Chem. Mater. 2005, 17, 1717-1721
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Surface Functionalization and Pore Size Manipulation for Carbons of Ordered Structure Zuojiang Li and Sheng Dai* Chemical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ReceiVed NoVember 22, 2004. ReVised Manuscript ReceiVed February 3, 2005
Covalent functionalization and pore size manipulation were successfully achieved on ordered mesoporous carbons, by using a method in which diazonium compounds were in-situ generated and reacted with the carbon surface. Aryl groups substituted on the 4-poisiton (Ar-R, R ) chlorine, ester, and alkyl) were covalently attached to the surface of hexagonally structured carbon, which was synthesized using ordered silica SBA-15 as template. The presence of these functional groups on the modified carbons was confirmed with Fourier transform infrared spectroscopy, thermogravimetric analysis, electron microscopy, and nitrogen adsorption. A relatively high grafting density of ∼1.5 µmol/m2 was achieved. Chemical modification considerably changed the pore width from 3.0 nm for the unmodified samples to ∼1.4 nm for modified samples, while the primary hexagonal structure and the unit parameter of ordered mesoporous carbon were retained after attachment of surface functionalities.
Introduction In the past several years, significant efforts have been devoted to the synthesis of mesoporous carbons with ordered porous structure, primarily due to their potential applications in separation, adsorption, and electronic devices.1 In most cases, ordered mesoporous carbons were synthesized by using silicas of ordered structures as the templates. Ryoo et al. first reported the highly ordered mesoporous carbon, which was synthesized by employing ordered silica MCM48 as template and sucrose as carbon precursor.1a The carbonization of sucrose inside pore channels of MCM-48 followed by removal of the silica framework rendered a carbon (CMK-1) with 3-D ordered pores (I41/a) and large surface area.1a,e Subsequently, ordered mesoporous carbons with different structures have been synthesized using a variety of templates and carbon precursors.2 The potential applications of these highly ordered carbons in separation, electronics, as well as sensing require a functionalized carbon surface with specific affinities and reactivities. Nevertheless, chemical modification of carbon surface is difficult because of the low reactivity of carbons. The conventional modifica* To whom correspondence should be addressed. Phone: (865) 576-7307. Fax: (865) 576-5235. E-mail:
[email protected].
(1) (a) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743. (b) Ryoo, R.; Joo, S. H.; Kruk, M.; Jaroniec, M. AdV. Mater. 2001, 13, 677. (c) Ryoo, R.; Park, I. S.; Jun, S.; Lee, C. W.; Kruk, M.; Jaroniec, M. J. Am. Chem. Soc. 2001, 123, 1650. (d) Lee, J.; Han, S.; Hyeon, T. J. Mater. Chem. 2004, 14, 478. (e) Kaneda, M.; Tsubakiyama, T.; Carlsson, A.; Sakamoto, Y.; Ohsuna, T.; Terasaki, O.; Joo, S. H.; Ryoo, R. J. Phys. Chem. B 2002, 106, 1256. (f) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, T. J. Am. Chem. Soc. 2000, 122, 10712. (2) (a) Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M. J. Phys. Chem. B 2002, 106, 4640. (b) Kim, T. W.; Park, I. S.; Ryoo, R. Angew. Chem., Int. Ed. 2003, 42, 4375. (c) Kruk, M.; Jaroniec, M.; Kim, T. W.; Ryoo, R. Chem. Mater. 2003, 15, 2815. (d) Vix-Guterl, C.; Saadallah, S.; Vidal, L.; Reda, M.; Parmentier, J.; Patarin, J. J. Mater. Chem. 2003, 13, 2535. (e) Li, Z. J.; Jaroniec, M. J. Phys. Chem. B 2004, 108, 824. (f) Tian, B. Z.; Che, S. N.; Liu, Z.; Liu, X. Y.; Fan, W. B.; Tatsumi, T.; Terasaki, O.; Zhao, D. Y. Chem. Commun. 2003, 2726.
