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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Nanoscale Spatial Distribution of Supported Nanoparticles Controls Activity and Stability in Powder Catalysts for CO Oxidation and Photocatalytic H 2 Evolution

Here we discuss, supported metal nanoparticles are essential components of high-performing catalysts, and their structures are intensely researched. In comparison, nanoparticle spatial distribution in powder catalysts is conventionally not quantified, and the influence of this collective property on catalyst performance remains poorly investigated. Here, we demonstrate a general colloidal self-assembly method to control uniformity of nanoparticle spatial distribution on common industrial powder supports. We quantify distributions on the nanoscale using image statistics and show that the type of nanospatial distribution determines not only the stability, but also the activity of heterogeneous catalysts. Widely investigated systems (Au–TiO 2 for CO oxidation thermocatalysis and Pd–TiO 2 for H 2 evolution photocatalysis) were used to showcase the universal importance of nanoparticle spatial organization. Spatially and temporally resolved microkinetic modeling revealed that nonuniformly distributed Au nanoparticles suffer from local depletion of surface oxygen, and therefore lower CO oxidation activity, as compared to uniformly distributed nanoparticles. Nanoparticle spatial distribution also determines the stability of Pd–TiO 2 photocatalysts, because nonuniformly distributed nanoparticles sinter while uniformly distributed nanoparticles do not. This work introduces new tools to evaluate and understand catalyst collective (ensemble) properties in powder catalysts, which thereby pave the way to more active and stable heterogeneous catalysts.

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

Thermodynamics and Mechanism of a Photocatalyzed Stereoselective [2 + 2] Cycloaddition on a CdSe Quantum Dot

Colloidal quantum dots (QDs) have shown promise over the last few decades for a range of applications including single photon emission, in vivo imaging and photocatalysis. Recent experiments demonstrated that QDs impart stereoselectivity to triplet excited-state [2+2] cycloaddition reactions of alkenes photocatalyzed by the QD through self-assembly of the reagent molecules on the QD surface, but these experiments did not reveal the precise geometries of surface-bound molecules or their interactions with surface atoms. Here, a theoretical mechanistic approach is used to study such interactions for [2+2] cycloadditions of 4-vinylbenzoic acid derivatives on CdSe QDs. Spin-polarized periodic density functional theory (DFT) and non-periodic DFT calculations are deployed to determine the origin of the selectivity for the syn diastereomer of the resultant tetrasubstituted cyclobutane product via atomistic modelling of the CdSe surface and substrates, and determination of the thermodynamic energies of reactions for each step, the intermolecular interactions between the substrates, and the triplet state reaction paths. The calculations indicate that reaction selectivity arises from preferred binding of pairs through intermolecular interactions of substrate molecules on the QD surface in a syn-precursor structure followed by dimerization after triplet excitation. Further, these mechanisms are generalizable to other metal-enriched QD surfaces that have a similar surface structure to CdSe, such as InSe or CdTe. Design principles for anti diastereomer derivatives are also discussed.

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

Multi-Electron Transfer at H-Terminated p-Si Electrolyte Interfaces: Large Photovoltages under Inversion Conditions

Here, photovoltages for hydrogen terminated p-Si(111) in an acetonitrile electrolyte were quantified with methyl viologen [1,1'-(CH 3 ) 2 -4,4'-bipyridinium](PF 6 ) 2 , abbreviated MV 2+ , and [Ru(bpy) 3 ](PF 6 ) 2 , where bpy is 2,2'-bipyridine, that respectively undergo two and three one-electron transfer reductions. The reduction potentials, E°, of the two MV 2+ reductions occurred at energies within the forbidden bandgap, while the three [Ru(bpy) 3 ] 2+ reductions occurred within the continuum of conduction band states. Bandgap illumination resulted in reduction that were more positive than that measured with a degenerately doped n + -Si demonstrative of a photovoltage, Vph, that increased in the order: MV 2+/+ (260 mV) < MV +/0 (400 mV) < Ru 2+/+ (530 mV) ~ Ru +/0 (540 mV) ~ Ru 0/– (550 mV). Pulsed 532 nm excitation generated electron-hole pairs whose dynamics were nearly constant under depletion conditions and increased markedly as the potential was raised or lowered. A long wavelength absorption feature assigned to conduction band electrons provided additional evidence for the presence of an inversion layer. Collectively, the data reveal that the most optimal photovoltage, as well at the longest electron-hole pair lifetime and the highest surface electron concentration, occur when E° lies energetically within the unfilled conduction band states where an inversion layer is present. The bell-shaped dependence for electron-hole pair recombination with the surface potential was predicted by the time-honored SRH model providing a clear indication that this interface provides access to all four bias conditions, i.e., accumulation, flat band, depletion, and inversion. The implications of these findings for photocatalysis applications and solar energy conversion are discussed.

