Self-assembled monolayers (SAMs) of 3,3′-thiobis(1-propanesulfonic 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 × 3√3) TBPS adlayer phase was observed at potentials E > 0.7 VRHE in TBPS-free electrolyte. At more cathodic potentials, the adlayer transforms irreversible to a disordered phase. In contrast, 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 α, to a high coverage stripe phase β, and finally to a disordered aggregate phase σa. The reverse cathodic sweep shows transitions from σa to β, back to σa, and to an ordered adlayer phase ω. All of these phases significantly differ from the (6 × 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.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films