- Email: [email protected]
Preparation of a carbon monolith with bimodal perfusion pores
Microporous and Mesoporous Materials 115 (2008) 618–623
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Preparation of a carbon monolith with bimodal perfusion pores Lan-Ying Xu a,b, Zhi-Guo Shi a, Yu-Qi Feng a,* a b
Department of Chemistry, Wuhan University, Wuhan 430072, China Department of Chemistry, Huanggang Normal University, Huangzhou 438000, China
a r t i c l e
i n f o
Article history: Received 21 September 2007 Received in revised form 21 February 2008 Accepted 4 March 2008 Available online 18 March 2008 Keywords: Carbon Monolith Bimodal perfusion pores Template
a b s t r a c t We have initiated a method for the synthesis of a carbon monolith with trimodal porous structure of macro/macro/mesopores by nano-casting and phase separation. In this method, a silica monolith was employed as the hard template. A mixture of styrene and divinylbenzene, in the presence of initiator and porogenic reagent (dodecanol), was filled into the void of the silica monolith template. After the subsequent polymerization, carbonization and removal of the silica template, a carbon monolith with bimodal perfusion pores was successfully prepared. The primary perfusion pores were resulted from the dissolution of the silica template and the secondary perfusion pores were originated from the phase separation in the polymerization. In addition to macropores, mesopores also existed in the skeletons of the carbon monolith. Such structure of the carbon monolith has been characterized by scanning electron microscopy, mercury porosimetry and nitrogen sorption. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Porous carbons obtained by nanocasting inorganic silicas have attracted wide interest during the past decade [1–11]. These carbons, characterizing by high specific surface area, uniform textural pores and large pore volume, have been applied in many areas including catalysis, separation, adsorption and energy storage, etc. [12,13]. And most of the carbons are in the form of powder with size ranging from 1 to 10 lm. However, under certain circumstance the powder-like morphology make it difficult to use these materials directly. For example, as an electrode of a double layer capacitor, carbon powders must be glued to form into a flat layer. Another example, as separation or adsorption media, carbon powders must be packed into a column. The fine powders may bring high back pressure, leading to opearational inconvenience. A possible way to circumvent these problems is to fabricate carbon materials into monolithic shape directly. Recently, several groups have reported the successful synthesis of monolithic carbons by nanocasting different templates including silica monoliths [14–18], particle arrays [19,20] and polymer foams [21,22]. By deliberate selection of proper templates, the morphology and pore structure of the final carbon monoliths can be easily controlled to cater for various applications. To date, according to their porous structure, two kinds of carbon monoliths have been fabricated. Zhao and Mokaya et al. have utilized ordered mesoporous silica aggregates, via chemical vapor depostion or polymerization followed by carbonization, to prepare
* Corresponding author. Fax: +86 27 68754067. E-mail address: [email protected] (Y.-Q. Feng). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.03.004
carbon monoliths [15,16]. The as-obtained carbon monoliths are featured with high surface area and exculsively mesoporous strucuture. In most case these materials do not have much practical value because the interior of the carbons cannot be easily accessible [21,22]. Recently, several groups reported carbon monoliths with hierarchical macro/mesoporous structure [17,18,21,22]. The presence of large macropores (also through pores or perfusion pores) facilitates rapid transport of species to the interior of monoliths, whereas mesopores guarantee large surface area for interaction. The well-arranged, open porous structure of this kind of carbon monolith render it much useful for a variety of purposes. In the present work we report a new carbon monolith with trimodal macro/macro/mesoporous structure. A simple and versatile procedure, coupling nanocasting and phase separation, was proposed for its synthesis. The as-said carbon monolith exhibits structural macropores, macroporous skeletons and mesopores. This carbon monolith with such novel pore structure may find applications in fast catalysis, fast separation, etc. 2. Experimental 2.1. Materials and reagents Tetramethoxysilane (TMOS) was obtained from the Chemical Factory of Wuhan University (Wuhan, China). Styrene, dodecanol, poly (ethylene glycol) (PEG, Mw = 10,000), ethanol, sodium hydroxide (NaOH), acetic acid and 2,20 -azobis (isobutyronitrile) (AIBN) of analytical grade were purchased from Shanghai General Chemical Reagent Factory (Shanghai, China). Divinylbenzene from Aldrich (Milwaukee, USA) was extracted with 5% (w/v) NaOH solution
L.-Y. Xu et al. / Microporous and Mesoporous Materials 115 (2008) 618–623
and AIBN was recrystallized in ethanol prior to use. Double-distilled water was used in all experiments. 2.2. Preparation of silica monolith A silica monolith was prepared according to the method reported previously [18]. Briefly, 6 mL of TMOS was added into 10 mL of acetic acid (pH 3) in the presence of PEG (1.0 g). The mixture was stirred vigorously under ice bath until a homogeneous solution was formed. Then it was transferred into a disposable polypropylene syringe and allowed to gel at 313 K for 24 h. Subsequently, the obtained wet gels were aged in situ for another 48 h and then washed sequentially with massive water and ethanol to remove any possible residues. Finally they were carefully dried and calcined at 873 K for 2 h (ramped at 2 K/min) to form rigid silica monolith for following use.
