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Novel and Facile Method for the Preparation of Monodispersed Titania Hollow Spheres Xinjian Cheng, Min Chen, Limin Wu,* and Guangxin Gu Department of Materials Science and the AdVanced Coatings Research Center of China Educational Ministry, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed December 18, 2005. In Final Form: February 26, 2006 The fabrication of monodispersed hollow spheres in varying sizes and shapes is very interesting and has a lot of potential applications. This paper provides a very simple route to preparing hollow titania spheres using polystyrene (PS) as a template. In this approach, the titania shells were first formed and the PS cores were dissolved subsequently, even synchronously, in the same medium; neither an additional dissolution nor a calcination process was needed to remove the PS cores. Transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and Barret-Emmet-Teller measurements were used to characterize the monodispersed hollow titania spheres. A possible formation mechanism of the hollow spheres was proposed.
Introduction The fabrication of monodispersed hollow spheres in varying sizes and shapes is currently one of the fastest growing areas of materials research since the pioneering works done by Kowalski and co-workers at Rohm and Haas.1,2 The resulting hollow nanoand microspheres are of great technological importance for their potential applications in catalysis, chromatography, protection of biologically active agents, fillers (for pigments or coatings), waste removal, and large bimolecular release systems.3-13 As shown in recent studies, a variety of chemical and physicochemical methods, including heterophase polymerization/combined with a sol-gel process,14,15 emulsion/interfacial polymerization strategies,16-18 a spray-drying method,19,20 self-assembly techniques,21-23 and a surface living polymerization process24-26 have been employed to prepare hollow spheres comprised of polymeric or ceramic materials. Two of them are particularly * Corresponding author. E-mail:
[email protected]. (1) Kowalski, A.; Vogel, M.; Blankenship, R. M. U.S. Patent 4,427,836, 1884. (2) Blankenship, R. M. U. S. Patent 5,494,971, 1996. (3) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (4) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (5) Kidambi, S.; Dai, J. H.; Bruening, M. L. J. Am. Chem. Soc. 2004, 126, 2658. (6) Wang, T.; Cohen, R. E.; Rubner, M. F. AdV. Mater. 2002, 14, 1534. (7) Wang, Y.; Cai, L.; Xia, Y. AdV. Mater. 2005, 17, 473. (8) Wang, Y.; Xia, Y. Nano Lett. 2004, 4, 2047. (9) Caruso, F. AdV. Mater. 2001, 13, 740. (10) Xu, X.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 7940. (11) Yang, Z.; Niu, Z.; Lu, Y.; Hu, Z.; Han, C. C. Angew. Chem., Int. Ed. 2003, 42, 1943. (12) Kim, J.; Yoon, S.; Yu, J. Chem. Commun. 2003, 790. (13) Li, Y.; Shi, J.; Hua, Z.; Chen, H.; Ruan, M.; Yan, D. Nano Lett. 2003, 3, 609. (14) Imhof, A. Langmuir 2001, 17, 3579. (15) Tissot, I.; Reymond, J. P.; Lefebvre, F.; Bourgeat-lami, E. Chem. Mater. 2002, 14, 1325. (16) Bruinsma, P. J.; Kim, A. Y.; Liu, J.; Baskaran, S. Chem. Mater. 1997, 9, 2507. (17) Fowler, C. E.; Khushalani, D.; Mann, S. Chem. Commun. 2001, 2028. (18) ] Rana, R. K.; Mastai, Y.; Gedanken, A. AdV. Mater. 2002, 14, 1414. (19) Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature 1999, 398, 223. (20) Iida, M.; Sasaki, T.; Watanable, M. Chem. Mater. 1998, 10, 3780. (21) Discher, B. M.; Won, Y. Y.; Ege, D. S.; Lee, J. C. M.; Battes, F. S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143. (22) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (23) Wendland, M. S.; Zimmerman, S. C. J. Am. Chem. Soc. 1999, 121, 1389. (24) Zhou, Q.; Wang, S.; Fan, X.; Advincula, R. C. Langmuir 2002, 18, 3324. (25) Perruchot, C.; Khan, M. A.; Kamitsi, A.; Armes, S. P.; von Werne, T.; Patten, T. E. Langmuir 2001, 17, 4479. (26) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 2001, 123, 7497.