14 SOLAR ENERGY↗
DOE OSTI2025

Fano Resonance in CO 2 Reduction Catalyst Functionalized Quantum Dots

Molecular catalyst functionalized semiconductor quantum dots (QDs) are a promising modular platform for developing novel hybrid photocatalysts. The interaction between adsorbed catalyst vibrations and the QD electron intraband absorption can influence the photophysical properties of both the QD and the catalysts and potentially their photocatalysis. In CdSe QDs functionalized by the CO 2 reduction catalyst, Re(CO) 3 (4,4’-bipyridine-COOH)Cl, we observe that the transient Fano resonance signal resulting from coupling of the catalyst CO stretching mode and the QD conduction band electron mid-IR intraband absorption appears on an ultrafast time scale and decays with the electron population, irrespective of the occurrence of photoreduced catalysts. The Fano asymmetry factor increases with an increase in the adsorbed catalyst number and a decrease in QD sizes. The latter can be attributed to an enhanced charge transfer interaction between the more strongly quantum-confined QD conduction band and catalyst LUMO levels. These results provide a more in-depth understanding of interactions in excited QD-catalyst hybrid photocatalysts.

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

Shell Thickness and Heterogeneity Dependence of Triplet Energy Transfer between Core–Shell Quantum Dots and Adsorbed Molecules

Quantum dot (QD)-sensitized triplet energy transfer (TET) has found promising applications in photon upconversion and photocatalysis. However, the underlying mechanism of TET in the QD-acceptor complex remains unclear despite the well-developed TET theory for the molecular donor–acceptor systems. Herein, the coupling strength of TET from CdSe/CdS core–shell QDs to 9-anthracene carboxylic acid (ACA) was studied by measuring the TET rate as a function of shell thickness with time-resolved photoluminescence. The change of TET-coupling strength with increasing shell thickness was further compared to those of electron and hole transfers from QDs so that we could test whether QD-sensitized TET is mediated by the charge transfer virtual state and can be considered as simultaneous electron and hole transfers as in molecular donor–acceptor systems. The measured coupling strength of TET from the CdSe/CdS QD decreases exponentially with the CdS shell thickness r: |V|(r) = |V|(0)e –βr , with an exponential decay factor β of 0.19 Å –1 , which is smaller than the sum of the measured decay factors for electron transfer to methyl viologen (0.18 Å –1 ) and hole transfer to phenothiazine (0.29 Å –1 ) from the same QD. This inconsistency is explained by the broadening of QD shell thicknesses in the distance dependence study, which significantly modifies the TET-coupling strength and driving force, resulting in a shallower distance dependence of the TET rate constants. This study sheds light on the fundamental mechanisms of QD-sensitized TET reactions.

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

Photoelectrochemical Imaging of Charge Separation between MoS 2 Triangles and Insulating SiO 2 Support

The role of the insulating support in photocatalysis is poorly understood. Using high-resolution photo-scanning electrochemical microscopy (photo-SECM), we observed significant spatial charge separation in few-layer-thick molybdenum disulfide (MoS 2 ) triangles attached to a SiO 2 substrate. Spatially resolved surface photovoltage (SPV) measurements revealed that photogenerated holes migrate from MoS 2 to the SiO 2 surface and travel laterally over distances exceeding 2 μm, driven by the builtin electric field of ~1.7 kV/cm. In thicker and less uniform flakes, the charge separation is dominated by internal driving forces within MoS 2 , without significant contribution from SiO 2 . These findings underscore the importance of insulator–semiconductor interactions for effective charge separation, suggesting a new strategy for optimizing photocatalytic systems.

