Anomalous Pore Expansion of Highly Monodispersed Mesoporous

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Anomalous Pore Expansion of Highly Monodispersed Mesoporous Silica Spheres and Its Application to the Synthesis of Porous Ferromagnetic Composite Mamoru Mizutani,* Yuri Yamada, Tadashi Nakamura, and Kazuhisa Yano* Toyota Central Research & DeVelopment Laboratories, Incorporated, Nagakute, Aichi 488-1192, Japan ReceiVed September 28, 2007. ReVised Manuscript ReceiVed May 14, 2008

In this paper, we report on the pore expansion of monodispersed mesoporous silica spheres (MMSS) by a swelling agent incorporation method, by which swelling agents such as various hydrocarbons were incorporated into mesopores. Mesopores were expanded to more than twice the size (5.5 nm) of the original one (2.5 nm). In this method, pore expansion of MMSS was achieved while radially ordered hexagonal mesopores with spherical morphology and high monodispersity were retained. In addition, we have successfully obtained highly monodispersed porous FePtsMMSS composites by incorporating FePt precursors into the pore-expanded MMSS. The composites had both ferromagnetic behavior with coercivity of 3.3 kOe at room temperature and mesoporosity with pore volume of 0.74 cm3g-1, which is enough to incorporate a variety of chemicals, nanoparticles, and proteins, such as drugs, quantum dots, enzymes, and DNA molecules. They have potential applications in drug delivery systems, gene delivery systems, magnetic hyperthermia treatments, and magnetic photonic crystals.

1. Introduction Periodic mesoporous silica, MCM-41, was discovered by the Mobil researchers1 and has been extensively studied by many researchers up to now.2 Ordered mesoporous silicas have been expected to be increasing use as new types of catalysts,3 adsorbents,4 and host materials.5 Controlling the morphologies of such materials is one of the major challenges that need to be solved to enable their use in industrial applications. Nonporous monodispersed silica spheres in the micron-sized range were first synthesized in 1968 by Sto¨ber, who employed a water, alcohol, ammonia, and tetraalkoxysilane system.6 Since then, this method has been modified by the addition of cationic surfactants and/or by the use of other organic solvents, and mesoporous silica spheres have now been synthesized.7 In our laboratory, we have succeeded in the synthesis of hexagonally ordered and well-defined highly monodispersed * To whom correspondence should be addressed. E-mail: mizutani@ mosk.tytlabs.co.jp; [email protected].

(1) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (b) 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. (2) (a) Sayari, A. Chem. Mater. 1996, 8, 1840. (b) Taguchi, A.; Schu¨th, F. Microporous Mesoporous Mater. 2005, 77, 1. (c) Hoffmann, F.; Cornelius, M.; Morell, J.; Fro¨ba, M. Angew. Chem., Int. Ed. 2006, 45, 3216. (3) (a) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995, 378, 159. (b) Reynhardt, J. P. K.; Yang, Y.; Sayari, A.; Alper, H. Chem. Mater. 2004, 16, 4095. (4) (a) Antochshuk, V.; Olkhovyk, O.; Jaroniec, M.; Park, I. S.; Ryoo, R. Langmuir 2003, 19, 3031. (b) Kang, T.; Park, Y.; Choi, K.; Lee, J. S.; Yi, J. J. Mater. Chem. 2004, 14, 1043. (5) (a) Benitez, M.; Bringmann, G.; Dreyer, M.; Garcia, H.; Ihmels, H.; Waidelich, M.; Wissel, K. J. Org. Chem. 2005, 70, 2315. (b) Tura, C.; Coombs, N.; Dag, O. Chem. Mater. 2005, 17, 573. (6) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.

mesoporous silica spheres (MMSS) from tetramethoxysilane and n-alkyltrimethylammonium bromide (CnTMABr).8 The pore sizes of the MMSS can be tuned by changing the lengths of the alkyl chains in the surfactant.9 However, the maximum pore sizes of the MMSS was limited to 2.5 nm. For mesoporous materials, the control of pore size is as important as controlling the particle size. Many methods have been reported that attempt to expand the pore sizes of mesoporous materials. The most commonly used technique is the introduction of a swelling agent into the structure-directing template, either in the preparation step10 or in the postsynthesis hydrothermal treatment.11 The diameters of the pores could easily be expanded, but it was uncertain whether the morphology was maintained or not.

