New Strategy Using Glycol-Modified Silane to Synthesize

Sep 17, 2009 - Toyota Central R&D Laboratories, Inc., Nagakute, Aichi, 480-1192 Japan. ‡ Division of Inorganic Chemistry I, University of Ulm, ...
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New Strategy Using Glycol-Modified Silane to Synthesize Monodispersed Mesoporous Silica Spheres Applicable to Colloidal Photonic Crystals Tadashi Nakamura,*,† Hisashi Yamada,† Yuri Yamada,† Alper G€urtanyel,‡ Sarah Hartmann,‡ Nicola H€using,‡ and Kazuhisa Yano† †

Toyota Central R&D Laboratories, Inc., Nagakute, Aichi, 480-1192 Japan, and ‡Division of Inorganic Chemistry I, University of Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany Received July 10, 2009. Revised Manuscript Received August 31, 2009

A new approach focusing on the reactivity of a silica source was developed for the particle size control of monodispersed mesoporous silica spheres (MMSSs). A glycol-modified silanes, tetrakis(2-hydroxyethyl) orthosilicate was chosen as the silica source and successfully applied in the surfactant-templated synthesis of MMSSs in a very dilute alkaline alcohol-water mixture. Due to its higher hydrolysis rate compared with tetraalkyl orthosilicates, it took less time for the primary particles to come out, resulting in the formation of small particles with diameters falling in the low submicrometer range. The resultant spheres possessed a well-ordered mesoporous structure, which was typically MCM41-type hexagonal. The MCM-48-type cubic spheres could also be obtained by changing the reaction condition. The monodispersity and particle size of the spheres were precisely controlled by the adjustment of the solvent composition in the methanol-ethanol-water system. Furthermore, ionic colloidal crystals exhibiting a well-defined stop band in the visible light region could be fabricated from the resultant MMSSs for the first time. This work encourages the further utilization of MMSSs in photonics as well as in catalysis or biochemistry.

Introduction Toward the practical application of mesoporous silicas, considerable efforts have been devoted to the control of their macroscopic morphologies, leading to various shapes such as spheres,1-3 fibers,4,5 films,6,7 or monoliths.8 We have recently succeeded in synthesizing highly monodispersed mesoporous silica spheres (abbreviated as MMSSs) with hexagonally ordered mesostructure.9 MMSSs are useful for the fundamental research on mesoporous materials, because the uniform size of MMSSs allows one to assess the catalytic activity or the diffusion kinetics in the mesopore channels with excluding the effect of the particle size.10-12 For example, the pure pore size effect on base catalysis was successfully elucidated by using amino-functionalized MMSSs with the same diameters and different pore sizes.10 The alternative availability of MMSSs is their possible application to colloidal photonic crystals (three-dimensional periodic arrays of *Corresponding author. E-mail: [email protected]. Fax: þ81-56163-6156. Tel: þ81-561-71-8071.

(1) B€uchel, G.; Gr€un, M.; Unger, K. K.; Matsumoto, A.; Tsutsumi, K. Supramol. Sci. 1998, 5, 253. (2) Gr€un, M.; Unger, K. K.; Matsumoto, A.; Tsutsumi, K. Microporous Mesoporous Mater. 1999, 27, 207. (3) Liu, S. Q.; Cool, P.; Collart, O.; Van Der Voort, P.; Vansant, E. F.; Lebedev, O. I.; Van Tendeloo, G.; Jiang, M. H. J. Phys. Chem. B 2003, 107, 10405. (4) Huo, Q. S.; Zhao, D. Y.; Feng, J. L.; Weston, K.; Buratto, S. K.; Stucky, G. D.; Schacht, S.; Schuth, F. Adv. Mater. 1997, 9, 974. (5) Wang, J. F.; Tsung, C. K.; Hong, W. B.; Wu, Y. Y.; Tang, J.; Stucky, G. D. Chem. Mater. 2004, 16, 5169. (6) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 7941. (7) Ogawa, M. Chem. Commun. 1996, 1149. (8) Melosh, N. A.; Lipic, P.; Bates, F. S.; Wudl, F.; Stucky, G. D.; Fredrickson, G. H.; Chmelka, B. F. Macromolecules 1999, 32, 4332. (9) Yano, K.; Fukushima, Y. J. Mater. Chem. 2004, 14, 1579. (10) Suzuki, T. M.; Yamamoto, M.; Fukumoto, K.; Akimoto, Y.; Yano, K. J. Catal. 2007, 251, 249. (11) Suzuki, T. M.; Nakamura, T.; Fukumoto, K.; Akimoto, Y.; Yano, K. J. Mol. Catal. A: Chem. 2008, 280, 224. (12) Suzuki, T. M.; Nakamura, T.; Sudo, E.; Akimoto, Y.; Yano, K. Microporous Mesoporous Mater. 2008, 111, 350. (13) Yamada, Y.; Nakamura, T.; Ishii, M.; Yano, K. Langmuir 2006, 22, 2444.

