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 peakintensities 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.

Spectroscopic characterization of the interaction of lithium with thin films of the ionic liquid 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide

“…However, as the present XPS results for [OMIm]Tf 2 N on copper are in good agreement with XPS studies in the literature 26,[29][30][31] and as there are no further chemical changes visible, bulk interactions between [OMIm]Tf 2 N and copper can be neglected. .…”

Electronic-State Changes of Ferrocene-Terminated Self-Assembled Monolayers Induced by Molecularly Thin Ionic Liquid Layers: A Combined Atomic Force Microscopy, X-ray Photoelectron Spectroscopy, and Ultraviolet Photoelectron Spectroscopy Study

“…Figure 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 123 They observed that the peak at approximately 7 eV starts to grow at a coverage greater than one monolayer and is shifted to a higher binding energy toward the bulk IL value, indicating that this peak is derived from the electronic states of the randomly oriented IL not directly in contact with the metal substrate. Because the peak positions of bulk UP-spectra of BMIM-TFSI and EMIM-TFSI are similar, 122 we consider that the peak at approximately 7 eV observed in this study can be attributed to the randomly oriented IL on ferrocene-terminated SAMs. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 ...…”

Theoretical reconstruction and elementwise analysis of photoelectron spectra for imidazolium-based ionic liquids

“….n ) in our calculation (see Table 1 and Fig. 1 ), which contribute to the peak at 283.52 eV (285.2 eV in the experiment 1,27 ), and carbon atoms directly attached to nitrogen atoms in the imidazoliumring (C . Notably, the calculated contributions of the carbon atoms C 1 -C 3 (see Fig.…”

“…In the experimental spectra this effect is indicated by a shift of the respective peak with the increasing length of the alkyl-chain. 1,27,57 Finally, we have to note that like for the N1s core levels the calculated energy difference between the C1s peaks attributed to the [Tf 2 …”

“…octyl for [OMIm]Tf 2 N) of the cation stick out of the ionic liquid surface. However, using the extremely surface sensitive metastable induced electron spectroscopy (MIES) 1 we have recently also found significant hints that the alkyl-chains in [OMIm]Tf 2 N do not completely cover the surface, but the [Tf 2 N] À anion may be still present there. The former result has been supported by parallel MIES studies of T. Iwahashi et al 39 Molecular dynamics simulations of the interface between imidazolium-based ionic liquids and vacuum/air by A. S. Pensado et al, 40 T. Yan et al, 41 and C. D. Wick et al 42 have found the alkyl-chain sticking out of the surface and have found the anion, e.g.…”