Journal of Sol-Gel Science and Technology 19, 275–278, 2000 c 2000 Kluwer Academic Publishers. Manufactured in The Netherlands. °
Vanadium Pentoxide Sol and Gel Mesophases J. LIVAGE Chimie de la Mati`ere Condens´ee, Universit´e Paris VI, Paris, France
[email protected]
O. PELLETIER AND P. DAVIDSON Physique des Solides, Universit´e Paris Sud, Orsay, France
Abstract. Colloidal suspensions of V2 O5 ribbon-like particles display optical textures typical of lyotropic nematic phases. Tactoids (small nematic droplets) and then isotropic phases are formed as these systems are diluted. Nematic suspensions can be oriented by applying a magnetic or an electric field. Such a liquid crystal behavior is mainly due to the highly anisotropic shape of vanadium oxide colloidal particles. Acid dissociation at the oxide/water interface gives rise to surface electrical charges and electrostatic repulsion should also be responsible for the stabilization of the nematic phases. Anisotropic xerogel layers are formed when these gels are deposited and dried onto flat substrates. X-ray diffraction patterns of such coatings exhibit a series of 00l harmonics due to the turbostratic stacking of the oxide particles. Dehydration is reversible and fluid mesophases are again obtained via a swelling process when water is added to the xerogel. Keywords:
1.
vanadium pentoxide, mesophase, nematic, liquid crystal
Introduction
The hydrolysis and condensation of metal alkoxides lead to the growth of colloidal particles that form sols or gels. Shaped materials can be obtained directly from these fluid phases allowing the powderless processing of glasses and ceramics. Thin films are currently deposited by spin or dip-coating and fibers are drawn from viscous gels. However sols and gels are only an intermediate step during the formation of sol-gel materials. Water and solvent molecules are removed upon drying in order to obtain xerogels or aerogels. Vanadium pentoxide gels V2 O5 ,nH2 O have been extensively studied during the past few years [1]. They exhibit both proton and electron conduction and thin films deposited from these gels have been used as antistatic coatings in the photographic industry [2], reversible cathodes for lithium batteries or counterelectrodes in electrochromic devices [3] and even conducting matrix for glucose biosensors [4].
The liquid crystal properties of aqueous suspensions of vanadium pentoxide colloids were first reported by Zocher in 1925 [5] but this was clearly established a few years ago only [6]. These properties are rather surprising as almost all liquid crystal materials are based on organic compounds. Very few inorganic mesophases have been described up to now [7, 8]. This paper describes the main properties of vanadium pentoxide mesophases showing that they follow Onsager’s theoretical model. Electrostatic interactions between ribbon-like oxide particles should be responsible for stabilization of the nematic colloidal phase. 2.
Formation of Vanadium Pentoxide Sols and Gels
Vanadium pentoxide gels are currently formed via the acidification of aqueous solutions of sodium metavanadate NaVO3 through a proton exchange resin. However several other syntheses such as the hydrolysis of
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vanadium oxo-alkoxides VO(OR)3 , the exothermic reaction of V2 O5 with hydrogen peroxide and even the very slow dissolution of V2 O5 in pure water, have been reported in the literature [1]. In all cases, a clear yellow solution is first formed that progressively turns red while its viscosity increases. A dark red sol or gel is obtained after few minutes, hours or even months depending on the experimental procedure. It remains stable for years when kept in a closed vessel in order to prevent water evaporation. A reversible sol-gel transition is observed for a vanadium concentration corresponding to V2 O5 ,250H2 O ([V] ≈ 0.2 mol/l). The 51 V NMR spectra of the aqueous precursors solutions (Fig. 1) show that they contain mainly decavanadate species [H2 V10 O28 ]4− (peaks at δ = −427, −512 and −532 ppm) with some smaller amount of di-oxo cation [VO2 ]+ (δ = −543 ppm). The intensity of these peaks progressively decreases as condensation proceeds showing that VV species are included in large polymeric species [9]. Quantitative measure-
Figure 1.
