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Preparation of Titania-Coated Polystyrene Particles in Mixed Solvents by Ammonia Catalysis Peng Wang,†,‡ Dong Chen,† and Fang-Qiong Tang*,† Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China ReceiVed January 12, 2006. In Final Form: February 24, 2006 A mixed-solvent method was developed to coat polystyrene (PS) spheres with smooth, homogeneous shells of amorphous titania by ammonia catalysis. The TEM images showed that, in the presence of ammonia, the thickness of titania shells could be controlled in the range of 8-65 nm by varying the concentration of titanium tetrabutoxide (TBOT) in the ethanol/acetonitrile mixed solvents with an appropriate volume ratio. The diffusion-controlled mechanism of the mixed solvents and the catalysis mechanism of ammonia were investigated. After the calcination of core-shell particles for 2 h at 500 °C, spherical hollow titania shells could be obtained, and the surfaces of the particles remained quite smooth and homogeneous. The XRD analysis indicated that calcination promoted the transformation of amorphous titania into an anatase phase.
Introduction Inorganic hollow spheres, together with their core/shell structures, have attracted much research interest in recent years owing to their potential applications in controlled delivery, catalysis, photochemical solar cells, and confined reactors.1-5 Furthermore, the structure, size, and composition of theses particles can be altered in a controllable way over a broad range to tailor their optical, electrical, thermal, mechanical, electrooptical, magnetic, and catalytic properties. Among those core/shell particles, titania-coated polymer spheres are one of the model systems for the study of encapsulation and hollow sphere preparation because titania-coated polymer spheres and the resulting hollow spheres are very useful as photonic band gaps (PBG),6 catalysts,7 and nanoreactors.3 Considerable effort has been devoted to the preparation of titaniacoated polymer spheres, and there are two main approaches: the first7-9 is to form a coating by the controlled reaction of titania precursors with polymer particles; the second,6,10-12 which is a combined layer-by-layer (LBL) technique, is to coat titania nanoparticles onto polymer particles by deposition. In terms of simplicity, the first method based on sol-gel precursors seems * Corresponding author. E-mail:
[email protected]. Phone: +8610-82543521. Fax: +86-10-62554670. † Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences. (1) Nishimura, S.; Arams, N.; Lewis, B. A.; Halaoui, L. I.; Mallouk, T. E.; Benkstein, K. D.; Lagemaat, J. V.; Frank, A. J. J. Am. Chem. Soc. 2003, 125, 6306. (2) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (3) Li, J.; Zeng, H. C. Angew. Chem., Int. Ed. 2005, 44, 4342. (4) Caruso, F. Chem.sEur. J. 2000, 6, 413. (5) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (6) Nakamura, H.; Ishii, M.; Tsukigase, A.; Harada, M.; Nakano, H. Langmuir 2005, 21, 8918. (7) Zhang, M.; Gao, G.; Zhao, D. C.; Li, Z. Y.; Liu, F. Q. J. Phys. Chem. B 2005, 109, 9411. (8) Imhof, A. Langmuir 2001, 17, 3579. (9) Yang, Z. Z.; Niu, Z. W.; Lu, Y. F. Angew. Chem., Int. Ed. 2003, 42, 1943. (10) Caruso, F.; Shi, X. Y.; Caruso, R. A.; Susha, A. AdV. Mater. 2001, 13, 740. (11) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (12) Wang, L.; Sasaki, T.; Ebina, Y.; Kurashima, K.; Watanabe, M. Chem. Mater. 2002, 14 4827.
