Effect of Microgravity on the Growth of Silica Nanostructures

Langmuir 2000, 16, 10055-10060 10055 Effect of Microgravity on the Growth of Silica Nanostructures David D. Smith,* Laurent Sibille,† Raymond J. Cronise, Arlon J. Hunt,‡ Steven J. Oldenburg,§ Daniel Wolfe,§ and Naomi J. Halas§ Biotechnology Science Group, Microgravity Sciences and Applications Department, NASA Marshall Space Flight Center, SD-48, Huntsville, Alabama 35812 Received May 3, 2000. In Final Form: August 30, 2000 The effect of microgravity on the growth of silica nanoparticles via the sol-gel route is profound. In four different recipes that typically produce silica nanoparticles in unit gravity, low-density gel structures were instead formed in microgravity. These observations suggest that microgravity reduces the particle growth rate, allowing unincorporated species to form aggregates and ultimately gel. Hence microgravity favors the formation of more rarefied structures, providing a bias toward diffusion-limited aggregation. Moreover, these results add to evidence that the growth of silica nanoparticles occurs not simply through monomer addition but by the attachment of smaller primary particles and aggregates. Introduction The process of the formation of structures from coagulating ensembles is fundamentally important since the collective behavior of the constituents often results in dramatically improved or unusual mechanical, thermal, chemical, and optical properties. Examples include colloidal dispersions, sol-gels, thixotropic clays, liquid crystals, ferrofluids, and colloidal crystals, which span the order parameter from random to quasi-fractalline to highly ordered (yet tenuous) crystals. These structures are typically characterized by weak collective interactions that result in mechanical softness and sensitivity to thermal fluctuations.1 Hence their formation is highly sensitive to gravity-induced perturbations such as flowinduced shear, sedimentation, convection, and hydrodynamic pressure. In this study we examine the effect of microgravity on the formation of silica structures, specifically particles and gels. There is previous evidence that the formation of this “soft” matter is altered in microgravity. The first commercially available products from space (still available from NIST) were the monodisperse latex sphere standards of Vanderhoff et al., who demonstrated that emulsion polymerization of latexes in space resulted in better monodispersity, increased uniformity, and reduced coagulation.2 In addition it has been hypothesized that in unit gravity, buoyancy-driven fluid flows and sedimentation deleteriously perturb sol-gel substructures prior to gelation, and these perturbations are “frozen” into the resulting microstructure.3,4 Hence, sol-gel pores may be expected to be smaller, more uniform, and less rough when * To whom correspondence should be addressed: (256) 544-8762 (fax); (256) 544-7778 (phone); [email protected]. † Universities Space Research Association. ‡ Lawrence Berkeley National Laboratory, Energy and Environment Division. § Department of Electrical and Computer Engineering and Department of Chemistry, Rice University. (1) Rajagopalan R. In Ordering and Phase Transitions in Charged Colloids; Arora, A. K., Tata, B. V. R., Eds.; VCH Publishers: New York, 1996; p v. (2) Vanderhoff J. W.; El-Aasser, M. S.; Micale, F. J.; Sudol, E. D.; Tseng, C. M.; Sheu, H. R. Polym. Prepr. 1987, 28, 455; Mater. Res. Soc. Symp. Proc. 1987, 87, 213. (3) Noever, D. A. Microgravity Sci. Technol. 1994, 3, 14. formed in microgravity. Wessling et al.5 have reported that the formation of polyurethane foams in low gravity reduced the average void size, increased the pore roundness, and narrowed the standard deviation in pore size. Using a shadowgraphic technique, Leontjev et al.6 observed fluid flows due to convection and sedimentation during the formation of polyacrylamide gels and deduced from electrophoretic separations that the resulting pore size distributions were narrower for gels formed in microgravity. More recently Zhu et al.7,8 have shown that colloidal crystals of poly(methyl methacrylate) (PMMA) formed in microgravity are an order of magnitude larger and that completely different polymorphs can result. Okubo et al. have studied the kinetics of the formation of colloidal silica particles (both