Langmuir 2002, 18, 7073-7076
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Synthesis and Characterization of Copper-Doped Carbon Aerogels Theodore F. Baumann,* Glenn A. Fox, and Joe H. Satcher, Jr. Lawrence Livermore National Laboratory, Livermore, California 94551
Noriko Yoshizawa, Ruowen Fu, and Mildred S. Dresselhaus Department of Electrical Engineering and Computer Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received May 2, 2002. In Final Form: June 18, 2002 We have prepared carbon aerogels (CAs) doped with copper through sol-gel polymerization of formaldehyde with the potassium salt of 2,4-dihydroxybenzoic acid, followed by ion exchange with Cu(NO3)2, supercritical drying with liquid CO2, and carbonization at 1050 °C under a N2 atmosphere. The materials were characterized by elemental analysis, nitrogen adsorption, high-resolution transmission electron microscopy, and X-ray diffraction. Results obtained indicate that this approach is an effective method for controlling both the amount and the distribution of a desired metal species within the carbon framework. We also found that carbonization of the copper-doped organic aerogels results in the formation of spherical copper nanoparticles within the carbon framework of the aerogel. The copper nanoparticles have a cubic crystalline structure and range in size from 10 to 50 nm. The Cu-doped CAs retain the overall open cell structure of metal-free CAs, exhibiting high surface areas and pore diameters in the micro- and mesoporic region.
Introduction Carbon aerogels (CAs) are novel mesoporous materials with many interesting properties, such as low mass densities, continuous porosities, high surface areas, and high electrical conductivity.1-4 These properties are derived from the aerogel microstructure, which is a network of interconnected primary particles with characteristic diameters between 3 and 25 nm. Because of their unusual chemical and textural characteristics, carbon aerogels are promising materials for use as electrode materials for super capacitors and rechargeable batteries, advanced catalyst supports, adsorbents, chromatographic packing, thermal insulators, and a variety of other applications.5-7 CAs are prepared through the sol-gel polymerization of resorcinol with formaldehyde in aqueous solution to produce organic gels that are supercritically dried and subsequently pyrolyzed in an inert atmosphere. To expand their potential applications, recent efforts have focused on modification of CAs through the use of dopants. One area of significant interest is in the incorporation of metal species into the carbon framework with the goal of modifying the structure, conductivity, and catalytic activity of the aerogel.8-15 Currently, syn(1) Pekala, R. W. J. Mater. Sci. 1989, 24, 3221. (2) Pekala, R. W.; Alviso, C. T.; Kong, F. M.; Hulsey, S. S. J. NonCryst. Solids 1992, 145, 90. (3) Pekala, R. W.; Alviso, C. T.; LeMay, J. D. In Chemical Processing of Advanced Materials; Hench, L. L., West, J. K., Eds.; J. Wiley & Sons: New York, 1992; p 671. (4) Kong, F. M.; LeMay, J. D.; Hulsey, S. S.; Alviso, C. T.; Pekala, R. W. J. Mater. Sci. 1993, 8, 3100. (5) Saliger, R.; Fischer, U.; Herta, C.; Fricke, J. J. Non-Cryst Solids 1998, 225, 81. (6) Yang, K. L.; Ying, T. Y.; Yiacoumi, S.; Tsouris, C.; Vittoratos, E. S. Langmuir 2001, 17, 1961. (7) Pekala, R. W.; Farmer, J. C.; Alviso, C. T.; Tran, T. D.; Mayer, S. T.; Miller, J. M.; Dunn, B. J. Non-Cryst Solids 1998, 225, 74. (8) Bekyarova, E.; Kaneko, K. Adv. Mater. 2000, 12, 1625. (9) Bekyarova, E.; Kaneko, K. Langmuir 1999, 15, 7119.