619
sectional fractures were cut off from the silica monolith and the carbon monoliths, and were deposited with gold for SEM analysis at 30 kV. The macropore size distributions of the carbon monoliths were measured with an Autopore IV 9510 mercury porosimeter (Micromeritics, USA). The monolithic carbon materials were directly used for this testing. Nitrogen sorption experiments were performed on a Coulter SA 3100 Plus surface area and pore size analyzer (Florida, USA) at 77 K. The monolithic samples were broken into pieces and then activated by evacuating in vacuum at 473 K for 10 h to remove any physically adsorbed substances before analysis. The specific surface area values were calculated according to the BET (Brunauer–Emmett–Teller) equation at P/Po between 0.05 and 0.2. The pore size distributions were calculated from the desorption branches of isotherms based on BJH (Barrett–Joyner–Halenda) model.
2.3. Preparation of carbon monoliths
3. Results and discussion
The carbon precursor solution, including 1 mL of styrene (monomer), 2 mL of divinylbenzene (crosslinker), 2 mL of dodecanol (porogenic agent) and 15 mg of AIBN (initiator), was degassed by ultrasonicator for 5 min. Afterwards, a silica monolith prepared above was immersed in the solution until no infiltration was observed (the void of the silica monolith was filled completely). Subsequently, the filled monolith was taken out and sealed for polymerization. After being kept at 343 K for 6 h, a polymer-silica composite monolith was formed and then carbonized at 1173 K for 5 h under inert atmosphere. Finally the carbon monolith (designated as C-1) was obtained after removal of the silica template by treating with a 1.0 mol L1 NaOH solution. For comparison, a carbon monolith (designated as C-2) was prepared as the same procedures of C-1 in parallel but in the absence of dodecanol. Additionally, two carbon monoliths prepared without silica monolith templates have also been synthesized, and their difference lies in the presence or absence of dodecanol in carbon precursors. They were designated as C-3 (in the presence of dodecanol) and C-4 (in the absence of dodecanol), respectively.
Fig. 1 shows photographs of the silica templates and the corresponding carbon monoliths. Apparently, the monolithic carbons are successfully prepared via nanocasting by utilizing silica monoliths as templates. In comparison with their templates, C-1 displays ca. 15% shrinkage while almost no shrinkage occurs on C-2. Since their silica templates can be molded by the holding vessel, the size and shape of the carbon monoliths can be tailored conveniently, which has been demonstrated extensively [23,24]. Silica residue in the carbon monoliths was studied by TG analysis. Only 0.4% ash, as shown in Fig. 2, was found, suggesting that the silica template could be efficiently removed by NaOH corrosion. Fig. 3a–d show the SEM micrographs of the silica template, C-1, C-2 and C-3. Obviously, the silica template, C-1 and C-2 display similar macroscopic structure of interconnected skeletons and interwoven through pores while C-3 demonstrates a dense surface. The formation of bi-continuous structure of the silica monolith is ascribed to synergic interaction of phase separation and sol–gel transition, as extensively clarified in many documents [25,26]. The bi-continuous structure of the silica monolith provides suitable channels for the impregnation of liquid. Hence, carbon precursor solution can easily penetrate into the pores and fill them. The subsequent polymerization gelated the in situ liquid mixture into a three-dimensional solid network. In the following carbonization process, the copolymer of styrene and divinylbenzene decomposed first to produce polymer segments and small molecules. As the temperature further increased, they were dehydrogenated and formed carbon residue occupying the voids of the silica monolith.
2.4. Characterization of carbon monoliths Thermogravimetry (TG) analysis was carried out on a Diamond DSC TG-DTA 6300 thermal analyzer (PE, USA) to determine the silica residue in the resulting carbon monoliths. The measurement was done on dried carbon materials in air with a heating ramp of 10 K/min. Scanning electron microscope (SEM) micrographs were obtained using a Quanta 200 instument (FEI, Holand). Cross-
Fig. 1. Representative photographs of (a) silica template (left) and the carbon monolith C-1 (right); (b) silica template (left) and the carbon monolith C-2 (right).
620
L.-Y. Xu et al. / Microporous and Mesoporous Materials 115 (2008) 618–623
Fig. 2. TGA curve of the carbon monolith.
