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102 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Micro-zoning in minerals of a Landes silicate inclusion

There is an increasing number of meteorites with chondritic bulk composition but completely different textures than the conventional chondrite groups. Winonaites, Acapulcoites and silicate inclusions in IAB-iron meteorites have in common coarse grain size, highly equilibrated mineralogy with frequent 120 deg triple junctions and they record a significantly lower degree of oxidation than ordinary chondrites. They all have equilibration temperatures, based on Ca-exchange among pyroxenes, of around 900 to 1100 deg C. However, on cooling disequilibrium features may develop: (1) Olivine in IAB-inclusions has lower Fa-content than equilibrium Fs-content of pyroxenes requires; (2) CaO-zoning in olivine was established at temperatures of around 500 deg C, several hundred degrees below pyroxene equilibration temperatures. Obviously, olivine responded faster to changes in fO2 (Fa in olivine) and temperature (Ca-zoning) than pyroxenes. Differences in diffusion coefficients can readily explain the observed trends. Here we report on much more subtle zoning features in pyroxenes. TEM-observations reveal large compositional gradients in Ca, Na, Cr, Ti and Fe within the first micrometer of cpx and opx crystals. In summary, the data reflect the complicated subsolidus history of a chondritic mineral assemblage that was in thermodynamic equilibrium at about 900 deg C and cooled slowly from this temperature whereby oxidation reactions and different closure temperatures for various minerals and elements played an important role. The oxidation of P dissolved in metal and formation of phosphate, which is thermodynamically stable at low temperatures, is suggested to be responsible for most of the observed zoning.

Eisenhour, D. D.↗
DOE OSTI2021

Chromoanagenesis from radiation-induced genome damage in Populus

Chromoanagenesis is a genomic catastrophe that results in chromosomal shattering and reassembly. These extreme single chromosome events were first identified in cancer, and have since been observed in other systems, but have so far only been formally documented in plants in the context of haploid induction crosses. The frequency, origins, consequences, and evolutionary impact of such major chromosomal remodeling in other situations remain obscure. Here, we demonstrate the occurrence of chromoanagenesis in poplar ( Populus sp . ) trees produced from gamma-irradiated pollen. Specifically, in this population of siblings carrying indel mutations, two individuals exhibited highly frequent copy number variation (CNV) clustered on a single chromosome, one of the hallmarks of chromoanagenesis. Using short-read sequencing, we confirmed the presence of clustered segmental rearrangement. Independently, we identified and validated novel DNA junctions and confirmed that they were clustered and corresponded to these rearrangements. Our reconstruction of the novel sequences suggests that the chromosomal segments have reorganized randomly to produce a novel rearranged chromosome but that two different mechanisms might be at play. Our results indicate that gamma irradiation can trigger chromoanagenesis, suggesting that this may also occur when natural or induced mutagens cause DNA breaks. We further demonstrate that such events can be tolerated in poplar, and even replicated clonally, providing an attractive system for more in-depth investigations of their consequences.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Electrochemically induced metal- vs. ligand-based redox changes in mackinawite: identification of a Fe 3+ - and polysulfide-containing intermediate

Under anaerobic conditions, ferrous iron reacts with sulfide producing FeS, which can then undergo a temperature, redox potential, and pH dependent maturation process resulting in the formation of oxidized mineral phases, such as greigite or pyrite. A greater understanding of this maturation process holds promise for the development of iron-sulfide catalysts, which are known to promote diverse chemical reactions, such as H + , CO 2 and NO 3 - reduction processes. Hampering the full realization of the catalytic potential of FeS, however, is an incomplete knowledge of the molecular and redox processess ocurring between mineral and nanoparticulate phases. Here, we investigated the chemical properties of iron-sulfide by cyclic voltammetry, Raman and X-ray absorption spectroscopic techniques. Tracing oxidative maturation pathways by varying electrode potential, nanoparticulate n(Fe 2+ S 2- ) (s) was found to oxidize to a Fe 3+ containing FeS phase at -0.5 V vs. Ag/AgCl (pH = 7). In a subsequent oxidation, polysulfides are proposed to give a material that is composed of Fe 2+ , Fe 3+ , S 2- and polysulfide (S n 2- ) species, with its composition described as Fe 2+ 1-3x Fe 3+ 2x S 2- 1-y (S n 2- ) y . Thermodynamic properties of model compounds calculated by density functional theory indicate that ligand oxidation occurs in conjunction with structural rearrangements, whereas metal oxidation may occur prior to structural rearrangement. These findings together point to the existence of a metastable FeS phase located at the junction of a metal-based oxidation path between FeS and greigite (Fe 2+ Fe 3+ 2 S 2- 4 ) and a ligand-based oxidation path between FeS and pyrite (Fe 2+ (S 2 ) 2- ).

