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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 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Ultrafast Two-Electron Transfer in a CdS Quantum Dot–Extended-Viologen Cyclophane Complex

Time-resolved optical spectroscopies revealmulti-electron transfer from the biexcitonic state of a CdS quantum dot toan adsorbed tetracationic compoundcyclobis(4,4'-(1,4-phenylene)bipyridin-1-ium-1,4-phenylene-bis(methylene))(ExBox 4+ )to formboth the ExBox 3+ ∙and the doubly reduced ExBox2(+∙)statesfrom a single laser pulse. Electron transfer in the single-exciton regime occurs in 1 ps.At higher excitation powers the second electron transfertakes~5ps, which leadsto a mixture of redox states of the acceptor ligand.The doubly reducedExBox2(+∙)state has a lifetime of ~10 ns, while CdS + ∙:ExBox 3+ ∙recombines with multiple time constants, the longest of which is ~300 μs. The long-lived charge separation and ability to accumulate multiple charges on ExBox 4+ demonstrate the potential of the CdS:ExBox4+complex to serve as a platform for two-electron photocataly-sis.

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DOE OSTI2016

Solution Structures of Highly Active Molecular Ir Water-Oxidation Catalysts from Density Functional Theory Combined with High-Energy X-ray Scattering and EXAFS Spectroscopy

The solution structures of highly active Ir water-oxidation catalysts are elucidated herein by combining density functional theory, high-energy X-ray scattering (HEXS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. We find that the catalysts are Ir dimers with mono-μ-O cores and terminal anionic ligands, generated in situ through partial oxidation of a common catalyst precursor. The proposed structures are supported by 1 H and 17 O NMR, EPR, resonance Raman and UV–vis spectra, electrophoresis, etc. Our findings are particularly valuable to understand the mechanism of water oxidation by highly reactive Ir catalysts. Importantly, our DFT-EXAFS-HEXS methodology provides a new in situ technique for characterization of active species in catalytic systems.

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DOE OSTI2016

Extended-Charge-Transfer Excitons in Crystalline Supramolecular Photocatalytic Scaffolds

Coupling among chromophores in molecular assemblies is responsible for phenomena such as resonant energy transfer and intermolecular charge transfer. These processes are central to the fields of organic photovoltaics and photocatalysis, where it is necessary to funnel energy or charge to specific regions within the system. As such, a fundamental understanding of these transport processes is essential for developing new materials for photovoltaic and photocatalytic applications. Recently, photocatalytic systems based on photosensitizing perylene monomimide (PMI) chromophore amphiphiles were found to show variation in hydrogen gas (H 2 ) production as a function of nanostructure crystallinity. The 2D crystalline systems form in aqueous electrolyte solution, which provides a high dielectric environment where the Coulomb potential between charges is mitigated. This results in relatively weakly bound excitons that are ideal for reducing protons. Furthermore, in order to understand how variations in crystalline structure affect H 2 generation, two representative PMI systems are investigated theoretically using a modified Holstein Hamiltonian. The Hamiltonian includes both molecular Frenkel excitations (FE) and charge-transfer excitations (CTE) coupled nonadiabatically to local intramolecular vibrations. Signatures of FE/CTE mixing and the extent of electron/hole separation are identified in the optical absorption spectrum and are found to correlate strongly to the observed H 2 production rates. The absorption spectral signatures are found to sensitively depend on the relative phase between the electron and hole transfer integrals, as well as the diabatic energy difference between the Frenkel and CT exciton bands. Our analysis provides design rules for artificial photosynthetic systems based on organic chromophore arrays.

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DOE OSTI2016

Direct Observation of a Charge-Transfer State Preceding High-Yield Singlet Fission in Terrylenediimide Thin Films

Singlet exciton fission (SF) in organic chromophore assemblies results in the conversion of one singlet exciton (S 1 ) into two triplet excitons (T 1 ), provided that the overall process is exoergic, i.e., E (S 1 ) > 2 E (T 1 ). In this paper, we report on SF in thin polycrystalline films of two terrylene-3,4:11,12-bis(dicarboximide) (TDI) derivatives 1 and 2 , which crystallize into two distinct π-stacked structures. Femtosecond transient absorption spectroscopy (fsTA) reveals a charge-transfer state preceding a 190% T 1 yield in films of 1 , where the π-stacked TDI molecules are rotated by 23° along an axis perpendicular to their π systems. In contrast, when the TDI molecules are slip-stacked along their N–N axes in films of 2 , fsTA shows excimer formation, followed by a 50% T 1 yield.