(7) (a) Gru¨n, M.; Bu¨chel, G.; Kumar, D.; Schumacher, K.; Bidlingmaier, B.; Unger, K. K. Stud. Surf. Sci. Catal. 2000, 128, 155. (b) Gru¨n, M.; Lauer, I.; Unger, K. K. AdV. Mater. 1997, 9, 254. (c) Schumacher, K.; Renker, S.; Unger, K. K.; Ulrich, R.; Chesne, A. D.; Spiess, H. W.; Wiesner, U. Stud. Surf. Sci. Catal. 2000, 129, 1. (d) Luo, Q.; Li, L.; Xue, Z.; Zhao, D. Stud. Surf. Sci. Catal. 2000, 129, 37. (8) (a) Yano, K.; Suzuki, N.; Akimoto, Y.; Fukushima, Y. Bull. Chem. Soc. Jpn. 2002, 75, 1977. (b) Yano, K.; Fukushima, Y. J. Mater. Chem. 2003, 13, 2577. (9) Yano, K.; Fukushima, Y. J. Mater. Chem. 2004, 14, 1579. (10) (a) Lefe`vre, B.; Galarneau, A.; Iapichella, J.; Petitto, C.; Renzo, F. D.; Fajula, F.; Bayram-Hahn, Z.; Skudas, R.; Unger, K. Chem. Mater. 2005, 17, 601. (b) Pauly, T. R.; Pinnavaia, T. J. Chem. Mater. 2001, 13, 987. (c) Kimura, T.; Sugahara, Y.; Kuroda, K. Chem. Commun. 1998, 559. (d) Ulagappan, N.; Rao, C. N. R. Chem Commun 1996, 2759. (e) Zhang, W.; Pauly, T. R.; Pinnavaia, T. J. Chem. Mater. 1997, 9, 2491. (11) (a) Hanrahan, J. P.; Copley, M. P.; Ryan, K. M.; Spalding, T. R.; Morris, M. A.; Holmes, J. D. Chem. Mater. 2004, 16, 424. (b) Sayari, A.; Hamoudi, S.; Yang, Y. Chem. Mater. 2005, 17, 212. (c) Sun, J.H.; Coppens, M.-O. J. Mater. Chem. 2002, 12, 3016. (d) Sayari, A.; Kruk, M.; Jaroniec, M.; Moudrakovski, I. L. AdV. Mater. 1998, 10, 1376. (e) Sun, J.-H.; Coppens, M.-O. Stud. Surf. Sci. Catal. 2002, 141, 85.

