pubs.acs.org/Langmuir © 2009 American Chemical Society
Concentric Hollow Nanospheres of Mesoporous Silica Shell-Titania Core from Combined Inorganic and Polymer Syntheses Guoliang Li,† E. T. Kang,†,* K. G. Neoh,† and Xinlin Yang‡,* †
Department of Chemical & Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore 119260, and ‡Key Laboratory of Functional Polymer Materials, Ministry of Education, Institute of Polymer Chemistry, Nankai University, Tianjin 300071, P. R. China Received March 3, 2009. Revised Manuscript Received March 16, 2009 Nearly monodispersed concentric hollow nanospheres with a mesoporous silica shell and anatase titania inner core were synthesized by the combination of sol-gel reaction and distillation-precipitation polymerization. The welldefined mesoporous concentric hollow nanospheres, comprising two nanostructured functional inorganics, can be used for confined catalytic reactions. The direct synthesis procedures can be readily extended to preparation of the concentric hollow nanospheres with multiple cores, or other functional concentric hollow nanospheres having different core-shell compositions.
Combined organic and inorganic approaches to the synthesis of hetero- or hybrid materials in a functional structure is of great interest.1 Hollow organic, inorganic, and hybrid micro- and nanospheres2-7 are widely used in catalysis, pigment encapsulation, chromatography, controlled release of drugs, and gene therapy.8-12 Efforts have been made constantly to broaden their applications by altering the properties of shell materials and by fabricating more sophisticated hollow structures, such as yolkshell nanostructures,13 hollow spheres with controllable surface pores,14 pH- or temperature-responsive hollow capsules,15,16 and concentric hollow nanospheres.17,18 In addition, the performance of functional spheres is further improved by their narrow dispersity in sizes.19,20 Titania (TiO2) has been widely used in photovoltaics and as a photocatalyst for decomposing toxic organic *To whom correspondence should be addressed. E-mail:
[email protected]. sg (E.T.K.);
[email protected] (X.L.Y.). (1) Kamata, K.; Lu, Y.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 2384–2385. (2) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453–457. (3) Lou, X. W.; Archer, L. A.; Yang, Z. C. Adv. Mater. 2008, 20, 3987–4019. (4) Xu, X.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 7940–7945. (5) Lou, X. W.; Wang, Y.; Yuan, C.; Lee, J. Y.; Archer, L. A. Adv. Mater. 2006, 18, 2325–2329. (6) Lu, X.; Au, L.; Mclellan, J.; Li, Z.; Marquez, M.; Xia, Y. N. Nano Lett. 2007, 7, 1764–1769. (7) Chen, j.; MaLellan, J. M.; Siekkinen, A.; Xiong, Y.; Li, Z. Y.; Xia, Y. N. J. Am. Chem. Soc. 2006, 128, 14776–14777. (8) Chen, J.; Wang, D.; Xi, J.; Au, L.; Siekkinen, A.; Warsen, A.; Li, Z.; Zhang, H.; Xia, Y.; Li, X. Nano Lett. 2007, 7, 1318–1322. 00 (9) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111–1114. (10) Ikeda, S.; Ishino, S.; Harada, T.; Okamoto, N.; Sakata, T.; Mori, H.; Kuwabata, S.; Torimoto, T.; Matsumura, M. Angew. Chem., Int. Ed. 2006, 45, 7063–7066. (11) Li, G. L.; Yang, X. Y.; Wang, B.; Wang, J. Y.; Yang, X. L. Polymer 2008, 49, 3436–3443. 00 (12) Shchukin, D. G.; Sukhorukov, G. B.; Mohwald, H. Angew. Chem., Int. Ed. 2003, 42, 4472–4475. (13) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711–714. (14) Im, S. H.; Jeong, U.; Xia, Y. N. Nat. Mater. 2005, 4, 671–675. (15) Sauer, M.; Streich, D.; Meier, W. Adv. Mater. 2001, 13, 1649–1651. (16) Zha, L.; Zhang, Y.; Yang, W. L.; Fu, S. K. Adv. Mater. 2002, 14, 1090– 1093. (17) Yang, M.; Ma, J.; Zhang, C.; Yang, Z.; Lu, Y. Angew. Chem., Int. Ed. 2005, 44, 6727–6729. (18) Li, G. L.; Yang, X. J. Phys. Chem. B 2007, 111, 12781–12786. (19) Chattopadhyay, P.; Shekunov, B. Y.; Yim, D.; Cipolla, D.; Boyd, B.; Farr, S. Adv. Drug Delivery Rev. 2007, 59, 444–453. (20) Shiga, K.; Muramatsu, N.; Kondo, T. J. Pharm. Pharmacol. 1996, 48, 891– 895.
