Formation Mechanism for Monodispersed Mesoporous Silica Spheres

Dec 22, 2006 - ... Silica Spheres and Its Application to the Synthesis of Core/Shell Particles ..... Tuning the Release of Anticancer Drugs from Magne...
4 downloads 7 Views 650KB Size
Download PDF
J. Phys. Chem. C 2007, 111, 1093-1100

1093

Formation Mechanism for Monodispersed Mesoporous Silica Spheres and Its Application to the Synthesis of Core/Shell Particles Tadashi Nakamura, Mamoru Mizutani, Hiroshi Nozaki, Noritomo Suzuki, and Kazuhisa Yano* Toyota Central R & D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan ReceiVed: July 28, 2006; In Final Form: October 10, 2006

A formation mechanism for monodispersed mesoporous silica spheres was investigated from the viewpoint of both particle growth and the progress of the condensation of the silica precursor. The particle growth of monodispersed mesoporous silica spheres was studied by TEM observation. The development of the mesopores was examined by in situ X-ray diffraction measurements. The condensation reaction of the silica precursor was analyzed by silicic acid titration and pH-conductivity measurements during the synthesis. It was found that small particles emerged suddenly after the commencement of the synthesis, and then the residual silica precursors reacted preferentially with the surface silanols on these existing particles. This led to the formation of monodispersed mesoporous silica spheres. Conversely, new small particles emerge throughout the synthesis in the case of a heterogeneous system. To confirm the preferential reaction of the silica precursors with the surface silanols, expansion of the existing particles was carried out by adding more silica precursor after the completion of the synthesis. It was found that the sizes of the particles were enlarged by the successive addition of more of the silica precursor. Surprisingly, the radial alignment of the hexagonal mesopores was still retained in the expanded particles. In addition, by the addition of a different silica precursor to the initial one, highly monodispersed core/shell mesoporous silica spheres possessing a hydrophilic core and a hydrophobic shell were successfully obtained for the first time.

Introduction Mesoporous silica, which was first discovered by researchers working for Mobil, has received much attention because of its uniform pore characteristics: it forms uniform mesopores with high specific surface areas.1,2 Many studies have been conducted on the synthesis and applications of mesoporous silicas as catalysts, as adsorbents, for use in separation columns, etc. The shapes of the particles have been controlled in various morphologies: fibers,3-5 sponge-like membranes,6,7 rod-like powders,8 films,9 and spheres.10-13 Among these particles, spheres are of great interest for applications in chromatography,14-17 cosmetics,18 and colloidal crystals.19,20 Spherical mesoporous silica has been synthesized under both acidic and basic conditions. Because spherical particles are obtained as a result of minimizing surface energy, it is very important to control the reaction rate. In acidic media, to reduce the reaction rate, syntheses were conducted under static conditions, mainly by the use of a nonionic surfactant.21,22 As alternative methods, aerosol-assisted or evaporation-induced selfassembly has been employed for the synthesis of spherical mesoporous silicas.23-25 Elsewhere, amine compounds such as alkyl amines and alkyl ammonium salts are often used as surfactants when the reaction takes place under basic conditions. In this case, a modified Sto¨ber method is employed, which enables us to make monodispersed silica spheres using the tetraalkoxysilane-ammonia-water-alcohol system.26-31 Recently, we have successfully synthesized monodispersed mesoporous silica spheres with ordered hexagonal regularity from tetramethoxysilane (TMOS) and n-alkyltrimethylammonium halide (CnTMAX) by precisely optimizing the concentrations * Corresponding author. E-mail: [email protected].

