Oxygen Layer Research Articles

Responses of Active Ammonia Oxidizers to Eutrophication and Oxygen Statuses in Taihu Freshwater Sediments

Here, we employed DNA-based stable isotope probing (SIP) and molecular biology methods to investigate active ammonia oxidizer communities in suboxic sediments (0 to –2 cm) at the micromolar oxygen level and layers (–2 to –5 cm) at nanomolar oxygen concentrations from meso-eutrophic and light-eutrophic locations in Taihu Lake. The results revealed that ammonia-oxidizing archaea (AOA) were less active in the anoxic layer of meso-eutrophic sites, while ammonia-oxidizing bacteria (AOB) were less active in suboxic sediments of light-eutrophic sites after 8 weeks of incubation. The active AOA in the meso- and light-eutrophic sediments belonged to the Nitrosopumilus, Nitrosotalea, and Nitrososphaera clusters and the Nitrosopumilus and Nitrososphaera clusters, respectively, with Nitrosopumilus cluster as the predominant AOA, which took up a higher ratio in the light-eutrophic and suboxic layers than their counterparts. The advantageous active AOB were numerically predominated by the Nitrosomonas cluster in the suboxic layers, and the Nitrosospira cluster in the anoxic layers, respectively, both of which were distributed in diverse frequencies in different eutrophication statuses. The role and community composition diversities of active ammonia oxidizers in freshwater sediments were attributed to the different eutrophication (including nitrogen and organic carbon content) and oxygen statuses.

Stereospecific Autocatalytic Surface Explosion Chemistry of Polycyclic Aromatic Hydrocarbons.

Autocatalytic processes are important in many fields of science, including surface chemistry. A better understanding of its mechanisms may improve the current knowledge on heterogeneous catalysis. The thermally induced decomposition of eight different polycyclic aromatic hydrocarbons (PAHs) on a saturated monolayer of atomic oxygen on a Cu(100) surface is studied using temperature-programmed reaction spectroscopy (TPRS), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). 9-Bromo-heptahelicene decomposes autocatalytically in a narrow temperature range into CO2 and H2O, while non-halogenated heptahelicene decomposes into the same products but does not show autocatalytic behavior. Fixation of the hydrocarbon to the surface via the organometallic bond after elimination of the bromine is identified as a prerequisite for the autocatalytic reaction mechanism. Of all the hydrocarbons studied, only those being sterically overcrowded decompose autocatalytically. Such an observation can be explained by facile dehydrogenation of the overcrowded PAHs. The reaction of such hydrogen with oxygen creates vacancies in the oxygen layer which act as active sites and catalyze further decomposition.

Influence of the Diagonal and Off-Diagonal Electron–Phonon Interactions on the Formation of Local Polarons and Their Band Structure in Materials with Strong Electron Correlations

For systems with strong electron correlations and strong electron–phonon interaction, we analyze the electron–phonon interaction in local variables. The effects of the mutual influence of electron–electron and electron–phonon interactions that determine the structure of local Hubbard polarons are described. Using a system containing copper–oxygen layers as an example, we consider the competition between the diagonal and off-diagonal interactions of electrons with the breathing mode as the polaron band structure is formed within a corrected formulation of the polaron version of the generalized tight-binding method. The band structure of Hubbard polarons is shown to depend strongly on the temperature due to the excitation of Franck–Condon resonances. For an undoped La2CuO4 compound we have described the evolution of the band structure and the spectral function from the hole dispersion in an antiferromagnetic insulator at low temperatures with the valence band maximum at point (π/2, π/2) to the spectrum with the maximum at point (π, π) typical for the paramagnetic phase. The polaron line width at the valence band top and its temperature dependence agree qualitatively with angle-resolved photoemission spectroscopy for undoped cuprates.

