Three-fold Coordinated Hollow Sites Research Articles

Oxygen adsorption on Fe(110) surface revisited

Abstract We performed density functional theory calculations of an oxygen adsorption on a Fe(110) surface to examine structural, electronic, and magnetic properties of O/Fe(110) systems. Oxygen adsorption in different on-surface sites was considered for O coverages varying between a 1/4 and 1 monolayer (ML). It has been found that an oxygen adsorption in long bridge sites is less favorable than in pseudo threefold hollow sites, although at low coverage (1/4 ML) the adsorption in both sites is nearly degenerated in energy. At higher coverages, oxygen atoms markedly prefer to bind in pseudo threefold coordinated hollows. This is in contrast to the experiments which suggested that the long bridge sites are most stable, and to earlier theoretical studies which reported the threefold coordinated hollow sites to be unstable. We show that the preference for an oxygen adsorption in the threefold hollow sites which results from calculations presented in this work can be reconciled with experimental observations of low-coverage adsorption of O in long bridge sites. Consequences of this change in the order of the stability of on-surface sites for the O atoms adsorption in subsurface sites at the O-precovered Fe(110) surface are briefly discussed.

Does methanol produce a stable methoxy species on Ru(0001) at low temperatures?

Soft X-ray photoelectron spectroscopy (SXPS) and energy-scanned photoelectron diffraction (PhD) have been used to study the surface species produced by exposure of Ru(0001) to methanol at ~150K. SXPS shows a single surface species is formed at sub-monolayer coverages with an O 1s peak binding energy of ~532.6eV, 2.8eV greater than that of chemisorbed atomic oxygen. O 1s PhD data from this species shows no significant modulations, in contrast to simulated PhD spectra from a methoxy species occupying a three-fold coordinated hollow site, as predicted by earlier density functional theory calculations, or atop or bridging sites. By contrast, PhD data from the O 1s of the atomic oxygen species in the Ru(0001)(2×1)–O phase are consistent with the oxygen atoms occupying ‘hcp’ hollow sites (above second-layer Ru atoms) at a RuO bondlength of 2.01±0.02Å, essentially identical to previous structure determinations of this phase. O 1s PhD recorded at normal emission from adsorbed CO are also consistent with the known CO atop adsorption species. We conclude that the methanol-derived surface molecular species is not methoxy in a well-defined local site on the surface, but is consistent with clusters of intact methanol identified in a recent infrared spectroscopy investigation.

Ordering transition of gases adsorbed on aC60surface: Monte Carlo simulations and lattice-gas models

A monolayer of ${\text{C}}_{60}$ molecules on a flat surface provides an unusual substrate for the adsorption of simple gases. Both the lattice constant and the corrugation are larger than is typical of most traditional surfaces. These differences give rise to different phenomena, such as unusual commensurate phases. This paper discusses the ordering transition of various gases corresponding to the filling of the honeycomb array of threefold coordinated hollow sites located between ${\text{C}}_{60}$ molecules. That transition is investigated with Monte Carlo simulations and analytical (lattice-gas) models. The value of the resulting transition temperature $({T}_{c})$ depends on the form assumed for the long-range interaction and the role of many-body effects, i.e., screening, due to gas molecules' close proximity to the ${\text{C}}_{60}$ layer. The effect of three-body interactions is found to be large in all cases considered.

A structural study of a C 3H 3 species coadsorbed with CO on Pd(1 1 1)

The combination of chemical-state-specific C 1s scanned-energy mode photoelectron diffraction (PhD) and O K-edge near-edge X-ray absorption fine structure (NEXAFS) has been used to determine the local adsorption geometry of the coadsorbed C 3H 3 and CO species formed on Pd(1 1 1) by dissociation of molecular furan. CO is found to adopt the same geometry as in the Pd(1 1 1)c(4 × 2)-CO phase, occupying the two inequivalent three-fold coordinated hollow sites with the C–O axis perpendicular to the surface. C 3H 3 is found to lie with its molecular plane almost parallel to the surface, most probably with the two ‘outer’ C atoms in equivalent off-atop sites, although the PhD analysis formally fails to distinguish between two distinct local adsorption sites.

The interface structure of n-alkylthiolate self-assembled monolayers on coinage metal surfaces

The current state of understanding of the structure of the metal/thiolate interface of n-alkylthiolate 'self-assembled monolayers' (SAMs) on Cu(111), Ag(111) and Au(111) is reviewed. On Cu(111) and Ag(111) there is now clear evidence that adsorbate-induced reconstruction of the outermost metal layer occurs to a less atomically-dense structure, with the S head-group atom bonded to four-fold and three-fold coordinated hollow sites, respectively, and that intermolecular interaction plays some role in the periodicity of the resulting SAMs. On the far more heavily-studied Au(111) surface, the detailed interface structure remains controversial, but there is growing evidence for the role of Au-adatom-thiolate moieties in the layer ordering.

