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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 1,585 records · Page 88
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Laser-Induced Photoreduction of Iron(III) Oxide Nanoparticles Enhanced by the Presence of Organic Chromophores

Light-induced electron transfer between chromophoric organic matter and Fe(III)-oxides lies at the heart of aqueous Fe(II) fluxes in the photic zone of natural systems. Understanding this photoreductive dissolution process is also essential for developing water purification techniques based on this class of materials. Previously, optical transient absorption spectroscopy (TAS) measurements revealed that sub-picosecond relaxation times of photo-excited rhodamine B (RhB) dye increased when sorbed onto hematite nanoparticles (HNPs), consistent with electron transfer to the oxide. Here, in the present study, we exploit time-resolved X-ray absorption spectroscopy (XAS) at the Fe K-edge to follow the Fe oxidation state for this same process to (i) confirm that RhB photoexcitation leads to interfacial electron transfer and Fe reduction; and (ii) quantify the lifetime of injected electrons as a function of solution conditions. Regardless of RhB dye availability and pH, direct band gap photoexcitation of HNPs yields an Fe(II)-like small polaronic absorption signature with a lifetime of ~ 1 ns, an order of magnitude longer than previously reported. However, when RhB is present at low pH under conditions where dye favorably interacts with the positively charged hematite surface, a second relaxation process approaching microsecond timescales is observed that likely represents back-reaction with the photoexcited adsorbed dye. At pH above neutral, the efficiency of the interfacial electron transfer is diminished by weaker interaction between sorbed dye and particle surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Elucidating Charge Separation Dynamics in a Hybrid Metal–Organic Framework Photocatalyst for Light-Driven H 2 Evolution

Metal organic frameworks (MOFs) have emerged as novel scaffolds for artificial photosynthesis due to their unique capability in incorporating homogeneous photosensitizer and catalyst to their robust heterogeneous matrix. Here, we report the charge separation dynamics between molecular Ru-photosensitizer and Pt-catalyst, both of which were successfully incorporated into a Zr-MOF that demonstrates excellent activity and stability for light-driven H 2 generation from water. Using optical transient absorption (OTA) spectroscopy, we show that charge separation in this hybrid MOF occurs via electron transfer (ET) from Ru-photosensitizer to Pt-catalyst. Using Pt L 3 -edge X-ray transient absorption (XTA) spectroscopy, we observed the intermediate reduced Pt site, directly confirming the formation of charge separated state due to ET from Ru-photosensitizer and unraveling their key roles in photocatalysis.

14 SOLAR ENERGY↗
DOE OSTI2024

Deciphering Charge Transfer Processes in Transition Metal Complexes from the Perspective of Ultrafast Electronic and Nuclear Motions

Chemical transformations in charge transfer states result from the interplay between electronic dynamics and nuclear reorganization along excited-state trajectories. Here, in this study, we investigate the ultrafast structural dynamics following photoinduced electron transfer from the metal-metal-to-ligand charge transfer state of an electron donor, a Pt dimer complex, to a covalently linked electron acceptor group using ultrafast time-resolved wide-angle X-ray scattering and optical transient absorption spectroscopy methods to disentangle the interdependence of the excited-state electronic and nuclear dynamics. Following photoexcitation, Pt-Pt bond formation and contraction takes up to 1 ps, much slower than the corresponding process in analogous complexes without electron acceptor groups. Because the Pt-Pt distance change is slow with respect to excited-state electron transfer, it can affect the rate of electron transfer. These results have potential impacts on controlling electron transfer rates via structural alterations to the electron donor group, tuning the charge transfer driving force.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Ligand Mediation of Vectorial Charge Transfer in Cu(I)diimine Chromophore–Acceptor Dyads