Vanadium Oxide Oligomers and Ordered Monolayers Supported on CeO2(111): Structure and Stability Studied by Density Functional Theory

Structures and stabilities of vanadium oxide oligomers as well as two candidate structures for a monolayer on the CeO2(111) surface have been studied by density functional theory, employing a genetic algorithm to determine the global energy minimum structures. These ceria-supported structures have predominantly 4-fold coordinated V5+ ions with V═O groups in common. The agglomeration of VO2 clusters deposited on the surface is a strongly exothermic process, particularly when ring structures with three or six VO2 units are formed that are commensurate with the close-packed surface-terminating oxygen layer. The VO2 and V2O5 monolayers feature larger coordination numbers (5, 6) of V and contain V atoms without V═O groups. Relative to oligomers, VO2 and V2O5 monolayer structures with and without oxygen defects are thermodynamically more stable. This, together with the fact that flat “monolayer” clusters are preferred to taller “bilayer” clusters, indicates the preference for a complete two-dimensional wetting ...

Investigation of the Na-M-O Phase Diagram and Preparation of Related High-Voltage P2 Cathodes for Na-Ion Batteries

As people’s energy demand increases, an increasing use of renewable sources is needed to mitigate the effects of global warming. These sources (such as solar and wind) are intermittent, thus energy storage is crucial for their widespread adoption. Although Li-ion batteries are dominating the market for portable electronic devices and electric vehicles because of their high energy density, their application in grid scale storage has been limited due to their high cost. Na, as an abundant and evenly distributed element on earth, is a low-cost alternative to Li which is more suitable for large-scale applications. A wide range of cathode materials have been proposed for Na-ion batteries, [1] and layered oxides of the type NaMO2 (M = transition metal) have attracted significant attention because of their high theoretical energy density and scalable synthesis method. Following the notation of Delmas,[2] there are different structures depending on the different oxygen stacking ordering, such as P2 and O3, where the letters stand for the different environment of Na ion while the number indicates the stacking order of oxygen layers. P2 structured NaxMO2 (0.67<x<0.75) have Na ions occupying two prismatic sites between ABBA stacked oxygen layers, and our previous research has identified that it has better mobility in general as compared to O3 [3], therefore higher promise for large reversible capacity. Some P2 materials have already shown high capacity, but the use of high-valent elements (Mn4+, Ti4+, Sn5+, Te6+) often resulted in relatively low average voltage, limiting their energy density [4]. Therefore, high voltage transition metal redox couples need to be used. In this work, using first principles calculations (DFT) we investigated in depth the phase diagram of Na-M-O, with M = Co, Ni, Mn, with a particular attention towards the stability of P2 structures for Mn-poor compositions. Thanks to these results we were recently able to synthesize by solid state methods new P2 structured Na-ion cathodes which showed high specific capacities (~140 mAh/g) at a high average discharge voltage of 3 V or higher. In our contribution, we will detail the synthesis and characterization of such cathodes, both from an electrochemical and structural standpoint. We employed several experimental techniques to probe the pristine material, and we studied in detail its oxidation made via desodiation. Employed techniques include galvanostatic cycling, GITT, ex situ and operando diffraction, SEM, SQUID and XANES. These promising results highlight the synergetic effect of different transition metals, and we will discuss how a deeper understanding of such effect (especially including further cation mixing) will guide us towards high-performance P2-type Na-ion battery cathodes. [1] Dipan Kundu, et al, Angew. Chem. Int. Ed. 2015, 54, 3431 – 3448 [2] R. Berthelot, D. Carlier, C. Delmas, Nat. Mater. 2011, 10, 74-80 [3] Yifei Mo, et al, Chem. Mater. 2014, 26, 5208−5214 [4] Clément R. et al., J. Electrochem. Soc., 2015, 162 (14) A2589-A2604

Analytical bond order potential for simulations of BeO 1D and 2D nanostructures and plasma-surface interactions

An analytical interatomic bond order potential for the Be–O system is presented. The potential is fitted and compared to a large database of bulk BeO and point defect properties obtained using density functional theory. Its main applications include simulations of plasma-surface interactions involving oxygen or oxide layers on beryllium, as well as simulations of BeO nanotubes and nanosheets. We apply the potential in a study of oxygen irradiation of Be surfaces, and observe the early stages of an oxide layer forming on the Be surface. Predicted thermal and elastic properties of BeO nanotubes and nanosheets are simulated and compared with published ab initio data.

