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Microporous Vanadium Pentaoxide. 2. Making Solids from Colloidal Microemulsions Sameer D. Desai and E. L. Cussler* Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received March 5, 1997. In Final Form: October 28, 1997X Vanadyl isopropoxide can be polymerized in microemulsions of Aerosol OT containing water within reverse micelles. After a fast hydrolysis, vanadium pentaoxide forms ribbons in an intramicellar process taking about 100 s. These ribbons then aggregate by an intermicellar agglomeration taking 104 s. Drying these clusters in the presence of the surfactant leads to solids with surface areas around 100 m2/g.
Introduction The key steps in making microporous solids are (i) creating the interfacial area and (ii) retaining the surface area upon drying. These key steps take place differently in two limiting cases. In the first case, exemplified by the packing of particles, these steps are sequential: the particle size synthesized is directly related to the surface area retained. In the second case, typified by the drying of fractal objects, these two process occur simultaneously. In this second case, factors like drying speeds, fractal dimensions, and reaction rates affect the final structure. This second scenario, although more complex in nature, can provide more ways to manipulate the final structure. We have used this second case to make microporous vanadium pentaoxide. When this oxide has a surface area of 100 m2/g, it has potential in catalysis and rechargeable batteries.1,2 In our previous paper, we detailed the kinetics of the reaction between vanadyl isopropoxide and water in a reverse microemulsion stabilized by the surfactant sodium dioctylsulfosuccinate (Aerosol OT).3 This reaction, complete within 100 ms, is the first step in a process for making the microporous vanadium pentaoxide. In this paper, we discuss the kinetics of subsequent steps for making this microporous material. The next step, the intramicellar conversion of the ions formed in the first step into ribbons, takes a few minutes. The following step, the intermicellar agglomeration of these ribbons into clusters, takes around an hour. These steps were studied with small-angle X-ray scattering and dynamic light scattering. The final step, drying these clusters, was analyzed with nitrogen adsorption. Before we describe the experimental details, we want to discuss qualitative aspects of the experimental results. The key experiment is the measurement by dynamic light scattering of the particle size vs time. This experiment shows a particle size which initially does not change with time, giving a time lag, and then which increases nonlinearly with time, giving a particle growth rate. We believe that the time lag is associated with the intramicellar particle growth. The lag time is inversely related to this intramicellar reaction rate, so this rate should be proportional to the alkoxide concentration. Thus higher alkoxide concentrations should give shorter lag times. X Abstract published in Advance ACS Abstracts, December 15, 1997.
(1) Livage, J. Chem. Mater. 1991, 3, 578. (2) Atlung, S.; West, K. J. Power Sources 1989, 26, 139. (3) Desai, Sameer D.; Cussler, E. L. Langmuir 1997, 13, 1498.
In the same spirit, we expect that the growth rate is associated with intermicellar agglomeration of colloidal V2O5. Many such agglomerations are second order, suggesting that
dN/dt ) -KN2
(1)
1 ) Kt N0
(2)
where N is the number concentration of clusters at any time t and K is the coagulation kernel, a rate coefficient of the aggregation.4 From a mass balance, we expect that
N ∝ [A]/Rd
(3)
were [A] is the total alkoxide concentration, a constant, R is the cluster size, and d is the fractal dimension. Thus
Rd ∝ [A]t
(4)
The growth rate should be proportional to the alkoxide concentration and suggests values for the fractal dimension. We will test these ideas with the experiments described below. More specifically, we outline the synthetic method for making V2O5 and the experimental techniques for its characterization. Then we present information regarding the growth of V2O5 clusters determined with these techniques. Next, we detail the washing and drying steps that lead to the final solid. Finally, we present data on the pore size distributions and surface areas as a function of washing and drying conditions. Experimental Section Sodium dioctyl sulfosuccinate (Aerosol OT, Sigma, 99%) and toluene (EM Science) were used as received. The vanadyl isopropoxide (Gelest) was distilled once; water was distilled twice. Two solutions were prepared, mixed, and placed into a cuvette for analysis. The first solution, the reverse microemulsion, contained toluene, water, and the surfactant. The second solution, prepared in a nitrogen-filled glovebox, combined 140200 µL of vanadyl isopropoxide with about 11 g of toluene. About 6 g of the microemulsion was filtered through a 0.45 µm filter and stirred in a reaction vial. About 2.5 g of the alkoxide solution was quickly injected with a syringe capped with a 0.45 µm filter. The solution immediately turned orange; 3 mL was quickly (4) Brinker, C. J.; Scherer, G. W. In Sol-Gel Science; Academic Press; San Diego, CA, 1990.
