Syntheses of ordered mesoporous silica by new hybrid template

Syntheses of ordered mesoporous silica by new hybrid template

Colloids and Surfaces B: Biointerfaces 38 (2004) 121–125 Syntheses of ordered mesoporous silica by new hybrid template Jinting Jiua,b,∗ , Ken-ichi Ku...

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Colloids and Surfaces B: Biointerfaces 38 (2004) 121–125

Syntheses of ordered mesoporous silica by new hybrid template Jinting Jiua,b,∗ , Ken-ichi Kurumadac , Lihua Peib , Masataka Tanigakia a

Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-Ku, Kyoto 606-8501, Japan b Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan c Graduate School of Environment and Information Science, Yokohama National University, Yokohama 240-8501, Japan Received 13 October 2003; accepted 26 April 2004 Available online 21 August 2004

Abstract Ordered mesoporous silica with macroscopic shape has been prepared with a hybrid template of gel and poly(ethylene oxide)106 –poly(propylene oxide)70 –poly(ethylene oxide)106 (pluronic F127) surfactant, where both water-soluble agar gel and pluronic F127 significantly affect the mesoporous structure and morphology of silica. The thermal analysis revealed the noticeable interaction between agar and F127, which contributes to the formation of homogenous hybrid template. In the hybrid template, agar gel contributed to the maintenance of morphology structure, while F127 was responsible for the formation of ordered porous structure in silica solids. © 2004 Elsevier B.V. All rights reserved. Keywords: Hybrid template; Gel; Surfactant; Mesoporous silica; Morphology

1. Introduction Using self-assembled supramolecular arrays of organic compounds as the structure-directing agents has been widely explored to synthesize mesostructured materials [1–3], especially mesoporous silica with various morphologies, such as particles, thin films, spheres, fibers, hollow tubes, etc. [4–6] since the first synthesis of mesoporous MCM-41 in 1992 [7]. However, the drastic shrinkage removing the template always destroys the mechanical properties of materials. Hence, it is important to pay great attention on developing new templates for the design and syntheses of ordered mesoporous solids with improved properties for practical applications. Ryoo et al. [8] has attempted to prepare ordered MCM-41 by mixture templates, in which two surfactants were simply mixed to control the pore size of materials. However, they did not mention the morphology and shrinkage of materials. In our previous work [9], gels with tangible macroscopic shapes and intriguing microscopic structures have been turned out to be suitable as supramolecular templates for constructing ∗

Corresponding author. Fax: +81 774 38 3508. E-mail address: [email protected] (J. Jiu).

0927-7765/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2004.04.010

of mesoporous materials, especially decreasing the shrinkage and maintaining the morphology structure due to the bulk properties of gel itself. However, ordered mesoporous solids have not been obtained only in the gel template with random network structure. We expected that periodic mesoporous materials would be fabricated when the random microscopic structure of gel can be ordered upon in the progress of mixture with inorganic species. Hence, we attempted to order the gel structure by the interaction between gel and surfactant, the latter can be dissolved and form ordered liquid crystalline structure. By incorporating with ordered liquid crystalline structure, the network of the hybrid of gel and surfactant is expected to be a good candidate for the fabrication of ordered mesoporous materials with various morphology and minimum shrinkage. Moreover, the hybrid template acting as structure-directing agent possesses both advantages of gel and surfactant. Based on the consideration above, we chose a water-soluble gel called agar which is nonionic polysaccharide consisted of a linear homopolymer of d-glucose units linked by a ␤(1,3)-glucosidic linkage, and pluronic F127 surfactant which easily formed ordered liquid crystalline structure in water solution, to fabricate mesoporous silica. It is known that agar gel had been extensively used as food addi-


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tive due to its health benefits, heat-gelling and water-binding properties [10]. It has to be mentioned that agar can form “high-set gel” above 80 ◦ C in water which is elastic and hard gel used mainly to maintain the shape and morphology of food. In the work, therefore, we have attempted to use agar gel and pluronic surfactant to form hybrid template for the preparation of ordered mesoporous materials in order to obtain silica solids with various morphology and minimum shrinkage.

2. Experimental procedure 2.1. Material Pluronic F127 surfactant is commercially available from BASF. Agar was provided by Tokyo Kasei. All the chemicals for sample preparations were used as provided.

Fig. 1. Thermogravimetric and corresponding derivative curves: (a, a ) neat pluronic F127; (b, b ) neat agar; (c, c ) hybrid template of F127 and agar; (d, d ) complex of the hybrid template and silica.

