Langmuir 2000, 16, 5295-5303
5295
A Detailed Study of the Synthesis of Aqueous Vanadium Pentoxide Nematic Gels O. Pelletier,† P. Davidson,*,† C. Bourgaux,‡ C. Coulon,§ S. Regnault,§ and J. Livage| Laboratoire de Physique des Solides, UMR 8502 CNRS, Baˆ t. 510, Universite´ Paris-Sud, 91405 Orsay Cedex, France, Laboratoire pour l’Utilisation du Rayonnement Electromagne´ tique, UMR 130 CNRS, Baˆ t. 209D, Universite´ Paris-Sud, 91405 Orsay Cedex, France, Centre de Recherche Paul Pascal, UP 8641 CNRS, Universite´ Bordeaux I, Av. Albert Schweitzer, 33600 Pessac, France, and Laboratoire de Chimie de la Matie` re Condense´ e, UMR 7574 CNRS, Universite´ Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex, France Received October 25, 1999. In Final Form: February 17, 2000 Vanadium pentoxide (V2O5) gels are suspensions of V2O5 ribbonlike particles dispersed in water. At volume fractions larger than 0.7%, they form a lyotropic nematic liquid-crystalline phase. We investigate here three different methods of synthesis (by ion-exchange, from peroxovanadic species, and by dissolution of V2O5 powders) and suggest a common chemical mechanism giving rise to these V2O5 ribbons. In addition, we investigate in detail the ion-exchange synthesis process by optical microscopy, time-resolved synchrotron small-angle X-ray scattering, rheology, and conductivity measurements. In particular, we relate the flocculation occurring during synthesis and the subsequent dispersion of V2O5 condensed moieties to their electric surface charge. The fractal structure of the transient flocculate is demonstrated. We then give a complete physical description of the formation of the ribbons and their organization in a nematic phase. Finally, we try to describe this polymerization process by borrowing concepts devised to understand the self-assembly of amphiphilic molecules.
I. Introduction Aqueous V2O5 gels are one of the best examples of materials synthesized by “chimie douce” (soft chemistry) techniques.1,2 These suspensions of V2O5 ribbons actually combine an unusual number of physical and chemical features such as liquid crystallinity, magnetic field alignment, gel elasticity, redox, and intercalation properties, for instance. V2O5 gels are currently being developed and used for applications in many fields ranging from the photographic industry to batteries and electrochromic displays.3,4 These colloids are comprised of ribbons 1 nm thick, about 25 nm wide, and about 300 nm long, whose structure (Figure 1) is closely related to that of orthorhombic V2O5.2,5,6 As early as 1925, Zocher suggested that these suspensions form a lyotropic nematic phase.7 This was only recently proved by microscopic observations in polarized light and X-ray scattering experiments on single domains.5,8,9 These materials are therefore one of the very few examples of mineral liquid crystals.10 These gels can be synthesized in many ways that essentially lead to similar materials. One of the most popular methods of synthesis consists of the acidification
Figure 1. Schematic representation of a V2O5 ribbon showing its detailed structure.
* Author for correspondence. † Laboratoire de Physique des Solides, Universite ´ Paris-Sud. ‡ Laboratoire pour l’Utilisation du Rayonnement Electromagnetique, Universite´ Paris-Sud. § Universite ´ Bordeaux I. | Universite ´ Pierre at Marie Curie.
of a NaVO3 solution by ion-exchange because no foreign ions are thus introduced.2 Previous studies of the synthesis of V2O5 gels have mostly focused on the determination of the chemical mechanism by which vanadium molecular species polymerize into ribbons.11-17 However, the mechanism of this synthesis still remains unclear from the
(1) Ditte, A. C. R. Acad. Sci. 1885, 101, 698. (2) Livage, J. Chem. Mater. 1991, 3, 578. (3) Guestaux, C.; Leaute, J.; Virey, C.; Vial, J. U.S. Patent 3 658 573, 1972. (4) Livage, J.; Baffier, N.; Pereira-Ramos, J. P.; Davidson, P. Mater. Res. Soc. Symp. Proc. 1995, 369, 179. (5) Davidson, P.; Bourgaux, C.; Schoutteten, L.; Sergot, P.; Williams, C.; Livage, J.; J. Phys. II 1995, 5, 1577. (6) (a) Bachmann, H. G.; Ahmed, F. R.; Barnes, W. H. Z. Kristallogr. 1960, 115, 110. (b) Yao, T.; Oka, Y.; Yamamoto, N. Mater. Res. Bull. 1981, 16, 669. (7) Zocher, H. Z. Anorg. Allg. Chem. 1925, 147, 91.
(8) Davidson, P.; Garreau, A.; Livage, J. Liq. Cryst. 1994, 16, 905. (9) Commeinhes, X.; Davidson, P.; Bourgaux, C.; Livage, J. Adv. Mater. 1997, 9, 900. (10) Gabriel, J. C. G.; Davidson, P. Adv. Mater. 2000, 12, 9, and references therein. (11) Lemerle, J.; Nejem, L.; Lefebvre, J. J. Inorg. Nucl. Chem. 1980, 42, 17. (12) Bailey, J. K.; Nagase, T.; Pozarnsky, G. A.; Mecartney, M. L. Mater. Res. Soc. Symp. Proc. 1992, 180, 759. (13) Bailey, J. K.; Pozarnsky, G. A.; Mecartney, M. L. J. Mater. Res. 1992, 7, 2530.
