Carbon 41 (2003) 1525–1532

D irect fabrication of mesoporous carbons using in-situpolymerized silica gel networks as a template

*Sangjin Han, Minsuk Kim, Taeghwan HyeonNational Creative Research Initiative Center for Oxide Nanocrystalline Materials and School of Chemical Engineering, Seoul National

University, Seoul 151-744, South Korea

Received 2 October 2002; received in revised form 5 February 2003; accepted 13 February 2003

Abstract

Mesoporous carbons were synthesized by in-situ polymerized silica gel networks as a template. The co-condensation ofcarbon precursor (sucrose) and silica precursor (sodium silicate) followed by heat treatment generated a carbon/silicananocomposite. After etching the silica template, mesoporous carbons were obtained. Under optimum synthesis conditions a

2mesoporous carbon with a high surface area of.800 m /g and a narrow pore size distribution centered at 3 nm wasproduced. The three-dimensionally interconnected silica structures effectively functioned as the template for the porouscarbon materials. 2003 Elsevier Science Ltd. All rights reserved.

Keywords: A. Porous carbon; B. Carbonization; C. BET surface area; D. Porosity

1 . Introduction synthetic opals as templates [11]. Kyotani and co-workersreported the synthesis of microporous carbon materials

Porous carbon materials have been applied in various using zeolites as templates [12–16]. Our research groupareas including gas separation, water purification, catalyst and several other groups have synthesized differentsupport, and battery electrodes [1,2]. As representative mesoporous carbon materials using mesoporous silicaporous carbons, activated carbons were synthesized materials as templates [17–24]. Some of these mesoporousthrough physical or chemical activation [3–5]. Most carbons exhibited excellent performance as electrode ma-activated carbons, however, are microporous (pore size,2 terials in EDLCs and in fuel cells. Our group also reportednm) and these carbon materials have limited application as the fabrication of mesocellular carbon foams with uniformadsorbents for large molecules and electrochemical double ultralarge|30 nm mesocells interconnected through uni-layer capacitor (EDLC) electrodes [6,7]. Tamai and co- form|15 nm windows [25]. One major disadvantage ofworkers reported the synthesis of mesoporous carbons using mesoporous silica materials as templates is the highthrough the steam invigoration of pitches homogenized production cost. We have synthesized mesoporous carbonwith organo–rare-earth metal complexes [8–10]. They materials using cheap commercial silica sols as templates.successfully applied these mesoporous carbons as adsor- Mesoporous carbons, designated as SMC1 carbons, with abents for bulky dyes and humic substances. wide range of pore sizes, from 10 to 100 nm, have been

Recently, various porous carbon materials have been fabricated using Ludox HS-40 aqueous silica sol as afabricated using different kinds of nanostructured silica template and resorcinol–formaldehyde gel as a carbonmaterials as templates. Zakhidov et al. synthesized macro- precursor [26]. When silica sol nanoparticles stabilized byporous carbons with close-packed|100 nm pores using surfactant were applied as templates, mesoporous carbons

with uniform pore sizes of|10 nm were fabricated [27].These SMC1 carbons have been shown to be good*Corresponding author. School of Chemical Engineering,adsorbents for large molecules, such as dyes and humicSeoul National University, San 56-1, Shilim-dong, Kwanak-gu,acids. Recently, Li and Jaroniec reported on the synthesisSeoul 151-744, South Korea. Tel.:182-2-880-7150; fax:182-2-of mesoporous carbons using mesophase pitch or poly-888-1604.

E-mail address: thyeon@plaza.snu.ac.kr(T. Hyeon). acrylonirile as a carbon precursor and silica sol or silica

0008-6223/03/$ – see front matter 2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S0008-6223(03)00072-1