51 V
ments show that once the gel is formed decavanadate polyanions correspond to 5% of the vanadium only. 95% of VV species have been transformed into vanadium oxide polymers. Transmission electron microscopy of the oxide colloids show that they are made of ribbon-like polymeric particles about 0.5 µm in length and 20 nm wide. This was confirmed by small angle X-ray scattering experiments performed on diluted sols (isotropic phase). They show that the scattered intensity varies as a q −2 power law at large angles and by a q −1 power law at smaller angles as expected for ribbon-like polymeric particles. The form factor deduced from these experimental results leads to 1 × 25 × 200 nm3 [10]. X-ray and electron diffraction experiments suggest that the structure of these ribbons is close to the layered structure of orthorhombic V2 O5 [11]. Each ribbon is ˚ [12]. made of two V2 O5 planes at a distance of 2.8 A Double chains of edge sharing [VO5 ] square pyramids are linked via corners to form a V2 O5 plane.
NMR spectra of the precursor solution during the formation of V2 O5 ,nH2 O gels.
Vanadium Pentoxide Sol and Gel Mesophases
3.
Nematic Order in Vanadium Pentoxide Mesophases
Vanadium pentoxide sols and gels appear birefringent when observed between crossed polarizers. They exhibit the optical textures typical of lyotropic nematic mesophases. Diluted sols display Schlieren textures with ±1/2 disclination lines whereas concentrated gels exhibit threaded textures typical of polymeric liquid crystals. Highly diluted sols (n > 600) are isotropic [13]. A biphasic region around n ≈ 500 lies between the nematic and isotropic phases in which spindle-shaped tactoids are observed [14]. Temperature has very little effect on these mesophases. The nematic ordering can be observed up to about 200◦ C in closed capillaries. The vanadium oxide system therefore appears to be athermal, a characteristic of Onsager’s model based on excluded volume interactions between charged particles [15]. Small-angle X-ray and neutron scattering experiments of nematic V2 O5 , nH2 O sols and gels show that the scattered intensity does not decrease regularly with the scattering vector q = 4π sin 2/λ. A broad maximum is observed that shifts toward smaller q values when the amount of water increases (Fig. 2(a)). This
Figure 2.
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suggests that the ribbon-like particles are aligned along a common direction, as expected for a nematic phase [16, 17]. The average distance “d” between these ribbons increases with “n” (Fig. 2(b)). This could be described as a swelling process. However two different slopes are observed showing that “d” varies as n 0.85 for high vanadium concentration and n 0.6 for more diluted fluid phases. This transition in the swelling behavior occurs when the distance “d” between ribbons becomes larger than the width of the ribbons [13]. The 1-D swelling behavior of flat ribbons is then first observed and is one of the distinctive features of a biaxial nematic phase [18]. However as d increases, the ribbons have more space to rotate freely. They then behave as rods and the 2-D swelling typical of an usual nematic phase is observed. Vanadium pentoxide nematic sols can be aligned by a magnetic field. These experiments were performed on dilute samples, close to the biphasic region (n ≈ 400). Nematic sols are placed in capillary flat tubes and observed in polarized light [19]. In the absence of magnetic field, such samples exhibit a typical threaded texture. Many disclination lines (topological defects) can be seen. Applying a small magnetic field (≈0.3 Tesla) removes all defects and leads to a fully aligned
Nematic V2 O5 ,nH2 O sols and gels (a) small angle neutron scattering pattern, (b) variation of the basal distance with n.
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nematic single domain. The vanadium oxide ribbons can be aligned along any direction by rotating the magnetic field. This alignment is preserved for several days after removing the magnetic field. As already observed for other polymeric liquid crystals, hydrodynamic instabilities occur during the reorientation of these single domains when the magnetic field is rotated, giving transient striped textures. Similar experiments performed on isotropic sols do not induce any birefringence. This shows that the orientation of the nematic phase requires some cooperative behavior of the ribbons. A single ribbon cannot be oriented by a magnetic field! Nuclear Magnetic Resonance experiments have been recently performed with D2 O enriched V2 O5 nematic sols. The spectrum obtained on deuterated water molecules shows a splitting which points to an anisotropic uniaxial dynamic behavior. On the time scale probed by NMR (10−6 s), water molecules undergo fast exchange between unoriented free sites between ribbons and adsorbed sites onto oriented ribbons. This is rather unexpected as the volume fraction of the ribbons in these suspensions is very small (5 × 10−3 ). Nevertheless, these experiments demonstrate how the nematic symmetry of the phase deeply affects the solvent microstructure [20]. 4.
