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

Area of Science:

• Surface science
• Materials science
• Spectroscopy

Background:

• Ionic liquids exhibit unique properties at interfaces, crucial for applications in catalysis and electronics.
• Understanding the behavior of ionic liquids at solid-liquid interfaces requires advanced surface-sensitive techniques.
• Barium fluoride (BaF2) surfaces present a charged substrate that influences interfacial molecular organization.

Purpose of the Study:

• To investigate the interfacial behavior of 1-butyl-3-methylimidazolium dicyanamide ([BMIM][DCA]) on bare and graphene-coated BaF2 (111) surfaces.
• To elucidate the role of graphene as a surface modifier in controlling ionic liquid adsorption.
• To determine the effect of graphene coating on the surface energy of BaF2.

Main Methods:

• Sum frequency generation (SFG) vibrational spectroscopy to probe molecular orientation and composition at the interface.
• Contact angle measurements to quantify surface wettability and surface energy.
• Utilizing bare and few-layer graphene-coated BaF2 (111) single crystal surfaces as substrates.

Main Results:

• SFG spectroscopy detected both [BMIM](+) cations and [DCA](-) anions on graphene-coated BaF2, while only [DCA](-) anions were observed on bare BaF2.
• [DCA](-) anions adsorbed onto the positively charged bare BaF2 surface.
• Graphene coating effectively shielded the charged BaF2 surface, enabling the co-existence of both ionic species at the interface.
• Contact angle measurements indicated a reduction in surface energy upon graphene deposition.

Conclusions:

• Graphene coating modifies the interfacial properties of BaF2, allowing for a more complete representation of the ionic liquid at the solid-liquid interface.
• The shielding effect of graphene is critical for observing both cation and anion interactions.
• Graphene deposition lowers the surface energy of BaF2, impacting interfacial phenomena.