tion of carbon surface involves oxidation with acids or ozone, during which the oxygenated functionalities such as carboxylic acid, esters, or quinones were generated on the carbon surface. The subsequent reaction of thionyl chloride with the carboxylic groups makes it possible to graft and further elaborate the surface properties.3 The drawback with this modification technique lies in the low degree of functionalization and the corrosion of the carbon surface during the oxidative treatment.4 Stimulated by the discovery of single-walled carbon nanotubes (SWNT),5 a number of methods were proposed to modify the surface of SWNTs,4a,6 including (a) noncovalent modification of SWNTs with polymers, amines, or organometallic compounds, (b) fluorination, (c) covalent modification via aryl diazonium or azomethine ylides chemistry, (d) modification of carbon surface with carbenes or nitrenes, and (e) reduction of carbon surface with lithium compounds.7-10 It is noteworthy that a significant degree of modification was reported on SWNTs through the routes of electrochemical reduction of diazonium salts9b or chemical (3) Kinoshita, K. Carbon Electrochemical and Physicochemical Properties; Wiley: New York, 1988. (4) (a) Bahr, J. L.; Tour, J. M. J. Mater. Chem. 2002, 12, 1952. (b) Martinhopkins, M. B.; Gilpin, R. K.; Jaroniec, M. J. Chromatogr. Sci. 1991, 29, 147. (5) (a) Ajayan, P. M.; Iijima, S. Nature 1992, 358, 23. (b) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (6) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853. (7) (a) Chen, R. J.; Zhan, Y. G.; Wang, D. W.; Dai, H. J. J. Am. Chem. Soc. 2001, 123, 3838. (b) Shim, M.; Javey, A.; Kam, N. W. S.; Dai, H. J. J. Am. Chem. Soc. 2001, 123, 11512. (c) Star, A.; Stoddart, J. F.; Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W.; Yang, X.; Chung, S. W.; Choi, H.; Heath, J. R. Angew. Chem., Int. Ed. 2001, 40, 1721. (8) Mickelson, E. T.; Chiang, I. W.; Zimmerman, J. L.; Boul, P. J.; Lozano, J.; Liu, J.; Smalley, R. E.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. B 1999, 103, 4318. (9) (a) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3823. (b) Bahr, J. L.; Yang, J. P.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536. (c) Dyke, C. A.; Tour, J. M. J. Am. Chem. Soc. 2003, 125, 1156.
10.1021/cm047954z CCC: $30.25 © 2005 American Chemical Society Published on Web 03/11/2005
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modification with diazonium compounds.9a,c As an efficient modification technique, the reaction between carbon surface and diazonium compounds can be dated back to the early 1990s when Delamar et al.11 modified glassy carbon by electrochemically reduction of diazonium salts in acetonitrile. Despite the intensive research on modification of SWNTs via diazonium chemistry, the studies on functionalization of ordered carbons are very limited,12 which seriously limits the applications of these novel ordered carbons. Here, we present the chemical functionalization of ordered carbons by using a solvent-free technique, in which the diazonium compound is in-situ generated and reacted with the carbon surface. The functionalized ordered carbons were studied with Fourier transform infrared (FTIR) spectroscopy, low temperature nitrogen adsorption, X-ray diffraction (XRD), and thermogravimetric analysis. A relatively high degree of grafting density was achieved by using this efficient modification method. Combination of the modified surface functionalities and ordered pore channels allows advanced applications of porous carbons in chemical analysis, separation, catalysis, and biotechnologies.12,13 Experimental Section Synthesis of Ordered Silica SBA-15. All of the chemicals used in the current research were purchased from Aldrich without further treatment. In a typical synthesis of ordered silica SBA-15, 16 g of block copolymer surfactant P123 (EO20PO70EO20) was dispersed in 500 mL of water in a plastic bottle, followed