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

Probing Photoelectrochemical Hydrogen and Oxygen Evolution at Individual Al:SrTiO 3 /Rh 2–y Cr y O 3 Photocatalyst Particles

Particle-based photocatalysts for overall water splitting convert solar energy into hydrogen fuel without the use of any photovoltaic devices. As such they have the potential to revolutionize renewable energy production on Earth. While proven efficiencies have reached 1.1%, further improvements of photocatalyst technology depend on a better understanding of energy loss mechanisms on the individual particle level. Here, we conduct the first in operando photoelectrochemical measurements on individual micrometer-sized Al-doped SrTiO 3 /Rh 2–y CryO 3 photocatalyst particles. We find that the photocatalyst particles behave mainly as water oxidation photoanodes reaching up to 0.5 mA cm −2 at 0.8 V versus RHE and a photovoltage of ∼1.00 V under 4.7 mW cm −2 ultraviolet illumination. This proves that charge separation in the unbiased Al:SrTiO 3 /Rh 2–y Cr y O 3 catalyst is driven by a junction at the n-semiconductor-liquid contact. While O 2 is detected symmetrically around photocatalyst particles, uneven H 2 evolution profiles reflect an irregular Rh 2–y Cr y O 3 cocatalyst distribution. Additionally, the H 2 evolution activity varies significantly between different photocatalyst particles. Furthermore, this suggests that performance gains are possible by better controlling catalyst composition on the microscale.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Facets control charge separation during photoelectrochemical water oxidation with strontium titanate (SrTiO 3 ) single crystals

The photocatalytic overall water splitting reaction provides a pathway to hydrogen fuel from sunlight. Photocatalysts must achieve the reaction without the application of an external bias, which requires an effective charge separation mechanism. Photolabeling studies and electrostatic simulations for the well-known CoOOH/Al:SrTiO 3 /Rh/Cr 2 O 3 photocatalyst suggest that charge separation is driven by work function differences at the (100) and (110) facets of SrTiO 3 , which are electron and hole selective, respectively. Here we use hydrogen annealed SrTiO 3–x single crystals to obtain the first quantitative assessment of the charge separation ability of the (100), or (110), or (111) facets during oxygen evolution. Under UV illumination (60 mW cm –2 ), the crystals exhibit variable water oxidation photocurrents (0.34, 0.82, 1.36 mA cm –2 at 1.23 V versus RHE) and photovoltage values of 1.40, 1.52 and 1.52 V for (100), (110), and (111) SrTiO 3–x , respectively. A surface photovoltage increase in that same order (0.31 V < 0.57 V < 0.67 V) is confirmed independently with vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPV) under 375 nm (1.91 mW cm –2 ) illumination. Mott Schottky measurements in aqueous K 3/4 [Fe(CN) 6 ] reveal facet-dependent flatband positions of –0.58, –0.71, and –0.74 V RHE for the (100), (110), and (111) crystals respectively. This confirms that the photoelectrochemical water oxidation performance of SrTiO 3–x crystals is controlled by the work function of each facet, which determines the electron transfer barrier height of the respective solid–liquid junctions. After correcting for differences in electron donor concentrations, barriers are found to increase in the order (100) < (111) < (110) and differ by as much as 0.16 eV, similar to an earlier prediction. Altogehter, these results explain the charge separation mechanism in SrTiO 3 photocatalysts and highlight the need for faceted semiconductor crystals as light absorbers in particle-based photocatalysts.

08 HYDROGEN↗
DOE OSTI2025

Rational Design of Methylated Triazine‐Based Linear Conjugated Polymers for Efficient CO 2 Photoreduction with Water

The development of semiconducting conjugated polymers for photoredox catalysis holds great promise for sustainable utilization of solar energy. In this work, a new family of porous methylated triazine‐based linear conjugated polymers is reported that enable efficient photoreduction of carbon dioxide (CO 2 ) with water (H 2 O) vapor, in the absence of any additional photosensitizer, sacrificial agents or cocatalysts. It is demonstrated that the key lies in the generation of methylated triazine linkages through a facile condensation reaction between benzamidine and acetic anhydride, which impedes the formation of conventional triazine‐based frameworks. It is also shown that regulating conjugated linear backbones with different lengths of electron‐donated benzyl units provides a facile means to modulate their optical properties and the exciton dissociation, thereby affording more long‐lived photogenerated charge carriers and boosting charge separation and transfer. A high‐performance carbon monoxide (CO) production rate of 218.9 µmol g −1 h −1 is achieved with ≈ 100% CO selectivity, which is accompanied by exceptional H 2 O oxidation to oxygen (O 2 ). It anticipates this new study will advance synthetic approaches toward polymeric semiconductors and facilitate new possibilities for triazine‐based conjugated polymers with promising potential in artificial photocatalysis.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Electrochemically Mediated Au–C(sp 2 ) Anchors for Molecular Electronics

