Chemisorbed Oxygen Atoms Research Articles

Thin Au/Ge Schottky diodes for detection of chemical reaction induced electron excitation

Ultra-thin film Au/ n-Ge Schottky diodes were used to detect electrons excited by chemisorption of atomic oxygen and atomic hydrogen. The absorption generated a chemicurrent in the diode as ballistic electrons excited by the reaction energy transfer traversed the Schottky barrier. The large-area metal-semiconductor contacts were manufactured under cleanroom conditions by evaporating Au on Ge(0 0 1). By annealing at temperatures between 370 and 400 K under ultrahigh vacuum, the sensitivity to reactive gases may be completely recovered even for samples which were stored under air atmosphere for 1 week or exposed to de-ionized water. The recovery process is explained by desorption of adsorbates and thermal decomposition of oxidized gold layers at the surface.

NO-CO catalytic reaction on a square lattice: the effect of the Eley-Rideal mechanism

Monte Carlo simulations have been used to explore the effects of the Eley-Rideal mechanism (reaction of CO molecule with already chemisorbed oxygen atom to produce CO2) on a simple Langmuir-Hinshelwood model for the NO-CO catalytic reaction on a square surface. The diffusion of the CO and N adatoms on the surface and desorption of CO from the surface are also introduced into the model. Without diffusion and desorption, the model generates a very small reactive window of the order of 0.033. The moment CO partial pressure (yCO) departs from zero, continuous production of CO2 and N2 starts. A first-order transition terminates the catalytic activity at yCO = 0.033 and the surface is poisoned with a combination of CO and N. However, the diffusion of the N atom and CO molecule shifts the transition point from 0.033 to higher values of yCO. The introduction of desorption of CO shows some interesting results. A very small desorption probability of CO (=0.01) increases the width of the reactive window to 0.12. However, this reactive window is separated by two transition points y1(≈0.2) and y2(≈0.32). For yCO y2) the surface is poisoned by a combination of O and N (CO and N). With further increase in desorption probability the width increases significantly.

Observation of excited electrons from nonadiabatic molecular reactions of NO and O 2 on polycrystalline Ag

Adsorption of ground state nitric oxide and molecular oxygen on Ag at 130 K were observed to produce excited electrons which were detectable as a “chemicurrent” in a large area ultrathin film Ag/Si(1 1 1) Schottky diode sensor. The charge carriers produced at the surface had sufficient lifetimes and energies to reach the Ag/Si interface (6–8 nm) and surmount the Schottky barrier (⩽0.5 eV). The detected current from nitric oxide exposure decreased with increasing coverage from an initial peak intensity of 1×10 −4 e −/incident molecule. A secondary peak (5×10 −5 e −/incident molecule) was observed at an exposure of approximately 22 ML. The signal decayed to the noise floor (∼100 fA) at longer exposures (>40 ML). With molecular oxygen exposure a smaller peak intensity of 1×10 −5 e −/incident molecule was observed, followed by a decay in signal to the noise floor at longer exposures (>10 ML). The signal from nitric oxide is attributed to the superposition of charge carriers produced by nonadiabatic adsorption of NO at 130 K on the Ag surface with carriers generated from the coverage dependent chemisorption of atomic oxygen produced during the formation and decomposition of (NO 2) dimers into atomic nitrous oxide. The latter is the cause of the observed secondary peak. The detected current from molecular oxygen exposure on the Ag surface at 130 K is consistent with molecular adsorption generating the detected current. The detection of this “chemicurrent” is direct experimental evidence of nonadiabatic energy transfer during molecular adsorption.

Commissioning of surface chemistry end-station in BL23SU of SPring-8

An experimental apparatus for surface reaction research is now under commissioning as an end-station of a soft X-ray beamline, BL23SU, in the SPring-8. A supersonic molecular beam (SSMB) technique is applied to control the translational-energy of incident molecules. Owing to keeping low pressure in vacuum chambers during molecular beam operation by differential-pumping systems, the suppression of gas diffusion into the beamline was sufficiently achieved. The translational-energy control and photoemission spectroscopic performance were checked by preliminary experiments concerning the silicon oxidation using a conventional X-ray source. The photoemission intensity from chemisorbed oxygen atoms on an Si(1 0 0) surface was enhanced with increasing the translational-energy of incident oxygen molecules up to 3 eV.

