Langmuir 2004, 20, 5157-5159
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Graphitic Mesostructured Carbon Prepared from Aromatic Precursors Chy Hyung Kim,† Dong-Keun Lee,‡ and Thomas J. Pinnavaia*,‡ Department of Chemistry, Chongju University, Chongju 360-764, South Korea, and Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 Received February 15, 2004. In Final Form: April 18, 2004 Mesostructured carbons with graphitic framework walls are conveniently prepared at ambient pressures through the replication of a mesostructured silica template using an aromatic hydrocarbon as the carbon precursor and a catalyst.
Porous carbon materials have been widely studied for many potential applications, including adsorption, catalysis, and energy storage.1-3 In addition, the electronic conductivity, corrosion resistance, surface properties, and high energy density of carbon at low operating temperatures make these materials useful for electrical applications,4-7 particularly if the carbon is in graphitic form. Conventional high surface area carbons are derived from cellulose, coal, petroleum pitches, methane, and synthetic polymers.3,8-11 Recently, mesostructured forms of carbon have been prepared through the templating of mesostructured silicas. In this approach a three-dimensional (3D) hexagonal silica mesostructure such as SBA-1512 or its highly cross-linked MSU-H analogue13 is replicated through the carbonization of a molecular precursor and subsequently dissolving away the silica in sodium hydroxide or hydrofluoric acid solution.14-16 Silica with a wormhole framework structure17 also has been shown to be an effective template for the preparation of mesostructured carbons.18 Sucrose, in combination with sulfuric acid as a catalyst, is perhaps the most commonly used precursor for the preparation of mesostructured carbons. However, the resulting carbon has little or no graphitic character even when processed at 900 °C. More recently, an ordered * Corresponding author. E-mail:
[email protected]. † Chongju University. ‡ Michigan State University. (1) Kyotani, T. Carbon 2000, 38, 269. (2) Rodriguez-Reinoso, F. Carbon 1998, 36, 159. (3) Yu, J. S.; Kang, S.; Yoon, S. B.; Chai, G. J. Am. Chem. Soc. 2002, 124, 9382. (4) Flandrois, S.; Simon, B. Carbon 1999, 37, 165. (5) Bessel, C. A.; Laubernds, K.; Rodriguez, N. M.; Baker, R. T. K. J. Phys. Chem. B 2001, 105, 1115. (6) Kinoshita, K. Carbon, Electrochemical and Physicochemical Properties; Wiley: New York, 1988. (7) Park, K.-W.; Ahn, K.-S.; Choi, J.-H.; Nah, Y.-C.; Kim, Y.-M.; Sung, Y.-E. Appl. Phys. Lett. 2002, 81, 907. (8) Yoshzawa, N.; Yamada, Y.; Furuta, T.; Shiraishi, M. Energy Fuels 1997, 11, 327. (9) Leboda, R.; Skubiszewska-Zieba, H.; Bogillo, V. I. Langmur 1997, 13, 1211. (10) Tamai, H.; Kakii, T.; Hirota, Y.; Kumamoto, T.; Yasuda, H. Chem. Mater. 1996, 8, 454. (11) Ruckenstein, E.; Hu Yun, H. H. Carbon 1998, 36, 269. (12) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (13) Kim, S. S.; Pauly, T. R.; Pinnavaia, T. J. Chem. Commun. 2000, 1661. (14) Kim, S. S.; Pinnavaia, T. J. Chem. Commun. 2001, 2418. (15) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712. (16) Fuertes, A. B. Microporous Mesoporous Mater. 2004, 67, 273. (17) Pauly, T. R.; Liu, Yu; Pinnavaia, T. J.; Billinge, Simon J. L.; Rieker, T. P. J. Am. Chem. Soc. 1999, 121, 8835. (18) Lee, J.; Yoon, S.; Oh, S. M.; Shin, C.-H.; Hyeon, T. Adv. Mater. 2000, 12, 359.
Figure 1. XRD patterns of Cscr, Cbnz, Cnpt, Catr, and Cpyr formed from sucrose, benzene, naphthalene, anthracene and pyrene, respectively: (A) the low-angle region and (B) the wide-angle region.
mesoporous carbon replica with graphitic character was prepared from acenaphthene as the precursor.19 The preparation of this graphitic derivative required the pretreatment of the silica template with aqueous AlCl3 in order to provide catalytic acid sites for polymerization to pitch, initially at 750 °C under autoclave conditions, followed by carbonization at 900 °C under vacuum. (19) Kim, T.-W.; Park, I.-S.; Ryoo, R. Angew. Chem., Int. Ed. 2003, 42, 4375.
