9704
J. Phys. Chem. C 2007, 111, 9704-9708
Raspberry-like Silica Hollow Spheres: Hierarchical Structures by Dual Latex-Surfactant Templating Route Xiaofeng Wu,†,‡ Yajun Tian,† Yanbin Cui,†,‡ Lianqi Wei,†,‡ Qi Wang,†,‡ and Yunfa Chen*,† Key Laboratory of Multiphase Reaction, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100080, China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing, 100049, China ReceiVed: January 31, 2007; In Final Form: May 10, 2007
Raspberry-like hollow SiO2 spheres were synthesized successfully through a dual latex-surfactant templating route. In this approach, cationic cetyltrimethylammonium (CTA+) micelles and microscale polystyrene (PS) latex were employed as structure-directing templates for constructing the inorganic hollow SiO2 spheres with hierarchical structures. The final product has been analyzed scanning electron microscopy (SEM), highresolution transmission electron microscopy (HRTEM), single-angle X-ray diffraction (SAXRD), BrunauerEmmett-Teller (BET) measurements, and Hg porosimetry measurements. The as-obtained hollow SiO2 spheres present a novel raspberry-like protruding surface morphology and possess hierarchical porous shells. Moreover, the factors that could impact the surface morphology and hierarchical porous structure are discussed, and the corresponding mechanism was proposed accordingly. This dual templating route is expected to be an effective means to pattern multiscale structures in the shell of the SiO2 microspheres; hence their potential applications could be greatly broadened.
Introduction Mesoporous silica materials with hierarchical morphologies and well-defined structures have attracted increasing attention due to their novel structures and extraordinary properties.1-7 Besides, studies about mesoporous materials are also favorable to deepen the insight into the biomineralization process.1,2,8,9 Taking hierarchical meso-macroporous materials as an example, the mesopores provide size and shape selectivity for guest molecules, while the coexisting macropores give short paths to enhance mass transfer, improve reaction efficiencies, and minimize blocking of the channels in the separation and catalysis fields.3,10-12 Among the various mesoporous materials, hollow spheres with mesoporous shells are of great significance because of their promising application in confined-space catalysis, biomolecule separation, enzyme immobilization, and controlled drug release.13-19 Many research efforts have been devoted to fabricate mesoporous spheres with hollow interiors, in which colloidal particles,13-16 vesicles,17,18 emulsions,2 and polymer aggregates19 have been employed in combination with some surfactant templates for dual templating preparation of hollow mesoporous silica spheres. Despite these excellent efforts, the synthesis of hollow silica spheres with hierarchically structured shells is still a challenging field. Recently, some progress has been made by a few research groups using emulsions as templates in combination with polymers or other organic molecules.20-22 In contrast, colloidal particle templating retains still a more simple and effective approach to achieve hollowstructured materials because of the controllable size and surface reactivity of colloidal particles. However, to our knowledge, few reports have been released on this approach to achieve the hierarchical mesoporous hollow silica spheres and facilely manipulate their surface morphologies. * Corresponding author. E-mail:
[email protected]. † Institute of Process Engineering, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
In the present work, we have successfully fabricated novel raspberry-like silica hollow spheres with both hierarchical meso-macroporous shells and interesting hierarchical protruding surface morphology by the dual templating route. In our preparation procedure, cationic cetyltrimethylammonium (CTA+) surfactants and polystyrene latex particles act as templates to construct the mesopores in shells and the hollow interiors of hollow silica spheres like the reported one.13 Differently, through changing the synthesis parameters, distinctive surface morphologies and hierarchical porous shells are facilely achieved. This novel material is potentially applied in drug delivery, superhydrophobic materials, absorption/separation, catalyst support, and so on, due to its unique surface morphology and the hierarchical porous shells. Experimental Section Material Synthesis. A typical procedure for synthesizing the raspberry-like hollow SiO2 microspheres is carried out as follows: 0.5 g of polystyrene (PS) monodisperse particles (synthesis details, Supporting Information) were dispersed into 68 mL of absolute ethanol by supersonic oscillation for 30 min, and the obtained suspension was added into 50 mL of 0.14 mol/L aqueous cetyltrimethylammonium bromide (Beijing Chemical Reagents Co. Ltd.) solution. Then the mixture was stirred at room temperature for 1 h and used for inorganic silica deposition. Following regulating the pH to 10-12 with 25 wt % ammonia (Beijing Chemical Reagents Co. Ltd.), 1.34-5.38 mL of tetraethyl orthosilicate (Beijing Yili Chemical Reagent Co. Ltd.) was added. Subsequently, the mixture was stirred at room temperature for 10 h and then hermetically aged at 80 °C for 48 h. After that, the mixture was centrifuged, and the resultant white product was washed by distilled water and centrifuged alternatively three times. Finally, after the white product was dried, the sample was heated to 550 °C in air with a heating rate of 1 °C/min and kept there for 2 h to remove the templates.