2002 DOI: 10.1021/la902498p

submicrometer particles),13 which are expected to find interesting applications in photonics. We have already demonstrated the fabrication of various types of close-packed colloidal crystalline arrays (or synthetic opals) using MMSSs13 and their derivative composites14-17 as building blocks. For the application of MMSSs to the colloidal photonic crystals, it is very important to control their particles size because the wavelength of the pseudophotonic band gap (or Bragg stop band) of the colloidal crystal is mainly determined by the size of the spheres. To obtain colloidal crystals exhibiting a stop band in the visible light region, the diameter of constituent spheres must be in the range of 100-350 nm according to the Bragg equation. However, our synthesis route for MMSSs led to particles with typical diameters of 400-1200 nm,9 and only supermicroporous particles less than 400 nm were obtained when a surfactant with a short alkyl-chain length was used as a template.18 Therefore, the development of new general method to control the particle size of MMSSs in the range between 100 and 350 nm is required. In addition, the ability to synthesize MMSSs with diameters of less than 200 nm would provide us the opportunity to fabricate not only close-packed colloidal crystals but also ionic colloidal crystals in which charged spheres are “loosely” packed in the liquid medium. Although there are several publications describing the synthesis of colloidal mesoporous spheres with diameters of less than 300 nm,19-21 close-packed or ionic colloidal photonic crystals with an optically high quality have not been fabricated from those spheres due to (14) Nakamura, T.; Yamada, Y.; Yano, K. J. Mater. Chem. 2006, 16, 2417. (15) Nakamura, T.; Yamada, Y.; Yano, K. Chem. Lett. 2006, 35, 1436. (16) Nakamura, T.; Yamada, Y.; Yano, K. J. Mater. Chem. 2007, 17, 3726. (17) Nakamura, T.; Yamada, Y.; Yano, K. Microporous Mesoporous Mater. 2009, 117, 478. (18) Yamada, Y.; Yano, K. Microporous Mesoporous Mater. 2006, 93, 190. (19) Fowler, C. E.; Khushalani, D.; Lebeau, B.; Mann, S. Adv. Mater. 2001, 13, 649. (20) Lai, C. Y.; Trewyn, B. G.; Jeftinija, D. M.; Jeftinija, K.; Xu, S.; Jeftinija, S.; Lin, V. S. Y. J. Am. Chem. Soc. 2003, 125, 4451. (21) Moller, K.; Kobler, J.; Bein, T. Adv. Funct. Mater. 2007, 17, 605.

Published on Web 09/17/2009

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Article Table 1. Properties of Particles Obtained from EGMS in Methanol-Water System

no.

