Sum frequency generation study on the orientation of room-temperature ionic liquid at the graphene–ionic liquid interface

The sum frequency generation (SFG) vibrational spectrum and contact angle of a room-temperature ionic liquid, 1-butyl-3-methylimidazolium methane sulfate, [BMIM][MS], at the graphene interface has been measured. The SFG spectra are dominated by the CH3 resonance of the anion. The methyl group of methane sulfate is oriented at an angle greater than 40 from the surface normal. The SFG data suggests the cation, [BMIM], is weakly oriented with the alkyl chain parallel to the graphene plane and the aromatic ring somewhat tilted from the surface plane. As the ionic liquid [BMIM][MS] wets the surface with a contact angle of 58 ± 2 , suggesting that the non-polar interaction of the alkyl chain and p–p interaction of the ring, to graphene dominate the surface interactions. 2011 Published by Elsevier B.V. Although pioneering work on graphene was awarded the 2010 Nobel Prize in physics [1], there are still many areas of its chemistry in need of exploration. This is especially true of its surface chemistry, since many of the applications envisioned for graphene involve surface interactions [2]. In the realm of energy systems for graphene such as dye-sensitized solar cells and super capacitors, room-temperature ionic liquids are considered the medium of choice. Thus, while the ionic liquid appears to be a promising medium, there is almost no molecular-level information as to their interaction with the solid graphene surface. Detailed experimental molecular information on the surface chemistry of graphene is limited however recent simulations have been conducted on the interaction of ion pair with the free graphene-like layer. The simulation indicates a specific orientation of the ion pair to the graphene face is preferred and that there is partial charge transfer between the ions and the graphene suggesting a relatively strong interaction [3]. Nonlinear optics of the graphene substrate has been performed using second harmonic generation (SHG) where the rotation around the surface normal produced a anisotropic SHG signal. Surface symmetry of the graphene, threefold, dominated over the fourfold symmetry of the substrate [4]. The interfacial interaction of the ionic liquid 1-butyl-3-methylimidazolium methyl sulfate, [BMIM][MS], with a graphene surface is studied using sum frequency generation vibrational spectroscopy and contact angle measurements. The results suggest a significant interaction of the ionic liquid dominated by anion adsorption.