A sum frequency generation study of the room-temperature ionic liquid-titanium dioxide interface

The interfacial structure between TiO2 and the room-temperature ionic liquids 1-butyl-3-methylimidazolium dicyanamide ([BMIM][DCA]) and 1-butyl-3-methylimidazolium methyl sulfate ([BMIM][MS]) was examined using sum frequency generation (SFG) vibrational spectroscopy and contact angle measurements. Vibrations corresponding to the normal modes of the [BMIM](+) cation and dicyanamide [DCA](-) were detected. The results suggest that the cation and the anion are present and oriented at the interface for [BMIM][DCA]. The molecular orientation of the ionic species was calculated using a simulation on the basis of the polarization dependence of the sum frequency spectra. The tilt of the C-3 axis of the methyl group from the butyl chain in [BMIM](+) ranges from 57 to 90 degrees from the surface normal for [BMIM][MS] and from 66 to 90 degrees for [BMIM][DCA]. The C-2 axis from the dicyanamide anion tilts at an angle of 53-90 degrees as a function of the twist. The imidazolium ring seems to lie nearly parallel to the TiO2 surface as suggested by the SFG spectra, contact angle results, and surface charge density calculations. Finally, specific adsorption of [DCA](-) was also observed, in contrast to a weak adsorption of [MS](-). The enhance charge adsorption could explain the efficiency of these compounds in dye-sensitized solar cells.