Terminal anchor groups play a key role in the stability and electronic properties of molecular junctions. Single molecule junctions typically consist of two preinstalled terminal anchors linking organic molecules to metal electrodes. Here, in this work, we show that p -terphenyl derivatives containing only a single terminal anchor show conductance features similar to junctions with two preinstalled terminal anchors. A set of p -terphenyl derivatives with one terminal anchor was prepared using automated chemical synthesis and characterized using single molecule electronics experiments, molecular dynamics (MD) simulations, bulk electrochemistry and spectroscopy, and nonequilibrium Green’s function-density functional theory (NEGF-DFT) calculations. Our results show that 4-amino- p -terphenyl (PPP) and related analogs exhibit a well-defined high conductance state that is diminished or absent in other p-terphenyl derivatives lacking a preinstalled amine terminal anchor or fluorine or methyl substitutions at the terminal para position. However, a low conductance state is observed in all amino- p -terphenyl derivatives with one preinstalled anchor due to molecular junctions formed by noncovalent dimeric π–π stacking interactions. The observed high conductance state diminishes upon the addition of reducing agents and is restored upon the addition of an oxidizing agent. Our results suggest that the high conductance state arises due to Au–C(sp 2 ) bond formation facilitated by a single electron oxidation event at the electrode surface. A series of control experiments with different anchor groups shows that primary amines play a key role in forming Au–C bonds for molecular junctions. Overall, these results suggest that Au–C bond formation gives rise to high conductance pathways in organic molecules containing only one preinstalled terminal anchor. Insights from this work can be leveraged in the design of molecular electronic devices, particularly in understanding the mechanisms of molecular binding and junction formation.

charge transport↗
DOE OSTI2026

A Monolithic Artificial Leaf for Solar Methanol Production from CO 2 and H 2 O

Methanol, an important liquid fuel and chemical feedstock, has yet to be produced using solar energy, H 2 O, and CO 2 as sole inputs in a standalone device. Here, this study directly addresses this longstanding challenge through presenting the first demonstration of unbiased solar methanol production from CO 2 and H 2 O with a monolithic artificial leaf design, surpassing the previous best energy efficiency in solar alcohol production by at least 1 order of magnitude. We first develop a new generation of photocathodes based on Si micropillar arrays and a cobalt tetraaminophthalocyanine molecular catalyst. By integrating a C 60 interlayer that facilitates unidirectional electron transfer through the semiconductor/catalyst interface, we realize a photovoltage of 500 mV, one of the highest recorded for single-junction Si-based photoelectrodes in aqueous CO 2 reduction, as well as unprecedented methanol formation with a Faradaic efficiency of 30% and a partial current density of 6.3 mA cm –2 . We further integrate the photocathode with a multijunction perovskite photovoltaic minimodule to afford a standalone solar fuel system, which demonstrates a light-to-methanol conversion efficiency of 0.8%, 32 times higher than the present record in light-to-alcohol conversion with an artificial leaf.

alcohols↗
DOE OSTI2022

Novel and effective surface passivation for high efficiency n- and p-type Silicon solar cell

