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Characterization of the Gold Metal-Solution Interface Using LEED, in situ EXAFS and SXS

Shijie Wu
Electrochemistry and Low Energy Electron Diffraction
EXAFS Setup, Model, and Cmposition of the interface
The three low index Au single crystal surfaces are known to reconstruct in an electrochemical cell. An understanding of the influence of reconstruction on the adsorption of cations and anions is very important to many areas of electrochemistry. I have applied both ex situ low energy electron diffraction (LEED) and in situ surface X-ray scattering (SXS) techniques in addition to traditional electrochemistry to study reconstruction of the Au surfaces in the presence of anions (Cl-,Br-) and small organic molecules (pyridine, bipyridine). I also used in situ EXAFS to study the coadsorption of Cu and anions (SO42-, Cl- Br-) at the Au/electrolyte interface.
LEED experiments have shown that the adsorption of Br- on Au(100) lifts the surface reconstruction at a small coverage . Very interesting structure have been observed for the Br- overlayer formed at this surface of gold.
Potential induced structural transformation (1x1)(1x23) between the unreconstructed (1x1) and the reconstructed (1x23) surfaces of Au(111) electrode in the presence of organic molecules was studied in situ using surface x-ray scattering (SXS) technique. It was shown that adsorption of small organic molecules such as pyridine and bipyridine inhibits this structural transformation. No direct correlation between the surface coverage and lifting of the reconstruction was observed.
Chronocoulometry and in situ X-ray absorption
Model Structure of Cu and Sulfate Coadsorbed on Au(111)
Chronocoulometry and in situ extended X-ray absorption fine structure(EXAFS) and X-ray absorption near edge structure(XANES) have been applied to study coadsorption of Cu with sulfate, chloride, and bromide ions at the Au(111) electrode surface. These anions (SO42-, Cl-, Br-) have great effect on the submonolayer deposition of Cu at the Au surface. On the other hand, the adsorption of anions at the interface is dramatically promoted by the adsorbed Cu atoms as well. Thermodynamic analysis of chronocoulometric data provided the surface excesses for copper and anions coadsorbed at the interface. A complete monolayer of Cu was formed at a potential close to 0 mV/SCE. Bulk deposition of Cu was observed at even lower potentials. The amount of Cu at the surface was reduced to about 2/3 of the maximum at about 100 mV. At potentials above 200 mV, Cu was completely stripped from the surface. Coadsorption of anions with Cu at the surface was observed throughout the entire potential range. Electrochemical and X-ray absorption data indicate that copper coadsorbed with sulfate forms a well ordered monolayer. In contrast copper coadsorbed with chloride or bromide forms a bilayer. in which the Cu atoms are adsorbed directly on the Au(111) substrate.. and the coadsorbed anions form the top layer. The copper adatoms are adsorbed in registry with respect to the overlayer of the anion and form an incommensurate structure with respect to the gold substrate. Near edge spectra suggest that the oxidation state of the adsorbed Cu atoms deviates from that of a purely metallic state and varies somewhat with the applied electrode potential.