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Direct synthesis of unimodal and bimodal nanoporous carbon
Microporous and Mesoporous Materials 74 (2004) 73–78 www.elsevier.com/locate/micromeso
Direct synthesis of unimodal and bimodal nanoporous carbon q Jiebin Pang a, Qingyuan Hu a, Zhiwang Wu a, J. Eric Hampsey a, Jibao He b, Yunfeng Lu a,* a b
Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, LA 70118, USA Microscopy Lab, Coordinated Instrumentation Facility, Tulane University, New Orleans, LA 70118, USA Received 1 March 2004; accepted 14 June 2004 Available online 20 July 2004
Abstract Unimodal and bimodal nanoporous (mesoporous) carbons have been synthesized by simply removing silica from carbon/silica nanocomposites. The sucrose/silica nanocomposites are directly prepared by the sol–gel process of tetraethyl orthosilicate (TEOS) with or without colloidal silica particles in the presence of carbon precursor molecules (i.e., sucrose). Subsequent carbonization converts the sucrose/silica nanocomposites into nonporous carbon/silica composites. Removal of the silica templates results in nanoporous carbons with high surface areas (e.g., >1500 m2 /g) and pore volumes (e.g., >1.0 cm3 /g). Using TEOS as the only silica source, nanoporous carbons with unimodal 2 nm worm-like pores are produced. Nanoporous carbons with bimodal 2 and 27 nm diameter pores have been prepared by using both the TEOS-derived silica network and the colloidal silica particles as templates. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Nanoporous carbon; Unimodal; Bimodal; Hierarchical; Templating synthesis
1. Introduction Porous carbon materials with high surface areas and pore volumes are of interest for energy storage, separation, catalysis, and other applications [1–5]. Templating techniques, which are generally based on carbonization of carbon precursors that are infiltrated within inorganic frameworks and subsequent removal of the inorganic frameworks to create porous networks, have been employed to synthesize porous carbon materials [4,6–9]. One of the most successful examples is the synthesis of mesoporous carbon using surfactanttemplated mesoporous silica frameworks as templates [4,6,7]. In this approach, the mesoporous silica templates are prepared by co-assembling silicates and surfactants into silicate/surfactant nanocomposites that contain ordered lyotropic liquid-crystalline mesophases. Removal of the surfactant results in mesoporous silica q Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2004.06.009. * Corresponding author. Tel.: + 1-504-865-5827; fax: +1-504-8656744. E-mail addresses: [email protected] (J. Pang), [email protected] (Y. Lu).
1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.06.009
with ordered hexagonal, cubic, or other mesostructured pore channels. Subsequent carbon precursor infiltration, carbonization, and silica removal processes lead to the formation of mesoporous carbon with ordered pore structure. This two-step process allows precise porestructure control through replicating the mesostructure of the silica templates. However, this process is tedious and has limitations, such as surfactant costs, incomplete infiltration of carbon precursors, formation of nonporous carbon on external surface of the mesoporous powders [10], and the difficulty in controlling their macroscopic morphologies [11,12]. To overcome these difficulties, recent research has developed direct pathways to synthesize nanoporous carbon [10,13,14]. These direct approaches often involve the formation of nanocomposites containing carbon precursors (e.g., cyclodextrin) and silica through the sol–gel process. Nanoporous carbon with high surface areas but relatively small pore diameters (e.g., from 1.6 to 4.0 nm) has been prepared after the carbonization and silica removal processes. Many applications such as chromatography and catalysis [15,16] require the synthesized nanoporous carbons to contain high surface areas, larger pore diameters, and interconnected hierarchical pores that allow rapid mass transport. Although nanoporous
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Scheme 1. Proposed nanoporous carbon formation mechanism.