Adsorption of methanol and methoxy on theα-Cr2O3(0001) surface

We present density functional theory calculations of methanol and methoxy adsorption at the Cr-terminatedα-Cr2O3(0001) surface. We report on the equilibrium geometries of the adsorbed methanol andmethoxy molecules, analyse the bonding to the surface, and discuss the dissociationenergetics of methanol into methoxy on the surface. We found that methanoladsorbs with its O atom situated above a threefold coordinated hollow site inthe surface O layer at a distance of 2.12 Å from the nearest-neighbour Cr atom,and with a calculated adsorption energy of 0.82 eV. For the methoxy moleculewe found the optimum adsorption geometry to be with the methoxy O on topof a Cr atom and with the CO-axis tilted away from the surface normal by∼55°. Methoxy is strongly bound to the surface with an estimated adsorption energy of3.3 eV.

Adsorption geometry of CN on Cu(111) and Cu(111)/O

The adsorption geometry of CN on Cu(1 1 1), both with and without predosing with oxygen, has been investigated using N K-edge near-edge X-ray absorption fine structure (NEXAFS) and C 1s and N 1s scanned-energy mode photoelectron diffraction (PhD). The NEXAFS shows clearly that adsorbed onto clean Cu(1 1 1) the C–N axis is closely parallel to the surface, but in the presence of coadsorbed oxygen the average orientation has the axis tilted by 25° away from the surface; this confirms a much earlier report of an oxygen-induced reorientation of CN on this surface based on vibrational spectroscopy. The PhD data show very weak modulations which are rather insensitive to the emission geometry, clearly implying a high degree of disorder or a local adsorption site well-removed from any position of high point group symmetry. The best-fit structure corresponds to the CN lying slightly displaced from the three-fold coordinated hollow sites but with the C and N atoms having single Cu atom nearest neighbours at distances of 1.98 ± 0.05 and 2.00 ± 0.05 Å respectively.

Site preference of CO chemisorbed on Pt(1 1 1) from density functional calculations

Chemisorption of carbon monoxide on monocoordinated and tricoordinated sites of Pt(1 1 1) is studied using various computational methods based on density functional theory and a series of cluster and periodic models. Calculated results for geometries and binding energies are provided. We demonstrate that both types of models, irrespective of the density functional approximation used, always favour CO adsorption at the threefold coordinated hollow site instead of on-top, monocoordinated CO, as already suggested in the paper of Feibelman et al. [J. Phys. Chem. B 105 (2001) 4018]. This is at variance with experimental evidence and indicates a possible limitation of common approximate density functional theory methods. It is shown that small clusters, that do not correctly describe the substrate environment of the active site, are not adequate models to obtain adsorption energies or adsorption energy differences. However, with increasing cluster size, cluster results are very close to results of periodic calculations. The new insight is that hybrid functionals including a part of the exact exchange decrease the energy difference between the two positions, suggesting a stabilization of the on top site relative to the threefold hollow site in the limit of extended models. Arguments are presented that the energetic preference of the threefold hollow site is due to an inadequate description of the HOMO–LUMO gap.

The local adsorption geometry of CH 3 and NH 3 on Cu( [formula omitted]): a density functional theory study

Total energy calculations in the generalised gradient approximation of density functional theory, using the castep computer code, have been used to determine the equilibrium geometries of NH 3 and CH 3 on Cu(1 1 1). Consistent with the results of both previous calculations and scanned-energy mode photoelectron diffraction (PhD) experiments, ammonia is found to adsorb in atop sites with an adsorption energy of approximately 0.7 eV. The Cu–N nearest-neighbour bondlength shows some coverage dependence, the best agreement with experiment being achieved at lower coverage. The adsorbed methyl species, however, shows the highest adsorption energy for a threefold coordinated hollow site, as found in a PhD experimental study, with a strong preference for the C–H bonds to point towards the nearest-neighbour Cu atoms. The results, which imply involvement of both the C and the H atoms in the substrate bonding, are consistent with previous theoretical studies of methyl adsorption on Ru(0 0 0 1) and Ni(1 1 1), but quite unlike those of similar studies on Pt(1 1 1) and other metal surfaces.

