Substitution Of Oxygen Research Articles

Electronic and optical properties of [formula omitted]-doped monolayer [formula omitted

The O-doping effects for monolayer molybdenum disulfide (MoS2) are systematically investigated by first-principle calculations. It is shown that the geometrical, electronic and optical properties are affected distinctively by the oxygen dopant. Structural analysis reveals a local contraction along c axis in the substituted cases. The substitution of oxygen for a sulfur atom in monolayer MoS2 leads to a transition from a direct K–K band gap to an indirect Γ–K band gap. And, the value of band gap decreases with increasing doping concentration. In addition, for the pure MoS2, strong covalent chemical bonds are formed on the Mo–S bonds, which is ascribed to the strong hybridization between Mo-4d and S-3p orbitals. After oxygen doping, the covalent bonding of Mo–S is distinctively weakened. More electrons are transferred from Mo to O because of the larger electronegativity of O, and the atomic populations of O atoms become larger than that of S atoms. Optical properties are also found to be affected distinctively by the oxygen dopant. An interesting blue-shift of the absorption threshold is observed in the O-doped systems.

Rhodium-catalysed asymmetric allylic arylation of racemic halides with arylboronic acids.

Csp(2)-Csp(2) cross-coupling reactions between arylboronic acid and aryl halides are widely used in both academia and industry and are strategically important in the development of new agrochemicals and pharmaceuticals. Csp(2)-Csp(3) cross-coupling reactions have been developed, but enantioselective variations are rare and simply retaining the stereochemistry is a problem. Here we report a highly enantioselective Csp(2)-Csp(3) bond-forming method that couples arylboronic acids to racemic allyl chlorides. Both enantiomers of a cyclic chloride are converted into a single enantiomer of product via a dynamic kinetic asymmetric transformation. This Rh-catalysed method uses readily available and inexpensive building blocks and is mild and broadly applicable. For electron-deficient, electron-rich or ortho-substituted boronic acids better results are obtained with racemic allyl bromides. Oxygen substitution in the allyl halide is tolerated and the products can be functionalized to provide diverse building blocks. The approach fills a significant gap in the methods for catalytic asymmetric synthesis.

Morphology and growth behavior of O2-free chemical bath deposited ZnS thin films

We investigate the role of reagent concentrations and ambient O2 on the morphology and growth behavior of ZnS thin films grown with the chemical bath deposition method. We investigate the role of substrate on film morphology, and find significant differences between films deposited on SiO2 versus Si. The films are also sensitive to dissolved O2 in the bath, as it causes a layer of SiO2 to form at the ZnS/Si interface during deposition. Degassing of solutions and an N2 atmosphere are effective to minimize this oxidation, allowing deposition of ZnS films directly onto Si. Under these conditions, we examine film properties as they relate to reagent bath concentrations. As the reagent concentrations are decreased, both the film roughness and growth rate decrease linearly. We also observe deformation and shifting of X-ray diffraction peaks that increases with decreasing reagent concentrations. The shifts are characteristic of lattice compression (caused by the substitution of oxygen for sulfur), and the deformation is characteristic of distortion of the lattice near crystal grain interfaces (caused by tensile stress from interatomic forces between neighboring crystal grains). At the weakest concentrations, the low roughness suggests a mixed growth mode in which both clusters and individual ZnS nanocrystallites contribute to film growth. With increasing reagent concentrations, the growth mode shifts and becomes dominated by deposition of clusters.

Ar plasma treated ZnON transistor for future thin film electronics

To achieve high-mobility and high-reliability oxide thin film transistors (TFTs), ZnON has been investigated following an anion control strategy based on the substitution of oxygen with nitrogen in ZnO. However, as nitrogen possesses, compared to oxygen, a low reactivity with Zn, the chemical composition of ZnON changes easily, causing in turn a degradation of both the performance and the stability. Here, we have solved the issues of long-time stability and composition non-uniformity while maintaining a high channel mobility by adopting the argon plasma process, which can delay the reaction of oxygen with Zn–O–N; as a result, owing to the formation of very fine nano-crystalline structure in stable glassy phase without changes in the chemical composition, the material properties and stability under e-radiation have significantly improved. In particular, the channel mobility of the ZnON TFTs extracted from the pulsed I−V method was measured to be 138 cm2/V s.

