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Journal ArticleDOI

Novel synthesis of porous carbons with tunable pore size by surfactant-templated sol-gel process and carbonisation.

04 Nov 2002-Chemical Communications (The Royal Society of Chemistry)-Iss: 22, pp 2722-2723

TL;DR: Surfactant-templated sol-gel polymerisation was explored to synthesize the resorcinol-formaldehyde gels without supercritical drying step, which were further carbonised to obtain porous carbons of a tunable pore size.

AbstractSurfactant-templated sol-gel polymerisation was explored to synthesize the resorcinol-formaldehyde (RF) gels without supercritical drying step, which were further carbonised to obtain porous carbons of a tunable pore size.

Topics: Supercritical drying (57%)

Summary (1 min read)

T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f

  • The pore size is primarily determined by the size of gel clusters, which is in turn controlled by the monomer/surfactant concentration.
  • It is thus very likely that, in acidic conditions, the cross-linking between gel clusters becomes more facilitated to produce a more rigid gel network, and the pore collapse or shrinking at both the drying step and subsequent carbonisation step is suppressed.
  • To test this possibility, the sol-gel polymerisation was performed at different pH by varying the R/F ratio in this work (samples C5-C7).
  • When the pore properties are compared for samples C5-C7, there appears a strong correlation between the porosity and pH of the sol-gel medium even if the carbon cluster size is comparable for the three because the monomer/surfactant concentration is the same.
  • Fig. 3(b) shows the pore size distribution of samples C3-C7 as calculated by the Barrett-Joyner-Halenda (BJH) method from the desorption branch of the isotherm.

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Novel synthesis of porous carbons with tunable pore size by
surfactant-templated sol–gel process and carbonisation
Kyu Tae Lee and Seung M. Oh*
School of Chemical Engineering and Research Centre for Energy Conversion & Storage, Seoul National
University, Seoul 151-744, Korea. E-mail: seungoh@plaza.snu.ac.kr; Fax: 82-2-872-5755;
Tel: 82-2-880-7074
Received (in Cambridge, UK) 16th August 2002, Accepted 4th October 2002
First published as an Advance Article on the web 21st October 2002
Surfactant-templated sol–gel polymerisation was explored
to synthesize the resorcinol–formaldehyde (RF) gels without
supercritical drying step, which were further carbonised to
obtain porous carbons of a tunable pore size.
Since Pekala reported the preparation of resorcinol–formal-
dehyde (RF) aerogels by sol–gel polymerisation and subsequent
supercritical drying,
1
RF aerogels have been widely explored as
an intermediate for mesoporous carbons.
2
Even if the aerogel-
derived carbons have a high surface area and large mesopore
volume, thereby applicable to adsorbents, electric double-layer
capacitors, and materials for chromatographic separation,
3
the
cost of supercritical drying is extremely expensive. As an
alternative less-expensive preparation method, solvent ex-
change or freeze-drying has been proposed.
4
Here, we report
another low-cost preparation method for RF xerogels, where a
normal drying is still applicable because the pore collapse can
be minimized during the water evaporation. The resulting RF
xerogels are carbonised to obtain porous carbons with tunable
pore size from macropores to mesopores.
The novel feature in this synthesis is the utilization of
surfactant-template in the sol–gel polymerisation reaction. The
surfactant molecules play two important roles in this synthesis.
Firstly, they form spherical micelles in water and act as a
template for the sol–gel polymerisation. The micro- to nano-
sized spherical gel clusters are formed within the micelles,
which are further three-dimensionally connected via a cross-
linking reaction. As a result, interstitial sites are generated that
become pores. Secondly, the surfactant molecules still adsorb
on the pore walls even after the polymerisation reaction, such
that they eventually lower the surface tension at the pore water–
resin interface. This minimizes the pore collapse during the
drying step.
The synthetic procedure is shown in Fig. 1. In detail, the
surfactant (cetyltrimethylammonium bromide, CTAB) is dis-
persed in deionised water. Then, the mixture of resorcinol (R),
formaldehyde (F), and sodium carbonate (as a catalyst) is added
to obtain surfactant-stabilized (templated) RF sols. Heating at
85 °C induces a polymerisation reaction inside the surfactant-
templated RF sols to generate spherical gel clusters. An
interconnection between the RF gel clusters is also possible
because their surface is still active for the cross-linking reaction.
The resulting RF gels are dried at 85 °C and further heat-treated
at 1000 °C for 3 h under argon atmosphere to obtain porous
carbons in monolithic shape.
Table 1 lists the synthetic conditions and physical properties
of the porous carbons. The RF gel prepared without the use of
surfactant-template carries a negligible pore structure and thus
the resulting carbon (C1) shows a specific surface area of < 1
m
2
g
21
. This is the result of pore collapse during the drying step
due to large capillary forces imposed at the liquid–vapor
interface.
5
Interestingly, however, when the surfactant-template
is used, the RF xerogels exhibit a rather stable pore structure
even after a normal drying. This feature must arise from the
reduction of surface tension by the adsorbed surfactant
molecules. The resulting carbons show a well-developed pore
structure as described below.
Fig. 2 shows the field-emission scanning electron micros-
copy (FE-SEM) images of one RF gel and three porous carbons.
The spherical gel clusters and pores made at the interstitial sites
can be recognized in Fig. 2(a). In this particular synthesis, the
size of the gel clusters is ca. 100 nm (Fig. 2(a)), but reduced to
ca. 65 nm by carbonisation (Fig. 2(b)). The pores made by the
Table 1 Synthetic conditions, relative molar ratios of reagents and physical properties of porous carbons
Sample R/mol F/mol CTAB/mol H
2
O/mol pH
Surface area/m
2
g
21
Cluster diameter
a
/
nm
Pore diameter
b
/
nm
Pore volume
c
/
cm
3
g
21
C1 1 2 0 5.62 6.3 < 1
C2 1 2 0.082 5.62 5.0 10 ca. 2000
C3 1 2 0.082 5.62 3 20 6.0 426 ca. 160 > 60 0.40
C4 1 2 0.082 5.62 3 30 6.0 422 ca. 65 31 0.51
C5 1 2 0.082 5.62 3 10
3
7.1 419 ca. 15 3.9 0.24
C6 1 50 0.082 5.62 3 10
3
6.7 461 ca. 15 6.5 0.35
C7 1 100 0.082 5.62 3 10
3
6.4 501 ca. 15 7.4 0.41
a
Cluster diameter was assessed from the FE-SEM and TEM images.
b
Pore diameter is the peak value in pore size distribution that was calculated by the
BJH method from the N
2
desorption isotherm. The micropores located inside carbon clusters are ignored.
c
Pore volume was calculated by the BJH method
from the N
2
desorption isotherm.
Fig. 1 The synthetic scheme for porous carbons. The detailed explanation is
given in the text.
This journal is © The Royal Society of Chemistry 20022722 CHEM. COMMUN., 2002, 2722–2723
DOI: 10.1039/b208052d

