Sum Frequency Generation Spectroscopy Study of an Ionic Liquid at a Graphene-BaF₂ (111) Interface

Abstract

Sum frequency generation (SFG) vibrational spectroscopy and contact angle measurements of an ionic liquid, 1-butyl-3-methylimidazolium dicyanamide [BMIM]­[DCA], at solid–liquid interfaces are reported. Bare solid single crystal BaF₂ (111) surface, a single and few layer graphene-coated BaF₂ (111) surface are used as the solid substrates. The SFG results indicate that both [BMIM]⁺ and [DCA]⁻ can be detected specifically on the graphene-coated BaF₂ (111) surface, without coating only [DCA]⁻ are observed. [DCA]⁻ anions are attracted to the positively charged BaF₂ (111) surface and occupy the first layer at the solid–liquid interface. The graphene coating shields the charged crystal surface and allows both cations and anions to exist at the interface. Furthermore, increase in the contact angle of BaF₂ surface after graphene layers deposition suggests that the graphene coating lowers the surface energy