10.1021/la9914155 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/05/2000
5296
Langmuir, Vol. 16, No. 12, 2000
physical point of view. In this work, we compare three different methods of synthesis: by ion-exchange, by reaction with H2O2, and by direct dissolution in water. Moreover, for the ion-exchange technique, we investigate in detail the formation of these ribbons by optical microscopy, time-resolved synchrotron small-angle X-ray scattering (SAXS), rheology, and conductivity measurements. On the basis of these results, we then suggest a scenario for the formation and growth of the ribbons and the subsequent gelation and nematic ordering of the suspensions. Finally, we describe experiments devised to probe the reversibility of the polymerization process. We try to interpret them by a tentative thermodynamic description drawn from an analogy with the self-assembly of surfactant molecules. II. Experimental Section II.1. Synthesis Methods. Synthesis by Ion-Exchange. This method is well documented.2 One hundred milliliters of a sodium metavanadate, NaVO3, solution of molarity ranging from 0.5 to 1 M (pH ≈ 9) is passed through a column filled with a bed of a proton exchange resin (Dowex 50 W-X2 50-100 mesh, ≈80 g). (Solutions of molarities less than 0.1 M do not polymerize, and solutions of molarities larger than 1 M polymerize inside the column.) The effluent solution (pH ≈ 2-3) turns yellow as soon as acidification occurs showing that the coordination of VV increases from four to six. This solution of vanadic acid, first clear, turns orange and turbid and then red upon aging while its viscosity progressively increases. A dark red gel is finally obtained after a few hours that remains stable for years when kept in a closed vessel. Synthesis from Peroxovanadic Species. Vanadium pentoxide gels can also be made via the reaction of the oxide powder with hydrogen peroxide.18,19 The dissolution reaction is highly exothermic so that dilute H2O2 solutions (10%) have to be used. The hydrogen peroxide is partially decomposed during dissolution leading to the release of oxygen gas. A clear orange solution is formed after about 10 min (pH ≈ 1.5). O2 still evolves slowly from the solution that turns deep red after about 2 h. Then, oxygen bubbling progressively stops, the coloration turns orange-yellow, and a red-brown gelatinous flocculate forms after a few hours. This flocculate swells spontaneously in the yellow mother solution giving rise to a homogeneous viscous dark red gel about 24 h later. Dissolution of V2O5 Powders in Water. It has already been shown that grinding amorphous V2O5, obtained by splat-cooling from the melt, with water, leads to the formation of vanadium pentoxide gels.20 According to literature, crystalline vanadium pentoxide does not dissolve into water and no crystalline V2O5‚ nH2O hydrate is known.21 However, experiments performed in our laboratory show that some reaction occurs when a crystalline V2O5 powder is left in the presence of water (pH ≈ 7) for a very long time. The solution turns light yellow after about 2 weeks, and its pH decreases slowly. A light red flocculate forms after a few months at the powder-water interface. This flocculate progressively grows in volume, and after about 1 year the whole sample is a red gel. II.2. Characterization Techniques. 51V NMR solution spectra were recorded on a Bruker AM300 spectrometer operating at 65.7 MHz, using a 90° pulse width of 16 µs, a relaxation delay of 1 s, and a spectral line width of 62.5 kHz. Neat VOCl3 is used as an external reference for chemical shifts (δ ) 0 ppm). All (14) Pozarnsky, G. A.; Wright, L.; McCormick, A. V. J. Sol.-Gel Sci. Technol. 1994, 3, 57. (15) Pozarnsky, G. A.; McCormick, A. V. Chem. Mater. 1994, 6, 380. (16) Pozarnsky, G. A.; McCormick, A. V. J. Mater. Chem. 1994, 4, 1749. (17) Pozarnsky, G. A.; McCormick, A. V. J. Mater. Sci Lett. 1996, 15, 1526. (18) Osterman, W. Wiss. Ind. Hamburg 1922, 1, 17. (19) Hibino, M.; Ugaji, M.; Jishimoto, A. K.; Kudo, T. Solid State Ionics 1995, 79, 239. (20) Gharbi, N.; R’kha, C.; Ballutaud, D.; Michaud, M.; Livage, J.; Audie`re, J. P.; Schifmacher, G. J. Non-Cryst. Solids 1981, 46, 247. (21) The´obald, F. Bull. Soc. Chim. Fr. 1975, 7-8, 1607.