By NaOH eroding, the silica template in the composite was removed and evolved as through pores of the carbon monolith. The carbon residue derived from copolymer became backbone of the carbon monolith. Therefore, the obtained carbon monoliths (C-1 and C-2) resemble their templates, displaying bi-continuous
structure of skeletons and macropores. No macropore in C-3, as shown in Fig. 3d further suggests that the bi-continuous structure in C-1 and C-2 is resulted from their silica monolith templates. High resolution SEM on C-1 and C-2 demonstrated that they have different fine structure. It can be seen from Fig. 4a that a secondary bi-continuous structure exists in the skeletons of C-1, while the skeletons of C-2 are very smooth (Fig. 4b). The macropores in the skeletons of C-1 are in the range of 130–180 nm. The formation of this secondary bicontinuous structure of C-1 can be ascribed to the existence of dodecanol in the carbon precursor solution [27,28]. In the preparation of C-1, 2 mL of dodecanol was added to the carbon precursor solution. Dodecanol was used as the porogenic solvent in this case. Its solubility is poor in monomer and crosslinker, especially in oligomers and polymers. With the polymerization of styrene and divinylbenzene, Gibbs free energy of the system increased. In the presence of dodecanol, phase separation occurred and led to the formation of transient bi-continuous polymer-rich phase and solvent-rich phase. The polymerization froze the transient bi-continuous two phases as permanent morphology. Dodecanol was finally removed by calcination to form macropores, while the copolymer was developed into carbon skeletons simultaneously. Therefore, a secondary bi-continuous morphology was presented in the skeletons of C-1. However, in the case of C-2 preparation, no phase separation occurred because of the absence of
Fig. 3. SEM micrographs of: (a) silica template, (b) carbon monolith C-1, (c) carbon monolith C-2 and (d) carbon monolith C-3.
L.-Y. Xu et al. / Microporous and Mesoporous Materials 115 (2008) 618–623
621
Fig. 4. High resolution SEM micrographs of the skeletons of: (a) C-1 and (b) C-2.
Fig. 5. Differential pore diameter distribution of C-1 and C-2 determined by mercury porosimetry measurements. Offset: C-1, 4.6 mL/g.
dodecanol. As a result, the skeletons of C-2 were very smooth, and no secondary bi-continuous morphology was obtained. Fig. 5 displays the macropore size distribution obtained from the mercury porosimetry for carbon monoliths of C-1 and C-2. Obviously, bimodal macropore size distribution (2.5 lm with the pore volume of ca. 6.4 mL/g and 0.16 lm with the pore volume of ca. 0.7 mL/g) can be observed for C-1, while unimodal macropore size distribution (2.5 lm with the pore volume of ca. 4.1 mL/g) is presented for C-2. The pores of 2.5 lm for C-1 and for C-2 are obviously primary through pores of these two materials, which were resulted from the elimination of the silica template. Meanwhile, the secondary macropores of 0.16 lm for C-1 are channels in the skeletons that were derived from phase separation of the polymer. However, this kind of pores cannot be observed in the skeletons of C-2. These results are in good agreement with the above SEM observation (Fig. 4a and b), which fully demonstrates the hierarchical multi-level through pores of C-1. In addition to macropores, mesopores also exists in these carbon monoliths. Fig. 6 shows the nitrogen adsorption–desorption isotherms of the silica template, C-1 and C-2, respectively. It can be seen that the silica monolith and C-2 exhibit the typical type IV adsorption characteristic, while the isotherm of C-1 belongs to type II. All the isotherms show capillary condensation at medium
relative pressures, indicating the existence of mesopores. Compared to the silica template, C-1 and C-2 show remarkable adsorption uptake at pressures below 0.05 Po, which is the signal of the existence of micropores. As for C-1, the adsorption keeps increasing with the increase in relative pressure up to Po, suggesting macropores are also present in its skeletons. The skeleton pore size distributions of the silica monolith template, C-1 and C-2 calculated from desorption branches of the isotherms are plotted in the inset of Fig. 6. Apparently, the majority of the skeleton pores lie in mesopore range. Carbon monoliths, C-3 and C-4, were synthesized in the absence of silica monolith, and their nitrogen adsorption–desorption isotherms are shown in Fig. 7. It can be seen that the isotherm of C4 belongs to type l and no uptake appears at medium relative pressures, which is the characteristic of microporous materials. The isotherm of C-3 is similar to that of C-4, except that when the relative pressure further increases to Po, a little adsorption increase can be observed, suggesting macropores are also present. However, it is clear that mesopores do not exist in both of the carbon materials. Therefore, it can be concluded that the mesopores in the carbon monoliths, C-1 and C-2, prepared with silica templates are totally derived from the silica templates. The surface area and pore structure parameters of the silica monolith, C-1 and C-2 are summaried in Table 1. As shown in Table 1, the BET surface area is 661 m2/g for C-1, while 805 m2/g for C-2. The higher surface area of C-2 is ascribed to the massive micropores in it. The micropore surface area for C-2 is 404 m2/g, which is greater than that of C-1 (208 m2/g). Obviously, the difference in the micropore surface area was resulted from the different amount of carbon sources of them. In the preparation of C-1, dodecanol was added to induce phase separation. The added dodecanol would be was evaporated in the subsequent carbonization process, which led to the decrease in amount of the carbon source of C-1 and thus resulted in much less micropores in C-1. 4. Conclusions An easy route was successfully developed to prepare novel carbon monolith with bimodal perfusion pores. The larger perfusion pores (2.5 lm) were resulted from the elimination of the silica monolith template and the smaller ones (0.16 lm) were derived from phase separation induced by dodecanol. In addition, mesop-
622
L.-Y. Xu et al. / Microporous and Mesoporous Materials 115 (2008) 618–623
Fig. 6. Nitrogen adsorption–desorption isotherms and corresponding pore size distribution curves (inset) for silica template (-N-), C-1 (-d-) and C-2 (-j-).