from aqueous silicates and from alkoxides) during parabolic aircraft flights (∼23 s of microgravity per parabola) using dynamic light scattering and transmission measurements and have found that the formation rate of silica particles is considerably reduced in microgravity.9 Stable silica nanoparticle dispersions may be formed either by polymerization of silicic acids in an aqueous system or through hydrolysis and condensation of silicon alkoxides (the sol-gel or Stöber route). These two routes are distinguished from one another by the mechanism of particle formation. Comparison of nuclear magnetic resonance (NMR) spectra obtained from Ludox, a commercial aqueous silicate, with acid-catalyzed silicon alkoxides has demonstrated that solutions of the former are dominated by monomers and tetrafunctionalized (4) Sibille, L.; Smith, D. D.; Cronise, R. J.; Noever, D. A.; Hunt, A. J. Proceedings of the Space Technology and Applications International Forum, 1st Conference on Commercial Development of Space, Albuquerque, NM, January 7-11, 1996; American Institute of Physics: Woodbury, N.Y., p 451. (5) Wessling, F. C.; McManus, S. P.; Mathews, J.; Patel, D. J. Spacecraft Rockets 1990, 27 (8), 324. (6) Leontjev, V. B.; Abdurakhmanov, Sh.D.; Levkovich, M. G. Proceedings of the AIAA Microgravity Science Symposium, Moscow, May 13-17, 1991; AIAA: Washington, DC, p 274. (7) Zhu, J. X.; Li, M.; Phan, S. E.; Russel, W. B.; Chaikin, P. M.; Rogers, R.; Meyer, M. 3rd Microgravity Fluid Physics Conference, 1996; American Institute of Physics: Woodbury, N.Y., p 397. (8) Zhu, J. X.; Li, M.; Rogers, R.; Meyer, W.; Ottewill, R. H.; STS-73 Space Shuttle Crew; Russel, W. B.; Chaikin P. M. Nature 1997, 387, 883. (9) Okubo, T.; Tsuchida, A.; Kobayashi, K.; Kuno, A.; Morita, T.; Fujishima, M.; Kohno, Y. Colloid Polym. Sci. 1999, 277, 474. 10.1021/la000643s CCC: $19.00 © 2000 American Chemical Society Published on Web 12/02/2000 10056 Langmuir, Vol. 16, No. 26, 2000 species, whereas di- and trifunctionalized species dominate for alkoxides.10 Moreover, comparison of small-angle X-ray scattering (SAXS) measurements of Ludox with acid- and base-catalyzed alkoxides shows that only aqueous silicate sols are uniform, whereas alkoxides generate fractal particles.11 As Brinker points out,10 these results illustrate that sols derived from aqueous silicates are fully hydrolyzed and grow by classical monomer addition resulting in uniform polymeric particles, whereas sols derived from silicon alkoxides grow through cluster aggregation and retain a fractal inner morphology even while the particles coarsen through surface tension reorganization. Two distinct regimes characterize particle growth: diffusion-limited, in which the transport of mass to the growing structure is the dominant limitation to growth, and reaction-limited, in which the efficiency of attachment limits the growth process. These two regimes are universal; the structures formed in one regime are strikingly similar even from vastly different material systems.12 In general, diffusion-limited conditions result in a reduction in the growth rate because there is a decrease in the frequency of collisions. Moreover, those species that do collide do not have the chance to attach in a manner that minimizes surface energy; i.e., exterior sites are favored. As a result, aggregates formed in diffusion-limited conditions are distinguished by lower fractal dimensions. Reactionlimited growth, on the other hand, is characterized by more compact structures. The sticking coefficient is small enough that species are able to sample attachment sites for energetically favorable configurations. In this study the formation of silica Stöber particles in microgravity is examined using transmission electron microscopy (TEM). Microgravity allows diffusion-limited conditions to persist in recipes that typically are reactionlimited, essentially expanding the parameter space under which diffusion-limited conditions prevail and providing us with a snapshot of the aggregation process that would not normally be accessible. In the case of silica nanostructures, microgravity provides a bias toward diffusionlimited cluster-cluster growth, altering structure formation and generally resulting in lower fractal dimensions. Methodology Four different recipes were developed in laboratory preparations using