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

e- injection from a photo-excited dye into the

conduction band of TiO2, followed by d, followed by dye regeneration

and hole transport to a counter electrode.

Study: ionic liquid-TiO2 interface for

1-butyl-3-methylimidazolium dicyanamide and methylsulfate ([BMIM][DCA], [BMIM][MS]).

http://lpi.epfl.ch/solarcellE.html

http://www.st-andrews.ac.uk/~osc/solar.html

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 CH3 and CH2 groups.

New formulation was developed for the dicyanamide anion via normal coordinate analysis.

Experimental details

TiO2 film :anatase nanoparticles (few nm in

diameter) at the surface of a CaF2 window.

The ionic liquid is dried to a

pressure of 5x10-5 Torr and

airless transferred to the SFG

cell.

Experimental results [BMIM][DCA] Anion spectra.

Cation spectra.

2000 2100 2200 23000.0

0.2

0.4

0.6

0.8

1.0 [BMIM][DCA] ssp

SF

G in

ten

sity

[a

.u.]

wavenumber [cm-1]

2000 2100 2200 23000.0

0.2

0.4

0.6

0.8

1.0 [BMIM][DCA] ppp

SF

G in

ten

sity

[a

.u.]

wavenumber [cm-1]

2700 2850 3000 3150 33000.0

0.4

0.8

1.2

1.6[BMIM][DCA] ssp

SF

G in

tens

ity [a

.u.]

wavenumber [cm-1]

2700 2850 3000 3150 33000.0

0.2

0.4

0.6

0.8 [BMIM][DCA] ppp

x2

SF

G in

ten

sity

[a

.u.]

wavenumber [cm-1]

2700 2850 3000 3150 33000.00

0.04

0.08

0.12

0.16

0.20

x8

[BMIM][DCA] sps

SFG

inte

nsity

[a.u

.]wavenumber [cm-1]

Orientation [BMIM][DCA]

• The tilt of the C3 axis of the terminal CH3 varies from 66º to 82º, for ranging from 0 º to 10 º.

• The tilt of the C2 axis for DCA varies between 53º to 90º for twist angles ranging from 0º to 40º.

0 30 60 900

1

2

3

4

=40°

=20°=0°

SF

G in

ten

sity

ra

tio(s

spS

S/s

spA

S)

DC

A

°

0 20 40 60 80 1000

15

30

45

60

=40°

=20°

=10°=0°

SF

G in

ten

sity

ra

tio(s

spS

S/s

psA

S)

me

thyl

°

Experimental results [BMIM][MS]

Anion and cation spectra are in the same wavenumber range.

2700 2850 3000 3150 33000.0

0.5

1.0

1.5[BMIM][MS] ssp

SF

G in

tens

ity [a

.u.]

wavenumber [cm-1]

2700 2850 3000 3150 33000.0

0.5

1.0

1.5[BMIM][MS] ppp

SF

G in

tens

ity [a

.u.]

wavenumber [cm-1]

2700 2850 3000 3150 33000.00

0.15

0.30

0.45

x3

SF

G in

tens

ity [a

.u.]

wavenumber [cm-1]

[BMIM][MS] sps

Orientation [BMIM][MS]

The tilt of the C3 axis from the terminal CH3 in the butyl chain varies from 57º to 90º for ranging from 0º to 20º.

0 30 60 900

4

8

12

16

20

=80°

=40°

=20°

=0°

SF

G in

tens

ity r

atio

(ssp

SS

/sps

AS

) m

ethy

l

°

Contact angle measurements in vacuum

[BMIM][DCA][BMIM][DCA] [BMIM][MS][BMIM][MS]

30°30° 23°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 CaF2 supported TiO2 films. Imee Martinez for assisting with contact angle

measurements.