to be the most attractive for forming core/shell particles. However, because of the high activity of titania precursors, this method is generally accompanied by a number of disadvantages, such as the formation of irregular coatings, aggregation of the coated particles, and low efficiency of controlling the coating thickness. Therefore, it is necessary for the formation of dense, smooth titania coatings on the surface of polymer colloid particles to control the hydrolysis rate and the diffusion rate of titania precursor in the reaction system. Recently, the use of mixed solvents has been developed into a new approach to synthesize and process materials. Varying the ratio of the mixed solvents affects not only the hydrolysis rate but also the diffusion rate of the precursor, both of which have great effects on the morphology of the particles. Many kinds of spherical inorganic particles have been synthesized by the hydrolysis of metal salts and alkoxides in mixed solvents.13-17 However, to the best of our knowledge, no report has been devoted to the preparation of colloid particles coated with titania in mixed solvents. In this work, for the first time we developed a mixedsolvent method to coat anionic PS spheres with titania by surface catalysis to promote condensation reactions. Although this is an example of coating by heterocoagulation, it led to unusually smooth, homogeneous titania shells whose thickness could be controlled in the range of 8-65 nm. The effects of volume ratios of ethanol to acetonitrile in the mixed solvent and the concentration of titanium tetrabutoxide (TBOT) on the morphology and thickness of titania coating were investigated. Moreover, the mechanism of coating by ammonia catalysis was also studied. By calcination, hollow titania spheres with a smooth, dense morphology were obtained. Experimental Section Materials. Titanium tetrabutoxide (TBOT), potassium persulfate (KPS), acetonitrile, and ammonia were purchased and used without further purification. Styrene was purified by distillation under reduced pressure. Ethanol was dehydrated by molecule sieves. (13) Chen H. I.; Chang, H. Y. Colloids Surf., A 2004, 242, 61. (14) Park, K. H.; Kim, D. K.; Kim, Ch. H. J. Ceram. Soc. 1997, 80, 743. (15) Choi, J. Y.; Kim, D, K. J. Sol.-Gel Sci. Technol.1999, 15, 231. (16) Chen, K. Y.; Chen, Y. W. J. Sol.-Gel Sci. Technol. 2003, 27, 111. (17) Mine, E.; Hirose, M.; Nagao, D.; Kobayashi, Y.; Konno, M. J. Colloid Interface Sci. 2005, 291, 162.
10.1021/la060112p CCC: $33.50 © 2006 American Chemical Society Published on Web 04/08/2006
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Figure 1. TEM images of titania-coated PS spheres (A) in the absence of ammonia and (B) in the presence of ammonia. Anionic PS Spheres. Anionic PS spheres of 235 nm diameter, used as core materials, were prepared by emulsifier-free emulsion polymerization using KPS as the anionic initiator, as described by Furusawa et al.18 Typically, under gentle stirring, 13 mL of styrene was added at room temperature to 120 mL of deionized water that was purged with nitrogen before the reaction. After 0.3 g of KPS was also added, the temperature was increased gradually to 70 °C, and the mixture was stirred for 24 h at 70 °C. The resulting PS spheres were recovered by centrifugation and washed three times with deionized water. After redispersion into deionized water, the sample was freeze dried for 24 h and then preserved for subsequent experiments. Coating PS Spheres with Titania. The coating reaction was processed in the mixed solvent of ethanol and acetonitrile at room temperature by hydrolyzing TBOT in the presence of ammonia. In a typical experiment, 0.024 g of PS spheres of 235 nm diameter were dispersed in 100 mL of ethanol/acetonitrile (3:1 v/v) and then mixed with 0.3 mL of ammonia at room temperature. Finally, a solution of 0.5 mL TBOT in 20 mL of ethanol/acetonitrile (3:1 v/v) was added to the above PS suspension under stirring. After reacting for 1 h, the composite particles were cleaned by three cycles of centrifugation and ultrasonic dispersion in ethanol. Hollow Titania Spheres Obtained by Calcination. The PS cores could be removed by calcination in a furnace. The suspension of titania-coated PS particles in ethanol was dried at 60 °C and then calcined in a furnace at 500 °C for 2 h in air. Characterization. A transmission electron microscope (TEM, JEOL-200CX) and a scanning electron microscope (SEM, Hitachi 4300) were used to observe the morphology of the particles before and after calcination. The samples calcined at 500 °C were also characterized by means of X-ray powder diffraction (XRD, D/max2400, Cu KR, λ ) 0.15418 nm). The process taking place during calcination was studied by thermogravimetric analysis and differential scanning calorimeter (TGA-DSC, STA 449C).
Results and Discussion Effect of Ammonia on the Formation of the Titania Coating. Figure 1 shows the TEM images of the titania-coated PS particles prepared with and without ammonia, respectively. Except for the ammonia, the rest of the concentrations are the same during the coating step for the two samples. It reveals that, in the presence of ammonia, smooth, homogeneous titania coatings are formed on the surface of PS spheres (Figure 1B). However, when the ammonia is substituted with doubly distilled water, a large amount of titania flocculation is formed and dispersed in the solution, and a very thin titania coating can be found on the surface of PS spheres (Figure 1A). It is supposed that the ammonia ions (NH4+) play a very important role in overcoming the generally weak repulsive barrier and drawing the negatively charged tTiO- species together. The formation mechanism of the titania coating can be visualized from Figure 2. First, the NH4+ species are formed in the reaction (18) Furusawa, K.; Norde, W.; Lyklema, J. Kolloid Z. Z. Polym. 1972, 250, 908.
