Search NASA⌕ Search

SEARCH · Search NASA

Results for “C 3 oxygenates”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Precursor selection in hybrid molecular beam epitaxy of alkaline-earth stannates

One of the challenges of oxide molecular beam epitaxy (MBE) is the synthesis of oxides containing metals with high electronegativity (metals that are hard to oxidize). The use of reactive organometallic precursors can potentially address this issue. To investigate the formation of radicals in MBE, we explored three carefully chosen metal-organic precursors of tin for SnO 2 and BaSnO 3 growth: tetramethyltin (TMT), tetraethyltin (TET), and hexamethylditin (HMDT). All three precursors produced single-crystalline, atomically smooth, and epitaxial SnO 2 (101) films on r-Al 2 O 3 (101¯2) in the presence of oxygen plasma. The study of growth kinetics revealed reaction-limited and flux-limited regimes except for TET, which also exhibited a decrease in the deposition rate with increasing temperature above ~800 °C. Contrary to these similarities, the performance of these precursors was dramatically different for BaSnO 3 growth. TMT and TET were ineffective in supplying adequate tin, whereas HMDT yielded phase-pure, stoichiometric BaSnO 3 films. Significantly, HMDT resulted in phase-pure and stoichiometric BaSnO 3 films even without the use of an oxygen plasma (i.e., with molecular oxygen alone). Furthermore, these results are discussed using the ability of HMDT to form tin radicals and therefore assisting with Sn → Sn 4+ oxidation reaction. Structural and electronic transport properties of films grown using HMDT with and without oxygen plasma are compared. This study provides guideline for the choice of precursors that will enable the synthesis of metal oxides containing hard-to-oxidize metals using reactive radicals in MBE.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Atomic and Electronic Structure in MgO–SiO 2

Understanding disordered structure is difficult due to insufficient information in experimental data. Here, we overcome this issue by using a combination of diffraction and simulation to investigate oxygen packing and network topology in glassy (g-) and liquid (l-) MgO–SiO 2 based on a comparison with the crystalline topology. We find that packing of oxygen atoms in Mg 2 SiO 4 is larger than that in MgSiO 3 , and that of the glasses is larger than that of the liquids. Moreover, topological analysis suggests that topological similarity between crystalline (c)- and g-(l-) Mg 2 SiO 4 is the signature of low glass-forming ability (GFA), and high GFA g-(l-) MgSiO 3 shows a unique glass topology, which is different from c-MgSiO 3 . We also find that the lowest unoccupied molecular orbital (LUMO) is a free electron-like state at a void site of magnesium atom arising from decreased oxygen coordination, which is far away from crystalline oxides in which LUMO is occupied by oxygen’s 3s orbital state in g- and l-MgO–SiO 2 , suggesting that electronic structure does not play an important role to determine GFA. We finally concluded the GFA of MgO–SiO 2 binary is dominated by the atomic structure in terms of network topology.

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

Polyhedral Distortions and Unusual Magnetic Order in Spinel FeMn 2 O 4

Spinel compounds AB 2 X 4 consist of both tetrahedral (AX 4 ) and octahedral (BX 6) environments with the former forming a diamond lattice and the latter a geometrically frustrated pyrochlore lattice. Exploring the fascinating physical properties and their correlations with structural features is critical in understanding these materials. FeMn 2 O 4 has been reported to exhibit one structural transition and two successive magnetic transitions. In this work, we report the polyhedral distortions and their correlations to the structural and two magnetic transitions in FeMn 2 O 4 by employing the high-resolution neutron powder diffraction. The cation distribution is found to be ($Mn_{0.9}^{2+}Fe_{0.1}^{3+}$)$_{A}$($Mn^{3+}Fe_{0.9}^{3+}Mn_{0.1}^{2+}$)$_{B}$O 4 . While large trigonal distortion is found even in the high-temperature cubic phase, the first-order cubic-tetragonal structural transition associated with the elongation of both tetrahedra and octahedra with shared oxygen atoms along the c axis occurs at T S ≈ 750 K, driven by the Jahn–Teller effect of the orbital active B-site Mn 3+ cation. Strong magnetoelastic coupling is unveiled at T N1 ≈ 400 K as manifested by the appearance of Néel-type collinear ferrimagnetic order, an anomaly in both tetrahedral and octahedral distortions, as well as an anomalous decrease of the lattice constants c and a weak anomaly of a. Upon cooling to T N2 ≈ 65 K, it evolves to a noncollinear ferrimagnetic order accompanied by the different moments at the split magnetic sites B1 and B2. Only one-half of the B-site Mn 3+ /Fe 3+ spins, i.e., the B2-site spins in the pyrochlore lattice, are canted, which is a unique magnetic order among spinels. The canting angle between A-site and B2-site moments is ~25°, but the B1-site moment stays antiparallel to the A-site moment even at 10 K. This noncollinear order is accompanied by a modification of the O–B–O bond angles in the octahedra without significant change in lattice constants or tetrahedral/octahedral distortion parameters, indicating a distinct magnetoelastic coupling. We demonstrate distinct roles of the A-site and B-site magnetic cations in the structural and magnetic properties of FeMn 2 O 4 . Our study indicates that FeMn 2 O 4 is a wonderful platform to unveil interesting magnetic order and to investigate their correlations with polyhedral distortions and lattice.

