Surface-enhanced Raman spectroscopy and electrochemistry at the copper vertical bar 4-phenylpyridine interface

The adsorption of 4-phenylpyridine (4-PhPy) on copper was examined by means of cyclic voltammetry and surface-enhanced Raman spectroscopy (SERS) in a wide range of electrode potentials. Electrochemical results revealed that a nodic dissolution of copper as well as the cathodic evolution of hydrogen a re inhibited in the presence of chemisorbed 4-PhPy molecules in both neutra l and acidic solutions. An abrupt change of the spectral pattern was observ ed in the SERS spectra of neutral solutions of 4-PhPy at potentials more ne gative than -0.6 V versus Ag/AgCl. An assignment of the spectrum by ab init io calculations enabled us to interpret the spectroscopic results in terms of potential-induced reorientation of the molecular plane with respect to t he surface. The SERS results revealed that at more negative potentials the adsorbed molecules stand vertically on the surface and self-assemble into m olecular stacks. In intermediate potentials, molecules are adsorbed in a fl at configuration. A vertical configuration reappears at positive potential values. (C) 2001 Elsevier Science Ltd. All rights reserved.