«Engineering» of a liquid-crystalline aqueous suspension of TiO 2 rutile nanorods Anisotropic photocatalytic properties

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1 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

2 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

3 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 Huang, Q.; Gao, L. Chemistry Letters 2003, 32,

4 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 nm 50 nm nanorods are the only species formed Polarised light microscopy Dessombz A. et al, JACS 2007, 129, but in a capillary, the suspension is not birefringent. Nematic fluctuations and depolarised scattering are observed. Very concentrated suspensions slowly flocculate 4

5 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

6 Study of mesogenic nanorods Transmission electronic microscopy High resolution 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

7 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

8 Phase diagram Isotropic Biphasic Nematic Nematic proportion Volume fraction (%) - No gelation - No other liquid-crystalline phase - Colloidal stability problems arise when increasing the volume fraction 8

9 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 ) q (nm -1 ) Azimuthal Angle (degrees) in good agreement with φ = 12% Estimate of the nematic order parameter S = 0.75 ±

10 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

11 Photocatalysis: Results Absorption (u.a.) 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 Wavelength (nm) Maximum degradation when the UV-light is polarised in a direction parallel to the c-axis of the nanoparticles 11

12 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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14 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

15 Influence of φ upon film quality and orientation Φ = 3 %, onset of nematic order Non-oriented poor films Oriented films 15