EGMS/mol/ L

Na/ Si

methanol weight fraction

precipitation timea/s

average diameter/ μm CV/ %

d100/ nm

pore size/nm

specific surface pore volume/ area/m2 g-1 cm3 g-1

TMb 0.0096 0.26 0.50 185 0.650 3.4 3.23 2.12 1155 0.68 c c d d d d EG1 0.0097 0.26 0.50 20 EG2 0.0100 0.26 0.40 20 0.208 15.0 3.41 2.28 1169 0.80 EG3 0.0100 0.13 0.40 30 0.137 14.5 3.40 2.22 1111 0.76 EG4 0.0100 0.065 0.40 20 0.140 16.1 3.65 2.21 958 0.65 EG5 0.0100 0.032 0.40 80 0.146 12.6 3.92 2.21 811 0.54 d d d EG6 0.0101 0.13 0.35 20 0.091 20.1 3.43 EG7 0.0066 0.13 0.40 40 0.106 15.6 3.56 2.37 1102 0.79 EG8 0.0050 0.13 0.40 73 0.116 11.1 3.74 2.49 1092 0.84 a Time that solution turns opaque after the addition of EGMS. b MMSSs obtained from TMOS under the optimum conditions. c Difficult to measure because of aggregation. d Not measured.

their either less sphericity or low monodispersity. It remains a formidable challenge to synthesize highly monodispersed mesoporous spheres that can serve as building blocks to form closepacked or ionic colloidal crystals reflecting visible light. On the other hand, we analyzed a formation mechanism for MMSSs and found that small primary particles emerged suddenly, and then, the residual silica precursors preferentially reacted with the surface silanols on these existing particles.22 According to the mechanism, it is required for the reduction of the particle size of MMSS to increase the number of initial nuclei. In our previous study,9 when a silica source with lower hydrolysis and condensation rates than tetramethyl orthosilicate (TMOS) was used, it took longer for the primary spheres to come out, resulting in the formation of larger spheres. This result indicates that the number of initial nuclei of MMSSs can be controlled by the hydrolysis and condensation rates of the silica precursor; higher reactivity leads to more nuclei and thus to smaller final spheres. Di Renzo et al. also reported that the particle size of MCM-41 was strongly affected by the rate of hydrolysis of the silica source, with faster hydrolysis giving smaller particles,23 although their products were not monodispersed spheres. It is therefore expected that the use of a silica source with higher reactivity than TMOS would allow us to synthesize MMSSs with smaller particle sizes. Glycol-modified silanes are promising candidates possessing a high hydrolysis rate compared with alkoxysilanes. Modifications of alkoxysilanes with diols or polyols such as ethylene glycol, glycerol, etc. was reported in the middle of the last century,24 but their hydrolytic instability prevented them from being widely used. Only in recent years, interest in this type of glycol-modified silanes has been increasing again,25,26 and they were successfully applied in the surfactant-templated synthesis of hierarchically organized silica monoliths.27-29 Herein, we describe a new strategy using an ethylene glycolmodified silane (EGMS), which is one of the glycol-modified silans, to synthesize MMSSs. It is found that the use of EGMS enables us to control the particle size as well as the mesostructure. Furthermore, we demonstrate that ionic colloidal crystals fabricated from the mesoporous spheres obtained exhibit a welldefined stop band in the visible light region. (22) Nakamura, T.; Mizutani, M.; Nozaki, H.; Suzuki, N.; Yano, K. J. Phys. Chem. C 2007, 111, 1093. (23) Di Renzo, F.; Testa, F.; Chen, J. D.; Cambon, H.; Galarneau, A.; Plee, D.; Fajula, F. Microporous Mesoporous Mater. 1999, 28, 437. (24) Mehrotra, R. C.; Narain, R. P. Indian J. Chem. 1967, 5, 444. (25) Sattler, K.; Hoffmann, H. Prog. Colloid Polym. Sci. 1999, 112, 40. (26) Meyer, M.; Fischer, A.; Hoffmann, H. J. Phys. Chem. B 2002, 106, 1528. (27) H€using, N.; Raab, C.; Torma, V.; Roig, A.; Peterlik, H. Chem. Mater. 2003, 15, 2690. (28) Brandhuber, D.; Torma, V.; Raab, C.; Peterlik, H.; Kulak, A.; H€using, N. Chem. Mater. 2005, 17, 4262. (29) Hartmann, S.; Brandhuber, D.; H€using, N. Acc. Chem. Res. 2007, 40, 885.

