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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 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Highly Active Rutile TiO 2 Nanocrystalline Photocatalysts

The controllable synthesis of rutile TiO 2 single crystal particles with the preferential orientation of {111} facets still remains a scientific and technological challenge. Here, we developed a facile route for fabrication of rutile TiO 2 nanorod crystals (RTiO 2 NRs) having high ratios of oxidative {111} to reductive {110} surfaces. RTiO 2 NRs were synthesized using a peroxo-titanium complex (PTC) approach, which was controlled by changing the Ti/H 2 O 2 ratio. The thus obtained RTiO 2 NRs revealed a high tendency to agglomerate through orientation-dependent attachment along the {110} facets. This resulted in an increased {111}/{110} surface ratio and led to a markedly improved photocatalytic activity of RTiO 2 NR aggregates. The reported findings illustrate the rich potential of the herein proposed facile and energy-efficient synthesis of nanostructured rutile TiO 2 -based photocatalysts.

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

Cocontinuous Nanostructures by Microphase Separation of Statistically Cross-Linked Polystyrene/Poly(2-vinylpyridine) Networks

Cocontinuous polymeric nanostructures have drawn considerable attention due to their ability to combine distinct, percolation-dependent properties of two different polymer domains. Randomly end-linked copolymer networks (RECNs) have previously been shown to support the formation of disordered cocontinuous nanostructures across wide composition windows in a robust way. However, achieving highly efficient linking of telechelic polymers with excellent end-group fidelity often requires complex synthetic routes. As an alternative, we study here statistically cross-linked copolymer networks (SCCNs) composed of polystyrene and poly(2-vinylpyridine) (PS and P2VP) with cross-linkable allyl pendent groups that are conveniently synthesized by controlled radical copolymerization. Via selective extraction of P2VP, coupled with gravimetry, small-angle X-ray scattering, and electron microscopy, we find disordered cocontinuous phases across wide composition ranges (up to ≈ 35 wt %), approaching values previously determined for RECNs. Remarkably, even for samples that appear to exhibit full percolation, a substantial fraction of P2VP (≈ 20–30 wt %) cannot be removed, which we ascribe to short strands between nearby cross-linkers that are physically embedded within PS domains. Here, the resulting PS porous monoliths with residual surface P2VP layers enable facile surface modification to resist protein adsorption and templating of porous gold nanostructures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Structure of Layers Formed by [2-(3,6-Disubstituted-9 H -carbazol-9-yl)ethyl]phosphonic Acids on Metal Oxides

[2-(9 H -Carbazol-9-yl)ethyl]phosphonic acid (2PACz) and its derivatives are being used extensively as hole-transport layers in organic and perovskite solar cells due to their ability to modify electrode work function, surface wettability, and in some cases, to improve active-layer adhesion, while minimizing interfacial energy losses. The orientation and coverage of surface modifiers significantly impact these functional properties; however, the detailed structure and packing in these overlayers are challenging to investigate, leading to a lack of understanding of how this structure and packing influence performance and limiting the ability to rationally design molecular modifiers. Here, we investigate monolayers of 2-(9 H -carbazol-9-yl)ethyl phosphonic acid derivatives (from here on referred to as X-2PACz) on indium tin oxide (ITO) and alpha phase aluminum oxide (α-Al 2 O 3 ) using a combination of X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and X-ray reflectivity (XRR). By correlating elemental ratios, molecular orientation, and electron density profiles, we directly quantify surface coverage, layer thickness, and molecular tilt across a series of chemically related monolayers. We find that 2PACz and the X-2PACz derivatives form dense monolayers on α-Al 2 O 3 and ITO, with similar surface coverages on either substrate that are inversely proportional to molecular steric bulk. These surface coverage results indicate that X-2PACz is sterically limited in its monolayer surface packing density, as opposed to site limited. Despite surface packing density differences between molecules, the NEXAFS data show a constant average molecular orientation of 61° to 65° between the plane of the carbazole and the substrate for all the molecules on both substrates. These results increase our general understanding of 2PACz derivatives as they become increasingly useful in high performance solar cells.