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Recently, we successfully fabricated a colloidal crystal from MMSS and its composite.12 The large-pore MMSS is very promising for the new types of colloidal crystals by the incorporation of large molecules, such as biomolecules and metal complexes. We have tried to achieve the direct synthesis of poreexpanded MMSS by using swelling agent such as 1,3,5trimethylbenzene (TMB). However, because the synthetic conditions of hexagonally ordered mesoporous silica with monodispersed morphology are very limited, monodispersed particles could still not be obtained. We then developed a new method to expand mesopores of MMSS while retaining the morphology, monodispersity, and radially highly ordered hexagonal mesopores. The pore size of MMSS was enlarged to 3.7 nm from 2.5 nm by the complete exchange of the surfactants. Preliminary results were presented in a previous communication.13 In the meantime, FePt with a chemically ordered fct L10 phase have attracted much attention, because the material exhibits higher magnetocrystalline anisotropy (Ku: 6.6-10 MJ/m3),14 saturation magnetization (Ms: 1140 emu/cm3),14 and better chemical stability. FePt nanoparticles have potential applications in data storage, permanent magnetic nanocomposites, and biomedicine.15 Usually, it is very difficult to handle nanoparticles, because they readily tend to aggregate. Incorporation of FePt nanoparticles into mesopores of MMSS is very promising, not only to prevent aggregation of the particles but also to achieve ease of handling. However, because the threshold size of FePt for ferromagnetism is more than 2.8-3.3 nm,16 it is necessary to prepare for mesoporous silica having pore size larger than that value. In this paper, we now report on the pore expansion of MMSS by newly developed swelling agent incorporation method. The pore sizes of MMSS samples were enlarged to more than twice the original value. We will also describe the mechanism for the pore-expansion process of the method. In addition, the incorporation of ferromagnetic FePt nanoparticles into the pore-expanded MMSS will be described. There are varieties of possible applications for MMSS materials, and the methods described here are very important for the incorporation of large molecules such as metal complexes and proteins into mesopores, for the size control of nanoparticles inside mesopores, and for the retention of pore space after the grafting of organic functional groups. 2. Experimental Section 2.1. Chemicals. All reagents and solvents were the highest grade available and used without further purification. 2.2. Synthesis of MMSS18. MMSS18 were prepared using octadecyltrimethylammonium Chloride (C18TMACl) as a template (12) Yamada, Y.; Nakamura, T.; Ishii, M.; Yano, K. Langmuir 2006, 22, 2444. (13) Mizutani, M.; Yamada, Y.; Yano, K. Chem. Commun. 2007, 1172. (14) Klemmer, T.; Hoydick, D.; Okumura, H.; Zhang, B.; Soffa, W. A. Scr. Met. Mater. 1995, 33, 1793. (15) Sun, S. AdV. Mater. 2006, 18, 393. (16) (a) Takahashi, Y. K.; Ohkubo, T.; Ohnuma, M.; Hono, K. J. Appl. Phys. 2003, 93, 7166. (b) Takahashi, Y. K.; Koyama, T.; Ohnuma, M.; Ohkubo, T.; Hono, K. J. Appl. Phys. 2004, 95, 2690.