Fig. 7. Nitrogen adsorption–desorption isotherms for C-3 (a) and C-4 (b).
Table 1 Structure parameters for the silica monolith and the corresponding carbon replicas of C-1 and C-2
Silica monolith C-1 C-2
Mean pore size (nm)
BET surface area (m2/g)
Micropore surface area (m2/g)
Total pore volume (mL/g)
Micropore volume (mL/g)
16.6 11.9 8.6
200 661 805
0 208 404
1.06 2.10 1.24
0 0.09 0.18
ores were also presented in the carbon skeletons to ensure high surface area of the material. Such novel structure of the carbon monolith is expected to facilitate fast mass transfer, and thus can be applied to a variety of applications such as separation science and catalysis. Acknowledgments This work was partly supported by Grants from the National Science Fund for Distinguished Young Scholars (Grant No.20625516)
L.-Y. Xu et al. / Microporous and Mesoporous Materials 115 (2008) 618–623
and the Nature Science Fund of Hubei Province (Grant: 2005ABA033). References [1] R. Ryoo, S.H. Joo, S.J. Jun, J. Phys. Chem. B 103 (1999) 7743. [2] R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Adv. Mater. 13 (2001) 677. [3] S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 122 (2000) 10712. [4] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. [5] S.S. Kim, T.J. Pinnavaia, Chem. Commun. (2001) 2418. [6] S.B. Yoon, J.Y. Kim, J.S. Yu, Chem. Commun. (2001) 559. [7] S.J. Han, T. Hyeon, Chem. Commun. (1999) 1955. [8] S.J. Han, T. Hyeon, Carbon 37 (1999) 1645. [9] J. Lee, K. Sohn, T. Hyeon, J. Am. Chem. Soc. 123 (2001) 5146. [10] J. Lee, K. Sohn, T. Hyeon, Chem. Commun. (2002) 2674. [11] J. Lee, J. Kim, T. Hyeon, Chem. Commun. (2003) 1138. [12] H.C. Foley, Microporous Mater. 4 (1995) 407. [13] T. Kyotani, Carbon 38 (2000) 269. [14] B.H. Han, W.Z. Zhou, A. Sayari, J. Am. Chem. Soc. 125 (2003) 3444.
623
[15] H.F. Yang, Q.H. Shi, X.Y. Liu, S.H. Xie, D.C. Jiang, F.Q. Zhang, C.Z. Yu, B. Tu, D.Y. Zhao, Chem. Commun. (2002) 2842. [16] Y.D. Xia, R. Mokaya, J. Phys. Chem. C 111 (2007) 10035. [17] A. Taguchi, J.H. Smatt, M. Linden, Adv. Mater. 15 (2003) 1209. [18] Z.G. Shi, Y.Q. Feng, L. Xu, S.L. Da, M. Zhang, Carbon 41 (2003) 2677. [19] O. Klepel, H. Straub, A. Garsuch, K. Bohme, Mater. Lett. 61 (2007) 2037. [20] X.Q. Wang, K.N. Bozhilov, P.Y. Feng, Chem. Mater. 18 (2006) 6373. [21] S. Alvarez, J. Esquena, C. Solans, A.B. Fuertes, Adv. Eng. Mater. 6 (2004) 897. [22] S. Alvarez, A.B. Fuertes, Mater. Lett. 61 (2007) 2378. [23] Z.G. Shi, Y.Q. Feng, L. Xu, S.L. Da, M. Zhang, Carbon 42 (2004) 1677. [24] L.F. Wang, S. Lin, K.F. Lin, C.Y. Yin, D.S. Liang, Y. Di, P.W. Fan, D.Z. Jiang, F.S. Xiao, Micropor. Mesopor. Mater. 85 (2005) 136. [25] H. Minakuchi, K. Nakanishi, N. Soga, N. Ishizuka, N. Tanaka, J. Chromatogr. A 797 (1998) 121. [26] K. Kataoka, O. Urakawa, H. Nishioka, Q. Tran-Cong, Macromolecules 31 (1998) 8809. [27] X.D. Huang, H.F. Zou, X.M. Chen, Q.Z. Luo, L. Kong, J. Chromatogr. A 984 (2003) 273. [28] L.Q. Lin, Y.C. Li, Q. Fu, L.C. He, J. Zhang, Q.Q. Zhang, Polymer 47 (2006) 3792.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Preparation of a carbon monolith with bimodal perfusion pores Lan-Ying Xu a,b, Zhi-Guo Shi a, Yu-Qi Feng a,* a b
Department of Chemistry, Wuhan University, Wuhan 430072, China Department of Chemistry, Huanggang Normal University, Huangzhou 438000, China
a r t i c l e
i n f o
Article history: Received 21 September 2007 Received in revised form 21 February 2008 Accepted 4 March 2008 Available online 18 March 2008 Keywords: Carbon Monolith Bimodal perfusion pores Template
a b s t r a c t We have initiated a method for the synthesis of a carbon monolith with trimodal porous structure of macro/macro/mesopores by nano-casting and phase separation. In this method, a silica monolith was employed as the hard template. A mixture of styrene and divinylbenzene, in the presence of initiator and porogenic reagent (dodecanol), was filled into the void of the silica monolith template. After the subsequent polymerization, carbonization and removal of the silica template, a carbon monolith with bimodal perfusion pores was successfully prepared. The primary perfusion pores were resulted from the dissolution of the silica template and the secondary perfusion pores were originated from the phase separation in the polymerization. In addition to macropores, mesopores also existed in the skeletons of the carbon monolith. Such structure of the carbon monolith has been characterized by scanning electron microscopy, mercury porosimetry and nitrogen sorption. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Porous carbons obtained by nanocasting inorganic silicas have attracted wide interest during the past decade [1–11]. These carbons, characterizing by high specific surface area, uniform textural pores and large pore volume, have been applied in many areas including catalysis, separation, adsorption and energy storage, etc. [12,13]. And most of the carbons are in the form of powder with size ranging from 1 to 10 lm. However, under certain circumstance the powder-like morphology make it difficult to use these materials directly. For example, as an electrode of a double layer capacitor, carbon powders must be glued to form into a flat layer. Another example, as separation or adsorption media, carbon powders must be packed into a column. The fine powders may bring high back pressure, leading to opearational inconvenience. A possible way to circumvent these problems is to fabricate carbon materials into monolithic shape directly. Recently, several groups have reported the successful synthesis of monolithic carbons by nanocasting different templates including silica monoliths [14–18], particle arrays [19,20] and polymer foams [21,22]. By deliberate selection of proper templates, the morphology and pore structure of the final carbon monoliths can be easily controlled to cater for various applications. To date, according to their porous structure, two kinds of carbon monoliths have been fabricated. Zhao and Mokaya et al. have utilized ordered mesoporous silica aggregates, via chemical vapor depostion or polymerization followed by carbonization, to prepare
* Corresponding author. Fax: +86 27 68754067. E-mail address: [email protected] (Y.-Q. Feng). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.03.004
carbon monoliths [15,16]. The as-obtained carbon monoliths are featured with high surface area and exculsively mesoporous strucuture. In most case these materials do not have much practical value because the interior of the carbons cannot be easily accessible [21,22]. Recently, several groups reported carbon monoliths with hierarchical macro/mesoporous structure [17,18,21,22]. The presence of large macropores (also through pores or perfusion pores) facilitates rapid transport of species to the interior of monoliths, whereas mesopores guarantee large surface area for interaction. The well-arranged, open porous structure of this kind of carbon monolith render it much useful for a variety of purposes. In the present work we report a new carbon monolith with trimodal macro/macro/mesoporous structure. A simple and versatile procedure, coupling nanocasting and phase separation, was proposed for its synthesis. The as-said carbon monolith exhibits structural macropores, macroporous skeletons and mesopores. This carbon monolith with such novel pore structure may find applications in fast catalysis, fast separation, etc. 2. Experimental 2.1. Materials and reagents Tetramethoxysilane (TMOS) was obtained from the Chemical Factory of Wuhan University (Wuhan, China). Styrene, dodecanol, poly (ethylene glycol) (PEG, Mw = 10,000), ethanol, sodium hydroxide (NaOH), acetic acid and 2,20 -azobis (isobutyronitrile) (AIBN) of analytical grade were purchased from Shanghai General Chemical Reagent Factory (Shanghai, China). Divinylbenzene from Aldrich (Milwaukee, USA) was extracted with 5% (w/v) NaOH solution
L.-Y. Xu et al. / Microporous and Mesoporous Materials 115 (2008) 618–623
and AIBN was recrystallized in ethanol prior to use. Double-distilled water was used in all experiments. 2.2. Preparation of silica monolith A silica monolith was prepared according to the method reported previously [18]. Briefly, 6 mL of TMOS was added into 10 mL of acetic acid (pH 3) in the presence of PEG (1.0 g). The mixture was stirred vigorously under ice bath until a homogeneous solution was formed. Then it was transferred into a disposable polypropylene syringe and allowed to gel at 313 K for 24 h. Subsequently, the obtained wet gels were aged in situ for another 48 h and then washed sequentially with massive water and ethanol to remove any possible residues. Finally they were carefully dried and calcined at 873 K for 2 h (ramped at 2 K/min) to form rigid silica monolith for following use.
