Destabilization of Ag nanoislands on Ag(100) by adsorbed sulfur

Sulfur accelerates coarsening of Ag nanoislands on Ag(100) at 300 K, and this effect is enhanced with increasing sulfur coverage over a range spanning a few hundredths of a monolayer, to nearly 0.25 monolayers. We propose that acceleration of coarsening in this system is tied to the formation of AgS(2) clusters primarily at step edges. These clusters can transport Ag more efficiently than can Ag adatoms (due to a lower diffusion barrier and comparable formation energy). The mobility of isolated sulfur on Ag(100) is very low so that formation of the complex is kinetically limited at low sulfur coverages, and thus enhancement is minimal. However, higher sulfur coverages force the population of sites adjacent to step edges, so that formation of the cluster is no longer limited by diffusion of sulfur across terraces. Sulfur exerts a much weaker effect on the rate of coarsening on Ag(100) than it does on Ag(111). This is consistent with theory, which shows that the difference between the total energy barrier for coarsening with and without sulfur is also much smaller on Ag(100) than on Ag(111).     

Adsorption of O, H, OH, and H2O on Ag(100)

The adsorption of H(2)O and its dissociation products, O, H, and OH, on Ag(100) has been studied using an ab initio embedding method. Results at different sites (atop, bridge, and hollow) are presented. The four-fold hollow site is found to be the most stable adsorption site for O, H, and OH, and the calculated adsorption energies are 87.1, 42.7, and 76.2 kcal mol(-1), respectively. The adsorption energy of water at the atop and bridge sites is almost identical with values of 11.1 and 12.0 kcal mol(-1), respectively. The formation of adsorbed OH species by adsorption of water on oxygen-precovered Ag(100) is predicted to be exothermic by 36 kcal mol(-1).     

Structure of CO2 adsorbed on the KCl(100) surface

The structure and dynamics of the adsorbate CO(2)/KCl(100) from a diluted phase to a saturated monolayer have been investigated with He atom scattering (HAS), low-energy electron diffraction (LEED), and polarization dependent infrared spectroscopy (PIRS). Two adsorbate phases with different CO(2) coverage have been found. The low-coverage phase is disordered at temperatures near 80 K and becomes at least partially ordered at lower temperatures, characterized by a (2√2×√2)R45° diffraction pattern. The saturated 2D phase has a high long-range order and exhibits (6√2×√2)R45° symmetry. Its isosteric heat of adsorption is 26 ± 4 kJ mol(-1). According to PIRS, the molecules are oriented nearly parallel to the surface, the average tilt angle in the saturated monolayer phase is 10° with respect to the surface plane. For both phases, structure models are proposed by means of potential calculations. For the saturated monolayer phase, a striped herringbone structure with 12 inequivalent molecules is deduced. The simulation of infrared spectra based on the proposed structures and the vibrational exciton approach gives reasonable agreement between experimental and simulated infrared spectra.     

Intramolecular resonances afterK-shell excitation of C2H2n adsorbed on Ag and Cu(100) surfaces

The near edge X-ray absorption fine structure (NEXAFS) at the C K-edge of C₂H₂, C₂H₄, C₂H₆ on Cu(100) and Ag(100) surfaces is studied. The oriented character of the molecules on the surface combined to the dipole selection rules for photon absorption allows an experimental assignment of the nature of the various final states. A careful fitting of the NEXAFS spectra reveals final states of C–H character and shake-up satellites of the C(1 s) to π* (C–C) transition. The asymmetric line shape of the C(1 s) to σ* (C–C) transition is described in a scattering model of the photoelectron from the molecular potential.     

CO adsorption on Ag(100) and Ag/MgO(100)

Theoretical studies of CO adsorption on a two-layer Ag(100) film and on a two-layer Ag film on a MgO(100) support are reported. Ab initio calculations are carried at the configuration interaction level of theory using embedding methods to treat the metal-adsorbate region and the extended ionic solid. The metal overlayer is considered in two different structures: where Ag-Ag distances are equal to the value in the bulk solid, and for a slightly expanded lattice in which the Ag-Ag distances are equal to the O-O distance on the MgO(100) surface. The calculated adsorption energy of Ag(100) on MgO(100) is 0.58 eV per Ag interfacial atom; the Ag-O distance is 2.28 A. A small transfer of electrons from MgO to Ag occurs on deposition of the silver overlayer. CO adsorption at an atop Ag site is found to be the most stable for adsorption on the two-layer Ag film and also for adsorption on Ag deposited on the oxide; CO adsorption energies range from 0.12 to 0.19 eV. The CO adsorption energy is reduced for the Ag/MgO system compared to adsorption on the unsupported metal film thereby providing evidence for a direct electronic effect of the oxide support at the metal overlayer surface. Expansion of the Ag-Ag distance in the two-layer system also reduces the adsorption energy.     