Photoemission properties of flat and rough GaAs surfaces with cesium and oxygen layers

The evolution of surface band bending and the probabilities of electron emission in a vacuum are studied under cesium and oxygen deposition on atomically flat and rough As-rich and Ga-rich GaAs (001) surfaces by means of photoreflectance spectroscopy and photoemission quantum-yield spectroscopy. On the rough surface, as compared to the flat one, the suppression of non-monotonic Cs-induced band bending variations is observed, along with the overall band bending increase. Multiple repeated activations of the GaAs (001) surface by cesium and oxygen followed by vacuum anneals led to an increase in the root-mean-square roughness from Rq ˜ 0.1-0.2 nm up to ˜ 3 nm. As a result, the probability of electron escape into vacuum decreased by half.

Semi-Solid Remelting of Magnesium Chips

Compact and loose magnesium chips were processed by means of remelting. The remelting was successfully performed using the new method of semi-solid melting, without the addition of flux, at temperatures between 580°C and 600°C. In this temperature range, the exothermic reaction between magnesium and the oxygen present in the surrounding atmosphere is avoided; in addition, the oxygen layer of the chips is stripped off by the particles of the semi-solid melt. Results show that more than 95% of the magnesium chips can be recovered as metal. Experiments were performed on different scales to obtain production parameters for the recycling process. Larger particle sizes of magnesium chips can be remelted faster than smaller ones. The ability to remelt at temperatures in the semi-solid region of alloys demonstrates the possibility of recovering virtually all of the metal from the chips.

Constraining the Final Fates of Massive Stars by Oxygen and Iron Enrichment History in the Galaxy

Abstract Recent observational studies of core-collapse supernovae suggest that only stars with zero-age main-sequence masses smaller than 16– explode when they are red supergiants, producing Type IIP supernovae. This may imply that more massive stars produce other types of supernovae or they simply collapse to black holes without giving rise to bright supernovae. This failed supernova hypothesis can lead to significantly inefficient oxygen production because oxygen abundantly produced in inner layers of massive stars with zero-age main-sequence masses around 20– might not be ejected into the surrounding interstellar space. We first assume an unspecified population of oxygen injection events related to massive stars and obtain a model-independent constraint on how much oxygen should be released in a single event and how frequently such events should happen. We further carry out one-box galactic chemical enrichment calculations with different mass ranges of massive stars exploding as core-collapse supernovae. Our results suggest that the model assuming that all massive stars with 9– explode as core-collapse supernovae is still most appropriate in explaining the solar abundances of oxygen and iron and their enrichment history in the Galaxy. The oxygen mass in the Galaxy is not explained when assuming that only massive stars with zero-age main-sequence masses in the range of 9–17 contribute to the galactic oxygen enrichment. This finding implies that a good fraction of stars more massive than should eject their oxygen layers in either supernova explosions or some other mass-loss processes.

Thermodynamic Stability of Molybdenum Oxycarbides Formed from Orthorhombic Mo2C in Oxygen-Rich Environments

Molybdenum carbide (Mo2C) nanoparticles and thin films are particularly suitable catalysts for catalytic fast pyrolysis (CFP) as they are effective for deoxygenation and can catalyze certain reactions that typically occur on noble metals. Oxygen deposited during deoxygenation reactions may alter the carbide structure leading to the formation of oxycarbides, which can determine changes in catalytic activity or selectivity. Despite emerging spectroscopic evidence of bulk oxycarbides, so far there have been no reports of their precise atomic structure or their relative stability with respect to orthorhombic Mo2C. This knowledge is essential for assessing the catalytic properties of molybdenum (oxy)carbides for CFP. In this article, we use density functional theory (DFT) calculations to (a) describe the thermodynamic stability of surface and subsurface configurations of oxygen and carbon atoms for a commonly studied Mo-terminated surface of orthorhombic Mo2C, and (b) determine atomic structures for oxycarbides with a Mo:C ratio of 2:1. The surface calculations suggest that oxygen atoms are not stable under the top Mo layer of the Mo2C(100) surface. Coupling DFT calculations with a polymorph sampling method, we determine (Mo2C)xOy oxycarbide structures for a wide range of oxygen compositions. Oxycarbides with lower oxygen content (y/x <=2) adopt layered structures reminiscent of the parent carbide phase, with flat Mo layers separated by layers of oxygen and carbon; for higher oxygen content, our results suggest the formation of amorphous phases, as the atomic layers lose their planarity with increasing oxygen content.

