Surface-enhanced raman scattering study on the structure of 2,2′-bipyridine adsorbed on an Ag electrode

Abstract

The surface-enhanced Raman scattering (SERS) spectra were measured for 5 m M 2,2′-bipyridine+0.1 M KX (X = CI, Br or I) and the assignments of the bands were discussed by comparing the surface spectra with the Raman and infrared spectra of Ag(I)(bpy) 2NO 3 and bpy itself. As the electrode potential was changed from +0.05 to −1.1 V (vs. Ag/AgCl), the SERS spectra showed a sequence of changes in their features indicating that a series of transformations are taking place between at least four kinds of molecular states (Type I, II, III and IV) of adsorbed bpy molecules on silver electrode surfaces. Near +0.05 V in the KCI electrolyte solution the adsorbates take on a structure of type I which gives a SERS spectrum almost identical with the Raman spectrum of Ag(I)(bpy) 2NO 3. A study of the effect of illumination by a 514.5 nm laser indicated that the type I adsorbate is a reaction product from bpy molecules and Ag atoms which were created by photoreduction of an AgCl layer produced during the anodization process of the electrode. When the electrode potential was changed to a more negative region (−0.2, −0.4 and −0.5 V for the KCl, KBr and KI electrolytes, respectively), the silver halide layers were electrochemically reduced forming a fresh Ag layer. On adsorbing on this layer the bpy molecules always assume a structure of type II, where the molecules again take on coordination and backbone structures similar to those of the argentous complex. In this state some of the out-of-plane vibrations of the pyridine moieties are observed to be specifically enhanced, which suggests that the molecular plane of the ligands lies almost flat on the electrode surface. On sweeping the electrode potential to more negative values, the type II adsorbate is generally converted to a type III species, which is characterized by its breathing vibration observed at 1025 cm −1. It was suggested that in this state the nitrogen atoms of the pyridine moieties are strongly chemisorbed to a silver atom on the electrode surface, forming a Lewis acid coordination structure. In the KCl and KBr electrolyte solutions the type III species is converted to a type IV adsorbate when the electrode potential approaches the pzc of the Ag electrode (about −0.91 V). In the latter species the adsorbed bpy molecules give rise to the breathing vibration near 1005 cm −1 suggesting that the molecules exist in a physisorbed state.