interesting and usually used to fabricate hollow spheres with homogeneous, dense layers. One is templating against colloid particles (including gold,27 CdS,28 Pb,7 and mesoscale ZnS29 or polymer beads30-32). In its typical procedure, template particles were coated in solution either by controlled surface precipitation of inorganic molecule precursors (silica, titania, etc.) or by direct surface reactions utilizing specific functional groups on the cores to create core-shell composites. The template particles were subsequently removed by selective dissolution in an appropriate solvent or by calcination at an elevated temperature in air to generate ceramic hollow spheres. Xia et al.33 prepared mesoscale titania hollow spheres by templating the sol-gel precursor solution against crystalline polystyrene (PS) beads. By infiltrating with the titania precursor solution the dried physically contacted PS beads between two glass substrates and exposing to the moisture in air, the precursor hydrolyzed into titania sol, which subsequently aggregated into a network of gel. The gel precipitated out, and a coating resulted around each PS bead, while the solvent evaporated. The titania-coated PS beads were immersed in toluene to dissolve the PS template, and the titania hollow spheres were obtained. Another method, termed the layer-by-layer (LbL) selfassembly technique, is becoming a very attractive topic of investigation since the pioneering work done by Caruso et al.3,9 The basis of this process is the electrostatic association between alternately deposited, oppositely charged species. Multilayered shells were assembled onto submicrometer-sized colloidal particles by the sequential adsorption of polyelectrolytes and oppositely charged nanoparticles. Upon calcination of the obtained core-shell particles, the uniform-sized hollow spheres of various diameters and wall thicknesses were generated for a variety of inorganic materials, including silica, iron oxide, titania, zeolite, clay, and inorganic heterocomposites. (27) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Chem. Commun. 1996, 731. (28) Wu, D.; Ge, X.; Zhang, Z.; Wang, M.; Zhang, S. Langmuir 2004, 20, 5192. (29) Velikov, K.; Blaaderen, A. V. Langmuir 2001, 17, 4779. (30) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693. (31) Tissot, I.; Novat, C.; Lefebvre, F.; Bourgeat-lami, E. Macromolecules 2001, 34, 5737. (32) Yang, M.; Ma, J.; Zhang, C.; Yang, Z.; Lu, Y. Angew. Chem., Int. Ed. 2005, 44, 2. (33) Zhong, Z.; Yin, Y.; Gates, B.; Xia, Y. AdV. Mater. 2000, 12, 206.
10.1021/la0534221 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/18/2006
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Figure 1. TEM images of the original PS particles (a) and all the coated spheres obtained at various amounts of ammonia: (b) 1.0 g, (c) 2.0 g, (d) 3.0 g, (e) 4.0 g. TBT ) 0.3 g. The scale bar is 1 µm for all the images.
Unfortunately, although the above pioneering works are very interesting, the preparation processes seem to be very timeconsuming. First, multistep processes are needed for the synthesis of core-shell composite particles. For example, surface functionalization of templating particles, exchange of the solvent, the coating reaction for the templating particles approach or repeated adsorption, centrifugation, a water wash, and redispersion cycles are the steps needed for the LbL method. Second, to obtain hollow spheres from the core-shell composite particles, removing the core particles by selective dissolution in addition to an appropriate solvent or by calcination at elevated temperature in air is indispensable. Therefore, how to develop facile and feasible methods to prepare hollow spheres remains a great challenge to materials scientists. In this paper, we describe a novel and facile technique to fabricate titania hollow spheres via a one-step process, which means the formation of titania shells and the dissolution of core particles occurs in the same medium. In comparison with other techniques, the primary differences and advantages of this new approach are: (i) monodispersed PS particles were prepared via a dispersion polymerization in an ethanol/water medium in which hydrolysis and condensation of tetra-n-butyl titanate (TBT) was carried out directly, and no exchange of solvent was needed; (ii) positively charged PS colloids were obtained by using the cationic monomer 2-(methacryloyl)ethyltrimethylammonium chloride (MTC) as a co-monomer,9,14 which ensured the generating titania sol could be rapidly captured by PS particles via electrostatic interaction and the homogeneous nucleation of titania could be
