Langmuir 2007, 23, 9527-9530
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Pickering Stabilization as a Tool in the Fabrication of Complex Nanopatterned Silica Microcapsules Stefan A. F. Bon* and Tao Chen Department of Chemistry, UniVersity of Warwick, CoVentry CV4 7AL, U.K. ReceiVed June 6, 2007. In Final Form: August 1, 2007 Complex silica-based microcapsules with nanopatterned features were made using Pickering stabilization as a fabrication tool. A sequential two-step liquid-liquid interface-driven assembly process was employed using Laponite clay discs and Laponite armored polystyrene latex particles as solids to stabilize emulsion droplets on two different length scales. The discotic Laponite particles and poly(diethoxysiloxane) were used as silica sources. The ethoxy groups of the poly(diethoxysiloxane) were removed via a triethylamine-catalyzed interfacial hydrolysis and sol-gel reaction. The organic components were removed via a calcination step. The two-stage templating route provided siliceous microcapsules of which the capsule walls were decorated on either the outside or inside with nanocapsules composed of Laponite clay.
Introduction Efforts are increasingly being made to design synthetic routes for the fabrication of inorganic materials with hierarchical structure on both nano- and microlength scales. Inspiration is drawn from nature where biomineralization in organisms, such as diatoms, leads to fascinating skeleton morphologies of complexity and aesthetics that are beyond existing accomplishments of material science.1 Nature’s biomineralized structures inspire scientists from areas such as micromechanics,2 electronics, optics, and sensing applications.3 One specific type of material that has received tremendous interest is hollow inorganic capsules, with applications as lightweight fillers, thermal and electrical insulators, confined reactors, controlled delivery containers, and catalysts. Their synthesis frequently relies on the use of a template, such as polymer spheres,4 vesicles,5 and emulsion droplets.6 The inorganic material is created/deposited via a range of strategies, including sol-gel reactions,6 heterocoagulation/precipitation,7 and the closely related layer-by-layer deposition.4 Not many reports, however, have dealt with strategies to enhance the morphological complexity of these capsules by the creation of hierarchical nanofeatures onto, or in, their shells. These tactics that allow for the fabrication of more advanced inorganic materials would logically create features that are currently achieved only with/from nature’s biomineralized skeletons.2,3 Schu¨th et al. describe the synthesis of silica-based hollow spheres of approximately 50 µm diameter that consist of intergrown platelike silica particles that were generated in situ as a mesostructure under the influence of surfactant.8 Sommerdijk et al. described the formation of uniformly shaped silica capsules with a multilamellar shell structure containing wormlike mesopores of 3 to 4 nm diameter via an emulsion-based templating route using the polymeric surfactant poly((ethylene oxide)76* Corresponding www.stefanbon.eu.
author.
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(1) Sumper, M.; Brunner, E. AdV. Funct. Mater. 2006, 16, 17. (2) Gebeshuber, I. C.; Crawford, R. M. Proc. Inst. Mech. Eng., Part J: J. Eng. Tribol. 2006, 220, 787. (3) Bao, Z.; Weatherspoon, M. R.; Shian, S.; Cai, Y.; Graham, P. D.; Allan, S. M.; Ahmad, G.; Dickerson, M. B.; Church, B. C.; Kang, Z.; Abernathy, H. W.; Summers, C. J.; Liu, M.; Sandhage, K. H. Nature 2007, 446, 172. (4) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111-1114. (5) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267. (6) Zoldesi, C. I.; Imhof, A. AdV. Mater. 2005, 17, 924-928. (7) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 43294335. (8) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu¨th, F. Science 1996, 273, 768-771.
