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Size-Controlled Syntheses of Nanoporous Silica Spherical Particles through a Microfluidic Approach Kota Shiba, Kumiko Kambara, and Makoto Ogawa* Graduate School of CreatiVe Science and Engineering, Graduate School of Science and Engineering, and Department of Earth Sciences, Waseda UniVersity, Nishiwaseda 1-6-1, Shinjukuku, Tokyo 169-8050, Japan
Monodispersed spherical particles of silica/surfactant hybrid with the particle size of 200-400 nm were successfully synthesized by a microfluidic reaction for nucleation and subsequent growth process under ambient condition and were transformed into nanoporous spherical particles with uniform pore size of ca. 2 nm and BET surface area of 500-1000 m2 g-1. Compared to the results obtained by conventional batch method using identical starting solution, the products obtained utilizing microfluidic syntheses showed narrower particle size distribution as a result of separating nucleation and growth processes. Introduction Spherical particles with controlled size and size distribution are promising materials for such uses as the building block of photonic crystals and chromatographic stationary phases,1-3 so that spherical particles of nanoporous silica have been prepared by utilizing various synthetic approaches including emulsion chemistry,4 morphology template,5 and spray drying.6 The Sto¨ber method,7,8 which was developed for the preparation of monodispersed silica spherical particles with controlled size, has been successfully applied to prepare surfactant templated nanoporous silica spherical particles with the particle size of tens of nanometers to a few micrometers.9-21 Thanks to the numerous experiments, it became possible to prepare nanoporous silica spherical particles with narrow particle size distribution; however, there still remains synthetic challenges toward larger and smaller particles, narrower size distribution, controlled pore size, and so on. The nanoporous silica particles with a size smaller than 400 nm are attractive because of the possible reflection of visible light when they form a close-packed structure. The applications of colloidal photonic crystals for visible light driven sensing by the change of the reflection upon adsorption of molecules are expected. Also, it is known that silica particles with a size smaller than a few hundreds of nanometer have biocompatibility, so their use as drug carriers can be expected. However, relatively polydispersed particles with the coefficient of variation (abbreviated as CV, and this value shows particle size uniformity) above 13% have been obtained in the size range.12,16,17,21 In order to prepare uniform-sized spherical particles with wide size range, it is essential to separate nucleation and growth processes, which intrinsically correlate with monodispersity of products. However, these two processes usually proceed simultaneously. It is known that reactions that occur within micrometersized channels and show characteristic features, such as negligible temperature and concentration gradients,22-24 have a potential to realize uniform reactions which directly correlate with the homogeneity of products, and such microfluidic reaction can be a method as useful as the spray drying and templating approaches for the production of uniform particles. Recently, emulsion-based microfluidic syntheses of nanoporous silica spheres have been reported by several groups. Luo et al. succeeded in the size-controlled syntheses over a wide range * To whom correspondence should be addressed. Tel: +81-3-52861511. Fax: +81-3-3207-4950. E-mail:
[email protected].
such as 50-350 µm,25-27 and Petsev et al. showed further possibility to obtain nanoporous silica particles with the size of below 20 µm.28 Jeong et al. also reported the size-controlled syntheses of nanoporous silica spherical particles having 15-35 µm in diameter by using microreactor.29 However, there are technical difficulties for fabricating spherical particles of nanoporous silica with submicrometer size by emulsion-based methodology. It is hard to make uniform submicrometer size emulsion droplets, because their sizes are mainly defined by the channel width or nozzle diameter of the microfluidic devices. Recently, we have reported microfluidic syntheses of monodispersed nanoporous titania spherical particles with the size of ca. 500 nm and CV of 5%.30 Whole processes were based on sol-gel reactions, and such size controlled syntheses were achieved by the separation of the nucleation in the microreactor and subsequent particle growth. Sol-gel-derived homogeneous reaction solution may lead to further control of particle size which has not achieved in the previous reports involving microfluidic syntheses of nanoporous silica particles. In this paper, we report the size controlled syntheses of nanoporous silica spherical particles with the size of 200-400 nm through a microfluidic approach. Experimental Section Tetraethoxysilane (abbreviated as TEOS) and hexadecyltrimethylammonium chloride (abbreviated as C16TAC) were obtained from Tokyo Kasei Kogyo Co., Ltd. and were used without further purification. Methanol (abbreviated as MeOH) and 28% aqueous ammonia solution were obtained from Kanto Chemical Co., Ltd. and were used without further purification. For the monodispersed particle preparation, a microreactor which had a Y-type junction and cross section of ca. 1 mm2 (YMC Inc.) was used in the present study. The partial hydrolysis of TEOS occurred in the microchannel and then the reaction solution containing preformed nuclei was poured into another solution for particle growth. The whole reaction process is shown in Figure 1. TEOS/MeOH mixed solution (solution A) and MeOH aqueous solution (solution B) flowed at 20 mL min-1 using a syringe pump (YMC Inc.) and were mixed within the Y-type junction microchannel which was made of PTFE. Total volume of solution A, B was 12.5 mL and the molar ratio of TEOS:H2O in the solution A + B was 1:2.7. Then, the mixture of the solutions A and B was poured into C16TAC/ MeOH aqueous ammonia solution (solution C) under magnetic
10.1021/ie100225b 2010 American Chemical Society Published on Web 07/21/2010
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Figure 1. Schematic of the reaction setup used in the present study.