10.1021/jp070811q CCC: $37.00 © 2007 American Chemical Society Published on Web 06/19/2007
Raspberry-like Silica Hollow Spheres
J. Phys. Chem. C, Vol. 111, No. 27, 2007 9705
Figure 1. SEM images of raspberry-like SiO2 hollow spheres obtained synthesized under different TEOS concentration: (A) 0.06 mol/L; (B) 0.12 mol/L; (C) 0.24 mol/L. (D) Representative TEM image of the bionic hollow spheres obtained from the same sample B. All samples are obtained at fixed pH 10.
Material Characterization. Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEM 2010F operated at 200 kV. A scanning electron microscope (SEM) of type JSM-6700F, JEOL, was also used to characterize the surface morphologies of the products using field emission. X-ray powder diffraction data were collected on the X’Pert Pro powder diffractometer. N2 absorption and desorption were conducted on a Quantachrome Autosorb-1-MP automated gas adsorption system, and the degassed temperature was 300 °C. The mercury porosimetry was carried out on an Autopore II 9220 (Micromeritics Co.USA). Results and Discussion Surface Morphologies of the Samples. The morphology of the final products shown in Figure 1 is microscale spherical particles with a papillar surface, which visually resembles that of raspberries. The presence of some crevasses suggests the hollow structure of the as-synthesized products. The dimension of the cavity of this hollow structure is almost equal to that of the PS spherical templates we used, clearly implying that it is replicated from the PS latex. Furthermore, it is very clear that the shells are built up with the aggregated small particles and present the protruding surface morphologies. This raspberrylike hollow microsphere possessing both nanoscale (submicrometer) and microscale structures represents typical hierarchical structure,23-26 and the resultant surface roughness is considered as one of the key factors to obtain superhydrophobicity when wetted.23-26 It is also found that the protruding morphologies of the obtained samples can be conveniently modified by changing the reaction parameters. SEM images in Figure 1A-C reveal the effects of different TEOS concentrations on the protruding morphologies. When TEOS concentration is 0.06 mol/L, the small particles with individual diameters of about 50 nm are packed loosely. The loose shells are vulnerable to collapse as
Figure 2. SEM (A-C) and TEM (D-F) images of SiO2 hollow spheres obtained under different pH values: (A,D) pH 9; (B,E) pH 12; (C,F) pH 13. The TEOS concentration is fixed at 0.12 mol/L.
