In the present work the growth and redox behavior of thin Au islands or films with various thicknesses (two to five layers) deposited on Ru(0001) was studied by x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). By exposure of atomic oxygen at room temperature, small oxidized gold nanoparticles are formed by the fragmentation of the metallic gold islands or film. For smaller exposures of atomic oxygen (<
80 L) only the gold islands are attacked, while the Ru(0001) surface is unharmed. With increasing thickness of the Au islands (or film), the rate of the Au oxide/Au nanoparticle formation and the number of formed nanoparticles decreases, while their size increases. To describe the thickness dependent oxidation and fragmentation process of the gold islands (or films), a shoveling mechanism is proposed where oxidized gold atoms are shoveled from the gold-ruthenium interface to the rim of the gold islands (films). The catalytic activity of these nanoparticles was investigated by CO oxidation experiments at room temperature. However no activity has been observed. Only the reduction of the Au oxide/Au nanoparticles occurs, while the shape and dispersion of the nanoparticles on the surface is retained.This change on the morphologies of the gold islands (or films) upon their oxidation or reduction is elucidated in the context of the theory of heterogeneous nucleation and epitaxial growth. Based on Young s equation in particular, the energy contributions of the interface energy, the strain energy and the surface free energies of the deposited material and the substrate are related to the growth behavior and the resulting morphology.In the second part of the present work the growth and redox behavior of metallic ruthenium structures on Au(111) were studied. Again the resulting morphologies upon oxidation and reduction of ruthenium are elucidated by the energy relation given by Young s equation. The deposition of ruthenium on the Au(111) surface leads to three dimensional growth of metallic ruthenium islands. These islands merge to a rough ruthenium film. By exposure of oxygen at 680 K the merged ruthenium islands rearrange to a rather flat ruthenium film with a unique perforated morphology. XPS measurements indicate that this perforated film is stabilized by a chemisorbed oxygen phase. By using typical Ru(0001) single crystal oxidation conditions (680 K, 5·10-5 mbar O2, 30 min) the ruthenium islands on Au(111) do only form a covering film of RuO2 if the former metallic ruthenium islands had a critical thickness of 10 monolayers Ru. RuO2 structures bound to the Au(111) surface are assumed to be not stable, so a metallic ruthenium buffer layer between the oxide and the gold substrate is necessary. To describe the transformation of the three dimensional Ru islands to the perforated ruthenium film with a chemisorbed oxygen phase, a mechanism is proposed based on the energy relation given by Young s equation.Finally a brief literature overview of other growth systems is given to further evaluate the general applicability of Young s equation.
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