LETTER pubs.acs.org/Langmuir
Synthesis of Hollow Silica Spheres with Hierarchical Shell Structure by the Dual Action of Liquid Indium Microbeads in VaporLiquidSolid Growth Jian-Tao Wang,†,§,|| Hui Wang,†,|| Xue-Mei Ou,† Chun-Sing Lee,‡ and Xiao-Hong Zhang*,† †
Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Center of Super-Diamond and Advanced Film (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China § Graduate University of the Chinese Academy of Sciences, Beijing 100049, China
bS Supporting Information ABSTRACT: Geometry-based adhesion arising from hierarchical surface structure enables microspheres to adhere to cells strongly, which is essential for inorganic microcapsules that function as drug delivery or diagnostic imaging agents. However, constructing a hierarchical structure on the outer shell of the products via the current microcapsule synthesis method is difficult. This work presents a novel approach to fabricating hollow microspheres with a hierarchical shell structure through the vaporliquidsolid (VLS) process in which liquid indium droplets act as both templates for the formation of silica capsules and catalysts for the growth of hierarchical shell structure. This hierarchical shell structure offers the hollow microsphere an enhanced geometry-based adhesion. The results provide a facile method for fabricating hollow spheres and enriching their function through tailoring the geometry of their outer shells.
’ INTRODUCTION Hollow-structured micro- and nanoscopic capsules are important in materials science for their potential applications as controlled release carriers for drug delivery and diagnostic imaging agents.110 Strong surface adhesion is critical for these applications. For example, with strong adhesion, drug-carrying particles could be intensively attached to mucosal surface, which results in a prolonged residence time and increases the drug concentration and absorption at the site of action. In contrast, weak adhesion might cause the drug vehicle to fall off the target tissue because of the cell turnover. 1114 Chemically modifying the surfaces of spheres with antigenantibody or lectin conjugates could improve their adhesion.13,15 In spite of that, the organic compounds might be easily damaged or degraded by the mucous layer, which would increase the potential toxicity and immunogenicity.16 Inspired by geckos, geometry-based adhesion is a promising property arising from hierarchical surface structures.1719 Recent research reveals that microspheres coated with silicon nanowires show significantly increased adhesion, which could be a potential next-generation adhesive drug-delivery system with strong stability even in degradable environments, such as the gastrointestinal system.14 Various hollow microspheres can be fabricated through sacrificial-core methods.2023 In a typical procedure, layers of various materials are coated onto the surfaces of polymeric or inorganic cores by controlled surface precipitation. The template r 2011 American Chemical Society
cores are subsequently removed by dissolution in the proper solvents or calcination in air to generate inorganic hollow spheres with porous or patterned shell structure. However, it is difficult to synthesize hollow microspheres with hierarchical shell structure and strong geometry-based adhesion by this method. In this letter, we report a facile method for the controlled synthesis of hollow microspheres with hierarchical shell structures composed of Si/SiO2 nanowire arrays grown uniformly on a patterned SiO2 spherical shell during a simple VLS process with the liquid indium microbead acting as both the template and catalyst.
’ EXPERIMENTAL SECTION Chemicals. Silicon monoxide (SiO) powder, (325 mesh, 99.9%) was purchased from Aldrich Chemistry. Indium power (>99.9%) was purchased from the Institute of Fine Chemicals (Tianjin, PR China). Synthesis of Hierarchically Structured Hollow Microspheres. The experiment was performed in a tube furnace at high temperature. After being loaded with a mixture of 0.5 g of SiO power and 0.5 g of indium power, an alumina boat was placed at the center of an alumina tube filled with N2 (with an inside diameter of 37.4 mm and a length of 1200 mm). The system was pumped to 100 Pa and then heated to 1350 °C at a rate of Received: April 20, 2011 Revised: June 4, 2011 Published: June 09, 2011 7996
dx.doi.org/10.1021/la201451x | Langmuir 2011, 27, 7996–7999
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Figure 1. Illustration of procedures for preparing hollow SiO2 spheres with hierarchical structure. 10 °C/min. After remaining at this temperature for 300 min, the system was then cooled naturally to room temperature. Gray products were obtained on a silicon wafer placed approximately 1821 cm downstream from the alumina boat. The temperature at the deposition zone was measured to be 700900 °C with a thermocouple. Characterization. As-deposited products were studied with scanning electron microscopy (SEM, Hitachi S-4300). The deposition products were dispersed in ethanol, and a drop of the dispersion was placed on a carboncoated copper grid for the measurement with transmission electron microscopy (TEM, JEOL JEM-2100, operated at 200 kV). Adhesion Test. The adhesion strength of the spheres was measured under an optical microscope (Leica DM 2500 M) with a brass cantilever (S1).