Aqueous Humour and Serum Levels of Nitric Oxide, Malondialdehyde and Total Antioxidant Status in Patients with Type 2 Diabetes with Proliferative Diabetic Retinopathy and Nondiabetic Senile Cataracts.

Diabetic retinopathy is associated with enhanced oxidative stress and/or reduction of antioxidant status. Several studies have examined the oxidative agents, such as nitric oxide (NO) and malondialdehyde (MDA), in the body fluids of patients with diabetes. However, total antioxidant status (TAS) in the aqueous humour of patients with diabetes has not been determined to date. To address this issue, we analyzed the aqueous humour and serum levels of NO, MDA and TAS in patients with type 2 diabetes and nondiabetic senile cataracts. This prospective study included 35 patients with type 2 diabetes and 35 age- and sex-matched healthy subjects in whom cataract surgery was indicated. Aqueous humour and serum MDA, NO and TAS levels were determined by spectrophotometric methods, respectively. The analysis of MDA levels in the serum and aqueous humour revealed no significant differences in any of the groups (p>0.05). At the level of aqueous humour, patients with type 2 diabetes had significantly increased NO levels, compared to the controls (p=0.003). The control group also presented significantly higher TAS levels than the subgroup with type 2 diabetes in serum (p=0.001). However, there were no significant differences in the TAS levels of aqueous humour and serum NO levels in the groups (p>0.05). Our results seem to demonstrate that the development of diabetic retinopathy is associated with high levels of aqueous humour NO and reduced serum antioxidant defenses. Therefore, inhibition of reactive oxygen species production and substitution of serum antioxidant status may be a therapeutic target for eye diseases associated with oxidative stress.

Activity studies of sesquiterpene oxides and sulfides from the plant Hyptis suaveolens (Lamiaceae) and its repellency on Ixodes ricinus (Acari: Ixodidae).

Hyptis suaveolens (Lamiaceae), a plant traditionally used as a mosquito repellent, has been investigated for repellent properties against nymphs of the tick Ixodes ricinus. Essential oils and volatile compounds of fresh and dried leaves, from plants originating from Laos and Guinea-Bissau, were identified by GC-MS and tested in a tick repellency bioassay. All the essential oils were strongly repellent against the ticks, even though the main volatile constituents differed in their proportions of potentially tick repellent chemicals. (+)/(-)-sabinene were present in high amounts in all preparations, and dominated the emission from dry and fresh leaves together with 1,8-cineol and α-phellandrene. 1,8-Cineol and sabinene were major compounds in the essential oils from H. suaveolens from Laos. Main compounds in H. suaveolens from Guinea-Bissau were (-)-sabinene, limonene and terpinolene. Among the sesquiterpene hydrocarbons identified, α-humulene exhibited strong tick repellency (96.8 %). Structure activity studies of oxidation or sulfidation products of germacrene D, α-humulene and β-caryophyllene, showed increased tick repellent activity: of mint sulfide (59.4 %), humulene-6,7-oxide (94.5 %) and caryophyllene-6,7-oxide (96.9 %). The substitution of oxygen with sulfur slightly lowered the repellency. The effects of the constituents in the oils can then be regarded as a trade off between the subsequently lower volatility of the sesquiterpene derivatives compared to the monoterpenes and may thus increase their potential usefulness as tick repellents.

Structure Stabilization by Mixed Anions in Oxyfluoride Cathodes for High-Energy Lithium Batteries.