block-(propylene oxide)29-block-(ethylene oxide)76.9 Yang and co-workers used hollow polymer latex cages with transverse hydrophilic channels connected to an interior hydrophilic surface as a template to prepare composite hollow spheres with titania pillars protruding from the outer shell surface, eventually creating double-shelled hollow spheres.10 Chen et al. used a single-step emulsion templating method creating budded mesoporouss silica capsules with the protruding buds formed from lamellar vesicular mesostructures.11 Fujiwara et al. used a water-in-oil-in-water double emulsion with the key presence of water-soluble polymers in the dispersed water phase as a template to create silica hollow spheres with pores exceeding 100 nm in diameter.12 Chen and co-workers used cationic cetyltrimethylammonium micelles and microscale polystyrene latex particles as dual templates to control the raspberry morphology of silica hollow spheres.13 We are interested in the design of complex colloid-based structures, which we refer to as supracolloidal structures, by the interface-driven assembly of solid nanoparticles, a phenomenon more commonly referred to as Pickering stabilization.14,15 Solid particles can adhere very strongly to a soft interface (e.g., liquidliquid or liquid-gas). Pieranski demonstrated that this process in essence is guided by the interplay of interfacial tensions, leading to practical irreversible adhesion of the particles to the interface with energies that are several orders of magnitude greater than the thermal energy, kBT.16 Taking the view of a bottom-up fabrication process, soft interfaces can be used to direct the assembly process of solid particles, a strategy that can lead to the creation of fascinating supracolloidal structures. Velev et al. pioneered the synthesis of capsules with permeable structures via the assembly of polystyrene latex spheres at the interface of emulsion droplets,17 which were later named colloidosomes by Dinsmore and co-workers.18 Lo¨bmann et al. prepared highly porous “honeycomb-structured” hollow titania (9) Sun, Q.; Kooyman, P. J.; Grossmann, J. G.; Bomans, P. H. H.; Frederik, P. M.; Magusin, P. C. M. M.; Beelen, T. P. M.; van Santen, R. A.; Sommerdijk, N. A. J. M. AdV. Mater. 2003, 15, 1097-1100. (10) Yang, M.; Ma, J.; Niu, Z.; Dong, X.; Xu, H.; Meng, Z.; Jin, Z.; Lu, Y.; Hu, Z.; Yang, Z. AdV. Funct. Mater. 2005, 15, 1523-1528. (11) Wang, J.; Xiao, Q.; Zhou, H.; Sun, P.; Yuan, Z.; Li, B.; Ding, D.; Shi, A.-C.; Chen, T. AdV. Mater. 2006, 18, 3284-3288. (12) Fujiwara, M.; Shiokawa, K.; Sakakura, I.; Nakahara, Y. Nano Lett. 2006, 6, 2925-2928. (13) Wu, X.; Tian, Y.; Cui, Y.; Wei, L.; Wang, Q.; Chen, Y. J. Phys. Chem. C 2007, 111, 9704-9708. (14) Pickering, S. U. J. Chem. Soc. 1907, 91, 307. (15) Colver, P. J.; Chen, T.; Bon, S. A. F. Macromol. Symp. 2006, 245-246, 34-41. (16) Pieranski, P. Phys. ReV. Lett. 1980, 45, 569.
10.1021/la7016769 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/15/2007
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Experimental Section
Figure 1. Schematic illustrations of the two-step liquid-liquid interface-driven assembly process of fabricating siliceous supracolloidal microstructures with nanopatterned features. (a) Laponite clay nanodiscs dispersed in water. NaCl (0.1 M) and styrene/ hexadecane are added, and the mixture is sonicated and polymerized. (b) Clay-armored polystyrene nanospheres of ca. 200 nm diameter. (c) Nanospheres assembled on a micrometer-sized droplet of poly(diethoxysiloxane) (PDEOS)/hexadecane and PDEOS hydrolyzed with the aid of triethylamine (TEA) to yield the targeted siliceous supracolloidal structure. (d) Targeted supracolloidal silica-based structures obtained after the calcinations at 600 °C, thereby removing soft organic matter.
