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Synthesis of micro-mesoporous bimodal silica nanoparticles using lyotropic mixed surfactant liquid-crystal templates
Microporous and Mesoporous Materials 91 (2006) 172–180 www.elsevier.com/locate/micromeso
Synthesis of micro-mesoporous bimodal silica nanoparticles using lyotropic mixed surfactant liquid-crystal templates Hiroshi Mori a, Masafumi Uota b, Daisuke Fujikawa b,c, Takumi Yoshimura Takeshi Kuwahara b,c, Go Sakai b,c, Tsuyoshi Kijima b,c,* a
b,c
,
Department of Chemical Science of Engineering, Miyakonojo National College of Technology, Miyakonojo 885-8567, Japan b CREST, Japan Science and Technology Corporation, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan c Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Miyazaki 889-2192, Japan Received 21 July 2005; received in revised form 30 October 2005; accepted 2 November 2005 Available online 18 January 2006
Abstract Micro-mesoporous bimodal silica nanoparticles with a particle diameter of as small as 40–90 nm have been synthesized by a two-step reaction based on the polymerization of silicate (TEOS) species confined to the mixed surfactant hexagonal-structured liquid-crystal (LC) templates of nonaethyleneglycol dodecylether (C12EO9) and polyoxyethylene (20) sorbitan monostearate (Tween60) or eicosaethyleneglycol octadecyl ether (C18EO20). After pre-aging for water-insolubilization, the LC phase was kept in contact with dilute aqueous solution of ammonium acetate to achieve full condensation of silicate species. The catalyst-free pre-aging treatment induces the separation of the LC phase into domains and the subsequent water-phase-in-contact process serves to maintain the hexagonal framework by removing the ethanol evolved through hydrolysis. On calcination the hexagonal array of mixed surfactant cylindrical micelles in the LC templates is converted into a hexagonal structure of mesopores with some irregularity in pore arrangement, separated by silica walls with irregularly arranged micropores. Throughout aging and calcination, the hexagonal framework of the silicate-loaded LC or condensed particles in the Tween60 based mixed surfactant system contract to yield silica nanoparticles with mesopores of 5 nm diameter and 2 nm-thick silica walls. In contrast, the framework in the other system remarkably expands to form mesopores of 4.6 nm diameter and silica walls of as thick as 4.7 nm, along with 1.5–2 times larger micropore volume and specific surface area. The striking difference between the structural parameters in both systems is arisen from the opposing effects due to the hydrophilic triple-branched or linear polyoxyethylene (PEO) chains of Tween60 and C18EO20 molecules. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Silica; Nanoparticles; Mixed surfactant; Liquid crystals; Micro-mesoporous; Bimodal
1. Introduction Since the discovery of mesoporous silica M41S [1] and FSM-16 [2], the surfactant templating approach has been extensively applied to the synthesis of various mesoporous materials including silica and other metal oxides because of their potential applications in catalysis, membrane filters * Corresponding author. Address: Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Miyazaki 889-2192, Japan. Tel.: +81 985 58 7311; fax: +81 985 58 2876. E-mail address: [email protected] (T. Kijima).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.11.033
and separation technology based on their large internal surface areas [3–8]. Especially, much and unremitting attention has been devoted to synthesize a wide range of mesoporous silica materials with various framework structures, pore sizes and morphologies [1–8]. Recent studies in this field are characteristically focused on the synthesis of hierarchical porous silica materials [9–34] as well as silica fine particles on a nanometer scale [35,36]. Bimodal porous structures that are formed of micro and mesopores were first suggested for mesoporous silica MCM-41 [9,10] and SBA-15 [11,12]. This stimulated many subsequent studies on the bimodal micro-mesoporous [13–21], double
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mesoporous [22,23] or macro-mesoporous [24,25] materials as well as trimodal porous ones [26–29] because the large and small pores play a cooperative role as gas or fluid paths and molecular adsorption sites or reservoirs, leading to highly functional catalyst supports and adsorbents. Nevertheless, all of these bimodal or trimodal pore structures so far reported are based on mesoporous silica particles as large as much more than 100 nm in size or monolithic solids. In contrast, small sized mesoporous silica particles are advantageous for catalyst supports and adsorbents because reactant molecules are rapidly transported into the interior space through the short mesopore channels. Hence singledomain mesoporous silica nanoparticles as small as 20– 100 nm in particle size have been synthesized by modifying the conventional method based on the hydrolysis of tetraethylorthosilicate (TEOS) mixed with aqueous solution of surfactant [30–35]. However, all of these nanomaterials were obtained as monomodal mesoporous silica by using cetyltrimethylammonium (CTA) cations as the template. It would be thus further desired to develop bimodal porestructured silica nanoparticles with such small particle sizes. On the other hand, the previous studies based on surfactant templating almost exclusively used a single surfactant among a variety of ionic or nonionic ones, except for several reports [20,21,29–35]. Mixed solutions of cationic surfactants and nonionic poly(ethyleneglycol) or triblockcopolymers were employed for the synthesis of monolithic trimodal porous silica or mesoporous silica nanoparticles [29,35]. Lyotropic mixtures of amphiphilic block copolymers of different lengths with hydrophilic linear poly(ethyleneoxide) (PEO) chains were also applied to their nanocasting into bimodal micro-mesoporous silica to formulate the dependence of the mesopore sizes and the microporosity on the lengths or sizes of the hydrophobic and the hydrophilic blocks, but the mesostructures were worm-type in morphology and several hundred nanometers or more in size [20,21]. On the other hand, our recent study demonstrated that equimolar amounts of favorably different-sized nonionic surfactant molecules with linear or triple-branched PEO chains are combined into cylindrical rod-like micelles to form a hexagonal liquid crystal (LC), which yielded platinum nanotubes through the reduction of Pt salts confined to the LC template [37]. This motivated us to apply this specific template system to the synthesis of nanostructured silica materials, along with a combined use of different-sized linear molecules. Here we report the first synthesis of micro-mesoporous bimodal silica nanoparticles with a hexagonal mesostructure and a particle diameter of 40–90 nm using the mixed surfactant LC templates of nonaethyleneglycol dodecylether (C12EO9) and polyoxyethylene (20) sorbitan monostearate (Tween60) or eicosaethyleneglycol octadecyl ether (C18EO20). To our best knowledge, this is the first example of the micro-mesoporous bimodal silica nanoparticles with a diameter of less than 100 nm. It is also remarked that all the wall thickness, specific surface area, and micropore volume of silica nanoparticles for the C12EO9/C18EO20 system
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are nearly two times greater than those for the C12EO9/ Tween60 system. The formation mechanism of such bimodal silica nanoparticles is proposed on the basis of the molecular parameters of surfactants. 2. Experimental 2.1. Materials The reagent-grate nonaethyleneglycol monododecyl ether (Sigma–Aldrich, C12EO9), polyoxyethylene(20) sorbitan monostearate (Kishida chemical, Tween60), eicosaethyleneglycol octadecyl ether (Sigma, C18EO20), and tetraethyl orthosilicate (Kishida chemical, TEOS) were used without further purification. The chemical structure of Tween60 is shown in Fig. 1. 2.2. Synthesis of silica nanoparticles In the TEOS/C12EO9/Tween60/H2O system, a mixture of C12EO9, Tween60, and H2O at a molar ratio of 1:1:60 was shaken for 20 min at 60 °C, followed by addition of TEOS. After the resulting solution being cooled to 20 °C, the resulting pasty LC phase was water-insolubilized by pre-aging for 1 h at 20 °C. The transparent pre-aged LC phases were then immersed in water of 250 times larger in volume than TEOS, followed by the addition of ammonium acetate of eight times larger in mole than C12EO9. The pH of the water phase after adding the acetate was 6.6. The large quantity of additional water was used to extract the ethanol produced by the hydrolysis of TEOS and to promote condensation reaction. The pre-aged material immersed in water was kept at 20 °C for 7d and the resulting fully aged soft gel was centrifuged, washed initially with water and then repeatedly with ethanol prior to vacuum dryness. The as-grown silica/surfactant composite materials thus obtained were calcined at 400 °C for 10 min to remove the organic species. The reaction in the TEOS/C12EO9/C18EO20/H2O system was carried out by a similar procedure using the TEOS/ C12EO9 ratio of 4 and C18EO20 instead of Tween60. For a comparative study, C12EO9-free LC phases in both systems were also prepared. 2.3. Characterization X-ray diffraction (XRD) patterns were measured at a scanning rate of 1 deg/min by the reflectance method on a Rigaku Multiflex using CuKa radiation. Transmission electron microscope (TEM) images were taken with a
Fig. 1. Chemical structure of Tween60.