’ RESULTS AND DISCUSSION In this synthesis process, first microspheres of liquid indium were formed and then acted as templates to arrange Fibonacci number patterns of SiO2 nanoparticles on their surfaces through stress-driven self-assembly. After the miscrospheres were completely covered with SiO2 nanoparticles (Figure 1AC), the liquid indium template was removed through evaporation and hollow SiO2 spheres comprising patterned SiO2 nanoparticles were produced. Then, indium vapor recondensed on the top of these SiO2 nanoparticles and acted as catalyst droplets to lead the growth of silicon nanowires and the formation of hierarchically structured hollow microspheres (Figure 1D,E). These microspheres show much stronger geometry-based adhesion than similarly sized SiO2 nanoparticles without nanowires on the surface. The hollow microspheres were synthesized in a tube furnace by thermal evaporation of a mixture of SiO and indium powder at 1350 °C under 100 Pa. The amount of indium powder in the evaporation source was carefully controlled to ensure that the indium source would be exhausted during the experiment. As a result of the source control, the growth process could be divided into sequential two steps: (1) the patterning of SiO2 nanoparticles on the indium microbead and (2) the growth of Si/SiO2 nanowires catalyzed by indium nanoparticles. The first step of the hollow SiO2 hierarchical structure fabrication involved the formation of indium microbeads and the self-assembly of SiO2 spheres on their surfaces. During the first step, the indium was first evaporated from the source because of its low melting point and then formed a liquid sphere with a size of several micrometers to be suspended in the vapor
Figure 2. SEM images and EDX elemental mappings revealing the patterning process of SiO2 nanoparticles on the indium microsphere. (A) Pentagonal patterns and (B) hexagonal patterns of SiO2 nanoparticles formed on the indium microsphere. (C) An indium microsphere fully wrapped by SiO2 nanoparticles. (D) A broken microsphere showing the hollow interior. EDX elemental mappings for (E) the outside and (F) the inside of two microspheres.
(Figure 1A and S2).24 Then, SiO was adsorbed by the liquid indium sphere and decomposed into Si and SiO2. Si was resultantly trapped in the indium microsphere whereas SiO2 nanoparticles floated on its surface because of their different solubility in liquid indium (Figure 2A and S3). The as-formed SiO2 nanoparticles were patterned by the surface stress of the liquid indium microsphere (Figure 2B).25 Finally, the liquid indium core would be removed through evaporation, and a hollow SiO2 sphere comprising patterned SiO2 nanoparticles was produced (Figure 2C). Scanning electron microscopy (SEM) images of the products at the beginning stage are shown in Figure 2AC. Figure 2A,B reveals that the patterned SiO2 nanoparticles have been formed on the surface of the indium droplet. (The chemical composition analysis is shown in Figure 2E and Supporting Information S3.) The patterning process can be attributed to the previously reported surface-stress-drove mechanism of SiO2 nanoparticles on the liquid metal droplet.24 The as-formed SiO2 nanoparticles were arranged in pentagonal patterns at first (Figure 2A) and then accumulated and finally patterned into hexagons driven by the surface stress (Figure 2B). It is proposed that in the situation of a controlled indium source there would be no more fresh 7997
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Figure 4. SEM image of the hierarchical sphere adhered on the surface of a vertically standing flat Si wafer. (The inset shows a cartoon of the experimental process.)
