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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 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Assembly and Photocarrier Dynamics of Heterostructured Nanocomposite Photoanodes from Multicomponent Colloidal Nanocrystals

Multicomponent oxides and their heterostructures are rapidly emerging as promising light absorbers to drive oxidative chemistry. To fully exploit their functionality, precise tuning of their composition and structure is crucial. Here, we report a novel solution-based route to nanostructured bismuth vanadate (BiVO 4 ) that facilitates the assembly of BiVO 4 /metal oxide (TiO 2 , WO 3 , and Al 2 O 3 ) nanocomposites in which the morphology of the metal oxide building blocks is finely tailored. Here, the combination of transient absorption spectroscopy-spanning from picoseconds to second time scales-and photoelectrochemical measurements reveals that the achieved structural tunability is key to understanding and directing charge separation, transport, and efficiency in these complex oxide heterostructured films.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Charged- and Multi-Exciton Dynamics in Colloidal Quantum Dot Molecules

Multicarrier states in quantum dots are confined to small volumes, resulting in increased nonradiative Auger recombination rates with implications for different optoelectronic applications. Recently, the fusion of two core-shell quantum dots into a dimer has provided a new physical landscape for multiexciton states, since the excitons may share a core (intradot, localized) or occupy different cores (interdot, segregated). Here, in this study, we employ transient absorption spectroscopy to investigate the multiexciton dynamics in coupled quantum dot dimers. We observe that multiexciton populations in the dimers live significantly longer in comparison to the parent monomers, in contrast to the single exciton regime. A kinetic model that accounts for the statistical differences between monomers and dimers reveals that, while intradot multiexcitons show Auger rates similar to the monomers, interdot states have reduced Auger rates. These results pave the way for the rational design of new quantum dot molecules with tailored multiexciton properties.

Auger recombination↗
DOE OSTI2019

Achieving High Selectivity for Alkyne Hydrogenation at High Conversions with Compositionally Optimized PdAu Nanoparticle Catalysts in Raspberry Colloid-Templated SiO 2

Improving the selectivity for catalytic hydrogenation of alkynes is a key step in upgrading feedstocks for olefin polymerization. Herein, dilute Pd x Au 1-x alloy nanoparticles embedded in raspberry colloidtemplated silica (x = 0.02, 0.04, and 0.09) are demonstrated to be highly active and selective for the gas-phase hydrogenation of 1-hexyne, exhibiting higher selectivity than pure Pd at high conversion. The conversion of 1-hexyne remains high even for the very low amounts of Pd in Pd 0.02 Au 0.98 . These catalysts are highly resistant to sintering—addressing a long-standing challenge in the use of Au-based catalysts. Clear evidence is presented that the addition of the second hydrogen to the halfhydrogenated intermediate is the rate-limiting step, and that the stability of the half-hydrogenated intermediate of the alkyne is higher than the half-hydrogenated alkene which explains the high selectivity even at high conversions. Moreover, of the three compositions investigated, optimum selectivity and activity is observed for the nanoparticles containing 4% Pd. The apparent activation energy for production of 1-hexene from 1-hexyne is measured to be 38 kJ mol -1 for the Pd 0.04 Au 0.96 catalysts, which is ~14 kJ mol -1 lower than for pure Pd. The hydrogenation is completely, but reversibly, suppressed by adding CO to the reactant mixture, indicating that the Pd centers are the active sites for reaction. The method of templating used in preparation of the catalysts is highly customizable and versatile. As a result, this study demonstrates that the composition of the nanoparticles as defined by the dilution ratio of Pd in Au and by the method used to make the supported catalyst are important tunable parameters that can be used to optimize activity and selectivity of bimetallic systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Grafted nanoparticles as soft patchy colloids: Self-assembly versus phase separation

We investigate the thermodynamic behavior of a model polymer-grafted nanoparticle (GNP) system on a fine lattice, using grand canonical Monte Carlo simulations, to compare and contrast the validity of two different models for GNPs: “nanoparticle amphiphiles” versus “patchy particles.” In the former model, continuous self-assembly processes are expected to dominate the system, whereas the latter are characterized by first-order phase separation into novel equilibrium phases such as “empty liquids.” We find that, in general, considering GNPs as amphiphiles within the framework of a recent mean-field theory [Pryamtisyn et al., J. Chem. Phys. 131, 221102 (2009)] provides a qualitatively accurate description of the thermodynamics of GNP systems, revealing either first-order phase separation into two isotropic phases or continuous self-assembly. Our model GNPs display no signs of empty liquid formation, suggesting that these nanoparticles do not provide a route to such phases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Micelle-induced depletion interaction and resultant structure in charged colloidal nanoparticle system

