An electrochemical approach to fabricate a heterogeneous mixed monolayer on planar polycrystalline Au and its characterization with Lateral Force Microscopy

Abstract

A method describing the assembly and characterization of a heterogeneous mixed monolayer composed of 2-aminoethane thiol (AET) and 11-mercaptoundecanoic acid (MUA) on a planar polycrystalline Au electrode is presented. A single component self-assembled monolayer (SAM) of AET was stable on the metal within a narrow range of potentials but at more negative polarizations, cyclic voltammetry (CV) and capacitance measurements confirmed reductive desorption of AET from the Au(1 1 1), Au(1 0 0) and Au(1 1 0) facets of the surface consecutively. By truncating the negative potential limit, desorption of AET occurred selectively from facets with Au(1 1 1) character and diffused away from the interface thus limiting oxidative readsorption on the return to positive potentials. The extent of oxidative readsorption was found to be dependent not only on the time spent at the desorption limit but also on electrode geometry and configuration. The largest degree of readsorption occurred on a planar electrode when set in a hanging meniscus configuration. As a result, a modified desorption procedure involving 20 truncated desorption pulses each with a duration of 1 min was utilized to limit readsorption. This produced monolayer voids at the Au(1 1 1) facets which were subsequently backfilled with MUA to create the heterogeneous mixed monolayer. Surface characterization was achieved with Electron Backscattered Diffraction (EBSD) which produced a map of a small section of the metal surface crystallography and with Lateral Force Microscopy (LFM) which verified MUA adsorption on the Au(1 1 1) facets and AET on all other low index planes.