Laboratoire de Physique du Solide d'orsay Laboratoire de Chimie de la Matière Condensée «Engineering» of a liquid-crystalline aqueous suspension of TiO 2 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 (LiMo 3 Se 3, imogolite) Semi-flexible ribbons (V 2 O 5 ) 2-d sheets (clays, H 3 Sb 3 P 2 O 14 ) Rod-like (boehmite, goethite) or disk-like (gibbsite) crystallites White pigment: Very large refraction index (n = 2.63), highly scattering, extensively used in paints and other formulations. Photovoltaïc material: «Large bandgap» semi-conducting material (3 ev, near UV). Produces an electric current upon UVabsorption. L D [001] Trusting L.Onsager s predictions Tetragonal P4 2 /mnm 2
Mineral synthesis: «Chimie douce» Chimie douce: Mineral synthesis technique comparable to organic polymerization. A polycondensation takes place in solution; it can be controlled by adjusting the ph and temperature. Olation Ti-OH + Ti-OH 2 Ti-OH-Ti + H 2 O Oxolation Ti-OH + Ti-OH Ti-O-Ti + H 2 O The synthesis takes place in very acidic solution HNO 3 15M, TiCl 4 3 M precursor At «high» temperature, for chimie douce Reflux synthesis in a flask, in an oil bath at 120 C In a «short» time, still for chimie douce The nanoparticles are collected and washed after 24 hours 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
X-ray diffraction Study of «seed» nanorods Transmission electron microscopy 2θ Rutile is the only phase observed Strong flow birefringence of suspensions sheared between glass slide and coverslip Length 100 ± 20 nm and diameter D = 12 ± 2 nm, leading to L/D 8. 50 nm 50 nm nanorods are the only species formed Polarised light microscopy Dessombz A. et al, JACS 2007, 129, 5904-5909 but in a capillary, the suspension is not birefringent. Nematic fluctuations and depolarised scattering are observed. Very concentrated suspensions slowly flocculate 4
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 HNO 3 acid solution at ph 1 (σ ~ 0.2 C.m -2 ). The second problem requires a change of the synthesis strategy Seeding Growth: 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 TiCl 4 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 rutile is the only phase formed. 75 nm Nanorods a little longer Mesogenic nanorods: L = 160 ± 40 nm and D = 15 ± 5 nm, leading to: L/D 11, which is just enough 2 nm [001] [110] Reticular planes are observed. The [001] growth direction is confirmed. 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 that sediment and form a LC phase with Schlieren texture 200 µm 200 µm Preparation of an aligned sample by sucking the suspension into a glass capillary. Extinction Maximum transmission 7
Phase diagram 1 0.8 Isotropic Biphasic Nematic 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 8
Small-angle X-ray scattering Shoulder at q = 0.2nm -1, leading to an inter-particle distance of d 40 nm SAXS pattern of an aligned nematic sample (φ = 12 %) Scattered Intensity (mm -1 ) Scattered intensity (mm -1 ) 10 5 10 4 1000 100 0.01 0.1 1 10 4 q (nm -1 ) 1000 150 200 250 300 350 Azimuthal Angle (degrees) in good agreement with φ = 12% Estimate of the nematic order parameter S = 0.75 ± 0.05 9
Drop of rutile suspension Oriented films and photocatalysis Spin-coating (glass, 6000 rpm) Stirring UV source 1 cm 1 cm Selection of an area where the nanoparticles are radially aligned Nice aligned films are only obtained from the nematic suspensions (strangely 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. 10
Photocatalysis: Results Absorption (u.a.) 0.35 0.3 0.25 0.2 0.15 0.1 The parallel orientation is the most favorable for methylene blue degradation. Light polarised parallel to the c-axis of rutile Light polarised perpendicular to the c-axis of rutile 0.05 0 620 630 640 650 660 670 680 690 700 Wavelength (nm) Maximum degradation when the UV-light is polarised in a direction parallel to the c-axis of the nanoparticles 11
Conclusion -By using a two-steps «chimie douce» synthesis, the aspect ratio of TiO 2 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. 12
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Remerciements Je tiens à remercier pour ce travail : Au LPS : Patrick Davidson Doru Constantin, Marianne Imperor Laboratoire de Physique du Solide d'orsay Laboratoire de Chimie de la Matière Condensée Au LCMCP, UPMC-Paris VI, où la chimie a été réalisée : Corinne Chanéac, David Chiche Jean-Pierre Jolivet Fabienne Warmont (UPMC, Paris) et Dominique Jalabert (Université d Orleans), pour la microscopie électronique A l ESRF : Pierre Panine 14
Influence of φ upon film quality and orientation Φ = 3 %, onset of nematic order Non-oriented poor films Oriented films 15