Surface Electronic Structure of [XMIm]Cl Probed by Surface‐Sensitive Spectroscopy

Abstract

We apply electron spectroscopy methods with different surface sensitivities to elucidate the DOS of the surface and the near-surface region of [XMIm]Cl (X=octyl, hexyl, butyl, and ethyl alkyl chain) ionic liquids. Using metastable induced electron spectroscopy (MIES) we are able to detect the density of states in front of the outermost surface, whereas ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) measurements provide lower surface sensitivity. The assignment of certain structures in the valence band spectra to particular atoms/functional groups of the ionic liquid based on DFT calculations and the reconstruction of PES spectra enables us to obtain information on the dominating groups at the surface, or in other words, on the molecular/ionic arrangement and orientation at the surface. From angular resolved XPS it is concluded that the alkyl chains dominate at the outermost surface. In agreement with this a decreasing chlorine signal is observed in the UPS spectra for ionic liquids with increasing alkyl chain length. The analysis of the MIES data shows that in case of [OMIm]Cl--in contrast to UPS and XPS--no Cl-induced features are visible in the MIES spectra at all and that the MIES spectra are dominated by the [OMIm](+) alkyl chain.

Ionic Liquids in Catalysis

“…Many established UHV-based surface science techniques have successfully contributed to an increasingly detailed understanding not only of the IL/ vacuum interface, but also of IL bulk properties. The applied methods include X-ray photoelectron spectroscopy (XPS) [106,107,, UV photoelectron spectroscopy [138,139,165,166], inverse photoelectron spectroscopy (IPES) [165,166], X-ray absorption spectroscopy (NEXAFS) [165], soft X-ray emission spectroscopy (SXES) [166], low energy ion scattering (LEIS) [141], metastable ion spectroscopy (MIES) [138,139], time-offlight secondary mass spectroscopy (TOF-SIMS) [140], Rutherford backscattering [167][168][169][170][171], high resolution electron energy loss spectroscopy (HREELS) [138], reflection absorption infrared spectroscopy (RAIRS) [172][173][174][175], direct recoil spectroscopy (DRS) [176], and scanning tunneling microscopy [177][178][179][180][181], to name only a few. In addition, an increasing number of theoretical studies and simulations have been conducted [101,[182][183][184][185][186][187][188][189][190].…”

The ionic liquid–vacuum outer atomic surface: a low-energy ion scattering study

“…24 Ionic liquids have sufficiently low vapour pressure that they can be studied at room temperature using standard UHV apparatus, opening up the possibility of using a wide range of surface science techniques to probe the molecular level structure of these liquid systems. 13,[25][26][27] The techniques that have been used to investigate the ionic liquid-vacuum surface (the region within which the properties of the ionic liquid are signicantly different from that of the bulk ionic liquid) can be separated into four different areas: spectroscopy, [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] scattering, [46][47][48][49][50][51][52][53][54][55][56][57][58][59] sticking probability/temperature programmed desorption, [60][61][62] and molecular dynamics (MD) simulations. 45,[63][64][65][66]…”

“…The most frequently postulated model, and therefore we conclude is the most probable model, for the ionic liquidvacuum surface, typically based upon studies on 1,3-dialkylimidazolium-based ionic liquids, is that the alkyl carbons are nearest to the vacuum, and that the charged components are nearest to the bulk liquid. 1,2,[28][29][30][31][32][33][34][35][36][37][38][39][45][46][47][48][52][53][54][55][56][57][58][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78] This model is the most probable, but is it truly representative of the complex ionic liquid-vacuum surface? In addition, will this model be sufficient to explain the surface properties of ionic liquids?…”

“…70 There is agreement between experiments 29,35,[46][47][48]52,59 and MD simulations 48,65,70,77 N] À ) using two different techniques, MAES and neutral impact collision ion scattering spectroscopy (NICISS), is that the anion is not located at the ionic liquid-vacuum outer atomic surface. 35,36,[53][54][55][56][57] In addition, coarse grain MD simulations on [ 4 ] the anion is indeed located at the ionic liquidvacuum outer atomic surface. 66 Unlike for cations, there is no contradictory evidence in the literature about the anion orientation for the ions nearest the vacuum.…”

“…Reichelt et al used NICISS for [C 6 C 1 Im]Cl to conclude that there are hexyl chains located nearer the vacuum than Cl À anions, and that the ionic liquidvacuum outer atomic surface is composed of cationic atoms 54. Ulbrich et al used MAES and calculations to conclude that Cl À is at the solid-vacuum outer atomic surface for CsCl but not at the ionic liquid-vacuum outer atomic surface for [C 8 C 1 Im]Cl 36. Our results show that Cl À is denitely present at the ionic liquid-vacuum outer atomic surface for [C 8 C 1 Im]Cl(Fig.…”