Mizutani et al. according to the literature.8,9 In a typical synthesis procedure of MMSS18, in brief, C18TMACl (3.52 g) and 1 M NaOH solution (2.28 mL) were dissolved in a water/methanol (40/60; w/w) solution (800 g). TMOS (1.32 g) was then added to the solution with vigorous stirring at 298 K. After 8 h of continuous stirring, the mixture was aged overnight. The white powder was then filtered and washed with distilled water, and dried at 318 K for 72 h. It is worth mentioning that surfactants inside MMSS18 should not be removed. 2.3. Pore Expansion of MMSS18 by Surfactant Exchange Method (the SUR method). In a typical treatment, 1 g of MMSS18 silica/surfactant composite was added to 60 mL of 0.1 M C22TMACl water/ethanol (50/50; v/v) solution. The mixture was sealed and placed in an oven at 373 K for 7 days without stirring. The resulting white powder was filtered out, washed with distilled water, and then dried. The powder was calcined in air at 823 K for 6 h to remove any remaining organic species. Hereafter, the samples obtained by the SUR method are denoted as PS(22)MS18. 2.4. Pore Expansion of MMSS18 by Swelling Agent Incorporation Method (the SWE method). In the SWE method, hydrophobic reagents such as 1,3,5-trimethylbenzene (TMB), 1,3,5triisopropylbenzene, anthracene, 1,10-phenanthoroline, benzene, cyclohexane, dodecane, hexane, and naphthalene were used. In a treatment, 1 g of MMSS18 silica/surfactant composite was added to 60 mL of a water/ethanol (50/50; v/v) solution including 2.25 g of a swelling agent. The subsequent steps were the same as described above in the SUR method, but the resulting white powder was washed with ethanol or ethanol/water mixture. The powder was calcined in air at 823 K for 6 h to remove any remaining organic species. The samples obtained by the SWE method are denoted as PS(swe)MS18 (“swe” represents the abbreviation of the swelling agent used for the treatment) hereafter. Incorporation of TMB was also conducted in a 0.1 M C22TMACl water/ethanol (50/50; v/v) solution. The sample obtained with this procedure is denoted as PS(22, TMB)MS18. 2.5. Synthesis of Monodispersed Porous Ferromagnetic Composite Spheres. Fe(NO3)3 · 9H2O and H2PtCl6 · 6H2O were used as the metal sources. In a typical synthesis, (pore expanded) MMSS was added to Fe(NO3)3 and H2PtCl6 solution (84:8:8 Si:Fe:Pt molar ratio) and the mixture was evaporated dry by means of a rotary evaporator on a steam bath. The yellow powder obtained was calcined at 800 °C for 4 h under a reducing atmosphere (H2/N2 ) 5/95, v/v). The reduction was conducted at higher temperature (usually 600 °C for bulky FePt) by considering the difficulties in crystallization of FePt inside mesopores. 2.6. Materials Characterization. We used a SIGMA-V (Akashi Seisakusyo) scanning electron microscope and a JEM2000EX (JEOL) transmission electron microscope to characterize the particle size and morphology. The average particle diameter was calculated from the diameters of 50 particles in an SEM picture. The standard deviation was also calculated, from which the particle diameter distribution was judged. TMB in the sample was analyzed by GC6890/5973N (Agilent Technologies) thermo desorption system gas chromatography mass spectrometry (TDS-GC/MS). The constituent phases were determined by X-ray diffraction using a RINT 2200 (Rigaku) diffractometer with Cu KR radiation. Magnetic properties were studied at room temperature in a VSM-3S-15 (TOEI INDUSTRY) instrument. The pore properties were analyzed using BELSORP-mini II (BEL JAPAN) nitrogen adsorption-desorption measurements. The specific surface areas were computed using the Brunauer, Emmett, and Teller (BET) multimolecular layer adsorption model, and the average pore sizes were calculated from the desorption branch by using the Barrett, Joyner, and Halenda (BJH) model.

Pore Expansion of Monodispersed Mesoporous Silica Spheres

Figure 1. SEM and TEM images of the pore-expanded MMSS samples; (a, c) PS(22)MS18, (b, d) PS(TMB)MS18.

3. Results and Discussions 3.1. Pore Expansion of MMSS by Swelling Agent Incorporation Method. We previously reported on the pore expansion of MMSS samples by surfactant exchange method (the SUR method).13 The surfactants inside mesopores (decyl, tetradecyl, and octadecyl trimethylammonium halides) were completely replaced by the surfactant with longer alkylchain length (dococyltrimethlyammonium chloride) during a solvothermal treatment. The pore size of MMSS18 was expanded to 3.6 nm from 2.5 nm while retaining spherical morphology and high monodispersity. The use of ethanol/ water mixture led to this anomalous pore expansion by suppressing the dissolution of silica source at an appropriate amount. We have extended this method by using a swelling agent, such as TMB instead of dococyltrimethlyammonium chloride. Consequently, mesopores of MMSS18 were expanded to more than twice the original size. Here, we made a comparison for the pore-expanded MMSS18 samples obtained by the SUR (PS(22)MS18) and SWE method (PS(TMB)MS18).13 Figure 1 shows the SEM and TEM images of PS(22)MS18 and PS(TMB)MS18, and Table 1 lists a summary of the results concerning the uniformities and pore properties of the samples. It is obvious from Figure 1 that both PS(22)MS18 and PS(TMB)MS18 samples retained high monodispersity and spherical morphology. Their average diameters before and after postsynthesis solvothermal treatment hardly changed, and they were 1.17 (MMSS18), 1.10 (PS(22)MS18), and 1.19 µm (PS(TMB)MS18), respectively, with coefficients of variation of less than 5%. In addition, it is observed from both TEM images that radial alignments of mesopores were still retained after substantial reorganization of mesopores during the pore expansion. The XRD patterns that were obtained for PS(22)MS18 and PS(TMB)MS18 showed the (100) diffraction peaks at 2θ ) 2.20 and 1.27°, which indicated the presence of a mesoporous structure. The d100 values calculated from 2θ were 4.02 and 6.96 Å, respectively. The changes in the pore properties of pore-expanded MMSS18 were confirmed by the results of nitrogen adsorption-desorption measurements. Figure 2 shows the nitrogen adsorption-desorption isotherms and the