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

Au 76 (SC 6 H 4 - p -CH 3 ) 42 Square Quantum Platelet: One-Dimensional Growth of Quantum Rods Turns 90 Degrees

The growth pattern of atomically precise nanoclusters (NCs) is of fundamental interest, and the structural effect on their photoluminescence (PL) is important owing to their PL in the second near-infrared (NIR-II) range (900–1700 nm optoelectronically or 1000–1700 nm biologically) that holds great promise for optoelectronic and biomedical applications. Because of the small energy gaps required for NIR-II emission, the PL performance of NIR-II luminophores is largely limited by nonradiative processes. In this work, we discovered a Au 76 (p-MBT) 42 (Au 76 ) (p-MBTH = p-methylbenzenethiol) nanocluster featuring a face-centered cubic (fcc) core in a square shape (edge length: 1 nm). This square quantum platelet can be viewed as a side-facet (010) growth of the Au 52 (p-MBT) 32 (Au 52 ) rod, as opposed to the (001) facet growth. We found that Au 76 exhibits bright emission centered at 970 nm with a PL quantum yield (PLQY) of 30% in solution under ambient conditions, which can be further enhanced to 40% when the solution is deaerated. X-ray crystallography analysis coupled with time-resolved spectroscopy revealed that the nearly doubled PLQY compared to Au 52 (18.3%) was resulted from shorter Au–Au bond lengths in Au 76 (average 2.835 Å) than that in Au 52 (3.04 Å). This work provides important insights into the design of highly luminescent NCs, which are promising for photovoltaics, photocatalysis, and optoelectronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Role of the Character of the Excited State of Singly Reduced Rh 2 (II,II) Intermediates on Photocatalytic Activity

Singly reduced intermediates have recently been implicated as photoactive intermediates in a number of important reactions; however, their photophysical properties remain poorly understood. A series of dirhodium(II,II) complexes, cis-[Rh 2 (p-R-Form) 2 (bncn) 2 ] 2+ (bncn = benzo[c]cinnoline; p-R-Form = N,N′-di-p-R-phenylformamidinate), where R = −OCH 3 (1), −CH 3 (2), −H (3), −F (4), −Cl (5), and −CF 3 (6), and their respective radical anions were prepared and their excited state properties were investigated. Complex 3 acted as a singlemolecule photocatalyst for H 2 production with red light. Substitution on the formamidinate ligands in 1−6 affected the energy of the Rh 2 (δ*)/Form(π,nb) highest occupied molecular orbital (HOMO), consistent with the metal/ligand-to-ligand charge transfer ( 1 ML-LCT) absorption maxima and the 3 ML-LCT excited state lifetime, ranging from 1.6 ns in 1 to 54 ns in 6 in CH 3 CN. The highest turnover number for photocatalytic H 2 evolution was observed for 3, and the lowest values were for 1 and 6. The radical anion, [Rh 2 ] − , formed during photocatalysis, was shown to absorb a photon and undergo a second reduction. The lifetimes of the doublet excited states of [3] − and [6] − were 0.49 and 0.24 ns, respectively. Calculations showed that the lowest energy excited state in [3] − was 2 ML-LCT, whereas that in [6] − was a bncn − → Rh 2 (σ*) ligand-to-metal charge transfer ( 2 LMCT) state. The 2 LMCT state stabilized across the series from [1] − to [6] − , pointing at its role in modulating the photophysical properties. This work highlights the importance of the reductive quenching of [Rh 2 ] − and the generation of the doubly reduced species to effectively catalyze hydrogen evolution.

Absorption↗
DOE OSTI2022

Oxo dicopper anchored on carbon nitride for selective oxidation of methane

Abstract Selective conversion of methane (CH 4 ) into value-added chemicals represents a grand challenge for the efficient utilization of rising hydrocarbon sources. We report here dimeric copper centers supported on graphitic carbon nitride (denoted as Cu 2 @C 3 N 4 ) as advanced catalysts for CH 4 partial oxidation. The copper-dimer catalysts demonstrate high selectivity for partial oxidation of methane under both thermo- and photocatalytic reaction conditions, with hydrogen peroxide (H 2 O 2 ) and oxygen (O 2 ) being used as the oxidizer, respectively. In particular, the photocatalytic oxidation of CH 4 with O 2 achieves >10% conversion, and >98% selectivity toward methyl oxygenates and a mass-specific activity of 1399.3 mmol g Cu −1 h −1 . Mechanistic studies reveal that the high reactivity of Cu 2 @C 3 N 4 can be ascribed to symphonic mechanisms among the bridging oxygen, the two copper sites and the semiconducting C 3 N 4 substrate, which do not only facilitate the heterolytic scission of C-H bond, but also promotes H 2 O 2 and O 2 activation in thermo- and photocatalysis, respectively.

