Charged Oxygen Species Research Articles

Optoelectronic properties of three-dimensional ZnO hybrid structure

Three-dimensional (3D) ZnO hybrid structure was fabricated by growing a ZnO buffer layer, a ZnO nanowire array, and a ZnO film continuously through the control of supersaturation conditions. Lower and upper ends of vertically aligned nanowires in this hybrid structure formed seamless interfacial contacts with the buffer layer and the film for current conduction. Photocurrent was generated only when the ultraviolet (UV) light (λ=350nm) was irradiated. This structure also exhibited different atmosphere-dependent responses to the UV light. The optoelectronic properties of the 3D structure are attributed to the photogenerated carriers and the surface reaction of negatively charged oxygen species in ZnO nanowires.

Study of adsorption of oxygen on β-Al 2O 3 + Au and β-Al 2O 3 + Pt : Work function measurements—proposition of a model

To improve the understanding of the electrochemical effects observed on an original potentiometric gas sensor, interactions of oxygen with the device were investigated. This gas sensor is made of a solid electrolyte (treated Na-β-alumina) associated with two metallic electrodes (gold and platinum) located in the same gas mixture. Adsorption of charged oxygen species, considered responsible for the electrical response developed by the sensor, was investigated by work function measurements. Results showed that charged oxygen species only form on partially gold or platinum covered solid electrolyte. Comparison of these results with those obtained in a previous calorimetric study of interactions between oxygen and the same materials suggests the existence of at least two different oxygen species adsorbed on the surface of the sensitive element. The first one, located on the solid electrolyte surface, is neutral and characterized by an endothermal reaction of formation. The second one is charged and probably produced at the gas/solid electrolyte/metallic electrode interface. A mechanism based on the concept of “three phase boundary” and similar to the “reverse spillover” phenomenon is proposed to account for the adsorption of these oxygen species.

From oxygen adsorption to the growth of thin oxides on silicon surfaces

Oxidation of silicon surfaces at relatively low temperatures is shown to go through several activated steps, in the form of configurations inert to further uptake of oxygen. Starting from room temperature adsorption, different configurations of oxygen atoms adsorbed on and in the Si(1 1 1) and Si(0 0 1) surfaces are found, with history and/or coverage dependent energy barriers connecting them. From well below to slightly above an effective oxide coverage of a monolayer, clustering of up to three oxygen atoms around one single silicon atom has been predicted for the Si(0 0 1) surface to represent one such energy minimum; this model is confirmed here experimentally. These and other clusters are shown to agglomerate into silicon dioxide islands before coalescing into a contiguous, inert layer upon higher oxygen supplies. Another problem addressed here is the presence of molecular adsorbates in the oxidation reaction path, an issue which is still debated in the literature. For the Si(1 1 1) surface a molecular, charged oxygen species has earlier been found at temperatures up to room temperature, but not for the Si(0 0 1) surface. This is confirmed in the present experiments, and new data for this state shows that it is highly mobile until quenched at a critical oxygen coverage. It is not the initial state of oxygen on silicon, and therefore not the precursor for atomic insertion of oxygen; rather, it is found to co-exists with atomic oxygen inserted in back-bonds, at a certain, low coverage regime in which parts of the Si(1 1 1) surface are still ordered.

Dynamics of charge transfer states on metal surfaces: the competition between reactivity and quenching.

The dynamics of excited states of adsorbates on surfaces caused by charge transfer is studied. Both negative and positive charge transfer processes are possible. In particular we are interested in positive charge transfer from a metal surface to molecular or atomic oxygen adsorbed on the surface. Once the negatively charged oxygen on the surface loses an electron it becomes chemically activated. The ability of this species to react depends on the quenching time or back transfer. The analysis of these processes is based on a set of diabatic potential energy surfaces each representing a different charged oxygen species. The dynamics is followed by solving the multichannel time-dependent Schrödinger equation or Liouville von Neumann equation. Due to the nonadiabatic character of these reactions large isotope effects are predicted.

In Situ Electrical Conductivity Study of Ni-V-O Catalysts Used in Oxidative Dehydrogenation of Propane

electrical conductivity of ni-v-o series catalysts was measured under different sequential gaseous atmospheres and at different temperatures, their conduction type was detemined and their catalytic behaviour in oxidative dehydrogenation of propane to propene was investigated. it was found that p-type semiconductor catalysts have a better catalytic behaviour than n-type ones. in reaction mixture, the higher the electrical conductivity of p-type semiconductor, the higher its catalytic activity, the lower electrical conductivity of n-type semiconductor, the higher its catalytic activity. these are due to different mechanism of the formation of the partial charged oxygen species on the surface of catalysts.

Non-adiabatic charge transfer process of oxygen on metal surfaces

The dynamics of charge transfer processes of oxygen on metal surfaces are investigated. The analysis is based on a set of diabatic potential energy surfaces, each representing a different charged oxygen species. Empirical universal potential energy functions have been constructed that mimic the oxygen–silver, oxygen–aluminum and oxygen–cesium systems. The differences between the work functions of these metals are reflected in the potential parameters. The dynamics are followed by solving the multichannel time-dependent Schrödinger equation starting from oxygen in the gas phase. Only the direct short time part of the dynamics is followed leading to the creation O − 2 in the gas phase as well as dissociative chemisorption. A large portion of the wave function is trapped in molecular chemisorption charged states. It is found that the position of the crossing seam between potentials has a profound influence on the outcome.