stirring. The total amount of MeOH and H2O in the starting solutions was 87.5 and 8.85 mL, respectively, where the molar ratio of TEOS:C16TAC:H2O:MeOH:ammonia was 1-10:0.4: 774:2627:72.16 The amount of each component in the starting solutions is shown in Table 1. After aging at room temperature for 20 h, the products were collected by vacuum filtration, washed with MeOH, and then dried at 333 K in air for a day. In order to remove surfactants, as-synthesized particles were calcined in air at 823 K for 10 h at a heating rate of 150 K h-1. Scanning electron micrographs (SEM) were obtained on a Hitachi S-2380N scanning electron microscope. Prior to the measurements, the samples were coated with gold. Average particle sizes and CV values were obtained by SEM observation for no less than 100 primary particles. Thermogravimetricdifferential thermal analysis (TG-DTA) curves were recorded on a Rigaku TG-8120 instrument at a heating rate of 10 K min-1 and using R-alumina as the standard material. Nitrogen adsorption/desorption isotherms of the calcined particles were measured at 77 K on a Belsorp Mini-instrument (Bel Japan Inc.). Prior to the measurements, the samples were dried at 393 K under nitrogen atmosphere for 3 h. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method31 using a linear plot over the range of P/P0 0.05-0.20. Pore size distributions of the calcined particles were derived from the nitrogen adsorption isotherms by the Barrett-JoynerHalenda (BJH) method.32 Pore volumes were determined from the amount of nitrogen adsorbed at P/P0 ) 0.90-0.95. Results and Discussion The SEM images of the products are shown in Figure 2. As shown there, monodispersed spherical particles composed of silica and C16TA+ with the size of 200-400 nm were successfully obtained. C16TAC was ionized to form C16TA+ in aqueous media and interacted with surface silanol groups which had negative charge under basic conditions. Thereafter, SiO2-C16TA+ hybrids were aggregated to form spherical particles composed of silica wall and C16TA+ as nanopore templates.33 The particle size was gradually decreased when the added amount of TEOS was increased, indicating that the number of nuclei per unit volume in the reaction increased with TEOS amount. The results are similar to those reported in the
Figure 2. SEM and TEM images of the as-synthesized products. Corresponding molar C16TAC/TEOS ratios in the starting solutions of each product are (a) 0.04, (b) 0.08, (c) 0.2, (d) 0.4. Table 2. BET Surface Area, BJH Pore Size, and Pore Volume of Each Product BET surface area (m2 g-1)
BJH pore size (nm)
pore volume (cm-1 g-1)
C16TAC/TEOS molar ratio
527 759 898 1049
1.9 1.9 2.0 1.9
0.33 0.44 0.55 0.61
0.04 0.08 0.2 0.4
literature;16,17 however, we emphasize here that the particles thus obtained were uniform in size compared to the spherical particles of nanoporous silica reported previously. SiO2-based yield of each product, which was calculated from the weight loss at around 593 K in the TG curve, reached 90%, suggesting that all the added TEOS were reacted to form SiO2-C16TA+ hybrids. Although similar high yields were obtained in the previously reported batch syntheses, it is possible to fabricate these hybrids continuously with the microreactor as long as the source materials were not consumed. It should be mentioned that the present microfluidic reaction and subsequent aging process were successfully applied for the size-controlled syntheses of nanoporous silica spherical particles as well as nanoporous titania spherical particles.30 The obtained SiO2-C16TA+ hybrids were heat-treated under air at 823 K for 10 h (heating rate of 2.5 K min-1) to eliminate C16TA+ and transform the hybrids into nanoporous particles. We confirmed that the morphology of the particles did not change during the calcination, while particle size shrinkage of ca. 5% was observed due to condensation reactions of surface silanol groups. Nitrogen adsorption/desorption measurements were conducted to evaluate the nanoporous structure. BET surface area, BJH pore size, and pore volume are shown in Table 2. On the basis of the nitrogen adsorption/desorption isotherms shown in Figure 3, BET surface area increased from 500 to 1000 m2 g-1 as C16TAC/TEOS ratio in the starting solutions increased. When the C16TAC/TEOS ratio increased, the amount of C16TA+ per each particle was increased, resulting in high surface area formation. We also calculated the molar C16TA+/