LEED STRUCTURE ANALYSES OF THE CLEAN AND FULLY HYDROGEN-COVERED W(110) and Mo(110) SURFACES

We present a comparative low energy electron diffraction (LEED) structure analysis for the clean as well as hydrogen-saturated (110) surfaces of tungsten and molybdenum. Both clean surfaces exhibit some contraction of the first interlayer spacing, i.e. -3.1% for W and -4.0% for Mo, whereas deeper layer distances remain practically bulklike. The only structural consequence of a hydrogen monolayer adsorbed is the reduction of the first interlayer distance contraction to approximately half its value of the clean surface, i.e. to -1.7% for W and -2.0% for Mo . Our results give no evidence for an adsorbate-induced reconstruction of either surface and thus discard the widely accepted model of a hydrogen-induced lateral shift of the top layer of W (110). Hydrogen itself is found to be adsorbed in threefold-coordinated hollow sites at a height of about 1.2 Å and 1.1 Å above the first substrate layer of W and Mo , respectively.

Structure determination of a [formula omitted] coadsorption phase on Ni(111)

The structure of the Ni(111)(2 × 2)OCO adsorption phase, formed by CO adsorption onto a Ni(111)(2 × 2)O phase, has been determined by scanned-energy mode photoelectron diffraction of both the C 1s and O 1s signals. Contrary to earlier interpretation of vibrational spectroscopic data, the CO molecules occupy the same threefold coordinated hollow sites which they adopt on the clean Ni(111) surface, with a NiC bondlength of 1.95 ± 0.04 A ̊ and a CO bondlength of 1.17 ± 0.03 A ̊ . Changes in bondlength and the vibrational amplitude of the CO molecule against the surface are related to the weaker CO-metal bonding in the presence of the atomic oxygen. The chemisorbed oxygen atoms occupy the same “fcc” hollow sites, directly above third layer Ni atoms, both before and after CO adsorption.

The structure of sodium adsorbed c(2 × 8)Ge(111) surface

We performed SEXAFS and core level photoemission studies of Na adsorbed on c(2 × 8)-Ge(111) surface at different coverage regimes. The c(2 × 8) LEED pattern of the clean Ge(111) surface changes to a (1 × 1) with higher background after Na adsorption. A plasmon feature appears on the low energy side of Na 2p core level which indicates metallization of the Na overlayer. We determined the NaGe bond length from the SEXAFS data using the theoretical phase shift of NaGe, calculated by FEFF program. SEXAFS model calculations show that Na atoms are adsorbed on threefold-coordinated hollow sites with a NaGe bond length of 2.9 Å and a second nearest neighbour distance of 3.4 Å to the Ge atoms in the second layer.

Acetylene adsorption on Cu(111) and stepped Cu(111): theoretical study

ASED MO and DV-Xalpha methods have been employed to describe the absorption of C2H2 on the Cu(111) and stepped Cu(111) surface. Our results show that all adsorbed C2H2 molecules are distorted because both the acetylene pi donation to the surface and substrate back-donation to the acetylene pi * orbital result in the equilibrium bonding geometry. On the Cu(111) surface, two stable configurations are found. One is on the bridge site with the C-C axis parallel to the surface; the other is two C atoms on the two symmetrically distinct threefold-coordinated hollow sites on the surface. The influence and mechanism of a monatomic step with (110) geometry on the binding energy and the structure of the adsorbate substrate complex have been investigated. The results show that two new types of chemisorption complex can be formed; the binding energies are largely increased.

Structural determination of the [formula omitted] surface using the normal incidence X-ray standing wave method

A full structural determination has been made of the Cu(111)(√3 × √3)-R30°-ClBr surface, which comprises a mixed ClBr overlayer with a nominal coverage of one third of a monolayer and a 1:1 stoichiometry, using the normal incidence X-ray standing wave (NIXSW) method. Two Bragg reflections were used, the (111) which uses reflecting planes parallel to, and the (1̄11) which are tilted at an angle of 70.5° to, the (111) surface. Using the (111) data, adsorbate-substrate outermost layer spacings were found to be 1.81 ± 0.05 Å for chlorine and 2.00 ± 0.05 Å for bromine assuming zero substrate relaxation. These results, coupled with the (1̄11) data, are consistent with a model in which both halogens, each of which has a coverage of 16 monolayer, occupy three-fold coordinated hollow sites on the surface. The adsorbates occupy primarily fcc hollows (directly above third-layer Cu atoms) but some occupation of hcp hollows (above second-layer Cu atoms) is also observed. For chlorine the fcc:hcp population ratio is 75%:25% (± 10%), while for bromine the rato is 80%:20% (± 10%). Possible origins of the minor species hcp hollow site occupation are discussed.