Photochemistry on ultrathin metal films: strongly enhanced cross sections for NO2 on Ag/Si(100)

The surface photochemistry of NO(2) on ultrathin Ag(111) films (5-60 nm) on Si(100) substrates has been studied. NO(2), forming N(2)O(4) on the surface, dissociates to release NO and NO(2) into the gas phase with translational energies exceeding the equivalent of the sample temperature. An increase of the photodesorption cross section is observed for 266 nm light when the film thickness is decreased below 30 nm despite the fact that the optical absorptivity decreases. For 4.4 nm film thickness this increase is about threefold. The data are consistent with a similar effect for 355 nm light. The reduced film thickness has no significant influence on the average translation energy of the desorbing molecules or the branching into the different channels. The increased photodesorption cross section is interpreted to result from photon absorption in the Si substrate producing electrons with no or little momenta parallel to the surface at energies where this is not allowed in Ag. It is suggested that these electrons penetrate through the Ag film despite the gap in the surface projected band structure.     

Die Kristallstruktur von Ag2TeO2(OH)4

Zusammenfassung Die Kristallstruktur von Ag₂TeO₂(OH)₄ wurde mit Hilfe von Einkristallmethoden bestimmt und nach der Methode der kleinsten Quadrate verfeinert. Die Gitterparameter der Elementarzelle (Fdd 2–C _2v ¹⁹ ) betragen:a=18,72;b=6,48;c=8,94 Å. Mit allen beobachteten Reflexen wurde einR-Wert von 8,50%, unter Einschluß derHamilton-Reflexe einR-Wert von 9,41% erhalten. Die Telluratome sind oktaedrisch von 6 Sauerstoffen umgeben, während die Silberatome stark verzerrt tetraedrisch von 4 Sauerstoffen umgeben sind. Zwei dieser Tetraeder bilden mit einem dazwischenliegenden Oktaeder eine geometrische Einheit. Diese Einheiten sind über Sauerstoffe, die zu zwei verschiedenen Tetraedern gehören, und über Wasserstoffbrücken zu einem dreidimensionalen Gerüst verknüpft.

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The crystal structure of Ag₂TeO₂(OH)₄

The crystal structure of Ag₂TeO₂(OH)₄ has been determined by single crystal X-ray methods and refined by the least squares method. The lattice parameters of the cell (Fdd 2–C _2v ¹⁹ ) are:a=18.72;b=6.48;c=8.94 Å. With all observed reflections anR-Value of 8.50% was obtained; if allHamilton-reflections are includedR=9.41%. The Te atoms are surrounded octahedrally by 6 oxygens, the 4 oxygens around the silver atoms form a distorted tetrahedron. Two of these tetrahedra form with an intermediate octahedron a geometric unit. These units are linked together to a three-dimensional structure by oxygens belonging to two different tetrahedra and by hydrogen bondings.

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Mit 5 Abbildungen     

Geometrical,spectroscopic, and magnetic properties of an oxygen atom adsorbed on the Ni(100) surface

Spin-polarized linear combinations of Gaussian-type orbital–model core potential–local spin density (LCGTO –MCP –LSD ) computations have been performed for oxygen chemisorption on a Ni(100) surface simulated by four different clusters. Results show that the oxygen atom chemisorbs preferentially on the fourfold hollow site with an equilibrium distance of 1.931 Å and a vertical vibrational frequency of 401 cm^?1. The corresponding experimental values are 1.960 Å and 423 or 430 cm^?1. A satisfactory agreement with experiment is also found for the adsorption energy (6.7 vs. 5.6 eV). The bridge position lies at only 0.4 eV above the fourfold hollow one. It is found that oxygen adsorption leaves the bare cluster total spin magnetic moment unchanged, but induces appreciable reductions of the local atomic moment on the surface nickel atoms. © 1993 John Wiley & Sons, Inc.     

Thermodynamic and structural properties of water adsorbed film on MgO (100) ionic surface

We have investigated by adsorption isotherms and neutron diffraction measurements, respectively the thermodynamic and structural properties of water physisorbed film on MgO (100) powder. Thanks to a high temperature thermal treatment, under vacuum, our MgO powder samples are characterized by a highly homogeneous (100) MgO surface. We have determined the structure of the (2D) water film physisorbed on such an ionic surface. This one is a commensurate P(2×3) structure which is very similar to the (110) planes of ice-VII. Recall that ice VII, which is stable at very high pressure, is characterized by a quite large density (d = 1.6).     

Electronic excited states of Si(100) and organic molecules adsorbed on Si(100)

The electronically excited states of the Si(100) surface and acetylene, benzene, and 9,10-phenanthrenequinone adsorbed on Si(100) are studied with time-dependent density functional theory. The computational cost of these calculations can be reduced through truncation of the single excitation space. This allows larger cluster models of the surface in conjunction with large adsorbates to be studied. On clean Si(100), the low-lying excitations correspond to transitions between the pi orbitals of the silicon-silicon dimers. These excitations are predicted to occur in the range 0.4-2 eV. When organic molecules are adsorbed on the surface, surface --> molecule, molecule --> surface, and electronic excitations localized within the adsorbate are also observed at higher energies. For acetylene and benzene, the remaining pipi* excitations are found to lie at lower energies than in the corresponding gas-phase species. Even though the aromaticity of 9,10-phenanthrenequinone is retained, significant shifts in the pipi* excitations of the aromatic rings are predicted. This is in part due to structural changes that occur upon adsorption.     