carbons with larger pore diameters have been synthesized using colloidal silica particles as templates [5,17– 19], the materials often exhibit isolated pore structures [5,17a]. Nanoporous carbons containing bimodal or hierarchical pore structures [7b,12,20] have been recently synthesized using porous silica as templates. However, the syntheses were based on the two-step process and required the precise control of the amount of carbon precursor infiltrated [7b,20]. It is also difficult to control the pore structure and pore connectivity of those randomly formed interparticle textural pores [7b]. This work reports the direct synthesis of nanoporous carbon with controllable bimodal framework pore structures, high surface areas, and high pore volumes. Scheme 1 illustrates our synthesis strategy. First, sucrose/silica nanocomposites are prepared by reacting tetraethyl orthosilicate (TEOS) in the presence of colloidal silica particles and carbon precursor molecules (i.e., sucrose) through the sol–gel process. Subsequent carbonization process converts the sucrose/silica nanocomposites into nonporous carbon/silica composites. Removal of the silica templates results in nanoporous carbons, with pore sizes determined by the sizes of the added silica particles and of the silica network produced from sol–gel reactions of TEOS. Since the colloidal silica particles are commercially available with different diameters (e.g., 20–500 nm), this synthesis process provides an efficient route to synthesize nanoporous carbons with controllable hierarchical pore structures. Also, the use of low-cost sucrose instead of the expensive cyclodextrin10 as the carbon precursor enables the production of nanoporous carbon in a large quantity. To the best of our knowledge, this is the first report of nanoporous carbons with interconnected hierarchical pore structures by a direct synthesis approach.
2. Experimental 2.1. Synthesis To understand the formation of bimodal pore structure, nanoporous carbons with unimodal, small pore sizes were prepared by adding different amounts of su-
crose (0.4–1.5 g) to a silicate solution prepared by mixing 2.08 g of TEOS (Aldrich) with water, ethanol and HCl at 60 °C for 4 h. The molar ratio of TEOS: H2 O:EtOH:HCl was maintained at 1:6:6:0.01. To synthesize nanoporous carbons with bimodal pore structure, 0.9 g of sucrose was added to a silicate solution prepared by mixing 2.08 g of TEOS in a similar condition but a different TEOS:H2 O:EtOH:HCl molar ratio of 1:20:2:0.1. Then, 0.3 or 0.6 g of colloidal silica solutions (Nissan Chemical Snowtex-50, 20–30 nm, 50 wt%) were added into the mixtures under stirring, followed by ultrasonication for 5 min. The clear sucrose/ silicate mixtures were then cast into plastic molds and allowed to dry in ambient conditions for several days to form transparent sol–gel monoliths. These monoliths were carbonized at 900 °C for 4 h in nitrogen atmosphere with a heating rate of 2 °C/min. The carbon/silica composites were then ground into powders and washed using 2 wt% HF to remove the silica. Nanoporous carbon was obtained by filtration and washing with H2 O. The complete removal of silica from the carbon/silica nanocomposites was confirmed by X-ray energy dispersive spectroscopy (EDS) analyses and thermogravimetric analyses (TGA). For comparison, nanoporous silica was also prepared by calcinating the carbon/silica nanocomposites in air at 600 °C for 15 h. 2.2. Characterization TGA of the samples were performed on a Thermal Analysis Hi-Res TGA 2950 instrument with a heating rate of 5 °C/min, a heating range from 30 to 1000 °C, and an oxygen flow rate of 80 cm3 /min. The nitrogen adsorption–desorption isotherms were measured at 77 K using a Micromeritics ASAP 2010 analyzer. Before measurement, the samples were degassed at 200 °C and below 1.33 Pa for several hours. The pore size was calculated by Barrett–Joyner–Halenda (BJH) method based on the adsorption branch of the nitrogen sorption isotherms. Transmission electron microscopy (TEM) micrographs were obtained on a JEOL 2010 microscope operated at 120 kV. EDS analyses were performed on an Oxford Link ISIS 6498 spectrometer attached to the TEM instrument. Scanning electron microscopy (SEM)
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studies were performed on a JEOL JSM-5410 microscope operated at 20 kV.
