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.