Adsorption of pyrazine on Au(111) and Ag(111) electrodes an ex situ XPS study

The adsorption of pyrazine on Au(111) and Ag(111) electrodes from aqueous solutions has been studied ex situ by X-ray photo-electron spectroscopy (XPS). The well known structure transition of pyrazine on Au(111) from flat to vertical is clearly reflected in a change in the N 1s binding energy, while no such change in orientation has been noted for pyrazine on Ag(111), for which a flat-lying molecule is assumed at all potentials. Furthermore, distinctly different electrochemical shifts for the N 1s level were observed for the two orientations in the case of pyrazine on Au(111). This suggests that the electrochemical shift of adsorbed molecules may be successfully employed for sensing the electrostatic potential variation inside the electrochemical double layer. The surface concentration of pyrazine on Au(111) as a function of potential has been determined from XPS intensities and is compared with coulometric data.