3. Results and discussion The nitrogen adsorption/desorption studies suggest that the carbon/silica nanocomposites before the removal of silica or carbon are nonporous (e.g., 3 m2 /g of surface area and 0.0008 cm3 /g of pore volume) to nitrogen at 77 K. Removal of the silica or carbon from the nanocomposites results in nanoporous carbon (see Table 1) or nanoporous silica, respectively. Table 1 lists the surface areas and pore volumes of the nanoporous carbon synthesized using different amounts of sucrose and colloidal silica particles while fixing the amount of TEOS used as 2.08 g. Fig. 1 shows the N2 sorption isotherms of the nanoporous carbons prepared using TEOS as the only silica source. The nanoporous carbons prepared using 1.5, 1.2, and 0.9 g of sucrose exhibit isotherms with high surface areas and pore volumes (also see Table 1). The absence of nitrogen uptake at the relative pressure larger than 0.45 indicates the formation of uniform pores and the absence of interparticle porosity. The pore diameters calculated using the BJH model are around 2.0 nm. As shown in Table 1, decreasing the amount of sucrose from 1.5 to 0.9 g leads to an increased surface area and pore volume from 1526 m2 /g and 0.789 cm3 /g to 2314 m2 /g and 1.305 cm3 /g, demonstrating that the sol–gel silica framework effectively acts as templates for the formation of nanopores. However, further decreasing the amount of sucrose to 0.4 g results in nanoporous carbon with type IV isotherms with weak hysteresis [21], indicating broadened pore size distributions. The lowest surface area of 1076 m2 /g and pore volume of 0.717 cm3 / g resulted when 0.4 g of sucrose was used. The effect of the sucrose amount on the porosity and pore structure of the nanoporous carbon is expected. Decreasing the amount of sucrose increases the volume fraction of silica within the carbon/silica nanocomposites, resulting in
Fig. 1. N2 sorption isotherms (adsorption: solid symbols; desorption: open symbols) for the nanoporous carbon obtained from different carbon/silica nanocomposites.
nanoporous carbon with more porosity after the removal of silica. However, when the sucrose content is too low or the silica content is too high, carbon with decreased porosity is obtained. As expected, the carbon and silica networks are corresponding structures; therefore, the surface area and pore volume of the nanoporous silica after the removal of carbon generally decrease when the amount of sucrose added is decreased. Fig. 2 shows the N2 sorption isotherms of the nanoporous carbons with bimodal pore structure prepared by incorporating 0.3 and 0.6 g of colloidal silica solutions, respectively. Both isotherms exhibit significant nitrogen uptakes at the relative pressure below 0.5 and between
Table 1 Synthetic compositions, specific surface areas, and pore volumes of the nanoporous carbon obtained from different carbon/silica composites Samples C-1 C-2 C-3 C-4 C-3-1a C-3-2b
Sucrose (g) 1.5 1.2 0.9 0.4 0.9 0.9
Surface area (m2 /g) 1526 1678 2314 1076 1732 (27) 1630 (42)
Pore volume (cm3 /g) 0.789 0.903 1.305 0.717 1.369 (0.184) 1.451 (0.283)
Addition of 0.3 ga and 0.6 gb of 20–30 nm sized colloidal silica solution. The values in the bracket are the surface areas and pore volumes of pore diameter between 20 and 35 nm.
Fig. 2. N2 sorption isotherms (adsorption: solid symbols; desorption: open symbols) of the bimodal nanoporous carbons. Inset: Corresponding BJH pore size distribution derived from the adsorption branch of the sample C-3-1.
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0.85 and 0.95, which are mainly attributed from the small nanopores templated by the TEOS-derived silica frameworks and from the large nanopores templated by the colloidal silica particles, respectively. The absence of significant nitrogen uptake at relative pressures near 1.0 suggests no appreciable interparticle textural pores exist. The BJH adsorption pore size distributions (inset of Fig. 2) consistently reveal a bimodal pore size distribution centered at 2.2 and 27 nm, respectively. The larger pore size (27 nm) agrees well with the size of the colloidal silica particles (20–30 nm), indicating the effective template of the colloidal silica particles. The significant nitrogen adsorption uptakes at the similar relative pressure of 0.9 illustrate that both samples have identical larger-pore sizes. The distinguishable hysteresis loops are attributed from the interconnected large pores and small pores [21,22]. The nanoporous carbon C-3-1 shows a sharp nitrogen desorption step at the relative pressure around 0.50, further indicating that the larger pores are interconnected with the smaller pores. Increasing the amount of silica particles added results in the nanoporous carbon C-3-2 with a gradual nitrogen desorption from the relative pressure of 0.75 to 0.5, which indicates the formation of pore windows between
the larger pores and therefore an improved pore interconnectivity [22]. Fig. 3 shows the representative TEM images of the nanoporous carbons. Although it is difficult to directly measure its pore size, the nanoporous carbon C-3 shows disordered, uniform-sized, worm-like pores (see Fig. 3(a)) that are similar to those of the corresponding nanoporous silica [23,24] prepared by using organic molecules as templates. The use of colloidal silica particles as the secondary templates leads to the formation of nanoporous carbon with a bimodal pore structure. The TEM image of the nanoporous carbon C-3-1 (Fig. 3(b)) shows the presence of the larger pores (20–30 nm pore diameter) uniformly distributed within the nanoporous carbon framework. Increasing the amount of colloidal silica added improves the interconnectivity of the large pores by forming pore windows between the larger pores [22] (Fig. 3(c,d)), which is consistent with the nitrogen sorption isotherm shown in Fig. 2. The TEM images clearly reveal the formation of small and large framework nanopores, which are quite different compared with those of the reported bimodal mesoporous carbon that contains small framework mesopores and large interparticle textural pores [7b].