Mixed-anion oxyfluorides (i.e., FeOxF2-x) are an appealing alternative to pure fluorides as high-capacity cathodes in lithium batteries, with enhanced cyclability via oxygen substitution. However, it is still unclear how the mixed anions impact the local phase transformation and structural stability of oxyfluorides during cycling due to the complexity of electrochemical reactions, involving both lithium intercalation and conversion. Herein, we investigated the local chemical and structural ordering in FeO0.7F1.3 at length scales spanning from single particles to the bulk electrode, via a combination of electron spectrum-imaging, magnetization, electrochemistry, and synchrotron X-ray measurements. The FeO0.7F1.3 nanoparticles retain a FeF2-like rutile structure but chemically heterogeneous, with an F-rich core covered by thin O-rich shell. Upon lithiation the O-rich rutile phase is transformed into Li-Fe-O(-F) rocksalt that has high lattice coherency with converted metallic Fe, a feature that may facilitate the local electronic and ionic transport. The O-rich rocksalt is highly stable over lithiation/delithiation and thus advantageous to maintain the integrity of the particle, and due to its predominant distribution on the surface, it is expected to prevent the catalytic interaction of Fe with electrolyte. Our findings of the structural origin of cycling stability in oxyfluorides may provide insights into developing viable high-energy electrodes for lithium batteries.

First-Principles Design and Analysis of an Efficient, Pb-Free Ferroelectric Photovoltaic Absorber Derived from ZnSnO3

Ferroelectric oxides, which exhibit a spontaneous and reversible electric polarization, have recently gained interest for photovoltaic applications, because this polarization can potentially facilitate exciton separation and carrier extraction while also generating open-circuit voltages orders-of-magnitude larger than the band gap. However, photovoltaic efficiencies in these materials are often limited by large band gaps and high hole effective masses. Developing a means to simultaneously reduce both without destroying the ferroelectric polarization could revolutionize photovoltaic technologies. In this work, we use first-principles computations to describe how chemical substitution of oxygen in the recently characterized ferroelectric ZnSnO3 with sulfur to form ZnSnS3 reduces the band gap to a near-optimal 1.3 eV while leaving the polarization virtually unchanged. Furthermore, we show that other key photovoltaic materials characteristics, such as hole effective mass, dielectric constant, and absorption c...

A Novel Glycolipid Antigen for NKT Cells That Preferentially Induces IFN-γ Production.

In this article, we characterize a novel Ag for invariant NKT (iNKT) cells capable of producing an especially robust Th1 response. This glycosphingolipid, DB06-1, is similar in chemical structure to the well-studied α-galactosylceramide (αGalCer), with the only change being a single atom: the substitution of a carbonyl oxygen with a sulfur atom. Although DB06-1 is not a more effective Ag in vitro, the small chemical change has a marked impact on the ability of this lipid Ag to stimulate iNKT cells in vivo, with increased IFN-γ production at 24 h compared with αGalCer, increased IL-12, and increased activation of NK cells to produce IFN-γ. These changes are correlated with an enhanced ability of DB06-1 to load in the CD1d molecules expressed by dendritic cells in vivo. Moreover, structural studies suggest a tighter fit into the CD1d binding groove by DB06-1 compared with αGalCer. Surprisingly, when iNKT cells previously exposed to DB06-1 are restimulated weeks later, they have greatly increased IL-10 production. Therefore, our data are consistent with a model whereby augmented and or prolonged presentation of a glycolipid Ag leads to increased activation of NK cells and a Th1-skewed immune response, which may result, in part, from enhanced loading into CD1d. Furthermore, our data suggest that strong antigenic stimulation in vivo may lead to the expansion of IL-10-producing iNKT cells, which could counteract the benefits of increased early IFN-γ production.

Influence of Oxygen, Tellurium, and Zinc Substitution on CdSe Nanoribbon: A First-Principles Investigation

The electronic transport property and band structure of pure cadmium selenide, oxygen, tellurium, and zinc substituted CdSe nanoribbon and defect structured CdSe nanoribbon are studied using density functional theory. The band structure of pure, oxygen, tellurium, and zinc substituted CdSe nanoribbon and defect structured nanoribbon shows a metallic behavior. The localization of charges in the valence band and conduction band is analyzed by the density of states spectrum. The electronic transport properties of CdSe nanoribbon are investigated in terms of transmission spectrum. The transmission can be modified with the substitution impurities such as oxygen, tellurium and zinc in the nanostructure.

ChemInform Abstract: Influence of Sulfur for Oxygen Substitution in the Solvolytic Reactions of Chloroformate Esters and Related Compounds

AbstractReview: 88 refs.