Pelletier et al. spectra are recorded with the same sample in order to make quantitative measurements. Flat-glass optical (Vitro Com) capillaries of thickness 100 µm were filled with V2O5 suspensions and flame-sealed. Their textures were observed in polarized light with a (Leitz) microscope. Images were recorded with a (JVC) CCD color video camera connected to a personal computer equipped with a standard image acquisition software. This software outputs images coded with 256 gray levels. The correlation functions of these images were then computed with homemade software (Commercial software gave identical results). SAXS experiments were performed on the D24 experimental station at the LURE synchrotron facility. This station, already described,22 uses a single bent Ge (111) monochromator, which provides a beam focused in the horizontal plane. This beam has a flux of about 1010 photons/s‚mm2. To reduce parasitic scattering, the beam path is kept under vacuum and antiparasitic slits are placed before the sample. The scattered X-ray intensity is recorded either with a gas-filled (Ar-CO2 or Xe-CO2), position-sensitive detector or with imaging plates. Typical exposure times were 10 min allowing the time-resolved study of the polymerization process. Samples were held in Lindemann glass capillary tubes of 1.5 mm diameter. Viscoelastic moduli were measured with a stress-imposed CarriMed CSL-100 rheometer with a cone-plane geometry. The shear applied was less than 1 Pa, which ensured that the measurements were done in the linear regime. Data acquisition took typically 10 min, which also allowed us to perform timeresolved studies. Measurements were performed at 1 Hz, but this choice of frequency had little influence on the results. Conductivity experiments were made with an apparatus already described.23 The conductivity at low frequency is dominated by electrode polarization effects that are negligible above 105 Hz. Sample conductivity is then obtained by a standard data treatment and does not depend on frequency. Light-scattering experiments have been made on a series of samples obtained by diluting a concentrated solution. To keep the pH constant under dilution, we used for solvent a solution of hexafluorophosphoric acid (H+PF6-) of the same pH as that of the initial solution (here, pH ) 2.6). The big PF6- anion was chosen to minimize adsorption effects at the surface of the polymers. As concentrated samples absorb light, only diluted samples (with a V2O5 volume fraction below 10-3) were examined. The incident light is a vertically polarized beam (λ ) 6471 Å) produced by a “Coherent” I90K laser. The correlator is a BI2030AT from Brookhaven instruments. The sample is introduced in cylindrical cells (diameter 8 mm) immersed in a Decalin bath used for both index matching and temperature control ((0.2 °C).
III. Results III.1. Comparison between Different Synthesis Methods. All three synthetic methods give rise to very similar suspensions that differ only in details such as their precise color. For instance, suspensions prepared from amorphous V2O5 have a brown-greenish color, which indicates a few percent of vanadium(IV). These suspensions, however, still display viscoelasticity and nematic textures in polarized light. III.1.1. X-ray Scattering. Upon drying in air under ambient conditions, the gels lead to V2O5‚nH2O xerogels (n ≈ 2). The X-ray diffraction patterns (not shown) of such xerogels deposited onto a flat substrate exhibit a series of 00l peaks typical of some preferred orientation (called “turbostratic”) of a layered structure. The basal distance, d, deduced from the position of these peaks does not significantly depend on the synthesis method. d ) 1.16, 1.20, and 1.17 nm for gels obtained, respectively, via ionexchange, from peroxovanadic species, and by direct dissolution in water of the crystalline powder. (22) Dubuisson, J. M.; Dauvergne, J. M.; Depautex, C.; Vachette, P.; Williams, C. Nucl. Instrum. Methods Phys. Res. 1986, A286, 636. (23) Soubiran, L. Ph.D. Thesis, University Bordeaux I, 1996.
Synthesis of Vanadium Pentoxide Nematic Gels
Figure 2. SAXS pattern of a V2O5, 208 H2O nematic suspension obtained by dissolution of amorphous V2O5 in water. The arrow points to the interference peak; a second order can also be observed. Inset: SAXS pattern of a V2O5, 2000 H2O isotropic suspension obtained by dissolution of amorphous V2O5 in water. The straight lines show the two asymptotic q-1 and q-2 regimes.
Figure 3. SAXS pattern of a V2O5, 120 H2O nematic suspension obtained by dissolution of crystalline V2O5 in water. The arrow points to the interference peak. Inset: SAXS pattern of a V2O5, 1200 H2O isotropic suspension obtained by dissolution of crystalline V2O5 in water. The straight lines show the two asymptotic q-1 and q-2 regimes.
Whatever the synthetic method, all nematic suspensions show a more or less well-defined SAXS interference peak (Figures 2, 3). This peak is known to arise from lateral interferences between ribbons in a plane perpendicular to their main axis.5 In other words, it is due to short-range positional correlations of the ribbons. In contrast, all isotropic suspensions show by SAXS the usual signature of ribbons (Figure 2, 3 insets). This form factor classically involves a q-1-dependence at small q and a q-2-dependence at large q.5 The crossover between these two regimes defines the width of the ribbons. As described in detail in section III.2.2., the average width of the ribbons obtained by ion-exchange is about 25 nm. The width obtained by direct dissolution in water lies around 60 nm, whereas that obtained by reaction with H2O2 is about 10 nm. III.1.2. NMR Experiments. Synthesis by Ion-Exchange. The kinetics of the formation of V2O5 gels was followed by 51V NMR. The yellow solution obtained just after ionexchange is placed in a tube inside the NMR spectrometer, and 51V NMR spectra are recorded at different times. The [H2V10O28]4- ion characterized by the three intense peaks at δ ≈ -530, -510, and -425 ppm is the main solute species with some traces of the [VO2]+ ion at δ ) -545
Langmuir, Vol. 16, No. 12, 2000 5297
Figure 4. 51V NMR spectra of the condensation of vanadic acid solutions showing [H2V10O28]4- and [VO2]+ solute species. Inset: Evolution with time of the amount of vanadium in solute species.
