Müller, Stefan Fritz

Metal-oxide/organic-acid interfaces play an important role in various applications ranging from hybrid materials to catalysis. The present work studies the structure and energetics of these interfaces on the atomic scale. Employing first-principles calculations and a novel structure search tool, the most stable interface structures are determined, finding a restructuring of magnetite (Fe3O4) surfaces, and identifying shape-influencing factors for titania (TiO2) nanoparticles. Finally, the facet dependence of mechanical interface properties is examined, yielding deeper insights in the origin of failure in hybrid materials.

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), a statistical physics-based method, makes it possible to match those properties to atomic arrangements, and thus identify the most favorable structures within the whole configuration space. Local properties at the material’s surface are also influenced by the atomic arrangement within a small number of surface layers. A typical example is surface segregation, where the atomic order at the surface differs from that of the bulk because of a difference in chemical potential. In this thesis, CE fits were performed to predict the most favorable surface configurations and 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. It is shown here by means of an analysis of Bader charges and the partial density of states that gold is stabilized in the topmost layer by a charge transfer from silver to the more electronegative gold. 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. This can be evaluated by calculating the response of the electronic work function to in-plane strain. The resulting coupling parameter was then expanded in a CE fit to examine the influence of the silver surface concentration and the atomic arrangement at the surface. 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, namely slight gold enrichment in the surface layer and silver enrichment in the subsurface layer. The obtained silver concentrations in the first layers match those of a ground state obtained in the surface CE. The first-principles results from this thesis combined with the cluster expansion technique help to shed new light on surface phenomena in the Ag-Au alloy. Such data are very difficult to acquire experimentally, as they take into consideration hundreds of thousands of atomic configurations. Here, we verify our calculations by performing a LEED structure analysis, which yields fairly good agreement with the first principles data.

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