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

Direct observation of ultrafast symmetry reduction during internal conversion of 2-thiouracil using Coulomb explosion imaging

The photochemistry of heterocyclic molecules plays a decisive role for processes and applications like DNA photo-protection from UV damage and organic photocatalysis. The photochemical reactivity of heterocycles is determined by the redistribution of photoenergy into electronic and nuclear degrees of freedom, initially involving ultrafast internal conversion. Most heterocycles are planar in their ground state and internal conversion requires symmetry breaking. To lower the symmetry, the molecule must undergo an out-of-plane motion, which has not yet been observed directly. Here we show using the example of 2-thiouracil, how Coulomb explosion imaging can be utilized to extract comprehensive information on this molecular deformation, linking the extracted deplanarization of the molecular geometry to the previously studied temporal evolution of its electronic properties. Particularly, the protons of the exploded molecule are well-suited messengers carrying rich information on its geometry at distinct times after electronic excitation. We expect that our new analysis approach centered on these peripheral protons can be adapted as a general concept for future time-resolved studies of complex molecules in the gas phase.

Chemical physics↗
DOE OSTI2023

pyGWBSE: a high throughput workflow package for GW-BSE calculations

Abstract We develop an open-source python workflow package, py GWBSE to perform automated first-principles calculations within the GW-BSE (Bethe-Salpeter) framework. GW-BSE is a many body perturbation theory based approach to explore the quasiparticle (QP) and excitonic properties of materials. GW approximation accurately predicts bandgaps of materials by overcoming the bandgap underestimation issue of the more widely used density functional theory (DFT). BSE formalism produces absorption spectra directly comparable with experimental observations. py GWBSE package achieves complete automation of the entire multi-step GW-BSE computation, including the convergence tests of several parameters that are crucial for the accuracy of these calculations. py GWBSE is integrated with Wannier90 , to generate QP bandstructures, interpolated using the maximally-localized wannier functions. py GWBSE also enables the automated creation of databases of metadata and data, including QP and excitonic properties, which can be extremely useful for future material discovery studies in the field of ultra-wide bandgap semiconductors, electronics, photovoltaics, and photocatalysis.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Delayed fluorescence from a zirconium(iv) photosensitizer with ligand-to-metal charge-transfer excited states

Advances in chemical control of the photophysical properties of transition-metal complexes are revolutionizing a wide range of technologies, particularly photocatalysis and light-emitting diodes, but they rely heavily on molecules containing precious metals such as ruthenium and iridium. While the application of earth-abundant ‘early’ transition metals in photosensitizers is clearly advantageous, a detailed understanding of excited states with ligand-to-metal charge transfer (LMCT) character is paramount to account for their distinct electron configurations. Here we report an air- and moisture-stable, visible lightabsorbing Zr(iv) photosensitizer, Zr( Mes PDP Ph ) 2 , where [ Mes PDP Ph ] 2- is the doubly deprotonated form of [2,6-bis(5-(2,4,6- trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. This molecule has an exceptionally long-lived triplet LMCT excited state (τ = 350 μs), featuring highly efficient photoluminescence emission (Ф = 0.45) due to thermally activated delayed fluorescence emanating from the higher-lying singlet configuration with significant LMCT contributions. Zr( Mes PDP Ph ) 2 engages in numerous photoredox catalytic processes and triplet energy transfer. Our exploration offers a blueprint for future photosensitizer development featuring early transition metals and excited states with significant LMCT contributions.

14 SOLAR ENERGY↗
DOE OSTI2021

Flow and extraction of energy and charge carriers in hybrid plasmonic nanostructures

Strong interactions of electromagnetic fields with plasmonic nanomaterials have been exploited in various applications. These applications have centred on plasmon-enhanced scattering rates in nearby molecules or plasmon-induced heating. A question that has emerged recently is whether it is possible to use plasmonic nanostructures in a range of hot electron (hole) applications, including photocatalysis, photovoltaics and photodetection. These applications require coupling of a plasmonic component, which amplifies the interaction of light with the material, to an attached non-plasmonic component that extracts this energy in the form of electronic excitations to perform a function. Here, in this Perspective, we discuss recent work in the emerging field of hybrid plasmonics. We focus on fundamental questions related to the nanoscopic flow of energy and excited charge carriers in these multicomponent materials. We also address critical misconceptions, challenges and opportunities that require more attention.