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Experimental Section Materials. TMOS (Tokyo Kasei), tetraethyl orthosilicate (TEOS, Wako Inc.), cetyltrimethylammonium chloride (C16TMACl, Tokyo Kasei), 1 M sodium hydroxide solution (Wako Inc.), methanol (Wako Inc.), ethanol (Wako Inc.), and sulfuric acid (Wako Inc.) were used without further purification. Ethylene glycol (EG, Wako Inc.) was used after distillation in the presence of anhydrous sodium sulfate (Wako Inc.). Synthesis of EGMS. TEOS and EG were reacted in a molar ratio of 1:4 in a dry argon atmosphere at 413 K.28 Ethanol, which is produced during the transesterification reaction, was continuously removed by distillation. When no more progress in the reaction could be observed, the remaining ethanol and TEOS were removed in vacuo. The resultant tetrakis(2-hydroxyethyl) orthosilicate is abbreviated as EGMS. Synthesis of Mesoporous Silica Spheres. In a typical synthesis procedure, 3.52 g of C16TMACl and 1.14 mL of 1 M sodium hydroxide solution were dissolved in 800 g of methanol/ water (40/60 = w/w) solution (methanol weight fraction: 0.4). Then 2.0 mL of EGMS was added to the solution with vigorous stirring at 298 K. The clear solution turned opaque in several tens of seconds after the addition of the silica source, and resulted in a white suspension. After 8 h of continuous stirring, the mixture was aged overnight. The product was collected from the suspension by either centrifugation or filtration, washed with distilled water three times, and dried at 318 K. The powder obtained was calcined in air at 823 K for 6 h to remove the organic species. Fabrication of Ionic Colloidal Crystals in Water. Calcined MMSSs were added to water with a small amount of ionexchange resin (Bio-Rad). The solution was then agitated, and impurity ions were purified. The product was ultrasonicated for approximately 10 h in order to obtain a fine dispersion. It was then suction-filtered using a PTFE filter with a pore diameter of 5 μm to remove aggregated components. The filtered dispersion was concentrated to approximately 29 vol % by heating at 333 K. The concentrated dispersion of MMSSs was injected into a capillary cell, comprising two glass substrates and two spacers sandwiched between them, and the cell was sealed by an epoxy resin. Doublestick tape (Nitto Denko Corporation, No. 5615, 150 μm in thickness) was used as the spacer. A colloidal crystal grew between the glass substrates. It should be noted that glass substrates had been treated with a concentrated sulfuric acid for two days, followed by rinsing with a copious amount of distilled water, before the capillary cell was constructed. Characterization. Powder X-ray diffraction measurements were performed with either a Rigaku RINT-2200 or a Rigaku RINT-TTR X-ray diffractometer. The particle morphology was determined with a SIGMA-V (Akashi Seisakusho) or a Hitachi S-3600N scanning electron microscope (SEM). The surface of each sample was coated with gold before the measurement. The average diameter was calculated from the diameters of more than 50 particles in a SEM picture. Since only parts of the SEM images are shown in the figures, particles not appearing in the figures were also examined. The coefficient of variation (CV), defined as the DOI: 10.1021/la902498p

2003

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Figure 3. N2 adsorption isotherms for particles synthesized from EGMS in methanol-water system. (a) EG2, (b) EG3, (c) EG4, and (d) EG5.

Figure 1. (a-e) SEM images and (f) TEM image of particles synthesized from EGMS in methanol-water system. (a) EG1, (b) EG2, (c) EG3, (d) EG5, (e) EG8, and (f) EG3.

Figure 4. (a) Time course of pH in the reaction solution during synthesis. (b) Enlarged part of (a) from 0 to 80 s. Reaction conditions are described in Table 1.

Figure 2. XRD patterns of particles synthesized from EGMS in methanol-water system. (a) EG2, (b) EG3, (c) EG5, and (d) EG8. ratio of the standard deviation to the average diameter was also calculated. Transmission electron micrographs were obtained with a JEOL-200CX TEM using an acceleration voltage of 200 kV. Nitrogen adsorption isotherms were measured with a Quantachrome Autosorb-1 at 77 K. The BET surface area was deduced from N2 isotherm analysis in the relative pressure range of 0.05 to 0.13. The pore size distributions were calculated by means of the BJH method. The transmission spectra for colloidal crystals were measured using a Shimadzu UV-3600 UV-vis-NIR spectrophotometer.

