Interaction of C2N2 with clean and oxygen dosed Cu(111) surface studied by AES,ELS and TDS measurements

The adsorption and surface reaction of cyanogen on clean and oxygen covered Cu(111) have been investigated. From electron energy loss measurements, thermal desorption spectroscopy and electron beam effects in Auger spectroscopy, it is proposed that cyanogen adsorbs dissociatively on Cu(111) at 300 K. The activation energy for the desorption was calculated to be 180 kJ/mol. Cyanogen adsorption onto oxygen predosed Cu(111) is inferred to produce the NCO surface species. This interpretation was aided by data of electron energy loss measurements and from HNCO adsorption onto Cu(111) at 300 K. A reaction began in the co-adsorbed layer above 400 K, yielding CO₂ and N₂.     

First principles investigation on the stabilization mechanisms of the polar copper terminated Cu2O(1 1 1) surface

The stabilization of the unstable, polar copper terminated Cu₂O(1 1 1) surface by reconstruction and hydroxylation was studied theoretically with static and molecular dynamics calculations at ab initio density functional theory (DFT) level. Surface reconstruction was investigated using extensive finite temperature molecular dynamics (MD) combined with a simulated annealing technique. Both the global minimum energy structure obtained during annealing the system at higher temperature (300 K) and the final ‘quenched’ structure which was obtained after cooling the system to 0 K show the expected reconstruction of the adsorbate-free surface. The copper atoms in the first layer and oxygen atoms in the second and third layers are markedly displaced, and the atomic planes merge together to form a uniform mixed layer, thereby minimizing the polarity of the surface. Surface hydroxylation by adsorption of OH⁻ or dissociated water was investigated using static optimization at 0 K. The results show that adsorption is exothermic and that the reconstruction characterizing the annealed OH-free surface does not occur in the presence of adsorbed OH. A surface coverage of 50% results in the surface structure that is the closest to the unrelaxed bulk terminated surface.     

Oxygen induced surface segregation of Cu on the Au0.7Cu0.3(100) surface

The oxygen induced surface segregation of Cu on the Au_0.7Cu_0.3(100) surface was investigated by means of LEED and AES techniques. The dissociative adsorption of O₂ did not take place on this clean surface for a long time exposure at least up to 10⁴ L, and so the oxygen was forcibly introduced onto the surface through a pre-deposition of few a layers of Cu and its successive oxidation. The oxygen coverage was controlled by a heat treatment, which leads the system to a thermal equilibrium state. For the clean surface, the segregation of Au was clearly observed and the surface concentration of Au was estimated to be about 86%, greater than the bulk concentration of 70%. At low coverages below 0.16 ML, no remarkable oxygen induced segregation of Cu was observed. But, above 0.2 ML, the surface concentration of Cu was proportional to the oxygen coverage. The (2 × 4) LEED pattern was observed in a wide range of oxygen coverage. The maximum intensity of the (2 × 4) was observed at about 0.45 ML.     

Elucidation of the nature of active oxygen in the reaction of low-temperature oxidation of CO on single crystal surfaces platinum and palladium

The temperature-programmed reaction (TPR) method, high-resolution electron energy loss spectroscopy (HREELS), and molecular beam method were used to elucidate the role surface reconstruction, subsurface oxygen (O_subs), and CO_ads concentration play in the low-temperature oxidation of CO on the Pt(100), Pt(410), Pd(111), and Pd(110) surfaces. The possibility of the formation of so-called hot oxygen atoms, which arise at the surface at the instant of dissociation of O_2ads molecules and can react with CO_ads at low temperatures (～150 K) to form CO₂, was examined. It was revealed that, when present in high concentration, CO_ads initiates the phase transition of the Pt(100)-(hex) reconstructed surface into the (1 × 1) non-reconstructed one and blocks fourfold hollow sites of oxygen adsorption (Pt₄-O_ads), thereby initiating the formation of weakly bound oxygen (Pt₂-O_ads), active in CO oxidation. For the Pt(410), Pd(111), and Pd(110) surfaces, the reactivity of O_ads with respect to CO was demonstrated to be dependent on the surface coverage of CO_ads. The ¹⁸O_ads isotope label was used to determine the nature of active oxygen reacting with CO at ～150–200 K. It was examined why a CO_ads layer produces a strong effect on the reactivity of atomic oxygen. The experimental results were confirmed by theoretical calculations based on the minimization of the Gibbs energy of the adsorption layer. According to these calculations, the CO_ads layer causes a decrease in the apparent activation energy E _act of the reaction due to changes in the type of coordination and in the energy of binding of O_ads atoms to the surface.     