Fig. 3. Representative TEM images of the nanoporous carbons C-3 (a), C-3-1 (b), and C-3-2 (c,d).
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Fig. 4. Representative SEM images of the nanoporous carbons C-3 (a) and C-3-2 (b).
The representative SEM images of the nanoporous carbons are shown in Fig. 4. Since the samples were ground before washing with HF to remove silica, the largely granular carpolite-like morphology with particle size in micrometer scale indicates the original samples are monoliths in nature. Since the carbon precursor molecules are directly incorporated with the silica template during the synthesis, this direct synthesis approach affords the macroscopical morphology control of the nanoporous carbons, such as monoliths [10] and films [25], for prospective applications. 4. Conclusions In conclusion, nanoporous carbon with uniform 2 nm pore diameter, high surface areas, and high pore volumes have been successfully prepared by direct formation of sucrose/silica nanocomposites, subsequent carbonization, and silica removal processes. Nanoporous carbons with bimodal 2 and 27 nm diameter pores have also been produced by using both the TEOSderived silica network and the colloidal silica particles as templates. This facile synthesis method enables the large scale production of nanoporous carbon with high surface areas, high pore volumes, and pore structure control for gas separation, catalysis, chromatography, and other applications. Acknowledgements The authors gratefully acknowledge the financial support of this work by NASA (Grant No. NAG-102070 and NCC-3-946), Office of Naval Research, Louisiana Board of Regents (Grant No. LEQSF (200104)-RD-B-09) and National Science Foundation (Grant No. NSF-DMR-0124765). References [1] S. Sircar, T.C. Golden, M.B. Rao, Carbon 34 (1996) 1–12. [2] G.L. Che, B.B. Lakshmi, E.R. Fisher, C.R. Martin, Nature 393 (1998) 346–349.
[3] J.W. Jiang, J.B. Klauda, S.I. Sandler, Langmuir 19 (2003) 3512– 3518. [4] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169–172. [5] J.S. Yu, S. Kang, S.B. Yoon, G. Chai, J. Am. Chem. Soc. 124 (2002) 9382–9383. [6] (a) R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743– 7746; (b) R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Adv. Mater. 13 (2001) 677–681; (c) J.S. Lee, S.H. Joo, R. Ryoo, J. Am. Chem. Soc. 124 (2002) 1156–1157. [7] (a) J. Lee, S. Yoon, S.M. Oh, C.H. Shin, T. Hyeon, Adv. Mater. 12 (2000) 359–362; (b) J. Lee, J. Kim, T. Hyeon, Chem. Commun. (2003) 1138– 1139. [8] T.J. Bandosz, K. Putyera, J. Jagiello, J.A. Schwarz, Carbon 32 (1994) 659–664. [9] T. Kyotani, T. Nagai, S. Inoue, A. Tomita, Chem. Mater. 9 (1997) 609–615. [10] B.H. Han, W.Z. Zhou, A. Sayari, J. Am. Chem. Soc. 125 (2003) 3444–3445. [11] C.Z. Yu, J. Fan, B.Z. Tian, D.Y. Zhao, G.D. Stucky, Adv. Mater. 14 (2002) 1742–1745. [12] A. Taguchi, J.H. Sm att, M. Linden, Adv. Mater. 15 (2003) 1209– 1211. [13] S. Han, M. Kim, T. Hyeon, Carbon 41 (2003) 1525–1532. [14] D. Kawashima, T. Aihara, Y. Kobayashi, T. Kyotani, A. Tomita, Chem. Mater. 12 (2000) 3397–3401. [15] R.A. Caruso, M. Antonietti, Adv. Funct. Mater. 12 (2002) 307– 312. [16] J.H. Smatt, S. Schunk, M. Linden, Chem. Mater. 15 (2003) 2354– 2361. [17] (a) S.J. Han, T. Hyeon, Chem. Commun. (1999) 1955–1956; (b) S.J. Han, K. Sohn, T. Hyeon, Chem. Mater. 12 (2000) 3337– 3341. [18] Z. Li, M. Jaroniec, J. Am. Chem. Soc. 123 (2001) 9208– 9209. [19] S.B. Yoon, K. Sohn, J.Y. Kim, C.H. Shin, J.S. Yu, T. Hyeon, Adv. Mater. 14 (2002) 19–21. [20] A.H. Lu, W. Schmidt, B. Spliethoff, F. Sch€ uth, Adv. Mater. 15 (2003) 1602–1606. [21] (a) K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603–619; (b) S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, second ed., Academic Press, New York, 1982 (Chapter 3). [22] J. Lee, K. Sohn, T. Hyeon, J. Am. Chem. Soc. 123 (2001) 5146– 5147.