avoided; (iii) hydrolysis and condensation of TBT was carried out in aqueous ammoniacal alcohol medium at 50 °C, in which PS particles were dissolved subsequently, even synchronously, and neither additional dissolution nor a calcination process was needed. Experimental Section Materials. Styrene (St) was purchased from Shanghai Chemical Reagent Co. (China) and purified by treating with 5 wt % aqueous NaOH to remove the inhibitor. An aqueous solution of MTC (76.7 wt %) was supplied by YanCheng Medical Chemical Co. (China). 2,2′-azobis(isobutyronitrile) (AIBN, 98%, Aldrich, U.S.A.) was recrystallized from tetrahydrofuran before use. Poly(vinylpyrrolidone) (PVP, K30), with a molecular weight of 30 000, TBT, absolute ethanol, and aqueous ammonia solution (28 wt %) were purchased from Shanghai Chemical Reagent Co. (China) and used as received. Ultrapure water (>17 MΩ cm-1) from a Milli-Q water system was used throughout the experiment. Synthesis of Positively Charged PS Spheres. Monodispersed positively charged PS spheres were synthesized by dispersion polymerization in an ethanol/water medium as follows: 10 g of water, 0.6 g of AIBN, 3 g of PVP, half of the amount of St (10 g), and 45 g of ethanol were charged into a 250-mL three-necked flask equipped with a mechanical stirrer, thermometer with a temperature controller, N2 inlet, Graham condenser, and a heating mantle. The reaction solution was deoxygenated by bubbling nitrogen gas at room temperature for about 30 min and then heated to 70 °C under a stirring rate of 100 rpm for 1.5 h, followed by the addition of the solution of the rest of the St (10 g), EtOH (45 g), and all of the MTC
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Figure 2. SEM images of the typical hollow titania spheres obtained at various amounts of ammonia: (a, b) 2.0 g; (c, d) 3.0 g; (e, f) 4.0 g. (0.4 g). The reaction was continued until the conversion of St reached 95% and then cooled to room temperature. Synthesis of Titania Hollow Spheres. Positively charged PS spheres dispersion in the amount of 5 g was diluted in 30 g of ethanol, then various amounts of aqueous ammonia solution and TBT solution in ethanol at room temperature were added dropwise under stirring within 30 min. The mixture was stirred for 1 h and then heated to 50 °C for another 1.5 h to obtain hollow titania spheres. Characterization. Transmission Electron Microscopy (TEM) ObserVation. A transmission electron microscope (TEM Hitachi H-800, Hitachi Corp.) was used to observe the morphologies of the obtained particles. The particle dispersions were diluted with ethanol and sonicated at 25 °C for 15 min and then dried onto carbon-coated copper grids before examination. Scanning Electron Microscopy (SEM) ObserVation. (i) A scanning electron microscope (SEM Philips XL30 apparatus) was employed to observe the morphologies of the obtained particles. The particle dispersions were diluted and dried on a cover glass and sputter-
coated with gold prior to examination. (ii) The typical hollow titania spheres were dehydrated and subsequently embedded in epoxy for microtoming. The obtained ultrathin sections were sputter-coated with gold, and then the morphologies of the broken hollow tatania spheres were observed by SEM. ThermograVimetric Analysis (TGA). TGA was performed using a Perkin-Elmer thermogravimetric apparatus (U.S.A.) under a stream of air. The dried PS particles, titania-coated composite particles, and hollow titania spheres were heated from 25 to 800 °C at a scan speed of 10 °C/min. Fourier Transform Infrared Spectroscopy (FTIR) Measurement. Fourier transform infrared spectra were taken on a Nexus 470 FTIR spectrometer (Nicolet, U.S.A.). The particle dispersions were centrifuged and washed with absolute ethanol, then dried at 100 °C and pressed into KBr pellets for FTIR measurement. Barrett-Emmett-Teller (BET) Analysis. The nitrogen adsorption measurements were performed at 77 K using an ASAP 2010 analyzer,
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Results and Discussion
Figure 3. TEM images of the coated spheres obtained at various amounts of TBT: (a) 0.1 g, (b) 0.3 g, (c) 0.5 g, (d) 0.8 g. The scale bar is 1 µm for all the images.
