Degree Of Oxygen Deficiency Research Articles

Trace element and C-S-Fe geochemistry of Early Cambrian black shales and associated polymetallic Ni-Mo sulfide and vanadium mineralization, South China: Implications for paleoceanic redox variation

Early Cambrian black shales are widely exposed along the southeastern margin of the Yangtze Platform in South China for a length of over 1600 km. The basal part of this sequence host a large spectrum of mineralization, including polymetallic Ni-Mo sulfide, vanadium, phosphorite, barite, as well as combustible shale (stone coal). In particular, the polymetallic Ni-Mo sulfide ore layer is stratigraphically equivalent but geographically separated from the V mineralization. In the Zhangjiajie region in Hunan province, the tenor of the polymetallic Ni-Mo sulfide mineralization in the northeastern segment decreases gradually and grades into vanadium mineralization in the southwestern segment. Based on the stratigraphic correlation, we compared the trace element composition and carbon–sulfur-iron system of the polymetallic Ni-Mo sulfide ore, vanadium ore, and their host black shales.The polymetallic Ni-Mo sulfide and V ores display similar PAAS-normalized REE distribution patterns with negative Ce and positive Y anomalies, similar to present-day seawater. The polymetallic sulfide ores show elevated seawater-like Y/Ho ratios (mean: 52.7 ± 6.8) and the V ores with an average Y/Ho ratio of 40.2 ± 3.8 reflect the influence of terrigenous material. Although both the polymetallic Ni-Mo sulfide ore and the V ore formed under anoxic conditions, the degree of oxygen deficiency was variable, i.e., euxinic condition for the polymetallic sulfide ore but euxinic and suboxic conditions for the vanadium ore. This model is supported by the more elevated redox-sensitive elements (e.g., U and Co) in the polymetallic Ni-Mo sulfide ores compared to the V ores. Excess sulfur of the polymetallic Ni-Mo sulfide ores with S/Fe ratios>1.15 suggests that they formed under iron-limited euxinic conditions. In contrast, the V ores formed under euxinic to suboxic conditions with S/Fe ratios close to or lower than 1.15. The host black shales display positive correlations for TOC and redox-sensitive elements, suggesting that they formed under suboxic conditions. We infer that the polymetallic Ni-Mo sulfide ores formed in a sediment-starved semi-restricted euxinic marine basin. In contrast, the V ores formed under both euxinic and intermittently suboxic to euxinic conditions at the slope of the basin with terrestrial input from enhanced continental weathering. Repeated transgressive–regressive episodes led to the formation of V ores interlayered with suboxic black shale with more subdued oxygen deficiency. The results of this study suggest that exploration for V mineralization should focus on the southeastward lateral stratigraphic equivalents of the polymetallic Ni-Mo sulfide mineralization.

Deep Proton Insertion Assisted by Oxygen Vacancies for Long‐Term Memory in VO2 Synaptic Transistor

AbstractReversible phase transformation of correlated oxides by field‐driven ionic process present opportunity to efficiently transduce between ionic transfer and electrical currents in insertion‐based reconfigurable transistors. However, the switching rate of insertion transistors is fundamentally limited by the slow rate of ionic insertion into the lattices of correlated oxides. Here, it is demonstrated that preformed oxygen vacancies in VO2−δ lattices strongly accelerate proton insertion by low gate voltage in synaptic transistors. As the degree of oxygen deficiency δ increases in VO2−δ transistors, the steepness of phase transformation and transconductance increase during the voltage sweep at the expense of the channel current modulation. Theoretical and experimental analyses reveal that the accelerated of H+ kinetics in the VO2−δ lattice occurs because immobile oxygen vacancies reduce the energy barrier to H+ migration. In an electronic synapse, this facile H+ migration in VO2−δ lattices renders “inscribed” memory by positioning the H+ neurotransmitter far from the electrolyte/VO2−δ interface. This discovery suggests a strategy to improve the learning and memory processes of artificial synaptic devices by controlling the density of intrinsic defects in the lattice framework to achieve efficient ion exchange.

Effect of the Oxygen Vacancies and Structural Order on the Oxygen Evolution Activity: A Case Study of SrMnO3-δ Featuring Four Different Structure Types.

