A Density Functional Theory Study of Oxygen Adsorption at Silver Surfaces: Implications for Nanotoxicity (original) (raw)
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Ab initio density functional study of O on the Ag(001) surface
Surface Science, 2003
The adsorption of oxygen on the Ag(100) is investigated by means of density functional techniques. Starting from a characterization of the clean silver surfaces oxygen adsorption in several modifications (molecularly, on-surface, sub-surface, Ag 2 O) for varying coverage was studied. Besides structural parameters and adsorption energies also work-function changes, vibrational frequencies and core level energies were calculated for a better characterization of the adsorption structures and an easier comparison to the rich experimental data.
Dissociation of molecular oxygen on unpromoted and cesium promoted Ag(110) surfaces
Surface Science, 2000
We have computed the binding energies for the molecular and atomic chemisorbed states of oxygen, and barriers between them, for a three-layer model cluster representative of the (110) face of silver as a function of cesium coverage. To increase computational efficiency, a one-electron potential was developed for the third layer of the silver atoms making up the model cluster. These calculations predict a barrier of~17 kcal/mol to dissociate chemisorbed O 2 , in good agreement with experiment. When a Cs atom is added to the cluster, the barrier for O 2 dissociation is significantly lowered. However, when two Cs atoms were added the barrier for dissociation increased relative to the barrier for the cluster with one adsorbed Cs atom. These calculations predict that the dissociation of chemisorbed molecular oxygen will have a Cs coverage dependence. This dissociation pattern is consistent with prior calculations on Ag(111) model clusters.
Adsorption of atomic oxygen on Ag(): a study based on density-functional theory
Surface Science, 2002
We present a theoretical study--based on first principles calculations--aimed at characterizing the surface reconstructions which occur at the Ag(0 0 1) surface when oxygen is dosed on it. We first model this system at different coverages using (1  1), c(2  2), and p(2  2) structures of oxygen atoms adsorbed on the hollow sites of the Ag(0 0 1) surface. The corresponding equilibrium geometries are obtained by accurate energy minimizations performed within density-functional theory in the local density or in the generalized gradient approximations. We then compare the energies of these structures with that of oxygen adsorbed on a (2 ffiffi ffi 2 p  ffiffi ffi 2 p ) missing-row reconstructed substrate, recently proposed to be the stable phase at low temperature on the basis of X-ray photo-electron diffraction experiments. We do find evidence that the surface structure might be stabilized by a missing-row reconstruction, though our predicted geometry differs from that previously proposed. Ó
Subsurface oxygen and surface oxide formation at Ag(111): A density-functional theory investigation
Physical Review B, 2003
To help provide insight into the remarkable catalytic behavior of the oxygen/silver system for heterogeneous oxidation reactions, purely sub-surface oxygen, and structures involving both on-surface and sub-surface oxygen, as well as oxide-like structures at the Ag(111) surface have been studied for a wide range of coverages and adsorption sites using density-functional theory. Adsorption on the surface in fcc sites is energetically favorable for low coverages, while for higher coverage a thin surface-oxide structure is energetically favorable. This structure has been proposed to correspond to the experimentally observed (4 × 4) phase. With increasing O concentrations, thicker oxide-like structures resembling compressed Ag2O(111) surfaces are energetically favored. Due to the relatively low thermal stability of these structures, and the very low sticking probability of O2 at Ag , their formation and observation may require the use of atomic oxygen (or ozone, O3) and low temperatures. We also investigate diffusion of O into the sub-surface region at low coverage (0.11 ML), and the effect of surface Ag vacancies in the adsorption of atomic oxygen and ozone-like species. The present studies, together with our earlier investigations of on-surface and surface-substitutional adsorption, provide a comprehensive picture of the behavior and chemical nature of the interaction of oxygen and Ag , as well as of the initial stages of oxide formation.