619
sectional fractures were cut off from the silica monolith and the carbon monoliths, and were deposited with gold for SEM analysis at 30 kV. The macropore size distributions of the carbon monoliths were measured with an Autopore IV 9510 mercury porosimeter (Micromeritics, USA). The monolithic carbon materials were directly used for this testing. Nitrogen sorption experiments were performed on a Coulter SA 3100 Plus surface area and pore size analyzer (Florida, USA) at 77 K. The monolithic samples were broken into pieces and then activated by evacuating in vacuum at 473 K for 10 h to remove any physically adsorbed substances before analysis. The specific surface area values were calculated according to the BET (Brunauer–Emmett–Teller) equation at P/Po between 0.05 and 0.2. The pore size distributions were calculated from the desorption branches of isotherms based on BJH (Barrett–Joyner–Halenda) model.
2.3. Preparation of carbon monoliths
3. Results and discussion
The carbon precursor solution, including 1 mL of styrene (monomer), 2 mL of divinylbenzene (crosslinker), 2 mL of dodecanol (porogenic agent) and 15 mg of AIBN (initiator), was degassed by ultrasonicator for 5 min. Afterwards, a silica monolith prepared above was immersed in the solution until no infiltration was observed (the void of the silica monolith was filled completely). Subsequently, the filled monolith was taken out and sealed for polymerization. After being kept at 343 K for 6 h, a polymer-silica composite monolith was formed and then carbonized at 1173 K for 5 h under inert atmosphere. Finally the carbon monolith (designated as C-1) was obtained after removal of the silica template by treating with a 1.0 mol L1 NaOH solution. For comparison, a carbon monolith (designated as C-2) was prepared as the same procedures of C-1 in parallel but in the absence of dodecanol. Additionally, two carbon monoliths prepared without silica monolith templates have also been synthesized, and their difference lies in the presence or absence of dodecanol in carbon precursors. They were designated as C-3 (in the presence of dodecanol) and C-4 (in the absence of dodecanol), respectively.
Fig. 1 shows photographs of the silica templates and the corresponding carbon monoliths. Apparently, the monolithic carbons are successfully prepared via nanocasting by utilizing silica monoliths as templates. In comparison with their templates, C-1 displays ca. 15% shrinkage while almost no shrinkage occurs on C-2. Since their silica templates can be molded by the holding vessel, the size and shape of the carbon monoliths can be tailored conveniently, which has been demonstrated extensively [23,24]. Silica residue in the carbon monoliths was studied by TG analysis. Only 0.4% ash, as shown in Fig. 2, was found, suggesting that the silica template could be efficiently removed by NaOH corrosion. Fig. 3a–d show the SEM micrographs of the silica template, C-1, C-2 and C-3. Obviously, the silica template, C-1 and C-2 display similar macroscopic structure of interconnected skeletons and interwoven through pores while C-3 demonstrates a dense surface. The formation of bi-continuous structure of the silica monolith is ascribed to synergic interaction of phase separation and sol–gel transition, as extensively clarified in many documents [25,26]. The bi-continuous structure of the silica monolith provides suitable channels for the impregnation of liquid. Hence, carbon precursor solution can easily penetrate into the pores and fill them. The subsequent polymerization gelated the in situ liquid mixture into a three-dimensional solid network. In the following carbonization process, the copolymer of styrene and divinylbenzene decomposed first to produce polymer segments and small molecules. As the temperature further increased, they were dehydrogenated and formed carbon residue occupying the voids of the silica monolith.
2.4. Characterization of carbon monoliths Thermogravimetry (TG) analysis was carried out on a Diamond DSC TG-DTA 6300 thermal analyzer (PE, USA) to determine the silica residue in the resulting carbon monoliths. The measurement was done on dried carbon materials in air with a heating ramp of 10 K/min. Scanning electron microscope (SEM) micrographs were obtained using a Quanta 200 instument (FEI, Holand). Cross-
Fig. 1. Representative photographs of (a) silica template (left) and the carbon monolith C-1 (right); (b) silica template (left) and the carbon monolith C-2 (right).
620
L.-Y. Xu et al. / Microporous and Mesoporous Materials 115 (2008) 618–623
Fig. 2. TGA curve of the carbon monolith.