the Stöber method.13,14 Silica Stöber particles grown on the ground are of good quality in the range 100-700 nm. However, at certain precursor concentration ratios the particles are either polydisperse, bimodal, rough, or partially aggregated. Hence the recipes were carefully chosen to examine these “failure conditions”, essentially spanning a large portion of the parameter space over which Stöber particles may be produced. In these laboratory preparations, the alkoxide was freshly distilled and the samples were sealed in quartz glass under N2 to prevent water in the atmosphere from influencing the reaction. The first recipe (R1) was a control sample chosen to produce the best possible particles in terms of monodispersity and sphericity. The second recipe (R2) was chosen to produce the smallest Stöber particles, which tend to be rough, irregular, and less monodisperse. The third recipe (R3) was chosen to produce a bimodal size distribution, while the fourth recipe (R4) was chosen to produce large irregular (nonspherical) particles. TEM images of the (10) Brinker, C. J. In The Colloid Chemistry of Silica; Bergna, H. E., Ed.; American Chemical Society: Washington, D.C, 1994; p 361. (11) Schaefer, D. W.; Martin, J. E.; Keefer; K. D. In Physics of Finely Divided Matter; Bocarra, N., Daoud, M., Eds.; Springer-Verlag: Berlin, 1985; p 31. (12) Lin, M. Y.; Linday, H. M.; Weitz, D. A.; Ball, R. C.; Klein, R.; Meakin, P. Nature 1989, 339, 360. (13) Stöber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (14) Averitt, R. D.; Sarkar, D.; Halas, N. J. Phys. Rev. Lett. 1997, 78, 4217. Smith et al. Table 1. Stoichiometry of Silica Sol-Gel Recipes recipe TEOS (mL) ethanol (mL) water (mL) NH4OH (mL) R1 R2 R3 R4 0.140 0.153 0.446 0.335 4.21 4.59 3.81 3.58 n.a. n.a. 0.576 0.420 0.654 0.245 0.170 0.665 laboratory-grown particles are shown in Figure 1. The stoichiometry of each recipe is shown in Table 1. Note that only recipes R3 and R4 contain additional water (the ammonium hydroxide reagent also contains water). For the space-flight experiment, each 5 mL recipe was divided into two parts and loaded into coupled polyurethane (Hydex) syringes separated by a breakable Parafilm seal to enable mixing of the reactants. The first part consisted of tetraethyl orthosilicate (TEOS) and half the ethanol, while the second part consisted of water, ammonium hydroxide (30% NH3), and the remaining ethanol. Each batch was also divided into ground and space samples, which were stoichiometrically identical. The only difference in the growth conditions between the ground and space samples was the presence or absence of gravity. The designated space samples (12 total, 3 per recipe) were then placed in the Gelation of Sols, Applied Microgravity Research (GOSAMR) hardware, and activated aboard the space shuttle orbiter (Mission STS-95) after microgravity conditions had been established. The GOSAMR hardware, built by 3M Corporation and refurbished for this experiment, essentially consists of a set of modules, each of which contains eight coupled syringe cartridges. Upon activation a battery-powered motor-driven lead screw with a reversing actuator drives the syringe cartridges back and forth, which mixes the solutions after breaking the barrier seals between them. Upon return of the flight samples, an ultraprobe sonicator was used to obtain diluted suspensions of the samples in ethanol, and these were allowed to evaporate onto carbon-coated copper TEM grids. TEM images were recorded with a Philips JEOL 2010. Results and Discussion Visual inspection revealed that each of the space-grown samples had formed marginally coherent low-density gels, and that these gels coexisted in the syringe with regions of solvent. Ground-grown samples, on the other hand, remained in suspension. The resulting ground control particles were slightly different in size and polydispersity from the laboratory preparations (shown in Figure 1); presumably because for the ground-grown (and spacegrown) samples the TEOS was not freshly distilled, the samples could not be sealed under N2 and/or the reaction vessel was Hydex rather than quartz. TEM images for recipes R1 and R2 are shown in Figure 2. Note that there is a dramatic difference between the ground-grown silica structures and those grown in microgravity. Whereas growth in unit gravity produces