We report the facile synthesis methods of four materials, with the general formula SrMnO3-δ, which have previously been synthesized in multiple steps, involving switching between different oxidizing and reducing gases, quenching, the use of zirconium metal as a reductant, etc. However, we have shown that it is possible to synthesize all of these materials by facile processes without unnecessary complications. In fact, we have found methods of synthesizing the oxygen-deficient phases in only one step. Given the diverse range of structures that are formed for SrMnO3-δ, we have investigated the correlations between the structural order and electrocatalytic activity for the oxygen evolution reaction (OER) of water splitting. We have uncovered a systematic trend in the OER activity, where the most oxygen-deficient compound, SrMnO2.5, which features square-pyramidal coordination geometry around manganese, shows the highest OER performance. The next OER activity belongs to SrMnO2.6, which contains both MnO5 trigonal bipyramids and MnO6 octahedra. SrMnO3(cubic), containing only corner-sharing MnO6 units, shows the third best OER performance. The least activity is observed in SrMnO3(hexagonal), featuring both face- and corner-sharing MnO6 octahedra. We have also studied the electrochemically active surface area, as well as the kinetics of OER for all four materials, and found that the trend in these properties is the same as the trend in the OER activity. These findings indicate that the electrocatalytic activity is correlated with the degree of oxygen deficiency, as well as the polyhedral connectivity.

Charge transport in epitaxial barium titanate films

The electrical conductivity of epitaxial $\mathrm{BaTi}{\mathrm{O}}_{3}$ films was studied by small-signal impedance spectroscopy at temperatures of 10--720 K using Pt-$\mathrm{BaTi}{\mathrm{O}}_{3}\ensuremath{-}\mathrm{SrRu}{\mathrm{O}}_{3}$ capacitors. The $\ensuremath{\sim}150$-nm-thick $\mathrm{BaTi}{\mathrm{O}}_{3}$ films possessed different lattice strains and degrees of oxygen deficiency. A crossover between the low-temperature hopping of small polarons and the high-temperature semiconductor- to metal-type behavior was demonstrated in all films. It was suggested that the small electron polarons originate from self-trapping at Ti in the stoichiometric tensile-strained film and from trapping at Ti next to the oxygen vacancy in the oxygen-deficient films. The conduction-band transport was ascribed to the thermally activated release of the trapped electrons. It was pointed out that the electronic release can mimic the motion of oxygen vacancies, which are actually immobile.

Charge transport properties of Ca2FeGaO6-δ and CaSrFeGaO6-δ: The effect of defect-order

The ability to achieve different charge-transport properties in oxygen-deficient perovskites, an important class of materials in the fuel cell technology, has been demonstrated through manipulation of the defect-order. This correlation has been investigated through the synthesis of a new non-centrosymmetric oxygen-deficient perovskite, CaSrFeGaO6-δ, as well as its centrosymmetric analogue, Ca2FeGaO6-δ. X-ray photoelectron spectroscopy and iodometric titration studies indicate the same degree of oxygen-deficiency in both compounds. Neutron diffraction shows the transformation of the defect-arrangement, leading to variation in the orientation of tetrahedral chains in the material structure. The variation in the defect-order results in remarkable changes in charge-transport properties, where CaSrFeGaO6-δ shows significantly greater conductivity than Ca2FeGaO6-δ at room temperature. Variable-temperature electrical conductivity in a wide temperature-range, 25 °C–800 °C, points to semiconducting behavior for both materials, where the superior conductivity of CaSrFeGaO6-δ persists in the entire temperature-range.

Enhanced electrical properties in BaSrFe2O6−δ (δ = 0.5): A disordered defect-perovskite

The effect of defect-order and oxygen stoichiometry on electrical properties of oxygen-deficient perovskites has been demonstrated through the synthesis of BaSrFe2O6−δ (δ = 0.5). Neutron diffraction experiments indicate that the defects, created due to oxygen deficiency, are distributed in a disordered fashion in this compound. This is in sharp contrast to the Ca-analog, CaSrFe2O6−δ (δ = 1), which is synthesized under identical conditions, but forms a defect-ordered structure with a different degree of oxygen-deficiency. These structural changes result in significant variation of magnetic and electrical properties. Neutron diffraction shows the absence of long-range magnetic order in BaSrFe2O6−δ (δ = 0.5), and magnetic susceptibility data point to a spin-glass transition. This is contrary to CaSrFe2O6−δ (δ = 1), which is known to be antiferromagnetically ordered. X-ray photoelectron spectroscopy and iodometric titrations indicate that BaSrFe2O6−δ (δ = 0.5) contains both trivalent and tetravalent Fe, whereas in CaSrFe2O6−δ (δ = 1) the Fe ions are almost exclusively trivalent. In addition, there is a drastic enhancement of the electrical conductivity in BaSrFe2O6−δ (δ = 0.5), by one order of magnitude, compared to CaSrFe2O6−δ (δ = 1), as demonstrated by variable temperature conductivity measurements in a wide temperature-range, 25–800 °C. Furthermore, a transition in the temperature-dependent semiconducting behavior is observed at 400 °C, where the upturn in conductivity reverses. These results indicate that the electrical properties of oxygen-deficient perovskites can be tailored by controlling the stoichiometry, distribution and ordering of defects.