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[23] (a) Y. Wei, D.L. Jin, T.Z. Ding, W.H. Shih, X.H. Liu, S.Z.D. Cheng, Q. Fu, Adv. Mater. 10 (1998) 313–316; (b) J.B. Pang, K.Y. Qiu, Y. Wei, Chem. Mater. 13 (2001) 2361– 2365.
[24] S. Polarz, M. Antonietti, Chem. Commun. (2002) 2593– 2604. [25] J.B. Pang, X. Li, D.H. Wang, Z.W. Wu, V.T. John, Z.Z. Yang, Y.F. Lu, Adv. Mater. 16 (2004) 884–886.
Direct synthesis of unimodal and bimodal nanoporous carbon q Jiebin Pang a, Qingyuan Hu a, Zhiwang Wu a, J. Eric Hampsey a, Jibao He b, Yunfeng Lu a,* a b
Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, LA 70118, USA Microscopy Lab, Coordinated Instrumentation Facility, Tulane University, New Orleans, LA 70118, USA Received 1 March 2004; accepted 14 June 2004 Available online 20 July 2004
Abstract Unimodal and bimodal nanoporous (mesoporous) carbons have been synthesized by simply removing silica from carbon/silica nanocomposites. The sucrose/silica nanocomposites are directly prepared by the sol–gel process of tetraethyl orthosilicate (TEOS) with or without colloidal silica particles in the presence of carbon precursor molecules (i.e., sucrose). Subsequent carbonization converts the sucrose/silica nanocomposites into nonporous carbon/silica composites. Removal of the silica templates results in nanoporous carbons with high surface areas (e.g., >1500 m2 /g) and pore volumes (e.g., >1.0 cm3 /g). Using TEOS as the only silica source, nanoporous carbons with unimodal 2 nm worm-like pores are produced. Nanoporous carbons with bimodal 2 and 27 nm diameter pores have been prepared by using both the TEOS-derived silica network and the colloidal silica particles as templates. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Nanoporous carbon; Unimodal; Bimodal; Hierarchical; Templating synthesis
1. Introduction Porous carbon materials with high surface areas and pore volumes are of interest for energy storage, separation, catalysis, and other applications [1–5]. Templating techniques, which are generally based on carbonization of carbon precursors that are infiltrated within inorganic frameworks and subsequent removal of the inorganic frameworks to create porous networks, have been employed to synthesize porous carbon materials [4,6–9]. One of the most successful examples is the synthesis of mesoporous carbon using surfactanttemplated mesoporous silica frameworks as templates [4,6,7]. In this approach, the mesoporous silica templates are prepared by co-assembling silicates and surfactants into silicate/surfactant nanocomposites that contain ordered lyotropic liquid-crystalline mesophases. Removal of the surfactant results in mesoporous silica q Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2004.06.009. * Corresponding author. Tel.: + 1-504-865-5827; fax: +1-504-8656744. E-mail addresses: [email protected] (J. Pang), [email protected] (Y. Lu).
