Morphological control of highly ordered mesoporous carbon

Morphological control of highly ordered mesoporous carbon

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved 45 Morphological control of highly ...

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Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

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Morphological control of highly ordered mesoporous carbon C. Yu \ J. Fan \ B. Tian \ F. Zhang \ G. D. Stucky ^* and D. Zhao '* ^Department of Chemistry, Fudan University, Shanghai, 200433, P. R. China. Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA. Highly ordered mesoporous carbon materials with fiber-like, plate-like, rod-like and donutlike morphologies have been synthesized by using mesoporous silica as the templates. 1. INTRODUCTION Recently, periodic mesoporous carbon materials CMK-X synthesized from ordered mesoporous silica templates [1,2] have attracted much attention because of their versatile uses in gas separation, catalysis, chromatography, and energy storage. [3,4]Such new mesostructured carbon materials were synthesized by using mesoporous silica with threedimensional (3-D) pore networks (MCM-48, SBA-1, and SBA-15) as hard templates and sucrose, furfuryl alcohol as suitable carbon sources. In the case of mesoporous carbon CMK-3, [2] the structure is exactly an inverse replica of its silica templates SBA-15 [5] without any structural transformation. For practical applications, the fabrication of desired morphologies is important as well as the control in composition, structure, porosity, etc. Mesostructured fibers, donuts [6] and crystal morphologies [7] have been obtained for silica powders synthesized by using commercial nonionic block copolymers as supramolecular templates. Until now, however, no successful morphology control in mesoporous carbon has been reported. Here we show a successful morphological control of hexagonally ordered mesoporous carbon materials with fiber-like, rod-like, plate-like and donut-like morphologies by using SBA-15 silica as the hard templates. 2. EXPERIMENTAL Highly ordered mesoporous SBA-15 silica with different morphologies were synthesized by using poly(ethylene oxidcs)-/?-poly(propylene oxides)-/?-poly(ethylene oxides) (PEO-PPOPEO) triblock copolymer EO20PO70EO20 (BASF) as the structure directing agent. Rod-like SBA-15 was synthesized using TMOS as a silica source at 313 K under static conditions. This work was supported by the NSF of China (Grant No. 29925309 and 29873012), Shanghai Sci. Tech. Committee (0152nm029), State Key Basic Res. Prog. (G2000048001), and the National Science Foundation of the Unites States under Grant DMR-9634396.

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Plate-like SBA-15 was synthesized using TMOS as a silica source at 303 K under static conditions. The fiber-like and donut-like SBA-15 was synthesized as reported in literature. [6] In a typical synthesis of SBA-15 rods, 2.0 g (0.34 mmol) of EO20PO70EO20 were dissolved in 60 g of 2.0 M HCl at 38 °C. To this solution, 3.3 g (22 mmol) tetramethyl orthosilicate (TMOS) were added under vigorous stirring. The final reactants molar composition is 0.015 EO20PO70EO20/5.5 HCl/150 H2O/I TEOS. After stirring for 6 min, the mixture was kept in static conditions at the same temperature for one day, then the mixture was transferred into an autoclave and heated at 100 °C for another 24 h. The solid products were collected by filtration and dried at room temperature in air. The resulting powders were calcined at 550 °C for 4 h in order to obtain mesoporous silica materials. The synthesis of mesoporous carbon materials from SBA-15 silica hard templates was performed with sucrose as described in literature. [2] The dissolution of silica with 10% HP resulted in mesoporous carbon materials. 3. RESULTS AND DISCUSSION Scanning electron microscopy (SEM) images of mesoporous SBA-15 with different morphologies are shown in Figure la, b, c, and d. The well-defined fiber-like, rod-like, platelike and donut-like morphologies confirm that desired morphologies of SBA-15 have been obtained in high yield. It is interesting to note that the stirring condition is essential to the morphology of SBA-15, which leads to fiber like (with stirring) and rod-like SBA-15 materials (without stirring). Furthermore, the synthesis temperature can be used in the aspect ratio control of highly ordered SBA-15 materials, as can seen in the cases of rod-like and plate-like SBA-15 synthesized with the same reactant compositions while the synthesis temperature is different (313 K for rods and 303 K for plates). X-ray diffraction (XRD), N2 sorption analysis results (Table 1) and transmission electron microscopy (TEM) images (data not shown) confirm that these silica templates are highly ordered hexagonal mesostructures. Table 1 Physical Data of Mesoporous Silica/Carbon with Sample Morphology J(IOO) /nm 9.06 Mesoporous Fibers Silica 8.74 Rods SBA-15 Plates 8.72 9.11 Donuts 7.82 Mesoporous Fibers Carbon 7.96 Rods C-SBA-15 6.98 Plates 8.03 Donuts

