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Comparative Study of Thymine Adsorption on Cu- and Ag-Adlayers on Au(111) Electrodes

The electronic attributes of an electrochemically deposited copper monolayer on Au(111) were compared to its bulk equivalent by XPS experiments. Due to the modified interaction between the Cu-Cu as well as Cu-Au atoms at the surface a core level shift to lower binding energies occurs, which exceeds the values found for surface core level shifts of Cu atoms on Cu(100) surfaces. It could be shown that the thymine orientation on a copper layer depends on the externally applied potential in analogy to the adsorption behaviour on Au(111) and Ag(111) single crystals. The transition between a chemisorbed and a physisorbed phase occurs around the potential of zero charge. Independently of the adsorption state of thymine the electron density around the surface copper atoms is reduced by the interaction between thymine molecules and the copper surface. At electrochemically deposited silver monolayers, bilayers as well as at bulk silver on Au(111) different orientations and adsorption states of thymine could be evaluated. A comparison between XPS and electrochemical data reveal an alteration of the thymine–Ag interaction within the chemisorbed thymine layer deposited on 1 ML Ag/Au(111). It is notable that the transformation between the chemisorbed and physisorbed thymine phase does not seem to be dependent on the thickness of the silver adlayers, while the potential of zero charge differs by about 200 mV. In contrast, the onset of the hydrogen evolution shifts to lower potentials with increasing thickness of the Ag layers.