S0743-7463(97)00246-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/20/1998
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Figure 1. Three routes to produce V2O5 solids from colloids. In route 1, the surfactant is removed before the material is dried. In route 2, the surfactant is present during film formation. In route 3, the material is dried in water. withdrawn, filtered through a 3 µm filter, and placed in a glass cuvette for analysis. The analysis was either by electron microscopy, X-ray scattering, or dynamic light scattering, each measured as a function of time. Electron micrographs were taken with a Hitachi S-900 field emission scanning electron microscope on samples prepared by coating the dried vanadium pentaoxide with 10 Å of platinum. Small-angle X-ray scattering (SAXS) measurements were performed with a modified Kratky camera equipped with a short flight tube and movable beam stop.5 Before each measurement, the excess surfactant was removed by three sequential centrifugations at 15 000 rpm and redispersion in either toluene or acetone. Finally, the vanadium pentaoxide was suspended in toluene, placed in a glass capillary, and sealed for SAXS. Dynamic light scattering was performed on a Coulter Model N4SD sub-micro particle size analyzer. In these measurements, excess surfactant gives no significant signal, and so did not need to be removed. However, the particle sizes reported are apparent values, determined using the Stokes-Einstein equation based on the viscosity of the continuous phase, toluene. In fact, the microemulsions had measured viscosities of as much as eight times that amount. We have used the constant toluene viscosity because this is the basis of the Stokes-Einstein equation, but we want to emphasize the approximation that this implies. The suspensions were made into microporous solids by the sequence of washing and drying steps outlined in Figure 1. In route 1, the colloidal suspension was centrifuged at 15 000 rpm, washed with acetone to remove the surfactant, and redispersed in acetone by sonication. After this procedure was repeated three times, the acetone was exchanged with toluene, and the suspension was dried under vacuum. In route 2, the steps were similar but their sequence was changed. After centrifugation, the colloid was washed and redispersed in toluene instead of acetone. In this case, the colloid still contained surfactant: the vanadium to surfactant ratio was about 0.5 (wt/wt). The suspension was cast as a thin film and vacuum dried. The dried film was then extracted with acetone to remove the surfactant, the acetone was exchanged with toluene, and the film was dried a second time. Route 3 was the same as route 1 except that before drying, the acetone was exchanged with water instead of toluene. Surface area and pore size distributions of these dried materials were measured on a porous materials automated BET sorptometer controlled by an IBM PC. The highest pressure analyzed was 0.3 times the critical pressure.