Effect of the Amount of Ammonia. First, monodispersed positively charged PS particles were prepared by dispersion polymerization on the basis of the procedures described in the experiments. The TEM image, as demonstrated in Figure 1a, showed that uniform spherical PS particles with an average diameter of about 1.30 µm were obtained (measured by TEM). In the subsequent coating process, ammonia and TBT were added to the PS suspension for the sol-gel reaction. Under this condition, all titania formed as the shell on the core particle via the ammonia-catalyzed hydrolysis and condensation of TBT. No free titania particles were found in the medium, which indicated the intelligent selection of cationic MTC as the comonomer.34 More interestingly, the PS particles were dissolved in the same medium subsequently, even simultaneously, depending upon the amount of ammonia. Figures 1b-e displayed the TEM images of the four samples obtained from different ammonia amounts. At an ammonia amount of 1.0 g, the PS particles could not be dissolved in the medium, and the composite particles with PS as the core and titania as the shell formed. The average diameter of the composite particles was 1.38 µm, as shown in Figure 1b. The surface roughness of the spheres compared with that of the original PS particles indicated the formation of titania shells surrounding the core particles. When the ammonia was increased to 2.0 g, most of the PS particles were dissolved in the medium. The TEM in Figure 1c showed that the spheres consisted of titania layers supported by incompletely dissolved PS particles.
Figure 4. SEM images of the typical hollow titania spheres obtained at various amounts of TBT: (a) 0.1 g, (b) 0.3 g, (c) 0.5 g, (d) 0.8 g. The ammonia amount was kept at 3 g in all the above formulations.
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Figure 5. SEM images of the typical broken hollow titania spheres obtained at different magnifications: (a) ×10 000, (b) ×20 000.
Figure 6. FTIR spectra of (a) PS particles, (b) titania-coated PS, and (c) hollow titania spheres.
Figure 7. TGA curves for the (a) PS particles, (b) selected titaniacoated PS particles, and (c) hollow titania spheres.
When the ammonia continued to be increased to 3.0 g, the high contrast of the shells with the cores denoted that the PS particles inside were completely dissolved and homogeneous and dense titania layers were formed, causing monodispersed hollow titania spheres, as demonstrated in Figure 1d. As the ammonia content was further increased to 4.0 g, although the PS cores were completely dissolved, the hollow titania spheres were disfigured, as shown in Figure 1e. The reason for this was not very clear, (34) Chen, M.; Zhou, S.; You, B.; Wu, L. Macromolecules 2005, 38, 64116417.
however, it was probably because higher ammonia content resulted in faster hydrolysis and condensation of TBT, causing nucleation of secondary titania particles, which caused decreasing titania to form as the shells. Thus, the almost monodispersed hollow titania spheres could be easily obtained by adjusting the amount of ammonia. Figure 2 further illustrated the SEM images of the hollow titania spheres prepared at the ammonia amounts of 2.0, 3.0, and 4.0 g, respectively. It was found that all the spheres maintained the spherical morphology and narrow size distribution with around 9% polydispersity, suggesting that the anomalistic or deformed spheres in Figure 1 were resulted from the electron beam from the high voltage of TEM. However, if the amount of ammonia was too high, for example, 4.0 g, some protuberances were clearly seen on the surfaces of the titania shells, probably because of the formation of the secondary particles as a result of faster hydrolysis and condensation of TBT at higher ammonia amounts, just as discussed in TEM. Effect of the Amount of TBT. Figure 3 demonstrates the effect of the amount of TBT on the hollow titania spheres at 3.0 g of ammonia. When less than 0.5 g of TBT was used, there did not exist any secondary nucleation of titania on the surfaces of the hollow spheres (see Figures 3a-c). However, when TBT content continued to be increased to 0.8 g, a lot of secondary nucleation of titania could be seen, and the hollow titania spheres were disfigured (see Figure 3d). These spheres were probably disfigured because the faster hydrolysis and condensation of TBT as a result of higher TBT content resulted in the nucleation of secondary titania particles, which contrarily decreased the wall thickness of the hollow spheres with increasing TBT content. These hollow spheres, with decreasing wall thickness, could not endure the electron beam of TEM, just as discussed above. Figure 4 further presents the SEM images of hollow titania spheres obtained at various TBT concentrations. The titania particles of secondary nucleation were observed clearly on the surfaces of hollow titania spheres prepared at high TBT content, for example, especially more than 0.8 g (see Figure 4d). To further confirm the hollow structure of the obtained titania spheres, the selected microspheres were dehydrated and then embedded in epoxy resin for microtoming. The obtained ultrathin sections were observed by SEM, as demonstrated in Figure 5, clearly indicating the hollow structure with a wall thickness of ∼40 nm and 9.1 and 10.3% for the variation coefficients of the diameter and the wall thickness, respectively. FTIR Analysis. To further verify the PS cores were removed in an ethanol/water/ammonia medium to form hollow titania spheres, the FTIR spectra of the PS particles, the titania-coated PS particles, and the hollow titania spheres were recorded and shown in Figure 6. The characteristic absorption bands at ∼3000
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Figure 8. Schematic illustration of the formation mechanism for hollow titania spheres: (a) titania-formed and PS-dissolved subsequently, and (b) PS-dissolved and titania-formed synchronously.
