A sum frequency generation study of the room temperature ionic liquid- titanium dioxide interface Cesar Aliaga and Steven Baldelli

Room temperature ionic liquids 100% ionic compounds liquid at room temperature. Conductive, thermally and electrochemically stable, negligible vapor pressure. Electrochemical applications: Batteries Fuel cells Double layer capacitors Photo-electrochemical solar cells

Ionic liquids and dye-sensitized solar cells, followed by d e - injection from a photo-excited dye into the conduction band of TiO 2, followed by dye regeneration and hole transport to a counter electrode. Study: ionic liquid-TiO 2 interface for 1- butyl-3-methylimidazolium dicyanamide and methylsulfate ([BMIM][DCA], [BMIM][MS]).

Method: Sum frequency generation Nonlinear spectroscopic technique sensitive only in a non centrosymmetric environment such as an interface.

Orientation analysis Derived from the principle of polarization dependence of the SFG spectra.. Compare the ratio of peak intensities of vibrational modes with theoretical curves of peak intensity ratio vs. orientational angle. The formulation exists for terminal CH 3 and CH 2 groups. New formulation was developed for the dicyanamide anion via normal coordinate analysis.

Experimental details TiO 2 film :anatase nanoparticles (few nm in diameter) at the surface of a CaF 2 window. The ionic liquid is dried to a pressure of 5 x Torr and airless transferred to the SFG cell.

Experimental results [BMIM][DCA] Anion spectra. Cation spectra.

Orientation [BMIM][DCA] The tilt of the C 3 axis of the terminal CH 3 varies from 66º to 82º, for  ranging from 0 º to 10 º. The tilt of the C 2 axis for DCA varies between 53º to 90º for twist angles  ranging from 0º to 40º.

Experimental results [BMIM][MS] Anion and cation spectra are in the same wavenumber range.

Orientation [BMIM][MS] The tilt of the C 3 axis from the terminal CH 3 in the butyl chain varies from 57º to 90º for  ranging from 0º to 20º.

Contact angle measurements in vacuum [BMIM][DCA][BMIM][MS] 30°23°

Results and discussion [DCA] - generates more SFG signal than [MS] -. Strong signal from dicyanamide was detected in the gas-liquid and liquid- metal electrode systems. The contact angle results, suggest that the cation seems to lie horizontally at the surface with the alkyl chain directed away from the surface. The inverse configuration would imply less wetting.

Acknowledgements Welch foundation (Grant E1531 ). Dr. S Baldelli. Dr. Mark M. Law and C. Hirose for the valuable discussions regarding the FG matrix theory. Nkeng Asong from Tufts University for the preparation of the CaF 2 supported TiO 2 films. Imee Martinez for assisting with contact angle measurements.