2.2. Preparation of hybrid templates In the typical synthesis, agar (1.8 g) was dissolved in water (27 g) at 50–60 ◦ C with vigorous stirring. Then F127 (3.6 g) was added in this solution for the formation of hybrids. The hot hybrid template was left for the following use. 2.3. Preparation of silica source HCl (1 g) aqueous solution (1 M) was added to a mixture of TEOS (20.8 g) and water (18 g). The mixture was stirred at room temperature to hydrolyze TEOS until a transparent solution was obtained. Subsequently, the silica source was poured into the above hybrid template solution for homogeneous mixing with vigorous stirring for 10 min. A transparent mixture solution was immediately obtained. The resulting solution was transferred into 90 ◦ C oven for gelation, during which the inorganic precursor was polymerized and transparent gel was obtained. The as-made gel was then calcined at 600 ◦ C for 6 h in air to remove the hybrid template. The samples were observed by transmission electron microscope (TEM, JEOL JEM-1010, 100 kV), X-ray diffraction patterns were obtained using Cu K␣ (λ = 0.15418 nm, 40 kV, 100 mA) in the range of 0.5–5◦ with a step angle 0.01◦ . Thermogravimetric analysis (TGA) was performed by Shimadzu TGA-50 at the heating rate of 10 ◦ C min−1 in air.

3. Results and discussion Fig. 1 shows the thermal behavior of neat pluronic F127 and agar template, pluronic F127/agar template and template/silica complex, respectively. In the cases of neat pluronic F127 and agar, the decomposition temperatures (maximum point of temperature at which the rate of the weight reduction is the largest) are 243 and 303 ◦ C, respectively, with a drastic weight loss in TG curve (lines a, a and

b, b , respectively). However, the decomposition temperature of the template of pluronic F127 and agar is 295 ◦ C (lines c, c ), which is located between that of neat pluronic F127 and agar. The result suggested the presence of interaction between these two species [11]. (Assuming that they are entirely free of the mutual interaction, two peaks are expected at the same temperature as the neat pluronic F127 and agar.) The results also agree with the molecular structure of agar, in which many OH groups inevitably interact with the hydrophilic part of F127 molecule by hydrogen bondings. These results suggested that hybrid template of agar gel and pluronic F127 had been formed in the mixture of the both compounds. Moreover, silica is considered to have high affinity to the hybrid template, which is indicated by the increase in the decomposition temperature of the organic template species over 300 ◦ C. The increase in the decomposition temperature in silica–template complex is to be related to the silica–template interaction in the molecular scale. Higher decomposition temperature is required to remove the hybrid template from the incorporated state in the silica–template complex by hydrogen bonding between silica and template with –OH groups [9]. Fig. 2 shows typical XRD patterns of the calcined silicas prepared using hybrid template of agar and pluronic F127 as the structure-directing agent aged at 90 ◦ C, which is benefit for the formation of higher ordered structure. The mesoporous silica shows one intense peak and a weak broad peak at 2θ values between 1◦ and 5◦ that can be indexed as (1 0 0) and (1 1 0) Bragg diffractions as seen in typical hexagonal mesoporous silica [12,13]. The spacing distance of d (1 0 0) is about 7.06 nm, which is agreed with the results of ordered mesoporous silica prepared with pluronic series surfactant [12,13]. The clear low-angle diffraction peaks indicate that mesoporous structure is preserved in the calcined silica solids. Further evidence for the mesostructure is provided by TEM images presented in Fig. 3a. The micrograph

J. Jiu et al. / Colloids and Surfaces B: Biointerfaces 38 (2004) 121–125

Fig. 2. Powder X-ray diffraction patterns of silica prepared using hybrid template of agar and F127 at 90 ◦ C aged.

shows well-ordered mesoporous structures seen in different areas, suggesting that the mesoporous silica is a highly ordered 3D hexagonal mesostructure. Moreover, it is worth noting that periodic mesoporous silica was not obtained only with pluronic F127 or agar gel even at the same preparation condition (Fig. 3b and c). These results suggest that the hybrid template agent played a key role for the formation of the ordered mesoporous structure. It is known that agar is a polymer of agarobiose, a disaccharide composed of d-galactose and 3,6-anhydro-l-galactose. Agar is insoluble in cold wa-

Fig. 3. The TEM micrographs of silica prepared using hybrid template of agar and F127 (a), neat F127 template (b) and neat agar (c) aged at 90 ◦ C.


