Orientational phase transition in a pyridine adlayer on gold(111) in aqueous solution studied by in situ infrared spectroscopy and scanning tunneling microscopy

The orientation and structure of pyridine adsorbed on a highly ordered Au(111) surface from 0.1 M. NaClO4 + x M (10(-6) less than or equal to x less than or equal to 10(-3)) pyridine aqueous solutions have been investigated as a function of applied potential by in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) and scanning tunneling microscopy (STM). Symmetric in-plane pyridine ring vibrations (A(1) modes) were observed in the SEIRA spectra at potentials positive of about -0.3 V versus SCE, while asymmetric in-plane ring vibrations (B-1 modes) were hardly detected. The symmetric ring-breathing mode showed a blue shift upon adsorption, indicating the adsorption via the N atom. The band intensities were found not to be proportional to the surface concentration (the relative Gibbs surface excess) reported in the literature. On the basis of the surface selection rule in SEIRAS, the results are explained in terms of the potential-dependent reorientation of pyridine. The molecule is flatly adsorbed on the surface at negative potentials, and its molecular plane rises up as the applied potential increases and the surface concentration increases. Flat-lying, tilted, and vertically standing pyridine molecules were observed at different potentials also by STM for the first time.