Dependence of the electron affinity of homoepitaxially grown CVD diamond on the amount of surface oxygen

Photoemission threshold energies have been measured for boron-doped homoepitaxially grown chemical-vapor-deposited (CVD) diamond with various degrees of the surface oxidation below 0.5 diamond (100) monolayer, or 7.8×10 14 O atoms/cm 2. In the case of the smooth surfaces, the observed threshold energy almost linearly increases with increasing amounts of chemisorbed oxygen atoms at low oxygen coverages below 0.1 monolayer and is followed by a saturation phenomenon. This behavior can be explained in terms of the Topping model, where the dipole–dipole interaction is taken into account and becomes more important at high dipole densities to reduce the increases in the work function, therefore, in the electron affinity of the diamond. On the other hand, in the case of substantial nonuniform oxidation due to the presence of significant amounts of particles abnormally grown, the threshold energy is almost unchanged at ∼5.4 eV for oxygen coverages below 0.3 monolayer, indicating the threshold energy is determined by surface area with low work functions or low electron affinities. The dipole-induced surface potential is discussed in relation to the changes in the electron affinity.

Pretreatment effects by aqua-regia solution on field emission of diamond film

Field emission of diamond film was enhanced after surface treatment using boiling aqua-regia solution. The current generated by the emission of electrons was distinctly increased and the threshold field was reduced from 18.1 to 13.8 V/μm by the treatment. The amount of C–C bonds was decreased, but the O–C one was increased by the treatment. The Fermi level at the treated surface was increased by 2.2 eV. This provides evidence that the enhancement of electron emission originated from the reduction of the work function, caused by the chemisorption of oxygen atoms at the surface of diamond during the aqua regia treatment. Thus, the effective electron affinity changed from positive to negative, leading to the reduction of the potential barrier height and width for electron emission at the surface of diamond.

Adsorption and Decomposition of NO on Lanthanum Oxide

The adsorption behavior of NO on La2O3, an effective catalyst for selective NO reduction with CH4 at temperatures above 800 K, depends upon the pretreatment as indicated by temperature-programmed desorption (TPD) and diffuse reflectance FTIR spectroscopy (DRIFTS). The use of isotopic 18O2 exchange and adsorption showed that oxygen adsorbed dissociatively by filling oxygen vacancies and that both oxygen vacancies and lattice oxygen were mobile at high temperature. Oxygen pair vacancies were assumed to be created by desorption of molecular oxygen and, upon cooling, a certain distribution of pair and single vacancies exists at the surface as the pair vacancies can rearrange due to oxygen ion migration. After La2O3 was pretreated at 973 K in He, exposure to NO at 300 K caused a brief reaction forming N2O, then gave three NO TPD peaks at 400, 700, and 800 K. The only O2 desorption occurred during the 800 K NO peak and gave an NO/O2 ratio near unity. Oxygen chemisorption prior to NO admission eliminated the formation of N2O during NO adsorption at 300 K, blocked the sites giving NO desorption at 700 K, but enhanced the NO and O2 peaks at 800 K. TPD after 15N16O adsorption on an La2O3 surface containing exchanged 18O lattice anions, but no chemisorbed O atoms, showed that both 15N16O and 15N18O desorbed at 400 K, but only 15N16O was present in the 700 and 800 K desorption peaks, and 16O2 again desorbed at 800 K. When both lattice exchange and chemisorption of 18O2 on the La2O3 surface were allowed before 15N16O adsorption, 15N18O was desorbed at 400 and 800 K while 16O2, 16O18O, and 18O2 were also desorbed at 800 K; thus the NO peak at 400 K involves exchange with surface lattice oxygen atoms, while the 800 K peak involves exchange with chemisorbed oxygen atoms. DRIFTS indicated the presence of anionic nitrosyl (NO−), hyponitrite (N2O2)2−, chelated nitrite (NO2−), nitrito (ONO−), and bridging and monodenate nitrate (NO3−) species. Consequently, the three NO TPD peaks were assigned as follows: 400 K, decomposition of nitrito, nitro, and bidentate nitrate species; 700 K, desorption from NO− and (N2O2)2− species; and 800 K, decomposition of monodenate nitrate species into NO and O2. A model of the La2O3 surface based on the (001) and (011) crystal planes is proposed to account for these different sites. Two types of oxygen pair vacancy sites with a different O–O separation appear to exist, with one forming (N2O2)2− species, and four additional sites—(1) an oxygen single vacancy, (2) a single vacancy and a lattice oxygen atom, (3) a coordinative unsaturated lattice oxygen atom, and (4) adjacent lattice oxygen atoms—are proposed to explain the formation of NO−, nitrito (M–ONO−), chelated nitrites, and bridging nitrate species, respectively. Among these species, (N2O2)2− was detected by DRIFTS under reaction conditions at 800 K and is most likely to be an active intermediate during NO decomposition. Monodentate nitrate species are also observed at 800 K, but are very stable and still present after purging at 800 K.

