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Hollow Titania Spheres with Movable Silica Spheres Inside Kai Zhang, Xuehai Zhang, Haitao Chen, Xin Chen, Linli Zheng, Junhu Zhang, and Bai Yang* Key Lab of Supramolecular Structure & Materials, College of Chemistry, Jilin University, 10 # Qianwei Road, Changchun, People’s Republic of China Received September 10, 2004. In Final Form: October 13, 2004 We demonstrate a flexible method for preparing hollow TiO2 nanospheres with movable silica nanoparticles inside (HTNMSNs). In this method, we used monodisperse silica-polystyrene core-shell nanospheres (SiO2-PS-CSNs) sulfonated as templates and prepared the composite shell consisting of TiO2 and sulfonated polystyrene (SPS) through adsorbing or depositing tetrabutyl titanate gel into the SPS shell. Finally the HTNMSNs were obtained after removal of all polymers in the composite nanospheres by dissolution or calcinations. We investigated the dependence of the morphologies of HTNMSNs on the thickness of PS shells and the size of SiO2 cores and prepared rare earth doped HTNMSNs by a sol-gel process.
Functional hollow nanospheres and microspheres have increasingly attracted interest due to their potential applications in catalysis, controlled delivery, artificial cells, light fillers, low dielectric constant materials, acoustic insulation, and photonic crystals. Various procedures have been developed to prepare functional hollow spheres and form a range of materials such as carbons, polymers, metals, and inorganic materials.1 One existing typical method for preparing these hollow spheres is based on selectively removing the cores in the spherical core-shell particles by a solvent or calcination. Meanwhile, layerby-layer (LbL), a simple and facile method for preparing these functional hollow spheres consisting of functional shells or interiors, has been successfully used to prepare various hollow spheres including gold, silver, CdS nanoparticles, and even biomolecules by using polymer or silica beads as templates.2 Other successful chemical and physical processes based on different templates were established to produce functional hollow spheres such as high bulk refractive index hollow ZnS spheres3 and hollow palladium spheres for catalysis.4 Hollow spheres equipped with functional microparticles in their cavities have been developed by some groups. Xia and co-workers synthesized polymer hollow spheres whose interiors were functionalized with movable gold microparticles, which is a novel functional hollow organic sphere structure.5 Other novel structures include tin metal particles which were encapsulated with spherical hollow carbon by Oh and his group.6 The hollow sphere structure * To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) Zhong, Z.; Yin, Y.; Gates, B.; Xia, Y. Adv. Mater. 2000, 12, 206-209. (b) Chen, Z.; Zhan, P.; Wang, Z.; Zhang, J.; Zhang, W.; Ming, N.; Chan, C. T.; Sheng, P. Adv. Mater. 2004, 16, 417-422. (c) Pavlyuchenko, V. N.; Sorochinskaya, O. V.; Ivanchev, S. S.; Klubin, V. V.; Kreichman, G. S.; Budtov, V. P.; Skrifvars, M.; Halme, E.; Koskinen, J. J. Polym. Sci. Polym. Chem. 2001, 39, 1435-1449. (d) Wu, D.; Ge, X.; Zhang, Z.; Wang, M.; Zhang, S. Langmuir 2004, 20, 5192-5195. (2) (a) Decher, G. Science 1997, 277, 1232-1237. (b) Liang, Z.; Susha, A.; Caruso, F. Chem. Mater. 2003, 15, 3176-3183. (c) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317-2328. (d) Caruso, F. Adv. Mater. 2001, 13, 11-22. (3) Velikov, K. P.; Blaaderen, A. Langmuir 2001, 17, 4779-4786. (4) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642-7643. (5) Kamata, K.; Lu, Y.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 23842385. (6) Lee, K. T.; Jung, Y. S.; Oh, S. M. J. Am. Chem. Soc. 2003, 125, 5652-5653.
