Journal of Physical Chemistry, Vol.98, No.28, 7083-7089, 1994
Electrochemical and 2nd-Harmonic Generation Studies of 2,2’-Bipyridine Adsorption at the Au(111) Electrode Surface
Adsorption of 2,2’-bipyridine (22BPY) at the Au(lll)-solution interface has been studied using cyclic voltammetry, ac voltammetry, chronocoulommetry, and second harmonic generation spectroscopy (SHG). The thermodynamic quantities describing the energetics of 22BPY adsorption at the Au(lll) surface such as the Gibbs surface excess, Gibbs energy of adsorption, and the electrosorption valency were determined from electrochemical measurements. The effect of 22BPY on the crystallographic and electronic structure of the Au(lll) surface was assessed with the help of SHG experiments. It was found that 22BPY adsorbs at the Au(lll) surface in a number of states corresponding to different orientations and/or conformations of this molecule. The 22BPY molecule assumes a flat orientation in which the two aromatic rings are parallel to the gold surface at the negatively charged interface. At the positively charged interface, the molecule assumes a vertical orientation with a coplanar cis configuration in which the two nitrogen atoms of the molecule face the metal surface. The transition from the flat to the vertical orientation is gradual and goes through a series of intermediate states, Adsorption of 22BPY has little effect on the crystallographic structure of the Au(lll) surface. A significant change of the phase angle between the real and imaginary components of the isotropic term of the SHG signal indicates, however, that mixing of the molecular orbitals of the 22BPY with the electronic states of the metal affects the electronic structure of the surface.
Keywords:ENHANCED RAMAN-SCATTERING;SOLUTION INTERFACE;METAL SURFACES;PYRIDINE;INSITU;RECONSTRUCTION;SPECTROSCOPY