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JEOL TEM-2010MX microscope operated at an acceleration voltage 200 kV. BET surface areas, BJH pore-size distributions, pore volumes, and t- and as-plots of the adsorption data were obtained using nitrogen gas adsorption–desorption isotherms with a Micromeritics Gemini 2375 instrument. In this study nitrogen gas 0.36 nm in molecular size was used as the adsorbate for characterizing the microporosity of samples as widely employed in many studies referring to microporosity of silica. According to Long and his co-workers, for example, significant micropore filling at low partial pressure occurs when the molecular size of adsorbate is <0.5 nm [10], although Maier and co-workers reported that reliable pore sizes in the micrometer range are obtained only with argon as the adsorbate [9]. 3. Results In the TEOS/C12EO9/Tween60/H2O system, the silica/ surfactant composite material, 1, was obtained by fullaging of the precursory LC phase with the TEOS/C12EO9 ratio of 4 at 20 °C. On calcination at 400 °C, the as-grown product, 1, was deorganized into surfactant-free silica, 1 0 . The TEM images of 1 0 indicated the formation of spherical silica nanoparticles with a diameter of 40–80 nm at high yields (Fig. 2A). Moreover, the TEM image of the spherical particles at a higher magnification clearly demonstrated a hexagonal mesoporous structure with a pore-to-pore distance of 7 nm, but with some irregularity in pore arrangement, along with irregularly arranged micropores of about 0.4 nm diameter (Fig. 2B). The former pore-to-pore distance was in fair agreement with 7.0 nm obtained from the definite XRD peak around 2h = 1.45° (d = 6.09 nm) for 1 0 by assuming a hexagonal structure (Fig. 3A). The
morphological observations were also consistent with the pore-size distribution curve obtained from the N2 adsorption isotherm for 1 0 indicating the presence of mesopores with a somewhat broad distribution of pore diameter centered at 5.0 nm, along with micropores of less than 2 nm in size (Fig. 4A and B). The latter pores were further characterized as micropores of 0.4 nm diameter from the plots of pore volume against thickness of N2 adsorbed layer in t-plot (Fig. 4C): the t-plot clearly indicates that the thickness of the N2 adsorption layer is changed around 0.2 nm in radius (P/P0 = 0.001). The t-plot data also gave a value of 0.09 cm3/g for the specific micropore volume of 1 0 . It was also confirmed from the XRD pattern in Fig. 3A that the framework of the silica nanoparticles is amorphous, as usually observed for mesoporous silica materials. The calcined product in the TEOS/C12EO9/Tween60/H2O system can be thus identified as an aggregate of micro-mesoporous bimodal silica nanoparticles with a diameter of 40–80 nm and a specific surface area of 515 m2 g1 (Table 1). Furthermore, the particle framework structure is characterized by a hexagonal but slightly modulated array of uniformly sized mesopores of 5 nm diameter separated 2 nm-thick silica walls with irregularly arranged micropores of 0.4 nm diameter and 0.09 cm3 g1 volume. The structural change during the pathway from the precursory LC phase to the final product was examined by X-ray diffraction. The XRD pattern of the pre-aged LC phase with the TEOS/C12EO9 molar ratio of 4 exhibited one strong peak at 2h = 1.26° (d = 7.01 nm) and a broad band around 2h = 2.43° resolved into two peaks with d = 3.71 and 3.63 nm, in contrast to two relatively weak peaks at 2h = 1.38° (d = 6.40 nm) and 2h = 2.35° (d = 3.76 nm) for the TEOS-free hexagonal LC phase (Fig. 3B). The former three peaks are attributed to the
Fig. 2. TEM images taken at different magnifications for the calcined silica (A, B) 1 0 in the TEOS/C12EO9/Tween60/H2O system and (C, D) 2 0 in the TEOS/C12EO9/C18EO20/H2O system. The images in the insets of B and D show the formation of irregularly arranged micropores.
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Fig. 3. XRD patterns of the solid and LC phases obtained in the (A, B) TEOS/C12EO9/Tween60/H2O and (C, D) TEOS/C12EO9/C18EO20/H2O systems. (a) As-grown product 1 or 2; (b) 400 °C-calcined product 1 0 or 2 0 ; (c) 1:1:60 mixture of C12EO9, Tween60 (or C18EO20) and H2O; (d(x)) pre-aged mixtures of TEOS, C12EO9, Tween60 (or C18EO20) and H2O at a mole ratio of x:1:1:60; (e) blank measurement.
Fig. 4. Nitrogen adsorption–desorption isotherms (A, D), pore-size distribution curves (B, E), and t-plots (C, F) for the calcined silica. Samples: (A–C) 1 0 in the TEOS/C12EO9/Tween60/H2O system and (D–F) 2 0 in the TEOS/C12EO9/C18EO20/H2O system.
1 0 0, 1 1 0, and 2 0 0 reflections for a hexagonal phase with a = 8.09 nm, while the latter two to the 1 0 0 and 1 1 0 ones for an analogue with a = 7.39 nm: the two a values corre-
spond to the diameter of rodlike micelles for each phase. The TEOS/C12EO9 molar ratio of 4 was selected because the pre-aged LC phase with this composition showed the
– 0.65 – 0.97 – 515 – 919 – 2.3 – 4.9 – 5.0 – 4.6 – 7.03 – 9.26
e
c
d
b
2
Obtained by assuming a hexagonal structure. Pore size (D) obtained from BJH pore-size distribution curves calculated using the adsorption branch. Wall thickness (d) obtained from d = a D/1.05 [41]. Surface area (m2/g). Calculated by the t-plot method.