Adsorption of oxygen on a Cu{110} surface with and without the influence of A keV Ne+ beam: Stage I: Coverages up to half a monolayer

Low Energy Ion Scattering has been used to study the interaction of molecular oxygen with a Cu{110} surface. The amount of adsorbed atomic oxygen was monitored by the 4 keV Ne⁺¦O reflection signal. In the first adsorption stage (coverage less than half a monolayer) the sticking probability varied proportional to the number of empty adsorption sites: S = S₀ (1 ? $\text{}\text{?}$gq). It turned out not to be influenced by the Ne⁺ bombardment. The initial sticking probability S₀ was found to be ≈ 0.24. In this first adsorption stage the oxygen-covered surface is reconstructed according to the “missing row” model, leading to a (2 × 1) LEED pattern.     

The oxidation of H2CO on a copper(110) surface

The oxidation of H₂C¹⁶O by adsorbed ¹⁸O was studied on an Cu(110) sample by temperature programmed reaction spectroscopy. Formaldehyde exchanged its oxygen with surface ¹⁸O upon adsorption to yield H₂C¹⁸O_(a) and ¹⁶O_(a). Formaldehyde was also oxidized by surface ¹⁶O and ¹⁸O atoms to H₂COO which subsequently released one of the hydrogen atoms to form HCOO. The evolution of H₂ from the Cu(110) surface was desorption limited, and the low pre-exponential factor for the recombination of the surface hydrogen atoms suggested stringent requirement on the trajectories of the colliding partners. The formate was very stable and dissociated at elevated temperatures to simultaneously yield H₂ and CO₂. The surface concentration of ¹⁸O exerted a pronounced affect on the activity of the oxidation of formaldehyde on Cu(110).     

Collective defect formation in the high-temperature superconductor YBa2Cu3O7 under the influence of adsorbed water molecules

The mechanisms for defect formation stimulated by the adsorption of water molecules in the surface of YBa₂Cu₃O₇ ceramic are studied, together with the types of defects and their distributions. It is found that a water layer physically bound to the surface reduces the rates of annihilation and capture of positrons, changes the amount of barium and copper on the surface by a factor of two, and inhibits diffusive jumps of nickel atoms. A layer of adsorbed water excites subthreshold formation of 10²¹ cm⁻³ interstitial Ba and Cu1 atoms and transitions of oxygen from O1 to O5, and vice versa in the volume of crystallites, and the migration of defects and accumulation of Ba atoms in the surface layer, which block diffusive jumps of Ni within the volume of the crystals. These effects are related to the excitation of collective, low-frequency weakly damped motion of heavy holes in the crystal volume when defects are formed on the surface by physically adsorbed H₂O molecules, which is accompanied by Coulomb repulsion of cations from intermediate layers into interstitials and the migration of defects in the field of the collective excitations. Zh. éksp. Teor. Fiz. 116, 586–603 (August 1999)     

Mechanisms for oxygen adsorption on the Si(110) surface studied by Auger electron spectroscopy

Oxygen adsorption on the Si(110) surface has been studied by Auger electron spectroscopy. For a clean annealed surface chemisorption occurs, with an initial sticking probability of ～6 × 10^?3. In this case the oxygen o_kll signal saturates and no formation of SiO₂ can be detected from an analysis of the Si L_2,3VV lineshape. With electron impact on the surface during oxygen exposure much larger quantities are adsorbed with the formation of an SiO₂ surface layer. This increased reactivity towards oxygen is due to either a direct effect of the electron beam or to a combined action of the beam with residual CO during oxygen inlet, which creates reactive carbon centers on the surface. Thus in the presence of an electron beam on the surface separate exosures to CO showed adsorption of C and O. For this surface subsequent exposure in the absence of the electron beam resulted in additional oxygen adsorption and formation of SiO₂. No adsorption of CO could be detected without electron impact. The changes in surface chemistry with adsorption are detectable from the Si L_2,3VV Auger spectrum. Assignments can be made of two main features in the spectra, relating to surface and bulk contributions to the density of states in the valence band.     

