H.P. Bonzel

Adsorption and decomposition of H2O on a K-covered Pt(111) surface

Abstract The adsorption of H2O on clean and K-covered Pt(111) was investigated by utilizing Auger, X-ray and ultra-violet photoemission spectroscopies. The adsorption on Pt(111) at 100–150 K was purely molecular (ice formation) in agreement with previous work. No dissociation of this adsorbed H2O was noted on heating to higher temperatures. On the other hand, adsorption of H2O on Pt(111) + K leads to dissociation and to the formation of OH species which were characterized by a work function increase, an O 1s binding energy of 530.9 eV and UPS peaks at 4.7 and 8.7 eV below the Fermi level. The amount of OH formed was proportional to the K coverage for θK > 0.06 whereas no OH could be detected for θ⩽ 0.06. Dissociation of H2O occurred already at T = 100 K, with a sequential appearance of O 1s peaks at 531 and 533 eV representing OH and adsorbed H2O, respectively. At room temperature and above only the OH species was observed. Annealing of the surface covered with coadsorbed K/OH indicated the high stability of this OH species which could be detected spectroscopically up to 570 K. The adsorption energy of H2O coadsorbed with K and OH on Pt(111) is increased relative to that of H2O on Pt. The work function due to this adsorbed H2O increases whereas it decreases for H2O on Pt(111). The energy shifts of valence and O1s core levels of H2O on Pt + K as deduced from a comparison of gas phase and adsorbate spectra are 2.8–4.2 eV compared to ≈ 1.3–2.3 eV for H2O on Pt (111). This increased relaxation energy shift suggests a charge transfer screening process for H2O on Pt + K possibly involving the unoccupied 4a1 orbital of H2O. The occurrence of this mode of screening would be consistent with the higher adsorption energy of H2O on Pt + K and with its high propensity to dissociate into OH and H.

No adsorption on Pt(111)

Abstract The interaction of NO with a Pt(111) surface was studied by means of XPS, LEED and AES at 120 K and 305 K. The observed changes in the valence and O(1s) and N(1s) core level spectra during NO adsorption and stepwise heating of the adsorbed layer as well as the estimated O : N atomic ratios supported molecular NO adsorption on Pt(111). The NO saturation coverage was found to increase from 0.23 at 305 K to 0.54 at 120 K. At both adsorption temperatures and at coverages less than 0.15 only a single adsorption state was populated characterized by an O(1s) binding energy of 530.6 eV and a dipole moment of 0.42 D, negative end outwards. At higher coverages, a second state characterized by an O(1s) binding energy of 532.5 eV appeared. These two states are associated with bridge and linearly bonded NO, respectively, in accordance with existing vibrational data. For high NO coverages achieved after adsorption at 120 K the XPS O(1s) spectra produce evidence of an adsorbed NO species characterized by an O(1s) binding energy of ~ 531.1 eV. This species may correspond to disordered NO molecules unobservable by infra-red spectroscopy.

Oxygen adsorption on Pt(110)-(1 × 2) and Pt(110)-(1 × 1)

Abstract The adsorption of oxygen on the reconstructed Pt(110)-(1 × 2) surface at 120–500 K was investigated by O 1s core level spectroscopy and by measurements of work function changes. Molecular and atomic oxygen could be distinguished by O 1s binding energies at 530.7 and 529.9 eV, respectively. The initial sticking coefficients were 0.9 for molecular oxygen at 120 K and 0.3 for atomic oxygen at 300 K, with maximum coverages of 1.35 and 0.35, respectively. Repeated low temperature O2 exposures and heating to 300 K resulted in a maximum coverage of 0.85 at 300 K. There was always a work function increase after oxygen adsorption; the dipole moment of initially adsorbed molecular oxygen was twice that of atomic oxygen. For comparison, oxygen was also adsorbed on the metastable Pt(110)-(1 × 1) surface at 120–250 K. The maximum coverage at 250 K was 0.8 on (1 × 1) compared to 0.35 on (1 × 2). All data are consistently explained by assuming that the adsorption sites on close-packed atomic rows in [1 1 0] direction have the highest, rate-determining activity for O2 adsorption and dissociation.

Adsorption of H2O on Ru(001). I, Bilayer and clusters

Abstract The adsorption of H 2 O on Ru(001) was investigated by photoemission spectroscopy (UPS and XPS), LEED and workfunction measurements in the temperature range 120–300 K. The H 2 O coverage was calibrated via O1s core level intensity by comparison with a p(2 × 2)-O layer corresponding to a coverage of 0.5. The O1s spectra of adsorbed H 2 O layers always showed a contribution near 531.3 eV binding energy although the main intensity was located at 532.7–533.9 eV depending on coverage. Annealing of adsorbed layers to 173–193 K caused an O1s double peak to appear with binding energies at 531.3 and 532.7 eV. These peaks are assigned to islands of a H 2 O bilayer whereby the binding energies 531.3 and 532.7 eV belong to the first and second H 2 O layer, respectively. Higher order H 2 O layers have an O1s binding energy of at least 533.2 eV. The layer-dependent binding energy of H 2 O is suggested to be due to a distance-dependent screening effect of the O1s core hole, aside from the first layer H 2 O molecules which are chemically bonded to Ru surface atoms via the oxygen lone pair orbital. The formation of perfect bilayer islands is thermally activated and does not occur during low temperature adsorption at 120 K. Up to 15% of the first bilayer of H 2 O molecules are found to dissociate after a slow anneal to 210 K whereas dissociation is negligible for a fast flash.