In this paper, we present the photoinduced charge separation dynamics of four molecular dyads composed of heteroleptic Cu(I)bis(phenanthroline) chromophores linked directly to the common electron acceptor naphthalene diimide. The dyads were designed to allow us to (1) detect any kinetic preference for directionality during photoinduced electron transfer across the heteroleptic complex and (2) probe the influence of excited-state flattening on intramolecular charge separation. Singular value decomposition of ultrafast optical transient absorption spectra demonstrates that charge transfer occurs with strong directional preference, and charge separation occurs up to 35 times faster when the acceptor is linked to the sterically blocking ligand. Further, the charge-separated state in these dyads is stabilized by polar solvents, resulting in dramatically longer lifetimes for dyads with minimal substitution about the Cu(I) center. Finally, this unexpected but exciting observation suggests a new approach to the design of Cu(I)bis(phenanthroline) chromophores that can support long-lived vectorial charge separation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Emergence of Rapid Oxygen Surface Exchange Kinetics during in Situ Crystallization of Mixed Conducting Thin Film Oxides

The oxygen surface exchange kinetics of mixed ionic and electronic conducting oxides (MIECs) play a vital role in the efficiency of intermediate-to-high-temperature electrochemical devices. While there is increasing interest in low-temperature preparation of MIEC thin films, the impact of the resultant varied degrees of crystallinity on the surface exchange kinetics has not been widely investigated. Herein, we probe the effect of crystallization on oxygen surface exchange kinetics in situ, by applying an optical transmission relaxation (OTR) approach during annealing of amorphous films. OTR enables contact-free, in situ, and continuous quantification of the oxygen surface exchange coefficient ( k chem ); we previously applied it to Pr x Ce 1– x O 2-δ and SrTi 1– x Fe x O 3-δ thin films. In this work, the OTR approach was successfully extended to other mixed conducting thin film compositions for the first time (i.e., perovskite SrTi 0.65 Co 0.35 O 3-δ and Ruddlesden–Popper Sr 2 Ti 0.65 Fe 0.35 O 4±δ ), as well as to Pr 0.1 Ce 0.9 O 2-δ , enabling quantification of the k chem of their native surfaces and comparison of the behavior of films with different final crystal structures. All thin films were prepared by pulsed laser deposition at 25 or 700–800 °C and subject to subsequent thermal treatments with simultaneous OTR monitoring of k chem . The surface roughness, grain size, and crystallinity were evaluated by scanning probe microscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Fluorite Pr 0.1 Ce 0.9 O 2-δ films grown at 25 °C did not exhibit an increase in k chem after annealing, as they were already crystalline as grown at 25 °C. For all other compositions, OTR enabled in situ observation of both the crystallization process and the emergence of rapid surface exchange kinetics immediately upon crystallization. Perovskite SrTi 0.65 Co 0.35 O 3-δ and Ruddlesden–Popper Sr 2 Ti 0.65 Fe 0.35 O 4±δ thin films grown at 25 °C exhibited at least 1–2 orders of magnitude enhanced k chem after annealing compared with highly crystalline thin films grown at 800 °C, indicating the benefits of in situ crystallization.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Real-Time Visualization of Active Species in a Single-Site Metal–Organic Framework Photocatalyst

Here in this work, we report a new single-site photocatalyst (Co-Ru-UIO- 67(bpy)) based on a metal-organic framework platform with incorporated molecular photosensitizer and catalyst. We show that this catalyst not only demonstrates exceptional activity for light-driven H 2 production but also can be recycled without loss of activity. Using the combination of optical transient absorption spectroscopy and in situ X-ray absorption spectroscopy, we not only captured the key Co I intermediate species formed after ultrafast charge transfer from the incorporated photosensitizer but also identified the rate-limiting step in the catalytic cycle, providing insight into the catalysis mechanism of these single-site metal-organic framework photocatalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Plasmon-Enhanced Greenhouse Selectivity for High-Temperature Solar Thermal Energy Conversion

The greenhouse effect arises when thermal radiation is forced to undergo absorption and re-emission many times before escaping, while sunlight transmits largely unimpeded. Although this effect is responsible for global warming, it is generally weak in solid-state materials because radiation can be easily overpowered by other modes of heat transfer. Here, we report on the use of infrared plasmonic nanoparticles to enhance the greenhouse effect in transparent mesoporous materials. Local surface plasmon resonances in transparent conducting oxide nanoparticles (TCO NPs) selectively shorten the mean free path of thermal photons while maintaining high solar transmittance. The addition of a small amount of TCO NPs (<0.1% by volume) nearly halves the heat losses at 700 °C. This leads to an experimentally demonstrated effective thermal emittance of ~0.17 at 700 °C, which is the lowest reported value to date, among all selective surfaces and transparent insulating materials measured at 650 °C or above. Furthermore, our results show that plasmon-enhanced greenhouse selectivity (PEGS) is a promising mechanism for spectral control of radiative heat transfer, and more specifically, for conversion of minimally concentrated sunlight into high-temperature heat.