Figure 5. Evolution of the 51V NMR spectrum of peroxovanadic acid solutions as a function of time: (a) just after dissolution of V2O5 in H2O2, (b) after 3 h, and (c) after 4 hours
ppm (Figure 4). The intensity of these NMR peaks progressively decreases as condensation proceeds suggesting that, after about 10 h, most of the vanadium forms large polymers rather than molecular species.15-17 Actually, quantitative experiments show (Figure 4 inset) that, after 2 days, only 5% of the vanadium is still in the molecular species [H2V10O28]4- and [VO2]+ and 95% of the vanadium has been transformed into V2O5 ribbons. Synthesis from Peroxovanadic Species. The 51V NMR spectrum of the orange solution obtained just after dissolution exhibits a single sharp peak around δ ) -695 ppm (Figure 5a) that can be assigned to the orange diperoxo anion [VO(O2)2]-. This peak progressively broadens and decreases in intensity with time while oxygen gas evolves from the solution. The solution turns red after about 3 h, and four new peaks appear. Three of them are typical of the diprotonated decavanadate species [H2V10O28]4-, while the fourth one at δ ) -539 ppm can be assigned to the red oxoperoxo cation [VO(O2)]+ (Figure
5298
Langmuir, Vol. 16, No. 12, 2000
Pelletier et al.
Figure 6. Series of photographs of the same vial during polymerization. Photographs were taken every 15 min after polymerization started except for the last one, which was taken after 24 h.
5b).24 The red coloration progressively disappears within about 1 h, while a small peak grows that should be due to [VO2]+. Only oxo species [VO2]+ and [H2V10O28]4- are observed after 4 h, before gelation occurs (Figure 5c). Quantitative measurements based on the surface of 51V NMR peaks show that the total amount of vanadium seen by NMR remains constant over a large period of time (≈4 h). The 51V NMR spectrum of the solution then progressively decreases in intensity as gelation goes on, which should be due to the formation of large condensed species. MAS NMR experiments have then been performed on these gels. They give a NMR signal similar to that already observed with V2O5‚nH2O gels synthesized from aqueous solutions. III.2. Detailed Study of the Ionic Exchange Synthesis. III.2.1. Visual Inspection and Optical Microscopy. The concentration and pH of the yellow solution flowing out of the column vary with time. It is therefore useful to fractionate the effluent solution in different vials. The concentration in vanadium(V) species is the largest in the two or three vials filled in the middle of the experiment. These vials are the only ones where V2O5 polymerization will take place. Shortly after these vials are filled with the effluent solution, a flocculation process starts and can be observed with the naked eye (Figure 6). Small solid red particles appear and start sedimenting. Oddly enough, this sediment is not compact and slowly fills the whole volume of the suspension. At room temperature, flocculation is complete after about 2 h and the flocculate persists for about 12 h. After about a day, the suspensions become completely homogeneous dark red gels. It is therefore clear that the system goes through a series of kinetic stages before reaching thermodynamic equilibrium. The flocculation can also be observed by optical microscopy (Figure 7). The small particles, easily seen as dark spots on the bright background, appear in increasing numbers (Figure 7a-e). However, these particles gradually disappear, and a homogeneous suspension is obtained after a day. One interest of this experiment is that the density autocorrelation function can be derived from a simple image treatment. We used this technique to look for a possible fractal organization of the flocculate as is often observed with out of equilibrium aggregation phenomena. The autocorrelation function obtained from Figure 7e is shown in Figure 8 in log-log representation. The existence of a fractal dimension df should induce a linear dependence with a slope p ) df - 2.25 Of course, the (24) Butler, A.; Clague, M. J.; Meister, G. E. Chem. Rev. 1994, 94, 625. (25) Chaikin, P. M.; Lubensky, T. C. Principles of Condensed Matter Physics; Cambridge University Press: New York, 1995.
Figure 7. Series of optical microscopy photographs of a flat capillary filled with the effluent solution during polymerization. Photographs were taken every 15 min after polymerization started except for the last one, which was taken after 24 h. (The capillary is 1 mm wide.)
sample finite size alters the autocorrelation function at large r. Nevertheless, the linear dependence at small r yields a fractal dimension of 1.85, a result that will be confirmed in section III.2.2. by SAXS. The liquid-crystalline behavior of the suspensions can easily be assessed by optical microscopy in polarized light. Surprisingly, the typical birefringence associated with nematic ordering appears progressively only after a few days of aging.