1526 S. Han et al. / Carbon 41 (2003) 1525–1532

gel as a template [28,29]. Recently, Kawashima et al. and 20 ml of water solution at 50|80 8C. After completereported the synthesis of mesoporous carbon through co- dissolution, HCl solution was added, producing a darkpolymerization of furfuryl alcohol and TEOS [30]. The brown solution. This reaction mixture was kept for 1 dayresults are very important in the respect that the silica in a convection oven at 50|80 8C to achieve evaporation oftemplate and carbon precursor were simultaneously residual water and further polymerization. For carboniza-formed, which eliminated the time-consuming synthesis of tion, the mixture was heated under nitrogen atmospherethe silica template. The synthetic procedure, however, used from room temperature to 2008C with a heating rate ofvery expensive silica and carbon precursors. For large- 38C/min, kept at that temperature for 12 h, and thenscale production of mesoporous carbons, cheap and en- increased to 8508C at a heating rate of 58C/min, andvironmentally-friendly precursors are more desirable. Here maintained at that temperature for 3 h. The resultingwe report on the synthesis of mesoporous carbons through carbon/silica composites were stirred in a 3 M NaOHthe co-polymerization of sodium silicate and sucrose. We solution for 5 h to remove the silica template and the finalhave investigated the pore characteristics of mesoporous carbon materials were retrieved by filtration. Elementalcarbons synthesized under various reaction conditions. and thermogravimetric analysis (TGA) results showed that

silica was successfully removed through this process.

2 . Experimental 2 .2. Characterization of carbon materials

2 .1. Preparation of porous carbons The nitrogen adsorption/desorption isotherms weremeasured at 77 K using a Micromeritics ASAP 2000

The overall synthetic procedure is shown in Fig. 1. system. Total surface area and pore volumes were de-Sucrose (99% A.C.S. reagent) and aqueous sodium silicate termined using the BET (Brunauer–Emmett–Teller) equa-solution (containing|14% NaOH and|27% SiO ) from tion and the single point method, respectively. The pore2

Aldrich and hydrochloric acid (37 wt.% in water) from range of 1.7|300 nm was analyzed by the Barrett–Joyner–Samchun Chemical were used. The key step in the current Halenda (BJH) method. The pore size distribution curvesynthetic procedure is the simultaneous polymerization of was obtained from the adsorption branch of the nitrogencarbon precursor (sucrose) and silica precursor (sodium isotherm using the BJH method.S(BET) is the surface areasilicate) to produce homogeneous silica/carbon calculated by the BET method.S(BJH) is the cumulativenanocomposites. In work by Kyotani and co-workers HCl adsorption surface area in the pore ranges of 1.7|300 nmsolution was added to the TEOS solution before adding the calculated by the BJH method.V(single)is the single pointcarbon precursor in order to pre-hydrolyze the silica total pore volume of pores,150 nm. V(BJH) is theprecursor solution. In contrast, HCl solution was here cumulative adsorption pore volume in the pore ranges ofpoured into the mixture containing sucrose and sodium 1.7|300 nm calculated by the BJH method. Thesilicate. Firstly, 50 g of sucrose was dissolved in the mesoporosities were calculated from the ratio ofS(BJH) tosolution mixture containing 20|50 ml of silicate solution S(BET). Transmission electron micrographs have been

Fig. 1. Schematic diagram for the synthesis of mesoporous carbon through in-situ polymerization of silicate and sucrose.

S. Han et al. / Carbon 41 (2003) 1525–1532 1527

obtained at 200 kV accelerating voltage. Elemental analysis templates, the resulted carbon materials also had micro-was performed on EA 1110 elemental analyzer and TGA pores originating from the carbonization process. However,was conducted under an air and N atmosphere at 58C/ the portion of micropore was decreased as the silica2

min from room temperature to 10008C on a TGA 2050 content increased. The mesoporosity was above 0.8 exceptsystem. Raman spectroscopy was carried out on a Jobin for the lowest silica content used in our experiments. Fig. 2Yvon T64000 spectrometer using an Ar ion laser (514 nm). shows the N adsorption/desorption isotherms at 77 K2

which show that the mesoporous carbons exhibited narrowa hysteresis loop in the isotherms, demonstrating the well-

3 . Results and discussion developed pore structure of the materials. When theamount of silica is lower, the adsorbed volume in the