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corresponding pore size distributions for MMSS18, PS(22)MS18, and PS(TMB)MS18. All of the isotherms are type IV curves. The isotherm of MMSS18 indicates a reversible adsorption-desorption process with a characteristic nitrogen condensation-evaporation step at a relative pressure of ca. 0.25, and the volume adsorbed was 616 cm3 (STP) g-1. Meanwhile, the isotherms of PS(22)MS18 and PS(TMB)MS18 exhibit the step at a relative pressure of ca. 0.45 and 0.6, and the volumes adsorbed are 728 and 1075 cm3 (STP) g-1, respectively. As shown in Figure 2b, the corresponding pore size distributions are very narrow, and it is obvious that PS(TMB)MS18 had the largest pore size among these samples. When the solvothermal treatment of MMSS was conducted under the condition in which both TMB and C22TMACl existed, the pore size of the sample (PS(22, TMB)MS18) expanded to 4.24 nm (Table 1), which was less than the value obtained using TMB only (PS(TMB)MS18). If surfactants exist in a solution, the solubility of TMB is expected to increase. It is assumed that this increase causes the decrease in the amount of TMB incorporated into hydrophobic surfactant micelles inside mesopores. From the above results, it was confirmed that mesopores could be expanded substancially by the SWE method. An investigation was then conducted to find out what types of reagents could satisfactorily expand the mesopores. In the SWE method, it is assumed that a swelling agent penetrates into hydrophobic parts of micelle rods inside mesopores, leading to pore expansion. And it is well-known that hydrophobic reagents are effective to enlarge mesopores in the preparation step,10 or in the postsynthesis hydrothermal treatment.11 Therefore, mainly hydrophobic reagents were examined. Table 2 lists the specific surface area, average pore size, and pore volume of PS(swe)MS18 samples. Tricyclic reagents such as anthracene and 1,10-phenanthoroline achieved the lowest pore expansion. The penetration of these molecules into micelle rods was assumed to be inhibited by rather bigger molecular sizes of these reagents. In cases when other reagents of smaller molecular sizes were used, pores were expanded more effectively. Among them, TMB was found to be the best to expand the mesopores of MMSS. In the SWE method, a water/ethanol mixture was used as a solvent. If the affinity of a reagent to the solvent is high, the reagent is expected to remain in the solvent but to penetrate into hydrophobic micelle rods. It is understood, therefore, that a hydrophobic reagent with an appropriate size is of great importance for the use as a swelling agent. The only exception was N,N-dimethyldecylamine (DMDA) which is highly soluble to the water/ethanol mixture. This reagent brought the second largest pore expansion next to TMB. It had been proposed that in the presence of water, the alkylamine self-assembled into an inverted cylindrical micelle inside the alkyltrimethylammonium micelle, being held via attractive hydrophobic forces.11 Therefore, it is likely that the pore expansion by DMDA proceeded through the different mechanism from that by the other hydrophobic reagents. 3.2. Mechanism of Pore Expansion. It is very important to understand how pores were expanded by the pore-

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Table 1. Particle Diameter and d100 Value of MMSS18 and the Pore-Expanded MMSS18 Samples sample MMSS18 PS(22)MS18 PS(22, TMB)MS18 PS(TMB)MS18

average particle diameter (µm)

coefficients of variation (%)

d100 (nm)