NEXAFS↗
DOE OSTI2026

Buried Interfaces in Organic Photocathodes for H 2 Evolution: Fermi-Level Pinning and Recombination

Herein, we demonstrate how Fermi-level pinning at buried contacts impacts solar fuel generation in all-polymer photocathodes by systematically comparing the effects of work function, hydroxyl coverage, and hydrogen evolution using chemically modified indium tin oxide (ITO) supports. Photovoltages and net photocathode performance are improved when the ITO is passivated using phosphonic acids, independent of work function, suggesting that the passivation reduces Fermi-level pinning at the buried interface arising from blended heterojunction interactions with surface metal hydroxyls. Transient photovoltage decay reveals differences in recombination mechanisms, supported by light intensity-dependent measurements. Briefly, nonpassivated, hydrophilic contacts exhibit trap-assisted recombination, while passivated, hydrophobic contacts follow bimolecular recombination. We then investigate changes in electroactivity of hole-transfer processes as a function of scan rate and repetitive cycling using a diffusion-controlled molecular redox probe, analogous to a hole-only device achieved via the electrolyte. The nonpassivated buried contacts exhibit higher overpotentials for oxidation, indicative of hole injection/extraction barriers. We observe irreversible electron transfer via the hole-transport level of the blended heterojunction and a strong cycle dependence, consistent with changes in the hole trap state density. Passivation results in more reversible redox behaviors, consistent with more Ohmic-like contacts. Collectively, these results provide context toward the realization of durable organic photoelectrodes with optimized photovoltages and net solar-to-hydrogen conversion efficiencies via fundamental understanding of the rates of carrier generation, recombination, and transport in high-dielectric aqueous environments and opportunities to characterize buried interfaces under device-relevant electric fields.

buried interfaces↗
DOE OSTI2022

On-Surface Single-Molecule Identification of Mass-Selected Cyclodextrin-Supported Polyoxovanadates for Multistate Resistive-Switching Memory Applications

Polyoxovanadate anions immobilized on conducting substrates are promising candidates for molecule-based memory technologies. However, strong electronic interactions with the substrate and the formation of larger agglomerates remain bottlenecks on the way to their technical application. We demonstrate that ion soft-landing of mass selected cyclodextrin- functionalized hexavanadates enables to deposit these host-guest complexes as single memory units on surfaces without their quaternary ammonium countercations from solution. The cyclodextrin provides a stabilizing “shell” for the dianions. Further, multistate switching evidenced by the change in molecular resistance has been detected using scanning tunneling microscopy and spectroscopy and attributed to reduction of individual vanadium (+V) centers.

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

Spectral Performance of Multilayer Amorphous Selenium and Selenium–Tellurium Photodetectors

The ability to robustly and with scalability detect single photons in the visible spectrum with wavelength resolution would transform many imaging applications. Theoretical studies propose an array of carbon nanotubes (CNTs) functionalized with semiconductor quantum dots (QDs) as a physical realization of such photon sensors. In this work, we report approaches to synthesize these CNT-QD nanostructures using DNA as a smart glue to connect CNTs to QDs.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Effect of Molecular Weight and Layer Thickness on Dielectric Breakdown Strength of Neat and Homopolymer Swollen Lamellar Block Copolymer Films

Designing next-generation lightweight pulsed power devices hinges on understanding the factors influencing the energy storage performance of dielectric materials. Polymer dielectric films have a quadratic dependence of energy storage on the voltage breakdown strength and strategies to enhance the breakdown strength are expected to yield a path toward high energy storage densities. Highly stratified lamellar block copolymer (L-BCP) films of model polystyrene-b-polymethylmethacrylate (PS-b-PMMA) exhibited as much as ~50% enhancement in breakdown voltage (E BD ), (225 % increase in stored energy density, U~ E BD 2 ), compared to unordered as-cast L-BCP films. Such an energy density using amorphous polymer is on par with industry-standard semi-crystalline biaxially oriented polypropylene (BOPP), and as such a notable development in the field. This work develops a deeper understanding of the molecular mechanisms of E BD enhancement in L-BCP films, due to combination of interface formation and spatial isolation of the chain ends into segregated zones. We further confirm the role of chain ends in the breakdown process blending a low M n L-BCP with matched M n homopolymers to attain same layer spacing as neat L-BCP of higher M n . E BD shows a significant decrease at low homopolymer fractions due to increased net chain end density within swollen ordered L-BCP domains in wet-brush regime, followed by increased E BD due to layer thickness increase via segregated “interphase layer” formation by excess homopolymers. Notably, E BD of homopolymer swollen L-BCPs is always lower than neat L-BCP of the same domain spacing, due to overall adverse chain ends contribution from homopolymers. These findings provide important selection rules for L-BCPs for designing next-generation flexible electronics with high energy density solid-state BCP film capacitors.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Inline Rolling Shear Alignment: Deposition and Long-Range Order of Block Polymer Templates in a Fast, Single-Step Process

