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

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Ionic liquids (ILs) are salts that are liquid below 100°C, with tunable properties.
• Understanding the electronic structure of ILs is crucial for their application in various fields.
• [EMIM][BF4] is a common and widely studied ionic liquid.

Purpose of the Study:

• To investigate the electronic structure of [EMIM][BF4] ionic liquid.
• To compare experimental ultraviolet photoelectron spectroscopy (UPS) data with theoretical calculations.
• To evaluate the performance of different computational methods in describing the electronic properties of ILs.

Main Methods:

• Experimental measurement of the ultraviolet photoelectron spectrum (UPS) of [EMIM][BF4] in both liquid and gas phases.
• Ab initio calculations, including density functional theory (DFT) with various functionals (PBE-D3, vdW-DF functionals C09, optPBE, optB88, CX).
• Analysis of the electronic structure, focusing on the valence band and HOMO-LUMO gap.

Main Results:

• The experimental UPS spectrum of [EMIM][BF4] shows a low binding energy tail around 7.4 eV, consistent with the HOMO-LUMO gap.
• Standard DFT calculations showed limitations in accurately describing the top of the valence band.
• Dispersion-corrected PBE-D3 calculations provided good agreement with the experimental structure.
• vdW-DF functionals (C09, optPBE, optB88, CX) demonstrated the best agreement with the experimental electronic structure.

Conclusions:

• The electronic structure of [EMIM][BF4] can be effectively studied using a combination of UPS and advanced computational methods.
• vdW-DF functionals are recommended for accurate theoretical descriptions of the electronic structure of ionic liquids like [EMIM][BF4].
• This research provides valuable insights for the design and application of ionic liquids in electronic devices and catalysis.