Scheme 1. Synthesis Scheme for Hollow Titania Spheres with Movable Silica Spheres Inside
prevents the aggregation of functional microparticles and provides a profitable space for the functional microparticles in the practical application. These works are revelatory for the preparation of functional hollow spheres. Although Xia fabricated ceramic hollow spheres whose interior surfaces could be functionalized with the prespecified and nanoscopic objects,7 there are few reports about the fabrication of hollow inorganic spheres with movable cores inside. From this viewpoint, we propose a flexible process for fabricating hollow TiO2 spheres with movable cores such as silica nanospheres inside (HTNMSNs). This method is useful to obtain interesting and controllable structural hollow inorganic spheres and movable inorganic particles inside with well-defined morphology. These results offer a powerful platform to construct double functional hollow nanospheres applied in catalysis, magnetics, electronics, and photonic crystals. Results and Discussion In our previous works, we have synthesized monodisperse and well-defined spherical silica-polystyrene coreshell nanospheres (SiO2-PS-CSNs) by emulsion polymerization.8 In this paper, the SiO2-PS-CSNs were used as templates in the preparation process of HTNMSNs. The average diameters of nanospheres range from 177 to 330 nm, and the size of SiO2 core and PS shell can be changed according to the demand for templates. Scheme 1 shows the schematic procedure for preparing HTNMSNs. The monodisperse SiO2-PS-CSNs were sulfonated and used as templates, and then the SiO2-PS-sulfonated-PS coreshell nanospheres (SiO2-PS-SPS-CSNs) were immersed into the solution of tetrabutyl titanate (TBT) in absolute ethanol and hydrolyzed to allow a shell of TiO2 to form. (7) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. Chem. Mater. 2001, 13, 11461148. (8) Zhang, K.; Chen, H. T.; Chen, X.; Chen, Z. M.; Cui, Z. C.; Yang, B. Macromol. Mater. Eng. 2003, 288, 380-385.
10.1021/la047736k CCC: $27.50 © 2004 American Chemical Society Published on Web 11/12/2004
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Figure 1. TEM images of (A) SiO2-PS-SPS-CSNs (insert: TEM image of SiO2-PS-TiO2-CSNs, the scale bar is 200 nm) and (B) HTNMSNs after calcination at 700 °C for 1 h (insert: diffraction image of HTNMSNs).
After the polystyrene layers were removed from the capsules by either calcination or dissolution with toluene, HTNMSNs were obtained. The thickness of TiO2 shells and the size of cavities and cores can be controlled through changing the sulfonation time and the size of SiO2-PSCSNs, as mentioned by Yang and co-workers.9 In this paper, SiO2-PS-CSNs with a mean diameter of 330 nm were used as an example. The SiO2-PS-CSNs have beautiful structures of core-shell and were sulfonated in concentrated sulfuric acid. The SiO2-PS-SPS-CSNs were obtained after separating via centrifugation and cleaning by absolute ethanol. Figure 1 shows transmission electron microscopy (TEM) images of SiO2-PS-SPS-CSNs and HTNMSNs after calcination at 700 °C for 1 h. As shown in the TEM image of Figure 1A, the SiO2-PS-SPS-CSNs are in spherical shape and the mean diameter is 4.2% bigger than that of SiO2-PS-CSNs; these observations depend on the facts that the sulfonation improves the hydrophilicity of the polymer shell layer and the mean diameter of CSNs increases from 330 to 344 nm under the swelling action of polar solvent. The sulfonation process did not deform the spheroid of nanospheres without crosslinking, which is useful for the control of structures and morphologies; at the same time, the polymer shell without cross-linking gives us a better chance to obtain HTNMSNs by dissolving polymer with solvent. Through adsorption and hydrolyzation, SiO2-PS-TiO2-CSNs with a three-layer structure were obtained, and the inset in Figure 1A shows their morphology. The monodisperse SiO2-PS-TiO2-CSNs well-dispersed in solvent have potential applications in photonic crystals. From Figure 1B, we can see the HTNMSNs with 233 nm size after calcination, which shrink 29% from that of SiO2-PS-CSNs, and the TiO2 shell is 28 nm in thickness. From the TEM image (Figure 1B), we find that the 145 nm SiO2 cores lie in the cavities of TiO2, and the TiO2 shells constituted by the TiO2 nanoparticles are rough and loose, which is clearly verified in the scanning electron microscopy (SEM) image, too. We thought the formation
