Surface Analysis of Ionic Liquids with and without Lithium Salt Using X‑ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) was applied to a neat ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [EMI+][Tf2N–] and its lithium salt solution at room temperature to clarify the composition and structure of its near-surface region. Core level peaks were recorded for Li 1s, N 1s, C 1s, F 1s, O 1s, S 2s, and S 2p. Valence band XPS spectra (0–40 eV binding energy) were also studied. The XPS spectra were analyzed using DV-Xα calculations. Results show that the planar type isomer of the EMI+ cation is dominant at the near-surface region of EMI-Tf2N. Results of XPS measurements show a spectrum of Li 1s in Li/EMI-Tf2N. The proposed models for the preferred orientation of the ions exhibit good agreement with results obtained from the DV-Xα calculations.