Size dependence in adsorption on model surfaces

Considering the chemisorption of atomic oxygen on Li(001) surface, the size dependence in adsorption on the model surfaces has been investigated. All-electron density functional calculations within the local density approximation have been applied to cluster models with increasing size. The influence of cluster size on the physics and chemistry of adsorption has been investigated.

Surface and subsurface oxygen on Pd(111)

The interaction of O2 with Pd(111) in the temperature range from 300K to 1000K was studied by molecular beam adsorption, thermal desorption (TDS), low energy electron diffraction (LEED), high-resolution X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS). Using a capillary array doser and high effective oxygen pressures, evidence was found for the formation of a densely packed chemisorbed oxygen adlayer saturating at ΘO close to 1 and separately for subsurface migration of oxygen at elevated temperatures, but not at room temperature and below. Up to completion of a p(2×2) oxygen adlayer at 0.25ML surface coverage, the dissociative sticking probability of oxygen into the chemisorbed state is high and masks the much slower diffusion into the bulk. Beyond 0.25ML surface coverage, the adsorption rate into the chemisorbed state becomes small and the influence of bulk migration detectable. Exposure of the sample to high oxygen dosages at 1000K fills up the subsurface reservoir and subsequent sticking measurements are no longer influenced by oxygen loss to the bulk. The subsurface oxygen could be distinguished in both XPS and off-specular HREELS. These latter techniques revealed that considerable concentrations of oxygen in the near-surface region can build up, even at lower temperatures (523K) and oxygen exposures (40L). In contrast to chemisorbed oxygen atoms on Pd(111), the subsurface species cannot be removed by reaction with CO.

Resonant ion stimulated desorption of O+ and H+: A probe of interactions of oxygen and hydrogen with TiO2(110)

The mechanism of noncollisional H+ and O+ sputtering from reduced, hydrogenated and oxygenated TiO2(110) surfaces has been investigated by low-energy noble-gas ions and electron bombardment. The noncollisional sputtering is found to be initiated by formation of the O 2s core hole via the quasiresonant charge exchange. Desorption of oxygen and hydrogen occurs from core-excited oxygen and hydroxyl group, respectively, which have an antibonding character with a long lifetime. To survive efficient reneutralization, ionization of oxygen should occur after bond breakage via the intraatomic Auger decay of the O 2s hole while hydrogen can be ionized in the course of the O–H bond breakage at the surface via the interatomic Auger decay of the O 2s hole. On the basis of these findings, the interaction of oxygen and hydrogen with the reduced TiO2(110) surface is investigated using resonant ion stimulated desorption (RISD) by He+. At the reduced surface, the chemisorbed oxygen atom either fills a vacancy site of bridging oxygen atoms or chemisorbs at a fivefold-coordinated Ti4+ site as an adatom. In the RISD experiment, the oxygen adatom is detected much more efficiently than the bridging oxygen. Such oxygen adatoms are found to be unstable on the TiO2(110) surface; they diffuse into the bulk at the oxygenated surface or segregate from the bulk at the reduced surface. Hydrogen, bonded to Ti at the reduced surface, tends to be reorganized to form hydroxyl group upon oxygenation.

Photoelectron spectra of small LaOn- clusters: decreasing electron affinity upon increasing the number of oxygen atoms

We present mass selected photoelectron spectra of small lanthanum oxide cluster anions LaO n - (n=1-5) which have been generated in a laser vaporization cluster source. The electron affinity of the lanthanum oxide clusters drops continuously with the number of chemisorbed oxygen atoms as revealed from the anion photoelectron spectra. The decreasing electron affinity behaves contrary to several other metal oxide clusters. The geometry of some of the measured clusters are discussed in comparsion with configuration interaction and density functional calculations using a Gaussian94 program package.

