 2 µs without external mass calibration; mass uncertainty in laser-ablation atomic absorption spectroscopy (LA-AAS) results from uncertainties in line area fitting, tabulated oscillator strengths, and partition functions as well as a <10% underrepresentation of the mass due to treating the plasma as a single thermodynamic equilibrium. The LA-AAS-measured mass pertains to the atoms in the probed charge states and does not include condensed or molecular species. The ablation efficiency at 300 mbar of helium varies significantly between the two focusing conditions used. A comparison of crater-derived masses and LA-AAS masses suggests that more defocused ablation may result in significant redeposition of material in the crater, distorting the conclusions from drilling studies and crater imaging. More focused ablation conditions result in crater-derived masses that exceed those measured with LA-AAS and suggest melt expulsion or phase explosion.

Merten, Jonathan [Arkansas State University, Jones↗
DOE OSTI2022

Connecting cation site location to alkane dehydrogenation activity in Ni/BEA catalysts

Ni-modified beta zeolite (Ni/BEA) catalysts activate carbon-hydrogen bonds in light alkanes, as demonstrated through isobutane reaction testing. Controlled synthesis of Ni/BEA allows for efficient introduction of ion-exchanged Ni sites at varying Ni loadings (0.43% - 1.8%). These catalysts exhibit site time yields (STY) for H2 production that increase with increasing Ni loading. A detailed analysis of secondary reactions and carbon deposition based on the relative molar flowrates of product C and H indicates that the observed increase in H2 STY with increasing Ni loading is likely attributed to both increasing alkane activation activity and increasing formation of hydrogen-deficient aromatic products retained within the catalyst pores. In situ diffuse-reflectance UV-visible-NIR absorbance and X-ray absorption spectroscopies indicate isolated, 4-coordinate Ni(2+) species for all loadings. Quantum mechanics/molecular mechanics modeling identifies two distinct Ni(2+) sites consistent with the structural characterization, but with differing relative stabilities due to their coordination environment. Computed reaction energetics for isobutane dehydrogenation demonstrate that the more stable Ni(2+) species at a six-membered 4Si-2Al ring, Ni-6MR, is less active for isobutane dehydrogenation than the less stable Ni(2+) at the five-membered 3Si-2Al ring, Ni-5MR. The differing local structure of the isolated cationic Ni sites in Ni/BEA offers a possible rationalization for the increased H2 STY observed at greater Ni loadings.

catalysts↗