14 SOLAR ENERGY↗
DOE OSTI2022

Toward Improved Charge Separation through Conformational Control in Copper Coordination Complexes

The continued development of solar energy as a renewable resource necessitates the design and study of new approaches to sustaining photodriven charge separation (CS). To this end, we present a bioinspired approach in which triggered conformational changes are used to control electron transfer (ET) events. We report, photo-induced conformational rearrangements of a ligand are translated into changes in the coordination geometry and environment about a bound metal ion. Taking advantage of the differential coordination properties of Cu I and Cu II , these dynamics facilitate intramolecular ET from Cu I to the ligand to create a CS state. The synthesis and photophysical characterization of CuCl(dpaa R ) (dpaa = dipicolylaminoacetophenone, with R = H and OMe) is presented. These ligands incorporate a fluorophore into their framework that gives rise to a twisted intramolecular charge transfer (TICT) excited state. Excited state ligand twisting provides a tetragonal coordination geometry capable of capturing Cu II in the CS state when an internal ortho-OMe binding site is present (as in dpaaOMe). We employ NMR, IR, EPR, and optical spectroscopies, X-ray diffraction, and electrochemical methods to establish the ground state properties of the Cu I and Cu II complexes. We then investigate the photophysical dynamics of these Cu I complexes via time-resolved photoluminescence (TR-PL), and optical transient absorption (OTA) spectroscopies. We show that relative to controls lacking a TICT-active ligand, the lifetime of the CS state is enhanced ~1000-fold. Further, the presence of the ortho-OMe substituent greatly enhances the lifetime of the TICT* state and biases the coordination environment toward Cu II . The presence of Cu I decreases photoinduced degradation from 14 to <2% but does not result in significant quenching via ET. Factors affecting CS in these systems are discussed, laying the groundwork for our strategy toward solar energy conversion.

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

Characterization of Deformational Isomerization Potential and Interconversion Dynamics with Ultrafast X-ray Solution Scattering

Dimeric complexes composed of d 8 square planar metal centers and rigid bridging ligands provide model systems to understand the interplay between attractive dispersion forces and steric strain in order to assist the development of reliable methods to model metal dimer complexes more broadly. [Ir 2 (dimen) 4 ] 2+ (dimen = para-diisocyanomenthane) presents a unique case study for such phenomena, as distortions of the optimal structure of a ligand with limited conformational flexibility counteract the attractive dispersive forces from the metal and ligand to yield a complex with two ground state deformational isomers. Here, in this work, we use ultrafast X-ray solution scattering (XSS) and optical transient absorption spectroscopy (OTAS) to reveal the nature of the equilibrium distribution and the exchange rate between the deformational isomers. The two ground state isomers have spectrally distinct electronic excitations that enable the selective excitation of one isomer or the other using a femtosecond duration pulse of visible light. We then track the dynamics of the nonequilibrium depletion of the electronic ground state population—often termed the ground state hole—with ultrafast XSS and OTAS, revealing a restoration of the ground state equilibrium in 2.3 ps. This combined experimental and theoretical study provides a critical test of various density functional approximations in the description of bridged d 8 –d 8 metal complexes. The results show that density functional theory calculations can reproduce the primary experimental observations if dispersion interactions are added, and a hybrid functional, which includes exact exchange, is used.