Dynamics of Silicon Oxidation

ABSTRACTThe rate-determining process in dry oxidation of silicon for thicker oxides (say above 10 nm) is probably the interstitial diffusion of oxygen molecules. For thinner oxides (a few nm), this simple picture is inadequate. The Deal-Grove model of oxidation kinetics fails. Oxide usually grows by an essentially layer-by-layer process, with growth at terraces, not steps, and perhaps oscillatory roughening. Isotope experiments show that interstitial and network oxygens exchange close to the Si/oxide interface and close to the oxide/gas interface. These results imply limiting mechanisms other than diffusion, probably involving charged oxygen species. We have made Density Functional (DFT) Generalised Gradient Approximation (GGA) calculations for a range of neutral and charged oxygen species to assess relative stabilities, diffusion mechanisms, and propensity for isotope exchange. These results identify acceptable mechanisms. We have also used Monte-Carlo methods to examine the consequences of the image interaction bias, and also the effects of the charge transfer processes (like tunnelling) by which charged species form. We comment on implications for oxide quality, especially on the relationship between growth processes and the charge and energy localisation components of breakdown.

Taurine conjugates in the lipid fraction of Euglena cells and their mitochondria

Summary: Dark-grown resting (non-dividing) cells of Euglena gracilis var. bacillaris and mutants W3BUL (with a proplastid remnant) and W10BSmL (lacking plastids) incubated with 35SO2- 4 form a series of labelled lipids which are low or absent in dividing cells. These lipids all release labelled taurine on mild acid-hydrolysis. Treatment of the labelled lipids with 2,4-dinitrofluorobenzene (DNFB) followed by acid hydrolysis does not yield labelled dinitrophenyltaurine (DNP-taurine), but treatment with DNFB after hydrolysis readily forms labelled DNP-taurine, indicating that taurine is linked to the lipids by at least the amino group. Illumination increases the labelling of these taurolipids in plastid-containing cells (wild-type and W3BUL) but has little effect in cells lacking plastids (W10BSmL); labelling is highest in W10 cells irrespective of illumination. This indicates that the presence of a plastid may exert a negative control on taurolipid formation which is relieved by light. The same series of labelled lipids is found in isolated purified mitochondria from mutant W10, indicating that this organelle is a site for taurolipid deposition. The formation of taurolipids under non-dividing conditions may be a response to nutritional stress and these negatively charged constituents (as well as the thylakoid sulpholipid) may serve to protect membranes by repelling deleterious negatively charged oxygen species.

The low temperature adsorption of oxygen onto Ni oxides

In this paper we report on the low-temperature, reversible chemisorption of diatomic oxygen onto NiO, grown in situ on single-crystal Ni substrates. One narrow (γ) and two broad (α 1, α 2) thermal desorption peaks are observed, that exhibit no isotopic mixing during desorption. α 1, and α 2 are high-temperature states, with α 1 existing to well above room temperature. Both a states induce a large work-function increase during adsorption. This work-function increase with adsorption and the absence of isotopic mixing with desorption are consistent with a negatively charged diatomic oxygen species lying above the NiO plane. The adsorption is believed to occur at Ni 2O 3 point defects, with α 1 and α 2 yielding a combined saturation O 2 coverage of 0.11 ± 0.03 monolay The low temperature y peak (125 K) induces a small positive Δφ increase with desorption, suggesting an O 2 species with a small positive charge. Possible adsorption sites for oxygen in the γ state are step edges and/or domain boundaries. The maximum O 2 coverage of this state is 0.12 ± 0.03 monolayers and diminishes with substrate annealing. For all three states, adsorption is not associated with OH or CO 3 contamination, and is independent of oxide epitaxy.

The low temperature adsorption of oxygen onto Ni oxides

Abstract In this paper we report on the low-temperature, reversible chemisorption of diatomic oxygen onto NiO, grown in situ on single-crystal Ni substrates. One narrow (γ) and two broad (α1, α2) thermal desorption peaks are observed, that exhibit no isotopic mixing during desorption. α1, and α2 are high-temperature states, with α1 existing to well above room temperature. Both a states induce a large work-function increase during adsorption. This work-function increase with adsorption and the absence of isotopic mixing with desorption are consistent with a negatively charged diatomic oxygen species lying above the NiO plane. The adsorption is believed to occur at Ni2O3 point defects, with α1 and α2 yielding a combined saturation O2 coverage of 0.11 ± 0.03 monolay The low temperature y peak (125 K) induces a small positive Δφ increase with desorption, suggesting an O2 species with a small positive charge. Possible adsorption sites for oxygen in the γ state are step edges and/or domain boundaries. The maximum O2 coverage of this state is 0.12 ± 0.03 monolayers and diminishes with substrate annealing. For all three states, adsorption is not associated with OH or CO3 contamination, and is independent of oxide epitaxy.

Low-pressure oxidation of silicon stimulated by low-energy electron bombardment

Abstract The mechanisms of silicon oxide growth stimulated by electronic bombardment have been studied. Silicon single crystals, after cleaning by ion bombardment in UHV, were submitted to a low-energy (<100eV) electron irradiation (∼ 1 mA cm−2), under low oxygen pressure (10−4 Torr) in the temperature range 25–700°C. Oxide thicknesses less than 10 nm were thereby achieved. There is a strong increase of the initial growth rate when the electron energy decreases to 10eV. The oxidation kinetics can be fitted by a direct logarithmic law: X = X 0 log [(t/τ) + 1] where X is the thickness of oxide grown during time t and U = X 0/τ is the initial growth rate. Using the results of 18O tracing experiments, the growth mechanism is explained by a step-by-step motion of charged oxygen species under a constant electric field.