Results and Discussion Synthesis of Mesoporous Silica Spheres from EGMS. Methanol-Water System. In the synthesis of MMSS using TMOS as a silica source, the highly uniform spherical shape and the hexagonally ordered mesostructure were simultaneously 2004 DOI: 10.1021/la902498p

achieved under limited conditions.9 The very dilute concentration of TMOS (0.0095 to 0.038 mol L-1) and C16TMACl (typically 0.012 mol L-1) and the appropriate methanol/water ratio were the keys to obtain MMSS of good quality. The optimum composition to synthesize MMSS from TMOS and C16TMACl was found to be 1 TMOS:1.27 C16TMACl:0.26 NaOH:1439 methanol:2560 H2O based on the molar ratio. First, we attempted to synthesize MMSS from EGMS and C16TMACl in a dilute alkaline methanol/water mixture based on this condition. The summarized results with regard to the macroscopic uniformity and pore characteristics of the products are listed in Table 1, and some of their electron micrograph images, XRD patterns, and N2 adsorption isotherms are shown in Figures 1-3, respectively. The product synthesized under the above optimum condition for TMOS (sample no. EG1) was an aggregated precipitate composed of spherical and nonspherical particles. The reduction of methanol weight fraction from 0.5 to 0.4 (no. EG2) improved the dispersity of the product, and the additional reduction of Na/Si ratio from 0.26 to 0.13 (no. EG3) enabled us to obtain nearly monodispersed spheres though the coefficient of variation (CV) was not sufficiently low. The average diameter of EG3 (ca. 140 nm) was quite smaller than that of spheres obtained from TMOS and C16TMACl.9 The XRD pattern of EG3 exhibited three wellresolved diffraction peaks corresponding to (100), (110), and (200), indicating the spheres have hexagonally ordered regularity, and radially aligned mesopores were observed in the TEM image of EG3 (Figure 1f). In addition, N2 adsorption isotherm confirmed that the spheres had a high specific surface area over 1000 m2 g-1 and a relatively large pore volume. These results verify that spheres obtained from EGMS have similar mesoporosity Langmuir 2010, 26(3), 2002–2007

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Article Table 2. Properties of Particles Obtained from EGMS in the Ethanol-Water System CV/ %

pore symmetry

pore size/ nm

EG9 0.0097 0.50 47 0.451 28.1 EG10 0.0098 0.45 34 0.426 17.6 EG11 0.0099 0.40 26 0.329 7.1 EG12 0.0101 0.35 25 0.211 8.8 EG13 0.0102 0.30 21 0.144 12.3 EG14 0.0201 0.35 16 0.221 20.8 EG15 0.0075 0.35 38 0.173 8.7 EG16 0.0050 0.35 78 0.195 4.9 a Time that solution turns opaque after the addition of EGMS. b Not measured.

hex hexþcub cub cub(þhex) hex(þcub) hex hex hex

no.

concentration of EGMS/mol L-1

ethanol weight fraction

precipitation timea/s

average diameter/μm

specific surface area/m2 g-1

pore volume/ cm3 g-1

b

b

b

2.06 2.03 2.05 2.30

1,138 1,164 1,186 1,120

0.75 0.72 0.77 0.82

b

b

b

2.16 2.15

1,105 1,092

0.74 0.74

Figure 5. SEM images of particles synthesized from EGMS in ethanol-water system. (a) EG10, (b) EG12, (c) EG15, and (d) EG16.

