Short communication

Templated synthesis of nanosized mesoporous carbons

Sonia Alvarez, Teresa Valdes-Solıs, Antonio B. Fuertes *

Department of Chemistry of Materials, Instituto Nacional del Carbon (CSIC), P.O. Box 73, E-33080 Oviedo, Spain

Received 26 February 2007; received in revised form 13 June 2007; accepted 29 June 2007

Available online 10 July 2007

Abstract

Mesoporous carbon materials formed by nanosized particles have been synthesized by means of a nanocasting technique based

on the use of mesostructured silica materials as templates. We found that the modification of the chemical characteristics of the

surfactant employed allows mesostructured silica materials with particle sizes<100 nm to be synthesised. The mesoporous carbons

obtained from these silica materials retain the structural properties of the silica used as template and consequently they have a

particle size in the 20–100 nm range. These carbons exhibit large BET surfaces areas (up to 1300 m2 g�1) and high pore volumes

(up to 2.5 cm3 g�1), a framework confined porosity made up of uniform mesopores (3.6 nm) and an additional textural porosity

arising from the interparticle voids between the sub-micrometric particles. The main advantage of nanometer-sized mesoporous

carbons in relation to the micrometer-sized carbons is that they have enhanced mass transfer rates, which is important for processes

such as adsorption or catalysis.

# 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; A. Structural materials; B. Sol–gel chemistry; D. Microstructure

1. Introduction

In recent years, mesoporous carbons with different structures have been synthesised by using mesostructured silica

materials as sacrificial templates. These carbon materials, which have large surface areas and a high mesopore volume,

have great potential for applications such as energy storage in supercapacitors [1], adsorption of molecules in liquid

phase [2], immobilization of biomolecules [3], catalysis [4], etc. The templating synthesis of mesoporous carbons

leads to materials with a porosity that is an inverse replica of that of the silica framework. So, depending on the

characteristics of the silica used as template, carbons with different structural properties (i.e., particle size,

morphology, pore size, etc.) can be prepared [5]. Widespread interest has been focussed on the control of the porosity

(pore size and pore size distribution) in templated mesoporous carbons [5,6]. For the above-mentioned applications,

the size and shape of the carbon particles play an important role in optimizing their use for specific applications.

Specifically, mesoporous carbons made up of nanoparticles <100 nm are of great importance for applications that

require high mass transfer rates. Indeed, these materials will exhibit enhanced diffusion rates within their pore network

due to the short diffusion paths (<100 nm). To mention just some potential applications where these nanosized

mesoporous carbons could be advantageous over micrometer-sized carbons (>1 mm), they could be used as

adsorbents for the recovery of large molecules (i.e., biomolecules), as catalytic supports or as electrodes in electrical

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Materials Research Bulletin 43 (2008) 1898–1904

* Corresponding author. Tel.: +34 985 11 90 90; fax: +34 985 29 76 62.

E-mail address: abefu@incar.csic.es (A.B. Fuertes).

0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2007.06.057

double-layer capacitors operating with organic electrolytes. However, in spite of the practical interest in mesoporous

carbons with sizes in the nanometer range (<100 nm), only a few studies have focussed on this objective [7]. Typically,

the templated mesoporous carbons reported in the literature are made up of relatively large particles (>1 mm) [5,8,9].

Obviously, they have larger diffusion paths compared to nanosized carbons, a drawback which limits their

applicability in the areas just mentioned. Accordingly, the main purpose of this work is to present a synthetic route to

mesoporous carbons made up of nanometric particles (<100 nm). One way to obtain carbon materials with these

structural characteristics is the use of nanosized porous silica materials as templates. Silica materials made up of

nanosized particles have been previously reported; i.e., HMS silica [10], MSU-4 [11] or, more recently, MCM-41

bimodal porous silica [12]. Here, we selected as template MSU-1 type silica [13]. The synthetic strategy reported here

is an innovative method for preparing carbon materials with a very small particle size (<100 nm), which in addition

exhibit a mesostructured skeleton derived from the structural pores of the silica used as template. We expect carbon

materials combining these structural properties (i.e., nanosized particles and a well-developed mesoporous network)

to be very useful for the above-mentioned applications.