Theoretical study of the effects of vacancy and oxygen impurity on Ti2GaC)**Project supported by the National Magnetic Confinement Fusion Science Program of China (Grant No. 2014GB104002), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA03030100), and the National Natural Science Foundation of China (Grant Nos. 11275156 and 11304324).

This paper presents the mono-vacancy formation and migration energies of each element Ti, Ga, and C in the MAX phase Ti2GaC, which are obtained by first principles calculations. We also calculate the formation energies of oxygen substituting for Ti, Ga, and C and two formation energies of oxygen interstitial in different sites. The results show that the formation energy of oxygen substituting for Ti is the highest, and the formation energies of the O substitution for Ga atoms decrease as the oxygen concentration increases. The two different formation energies of one oxygen interstitial show that the stable site for the oxygen interstitial is at the center of the triangle composed by three Ga atoms. The effects of vacancy, oxygen substitution, and the interstitial on the electronic properties of Ti2GaC are also discussed in light of the density of states and the electron charge density.

Influence of oxygen on characteristics of Zn(O,S) thin films deposited by RF magnetron sputtering

Zn(O,S) thin films were successfully deposited by reactive sputtering using Ar and O2 gas mixtures at 473 K. X-ray diffraction patterns revealed that the well crystallized Zn(O,S) films were deposited with increasing oxygen concentration in O2/Ar, resulting in a shift of the Zn peak of 28.5° to a higher angle, closer to the ZnO peak of 34.4°. Zn(O,S) films were composed of grains agglomerated from small particles, which grew gradually with increasing oxygen concentration. The depth profiles and energy dispersive spectroscopy results of the films indicated that the O/(O+S) ratio increased from 0.04 to 0.81, and all Zn(O,S) films were Zn rich with uniform concentrations of each component. X-ray photoelectron spectroscopy revealed that, as the oxygen concentration increased to 2%, the ZnS films were transformed to Zn(O,S) films via substitution of oxygen for sulfur.

Emission properties and temperature dependence of Cu+ luminescence in the CaO–CaF2–P2O5 ternary glass system co-doped with CuO and SnO

Co-doping of the (50−x)CaO–xCaF2–50P2O5 (x=0,10, and 20) ternary glass system with CuO and SnO has been realized by the melt-quenching technique with the purpose of investigating on material photoluminescence (PL). The glasses display tunable luminescence by variation of excitation wavelength in connection with the distinct excitation/emission properties of Cu+ ions and Sn centers. Substitution of CaO by CaF2 appeared to support the Cu+ luminescence process. Analyses of emission decay dynamics suggest that both luminescent species exist primarily coordinated to oxygen atoms in the oxyfluorophosphate hosts. Further, the effect of temperature on Cu+ PL was evaluated from room temperature to 300°C. An increase in the activation energy for the thermal quenching of Cu+ PL was observed upon the substitution of oxygen by fluorine ions. Accordingly, the Cu+-doped oxyfluorophosphate matrix shows potential as thermally-stable glass for optical device applications (e.g. lighting, solar cells).