By NaOH eroding, the silica template in the composite was removed and evolved as through pores of the carbon monolith. The carbon residue derived from copolymer became backbone of the carbon monolith. Therefore, the obtained carbon monoliths (C-1 and C-2) resemble their templates, displaying bi-continuous
structure of skeletons and macropores. No macropore in C-3, as shown in Fig. 3d further suggests that the bi-continuous structure in C-1 and C-2 is resulted from their silica monolith templates. High resolution SEM on C-1 and C-2 demonstrated that they have different fine structure. It can be seen from Fig. 4a that a secondary bi-continuous structure exists in the skeletons of C-1, while the skeletons of C-2 are very smooth (Fig. 4b). The macropores in the skeletons of C-1 are in the range of 130–180 nm. The formation of this secondary bicontinuous structure of C-1 can be ascribed to the existence of dodecanol in the carbon precursor solution [27,28]. In the preparation of C-1, 2 mL of dodecanol was added to the carbon precursor solution. Dodecanol was used as the porogenic solvent in this case. Its solubility is poor in monomer and crosslinker, especially in oligomers and polymers. With the polymerization of styrene and divinylbenzene, Gibbs free energy of the system increased. In the presence of dodecanol, phase separation occurred and led to the formation of transient bi-continuous polymer-rich phase and solvent-rich phase. The polymerization froze the transient bi-continuous two phases as permanent morphology. Dodecanol was finally removed by calcination to form macropores, while the copolymer was developed into carbon skeletons simultaneously. Therefore, a secondary bi-continuous morphology was presented in the skeletons of C-1. However, in the case of C-2 preparation, no phase separation occurred because of the absence of
Fig. 3. SEM micrographs of: (a) silica template, (b) carbon monolith C-1, (c) carbon monolith C-2 and (d) carbon monolith C-3.
L.-Y. Xu et al. / Microporous and Mesoporous Materials 115 (2008) 618–623
621
Fig. 4. High resolution SEM micrographs of the skeletons of: (a) C-1 and (b) C-2.
Fig. 5. Differential pore diameter distribution of C-1 and C-2 determined by mercury porosimetry measurements. Offset: C-1, 4.6 mL/g.
dodecanol. As a result, the skeletons of C-2 were very smooth, and no secondary bi-continuous morphology was obtained. Fig. 5 displays the macropore size distribution obtained from the mercury porosimetry for carbon monoliths of C-1 and C-2. Obviously, bimodal macropore size distribution (2.5 lm with the pore volume of ca. 6.4 mL/g and 0.16 lm with the pore volume of ca. 0.7 mL/g) can be observed for C-1, while unimodal macropore size distribution (2.5 lm with the pore volume of ca. 4.1 mL/g) is presented for C-2. The pores of 2.5 lm for C-1 and for C-2 are obviously primary through pores of these two materials, which were resulted from the elimination of the silica template. Meanwhile, the secondary macropores of 0.16 lm for C-1 are channels in the skeletons that were derived from phase separation of the polymer. However, this kind of pores cannot be observed in the skeletons of C-2. These results are in good agreement with the above SEM observation (Fig. 4a and b), which fully demonstrates the hierarchical multi-level through pores of C-1. In addition to macropores, mesopores also exists in these carbon monoliths. Fig. 6 shows the nitrogen adsorption–desorption isotherms of the silica template, C-1 and C-2, respectively. It can be seen that the silica monolith and C-2 exhibit the typical type IV adsorption characteristic, while the isotherm of C-1 belongs to type II. All the isotherms show capillary condensation at medium
relative pressures, indicating the existence of mesopores. Compared to the silica template, C-1 and C-2 show remarkable adsorption uptake at pressures below 0.05 Po, which is the signal of the existence of micropores. As for C-1, the adsorption keeps increasing with the increase in relative pressure up to Po, suggesting macropores are also present in its skeletons. The skeleton pore size distributions of the silica monolith template, C-1 and C-2 calculated from desorption branches of the isotherms are plotted in the inset of Fig. 6. Apparently, the majority of the skeleton pores lie in mesopore range. Carbon monoliths, C-3 and C-4, were synthesized in the absence of silica monolith, and their nitrogen adsorption–desorption isotherms are shown in Fig. 7. It can be seen that the isotherm of C4 belongs to type l and no uptake appears at medium relative pressures, which is the characteristic of microporous materials. The isotherm of C-3 is similar to that of C-4, except that when the relative pressure further increases to Po, a little adsorption increase can be observed, suggesting macropores are also present. However, it is clear that mesopores do not exist in both of the carbon materials. Therefore, it can be concluded that the mesopores in the carbon monoliths, C-1 and C-2, prepared with silica templates are totally derived from the silica templates. The surface area and pore structure parameters of the silica monolith, C-1 and C-2 are summaried in Table 1. As shown in Table 1, the BET surface area is 661 m2/g for C-1, while 805 m2/g for C-2. The higher surface area of C-2 is ascribed to the massive micropores in it. The micropore surface area for C-2 is 404 m2/g, which is greater than that of C-1 (208 m2/g). Obviously, the difference in the micropore surface area was resulted from the different amount of carbon sources of them. In the preparation of C-1, dodecanol was added to induce phase separation. The added dodecanol would be was evaporated in the subsequent carbonization process, which led to the decrease in amount of the carbon source of C-1 and thus resulted in much less micropores in C-1. 4. Conclusions An easy route was successfully developed to prepare novel carbon monolith with bimodal perfusion pores. The larger perfusion pores (2.5 lm) were resulted from the elimination of the silica monolith template and the smaller ones (0.16 lm) were derived from phase separation induced by dodecanol. In addition, mesop-
622
L.-Y. Xu et al. / Microporous and Mesoporous Materials 115 (2008) 618–623
Fig. 6. Nitrogen adsorption–desorption isotherms and corresponding pore size distribution curves (inset) for silica template (-N-), C-1 (-d-) and C-2 (-j-).
