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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Confinement of Lewis Acid–Base Sites by Microporous Silica Layers on Titania for Enhanced Alkanol Dehydration Reactivity

Alkanol dehydration offers a pathway to upgrade biomass-derived short-chain oxygenates into alkenes, essential chemical building blocks widely used in industrial applications. Transition metal oxides with Lewis acid-base site pairs are attractive catalysts due to their high reactivity and cost-effectiveness. This work demonstrates a synthetic pathway to manipulate local environments around active Lewis acid-base pairs in anatase TiO 2 to enhance their reactivity in alkanol dehydration. Microporous SiO 2 layers with an average pore diameter of ~0.6 nm and a controlled thickness of 0.8-33 nm are deposited on anatase TiO 2 powders by using a molecular templated SiO 2 deposition method. The Lewis acid-base strength of accessible Ti-O pairs remains unchanged, as shown by temperature-programmed surface reactions of surface-bound formic acid-derived species and temperature-programmed desorption of pyridine. However, measured alkanol dehydration rates on confined Ti-O pairs are much higher (by up to 7-fold) than those on TiO 2 . The extent of rate enhancements depends on the reactant size and functional group positioning, suggesting that the rate enhancements reflect the interactions between the guest molecules (reactants and transition states) and the surrounding SiO 2 micropore environments. By providing a detailed synthetic procedure to tailor the local environments around active sites in bulk oxides, this approach offers an additional avenue for enhancing catalytic performance.

09 BIOMASS FUELS↗
DOE OSTI2026

Development of Composite Photocatalyst Materials that are Highly Selective for Solar Hydrogen Production and their Evaluation in Z-Scheme Reactor Designs

The key technology gap preventing a vertically stacked dual-bed particle suspension reactor from achieving the DOE MYRD&D ultimate cost target for H 2 production remains the lack of materials in particle form factor that exhibit ≥10% solar-to-H 2 energy conversion (STH) efficiency as a suspension. Therefore, our project goals centered around strategies to increase the STH efficiency by enhancing photophysical properties of perovskite oxide particles including increased visible-light absorption, increased selectivity for electrocatalysis of the H 2 evolution reaction (HER) and the O 2 evolution reaction (OER) through development of ultrathin oxide coatings, correlating composition and structure to function, and improving understanding of multiscale transport and kinetic processes.

08 HYDROGEN↗
DOE OSTI2020

Chemical approaches to artificial photosynthesis: A molecular, dye-sensitized photoanode for O 2 production prepared by layer-by-layer self-assembly

We describe here the preparation of a family of photoanodes for water oxidation that incorporate an electron acceptor–chromophore–catalyst in single molecular assemblies on nano-indium tin oxide (nanoITO) electrodes on fluorine-doped tin oxide (FTO). The assemblies were prepared by using a layer-by-layer, Atomic Layer Deposition (ALD), self-assembly approach. In the procedure, addition of an electron acceptor viologen derivative followed by a Ru II (bpy) chromophore and a pyridyl derivative of the water oxidation catalyst [Ru(bda) (L) 2 ] (bda = 2,2'-bipyridine-6,6'-dicarboxylate) 2 , were linked by ALD by addition of the bridge precursors TiO 2 , ZrO 2 , and Al 2 O 3 as the bridging groups giving the assemblies, FTO|nanoITO|–MV 2+ –ALD MO 2 –RuP2 2+ –ALD M'O 2 –WOC. In a series of devices, the most efficient gave water oxidation with an incident photon to current efficiency of 2.2% at 440 nm. Transient nanosecond absorption measurements on the assemblies demonstrated that the slow step in the intra-assembly electron transfer is the electron transfer from the chromophore through the viologen bridge to the nanoITO electrode.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

AlN quasi-vertical Schottky barrier diode on AlN bulk substrate using Al 0.9 Ga 0.1 N current spreading layer