Hightemperature electrical conductivity of solid solutions in the system NaNbO3 — nDnB3O9

Electric conduction of specimens in the system NaNbO3-NdNb3O9 is studied in the range 250 –1400°C. The anomalous behavior of the electrical conductivity as a function of the temperature in the region of high temperatures is associated with the chemisorption of oxygen atoms at the surface of grains in polycrystalline dielectrics

Ab initio cluster model comparative study of atomic oxygen and sulfur chemisorption on Pt(1 1 1) surface: relevance to heterogeneous catalysis

Chemisorption of atomic oxygen and sulfur on Pt(1 1 1) has been studied by means of the ab initio cluster model approach. For both adsorbates, we consider chemisorption on the threefold open site of the Pt(1 1 1) surface which is represented by a Pt 25 cluster model having 12 atoms in the first layer, six in the second and seven in the third. The ab initio molecular orbital Hartree–Fock method and explicitly correlated wavefunctions are used to obtain a reliable estimate of the interaction energy, vibrational frequency for the normal mode of the adsorbate above the surface, and the equilibrium geometry. The chemisorption bond is analyzed using different theoretical techniques including the constrained space orbital variation, CSOV, method, the analysis of dipole moment curves and the use of projection operators. The influence of electronic correlation effects is analyzed using multireference configuration interaction, MRCI, techniques and also, by using post-Hartree–Fock correlation functionals.

Adsorption/Desorption of H2 and O2 at a p-Cr2O3 Surface: Electrical Conductivity Studies

The behaviour of H2 and O2 flows over a porous p-Cr2O3 pellet have been studied by means of an in situ electrical conductivity technique. Changes in the electrical conductivity associated with such gas flows were attributed to changes in the concentration of positively charged holes in the material studied. In the presence of an H2 flow, the electrical conductivity of the p-Cr2O3 surface first increased and then decreased sharply. The initial increase was attributed to the chemisorption of H2 leading to surface hydroxylation. The formation of O–H bonds during this process resulted in a decrease in the electron density at the surface and thereby increased the contribution of positively charged holes towards the total conductivity. In contrast, the subsequent sharp decrease in conductivity was explained by the elimination of chemisorbed oxygen which had originally been adsorbed dissociatively at the surface. During this process, elimination of chemisorbed oxygen atoms led to the presence of electrons in the chromia lattice which generated a sharp decrease in the hole concentration via an electron–hole recombination process. In an O2 atmosphere, the conductivity of the deoxygenated chromia increased exponentially to a maximum value and then decreased until it reached a value similar to that for stoichiometric chromia. The increase in conductivity observed in this case was assigned to the adsorption of oxygen on the chromia surface while the subsequent decrease was attributed to hydrogen desorption. The kinetics of the four processes observed, i.e. hydrogen adsorption, oxygen desorption, oxygen adsorption and hydrogen desorption, have been analyzed.

Electronic structure investigation of Ag(110)/1 × 2-O surface

Ag(110)/1 × 2-O surface induced by chemisorption of atomic oxygen was newly found and investigated by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS). As results, it was suggested that the substrate was reconstructed with1 × 2 periodicity by LEED analysis. The oxygen coverage of theAg(110)/1 × 2-O surface was 0.5 ML by XPS measurements. We then propose that theAg(110)/1 × 2-O surface is reconstructed to a1 × 2 missing row structure where every second [¯110] Ag row is missing. ARUPS measurements along [001] and [¯110] directions on theAg(110)/1 × 2-O and2 × 1-O surfaces were performed. The interaction energies between Ag and O atoms were determined by comparing an LCAO calculation with the experimental results of the2 × 1-O surface. Several candidates for the1 × 2-O structure model were given by calculations using the interaction energies.

A Model of Homogeneous Bubble Nucleation of CO Bubbles in Fe–C–O Melts

Homogeneous CO gas bubble formation in Fe–C–O melts has been studied by assuming that the bubble formation is initiated by the clustering process of CO molecules formed by the reaction of chemisorbed oxygen at the surface of the cluster with carbon atoms impinging on the surface. The electrostatic energy due to dipole–dipole interaction between the chemisorbed oxygen atoms at the surface layer of the cluster was included in the free energy for the bubble formation in a melt system characterized with high surface tension and low vapor pressure. The calculated values of supersaturation for bubble formation, which turn out to be crucially dependent on the oxygen activity, provide the upper bound values of the supersaturation for the CO bubble formation in an iron melt.