The evolution of the interaction and the resultant structure in the mixed system of anionic silica nanoparticles (Ludox LS30) and non-ionic surfactant decaethylene glycol monododecylether (C12E10), undergoing phase separation, have been studied using small-angle neutron scattering and dynamic light scattering. The measurements have been carried out for a fixed concentration of nanoparticle (1 wt. %) with varying concentration of surfactant (0 to 1 wt. %), in the absence and presence of an electrolyte. It is found that the micelles of non-ionic surfactant adsorb on the nanoparticle in the absence of electrolyte (form stable system), whereas these micelles become non-adsorbing in the presence of electrolyte (show phase separation). The phase separation arises because of C12E10 micelles, causing depletion interaction between nanoparticles and leading to their aggregation. The interaction is modeled by double Yukawa potential accounting for attractive depletion as well as repulsive electrostatic forces. Both the interactions (attraction and repulsion) are found to be of long-range. The nanoparticle aggregation (phase separation) is governed by the increase in the magnitude and the range of the depletion attraction with the increase in the surfactant concentration. The nanoparticle aggregates formed are quite large in size (order of micron) and are characterized by the surface fractal having simple cubic packing of nanoparticles within the aggregates.

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

The role of collective elasticity on activated structural relaxation, yielding, and steady state flow in hard sphere fluids and colloidal suspensions under strong deformation

In this work, we theoretically study the effect of external deformation on activated structural relaxation and aspects of the nonlinear mechanical response of glassy hard sphere fluids in the context of elastically collective nonlinear Langevin equation theory. This microscopic force-based approach describes activated relaxation as a coupled local–nonlocal event involving caging and longer range collective elasticity, with the latter becoming more important and ultimately dominant with increasing packing fraction under equilibrium conditions. The central new question we address is how this physical picture of activated relaxation, and the relative importance of local caging vs collective elasticity physics, depends on external deformation. Theoretical predictions are presented for deformation-induced enhancement of mobility, the onset of relaxation speed up at remarkably low values of stress, strain, or shear rate, apparent power law thinning of the steady state structural relaxation time and viscosity, a non-vanishing activation barrier in the shear thinning regime, an apparent Herschel–Bulkley form of the rate dependence of the steady state shear stress, exponential growth of different measures of a dynamic yield or flow stress with the packing fraction, and reduced fragility and dynamic heterogeneity under deformation. The results are contrasted with experiments and simulations, and qualitative or better agreement is found. An overarching conclusion is that deformation strongly reduces the importance of longer range collective elastic effects relative to the local caging aspect for most, but not all, physical questions, with deformation-dependent fragility and dynamic heterogeneity phenomena being qualitatively sensitive to collective elasticity. Overall, nonlinear rheology is predicted to be a more local problem than quiescent structural relaxation, albeit with deformation-modified activated processes still important.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Increase in the resolution of the method of surface nanostructuring by femtosecond-laser impact through a layer of colloidal microparticles

It is experimentally demonstrated that by converting a few percent of the fundamental frequency pulse energy into the second harmonic one can obtain surface structures using dielectric microspheres with the dimension of the order of the second harmonic wavelength. Structuring by means of the second harmonic solely requires a significantly greater energy density in the pulse than using bichromatic irradiation. It is impossible to obtain structures using only the fundamental frequency radiation pulses, because radiation at the fundamental frequency is not focused by the system of microspheres. Therefore, the use of bichromatic pulses allows an essential increase in the structure recording density. Theoretical calculations demonstrating the possibility of further improvement of the structure recording density by using nonspherical microlenses of a smaller size made of a material with a higher refractive index are presented. (extreme light fields and their applications)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Photofragmentation of colloidal solutions of gold nanoparticles under femtosecond laser pulses in IR and visible ranges

The specific features of photofragmentation of sols of gold nanoparticles under focused femtosecond laser pulses in IR (1030 nm) and visible (515 nm) ranges is experimentally investigated. A high photofragmentation efficiency of nanoparticles in the waist of a pulsed laser beam in the visible range (at moderate radiation scattering) is demonstrated; this efficiency is related to the excitation of plasmon resonance in nanoparticles on the blue shoulder of its spectrum, in contrast to the regime of very weak photofragmentation in an IR-laser field of comparable intensity. Possible mechanisms of femtosecond laser photofragmentation of gold nanoparticles are discussed. (extreme light fields and their applications)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