20 a
C12EO9/Tween60/H2O TEOS/C12EO9/Tween60/H2O C12EO9/C18EO20/H2O TEOS/C12EO9/C18EO20/H2O
6.40 7.01 5.52 6.40
3.76 3.71 3.45 3.34
3.20 3.63 2.95 3.23
7.39 8.09 6.37 7.39
1
– 6.69 – 7.88
– 7.72 – 9.10
10
– 6.09 – 8.02
Total-pore volume (cm3/g) Sd dc (nm) Db (nm) aa (nm)
Mesoporous silica sample (400 °C-calcined)
d100 (nm) aa (nm) d100 (nm)
As-grown silica sample
aa (nm) d200 (nm) d100 (nm) d100 (nm)
Pre-aged LC System
Table 1 Structural properties of mesoporous silica prepared using mixed surfactant liquid-crystal templates
– 0.09 – 0.18
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Micropore volumee (cm3/g)
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strongest 1 0 0 reflection among those obtained with the ratios raging from 2 to 6 using the same aging time of 1 h (Fig. 3B). Furthermore, the intensity of the 1 0 0 peak for the pre-aged phase increased with an increase of aging time up to 3–4 h, but it decreased markedly for longer aging and the condensation of TEOS was also decelerated or rather terminated, as suggested from the observation that the pre-aged product remained transparent. This is probably because the initial condensation of TEOS promotes the ordering of LC phase but the ethanol produced in excess by the hydrolysis of TEOS acts to suppress additional polymerization and make the LC structure disordered, as suggested by Attard et al. for a similar but single surfactant system [38]. The relatively small amount of water initially added in the LC template would cause partial condensation of TEOS species that is effective only for the insolubilization of the LC phase in water. Therefore, the pre-aged LC was further immersed in a large amount of water with two aims to extract the ethanol evolved by hydrolysis in the template phase and supply with water enough for the complete hydrolysis of TEOS. The volume of the pre-aged LC was little affected by immersing in water with ammonium acetate added, indicating no significant swelling of the pre-aged LC phase: The total volume of the aged sample placed in a glass tube was macroscopically monitored on a cubic millimeter scale with a reading error of 5% (vol/vol). Thermogravimetric analysis showed that ammonium acetate serves to reduce the content of surfactant species remaining unremoved even after washing with ethanol. Relative to the pre-aged LC phase, the as-grown silica fully aged at 20 °C, 1, exhibited a remarkable decrease in intensity of the XRD peak centered at 2h = 1.32° (d = 6.69 nm) (Fig. 3A). This indicates that the framework structure becomes much less ordered by the developed condensation. In contrast, the 400 °C calcined solid, 1 0 , showed a much stronger XRD peak around 2h = 1.45° (d = 6.09 nm), suggesting that additional condensation at elevated temperature serves to improve the long-periodic order of the framework structure (Fig. 3A). In the TEOS/C12EO9/C18EO20/H2O system, a similar reaction was performed using C18EO20 in place of Tween60. The resulting silica was found to be an aggregate of nanoparticles bimodal but with greater wall thickness, micropore volume and specific surface area, compared with the final product in the Tween60 based mixed surfactant system. The LC phase pre-aged at 20 °C for 1 h exhibited XRD peaks attributed to a hexagonal phase with a = 7.39 nm (Fig. 3D). On keeping at 20 °C for 7d after addition of water and ammonium acetate, the pre-aged LC phase was converted into a solid phase, 2, characterized by the XRD peak around 2h = 1.12° (d = 7.88 nm), as shown in Fig. 3C. The pre-aged LC phase showed no significant swelling during immersion in water, as in the Tween60 based mixed surfactant system. On calcination of 2 at 400 °C, the resulting product, 2 0 , gave TEM images indicating the formation of spherical silica nanoparticles of
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40–90 nm in size and having a mesoporous structure with a pore-to-pore diameter of 7–10 nm and irregularly arranged micropores of 0.3 nm diameter (Fig. 2C and D). From the XRD pattern for 2 0 , a more definite value of 9.3 nm was obtained for the mesopore-to-mesopore distance by assuming a hexagonal structure, although the channels have some irregularity in pore arrangement (Fig. 3C). Moreover, the pore-size distribution curve and the t-plot for 2 0 also revealed the development of mesopores of 4.6 nm diameter but with a broad distribution ranging from 3 to 8 nm (Fig. 4D and E) and micropores of 0.4 nm diameter (Fig. 4D–F). The specific micropore volume of 2 0 obtained by the t-plot was 0.18 cm3/g. The structural parameters thus obtained for the C18EO20 based mixed surfactant system are summarized in Table 1, along with those for the Tween60 based mixed surfactant system. The specific micropore volumes of 0.09 and 0.18 cm3/g evaluated by the t-plot data for 1 0 and 2 0 are 1.8 and 1.4 times larger than 0.05 and 0.13 cm3/g obtained by the as-plot data for them, respectively (Fig. S1). Nevertheless it should be noted that both the t- and as-plot data concordantly reveal that the micropore volume of 2 0 is about two times or more larger than that of 1 0 . 4. Discussion The catalyst-free polymerization of TEOS within the Tween60 and the C18EO20 based mixed surfactant LC templates were found to serve for the separate growth of micro-mesoporous silica nanoparticles with a particle size of as small as 40–80 or 40–90 nm, as described above. This is in marked contrast to the growth of monolithic or macro-sized mesoporous silica from single or mixed poly(ethylene oxide) alkylether based LC templates by acid-catalyzed polymerization of alkoxide [20,21,38]. Furthermore, the sizes of the silica nanoparticles were also in close agreement with 57 and 69 nm for the domain size of the pre-aged Tween60 and C18EO20 based LCs, respectively, evaluated within the error limit of about 20 nm using Scherrer’s equation from the profiles of their main XRD peaks. The single surfactant TEOS/C12EO9/H2O system also yielded the most highly ordered hexagonal structure with a = 8.03 nm and with a domain size of 79 nm for the presaged LC phase with the same TEOS/surfactant molar ratio as that in the TEOS/C12EO9/Tween60/H2O system (Fig. S2). These facts suggest with that the catalyst-free pre-aging produces much less polymerized silicates to induce the separation of the 2d-hexagonal LC template into domains, in contrast to the acid-catalyzed polymerization yielding highly developed Si–O networks available for forming large sized silicate networks [39,40]. The subsequent polymerization of the pre-aged LC would yield fully condensed individual silica nanoparticles. It is further remarked that there is a striking contrast between the structural parameters other than particle size for the final products in the present two systems, as can be seen in Table 1. First, the silica wall thickness of
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Fig. 5. Lattice constant, a, at different stages of silicate condensation, obtained by assuming a hexagonal structure: (s) TEOS/C12EO9/ Tween60/H2O system; (d) TEOS/C12EO9/C18EO20/H2O system.