spherical carbon clusters are 31 nm in diameter (Fig. 2(b)) and
the BrunauerEmmettTeller (BET) surface area is 422 m
2
g
21
.
In this preparation, the pore size is primarily determined by
the size of gel clusters, which is in turn controlled by the
monomer/surfactant concentration. As the micellar size be-
comes smaller, the size of gel clusters and thus the carbon pore
size decrease with a dilution of the monomer/surfactant content
(Fig. 2(b)(d)).
The solgel polymerisation of resorcinol (R) with formal-
dehyde (F) is composed of two consecutive reactions: addition
and condensation with the latter acid-catalysed to produce a
cross-linked polymer network.
4
It is thus very likely that, in
acidic conditions, the cross-linking between gel clusters
becomes more facilitated to produce a more rigid gel network,
and the pore collapse or shrinking at both the drying step and
subsequent carbonisation step is suppressed. To test this
possibility, the solgel polymerisation was performed at
different pH by varying the R/F ratio in this work (samples C5
C7). As listed in Table 1, the solgel reaction medium becomes
more acidic within the pH range of 7.16.4 with an increase in
the formaldehyde content. When the pore properties are
compared for samples C5C7, there appears a strong correlation
between the porosity and pH of the solgel medium even if the
carbon cluster size is comparable for the three because the
monomer/surfactant concentration is the same. The RF gels and
carbons prepared under more acidic conditions exhibit larger
surface areas, pore diameters and pore volumes. This must be
the result of rigidity enhancement by the acid-catalysed cross-
linking reaction between the gel clusters.
Fig. 3(a) presents the nitrogen adsorption and desorption
isotherm for sample C7. This porous carbon as well as the others
shows a type IV isotherm with a hysteresis loop that is
associated with capillary condensation in mesopores and with a
plateau at high relative pressure, indicating the dominance of
mesopores in these samples. Fig. 3(b) shows the pore size
distribution of samples C3C7 as calculated by the Barrett
JoynerHalenda (BJH) method from the desorption branch of
the isotherm. It is seen that the pore size is uniform in each
sample and the size can be controllable within the mesopore
range.
Notes and references
1 R. W. Pekala, J. Mater. Sci., 1989, 24, 3221.
2 R. Saliger, V. Bock, R. Petricevic, T. Tillotson, S. Geis and J. Fricke, J.
Non-Cryst. Solids, 1997, 221, 144; A. W. P. Fung, Z. H. Wang, K. Lu, M.
S. Dresselhaus and R. W. Pekala, J. Mater. Res., 1993, 8, 1875; S. Han
and T. Hyeon, Chem. Commun., 1999, 1955.
3 R. Ryoo, S. H. Joo, M. Kruck and M. Jaroniec, Adv. Mater., 2001, 13,
677.
4 C. Lin and J. A. Ritter, Carbon, 1997, 35, 1271; H. Tamon, H. Ishizaka,
T. Yamamoto and T. Suziki, Carbon, 1999, 37, 2049.
5 C. J. Brinker and G. W. Scherer, J. Non-Cryst. Solids, 1985, 70, 301.
Fig. 2 FE-SEM images of porous materials: (a) the RF precursor gel for C4
before carbonisation; (b) C4; (c) C2; (d) C5. The images were obtained with
a JEOL JSM 6700F and 6330F. Note the uniform-sized spherical clusters
and pores formed at the interstitial sites.
Fig. 3 (a) The N
2
adsorptiondesorption isotherm of mesoporous carbon
(C7). (b) The pore size distribution of C3 (dashdotdot line), C4 (dashdot
line), C5 (solid line), C6 (dotted line) and C7 (dashed line) calculated from
the desorption branch of the nitrogen isotherm. The isotherms were
measured at 77 K with a Micrometrics ASAP2000 Gas Adsorption
Analyser.
2723
CHEM. COMMUN., 2002, 2722 2723
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1,496 citations


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Frequently Asked Questions (1)
Q1. What have the authors contributed in "Novel synthesis of porous carbons with tunable pore size by surfactant-templated sol–gel process and carbonisation" ?

Here, the authors report another low-cost preparation method for RF xerogels, where a normal drying is still applicable because the pore collapse can be minimized during the water evaporation. The microto nanosized spherical gel clusters are formed within the micelles, which are further three-dimensionally connected via a crosslinking reaction. The resulting RF gels are dried at 85 °C and further heat-treated at 1000 °C for 3 h under argon atmosphere to obtain porous carbons in monolithic shape.