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DOE OSTI2016

Influence of Anion Delocalization on Electron Transfer in a Covalent Porphyrin Donor–Perylenediimide Dimer Acceptor System

Photodriven electron transfer from a donor excited state to an assembly of electronically coupled acceptors has been proposed to enhance charge transfer efficiency in functional organic electronic materials. However, the circumstances under which this may occur are difficult to investigate in a controlled manner in disordered donor–acceptor materials. Here we investigate the effects of anion delocalization on electron transfer using zinc meso-tetraphenylporphyrin (ZnTPP) as a donor and a perylene-3,4:9,10-bis(dicarboximide) dimer as the acceptor (PDI 2 ). The PDI units of the dimer are positioned in a cofacial orientation relative to one another by attachment of the imide group of each PDI to the 4- and 5-positions of a xanthene spacer. Furthermore, the distal imide group of one PDI is linked to the para-position of one ZnTPP phenyl group to yield ZnTPP-PDI 2 . The data for the dimer are compared to two different ZnTPP-PDI monomer reference systems designed to probe electron transfer to each of the individual PDI molecules comprising PDI 2 . The electron transfer rate from the ZnTPP lowest excited singlet state to PDI 2 is increased by 50% relative to that in ZnTPP-PDI, when the data are corrected for the statistics of having two electron acceptors. Femtosecond transient IR absorption spectroscopy provides evidence that the observed enhancement in charge separation results from electron transfer producing a delocalized PDI 2 anion.

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DOE OSTI2016

Importance of Reducing Vapor Atmosphere in the Fabrication of Tin-Based Perovskite Solar Cells

Tin-based halide perovskite materials have been successfully employed in lead-free perovskite solar cells, but the tendency of these materials to form leakage pathways from p-type defect states, mainly Sn 4+ and Sn vacancies, causes poor device reproducibility and limits the overall power conversion efficiencies. Here, we present an effective process that involves a reducing vapor atmosphere during the preparation of tin-based halide perovskite solar cells to solve this problem, using MASnI 3 , CsSnI 3 and CsSnBr 3 as the representative absorbers. This process enables the fabrication of remarkably improved solar cells with power conversion efficiencies of 3.89%, 1.83% and 3.04% for MASnI 3 , CsSnI 3 and CsSnBr 3 respectively. The reducing vapor atmosphere process results in more than 20% reduction of Sn 4+ /Sn 2+ ratios, which leads to greatly suppressed carrier recombination, to a level comparable to their lead-based counterparts. These results mark an important step towards a deeper understanding of the intrinsic tin-based halide perovskite materials, paving the way to the realization of low-cost and lead-free Sn-based halide perovskite solar cells.

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DOE OSTI2017

Slow Equilibration between Spectroscopically Distinct Trap States in Reduced TiO 2 Nanoparticles

Understanding the nature of charge carriers in nanoscale titanium dioxide is important for its use in solar energy conversion, photocatalysis, and other applications. UV-irradiation of aqueous, colloidal TiO 2 nanoparticles in the presence of methanol gives highly reduced suspensions. Two distinct types of electron traps were observed and characterized by EPR and optical spectroscopies. The relative populations of the states depend on temperature, indicating a small energy difference, $ΔH$° = 3.0 ± 0.6 kcal/mol (130 ± 30 meV). Interconversion between the electron traps occurs slowly over the course of minutes to hours within the temperature range studied here, 0-50 °C. The slow time scale implies that interconversion involves changes in structure or stoichiometry, not just the movement of electrons. This occurrence of slow structural modification with changes in trap state occupancy is likely a general feature of reduced TiO 2 systems at thermodynamic equilibria or photostationary states and should be considered in the design of TiO 2 -containing devices.

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DOE OSTI2017

Exciton Absorption Spectra by Linear Response Methods: Application to Conjugated Polymers

The theoretical description of the time-evolution of excitons requires, as an initial step, the calculation of their spectra, which has been inaccessible to most users due to the high computational scaling of conventional algorithms and accuracy issues caused by common density functionals. Previously (J. Chem. Phys.2016, 144, 204105), we developed a simple method that resolves these issues. Our scheme is based on a two-step calculation in which a linear-response TDDFT calculation is used to generate orbitals perturbed by the excitonic state, and then a second linear-response TDDFT calculation is used to determine the spectrum of excitations relative to the excitonic state. Herein, we apply this theory to study near-infrared absorption spectra of excitons in oligomers of the ubiquitous conjugated polymers poly(3-hexylthiophene) (P3HT), poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), and poly(benzodithiophene-thieno[3,4-b]thiophene) (PTB7). For P3HT and MEH-PPV oligomers, the calculated intense absorption bands converge at the longest wavelengths for 10 monomer units, and show strong consistency with experimental measurements. The calculations confirm that the exciton spectral features in MEH-PPV overlap with those of the bipolaron formation. In addition, our calculations identify the exciton absorption bands in transient absorption spectra measured by our group for oligomers (1, 2, and 3 units) of PTB7. For all of the cases studied, we report the dominant orbital excitations contributing to the optically active excited state–excited state transitions, and suggest a simple rule to identify absorption peaks at the longest wavelengths. We suggest our methodology could be considered for further developments in theoretical transient spectroscopy to include nonadiabatic effects, coherences, and to describe the formation of species such as charge-transfer states and polaron pairs.

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DOE OSTI2017

In Situ Synthesis of Highly Dispersed and Ultrafine Metal Nanoparticles from Chalcogels

We report a unique reaction type for facile synthesis of ultrafine and well-dispersed Pt nanoparticles supported on chalcogel surfaces. The nanoparticles are obtained by in-situ Pt 2+ reduction of a chalcogel network formed by the metathesis reaction between K 2 PtCl 4 and Na 4 SnS 4 . The rapid catalytic ability of the chalcogelsupported Pt nanoparticles is demonstrated in a recyclable manner by using 4-nitrophenol reduction as a probe reaction.

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