microspheres via the liquid-phase deposition of TiO2 onto Pickering emulsion droplets stabilized with polystyrene latex particles.19 Epoxy polymeric microrods were used as building blocks to produce hairy colloidosomes by Paunov, which were reinforced or “scaffolded” by gelation of the interior liquid phase.20 Russell et al. demonstrated the fabrication of capsules made by directed self-assembly of wild-type cowpea mosaic virus bionanoparticles and subsequent scaffolding via crosslinking with gluteraldehyde.21 We used Pickering-stabilized emulsion droplets as polymerization vessels for the preparation of TiO2 organic/ inorganic hollow microcapsules,22 raspberry-shaped interpenetrating polymer network reinforced microcapsules,23 and Laponite clay armored polymer nanospheres.24,25 Herein, we would like to show that Pickering stabilization can be used as a tool in the fabrication of complex nanopatterned silica-based microcapsules. The two-stage templating of emulsion droplets via an interface-driven assembly process using Laponite nanodiscs and poly(diethoxysiloxane) as silica sources leads to raspberry-shaped microcapsules covered with hollow nanospheres (Figure 1). (17) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2374. (18) Dinsmore, M. F. H. A. D.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2003, 298, 1006. (19) Strohm, M. S. H.; Bertling, J.; Lo¨bmann, P. J. Mater. Sci. 2003, 38, 1605. (20) Noble, P. F.; Cayre, O. J.; Alargova, R. G.; Velev, O. D.; Paunov, V. N. J. Am. Chem. Soc. 2004, 126, 8092. (21) Russell, J. T.; Lin, Y.; Boeker, A.; Su, L.; Carl, P.; Zettl, H.; He, J.; Sill, K.; Tangirala, R.; Emrick, T.; Littrell, K.; Thiyagarajan, P.; Cookson, D.; Fery, A.; Wang, Q.; Russell, T. P. Angew. Chem., Int. Ed. 2005, 44, 2420. (22) Chen, T.; Colver, P. J.; Bon, S. A. F. AdV. Mater. 2007, DOI:// adma.200602447. (23) Bon, S. A. F.; Cauvin, S.; Colver, P. J. Soft Matter 2007, 3, 194. (24) Cauvin, S.; Colver, P. J.; Bon, S. A. F. Macromolecules 2005, 38, 7887. (25) Bon, S. A. F.; Colver, P. J. Langmuir 2007, 23, 8316-8322.
Materials. Clay (Laponite RD, with a lateral diameter of ca. 25-35 nm and a thickness of 1 nm, Fclay ) 2.57 g cm-3), with an ideal chemical formula of [Si8(Mg5.45Li0.4)O20(OH)4]Na0.7, was kindly donated by Rockwood Additives Ltd. 2,2′-Azobis(2,4-dimethyl valeronitrile) (V-65) was donated by Wako Chemicals. Polydiethoxysiloxane (PDEOS) was purchased from Gelest. All other chemicals were of analytical grade and were obtained from Aldrich. Styrene was passed over a short column of basic alumina prior to use to remove the inhibitor. Equipment. A Branson 450 W digital sonifier was used for emulsification in the first assembly step (Figure 1). An Ultra Turrax T25 basic mixer (Ika Werka) was used for the emulsification in step two (Figure 1). FE-SEM measurements were performed on a Zeiss supra 55VP FEGSEM. Procedures. Preparation of Laponite Clay-Stabilized Latex Spheres (Step 1, Figure 1). In a typical experiment, 0.5 g of Laponite clay discs were dispersed in 100 mL of a 0.1 M aqueous solution of NaCl using sonication for 4 min at 70% amplitude with a 30 s wait every minute. V-65 (0.05 g) was dissolved in 5.0 g of styrene containing 4.0 wt % n-hexadecane as a hydrophobe; subsequently, this was mixed with the clay dispersion. A stable Pickering miniemulsion was generated via sonication for 6 min at 70% amplitude with a 30 s wait every minute. The resulting miniemulsion was poured into a 250 mL round-bottomed flask, which was sealed using a rubber seal, and N2 was bubbled through it for 20 min. The reaction mixture was polymerized at 51 °C overnight under gentle magnetic stirring. Preparation of Nanopatterned Silica Microcapsules (Step 2, Figure 1). In a typical experiment, 5.0 mL of the prepared Laponite armored nanosphere dispersion in water (containing approximately 0.26 g of nanospheres) diluted with 5.0 mL of deionized water was used as the aqueous phase. PDEOS (2.0 g) was dissolved in 2.0 g of n-hexadecane, and then this was mixed with the water phase. A stable Pickering emulsion was generated via agitation by the Ultra Turrax at 22 000 rpm for 3 min. One droplet of triethylamine (TEA) was added to the resulting emulsion. The mixture was put on a rolling device at room temperature for about 3 days.