Different Wa /nm 6.8 6.6 6.3 7.8 3.8 3.9 3.9 3.4

Morphologies wj S /nm /m^g"' 7.0 916 6.9 811 910 6.5 983 7.7 1746 4.6 1899 4.5 1738 4.6 1558 4.3

V /cm"^g'' 1.21 0.99 1.09 1.44 1.48 1.65 1.45 1.22

Notes: d{\00) is d-spacing of mesoporous silica/carbon calculated from XRD data; Wa and wj are pore size of mesoporous silica/carbon calculated using BdB model from adsorption and dcsorption branch, respectively. S is BET surface area, and V is pore volume.

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Fig. 1. SEM images (a), (b), (c) and (d) of hexagonal mesoporous silica SBA-15 materials with fiberlike, rod-like, plate-like and donut-likc morphology, respectively. SEM images (e), (f), (g) and (h) and TEM images (i), (j), (k) and (1) of hexagonal mesoporous carbon with fiber-like, rod-like, plate-like and donut-like morphology, respectively. SEM images were obtained with a JEOL 6300-F microscope using 3.0 KV acceleration voltages. TEM micrographs were obtained with a JOEL 2000 transmission electron microscope operating at 200KV. XRD patterns and N2 sorption isotherms of mesoporous carbon materials synthesized from SBA-15 fibers (C-SBA-15) are shown in Figure 2a and 2b, respectively. The well-resolved three diffraction peaks can be indexed to 2D hexagonal structure (p6mm), indicating that the carbon products are faithful replication of their silica templates. Other C-SBA-15 synthesized from rod-like, plate-like and donut-like SBA-15 templates show similar XRD patterns and N2 sorption curves, and the physical data can be found in Table 1. Moreover, SEM (Figure le, f, g, h) and TEM images (Figure li, j , k, 1) confirm that the replication of these mesoporous

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carbon materials is strict in both mesostructure and morphology compared to their silica templates. It is noted that the length and curvature of mesopore channel within these C-SBA15 materials are different; and these mesoporous carbon materials with controlled morphologies have large surface area (up to 1900 m^g"') and pore volumes (up to 1.65 cm"^g').

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Fig. 2. (a) XRD patterns; (b) N2 adsorption-dcsorption isotherm plots and pore size distribution curve (inset) of mesoporous carbon materials C-SBA-15 with fiber-like morphology. The X-ray data were collected on a Scintag PADX diffractometer using Cu Ka radiation. The isotherms were measured using a Micromcritics ASAP 2000 system. The samples were degassed at 200 °C overnight on a vacuum line.

4. CONCLUSIONS In conclusion, highly ordered hexagonal mesoporous carbon materials with fiber-like, rodlike, plate-like and donut-like morphologies have been synthesized. Such ordered mesoporous carbon materials with uniform morphology, various pore parameter and extra large porosity may have potential use in catalysis and as nano-reactors for hydrophobic precursors. REFERENCES 1. R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. 2. S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Tcrasaki, J. Am. Chem. Soc. 122(2000) 10712. 3. R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Adv. Mater. 13 (2001) 677. 4. S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. 5. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Frederickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. 6. D. Zhao, J.Y. Sun, Q.Z. Li, G.D. Stucky, Chem. Mater. 12 (2000) 275. 7. C. Yu, B. Tian, J. Fan, G.D. Stucky, D. Zhao, J. Am. Chem. Soc. 124 (2002) 4556.