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

High-Temperature 2D Optical Relaxation Visualizes Enhanced Oxygen Exchange Kinetics at Metal-Mixed Conducting Oxide Interfaces

Solid-state heterointerfaces are of interest for emergent local behavior that is distinct from either bulk parent compound. One technologically relevant example is the case of mixed ionic/electronic conductor (MIEC)–metal interfaces, which play an important role in electrochemistry. Metal–MIEC composite electrodes can demonstrate improved catalytic activity vs single-phase MIECs, improving fuel cell efficiency. Similarly, MIEC surface reaction kinetics are often evaluated using techniques that place metal current collectors in contact with the surface under evaluation, potentially altering the response vs the native surface. Here, techniques enabling direct and local in situ observation of the behavior at and around such heterointerfaces are needed. Here, we develop a spatially resolved optical transmission relaxation (2D-OTR) method providing continuous evaluation of local, high-temperature, controlled atmosphere defect kinetics across a ~1 cm2 sample area simultaneously in a contact-free manner. We apply it to observe the spatial variance of oxygen incorporation and evolution rates at ~525–620 °C, in response to step changes in oxygen partial pressure, on MIEC SrTi 0.65 Fe 0.35 O 3–x films as a function of distance from porous Pt and Au layers. Using this model geometry, we find significant enhancements in kinetics adjacent to the metals that decay over a few millimeter distance. To extract kinetic parameters, we fit the short-term optical data (initial portion of relaxations) with an exponential decay function appropriate for surface-exchange-limited kinetics, yielding apparent surface exchange coefficients (k chem ) with spatial resolution, decreasing with distance from the metal. To understand the kinetic processes governing the complete (long-term) optical relaxations, we performed COMSOL simulations, which demonstrated that a combination of laterally varying k chem and in-plane diffusion controls the observed kinetics over the full time range. Further support for spatially varying kchem comes from demonstrations of changing surface and bulk chemistry vs distance from the metal–MIEC interface, by X-ray photoelectron and optical absorption spectroscopies, respectively. Although microporous Pt and Au are not excellent electrodes in isolation, both metals exert a synergistic effect on the oxygen surface exchange rate in the presence of the mixed conducting film.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Elucidating the active phases of CoOx films on Au(111) in the CO oxidation reaction

Abstract Noble metals supported on reducible oxides, like CoO x and TiO x , exhibit superior activity in many chemical reactions, but the origin of the increased activity is not well understood. To answer this question we studied thin films of CoO x supported on an Au(111) single crystal surface as a model for the CO oxidation reaction. We show that three reaction regimes exist in response to chemical and topographic restructuring of the CoO x catalyst as a function of reactant gas phase CO/O 2 stoichiometry and temperature. Under oxygen-lean conditions and moderate temperatures (≤150 °C), partially oxidized films (CoO x<1 ) containing Co 0 were found to be efficient catalysts. In contrast, stoichiometric CoO films containing only Co 2+ form carbonates in the presence of CO that poison the reaction below 300 °C. Under oxygen-rich conditions a more oxidized catalyst phase (CoO x>1 ) forms containing Co 3+ species that are effective in a wide temperature range. Resonant photoemission spectroscopy (ResPES) revealed the unique role of Co 3+ sites in catalyzing the CO oxidation. Density function theory (DFT) calculations provided deeper insights into the pathway and free energy barriers for the reactions on these oxide phases. These findings in this work highlight the versatility of catalysts and their evolution to form different active phases, both topological and chemically, in response to reaction conditions exposing a new paradigm in the catalyst structure during operation.