with those from TMOS and that the use of EGMS is effective to decrease the diameter of mesoporous silica spheres down to the low submicrometer range. The reason for the decrease of the mean size of spheres is assumed to be due to the high hydrolysis and condensation rates of EGMS compared with TMOS. In fact, when EGMS was used as a silica source, it took about 30 s for primary particles to come out (Table 1), which was shorter than about 200 s for TMOS.9 In order to directly prove the high reactivity of EGMS, the change in pH of the reaction solution for each silica source was measured versus time. As described in our previous study,22 the pH change of the solution corresponds to the formation process of MMSSs. Figure 4 shows how the value of pH changes after the commencement of the synthesis (= addition of the silica source). Even though the Na/Si ratio was lower for EGMS, the initial decrease of the pH value, corresponding to the hydrolysis and condensation rates of the silica source by sodium hydroxide, was much more pronounced for EGMS than for TMOS. The higher hydrolysis rate for EGMS is assumed to lead to more initial nuclei thus to smaller final spheres. To moderate the high reactivity of EGMS, the effect of the reduction of the Na/Si ratio (nos. EG4 and EG5) or the EGMS concentration (nos. EG7 and EG8) on the morphology and monodispersity of the spheres was investigated. Under those reaction conditions, although nearly monodispersed spheres with diameter of 100-150 nm were obtained, the values of CV remained to be more than 10%. In addition, the reduction of the Na/Si ratio or the EGMS concentration resulted in the broadening of the diffraction peak for (100) plane and the disappearance of higher order peaks (for (110), (200) planes) in the XRD patterns, indicating that the ordered domain sizes became smaller. The small ordered domain size also led to the Langmuir 2010, 26(3), 2002–2007

Figure 6. XRD patterns of particles synthesized from EGMS in ethanol-water system. (a) EG9, (b) EG12, (c) EG13, (d) EG14, and (e) EG15.

decrease in the pore volume and specific surface area. These results indicate that the value of Na/Si ratio should be not less than 0.13 for the synthesis of mesoporous silica spheres possessing highly ordered hexagonal regularity. Ethanol-Water System. Next, the effects of the cosolvent and its concentration on the particle size and the monodispersity were investigated in detail. Ethanol, which was typically employed in the modified St€ober method, was used in place of methanol. The concentration of C16TMACl and the Na/Si ratio were kept constant at 0.012 mol L-1 and 0.13, respectively. The properties of the products synthesized in ethanol-water cosolvent system are summarized in Table 2, and several SEM images and XRD patterns of the particles are shown in Figures 5 and 6, respectively. In the ethanol-water system, the average diameter decreased from 450 to 150 nm with decreasing ethanol weight fraction from 0.50 to 0.30, and nearly monodispersed spheres were obtained at the ethanol weight fraction of 0.4 or 0.35. Larger spheres were obtained at the same alcohol weight fraction in the ethanol-water system than in the methanol-water system. This is because the more hydrophilic cosolvent system leads to the smaller particle size in the synthesis of mesoporous silica.30 (30) Zhang, W.; Pauly, T. R.; Pinnavaia, T. J. Chem. Mater. 1997, 9, 2491.

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Figure 7. TEM images and corresponding selected area electron diffraction patterns of EG12: (a) along the [111] direction and (b) along the [110] direction.

Figure 8. Relationship between ethanol weight fraction and average diameter. The total alcohol weight fraction is kept constant at 0.40.