Abstract An aluminum nitride (AlN) quasi-vertical Schottky barrier diode (SBD) was fabricated on an AlN bulk substrate. An undoped AlN layer, a Si-doped Al 0.9 Ga 0.1 N current spreading layer and an AlN buffer layer were grown by plasma-enhanced molecular beam epitaxy. The epitaxial AlN layer was etched down to the n-Al 0.9 Ga 0.1 N layer to form an Ohmic contact. Ni/Au and V/Al/Ni/Au were deposited on the top AlN layer as Schottky contacts and on the exposed n-Al 0.9 Ga 0.1 N layer as Ohmic contacts, respectively. The Ohmic characteristics on the n-Al 0.9 Ga 0.1 N layer, capacitance–voltage ( C – V ) and current–voltage ( I – V ) characteristics of the AlN SBD were investigated.

Physics↗
DOE OSTI2023

CO Oxidation Catalyzed by Au Dispersed on SBA-15 Modified with TiO2 Films Grown via Atomic Layer Deposition (ALD)

It has been established that gold, when in nanoparticle (NP) form and in contact with reducible oxides, can promote oxidation reactions under mild conditions. Here, we report results from our exploration of the catalytic oxidation of carbon monoxide using catalysts where Au NPs were combined with thin titanium oxide films deposited on SBA-15 using atomic layer deposition (ALD). Both orders of deposition, with TiO2 added either before or after Au dispersion, were tested for two titania film thicknesses amounting to about half and full TiO2 monolayers. The resulting catalysts were characterized using various techniques, mainly electron microscopy and N2 adsorption–desorption isotherms, and the kinetics of the oxidation of CO with O2 were followed using infrared absorption spectroscopy. A synergy between the Au and TiO2 phases as it relates to the bonding and conversion of CO was identified, the tuning of which could be controlled by varying the synthetic parameters. The ALD of TiO2 films proved to be an effective way to maximize the Au-TiO2 interface sites, and with that help with the activation of molecular oxygen.

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

Formation of a Boron-Oxide Termination for the (100) Diamond Surface

A boron-oxide termination of the diamond (100) surface has been formed by depositing molecular boron oxide B 2 O 3 onto the hydrogen-terminated (100) diamond surface under ultrahigh vacuum conditions and annealing to 950 °C. The resulting termination is highly oriented and chemically homogeneous, although further optimization is required to increase the surface coverage beyond the 0.4 monolayer coverage achieved here. This work demonstrates the possibility of using molecular deposition under ultrahigh vacuum conditions for complex surface engineering of the diamond surface, and may be a first step in an alternative approach to fabricating boron doped delta layers in diamond.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Vapor-to-glass preparation of biaxially aligned organic semiconductors

Physical vapor deposition (PVD) provides a route to prepare highly stable and anisotropic organic glasses that are utilized in multi-layer structures such as organic light-emitting devices. While previous work has demonstrated that anisotropic glasses with uniaxial symmetry can be prepared by PVD, here, we prepare biaxially aligned glasses in which molecular orientation has a preferred in-plane direction. With the collective effect of the surface equilibration mechanism and template growth on an aligned substrate, macroscopic biaxial alignment is achieved in depositions as much as 180 K below the clearing point TLC−iso (and 50 K below the glass transition temperature Tg) with single-component disk-like (phenanthroperylene ester) and rod-like (itraconazole) mesogens. The preparation of biaxially aligned organic semiconductors adds a new dimension of structural control for vapor-deposited glasses and may enable polarized emission and in-plane control of charge mobility.

Chemistry↗
DOE OSTI2020

Boltzmann machine modeling of layered MoS 2 synthesis on a quantum annealer

Two-dimensional layered materials are usually synthesized by chemical vapor deposition (CVD). Here we use the Restricted Boltzmann Machine (RBM) and its enhancement called Limited Boltzmann Machine (LBM) on a quantum computer to model the chemical vapor deposition (CVD) growth data for a MoS 2 monolayer. The CVD data for Boltzmann machines are generated by reactive molecular dynamics (RMD) simulations. The Boltzmann machines give insight into the semiconducting (2H) and metallic (1T) phases and defects generated during the CVD growth of MoS 2 . We compare the performances of the two types of Boltzmann machine models, i.e. RBM versus augmented topologies with limited intra-layer coupling between the hidden units (LBM). We show that limited topologies have a performance advantage over restricted topologies. We examine connectivity within our topological variants, explore hardware qubit mapping schemes, and discuss what performance differences may imply about locality within the data without prior knowledge.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Selective Hydration of Rutile TiO 2 as a Strategy for Site-Selective Atomic Layer Deposition