Synthesis of M(NpO4)2·nH2O (M = Mg, Ca, Sr, Ba) Using Н3ВО3 Solutions and Properties of These Salts

Two procedures for preparing the compounds M(NpO4)2·nH2O (M = Mg, Ca, Sr, Ba) using boric acid were suggested. In the first procedure, samples of freshly prepared salts M3(NpO5)2·nH2O (M = Ca, Sr, Ba) are treated with excess 0.5 M H3BO3 with vigorous stirring. In the process, the initially light green salts rapidly transform into black products of the general composition M(NpO4)2·nH2O. In the second procedure, a measured volume of a Np(VII) solution with a known LiOH concentration was added to excess 0.5 M H3BO3 solution containing a calculated amount of Mg, Ca, Sr, or Ba nitrate. The reaction yields black precipitates of the same compounds as in the previous case. After washing with water and drying in an oxygen stream, the final products contain a small impurity of Np(VI). The IR spectra suggest that the compounds obtained are structurally related to the previously studied salts MNpO4 (M = K–Cs), i.e., in their lattices there are neptunium–oxygen layers built of NpO23+ cations and bridging O atoms. New data on the properties of the compounds M3(NpO5)2·nH2O with M = Ca, Sr, and Ba were also obtained.

Experimental results of near real-time protection system for plasma facing components in Wendelstein 7-X at GLADIS

One of the aims of stellarator Wendelstein 7-X (W7-X), is to investigate steady state operation, for which power exhaust is an important issue. The predominant fraction of the energy lost from the confined plasma region will be absorbed by an island divertors, which is designed for steady state operation. In order to protect the divertor targets from overheating, 10 state-of-the-art infrared endoscopes will be installed at W7-X. In this work, we present the experimental results obtained at the high heat flux test facility GLADIS (Garching LArge DIvertor Sample test facility in IPP Garching) [] during tests of a new plasma facing components (PFCs) protection algorithm designed for W7-X. The GLADIS device is equipped with two ion beams that can generate a heat load in the range from 3 MWm−2 to 55 MWm−2. The algorithms developed at W7-X to detect defects and hot spots are based on the analysis of surface temperature evolution and are adapted to work in near real-time. The aim of this work was to test the near real-time algorithms in conditions close to those expected in W7-X. The experiments were performed on W7-X pre-series tiles to detect CFC/Cu delaminations. For detection of surface layers, carbon fiber composite (CFC) blocks from the divertor of the Wendelstein 7-AS stellarator were used to observe temporal behavior of fully developed surface layers. These layers of re-deposited materials, like carbon, boron, oxygen and iron, were formed during the W7-AS operation. A detailed analysis of the composition and their thermal response to high heat fluxes (HHF) are described in []. The experiments indicate that the automatic detection of critical events works according to W7-X PFC protection requirements.

Chemisorbed Oxygen at Pt(111): a DFT Study of Structural and Electronic Surface Properties

Simulations based on density functional theory are used to study the electronic and electrostatic properties of a Pt(111) surface covered by a layer of chemisorbed atomic oxygen. The impact of the oxygen surface coverage and orientationally ordered interfacial water layers is explored. The oxygen adsorption energy decreases as a function of oxygen coverage due to the lateral adsorbate repulsion. The surficial dipole moment density induced by the layer of chemisorbed oxygen causes a positive shift of the work function. In simulations with interfacial water layers, ordering and orientation of water molecules strongly affect the work function. It is found that the surficial dipole moment density and charge density are roughly linearly dependent on the oxygen surface coverage. Moreover, we found that water layers exert only a small impact on the surface charging behavior of the surface.