of the reactants and the water/alcohol ratios.32-35 In these particles, hexagonally ordered mesopores were uniformly aligned radially from the center to the surface of each sphere. It is of great interest to understand how these homogeneous spheres with their novel internal structures are formed. The formation mechanism of mesoporous silicas has been investigated by many researchers. The development of uniform mesopores was first explained by a liquid crystal templating mechanism,1 and then by a cooperative templating mechanism.36,37 These syntheses were mostly conducted above the critical micelle concentrations of the surfactants. The changes from spherical micelles to rod-like micelles were analyzed by EPR using spin probes or by small-angle X-ray scattering measurements.38-44 In addition, cryo-TEM and laser scattering measurements were carried out to study particle growth.34,45 However, because the mesoporous silicas have been synthesized under various reaction conditions, no single mechanism can interpret the nucleation, the growth of the particles, and the development of uniform mesopores. Regarding the formation of spherical mesoporous silicas, Ostafin and co-workers proposed that particle growth was achieved by the addition of monomers and oligomers to the surface.46 Tendeloo and co-workers reported that the Ia3d cubic structure was observed at the center of the spherical silica particles and that radially aligned mesopores were developed from this cubic structure (according to the results of electron diffraction measurement).47-49 It was indicated that the structure of the central core governed the mesopore alignment. On the contrary, Rankin and co-workers proposed that a parallel arrangement of surfactants at the surfaces of the particles led to a radially aligned mesoporous structure, because the Ia3d cubic structure was not observed in their particles.50 In terms

10.1021/jp0648240 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/22/2006

1094 J. Phys. Chem. C, Vol. 111, No. 3, 2007 of mesopore development, they mentioned that particles with disordered pores were generated initially, and then those disordered pores were gradually converted to ordered mesopores. Until now, there have been few reports that describe the formation mechanism of monodispersed mesoporous silica spheres in detail. Because of this monodispersity of particle size, the particle growth of monodispersed mesoporous silica spheres can be observed clearly and accurately. Here, we describe a formation mechanism for monodispersed mesoporous silica spheres with radially aligned mesopores. Particle growth was examined by TEM and compared to that of a heterogeneous system. In situ XRD analysis was carried out to study the development of the mesopores. Silicic acid titration and pHconductivity measurements were carried out during the synthesis to analyze the reaction. A formation mechanism for the monodispersed mesoporous silica spheres is proposed that takes into account all of the data obtained from the aforementioned experiments. In addition, highly monodispersed core/shell mesoporous silica spheres possessing a hydrophilic core and a hydrophobic shell were successfully obtained for the first time based on the proposed mechanism. Experimental Section Synthesis. Hexadecyltrimethylammonium chloride (C16TMACl) and TMOS (Tokyo Kasei), 1 M sodium hydroxide solution, and methanol (Wako Inc.) were used without further purification. In a typical synthesis, 3.52 g of C16TMACl and 2.28 mL of 1 M sodium hydroxide solution were dissolved in 800 g of methanol/water (50/50 ) w/w) solution. Next, 1.32 g of TMOS was added to the solution with vigorous stirring at 298 K. After the addition of TMOS, the clear solution suddenly turned opaque and resulted in a white precipitate. In the case of core/shell mesoporous silica synthesis, double the molar amount of mercaptopropyltrimethoxysilane (MPTMS)/TMOS ()20/80 mol/mol) mixture was again added to the solution 1 h later. After 8 h of continuous stirring, the mixture was aged over night. The white powder was then filtered and washed at least three times with distilled water, and then dried at 318 K for 72 h. The powder that was obtained was calcined in air at 823 K for 6 h, or in the case of core/shell particles, extracted in acidic ethanol at 333 K for 3 h to remove the organic species. Characterization. Transmission electron microscopy (TEM) was performed on a Jeol-200CX TEM using an acceleration voltage of 200 kV. A small amount of the reaction solution was dropped on a carbon-coated copper grid every 10 s for 10 min during the synthesis. The liquid portion immediately passed through the membrane, and only solid particles remained on the grid, quenching the particle growth. In situ XRD analysis was carried out with a Rigaku Rint-2200 X-ray diffractometer using Cu KR radiation. The reaction solution was injected into a cell with polyimide “Kapton” windows placed 2 µm apart. Scanning electron micrographs (SEMs) were obtained using a SIGMA-V (Akashi Seisakusho). The surfaces of the samples were coated with gold before the measurements. The average particle diameter was calculated from the diameters of 50 randomly selected particles in an SEM picture. Because only parts of the SEM samples are shown in the figures, particles that do not appear in the figures were also examined. The standard deviation was also calculated, from which the particle diameter distribution was determined. 68.27% of the data are within the range of the mean plus or minus one standard deviation. Thus, a smaller standard deviation indicates that the particle diameter distribution is in a narrower range. The concentration of silicic acid that was dissolved in the reaction