Synthesis of Vanadium Pentoxide Nematic Gels
Figure 8. Autocorrelation function derived from Figure 7e.
Figure 9. Rise of the integrated SAXS intensity as a function of time, at room temperature, for four samples from different vials filled during the same V2O5 gel synthesis. (Note the break in the time axis.)
III.2.2. Time-Resolved Small-Angle X-ray Scattering. To obtain microscopic information about the nature of the flocculate, we have performed in situ time-resolved SAXS studies of the suspension during the polymerization process. The kinetics involved are indeed slow enough to be explored with synchrotron radiation. Two types of information can be thus derived:26 we use the integrated SAXS intensity to monitor semiquantitatively the evolution of the polymerization reaction; the dependence of the SAXS signal with the scattering vector modulus, q ) 4π sin θ/λ (where λ is the wavelength and 2θ the scattering angle) is related to the shape of the scattering objects. As expected, flocculation induces a huge (and exponential) increase of the integrated SAXS intensity as depicted in Figure 9. After a few hours, the integrated intensity saturates and even slightly decreases after a day. This stage corresponds to the homogenization of the suspension as observed optically. The q-dependence of the SAXS signals should tell us more about the scattering objects at the different stages of the synthesis, but this dependence is difficult to analyze. This is especially true at the beginning of the process when the q-dependence is very strong at small q and much weaker at larger q. As polymerization proceeds, the dependence at small q becomes weaker whereas the dependence at large q becomes stronger until the usual form factor of the ribbons is finally observed. (26) Guinier, A.; Fournet, G. Small angle scattering of X-rays; WileyInterscience: New York, 1955.
Langmuir, Vol. 16, No. 12, 2000 5299
Figure 10. Evolution with time of the two exponents used to describe the q-dependence of the SAXS signal. Filled squares correspond to the low-q-regime and filled circles to the higher q-regime. (Synthesis performed at 40 °C to improve kinetics.)
Figure 11. q-dependence of the SAXS signal recorded at the maximum flocculation (empty squares) as described by a power law q-1.8 (straight solid line) on the whole q-range (almost two decades), which suggests a fractal organization.
Since we do not have any structural model for the scattering units at the early stages of polymerization, we decided to characterize each SAXS curve by two exponents describing the q-dependence at small and large q, respectively. These two exponents must eventually tend toward the values of -1 and -2, which apply to ribbons. The evolution of these two exponents during polymerization is shown in Figure 10. These two curves cross when flocculation is at its maximum, and the SAXS curve can then be described with a single exponent -1.8. (Figure 11) over the whole q-range. This exponent is remarkably close to that obtained above from optical microscopy at precisely the same stage of the flocculation process, which confirms the fractal organization of the flocculate. Indeed, this self-similar structure is observed both in the 10-100 nm range and in the 10-100 µm range. When the two exponents describing the SAXS curves have about reached their final -1 and -2 values typical of ribbons, the position of the crossover between these two regimes gives us the width of the ribbons. The width actually increases with time from about 3 nm to about 25 nm after 7 h (Figure 12) at room temperature. Even after 10 h, the SAXS signal is still well described by the form factor of isolated ribbons. This shows that positional correlations are still very weak. However, aged samples clearly show a peak of positional short-range correlations.5 In fact, such correlations appear very slowly. For instance, the SAXS curve of a 1 day old suspension barely shows a shoulder where the interference peak is
5300
Langmuir, Vol. 16, No. 12, 2000
Figure 12. Evolution with time of the ribbon width (same synthesis as Figure 9; note the break in the time axis).
Figure 13. Evolution with time of the viscoelatic moduli G′, G′′ as polymerization proceeds (synthesis performed at room temperature).
usually found. Such a well-developed peak can only be seen in suspensions at least a few days old. III.2.3. Influence of Temperature and Shear. In this subsection, we briefly describe the effect of temperature and shear flow on the synthesis of colloidal V2O5 suspensions. We have tried to heat the samples in order to increase their polymerization rate. Indeed, raising the temperature to 50 °C increases the flocculation rate by roughly a factor of 2 without altering the structure of the flocculate. Also, once the ribbons are formed, their width increases much faster. Moreover, positional correlations appear sooner. However, samples heated to 80 °C during the early stages of polymerization do not have a fractal organization, do not evolve, and therefore do not disperse in suspension any more. We have also sheared the suspensions in a Couette shear cell5 either during all the polymerization process or after about 10 h to try to enhance the nematic ordering. Unfortunately, these experiments were disappointing because no effect of shear could be detected. This confirms the previous observation that, even though they look similar to older ones, suspensions 10 h old have not yet completed their structural evolution. III.2.4. Rheological Measurements. Rheological experiments were performed to characterize the mechanical properties of the suspensions. The elastic G′ and viscous G′′ moduli have been measured during the polymerization of the ribbons (Figure 13). Both moduli suddenly increase by about 3 orders of magnitude after about 8 h of reaction, which corresponds to the stage where the ribbons are formed. After a day, G′ is an order of magnitude larger
Pelletier et al.