Figure 3. SEM and TEM images of the products in the second step. (A) Indium nanoparticle formed on top of the SiO2 nanoparticle. (B, C) Growth of the 1D nanostructure with indium nanoparticles as the catalyst. An inset cartoon in part C reveals the hollow microsphere. (D) TEM image of the 1D nanowire in the white square in part C, which shows a spherical indium droplet on the tip of the nanowire.
supply of indium vapor to the liquid sphere after the exhaustion of the indium. This condition leads to the shrinking of the liquid indium droplet that facilitates this pattern transformation. Finally, the amount and density of SiO2 particles on the sphere would increase and eventually cover the entire surface of the sphere (Figure 2C). Figure 2D shows one of these shells that had been broken during sample preparation, in which the indium core was already removed and the hollow structure was formed. Energy dispersive X-ray (EDX) elemental mappings of an intact shell and a broken one are shown in Figure 2E,F, respectively. These results show that the surface of the sphere consists mainly of Si and O whereas inside the shell there appears to be a thin layer of indium. In the second stage, the indium microsphere wrapped with SiO2 nanoparticles would continue to evaporate because of the absence of indium vapor in the tube. The indium vapor that escaped from the SiO2 cage could recondense on the tip of the SiO2 nanoparticles (Figure 1D), forming liquid nanoparticles and leading the Si nanowires to grow, following the VLS process (Figure 1E). SEM and TEM images of the hierarchical structures in the second step are shown in Figure 3. The images reveal that an indium nanoparticle eventually accumulates on the tip of each SiO2 nanoparticle (Figure 3A). The formation of the indium nanoparticle on the SiO2 nanoparticle can be viewed as a condensing process in which a nanosized droplet (In) would grow on a nonwettable nanosized substrate (SiO2). It is proposed that indium vapor evaporated from the wrapped indium microsphere would recondense on the surfaces of SiO2 nanoparticles. Liquid indium would prefer to form droplets instead of a film becauase liquid indium does not wet SiO2. Consequently, the indium droplet would be located at the tip of the SiO2 nanoparticle
because of the largest contact angle on that site. Moreover, the droplet size would be limited by that of the SiO2 nanoparticles. A bundle of Si/SiO2 composite nanowires would then vertically grow from each of the indium nanodroplets following a VLS process as SiO vapor is adsorbed into the patterned indium droplet throughout the growing process (Figure 3AC).26 Therefore, each of the indium nanoparticles dominates the growth of one nanowire in the VLS mechanism (Figure 3D), and the patterned indium nanoparticles would induce the array of Si/SiO2 nanowires to grow uniformly on the patterned SiO2 spherical shell. TEM and EDX studies reveal that the 1D nanowire has a singlecrystal Si core and a radially amorphous silica nanowire shell (Figures S4 and S5). The adhesion of the hollow sphere with a hierarchical shell was measured and compared with that of the smooth, solid SiO2 balls. The average adhesion force of each hierarchical particle on the Si wafer was estimated to be 9.1 105 N, which is equivalent to 500 times the weight of solid SiO2 microspheres with the same volume. Figure 4 shows an SEM image of the hierarchical particle adhered on a vertically standing Si wafer. The strong adhesion of the particle actually originated from a few anchored points. It could also be found that the hierarchical hollow spheres tightly attached to the Si wafer no matter how the wafer was rotated. In contrast, the solid SiO2 microspheres with smooth surfaces quickly fell off of the Si wafer even when the wafer was rotated several degrees.
’ CONCLUSIONS We demonstrate a simple metal-catalyzed VLS method for the synthesis of hollow microscopic capsules with a hierarchical shell structure to enhance their geometry-based adhesion. The growth process is based on the self-assembly of nanoparticles on liquid droplets and liquid-metal-nanoparticle-catalyzed VLS growth in which the catalyst droplet acts as (1) a spherical liquid template to pattern the growth of SiO2 nanoparticles and (2) a nanosized catalyst to enable the growth of 1D nanowires on the patterned SiO2 nanoparticles. Results show that the produced hierarchically structured shell offers the hollow SiO2 spheres much stronger surface adhesion compared to that in solid SiO2 microspheres with smooth surfaces. In this approach, the hierarchical 7998
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Langmuir shell structure could be constructed to enrich the function of the microspheres. Moreover, the pattern-transformation processes of SiO2 nanoparticles on the liquid metal microbead could also be useful for adjusting the permeability of the hollow microspheres. This facile growth process provides an efficient method for fabricating similar functional hierarchical nanostructures for the other semiconductors and oxides.
’ ASSOCIATED CONTENT
bS
Supporting Information. Adhesive force measurements, TEM, SEM, EDX, and XRD. This material is available free of charge via the Internet at http://pubs.acs.org.