shown in the upper part of Figure 1A. If TEOS concentration is added to 0.12 mol/L, the packed small particles present a more close arrangement, resulting in sound shells of hollow spheres. Figure 1D shows TEM pictures of the same sample, and further confirms the typical shell structure comprised of the aggregated small particles. The dimension of the small particles increases up to about 90 nm. However, a too-high TEOS concentration could result in homogeneous precipitation of inorganic species. The SEM picture in Figure 1C suggests the presence of some freestanding small particles when the TEOS concentration is up to 0.24 mol/L, and their diameters are around 350 nm, almost equal to that of the aggregated particles building up the shells of hollow spheres. In addition, pH values play an important role in controlling the precipitation of silica and surface morphologies of hollow spheres. Figure 2 shows SEM (A-C) and TEM (D-F) pictures obtained at different pH values. It can be seen in Figure 2A,D that no apparently papillar morphology is identified for the samples obtained at pH 9. Simultaneously, some irregular small particles are also apparently identified, possibly attributed to the induced precipitation of silica by CTA+ ions in solution. If pH is elevated to 10-12, the surfaces of the samples present the typical protruding surface morphologies as shown in Figure 1B,D and Figure 2B,E. Unexpectedly, when pH J13, the protruding morphology inversely disappears. The most possible reason behind these phenomena could be the matching rates between silica precipitation from solution and absorption of freestanding CTA+ ion at the proper range of pH values. By heteronucleation at the solid-liquid interface, the silica particles grow on the
9706 J. Phys. Chem. C, Vol. 111, No. 27, 2007
Figure 3. HRTEM images of hollow SiO2 spheres obtained at high pH value of (A) pH 9, (B) pH 12, and (C) pH 13. The TEOS concentration is fixed at 0.12 mol/L.
surface of the templating PS particles, and freestanding CTA+ ions in solution are simultaneously adsorbed on their outer surface and limit the further growth of small silica particles by steric hindrance of CTA+ organic chains. Under the condition of low pH value, the hydrolyzed species of precursors could be insufficient for heteronucleation and dominantly induced to nucleate by CTA+ ions in solution at an early reaction stage. In fact, the aggregated irregular nanoparticles are obviously discerned at low pH value (pH 9) as are marked by arrows in Figure 2A,D. When the concentration of hydrolyzed species of precursors is up to the supersaturation required by heteronucleation, the quantity of the remaining freestanding CTA+ ions could be insufficient to limit the growth of small silica particles on the surface of PS particles, resulting in smooth shells of the final product. However, at pH ≈13, too-fast hydrolysis and condensation rate of precursors could exceed the adsorption rate of CTA+ ions from solution to the solid-liquid interface of the anchored small silica particles, which results in the complete aggregation and fusion of small silica particles into smooth shells of the final samples. Hierarchical Porosity of the Bionic Samples. In particular, irregular interparticle macropores are apparently identified at the proper range of pH 10-12 as are shown in Figure 1 and Figure 2B,E. In order to avoid the structural collapse of hollow spheres and small particles derived from the homogeneous nucleation, the proper CTAB concentration of 0.12 mol/L is preferential to obtain the products for further sample characterization. The representative TEM pictures in Figure 1D and Figure 2E show more clearly these macropores derived from the interconnected small silica particles comprising the hollow spherical shells. In order to further investigate the shell structure of the as-synthesized hollow spheres, HRTEM technique was applied and the representative results are disclosed in Figure 3. For the sample in Figure 3B, it is found that the mesopores with diameters around 2 nm exist within an individual papilla,