Nakamura et al. solution during the synthesis was determined spectrophotometrically by the molybdenum yellow method. Small portions of the reaction solution were periodically analyzed during the synthesis. Results and Discussion Particle Growth. Monodispersed mesoporous silica spheres (hereafter abbreviated as MMSS) were obtained from tetraalkoxysilane and alkyltrimethylammonium halide under very dilute concentrations.32-34 In our previous report,34 particle growth of MMSS was measured in situ with a laser diffraction particle size analyzer. Commencing just short of 3 min from the start of the experiment, the particles appeared and grew rapidly, reaching their final size within 15 s. On the other hand, the scattering intensity, which corresponds to the number of particles, increased gradually, reaching a maximum 6 min later. We concluded from these results that the particles were generated successively (not all at once), growing rapidly to the same maximum size sequentially during the synthesis. However, this mechanism is not entirely understandable. It is acceptable to consider that simultaneous generation, followed by propagation and subsequent termination of each and every particle, leads to the formation of monodispersed particles. In addition, in the case of the laser diffraction measurements, multiple scattering at high scattering intensity ()high particle concentration) causes an underestimation of both the particle size and the number that are present. To address the formation mechanism of MMSS in a different manner, particle growth was captured on TEM images during the synthesis. Figure 1 shows some of these images. Small particles (ca. 140 nm) suddenly emerged 144 s into the process, and grew to their final size (ca. 500 nm) in 600 s. TEM observations confirmed that the primary-generated small particles grew homogeneously into larger particles, leading to the formation of monodispersed spherical particles. The results of both the aforementioned laser diffraction measurements and the TEM observations support the conclusion that the small particles suddenly emerged in the reaction solution. Figure 2 shows the time course of the average particle diameter of the MMSS. An extrapolation of the curve in the early stages (dashed line) also supports the notion that particles emerged suddenly during the synthesis. It should also be mentioned that particles as small as 15 nm can be detected by the laser scattering analyzer. Although particles of about 140 nm in diameter happened to be captured on the TEM grid as primary particles, it is uncertain whether this was the actual size of all of the primary particles. It is very difficult to identify their actual size, because a finite measuring time is necessary for any measurement, and the particles may grow slightly during drying on the TEM grid after sampling. The particle growth of mesoporous silica spheres that are heterogeneous in size was also examined. By simply changing the methanol-to-water ratio from 50/50 to 60/40 in the reaction solution, particles with various sizes were obtained.34 Figure 3 shows some of the TEM images that were captured. Small particles suddenly emerged in the reaction solutions in a way similar to that of monodispersed particles. However, these small particles could exist at any time in the reaction solution, suggesting that new small particles were successively generated throughout the synthesis. It was concluded from the above results that the primary-generated small particles grow homogeneously into bigger particles (without the generation of any new particles) in the case of reactions that lead to monodispersed particles. On the contrary, new particles are generated throughout the synthesis for heterogeneous particles. The formation mechanism will be discussed in the section that covers the condensation reaction.

Formation Mechanism for Monodispersed Silica Spheres

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1095

Figure 1. Transmission electron micrograph (TEM) images of samples obtained at different times, (a) 130, (b) 144, (c) 150, (d) 160, (e) 200, (f) 240, (g) 360, and (h) 600 s after the commencement of the experiment. The scale bar represents 500 nm.

Figure 2. Time course of the average particle diameter. The average particle diameters were obtained by measuring particles on each SEM image at different times.

Internal Structure. In our previous report, the radial alignment of the mesopores in the MMSS was confirmed indirectly by the partial introduction of platinum.33,34 To obtain a direct image of the internal structure of an MMSS, a particle embedded in epoxy resin was sectioned using an electron beam, and then observed by TEM. Figure 4 shows the TEM image of the MMSS. A structure consisting of bundled mesopores can be clearly seen near the surface of the hemisphere. Meanwhile, a hexagonal structure is observed at the center of the hemisphere. The hexagonal structure at the center of the particle is also confirmed from an electron diffraction image of an MMSS in Figure 5, although the image is somewhat ambiguous. From the above results, it was concluded that mesopores are aligned radially from the center to the surface of the MMSS. Our result, whereby a hexagonal structure was observed at the center, agrees

