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