3 .1. The synthesis of mesoporous carbons under various region of mesopore and macropore was relatively small,sucrose /SiO ratios indicating a higher portion of micropores. All the iso-2

therms exhibited a rapid increase in theP/P range of0

Mesoporous carbons were synthesized under various 0.9|1.0. This phenomenon originated from the macroporessucrose/silica ratios. The molar ratio of sucrose to HCl of carbon considered to result from the secondary silicawas fixed at 0.3 and the reaction temperature was set at particle through sol–gel reaction in the acidic condition. In70 8C. The pore characteristics of mesoporous carbons Fig. 3, pore size distributions (PSDs) obtained from theproduced using different sucrose to silica molar ratios are adsorption branch of the nitrogen isotherm using the BJHshown in Table 1. When the carbons were synthesized method are shown. With sucrose/SiO ratio of 0.65, the2

using a sucrose/SiO molar ratio of 0.65, the increased maximum value and broadness of the pore size distribution2

silica content in the sucrose–silica composite induced increased with higher silica content. These results indi-higher surface area and pore volume of the resulting cated that higher amounts of silica precursor induced amesoporous carbon, demonstrating that the in-situ formed thicker silica gel network, resulting in the formation ofsilica gel network has effectively worked as a template. carbons with larger pore diameters. It is well known thatWhen too much silica was used, it did not effectively three-dimensional gel network structures are formed fromfunction as a template, resulting in low surface area and the sol–gel polymerization of silica in acidic media [33].pore volume. When carbon was synthesized without using The isotherms and PSD graphs demonstrated that thesilica templates, nearly nonporous carbon with surface area three-dimensional network structures of silica were effec-

2of ,5 m /g was produced. Some papers report that carbon tively utilized as a template for the generation of pores inmaterials produced from sucrose have a high surface area the carbon materials. As shown in Fig. 3e and f, whenwithout using templates. However, even in these carbons, excessively high amounts of silica were used in themost micropores were generated through the activation synthesis, carbons with low surface area and pore volumeprocess [31,32]. In the case of using silica materials as were produced.

Table 1The pore characteristic of mesoporous carbon depending on sucrose/SiO ratio2

iSucrose/SiO S(BET) S(BJH) S(micro) V(single) V(BJH) V(meso) V(macro) Mesoporosity2a 2 b 2 c 2 d 3 e 3 f 3 g 3 hmol. ratio (m /g) (m /g) (m /g) (cm /g) (cm /g) (cm /g) (cm /g)

j` 4.25 1.67 2.58 0.0024 0.0020 0.0008 0.0012 0.391.62 326 200 126 0.23 0.19 0.16 0.03 0.611.08 437 357 80 0.42 0.42 0.34 0.08 0.820.81 813 738 75 0.91 0.93 0.78 0.15 0.910.65 856 728 128 1.17 1.15 0.89 0.26 0.850.47 443 370 73 0.56 0.57 0.46 0.11 0.84

k0.39 307 319 N.D. 0.53 0.61 0.44 0.17 1.04a Sucrose/HCl ratio is fixed at 0.30 and the reaction temperature set at 708C.b S(BET) is the total surface area calculated by the BET method.c S(BJH) is the cumulative adsorption surface area (pores 1.7|300 nm) calculated by the BJH method.d S(micro) is the surface area of micropore calculated by the difference ofS(BET) andS(BJH).e V(single) is the single point total pore volume (pores,150 nm).f V(BJH) is the cumulative adsorption pore volume (pores 1.7|300 nm) calculated by the BJH method.g V(meso) is the cumulative adsorption mesopore volume (1.7,pores,50 nm) calculated by the BJH method.h V(macro) is the cumulative adsorption macropore volume (50,pores,300 nm) calculated by the BJH method.i Mesoporosity is defined as the ratio ofS(BJH)/S(BET).j` means that carbon materials were produced without adding silica template.

k N.D means ‘‘Not detected (below the detection limit of the instrument)’’.