well with that obtained by Rankin,44 even though Tendeloo and co-workers observed a cubic structure.48,49 The overall mesoporous structure within a particle is highly dependent on the synthesis conditions. Therefore, it is reasonable to consider that the same hexagonal structure was obtained throughout the reaction. Changes in the mesophase after primary nucleation are incomprehensible. Figure 6 shows TEM images of the internal structures of both primary and growing particles. It is surprising that hexagonal structures can be clearly seen in a primary particle. The bundles of mesopores appear to be aligned in three directions from the center to the surface of the particle. Meanwhile, radially aligned mesopores are observed on all surfaces of the growing particle. The mesopore alignment changes from three distinct directions to omni-directional during the course of the particle growth. This also supports the assumption that parallel alignment of surfactants at the surface of the particles leads to a radially aligned mesoporous structure.50 Development of Mesoporous Regularity. A hexagonal structure was observed in the primary particles by TEM observation during the synthesis. However, there is a possibility that the mesopores became ordered after they were sampled on the TEM grid. Therefore, the development of the mesoporous regularity of the MMSS was studied in situ by monitoring the X-ray diffraction intensity of the reaction solution at 2θ ) 2.25°, which corresponds to diffraction from the 100 plane of the hexagonal structure. Figure 7 shows the time course of the X-ray diffraction intensity during the synthesis. The time at which the intensity began to increase corresponds to that at which the

1096 J. Phys. Chem. C, Vol. 111, No. 3, 2007

Nakamura et al.

Figure 3. TEM images of samples obtained at different times, (a) 120, (b) 240, (c) 540, and (d) 720 s after the commencement of the experiment for a heterogeneous system. The scale bar represents 2 µm.

Figure 4. Transmission electron micrograph (TEM) image of a sample embedded in an epoxy resin. The embedded sample was cut by an electron beam.

Figure 6. TEM images of the samples obtained (a) 144 and (b) 250 s after the commencement of the experiment.

Figure 7. Time course of X-ray diffraction intensity of reaction solution at 2θ ) 2.25° during synthesis.

Figure 5. Electron diffraction image of the sample.

particles emerged, indicating that the hexagonal regularity was developed at the same time as the primary particles were generated. In addition, the intensity almost reached a maximum 600 s after the start of the experiment. To confirm the hexagonally ordered structures of the particles, a powder XRD pattern of the particles that were obtained after 600 s was examined (Supporting Information). The pattern indicates that

the particles had ordered hexagonal regularity. It is concluded from these results that hexagonally ordered mesopores were already formed in the primary particles, although Rankin and co-workers suggested that the primary-generated disordered pores were gradually converted to ordered mesopores. Condensation Reaction. Mesoporous silica is assembled from surfactants and oligomeric silica precursors, which are formed by the condensation of monomeric silica precursors: tetramethoxysilane (TMOS) was used in this study. After the commencement of the experiment, MMSS emerged suddenly in the reaction solution. Therefore, it is very important to understand how the condensation proceeds during the synthesis to investigate the formation mechanism. Figure 8 shows the result of silicic acid titration during the synthesis. Before precipitation, the concentration of silicic acid decreased slightly but remained almost unchanged. After precipitation, it decreased rapidly, and reached a constant value. Before carrying out this

Formation Mechanism for Monodispersed Silica Spheres

Figure 8. Time course of Si concentration in the reaction solution during synthesis. The dashed-line curve represents the calculated value assuming a first-order reaction.

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1097

Figure 12. TEM of platinum incorporated particles obtained by two TMOS additions.

Figure 13. Schematic illustration of the formation of core/shell MMSS.

Figure 9. Time course of pH and conductivity in the reaction solution during synthesis.

Figure 14. An SEM image of the silica/mercaptopropyl-modified silica core/shell MMSS.

Figure 10. Proposed mechanism for the formation of monodispersed mesoporous silica spheres. Progress of TMOS condensation is described above. Precipitation of particles is shown below. Short lines represent TMOS. Zig-zag lines represent oligomeric TMOS.