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

Visualization of dynamic polaronic strain fields in hybrid lead halide perovskites

Excitation localization involving dynamic nanoscale distortions is a central aspect of photocatalysis, quantum materials and molecular optoelectronics. Experimental characterization of such distortions requires techniques sensitive to the formation of point-defect-like local structural rearrangements in real time. Here, we visualize excitation-induced strain fields in a prototypical member of the lead halide perovskites via femtosecond resolution diffuse X-ray scattering measurements. Here, this enables momentum-resolved phonon spectroscopy of the locally distorted structure and reveals radially expanding nanometre-scale strain fields associated with the formation and relaxation of polarons in photoexcited perovskites. Quantitative estimates of the magnitude and shape of this polaronic distortion are obtained, providing direct insights into the dynamic structural distortions that occur in these materials. Optical pump–probe reflection spectroscopy corroborates these results and shows how these large polaronic distortions transiently modify the carrier effective mass, providing a unified picture of the coupled structural and electronic dynamics that underlie the optoelectronic functionality of the hybrid perovskites.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Ultrafast hot-hole injection modifies hot-electron dynamics in Au/p-GaN heterostructures

A fundamental understanding of hot-carrier dynamics in photo-excited metal nanostructures is needed to unlock their potential for photodetection and photocatalysis. Despite numerous studies on the ultrafast dynamics of hot electrons, so far, the temporal evolution of hot holes in metal-semiconductor heterostructures remains unknown. Here, we report ultrafast (t < 200 fs) hot-hole injection from Au nanoparticles into the valence band of p-type GaN. The removal of hot holes from below the Au Fermi level is observed to substantially alter the thermalization dynamics of hot electrons, reducing the peak electronic temperature and the electron-phonon coupling time of the Au nanoparticles. First-principles calculations reveal that hot-hole injection modifies the relaxation dynamics of hot electrons in Au nanoparticles by modulating the electronic structure of the metal on timescales commensurate with electron-electron scattering. Overall, these results advance our understanding of hot-hole dynamics in metal-semiconductor heterostructures and offer additional strategies for manipulating the dynamics of hot carriers on ultrafast timescales. Photo-excited gold nanoparticles are shown to provide ultrafast and efficient hot-hole injection to the valence band of p-type GaN, substantially altering hot-electron dynamics in the nanoparticles and forming a basis to design hot-hole-based optoelectronics.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Accessing monomers from lignin through carbon–carbon bond cleavage

Lignin, the heterogeneous aromatic macromolecule found in the cell walls of vascular plants, is an abundant feedstock for the production of biochemicals and biofuels. Here, many valorization schemes rely on lignin depolymerization, with decades of research focused on accessing monomers through C–O bond cleavage, given the abundance of β–O–4 bonds in lignin and the large number of available C–O bond cleavage strategies. Monomer yields are, however, invariably lower than desired, owing to the presence of recalcitrant C–C bonds whose selective cleavage remains a major challenge in catalysis. In this Review, we highlight lignin C–C cleavage reactions, including those of linkages arising from biosynthesis (β–1, β–5, β–β and 5–5) and industrial processing (5–CH 2 –5 and α–5). We examine multiple approaches to C–C cleavage, including homogeneous and heterogeneous catalysis, photocatalysis and biocatalysis, to identify promising strategies for further research and provide guidelines for definitive measurements of lignin C–C bond cleavage.

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

Single crystal structure and photocatalytic behavior of grafted uranyl on the Zr-node of a pyrene-based metal–organic framework

Accurately characterizing actinide oxides bound to metal–organic frameworks (MOFs) is important for designing MOFs as radioactive waste sorbents and catalytic supports. Here, the zirconium MOF NU-1000 was post-synthetically modified through solvothermal deposition to include the uranyl (UO 2 2+ ) ion and characterized via single-crystal X-ray diffraction. Additonally, bond lengths derived from the diffraction pattern and Raman spectroscopy indicate that UO 2 2+ maintains its chemical properties upon deposition, while alcohol oxidation photocatalysis reveals photo-interactions between the pyrene linkers and the UO 2 2+ ion.

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

Transforming energy using quantum dots

Transforming and directing the flow of energy from one form (e.g., sunlight, electricity, etc.) to other useful forms of energy (e.g., electricity, chemical bonds, light, etc.) in an efficient and controllable manner is critical to meet the increasing energy demands and build a sustainable society. In search of such energy mediators, colloidal semiconductor nanocrystals, or quantum dots (QDs) are promising building blocks for building and designing systems that can efficiently capture light and convert and direct that energy into other useful forms of energy. In this work, we summarize recent advances using QDs in energy conversion architectures with the express goal of converting optical energy to other forms of energy, including electricity (i.e., photovoltaics) , photons with different energies (i.e., photo up- or downconversion), and chemical bonds (i.e., photocatalysis). The advantages of employing QDs over molecular chromophores in absorbing and then directing and converting optical energy are highlighted. Finally, we discuss ongoing challenges as well as unique opportunities associated with the use of QDs for transforming energy.

30 DIRECT ENERGY CONVERSION↗