Structural Change in 4-Pyridineethanethiolate Self-Assembled Monolayers on Au(111) Induced by Protonation of Pyridine Ring in Electrolyte Solutions

Abstract

Self-assembled monolayers (SAMs) of 4-pyridineethanethiol (4-PyEtS) formed on a Au surface was investigated by cyclic voltammetry (CV), surface enhanced infrared spectroscopy (SEIRAS), and in situ scanning tunneling microscopy (STM). On the basis of reductive desorption for 4-PyEtS on Au(111), the peak potential was observed at -0.86 V vs. Ag/AgCl (sat. KCl). The surface excess was calculated to be 5.4 × 10-10 mol cm-2 from the CV. High-resolution STM images revealed that p(5 × √3 R−30å) lattice including two molecules was found in 0.1 M NaClO4, whereas the 4-PyEtS-SAMs formed p(10 × √3 R−30å) lattice constituted from four molecules in 0.05 M HClO4. By the analysis of SEIRA spectra, the b axis of pyridine ring of 4-PyEtS-SAMs formed on an evaporated Au film electrode in 0.1 M NaClO4 was tilted to the electrode. In contrast, that in 0.1 M HCl was parallel to the surface.