Chemical reduction-induced oxygen deficiency in Co3O4 nanocubes as advanced anodes for lithium ion batteries

Cobalt (II, III) oxide (Co3O4) nanostructures have attracted much attention as a candidate for anode materials in lithium-ion batteries (LIBs) due to its unique physical/chemical properties and high specific capacity. However, critical issues, such as low electronic conductivity, inefficient ionic diffusion, and large volume change during battery operation, have to be addressed before practical applications. In this work, we report a facile chemical reduction approach to modify Co3O4 nanocubes by NaBH4 solution treatment. The microstructure and chemical composition analysis indicate that the modified Co3O4 nanocubes are oxygen deficient and other secondary phases are not formed during the processing. The degree of oxygen deficiency can be well controlled by altering the concentration of NaBH4 solution. When evaluated as the anodes for LIBs, the optimized sample shows a reversible capacity of 873.5 mAhg−1 after 50 cycles at a current density of 0.1 Ag−1, and a charge-discharge capacity of 569.1 mAhg−1 at a higher current density of 5 Ag−1. The balanced oxygen deficiency and crystallinity in the chemically reduced nanostructured electrodes are supposed to be responsible for the improved lithium storage properties. We believe that the facile solution-based chemical reduction strategy provides an alternative way to control the defects in different functional nanostructures for broader applications.

Oxygen stoichiometry controlled sharp insulator-metal transition in highly oriented VO2/TiO2 thin films

The insulator-metal transition (IMT) in vanadium dioxide (VO2) which occurs above room temperature (67 °C) is highly sensitive to atomic defects caused by oxygen stoichiometry. The strained growth and the degree of oxygen deficiency in VO2 epitaxial films result in lowering of transition temperature below room temperature as well as the broadening of transition parameters such as transition width and hysteresis width, which limit its application potential. Here we demonstrate the growth of highly oriented strain-relaxed VO2 thin films on (001)-oriented TiO2 substrates at various oxygen partial pressures, exhibiting the narrow transition and hysteresis width. The cross-sectional transmission electron microscopy and x-ray diffraction analyses of the films reveal the highly oriented growth of insulating monoclinic VO2. The IMT parameters associated with temperature-dependent phase transition vary with the oxygen partial pressure used during the deposition. The presence of multiple and mixed valence states of vanadium in the films was confirmed by Raman and XPS analyses. We have achieved a narrow transition width (2.3 °C) and hysteresis width (1.2 °C) through controlling the oxygen stoichiometry during the growth of VO2/TiO2 films.

Effects of partial pressure of oxygen on sintering of a ZrO2-30 mol% Ti powder mixture

A powder mixture of ZrO2-30mol% Ti was sintered at 1500°C/h under various controlled partial pressures of oxygen (PaO2). The microstructure of each sintered sample was characterized using x-ray diffractometry and analytical transmission electron microscopy/energy-dispersive spectroscopy. TiO and highly oxygen-deficient zirconia were found after sintering at PaO2 ~2.1×10−4atm, which implies that the oxidation–reduction reaction is a controlling mechanism when there is a trace of residual oxygen. When the PaO2 was increased to 1.05×10−1atm, the dissolution of residual oxygen and nitrogen in the chamber into the titanium led to the formation of TiO(N) with nitrogen in solid solution. It was inferred that the oxygen in TiO(N) was also supplied by zirconia since the zirconia became oxygen-deficient in the sintered composites. After sintering in air (PaO2 ~2.1×10−1atm), TiO2 and TiN were formed along with nearly stoichiometric zirconia, indicating that oxygen and nitrogen in air played a major role in the oxidation and nitridization reaction of titanium. The degree of oxygen deficiency x in ZrO2−x decreased with increasing oxygen partial pressure, which led to an increased volume fraction of the monoclinic phase.

Optimal levels of oxygen deficiency in the visible light photocatalyst TiO2−x and long-term stability of catalytic performance

The dependence of the visible light-responsive photocatalytic activity of oxygen deficient TiO2 (TiO2−x) prepared by Ar/H2 plasma surface treatment on the degree of oxygen deficiency (x) was assessed to determine the deficiency region associated with highest performance. The highest activity was obtained at x=0.06 (TiO1.94). The maximum visible light activity for this material, estimated from the formaldehyde (HCHO) removal rate, was three times higher than that exhibited by nitrogen-doped TiO2 (TiO2−xNx). The catalytic ability was found to decrease over the first week after fabrication of the material, after which it became stable, and the performance of TiO2−x at this point was found to be nearly equal to that of TiO2−xNx. The results of ab initio calculations of density of states for TiO2−x suggest that new oxygen deficiency states emerge at almost the exact center between the valence and conduction bands when x>0.06, which increases the recombination rate between electrons and holes. Therefore the declining performance of TiO2−x at larger x values is attributed to the emergence of new oxygen deficient states.