Wu et al. and the arrangement of the mesopores is intuitively disordered. The small-angle XRD pattern (Figure 2S, Supporting Information) presents one broad and weak peak corresponding to a d spacing of 4 nm, indicative of disordered short-range-ordering structure, which can arise from the lacking of long-range crystallographic order.27 The nitrogen adsorption-desorption isotherm in Figure 4a reveals a type IV physisorption isotherm with an increase in nitrogen uptake at high relative pressure p/p0 ) 0.9-1.0 and a wide hysteresis loop at p/p0 ) 0.3-1.0, indicating a typical mesoporous solid with bigger pores. In the corresponding pore diameter distribution (inset in Figure 4a) appears a sharp peak around 2 nm, which well agrees with the pore size within the nanopapillae observed by HRTEM in Figure 3B. The specific surface area and the average pore diameter were estimated to be about 829 m2/g and 2.7 nm, respectively, using the Brunauer-Emmett-Teller (BET) and Barrett-JonerHalenda (BJH) methods. These mesopores about 2 nm are typical in bulk synthesis of silica templated by CTA+ cylindrical vesicles.28-30 However, there is no obvious peak at the macropore range over 50 nm. For learning the whole pore size distribution of the as-synthesized porous materials, Hg porosimetry was carried out, and the corresponding results are shown in Figure 4b. Obviously, the pore distribution curve (inserted chart) shows a broad and weak peak about 60 nm, which is most probably assigned to the voids that are left by clustered small particles. The peak near 1 µm may be caused by the hollow interior of spheres and the interstices derived from their agglomeration. Consequently, the shells of the as-synthesized raspberry-like SiO2 hollow spheres contain two types of porosity systems, which mimic the porosity of the diatom in a way and present a feature of hierarchically porous structure.10-12,20-22 However, when the pH value is outside the range of 10-12, the interparticle pores disappear obviously as shown in Figure 2A,D and Figure 2C,F. For the sample obtained at pH 9, there are no obviously mesopores identified in a high-resolution TEM image (Figure 3A), indicative of the absence of mesopores in shells of hollow spheres. It can be deduced, at the solid-liquid interface of colloidal PS particles, that there are no sufficiently adsorbed CTA+ cylindrical vesicles to construct the mesopores because of the consumption of CTA+ ions by induced nucleation and precipitation in solution. However, when pH ≈13, single mesoporous shells with pore diameters of 2 nm are identified using high-resolution TEM, and the corresponding result is provided in Figure 3C. BET data show the typical mesoporous characteristics (Figure 3S, Supporting Information). These results are in agreement with those reported that have been obtained in concentrated aqueous ammonia.13 The specific surface area is high, up to 1457 m2/g, and the average pore
Figure 4. (a) N2 adsorption-desorption isotherms and pore diameter distribution (inset) of the as-synthesized raspberry-like hollow SiO2 spheres. (b) Hg intrusion-extrusion isotherms of raspberry-like SiO2 hollow spheres and pore size distribution plot (inset) from intrusion data.
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Figure 5. Schematic picture of suggested formation process of biomimetic raspberry-like hollow SiO2 microspheres.
diameter is 2.46 nm. This suggests that the CTA+ cylindrical vesicles act as only templates for forming the mesopores at extremely high pH value. Formation Mechanism. On the basis of the above results, the formation mechanism of the hierarchically structured silica hollow spheres with raspberry-like protruding morphology and hierarchical porosity could be proposed as is shown in Figure 5. In aqueous solution above the critical micelle concentration (cmc), CTA+ surfactant ions are first assembled into cylindrical structure and further self-assembled into a periodic hexagonal mesophase at a chosen CTAB concentration.31 It can be rationally deduced that these hexagonal micelles and their assemblies are further assembled disorderedly on the surface of PS colloidal particles by organic affinity and form corporately complicated molds, in which inorganic silica species nucleate and grow into subunits (small SiO2 particles) building up the shells of the bigger particles. Specifically, the more detailed formation process of small silica particles on the solid-liquid interface of PS particles