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Fig. 2. The effect of sucrose/SiO ratio on N adsorption–desorption isotherm at 77 K. The reaction temperature is 708C and sucrose/HCl2 2

ratio is fixed at 0.30 (a) the ratio of sucrose/SiO is 1.62, (b) 1.08, (c) 0.81, (d) 0.65, (e) 0.47, and (f) 0.39.2

3 .2. The synthesis of mesoporous carbons under various the silica structure cannot act effectively as the templatesucrose /HCl ratios because well-developed three-dimensional silica networks

cannot be generated. As a result, the optimum amount ofMesoporous carbons were synthesized using various acid catalyst is very important to obtain mesoporous

sucrose/HCl ratios. The molar ratio of sucrose to silica carbons with well-interconnected pore structures. Table 2was fixed at 0.65 and the reaction temperature was set at shows the pore characteristics of carbon using various70 8C. In the current synthesis procedure, the amount of sucrose/HCl molar ratios. Using a sucrose/HCl ratio of

2acid catalyst is very important because it controls the 0.30, mesoporous carbon with a surface area of 856 m /g3polymerization rates of both sucrose and silicate. If the and pore volume of 1.15 cm /g was produced. Using

polymerization rate of silica is faster than that of sucrose, reaction mixtures with either higher or lower sucrose/HClthe carbon structure in the carbon/silica composite will be molar ratios, porous carbons with much lower surfacevery weak, resulting in the collapse of the carbon structure areas and pore volumes were generated, demonstrating thatafter removing the silica template. In the reverse is true, the optimum sucrose/HCl molar ratio is about 0.3 in this

S. Han et al. / Carbon 41 (2003) 1525–1532 1529

Fig. 3. The effect of sucrose/SiO ratio on pore size distribution calculated from the adsorption branch of the nitrogen isotherm by the BJH2

method. The reaction temperature is 708C and sucrose/HCl ratio is fixed at 0.30 (a) the ratio of sucrose/SiO is 1.62, (b) 1.08, (c) 0.81, (d)2

0.65, (e) 0.47, and (f) 0.39.

case. N adsorption/desorption isotherms of these carbons hibited similar surface area and pore volume. These results2

and the pore size distributions were similar to those shown indicate that the reaction temperature has very little effectin Figs. 2 and 3. on the total surface areas and pore volumes of carbons

under the synthesis condition. But, the formation of3 .3. The synthesis of mesoporous carbon under various macropores was affected by the reaction temperaturereaction temperatures because higher reaction temperature induced a large

macropore volume. As mentioned above, macropores wereMesoporous carbons were synthesized under various considered to result from the secondary silica particle

reaction temperatures. The molar ratio of sucrose to silica through the sol–gel reaction in the acidic condition. It wasand the molar ratio of sucrose to HCl were fixed at 0.65 shown that the formation of a secondary silica particle wasand 0.30, respectively, and Table 3 shows the pore facilitated at the higher reaction temperature. Fig. 4 showscharacteristics of carbons produced. These carbons ex- the N adsorption/desorption isotherm of the resulting2

1530 S. Han et al. / Carbon 41 (2003) 1525–1532

Table 2The pore characteristic of mesoporous carbon depending on sucrose/HCl ratio

iSucrose/HCl S(BET) S(BJH) S(micro) V(single) V(BJH) V(meso) V(macro) Mesoporositya 2 b 2 c 2 d 3 e 3 f 3 g 3 hmol. ratio (m /g) (m /g) (m /g) (cm /g) (cm /g) (cm /g) (cm /g)

0.61 549 453 96 0.66 0.64 0.63 0.01 0.830.30 856 728 128 1.17 1.15 0.89 0.26 0.850.20 546 519 27 0.65 0.65 0.55 0.10 0.95

a Sucrose/SiO ratio is fixed at 0.65 and the reaction temperature set at 708C.2b S(BET) is the total surface area calculated by the BET method.c S(BJH) is the cumulative adsorption surface area (pores 1.7|300 nm) calculated by the BJH method.d S(micro) is the surface area of micropore calculated by the difference ofS(BET) andS(BJH).e V(single) is the single point total pore volume (pores,150 nm).f V(BJH) is the cumulative adsorption pore volume (pores 1.7|300 nm) calculated by the BJH method.g V(meso) is the cumulative adsorption mesopore volume (1.7,pores,50 nm) calculated by the BJH method.h V(macro) is the cumulative adsorption macropore volume (50,pores,300 nm) calculated by the BJH method.i Mesoporosity is defined as the ratio ofS(BJH)/S(BET).