Exchange coupling-induced uniaxial anisotropy in La0.7Sr0.3MnO3 thin films

Establishing a deeper understanding of the anisotropy in manganites is useful for tailoring their magnetic properties for device applications. Here we showed that ferromagnetic La0.7Sr0.3MnO3 (LSMO) thin films, epitaxially grown on SrTiO3 (STO) substrates, exhibited unexpected double-shifted magnetization curves originating from the competition between common biaxial and emergent uniaxial anisotropies. This emergent uniaxial anisotropy could be induced by exchange coupling between the ferromagnetic LSMO and an antiferromagnetic LSMO dead layer at the LSMO/STO interface, which could be manipulated by the degree of oxygen deficiency.

Stoichiometric Study of the Dielectric and Magnetic Properties in Charge Frustrated System LuFe2O4

We report dielectric and magnetic properties of a single crystal of LuFe 2 O 4 with different degrees of oxygen deficiency. The oxygen stoichiometry was monitored with the gas mixture ratio of CO 2 and CO during the crystal growing process. A sample having high resistivity of about 10 kΩ cm sample was found at room temperature. The relation between the control of the charge frustration and the completeness of the charge ordering is discussed.

Influence of oxygen deficiency in indium tin oxide on the performance of polymer light-emitting diodes

We investigated effects of oxygen deficiency in the indium tin oxide (ITO) on the performance of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV)-based polymer light-emitting diodes, in which the ITO anode was deposited by radio frequency magnetron sputtering at different oxygen flow rates. We found that the degree of oxygen deficiency in the ITO films can affect the device performance significantly and is a source of current leakage. At the optimal oxygen flow rate, the leakage current of devices can be reduced and the balance between hole and electron fluxes can be promoted in the MEH-PPV layer to improve device efficiency.

Reaction Between Titanium and Zirconia Powders During Sintering at 1500°C

Zirconia–titanium (ZrO2–Ti) composites have been considered potential thermal barrier graded materials for applications in the aerospace industry. Powder mixtures of Ti and 3 mol% Y2O3 partially stabilized ZrO2 in various ratios were sintered at 1500°C for 1 h in argon. The microstructures of the as‐sintered composites were characterized by X‐ray diffraction and transmission electron microscopy/energy‐dispersive spectroscopy. Ti reacted with and was mutually soluble in ZrO2, resulting in the formation of α‐Ti(O, Zr), Ti2ZrO, and/or TiO. These oxygen‐containing phases extracted oxygen ions from ZrO2, whereby oxygen‐deficient ZrO2 was generated. For relatively small Ti/ZrO2 ratios, specimens with ≤30 mol% Ti, TiO were formed as oxygen could be sufficiently supplied by excess ZrO2. For the specimens with ≥50 mol% Ti, lamellar Ti2ZrO was precipitated in α‐Ti(Zr, O), with no TiO being found. Both m‐ZrO2−x and t‐ZrO2−x were found in specimens with ≤50 mol% Ti; however, only c‐ZrO2−x was formed in the specimen with 70 mol% Ti. As ZrO2 was gradually dissolved into Ti, yttria was retained in ZrO2 because of the very limited solubility of yttria in α‐Ti(O, Zr) or TiO. The concentration of retained yttria and the degree of oxygen deficiency in ZrO2 increased with the Ti content. The complete dissolution of ZrO2 into Ti was followed by the precipitation of Y2Ti2O7 in the specimen with 90 mol% Ti.

Magnetic properties of Ho0.1Sr0.9CoO3− perovskites (0.15⩽x⩽0.49)

Abstract The cobalt-based perovskite Ho 0.1 Sr 0.9 CoO 3− x ( x =0.15, 0.27, 0.31, 0.42, 0.49) has been studied by means of neutron diffraction as well as bulk susceptibility and magnetization measurements over the temperature range of 10– 300 K . It is found that the degree of oxygen deficiency not only strongly affects the magnetic ordering temperature, but also influences the magnetic and crystal structures.