Chen, Hao (ORCID:0000000337598352)↗
DOE OSTI2021

Identification of the acetaldehyde oxide Criegee intermediate reaction network in the ozone-assisted low-temperature oxidation of trans -2-butene

Uni- and bi-molecular reactions involving Criegee intermediates (CIs) have been the focus of many studies due to the role these molecules play in atmospheric chemistry. The reactivity of CIs is known to strongly depend on their structure. The reaction network of the second simplest CI, acetaldehyde oxide (CH 3 CHOO), is investigated in this work in an atmospheric pressure jet-stirred reactor (JSR) during the ozonolysis of trans-2-butene to explore the kinetic pathways relevant to atmospheric chemistry and low-temperature combustion. The mole fraction profiles of reactants, intermediates, and final products are determined by means of molecular-beam mass spectrometry in conjunction with single-photon ionization employing tunable synchrotron-generated vacuum ultraviolet radiation. A network of CI reactions is identified in the temperature region below 600 K, characterized by CI addition to trans-2-butene, water, formaldehyde, formic acid, and methanol. No sequential additions of the CH 3 CHOO CI are observed, in contrast with the reactivity of the simplest CI (H 2 COO) and the earlier observation of an extensive reaction network with up to four H 2 COO sequential additions (Phys. Chem. Chem. Phys., 2019, 21, 7341–7357). Experimental photoionization efficiency scans recorded at 300 K and 425 K and ab initio threshold energy calculations lead to the identification and quantification of previously elusive intermediates, such as ketohydroperoxide and hydroperoxide species. Specifically, the C 4 H 8 + O 3 adduct is identified as a ketohydroperoxide (KHP, 3-hydroperoxybutan-2-one, CH 3 C([double bond, length as m-dash]O)CH(CH 3 )OOH), while hydroxyacetaldehyde (glycolaldehyde, HCOCH 2 OH) formation is attributed to unimolecular isomerization of the CIs. Other hydroperoxide species such as methyl hydroperoxide (CH 3 OOH), ethyl hydroperoxide (C 2 H 5 OOH), butyl hydroperoxide (OOH), hydroperoxyl acetaldehyde (HOOCH 2 CHO), hydroxyethyl hydroperoxide (CH 3 CH(OH)OOH), but-1-enyl-3-hydroperoxide, and 4-hydroxy-3-methylpentan-2-one (HOCH(CH 3 )CH(CH 3 )C([double bond, length as m-dash]O)CH 3 ) are also identified. Detection of additional oxygenated species such as methanol, ethanol, ketene, and aldehydes suggests multiple active oxidation routes. Finally, these results provide additional evidence that CIs are key intermediates of the ozone-unsaturated hydrocarbon reactions providing critical inputs for improved kinetics models.

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

Epitaxial growth of hexagonal BaNi O 3–δ thin films on SrTi O 3 (111) substrates

Transition metal oxides containing nickel species in oxidation states higher than 3+ often exhibit high catalytic activity, which makes them promising for applications as advanced electrocatalysts for water splitting and fuel cells. Here, we examine the structure and properties of BaNiO 3 (BNO) thin films, containing formally Ni 4+ , grown on SrTiO 3 (111) substrates using oxygen-plasma–assisted molecular beam epitaxy. X-ray diffraction and scanning transmission electron microscopy measurements reveal that BNO films have a hexagonal structure with $c$- and $a$-axes of mixed textures showing epitaxial relationships BNO (0001) ∥ SrTiO 3 (111) and BNO (10$\overline{1}$0) ∥ SrTi O 3 (111), respectively. The formation of the $a$-axis texture is dominant due to the smaller lattice mismatch with the substrate. Here, density functional theory calculations confirm that the hexagonal BNO film with the $a$-axis texture is energetically more favorable than the competing $c$-axis texture. Detailed spectroscopy data analysis indicates that hexagonal BNO films contain mixtures of Ni 2+ , Ni 3+ , and Ni 4+ species, with Ni 4+ being dominant. Our study provides insights into stabilizing Ni 4+ in complex oxides, which is important for further exploration of the potential of materials containing Ni 4+ .