The ethanol weight fraction affected not only the particle size but also the mesoporous structure. Judging from the XRD patterns in Figure 6a, the mesoporous structure sequentially changed from an MCM-41-type two-dimensionally hexagonal to an MCM-48-type cubic to an MCM-41-type two-dimensionally hexagonal with decreasing ethanol weight fraction. Figure 7 shows the TEM images and corresponding selected-area diffraction patterns of spheres synthesized at the ethanol weight fraction of 0.35 (no. EG12). The [111] direction image (Figure 7a) is similar to that of MCM-48 reported previously by Monnier et al.,31 and the electron diffraction is consistent with the Ia3d symmetry. Although a shoulder in the XRD pattern for EG12 suggests that EG12 may contain a small fraction of two-dimensionally hexagonal structure, its TEM image indicates that the mesoporous structure of EG12 is almost identical to MCM-48. Schumacher et al. reported the synthesis of spherical MCM-48 particles,32-34 but the mean diameter of their products was typically around 500 nm. The successful synthesis of around 200 nm monodispersed colloidal spheres with almost MCM-48type cubic structure is quite noteworthy. The concentration of EGMS also affected the mesoporous structure. When the concentration of EGMS was increased (no. EG14) or decreased (no. EG15 and EG16) under otherwise same conditions as EG12, the spheres obtained exhibited a hexagonal symmetry in their XRD patterns, implying that the conditions for the synthesis of MCM-48 spheres are limited. The mechanism for the formation of MCM-48 structure at certain conditions is unclear at present. By the way, it is noteworthy that the spheres synthesized at the EGMS concentration of 0.0050 mol L-1 and the ethanol ratio of 0.35 (no. EG15) had highly uniform spherical shape (CV e 5%) and that they had a high specific surface area (1092 m2 g-1) and a large pore volume (0.74 cm3 g-1). This result demonstrates that the use of EGMS allows one to synthesize highly monodispersed mesoporous silica spheres with diameter of less than 300 nm. Methanol-Ethanol-Water System. As noted above, the spheres obtained in the ethanol-water system were larger than those in the methanol-water system at the same alcohol weight fraction. It is therefore expected that the particle size can be controlled by changing the methanol/ethanol ratio in the methanol-ethanol-water system. To verify the assumption, we then attempted to synthesize mesoporous silica spheres in the

methanol-ethanol-water system. The total alcohol weight fraction (= the weight fraction of sum of methanol and ethanol) was kept constant at 0.40, and only the methanol/ethanol ratio was varied. The relationship between the ethanol weight fraction and the average diameter is shown in Figure 8. As expected, the higher the ethanol weight fraction in the total alcohol was, the larger the mean diameter of spheres became, confirming that the particle size of MMSSs can be controlled by the adjustment of the solvent composition in the methanol-ethanol-water system. The properties of particles synthesized in the methanol-ethanol-water system are summarized in the Supporting Information. The alternative strategy using another glycol-modified silane to control the particle size of MMSSs is also described in the Supporting Information. Fabrication and Characterization of Ionic Colloidal Crystals. We have been working on close-packed colloidal crystals consisting of MMSSs or their derivative composites,13-17 but an ionic colloidal crystal is also attractive because its stop band wavelength can be tuned more easily by just changing the volume fraction of the constituent spheres in the liquid medium. The successful synthesis of MMSSs with diameters in the low submicrometer range provides us the opportunity to fabricate ionic colloidal crystals whose stop band is in the visible light region. Ionic colloidal crystals in water were fabricated using EG15 (average particle diameter: 0.173 μm). Dispersions containing three different contents of spheres (29, 21, and 14 vol %) were used for the fabrication of ionic colloidal crystals. As shown in the transmission spectra (Figure 9), the resultant ionic colloidal crystals exhibited well-defined stop bands whose wavelengths were 501, 568, and 631 nm for the sphere contents of 29, 21, and 14 vol %, respectively. Corresponding to their stop bands, the resultant ionic colloidal crystals showed uniform structural colors, which were blue, green, and red (photographs in Figure 9). These results confirm that MMSSs synthesized from EGMS can serve as building blocks for the fabrication of the ionic colloidal crystal with an optically good quality. We assume that the ionic colloidal crystal has a body-centered cubic (BCC) structure and that its (110) plane contributes to the stop band, because it is wellknown that silica colloids tend to have a BCC structure with its (110) plane oriented parallel to the substrate in most cases.35 The wavelength of the stop band of the BCC ionic colloidal crystal (λ) is given by the following equation (eq 1):

(31) Monnier, A.; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (32) Schumacher, K.; Gr€un, M.; Unger, K. K. Microporous Mesoporous Mater. 1999, 27, 201. (33) Schumacher, K.; du F. von Hohenesche, C.; Unger, K. K.; Ulrich, R.; Du Chesne, A.; Wiesner, U.; Spiess, H. W. Adv. Mater. 1999, 11, 1194. (34) Schumacher, K.; Ravikovitch, P. I.; Du Chesne, A.; Neimark, A. V.; Unger, K. K. Langmuir 2000, 16, 4648.