In this report the feasibility of a site-selective hydration strategy that enables site-selective atomic layer deposition (ALD) is investigated among four rutile TiO 2 facets [(110), (100), (101) and (001)] and their most prevalent step edges. First-principles simulations of asymmetric slab models were utilized to create accurate representations of pristine terrace and step edge sites. The adsorption free energies for molecular and dissociative adsorption of H 2 O were calculated to evaluate this strategy as a viable route to step edge selectivity. We predict that selective hydroxylation is possible on the 110 and 001 step edges and further computationally evaluate three metalorganic ALD precursors for their compatibility with the selective hydration strategy. Experimental evidence for delayed nucleation of ALD on rutile (001), (110), and (100) TiO2 single crystals corroborates predictions of the dehydration of the surface and suggests the possibility of site-selective ALD.

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

Over What Length Scale Does an Inorganic Substrate Perturb the Structure of a Glassy Organic Semiconductor?

While the bulk structure of vapor-deposited glasses has been extensively studied, structure at buried interfaces has received little attention, despite being important for organic electronic applications. To learn about glass structure at buried interfaces, we study the structure of vapor-deposited glasses of the organic semiconductor DSA-Ph (1,4-di-[4-(N,N-diphenyl)amino]styrylbenzene) as a function of film thickness; the structure is probed with grazing incidence X-ray scattering. Here, we deposit on silicon and gold substrates and span a film thickness range of 10–600 nm. Our experiments demonstrate that interfacial molecular packing in vapor-deposited glasses of DSA-Ph is more disordered compared to the bulk. At a deposition temperature near room temperature, we estimate ~8 nm near the substrate can have modified molecular packing. Molecular dynamics simulations of a coarse-grained representation of DSA-Ph reveal a similar length scale. In both the simulations and the experiments, deposition temperature controls glass structure beyond this interfacial layer of a few nanometers.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electrodeposition of Zinc in Aqueous Electrolytes Containing High Molecular Weight Polymers

Strong polarization of the ion distribution in liquid electrolytes subjected to potential differences exceeding the thermal voltage, V T = kT/e, produces a hydrodynamic instability termed electroconvection at ion-selective interfaces. Electroconvection is desirable in some situations (e.g., electrodialysis) because it promotes mixing in a stagnant electrolyte layer, enhancing the ion flux at a fixed potential difference. It is undesirable in others (e.g., electrodeposition of metals) where early experiments show that convective fluid rolls associated with hydrodynamic instability in bounded electrolytes produce preferential metal deposition at localized regions on an electrode. Such localized deposition drives the family of morphological instabilities loosely termed dendritic electrodeposition. We experimentally investigate the effect of ultrahigh molecular weight polymer additives on the onset conditions and physical characteristics of both instabilities. Direct observations of electrodeposit morphology and tracer particle motions used in tandem with indirect electrokinetic measurements reveal that even at moderate concentrations, the polymer additives have a large beneficial effect in extending the range of electric potentials where stable electrodeposition is observed. Additionally, we report that at polymer concentrations above the entanglement threshold, high molecular weight polymers impart elasticity to liquid electrolytes which dampen electroconvective flow at a cation-selective interface but have at most a minimal effect on the bulk ionic conductivity of the liquid.