1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.06.009
with ordered hexagonal, cubic, or other mesostructured pore channels. Subsequent carbon precursor infiltration, carbonization, and silica removal processes lead to the formation of mesoporous carbon with ordered pore structure. This two-step process allows precise porestructure control through replicating the mesostructure of the silica templates. However, this process is tedious and has limitations, such as surfactant costs, incomplete infiltration of carbon precursors, formation of nonporous carbon on external surface of the mesoporous powders [10], and the difficulty in controlling their macroscopic morphologies [11,12]. To overcome these difficulties, recent research has developed direct pathways to synthesize nanoporous carbon [10,13,14]. These direct approaches often involve the formation of nanocomposites containing carbon precursors (e.g., cyclodextrin) and silica through the sol–gel process. Nanoporous carbon with high surface areas but relatively small pore diameters (e.g., from 1.6 to 4.0 nm) has been prepared after the carbonization and silica removal processes. Many applications such as chromatography and catalysis [15,16] require the synthesized nanoporous carbons to contain high surface areas, larger pore diameters, and interconnected hierarchical pores that allow rapid mass transport. Although nanoporous
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Scheme 1. Proposed nanoporous carbon formation mechanism.
carbons with larger pore diameters have been synthesized using colloidal silica particles as templates [5,17– 19], the materials often exhibit isolated pore structures [5,17a]. Nanoporous carbons containing bimodal or hierarchical pore structures [7b,12,20] have been recently synthesized using porous silica as templates. However, the syntheses were based on the two-step process and required the precise control of the amount of carbon precursor infiltrated [7b,20]. It is also difficult to control the pore structure and pore connectivity of those randomly formed interparticle textural pores [7b]. This work reports the direct synthesis of nanoporous carbon with controllable bimodal framework pore structures, high surface areas, and high pore volumes. Scheme 1 illustrates our synthesis strategy. First, sucrose/silica nanocomposites are prepared by reacting tetraethyl orthosilicate (TEOS) in the presence of colloidal silica particles and carbon precursor molecules (i.e., sucrose) through the sol–gel process. Subsequent carbonization process converts the sucrose/silica nanocomposites into nonporous carbon/silica composites. Removal of the silica templates results in nanoporous carbons, with pore sizes determined by the sizes of the added silica particles and of the silica network produced from sol–gel reactions of TEOS. Since the colloidal silica particles are commercially available with different diameters (e.g., 20–500 nm), this synthesis process provides an efficient route to synthesize nanoporous carbons with controllable hierarchical pore structures. Also, the use of low-cost sucrose instead of the expensive cyclodextrin10 as the carbon precursor enables the production of nanoporous carbon in a large quantity. To the best of our knowledge, this is the first report of nanoporous carbons with interconnected hierarchical pore structures by a direct synthesis approach.
2. Experimental 2.1. Synthesis To understand the formation of bimodal pore structure, nanoporous carbons with unimodal, small pore sizes were prepared by adding different amounts of su-
crose (0.4–1.5 g) to a silicate solution prepared by mixing 2.08 g of TEOS (Aldrich) with water, ethanol and HCl at 60 °C for 4 h. The molar ratio of TEOS: H2 O:EtOH:HCl was maintained at 1:6:6:0.01. To synthesize nanoporous carbons with bimodal pore structure, 0.9 g of sucrose was added to a silicate solution prepared by mixing 2.08 g of TEOS in a similar condition but a different TEOS:H2 O:EtOH:HCl molar ratio of 1:20:2:0.1. Then, 0.3 or 0.6 g of colloidal silica solutions (Nissan Chemical Snowtex-50, 20–30 nm, 50 wt%) were added into the mixtures under stirring, followed by ultrasonication for 5 min. The clear sucrose/ silicate mixtures were then cast into plastic molds and allowed to dry in ambient conditions for several days to form transparent sol–gel monoliths. These monoliths were carbonized at 900 °C for 4 h in nitrogen atmosphere with a heating rate of 2 °C/min. The carbon/silica composites were then ground into powders and washed using 2 wt% HF to remove the silica. Nanoporous carbon was obtained by filtration and washing with H2 O. The complete removal of silica from the carbon/silica nanocomposites was confirmed by X-ray energy dispersive spectroscopy (EDS) analyses and thermogravimetric analyses (TGA). For comparison, nanoporous silica was also prepared by calcinating the carbon/silica nanocomposites in air at 600 °C for 15 h. 2.2. Characterization TGA of the samples were performed on a Thermal Analysis Hi-Res TGA 2950 instrument with a heating rate of 5 °C/min, a heating range from 30 to 1000 °C, and an oxygen flow rate of 80 cm3 /min. The nitrogen adsorption–desorption isotherms were measured at 77 K using a Micromeritics ASAP 2010 analyzer. Before measurement, the samples were degassed at 200 °C and below 1.33 Pa for several hours. The pore size was calculated by Barrett–Joyner–Halenda (BJH) method based on the adsorption branch of the nitrogen sorption isotherms. Transmission electron microscopy (TEM) micrographs were obtained on a JEOL 2010 microscope operated at 120 kV. EDS analyses were performed on an Oxford Link ISIS 6498 spectrometer attached to the TEM instrument. Scanning electron microscopy (SEM)
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studies were performed on a JEOL JSM-5410 microscope operated at 20 kV.
