Coadsorption Phase Research Articles

A low-energy electron diffraction study of potassium - oxygen coadsorption phases on Ni(111)

The coadsorption of potassium and oxygen on a Ni(111) surface has been studied by low-energy electron diffraction (LEED). The initial potassium-induced structures on the bare and the oxygen-monolayer-covered surface exhibit similar LEED ring patterns, which may be attributed to the strong dipole - dipole repulsive interaction in both cases. On the other hand, a small amount of pre-adsorbed oxygen (θ0<0.1) prevents the formation of the potassium ring phase, suggesting an attractive K - O interaction. Once all of the pre-adsorbed oxygen is attached to the potassium adatoms, further potassium adsorption produces the same phases as on the oxygen-free surface. For intermediate O precoverages, a gradual structural transformation from a p(2 × 2) to a phase is observed upon K adsorption. Potassium reduces the critical oxygen coverage for the completion of this phase transformation, suggesting that (disordered) K mixes in the O layer. At higher O precoverages, coadsorption results in a new c(4 × 2) ordered phase in a structure which probably involves both K and O.

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.

Full structure determination of an alkali-metal/CO coadsorption phase for Co{101-bar0}-c(2 x 2)-(K+CO).

A structural analysis of a c(2\ifmmode\times\else\texttimes\fi{}2)-(K+CO) coadsorption phase on Co{101\ifmmode\bar\else\textasciimacron\fi{}0} by low-energy electron diffraction has yielded a full picture of the geometry of a model alkali-metal CO coadsorption system. Potassium atoms are located in fourfold hollow sites above second-layer Co atoms, and CO molecules are displaced by 0.5\ifmmode\pm\else\textpm\fi{}0.2 \AA{} from the high-symmetry Co short bridge sites near the center of gravity of the three neighboring K adatoms. The CO causes a substantial increase, by 0.4 \AA{}, in the K-Co nearest-neighbor bond length, and K adatoms are located with their centers above the O atoms in the neighboring CO. This has important implications for the bonding interactions within the adlayer.

Structural Analysis of a Cs + O Coadsorption Phase on a Ru(0001) Surface

The adsorption of oxygen onto a Ru(0001)-(2 x 2)-Cs phase with coverage ΘCs = 0.25 gives rise to the formation of a Ru(0001)-(2√3 x 2√3)R30°-3Cs-20 coadsorption structure with three cesium and two oxygen adatoms in the unit cell. A low-energy electron-diffraction (LEED) analysis reveals that oxygen is located in threefold hollow sites on the Ru surface while the Cs atoms remain near on-top sites; this site was also found in the pure Ru(0001)-(2 x 2)-Cs phase. The oxygen-Ru bond length of (2.14 ± 0.05) Å is longer by 0.11 A than in the Ru(0001)-(2x2)-0 phase, indicating that the Ru-O bond is weakened in the presence of coadsorbed Cs. The Cs hard sphere radius of (1.75 ± 0.07) Å is close to the ionic Pauling radius of 1.69 Å

Local geometrical structure of a [formula omitted] Co-adsorption phase on Al(111): atop bonding due to chemical heterogeneity

Normal incidence X-ray standing wavefield absorption (NIXSW) including site triangulation, has been used to study the local geometry of O and Na when O chemisorbs onto the Al(111)(√3 × √3)R30°-Na surface alloy phase. The favoured geometry places O atop the surface Na atoms, a result which has no parallel on elemental surfaces and which must be attributed to the chemical and electronic heterogeneity of the surface alloy. This structure rationalises earlier implications of local NaO bonding from core level photoelectron spectroscopy.

Structure determination of an alkali metal-CO coadsorption phase: Ni(111)-K/CO.

The structure of a Ni(111)- $(2\ifmmode\times\else\texttimes\fi{}2)$--K $/n$CO $(n\ensuremath{\approx}2)$ coadsorption phase has been determined using K $2p$, C $1s$, and O $1s$ scanned energy mode photoelectron diffraction, and compared with results from pure CO and K adsorption phases having similar coverages. Coadsorbed K has little influence on the local hollow site geometry of the adsorbed CO molecules; the K remains in the atop site of the pure K layer, but the K-Ni bond length increases substantially (by $0.15\ifmmode\pm\else\textpm\fi{}0.05$ \AA{}), as does the Ni-Ni outer layer spacing. The results conflict with aspects of current interpretations of spectroscopic data from such systems.

Quantitative structural study of an Na–O coadsorption phase on Al(111) using X-ray standing waves

Using normal-incidence X-ray standing wavefield absorption (NIXSW) including site triangulation via measurements at (111) and (text-decoration:overline111) reflections of both the O and Na X-ray absorption, we have investigated the local adsorption sites produced by O chemisorption onto a room-temperature-prepared Al(111)(√3 ×√3)R 30°-Na surface. The structure of the pure alkali-metal phase has been shown previously, by SEXAFS and NIXSW, to involve Na substitution of top-layer Al atoms, and these sites are unchanged in the early stages of O chemisorption. The new NIXSW results indicate that in this stage the O atoms adsorb atop the Na atoms with an NaνO spacing of ca. 2.05 A. This site provides a clear rationalisation for the results of Na and Al 2p core-level spectroscopy which indicate that the dominant O bonding is to the Na atoms and not to the Al, as in O chemisorption on the clean Al(111) surface. An alternative interpretation of the NIXSW data based on adsorption to near-hcp hollow sites is difficult to reconcile with the very short NaνO bond length associated with this site, and with the implied inequivalence of hcp and fcc sites despite the local NaνO bonding found in photoemission.