The project objective was to develop a novel Si surface passivation method using chalcogens, sulfur (S) and/or selenium (Se), as passivating elements, to withstand industry-standard high temperature contacting and metallization schemes for p-type Si based passivated emitter and rear contact (p-PERC) solar cells. The back surface passivation of PERC cells has been improved drastically with the invention and successful application of an Al 2 O 3 passivation layer. However, the front n + diffused junction surface is still poorly passivated by the standard amorphous silicon nitride (SiNx) anti-reflection coating (ARC) layer. This project sought to address the passivation challenges of both front n+ emitter and undiffused p-Si back surface. Improved p-PERC solar cell performance with open circuit voltage (V OC ) > 680 mV and efficiency of 22% were targeted to validate superior defect passivation properties as compared to standard SiO 2 / Al 2 O 3 passivation. During this project, we systematically investigated process-structure-properties-performance relationships of this novel advanced defect passivation approach. The S/Se passivation was carried out by reacting industrial Czochralski (Cz) Si wafers in H 2 S and H 2 Se gases in a chemical vapor deposition (CVD) reactor at temperatures up to 700°C. After an exhaustive optimization of the process parameters (temperature, time, and gas concentration), we established an optimized process and demonstrated extremely low surface recombination velocities (SRVs) of 1.5 cm/s and 8 cm/s on n-type and p-type Si, respectively, by S-passivation. In-depth surface and interface characterization were performed using soft x-ray and photoelectron spectroscopies (XPS, UPS, XES), combined with capacitance-voltage-frequency (C-V-f) measurements, to decipher the surface chemical/electronic structure and interface defect state densities. These measurements provided critical understanding of the defect passivation mechanism and elucidated the presence of surface S-Si bonds, a reduction of surface dipoles, and low interface state densities (D it ) < 10 11 cm -2 ev -1 . We also found that the Se-passivation is inferior to the S-passivation (by at least one order of magnitude in SRV). Application of the optimized S-passivation to the n+ diffused emitter surface led to a low surface recombination current density, J0 ≈ 40 fA/cm 2 (~ 1/4 of the industry-standard SiNx-passivation), and high implied V OC (686 mV) in p-PERC solar cell structures. The S-passivation process also was found to improve the bulk quality of the p-type Si, better than the SiO 2 or Al 2 O 3 passivation processes. After successful demonstration of efficient passivation of Si surface defects by S, we extensively studied the air, thermal, and illumination stability of the passivation structure. S-passivation itself degrades in air due to competing reactions with moisture and oxygen to form oxides, which can be eliminated by a SiNx capping layer (also acting as a anti-reflective coating). After SiNx process optimization, we demonstrated illumination and thermally stable S-passivation with SRV < 5 cm/s and J 0 < 80 fA/cm 2 . These enhancements in Si passivation, incorporated into p-PERC cells, achieved an efficiency of 19.93% with V OC = 649 mV, using manufacturing metallization and contacting schemes. The low cell performance (cell V OC is much less than the implied V OC = 686 mV, anticipated from surface passivation) was identified due to degradation of S-passivation during the metal firing step (out-diffusion of S from the Si interface to the SiNx surface). The S-passivation of Si surfaces shows significant promise with excellent passivation quality, essential for high performance (high V OC , high efficiency) solar cells. Integration of this innovative defect passivation into devices, however, demands further development of the capping layer, low temperature (<700°C) metallization process, and/or engineering of advanced device structures. Surface passivation-dominated advanced Si solar cells, such as tunnel oxide passivated contacts and Si heterojunctions, are increasingly of interest due to their high-performance potential and will have a growing photovoltaic market share in the near future.

14 SOLAR ENERGY↗
DOE OSTI2021

Comprehensive analysis of differentially expressed mRNA and circRNA in Ankylosing spondylitis patients’ platelets

Highlights: • 4996 mRNAs and 2942 circRNAs were identified differentially expressed in platelets of AS patients. • Several platelet-derived immune mediators were upregulated to regulate the interaction between platelets and immune cells. • Two downregulated circRNA were identified, and the corresponding circRNA-miRNA-mRNA regulatory network was constructed. • mRNAs and circRNAs found herein could be used as diagnostic biomarkers or new therapeutic targets. Ankylosing spondylitis (AS) is a chronic inflammatory disease significantly decreasing the quality of life. Platelets play an important and active role in the development of AS. Accumulating evidence demonstrated platelets contain diverse RNA repository inherited from megakaryocytes or microvesicles. Platelet RNAs are dynamically affected by pathological conditions and could be used as diagnostic or prognostic biomarkers. However, the role of the platelet RNAs in AS is elusive. In this study, we compared mRNA and circRNA profiles in platelets between AS patients and healthy controls using RNA sequencing and bioinformatic analysis, and found 4996 mRNAs and 2942 circRNAs were differently expressed. The significantly over-expressed mRNAs in AS patients are involved in platelet activity, gap junction, focal adhesion, rap1 and toll and Imd signaling pathway. The previous identified platelet-derived immune mediators such as P2Y1, P2Y12, PF4, GPIbα, CD40L, ICAM2, CCL5 (RANTES), TGF-β (TGF-β1 and TGF-β2) and PDGF (PDGFB and PDGFA) are also included in these over expressed mRNAs, implying these factors may trigger inflammatory cascades and promote the development of AS. Additionally, we found two down-regulated circRNA (circPTPN22 and circFCHSD2) from the intersection analyses of platelets and spinal ligament tissues of AS patients. The circRNA-miRNA-mRNA regulatory network of these two circRNAs was constructed, and the target mRNAs were enriched in Th17 cell differentiation, inflammatory bowel disease, cell adhesion molecules, cytokine-cytokine receptor interaction, Jak-STAT and Wnt signaling pathway, all these pathways participate in the bone remodeling and pro/anti-inflammatory immune regulation in AS. Then, qRT-PCR was performed to validate the expression of selected key mRNAs and circRNAs and the results demonstrated that the expression levels of P2Y12, GPIbα, circPTPN22 and circFCHSD2 were consistent with the sequencing analysis. In addition, the high expression of five predicted miRNAs interacting with circPTPN22 and circFCHSD2 were also detected in AS by qRT-PCR. Taken together, our study presents a comprehensive overview of mRNAs and circRNAs in platelets in AS patients and offers new insight into the mechanisms of platelet involving in the pathogenesis of AS. The mRNAs and circRNAs identified in this study may serve as candidates for diagnosis and targeted treatment of AS.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2024