We report the first demonstration of a rapid, single-step technique to align block polymer, thin-film nanostructures that is amenable to continuous speeds ≥10 mm/s. This method, termed Inline Rolling Shear Alignment (IRSA), uses an elastomeric roller coupled with an electric hysteresis brake to shear films before the casting solvent fully evaporates, and it can enable high throughput, large-area manufacturing of patterns such as nanowire templates. IRSA is adaptable to various polymer systems including diblock and triblock polymers, and a post-shear annealing step can yield high-quality nanostructures with orientation parameters >0.99 and defect densities as low as 5 defect-pairs/μm 2 . To demonstrate the effectiveness of IRSA, platinum nanowires were produced from this rapid deposition and alignment approach using poly(styrene-b-2-vinylpyridine), followed by open-air, atmospheric-pressure plasma etching. Furthermore, this integrated concept, with 50x faster operation than existing processes, could be leveraged for cost-effective manufacturing of macroscopically aligned nanostructures in roll-to-roll systems.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Two-Step Thermoresponsive Ultrafiltration Membranes from Polymerization of Lyotropic Liquid Crystals

Here, in this study, we present the fabrication of a two-step thermoresponsive ultrafiltration (UF) membrane through polymerization of a lyotropic liquid crystal (LLC). A mixture of commercially available Pluronic F 127 block copolymer, water (containing ammonium persulfate as the initiator), and polymerizable oil (n-butyl acrylate/ethylene glycol dimethacrylate) is used to create an LLC with a lamellar structure, as characterized by cross-polarized light microscopy and atomic force microscopy. Differential scanning calorimetry is employed to evaluate the thermoresponsive behavior of the polymerized LLC (polyLLC). Two-step thermoresponsiveness (~35 and ~50 °C) of the polyLLC is observed due to the lower critical solution temperature (LCST) of F 127 and melting of the crystalline structure of the polyethylene oxide (PEO) chains of the F 127 surfactant. In the next step, the obtained mesophase is cast on a nonwoven polyester support sheet followed by thermal polymerization. The hydration capacity, water flux, water flux recovery after fouling, and molecular weight cutoff (MWCO) of the obtained membrane are evaluated at different temperatures to examine its thermoresponsiveness. The experimental results reveal that the UF membrane has a reversible thermoresponsive behavior at the LCST and PEO melting of polyLLC. Additionally, the cleaning efficiency of the fouled membrane can be enhanced by using its thermoresponsive behavior, resulting in an extended lifetime of the product. Furthermore, the MWCO of the membrane can be altered with temperature due to the pore size change with temperature stimulus.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Organic Modifiers Promote Furfuryl Alcohol Ring Hydrogenation via Surface Hydrogen-Bonding Interactions

Interactions between surface adsorbed species can affect catalyst reactivity, and thus, the ability to tune these interactions is of considerable importance. Deposition of organic modifiers provides one method of intentionally introducing controllable surface interactions onto catalyst surfaces. In this study, Pd/Al2O 3 catalysts were modified with either thiol or phosphonic acid (PA) ligands and tested in the hydrogenation of furanic species. The thiol modifiers were found to inhibit ring hydrogenation (RH) activity, with the degree of inhibition trending with the thiol surface coverage. This suggests that thiols do not strongly interact with the reactants and simply serve to block active sites on the Pd surface. PAs, on the other hand, were found to enhance RH when furfuryl alcohol (FA) was used as the reactant. Density functional theory calculations suggested that this enhancement was due to hydrogen-bonding interactions between FA-derived surface intermediates and PA modifiers. Here, installation of hydrogen-bonding groups on the Pd surface served to preferentially stabilize RH product states. Furthermore, the promotional effect on the RH of FA was observed to be greater when a higher-coverage PA was used, providing a rate more than twice that of the unmodified Pd/Al2O 3 . The results of this work suggest that organic ligands can be designed to impart tunable surface interactions on heterogeneous catalysts, providing an additional method of controlling catalytic performance.