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pollutants.21-24 The strong oxidizing power of irradiated TiO2 can also be used to eliminate tumor cells in cancer treatment and as new tools for gene therapy.25,26 On the other hand, silica materials, with safety approval from the US Food and Drug Administration (FDA), have a wide range of applications.27,28 The integration of these two different inorganic materials (TiO2 and SiO2) into a functional hybrid micro- or nanostructure via direct synthesis will have interesting applications. Titania is commonly deposited on silica particles or flat wafers to serve as a heterocatalyst. The fabrication of more sophisticated hollow silica nanospheres with an inner titania core will require appropriate interfacial materials and procedures for integrating the two inorganic materials. In this report, we describe the preparation of nearly monodispersed concentric hollow nanospheres, comprising of a mesoporous silica shell, void cavity, and mesoporous anatase titania core, via combined inorganic and polymer syntheses (Scheme 1). The mesoporous silica shells act as barriers to the aggregation of titania, while allowing molecular diffusion into the interior of hollow spheres. The void thus allows confined reactions to be catalyzed by the surface of the titania nanocore. The first synthesis step involved the preparation of cross-linked poly(methacrylic acid) (PMAA) core nanospheres via distillation-precipitation polymerization in the presence of ethylene glycol dimethylacrylate (EGDMA, a cross-linking agent).29-32 The next step involved the synthesis of PMAA/TiO2 composite (21) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M. Nature (London) 2008, 452, 301–310. (22) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 2243–2245. (23) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891–2959. (24) Akbal, F. Environ. Prog. 2005, 24, 317–322. (25) Paunesku, T.; Rajh, T.; Wiederrecht, G.; Maser, J.; Vogt, S.; Stojicevic, N.; Protic, M.; Lai, B.; Oryhon, J; Thurnauer, M.; Woloschak, G. Nat. Mater. 2003, 2, 343–346. (26) Blake, D. M.; Maness, P. C.; Huang, Z.; Wolfrum, E. J.; Huang, J.; Jacoby, W. A. Sep. Purif. Methods 1999, 28, 1–50. (27) Ma, Y.; Qi, L.; Ma, J.; Wu, Y.; Liu, Q.; Cheng, H. Colloids Surf., A: Physicochem. Eng. Aspects 2003, 229, 1–8. (28) Yoon, T. J. Angew. Chem., Int. Ed. 2005, 44, 1068–1071. (29) Bai, F.; Yang, X.; Li, R.; Huang, B.; Huang, W. Polymer 2006, 47, 5775– 5784. (30) Li, G. L.; Yang, X.; Bai, F. Polymer 2007, 48, 3074–3081. (31) Li, R.; Yang, X. L.; Li, G. L.; Li, S. N.; Huang, W. Q. Langmuir 2006, 22, 8127–8133. (32) Li, G. L.; Lei, C.; Wang, C.; Neoh, K. G.; Kang, E. T.; Yang, X. Macromolecules 2008, 41, 9487–9490.
Published on Web 3/23/2009
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Letter Scheme 1. Combined Polymerization and Sol-Gel Reactions for the Preparation of Nearly Monodispersed Concentric Hollow Nanospheres Composed of Mesoporous Silica Shells and Anatase Titania Inner Cores
nanospheres via the sol-gel process,33-35 using the cross-linked PMAA nanospheres as the template cores. In the subsequent step, the PMAA/TiO2 composite nanospheres were coated with a uniform PMAA layer via distillation-precipitation polymerization to produce the PMAA/TiO2@PMAA core-shell particles. Finally, coating of the core-shell particles with an outer silica shell, derived from the sol-gel reaction of the tetraethyl orthosilicate (TEOS) precursor, produced the PMAA/TiO2 @PMAA@SiO2 trilayer hybrid nanospheres.10 Nearly monodispersed concentric hollow spheres, consisting of a mesoporous silica shell and an anatase titania inner core, were obtained after selective removal of the polymer templates. The carboxylic acid groups, located on the surface and interior networks of the cross-linked nanosphere, were used for the subsequent functionalization via sol-gel reaction.33-35 The field-emission scanning electron microscopy (FESEM) image of the PMAA nanospheres in Figure 1a indicates that the polymeric particles have an average diameter of 63 nm and a polydispersity index (PDI) of 1.03. The particle size can be adjusted by varying the monomer concentration and polymerization time (Supporting Information). After hydrolysis of the titanium tetrabutoxide (TBOT)-coated