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

Controlled Synthesis of SnO 2 Nanocrystals with Tunable Band Gaps

Tin(IV) oxide nanocrystals (SnO 2 NCs) have significant potential in various applications, with their performance closely related to their band gap. The band gap is influenced by the size and shape of the NCs, which can be precisely controlled by adjusting reaction conditions. In this study, we present deliberately designed synthesis protocols to produce high-quality SnO 2 NCs with tunable band gaps using different methods. Key factors affecting the synthesis include control of the oxidizing agent, reaction temperature, solvent selection, and reaction time optimization. The resulting NCs were characterized by using TEM, XRD, XPS, and optical spectroscopy. Notably, SnO 2 NCs synthesized by controlling the oxidizing agent (air injection) in a hot organic solution were smaller in size and exhibited abundant oxygen vacancies. In contrast, extending the reaction time or using ethanol as a solvent in hydrothermal systems facilitated larger spherical or rod-like SnO 2 NCs with fewer oxygen vacancies. Further analysis of the band gap and valence band maximum energy revealed that the abundant vacancies in SnO 2 NCs synthesized with the air-controlled hot organic solution method resulted in a narrower band gap and an upshifted valence band. These synthetic strategies illustrate the potential for deliberately designing SnO 2 NCs with optimized electronic structures for various applications.

band gap↗
DOE OSTI2018

Orbital controlled band gap engineering of tetragonal BiFeO 3 for optoelectronic applications

Bismuth ferrite BiFeO 3 (BFO) is an important ferroelectric material for thin-film optoelectronic sensing and potential photovoltaic applications. Its relatively large band gap, however, limits the conversion efficiency of BFO absorber-based PV devices. In this study, based on density functional theory calculations we demonstrate that with well-designed Fe-site elemental substitution, tetragonal BFO can exhibit a much lower fundamental band gap than conventional rhombohedral BFO without forming in-gap electronic states and unravel the underlying mechanisms. Cation atomic size, electronegativity, and crystallographic symmetry are evidenced as critical parameters to tailor the metal 3d – oxygen 2p orbital interactions and thus intrinsically modify electronic structure, particularly, the shape and character of the valence and conduction band edges. With reduced band gap, improved mobility, and uncompromised ferroelectric and magnetic ground states, the present results provide a new strategy of designing high symmetry BFO for efficient optoelectronic applications.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2024

Out-of-equilibrium dynamics of a grid-like Fe( II ) spin crossover dimer triggered by a two-photon excitation

The application of two-photon excitation (TPE) in the study of light-responsive materials holds immense potential due to its deeper penetration and reduced photodamage. Despite these benefits, TPE has been underutilised in the investigation of the photoinduced spin crossover (SCO) phenomenon. Here, we employ TPE to delve into the out-of-equilibrium dynamics of a SCO Fe II dimer of the form [Fe II (HL) 2 ] 2 (BF 4 ) 4 ·2MeCN (HL = 3,5-bis{6-(2,2'-bipyridyl)}pyrazole). Optical transient absorption (OTA) spectroscopy in solution proves that the same dynamics take place under both one-photon excitation (OPE) and TPE. The results show the emergence of the photoinduced high spin state in less than 2 ps and with a lifetime of 147 ns. Time-resolved photocrystallography (TRXRD) reveals a single molecular reorganisation within the first 500 ps following TPE. Additionally, variable temperature single crystal X-ray diffraction (VTSCXRD) and magnetic susceptibility measurements confirm that the thermal transition is silenced by the solvent. While the results of the OTA and TRXRD utilising TPE are intriguing, the high pump fluencies required to excite enough metal centres to the high spin state may impair its practical application. Nonetheless, this study sheds light on the potential of TPE for the investigation of the out-of-equilibrium dynamics of SCO complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Nonequilibrium phonon effects in midinfrared quantum cascade lasers

We investigate the effects of nonequilibrium phonon dynamics on the operation of a GaAs-based midinfrared quantum cascade laser over a range of temperatures (77–300 K) via a coupled ensemble Monte Carlo simulation of electron and optical-phonon systems. Nonequilibrium phonon effects are shown to be important below 200 K. At low temperatures, nonequilibrium phonons enhance injection selectivity and efficiency by drastically increasing the rate of interstage electron scattering from the lowest injector state to the next-stage upper lasing level via optical-phonon absorption. As a result, the current density and modal gain at a given field are higher and the threshold current density lower and considerably closer to experiment than results obtained with thermal phonons. By amplifying phonon absorption, nonequilibrium phonons also hinder electron energy relaxation and lead to elevated electronic temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