Results and Discussion We performed a sequential two-step liquid-liquid interfacedriven assembly process for the fabrication of our target structures (Figure 1). The first step operates on the nanoscale and comprises the creation of Laponite nanodisc-stabilized submicrometer droplets of oil, in the present case, styrene. To provide a scaffold and warrant stability throughout the second step of our assembly process, these nanodroplets, created via sonication, were solidified using a free radical Pickering miniemulsion polymerization process.24,25 Stable polystyrene latexes armored with Laponite clay discs were obtained with an average particle diameter of approximately 200 nm according to dynamic light scattering measurements. FE-SEM analysis clearly shows that Laponite clay discs are present on the surface of the polymer particles (Figure 2a). Our recently reported detailed study on Pickering miniemulsion polymerization using Laponite clay discs as a stabilizer clearly demonstrated that the clay discs predominantly lie “flat” on the surface of the latex particles.25 The second step progresses on the microscale and involves the organization of the armored polystyrene nanospheres onto the surface of micrometer-sized oil droplets that contain poly(diethoxysiloxane) (PDEOS) as the silica precursor. The Pickering-stabilized micrometer-sized droplets were created using an Ultra Turrax mixer. Typical oil-phase mixtures consisted of a PDEOS/hexadecane weight ratio of 1:1 (average F ≈ 0.92 g cm-3). An oil/water weight ratio of 4:10 was used with the Laponite armored latex (average F ≈ 1.19 g cm-3, average dpart ) 200 nm). Upon addition of base (i.e., triethylamine) to the continuous aqueous phase, the reactive -Si(OCH2CH3) groups
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Figure 2. FE-SEM images of self-assembled silica-based supracolloidal structures. (a) Laponite clay armored polystyrene latex spheres (scale bar ) 200 nm). (b) Supracolloidal microstructures obtained after a two-step assembly process (scale bar ) 2 µm). (c) Microtomed sections of single microcapsules clearly showing a rigid silica wall with a thickness of approximately 200 nm, covered with essentially a monolayer of our Laponite armored latex particles on the surface of a microcapsule shell (scale bar left ) 400 nm, right ) 200 nm).
in poly(diethoxysiloxane) near the droplets’ interface hydrolyze, thereby reacting with themselves and with the Laponite clay armored nanospheres. This yields our targeted supracolloidal silica-based microcapsules with diameters in the range of 5-15 µm (Figure 2b). As a crude estimate, the diameter of the assembled structures can be predicted from the following expression (Supporting Information)22,23
doil ) πCoV
( )( )
moil Fpart d mpart Foil part
(1)
in which CoV expresses the ratio of the effective area covered by the Laponite armored latex spheres and the total area of the oil droplet, mpart and moil are the quantities of Pickering latex particles and oil used, Fpart and Foil are the densities of the building blocks and the oil phase, and dpart and doil are the diameters of the latex building blocks and the resulting Pickering-stabilized droplets, respectively. The assembly of the clay armored polystyrene particles onto the microdroplets results in nearly fully covered supracolloidal structures, as seen from Figure 2b. The observed sizes ranging from 5 to 15 µm are in rough agreement with the calculated average value for the diameter using eq 1 (i.e., doil ) 6.5 µm). The observed broadness of the particle size distribution is a direct cause of our emulsification process. There is no evidence of coalescence and collapsed structures. We also noticed that nonspherical microstructures were obtained using our emulsification process (Supporting Information). The incidental and singular creation of nonspherical particles via Pickering stabilizers has been reported.26-29 The hydrolysis of PDEOS forming cross-linked polysiloxane is localized at the interface of the assembled structures. This generates a silica-based shell, which enhances the robustness of (26) Subramaniam, A. B.; Abkarian, M.; Mahadevan, L.; Stone, H. A. Nature 2005, 438, 930. (27) Subramaniam, A. B.; Mejean, C.; Abkarian, M.; Stone, H. A. Langmuir 2006, 22, 5986. (28) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622. (29) Binks, B. P.; Clint, J. H.; Mackenzie, G.; Simcock, C.; Whitby, C. P. Langmuir 2005, 21, 8161.