Figure 14. Evolution with time of the electrical conductivity as polymerization proceeds (synthesis performed at room temperature).
than G′′ and both moduli have reached the values measured with gels several months old. Similarly, the yield stress (about 20 Pa) measured after a day is the same as that measured after a few months. Even though the linear rheologies of fresh and aged suspensions are rather similar, their nonlinear behaviors are quite different. For instance, we observed that the gel reconstruction after a large stress is a very fast process for aged gels whereas it is very slow for fresh ones. III.2.5. Conductivity Measurements. The most intuitive way to explain the sequential flocculation and dispersion of the particles is to assume that their surface electric charge evolves as polymerization proceeds. The fast flocculation would result from a large decrease of surface charge, whereas the subsequent slow dispersion of the flocculate would be due to a slow rise of the surface charge. The electric conductivity of the suspensions was therefore measured as a function of time (Figure 14). As expected, the conductivity sharply drops by a factor of 2 during the first 2 h while the system flocculates, and then it rises very slowly again over a day while the flocculate swells. These experimental results prove that surface electric charges evolve during synthesis. III.3. Tests of the Possible “Living” Character of V2O5 Ribbons. In this respect, the reversibility of the self-assembly process first needs to be checked. V2O5 ribbons are only stable in a narrow pH range (0.5 < pH < 2.5). It is therefore possible to dissolve the ribbons through a small pH change and study their self-assembly as the pH is brought back into the range of stability. However, this method suffers from the drawback of adding foreign ions to the suspension as the pH is adjusted back and forth with acid and base solutions. This raises the ionic strength, which is known to bring about flocculation of the suspension.27 Nevertheless, the pH of a V2O5 suspension was lowered by adding a few drops of a concentrated HCl solution until the solution was completely colorless. This is the sign that no V2O5 ribbon is left in the solution. Then, the pH of the solution was increased back to its initial value by adding a few drops of a concentrated NaOH solution. After a few days, solid reddish particles formed, which were collected and examined by wide-angle X-ray scattering. The most salient feature of this diffraction pattern (not shown) is a diffraction line that corresponds to a distance of about 1 nm. More detailed information can also be obtained using light-scattering techniques that are known to be very (27) Pelletier, O.; Davidson, P.; Bourgaux, C.; Livage, J. Europhys. Lett. 1999, 48, 53.
Synthesis of Vanadium Pentoxide Nematic Gels
Langmuir, Vol. 16, No. 12, 2000 5301
Figure 15. Typical light-scattering intensity (normalized to that of benzene) as a function of wave vector modulus q. V2O5 volume fraction Φ ) 2 × 10-4. The solid line shows the fit with the form factor of an infinite cylinder (given in the text).
efficient to investigate the assembly of large aggregates. Figure 15 shows, for a gel (several months old) diluted with a HPF6 solution, the scattered intensity as a function of q. A clear q-dependence is observed that saturates at the smallest wave vectors. At larger q, a crossover is seen toward a regime where the intensity varies as 1/q, in agreement with the SAXS experiments. From the position q* of this crossover, a characteristic length ξ ) 2π/q* for the ribbons can be estimated. More precisely, we have used the approximate expression of the form factor of a ribbon (or a cylinder) when q , 1/d (d being the largest transverse size).28
P(q) )
∫0qξsinx xdx - (qξ2 sin(qξ2 ))
2 qξ
2
Figure 16. Light-scattering intensity (normalized to that of benzene) extrapolated at q ) 0 as a function of the V2O5 volume fraction Φ: (a) general variation and (b) detailed for diluted samples. The solid line shows the fit with a power law I ∝ (Φ - Φ*)x, with Φ* ) 2.65 × 10-5 and x ) 0.66.
The fit shown as a continuous line gives ξ ) 291 nm. Note that this characteristic size can be interpreted either as the contour length of the ribbons or as their persistence length. In both cases, the shape of I(q) confirms that (V2O5)n ribbons are quite rigid objects. The same experiment can be repeated along a dilution line in order to study the influence of the volume fraction. Figure 16a shows the effect of dilution on the intensity extrapolated at q ) 0 (i.e., on the saturation value found at low q). A crossover is observed at a finite volume fraction (φ* ≈ 2.5 × 10-5). This effect is better seen in Figure 16b, which gives details close to φ*. The continuous line is a fit, for φ > φ* with the empirical form
I(φ) ) C (φ - φ*)x We obtain from this fit φ* ) 2.65 × 10-5, x ) 0.66. At the same time, we deduce from the q-dependence of the scattered intensity that ξ ≈ 300 nm along the dilution line above φ*. Below φ*, the scattered intensity becomes very weak and independent of q. We conclude that large aggregates are no longer present. IV. Discussion IV.1. Suggested Chemical Mechanism for the Formation of V2O5 Gels. Strikingly, it seems altogether that the three different methods of synthesis provide very similar colloidal objects, which suggests a common chemical mechanism (already partially discussed in ref 2) for the formation of these ribbons. (28) Berne, B. J.; Pecora, R. Dynamic light scattering; John Wiley and sons: New York, 1975.