4.9 nm for 2 0 in the C18EO20 based mixed surfactant system is greater than twice 2 nm for 1 0 in the other system. Second, the specific surface area and specific micropore and total-pore volumes of silica nanoparticles for the former system are also 1.5–2 times larger than those for the other, respectively. Third, the pore-to-pore distance, a, corresponding to the lattice constant obtained by assuming a hexagonal structure for the LCs or silica nanoparticles in both systems exhibits opposing dependences on the stage of silicate condensation after the introduction of TEOS into the silicate-free LC phase, as shown in Fig. 5. The pore-topore distance in the Tween60 based mixed surfactant system shows a tendency to decrease successively in two stages of full-aging and calcination, whereas the pore-to-pore distance in the C18EO20 based system shows a marked increase over the entire stage of silicate condensation including a slight but further increase even at the calcination stage. Contrary to the latter system using different-sized linear chain molecules, framework contraction was observed for the single surfactant TEOS/C12EO8/H2O system [38]. On the basis of the above observations and considerations, we can propose a model for the pathway from the precursory mixed surfactant LC to micro-mesoporous bimodal silica nanoparticles in the Tween60 and C18EO20 based mixed surfactant systems, as schematically shown in Scheme 1. The different sized C12EO9 and Tween60 or C18EO20 molecules in each mixed system would be combined into TEOS-loaded cylindrical rod like micelles to form a hexagonal LC, similarly to the TEOS-free mixed surfactant LC previously analyzed by structural calculation [37]. The TEOS species loaded within the aqueous shell of cylindrical micelles would undergo slight condensation into poorly condensed Si–O polymers, leading to a LC phase with a highly ordered hexagonal array of silicate-loaded cylindrical micelles. The partial condensation of TEOS species would also accompany the separation of the LC phase into domains 40–90 nm in size, as discussed above on the catalyst-free aging process. On contact with a large amount of water, the silicate-loaded LC domains would be further developed into highly polymerized silica nanoparticles with a much less ordered array of mesopores through additional
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Scheme 1. Schematic models for the pathway to micro-mesoporous bimodal silica nanoparticles from the precursory LC through the LC separation into domains depending on the degree of silicate condensation. The top illustrations (a) and (b) indicate the conformational change of surfactant molecules along with the dimensional change of silicate layer (purple or blue band) for the Tween60 and the C18EO20 based mixed surfactant systems, respectively. The as-grown samples 1 and 2 correspond to the fully aged phase and the calcined ones 1 0 and 2 0 the micro-mesoporous nanoparticles in the illustrations.
condensation of silicate species. In this process, the water phase serves to maintain the hexagonal framework through the removal of the ethanol evolved by hydrolysis of TEOS species. The as-grown solid thus formed would be converted into micro-mesoporous silica nanoparticles with a hexagonal but slightly modulated array of mesopores as a result of the complete condensation of silicate species on calcination. The silica walls of relatively small thickness formed in the Tween60 based mixed surfactant system could be thus explained by assuming that the growth of Si–O polymers within the aqueous shell induces partial exclusion of the silicate moiety from the triple-branched PEO chain assembly, leading to a decrease in the thickness of silicate-condensation layer. Because every three PEO chains bound to sorbitan ring are highly restricted in conformational change so that the silicate particles enlarged by additional condensation cannot totally remain within the PEO pocket. The structural rearrangement, in connection with the release of water and ethanol due to silicate condensation, would also cause the framework contraction of the hexagonal-structured LCs or incompletely con-
densed silica nanoparticles. On the other hand, the PEO single chain of C18EO20 molecule is much more free in conformational change than the PEO triple chain of Tween60 because the former is branch-free and nearly three times longer than the individual PEO chain of the latter. Accordingly, in the C18EO20 based mixed surfactant system, 3d polymerization of silicate species initiated by the formation of Si–O linear oligomers proceeds so as to change the conformation of PEO chain from their initial folded state into their more extended state, which would also accompany the displacement of Si–O clusters and/or un-reacted TEOS species toward the outside of cylindrical micelle. Such change in state of silicate-surfactant hybridized assemblies would result in the framework expansion of the hexagonalstructured LCs or incompletely condensed silica nanoparticles. The structural rearrangement thus induced would further result in the formation of hexagonal-structured silica nanoparticles with mesopores of smaller size and walls of more than two times larger thickness than those for the Tween60 based system. The two times or more larger micropore volume of silica nanoparticles for the C18EO20
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based system than that for the other would be due to the large difference between the effective volumes of hydrophilic PEO chains penetrated into silica walls, as suggested by Smarsly et al. [20,21]. The results for the present C18EO20 based system are significantly different from those for the acid-catalyzed condensation at 60 °C in the TEOS/ C12EO8/C18EO20/H2O system: the latter system yielded micro-mesoporous bimodal silica nanoparticles with worm-type array of mesopores as small as 2.6 nm in diameter and with a particle size of several hundred nanometers or more [20]. This difference likely depends on whether the polymerization process is catalyst-free or acid-catalyzed and the process for ethanol removal is based on aging in contact with water phase or evaporation in vacuum during pre-aging. 5. Conclusions We demonstrated the first synthesis of micro-mesoporous bimodal silica nanoparticles with a particle diameter of as small as 40–90 nm by a two-step reaction based on the pre and further polymerization of silicate species confined to the mixed surfactant hexagonal-structured LC templates of C12EO9 and Tween60 or C18EO20. After preaging for water-insolubilization, the silicate species loaded in the LC phase was fully condensed by keeping in contact with dilute aqueous solution of ammonium acetate. The catalyst-free pre-aging process induced the separation of the LC phase into domains and the subsequent waterphase-in-contact process served to maintain the hexagonal framework by removing the ethanol evolved through hydrolysis, leading to the bimodal nanoparticles. The silica nanoparticles thus prepared in both systems have slightly different pore sizes of 5 nm, but the wall thickness, specific surface area and specific micropore and total-pore volumes of nanoparticles for the C18EO20 based system are 1.5 times or more larger than those for the other system, respectively. The relatively larger structural parameters for the former system are attributed to the larger effective volume of hydrophilic PEO long chains penetrated into silica walls. The silicate-surfactant hybridized framework in this system expands over the entire stage of silicate condensation, mainly due to the unfolding of PEO long linear chains of C18EO20 molecules. In the Tween60 based system, on the other hand, the growth of Si–O polymers within the aqueous shell induces partial exclusion of the silicate moiety from the triple-branched PEO chain pocket, leading to a marked decrease in the thickness of silicatecondensation layer as well as the framework contraction. The present micro-mesoporous silica nanoparticles would be promising as catalyst supports for their small sizes available for the diffusion of reactant molecules. Acknowledgements This study was supported by Grant-in-Aids for the CREST of Japan Science and Technology Corporation
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Synthesis of micro-mesoporous bimodal silica nanoparticles using lyotropic mixed surfactant liquid-crystal templates Hiroshi Mori a, Masafumi Uota b, Daisuke Fujikawa b,c, Takumi Yoshimura Takeshi Kuwahara b,c, Go Sakai b,c, Tsuyoshi Kijima b,c,* a
b,c
,
Department of Chemical Science of Engineering, Miyakonojo National College of Technology, Miyakonojo 885-8567, Japan b CREST, Japan Science and Technology Corporation, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan c Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Miyazaki 889-2192, Japan Received 21 July 2005; received in revised form 30 October 2005; accepted 2 November 2005 Available online 18 January 2006
