Comparison between polycrystalline Au and single-crystalline Au(1 1 1) electrodes as the substrate of a cationic organic monolayer based on their anion dependent redox activities

Abstract

The structures of organic monolayers immobilized on a metal electrode and their functions depend on the substrate surface chemistry and morphology. A better defined crystalline flat electrode usually provides a structurally more ordered organic monolayer, although the use of a single-crystalline electrode costs more for mass production. Because the monolayer structures at both molecular and mesoscopic levels are critical to the functions of the modified electrode, in-depth experimental studies to clarify the relationship between the functions and the surface morphology of the electrode are crucial. In this work, we compared polycrystalline Au (poly-Au) and single-crystalline Au(111) electrodes as the substrates for a redox-active, self-assembled monolayer (SAM) of an alkyl-viologen thiol, 1-(10-mercaptodecyl)-1′-methyl-4,4′-bipyridinium dibromide (MeVC10-SH 2Br−). The voltammetric and electroreflectance measurements were performed at a mirror-polished poly-Au electrode and a flame-annealed single-crystalline Au(111) electrode after their surface modification with a MeVC10-SH monolayer. We found significant differences in the anion effects on the voltammetric behavior between the two substrates, especially in the dependence on anion hydrophobicity. The electroreflectance spectra revealed a tendency that a greater abundance of the viologen radical cation dimers at a Au(111) than that at a poly-Au. A considerable Br− adsorption at the SAM-modified Au(111) electrode was observed because of the loose packing of immobilized MeVC10-SH molecules. We discussed the origins of the contrasting behavior resulting from the difference of the surface.