Figure 17. Schematic diagram showing the regions of stability of the various vanadium species as a function of pH and vanadium molarity.
Whatever the synthesis method, 51V NMR spectra of VV solutions show that only two VV species, [H2V10O28]4- and [VO2]+, are observed in solution around pH ≈ 2 before gelation occurs. These species are known to be stable in aqueous solutions within a given range of pH (Figure 1729). They transform reversibly when changing the pH and should not behave as molecular precursors for the formation of a neutral vanadium oxide network. The protons of the decavanadic acid are too acidic to allow condensation, (29) Pope, M. T.; Dale, B. W. Q. Rev. Chem. Soc. 1968, 22, 527.
5302
Langmuir, Vol. 16, No. 12, 2000
whereas [VO2]+ cations have no V-OH bonds. We may then assume that some neutral species forms as an intermediate during the dissociation of the decavanadate anion into the [VO2]+ cation. This species readily gives rise to condensation and to the formation of an oxide network so that its lifetime is too short or its concentration is too small to be observed by 51V NMR. Such a neutral molecular precursor could be written as [VO(OH)3(OH2)2]0. One water molecule lies along the “z” axis, opposite to the VdO double bond, while the other one lies in the equatorial plane. Condensation can occur only within the xy plane leading to a two-dimensional vanadium oxide network. However, the x and y directions are not equivalent toward condensation. Olation occurs along the H2O-V-OH direction, while oxolation goes on along the HO-V-OH direction. The anisotropic ribbonlike shape of the particles then probably results from the difference between these two reactions in terms of thermodynamic balance. Vanadium pentoxide gels are then obtained. However, as long as this oxide is not completely dried via heating, some chemical reactions may still occur at the oxidewater interface. Polar water molecules are adsorbed and dissociated onto VV surface sites giving V-OH groups. In aqueous suspensions, the surface of vanadium oxide particles is then fully hydroxylated. Moreover owing to the strong polarizing power of V5+ ions, these V-OH groups exhibit acid properties and dissociate into water so that the pH of the aqueous medium remains very low (pH ≈ 2). Besides, the ribbons bear a large electrical charge of an estimated linear density of about 2e-/nm. The dissolution of crystalline V2O5 should also be due to similar reactions at the oxide/water interface. Bridging oxygens become protonated giving hydroxy bridges V-OH-V that should be weaker than V-O-V bonds. The structure of crystalline V2O5 is made of double chains of edge sharing [VO5] pyramids linked together via corners. Chemical bonds through corners should be destroyed faster than the bonds through edges. Their dissociation then leads to the formation of chain species similar to those formed via polymerization reactions. The fact that amorphous and crystalline V2O5 powders dissolve in water is quite significant. It clearly demonstrates that V2O5 gels are thermodynamically stable phases with respect to both the V2O5 amorphous and crystalline condensed states in the presence of water. IV.2. Why and How Do V2O5 Gels Form?: a Chronologic Description. In this section, we try to summarize, for the ion-exchange synthesis, the sequence of events during which V2O5 ribbons are formed and organize into nematic gels. When the pH of the effluent solution is lowered, the vanadium molecular species immediately start undergoing fast olation reactions that yield linear, probably flexible, polymers. Thus, charged molecular species, such as decavanadic acid [H2V10O28]4-, disappear, and neutral condensed moieties are formed. This process considerably reduces the electric conductivity of the suspension as confirmed by the conductivity measurements. Such polymers also probably experience strong van der Waals attractions since the Hamaker contrast between vanadium species and water is large.27,30 These attractions lead to aggregation of the polymer coils and therefore to the flocculation of the suspensions. After 2 h at room temperature, flocculation is at its maximum and the structure of the flocculate is self-similar with a fractal dimension df ≈ 1.85. A similar fractal dimension df ≈ 1.95 was observed for gels of iron hydroxide rigid (30) Israelachvili, J. H. N. Intermolecular and Surface Forces; Academic Press: New York, 1985.
Pelletier et al.
