Scanning Tunneling Microscopy and Cyclic Voltammetry Study of Self-Assembled 3,3'-Thiobis(1-propanesuIfonic acid, sodium salt) Monolayers on Au(111) Electrodes

摘要

Self-assembled monolayers (SAMs) of 3,3'-thiobis(1-propane-sulfonic acid, sodium salt) (TBPS) on Au(111) electrodes have been characterized by scanning tunneling microscopy and cyclic voltammetry in aqueous perchloric acid solutions. TBPS exhibits an adsorption behavior typically observed for dialkyl sulfides including intact adsorption and low coverage phases with molecules predominantly lying flat on the surface. On the other hand, an untypical chemical bond and well-ordered domains were determined which resemble the characteristics of alkenethiol SAMs. When the adlayer was prepared at its open circuit potential (OCP), a (6 x 3√3) TBPS adlayer phase was observed at potentials E > 0.7 V_(rhe) in TBPS-free electrolyte. At more cathodic potentials, the adlayer transforms irreversible to a disordered phase. Incontrast, in situ STM studies in TBPS-containing electrolyte reveal a very complex, potential-dependent adsorption behavior. With increasing electrode potential, the structure of the adlayer transforms in sequence from a disordered phase σ, to a low coverage stripe phase a, to a high coverage stripe phase β, and finally to a disordered aggregate phase σ_a. The reverse cathodic sweep shows transitions from σ_a to β, back to σ_a, and to an ordered adlayer phase y. All of these phases significantly differ from the (6 x 3√3) phase and are not transient phases at OCP. This behavior is attributed to the influence of the electrode potential on intermolecular and molecule-substrate interactions as well as on the TBPS coverage. Furthermore, the cathodic deposition of Au-TBPS complexes results in the formation of Au islands with fractal morphology.