Abstract
In this work [0], we have applied the DFT-based delta Kohn–Sham (ΔKS) method to ion pairs in a vacuum to obtain X-ray photoelectron spectra of corresponding ionic liquids (IL). On the example of forty ion pairs, we demonstrate how the core level binding energy (BE) values can be calculated and used to plot theoretical spectra at a low computational cost. Furthermore, we compare the ΔKS results, 1s Kohn–Sham orbital energies, and atomic charges against the experimental X-ray photoelectron data. Recently, in connection to the electrochemical application in the supercapacitors, we have measured spectra for EMImBF4 and EMImB(CN)4 ionic liquids at the carbon–IL interface [1–3]. Other experimental spectra were obtained from the literature [4,5]. Both the ΔKS BE values and the 1s Kohn–Sham orbital energies show a correlation, yet with a different order of the BEs assigned to specific atoms. We find that neither DDEC6 nor Bader charges correlate with the experimental data. Thus, the DFT calculations of 1s Kohn–Sham orbital energies provide the fastest way of predicting the XPS spectra. However, more detailed experimental studies are required to resolve the right order of the BE values and its relation to the atomistic structure of the ILs. The ΔKS calculations provide the most precise estimations of the BEs. Herewith, they also demand more resources and cause computational difficulties discussed in the presentation. Besides the prediction power, a robust computational method can help in intepretating experimental data when the appropriate reference values are either not available nor directly applicable. Thus, the ΔKS method can find its application in various fields of physics and chemistry where the XPS is used for resolving electronic and geometric structures of pure ILs, their mixtures, and at interfaces.
In this work, we have applied the DFT-based delta Kohn–Sham (ΔKS) method to ion pairs in a vacuum to obtain X-ray photoelectron spectra of corresponding ionic liquids (IL). On the example of forty ion pairs, we demonstrate how the core level binding energy (BE) values can be calculated and used to plot theoretical spectra at a low computational cost. Furthermore, we compare the ΔKS results, 1s Kohn–Sham orbital energies, and atomic charges against the experimental X-ray photoelectron data. Recently, in connection to the electrochemical application in the supercapacitors, we have measured spectra for EMImBF4 and EMImB(CN)4 ionic liquids at the carbon–IL interface [1–3]. Other experimental spectra were obtained from the literature [4,5]. Both the ΔKS BE values and the 1s Kohn–Sham orbital energies show a correlation, yet with a different order of the BEs assigned to specific atoms. We find that neither DDEC6 nor Bader charges correlate with the experimental data. Thus, the DFT calculations of 1s Kohn–Sham orbital energies provide the fastest way of predicting the XPS spectra. However, more detailed experimental studies are required to resolve the right order of the BE values and its relation to the atomistic structure of the ILs. The ΔKS calculations provide the most precise estimations of the BEs. Herewith, they also demand more resources and cause computational difficulties discussed in the presentation. Besides the prediction power, a robust computational method can help in intepretating experimental data when the appropriate reference values are either not available nor directly applicable. Thus, the ΔKS method can find its application in various fields of physics and chemistry where the XPS is used for resolving electronic and geometric structures of pure ILs, their mixtures, and at interfaces.
[0] M. Lembinen, E. Nõmmiste, H. Ers, B. Docampo‐Álvarez, J. Kruusma, E. Lust, V.B. Ivaništšev, Calculation of core‐level electron spectra of ionic liquids, Int. J. Quantum Chem. 120 (2020). https://doi.org/10.1002/qua.26247.
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