Growth and interaction of gold on the faceted rhenium (12-31) surface
Creators: name: Baier, Robert Christoph Wolfgang
Publisher: name: No Publisher Supplied
Year: 2008

description: Heterogeneous catalysts are a field of major interest is in surface science. The development of new supported model catalysts with a narrow size distribution is a central aspect of this research. One possibility to create narrow size distributions is the use of faceted surface structures as self assembled templates on which to grow metal nanoclusters. In this thesis, a faceted rhenium single crystal is used as a template for gold growth;mthe facets were induced by oxygen. The orientation of the planar rhenium crystal is (12-31), whereby the facets consist of two sided ridges with (11-21) and (01-10) orientations. The way of growth and the infuence of gold on the faceted rhenium crystal is studied by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Also, the infuence of gold on a planar Re (12-31) surface is studied.It was found that gold grows in Volmer-Weber growth mode on the faceted rhenium substrate. For coverages of 0.4 ML and 0.8 ML gold formed 2D islands on top of the ridges, whereby for 1.4 ML coverage 3D islands were observed. Annealing of the gold covered surface structure led to drastic changes in the surfaces morphology. After the first annealing to a temperature of approximately 870 K for approximately 30 minutes gold atoms wet the rhenium template. Further annealing of the surface led to the development of a three sided pyramidal structure; the orientation of the pyramids still contains the orientation of the original two sided ridges. Continued annealing to temperatures of approximately 970 K developed a totally new surface structure. Two sided ridges were found again on the surface, but with a different orientation and shape than the original template structure. Finally, after annealing above 1200 K, all surface structures were destroyed and gold was desorbed from the surface. The morphological change is attributed to gold induced faceting on the oxygen induced surface structure.The pyramid structure was examined more detailed. Thereby a coverage dependency in the development of the facets was found. For 0.4 ML one of the pyramids sides is underdeveloped. For 0.8 ML and 1.4 ML all sides of the pyramids structure were fully developed; small destruction of the pyramids for 1.4 ML coverage is attributed to beginning destruction of the original oxygen induced ridges. The orientations of the pyramids facets were determined as (0-110), (11-20) and (12-32). Analysis of the facets atomic structureled to the suspicion, that the (1232) facet is covered with a gold overlayer whereas the (0110) facet is not covered with gold.Also the influence of gold on planar Re (1231) was examined. After annealing of the gold covered planar crystal (Coverage: 0.8 ML) two sided ridges were found on the surface. The orientation of this ridges was rotated by around 90° with respect to the oxygen induced ridges. It is believed that gold has the ability to induce faceting on the planar Re (12-31) surface.Keywords: adsorbate-induced faceting, oxygen-induced faceting, gold-induced faceting, rhenium, gold, oxygen, two sided ridges, three-sided pyramids