Adsorption behavior of 11-mercapto-1-undecanol on Au(111) electrode in an electrochemical system

In-situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were used to study the phase evolution of 11-mercapto-1-undecanol (MUO) adlayer on an Au(111) electrode. The effect of various electrolytes, including HClO ₄ and H₂SO₄, on the adsorption behavior was studied. The MUO adsorption was found to initiate mainly at the intersectional corner of herringbone rows of an Au(111) reconstruction structure in both of the electrolytes. The following growth of an adsorbed cluster develops first along the face-centered-cubic (fee) position of the herringbone structure. In the HClO₄ solution, the MUO molecule is first adsorbed in a flat-lying orientation when the dose concentration of MUO is low, growing to an ordered domain of striped structure (β phase) with a molecular arrangement of (12 × √3). When the surface coverage becomes high, the hydrocarbon chains of MUO lift off from the Au(111) plane, forming a more condensed and saturated phase, the φ phase, identified as (√3 × √3)R30°. At a high dose concentration of MUO, however, the striped phase does not appear. Due to the fast adsorption of thiol groups at high dose concentrations, the hydrocarbon chains-gold interaction is inhibited, and therefore, a flat-lying orientation of MUO molecules cannot be obtained. In the H₂SO₄ solution, the striped phase does not form even at a low dose concentration and, instead, the φ phase appears directly in the low-coverage stage. The distinct phenomena observed for the two electrolytes are attributed to the different interactions of anionic ions with the gold surface. It has been shown that sulfate ions adsorb more strongly than perchlorate ions on a gold surface. The strongly adsorbed sulfate ions in the electrical double layer are supposed to resist the direct contact of hydrocarbon chain with the gold surface which also prevents the formation of a flat-lying orientation.