The adsorption of oxygen on Cu(210)

The system Cu(210)-O₂ has been examined using LEED and AES, combined with optical simulation of diffraction patterns to investigate the detailed structure of the adsorbed layer. Exposure at 300 K and 5 × 10^?9 Torr resulted in LEED patterns showing pronounced streaks. The corresponding structures are believed to require an adsorption mechanism in which O₂ dissociation can occur only at a limited number of surface sites and in which O atoms after dissociation diffuse over quite large distances (?10 nm) before becoming chemisorbed. Heating these structures to 500–600 K produced a sharp (2 × 1) pattern; this step is thought to involve equilibration of the adsorbed layer. Further combinations of exposure (?1 × 10^?6Torr) and heating (up to 500 K) resulted in a series of (2 × 1) and (3 × 1) patterns, while heating to 800 K at any stage of the oxygen interaction regenerated the clean surface.     

Molecular and atomic adsorption states of oxygen on Cu(111) at 100–300 K

Adsorption states of oxygen on Cu(111) at 100–300 K were investigated by means of HREELS. Two molecular species were characterized by different OO stretching frequencies (v(OO)) at 610 cm⁻¹ and 820–870 cm⁻¹, which are assigned to the peroxo-like species (O²⁻₂) adsorbed in a bridged form and the one in a bidentate form bound on an atop site, respectively. The bridged peroxo species is preferred at the low coverage and the atop peroxo species becomes dominant at the higher coverage. In addition to the peaks due to the molecular oxygen, a peak assigned to v(CuO) of atomic oxygen was observed at 370 cm⁻¹ at the high coverage. The frequency of this mode was higher than the frequency reported for Cu(111) exposed to oxygen above 300 K, indicating that the adsorption state of atomic oxygen formed at 100 K is different from that above 300 K. The v(OO) modes became faint after annealing to 170 K because of O₂ dissociation. The v(CuO) mode of the atomic oxygen formed at 100 K remained up to 230 K and disappeared after annealing to 300 K. No desorption of O₂ was detected on annealing to 300 K. It was also found that vibrational spectra for adsorbed NH₃ are influenced by the adsorption states of atomic oxygen on Cu(111).     

A study of H2S adsorption kinetics on the clean and oxygen covered (110) surface of copper

The very low pressure adsorption kinetics of H₂S on the clean and oxygen covered Cu(110) face have been examined by Auger Electron Spectroscopy (AES) and Mirror Electron Microscopy (MEM, used for continuous surface potential variations of the copper surface). The AES experimental curves on the clean copper face have been interpreted using a model of island growth by surface diffusion. The presence of an adsorbed oxygen layer on the copper surface changes notably the induction times observed on both AES and MEM measurements.     

Adsorption and decomposition of HNCO on Cu(111) surface studied by auger electron,electron energy loss and thermal desorption spectroscopy

No detectable adsorbed species were observed after exposure of HNCO to a clean Cu(111) surface at 300 K. The presence of adsorbed oxygen, however, exerted a dramatic influence on the adsorptive properties of this surface and caused the dissociative adsorption of HNCO with concomitant release of water. The adsorption of HNCO at 300 K produced two new strong losses at 10.4 and 13.5 eV in electron energy loss spectra, which were not observed during the adsorption of either CO or atomic N. These loses can be attributed to surface NCO on Cu(111). The surface isocyanate was stable up to 400 K. The decomposition in the adsorbed phase began with the evolution of CO₂. The desorption of nitrogen started at 700 K. Above 800 K, the formation of C₂N₂ was observed. The characteristics of the CO₂ formation and the ratios of the products sensitively depended on the amount of preadsorbed oxygen. No HNCO was desorbed as such, and neither NCO nor (NCO)₂ were detected during the desorption. From the comparison of adsorption and desorption behaviours of HNCO, N, CO and CO₂ on copper surfaces it was concluded that NCO exists as such on a Cu(111) surface at 300 K. The interaction of HNCO with oxygen covered Cu(111) surface and the reactions of surface NCO with adsorbed oxygen are discussed in detail.     

