Surface Spectroscopy of Room-temperature Ionic Liquids on a Platinum Electrode:  A Sum Frequency Generation Study

Abstract

Sum frequency generation vibrational spectroscopy (SFG) and cyclic voltammetry (CV) measurements have been conducted on the ionic liquid/platinum electrode system. The room-temperature ionic liquid investigated is 1-butyl-3-methylimidazolium [BMIM] + with [PF 6 ] - or [BF 4 ] - anions. SFG spectra are taken in situ under potential control from 2750 to 3300 cm -1 (C−H stretching region). Polarization-dependent SFG spectra are used to extract orientation information for the cation at the electrode surface. The SFG spectra indicate that the cation changes orientation as the electrode potential is changed within the double-layer region. The plane of the imidazolium ring tips from 35° at positive surface charge to ∼60° from the surface normal at negative surface charge. Further, the orientation change is different for [BMIM][PF 6 ] than for [BMIM][BF 4 ]. A model for ions at the surface is presented based on these spectroscopic and electrochemical measurements.