Thio-oxynitride phosphate glass electrolytes prepared by mechanical milling

Abstract

Understanding the Electrochemical Behavior of Vanadium Dioxyfluoride

After intensive investigation of light transition metal oxides as electrodes for lithium ion rechargeable batteries oxides new chemistries based on different ligands are needed to fulfill performance requirements in highly energy demanding devices. In this context transition metal (TM) fluorides have been investigated as both convertible and intercalation electrodes. In the latter case the main drawback is the high insulating behavior arising from the high ionic character of M-F bond. However, the increment of intercalation potential with respect to similar oxides can be advantageous. In some cases the intercalation potential has been in fact reported to exceed the electrochemical stability window of the commonly used liquid electrolytes. The partial substitution of oxygen by fluorine may have a more controlled effect on potential rising. Proper doping with fluorine may produce an increase of the intercalation potential high enough to get a significant increase of specific energy without exceeding the high voltage stability limit of the electrolyte. Nonetheless, stoichiometric control is not easily achieved. On the other hand, crystal chemistry differences between oxygen and fluorine may provide new crystalline structure with relevant topotactical features for intercalation. Note that among TM fluorides a large variety of structures are found, ranging from crystal structure with isolated M-F6 octahedra to true tridimensional structures.Following our work on cryolites Li3MF6 (M=Fe and V) we report now part of our research on oxyfluorides presenting a very interesting example that fills the gap in the MO2F compounds of Group 5 elements: V, Nb and Ta. Up to now NbO2F and TaO2F were known, but no evidences of the existence of VO2F were found. Even although intercalation of lithium into NbO2F has been reported, vanadium is much more interesting owing to its lightness. We have found a synthesis procedure to obtain pure samples of VO2F. Vanadium dioxyfluoride is isostructural with TiO2F exhibiting a hexagonal structure related to the ReO3 structure. The investigation of chemical composition, structure and oxidation state of vanadium has been completed with a study of its electrochemical behavior. A long discharge from 4 to 1.5 volts develops ca. 450 mAh/g. The voltage profile during the whole first discharge – charge cycle and further cycling (reversible capacity of 350mAh/g) indicates that VO2F suffers an irreversible phase transformation. The investigation of the phase formed down to 1.5 V is now under progress. However, we are also focusing on the high voltage region where a kinetically limited reaction is likely occurring. Between 4 and 2.25 V a reversible cell capacity of 250 mAh/g is developed. This result is outstanding inasmuch as kinetics limitation arising from the highly insulating character of fluorides and many oxyfluorides tend to inhibit the intercalation of lithium, favoring instead convertible reactions at lower voltage.

Effects of non-metal dopants and defects on electronic properties of barium titanate as photocatalyst

Design of novel photocatalyst through native defect and doping is a simple and effective approach to improve its performance in photocatalytic reaction, such as solar-driven hydrogen production. In this work, we present first-principles calculations on the engineering of the electronic structure of BaTiO3 by creating defects and introducing dopants for its application in photocatalytic water-splitting. We show that native vacancies may change its conducting character, but is difficult to reduce its band gap. The reduction of band gap can be achieved by hydrogenation at high density and N substitution of oxygen, resulting in enhanced sun-light absorption. We further show that intermediate bands are created by substituting Ba with NH-pair and inserting interstitial N. These intermediate bands play an important role in the visible-light absorption. We expect that BaTiO3 with band structure modified by hydrogenation, N-doping, and NH-codoping can be applicable in solar-energy harvesting with improved efficiency.

Effect of Nitrogen and Fluorine Co-substitution on the Structure and Magnetic Properties of Cr2 O3.

First-principles density functional calculations were carried out to determine the structure as well as electronic and magnetic properties of N and F co-substituted Cr2 O3 . The formation of strong CrN bonds upon substitution of oxygen with nitrogen leads to large distortions in the local structure and changes in magnetic moments, which are partly compensated by co-substitution with fluorine. The effects of spin-orbit coupling are relatively weak, but its combination with local structural distortions gives rise to canting of spins and an overall magnetic moment in N, F co-substituted Cr2 O3 . Experimentally, we observe spin canting in N, F co-substituted Cr2 O3 with considerable enhancement in the coercive field at low temperatures.

Application of the GGA-1/2 excited-state correction method to p-electron defective states: the special case of nitrogen-doped TiO2

One of the issues in applying the generalized gradient approximation–1/2 (GGA-1/2) quasiparticle approximation to correct the excited-state properties in defective semiconductors is determining the cutoff to the self-energy function at the point defect. Pure and large supercells of N-doped rutile TiO2 were studied using this method to correct excited states and find the correct position of the defect states. A relation between the Bader charge at the defect and the self-energy cutoff parameter of the GGA-1/2 method is shown, with the cutoff value in its p-orbital, which optimizes the position of the defect levels. It was found that, upon nitrogen substitution and associated oxygen vacancy formation, an impurity level at 0.40–0.52 eV above the valence band maximum appears. The same result was obtained upon nitrogen substitution and associated background charge. Finally, the method was also applied to other p-orbital defect systems like Si:X (X = N, P, B) to validate the method.

Polymorphic phase study on nitrogen-doped TiO2 nanoparticles: effect on oxygen site occupancy, dye sensitized solar cells efficiency and hydrogen production

In this work we show that phase formation and oxygen substitution can be controlled by the source of nitrogen used during the synthesis of TiO2 nanoparticles.