Sum frequency generation spectroscopy study of an ionic liquid at a graphene-BaF2 (111) interface.

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.