36 MATERIALS SCIENCE↗
DOE OSTI2022

Engineering metal-oxide interface by depositing ZrO 2 overcoating on Ni/Al 2 O 3 for dry reforming of methane

Zirconium oxide (ZrO 2 ) was deposited onto Ni/Al 2 O 3 catalyst as overcoating by atomic layer deposition (ALD) for dry reforming of methane (DRM). High-temperature heating during H-2-reduction could transform the ALD-prepared ZrO 2 thin film to tetragonal phase and crack the encapsulating layer on Ni sites, which constructed a beneficial Ni-ZrO x interface. Here, interfacial surface oxygen vacancies on ZrO 2 overcoating were induced by the partial reduction of ZrO 2 surface during high-temperature H 2 reduction, with the assistance of Ni. During DRM, the interfacial oxygen vacancies enhanced CO 2 activation by dissociating CO 2 and releasing active O, thereby limiting carbon formation. For DRM at 700 °C and 800 °C, Ni/Al 2 O 3 with 5 cycles of ZrO 2 ALD overcoating enhanced both activity and stability significantly. For a 100-h DRM test at 600 °C, no deactivation was observed for the Ni/Al 2 O 3 catalyst with 10 cycles of ZrO 2 ALD overcoating, as compared to 59% relative activity loss of Ni/Al 2 O 3 .

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

Revealing the Chemical State of Palladium in Operating In 2 O 3 Gas Sensors: Metallic Pd Enhances Sensing Response and Intermetallic In x Pd y Compound Blocks It

The sensing response of metal oxides activated with noble metal nanoparticles is significantly influenced by changes to the chemical state of corresponding elements under operating conditions. Here, a PdO/rh-In 2 O3 consisting of PdO nanoparticles loaded onto rhombohedral In 2 O 3 was studied as a gas sensor for H 2 gas (100–40000 ppm in an oxygen-free atmosphere) in the temperature range of 25–450 °C. The phase composition and chemical state of elements were examined by resistance measurements combined with synchrotron-based in situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy. As found, PdO/rh-In 2 O 3 undergoes a series of structural and chemical transformations during operation: from PdO to Pd/PdH x and finally to the intermetallic In x Pd y phase. The maximal sensing response (R N2 /R H2 ) of ~5 · 10 7 towards 40000 ppm (4 vol %) H 2 at 70 °C is correlated with the formation of PdH 0.706 /Pd. The In x Pd y intermetallic compounds formed around 250 °C significantly decrease the sensing response.

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

Arsenic activation and compensation in single crystal CdTe bilayers

In state-of-the art polycrystalline CdTe photovoltaics, group-V dopant activation is about 2%. Low activation can create electronic defects and lead to recombination and band tail losses. To develop methods to overcome this limitation, dopant activation was systematically investigated using molecular beam epitaxy (MBE) grown single crystal bilayers of As-doped CdTe on undoped CdTe. Results suggest multiple paths for improved As-activation in polycrystalline CdTe-based devices. It was found that the carrier concentration in this MBE material saturated at ∼3 × 1016 cm−3, with high levels (>50%) of As-activation possible. High activation could be achieved with a post-growth activation temperature of ∼450 °C, when the initial doping level was below the saturation level. However, at typical polycrystalline As incorporation levels (>5 × 1016 cm−3), the excess As is inactive or compensating, requiring elevated temperatures (500–600 °C) to achieve high activation. Oxygen in the annealing ambient was detrimental, while the effect of CdCl2 in the ambient is more case-dependent. A 575 °C activation anneal was combined with a 450 °C CdCl2 treatment to better understand the implications for polycrystalline CdTe. Interestingly, on highly doped samples, processes ending with a high temperature step displayed high activation, while those ending at 450 °C significantly reduced the carrier concentration (with or without CdCl2 in the ambient). Low activation can be restored with another high temperature anneal, allowing reproducible toggling between high and low activation based on the final temperature. Photoluminescence revealed the presence of donor–acceptor pairs in the low activation state that appear to be associated with a compensating defect.

