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Grain Size Control of Mesoporous Silica and Formation of Bimodal Pore Structures Kenichi Ikari, Keisei Suzuki, and Hiroaki Imai* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received July 1, 2004. In Final Form: September 14, 2004 The architecture of mesoporous silica was successfully controlled by adjusting the concentrations of a cationic surfactant and ammonia. An excess amount of the surfactant suppressed the grain growth and then induced the formation of small grains with a diameter below 20 nm. Consequently, assembly of the small-sized grains produced a bimodal pore structure consisting of framework mesopores of 2-3 nm and textural mesopores ranging over 10-100 nm.
Introduction Since the first synthesis of MCM-41,1,2 mesoporous silica has attracted much interest among the scientific community. Because the pore size distribution is closely related to the functional capability, the control of the pore structure over a broad range from nano- to micrometer scales is very important for practical applications of the M41S family in chromatography, catalysis, optical devices, chemical sensors, and drug delivery. Fundamentally, mesoporous silicas have a unimodal distribution of pore size, which is characterized by surfactants as a templating agent. The pore size of the M41S family is restricted in the range of 2-10 nm by the size of the surfactant micelle. Zhao et al. developed the synthesis of a SBA silica series that possesses large pores up to 30 nm using a triblock copolymer, a star diblock copolymer, and oligomeric surfactants as templating agents.3,4 Recently, various types of mesoporous silica with a bimodal pore size distribution have been reported.5-11 Wang et al. reported the synthesis of bimodal mesoporous silica (3 and 19 nm) using ammonia as a catalyst.5 Haskouri et al. produced bimodal mesoporous pure and doped silica (3 and 25-70 nm) from atrane complexes as an inorganic precursor.8 Suzuki et al. found a structural change from a unimodal pore structure to a bimodal one with an increase in the surfactant concentration.10 Commonly, the sources of small and large mesopores in a * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10843. (3) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chemelka, B. F.; Stucky, G. D. Science 1998, 279, 548-552. (4) Zhao, D.; Huo, Q.; Feng, J.; Chemelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024-6036. (5) Wang, X.; Dou, T.; Xiao, Y. Chem. Commun. 1998, 9, 1035-1036. (6) Bagshaw, S. A. Chem. Commun. 1999, 18, 1785-1786. (7) Sun, J.; Shan, Z.; Maschmeyer, T.; Moulijn, J. A.; Coppens, M. O. Chem. Commun. 2001, 24, 2670-2671. (8) Haskouri, J. El.; de Za´rate, D. O.; Guillem, C.; Latorre, J.; Calde´s, M.; Beltra´n, A.; Beltra´n, D.; Descalzo, A. B.; Rodriguez, G.; Martı´nezMan˜ez, R.; Marcos, M. D.; Amoro´s, P. Chem. Commun. 2002, 4, 330331. (9) Yuan, Y.; Blin, J. L.; Su, B. L. Chem. Commun. 2002, 5, 504-505. (10) Suzuki, K.; Ikari, K.; Imai, H. J. Mater. Chem. 2003, 13, 18121816. (11) Sun, J.; Shan, Z.; Maschmeyer, T.; Coppens, M. O. Langmuir 2003, 19, 8395-8402.
bimodal architecture (framework and textural mesopores) have been tentatively ascribed to the surfactant micelles and the interparticle spacing of silica grains, respectively. Pore size and pore size distribution of bimodal mesoporous silica have previously been controlled. The formation mechanism of the bimodal architectures and the structural origin of textural mesopores have not been sufficiently understood, while Sun et al. reported a scaffolding mechanism using cetyltrimethylammonium bromide and triblock copolymer to control small and large mesopore sizes.11 In this study, we found that the grain size of mesoporous silica was controlled by varying the concentrations of the surfactants and catalysts and that a bimodal architecture consisting of small and large mesopores was simultaneously produced. The size of large textural mesopores showed variation in a wide range of 10-100 nm. On the basis of electrostatic interaction,12 we discuss the variation of the grain size of mesoporous silica and the formation of textural mesopores. Experimental Section In a typical synthesis procedure, cetyltrimethylammonium chloride (CTAC, Kanto Chemical) was dissolved in 30 g of water with stirring for 1 h at room temperature, and then tetraethoxysilane (TEOS, Kanto Chemical) was added to the solution. After the solution had been stirred for 5 min, 14.7 M ammonia water (NH4OH, 28 wt %, Junsei Chemical) was mixed in to promote gelation. The molar ratio for the precursor solution was 1:0.241.94:0.49-5.89:106 TEOS/CTAC/NH4OH/H2O. The resultant gel was aged at room temperature for 24 h, dried at 333 K in air for 24 h, and finally calcined in air at 873 K for 3 h to remove the organic compounds. X-ray diffraction (XRD) patterns were recorded on a Rigaku RAD-C system with Cu KR radiation. Transmission electron microscopy (TEM) images were obtained using a Philips TECNAI F20. Nitrogen adsorption and desorption isotherms were obtained at 77 K with a Micromeritics TriStar 3000 using samples pretreated over 2 h at 433 K. The pore size distribution and the specific surface area were calculated by the BJH and BET methods, respectively.