Table 1. Amount of Each Component in the Starting Solutions solution A
solution B
solution C
TEOS (mL)
MeOH (mL)
H2O (mL)
MeOH (mL)
H2O (mL)
MeOH (mL)
C16TAC (g)
28% NH3aq (g)
C16TAC/TEOS molar ratio
1.84 0.920 0.368 0.184
10.66 11.58 12.13 12.32
0.395 0.197 0.079 0.040
12.11 12.30 12.42 12.46
8.455 8.653 8.771 8.810
64.73 63.62 62.95 62.72
0.1056
3.6
0.04 0.08 0.2 0.4
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Figure 3. Nitrogen adsorption/desorption isotherms of calcined products synthesized with various molar C16TAC/TEOS ratio in the starting solution: (O) adsorption isotherm and (×) desorption isotherm.
Figure 4. Variation of particle size, CV value, BET surface area and molar C16TA+/SiO2 ratio of the products as a function of molar C16TAC/TEOS ratio in the starting solution: (upper) O, average particle diameter; 0, coefficient of variation; (lower) O, BET surface area; 0, molar C16TA+/ SiO2 ratio of the products.
SiO2 ratio of the products from the weight loss on TG curve at around 423-873 K, which can be attributed to the thermal decomposition of C16TA+ (Figure 4). As can be seen in Figure 4, the molar C16TA+/SiO2 ratio increases with the molar C16TAC/TEOS ratio in the starting solutions, and this fact can be regarded as the cause of the surface area differences. BJH pore size distributions shown in Figure 5 indicate that nanoporous silica spheres obtained in different compositions of the starting solutions possess pore size of ca. 2 nm in diameter. There are further possibilities to obtain monodispersed silica spherical particles with controlled pore size in which another template of different alkyl chain length is used.
Figure 5. BJH pore size distribution of calcined products synthesized with various molar C16TAC/TEOS ratio in the starting solution.
It is worth mentioning that BET surface area and C16TA+/ SiO22 ratio are higher and CV value is lower if compared with those of the analogous nanoporous silica spherical particles reported previously.16 The microfluidic approach used in this study led to homogeneous nucleation. In the present syntheses, partial hydrolysis of TEOS was conducted in the microreactor, following the formation of uniform-sized nuclei, then these nuclei were grown under the presence of C16TA+ to form SiO2-C16TA+ hybrids while above-mentioned nucleation and growth processes occurred at the same time without using the microreactor, leading to the formation of relatively polydispersed particles. It is possible to synthesize larger sized monodispersed nanoporous silica spheres without using microreactor, because it can be thought that to reach the supersaturated concentration of silica species is rather slow so that nucleation and growth processes are more or less separated.34,35 Taking into account the fact that particle volume is proportional to particle size, it is possible to say that particle growth becomes difficult when the particle growth proceeds, since an extra amount of source materials are required. If a number of nuclei are formed at the initial stage of reaction, there must remain a few source materials and the final size of the product becomes smaller. Consequently, the particle size difference between initially formed particle and finally formed particle in the reaction, i.e., the polydispersity of the product, is noticeable compared to that of the case where relatively a small number of nuclei are formed initially. (In this case, larger particles are formed.) This is because the growth of particle becomes slower as the particle becomes larger due to further consumption of the source materials, indicating that monodispersed particles can be obtained as the average particle size becomes larger. Further investigation is required to elucidate the reaction mechanism, and then monodispersed particles of various compositions will be achieved. Conclusions In conclusion, by preparing nuclei within the microreactor, which has a Y-type junction channel, and subsequent aging them in C16TA+ MeOH aqueous ammonia solution, we can successfully obtain monodispersed nanoporous silica spherical particles with a size of 200-400 nm and CV value of 7-12%.