Abstract Micro-mesoporous bimodal silica nanoparticles with a particle diameter of as small as 40–90 nm have been synthesized by a two-step reaction based on the polymerization of silicate (TEOS) species confined to the mixed surfactant hexagonal-structured liquid-crystal (LC) templates of nonaethyleneglycol dodecylether (C12EO9) and polyoxyethylene (20) sorbitan monostearate (Tween60) or eicosaethyleneglycol octadecyl ether (C18EO20). After pre-aging for water-insolubilization, the LC phase was kept in contact with dilute aqueous solution of ammonium acetate to achieve full condensation of silicate species. The catalyst-free pre-aging treatment induces the separation of the LC phase into domains and the subsequent water-phase-in-contact process serves to maintain the hexagonal framework by removing the ethanol evolved through hydrolysis. On calcination the hexagonal array of mixed surfactant cylindrical micelles in the LC templates is converted into a hexagonal structure of mesopores with some irregularity in pore arrangement, separated by silica walls with irregularly arranged micropores. Throughout aging and calcination, the hexagonal framework of the silicate-loaded LC or condensed particles in the Tween60 based mixed surfactant system contract to yield silica nanoparticles with mesopores of 5 nm diameter and 2 nm-thick silica walls. In contrast, the framework in the other system remarkably expands to form mesopores of 4.6 nm diameter and silica walls of as thick as 4.7 nm, along with 1.5–2 times larger micropore volume and specific surface area. The striking difference between the structural parameters in both systems is arisen from the opposing effects due to the hydrophilic triple-branched or linear polyoxyethylene (PEO) chains of Tween60 and C18EO20 molecules. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Silica; Nanoparticles; Mixed surfactant; Liquid crystals; Micro-mesoporous; Bimodal
1. Introduction Since the discovery of mesoporous silica M41S [1] and FSM-16 [2], the surfactant templating approach has been extensively applied to the synthesis of various mesoporous materials including silica and other metal oxides because of their potential applications in catalysis, membrane filters * Corresponding author. Address: Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Miyazaki 889-2192, Japan. Tel.: +81 985 58 7311; fax: +81 985 58 2876. E-mail address: [email protected] (T. Kijima).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.11.033
and separation technology based on their large internal surface areas [3–8]. Especially, much and unremitting attention has been devoted to synthesize a wide range of mesoporous silica materials with various framework structures, pore sizes and morphologies [1–8]. Recent studies in this field are characteristically focused on the synthesis of hierarchical porous silica materials [9–34] as well as silica fine particles on a nanometer scale [35,36]. Bimodal porous structures that are formed of micro and mesopores were first suggested for mesoporous silica MCM-41 [9,10] and SBA-15 [11,12]. This stimulated many subsequent studies on the bimodal micro-mesoporous [13–21], double
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mesoporous [22,23] or macro-mesoporous [24,25] materials as well as trimodal porous ones [26–29] because the large and small pores play a cooperative role as gas or fluid paths and molecular adsorption sites or reservoirs, leading to highly functional catalyst supports and adsorbents. Nevertheless, all of these bimodal or trimodal pore structures so far reported are based on mesoporous silica particles as large as much more than 100 nm in size or monolithic solids. In contrast, small sized mesoporous silica particles are advantageous for catalyst supports and adsorbents because reactant molecules are rapidly transported into the interior space through the short mesopore channels. Hence singledomain mesoporous silica nanoparticles as small as 20– 100 nm in particle size have been synthesized by modifying the conventional method based on the hydrolysis of tetraethylorthosilicate (TEOS) mixed with aqueous solution of surfactant [30–35]. However, all of these nanomaterials were obtained as monomodal mesoporous silica by using cetyltrimethylammonium (CTA) cations as the template. It would be thus further desired to develop bimodal porestructured silica nanoparticles with such small particle sizes. On the other hand, the previous studies based on surfactant templating almost exclusively used a single surfactant among a variety of ionic or nonionic ones, except for several reports [20,21,29–35]. Mixed solutions of cationic surfactants and nonionic poly(ethyleneglycol) or triblockcopolymers were employed for the synthesis of monolithic trimodal porous silica or mesoporous silica nanoparticles [29,35]. Lyotropic mixtures of amphiphilic block copolymers of different lengths with hydrophilic linear poly(ethyleneoxide) (PEO) chains were also applied to their nanocasting into bimodal micro-mesoporous silica to formulate the dependence of the mesopore sizes and the microporosity on the lengths or sizes of the hydrophobic and the hydrophilic blocks, but the mesostructures were worm-type in morphology and several hundred nanometers or more in size [20,21]. On the other hand, our recent study demonstrated that equimolar amounts of favorably different-sized nonionic surfactant molecules with linear or triple-branched PEO chains are combined into cylindrical rod-like micelles to form a hexagonal liquid crystal (LC), which yielded platinum nanotubes through the reduction of Pt salts confined to the LC template [37]. This motivated us to apply this specific template system to the synthesis of nanostructured silica materials, along with a combined use of different-sized linear molecules. Here we report the first synthesis of micro-mesoporous bimodal silica nanoparticles with a hexagonal mesostructure and a particle diameter of 40–90 nm using the mixed surfactant LC templates of nonaethyleneglycol dodecylether (C12EO9) and polyoxyethylene (20) sorbitan monostearate (Tween60) or eicosaethyleneglycol octadecyl ether (C18EO20). To our best knowledge, this is the first example of the micro-mesoporous bimodal silica nanoparticles with a diameter of less than 100 nm. It is also remarked that all the wall thickness, specific surface area, and micropore volume of silica nanoparticles for the C12EO9/C18EO20 system
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are nearly two times greater than those for the C12EO9/ Tween60 system. The formation mechanism of such bimodal silica nanoparticles is proposed on the basis of the molecular parameters of surfactants. 2. Experimental 2.1. Materials The reagent-grate nonaethyleneglycol monododecyl ether (Sigma–Aldrich, C12EO9), polyoxyethylene(20) sorbitan monostearate (Kishida chemical, Tween60), eicosaethyleneglycol octadecyl ether (Sigma, C18EO20), and tetraethyl orthosilicate (Kishida chemical, TEOS) were used without further purification. The chemical structure of Tween60 is shown in Fig. 1. 2.2. Synthesis of silica nanoparticles In the TEOS/C12EO9/Tween60/H2O system, a mixture of C12EO9, Tween60, and H2O at a molar ratio of 1:1:60 was shaken for 20 min at 60 °C, followed by addition of TEOS. After the resulting solution being cooled to 20 °C, the resulting pasty LC phase was water-insolubilized by pre-aging for 1 h at 20 °C. The transparent pre-aged LC phases were then immersed in water of 250 times larger in volume than TEOS, followed by the addition of ammonium acetate of eight times larger in mole than C12EO9. The pH of the water phase after adding the acetate was 6.6. The large quantity of additional water was used to extract the ethanol produced by the hydrolysis of TEOS and to promote condensation reaction. The pre-aged material immersed in water was kept at 20 °C for 7d and the resulting fully aged soft gel was centrifuged, washed initially with water and then repeatedly with ethanol prior to vacuum dryness. The as-grown silica/surfactant composite materials thus obtained were calcined at 400 °C for 10 min to remove the organic species. The reaction in the TEOS/C12EO9/C18EO20/H2O system was carried out by a similar procedure using the TEOS/ C12EO9 ratio of 4 and C18EO20 instead of Tween60. For a comparative study, C12EO9-free LC phases in both systems were also prepared. 2.3. Characterization X-ray diffraction (XRD) patterns were measured at a scanning rate of 1 deg/min by the reflectance method on a Rigaku Multiflex using CuKa radiation. Transmission electron microscope (TEM) images were taken with a
Fig. 1. Chemical structure of Tween60.