14 SOLAR ENERGY↗
DOE OSTI2022

Structural and Bonding Analysis in Monomeric Actinide(IV) Oxalate from Th(IV) to Pu(IV): Comparison with the An(IV) Nitrate Series

In this work, single-crystal X-ray diffraction (SC-XRD) structures and Raman spectra of a series of new isomorphous molecular An(IV)-oxalate compounds (Th, U, Np, and Pu) are reported. These complexes are crystallized with cobalt(III) hexamine ([Co(NH 3 ) 6 ] 3+ ) as the counter cations, [Co(NH 3 ) 6 ] 2 [An(C 2 O 4 ) 5 ]·4H 2 O, revealing five bidentate nonbridging oxalate ligands in the first coordination sphere (CN = 10). The nonbridging oxalate is rather uncommon for An(IV)-oxalate systems, which are widely characterized as polymeric compounds. Density functional theory (DFT) calculations were performed to examine the bonding between An(IV) cations and oxalate ligands. For comparison, we also report results obtained for the An(IV)-hexanitrate series, [(C 2 H 5 ) 4 N] 2 [An(NO 3 ) 6 ] (with An = Th, U, Np, Pu, and Ce), which consists of O-donor ligands as well but with a larger coordination number (CN = 12). The bonding analysis confirms that the actinide–oxygen bond is predominantly ionic with a minor increase in covalency from Th to U and slight variations from U to Pu. Further comparison showed that the charge transfer increases slightly when increasing the number of anions in the coordination sphere (C 2 O$_4^{2–}$: CN = 10; NO$_3^–$: CN = 12), but covalent effects as indicated by the amount of internuclear electron density accumulation are small and similar for oxalate and nitrate.

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

Redox and mineral controls on Fe and Ti isotopic fractionations during calc-alkaline magmatic differentiation

Titanium and Fe isotopic compositions of lavas from a calc-alkaline differentiation suite and corresponding mineral separates from the Rindjani Volcano, Indonesia show that Fe and Ti isotopic fractionations between minerals and melts are lower than those recorded in other suites at all stages of differentiation. Here, the limited isotopic fractionation for Ti is likely due to low-Ti magnetite and clinopyroxene being the dominant carriers of Ti in Rindjani lavas, as these minerals are thought to have limited equilibrium Ti isotopic fractionation relative to silicate magmas. Other magmatic differentiation suites controlled by removal of Ti-rich magnetite and characterized by a lesser role of clinopyroxene have larger Ti isotopic fractionations. This effect is an indirect consequence of the elevated Fe 3+ /Fe 2+ ratio of calc-alkaline magmas such as Rindjani, which promotes Fe 3+ incorporation into magnetite at the expense of Fe 2+ -Ti 4+ pairs, such that increased oxygen fugacity will subdue Ti isotopic fractionation in global magmatic series. Similarly, we find negligible Fe isotopic fractionation in Rindjani bulk rocks and mineral separates, unlike previous studies. This is also likely due to the oxidized nature of the Rindjani differentiation suite, which leads to similar Fe 3+ /Fe 2+ ratios in melt and minerals and decreases overall mineral-melt Fe fractionation factors. Paired Ti and Fe isotopic analyses may therefore represent a powerful tool to assess oxygen fugacity during differentiation, independent from Fe 3+ determinations of erupted samples.

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

Microwave-assisted catalytic gasification of mixed plastics and corn stover for low tar, hydrogen-rich syngas production

