Fcc Threefold Hollow Site Research Articles

Location of hydrogen adsorbed on Rh(111) studied by low-energy electron diffraction and nuclear reaction analysis

The structures of clean and hydrogen-adsorbed Rh(111) surfaces were investigated by dynamical low-energy electron-diffraction (LEED) analysis. Exposure of ${\mathrm{D}}_{2}$ induced no additional LEED patterns except for $(1\ifmmode\times\else\texttimes\fi{}1)$. Surface-layer relaxation occurs vertically on both clean and D-saturated surfaces. On the clean surface, the interlayer distance between the first and second layers $({d}_{12})$ is smaller by $1.2(\ifmmode\pm\else\textpm\fi{}0.6)%$ than the corresponding bulk distance of $2.194\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. On the other hand, the contraction of ${d}_{12}$ is removed on the D-saturated surface. Detailed LEED analysis demonstrates that the D atoms are adsorbed on the fcc threefold hollow sites. The absolute saturation coverage of H on Rh(111) was determined to be 0.84 ML by nuclear reaction analysis (NRA). Moreover, the zero-point vibrational energy of H was derived from the analysis of the NRA resonance profile, which is discussed in comparison with the results of high-resolution electron-energy-loss spectroscopy.

Initial stages of oxidation of Cu(1 1 1)

Several surface analysis techniques were combined to study the initial stages of oxidation of Cu(1 1 1) surfaces exposed to O 2 at low pressure (<5 × 10 −6 mbar) and room temperature. Scanning tunneling microscopy (STM) results show that the reactivity is governed by the restructuring of the Cu(1 1 1) surface. On the terraces, oxygen dissociative adsorption leads to the formation of isolated O adatoms and clusters weakly bound to the surface. The O adatoms are located in the fcc threefold hollow sites of the unrestructured terraces. Friedel oscillations with an amplitude lower than 5 pm have been measured around the adatoms. At step edges, surface restructuring is initiated and leads to the nucleation and growth of a two-dimensional disordered layer of oxide precursor. The electronic structure of this oxide layer is characterised by a band gap measured by scanning tunneling spectroscopy to be ∼1.5 eV wide. The growth of the oxide islands progresses by consumption of the upper metal terraces to form triangular indents. The extraction of the Cu atoms at this interface generates a preferential orientation of the interface along the close-packed directions of the metal. A second growth front corresponds to the step edges of the oxide islands and progresses above the lower metal terraces. This is where the excess Cu atoms extracted at the first growth front are incorporated. STM shows that the growing disordered oxide layer consists of units of hexagonal structure with a first nearest neighbour distance characteristic of a relaxed Cu–Cu distance (∼0.3 nm), consistent with local Cu 2O(1 1 1)-like elements. Exposure at 300 °C is necessary to form an ordered two-dimensional layer of oxide precursor. It forms the so-called “29” superstructure assigned to a periodic distorted Cu 2O(1 1 1)-like structure.

Long-range ordering of methylidyne on Pt{110}(1×2)

A c(2×4) LEED pattern is observed for methylidyne (CH) chemisorbed on Pt{110}(1×2) at a saturation coverage of 0.25 ML. Density functional calculations reveal that methylidyne is preferentially adsorbed in the fcc three-fold hollow site on the {111} microfacet of the reconstructed surface. A structure for the ordered overlayer is thus proposed, and both through-metal and through-space interactions are considered as possible causes for this unexpected long-range coherence. We argue that entropic effects may be implicated.

The determination of the local adsorption structure ofSO2/Ni(111): multiple-scattering cluster studies

The multiple-scattering cluster (MSC) method has been used to calculate thesulphur 1s near-edge x-ray absorption fine structure of SO2 adsorbed onNi(111). Our investigation confirms the flat-lying adsorption structure ofSO2/Ni(111) and shows for the first time that the fcc threefold hollow site isthe most preferable adsorption site. It has been shown that the O-Sintramolecular bond length is elongated by 0.07 Å, the ∠OSO bond angle is reduced by10° after adsorption and the adsorption height is 2.0±0.2 Å. Furthermore,R-factor analysis exhibits a local minimum in the vicinity of the fcc threefoldhollow site, which is partly in agreement with the experimental result.

THE LOCAL ADSORPTION STRUCTURE OF SO2/Ni(111):THEMULTIPLE-SCATTERING CLUSTER STUDIES

The multiple-scattering cluster (MSC) method has been used to calculate the sulphur 1s near edge X-ray absorption fine structure of SO2 adsorbed on the Ni(111). The local adsorption structure of SO2/Ni(111) was determined by a direct comparison between MSC calculation and the experimental NEXAFS spectra. This study confirms the flat-lying structure and shows that the fcc three-fold hollow site is the most preferable adsorption site.It has been shown that the O—S intramolecular bond length is elongated by (0.007±0.002)nm,∠OSO bond angle is reduced by 10° after adsorption,and the adsorption heihgt is (0.20±0.02)nm.