2. Experimental

The synthetic scheme used to prepare the mesostructured silica materials is based on that reported by Bagshaw [13]

for the synthesis of MSU-1 silica. However, we have introduced here several modifications in this methodology related

to (a) the type of surfactant used, (b) the starting mole ratio surfactant/silica precursor and (c) the aging period. Thus,

the surfactants employed as structure directing agents are: Brij 58 (C16EO20, C16H33–(OCH2CH2)20–OH), Brij 76

(C18EO10, C18H37–(OCH2CH2)10–OH), Brij 78 (C18EO20, C18H37–(OCH2CH2)20–OH) and Brij 98 (C18H35EO20,

C18H35–(OCH2CH2)20–OH). A single EO (OCH2CH2) group contributes slightly more to hydrophilicity than a single

methylene contributes to hydrophobicity, so that the hydrophobic character of these surfactants decreases following

the sequence Brij 76� Brij 78 � Brij 98 > Brij 58 [14]. The synthesis consisted on the addition of the silica source

(TEOS = tetraethylortosilicate), under stirring, to an aqueous solution containing the surfactant that is maintained

under vigorous stirring (40 8C, 2 h). In a second step, dissolution of NaF 0.25 M is added and the mixture was

maintained under stirring (35 8C, 18 h). The resulting solid is washed with water, dried and calcined in air at 600 8C(2 8C/min, 4 h). The starting mole ratio was TEOS:Brij:NaF:H2O = 1:0.070:0.05:207, except in the case of Brij 58

(TEOS:Brij 58:NaF:H2O = 1:0.076:0.05:75), due to its higher hydrophilic character.

The synthesis of the carbons was performed according to the procedure reported elsewhere [15]. Briefly, the

mesostructured silica was impregnated with p-toluene sulfonic acid (0.5 M in ethanol) for 1 h, filtered, washed with a

small volume of ethanol and dried at 80 8C. Afterwards, a volume of furfuryl alcohol equal to the silica structural pore

volume was added. The impregnated sample was cured in air (12 h, 80 8C) in order to polymerise the furfuryl alcohol

into polyfurfuryl alcohol, which was then carbonised under N2 at 800 8C (2 8C/min, 1 h). The resulting carbon–silica

composite was immersed in 48% HF (room temperature, 15 h) to remove the silica template. The carbon obtained was

washed and then dried in air at 120 8C.

Small angle X-ray diffraction (XRD) spectra were obtained on a Siemens D5000 diffractometer operating at 40 kV

and using Cu Ka radiation (l = 0.15406 nm). Nitrogen adsorption isotherms were performed at �196 8C on a

Micromeritis ASAP 2010 gas analyser. The BET surface area was deduced from the isotherm analysis in the relative

pressure range of 0.04–0.20. The total pore volume was calculated for the amount adsorbed at a relative pressure of

0.99. Pore size distributions (PSD) were calculated by applying the Kruk–Jaroniec–Sayari (KJS) method to the

adsorption branch [16]. The pore volume of the structural pores (Vstr), the textural pores (Vtex) and the external surface

area (Sext) were estimated using the as-plot method. Micrographs SEM and TEM were obtained by a Zeiss microscope

(DSM 942) and a JEOL microscope (JEM-2000 EX II) operating at 160 kV, respectively.

