Valence electronic structure of [EMIM][BF4] ionic liquid: photoemission and DFT+D study

I Kuusik; M Berholts; J Kruusma; V Kisand; A Tõnisoo; E Lust; E Nõmmiste

doi:10.1039/c8ra05865b

Valence electronic structure of [EMIM][BF₄] ionic liquid: photoemission and DFT+D study

RSC Adv. 2018 Aug 28;8(53):30298-30304. doi: 10.1039/c8ra05865b. eCollection 2018 Aug 24.

Authors

I Kuusik¹, M Berholts^{1

2}, J Kruusma³, V Kisand¹, A Tõnisoo¹, E Lust³, E Nõmmiste¹

Affiliations

¹ Institute of Physics, University of Tartu W. Ostwaldi 1 50411 Tartu Estonia.
² Dept. of Physics and Astronomy, University of Turku FIN-20014 Turku Finland.
³ Institute of Chemistry, University of Tartu Ravila 14a 50411 Tartu Estonia.

Abstract

The ultraviolet photoelectron spectrum (UPS) of the [EMIM][BF₄] ionic liquid was recorded and analyzed. Together with the gas-phase UPS spectrum of the [EMIM][BF₄] vapor and ab initio calculation methods, detailed insight into the electronic structure of this simple ionic liquid is possible. The low binding energy tail in the UPS spectrum is about 7.4 eV, in agreement with previous estimations of the HOMO-LUMO gap of the [EMIM][BF₄] ion-pair. The bulk ab initio calculations are able to explain most of the features in the spectrum. However, DFT consistently lacks accuracy in the description of the top of the valence band. The dispersion corrected PBE calculation (PBE-D3) did offer very good agreement with the experimental structure, but the recently-developed vdW-DF functionals C09, optPBE, optB88 and CX were found to offer the best agreement in terms of the electronic structure.