carbon materials at the different reaction temperatures. Fig. to be responsible for this lower carbonization yield. TGA4 also shows that the adsorbed volumes in theP/P range experiment under an air flow showed that the silica content0

of 0.9|1.0 increased more rapidly at higher reaction in the carbon/silica composite was 44 wt.%, when atemperature. sucrose/SiO mol. ratio of 0.65 was used in the synthesis.2

Elemental analysis showed that the carbon content after3 .4. Thermogravimetric analysis, elemental analysis, and removing silica templates in all the carbon/silica compos-Raman spectroscopy ites is higher than 90 wt.%, indicating that the silica

template was successfully removed.In order to investigate carbonization yields and SiO /C In order to determine the effect of carbonization on the2

ratios, TGA experiments were conducted. Generally, suc- nanometer-scale space such as the three-dimensional silicarose is polymerized through a dehydration reaction by acid network structure on the degree of crystallization, Ramancatalyst or thermolysis followed by heat treatment in an spectroscopy was performed. Both the carbon materialsinert atmosphere, resulting in the formation of carbon. The with and without silica templates exhibited nearly the same

21calculated carbonization yield of sucrose is 44 wt.%, intensities of the D-band (1350 cm ) and G-band (158021because 1 mole of sucrose contains 12 moles of carbon cm ), demonstrating that both carbons are amorphous.

atoms. However, TGA results showed that the carboniza- The results could be explained by the low carbonizationtion yield was 39 wt.%, which is lower than the calculated temperature and randomly oriented silica template struc-value. The residual water in the polymerized sucrose seems ture.

Table 3The pore characteristic of mesoporous carbon depending on reaction temperatures

iReaction temperature S(BET) S(BJH) S(micro) V(single) V(BJH) V(meso) V(macro) Mesoporositya 2 b 2 c 2 d 3 e 3 f 3 g 3 h(8C) (m /g) (m /g) (m /g) (cm /g) (cm /g) (cm /g) (cm /g)

50 862 722 140 1.01 0.91 0.78 0.13 0.8460 966 789 177 1.12 1.09 0.94 0.15 0.8270 856 728 128 1.17 1.13 0.89 0.26 0.8580 828 726 102 1.13 1.18 0.91 0.27 0.88

a Sucrose/SiO ratio is fixed at 0.65 and sucrose/HCl ratio at 0.30.2b S(BET) is the total surface area calculated by the BET method.c S(BJH) is the cumulative adsorption surface area (pores 1.7|300 nm) calculated by the BJH method.d S(micro) is the surface area of micropore calculated by the difference ofS(BET) andS(BJH).e V(single) is the single point total pore volume (pores,150 nm).f V(BJH) is the cumulative adsorption pore volume (pores 1.7|300 nm) calculated by the BJH method.g V(meso) is the cumulative adsorption mesopore volume (1.7,pores,50 nm) calculated by the BJH method.h V(macro) is the cumulative adsorption macropore volume (50,pores,300 nm) calculated by the BJH method.i Mesoporosity is defined as the ratio ofS(BJH)/S(BET).

S. Han et al. / Carbon 41 (2003) 1525–1532 1531

Fig. 4. The effect of reaction temperature on N adsorption–desorption isotherm at 77 K. The sucrose/SiO ratio is 0.65 and sucrose/HCl2 2

ratio is fixed at 0.30 (a) The reaction temperature is 508C, (b) 608C, (c) 708C, and (d) 808C.

4 . Conclusion various reaction conditions. As the relative silica contentincreases in the sucrose–silica composite, the surface area

We have synthesized mesoporous carbons from the co- and pore volume of the resulting mesoporous carbon arepolymerization of cheap silica and carbon precursors increased. The optimum sucrose to HCl molar ratio was(sodium silicate and sucrose), followed by the carboniza- found to be 0.3. The formation of macropores was affectedtion and removal of the silica template. We have optimized by the reaction temperature. Under the best synthesisthe synthesis procedure by synthesizing carbons under condition, mesoporous carbon with a surface area of 850

2 3m /g, pore volume of 1.5 cm /g, and well-interconnectedpores of about 3 nm was obtained. Fig. 5 shows thetransmission electron micrograph of the resulting mesopor-ous carbon with the well-developed pore structure.

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