Synthesis, characterization, and electrochemical application of Ca(OH)2-, Co(OH)2-, and Y(OH)3-Coated Ni(OH)2 tubes

We report on the synthesis, characterization, and electrochemical application of Ca(OH)2-, Co(OH)2-, and Y(OH)3-coated Ni(OH)2 tubes with mesoscale dimensions. These composite tubes were prepared via a two-step chemical precipitation within an anodic alumina membrane under ambient conditions. The morphology and structure of the as-synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) equipped with energy dispersive spectroscopy (EDS). The results showed that the size of the tubes was of mesoscale dimension and the proportion of the tube morphology was about 95%. The as-prepared composite tubes were further investigated as the positive-electrode materials of rechargeable alkaline batteries. Electrochemical measurements revealed that the Ni(OH)2 tubes coated with Ca(OH)2, Co(OH)2, and Y(OH)3 exhibited superior electrode properties including high discharge capacity, excellent high-temperature and high-rate discharge ability, and good cycling reversibility. The mechanism analysis suggests that both the coated layers and the unique hollow-tube structures play an indispensable role in optimizing the electrochemical performance of nickel hydroxide electrodes.     

First observation of capping/uncapping by a ligand of a Zn porphyrin adsorbed on Ag(100)

A significant first step towards creation of catalytically active porphyrin-functionalised metal surfaces has been achieved.     

UPS study of cyanogen adsorbed on Pt(100)

Ultraviolet photoelectron spectra for adsorbed cyanogen on Pt(100) are presented and discussed in terms of possible models for the different adsorption states detected by other surface techniques. Taking the gas phase spectra of C₂N₂ and HCN as guidance interpretation of the various cyanogen induced features is attempted as follows: A prominent peak 6 eV below the Fermi level is ascribed to the overlapping π and nitrogen lone-pair orbitals, whereas a weak, broad feature around 16 eV is assigned to CC π bonds present in the absorbate layer. A feature within the Pt d-band region at 3 eV is tentatively associated with the “back-bonding” from filled metal d-band states into empty 2π^* states of the absorbate.     

Effects of elastomer on morphology,flammability and rheological properties of HIPS/PS‐encapsulated Mg(OH)2 composites

The effects of elastomer type on morphology, flammability and rheological properties of high‐impact polystyrene/Mg(OH)₂ based on encapsulated by polystyrene have been investigated. The ternary composites characterized by cone calorimetry, horizontal burning rate, limiting oxygen index (LOI), rheology and SEM. Morphology was controlled using poly[styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene] triblock copolymer (SEBS) or the corresponding maleinated SEBS (SEBS‐g‐MA). As revealed by SEM observations, composites of HIPS/SEBS/Mg(OH)₂ exhibit separation of the filler and elastomer and good adhesion between SEBS and the filler, whereas composites of HIPS/SEBS‐g‐MA/Mg(OH)₂ exhibit encapsulation of the filler by SEBS‐g‐MA. The flame retardant and rheological properties of ternary composites were strongly dependent on microstructure. The rheological test showed that the composites with encapsulation structure exhibit a stronger solid‐like response at low frequency than those of the composites with separate dispersion structure. The combustion tests showed that the composites with encapsulation structure showed higher flame retardant properties than those of separate dispersion structure at optimum use level of SEBS‐g‐MA. However, with the increase of the content of SEBS‐g‐MA, the flame retardancy of the composite declined somewhat which can be explained that the SEBS‐g‐MA coating acts as a heat and mass transfer barrier due to the formation of encapsulation structure. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2023–2030, 2007     

Crystal Structure of a Silver Phosphate-Tellurate: Te(OH)6 · 2Ag2HPO4

Crystal structure of a new silver phosphate-tellurate: Te(OH)₆ · 2Ag₂HPO₄ has been solved using 2534 X-ray reflections with a final R value: 0.048. This salt is monoclinic, P2₁/n with a bimolecular unit cell:a = 5.950(3), b = 20.52(1), c = 5.829(3) Å, β = 119.89(5°). As in the already described phosphate-tellurates the main feature of this crystal structures is the coexistence in the same crystal of two different types of anions: TeO₆ octahedra and PO₄ tetrahedra.     

Spectroscopy of ultrathin epitaxial rutile TiO(2)(110) films grown on W(100)

Epitaxial ultrathin titanium dioxide films of 0.3 to approximately 7 nm thickness on a metal single crystal substrate have been investigated by high resolution vibrational and electron spectroscopies. The data complement previous morphological data provided by scanned probe microscopy and low energy electron diffraction to provide very complete characterization of this system. The thicker films display electronic structure consistent with a stoichiometric TiO(2) phase. The thinner films appear nonstoichiometric due to band bending and charge transfer from the metal substrate, while work function measurements also show a marked thickness dependence. The vibrational spectroscopy shows three clear phonon bands at 368, 438, and 829 cm(-1) (at 273 K), which confirms a rutile structure. The phonon band intensity scales linearly with film thickness and shift slightly to lower frequencies with increasing temperature, in accord with results for single crystals.