PMAA nanospheres in the sol-gel reaction, the size of the PMAA/TiO2 composite nanospheres increased to about 85 nm without a significant change in PDI (Table 1). The chemical composition of the PMAA/TiO2 nanospheres was confirmed by X-ray photoelectron spectroscopy (XPS) analysis (Supporting Information, Figure S1). After distillation-precipitation polymerization of methacrylic acid on the PMAA/TiO2 composite particles in the presence of EGDMA, a uniform PMAA polymeric shell is formed and is discernible in the transmission electron microscopy (TEM) image of Figure 1b. The average particle size of the resultant core-shell nanospheres increases to 137 nm with a PDI of 1.01. Thus, the thickness of the newly formed PMAA shell is around 26 nm (Table 1). Subsequently, an outer silica layer is coated directly on the PMAA/ TiO2@PMAA core-shell particles via condensation hydrolysis of TEOS in the sol-gel process, with an added surfactant (cetyltrimethyl ammonium bromide, or CTAB), to result in the PMAA/TiO2@PMAA@SiO2 trilayer hybrid nanospheres of uniform shape and size, as shown in the TEM image of Figure 1c. The (33) Yang, M.; Ma, J.; Niu, Z.; Dong, X.; Xu, H.; Meng, Z.; Jin, Z.; Lu, Y.; Hu, Z.; Yang, Z. Z. Adv. Funct. Mater. 2005, 15, 1523–1528. (34) Li, G. L.; Liu, G.; Kang, E. T.; Neoh, K. G.; Yang, X. L. Langmuir 2008, 24, 9050–9055. (35) Khaled, S. M.; Sui, R.; Charpentier, P. A.; Rizkalla, A. S. Langmuir 2007, 23, 3988–3995.
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PMAA interlayer with a lower contrast is discernible between the PMAA/TiO2 composite core and the silica shell. The particle size increased significantly to about 210 nm in diameter. Therefore, the PMAA interlayer with carboxylic functional groups bridged the two inorganic materials of titania and silica. Narrowly dispersed concentric hollow nanospheres comprising a mesoporous silica shell and a titania inner core are obtained by selective removal of the PMAA interlayer, the polymeric backbones in the core, and the CTAB surfactant in the shell36-38 upon calcination of the PMAA/TiO2@PMAA@SiO2 trilayer hybrid nanospheres, as illustrated in Scheme 1. The TEM (Figure 1d) and FESEM (inset of Figure 1d) images reveal the unique structure of a SiO2 outer shell of about 206 nm in diameter and an inner TiO2 core of about 50 nm in diameter. Thus the titania inner core has shrunk from about 85 nm in the original PMAA/TiO2 nanosphere to about 50 nm after calcination. The so-obtained TiO2 nanoparticle in the cavity is mesoporous and of pure anatase (Supporting Information, Figure S2). The disappearance of PMAA characteristic absorption peaks in the Fourier transform infrared (FT-IR) spectrum of the concentric hollow nanospheres (Supporting Information, Figure S3) confirms that the PMAA templates have been completely removed from the PMAA/ TiO2@PMAA@SiO2 trilayer hybrid structure. The mesoporous structure of the silica is discernible in the TEM image of Figure 1e. The pores in the silica shells are interconnected through pores. As characterized by the nitrogen adsorption-desorption isotherms (Supporting Information, Figure S4), the porous silica structure has a specific surface area (SSA) of 546 m2/g, a mean pore size of 2 nm, and a pore volume of 0.51 cm3/g. Thus, the mesoporous structure allow the transport of molecules to and from the interior void, and is crucial to catalysis, delivery, and other applications. The open environment may not be ideal for performing specific chemical reactions or separations. It may be desirable to confine a specific reactant and catalyst in micro- or nanocages (nanoreactors).39-42 To explore potential applications of the (36) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature (London) 1992, 359, 710–712. (37) Deng, Y.; Qi, D.; Deng, C.; Zhang, X.; Zhao, D. J. Am. Chem. Soc. 2008, 130, 28–29. (38) Gorelikov, I.; Matsuura, N. Nano Lett. 2008, 8, 369–373. (39) Moore, J. S. Nature (London) 1995, 374, 495–496. :: :: (40) Dahne, L.; Leporatti, S.; Donath, E.; Mohwald, H. J. Am. Chem. Soc. 2001, 123, 5431–5436. (41) Gil, P. R.; del Mercato, L. L.; del_Pino, P.; Muooz-Javier, A.; Parak, W. J. Nanotoday 2008, 3, 12–21. :: (42) Kreft, O.; Prevot, M.; Mohwald, H.; Sukhorukov, G. B. Angew. Chem., Int. Ed. 2007, 46, 5605–5608.