Results and Discussion TEM images (Figure 1) show that the grain size of mesoporous silica obviously depended on the CTAC and NH4OH concentrations of the precursor solutions ([CTAC] (12) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, P. F.; Schuth, F. Stucky, G. D. Chem. Mater. 1994, 6, 1176-1191.
10.1021/la0483717 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/03/2004
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Figure 1. TEM images of mesoporous silica particles produced at various conditions. The conditions of the precursor solutions for the samples are schematically represented.
and [NH4OH]). The silica grains grew up to several hundreds of nanometers with relatively high [NH4OH] and low [CTAC] (samples E, H, and I). An increase in [CTAC] and a decrease in [NH4OH] reduced the grain size, and monodispersed nanograins of about 10 nm in diameter (samples B, C, D, and G) were finally obtained from the solutions at 0.5-1.0 M CTAC and 0.26 M NH4OH. Variations of nitrogen isotherms with [CTAC] and [NH4OH] are shown in Figure 2a,b, respectively. The type IV isotherms exhibited two inflections at P/P0 ) 0.4 and > 0.7, which are ascribed to a capillary condensation step. A steep increase at a low pressure indicates the presence of typical framework mesopores of about 3 nm due to the CTAC micelles as shown in Figure 3. The gradual increase and hysteresis loop of the isotherms in the region of P/P0 > 0.7 implied the coexistence of textural mesopores assignable to the interparticle spacing ranging over 10100 nm. A unimodal pore structure was obtained from large grains with a diameter above 100 nm (samples E, H, and I) at high [NH4OH] and low [CTAC] conditions. A reduction of the grain size with an increase in [CTAC] and a decrease in [NH4OH] accompanied the formation of the textural mesopores. Therefore, a bimodal pore structure consisting of framework and textural pores was achieved with the nanograins of mesoporous silica. The diameter of the textural mesopores decreased with a reduction of the grain size of the mesoporous silica. This fact corresponds to the variation in interparticle spacing with the change in the grain size. Although the size of the framework mesopores did not change, their volume
decreased when the grain size was reduced below 50 nm. Figure 4 shows the low-angle XRD patterns of various mesoporous silicas. Distinct diffraction peaks, which were indexed as (100), (110), and (200) of a two-dimensional hexagonal regular array, were observed for samples E, H, and I. This means that the framework mesopores were highly ordered in large grains. On the other hand, the degradation of the hexagonally ordered mesopores was indicated by the broadening of the XRD peaks with a reduction of the grain size. The degradation of the hexagonal array also occurred with a treatment in hot water at 373 K, as with ordinary MCM-41 samples.13 Figure 5 shows the grain size and the diameter of textural mesopores depending on [CTAC] and [NH4OH]. The architectures of the mesoporous silicas are classified into three categories according to the sizes of the grain and the textural mesopores. In category 1, large grains containing the hexagonal array of the framework mesopores were produced without textural mesopores. In categories 2 and 3, small monodisperse grains were observed in the presence of textural mesopores. The textural pore size corresponding to the interparticle space was smaller than the grain size in category 2. The diameter of the textural mesopores was considerably larger than that of the grains in category 3 (Figure 1C,D). As shown in Figure 6, the pore volume of the framework mesopores was about 1.0 cm3 g-1. Thus, the total pore volume increased with increasing the volume of the textural mesopores, and the maximum value, 3.4 cm3 g-1, including (13) Kim, J. M.; Ryoo, R. Bull. Korean Chem. Soc. 1996, 17, 66-68.
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Figure 2. Nitrogen adsorption-desorption isotherms of calcined mesoporous silica particles (curves are shifted for clarity): (a) samples E-G ([NH4OH] ) 1.54 M and [CTAC] ) 0.13, 0.25, and 0.51 M, respectively) and (b) samples B, F, and I ([CTAC] ) 0.25 M and [NH4OH] ) 0.26, 1.54, and 3.07 M, respectively).
Figure 3. Pore size distributions from the desorption branches: (a) samples E-G ([NH4OH] ) 1.54 M and [CTAC] ) 0.13, 0.25, and 0.51 M, respectively) and (b) samples B, F, and I ([CTAC] ) 0.25 M and [NH4OH] ) 0.26, 1.54, and 3.07 M, respectively).