CO adsorption on Pd(1 1 1): a high-resolution core level photoemission and electron energy loss spectroscopy study

By combining high-resolution X-ray photoelectron and electron energy loss spectroscopies a comprehensive analysis of the adsorption of CO on Pd(1 1 1) at 300 K has been performed. The characteristic fingerprints for various CO–Pd(1 1 1) bonding configurations have been identified from the decomposition analysis of the adsorbate C 1s and the substrate Pd 3d 5/2 core-level photoemission spectra obtained after CO adsorption at 120 K. The c(4×2) structure at 0.5 monolayer (ML) and the (2×2)-3CO structure at 0.75 ML formed at low temperature have been used for calibration purposes. The core-level results are consistent with CO adsorbing in a mixture of fcc and hcp threefold hollow sites in the c(4×2) structure and of hollow and on-top sites in the (2×2) structure, as reported in the literature. For CO adsorption at 300 K, a different site occupation is evidenced by the presence of two components in the C 1s and Pd 3d 5/2 core-level and C–O stretching vibration lineshapes. At coverages up to 0.1 ML only fcc threefold hollow sites in a ( 3 × 3 )R30° structure are occupied. The population of these sites continues with increasing the CO coverage up to 0.32 ML, but in addition adsorption of CO in bridge sites takes place. The latter sites become predominant at the 300 K saturation coverage of 0.5 ML, and are considered to form domain boundaries in a poorly ordered ( 3 × 3 )R30° phase with split low energy electron diffraction reflexes. Between 0.32 and 0.5 ML the domain boundary regions grow at the expense of the ( 3 × 3 )R30° domains.

A theoretical study of methylidyne chemisorption on Ni(111) and Co(0001) surfaces

Methylidyne is a key intermediate in hydrocarbon growth reactions over transition metal surfaces. However, experimental data characterizing its interaction with various surfaces are scarce. Therefore, to deepen our understanding of the chemisorption of methylidyne and to quantify its role in the catalytic formation of hydrocarbons, we have calculated the binding energy of methylidyne on Ni(111) and Co(0001) surfaces using density-functional theory within the generalized gradient approximation and the full-potential linear augmented planewave (FP-LAPW) method. The dependence of the binding energy on both the adsorption site and the surface coverage was significant on both surfaces. The binding energy of CH adsorbed in the fcc threefold hollow site of Ni(111) decreased by 1.1eV as the surface coverage increased from 0.25 to 1.0ML, whereas the increase of 0.74eV observed over Co(0001) for the hcp threefold hollow site was slightly less pronounced. The density-of-state plots revealed that rehybridization of the carbon atom interacting with the surface occurred, and the electronic changes induced upon chemisorption were similar for both surfaces.

An STM, AES and LEED study of the segregated sulfur on Pd(111)

At elevated temperatures, 700 K, sulfur segregates to the Pd(111) surface. The coverages as measured by Auger electron spectroscopy (AES) are in the range of 0.1 to 0.25 monolayers. The LEED pattern represents a clear ( 7 × 7 ) R 19.1-S° structure. With the scanning tunneling microscope (STM) the 7 structure is found on parts of the surface only. The position of the adsorbed S atoms is the fcc threefold hollow site. Beside the 7 structure, part of the surface is covered by a disordered layer of sulfur. A sizeable part of the surface is covered by sulfur in a (2 × 2) structure. The adsorbate in that structure is mesoscopically ordered in triangular 2D clusters with a rather narrow size distribution. The triangles contain between 15 to 66 sulfur atoms. The 〈110〉 edges of the Pd(111) surface are ‘decorated’ with sulfur, also in a (2 × 2) structure. These structures run parallel to the steps with a width of 4 to 5 atoms.

The electronic structure of the surface methoxy species on Cu{111}

Using model calculations for a Cu 25OCH 3 cluster the bonding interaction between the methoxy species and a Cu{111} surface has been investigated. In agreement with recent photoelectron diffraction measurements, OCH 3 is found to adsorb preferentially in the fcc three-fold hollow site with its OC axis perpendicular to the surface. The corresponding vibrational spectrum — measured with high resolution electron energy loss spectroscopy — is compatible with this bonding geometry. Moreover, on the basis of the known structure, selection rules are used to definitively assign the adsorbate-induced features in the photoemission spectrum. There is good agreement between the measured ionisation potentials and those obtained from the model calculations via Koopmans' theorem. Taken together, the results confirm a bonding picture in which charge transfer of metal electrons fills the 2e orbitals of the adsorbed OCH 3 species, leading to a largely ionic bond.