Making microporous vanadium pentaoxide can be idealized as four sequential steps. The first step, making vanadium polyions, was detailed in the earlier paper.3 The second and third steps produce suspensions of clustered V2O5 ribbons. These steps, which usually are finished in about an hour, are described first in this section. The final drying step, which can take longer, is described subsequently. Cluster Growth. Vanadium pentaoxide clusters were made by mixing an Aerosol OT microemulsion of water in toluene with a solution of vanadyl isopropoxide in toluene. The reaction mixture had a limited range of useful compositions at room temperature. The highest surfactant concentration (40%) was limited by adequate mixing, i.e., by the viscosity. The lowest concentrations of surfactant (24%) and of water (5%) were dictated by the phase diagram of the mixture.6 The lowest alkoxide concentration (0.37%) was due to the stability of precursor solution, while the upper concentration (0.53%) was dictated by the maximum size which could be measured by the particle size analyzer. Typical small-angle X-ray scattering measurements obtained from the reacting microemulsion are shown in Figure 2. The Porod slope equaled (-2.0) 5 min after mixing the reagent solutions. Two-dimensional sheets and random coil polymers both have a fractal dimension of 2.0, and hence a Porod slope of (-2.0).4 Although vanadium pentaoxide can form long ribbons that wind into coils, the persistence length of these ribbons is much
(5) Kaler, E. W. Surfactant Microstructures. Ph.D. Thesis, University of Minnesota, 1982.
(6) Candau, F.; Leong,Y. S.; Pouyet, G.; Candau, S. J. Colloid Interface Sci. 1984, 101, 93.
Figure 2. Scattering profiles of microemulsion base vanadia colloids. These plots give the Porod slope and thus fractal dimension of the colloids 5 min (a) and 3 h (b) after the reaction begins. The slope is -2.0 in (a) and -2.5 in (b). The original solution contained 4.9% water, 36% surfactant, and 0.55% alkoxide, all in weight percents. The remainder of the solution is toluene.
Results and Discussion
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Figure 3. Ribbon structure of vanadia colloids. Scanning electron micrographs show microemulsion-based vanadium pentaoxide displays a ribbon structure. The solution concentration is given in Figure 2. The reaction mixture was extracted in acetone before drying for scanning electron microscopy.
larger than the size scale probed by SAXS.7 The measured fractal dimension seems to be a reflection of the two dimensional character of the ribbons formed within the first few minutes of the reaction. Electron micrographs of this intermediate material show ribbon-like structures, in agreement with the scattering data (Figure 3). After 3 h, the Porod slope increases to about 2.5, and hence the fractal dimension rises. Two possible reasons for this effect are ribbon stacking to form dense structures or ribbon clustering to form looser flocs. The ribbon stacking echos similar aggregation of silica spheres, which can show a shift in Porod slope corresponding to the primary particle size. However, computer simulations have shown that both kinetic and diffusion limited aggregation process result in materials with fractal dimensions in this range.8-11 As a result, this changed slope does not by itself permit a definitive conclusion. The kinetics of cluster growth were also studied by dynamic light scattering to further clarify the aggregation mechanism. The measured particle size is plotted vs time in Figure 4. The plot of diameter squared vs time shows a lag time followed by a linear variation. This lag time decreases as the initial alkoxide concentration is increased, as shown in Figure 5. This decrease is roughly proportional to the square of the alkoxide concentration. The growth rates, as determined by the slopes of the diameter (7) Davidson, P.; Bourgaux,C.; Schouttleten, L.; Sergot, P.; Williams, C.; Livage, J. J. Phys. II 1995, 5, 1577. (8) Witten, T. A.; Sander, L. M. Phys. Rev. Lett. 1981, 47, 1400. (9) Meakin, P. Phys. Rev. Lett. 1983, 51, 1119. (10) Keefer, K. D. In Better Ceramics through Chemistry II; Brinker, C. J., Coark, D. E., Ulrich, D. R., Eds.; Materials Research Society: Pittsburgh, PA, 1986, p 295. (11) Martin, J. E.; Hurd, A. J. J. Appl. Crystallogr., 1987, 20, 61.