Figure 15. Properties of the silica/mercaptopropyl-modified silica core/ shell MMSS. (a) XRD pattern. (b) Nitrogen adsorption isotherm. Inset: Pore size distribution.

Figure 11. SEM images of expanded particles obtained by the different TMOS addition times, (a) 0, (b) 2, and (c) 4, and schematic illustration of the particle growth. Standard deviations are in parentheses.

experiment, we had expected that the silicic acid concentration ()TMOS concentration) would have decreased even before precipitation due to the condensation of TMOS. The oligomeric silica precursors might have been hydrolyzed, because silicic acid titration was carried out under strongly acidic conditions. In the synthesis, almost all of the TMOS precipitated as MMSS ()approximately 100% yield). Nevertheless, the silicic acid

concentration obtained by titration remained at over 1.2 mmol/ L, even at the end of the reaction. It is assumed that silicic acid monomers were slightly eluted from the particles during the titration due to an exchange reaction between the cationic surfactant inside the MMSS and protons, with subsequent hydrolysis of the silica. MMSS is synthesized from TMOS and a surfactant under very specific conditions.34 Generally, in the case where an alkyltrimethylammonium salt is used as the surfactant, about 15 mol % of the surfactant with respect to the TMOS is used to form the mesoporous silica (i.e., ratio of surfactant to TMOS ) 0.15:1.0). In this work, we used a molar amount of surfactant that was more than 8 times higher than is usually used to form mesopores (i.e., ∼1.3 times the concentration of the TMOS), even though the concentration was very low. Therefore, because of the excess amount of surfactant, the reaction can be assumed

1098 J. Phys. Chem. C, Vol. 111, No. 3, 2007

Nakamura et al.

Figure 16. TEM images of the platinum incorporated silica/mercaptopropyl-modified silica core/shell MMSS.

to be first-order with respect to TMOS concentration. In this case, the concentration of TMOS ()[TMOS]) can be expressed by the following equation:

[TMOS] ) [TMOS]0 e-kt where [TMOS]0 is the initial concentration of TMOS, k is the rate constant, and t is time. Here, the rate constant (k) can be readily calculated from the equation t1/2 ) ln 2/k, where t1/2 represents the half-life. The calculated TMOS concentration is also plotted in Figure 8. The calculated data coincide with the experimental data, indicating that the reaction proceeded as a first-order reaction with respect to TMOS concentration. In this study, the synthesis was conducted under basic conditions. When sodium hydroxide is used for the hydrolysis of TMOS, the values of pH and conductivity are expected to change. Figure 9 shows how the values of pH and conductivity change during the synthesis. After the commencement of the synthesis ()addition of TMOS), both the values of pH and conductivity decreased rapidly. This indicates that the hydrolysis of TMOS by sodium hydroxide took place very quickly. Next, the value of pH and conductivity remained constant until precipitation began, suggesting that no ionic reaction occurred. Therefore, hardly any reaction between the silica precursors and the surfactants is anticipated. It is assumed that only the condensation of TMOS proceeded during this period. After precipitation began, the value of pH increased and the conductivity gradually decreased, reaching a constant value after 600 s. An exchange reaction between the sodium ions and the surfactant cations occurred, and the surfactants combined with the silica precursors to form mesoporous silica. As a result, the amount of ionic molecules in the reaction solution decreased during precipitation, leading to the decrease in the conductivity in the solution. Because the surfactant, hexadecyltrimethylammonium chloride, exhibits weak acidity, it is assumed that a decrease of the soluble surfactant molecules in the solution caused the increase in the value of pH. The results of both silicic acid titration and pH-conductivity measurements demonstrate that the synthesis terminated in about 600 s, agreeing well with the results of both TEM and in situ XRD analysis. From the aforementioned results, a formation mechanism for the MMSS is proposed, as shown in Figure 10. The surfactant molecules are drawn as individual molecules rather than as micelles, because MMSS was only obtained under very low surfactant concentrations, which are assumed to be less than the critical micelle concentration.32-34 Hydrolyzed TMOS monomers condense to form oligomeric silica precursors. When the oligomeric silica precursors reach a certain size, these precursors and surfactants are assembled into small MMSS with