09 BIOMASS FUELS↗
DOE OSTI2023

Tailoring the Near-Surface Environment of Rh Single-Atom Catalysts for Selective CO 2 Hydrogenation

For this study, we used a combination of experimental spectroscopies, density functional theory calculations, and CO 2 hydrogenation studies to investigate the effects of modifying single-atom Rh 1 /TiO 2 catalysts with functionalized phosphonic acid monolayers. We found that the deposition of specific amine-functionalized ligands resulted in an ~8× increase in site-specific CO 2 reduction turnover frequency at 150 °C and a ~2× increase at 250 °C. On-stream stability also improved following ligand deposition. The effect of the modifier on reactivity was highly sensitive to the proximity of the amine functional group to the surface, which was controlled by adjusting the length of the phosphonic acid tail. Furthermore, deposition of alkyl phosphonic acids without an amine functional group resulted in blocked CO 2 adsorption and a near-complete loss of catalytic activity. Infrared spectroscopy studies suggested that the amine group provided binding sites for CO 2 that enabled hydrogenation when the amine was positioned near a Rh1 site. Phosphonic acid-modified catalysts also exhibited high selectivity to CO over the series product methane; the selectivity effect was traced to modification of the Rh1 sites to favor CO desorption. Phosphonic acid deposition resulted in 80–90% loss of accessible Rh1 sites, likely due to blocking by tail groups. However, even with the loss of sites, under low-temperature reaction conditions, the rates of CO 2 hydrogenation were improved with the coatings, indicating that the remaining sites are highly efficient. Organic functionalization of the supports for atomically dispersed catalysts offers the opportunity to precisely control the positioning of functional groups in the vicinity of a well-defined active site, potentially enabling an additional level of control over active site design.

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

Semiconductor Quantum Dots Are Efficient and Recyclable Photocatalysts for Aqueous PET-RAFT Polymerization

This Letter describes the use of CdSe quantum dots (QDs) as photocatalysts for photoinduced electron transfer reversible addition–fragmentation chain transfer (PET-RAFT) polymerization of a series of aqueous acrylamides and acrylates. In this work, the high colloidal solubility and photostability of these QDs allowed polymerization to occur with high efficiency (>90% conversion in 2.5 h), low dispersity (PDI < 1.1), and ultralow catalyst loading (<0.5 ppm). The use of protein concentrators enabled the removal of the photocatalyst from the polymer and monomer with tolerable metal contamination (8.41 ug/g). These isolated QDs could be recycled for four separate polymerizations without a significant decrease in efficiency. By changing the pore size of the protein concentrators, the QDs and polymer could be separated from the remaining monomer, allowing for the synthesis of block copolymers using a single batch of QDs with minimal purification steps and demonstrating the fidelity of chain ends.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Local Environment Affects the Activity of Enzymes on a 3D Molecular Scaffold

The ability to coordinate and confine enzymes presents an opportunity to affect their performance and to create chemically active materials. Recent studies show that polymers and biopolymers can be used to scaffold enzymes, and that can lead to the modulated biocatalytic efficiency. In this work, we investigated the role of microenvironments on enzyme activity using a well-defined molecular scaffold. An enzyme, glucose oxidase (GO x ), was positioned at different locations of a three-dimensional (3D) octahedral DNA scaffold (OS), allowing the enzyme’s polyanionic environments to be altered. Using electrical sensing, based on a bipolar junction transistor, we measured directly and in real-time the enzyme’s proton generation at these different microenvironments. We found a 200% enhancement of immobilized enzyme over free GOx and about a 30% increase in catalytic rates when the enzyme was moved on the same molecular scaffold to a microenvironment with a higher local concentration of polyanions, which suggests a role of local pH on the enzymatic activity.

77 NANOSCIENCE AND NANOTECHNOLOGY↗