In Situ Video-STM Study of Adlayer Structure and Surface Dynamics at the Ionic Liquid/Au (111) Interface

The molecular arrangement and surface dynamics of Au (111) electrodes in 1-hexyl-3-methylimidazolium chloride ([HMIm][Cl]) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIm][TFSA]) were investigated by in situ high-speed scanning tunneling microscopy (video-STM). These measurements provide direct insights into the potential-dependent adlayer structure of adsorbed [HMIm] + cations and Cl – anions on the Au (111) surface. At low electrode charge densities highly dynamic structures with only local order are observed. After changing the potential in the negative direction, stripe-like adlayers structures are observed, which are attributed to [HMIm] + in a planar adsorption geometry. These can be described by simple commensurate (√3 × 3) and a (√3 × √13) structures in the presence of Cl – and [TFSA] − counterions, respectively, indicating that the in-plane arrangement of the cations is affected by epitaxial effects and coadsorbed anions. Upon a further decrease in potential, a transition to a more close-packed (√3 × 2) adlayer phase is observed in [HMIm][TFSA], in which the cations are partly oriented away from the metal surface.