should adhere to the liquid-crystal templating mechanism like that of the mesoporous silica materials.28-30 With the growth of the silica nanoparticles, the anchored CTA+ molecular chains act as the regulators and limit their further growth on the surface of PS colloidal particles by steric hindrance at the proper pH value (10-12), resulting in anchoring of silica nanoparticles on the surface of PS particles and protruding morphology. After aging, the raspberry-like hollow SiO2 microspheres are obtained through removal of the organic templates. Conclusions On all accounts, by a dual latex-surfactant templating route, we have successfully synthesized raspberry-like SiO2 hollow spheres, which are characteristic with hierarchical structures with protruding surface morphology, and their meso-macroporous shells. It is also found that the TEOS concentration has an effect on the size of the aggregated silica particles building up the shells of hollow spheres, and a too-low TEOS concentration results in the loose shells of hollow spheres. The pH value plays a key role in forming the papillar surface morphology and the hierarchical porous shells of hollow spheres. Only in the range of pH 10-12 are the hierarchical hollow spheres obtained. Consequently, the surface morphologies and shell porosity of these hollow silica spheres can be adjusted by changing the parameters. The shells of the bionic hollow spheres present hierarchical porous channels derived from different porosity systems, which are desired to broaden the applications in hydrophobic materials, adsorption/separation, drug delivery
systems, and catalysis. This dual latex-surfactant templating route provides a convenient way to design hollow mesoporous materials. Acknowledgment. The research is supported by the National Natural Science Foundation of China (No. 20221603). Supporting Information Available: Details of synthesizing PS colloidal particles, small-angle XRD data of the bionic hollow silica spheres, and BET data of mesoporous hollow spheres obtained at a extreme pH value. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lin, H. P.; Mou, C. Y. Science 1998, 282, 765-767. (2) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu¨th, F. Science 1998, 282, 768-771. (3) Holland, B. T.; Abrams, L.; Stein, A. J. Am. Chem. Soc. 1999, 121, 4308-4309. (4) Karkamkar, A. J.; Kim, S. S.; Mahanti, S. D.; Pinnavaia, T. J. AdV. Funct. Mater. 2004, 14, 507-512. (5) Khalil, A. S. G.; Konjhodzic, D.; Marlow, F. AdV. Mater. 2005, 18, 1055-1058. (6) Yang, M.; Ma, J.; Zhang, C. L.; Yang, Z. Z.; Lu, Y. F. Angew. Chem., Int. Ed. 2005, 44, 6727-6730. (7) Wang, B.; Chi, C.; Shan, W.; Zhang, Y. H.; Ren, N.; Yang, W. L.; Tang, Y. Angew. Chem., Int. Ed. 2006, 45, 2088-2090. (8) Mann, S.; Ozin, G. A. Nature 1996, 382, 313-318. (9) Mann, S. J. Mater.Chem. 1995, 5, 935-946. (10) Lind, A. J.; Hohenesche, C. F.; Smatt, J. H.; Linden, M.; Uger, K. K. Microporous Mesoporous Mater. 2003, 66, 219-227. (11) Blin, J. L.; Le´onard, A.; Yuan, Z. Y.; Gigot, L.; Vantomme, A.; Cheetham, A. K.; Su, B. L. Angew. Chem., Int. Ed. 2003, 42, 28722875. (12) Lebeau, B.; Fowler, C. E.; Mann, S.; Farcet, C.; Charleux, B.; Sanchez, C. J. Mater. Chem. 2000, 10, 2105-2108. (13) Tan, B.; Rankin, S. E. Langmiur 2005, 21, 8180-8187. (14) Yin, Y. D.; Lu, Y.; Gates, B.; Xia, Y. N. Chem. Mater. 2001, 13, 1146-1148. (15) Rhodes, K. H.; Davis, S. A.; Caruso, F.; Zhang, B. J.; Mann, S. Chem. Mater. 2000, 2832-2834. (16) Arnal, P. M.; Weidenthaler, C.; Schu¨th, F. Chem. Mater. 2006, 18, 2733-2739. (17) Djojoputro, H.; Zhou, X. F.; Qiaok, S. Z.; Wang, L. Z.; Yu, C. Z.; Lu, G. Q. J. Am. Chem. Soc. 2006, 128, 6320-6321. (18) Yeh, Y. Q.; Chen, B. C.; Lin, H. P.; Tang, C. Y. Langmiur 2006, 22, 6-9. (19) Zhu, Y. F.; Shi, J. L.; Chen, H. R.; Shen, W. H.; Dong, X. P. Microporous Mesoporous Mater. 2005, 84, 218-222. (20) Sun, Q. Y.; Magusin, P. C. M. M.; Mezari, B.; Pierre, P.; Santen, R. A. V.; Sommerdijk, N. A. J. M. J. Mater. Chem. 2005, 15, 256259. (21) Wang, J. G.; Xiao, Q.; Zhou, H. J.; Sun, P. C.; Yuan, Z. Y.; Li, B. H.; Ding, D. T.; Shi, A. C.; Chen, T. H. AdV. Mater. 2006, 18, 32843288. (22) Fujiwara, M.; Shiokawa, K.; Sakakura, I.; Nakahara, Y. Nano Lett. 2006, 6, 2925-2928.
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