hexagonal regularity, which then emerge in solution. Any residual silica precursors then react preferentially with the surface silanols on the existing particles, preventing the generation of new particles. This leads to the formation of MMSS. It is not possible that the particles had grown by the co-aggregation of smaller particles, because the mesopores were aligned radially from the center to the surface of the particles. Silica precursors contain a fair amount of silanols that dissociate to Si-O- and protons. In consequence, they are negatively charged. By contrast, surfactants have a positive charge. Therefore, silica precursors and surfactants can contact each other throughout the reaction. Silica precursors are much more stable in solution than they are as precipitates when their size is small. However, when silica precursors attain a certain size by oligomerization, they are forced to precipitate as an organic-inorganic composite. The reason why no further nucleation of particles occurs after the precipitation of the primary particles under the conditions that we used is unknown at present. Monodispersed particles are obtained under very specific reaction conditions, in which the surfactant concentration, the methanol-to-water ratio, and the TMOS concentration have a great influence. Further study is needed to clarify these observations. Particle Size Expansion. It is expected from the mechanism proposed above that the sizes of the particles could be enlarged by the addition of further TMOS. To confirm this, equimolar amounts of TMOS were added every hour for 4 h after the completion of the initial reaction ()1 h later). The addition of further surfactant was not necessary because an excess amount of surfactant is used under our experimental conditions. Figure 11 shows SEM images of the particles that were obtained after two and four additions of TMOS to the initial reaction mixture. The diameters of the particles have clearly increased upon the addition of TMOS, while retaining their monodispersed characteristics (standard deviation in parentheses). This result supports the notion that the additional TMOS reacts preferentially with the surface silanol groups on the existing particles rather than generating new particles by reacting with each other. It is questionable whether the radial alignment of the mesopores is retained or not after the expansion of the particles. Figure 12 shows a TEM image of platinum-loaded enlarged particles after two TMOS additions. Surprisingly, radially aligned mesopores can be clearly seen inside the enlarged MMSS. Synthesis of Core/Shell Particles. On the basis of the above results, it is assumed that MMSS with a core/shell structure can be obtained by causing a different type of silica precursor to the original precursor to react with the existing particles, as illustrated in Figure 13. A synthesis was conducted in which double the molar amount of a mercaptopropyltrimethoxysilane

Formation Mechanism for Monodispersed Silica Spheres (MPTMS)/TMOS ()20/80 mol/mol) mixture was added to a solution including particles obtained from TMOS. Figure 14 shows an SEM image of the particles obtained. The average diameter was 0.73 µm, and the standard deviation was 4.3%, indicating that the particles were highly monodispersed. The XRD pattern and the nitrogen adsorption isotherm (the desorption branch was identical to the adsorption branch) are shown in Figure 15. The XRD pattern exhibits a sharp peak assigned to the (100) plane, as well as higher-order diffraction peaks, indicating that the materials possessed an ordered hexagonal regularity. From the nitrogen adsorption isotherm, the specific surface area is calculated to be 998 m2/g. The pore size distribution curve, as determined by the BJH method, shows two peaks at 17.3 and 18.8 Å. These diameters are assumed to be those of the shell and of the core portion, respectively. It is anticipated that the particles should possess a silica core/ mercaptopropyl-modified silica shell structure, more specifically, a hydrophilic core/hydrophobic shell structure. To confirm this, platinum was incorporated into the particles. An egg-type structure can be clearly seen in Figure 16. The platinum (dark portion) is concentrated in the hydrophilic core region. Because platinum was incorporated into the mesopores by using an aqueous solution of a tetra-ammine complex as a precursor, the platinum particles (black part) only existed in the hydrophilic core portions. From these results, it is obvious that MMSS with a core/shell structure could be successfully synthesized by changing the type of the additive silica precursor. Conclusions An investigation into the formation mechanism of monodispersed mesoporous silica spheres was conducted from the viewpoint of both particle growth and the progress of the condensation reaction. It was found that, under certain reaction conditions, small particles emerged suddenly after the commencement of the synthesis, and then these particles grew homogeneously to their final size. On the contrary, new small particles were observed throughout the synthesis when the reaction conditions favored a heterogeneous system. Hexagonal structures formed in the primary particles as they were generated under our reaction conditions. This result was confirmed by both TEM observations and in situ XRD measurements. The condensation proceeded through a first-order reaction with respect to the TMOS concentration under the conditions where there was an excess amount of surfactant. From the above results, a formation mechanism for monodispersed mesoporous silica spheres is proposed as follows. At first, hydrolyzed TMOS monomers condense to form oligomeric silica precursors. When the oligomeric silica precursors reach a certain size, these precursors and surfactants are assembled into small monodispersed mesoporous silica spheres, which then emerge in solution. The residual silica precursors then react preferentially with the surface silanols on the particles that have been generated, preventing any particles from forming. This leads to the formation of monodispersed mesoporous silica spheres. To verify the mechanism proposed above, particle-size expansion was demonstrated by adding additional silica precursor after the completion of the initial reaction. Surprisingly, the radial alignment of the hexagonal mesopores was still retained in the expanded particles. In addition, highly monodispersed core/shell mesoporous silica spheres possessing a hydrophilic core and a hydrophobic shell were obtained for the first time, by the addition of a different type of silica precursor to the initial one. In the case of these core/shell mesoporous silica spheres, different types of guest molecules can be selectively incorporated