3. Results and discussion

The morphology and the size of the silica particles prepared by means of sol–gel technique are controlled by the

nucleation and growing kinetics [17]. When the silica synthesis takes place at pH � 7, the hydrolyzed species from the

silica precursor get up a spontaneous condensation that will cause the mixing of the hydrolysis and condensation step

[18]. This implies a cluster–cluster growth that leads to nanometric silica particles. These particles are connected each

other, in order to be stabilized, which is evidenced by the structure with large aggregates as revealed by SEM

S. Alvarez et al. / Materials Research Bulletin 43 (2008) 1898–1904 1899

S. Alvarez et al. / Materials Research Bulletin 43 (2008) 1898–19041900

Fig. 1. SEM and TEM images obtained for the mesostructured silica S78 (a, c and e) and for the corresponding templated carbon C78 (b, d and f).

inspection (Fig. 1a). On the other hand, the final nanostructure and morphology of the mesostructured silica particles

are highly dependent of the interaction between the silica oligomers and the EO groups (present in the molecules of

surfactant) through hydrophilic H-bonding. Therefore, the decrease in the hydrophobic character of the surfactant

implies a growing in the hydrogen bonds, which makes clear that each micelle will be formed by a higher number of

silanol molecules aggregates and it will have (and so the silica particles) a larger size [19]. This suggests a pathway to

control the size of silica particles, which can be tailored by modifying the chemical characteristics of the surfactant

employed. The TEM microphotographs obtained for the silica samples show that they consist in nanoparticles with

sizes in the 20–100 nm range (Fig. 1c) and contain an internal porosity that is made up by mesopores. This porosity

exhibits a wormhole structure consisting in disordered interconnected channels with a regular diameter (Fig. 1e). This

structure is confirmed by the low range angles XRD patterns that exhibit well defined peak at 2u � 1.38 (Fig. 2), which

is consistent with a wormhole pore structure characteristic of the MSU-1 silica [13].

The nitrogen sorption isotherms for the silica samples are shown in Fig. 3a. They exhibit a typical isotherm type IV

with hysteresis loops and a capillarity condensation step at relative pressure�0.4–0.7 which is typical of mesoporous

materials. This is confirmed by PSD (Fig. 3c), that show that the porosity of these materials consist of mesopores with a

size centred at �4.3 nm, except in the case of silica S76, which has the highest pore size (10 nm). This can be

explained by the great number of H-bonding between the silica precursor and the Brij 76 molecules because of its

hydrophobic character, which leads to an increase in the micellar core size [20]. The nitrogen sorption isotherms also

show a large nitrogen adsorption uptake for p/po > 0.9, which reveals the presence of a complementary porosity

arising from the interparticle voids between the silica nanoparticles, which is normally denoted as textural porosity.

These results are coherent with the morphology illustrated by the SEM and TEM images (Fig. 1a, c and e). The textural

characteristics of the silica samples are listed in Table 1. The results deduced from the analysis of nitrogen isotherms

by means of the as-plot technique show that although the structural pore volume is similar for all the samples, as the

particle size diminishes the textural pore volume increases from 0.7 cm3 g�1 for S58 to 1.1 cm3 g�1 for S76. This

change occurs parallel to the variation observed for the external surface area (Table 1).

S. Alvarez et al. / Materials Research Bulletin 43 (2008) 1898–1904 1901

Fig. 2. Low angle XRD patterns for the silica templates (S58 and S98) and the corresponding carbon samples (C58 and C98).

The silica materials were used as sacrificial templates to prepare mesoporous carbons. The synthesis procedure

employed here allows obtaining carbons which retain the morphology and particle size of the silica precursor. Thus,

the templated carbons are made up of nanoparticles between 20 and 100 nm, as it can be deduced from the SEM and

TEM images shown in Figs. 1b, d and f. These nanoparticles have a particle diameter similar to that of the silica used

as templates, which proves that the size of the silica particles has been successfully preserved in the templated

carbons. The TEM image shown in Fig. 1f reveals that the carbons contain a wormhole porous structure, which is

analogous to that of silica. This is confirmed by the low angle XRD patterns obtained for the carbon samples, which

contain a well-defined diffraction peak (2u � 1.38) as shown in Fig. 2. These results demonstrate that not only is the

S. Alvarez et al. / Materials Research Bulletin 43 (2008) 1898–19041902

Fig. 3. Nitrogen sorption isotherm and pore size distribution of the silica samples prepared with different surfactants (a and c) and their

corresponding templated carbons (b and d). In (a) the isotherms are shifted vertically by 250, 150 and 150 cm3 STP g�1. In (b) the shifts are 200, 50

and 350 cm3 STP g�1.

particle size of silica preserved in the templated carbons but also the internal porous structure. In short, it is

demonstrated that mesoporous carbon nanoparticles are successfully obtained by applying our synthetic methodology.