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JEOL TEM-2010MX microscope operated at an acceleration voltage 200 kV. BET surface areas, BJH pore-size distributions, pore volumes, and t- and as-plots of the adsorption data were obtained using nitrogen gas adsorption–desorption isotherms with a Micromeritics Gemini 2375 instrument. In this study nitrogen gas 0.36 nm in molecular size was used as the adsorbate for characterizing the microporosity of samples as widely employed in many studies referring to microporosity of silica. According to Long and his co-workers, for example, significant micropore filling at low partial pressure occurs when the molecular size of adsorbate is <0.5 nm [10], although Maier and co-workers reported that reliable pore sizes in the micrometer range are obtained only with argon as the adsorbate [9]. 3. Results In the TEOS/C12EO9/Tween60/H2O system, the silica/ surfactant composite material, 1, was obtained by fullaging of the precursory LC phase with the TEOS/C12EO9 ratio of 4 at 20 °C. On calcination at 400 °C, the as-grown product, 1, was deorganized into surfactant-free silica, 1 0 . The TEM images of 1 0 indicated the formation of spherical silica nanoparticles with a diameter of 40–80 nm at high yields (Fig. 2A). Moreover, the TEM image of the spherical particles at a higher magnification clearly demonstrated a hexagonal mesoporous structure with a pore-to-pore distance of 7 nm, but with some irregularity in pore arrangement, along with irregularly arranged micropores of about 0.4 nm diameter (Fig. 2B). The former pore-to-pore distance was in fair agreement with 7.0 nm obtained from the definite XRD peak around 2h = 1.45° (d = 6.09 nm) for 1 0 by assuming a hexagonal structure (Fig. 3A). The
morphological observations were also consistent with the pore-size distribution curve obtained from the N2 adsorption isotherm for 1 0 indicating the presence of mesopores with a somewhat broad distribution of pore diameter centered at 5.0 nm, along with micropores of less than 2 nm in size (Fig. 4A and B). The latter pores were further characterized as micropores of 0.4 nm diameter from the plots of pore volume against thickness of N2 adsorbed layer in t-plot (Fig. 4C): the t-plot clearly indicates that the thickness of the N2 adsorption layer is changed around 0.2 nm in radius (P/P0 = 0.001). The t-plot data also gave a value of 0.09 cm3/g for the specific micropore volume of 1 0 . It was also confirmed from the XRD pattern in Fig. 3A that the framework of the silica nanoparticles is amorphous, as usually observed for mesoporous silica materials. The calcined product in the TEOS/C12EO9/Tween60/H2O system can be thus identified as an aggregate of micro-mesoporous bimodal silica nanoparticles with a diameter of 40–80 nm and a specific surface area of 515 m2 g1 (Table 1). Furthermore, the particle framework structure is characterized by a hexagonal but slightly modulated array of uniformly sized mesopores of 5 nm diameter separated 2 nm-thick silica walls with irregularly arranged micropores of 0.4 nm diameter and 0.09 cm3 g1 volume. The structural change during the pathway from the precursory LC phase to the final product was examined by X-ray diffraction. The XRD pattern of the pre-aged LC phase with the TEOS/C12EO9 molar ratio of 4 exhibited one strong peak at 2h = 1.26° (d = 7.01 nm) and a broad band around 2h = 2.43° resolved into two peaks with d = 3.71 and 3.63 nm, in contrast to two relatively weak peaks at 2h = 1.38° (d = 6.40 nm) and 2h = 2.35° (d = 3.76 nm) for the TEOS-free hexagonal LC phase (Fig. 3B). The former three peaks are attributed to the
Fig. 2. TEM images taken at different magnifications for the calcined silica (A, B) 1 0 in the TEOS/C12EO9/Tween60/H2O system and (C, D) 2 0 in the TEOS/C12EO9/C18EO20/H2O system. The images in the insets of B and D show the formation of irregularly arranged micropores.
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Fig. 3. XRD patterns of the solid and LC phases obtained in the (A, B) TEOS/C12EO9/Tween60/H2O and (C, D) TEOS/C12EO9/C18EO20/H2O systems. (a) As-grown product 1 or 2; (b) 400 °C-calcined product 1 0 or 2 0 ; (c) 1:1:60 mixture of C12EO9, Tween60 (or C18EO20) and H2O; (d(x)) pre-aged mixtures of TEOS, C12EO9, Tween60 (or C18EO20) and H2O at a mole ratio of x:1:1:60; (e) blank measurement.
Fig. 4. Nitrogen adsorption–desorption isotherms (A, D), pore-size distribution curves (B, E), and t-plots (C, F) for the calcined silica. Samples: (A–C) 1 0 in the TEOS/C12EO9/Tween60/H2O system and (D–F) 2 0 in the TEOS/C12EO9/C18EO20/H2O system.
1 0 0, 1 1 0, and 2 0 0 reflections for a hexagonal phase with a = 8.09 nm, while the latter two to the 1 0 0 and 1 1 0 ones for an analogue with a = 7.39 nm: the two a values corre-
spond to the diameter of rodlike micelles for each phase. The TEOS/C12EO9 molar ratio of 4 was selected because the pre-aged LC phase with this composition showed the
– 0.65 – 0.97 – 515 – 919 – 2.3 – 4.9 – 5.0 – 4.6 – 7.03 – 9.26
e
c
d
b
2
Obtained by assuming a hexagonal structure. Pore size (D) obtained from BJH pore-size distribution curves calculated using the adsorption branch. Wall thickness (d) obtained from d = a D/1.05 [41]. Surface area (m2/g). Calculated by the t-plot method.
