Surface-enhanced Raman spectroscopy and electrochemistry of 2,2′-bipyridine adsorbed at copper electrode

Abstract

The adsorption of 2,2′-bipyridine (22BPY) on the surface of polycrystalline copper electrode was studied by means of surface-enhanced Raman spectroscopy (SERS) and cyclic voltammetry in a wide range of electrode potential. Experiments were carried out in both acidic and neutral solutions, as well as in presence of two kinds of supporting electrolyte (KCl and LiClO 4) in order to examine the influence of molecule protonation and specifically adsorbed anions. It was found that a molecule orientation with respect to the surface changes from perpendicular to more tilted in the neutral perchlorate solution followed by the conformation change from cis to trans in strongly negative potentials. This conformational transition occurs gradually, probably through a series of intermediate states. The SERS features due to trans conformers are uniquely observed in presence of perchlorate anions. In the presence of chloride the molecular plane is at least slightly inclined towards the surface in the whole potential range. In acidic solutions the SERS results indicate that 22BPY adsorbs in non-protonated form independent on the present anion with the molecular plane tilted although it becomes more perpendicular with respect to the surface at negative potentials. The cyclic voltammetry measurements revealed that the hydrogen evolution is inhibited in presence of 22BPY in both neutral and acidic solutions. It suggests strong adsorption of this molecule in a wide range of electrode potentials.