In-situ time-of-flight neutron diffraction study of the structure evolution of electrode materials in a commercial battery with LiNi0.8Co0.15Al0.05O2 cathode

A commercial lithium-ion battery with LiNi0.8Co0.15Al0.05O2 (NCA) cathode has been studied in situ using high-intensity and high-resolution neutron diffraction. Structure and phase composition of the battery electrodes have been probed during charge-discharge in different cycling modes. The dependence of the anode composition on the charge rate has been determined quantitatively. Different kinetics of Li (de)intercalation in the graphite anode during charge/discharge process have been observed. Phase separation of the cathode material has not been detected in whole voltage range. Non-linear dependencies of the unit cell parameters, atomic and layer spacing on the lithium content in the cathode have been observed. Measured dependencies of interatomic spacing and interlayer spacing, and unit cell parameters of the cathode structure on the lithium content could be qualitatively explained by several factors, such as variations of oxidation state of cation in oxygen octahedra, Coulomb repulsion of oxygen layers, changes of average effective charge of oxygen layers and van der Waals interactions between MeO2-layers at high level of the NCA delithiation.

A study of adventitious contamination layers on technically important substrates by photoemission and NEXAFS spectroscopies

This paper addresses the study of adventitious contamination layers on the technically important substrates by X-ray and UV photoemission (XPS and UPS) as well as near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. This investigation draws attention with regard to its application in the electronic devices of lower dimension. The Au, Pt, Ag, Cu, H passivated and SiO2 coated Si (100) are used as substrates. The XPS studies reveal that the composition of the adventitious contamination layer on studied noble substrates is distinct from those of non-metallic (H passivated and SiO2 coated Si) substrates. The ratio of the composition is different for various noble metal substrates. Furthermore, chemisorbed oxygen layer is found to present on Pt, Ag and Cu substrates in contrast to Au one. Our UPS results expose that the valence electronic structure, as well as workfunction of noble metal surfaces, are modified due to the contamination layer onto the substrate surfaces. The C K-edge NEXAFS spectra from the contamination layers on several substrates are presented to exhibit that the composition of contamination layer is different on different substrates whereas angle dependent NEXAFS data suggests that adventitious contamination layers on all substrates are amorphous in nature.

Charge-Transfer-Induced Lattice Collapse in Ni-Rich NCM Cathode Materials during Delithiation

Ni-rich LiNixCoyMnzO2 (NCM) cathode materials have great potential for application in next-generation lithium-ion batteries owing to their high specific capacity. However, they are subjected to severe structural changes upon (de)lithiation, which adversely affects the cycling stability. Herein, we investigate changes in crystal and electronic structure of NCM811 (80% Ni) at high states of charge by a combination of operando X-ray diffraction (XRD), operando hard X-ray absorption spectroscopy (hXAS), ex situ soft X-ray absorption spectroscopy (sXAS), and density functional theory (DFT) calculations and correlate the results with data from galvanostatic cycling in coin cells. XRD reveals a large decrease in unit cell volume from 101.38(1) to 94.26(2) A3 due to collapse of the interlayer spacing when x(Li) < 0.5 (decrease in c-axis from 14.469(1) A at x(Li) = 0.6 to 13.732(2) A at x(Li) = 0.25). hXAS shows that the shrinkage of the transition metal–oxygen layer mainly originates from nickel oxidation. sXAS, ...

Low Energy Electron Induced C–H Activation Reactions in Methane Containing Ices

Conversion of alkanes to functionalized compounds is a highly sought after goal. Under high reaction temperatures, desired intermediate products like alcohols, aldehydes, and carboxylic acids are more easily oxidized to CO2 than the starting alkane, which makes it difficult to recover these functionalized compounds. Here, we use electrons with energy <20 eV to initiate reactions in pure methane and mixed layers of methane and oxygen condensed on a gold substrate at 15 K. Observation of ethane (in pure and mixed films) and methanol and formaldehyde (in mixed films) indicates methane activation by electrons. From the formation threshold and energy dependence of product yields, electronic excitation of reactant/s followed by dissociation into neutral radicals appears to be the reaction initiating step. The results demonstrate the utility of low energy electrons in bringing about functionalization in the simplest alkane.

