Surface Electronic Structure of Imidazolium-Based Ionic Liquids Studied by Electron Spectroscopy

Abstract

We have measured the bulk and surface electronic structure of several ionic liquids of alkyl-imidazolium cations with different alkyl chains (EMIm, BMIm, HMIm, OMIm) and bis(trifluoromethylsulfonyl)imide anions (Tf₂N) by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS He I and He II) as well as metastable induced electron spectroscopy (MIES). The results are compared with the densities of states (DOS) calculated by density functional theory (DFT). By XPS we found the stoichiometry of the respective ILs reproduced, and different carbon atom positions reflected by the splitting of the C(1s) - from the trifluor-methyl groups (CF₃) of the anion with highest binding energy to the alkyl chains of the cation with the lowest one. Furthermore with increasing alkyl chain length the peak related to the alkyl group appears at slightly lower binding energy. The more bulk-sensitive XPS spectra reveal only minor differences in the valence band structure for the studied ionic liquids, whereas the more surface-sensitive methods UPS and especially MIES display distinct changes in the peak-intensities for varying the alkyl chain length. This is a strong indication for either a non-stoichiometric composition of the upmost molecular layer of the IL-surface and/or a reorientation of the cations, probably turning the alkyl chains to the surface. [DOI: 10.1380/ejssnt.2010.241]