The adsorption of carbon monoxide on Cu(110)-Ni surfaces prepared by dissociation of nickel carbonyl

Cu(110)-Ni surface alloys were prepared by dissociation of nickel carbonyl on clean Cu(110). The adsorption of CO is reversible in the temperature region of 22–200°C and the pressure range of 5 × 10^?8-0.7 Torr, as monitored with ellipsometry and AES. The amount of adsorbed CO depends on the amount of preadsorbed oxygen but not on the amount of carbon present at the surface. The isosteric heat of adsorption decreases from 31 ± 3kcal/mole to 18 ± 2 kcal/mole with increasing CO coverage (up to θ = 0.14θ_max) but is constant for higher coverages (up to θ = 0.4θ _max).     

Evolution of the magnesium surface during oxidation studied by Metastable Impact Electron Spectroscopy

The evolution of a polycrystalline magnesium surface during oxidation at room temperature has been studied by Metastable Impact Electron Spectroscopy (MIES). This technique allowed us to follow the metal-to-insulator transformation of the top layer of the surface. An electronic signal corresponding to a metallic behavior of the surface evidences the presence of under-stoichiometric MgO species on the surface. The total covering by oxygen of the Mg surface uppermost layer, obtained at around 10 L of oxygen deposition, does not correspond to a fully insulating surface. An insulating surface is obtained after 30 L of oxygen deposition. Depositions of CO₂ on a clean and a preoxidized polycrystalline Mg surface have been analyzed to give information about the composition of the surface and its evolution. CO₂ adsorption in the form of CO₃²⁻ compounds on preoxidized Mg is more efficient than on clean Mg. Oxygen species, corresponding to chemisorbed oxygen less bounded than oxygen in the MgO lattice, allows the formation of CO₃²⁻. Therefore, it is concluded that during oxygen deposition at room temperature, MgO islands and chemisorbed oxygen species coexist on the surface. Moreover, the larger the oxygen predeposition is, the less CO₃²⁻ compounds are formed, meaning a decrease of available chemisorbed oxygen sites. From oxidation measurements at high temperature (420 K), we show that MgO islands and uncovered Mg domain coexist. Further, no under-stoichiometric compound features have been observed. The high temperature allows the direct formation of oxide MgO species in islands.     

First principle calculations of benzotriazole adsorption onto clean Cu(1 1 1)

The adsorption of benzotriazole (BTAH or C₆N₃H₅) on a Cu(1 1 1) surface is investigated by using first principle density functional theory calculations (VASP). It is found that BTAH can be physisorbed (<0.1 eV) or weakly chemisorbed (∼0.43 eV) onto Cu(1 1 1), and the chemical bond is formed through nitrogen sp² lone pairs. The weak chemisorption can be stabilized by reaction with neighboring protonphilic radicals, like OH⁻. Furthermore, the geometries and associated energies of intermolecular hydrogen bonds between adsorbates on Cu(1 1 1) are also calculated. A model of the first layer of BTAH/BTA⁻ on Cu(1 1 1) surface is developed based on a hydrogen bond network structure.     

A first principle study on the magnetic properties of Cu2O surfaces

Spin-polarized density functional theory calculations were performed to investigate the magnetism of bulk and Cu₂O surfaces. It is found that bulk Cu₂O, Cu/O-terminated Cu₂O(111) and (110) surfaces have no magnetic moment, while, the O-terminated Cu₂O(100) and polar O-terminated Cu₂O(111) surfaces have magnetism. For low index surfaces with cation and anion vacancy, we only found that the Cu vacancy on the Cu₂O(110) Cu/O-terminated surface can induce magnetism. For atomic and molecular oxygen adsorption on the low index surfaces, we found that atomic and molecular oxygen adsorption on the Cu-terminated Cu₂O(110) surface is much stronger than on the Cu/O-terminated Cu₂O(111) and Cu-terminated Cu₂O(100) surfaces. More interesting, O and O₂ adsorption on the surface of Cu/O terminated Cu₂O(111) and O₂ adsorption on the Cu-terminated Cu₂O(110) surface can induce weak ferromagnetism. In addition, we analysis origin of Cu₂O surfaces with magnetism from density of state, the surface ferromagnetism possibly due to the increased 2p–3d hybridization of surface Cu and O atoms. This is radically different from other systems previously known to exhibit point defect ferromagnetism, warranting a closer look at the phenomenon.     

