Surface properties of silver-gold alloys – a quantum mechanics-based approach combining theory and experiment

A prominent tool in computational materials modeling is density functional theory (DFT), which allows one to calculate macroscopic material properties from the atomic scale and based solely upon physical principles. The cluster expansion (CE) makes it possible to match those properties to atomic arrangements, and identify the most favorable structures within the whole configuration space.

In this thesis, CE fits were performed to analyze the segregation behavior at flat and stepped Ag-Au surfaces. The Ag-Au system was of particular interest here, since small amounts of silver that remain after the fabrication process may explain the origin of the high catalytic reactivity of the sponge-like nanoporous gold.

Interestingly, gold segregation to the topmost layer of the adsorbate-free Ag-Au surfaces was obtained in this work, whereas numerous experimental and theoretical studies from the past report silver surface segregation. In a next step, it is revealed that for oxygen-covered Ag-Au surfaces, silver impurities are drawn to the surface layer. The special case of an infinite oxide chain on the stepped Au(321) surface with Ag impurities is characterized by an analysis of the bonding characters and the electronic surface structure.

Furthermore, the electromechanical coupling behavior at the Ag-Au (111) surface is studied. In summary, a strong influence of the surface layer composition on the coupling parameter is found for the Ag-Au alloy surface.

Finally, the atomic structure composition of the adsorbate-free Ag-Au (111) surface is characterized experimentally via low-energy electron diffraction (LEED). The LEED structure analysis indicates good agreement with the calculated segregation behavior.

The DFT results from this thesis combined with the CE technique help to shed new light on surface phenomena in the Ag-Au alloy. Such data are difficult to acquire experimentally, as they take into consideration hundreds of thousands of atomic configurations.