theses of metal-doped carbon aerogels involve the addition of the desired metal salt to the sol-gel polymerization reaction. Such a technique relies on the precipitation or crystallization of the metal salts within the pore network. While this approach has been used to prepare a variety of new materials, it is not reliable for the homogeneous distribution of metal ions throughout the aerogel. Our goal was to develop a method that would allow us to control both the amount and the distribution of a desired metal species within the carbon framework. Our strategy was the use of a resorcinol derivative containing an ionexchange moiety that could be polymerized using sol-gel techniques to produce metal-doped aerogels. As a result, each repeat unit of the organic polymer would contain a binding site for metal ions, ensuring a uniform dispersion of the dopant. Toward this goal, we have developed a new method for the preparation of metal-doped carbon aerogels in which resorcinol is replaced with the potassium salt of 2,4-dihydroxybenzoic acid in the sol-gel process, producing K+-doped hydrogels. The potassium ions in the gel can be replaced with the desired metal ion through an ion-exchange process, and the gels can then be dried and carbonized to generate novel metal-doped CAs. In this report, we present the synthesis of copper-doped carbon aerogels prepared by this method.16 One of the unique features of these novel materials is that carbonization of (10) Maldonado-Ho´dar, F. J.; Moreno-Castilla, C.; Rivera-Utrilla, J.; Hanzawa, Y.; Yamada, Y. Langmuir 2000, 16, 4367. (11) Maldonado-Ho´dar, F. J.; Ferro-Garcı´a, M. A.; Rivera-Utrilla, J.; Moreno-Castilla, C. Carbon 1999, 37, 1199. (12) Moreno-Castilla, C.; Maldonado-Ho´dar, F. J.; Rivera-Utrilla, J.; Rodrı´guez-Castello´n, E. Appl. Catal., A 1999, 183, 345. (13) Moreno-Castilla, C.; Maldonado-Ho´dar, F. J. Phys. Chem. Chem. Phys. 2000, 2, 4818. (14) Moreno-Castilla, C.; Maldonado-Ho´dar, F. J.; Carrasco-Marin, F.; Rodriguez-Castellon, E. Langmuir 2002, 18, 2295. (15) Miller, J. M.; Dunn, B. Langmuir 1999, 15, 799. (16) Yoshizawa, N.; Fu, R.; Dresselhaus, M. S.; Dresselhaus, G.; Satcher, J. H.; Baumann, T. F. Materials Research Society Symposium Proceedings, Nov 2001, Boston.
10.1021/la0259003 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/03/2002
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the copper-doped organic aerogels results in the formation of spherical copper nanoparticles within the carbon framework. The preparation of these materials and the characterization of their microstructure will be discussed. Experimental Section Sample Preparation. Metal-loaded organic aerogels (DF aerogels) were synthesized using methods analogous to those developed by Pekala for the preparation of resorcinol-formaldehyde (RF) aerogels.1 In a typical experiment, a suspension of 2,4-dihydroxybenzoic acid (2.9 g, 18.8 mmol) in distilled water (100 mL) was treated with K2CO3 (1.29 g, 9.4 mmol) with vigorous stirring. The reaction solution became clear after 0.5 h, when all of the acid was neutralized. Formaldehyde (2.98 g, 37 mmol) was then added to the solution, followed by the catalyst, K2CO3 (26 mg, 0.188 mmol). The clear solution was poured into glass molds that were then sealed, and the mixture was allowed to cure for 24 h at room temperature and 72 h at 80 °C. The resultant K+-loaded hydrogels were obtained as dark red, transparent monoliths. For preparation of the Cu2+-loaded organic gels, the K+-doped hydrogels were soaked in a 0.1 M aqueous solution of Cu(NO3)2 for 24 h; this procedure was repeated three times. Both the K+- and Cu2+-loaded hydrogels were washed with acetone until the water was completely exchanged and then dried with supercritical CO2 (Tc ) 31.1 °C, Pc ) 7.4 MPa). The above formulation generated DF aerogels with densities between 200 and 250 mg/cm3. Carbonization of the metal-loaded organic aerogels was performed at 1050 °C for 3 h under a N2 atmosphere. Following pyrolysis, the densities of the dark brown monoliths were 350-400 mg/cm3. Characterization. Surface area determination and pore volume and size analysis were performed by Brunauer-EmmettTeller (BET) and Barrett-Joyner-Halenda (BJH) methods using an ASAP 2000 surface area analyzer (Micromeritics Instrument Corp.).17 Samples of approximately 0.1 g were heated to 150 °C under vacuum (10-5 Torr) for at least 24 h to remove all adsorbed species. Nitrogen adsorption data were then taken at five relative pressures from 0.05 to 0.20 at 77 K to calculate the surface area by BET theory. Bulk densities of the DF and carbonized aerogels were determined by measuring the dimensions and mass of each monolithic sample. Elemental analyses were performed by Galbraith Laboratories, Knoxville, TN. High-resolution transmission electron microscopy (HRTEM) of the metal-doped aerogels was performed on a JEOL JEM-200CX operating at 200 keV. The images were taken under BF (bright field) conditions and slightly defocused to increase contrast. The X-ray diffraction (XRD) pattern was recorded using a Rigaku 300 X-ray diffractometer with the following measurement conditions: high voltage, 60 kV; current, 300 mA; divergence slit, 1°; scatter slit, 1°; receiving slit, 0.3°; scan mode, continuous; scan type, standard; axis, 2θ/θ-refl; scan, 5-85°; scan speed, 10°/min; sampling interval, 0.05°.