Oxygen adsorption on Ag(111): A density-functional theory investigation
Physical Review B, 2002
The oxygen/silver system exhibits unique catalytic behavior for several large-scale oxidation ͑and partial oxidation͒ industrial processes. In spite of its importance, very little is known on the microscopic level concerning the atomic geometry and chemical nature of the various O species that form. Using densityfunctional theory within the generalized gradient approximation, the interaction between atomic oxygen and the Ag͑111͒ surface is investigated. We consider, for a wide range of coverages, on-surface adsorption as well as surface-substitutional adsorption. The on-surface fcc-hollow site is energetically preferred for the whole coverage range considered. A significant repulsive interaction between adatoms is identified, and on-surface adsorption becomes energetically unstable for coverages greater than about 0.5 monolayer ͑ML͒ with respect to gas-phase O 2 . The notable repulsion even at these lower coverages causes O to adsorb in subsurface sites for coverages greater than about 0.25 ML. The O-Ag interaction results in the formation of bonding and antibonding states between Ag 4d and O 2p orbitals where the antibonding states are largely occupied, explaining the found relatively weak adsorption energy. Surface-substitutional adsorption initially exhibits a repulsive interaction between O atoms, but for higher coverages switches to attractive, towards a (ͱ3ϫͱ3)R30°structure. Scanning tunneling microscopy simulations for this latter structure show good agreement with those obtained from experiment after high-temperature and high-O 2 -gas-pressure treatments. We also discuss the effect of strain and the found marked dependence of the adsorption energy on it, which is different for different kinds of sites.
Structure and dynamics of oxygen adsorbed on Ag(100) vicinal surfaces
Physical Review B, 2004
The structure and dynamics of atomic oxygen adsorbed on Ag(410) and Ag(210) surfaces has been investigated using density functional theory. Our results show that the adsorption configuration in which O adatoms decorate the upper side of the (110) steps forming O-Ag-O rows is particularly stable for both surfaces. On Ag , this arrangement is more stable than other configurations at all the investigated coverages. On Ag(410), adsorption on the terrace and at the step edge are almost degenerate, the former being slightly preferred at low coverage while the latter is stabilized by increasing the coverage. These findings are substantiated by a comparison between the vibrational modes, calculated within density-functional perturbation theory, and the HREEL spectrum which has been recently measured in these systems.
Research on Chemical Intermediates, 2004
The electronic structure of atomic oxygen adsorbed species is studied by means of the density functional theory in the context of the ethylene epoxidation on the silver surface. The adsorbed oxygen species are modeled by the Ag 2 O molecule either in its closed ( 1 A 1 / or openshell states ( 3 B 1 and 1 B 1 ). In both open-shell states the 1s level appears to be lower than that in 1 A 1 by about 2 eV. This is apparently a sequence of the separation of electron pair, occupying the ¼ ¤ -type highest occupied molecular orbital (HOMO), decreasing the electron density at the oxygen center. Such variation of the O 1s level for closed and open-shell Ag 2 O states seems to explain the X-ray photoelectron spectroscopy (XPS) data concerning two distinct atomic oxygen species on silver surface having the O 1s binding energy of about 528 and 530 eV, called nucleophilic and electrophilic oxygen, respectively. The X-ray absorption O K-edge spectra (XANES) calculated for two types of the Ag 2 O states by means of multiple-scattered-X®-based approach appears to be in a qualitative agreement with those experimentally recorded for nucleophilic and electrophilic oxygen.
Sub-surface incorporation of oxygen on Ag(001) during molecular dissociation
Chemical Physics Letters, 1999
We report evidence for the occurrence of sub-surface oxygen incorporation during the dissociation process of Ž. admolecules. The process is observed for O chemisorbed on Ag 001 at a surface temperature of 100 K, where thermally 2 activated sub-surface diffusion is inactive. The phenomenon occurs both when dissociation is induced thermally or by electron bombardment. EELS peaks corresponding to the vibration of adatoms and of sub-surface oxygen are observed.
Properties of Adsorbed Oxygen Forms on a Defective Ag(111) Surface. DFT Analysis
Journal of Structural Chemistry - J STRUCT CHEM-ENGL TR, 2002
A cluster model of an Ag12–3O (ASV) adsorption center using layered silver oxide as a prototype is proposed. The model includes a cation vacancy V on the Ag(111) surface and oxide type subsurface oxygen atoms Oox. Density functional theory (DFT) (B3LYP/LANL1MB approximation) is used to analyze the electronic structure of ASV and oxygen adsorption on this center, ASV+O ? AS–O. As shown by the calculations, the adsorbed oxygen is associated with the subsurface oxygen atoms Oss to form structures similar to metal ozonides — Ag–Oss–Oep–Oss–Ag–Oox–Ag, containing electrophilic oxygen Oep along with the oxide oxygen Oox. The optical spectra of the ASV and AS–O centers were calculated by the configuration interaction method with single excitations (CIS). For ASV, the most intense absorption bands were obtained in the region 500-700 nm. Oxygen association is accompanied by a sharp decrease in spectrum intensity in the range 600-700 nm and an increase in the intensity of the peak at 500 nm. V...