Surface characterization of imidazolium ionic liquids by high-resolution Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy

• Published in: Surface science Vol. 604; no. 3; pp. 464 - 469
• Main Authors: Hashimoto, Hiroki, Ohno, Atsushi, Nakajima, Kaoru, Suzuki, Motofumi, Tsuji, Hiroshi, Kimura, Kenji
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 15-02-2010; Elsevier
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; High energy ion scattering (HEIS); Liquid surfaces; Physics; X-ray photoelectron spectroscopy; Liquid surfaces; High energy ion scattering (HEIS); X-ray photoelectron spectroscopy; Liquid state; RBS; Ion scattering; X-ray photoelectron spectra
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/j.susc.2009.12.023

The surface composition of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM] [PF
6]) and 1-butyl-3-methylimidazolium dicyanamide ([BMIM] [DCA]) are studied by high-resolution Rutherford backscattering spectroscopy. Although [BMIM] [PF
6] is almost stoichiometric up to the topmost molecular layer, considerable deviation from the theoretical stoichiometry is observed for [BMIM] [DCA] in a surface layer of ∼1.5
nm thickness. Nitrogen is almost completely depleted in this layer while carbon is enhanced. In addition, there are oxygen impurities of ∼3
×
10
14
atoms/cm
2 in this surface layer. With the help of X-ray photoelectron spectroscopy measurements it is concluded that the surface of [BMIM] [DCA] is covered by ∼1.7
×
10
14
molecules/cm
2 of esters and/or carboxylic acids. These contaminant molecules have a preferred orientation, i.e. the carbonyl groups are on the surface of [BMIM] [DCA] and the alkyl chains are pointing towards vacuum. The origin of the contamination layer could be the surface segregation of bulk impurities.