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1099 into their cores and shells by designing an appropriate combination of core and shell properties. Work is underway to synthesize a variety of monodispersed mesoporous core/shell silica spheres and to explore potential new applications of these materials. Acknowledgment. This research was partially supported by the Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B), 17310079, 2006. Supporting Information Available: Powder XRD pattern of the particles that were obtained after 600 s. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (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. (3) Yang, P.; Zhao, D.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1998, 10, 2033. (4) Bruinsma, P. J.; Kim, A. Y.; Liu, J.; Baskaran, S. Chem. Mater. 1997, 9, 2507. (5) Lin, H.-P.; Yang, L.-Y.; Mou, C.-Y.; Lee, H.-K.; Liu, S.-B. Stud. Surf. Sci. Catal. 2000, 129, 7. (6) Zhao, D.; Yang, P.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 1174. (7) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Chem. Mater. 2000, 12, 275. (8) Shio, S.; Kimura, A.; Yamaguchi, M.; Yoshida, K.; Kuroda, K. Chem. Commun. 1998, 2461. (9) Tolbert, S. H.; Schaffer, T. E.; Feng, J.; Hansma, P. K.; Stucky, G. D. Chem. Mater. 1997, 9, 1962. (10) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. Stud. Surf. Sci. Catal. 2000, 129, 37. (11) Blin, J. L.; Le´onard, A.; Su, B. L. Chem. Mater. 2001, 10, 3772. (12) Yang, H.; Vovk, G.; Coombs, N.; Sokolov, I.; Ozin, G. A. J. Mater. Chem. 1998, 8, 743. (13) Boissie`re, C.; Lee, A.; Mansouri, A. E.; Larbot, A.; Prouzet, E. Chem. Commun. 1999, 2047. (14) Boissie`re, C.; Kummel, M.; Persin, M.; Larbot, A.; Prouzet, E. AdV. Funct. Mater. 2001, 11, 129. (15) Gallis, K. W.; Araujo, J. T.; Duff, K. J.; Moore, J. G.; Landry, C. C. AdV. Mater. 1999, 11, 1452. (16) Kurganov, A.; Unger, K. K.; Issaeva, T. J. Chromatogr., A 1996, 753, 177. (17) Nassivera, T.; Eklund, A. G.; Landry, C. C. J. Chromatogr., A 2002, 973, 97. (18) Lee, M.-H.; Oh, S.-G.; Moon, S.-K.; Bae, S.-Y. J. Colloid Interface Sci. 2001, 83, 240. (19) Yamada, Y.; Nakamura, T.; Ishii, M.; Yano, K. Langmuir 2006, 22, 2444. (20) Nakamura, T.; Yamada, Y.; Yano, K. J. Mater. Chem. 2006, 16, 2417. (21) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. Chem. Mater. 1998, 10, 1623. (22) Yang, H.; Vovk, B.; Coombs, N.; Sokolov, I.; Ozin, G. A. J. Mater. Chem. 1998, 8, 743. (23) Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature 1999, 398, 223. (24) Bore, M. T.; Rathod, S. B.; Ward, T. L.; Datye, A. K. Langmuir 2003, 19, 256. (25) Rao, G. V. R.; Lo´pez, G. P.; Bravo, J.; Pham, H.; Datye, A. K.; Xu, H.; Ward, T. L. AdV. Mater. 2004, 14, 1301. (26) Bu¨chel, G.; Gru¨n, M.; Unger, K. K.; Matsumoto, A.; Tsutsumi, K. Supramol. Sci. 1998, 5, 253. (27) Gru¨n, M.; Bu¨chel, G.; Kumar, D.; Schumacher, K.; Bidlingmajer, B.; Unger, K. K. Stud. Surf. Sci. Catal. 2000, 128, 155. (28) Unger, K. K.; Kumar, D.; Gru¨n, M.; Bu¨chel, G.; Lu¨dtke, S.; Adam, T.; Schumacher, K.; Renker, S. J. Chromatogr., A 2000, 892, 47. (29) Gru¨n, M.; Lauer, I.; Unger, K. K. AdV. Mater. 1997, 9, 254. (30) Liu, S.; Cool, P.; Collart, O.; Voort, P. V. D.; Vansant, E. F.; Lebedev, O. I.; Tendeloo, G. V.; Jiang, M. J. Phys. Chem. B 2003, 107, 10405. (31) Martin, T.; Galarneau, A.; Renzo, F. D.; Fajula, F.; Plee, D. Angew. Chem., Int. Ed. 2002, 41, 2590. (32) Yano, K.; Fukushima, Y. Bull. Chem. Soc. Jpn. 2002, 75, 1977. (33) Yano, K.; Fukushima, Y. J. Mater. Chem. 2003, 13, 2577. (34) Yano, K.; Fukushima, Y. J. Mater. Chem. 2004, 14, 1579.