Results and Discussion Our objective was to establish a new method for the synthesis of metal-doped CAs that allows for better control over the amount and distribution of the metal species. Our strategy was to utilize resorcinol derivatives containing ion-exchange sites in the sol-gel polymerization process. We chose to start this work with 2,4-dihydroxybenzoic acid (2,4-DHBA) because it is an inexpensive, commercially available starting material and, more importantly, the carboxylate moiety would serve as the ion-exchange site for the incorporation of metal species into the gel framework. One concern in using resorcinol derivatives, however, was whether they would retain the reactivity of the parent resorcinol molecule. In the preparation of RF gels, resorcinol reacts with formaldehyde under alkaline conditions to form a mixture of addition and condensation products that then react further to form the cross-linked gel network. Due to the strongly activating nature of the hydroxy groups, the electrophilic substitution reactions occur at the 2,4,6 ring positions of
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resorcinol. With 2,4-DHBA, one of these ring positions is now occupied by a carboxylate moiety, which is deactivating with regard to electrophilic aromatic substitution. Nevertheless, sol-gel polymerization of the potassium salt of 2,4-DHBA with formaldehyde affords dark red, transparent monoliths. Clearly, the benzene ring of 2,4DHBA was still reactive enough to allow for the basecatalyzed gelation.18 Copper ions were introduced into the organic gel network through ion exchange. The monolithic wet gels containing potassium ions were soaked in an aqueous Cu(NO3)2 solution. To ensure that there was no precipitation or crystallization of residual copper nitrate in the pores of the gel, the monoliths were washed with water following ion exchange. On the basis of the elemental analysis of the dried aerogels, the exchange appears to be nearly complete, despite the fact that this process relies on the exchange of two potassium ions for one copper ion. The potassium content (6%) in the K+-loaded DF aerogel was lower than would be expected if every repeat unit in the polymer contained a potassium ion (19%). This indicates that some of the carboxylate moieties may have been protonated following gelation during the water wash. Following ion exchange, less than 0.01% of the K+ ions remained in the Cu2+-loaded DF aerogel, and assuming an exchange of two potassium ions for one copper ion, the incorporation of copper ions was over 90% complete. To determine the role that the carboxylate moiety plays in the metal ion incorporation, we attempted to prepare Cu2+-doped organic aerogels by two alternative methods using the parent resorcinol molecule in the sol-gel reaction. The first method used the technique reported in the literature in which the desired metal salt is added to the sol-gel mixture prior to gelation.8,10 We were unable to make RF gels when Cu(NO3)2 was added to the sol-gel solution, even with low Cu2+ concentrations. A possible explanation for this observation could be Cu2+ oxidation of resorcinol in solution during the cure cycle. The other method employed was treatment of wet RF gels with 0.1 M aqueous solutions of Cu(NO3)2 over an extended period of time. This method, however, was also ineffective as most, if not all, of the Cu(NO3)2 impregnated in the pores of the gels washed out of the gel during the acetone exchange step. Analysis of the dried materials resulting from this method showed that very little (