3. Results and discussion The nitrogen adsorption/desorption studies suggest that the carbon/silica nanocomposites before the removal of silica or carbon are nonporous (e.g., 3 m2 /g of surface area and 0.0008 cm3 /g of pore volume) to nitrogen at 77 K. Removal of the silica or carbon from the nanocomposites results in nanoporous carbon (see Table 1) or nanoporous silica, respectively. Table 1 lists the surface areas and pore volumes of the nanoporous carbon synthesized using different amounts of sucrose and colloidal silica particles while fixing the amount of TEOS used as 2.08 g. Fig. 1 shows the N2 sorption isotherms of the nanoporous carbons prepared using TEOS as the only silica source. The nanoporous carbons prepared using 1.5, 1.2, and 0.9 g of sucrose exhibit isotherms with high surface areas and pore volumes (also see Table 1). The absence of nitrogen uptake at the relative pressure larger than 0.45 indicates the formation of uniform pores and the absence of interparticle porosity. The pore diameters calculated using the BJH model are around 2.0 nm. As shown in Table 1, decreasing the amount of sucrose from 1.5 to 0.9 g leads to an increased surface area and pore volume from 1526 m2 /g and 0.789 cm3 /g to 2314 m2 /g and 1.305 cm3 /g, demonstrating that the sol–gel silica framework effectively acts as templates for the formation of nanopores. However, further decreasing the amount of sucrose to 0.4 g results in nanoporous carbon with type IV isotherms with weak hysteresis [21], indicating broadened pore size distributions. The lowest surface area of 1076 m2 /g and pore volume of 0.717 cm3 / g resulted when 0.4 g of sucrose was used. The effect of the sucrose amount on the porosity and pore structure of the nanoporous carbon is expected. Decreasing the amount of sucrose increases the volume fraction of silica within the carbon/silica nanocomposites, resulting in
Fig. 1. N2 sorption isotherms (adsorption: solid symbols; desorption: open symbols) for the nanoporous carbon obtained from different carbon/silica nanocomposites.
nanoporous carbon with more porosity after the removal of silica. However, when the sucrose content is too low or the silica content is too high, carbon with decreased porosity is obtained. As expected, the carbon and silica networks are corresponding structures; therefore, the surface area and pore volume of the nanoporous silica after the removal of carbon generally decrease when the amount of sucrose added is decreased. Fig. 2 shows the N2 sorption isotherms of the nanoporous carbons with bimodal pore structure prepared by incorporating 0.3 and 0.6 g of colloidal silica solutions, respectively. Both isotherms exhibit significant nitrogen uptakes at the relative pressure below 0.5 and between
Table 1 Synthetic compositions, specific surface areas, and pore volumes of the nanoporous carbon obtained from different carbon/silica composites Samples C-1 C-2 C-3 C-4 C-3-1a C-3-2b
Sucrose (g) 1.5 1.2 0.9 0.4 0.9 0.9
Surface area (m2 /g) 1526 1678 2314 1076 1732 (27) 1630 (42)
Pore volume (cm3 /g) 0.789 0.903 1.305 0.717 1.369 (0.184) 1.451 (0.283)
Addition of 0.3 ga and 0.6 gb of 20–30 nm sized colloidal silica solution. The values in the bracket are the surface areas and pore volumes of pore diameter between 20 and 35 nm.