Figure 2. Scheme for depositing the titania coating onto the PS surface by the surface catalysis of ammonia.
system, as shown in eqs 1 and 3. Then the NH4+ species can be held by the counter-charged surface of PS spheres terminating in the -SO4- groups. Finally, with the accumulation of NH4+ species on the surface of the PS spheres, the negatively charged tTiO- species are adsorbed by the NH4+ species onto the surface of the PS spheres, and then the condensation process by the catalysis of NH4+ species will occur easily on the surface of PS spheres, resulting in core-shell particles (eq 4 in Figure 2). However, in the absence of ammonia, because there is no accumulation of NH4+ species on the surface of PS spheres, the condensation will not occur exactly on the surface of PS spheres but will arise randomly in the reaction system, so the smooth, homogeneous titania coatings hardly form on the surface of PS spheres, as shown in Figure 1A. Therefore, it can be concluded that, in the reaction system, ammonia acts as a catalyst to promote the formation of a smooth, homogeneous titania coating on the surface of PS spheres. Effect of Ethanol/Acetonitrile Volume Ratios on the Formation of the Titania Coating. Figure 3 shows the TEM images of PS spheres and titania-coated particles prepared at various ethanol/acetonitrile volume ratios (120 mL total volume). The amounts of TBOT, ammonia, and PS spheres were 0.12 mL, 0.08 mL, and 0.024 g, respectively, and they remained constant in all of the coating reactions. The titania coatings (Figure 3B) on the PS spheres are very thin and rough for 9:1 ethanol/ acetonitrile. The smooth, homogeneous titania coatings (Figure 3C) can be achieved when the volume ratio is decreased to 3:1. However, when the ethanol/acetonitrile volume ratio is 1:1, necks between particles (Figure 3D) are formed. As a control, we also investigated the morphology of the titania coating in the absence of acetonitrile. The results show that there is no obvious formation of titania coatings when acetonitrile is not involved in the reaction
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Figure 3. TEM images of (A) PS spheres and titania-coated PS spheres obtained at different ethanol/acetonitrile volume ratios of (B) 9:1, (C) 3:1, and (D) 1:1.
system. These differences in morphology indicate that the ethanol/ acetonitrile volume ratio has a great effect on the formation of titania coatings. For the formation of smooth, homogeneous titania coatings, the appropriate ethanol/acetonitrile volume ratio is 3:1. This effect of volume ratios of mixed solvents on the formation of titania coatings can be explained by the diffusion rate of TBOT and its hydrolysate onto the surface of PS spheres in the mixed solvents. In the presence of ammonia, the hydrolysis of TBOT molecules will result in the formation of the hydrolysates of TBOT that coexist with TBOT in the solution. The TBOT molecules and their hydrolysates have a high solubility in ethanol, indicating the relatively strong interaction between the solute and the solvent molecules, whereas acetonitrile is just the reverse. As a result, when a certain amount of acetonitrile is added to the TBOT solution containing ethanol, the interaction between the solute of TBOT and their hydrolysate molecules and the mixed solvents of ethanol and acetonitrile will be weakened. Therefore, TBOT and their partial hydrolysates in the mixed solvent can easily diffuse onto the surface of PS spheres and then hydrolyze and condense by the catalysis of NH4+ ions to form a titania coating rather than titania particles dispersing in solution. By regulating the acetonitrile/ethanol volume ratio, an appropriate diffusion rate for TBOT and their partial hydrolysates can be obtained to promote the 3D polymerization reaction on the surface of PS spheres. Therefore, a smooth, homogeneous coating can be formed. However, at higher acetonitrile content, necks between particles will be formed because of the higher diffusion rate of TBOT molecules and their hydrolysates to the surfaces of PS spheres. Control over the Thickness of Titania Shells. When the ethanol/acetonitrile volume ratio is constant at 3:1, the thickness of titania shells strongly depends on the concentration of TBOT in the mixed solvent. This is because at such a volume ratio almost all TBOT molecules in the mixed solvent can easily diffuse onto the surface of PS spheres and hydrolyze to form a titania coating. In the experiment, we also find that it is most convenient and reproducible to control the thickness of titania coatings by
adjusting the concentration of TBOT when the concentration of PS spheres is 0.20 g/L. By simply altering the concentration of TBOT from 1.5 to 19 mM, the thickness of the deposited titania coating varies from ∼8 to ∼70 nm, which can be confirmed by TEM images (Supporting Information). The theoretical values of the thickness of titania shells can be calculated from eq 5