Figure 4. Growth kinetics of V2O5 in a microemulsion. The raw data in (a) are replotted in (b) and (c). The linear variation of the diameter squared vs time suggests the growing clusters have a fractal dimension of 2. The solution concentrations are the same as Figure 2, except the alkoxide concentration was 0.44%.
squared vs time plots, vary linearly with the initial alkoxide concentration, as shown in Figures 5 and 6. These lag times in Figures 4-6 may be due to intermicellar monomer additions or to intramicellar particle growth. In our earlier work,3 we showed with experiments using a stopped flow reactor that monomer addition was fast, complete in seconds. This is much faster than the observed lag times. The data in Figures 4-6 are more consistent with intramicellar particle growth followed by intermicellar particle agglomeration. The inverse second-order dependence of the lag time on alkoxide concentration seems analogous to the second-order dependence seen in the condensation of silica procursors.4 Indeed, in related studies with the stopped flow reactor,3,12 we observed evidence of chemical changes during this lag time when the particle size is constant. This supports the association of the lag time with an intramicellar process. (12) Desai, Sameer D. Microemulsion Guided Synthesis of Microporous Vanadium Pentoxide. Ph.D. Thesis, University of Minnesota, 1996.
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Figure 5. Effect of initial alkoxide concentration on growth rate. The data show two separate steps, a lag time and a growth stage. The initial alkoxide concentration is inversely proportional to the lag time and directly proportional to the growth rate. The alkoxide concentrations from top to bottom are 0.55, 0.44, and 0.38, all in weight percent. Other concentrations are given in Figure 2.
Desai and Cussler
Figure 7. Effect of water concentration on particle growth rate. When the water concentration in the initial microemulsion is doubled, the growth rate is doubled. These solutions contained 0.55% alkoxide and 36% surfactant.
Figure 6. Effect of initial alkoxide concentration on the growth rate. The growth rate, i.e., the slope of the diameter squared vs time plot, is linearly proportional to the initial alkoxide concentration. These solutions contained 4.9% water and 36% surfactant.
The particle growth rates measured in these experiments imply intermicellar agglomeration of fractal clusters. The linear variation of the diameter squared vs time suggests a fractal dimension close to 2, consistent with the SAXS data and with eq 4. The linear variation of growth rate with alkoxide concentration in Figures 5 and 6 is also consistent with this equation. At the same time, all the experimental evidence is not explained by this simple picture. In particular, the growth rate increases with increased water concentration, as shown by the results in Figure 7. This is inconsistent with our experiments at short times described earlier.3 These earlier experiments imply that for the concentrated microemulsions used here, increasing the water concentration increases the number of micelles. Increasing the number of micelles would seem to imply increasing micellar agglomeration. However, increasing the number of micelles should not affect the concentration of ribbons and hence the rate constant of this second-order reaction. That such an effect does occur indicates chemical complexities not included in our analysis. Cluster Drying. We now turn from studies of cluster growth kinetics to measurements of cluster properties
Figure 8. Pore size distributions of microporous vanadium pentaoxide. The average pore size of materials made through route 1 (a) is 60 nm whereas the route 2 (b) results in materials with an average pore size of 6 nm. Solution concentrations are given in Figure 2.
after drying. Table 1 summarizes the microstructure of the materials made by the three routes shown in Figure 1. The most striking feature in Table 1 is the effect of the surfactant. If the colloid is washed with acetone before drying (route 1), the film contains about 5 g of surfactant per 100 g vanadium pentaoxide. If the colloid is washed only with toluene before drying (route 2), the resulting film can have 50 g of surfactant per 100 g of vanadium pentaoxide. The microstructures of these solids have sharp similarities and differences. The surface areas in both the route 1 and 2 materials are close to 80 m2/g. However, the porosity of the route 1 material is a factor of 2 higher than the porosity of the route 2 material. The (13) Hirashima, H.; Gengyo, M.; Kojima, C.; Imai, H. J. Non-Crystal. Solids 1995, 186, 54.
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Table 1. Microstructural Properties of Vanadium Pentoxide Materials route
AOT/V2O5
area (m2/g)
porositya
pore size (nm)
XRD
1. film made after surfactant removed 2. film made before surfactant removed 3. dried in water after surfactant removed
0.05 0.50 0.05
87 83