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These nanoporous particles had uniform pore size of ca. 2 nm and controllable BET surface area (500-1000 m2 g-1) depending on the molar C16TAC/TEOS ratio in the starting solutions. The present microfluidic approach played an important role in the separation of nucleation and growth process concerning particle formation with uniform size. Acknowledgment This work was supported by Waseda University financially as a special research project. Literature Cited (1) Overbeek, J. T. G. Monodisperse Colloidal Systems, Fascinating and Useful. AdV. Colloid Interface Sci. 1982, 15, 251. (2) Wang, Y.; Price, A. D.; Caruso, F. Nanoporous Colloids: Building Blocks for a New Generation of Structured Materials. J. Mater. Chem. 2009, 19, 6451. (3) Marlow, F.; Muldarisnur; Sharifi, P.; Brinkmann, R.; Mendive, C. Opals: Status and Prospects. Angew. Chem., Int. Ed. 2009, 48, 6212. (4) Huo, Q.; Feng, J.; Schu¨th, F.; Stucky, G. D. Preparation of Hard Mesoporous Silica Spheres. Chem. Mater. 1997, 9, 14. (5) Yang, S. M.; Coombs, N.; Ozin, G. A. Micromolding in Inverted Polymer Opals (MIPO): Synthesis of Hexagonal Mesoporous Silica Opals. AdV. Mater. 2000, 12, 1940. (6) Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Aerosol-Assisted Self-Assembly of Mesostructured Spherical Nanoparticles. Nature 1999, 398, 223. (7) Sto¨ber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62. (8) Bogush, G. H.; Tracy, M. A.; Zukoski, C. F. Preparation of Monodisperse Silica Particles: Control of Size and Mass Fraction. J. NonCryst. Solid. 1988, 104, 95. (9) Gru¨n, M.; Lauer, I.; Unger, K. K. The Synthesis of Micrometerand Submicrometer-Size Spheres of Ordered Mesoporous Oxide MCM41. AdV. Mater. 1997, 9, 254. (10) Yano, K.; Suzuki, N.; Akimoto, Y.; Fukushima, Y. Synthesis of Mono-Dispersed Mesoporous Silica Spheres with Hexagonal Symmetry. Bull. Chem. Soc. Jpn. 2002, 75, 1977. (11) Yano, K.; Fukushima, Y. Particle Size Control of Mono-Dispersed Super-Microporous Silica Spheres. J. Mater. Chem. 2003, 13, 2577. (12) Yano, K.; Fukushima, Y. Synthesis of Mono-Dispersed Mesoporous Silica Spheres with Highly Ordered Hexagonal Regularity Using Conventional Alkyltrimethylammonium Halide as a Surfactant. J. Mater. Chem. 2004, 14, 1579. (13) Yano, K. Synthesis of Mono-dispersed Spherical Mesoporous Silica. R&D ReV. Toyota CRDL 2004, 40, 28. (14) Shimura, N.; Ogawa, M. Growth of Nanoporous Silica Spherical Particles by the Sto¨ber Method Combined with Supramolecular Templating Approach. Bull. Chem. Soc. Jpn. 2005, 78, 1154. (15) Nakamura, T.; Mizutani, M.; Nozaki, H.; Suzuki, N.; Yano, K. Formation Mechanism for Monodispersed Mesoporous Silica Spheres and Its Application to the Synthesis of Core/Shell Particles. J. Phys. Chem. C 2007, 111, 1093. (16) Kambara, K.; Shimura, N.; Ogawa, M. Larger Scale Syntheses of Surfactant-Templated Nanoporous Silica Spherical Particles by the Sto¨ber Method. J. Ceram. Soc. Jpn. 2007, 115, 315.
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ReceiVed for reView January 30, 2010 ReVised manuscript receiVed May 17, 2010 Accepted June 22, 2010 IE100225B
