Adsorption and photon-driven charge transfer of pyridine on a cobalt electrode analyzed by surface enhanced Raman spectroscopy and relevant theories

Surface enhanced Raman spectroscopy (SERS) has been applied to study the interaction of pyridine with a cobalt electrode surface. A set of good quality SERS spectra was obtained by using an appropriate surface-roughening procedure and a highly sensitive confocal Raman microscope. The surface enhancement factor was about three orders of magnitude calculated from experimental data and analyzed by the relevant theories. The electromagnetic contribution accounts for two out of the three orders of magnitude mainly through the lightning-rod effect. The remaining enhancement originates from the photon-driven charge transfer mechanism. The SERS intensity-potential profile shows the existence of two charge transfer processes. One is the excitation of the metal 4s orbital to the mixing orbital of the metal 4px and π-type 3b1 of pyridine, and the other is from the metal 3d orbital to the π-type 2a2 orbital of pyridine. Furthermore, the spectral difference for the adsorbed pyridine on cobalt and silver surfaces indicates that the chemical interaction of pyridine with the former is considerably stronger than the latter.