25 ENERGY STORAGE↗
DOE OSTI2022

Growth of ultrathin Bi 2 Se 3 films by molecular beam epitaxy

Bi 2 Se 3 is a widely studied 3D topological insulator having potential applications in optics, electronics, and spintronics. When the thickness of these films decreases to less than approximately 6 nm, the top and bottom surface states couple, resulting in the opening of a small gap at the Dirac point. In the 2D limit, Bi 2 Se 3 may exhibit quantum spin Hall states. However, growing coalesced ultrathin Bi 2 Se 3 films with a controllable thickness and typical triangular domain morphology in the few nanometer range is challenging. Here, we explore the growth of Bi 2 Se 3 films having thicknesses down to 4 nm on sapphire substrates using molecular beam epitaxy that were then characterized with Hall measurements, atomic force microscopy, and Raman imaging. We find that substrate pretreatment—growing and decomposing a few layers of Bi 2 Se 3 before the actual deposition—is critical to obtaining a completely coalesced film. In addition, higher growth rates and lower substrate temperatures led to improvement in surface roughness, in contrast to what is observed for conventional epitaxy. Altogether, coalesced ultrathin Bi 2 Se 3 films with lower surface roughness enable thickness-dependent studies across the transition from a 3D-topological insulator to one with gapped surface states in the 2D regime.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Surface stability of SrNbO 3+δ grown by hybrid molecular beam epitaxy

4d transition metal oxides have emerged as promising materials for numerous applications including high mobility electronics. SrNbO 3 is one such candidate material, serving as a good donor material in interfacial oxide systems and exhibiting high electron mobility in ultrathin films. However, its synthesis is challenging due to the metastable nature of the d 1 Nb 4+ cation and the limitations in the delivery of refractory Nb. To date, films have been grown primarily by pulsed laser deposition (PLD), but development of a means to grow and stabilize the material via molecular beam epitaxy (MBE) would enable studies of interfacial phenomena and multilayer structures that may be challenging by PLD. To that end, SrNbO 3 thin films were grown using hybrid MBE for the first time using a tris(diethylamido)(tert-butylimido) niobium precursor for Nb and an elemental Sr source on GdScO 3 substrates. Varying thicknesses of insulating SrHfO 3 capping layers were deposited using a hafnium tert-butoxide precursor for Hf on top of SrNbO 3 films to preserve the metastable surface. Grown films were transferred in vacuo for x-ray photoelectron spectroscopy to quantify elemental composition, density of states at the Fermi energy, and Nb oxidation state. Ex situ studies by x-ray absorption near edge spectroscopy and scanning transmission electron microscopy illustrate that the SrHfO 3 capping plays an important role in preserving the crystalline quality of the material and the Nb 4d 1 metastable charge state under atmospheric conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

PDADMAC/PSS Oligoelectrolyte Multilayers: Internal Structure and Hydration Properties at Early Growth Stages from Atomistic Simulations

We analyze the internal structure and hydration properties of poly(diallyl dimethyl ammonium chloride)/poly(styrene sulfonate sodium salt) oligoelectrolyte multilayers at early stages of their layer-by-layer growth process. Our study is based on large-scale molecular dynamics simulations with atomistic resolution that we presented recently [Sánchez et al., Soft Matter 2019, 15, 9437], in which we produced the first four deposition cycles of a multilayer obtained by alternate exposure of a flat silica substrate to aqueous electrolyte solutions of such polymers at 0.1M of NaCl. In contrast to any previous work, here we perform a local structural analysis that allows us to determine the dependence of the multilayer properties on the distance to the substrate. We prove that the large accumulation of water and ions next to the substrate observed in previous overall measurements actually decreases the degree of intrinsic charge compensation, but this remains as the main mechanism within the interface region. We show that the range of influence of the substrate reaches approximately 3 nm, whereas the structure of the outer region is rather independent from the position. This detailed characterization is essential for the development of accurate mesoscale models able to reach length and time scales of technological interest.