1.17 1.10

4.4 4.4

3.61 4.02

1.19

3.8

6.96

expansion treatment. We previously reported that the surfactants inside the mesopores had exchanged completely with the longer alkyl-chain length surfactants in solution during the SUR method.13 It has been suggested that a strong hydrophobic interaction between surfactants with longer alkyl-chain lengths leads to this complete exchange. A silica source and quaternary ammonium surfactants are bound electrostatically in mesoporous silicas. If identical numbers of surfactants with shorter alkyl-chain lengths are replaced by surfactants with longer alkyl-chain lengths, the pore sizes of the mesoporous silica should increase. The condensation of silanol groups in the as-prepared MMSS is incomplete, because the synthesis of the MMSS was conducted at 25 °C. Therefore, under solvothermal conditions, small quantities of silica dissolved in a water/ethanol mixture (as

Figure 2. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distributions of MMSS18 and pore-expanded MMSS18 samples. Table 2. Properties of MMSS18 and Pore-Expanded MMSS18 Samples sample

specific surface area (m2 g-1)

average pore size (nm)

pore volume (cm3 g-1)

original MMSS18 PS(anthracene)MS18 PS(1,10-Phen)MS18a PS(benzene)MS18 PS(TiPB)MS18b PS(cyclohexane)MS18 PS(dodecane)MS18 PS(hexane)MS18 PS(naphthalene)MS18 PS(DMDA)MS18c PS(TMB)MS18

1240 1130 1110 1110 1000 1110 1120 1150 1050 1070 910

2.46 3.04 3.06 3.27 3.72 3.91 3.99 4.55 5.03 5.16 5.46

0.95 1.09 1.06 1.11 1.40 1.20 1.48 1.50 1.21 1.84 1.66

c

a 1,10-Phen: 1,10-phenanthoroline. DMDA: N,N-dimethyldecylamine.

b

TiBP: 1,3,5-triisopropylbenzene.




1240 1120 910 910

2.46 3.58 4.24 5.46

0.95 1.13 1.32 1.66

confirmed by the molybdenum-yellow method). This leads to a reorganization of the mesopores of MMSS, as illustrated in Scheme 1 (left arrow). Meanwhile, if TMB is incorporated into mesopores of MMSS18, TMB should be detected in PS(TMB)MS18. The TMB inside the PS(TMB)MS18 before it was calcined were analyzed by TDS-GC/MS. The TDS-GC/MS spectra of PS(TMB)MS18 show large TMB peak (m/z ) 120). On the contrary, the TDS-GC/MS spectra of PS(TMB)Sto¨ber, which was Sto¨ber silica that was treated with TMB, exhibited small TMB peaks. The peak area of PS(TMB)MS18 was 40 times or more as large as that of PS(TMB)Sto¨ber. From these results, it is confirmed that the TMB in solutions had been incorporated into the mesopores. It is assumed that a strong hydrophobic interaction between surfactant’s alkyl chain and TMB leads to this incorporation. Once TMB is incorporated into the mesopores, the pore sizes of the mesoporous silica should increase. The reorganization of the mesopores of MMSS by dissolution and redeposition of silica components leads to the pore expansion of MMSS as illustrated in Scheme 1 (right arrow). 3.3. FePt-MMSS Composites. We have succeeded in the synthesis of highly monodispersed superparamagnetic composite spheres by incorporating iron oxides such as γ-Fe2O3 and ε-Fe2O3 into MMSS16.17 When we consider the result that pore size of PS(TMB)PS18 was 5.5 nm, it is impossible to obtain ferromagnetic composites by incorporating the iron oxides, because the threshold sizes of these oxides for ferromagnetism are more than 10 nm. We then incorporated FePt into MMSS for the synthesis of ferromagnetic composite spheres because of its lower threshold sizes (2.8-3.3 nm) for ferromagnetism.18 Figure 3 shows SEM images of FePt incorporated samples. When MMSS18, which had the smallest pore size, was used as a host, some nanoparticles are observed on the surface of the particles. On the contrary, no small particles on the surfaces are observed for FePt-PS(22)MS18 and FePt-PS(TMB)MS18. From the above results, it is obvious that large MMSS mesopores play a significant role in keeping FePt nanoparticles inside MMSS particles. The Fe/Pt ratio determined from the EDX analysis on SEM were ca. 0.45 for all samples, suggesting that some of Fe component evaporated during the reduction process conducted at high temperature.