An optical emission spectroscopy study of a reactive magnetron sputtering Ar-O 2 discharge modulated at low frequency

Recent studies have shown that the target poisoning often encountered during sputter-deposition of oxides could be controlled by means of a low-frequency modulation of the discharge power. In order to understand the basic mechanisms resulting in improvement of the deposition rate of stoichiometric oxides such as TiO 2, optical emission spectroscopy (OES) measurements have been carried out in both stable and unstable sputtering conditions. The current study shows that stoichiometric coatings can be synthesised even if they are periodically submitted to an under-stoichiometric flow of particles at the end of the step ‘high’ of the modulation. This suggests the existence of a characteristic time compatible with the diffusion kinetics of chemisorbed oxygen atoms m the under-stoichiometric layer of the deposit created.

Oxygen adsorption on MoN precovered Fe-3.5%MoN(100) single crystals

The two-dimensional MoN surface compound has been prepared on the Fe 3.5%MoN(100) surface by means of surface cosegregation of molybdenum and nitrogen. The MoN precovered Fe-3.5%MoN(100) surface shows a sharp (1 × 1) low energy electron diffraction (LEED) pattern. The structure of the MoN surface compound has been determined by X-ray photoelectron diffraction to consist of a single MoN surface layer plus an additional Mo subsurface layer. AES and LEED have been utilized to study the dissociative oxygen adsorption on the MoN precovered Fe-3.5%MoN(100) surface. The experiments have been conducted at oxygen pressures ranging from 5 × 10 −9 to 1 × 10 −6 mbar and temperatures between 400 and 550°C. It has been found that chemisorbed oxygen atoms replace nitrogen atoms of the MoN surface compound and cause a reduction of the Mo surface concentration. After ∼ 100 l O 2 the initial exchange process is largely completed and leads to an oxygen and molybdenum rich (Mo x Fe 1 − x )(O y N 1 − y ) surface phase. The (Mo x Fe 1 − x )(O y N 1 − y ) covered substrate shows a sharp (1 × 1) LEED pattern indicating epitaxial stabilization of the (Mo x Fe 1 − x )(O y N 1 − y ) surface phase on the Fe-3.5%Mo(100) surface. It is shown that the structure of the (Mo x Fe 1 − x )(O y N 1 − y ) surface phase is related to the (100) plane of the rocksalt structure. Further oxidation causes the formation of inhomogeneous iron oxide layers on MoN precovered and sputter cleared Fe-3.5%MoN(100) surfaces as well.

Voltammetric studies of electrochemical processes at the interface Pt|YSZ between 300 and 600°C

Abstract Electrochemical processes occurring at the interface platinum|yttria-stabilized zirconia were investigated by cyclic voltammetry and potentiostatic current-potential curves, taken under steady-state conditions, in different mixtures of oxygen and nitrogen at temperatures between 300 and 600°C. Point or paste electrodes were used. The exchange current of the rate-determining step of the oxygen reduction becomes very small below 500°C. Therefore, the equilibrium of the oxygen electrode is no longer established. A layer of chemisorbed oxygen atoms is formed anodically on the contact surface of the point electrode, not by the dissociation of O 2 at the three-phase boundary. The shape of the cyclic voltammograms is discussed.

The development and functions of silver in water purification and disease control

The use of silver to control putrefaction of liquids and as a mitigate to the incursion and spread of disease can be traced to ancient times. In recent decades, studies have revealed the biochemical reactions of ionic silver that result in the inactivation of bacteria, fungi, protozoa, spirochetes, viruses, etc. not protected by proteinaceous membranes. The unique chemisorption of atomic oxygen on the surface of silver in both gaseous and aqueous media combined with freedom of movement of oxygen throughout the crystal lattice of the silver provides a reservoir of oxygen and thus sets silver apart in its catalytic activity. Studies have revealed that bulk silver in an oxygen-charged aqueous media will catalyze the complete destructive oxidation of microorganisms. However, the broad use of silver as a powerful clinical tool is still in the future because its full range of activity remains to be elucidated.