10.1021/la049602c CCC: $27.50 © 2004 American Chemical Society Published on Web 05/28/2004
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Langmuir, Vol. 20, No. 13, 2004
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Figure 2. TEM images of mesostructured Cbnz, Cnpt, Catr, and Cpyr carbons.
We report here alternative methods for forming graphitic mesostructured carbon from aromatic molecular precursors without the need for extensive template modification or for the processing of the reaction mixture under autoclave conditions. The aromatic precursors, which include naphthalene, anthracene, pyrene, and even benzene, afford derivatives with improved graphitic character and a higher electrical conductivity in comparison to carbon formed from a nonaromatic precursors such as sucrose. Mesostructured MSU-H silica,13 an analogue of hexagonal SBA-15 with p6mm symmetry, was prepared at 60 °C from a reaction mixture of nonionic Pluronic P123 surfactant as the structure-directing porogen, sodium silicate as the silica source, and sodium sulfate as a mineralizer. The 1.00 SiO2:0.011 P123:0.50 Na2SO4 molar mixture was buffered to a pH of ∼6.5 with acetic acid. After a reaction time of 24 h, the as-made product was washed free of salt and calcined at 550 °C for 6 h to remove the surfactant. The carbon replicas derived from naphthalene, anthracene, and pyrene, were denoted Cnpt, Catr, and Cpyr, respectively. These products were obtained by impregnating 0.70 g of MSU-H silica with 5-mL aliquots of 0.31 M naphthalene, 0.05 M anthracene, and 0.16 M pyrene in acetone containing 0.14 M sulfuric acid. After each 5-mL addition of acetone solution, the silica was heated at 160 °C in a loosely covered vial for 1 h to initiate carbonization. The impregnation and carbonization procedures were repeated until the entire precursor solution was consumed. The total loading of naphthalene, anthracene, and pyrene was 0.86, 0.36, and 0.71 g/g of silica, respectively. The
resulting composites were each heated in a stoppered vial at 400 °C for 4 h and then further carbonized under nitrogen at 750 °C (naphthalene), 800 °C (anthracene) or 850 °C (pyrene) for a 4-h period. The composites were then suspended in 25% HF overnight to dissolve the silica template and obtain the pure carbon replicas. For comparison purposes, an analogous procedure was used to prepare a carbon replica using sucrose as a precursor, denoted Cscr, but in this case the impregnation solvent was water and the sucrose loading was 2.0 g/g of silica. The carbon replica made from benzene, denoted Cbnz, required a different synthesis procedure, in part, because of its high volatility. Thus, MSU-H silica (1.0 g) was impregnated with Fe(CO)5 (0.25 g) in degassed benzene solution (4.75 g), and the benzene was allowed to evaporate. The intent of this step was to provide iron nanoparticles in the mesopores to facilitate carbonization in a subsequent step wherein the impregnated silica was placed in a quartz glass tube and exposed to a flow of nitrogen saturated with benzene vapor at 800 °C for 6 h. The resulting carbon-mesostructured silica composite was suspended in 25% HF overnight to remove the silica template. Figure 1 shows the powder X-ray diffraction patterns for the mesostructured carbons in the low- and wide-angle regions. The low-angle patterns reveal substantial differences in the long-range framework order. The best framework order is obtained with anthracene, benzene, and sucrose as carbon precursors. Although the carbons made from naphthalene and pyrene show lower long-range framework order, they have greater graphitic structure, as judged by the intensity and line widths of the wide-
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Figure 3. Nitrogen adsorption-desorption isotherms of Cscr, Cbnz, Cnpt, Catr, and Cpyr carbons. Table 1. Textural Properties of Mesostructured Carbon Replicas of MSU-H
carbon
carbon source
Cscr Cnpt Catr Cpyr Cbzn
sucrose naphthalene anthracene pyrene benzene
d100 pore spacing sizea Vtot SBET conductivityb (nm) (nm) (cm3 g-1) (m2 g-1) (S/cm) 9.6 10.5 10.0 10.0 10.7
3.1 2.7 2.8 2.6 2.4
0.79 1.28 0.90 0.97 1.00
868 889 826 598 685
0.0016 0.0079 0.116 0.136 1.55
a Pore sizes were determined from the nitrogen adsorption isotherm using the BJH model. b The dc conductivity was determined for powders pressed into a quartz glass tube at an applied pressure of 6.0 MPa.