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

Energetic Tug-of-War between Pt and Leaky TiO 2 : Positive and Negative Effects on the Function of Molecularly-Modified p -Si(111)|TiO 2 |Pt Photocathodes

Photoelectrochemical solar energy conversion to chemical products is a promising avenue towards renewable fuels, and TiO 2 deposition on semiconductor photocathodes is a widespread surface protection strategy. However, the electronic, thermodynamic, and kinetic effects of protection layers on interfacial energetics remains poorly understood. Here, electrochemical data (photocurrent onset and flatband potentials) of p-Si(111)|TiO 2 |Pt is collected in contact with an outer-sphere redox couple. We find that onset potentials shift negatively as a function of increasing oxide thickness, but this effect is not reflected in the flatband potentials. This reveals the mechanism of these shifts is kinetic rather than thermodynamic,and suggests that hole leakage through the TiO 2 is responsible. When Pt nanoparticles are deposited onto the TiO 2 , positive shifts in both the onset and flatband potentials are observed, revealing Pt catalysts are beneficial for more than just increased catalytic rates. These results highlight the importance of understanding the effects of surface modifications on the energetics at the interface in both catalytic and simple electron transfer systems.

14 SOLAR ENERGY↗
DOE OSTI2020

Substrate-Dependent Study of Chain Orientation and Order in Alkylphosphonic Acid Self-Assembled Monolayers for ALD Blocking

For years, many efforts in area selective atomic layer deposition (AS-ALD) have focused on trying to achieve high-quality self-assembled monolayers (SAMs), which have been shown by a number of studies to be effective for blocking deposition. In this work, we show that in some cases where a densely packed SAM is not formed, significant ALD inhibition may still be realized. The formation of octadecylphosphonic acid (ODPA) SAMs was evaluated on four metal substrates: Cu, Co, W, and Ru. The molecular orientation, chain packing, and relative surface coverage were evaluated using near-edge X-ray absorption fine structure (NEXAFS), Fourier transform infrared (FTIR) spectroscopy, and electrochemical impedance spectroscopy (EIS). ODPA SAMs formed on Co, Cu, and W showed strong angular dependence of the NEXAFS signal whereas ODPA on Ru did not, suggesting a disordered layer was formed on Ru. Additionally, EIS and FTIR spectroscopy confirmed that Co and Cu form densely packed, “crystal-like” SAMs whereas Ru and W form less dense monolayers, a surprising result since W-ODPA was previously shown to inhibit the ALD of ZnO and Al 2 O 3 best among all the substrates. This work suggests that multiple factors play a role in SAM-based AS-ALD, not just the SAM quality. Therefore, metrological averaging techniques (e.g., WCA and FTIR spectroscopy) commonly used for evaluating SAMs to predict their suitability for ALD inhibition should be supplemented by more atomically sensitive methods. Finally, it highlights important considerations for describing the mechanism of SAM-based selective ALD.

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

Construction of Lattice Strain in Bimetallic Nanostructures and Its Effectiveness in Electrochemical Applications

Bimetallic nanocrystals (NCs), associated with various surface functions such as ligand effect, ensemble effect, and strain effect, exhibit superior electrocatalytic properties. The stress-induced surface strain effect can alter binding strength between the surface active sites and reactants as well as their intermediates, and the electrochemical performance of bimetallic NCs can be significantly facilitated by the lattice-strain modification via their morphologies, sizes, shell-thickness, surface defectiveness as well as compositions. Here in this review, an overview of fundamental principles, characterization techniques, and quantitative determination of the surface lattice strain is provided. Various strategies and synthesis efforts on creating lattice-strain-engineered bimetallic NCs, including the de-alloying process, atomic layer-by-layer deposition, thermal treatment evolution, one-pot synthesis, and other efforts are also discussed. It is further outlined how the lattice strain effect promotes electrochemical catalysis through the selected case studies. The reactions on oxygen reduction reaction, small molecular oxidation, water splitting reaction, and electrochemical carbon dioxide reduction reactions are focused. In particular, studies of lattice strain arisen from core–shell nanostructure and defectiveness are highlighted. Lastly, the potential challenges are summarized and the prospects of lattice-strain-based engineering on bimetallic nanocatalysts with suggestion and guidance of the future electrocatalyst design are envisioned.

36 MATERIALS SCIENCE↗