The challenge for efficient management of post-consumer plastic and biomass waste has grown over the past few decades due to their dramatic increases. In comparison to conventional gasification, microwave-assisted co-gasification of plastics and corn stover offers many benefits, including increased H 2 yield and gas components compared to unfavorable char/tar. Nonetheless, for future commercialization of the process and ease of product separation, further reduction of the undesirable tar is necessary, which can be achieved over the catalytic route. Here, in this work, we studied the catalytic effect of magnetite for microwave-assisted co-gasification of corn stover and plastic to make syngas with higher H 2 and lower tar selectivity over non-catalytic conditions. A 1:1:1 ratio of plastic-corn stover-magnetite was used to evaluate the reaction parameters such as temperature, space velocity, heating media, and catalytic cycles under gasification conditions. In comparison with the microwave non-catalytic route, a 100% increase in the total H 2 yield with 76% higher H 2 production efficiency (mmol/kWh) was achieved in the presence of the magnetite catalyst, while reducing the overall tar formation from 9% to 2%. When magnetite was reduced in situ during the reaction, it coupled with microwave and delivered oxygen radicals that cracked down plastic and corn stover intermediates generated from the synergistic effect under microwave heating. Soon after the oxygen transfer process initiated, magnetite reached its final oxidation state consisting of microwave-active Fe and Fe 3 C phases that continued coupling with microwaves along with the generated graphitic carbon to maintain the heat necessary to further reduce the generated tar and make additional gaseous products, as confirmed by XRD, Raman, and TGA analyses.

08 HYDROGEN↗
DOE OSTI2020

Favorable Redox Thermodynamics of SrTi 0.5 Mn 0.5 O 3–δ in Solar Thermochemical Water Splitting

Two-step, solar thermochemical splitting of water using nonstoichiometric redox-active metal oxides has emerged as an intriguing approach for large-scale hydrogen production. Perovskites have been proposed as alternatives to state-of-the-art fluorite CeO 2–δ because of their potential for lowering reduction temperature while maintaining high fuel productivity. Guided by computational insights, we explore the thermodynamic properties and water splitting efficacy of the cubic perovskite SrTi 0.5 Mn 0.5 O 3–δ (STM55). Thermogravimetric analysis is performed under controlled oxygen partial pressures (pO 2 ) and temperatures up to 1500 °C, from which both the enthalpy and entropy of reduction as a function of oxygen nonstoichiometry are determined. STM55 provides an attractive combination of moderate enthalpy, 200–250 kJ (mol-O) –1 , and high entropy, with unusual δ dependence. Using a water splitting cycle in which the material is thermally reduced at 1350 °C (pO 2 , ~10 –5 atm) and subsequently exposed to steam at 1100 °C (steam partial pressure of pH 2 O = 0.4 atm), we demonstrate a hydrogen yield of 7.4 mL g –1 . Through both half-cycles, the material remains largely in quasi-equilibrium with the gas phase, as reflected in the agreement of the measured data with predicted profiles based on the thermodynamic data. This behavior indicates rapid surface and bulk diffusion kinetics. Cyclic operation showed the material to be free of degradation and always resulted in a 2:1 yield of H 2 /O 2 . Altogether, STM55 provides outstanding performance characteristics for thermochemical hydrogen production.

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

Self-Supported Branched Poly(ethylenimine) Monoliths from Inverse Template 3D Printing for Direct Air Capture

3D-printed inverse templates are combined with ice templating to develop self-supported branched poly(ethylenimine) monoliths with regular channels of varying channel density and ordered macropores. A maximum uptake of 0.96 mmol of CO 2 /g of monolith from ambient air containing 45.5% RH is achieved from dynamic breakthrough experiments, which is a 31% increase compared to the CO 2 uptake from adsorption under dry conditions for the same duration. The breakthrough experiments show characteristics of internal mass-transfer limitations. The cyclic dynamic breakthrough experiments indicate stable operation without significant loss in CO 2 uptake across eight cycles. Moreover, the self-supported monolith shows minimal loss in adsorption capacity (7.7%) upon exposure to air containing 21% oxygen at 110 °C, in comparison to a conventional sorbent consisting of poly(ethylenimine) impregnated on Al 2 O 3 (18.9%). The monoliths exhibit good mechanical stability, contributed by elastic deformation, corresponding to up to 74% strain and lower pressure drop compared to many existing monoliths in the literature.