angle Bragg reflections. Sucrose as a carbon precursor shows little or no graphic structure. The transmission electron microscopy (TEM) images of the carbons prepared from aromatic precursors are shown in Figure 2. In accord with the X-ray diffraction (XRD) results, Catr exhibits excellent hexagonal framework order. The remaining carbons formed from aromatic precursors show evidence for bundles of rods. For Cnpt and Cpyr carbons, the rods are separated from the bundles, explaining why a lower degree of hexagonal order is observed for these products by XRD. No iron was found for the Cbnz sample by EDX analysis, indicating that iron formed through the thermal decomposition of Fe(CO)520 was removed when the silica was dissolved in 25% HF. Nitrogen adsorption-desorption isotherms for the mesostructured carbons are shown in Figure 3. The BET surface areas, pore volumes, and other textural properties of the replicas are summarized in Table 1. The high degree of hexagonal order observed in the XRD patterns of Cscr and Catr is reflected in the well-expressed adsorption step for these two products in the partial pressure range 0.350.55. Cnpt and Cpyr, which showed lower hexagonal ordering (20) Liu, X.-Y.; Huang B.-C.; Coville, N. J. Carbon 2002, 40, 2791.
in the XRD and the separation of nanorods in the TEM, do not exhibit a well-defined step in the nitrogen isotherms. Nevertheless, the products are all mesoporous with pore sizes centered between 2.4 and 2.8 nm, as indicated by the BJH pore size distributions shown in the inset to Figure 3. Table 1 also provides the dc conductivities of the mesostructured carbons in pressed powder form. A Keithley 236 source unit and four-probe dc method was used to obtain the conductivities at an applied current of 0.0110 mA and a maximum applied voltage of 1.0 V. As we anticipated on the basis of differences in graphitic character, the Cnpt, Catr, and Cpyr carbons exhibit conductivities (0.08-0.14 S/cm) that are substantially larger than the Cscr carbon formed from sucrose (0.0016 S/cm). An even higher conductivity (1.56 S/cm) was found for Cbzn formed from benzene. For comparison purposes we also determined the dc conductivity of a synthetic graphite powder (Aldrich) with a surface area of 16 m2/g. The value observed for this latter carbon in pressed powder form was 44.4 S/cm. Thus, the mesostructured carbons are less conductive than a conventional graphite powder compacted under equivalent conditions. Also, the conductivities are lower than the value reported for a mesostructured graphitic carbon prepared from acenaphthene through autoclave processing19 and different compaction pressures were used to prepare the sample. There is no doubt that the electrical conductivity of a mesostructured carbon can be substantially improved by increasing the graphitic character of the framework walls. Even though Catr and Cscr have very similar framework structures and textural properties, as judged by XRD and N2 adsorption measurements, the conductivity of Catr is 73 times as large as the conductivity of Cscr. The main difference in these two mesostructures lies in the degree of graphitic character of the framework walls. However, the degree of graphitic character of the framework walls clearly is not the only factor influencing the dc conductivity. For instance, the conductivity of Cbzn is 1000 times as large as the conductivity of Cscr, even though the degree of graphitic character of the framework walls in Cbzn is not substantially different from the mesostructured carbons formed from the other aromatic precursors (cf. Figure 1B). The above results demonstrate that mesostructured carbons with graphitic framework walls can be conveniently prepared at ambient pressures through the impregnation of a silica template with an aromatic precursor and a catalyst. It is also clear that the electrical conductivity of resulting carbons in packed powder form is determined by the conductance of the individual grains and, equally importantly, the contacts between the grains. Precursors that promote the formation of graphitic framework walls are highly desirable in optimizing conductivity, but attention also must be given to mediating the grain size and grain interfaces of these materials for the future applications of these materials in electrochemical sensing and related materials applications. Acknowledgment. The support of this research through NSF-CRG Grant CHE-0211029 is gratefully acknowledged. LA049602C
