" ENGINEERING " OF A LIQUID-CRYSTALLINE AQUEOUS SUSPENSION OF TIO 2 RUTILE NANORODS
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Laboratoire de Physique Laboratoire de Chimie
du Solide d'Orsay de la Matière Condensée
« Engineering » of a liquid-crystalline
aqueous suspension of TiO2 rutile nanorods
Anisotropic photocatalytic properties
Arnaud Dessombz, David Chiche: PhD students
Corinne Chanéac, Jean-Pierre Jolivet: Lab. de Chimie de la Matière
Condensée, Université Paris 6
Pierre Panine: ESRF, ID02, Grenoble
1
Why rutile ?
Colloidal liquid-crystalline suspensions of mineral nanoparticles
Stiff 1-d polymers and nanotubes (LiMo3Se3, imogolite)
Semi-flexible ribbons (V2O5)
2-d sheets (clays, H3Sb3P2O14)
Rod-like (boehmite, goethite) or disk-like (gibbsite) crystallites
White pigment: Very large refraction Photovoltaïc material:
index (n = 2.63), highly scattering, « Large bandgap »
extensively used in paints and other semi-conducting
formulations. material (3 eV, near UV).
Produces an electric L
current upon UV-
absorption.
D
[001]
Trusting L.Onsager’s predictions … Tetragonal
P42/mnm 2
Mineral synthesis: « Chimie douce »
Chimie douce: Mineral synthesis
technique comparable to organic The synthesis takes place in very acidic
polymerization. A polycondensation solution…
takes place in solution; it can be HNO3 15M, TiCl4 3 M precursor
controlled by adjusting the pH and
temperature. At « high» temperature, for chimie douce…
Reflux synthesis in a flask,
in an oil bath at 120°C
Olation
Ti-OH + Ti-OH2 → Ti-OH-Ti + H2O In a « short » time, still for chimie douce
The nanoparticles are collected and
Oxolation washed after 24 hours
Ti-OH + Ti-OH → Ti-O-Ti + H2O
Jolivet, J. P. Metal Oxide Chemistry and Synthesis. From Solution to Solid State, Wiley, Chichester 2000.
Huang, Q.; Gao, L. Chemistry Letters 2003, 32, 638–639. 3
Study of « seed » nanorods
X-ray diffraction Transmission electron
microscopy
2θ
50 nm 50 nm
ÆRutile is the only
phase observed Ænanorods are the only species formed
Polarised light
microscopy
… but in a capillary, the
Strong flow birefringence of suspension is not
suspensions sheared birefringent. Nematic
between glass slide and fluctuations and
coverslip … depolarised scattering
are observed.
Length 100 ± 20 nm and diameter Very concentrated suspensions
D = 12 ± 2 nm, leading to L/D ≈ 8. slowly flocculate …
4
Dessombz A. et al, JACS 2007, 129, 5904-5909
Improvement of the process ?
Liquid-crystalline ordering is not far…
But two problems arise:
- particle aggregation
increase of surface charge
- particles too short
increase of L/D ratio
The synthesis must be improved and new steps be introduced
The first problem may be fixed by washing the nanoparticles more
thoroughly and by stabilizing them in a more complexing medium,
like a HNO3 acid solution at pH 1 (σ ~ 0.2 C.m-2).
The second problem requires a change of the synthesis strategy
ÆSeeding
ÆGrowth: after a « short » time, a small amount of reagent is
introduced in order to avoid a new nucleation step:
after 24 hours, new introduction of the TiCl4 precursor at 0.3 M
5
Study of mesogenic nanorods
Transmission electronic
microscopy
High resolution
20 30 40 50 60 70 d=2,9Å
2θ
X-ray diffraction confirms that
[001]
rutile is the only phase formed.
[110]
75 nm
Æ Nanorods a little longer … 2 nm
ÆReticular planes are
Mesogenic nanorods: observed. The [001]
L = 160 ± 40 nm and D = 15 ± 5 nm, growth direction is
leading to: L/D ≈ 11, which is just confirmed.
enough …
6
Polarised light microscopy
With this L/D ~ 11, as φ increases from 3% to 12% :
75 µm 100 µm
100 µm
Appearance of nematic droplets with Schlieren texture
that sediment and form a LC phase
200 µm 200 µm
Preparation of an aligned sample
by sucking the suspension into a
glass capillary.
Extinction Maximum transmission
7
Phase diagram
1
Isotropic
Biphasic
Nematic
0.8
Nematic proportion
0.6
0.4
0.2
0
0 2 4 6 8 10 12 14 16
Volume fraction (%)
- No gelation
- No other liquid-crystalline phase
- Colloidal stability problems arise when increasing the volume fraction
8Small-angle X-ray scattering
Shoulder at q = 0.2nm-1, leading to an
inter-particle distance of d ≈ 40 nm
Scattered Intensity (mm-1)
105
Æin good agreement
104 with φ = 12%
1000
100
0.01 0.1 -1 1
q (nm )
Estimate of the
Scattered intensity (mm-1)
104
nematic order
parameter
S = 0.75 ± 0.05
SAXS pattern of an aligned 1000
150 200 250 300 350
nematic sample (φ = 12 %) Azimuthal Angle (degrees)
9Oriented films and photocatalysis
Drop of rutile
suspension
Selection of an area where the
Spin-coating nanoparticles are radially aligned
(glass, 6000
rpm) 1 cm
1 cm
UV source Nice aligned films are only obtained from
the nematic suspensions (strangely
Stirring enough …)
Polariser // UV lamp at λ = 365 nm
┴
Quartz polariser (1 cm²)
oriented film
Methylene blue solution
Photocatalysis experiment:
The degradation of methylene blue by a rutile film under polarised UV light
is followed by monitoring the absorption peak at 665 nm.
10Photocatalysis: Results
0 .35
0 .3 The parallel orientation is the most favorable
for methylene blue degradation.
0 .25
Absorption (u.a.)
0 .2 zLight polarised parallel to the c-axis of rutile
0 .15 Light polarised perpendicular to the c-axis of rutile
0 .1
0 .05
Maximum degradation when
0 the UV-light is polarised in a
6 20 6 30 6 40 6 50 6 60 6 70 6 80 6 90 7 00
direction parallel to the c-axis
Wavelength (nm)
of the nanoparticles
11Conclusion
-By using a two-steps « chimie douce » synthesis, the aspect ratio of TiO2
rutile nanoparticles could be adjusted to obtain a nematic phase; once
again, Onsager was right !
- However, probably due to large van der Waals interactions, colloidal
suspensions of rutile are only marginally stable.
- The nematic cooperative order allowed us to produce aligned thin films.
We showed that the rutile photocatalytic efficiency is maximum when the
UV-light is polarised parallel to the c-axis of the nanorods.
- The anisotropy of (photo-)conductivity of rutile nanorods is now under
investigation.
- The plasmon resonances of these nanorods are also being studied.
1213
Remerciements
Je tiens à remercier pour ce travail :
Au LPS :
Patrick Davidson
Laboratoire de Physique
Doru Constantin, Marianne Imperor du Solide d'Orsay
Au LCMCP, UPMC-Paris VI, où la chimie a été réalisée :
Corinne Chanéac, David Chiche
Laboratoire de Chimie Jean-Pierre Jolivet
de la Matière Condensée
Fabienne Warmont (UPMC, Paris) et Dominique Jalabert
(Université d’Orleans), pour la microscopie électronique
A l’ESRF :
Pierre Panine
14Influence of φ upon film quality and orientation
Φ = 3 % , onset of nematic order
Non-oriented poor films Oriented films
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