ter but dissolves to give random coils in hot water to form a clear, stable and firm gel [10]; therefore the disordered agar gel is not responsible for the ordered structure in resultant solids. Generally, the micellar structure of F127 can work as the template for the preparation of ordered mesoporous materials [14,15]. In the present case of neat F127, ordered mesoporous structure was not obtained suggesting that the concentration of pluronic F127 was not enough for the formation of ordered liquid crystalline structure. Comparatively, ordered mesoporous structure could be obtained in the hybrid template of gel and surfactant, which was expected that the incorporation of pluronic F127 in agar gel efficiently promoted the formation of the ordered hybrid template. And, it is also related to the decrease of c.m.c. due to the presence of agar gel as proposed in the extensive observation of system of polymer and surfactant [16–18]. Most of those works support that the complex is consisted of spherical, monodispersed micelles adsorbed onto the polymer coil in the “bead-and-necklace” arrangement [18]. In the presence of the polymer, adsorbed micelles form at a critical aggregation concentration (c.a.c.) which is lower than the conventional c.m.c. observed in the absence of polymer. These results revealed that the formation of ordered liquid crystalline structure of pluronic F127 in the hybrid template was also promoted by the presence of agar gel. Hence, the mutual promotion between agar and F127 dedicate the hybrid template into the fabrication of ordered mesoporous silica. On the other hand, agar is known to be dissolved into boiling water and form a very porous gel with up to 99.5% water. The porous and “free” structure is benefit for the permeation of the micelle of F127 resulting in the formation of F127/gel complex. In order to observe the mechanical properties of mesoporous silica, the sample was treated in boiling water for 2 h. Fig. 4 shows the TEM and electron diffraction (ED) image of the sample. It is clear that the ordered mesoporous structure is still kept indicating that the thermal property of the materials is strong enough to resist the thermal impact. The ED pattern of Fig. 4a also confirms the mesoporous structure is still hexagonal. Due to the various thickness of sample along arrow (in Fig. 4a), two diffraction patterns have been observed which corresponded to (0 0 1) and (1 0 0) plane with strong hexagon and circle diffraction spots, respectively (Fig. 4b) [19]. These results revealed that the silica prepared with the hybrid template still kept bulk and hexagonally ordered mesoporous structure. Fig. 5 shows the SEM images of samples before and after calcinations at 600 ◦ C for 6 h. It is large bulk structure but powder sample has been obtained even after the sample has been calcined for 6 h at 600 ◦ C. Also it is worth to be noted that there is no clear difference in the morphology indicating that there is no clear shrinkage removing the hybrid template by calcinations. Moreover, the macroscopic shape of sample before and after calcinations also confirmed that there was no shrinkage caused by calcinations except that the color of sample changed from bright yellow into no color before and after calcinations, respectively (insert Fig. 5). The thickness


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Fig. 4. The TEM micrographs of silica treated in boiling water for 2 h (a) and electron diffraction of a (b).

Fig. 5. The SEM micrographs of silica before and after calcination at 600 ◦ C for 6 h.

of sample is about 1.3 mm and transparent before and after calcinations, which is similar to the result of mesoporous silica prepared with the hybrid template of curdlan and F127 in our laboratory. Also, it is very interesting that different morphology silica samples can be obtained by coating the hybrid/silica complex gel on the surface of substrates with various shapes. The successful points to be stressed in the present work are: (1) constructing stable mesophase by incorporating agar gel with pluronic F127, and (2) reducing the shrinkage of bulk sample that improved the mechanical properties of materials in the removal of the organic templates by the calcinations. The ordered mesophases is preferably formed when both agar and pluronic F127 are used as the structure-directing agents. The stability of a mesostructure depends on various factors, such as, the extent of condensation or crystallization of the inorganic materials, the shrinkage of structure during the removal the templates. The former is mostly achieved by adjusting the pH and reaction temperature. The latter is still challenging for the preparation of monolithic mesoporous materials. Generally, the macroscopic shape of porous materials undergoes some shrinkage in removing of the templates [20,21]. In our experiments, the shrinkage was effectively restricted (Fig. 5) by using the stable and firm agar gel as the template compared to the case where surfactant template is

used to obtain powders as the resultant product. These results suggest that the gel is indispensable for keeping macroscopic shape in the hybrid template and decreasing the shrinkage of materials. From the practical viewpoint, the usefulness of the hybrid template of surfactant and gel is interesting in that it effectively contributes to obtaining bulk and ordered materials.

4. Conclusion The use of the hybrid template of agar and pluronic F127 surfactant as structure-directing agent allows the formation of mesoporous silica with macroscopically bulk and ordered structure. Considering the common chemical structure of the pluronic series, this hybridizing method can be applicable to other pluronic surfactants. We anticipate that with the hybrid template, the synthesis of macroscopically ordered non-siliceous oxides will also be possible using normal inorganic salts with the help of gel working as the frame for the macroscopic shaping. It should be remarked that using the hybrid template of surfactant species and gel is a practically useful method to produce bulk mesoporous materials effectively avoiding macroscopic shrinkage or cracking.

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Acknowledgement The authors appreciate the support of Japan Society for Promotion of Science (JSPS) for Dr. J. Jiu’s research in Japan. They also gratefully acknowledge Ms. K. Yamanaka for the continuous technical instruction in our TEM observations. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] B.R. Heywood, S. Mann, Adv. Mater. 6 (1994) 9. [3] K. Nakanishi, H. Minakuchi, N. Soga, N. Tanaka, J. Sol-Gel Sci. Technol. 13 (1998) 163. [4] Z. Zhang, B. Tian, S. Shen, et al., Chem. Lett. (2002) 584. [5] S. Che, S. Kamiya, O. Terasaki, T. Tatsumi, J. Am. Chem. Soc. 123 (2001) 12089. [6] J.M. Kim, S.K. Kim, R. Ryoo, Chem. Commun. (1998) 259. [7] J.S. Beck, J.C. Vartuli, W.J. Roth, et al., J. Am. Chem. Soc. 114 (1992) 10834.


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