( ) WTiO2
+
FTiO2 Dcom ) DPS WPS FPS
WPS FPS
1/3
(5)
where DPS and Dcom are the diameters of the PS cores and the resulting core-shell particles, respectively. WTiO2 is the weight of the titania coating, which can be calculated from the amount of TBOT used in the reaction, and WPS is the weight of the PS spheres. Additionally, FPS (1.05 g/mL) and FTiO2 (2.9 g/mL) are the densities of the PS cores and titania coating, respectively. When the concentration of TBOT is altered from 1.5 to 19 mM, the theoretical values of the thickness of the titania shells will range from 8 to 65 nm by the calculation using eq 5, which is a very close match to the experimental values mentioned above. Such excellent agreement can also be confirmed by the calculated and experimental curves of shell thickness versus TBOT concentration (Supporting Information). When the concentration of TBOT is reduced to less than 1.5 mM, the titania sols derived from the precursor seem to be insufficient to form a complete shell on the surface of each PS bead, which are indicated in the TEM image shown in Figure 3B. As the concentration of TBOT is increased to more than 19 mM, the surface of each PS bead is coated with a complete, homogeneous, smooth shell of titania, but a significant number of titania solid particles are also found in the final product as a result of unsuppressed homogeneous nucleation. Hollow Titania Spheres by Calcination and Characterization. The PS cores can be completely removed by calcination
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Figure 4. (A) SEM micrograph of the calcined titania-coated PS sample containing some artificially crushed spheres and (B) TEM image of titania-coated spheres calcined at 500 °C.
Figure 5. (A) EDX spectrum and (B) XRD pattern of titania-coated PS particles calcined at 500 °C.
ratio is 1:2.66. The higher amount of oxygen than the theoretical value in TiO2 can be explained by the interference of oxygen existing in alumina on the surface of aluminum foil. The XRD pattern in Figure 5B shows that the titania shells are transformed to the anatase phase (JCPDS file no. 21-172). The results of the thermal analysis (TG-DSC) of coated particles are illustrated in Figure 6. The first weight loss due to the release of water is in the range of 60-250 °C, whereas the second weight loss between 265 and 450 °C is attributed to the decomposition of cores particles. The exothermic peak in the DSC curve at 480 °C corresponds to the crystallization of amorphous titania into anatase, as confirmed by XRD.
Conclusions Figure 6. TG-DSC curves of calcined titania-coated PS particles.
of the coated particles at elevated temperatures, which can be confirmed by FTIR analysis (not shown). The morphology of hollow titania is characterized by SEM and TEM. To observe the hollow structure of titania spheres by SEM, the calcined sample was artificially crushed by a nipper on the aluminum foil prior to the measurement. As shown in Figure 4A, the uncrushed particles retain their spherical morphology despite the calcination at 500 °C. Moreover, the surface of the particles is quite smooth and homogeneous. It can also be seen from the crushed particles that the hollow structure of spherical particles forms after the PS cores are removed from the coated particles by calcination at 500 °C. In Figure 4B, intact, hollow titania particles can be seen from the TEM image of titania-coated PS spheres calcined at 500 °C, which is consistent with the SEM result in Figure 4A. The EDX spectrum and XRD pattern of titania-coated PS spheres calcined at 500 °C are showed in Figure 5.The EDX spectrum of the sample (Figure 5A) reveals that, after calcination, only two elements of titanium and oxygen are left and the atomic
Stable core-shell particles consisting of PS cores and titania shells were prepared through the hydrolysis of TBOT by the catalysis of ammonia ions in the mixed solvents that were in control of the diffusion rate of the precursors of titania. The results showed that the coatings were very smooth and homogeneous when the ethanol/acetonitrile volume ratio was 3:1. By adjusting the concentration of TBOT in the range of 1.5-19 mM, the thickness of the titania coating could be varied from 8 to 65 nm, which was very consistent with the theoretical value. The calcination at 500 °C for 2 h could produce dense, uniformed shells of titania consisting of small anatase crystallites. Acknowledgment. The current investigations were financially supported by the Hi-Tech Research and Development Program of China (863) (2002AA302108) and the National Natural Science Foundation of China (60372009, 20301015). Supporting Information Available: TEM images of titaniacoated PS spheres with different shell thicknesses. Calculated and experimental curves of shell thickness versus TBOT concentration. This material is available free of charge via the Internet at http://pubs.acs.org. LA060112P