Direct adsorbate-structure determination by scanned-energy-mode photoelectron diffraction.

A method for the transformation of scanned-energy-mode photoelectron diffraction spectra from adsorbates into real-space images of the nearest-neighbor substrate atoms is proposed. The approach is analogous to Fourier-transform mapping but takes into account the atomic scattering phase shifts. As an example, the adsorption of atomic oxygen on Ni{111} in an fcc threefold-hollow site is discussed using simulated photoelectron diffraction data. It is found that the position of the emitter relative to the surface can be directly determined to within 0.2 A\r{}.

Ethylidyne on Pt(111): Determination of adsorption site, substrate relaxation and coverage by automated tensor LEED

The adsorption of ethylidyne on Pt(111) is investigated by low-energy electron diffraction (LEED). An automated search program for LEED structure analysis is used to determine the adsorption geometry from comparison of experiment and theory. The fcc threefold hollow site is preferred by the fit. The carbon-carbon bond distance is 1.49 Å and the carbon-platinum bond length 1.91 Å. A buckling within the top two substrate layers is detected which corresponds to a local expansion of the first-to-second layer distance underneath the adsorbate by 0.08 Å as compared to the bulk value. The three substrate atoms forming the hollow site are closer to one another than in the bulk structure by 0.11 Å. A tilt of the carbon-carbon bond by 6° seems to occur and could be indicative of a wagging vibration of this bond. The hydrogen atoms do not contribute to the diffraction that yields the ordered pattern as indicated by the absence of hydrogen related features in the experimental I– V curves, suggesting relatively free rotation of the methyl group about the carbon-carbon axis. Additionally, a distinction between a p(2 × 2) and an average of three domains of a p(2 × 1) structure, which both give rise to the same (2 × 2) LEED pattern, is possible. The p(2 × 2) is favored in the structural analysis, giving an ethylidyne coverage of 0.25 monolayers.

Surface structure of sqrt 3 x sqrt 3 R30 degrees Cl/Ni(111) determined using low-temperature angle-resolved photoemission extended fine structure.

A surface structural study of the \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 R30\ifmmode^\circ\else\textdegree\fi{} Cl/Ni(111) adsorbate system was made using low-temperature angle-resolved photoemission extended fine structure. The experiments were performed along two emission directions, [111] and [110], and at two temperatures, 120 and 300 K. The multiple-scattering spherical-wave analysis determined that the Cl atom adsorbs in the fcc threefold hollow site, 1.837(8) \AA{} above the first nickel layer, with a Cl-Ni bond length of 2.332(6) \AA{}, and an approximate 5% contraction between the first and the second nickel layers (the errors in parentheses are statistical standard deviations only).

Adsorption position of oxygen on the Pt(111) surface

The adsorption position of oxygen on the clean Pt(111) surface has been determined by means of the transmission channeling technique. Oxygen adsorbs in a well ordered p(2 × 2) overlayer structure at temperatures 200 ⩽ T ⩽ 350 K. From an analysis of the angular scans along the [111], [110] and [100] axial directions it is concluded that the O atoms are adsorbed in the fcc three-fold hollow site exclusively at a height of 0.85 ± 0.06 Å above the Pt surface layer. From a narrowing of the [111] angular O scan, the O RMS displacement parallel to the surface is found to be 0.16 ±0.03 Å.

Adsorption position of oxygen on the Pt(111) surface

The adsorption position of oxygen on the clean Pt(111) surface has been determined by means of the transmission channeling technique. Oxygen adsorbs in a well ordered p(2 X 2) overlayer structure at temperatures 200 ≤ T ≤ 350 K. From an analysis of the angular scans along the [111], [110] and [100] axial directions it is concluded that the O atoms are adsorbed in the fcc three-fold hollow site exclusively at a height of 0.85 ±0.06 Å above the Pt surface layer. From a narrowing of the [111] angular O scan, the O RMS displacement parallel to the surface is found to be 0.16±0.03 Å.

Adsorption position of deuterium on the Pt(111) surface determined with transmission channeling

By means of the transmission-channeling technique, the adsorption position of deuterium (D) on the clean Pt(111) surface is determined. Deuterium adsorbs in a well ordered (1 × 1) overlayer at temperatures ⩽ 200 K. From an analysis of the angular scans around the [111], [110] and [100] axial directions it is concluded that D adsorbs in the fcc three-fold hollow site at 0.58 Å above the surface. From a narrowing of the [111] angular D scan, the D RMS displacement parallel to the surface is found to be 0.29 Å. It is furthermore concluded that D does not occupy sub-surface octahedral sites on the Pt(111) surface.