The adsorption and coadsorption of sulfur and oxygen on the Ir(110) surface was investigated by scanning tunneling
microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). The clean
Ir(110) surface forms alternating (331) and (33-1) minifacets, resulting in a mesoscopically rippled surface. Upon
chemisorption of sulfur or oxygen and subsequent annealing, the surface structure is changed. In the following, the
results concerning sulfur and oxygen adsorption will be summarized before addressing the coadsorption system.
Sulfur adsorption: At sulfur coverages of 0.1-0.2 ML, the Ir(110) surface adopts a (1x2) missing-row configuration
similar to clean Au(110) and Pt(110). The sulfur-stabilized Ir(110)-(1x2) does not show any evidence for the
preference of (111) faceted steps, and consequently does not form a mesoscopic fish-scale pattern. The latter was
observed on the (110) surfaces of Au and Pt, and was found to be driven by the preference for (111) step facets. On
Ir(110), no such preference seems to exist, since (331) step facets are frequently observed. With respect to the
adsorbed sulfur, no extended islands are observed, indicating repulsive adsorbate-adsorbate interactions.
At sulfur coverages near 0.5 ML, a p(2x2) structure with p2mg (glide-plane) symmetry is observed. The adsorption site
and structural model derived by STM are compatible with an earlier LEED analysis of that structure: S adsorbs in
threefold coordinated fcc hollow sites above the (111) facets formed by the non-missing substrate rows.
At coverages higher than 0.5 ML, a c(2x4) LEED pattern with additional faint streaks in the [-110] azimuth is observed.
STM reveals that the streaks are due to pairs of sulfur atoms (dimers, for brevity) in a second adsorbate layer, that can
be desorbed by heating to 1100 K. A structural model is derived on the basis of the STM results, showing the dimer
atoms in on-top positions over sulfur atoms of the first adsorbate layer. When the surface is completely covered by the
dimers, the surface is saturated at 0.75 ML.
Oxygen adsorption: In agreement with earlier reports, oxygen adsorption and subsequent annealing to 700-900 K results
in an unreconstructed (1x1) surface, covered by a c(2x2)-O overlayer at 0.5 ML coverage.
Coadsorption of oxygen on an S-precovered surface (S-coverage below 0.5 ML) leads to a phase separation of the
adsorbates (competitive adsorption). At low coverages, oxygen forms a p(2x2)-O phase, whereas at higher
O-coverages a compression into a (1x2)-O phase is observed. Postannealing the (1x2)-O phase at 900 K in vacuum
leads to a reduction of the sulfur concentration, indicating sulfur oxidation. Interestingly, the p(2x2)-O phase does not
seem to be reactive, according to the AES results. A possible explanation may be that the more densely packed
(1x2)-O phase can be regarded as an activated structure. This is also supported by the STM results.
At S-coverages above 0.5 ML, the surface is completely poisoned with respect to oxygen adsorption. Nevertheless,
heating the sulfur saturated Ir(110)-c(2x4)-S structure in an oxygen atmosphere, the sulfur concentration gradually
drops to zero. At intermediate stages of this oxidation process, island formation is observed by STM, but the underlying
formation processes remain to be resolved.
| Identifer | oai:union.ndltd.org:uni-osnabrueck.de/oai:repositorium.ub.uni-osnabrueck.de:urn:nbn:de:gbv:700-2000092666 |
| Date | 26 September 2000 |
| Creators | Kuntze, Jens |
| Contributors | Prof. Dr. Werner Heiland, Prof. Dr. Eckard Rühl |
| Source Sets | Universität Osnabrück |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/zip, application/gzip |
| Rights | http://rightsstatements.org/vocab/InC/1.0/ |
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