LETTER
(20) Liu, H. Y.; Chen, D.; Li, L. L.; Liu, T. L.; Tan, L. F.; Wu, X. L.; Tang, F. Q. Angew. Chem., Int. Ed. 2011, 50, 891. (21) Shiomi, T.; Tsunoda, T.; Kawai, A.; Mizukami, F.; Sakaguchi, K. Chem. Commun. 2007, 42, 4404. (22) Nie, Z.; Xu, S.; Seo, M.; Lewis, P. C.; Kumacheva, E. J. Am. Chem. Soc. 2005, 127, 8058. (23) Kim, J. W.; Utada, A. S.; Fernandez-Nieves, A.; Hu, Z.; Weitz, D. A. Angew. Chem., Int. Ed. 2007, 46, 1819. (24) Li, C. R.; Ji, A. L.; Gao, L.; Cao, Z. X. Adv. Mater. 2009, 21, 4652. (25) Wang, H.; Zhang, X. H.; Ma, D. D. D.; Lee, S. T. Appl. Phys. Lett. 2008, 93, 023119. (26) Wang, H.; Zhang, X. H.; Meng, X. M.; Zhou, S. M.; Wu, S. K.; Shi, W. S.; Lee, S. T. Angew. Chem., Int. Ed. 2005, 44, 6934.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: þ86-10-82543510. )
Author Contributions
These authors contributed equally to this work.
’ ACKNOWLEDGMENT The work was partially supported by the National Basic Research Program of China (973 Program) (grant nos. 2007CB936000 and 2010CB934500) and the National Natural Science Foundation of China (grant nos. 50825304, 50972150, and 20971128). ’ REFERENCES (1) Seiffert, S.; Thiele, J.; Abate, A. R.; Weitz, D. A. J. Am. Chem. Soc. 2010, 132, 6606. (2) Plush, S. E.; Woods, M.; Zhou, Y. F.; Kadali, S. B.; Wong, M. S.; Sherry, A. D. J. Am. Chem. Soc. 2009, 131, 15918. (3) Akartuna, I.; Tervoort, E.; Studart, A. R.; Gauckler, L. J. Langmuir 2009, 25, 12419. (4) Sun, B. J.; Shum, H. C.; Holtze, C.; Weitz, D. A. ACS Appl. Mater. Interfaces 2010, 2, 3411. (5) Ni, D.; Wang, L.; Sun, Y.; Guan, Z.; Yang, S.; Zhou, K. Angew. Chem., Int. Ed. 2010, 49, 4223. (6) Kaiser, A.; Liu, T. T.; Richtering, W.; Schmidt, A. M. Langmuir 2009, 25, 7335. (7) Stadler, B.; Chandrawati, R.; Goldie, K.; Caruso, F. Langmuir 2009, 25, 6725. (8) Im, S. H.; Jeong, U.; Xia, Y. Nat. Mater. 2005, 4, 671. (9) Schacht, S.; Huo, Q.; Voigt-Martin, I. G. G.; Stucky, D.; Sch€uth, F. Science 1996, 273, 768. (10) Caruso, F.; Caruso, R. A.; M€ohwald, H. Science 1998, 282, 1111. (11) Lee, J. W.; Park, J. H.; Robinson, J. R. J. Pharm. Sci. 2000, 89, 850. (12) Norris, D. A.; Puri, N.; Sinko, P. J. Adv. Drug Delivery Rev. 1998, 34, 135. (13) Ponchel, G.; Irache, J. M. Adv. Drug Delivery Rev. 1998, 34, 191. (14) Fischer, K. E.; Aleman, B. J.; Tao, S. L.; Daniels, R. H.; Li, M. E.; B€unger, M. D.; Nagaraj, G.; Singh, P.; Zettl, A.; Desai, T. A. Nano Lett. 2009, 9, 716. (15) Gabor, F.; Wirth, M. STP Pharma. Sci. 2003, 13, 3. (16) Bies, C.; Lehr, C. M.; Woodley, J. F. Adv. Drug Delivery Rev. 2004, 56, 425. (17) Mahdavi, A.; Ferreira, L.; Sundback, C.; Nichol, J. W.; Chan, E. P.; Carter, D. J. D.; Bettinger, C. J.; Patanavanich, S.; Karp, J. M. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2307. (18) Lee, H.; Lee, B. P.; Messersmith, P. B. Nature 2007, 448, 338. (19) Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Nature 2002, 405, 681. 7999
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