by addition of 100 g of concentrated HCl (35 wt %). Stirring was continued until a clear solution was obtained. Next, 34.5 g of tetraethoxysilane (TEOS) was added to the plastic bottle under vigorous stirring at 35 °C, and the stirring was continued for 2 h. The bottle was then put into oven at 35 °C for 24 h, followed by hydrothermal treatment at 100 °C for another 24 h. The product was filtered, and the surfactant was removed through calcinations in air at 550 °C for 10 h, using a heating rate of 1 °C/min. Synthesis of Ordered Carbon C15. Three grams of ordered silica SBA-15 was well mixed in ethanol with 3.0 g of fine powder of ground mesophase pitch (Mitsubishi). After ethanol was gradually evaporated, the mixture was put into a tube furnace at 360 °C for 2 h under nitrogen protection, followed by the structure stabilization of pitch at 220 °C in air for 10 h. The carbonization was carried out at 800 °C in nitrogen atmosphere. Finally, the silica template was removed by diluted hydrogen fluoride acid or concentrated NaOH solution, and the obtained carbon was denoted as C15. Modification of Ordered Carbon C15. Caution: Nitrogen gas is produced in the reaction. The reaction mixture can explode if heated or not well controlled. The outlet vent should always be open. (10) (a) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760. (b) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y. S.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (c) Chen, Y.; Haddon, R. C.; Fang, S.; Rao, A. M.; Lee, W. H.; Dickey, E. C.; Grulke, E. A.; Pendergrass, J. C.; Chavan, A.; Haley, B. E.; Smalley, R. E. J. Mater. Res. 1998, 13, 2423. (11) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883. (12) (a) Li, Z. J.; Del Cul, G. D.; Yan, W. F.; Liang, C. D.; Dai, S. J. Am. Chem. Soc. 2004, 126, 12782. (b) Jun, S.; Choi, M.; Ryu, S.; Lee, H. Y.; Ryoo, R. Nanotechnol. Mesostruct. Mater. 2003, 146, 37. (13) (a) Li, Z. J.; Jaroniec, M. Anal. Chem. 2004, 76, 5479. (b) Liang, C. D.; Dai, S.; Guiochon, G. Anal. Chem. 2003, 75, 4904. (c) Liang, C. D.; Hong, K. L.; Guiochon, G.; Mays, J. W.; Dai, S. Angew. Chem., Int. Ed. 2004, 43, 5785.
Li and Dai
Figure 1. Comparison of the thermogravimetric curves for unmodified carbon C15 and the modified samples C15-Cl, C15-Est, and C15-Buty. The weight change curve for the unmodified sample C15 was upshifted a value to offset the influence of adsorbed water at temperatures below 100 °C.
The chemical modification processes was carried out under nitrogen protection. In a typical modification process, ∼0.12 g of ordered carbon C15 was first dried overnight at 120 °C in a branched flask and then mixed with 1.2 g of 4-substituted anilines (4-chloroaniline, 4-tert-butylaniline, methyl 4-aminobenzoate). Later, 4 mL of isoamyl nitrite was added dropwise with a syringe under vigorous stirring. The stirring was continued for 1 h at room temperature, and the reaction temperature was gradually raised to 70 °C. Finally, the reaction was continued at 70 °C for 2 h. The reaction mixture was repeatedly washed with dimethyl formamide (DMF) and filtered. The obtained carbons were finally washed and filtered with hot chloroform three times and dried at 90 °C overnight. The obtained samples were denoted as C15-Cl, C15Buty, and C15-Est for 4-chloroaniline, 4-tert-butylaniline, and methyl 4-aminobenzoate modified samples, respectively. FTIR, TGA, SEM/TEM, and Adsorption Measurement. The Fourier transform infrared spectra for the unmodified and modified samples were measured on a DIGILAB FTS 7000 instrument under attenuated total reflection (ATR) mode using a diamond module. Thermogravimetric analysis (TGA) was carried out on a TGA 2950 (TA Instruments) instrument using a heating rate of 10 °C/min in nitrogen atmosphere. Low-temperature nitrogen adsorption isotherms were measured at 77 K on a Quantachrome Autosorb-1 volumetric analyzer. Samples were degassed at 105 °C in high vacuum (
