Adsorption of Solvent Cations on Au(111) and Au(100) in Alkylimidazolium-Based Ionic Liquids – Worm-Like versus Micelle-Like Structures

Abstract

By employing high resolution in-situ STM, the adsorption of alkylimidazolium-based cations of EMI⁺, PMI⁺, BMI⁺ and OMI⁺ on Au(111) and Au(100) surfaces are investigated systematically. The cation adsorption on both Au(111) and Au(100) are composed of double rows arising from counter-facing imidazolium-based cation pairs. On Au(100), the double rows associated with the four cations show micelle-like appearance along the two √ 2 directions of the Au(100) surface lattice units. The width of the double rows varies depending on the side chain length of the cations, but is constrained by the periodicity along the √ 2 directions. Anions of BF₄^-, PF₆^-, CF₃SO₃^- and Tf₂N^- do not influence the micelle-like adsorption structure. On Au(111), the double rows are formed only when the terraces are etched to several atoms wide. Most likely, the underneath Au surface experiences restructuring to accommodate the double row structure, and the worm-like orientation of the double rows is the consequence of strain release. Both the micelle-like and worm-like adsorption structures would be lifted upon cathodic potential excursions when the surfaces are driven to undergo ordinary Au(100)-hex and Au(111)-(√ 3 × 22) reconstructions. These results reveal that the ordered micelle-like structure on Au(100) and the irregular worm-like structure on Au(111) are of the same nature.