Stöber particles, growth in microgravity favors loose gel structures. In fact for recipes R1 and R2 it was difficult to find any Stöber particles at all in the space-grown samples. However, all space-grown samples did form gels, and these gels had a common form and scale that was nearly recipe-independent. The particles making up the backbone of the gel were elongated with diameters of approximately 10 nm and lengths of about 50 nm. These gels are similar to the structures observed by Yoshida, who added a calcium salt and a sodium hydroxide solution to a polymerizing silicic acid sol and heated the mixture in an autoclave to effect nonuniform particle growth.15 For the recipes containing added water, R3 and R4, the particles making up the gel backbone were slightly wider and less elongated. In addition, a few large spheres coexisted with the more prevalent gel structure as shown in Figure 3. These spheres tended to be smaller (in some (15) Yoshida, A. In The Colloid Chemistry of Silica; Bergna, H. E., Ed.; American Chemical Society: Washington, D.C., 1994; p 51. Langmuir, Vol. 16, No. 26, 2000 10057 Microgravity and Silica Nanostructure Growth Figure 1. Four recipes R1-R4, prepared in laboratory glassware. cases only about half as large) and have much larger size distributions than Stöber particles formed on the ground. As shown by Bogush and Zukowski,16,17 the coexistence of large monodisperse spheres (50-250 nm) with smaller (∼10 nm) aggregating primary particles implies that the growth of silica Stöber particles does not occur by the classical nucleation and growth model, where nucleation occurs over a span of time limited by the decreasing availability of monomer. Rather Bogush and Zukowski deduced that nucleation of particles proceeds continuously throughout the reaction. The smaller primary particles form by the classical monomer addition growth mechanism and then aggregate because of their small size, until they become colloidally stable. Bogush and Zukoski propose that the resulting stable aggregates are the building blocks for the formation of Stöber particles, collecting smaller aggregates and newly formed particles as they are transported through the solution. Therefore, in this view, reaction-limited conditions must persist to maintain smooth spherical particles. The final structure then (16) Bogush, G. H.; Zukoski, C. F. In Proceedings of the 44th Annual Meeting of the Electron Microscopy Society of America; Bailey, G. W., Ed.; San Francisco Press: San Francisco, CA, 1986; p 846. (17) Bogush, G. H.; Zukoski, C. F. In Ultrastructure Processing of Advanced Ceramics; Mackenzie, J. D., Ulrich, D. R., Eds.; Wiley: New York, 1988; p 477. coarsens through surface tension reorganization to form the resulting Stöber particle. In contradiction to Bogush and Zukoski, Harris et al.18,19 and also van Blaaderen and Vrij20 have argued that if growth continued to occur through aggregation of subparticles, smooth spherical particles cannot result. In their view, Stöber particles initially grow by aggregation of subparticles but monomer addition later fills in the nonuniformities, resulting in a smooth particle. The irregular shape of smaller Stöber particles is a remnant of the aggregation mechanism not yet enveloped by the subsequent monomer growth. The size difference between the subparticles and the resulting Stöber particles certainly supports this view, since this difference is likely too small to yield a smooth surface even when reactionlimited conditions prevail. Furthermore, although only a few Stöber particles formed in microgravity, those that did form were smooth. Hence, the fact that smooth (18) Harris, M. T.; Basaran, O. A.; Byers, C. H. In Ultrastructure Processing of Advanced Ceramics; Mackenzie, J. D., Ulrich, D. R., Eds.; Wiley: New York, 1988; p 843. (19) Harris, M. T.; Brunson, R. R.; Byers, C. H. J. Non-Cryst. Solids 1990, 121, 397. (20) Van Blaaderen, A.; Vrij, A. In The Colloid Chemistry of Silica; Bergna, H. E., Ed.; American Chemical Society: Washington, D.C., 1994; p 83. 