SPring-8, the large synchrotron radiation facility

Date & Time: 11:00-12:00, January 11, 2006

Speaker: Dr. Frank Uwe Renner
Affiliation: ESRF

Abstract
　　For the understanding of corrosion as well as catalytic or etching processes, knowledge about the detailed structure of metal surfaces is required. Binary metal alloys are in general of special interest because their properties can be tailored by their composition. However, in our natural environment, all materials are exposed to wet conditions, leading to the formation of a thin layer of aqueous electrolyte and to electrochemical corrosion. Despite of keen interest, there is very little understanding of electrochemical processes on alloy surfaces on the atomic scale. This is mainly due to the lack of techniques that can resolve atomic structures in the presence of the electrolyte. We are reporting about an in-situ X-ray investigation with controlled electrical potential of the corrosion of Cu₃Au(111) in 0.1M H₂SO₄ electrolyte. Cu-Au alloys can be used to produce nano-porous Au layers with many anticipated applications at potentials above the critical potential Ec for dealloying. During the initial electrochemical corrosion below Ec an ultra-thin metallic passivation layer is formed with the selective dissolution of Cu from the surface. Our results identify it as ultra-thin epitaxial Cu_xAu_1-x(111), which forms on the surface at a potential where Cu dissolution starts. However, with further increasing electrode potentials (elevated overpotentials) pure Au islands are formed. The thickness of the layer is changing from a few atomic monolayers for the initial ultra-thin layer to several nanometers for the pure Au islands. We used single crystals, which were free of stacking faults, and thus could observe that the pseudo-fcc metallic passivation film grows with a detailed reversed stacking sequence with a controlled slow increase in potential. The results are compared with the structural evolution in the presence of Cl- ions and with experiments on Cu₃Au(100). The information is completed by ex-situ AFM images of the etched surfaces. In conclusion, we could follow the potential controlled evolution of the structure and chemical composition of a binary alloy surface in contact with electrolyte in-situ and with atomic resolution.