Fig. 2. N2 sorption isotherms (adsorption: solid symbols; desorption: open symbols) of the bimodal nanoporous carbons. Inset: Corresponding BJH pore size distribution derived from the adsorption branch of the sample C-3-1.
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0.85 and 0.95, which are mainly attributed from the small nanopores templated by the TEOS-derived silica frameworks and from the large nanopores templated by the colloidal silica particles, respectively. The absence of significant nitrogen uptake at relative pressures near 1.0 suggests no appreciable interparticle textural pores exist. The BJH adsorption pore size distributions (inset of Fig. 2) consistently reveal a bimodal pore size distribution centered at 2.2 and 27 nm, respectively. The larger pore size (27 nm) agrees well with the size of the colloidal silica particles (20–30 nm), indicating the effective template of the colloidal silica particles. The significant nitrogen adsorption uptakes at the similar relative pressure of 0.9 illustrate that both samples have identical larger-pore sizes. The distinguishable hysteresis loops are attributed from the interconnected large pores and small pores [21,22]. The nanoporous carbon C-3-1 shows a sharp nitrogen desorption step at the relative pressure around 0.50, further indicating that the larger pores are interconnected with the smaller pores. Increasing the amount of silica particles added results in the nanoporous carbon C-3-2 with a gradual nitrogen desorption from the relative pressure of 0.75 to 0.5, which indicates the formation of pore windows between
the larger pores and therefore an improved pore interconnectivity [22]. Fig. 3 shows the representative TEM images of the nanoporous carbons. Although it is difficult to directly measure its pore size, the nanoporous carbon C-3 shows disordered, uniform-sized, worm-like pores (see Fig. 3(a)) that are similar to those of the corresponding nanoporous silica [23,24] prepared by using organic molecules as templates. The use of colloidal silica particles as the secondary templates leads to the formation of nanoporous carbon with a bimodal pore structure. The TEM image of the nanoporous carbon C-3-1 (Fig. 3(b)) shows the presence of the larger pores (20–30 nm pore diameter) uniformly distributed within the nanoporous carbon framework. Increasing the amount of colloidal silica added improves the interconnectivity of the large pores by forming pore windows between the larger pores [22] (Fig. 3(c,d)), which is consistent with the nitrogen sorption isotherm shown in Fig. 2. The TEM images clearly reveal the formation of small and large framework nanopores, which are quite different compared with those of the reported bimodal mesoporous carbon that contains small framework mesopores and large interparticle textural pores [7b].
Fig. 3. Representative TEM images of the nanoporous carbons C-3 (a), C-3-1 (b), and C-3-2 (c,d).
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Fig. 4. Representative SEM images of the nanoporous carbons C-3 (a) and C-3-2 (b).
The representative SEM images of the nanoporous carbons are shown in Fig. 4. Since the samples were ground before washing with HF to remove silica, the largely granular carpolite-like morphology with particle size in micrometer scale indicates the original samples are monoliths in nature. Since the carbon precursor molecules are directly incorporated with the silica template during the synthesis, this direct synthesis approach affords the macroscopical morphology control of the nanoporous carbons, such as monoliths [10] and films [25], for prospective applications. 4. Conclusions In conclusion, nanoporous carbon with uniform 2 nm pore diameter, high surface areas, and high pore volumes have been successfully prepared by direct formation of sucrose/silica nanocomposites, subsequent carbonization, and silica removal processes. Nanoporous carbons with bimodal 2 and 27 nm diameter pores have also been produced by using both the TEOSderived silica network and the colloidal silica particles as templates. This facile synthesis method enables the large scale production of nanoporous carbon with high surface areas, high pore volumes, and pore structure control for gas separation, catalysis, chromatography, and other applications. Acknowledgements The authors gratefully acknowledge the financial support of this work by NASA (Grant No. NAG-102070 and NCC-3-946), Office of Naval Research, Louisiana Board of Regents (Grant No. LEQSF (200104)-RD-B-09) and National Science Foundation (Grant No. NSF-DMR-0124765). References [1] S. Sircar, T.C. Golden, M.B. Rao, Carbon 34 (1996) 1–12. [2] G.L. Che, B.B. Lakshmi, E.R. Fisher, C.R. Martin, Nature 393 (1998) 346–349.
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