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O: Fachverband Oberflächenphysik

O 64: Solid/liquid interfaces I (focussed session)

O 64.6: Vortrag

Donnerstag, 25. März 2010, 12:30–12:45, H33

Adsorption and Phase Transition of Heptylviologen on HOPG observed by EC-STM — •Knud Gentz and Klaus Wandelt — Institut für Physikalische und Theoretische Chemie der Uni Bonn, Wegelerstraße 12, 53115 Bonn

Heptyl Viologen (1,1’-diheptyl-4,4’-bipyridin, HV) has been the focus of several publications which derived possible structures of the adsorbate films and it’s behavior during phase transitions by means of cyclic voltammetry and chronoreflectometry. The purpose of this contribution is to verify these proposed structures as well as the behavior upon adsorption and phase changes by scanning tunneling microscopy. We adsorbed Heptylviologen on an HOPG electrode surface from a supporting electrolyte of sulfuric acid and potassiumchloride (-bromide). In the CV the characteristic voltammetric spikes of the surface phase transition were observed both in the presence of chloride as well as bromide. The STM images clearly show a stripe structure on the surface in the regime negative of the spike voltammetric peaks indicating the presence of the radical cationic phase (HV^•+) stabilized by π–π-interaction of the individual molecules. A single row and a double row structure were found showing a herringbone-like arrangement of the molecules. Above the spike peak no ordered layer was observed. The proposed dimers of the radical cation were not found on the surface at any given potential, instead we observed long-chained oligomers. In the presence of bromide an insoluble viologen bromide is formed on the surface hampering the stabilization of the surface phase which causes the spike peak to fade with progressing voltammetric cycles.