Conclusion
The formation of nematic lyotropic inorganic sols and gels requires that colloidal oxide particles have an anisotropic shape. The ribbon-like shape of vanadium pentoxide polymers is related to the chemistry involved in their synthesis. As shown by 51 V NMR, the sol-gel solution contains anionic [H2 V10 O28 ]4− and cationic [VO2 ]+ species. Both of them lead to the precipitation of crystalline salts such as Na4 [H2 V10 O28 ] or VOPO4 ,2H2 O when counter ions (Na+ , PO3− 4 ) are added to the solution. An oxide network should then be formed via the condensation of neutral precursors such as [VO(OH)3 (OH2 )2 ]0 . Two different condensation processes occur in the equatorial plane, oxolation along the HO V OH direction and olation along the H2 O V OH direction, leading to the formation of long polymeric particles made of chains of edge sharing [VO5] square pyramids [21]. Moreover, these particles bear some negative charge (≈0.2e− per V2 O5 ) arising from the acid dissociation of V5+ OH groups.
The formation of nematic phases should then be due to some subtle balance between electrostatic, van der Waals and excluded volume interactions. Besides the basic interest of vanadium pentoxide gels as one of the very few examples of inorganic liquid crystals, the formation of nematic phases leads to solgel materials with specific properties. The nematic order can be preserved upon drying and vanadium oxide xerogels exhibit some preferred orientation due to the stacking of the ribbon-like oxide particles. They behave as a host structure toward the intercalation of guests species and many new materials have been synthesized via the sol-gel route, especially for the realization of cathodes in lithium batteries [1]. References 1. J. Livage, Chem. Mater. 3, 578 (1991). 2. C. Guestaux, J. Leaute, C. Virey, and J. Vial, US patent 3,658,573, April 1972. 3. R. Baddour, J.P. Pereira-Ramos, R. Messina, and J. Perichon, J. Electroanal. Chem. 277, 359 (1990). 4. V. Glezer and O. Lev, J. Am. Chem. Soc. 115, 2533 (1993). 5. H. Zocher, Z. Anorg. Allg. Chem. 147, 91 (1925). 6. P. Davidson, A. Garreau, and J. Livage, Liq. Cryst. 16, 905 (1994). 7. P. Davidson, P. Batail, J.C. Gabriel, J. Livage, C. Sanchez, and C. Bourgaux, Prog. Polym. Sci. 22, 913 (1997). 8. A.S. Sonin, J. Mater. Chem. 8, 2557 (1998). 9. G.A. Pozarnsky and A.V. McCornick, Chem. Mater. 6, 380 (1994). 10. O. Pelletier, C. Bourgaux, O. Diat, P. Davidson, and J. Livage, Eur. Phys. J. E 2, 191 (2000). 11. J.J. Legendre and J. Livage, J. Colloid Interf. Sci. 94, 75 (1983). 12. T. Yao, Y. Oka, and N. Yamamoto, Mat. Res. Bull. 27, 669 (1992). 13. P. Davidson, C. Bourgaux, L. Schoutteten, P. Sergot, C. Williams, and J. Livage, J. Phys. II France 5, 1577 (1995). 14. J.H.L. Watson, W. Heller, and W. Wojtowicz, Science 109, 274 (1949). 15. L. Onsager, Ann. N.Y. Acad. Sci. 51, 627 (1949). 16. P. Aldebert, H.W. Haesslin, N. Baffier, and J. Livage, J. Colloids Interface Sci. 98, 478 (1984). 17. N. Baffier, P. Aldebert, J. Livage, and H.W. Haesslin, J. Colloids Interface Sci. 141, 467 (1991). 18. O. Pelletier, C. Bourgaux, O. Diat, P. Davidson, and J. Livage, Eur. Phys. J. B 12, 541 (1999). 19. X. Commeinhes, P. Davidson, C. Bourgaux, and J. Livage, Adv. Mater. 9, 900 (1997). 20. O. Pelletier, P. Sotta, and P. Davidson, J. Phys. Chem. B 103, 5427 (1999). 21. J. Livage, Coord. Chem. Rev. 178–180, 999 (1998).
