Characterization of Ionic Liquid-Graphene Interface: Sum Frequency Generation Spectroscopy and Electrochemical Studies
Xu, Siyun 1987-
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The characterization of ionic liquids near neutral and charged graphene surfaces was performed using four complementary surface techniques: sum frequency generation (SFG) spectroscopy, contact angle measurement, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The SFG results indicate that only the 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 ionic liquid/ graphene interface. The SFG results show the presence of both cations and anions at the charged graphene surface. The anion signal increased with the external potential growth and the cation signal had little influence on the external potential. Results from the contact angle measurement reveal the increase in the contact angle of the ionic liquid on the BaF2 surface after the graphene layer deposition, suggesting that the graphene coating modifies the wetting behavior of BaF2 substrate and lowers the surface energy. The electrochemical results suggest that ions form a stable double-layer at the charged graphene surface and the amount of cations increases with the reduction of potential. These measurements demonstrate that ions of the ionic liquid adsorb to the graphene electrode surface to generate a Helmholtz-like electric double-layer, where anions are located at the inner Helmholtz layer and cations are distributed at the outer Helmholtz layer.