Gas sensing properties of WO 3 thick film for NO 2 gas dependent on process condition

The thick film sensor of tungsten oxide for NO 2 gas detection was fabricated by changing the process conditions, which were firing temperature and annealing atmosphere. The origin of sensor property variation was interpreted by the characterization of crystallite size, specific surface area and microstructure. The degree of oxygen deficiency in the WO 3 sensor also affected sensor properties and the optimum oxygen content of WO 3 was necessary to get high sensitivity for NO 2. The WO 3 thick film fired at 700°C and operated at 100°C showed excellent sensor properties, and its operation temperature is lower than that from other reports.

Effects of severe oxygen depletion on macrobenthos in the Pomeranian Bay (southern Baltic Sea): a case study in a shallow, sublittoral habitat characterised by low species richness

Severe oxygen depletion was detected in shallow parts of the Pomeranian Bay (southern Baltic Sea) for the first time in July/August 1994. A combination of extraordinary meteorological and hydrographical conditions along with generally high nutrient loads in this coastal area is thought to have led to extensive hypoxia/anoxia. Effects of this event on the macrobenthos were studied by comparing the community structure before and after summer 1994 at four sites which differed in degree of oxygen deficiency. Observed changes in the macrobenthos at three stations were attributed mainly to the hypoxia/anoxia event in summer 1994. Macrobenthos recovery after the oxygen depletion did not follow the commonly described succession pattern after a disturbance event, which is characterised by a mass recruitment of opportunistic species together with a rapid species turnover. At station 2, the most severely affected site, species number, total abundance, and total biomass of macrobenthos decreased significantly after the oxygen deficiency event, and recolonisation was still not complete two years later. Stations 1 and 3, which were moderately affected, showed almost complete recovery with respect to species composition and abundance within two years, but biomass was still lower. While recolonisation by juvenile and adult stages from nearby unimpacted coastal areas occurred at stations 1 and 3, succession was dominated by post-larval colonisation from planktonic dispersal at station 2. Station 4 was not affected by oxygen depletion and showed only small variation of community structure. The generally very slow recovery of amphipods at all three affected stations and even of the oligochaetes ( Tubificoides ( Peloscolex) benedeni and Heterochaeta ( Tubifex) costata) at station 2 further underline the severity of this oxygen deficiency in Pomeranian Bay sediments.

High-resolution proton nuclear magnetic resonance studies of urine from asphyxiated newborn infants

High-resolution proton nuclear magnetic resonance spectroscopy was used to study human urine obtained from 10 normal babies and twenty babies with various degrees of neonatal asphyxia, respiratory distress syndrome (RDS), and meconium aspiration syndrome (MAS). All sick babies showed different degrees of oxygen deficiency, indicated by an obvious increase of the lactate signal level in the urine spectra. Changes in the concentration of other urinary metabolites produced from the citric acid cycle were also observed. In extremely serious cases, the signals of some of the major components, including citrate, alpha-ketoglutarate, and succinate, simply disappeared. The spectra of urine, serum, and CSF of an infant suffering from SIDS showed common characteristics of the metabolites.

Dielectric Relaxation and Hopping of Electrons in ErFe2O4

We report the frequency and the temperature dependence of the dielectric constant of polycrystalline ErFe 2 O 4-δ with different degrees of oxygen deficiency δ. A large dielectric relaxation, with the order of magnitude of 10 4 and nearly of the Debye type, is observed in the magnetically ordered states within the temperature and frequency ranges of 150 K to 290 K and 1 kHz to 3 MHz, respectively. The dispersion is larger in the sample with more oxygen deficiency. From the temperature dependence of the characteristic frequency, we conclude that the elementary process of the dispersion is related to electron hopping between Fe 2+ and Fe 3+ ions. A large value of the dielectric constant is consistent with the existence of spontaneous polarization in the magnetic phases of this oxide. We propose that the motion of the polarization domain boundaries is a determining factor of low-frequency dielectric constant.

Formation of oxygen-deficient water mass in Omura Bay

Omura Bay is a typical enclosed inner bay. Thermal stratification begins gradually in the central part of the bay in May when the water temperatures difference between the surface and the bottom shows a tendency of increase. Oxygen-deficient water in the bottom layer is constantly formed every summer. In August, the degree of oxygen-deficiency is the highest. Nutrients(PO 4-P and NH 4-N)are released from the bottom sediments under these anaerobic conditions. The water circulation begins in October or November. The anaerobic condition of the water is sometimes influenced by wind. Low dissolved oxygen concentration in the surface water was observed when an upwelling of oxygen-deficient water occurred due to a drift current. Formation of oxygen-deficient water mass during the summer influences marine organisms in the bay. Marine resources are damaged by the upwelling of the water. The circulation of the water mass in autumn accelerates the occurrences of red tides.