Figure 3. FE-SEM images of complex nanopatterned silica microstructures. (a) siliceous microstructures with clay nanocapsules on the outer surface after calcination at 600 °C for 6 h (scale bar ) 1 µm). (b) Siliceous microstructures with clay nanocapsules on the inner surface after calcination at 600 °C for 6 h (scale bar ) 2 µm). (c) Microtomed section of a single microcapsule with clay nanocapsules on the outside (scale bar ) 400 nm). (d) Microtomed section of a single microcapsule with clay nanocapsules on the inside (scale bar ) 400 nm).
the microcapsules. Indeed, no broken microcapsules were observed in FE-SEM analysis. Cross-sectional analysis of microtomed supracolloidal structures shows that hollow capsules are indeed obtained and have a rigid silica wall with a thickness of approximately 200 nm, covered with essentially a monolayer of our Laponite armored latex particles (Figure 2c). A final calcination step at high temperatures (600 °C) was undertaken to remove soft organic material to obtain the siliceous skeleton exclusively. Corresponding FE-SEM studies of the capsules annealed at 600 °C for 6 h showed the structural integrity of the microcapsules and that coalescence and cracking of the individual supracolloidal structure did not take place (Figure 3a). After microtoming, it clearly shows (Figure 3c) that the nanospheres at the surface of the microcapsules are hollow, as expected. Note that when we fabricated our microcapsules using non-armored latex particles in the second step, these nanopatterned features were not present (Supporting Information) We tried to invert the second assembly step, meaning that now the oil phase forms the continuous phase. We adjusted the ratio of oil/water from 4:10 to 8:3 in order to prepare an inverse Pickering emulsion. The obtained microcapsules now should have the Laponite armored polystyrene particles on the inside of the capsule wall. FE-SEM images of these structures annealed at 600 °C for 6 h are shown in Figure 3b,d. It can be clearly seen that nanocapsules indeed are present on the inner surface of microcapsules, and it should be noted that some microcapsules were fused together as a result of the sol-gel process.
Conclusions We showed that complex silica-based microcapsules with nanopatterned features can be obtained using Pickering stabilization as a fabrication tool. We demonstrated this using a sequential two-step liquid-liquid interface-driven assembly process. This provided siliceous microcapsules of which the silica capsule wall was decorated on either the outside or the inside with nanocapsules composed of Laponite clay. We envisage that our Pickering fabrication strategy can be widely employed to create complex inorganic materials with hierarchical structures at their interface. Acknowledgment. We thank Steve York (Physics Department) and Catheline Colard for their help with FE-SEM imaging,
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Richard Cave for Figure 1, and Patrick Colver for the synthesis of the Laponite armored polystyrene latex nanospheres. Supporting Information Available: Low-magnification FESEM images to support Figures 2b and 3a,b, FE-SEM images of the
Letters fabrication of siliceous microcapsules using non-armored latex particles as the Pickering stabilizer, and the derivation of eq 1. This material is available free of charge via the Internet at http://pubs.acs.org. LA7016769