1100 J. Phys. Chem. C, Vol. 111, No. 3, 2007 (35) Yamada, Y.; Yano, K. Microporous Mesoporous Mater. 2006, 93, 190. (36) Huo, Q.; Margolese, D. I.; Ciesra, U.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (37) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinki, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138. (38) Zhang, J.; Luz, Z.; Goldfarb, D. J. Phys. Chem. B 1997, 101, 7087. (39) Zhang, J.; Luz, Z.; Zimmermann, H.; Goldfarb, D. J. Phys. Chem. B 2000, 104, 279. (40) Baute, D.; Frydman, V.; Zimmermann, H.; Kababya, S.; Goldfarb, D. J. Phys. Chem. B 2005, 109, 7807. (41) Firouzi, A.; Atef, F.; Oertli, A. G.; Stucky, G. D.; Chmelka, B. F. J. Am. Chem. Soc. 1997, 119, 3596. (42) Monnier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouz, A.; Janicke, J.; Chmelka, B. F. Science 1993, 261, 1299.

Nakamura et al. (43) O’Brien, S.; Francis, R. J.; Fogg, A.; O’Hare, D.; Okazaki, N.; Kuroda, K. Chem. Mater. 1999, 11, 1822. (44) Khodakov, A. Y.; Zholobenko, V. L.; Impe´ror-Clerc, M.; Durand, D. J. Phys. Chem. B 2005, 109, 22780. (45) Ruthstein, S.; Schmidt, J.; Kesselman, E.; Talmon, Y.; Goldfarb, D. J. Am. Chem. Soc. 2006, 128, 3366. (46) Nooney, R. I.; Thirunavukkarasu, D.; Chen, Y.; Josephs, R.; Ostafin, A. E. Chem. Mater. 2002, 14, 4721. (47) Pauwels, B.; Tendeloo, G. V.; Thoelen, C.; Rhijn, W. V.; Jacobs, P. A. AdV. Mater. 2001, 13, 1317. (48) Lebedev, O. I.; Tendeloo, G. V.; Collart, O.; Cool, P.; Vansant, E. F. Solid State Sci. 2004, 6, 489. (49) Tendeloo, G. V.; Lebedev, O. I.; Collart, O.; Cool, P.; Vansant, E. F. J. Phys.: Condens. Matter 2003, 15, S3037. (50) Tan, B.; Rankin, S. E. J. Phys. Chem. B 2004, 108, 20122.