20 a
C12EO9/Tween60/H2O TEOS/C12EO9/Tween60/H2O C12EO9/C18EO20/H2O TEOS/C12EO9/C18EO20/H2O
6.40 7.01 5.52 6.40
3.76 3.71 3.45 3.34
3.20 3.63 2.95 3.23
7.39 8.09 6.37 7.39
1
– 6.69 – 7.88
– 7.72 – 9.10
10
– 6.09 – 8.02
Total-pore volume (cm3/g) Sd dc (nm) Db (nm) aa (nm)
Mesoporous silica sample (400 °C-calcined)
d100 (nm) aa (nm) d100 (nm)
As-grown silica sample
aa (nm) d200 (nm) d100 (nm) d100 (nm)
Pre-aged LC System
Table 1 Structural properties of mesoporous silica prepared using mixed surfactant liquid-crystal templates
– 0.09 – 0.18
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Micropore volumee (cm3/g)
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strongest 1 0 0 reflection among those obtained with the ratios raging from 2 to 6 using the same aging time of 1 h (Fig. 3B). Furthermore, the intensity of the 1 0 0 peak for the pre-aged phase increased with an increase of aging time up to 3–4 h, but it decreased markedly for longer aging and the condensation of TEOS was also decelerated or rather terminated, as suggested from the observation that the pre-aged product remained transparent. This is probably because the initial condensation of TEOS promotes the ordering of LC phase but the ethanol produced in excess by the hydrolysis of TEOS acts to suppress additional polymerization and make the LC structure disordered, as suggested by Attard et al. for a similar but single surfactant system [38]. The relatively small amount of water initially added in the LC template would cause partial condensation of TEOS species that is effective only for the insolubilization of the LC phase in water. Therefore, the pre-aged LC was further immersed in a large amount of water with two aims to extract the ethanol evolved by hydrolysis in the template phase and supply with water enough for the complete hydrolysis of TEOS. The volume of the pre-aged LC was little affected by immersing in water with ammonium acetate added, indicating no significant swelling of the pre-aged LC phase: The total volume of the aged sample placed in a glass tube was macroscopically monitored on a cubic millimeter scale with a reading error of 5% (vol/vol). Thermogravimetric analysis showed that ammonium acetate serves to reduce the content of surfactant species remaining unremoved even after washing with ethanol. Relative to the pre-aged LC phase, the as-grown silica fully aged at 20 °C, 1, exhibited a remarkable decrease in intensity of the XRD peak centered at 2h = 1.32° (d = 6.69 nm) (Fig. 3A). This indicates that the framework structure becomes much less ordered by the developed condensation. In contrast, the 400 °C calcined solid, 1 0 , showed a much stronger XRD peak around 2h = 1.45° (d = 6.09 nm), suggesting that additional condensation at elevated temperature serves to improve the long-periodic order of the framework structure (Fig. 3A). In the TEOS/C12EO9/C18EO20/H2O system, a similar reaction was performed using C18EO20 in place of Tween60. The resulting silica was found to be an aggregate of nanoparticles bimodal but with greater wall thickness, micropore volume and specific surface area, compared with the final product in the Tween60 based mixed surfactant system. The LC phase pre-aged at 20 °C for 1 h exhibited XRD peaks attributed to a hexagonal phase with a = 7.39 nm (Fig. 3D). On keeping at 20 °C for 7d after addition of water and ammonium acetate, the pre-aged LC phase was converted into a solid phase, 2, characterized by the XRD peak around 2h = 1.12° (d = 7.88 nm), as shown in Fig. 3C. The pre-aged LC phase showed no significant swelling during immersion in water, as in the Tween60 based mixed surfactant system. On calcination of 2 at 400 °C, the resulting product, 2 0 , gave TEM images indicating the formation of spherical silica nanoparticles of
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40–90 nm in size and having a mesoporous structure with a pore-to-pore diameter of 7–10 nm and irregularly arranged micropores of 0.3 nm diameter (Fig. 2C and D). From the XRD pattern for 2 0 , a more definite value of 9.3 nm was obtained for the mesopore-to-mesopore distance by assuming a hexagonal structure, although the channels have some irregularity in pore arrangement (Fig. 3C). Moreover, the pore-size distribution curve and the t-plot for 2 0 also revealed the development of mesopores of 4.6 nm diameter but with a broad distribution ranging from 3 to 8 nm (Fig. 4D and E) and micropores of 0.4 nm diameter (Fig. 4D–F). The specific micropore volume of 2 0 obtained by the t-plot was 0.18 cm3/g. The structural parameters thus obtained for the C18EO20 based mixed surfactant system are summarized in Table 1, along with those for the Tween60 based mixed surfactant system. The specific micropore volumes of 0.09 and 0.18 cm3/g evaluated by the t-plot data for 1 0 and 2 0 are 1.8 and 1.4 times larger than 0.05 and 0.13 cm3/g obtained by the as-plot data for them, respectively (Fig. S1). Nevertheless it should be noted that both the t- and as-plot data concordantly reveal that the micropore volume of 2 0 is about two times or more larger than that of 1 0 . 4. Discussion The catalyst-free polymerization of TEOS within the Tween60 and the C18EO20 based mixed surfactant LC templates were found to serve for the separate growth of micro-mesoporous silica nanoparticles with a particle size of as small as 40–80 or 40–90 nm, as described above. This is in marked contrast to the growth of monolithic or macro-sized mesoporous silica from single or mixed poly(ethylene oxide) alkylether based LC templates by acid-catalyzed polymerization of alkoxide [20,21,38]. Furthermore, the sizes of the silica nanoparticles were also in close agreement with 57 and 69 nm for the domain size of the pre-aged Tween60 and C18EO20 based LCs, respectively, evaluated within the error limit of about 20 nm using Scherrer’s equation from the profiles of their main XRD peaks. The single surfactant TEOS/C12EO9/H2O system also yielded the most highly ordered hexagonal structure with a = 8.03 nm and with a domain size of 79 nm for the presaged LC phase with the same TEOS/surfactant molar ratio as that in the TEOS/C12EO9/Tween60/H2O system (Fig. S2). These facts suggest with that the catalyst-free pre-aging produces much less polymerized silicates to induce the separation of the 2d-hexagonal LC template into domains, in contrast to the acid-catalyzed polymerization yielding highly developed Si–O networks available for forming large sized silicate networks [39,40]. The subsequent polymerization of the pre-aged LC would yield fully condensed individual silica nanoparticles. It is further remarked that there is a striking contrast between the structural parameters other than particle size for the final products in the present two systems, as can be seen in Table 1. First, the silica wall thickness of
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Fig. 5. Lattice constant, a, at different stages of silicate condensation, obtained by assuming a hexagonal structure: (s) TEOS/C12EO9/ Tween60/H2O system; (d) TEOS/C12EO9/C18EO20/H2O system.