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of tiny gaps in the TiO2 shell depends on the shrinkage and crystal inversion of the TiO2 gel in the process of calcination. The tiny gaps in the TiO2 shell are useful for potential applications when the silica cores are replaced by other functional particles, for example, catalysis, magnetic, and electric applications. The diffraction image (the insert of Figure 1B) and the X-ray diffraction (XRD) pattern (Figure 2A) prove the structure of anatase of the TiO2 shell; the presence of the SiO2 core did not affect the crystal structure, and it would not affect the application of TiO2 in photocatalytic material, which is important for constructing double functional nanospheres. To broaden the synthesis process to other functional materials as core or shell, rare earth doped HTNMSNs were prepared. The complex of Eu with fluorescence was doped into the mixture of TBT and ethanol, and the products were calcined at 500 °C for 4 h. Figure 2B shows the photoluminescence (PL) spectrum obtained for the Eu-doped HTNMSNs; Eu3+ transitions with the maximum at 613 nm (5D0 f 7F4) are well resolved in the spectrum. The sol-gel process of TiO2 helped the successful composition of the complex of Eu, and it is very important that the process is appropriate for many other inorganic nanoparticles which can be prepared or blended by the sol-gel method. Various hollow nanospheres with movable functional cores can be obtained when we use other inorganic/organic CSNs as templates or inorganic nanoparticles as hollow shells. Considering the functional nanoparticles which cannot be dealt with by calcination, we used an etching process via a solvent to remove the polymer shell. Figure 3 shows TEM and SEM images of HTNMSNs etched by toluene; the mean size of HTNMSNs is 344 nm with a shell of 40 nm in thickness, and shrinkage of the TiO2 shell did not occur. Cavities of TiO2 formed, and the cores of silica remained in the cavities after the layers of polystyrene were dissolved by toluene. The shells of TiO2 are compact and smooth; however, some broken shells are seen from the TEM and SEM images (Figure 3) and the silica cores can be seen through the broken area (the arrow). This problem was expected to be improved through adjusting the conditions of reaction. But the dissolution method does not adapt to obtain the Eu-doped HTNMSNs; the large amount of hydroxyl in the TiO2 gel shell quenched the luminescence intensity. At the same time, we found that a part of the PS chains inside the TiO2 cavities cannot be removed by toluene because of the covalent bond between PS chains and silica cores. The thermogravimetric analysis (TGA) spectrum of HTNMSNs after dissolution by toluene proved 27.91% weight loss of the sample from 329 to 415 °C for the decomposition of remnants of PS in the HTNMSNs.
Figure 2. XRD pattern of HTNMSNs after calcination at 700 °C for 1 h (A) and PL spectra of Eu-doped HTNMSNs (B).
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tion or solvent dissolving. This special structural material offers the possibility to construct double functional hollow spheres. Finally, the process was attempted to synthesize other functional materials, such as fluorescent Eu-doped HTNMSNs. We believe that this method could be extended to prepare other functional structures and materials and open up a variety of applications in catalysis, magnetics, electronics, and photonic crystals. Figure 3. TEM and SEM images of HTNMSNs after dissolving by toluene. (The arrow shows the silica core in the cavity of the hollow TiO2 sphere.)
Acknowledgment. This work was supported by the Special Funds for Major State Basic Research Projects (No. 2002CB613401) and the National Natural Science Foundation of China (No. 200340062).
Conclusions In conclusion, by means of sulfonation and further adsorbed TBT solution, we have successfully prepared monodisperse HTNMSNs with different sizes via calcina-
Supporting Information Available: The detailed synthetic procedure for HTNMSNs, TEM image of the SiO2PS-CSNs, SEM image of HTNMSNs after calcination, and TGA results of HTNMSNs etched by toluene. This material is available free of charge via the Internet at http://pubs.acs.org.
(9) Yang, Z.; Niu, Z.; Lu, Y.; Hu, Z.; Han, C. C. Angew. Chem., Int. Ed. 2003, 42, 1943-1945.
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