2006 DOI: 10.1021/la902498p

λ ¼ 1:436dneff =φ1=3

ð1Þ

where d is the mean diameter of the spheres, neff is the mean refractive index of the colloidal crystal, and φ is the volume (35) Shinohara, T.; Yoshiyama, T.; Sogami, I. S.; Konishi, T.; Ise, N. Langmuir 2001, 17, 8010.

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Article Table 3. Comparison of the Stop Band Wavelengths (λ) Measured by the Transmission Spectra and Those Calculated from eqs 1-3 calculated values φ

λ measured by transmission spectra/nm

neff

λ/nm

0.29 0.21 0.14

501 567 631

1.35 1.34 1.34

505 561 640

applied to a facile gel-immobilized colloidal crystal laser, whose oscillation occurs within the stop band of the colloidal crystal.37 Figure 9. Transmission spectra and corresponding photographs of ionic colloidal crystals containing different contents of spheres. (a) 29, (b) 21, and (c) 14 vol %.

fraction of spheres. The mean refractive index neff can be calculated from the following equation (eq 2): neff 2 ¼ nsphere 2 φ þ nwater 2 ð1 -φÞ

ð2Þ

where nsphere and nwater are the refractive indices of sphere and water, respectively. Since pores of MMSSs are filled with water in the ionic colloidal crystal, nsphere is expressed by the following equation (eq 3): nsphere 2 ¼ nsilica 2 Vsilica þ nwater 2 Vpore

ð3Þ

where nsilica is the refractive index of silica and Vsilica and Vpore are volume fractions of silica and pore in MMSSs, respectively. Using the known refractive index values, nwater =1.33, nsilica=1.46, and the density of the silica wall of MMSSs, 1.8 g cm-3 which was assumed on the basis of the result reported by Floquet et al.,36 the wavelength of the stop band λ was calculated from eqs 1-3 and compared with the measured ones in Table 3. The calculated values agreed closely with the ones measured by the transmission spectra, indicating that the stop band can be ascribable to the periodicity of the ionic colloidal crystal with the BCC structure. Likewise, a close-packed colloidal crystal whose stop band is in the visible rage can be fabricated from MMSSs synthesized from EGMS [Supporting Information]. Recently, we have demonstrated that MMSSs synthesized from EGMS have been successfully (36) Floquet, N.; Coulomb, J. P.; Llewellyn, P. L.; Andre, G.; Kahn, R. Stud. Surf. Sci. Catal. 2007, 160, 71.

Langmuir 2010, 26(3), 2002–2007

Conclusions MMSSs with diameters falling in the low submicrometer range were successfully synthesized using EGMS and C16TMACl as a silica source and a surfactant, respectively. The higher reactivity of EGMS compared with TMOS shortened the time that primary particles came out, resulting in the formation of small spheres. The MMSSs had highly uniform spherical morphology and their size could be controlled by the adjustment of the solvent composition in the methanol-ethanol-water system. Under most reaction conditions, mesoporous silica spheres with an MCM-41-type hexagonal pore symmetry were obtained, whereas ones with an MCM-48-type cubic pore symmetry were also obtained at certain reaction conditions. Furthermore, the mesoporous spheres obtained in this study could be self-assembled into ionic colloidal crystals in water due to their high monodispersity, and the resultant colloidal crystal exhibited a well-defined stop band in the visible light region. We believe that the wide range control of the particles size attained herein encourages the further utilization of MMSSs in various areas including catalysis, chemical sensors, and photonics. Supporting Information Available: (1) Properties of particles obtained from EGMS in methanol-ethanol-water system. (2) Synthesis of mesoporous silica spheres from tetrakis(3-hydroxypropyl) orthosilicate (TGMS). (3) Fabrication and characterization of a close-packed colloidal crystalline array. This material is available free of charge via the Internet at http://pubs.acs.org. (37) Yamada, H.; Nakamura, T.; Yamada, Y.; Yano, K. Adv. Mater. DOI: 10.1002/adma.200900721.

DOI: 10.1021/la902498p

2007