the volume of 2.4 cm3 g-1 for the textural mesopores, was obtained at [CTAC] ) 1.01 M and [NH4OH] ) 0.26 M. The results of this work clarified that the grain size of mesoporous silica is one of the key factors for the formation of the bimodal pore structure. The grain size was highly influenced by [CTAC] and [NH4OH]. The unimodal pore structure was formed by relatively large grains containing the framework mesopores with hexagonal arrangement. A bimodal structure consisting of framework and textural mesopores was achieved by assembly of monodispersive small grains with a diameter below 50 nm. The hexagonal arrangement of the framework mesopores was degraded with a reduction of the diameter. Because textural mesopores are produced as an interparticle space in the small grains, the pore size distribution depended on the size and dispersivity of silica grains. The variation of the grain sizes with [CTAC] and [NH4OH] was considered from the balance of anionic and cationic components. Silicate anions generated by the hydrolysis of TEOS are negatively charged under a basic condition. On the other hand, cetyltrimethylammonium (CTA) ions are positively charged in the solution. The cationic surfactants (S+) and anionic silicate species (I-) are organized as a CTA-silicate composite through electrostatic interaction. The growth of the mesostructured composite is achieved by a balanced assembly of the positively charged CTA and negatively charged silicate. The results indicated that the grain size decreased with
an increase in [CTAC]. An excess amount of CTA would cover the CTA-silicate composite as neutral species CTAC (CTA+Cl-) and restrict the grain growth. Small grains of 10-20 nm in diameter were then produced with high [CTAC]. We already reported that the presence of a block copolymer induced a reduction of the grain size of mesoporous silica prepared with CTA micelles.14 An excess amount of CTA exhibits a similar effect to that of the nonionic surfactant. However, the order of the mesostructure was degraded with a decrease in the grain size. The mesostructure may be deformed with the strain as a result of the high surface energy of the small grain. Because the complete hydrolysis of TEOS under basic conditions was effective in the improvement of the hexagonal array, residual alkoxy groups of TEOS affected the ordered assembly of the mesostructure. A reduction of the grain size was also observed with a decrease in the concentration of NH4OH, which was added as a catalyst for the hydrolysis of TEOS. The pH value of the solution corresponding to [NH4OH] changed from pH 11.3 ([NH4OH] ) 0.26 M) to pH 12.1 (3.07 M). The solubility of silicate anions increased 10-fold with increasing the pH in this pH region; however, the absolute value was not exactly estimated.15 Therefore, the decrease in [NH4OH] (14) Suzuki, K.; Ikari, K.; Imai, H. J. Am. Chem. Soc. 2004, 126, 462-463. (15) Stumm, W.; Morgan, J. J. Aquatic Chemistry, 3rd ed.; WileyInterscience: New York, 1996; Chapter 7.
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Figure 6. Variation of the volumes of framework mesopores, textural mesopores, and total mesopores as a function of [CTAC] ([NH4OH] ) 0.26 M).
Figure 4. XRD patterns of mesoporous silica particles: (a) samples E-G ([NH4OH] ) 1.54 M and [CTAC] ) 0.13, 0.25, and 0.51 M, respectively) and (b) samples B, F, and I ([CTAC] ) 0.25 M and [NH4OH] ) 0.26, 1.54, and 3.07 M, respectively).
Figure 5. Categorization of mesoporous silica according to the sizes of the silica grains and the textural pores.
caused a decrease in the amount of the negatively charged silicate species in the solution and then resulted in the presence of excess positively charged CTA. As a consequence, the residual CTA covered the composite grains and reduced the grain size in a similar manner as [CTAC] increased. Nonionic silicate species would exist as a polymerized chain in the solution and be incorporated into the silica wall of the composite. This assumption is
Figure 7. Pore size distributions of samples A-D ([NH4OH] ) 0.26 M and [CTAC] ) 0.13, 0.25, 0.51, and 1.01 M, respectively).
supported by an expansion of the periodic lattice of the framework mesopores with a decrease in [NH4OH], which was estimated from the XRD patterns shown in Figure 4b. The formation of textural mesopores was derived in the presence of small grains induced by the suppression of the growth with an excess of CTA. In general, the diameter of the textural mesopores was smaller than the diameter of the grains. In category 3, however, the textural pores were much larger than the grains. At the conditions of a high [CTAC] and a low pH in this category, a large amount of CTA was a surplus to the charge balance between the negative silicate and the positive surfactant. Then, CTA is deduced to exist as a CTAC aggregate with a diameter of about 50 nm, which causes the formation of large textural mesopores. When the [CTAC] increased from 0.13 to 1.01 M with a low amount of ammonia, the textural mesopore sizes increased from 10 to 35 nm (Figure 7). At the same time, the full width at half-maximum of textural mesopores increased while both the grain sizes and the grain size distribution did not change. Similar variation of the textural mesopore sizes has previously been observed with addition of nonionic surfactant.11 Thus, an excess amount of CTAC behaves as a nonionic entity and expands the textural mesopore sizes.
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Conclusions We successfully controlled the grain size of mesoporous silica by adjusting the CTAC and NH4OH concentrations. Subsequently, the pore architecture was varied from a unimodal to a bimodal structure consisting of framework and textural mesopores. The controllable structure with a change in the grain size is promising in wide applications with adjustable pore sizes.
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Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research (No. 15560587) and the 21st Century COE program “KEIO Life Conjugate Chemistry” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. LA0483717