Scheme 1. Schematic Representation of Pore Expansion of MMSS

Pore Expansion of Monodispersed Mesoporous Silica Spheres

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Figure 3. SEM images of FePt-MMSS composites: (a) FePt-MMSS18, (b) FePt-PS(22)MS18, and (c) FePt-PS(TMB)PS18. Table 3. Adsorption Properties of FePt Composites sample




FePtsMMSS18 FePtsPS(22)MS18 FePtsPS(TMB)MS18

810 680 580

2.14 3.28 4.54

0.50 0.56 0.74

Table 3 lists the adsorption properties of FePt composites. The specific surface area, average pore diameter, and pore volume decreased with the incorporation of FePt nanoparticles (compare adsorption properties in Tables 1 and 3). However, even after the incorporation of FePt nanoparticles, the composites adsorbed a certain amount of nitrogen. Among these, FePtsPS(TMB)MS18 had the largest pore volume of 0.74 cm3 g-1, which is still enough to incorporate other chemicals, nanoparticles, and proteins, such as drugs, quantum dots, enzymes, and DNA molecules. Figure 4 shows TEM images of FePt composites, and the internal structures of the composites are clearly visualized by the incorporation of FePt nanoparticles. The dark parts of the micrograph represent FePt nanoparticles, which were incorporated into the mesopores. It is confirmed that the mesopores are aligned radially from the center to the outside of the spherical particles in all the composites. FePt nanoparticles are uniformly dispersed in the mesopores of FePtPS(22)MS18 and FePt-PS(TMB)MS18; however, some large nanoparticles are observed outside of MMSS for FePt-MMSS18. The result agrees well with that of SEM (Figure 3). It can be seen that the sizes of FePt nanoparticles are slightly larger in FePt-PS(TMB)MS18 than in FePtPS(22)PS18. The powder X-ray diffraction (XRD) patterns of FePt composites are shown in Figure 5. Several broad peaks are seen in the pattern, at 2θ between 20 and 70°. The resulting XRD pattern exactly matches the tetragonal FePt pattern represented at the bottom of the figure by the black bars (JCPDS 65-9121). These broad XRD peaks indicate that the sizes of the FePt particles are in nanometer region. However, it can be seen that sharp peaks overlap with broader peaks in the pattern of FePt-MMSS18. These sharp peaks could be attributed to the large FePt nanoparticles formed outside MMSS observed in SEM and TEM images. If it is assumed that the FePt nanoparticles are spheres, the average FePt particle diameters in FePt-PS(22)MS18 and FePt-PS(TMB)MS18 are calculated to be 4.1 and 5.5 nm, respectively, according to the Scherrer equation.19 These (17) Nakmura, T.; Yamada, Y.; Yano, K. J. Mater. Chem. 2006, 16, 2417. (18) Weller, D.; Moser, A.; Folks, L.; Best, M. E.; Lee, W.; Toney, M. F.; Schwickert, M.; Thiele, J.-U.; Doerner, M. F. IEEE Trans. Magn. 2000, 36, 10. (19) Scherrer, P. Go¨ttinger Nachr. 1918, 2, 98.