Adsorption and dissociation of O2 on CuCl(1 1 1) surface: A density functional theory study

The adsorption and dissociation of O₂ on CuCl(1 1 1) surface have been systematically studied by the density functional theory (DFT) slab calculations. Different kinds of possible modes of atomic O and molecular O₂ adsorbed on CuCl(1 1 1) surface and possible dissociation pathways are identified, and the optimized geometry, adsorption energy, vibrational frequency and Mulliken charge are obtained. The calculated results show that the favorable adsorption occurs at hollow site for O atom, and molecular O₂ lying flatly on the surface with one O atom binding with top Cu atom is the most stable adsorption configuration. The O-O stretching vibrational frequencies are significantly red-shifted, and the charges transferred from CuCl to oxygen. Upon O₂ adsorption, the oxygen species adsorbed on CuCl(1 1 1) surface mainly shows the characteristic of the superoxo (O₂⁻), which primarily contributes to improving the catalytic activity of CuCl, meanwhile, a small quantity of O₂ dissociation into atomic O also occur, which need to overcome very large activation barrier. Our results can provide some microscopic information for the catalytic mechanism of DMC synthesis over CuCl catalyst from oxidative carbonylation of methanol.     

Effect of oxygen gas pressure on orientations of Cu2O nuclei during the initial oxidation of Cu(100), (110) and (111)

The orientations of oxide nuclei during the oxidation of Cu(100), (110) and (111) surfaces have been examined by in situ transmission electron microscopy. Our results indicate that the epitaxial nucleation of oxide islands on these surfaces cannot be maintained for a whole range of oxygen gas pressure varying from 10^? 5 Torr to 750 Torr. The critical oxygen gas pressure, pO₂, leading to the transition from nucleating epitaxial to non-epitaxial oxide nuclei shows a dependence on the crystallographic orientations of the Cu substrates with pO₂⁽¹⁰⁰⁾ > pO₂⁽¹¹¹⁾ > pO₂⁽¹¹⁰⁾. By fitting the experimentally determined critical oxygen pressures to a kinetic model, we find that such dependence can be attributed to the effect of surface orientations of the Cu substrates on the oxygen surface adsorption and diffusion, which dominate the kinetic processes of oxide nucleation.     

二维全同Nb4团簇在Cu(100)表面的结构稳定性和电子性质

使用基于密度泛函理论的第一性原理赝势法和超原胞模型，研究了吸附在Cu(100)表面上的二维有序排列的幻数团簇Nb₄的结构稳定性及其电子结构性质.计算表明，四面体结构和平面的菱形结构的Nb₄团簇都可以稳定地吸附在Cu(100)表面上，这个体系很可能有重要的应用前景.在Cu(100)表面上，菱形结构的Nb₄比四面体结构的Nb₄更稳定，从Nb₄团簇的四面体结构到菱形结构，需经过的势垒高度约为0.94eV/团蔟.电子结构的计算表明，在Nb₄吸附后，Cu(100)表面与Nb₄团簇间有明显的电荷重新分布，表面Cu原子的电子态密度也明显改变. 关键词： 4团簇')" href="#">Nb₄团簇 有序排列 结构稳定性 从头计算     

The oxidation of a Cu(100) single crystal studied by angle-resolved photoemission using a range of photon energies

The changes which occur in the angle-dispersive photoemission spectra of a Cu(100) single crystal with increasing exposure to oxygen are described. They are interpreted in terms of an initial chemisorption stage followed by a gradual incorporation of oxygen and reconstruction of the surface region to a “Cu₂O” type layer. The use of different photon energies and angleresolving methods facilitates an identification of the various stages by emphasising the appropriate spectral features.