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

Oxidation Temperature-Dependent Electrochemical Doping of WO 3 Deposited via Atomic Layer Deposition

Silicon-based photoelectrochemical devices show promise for the performance of light-driven CO 2 reduction but suffer from instability under photoelectrochemical conditions relevant to CO 2 reduction. Coating silicon electrodes with thin layers of metal oxides has shown promise to passivate unstable silicon surfaces, and many different metal oxides can be deposited on silicon using various techniques. In this study, we investigate the fundamental photoelectrochemical performance of WO 3 -coated silicon photoelectrodes, which were generated by oxidation of W-metal films deposited via atomic layer deposition on both degenerately doped (nSi + ) and low-doped (pSi) silicon. Two different oxidation temperatures were investigated (400 and 600 °C), and it was found that the monoclinic phase of WO 3 predominates at both temperatures but that more grain boundaries are present in the 600 °C film. From X-ray photoelectron spectroscopy, the stoichiometry of both films was found to be 1:3 W:O, and low electron energy loss experiments indicate band gaps of 3.0 and 3.1 eV for 400 and 600 °C films, respectively. Cyclic voltammetry experiments showed that the electron transfer kinetics increased after continued redox cycling, particularly for the material produced at 400 °C. X-ray photoelectron spectra suggest that the observed increase in electrode conductivity is due to the formation of oxygen vacancies in the film. Electrochemical impedance spectroscopy indicated that charge transport through the films was impacted by the grain boundaries that formed during oxidation of the film. Photoelectrochemical studies on pSi/WO 3 electrodes were highly variable, only producing a photocurrent and photovoltage with some samples. Our best sample, formed at 400 °C, produced a photovoltage of 180 mV, which is lower than what has previously been reported for WO 3 -coated silicon (500 mV). We hypothesize that the variability in photoelectrochemical experiments arose from a roughened WSiO x interface that is generated during film preparation. WO 3 shows promise as a metal oxide coating for silicon, but our results suggest that formation of a high-quality interface between Si and WO 3 is vital for best performance.

Charge transfer↗
DOE OSTI2024

Materials Science of the Interstitial Doping Process

Particle accelerators are an increasingly important tool for frontier science. Growing initial and operating costs are a significant barrier for upgrades and for new machines. While everything matters, the major cost contributor is the SRF cavities and their ancillary facilities (e.g., cryoplant). Accordingly, the accelerator science community devotes much R&D effort to improving their energy efficiency (increased Q o ) and gradient (E acc ). While improved gradient is at the forefront for certain machines (ILC), improved quality factor has broader impact, benefitting all SRF applications. An important opportunity for accelerator science and technology to move forward arose in the course of building the LCLS-II, the second generation Linac Coherent Light Source at SLAC. At more or less the same time, researchers at Fermilab discovered that introducing a small amount of nitrogen to the niobium surface could improve the mid-range quality factor as much as three-fold. A firm resolution of how nitrogen confers its benefit attracts much current research interest. The key elements of the “nitrogen doping” process were vacuum bake at 800 °C, brief exposure to mTorr of nitrogen at several hundred degrees followed by electropolish (EP) to remove several microns from the surface to eliminate unwanted nitrides: While the process as a whole was novel, the comprising unit operations are familiar to the accelerator community. It was judged reasonable to adopt it as a cost-reduction technology for LCLS-II. Researchers carried out a program of varying process parameters and measuring performance in single-cell cavities, leading to a consensus stable protocol for the project. The transition to vendor fabrication and multiple niobium sources has presented unforeseen challenges of performance variation evidently not connected to anything that could be incorporated in a purchase specification. Moving beyond the high temperature N-doping process above, researchers reported a simplified process consisting entirely of tens of hours anneal in a N atmosphere at low temperatures (~120°C – 160°C) after 800 °C UHV bake. These processes typically yield a few-nm doped layer while the high temperature process yields at least a few to many micron doped layer. Even more recently, oxygen has been used to dope instead of nitrogen which leaves a few-µm O-alloyed layer upon vacuum annealing for 300 °C for ~3 hours. The investigation of these materials is just beginning, but the process simplification they may offer is surely attractive. The very low quantity of material that appears to be significant in the “infusion” process indicates the need for very careful control of gas species available for diffusion into the surface during low temperature treatment, both for process control and research to characterize the underlying dynamics. Oxygen alloying offers the further opportunity to utilize the decomposition of the surface native oxide as the dopant source. It is necessary to understand and (thus) manage this process. We have been supported by the Department of Energy Offices of High Energy Physics and Nuclear Physics to pursue this goal.

36 MATERIALS SCIENCE↗