values are the same or a little bit larger than the average pore sizes of PS(22)MS18 (3.6 nm) and PS(TMB)MS18 (5.5 nm), mentioning that some mesopores were collapsed by the growth of FePt nanoparticles when the mesopore size was not big enough. From these results, it is obvious that the particle size of FePt can be controlled by the size of mesopores of MMSS. It was expected that both FePt-PS(22)MS18 and FePtPS(TMB)PS18 had ferromagnetic behavior because the sizes of the FePt nanoparticles inside mesopores were larger than the threshold size of FePt for ferromagnetism.18 The magnetism of the samples was studied by vibrating sample magnetometer (VSM). Figure 6 shows magnetization curves of FePt-MMSS18, FePt-PS(22)MS18, and FePt-PS(TMB) MS18. In each curve, although a hysteresis loop is observed because of ferromagnetic behavior of the composites, a decrease in the magnetization around zero magnetic fields suggests the existence of superparamagnetic components. The coercivities of the composites were 2.7 and 3.3 kOe for FePt-PS(22)MS18 and FePt-PS(TMB)MS18, respectively. Although the pore size of MMSS18 was much smaller than the threshold size of ferromagnetism for FePt nanoparticles, a hysteresis loop is observed for the curve for FePt-MMSS18. It was assumed that the FePt nanoparticles outside MMSS (Figures 3 and 4) contributed to the ferromagnetism for FePt-MMSS18. The coercivities obtained for FePt-PS(22)MS18 and FePt-PS(TMB)MS18 related well to the sizes of FePt nanoparticles calculated according to the Scherrer equation. The degree of magnetization for FePt-PS(22)MS18 was a little higher than that for FePt-PS(TMB)MS18. It is likely that the existence of small superparamagnetic FePt nanoparticles may cause this unexpected result. Conclusions In conclusion, we have succeeded in anomalous pore expansion of monodispersed mesoporous silica spheres (MMSS) by a swelling agent incorporation method. The pore size was expanded to 5.5 from 2.5 nm, while retaining the morphology, monodispersity and a radially, hexagonally ordered mesopores. This was achieved by the solvothermal treatment of MMSS using a hydrophobic swelling agent such as TMB in a water/ethanol mixed solvent system. It was found that the swelling agent penetrated into micelle rod inside mesopores. We have then been successful in obtaining highly monodispersed ferromagnetic composite spheres by the use of the pore-expanded MMSS. When MMSS with large pore size was used, FePt nanoparticles were uniformly dispersed in mesopores without deposition on the surface of MMSS. These composites have not only ferromagnetic

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Figure 4. TEM images of FePt-MMSS composites: (a) FePt-MMSS18, (b) FePt-PS(22)MS18, and (c) FePt-PS(TMB)PS18.

Figure 6. Magnetization curves of FePtsMMSS composites: (a) FePtPS(22)MS18, (b) FePt-PS(TMB)MS18. and (c) FePt-MMSS18.

Figure 5. XRD patterns of FePt-MMSS composites: (a) FePt-MMSS18, (b) FePt-PS(22)MS18, and (c) FePt-PS(TMB)PS18. The tetragonal FePt pattern is represented at the bottom by the black bars (JCPDS 65-9121).

behavior with a coercivity at room temperature but also mesoporosity with large pore volume. With the method, it will be possible to control the pore size, particle size, and content of FePt nanoparticles in the composites. Because the composites still have large surface areas and pore volumes, a variety of nanoparticles and proteins, such as quantum dots, enzymes, and DNA mol-

ecules, can be incorporated. They have potential applications in drug delivery systems, gene delivery systems, magnetic hyperthermia treatments, and magnetic photonic crystals. We are currently preparing colloidal crystals from the ferromagnetic composites to create new photomagnetic devices. Acknowledgment. This research was partly supported by the Japan Society for the Promotion of Science (JSPS), by a Grantin-Aid for Scientific Research (B), 17310079, 2006. The authors also thank Kazuo Okamoto (MALDI-TOF/MS spectra), Ayako Ohshima (TDS-GC/MS spectra), Yusuke Akimoto (TEM images), and Shuji Kajiya (ESI-LC/MS spectra). CM702792E

Anomalous Pore Expansion of Highly Monodispersed Mesoporous

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