Templated synthesis of nanosized mesoporous carbons

Article abstract of DOI:10.1016/j.materresbull.2007.06.057

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.

Full text of DOI:10.1016/j.materresbull.2007.06.057

Materials Research Bulletin 43 (2008) 1898–1904
www.elsevier.com/locate/matresbu
Short communication
Templated synthesis of nanosized mesoporous carbons
´
Sonia Alvarez, Teresa Vald e´ s-Sol ´ı s, Antonio B. Fuertes
*
Department of Chemistry of Materials, Instituto Nacional del Carb o´ n (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
2
À1
particle size in the 20–100 nm range. These carbons exhibit large BET surfaces areas (up to 1300 m g ) and high pore volumes
(
3
up to 2.5 cm g ), a framework confined porosity made up of uniform mesopores (3.6 nm) and an additional textural porosity
À1
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
*
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

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S. Alvarez et al. / Materials Research Bulletin 43 (2008) 1898–1904
1899
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 (C EO , C H –(OCH CH ) –OH), Brij 76
1
6
20
16 33
2
2 20
(
C H –(OCH CH ) –OH). A single EO (OCH CH ) group contributes slightly more to hydrophilicity than a single
C EO , C H –(OCH CH ) –OH), Brij 78 (C EO , C H –(OCH CH ) –OH) and Brij 98 (C H EO ,
18 10 18 37 2 2 10 18 20 18 37 2 2 20 18 35 20
1
8
35
2
2 20
2
2
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:H O = 1:0.070:0.05:207, except in the case of Brij 58
2
(
TEOS:Brij 58:NaF:H O = 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
2
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 N at 800 8C (2 8C/min, 1 h). The resulting carbon–silica
2
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 À196 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
adsorption branch [16]. The pore volume of the structural pores (V ), the textural pores (V ) and the external surface
.99. Pore size distributions (PSD) were calculated by applying the Kruk–Jaroniec–Sayari (KJS) method to the
str
tex
area (S ) were estimated using the a -plot method. Micrographs SEM and TEM were obtained by a Zeiss microscope
ext
s
(
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

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1900
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).

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S. Alvarez et al. / Materials Research Bulletin 43 (2008) 1898–1904
1901
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/p > 0.9, which reveals the presence of a complementary porosity
o
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 a -plot technique show that although the structural pore volume is similar for all the samples, as the
s
3
À1
3
À1
particle size diminishes the textural pore volume increases from 0.7 cm g for S58 to 1.1 cm g for S76. This
change occurs parallel to the variation observed for the external surface area (Table 1).
Fig. 2. Low angle XRD patterns for the silica templates (S58 and S98) and the corresponding carbon samples (C58 and C98).

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S. Alvarez et al. / Materials Research Bulletin 43 (2008) 1898–1904
1902
Fig. 3. Nitrogen sorption isotherm and pore size distribution of the silica samples prepared with different surfactants (a and c) and their
3
corresponding templated carbons (b and d). In (a) the isotherms are shifted vertically by 250, 150 and 150 cm STP g . In (b) the shifts are 200, 50
À1
3
and 350 cm STP g
À1
.
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

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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
2
À1
3
(cm g
À1 a
b
Code
S
BET (m g
)
V
p
)
dKJS (nm)
s
a -plot results
3
À1 c
3
À1 d
2
À1 e
V
str (cm g
)
Vstr/V
p
V
tex (cm g
)
Vtex/V
p
S
ext (m g
)
S76
S78
S98
S58
720
710
630
670
1.6
1.5
1.2
1.2
10
4.6
0.4
0.3
0.3
0.4
0.25
0.20
0.25
0.33
1.1
1.2
0.9
0.7
0.69
0.80
0.75
0.58
560
445
340
380
4.3
4.1
C76
C78
C98
C58
1310
1730
1360
1275
1.6
2.5
1.9
1.4
3.5
3.6
3.7
3.5
0.7
1.2
1.0
1.0
0.44
0.48
0.53
0.71
0.9
1.2
0.9
0.4
0.56
0.48
0.47
0.28
280
300
230
150
a
Total pore volume.
b
c
d
e
Maximum of the PSD for the structural pores.
Pore volume of the structural pores.
Pore volume of the textural pores.
External surface area.
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/p ꢁ 0.5, characteristic of materials with framework confined mesopores. The PSD obtained for these samples
o
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/p > 0.9 observed in Fig. 3b
o
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
2
À1
3
À1
surfaces areas (1275–1730 m g ) and high pore volumes (1.4–2.5 cm g ). The results deduced from the a -plot
s
technique show that the structural pores are around 50% of the pore volume of carbon samples. Consistent with their
2
À1
small particle size these materials have large external surface areas of up to 300 m g
. Conclusions
In summary, we have illustrated a nanocasting route for successfully fabricating mesoporous carbons made up of
.
4
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
2
mesoporous carbons. Such carbons exhibit large surface areas (up to 1700 m g ), high pore volumes (up to
À1
3
.5 cm g ) and a structural porosity made up of uniform mesopores of around 3.6 nm. The particle size of
À1
2
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. Vald e´ s-Sol ´ı s thanks the
CSIC-ESF for the award of an I3P post-doctoral contract.
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