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Letter
Figure 1. FESEM and TEM micrographs of the (a) PMAA template cores, (b) PMAA/TiO2@PMAA core-shell nanospheres, (c) PMAA/ TiO2@PMAA@SiO2 trilayer hybrid nanospheres, (d) hollow silica nanospheres with an inner titania core after removal of the polymeric templates, (e) mesoporous silica shell under higher magnification, and (f ) hollow core-shell nanostructures containing multiple titania cores.
monodispersed concentric hollow silica nanospheres as reaction cages or nanoreactors for catalysis, UV-induced photocatalytic decomposition of methyl orange (MO) by the anatase TiO2 core were carried out, as illustrated in Figure 2a, after allowing MO to diffuse into the nanospheres. Figure 2b shows the photocatalytic decomposition of MO in the concentric hollow reactors with an apparent first-order rate constant (Ka) of 6.8 10-4 min-1. No significant decomposition of MO was observed in the absence of the concentric hollow nanospheres under similar irradiation conditions. The observed rate constant for the concentric (43) Wang, X. H.; Li, J. G.; Kamiyama, H.; Moriyoshi, Y.; Ishigaki, T. J. Phys. Chem. B 2006, 110, 6804–6809.
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hollow nanospheres as photocatalysts seems to be lower than those reported for naked and doped TiO2 nano- and microparticles.43-45 The lower reaction rate observed in the present work is probably due to the low content of mesoporous anatase titania in the hollow nanospheres. To encapsulate more titania inner cores within a single silica shell, one can control the volume and rate of acetonitrile distillation during the synthesis of PMAA/TiO2@ PMAA core-shell spheres, leading to the formation of multiple PMAA/TiO2 cores encapsulated within a single PMAA shell. (44) Li, Y. J.; Li, X. D.; Li, J. W.; Yin, J. Water Res. 2006, 40, 1119–1126. (45) Wang, H.; Miao, J. J.; Zhu, J. M.; Ma, H. M.; Zhu, J. J.; Chen, H. Y. Langmuir 2004, 20, 11738–11747.
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Letter Table 1. Size and Size Distribution of the Multilayer Hybrid Nanospheres and Concentric Hollow Nanospheres of Mesoporous Silica Shell-Titania Core samples
particle size (nm)
PDIa
shell thickness (nm)
PMAA PMAA/TiO2 PMAA/TiO2@PMAA PMAA/TiO2@PMAA@SiO2 concentric hollow TiO2 core-SiO2 shell spheres a Polydispersity index.
63 85 137 210 206
1.03 1.02 1.01 1.01 1.01
9 26 36 30
Figure 2. (a) Concentric hollow nanospheres of mesoporous silica shell-titania core as reaction cages for photocatalysis, and (b) the rate of TiO2-catalyzed photodegradation of MO in the concentric hollow silica cages. (Ka is the apparent first-order rate constant.)
Subsequent coating of silica and removal of polymeric templates can result in hollow silica cages containing multiple titania cores, as shown in the TEM image of Figure 1f. In addition, the size of the titania inner core, the size of the void, and the thickness of silica shell can be tuned through simple adjustments of the precursors (TBOT, MAA, and TEOS) concentration and/or reaction time. Mesoporous concentric hollow silica nanoreactors with other important metal cores can also be prepared. For example, hydroxylated Fe3O4 magnetic nanoparticles can be used as the core templates for preparing the initial core-shell nanospheres. In conclusion, nearly monodispersed concentric hollow nanospheres with mesoporous silica shell and anatase titania core have been prepared by the combination of sol-gel reaction and distillation-precipitation polymerization. The PMAA polymer with carboxylic functionalities acted initially as a medium for
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bridging the two different inorganic materials of titania and silica to produce the multilayered hybrid nanospheres of PMAA/ TiO2@PMAA@SiO2, and then as a sacrificial material in the preparation of concentric hollow nanospheres from the corresponding hybrid nanospheres. Narrowly dispersed and mesoporous concentric hollow nanospheres can be used as cages/reactors for photocatalytic reactions. The synthesis procedure can be readily extended to the fabrication of concentric hollow nanospheres with multiple cores or with other inorganic functional cores. Supporting Information Available: Detailed experimental procedures and characterizations of the nanostructures by XPS, XRD, TEM, FT-IR, and BET nitrogen adsorptiondesorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.
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