Fig. 7. Nitrogen adsorption–desorption isotherms for C-3 (a) and C-4 (b).
Table 1 Structure parameters for the silica monolith and the corresponding carbon replicas of C-1 and C-2
Silica monolith C-1 C-2
Mean pore size (nm)
BET surface area (m2/g)
Micropore surface area (m2/g)
Total pore volume (mL/g)
Micropore volume (mL/g)
16.6 11.9 8.6
200 661 805
0 208 404
1.06 2.10 1.24
0 0.09 0.18
ores were also presented in the carbon skeletons to ensure high surface area of the material. Such novel structure of the carbon monolith is expected to facilitate fast mass transfer, and thus can be applied to a variety of applications such as separation science and catalysis. Acknowledgments This work was partly supported by Grants from the National Science Fund for Distinguished Young Scholars (Grant No.20625516)
L.-Y. Xu et al. / Microporous and Mesoporous Materials 115 (2008) 618–623
and the Nature Science Fund of Hubei Province (Grant: 2005ABA033). References [1] R. Ryoo, S.H. Joo, S.J. Jun, J. Phys. Chem. B 103 (1999) 7743. [2] R. Ryoo, S.H. Joo, M. Kruk, M. Jaroniec, Adv. Mater. 13 (2001) 677. [3] S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 122 (2000) 10712. [4] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. [5] S.S. Kim, T.J. Pinnavaia, Chem. Commun. (2001) 2418. [6] S.B. Yoon, J.Y. Kim, J.S. Yu, Chem. Commun. (2001) 559. [7] S.J. Han, T. Hyeon, Chem. Commun. (1999) 1955. [8] S.J. Han, T. Hyeon, Carbon 37 (1999) 1645. [9] J. Lee, K. Sohn, T. Hyeon, J. Am. Chem. Soc. 123 (2001) 5146. [10] J. Lee, K. Sohn, T. Hyeon, Chem. Commun. (2002) 2674. [11] J. Lee, J. Kim, T. Hyeon, Chem. Commun. (2003) 1138. [12] H.C. Foley, Microporous Mater. 4 (1995) 407. [13] T. Kyotani, Carbon 38 (2000) 269. [14] B.H. Han, W.Z. Zhou, A. Sayari, J. Am. Chem. Soc. 125 (2003) 3444.
623
[15] H.F. Yang, Q.H. Shi, X.Y. Liu, S.H. Xie, D.C. Jiang, F.Q. Zhang, C.Z. Yu, B. Tu, D.Y. Zhao, Chem. Commun. (2002) 2842. [16] Y.D. Xia, R. Mokaya, J. Phys. Chem. C 111 (2007) 10035. [17] A. Taguchi, J.H. Smatt, M. Linden, Adv. Mater. 15 (2003) 1209. [18] Z.G. Shi, Y.Q. Feng, L. Xu, S.L. Da, M. Zhang, Carbon 41 (2003) 2677. [19] O. Klepel, H. Straub, A. Garsuch, K. Bohme, Mater. Lett. 61 (2007) 2037. [20] X.Q. Wang, K.N. Bozhilov, P.Y. Feng, Chem. Mater. 18 (2006) 6373. [21] S. Alvarez, J. Esquena, C. Solans, A.B. Fuertes, Adv. Eng. Mater. 6 (2004) 897. [22] S. Alvarez, A.B. Fuertes, Mater. Lett. 61 (2007) 2378. [23] Z.G. Shi, Y.Q. Feng, L. Xu, S.L. Da, M. Zhang, Carbon 42 (2004) 1677. [24] L.F. Wang, S. Lin, K.F. Lin, C.Y. Yin, D.S. Liang, Y. Di, P.W. Fan, D.Z. Jiang, F.S. Xiao, Micropor. Mesopor. Mater. 85 (2005) 136. [25] H. Minakuchi, K. Nakanishi, N. Soga, N. Ishizuka, N. Tanaka, J. Chromatogr. A 797 (1998) 121. [26] K. Kataoka, O. Urakawa, H. Nishioka, Q. Tran-Cong, Macromolecules 31 (1998) 8809. [27] X.D. Huang, H.F. Zou, X.M. Chen, Q.Z. Luo, L. Kong, J. Chromatogr. A 984 (2003) 273. [28] L.Q. Lin, Y.C. Li, Q. Fu, L.C. He, J. Zhang, Q.Q. Zhang, Polymer 47 (2006) 3792.