rods,32 but the SAXS signal of the V2O5 condensed moieties, at this stage of synthesis, is not that of rigid rods. In fact, the value df ≈ 1.85 compares quite well with the predictions of the cluster-cluster aggregation models31 that traditionally apply to such phenomena. In three dimensions, according to the aggregation process, these models predict a fractal dimension between 1.78 and 1.91. This therefore suggests that the flocculation results from a random and isotropic aggregation of vanadium pentoxide species. However, slower oxolation reactions start taking place and give their typical shape to the ribbons. Linear polymers are still reactive and start assembling into ribbons, probably like zippers. It should be here noted that previous cryo-TEM experiments have indeed shown ribbons frayed into individual threads.13 During this stage, olation reactions are probably still active as long as there are fair amounts of molecular species left in solution. The ribbon width grows from about 3 nm to its final value of about 25 nm 12 h after synthesis. At the same time, water molecules adsorb and dissociate at the oxide/water interface, giving rise to hydroxylated ribbons. These V-OH groups exhibit acid properties so that V2O5 ribbons acquire a negative electric charge, a phenomenon detected by conductivity measurements. Then, repulsive electrostatic interactions build up between ribbons and outweigh the van der Waals attractions leading to the dispersion of the flocculate. At this stage (about 12 h after synthesis), the suspension is now a dark red homogeneous gel comprised of entangled V2O5 ribbons dispersed in water and therefore it starts displaying viscoelastic properties evidenced by rheological measurements. In a subsequent evolution step 1 or 2 days long, owing to the electrostatic repulsions, the ribbons start developing positional short-range correlations. Finally, after several days, long-range orientational order sets in as evidenced by the appearance of permanent birefringence and nematic textures at rest. It is interesting to note that the two last steps happen even though the system has gelled. This demonstrates that the gelation of the suspension freezes only a few degrees of freedom and that the other degrees of freedom can still take a part in the orientational and positional rearrangements. Very anisotropic particles are indeed known to require only a few contacts per particle to build up gels that moreover have usually low fractal dimensions.31 IV.3. Self-Assembly of V2O5 Ribbons. The scenario described above still raises a number of open questions, for instance, that of the reversibility of the ribbon polymerization process. First, let us discuss the test that consists of adjusting the pH out of and back in the stability domain of the ribbons. The diffraction line observed with the precipitate thus produced can also be found in the diffraction pattern of the V2O5 flocs obtained by directly raising the ionic strength with a NaCl solution.33 The distance of ca. 1 nm is the typical stacking period of the ribbons in these flocs. The observation of this diffraction line therefore strongly suggests the self-assembly in solution of V2O5 ribbons as the pH was brought back around 2. Flocculation has then taken place as expected at this ionic strength. Now, if V2O5 ribbons can indeed be described as “living polymers”, we then expect to find a “critical aggregate concentration” (cac) that would be equivalent to the (31) Jullien, R.; Botet, R. Aggregation and Fractal Aggregates, World Scientific: River Edge, NJ, 1987. (32) (a) Philipse, A. P. Langmuir 1996, 12, 1127. (b) Philipse, A. P.; Wierenga, A. M. Langmuir 1998, 14, 49. (33) Pelletier, O.; Davidson, P.; Bourgaux, C.; Livage, J. Progress Colloid Polym. Sci. 1999, 112, 121.
Synthesis of Vanadium Pentoxide Nematic Gels
“critical micellar concentration” (cmc) of a solution of surfactants.34 Such a cac line is actually observed in the diagram shown in Figure 17. The light-scattering experiments also lead to the same conclusion: V2O5 aggregates can only be found at concentrations above 10-3 M (at pH ≈ 2), a value that may be interpreted as the cac. From a more quantitative point of view, the theory of living polymers predicts the simple scaling laws for the scattered light intensity, I(φ) ∼ (φ - φ*)1.5, and for the length ξ of the aggregates, ξ ∼ (φ - φ*)0.5.34,35 In the present case, a different behavior is observed. Figure 16 shows a dependence of the scattered intensity with an exponent 0.66, and ξ is approximately constant. This distinctive behavior most probably arises from the fact that these scaling laws are only valid for cylindrical neutral aggregates whereas vanadium molecular species aggregate into charged ribbons. A statistical physics study of the self-assembly of ribbonlike aggregates is presently in progress in our laboratories. V. Conclusion In this work, we have shown that the three different synthesis methods investigated all lead to similar liquidcrystalline gels. Most importantly, the V2O5 colloidal gel is actually the stable thermodynamic state with respect (34) Ben-Shaul A.; Gelbart, W. M. Micelles, Membranes, Microemulsions and Monolayers; Gelbart, W. M., Ben-Shaul A., Roux D., Eds; Springer-Verlag: New York, 1994. (35) Nallet, F. Private Communication.
Langmuir, Vol. 16, No. 12, 2000 5303
to both amorphous and crystalline V2O5 powders in the presence of water. On the basis of different experimental results, we have proposed the following scenario for the formation of these gels by the ion-exchange technique. Fast-olation condensation reactions lead to the formation of linear flexible V2O5 chains. These isotropic coils that only bear a small electric charge (if any) aggregate into a transient fractal structure owing to van der Waals attractive interactions. These linear polymers then assemble into ribbons through slower oxolation reactions and acquire a negative electric charge by the ionization of V-OH surface groups. Repulsive electrostatic interactions then appear and are responsible for both colloidal stability and liquid-crystalline order. Besides, even though the reversibility of the polymerization process is likely in the right conditions, the theories of the micellar selfassembly of surfactants do not describe it quantitatively. This problem is presently being examined in our laboratories, and we hope that the concepts developed in the field of complex fluids may also be relevant to describe some typical features of the new “chimie douce”. Acknowledgment. The authors are indebted to R. Botet, F. Nallet, and D. Roux for helpful and pleasant discussions and to J. Leng for help during the rheology experiments. Part of this work was done in the collaboration frame of the CNRS-GDR 690 “FORMES”. LA9914155