Characterization of Transferred Graphene Based Solar Cell

Graphene is a two-dimensional material which is flexible and has a high conductivity. We grow a single layer graphene through chemical vapor deposition (CVD). Graphene which is grown by CVD has single layer and large specific surface area about 2620 m2/g, theoretically. Among the properties of graphene, high electrical conductivity and transparency are important advantages as an emerging material of solar cell. Using Schottky barrier at the interface between graphene and Si, solar cells have been developed. In order to enhance the solar cell efficiency, IV characteristics of the Schottky junction have been studied. We use CVD graphene grown on copper foil and silicon wafer (n-doped <100>, 1-10 Ohm·cm, 300 nm oxide, LG Siltron). First, (5x5 mm2) window pattern is exposed on silicon through photolithography. The oxygen layer of the patterned area is removed using BOE (7:1 water:HF) for 4 minutes. The photoresist is removed in acetone at least 10h. A square shape electrode with Ti (10 nm)/Au (90 nm) is deposited on silicon oxide. Ag (300 nm) is deposited on backside of silicon and annealed to have an Ohmic contact. After all these processes are finished, graphene was transferred. Doping on graphene is one way to increase the efficiency of Schottky junction solar cell. We compare none-doped graphene with chemically doped graphene by HNO3 and bis (trifluoromethanesulfonyl) amide. This could be an alternative for the cheap and environmentally friendly solar cell in the future.

Fe-Ti oxide micro-inclusions in clinopyroxene of oceanic gabbro: Phase content, orientation relations and petrogenetic implication

Fe-Ti-oxide micro-inclusions in clinopyroxene of oceanic gabbro from the mid Atlantic ridge have been studied using electron backscatter diffraction and electron probe microanalyses. A first generation of Fe-Ti-oxide inclusions occurs as needles or elongated plates lying in the (010) plane of the clinopyroxene host. The inclusions show distinct elongation directions following “irrational” planes either nearly parallel to the “c” or nearly parallel to the “a” axis of the clinopyroxene host. The habit planes correspond to inclusion-host interfaces, where densely packed oxygen layers of the inclusion and host phases are coherent across the interface. Both inclusion types have distinct crystallographic orientation relations to the host, which are determined by the nearly parallel alignment of densely packed oxygen layers in the inclusions and the clinopyroxene host. Based on the angle between the two elongation directions primary formation of the inclusion at 800° to 900°C was inferred. Initially, homogeneous Ti-bearing magnetite was precipitated together with titanian pargasite lamellae due to reaction of early magmatic clinopyroxene with late magmatic fluid or melt. After cooling below 600°C the Ti-magnetite decomposed into an oriented magnetite+ulvospinel intergrowth. Late stage hydrothermal alteration leads to the corrosion of the magnetite-ulvospinel inclusions and partial replacement by ilmenite as well as newly precipitated homogeneous ilmenite plates that are closely associated with actinolite lamellae lying in the (100) plane of the clinopyroxene host.

Structure of the Clean and Oxygen-Covered Cu(100) Surface at Room Temperature in the Presence of Methanol Vapor in the 10-200 mTorr Pressure Range.

Using ambient pressure X-ray photoelectron spectroscopy (APXPS) and high pressure scanning tunneling microscopy (HPSTM), we show that in equilibrium with 0.01-0.2 Torr of methanol vapor, at room temperature, the Cu(100) surface is covered with methoxy species forming a c(2 × 2) overlayer structure. In contrast, no methoxy is formed if the surface is saturated with an ordered oxygen layer, even when the methanol pressure is 0.2 Torr. At oxygen coverages below saturation, methanol dissociates and reacts with the atomic oxygen, producing methoxy and formate on the surface, and formaldehyde that desorbs to the gas phase. Unlike the case of pure carbon monoxide and carbon dioxide, methanol does not induce the restructuring of the Cu(100) surface. These results provide insight into catalytic anhydrous production of aldehydes.