10058 Langmuir, Vol. 16, No. 26, 2000 Smith et al. Figure 2. TEM photos of silica Stöber particles formed from recipes R1 (A and B) and R2 (C and D) on the ground (A and C) and in microgravity (B and D). Low-density gels were always observed in microgravity, but only in recipes R3 and R4 could Stöber particles frequently be found. particles are obtained even in the absence of reactionlimited conditions further supports this view. Thus, it is not valid to consider silica sol-gels as either particulate or polymeric; they are both. The small ∼10 nm subparticles are primarily polymeric, representing the solubility limit of the molecule as a result of its increasing size and degree of cross-linking.21 As pointed out by Bailey and Mecartney,22 upon falling out of solution these polymeric precursors collapse, ultimately resulting in a compact particle due to continued hydrolysis and condensation. On the other hand, Stöber particles are at least to some extent particulate, initiated from stable “seeds” formed by aggregation of subparticles (or soluble microgels, in Bailey and Mecartney’s view), and later smoothed out due to continued addition of monomers (and other soluble species). Colloid stability therefore plays an important role in silica particle formation and morphology to the extent that it determines the size of the aggregates (or microgels) that constitute and augment the particle early in the (21) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (22) Bailey, J. K.; Mecartney, M. L. Colloids Surf. 1992, 63, 151. growth process. According to the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory,23 colloid stability is greatly affected by ionic strength, and the presence of water stabilizes these constitutive aggregates at smaller radii. Hence particle nucleation and growth is more readily established in the presence of water, which in part explains the greater population of Stöber particles in the watercontaining recipes R3 and R4. In addition, the kinetics of the formation of these aggregates depends on the degree to which the system is diffusion-limited or reaction-limited. Hence microgravity results in a decrease in the rate of formation of these constitutive aggregates due to a bias toward diffusion-limited conditions. Significant monomer depletion (into subparticles and smaller soluble species) then occurs faster than the time it takes for stable aggregates to form and initiate Stöber particle growth, leading to a preponderance of unstable subparticles and aggregates that eventually compose the loosely formed gel that we observe. Although colloid stability is not affected by microgravity directly, one may be tempted to think that microgravity (23) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of the Stability of Lyophobic Colloids; Elsevier: New York, 1948. Microgravity and Silica Nanostructure Growth Langmuir, Vol. 16, No. 26, 2000 10059 Figure 3. TEM photos of silica particles formed from recipes R3 (A and B) and R4 (C and D) on the ground (A and C) and in microgravity (B and D). The photos suggest that cluster aggregation is an important mechanism for the formation of Stöber particles; i.e., the basic building block for the formation of a particle is a smaller particle or aggregate. affects stability indirectly, in that reduced mass transport results in structures that are more extended and hence less stable, resulting in an increase in the critical radius for a stable seed. However, the fact that a few Stöber particles did form in microgravity (Figure 2) and that they were not larger in size (they were generally smaller) than those formed on the ground counters such a conclusion. Thus, microgravity probably does not increase appreciably the critical seed radius, but rather results in an increase in the average time it takes to form such a nucleus owing to decreased mass transport. Conceptually the resulting gel is not unlike an infinite Stöber particle where monomers and other soluble species are depleted before they can fill in the voids, resulting in a nonuniformity somewhat reminiscent of the fractal remnants formed early in the growth of (noninfinite) Stöber particles themselves. Since diffusion is present at any level of gravity, whereas buoyancy-driven convection is not, the effect of microgravity on Stöber particle growth can be understood most readily through its effect on diffusion. According to the Stokes-Einstein relation, the diffusion coefficient Dm for a particle of mass m undergoing Brownian diffusion is Dm ∝ 1 1 ) reff m1/d(g) (1) where reff is the effective radius of the particle and d(g) is the mass fractal dimension (which depends in