Reliable operation of Cr 2 O 3 :Mg/ $β$-Ga 2 O 3 p–n heterojunction diodes at 600 °C

Beta gallium oxide (β-Ga 2 O 3 )-based semiconductor heterojunctions have recently demonstrated improved performance at high voltages and elevated temperatures and are, thus, promising for applications in power electronic devices and harsh environment sensors. However, the long-term reliability of these ultra-wideband gap (UWBG) semiconductor devices remains barely addressed and may be strongly influenced by chemical reactions at the p–n heterojunction interface. Here, we experimentally demonstrate operation and evaluate the reliability of Cr 2 O 3 :Mg/β-Ga 2 O 3 p–n heterojunction diodes during extended operation at 600 °C, as well as after 30 repeated cycles between 25 and 550 °C. The calculated pO 2 -temperature phase stability diagram of the Ga-Cr-O material system predicts that Ga 2 O 3 and Cr 2 O 3 should remain thermodynamically stable in contact with each other over a wide range of oxygen pressures and operating temperatures. The fabricated Cr 2 O 3 :Mg/β-Ga 2 O 3 p–n heterojunction diodes show room-temperature on/off ratios >104 at ±5 V and a breakdown voltage (V Br ) of -390 V. The leakage current increases with increasing temperature up to 600 °C, which is attributed to Poole–Frenkel emission with a trap barrier height of 0.19 eV. Over the course of a 140-h thermal soak at 600 °C, both the device turn-on voltage and on-state resistance increase from 1.08 V and 5.34 mΩ cm 2 to 1.59 V and 7.1 mΩ cm 2 , respectively. This increase is attributed to the accumulation of Mg and MgO at the Cr 2 O 3 /Ga 2 O 3 interface as observed from the time-of-flight secondary ion mass spectrometry analysis. These findings inform future design strategies of UWBG semiconductor devices for harsh environment operation and underscore the need for further reliability assessments for β-Ga 2 O 3 -based devices.

36 MATERIALS SCIENCE↗
DOE OSTI2025

ZnTiN 2 as an Electron-Selective, Protective Layer on Si Photocathodes

Photoelectrochemical production of fuels requires photoelectrodes that efficiently convert sunlight to electrochemical energy by producing photovoltage and photocurrent and maintain this ability over time under a variety of pH, illumination, and applied bias conditions. Work in the photovoltaic community has demonstrated that interfaces with high charge carrier selectivity provide high photovoltages. This offers a co-design opportunity to create semiconductor photoelectrodes with contact layers that are both carrier-selective and offer protection from degradation in aqueous solutions. In this work, we explored the ternary nitride ZnTiN 2 as an electronselective, protective layer for Si-based photocathodes. We demonstrated that ZnTiN 2 formed a heterojunction with p-type Si that facilitated electron movement toward the ZnTiN 2 surface for light-driven reduction reactions. Across a variety of electrolyte conditions, ZnTiN 2 /Si produced an open circuit voltage of ca. 400 mV vs the solution potential, while bare Si produced 220−480 mV vs the solution potential depending on conditions. ZnTiN 2 was also shown to protect Si over 72 h at open circuit in the dark in 0.1 M KHCO 3 aqueous solution at pH 10.5, with a 2.4% loss in open circuit voltage compared to a 17% loss for unprotected Si. A protective effect was also observed under illumination during methyl viologen reduction at pH 3.5 for 21 h, with a 2.5% loss in open circuit voltage observed for ZnTiN 2 /Si compared to a 25% loss in open circuit voltage for unprotected Si under the same conditions. Elemental characterization revealed the presence of oxides on the surface of ZnTiN 2 that are consistent with the Pourbaix diagram after photoelectrochemical operation; these oxides appeared to support durability without hindering charge carrier extraction to drive electrochemical work. This work highlights the promise of ZnTiN 2 for durable photoelectrochemical applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