cm-1 for the C-H stretch, ∼1490 cm-1 for the aromatic C-C stretch, and 1700-2000 cm-1 for the aromatic overtones were clearly seen on the spectra of PS and titania-coated PS particles but disappeared in the spectrum of the hollow titania spheres, indicating that PS had completely been dissolved out. TGA Analysis. The thermogravimetric curves of the PS particles, the titania-coated PS composite particles, and the hollow titania spheres were shown in Figure 7. For the PS particles, there were two main temperature regions of weight loss; the weight loss below 270 °C could be attributed to the evaporation of physically absorbed water and residual solvent in the samples, and the weight loss in the temperature region of 270-390 °C could be resulted from the decomposition of PS. There were almost no residuals at much higher temperatures. For the titaniacoated PS composite particles, three weight loss stages (below 270, 270-390, and 390-490 °C), were observed, corresponding to the evaporation of physically adsorbed water, the decomposition of PS, and the decomposition of titania-bonded groups such as -OH and/or unhydrolyzed -OR, respectively. For the hollow titania spheres, the main weight loss stages occurred in the regions of below 270 and 390-490 °C, indicating that almost all the PS was removed. Porosity and Pore Size. The specific surface areas of the hollow titania spheres obtained at various TBT content were determined by the BET method and were 41, 36, and 34 m2/g, corresponding to 0.3, 0.5, and 0.8 g of TBT content, respectively, while the average pore size of the hollow titania spheres obtained at 0.3, 0.5, and 0.8 g of TBT were 17, 21, and 18 nm, respectively. That is, no obvious changes in specific surface area and porosity were observed with the wall thickness of titania, suggesting that the titania shell coated on the surfaces of PS cores was very dense.35 Formation Mechanism. To further understand the formation process of the hollow titania spheres, a control experiment omitting TBT in the formulation was carried out as follows: The same amount of PS suspension was heated to 50 °C in the presence of 3 g of ammonia for 1 h and then examined by TEM. No (35) Yuan, J.; Zhou, S.; You, B.; Wu, L. Chem. Mater. 2005, 17, 3587-3594.
particles were observed, indicating that PS particles were absolutely dissolved in the medium. Based on the above research, the possible formation mechanism of hollow titania spheres was proposed, as indicated by the schematic diagram in Figure 8. In the ethanol/water/ammonia medium at 50 °C, all titania formed as the shell on the PS core particle via the ammonia-catalyzed hydrolysis and condensation of TBT. After the coating of titania, the PS cores were dissolved subsequently, as shown in Figure 7a, or the PS cores almost began to dissolve synchronously during the coating process, as seen in Figure 7b. These dissolved PS macromolecule chains and their aggregates diffused out through the porous titania shells, forming hollow titania spheres. When high ammonia content or TBT content was used, the PS core particles were unable to capture the rapidly forming titania particles as a result of the fast hydrolysis and condensation of TBT, causing some nucleation of the secondary titania particles.
Conclusion In this study, a novel and facile approach to fabricate monodispersed hollow titania spheres on the basis of a dispersion polymerization/sol-gel process was proposed in which monodispersed positively charged PS particles with MTC as the comonomer were first prepared by dispersion polymerization. The titania shells were then coated on the PS particles via an ammoniacatalyzed hydrolysis and condensation of TBT. The PS particles were dissolved subsequently, even synchronously, during the coating process in the same medium. This technique presented a new paradigm in the preparation of hollow spheres. Based on this technique, other inorganic hollow spheres with various shell compositions (e.g., SiO2, Al2O3, V2O3, SnO2, and so on) could also be prepared. The related research is currently under way. Acknowledgment. We thank National “863” Foundation, Shanghai Special Nano Foundation, the Key Project of China Educational Ministry, the Doctoral Foundation of University, and Trans-century Outstanding Talented Person Foundation of China Educational Ministry for financial support for this research. LA0534221