some manner on gravitational acceleration g). Note that the diffusion coefficient only depends on gravity indirectly, through the dependence of the fractal dimension on gravitationally dependent transport mechanisms (convection and sedimentation). Various experiments and computer simulations have demonstrated that in general both diffusion-limited conditions and cluster-cluster aggregation produce more extended structures than reactionlimited monomer growth conditions, resulting in smaller fractal dimensions.24,25 Because in the Stöber route the dominant growth mechanism changes with substructure size, the bias toward diffusion-limited conditions obtained in microgravity leads to a larger decrease in d for larger substructures; i.e., the fractal dimension of the small subparticles (and smaller species) is not decreased ap(24) Meakin, P. In On Growth and Form; Stanley, H. E., Ostrowsky, N., Eds.; Martinus-Nijhoff: Boston, MA, 1986; p 111. (25) Meakin, P. Annu. Rev. Phys. Chem. 1988, 39, 237. 10060 Langmuir, Vol. 16, No. 26, 2000 preciably compared to that of the large aggregates. Accordingly, from eq 1 it can be seen that the diffusion coefficient is reduced in microgravity, and to a greater degree for larger substructures (aggregates). Hence the collision frequency of larger substructures, and consequently the Stöber particle induction rate, is reduced in microgravity. For Stöber particles that do manage to form, however, monomer addition again becomes important and the diffusion coefficient of incorporating species increases. The effect of microgravity is therefore to further increase the difference in the growth rates for different growth mechanisms. Equivalently, microgravity suppresses the coagulation of subparticles and aggregates more dramatically than it does their formation through addition of monomers and other small soluble species. Conclusion The importance of the aggregation of unstable clusters and subparticles to the formation and growth of silica Stöber particles makes the effect of microgravity on Stöber particle growth profound. Rather than simply retarding structure growth (in this case a silica sol) as would be expected for a singular growth mechanism, a pathway to an entirely different structure becomes available. Microgravity favors diffusion-limited conditions, which slows the formation of stable particle-forming aggregates. Monomers are consumed more by unstable subparticles and aggregates than by Stöber particles. Eventually cluster-cluster aggregation is the only remaining growth mechanism which yields more extended structures, leading to a decrease in the fractal dimension and ultimately to gel formation. We observed gels in microgravity at TEOS Smith et al. concentrations as low as 2.8%. These results suggest that microgravity favors the formation of more rarefied solgel structures, providing a bias toward diffusion-limited aggregation. Indeed the softest of matter may only be achievable in microgravity, where entropic reductions and perturbations to structure formation are minimized. Notably, our results are strikingly different than those of Vanderhoff et al., who achieved improved monodispersity in latex particles.2 The difference arises because their experiments involved emulsion polymerizations, which are stabilized in microgravity. Our future experiments in microgravity will investigate particle formation through emulsions, as well as seeded growth experiments to promote monodisperse silica particle growth. In addition, this study demonstrates that experiments in microgravity may help to clarify the mechanisms involved where routes to multiple structures are possible, i.e., to better resolve competing mechanisms. For example, a comparison of the growth of particles formed from aqueous silicates in microgravity with those formed from silicon alcoxides may provide insight into the relative importance of cluster addition mechanisms. Acknowledgment. The authors gratefully acknowledge discussions with W. Ford, M. Ayers, M. S. Paley, and D. B. Wurm, as well as the contribution of J. Glenn, who performed the microgravity experiments. Funding support was provided by the Space Product Development office at Marshall Space Flight Center, NASA, the Office of Naval Research, the National Science Foundation (ECS-9801707), and the Robert A. Welch Foundation. LA000643S