Introduction
X-ray photoelectron spectroscopy (XPS) is a powerful tool for studying the electronic structure of solids, liquids, gases as well as of structures formed at interfaces. It also helps in resolving the atomistic structure by comparing the measured core-level binding energies (BEs) to the reference values.1 When the appropriate reference values are either not available or not directly applicable, calculations offer a solution for that problem. Among possible methods, density functional theory (DFT)2computations allow analysing electronic and atomistic structures along with predicting BEs for a theoretical XPS spectrum.
Recently, we have applied the XPS method to study the carbon–ionic liquid (IL) interface properties in connection to its application in the supercapacitors.3–5 ILs have a variety of
appealing properties,6 yet there are not many XPS reference values for that class of compounds. Villar-Garcia et al. conducted experimental work on bulk ILs with imidazolium-based cations with varying anions, covering over 20 ILs.7Men et al. investigated bulk ILs based on pyrrolidinium cation and various anions.8 Foelske-Schmitz et al.studied EMImBF4, EMImB(CN)4, and EMImTFSI ILs at the carbon electrode.9 Kruusmaet al. also focused on EMImBF4 and EMImB(CN)4 characterisation near electrochemically charged carbon electrode.3–5
Although the DFT methods are well suited for ILs structure and property calculations,10,11 computational works on XPS of ILs are sparse. It is surprising as the calculation of the X-ray photoelectron core-level spectra is one of the examples where an ion pair is sufficient for modelling the bulk IL.12Generally, the solvate shell causes changes in the electronic and atomistic structures affecting the infrared, ultraviolet and other spectra. The core-level electron spectrum is influenced by the solvation shell to a lesser degree. As the core-levels are determined mostly by the chemical bonding within a molecular entity, hypothetically the BEs should correlate with the atomic charges.13 Fogartyet al. found such correlation for S(1s) electrons (R 2 = 0.98) and N(1s) electrons (R 2 = 0.94) between the experimental BEs and the computed atomic charges.14,15 Kruusma et al. calculated C(1s) electrons’ Kohn–Sham orbital energies and used their values for fitting the experimental spectra.3Similarly, Reinmöller et al. used Kohn–Sham orbital energies to calculate the BEs for the XPS spectra.16,17
In this study, we have applied for the first time the delta Kohn–Sham (ΔKS) method for obtaining the BEs of ion pairs. Despite doubts regarding charge transfer in ionic liquids modelled with core-hole,18 this article demonstrates a good agreement between the available experimental XPS data and the calculated ΔKS BEs. Furthermore, we present and discuss correlations found between the ΔKS BE values, 1s Kohn–Sham orbital energies and atomic charges.