4.9 nm for 2 0 in the C18EO20 based mixed surfactant system is greater than twice 2 nm for 1 0 in the other system. Second, the specific surface area and specific micropore and total-pore volumes of silica nanoparticles for the former system are also 1.5–2 times larger than those for the other, respectively. Third, the pore-to-pore distance, a, corresponding to the lattice constant obtained by assuming a hexagonal structure for the LCs or silica nanoparticles in both systems exhibits opposing dependences on the stage of silicate condensation after the introduction of TEOS into the silicate-free LC phase, as shown in Fig. 5. The pore-topore distance in the Tween60 based mixed surfactant system shows a tendency to decrease successively in two stages of full-aging and calcination, whereas the pore-to-pore distance in the C18EO20 based system shows a marked increase over the entire stage of silicate condensation including a slight but further increase even at the calcination stage. Contrary to the latter system using different-sized linear chain molecules, framework contraction was observed for the single surfactant TEOS/C12EO8/H2O system [38]. On the basis of the above observations and considerations, we can propose a model for the pathway from the precursory mixed surfactant LC to micro-mesoporous bimodal silica nanoparticles in the Tween60 and C18EO20 based mixed surfactant systems, as schematically shown in Scheme 1. The different sized C12EO9 and Tween60 or C18EO20 molecules in each mixed system would be combined into TEOS-loaded cylindrical rod like micelles to form a hexagonal LC, similarly to the TEOS-free mixed surfactant LC previously analyzed by structural calculation [37]. The TEOS species loaded within the aqueous shell of cylindrical micelles would undergo slight condensation into poorly condensed Si–O polymers, leading to a LC phase with a highly ordered hexagonal array of silicate-loaded cylindrical micelles. The partial condensation of TEOS species would also accompany the separation of the LC phase into domains 40–90 nm in size, as discussed above on the catalyst-free aging process. On contact with a large amount of water, the silicate-loaded LC domains would be further developed into highly polymerized silica nanoparticles with a much less ordered array of mesopores through additional
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Scheme 1. Schematic models for the pathway to micro-mesoporous bimodal silica nanoparticles from the precursory LC through the LC separation into domains depending on the degree of silicate condensation. The top illustrations (a) and (b) indicate the conformational change of surfactant molecules along with the dimensional change of silicate layer (purple or blue band) for the Tween60 and the C18EO20 based mixed surfactant systems, respectively. The as-grown samples 1 and 2 correspond to the fully aged phase and the calcined ones 1 0 and 2 0 the micro-mesoporous nanoparticles in the illustrations.
condensation of silicate species. In this process, the water phase serves to maintain the hexagonal framework through the removal of the ethanol evolved by hydrolysis of TEOS species. The as-grown solid thus formed would be converted into micro-mesoporous silica nanoparticles with a hexagonal but slightly modulated array of mesopores as a result of the complete condensation of silicate species on calcination. The silica walls of relatively small thickness formed in the Tween60 based mixed surfactant system could be thus explained by assuming that the growth of Si–O polymers within the aqueous shell induces partial exclusion of the silicate moiety from the triple-branched PEO chain assembly, leading to a decrease in the thickness of silicate-condensation layer. Because every three PEO chains bound to sorbitan ring are highly restricted in conformational change so that the silicate particles enlarged by additional condensation cannot totally remain within the PEO pocket. The structural rearrangement, in connection with the release of water and ethanol due to silicate condensation, would also cause the framework contraction of the hexagonal-structured LCs or incompletely con-
densed silica nanoparticles. On the other hand, the PEO single chain of C18EO20 molecule is much more free in conformational change than the PEO triple chain of Tween60 because the former is branch-free and nearly three times longer than the individual PEO chain of the latter. Accordingly, in the C18EO20 based mixed surfactant system, 3d polymerization of silicate species initiated by the formation of Si–O linear oligomers proceeds so as to change the conformation of PEO chain from their initial folded state into their more extended state, which would also accompany the displacement of Si–O clusters and/or un-reacted TEOS species toward the outside of cylindrical micelle. Such change in state of silicate-surfactant hybridized assemblies would result in the framework expansion of the hexagonalstructured LCs or incompletely condensed silica nanoparticles. The structural rearrangement thus induced would further result in the formation of hexagonal-structured silica nanoparticles with mesopores of smaller size and walls of more than two times larger thickness than those for the Tween60 based system. The two times or more larger micropore volume of silica nanoparticles for the C18EO20
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based system than that for the other would be due to the large difference between the effective volumes of hydrophilic PEO chains penetrated into silica walls, as suggested by Smarsly et al. [20,21]. The results for the present C18EO20 based system are significantly different from those for the acid-catalyzed condensation at 60 °C in the TEOS/ C12EO8/C18EO20/H2O system: the latter system yielded micro-mesoporous bimodal silica nanoparticles with worm-type array of mesopores as small as 2.6 nm in diameter and with a particle size of several hundred nanometers or more [20]. This difference likely depends on whether the polymerization process is catalyst-free or acid-catalyzed and the process for ethanol removal is based on aging in contact with water phase or evaporation in vacuum during pre-aging. 5. Conclusions We demonstrated the first synthesis of micro-mesoporous bimodal silica nanoparticles with a particle diameter of as small as 40–90 nm by a two-step reaction based on the pre and further polymerization of silicate species confined to the mixed surfactant hexagonal-structured LC templates of C12EO9 and Tween60 or C18EO20. After preaging for water-insolubilization, the silicate species loaded in the LC phase was fully condensed by keeping in contact with dilute aqueous solution of ammonium acetate. The catalyst-free pre-aging process induced the separation of the LC phase into domains and the subsequent waterphase-in-contact process served to maintain the hexagonal framework by removing the ethanol evolved through hydrolysis, leading to the bimodal nanoparticles. The silica nanoparticles thus prepared in both systems have slightly different pore sizes of 5 nm, but the wall thickness, specific surface area and specific micropore and total-pore volumes of nanoparticles for the C18EO20 based system are 1.5 times or more larger than those for the other system, respectively. The relatively larger structural parameters for the former system are attributed to the larger effective volume of hydrophilic PEO long chains penetrated into silica walls. The silicate-surfactant hybridized framework in this system expands over the entire stage of silicate condensation, mainly due to the unfolding of PEO long linear chains of C18EO20 molecules. In the Tween60 based system, on the other hand, the growth of Si–O polymers within the aqueous shell induces partial exclusion of the silicate moiety from the triple-branched PEO chain pocket, leading to a marked decrease in the thickness of silicatecondensation layer as well as the framework contraction. The present micro-mesoporous silica nanoparticles would be promising as catalyst supports for their small sizes available for the diffusion of reactant molecules. Acknowledgements This study was supported by Grant-in-Aids for the CREST of Japan Science and Technology Corporation
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