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 [PF6]- or [BF4]- 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][PF6] than for [BMIM][BF4]. A model for ions at the surface is presented based on these spectroscopic and electrochemical measurements.

The interface between Au(100) and 1-butyl-3-methyl-imidazolium-bis(trifluoromethylsulfonyl)imide

“…Also for BMITf 2 N, we assume a flat orientation of the BMI + cation layer, which is generated and stabilized by π−π interactions and van-der-Waals forces between the alkyl chains. Sum-frequency-generation-spectroscopy (SFG) measurements of the interface between imidazolium-based ILs and polycrystalline platinum confirm this presumption of a flat lying imidazolium ring at negative potentials [44,45].…”

Why Use Ionic Liquids for Electrodeposition?

“…A study using imidazolium bistriflamide, (F 3 CSO 2 ) 2 N − and BF 4 − salts suggested that a model of alternating anion and cation layers may be applicable to the data [29,30]. Baldelli [31,32] concluded that the double layer is one ion layer thick using sum frequency generation spectroscopy and electrochemistry to probe the electric field at the ionic liquid/electrode interface. The double layer capacitance in an ionic liquid is considerably smaller than in an aqueous solution and less than that predicted by having a perfect Helmholtz layer at the interface, which could result from the presence of ion pairs at the electrode surface at all potentials.…”

Spotlight on ionic liquids

“…A wide variety of experimental, theoretical, and simulation methods have been applied to the study of ILs at interfaces. [107][108][109][110][111][112][113][114][115][116][117][118][119][120][121][122] Santos and Baldelli 112 have used interfacial sumfrequency generation ͑SFG͒ to show that at IL interfaces with gases or vacuum, alkyl chains on either the cations or the anions extend from the surface into the gas or vacuum. Rutherford backscattering spectra for the ͓bmim͔ + / ͓PF 6 ͔ − -vacuum interface also show the cation butyl group protruding from the bulk liquid into vacuum.…”