Fig. 3b shows the nitrogen sorption isotherms measured for the carbon materials. They exhibit a hysteresis loop at

p/po � 0.5, characteristic of materials with framework confined mesopores. The PSD obtained for these samples

reveals that these mesopores have a uniform size centred around 3.6 nm (Fig. 3d). These structural mesopores are

originated by the removal of the silica framework. The large nitrogen uptake for p/po > 0.9 observed in Fig. 3b

confirms that these carbons, besides their structural porosity, have a complementary (textural) porosity originated by

the interparticle voids between the carbon nanoparticles, which is coherent with the microphotographs shown in

Fig. 1b, e and f. The textural characteristics for the templated carbons are listed in Table 1. They have large BET

surfaces areas (1275–1730 m2 g�1) and high pore volumes (1.4–2.5 cm3 g�1). The results deduced from the as-plot

technique show that the structural pores are around 50% of the pore volume of carbon samples. Consistent with their

small particle size these materials have large external surface areas of up to 300 m2 g�1.

4. Conclusions

In summary, we have illustrated a nanocasting route for successfully fabricating mesoporous carbons made up of

nanoparticles with a size in the 20–100 nm range as deduced by TEM inspection. The structural characteristics (i.e.,

morphology, particle size and the wormhole porosity) of the silica materials used as templates are retained by the

mesoporous carbons. Such carbons exhibit large surface areas (up to 1700 m2 g�1), high pore volumes (up to

2.5 cm3 g�1) and a structural porosity made up of uniform mesopores of around 3.6 nm. The particle size of

synthesised carbons can be tuned within a certain range by modifying the chemical characteristics of surfactant used in

the synthesis of the silica template. Due to their small particle size, we anticipate that these mesoporous carbons will be

of great interest in specific applications where fast mass transfer and easy accessibility to the sites in the structural

mesopores is important, i.e., as catalytic supports in fast reactions and as adsorbents for large molecules.

Acknowledgements

The financial support for this research work provided by the Spanish MCyT (MAT2005-00262) is gratefully

acknowledged. S. Alvarez thanks the Spanish MCyT for her FPI (BES-2003-0134) grant. T. Valdes-Solıs thanks the

CSIC-ESF for the award of an I3P post-doctoral contract.

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S. Alvarez et al. / Materials Research Bulletin 43 (2008) 1898–1904 1903

Table 1

Textural characteristics of the silica materials and the templated carbons

Code SBET (m2 g�1) Vp (cm3 g�1)a dKJS (nm)b as-plot results

Vstr (cm3 g�1)c Vstr/Vp Vtex (cm3 g�1)d Vtex/Vp Sext (m2 g�1)e

S76 720 1.6 10 0.4 0.25 1.1 0.69 560

S78 710 1.5 4.6 0.3 0.20 1.2 0.80 445

S98 630 1.2 4.3 0.3 0.25 0.9 0.75 340

S58 670 1.2 4.1 0.4 0.33 0.7 0.58 380

C76 1310 1.6 3.5 0.7 0.44 0.9 0.56 280

C78 1730 2.5 3.6 1.2 0.48 1.2 0.48 300

C98 1360 1.9 3.7 1.0 0.53 0.9 0.47 230

C58 1275 1.4 3.5 1.0 0.71 0.4 0.28 150

a Total pore volume.b Maximum of the PSD for the structural pores.c Pore volume of the structural pores.d Pore volume of the textural pores.e External surface area.

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