Nucleation and growth of CoSi2 on Si(100) studied by scanning tunneling microscopy

Abstract The initial stages of CoSi 2 formation on the Si(100) surface are investigated by scanning tunneling microscopy (STM). We find a quasi-periodical reconstruction of the Si surface for very low Co coverages of 0.01 ML which is similar to the Ni induced (2 × 8) structure. At higher Co coverage, in reactive deposition epitaxy, the formation of qualitatively different two and three-dimensional islands is observed. We have evidence that the former are Si terminated, with Co probably being positioned in substitutional sites beneath the island. The growth of the 3D CoSi 2 islands is connected with substantial mass transport from the substrate into the islands to enable the silicide formation. Their elongated shape is attributed to strain and they occur in different epitaxial relations to the substrate. CoSi 2 islands in (100) orientation are identified by the c(2 × 2) surface lattice with mixed Co and Si occupation. Simultaneous deposition of Co and Si up to 30 ML results in the formation of CoSi 2 island clusters and a rough surface. The roughness exponent β = 0.66 is in agreement with an existing Monte-Carlo simulation.

Morphology of periodic surface profiles below the roughening temperature: aspects of continuum theory

Abstract Using a simple analytical form of the anisotropic surface free energy, γ(ϑ), the quasi-stationary shape of one-dimensional periodic surface profiles below the roughening temperature as well as the kinetics of profile decay are calculated numerically. With the macroscopic surface being close to a cusp orientation in γ(ϑ), all profiels exhibit extended flat regions at the top and bottom (facets). These same regions and their adjacent rounded parts are near equilibrium, as a comparison with the truncated equilibrium shape of small particles for the same γ(ϑ) shows. The facet size of the profile depends on the total anisotropy of γ(ϑ) in the zone of interest, and on the amplitude of the profile. These dependences can also be simulated by the equilibrium construction. The γ(ϑ) function used in this study is examined in relation to the more fundamental expression for the anisotropic surface free energy and also experimental data for Pb. It is found to fit these experimental data very well over a large range of orientations.

Temperature-dependent morphologies of gold surfaces

Abstract Periodic surface profiles on Au{111} and Au{100} single crystals annealed at 1023 K under ultra-high vacuum conditions were studied ex situ by scanning tunneling microscopy (STM). The shapes exhibited several facets in each case separated from each other by sharp edges. Aside from the main {111} and {100} facets, extra facets at 3.7° and 11.7° for Au{111}〈112〉 and at 8.0° for Au{100}〈110〉 were observed. These orientations seem to represent stable surface phases, in the sense of “magic” orientations due to the influence of surface reconstruction.

Site-specific core level spectroscopy of CO and NO adsorption on Pt(110)(1×2) and (1×1) surfaces

The adsorption of CO and NO on the (1×2) and (1×1) modifications of the Pt(110) surface was studied by x-ray photoemission spectroscopy, LEED and work-function change measurements. The O(1s) binding energy of adsorbed CO is site-specific and differentiates between on-top and bridge adsorbed species. CO adsorption on Pt(110)(1×2) at 120 K occurred sequentially into on-top and bridge sites yielding an orderedc(8×4) layer at the maximum coverage. At 300 K only on-top bonded CO was present after CO adsorption on the (1×2) surface. CO adsorption on the (1×1) surface at 120 K showed a transient bridge adsorbed CO and on-top CO at saturation, with an ordered (2×1)p1g1 LEED pattern. Heating the (2×1)p1g1 CO layer to ∼400 K also showed this transient bridge CO species. Work function changes generally correlated with the appearance of different CO species but were complex in detail. The findings for CO adsorption are consistent with the missing row model of the (1×2) surface.Parallel data for NO adsorption on (1×2) and (1×1) surfaces at 120 K were less informative than those for CO because O(1s) spectra showed single broad peaks. Peak contributions due to bridge and on-top bonded NO could be estimated.

Step energetics of Pb(111) vicinal surfaces from facet shape

Abstract The step stiffness, step interaction and kink formation energies of steps vicinal to (111) have been derived from the equilibrium shape of a (111) facet on a small three-dimensional Pb crystal imaged by scanning tunneling microscopy. In addition, the azimuthal dependence of the step free energy for vicinal (111) surfaces has been obtained by an ‘inverse’ Wulff construction from the facet shape. Most of these energetic quantities can be quoted for both types of step (A- and B-steps). By also taking into account the curved portion of the vicinal surface, consisting primarily of B-steps, the entropic and total step interaction energy of B-steps can be separately obtained.

Interaction of NO with potassium promoted Pt(111)

Abstract The interaction of NO with a K-covered Pt(111) surface was studied at various adsorption temperatures ranging from 120 to 570 K by means of XPS, UPS and AES and compared to NO adsorption on clean Pt(111). It was found that NO adsorption is preferentially molecular at potassium coverages θK K NO layer to > 300 K leads to NO dissociation associated with a loss of surface nitrogen. The XPS and UPS spectra provide evidence for this dissociation and in addition for the formation of an NO2 species with O(1s) and N(1s) peaks at 532.8 and 404.0 eV, respectively. The probability for the formation of K-promoted NO2 increases with rising NO adsorption temperature and potassium coverage. This NO2 species is negatively charged due to the enhanced electron donation into its 6a1 molecular orbital; it starts to decompose at about 450 K.