Structure determination for coadsorbed molecular fragments using chemical shift photoelectron diffraction.

Chemical shift photoelectron diffraction has been applied to the model system of coadsorbed ${\mathrm{PF}}_{\mathit{x}}$(x=1,2,3) species on Ni(111), produced by electron beam fragmentation of a ${\mathrm{PF}}_{3}$ overlayer, to determine quantitatively and independently the local adsorption site of each species. ${\mathrm{PF}}_{3}$ is found to occupy atop adsorption sites above the outermost Ni layer atoms at a P-Ni bond length of (2.07\ifmmode\pm\else\textpm\fi{}0.05) \AA{}, ${\mathrm{PF}}_{2}$ occupies bridge sites at a spacing of (1.70\ifmmode\pm\else\textpm\fi{}0.05) \AA{}, while the PF species occupy both types of threefold coordinated hollow sites, maintaining a constant P coordination throughout the fragmentation.

The local adsorption structure of acetylene on Cu(lll)

A quantitative structural study of the adsorption of acetylene on Cu(111) has been carried out by scanned energy mode photoelectron diffraction from the C 1s core levels. The molecule is found to be adsorbed with the CC axis almost parallel to the surface with the carbon atoms occupying the two symmetrically distinct three-fold coordinated hollow sites on the surface. The C to top Cu atom layer spacing appears to be slightly smaller (1.38 ± 0.03 A ̊ ) above the “fcc” hollow site (above a third-layer Cu atom) than above the “hcp” hollow site (1.44 ± 0.03 A ̊ ) . The best fit to theory is obtained with the C atoms exactly in the three-fold symmetric sites of the substrate which constrains the CC bondlength to a value (1.48 ± 0.10 A ̊ ) substantially larger than that of the free molecule. This result is consistent with previously published vibrational spectroscopy from this adsorption system which indicates significant lowering of the CC bond order.

Threefold-coordinated hollow adsorption site for Ni(111)-c(4 x 2)-CO: A surface-extended x-ray-absorption fine-structure study.

Surface-extended x-ray-absorption fine-structure studies on the Ni(111)-c(4\ifmmode\times\else\texttimes\fi{}2)-CO system show that CO adsorbs in threefold-coordinated hollow sites. This result is in conflict with the adsorption-site determination via molecular vibrational frequencies, which for this system led to an assignment of a bridge site.

Chemisorption and vibration of hydrogen on Cu(111)

Ab initio Pseudopotential calculations for hydrogen adsorbed on Cu(111) are performed. The minimum energy adsorption site is found to be the threefold coordinated hollow site. This result contradicts the conclusion of McCash et al., based on EELS and RAIRS experiments. Based on our calculated vibrational frequencies for hydrogen adsorbed at the hollow site and at the bridge site, we suggest a reinterpretation of the main features of the experimental vibrational spectra. Our results are in agreement with the LAPW calculations of Feibelman and Hamann and with effective medium theory (EMT) calculations.

Threefold-coordinated hollow adsorption site for c(4 x 2)-NO/Ni(111): A surface-extended x-ray-absorption fine-structure study.

Surface-extended x-ray-absorption fine-structure studies on the c(4\ifmmode\times\else\texttimes\fi{}2)-NO/Ni(111) system yield a nearest-neighbor O-Ni distance of 2.68\ifmmode\pm\else\textpm\fi{}0.05 A\r{} and show that NO adsorbs in threefold-coordinated hollow sites. This result casts doubt on the generally used method of adsorption site determination via molecular vibrational frequencies, which for this system led to an assignment of a twofold bridge site.

The structure of mercaptide on Cu(111): a case of molecular adsorbate-induced substrate reconstruction

The structure of methyl mercaptide (CH 3S-) on Cu(111) has been investigated by surface EXAFS above the S K-edge and by normal incidence standing X-ray wavefield absorption, monitoring the same S K-shell absorption around the (111) Bragg reflection. SEXAFS analysis reveals the presence of C backscattering at grazing incidence but not at normal incidence, indicating that the C-S axis is perpendicular to the surface. Both SXW and SEXAFS indicate that the adsorbed molecule partially penetrates the top copper atomic layer leading to substantial motion of these top layer copper atoms parallel to the surface. The combination of the two measurements indicates there is very little expansion of the top metal layer spacing. These results are best fitted by a model in which the mercaptide occupies the hcp three-fold coordinated hollow sites with a movement of the neighbouring copper atoms parallel to the surface of 0.6 Å. This result is discussed briefly in the context of the observed stability of noble metal fcc (111) surfaces.