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Silica-Templated Continuous
Mesoporous Carbon Films by a
Spin-Coating Technique**
By Jiebin Pang, Xuan Li, Donghai Wang, Zhiwang Wu,
Vijay T. John, Zhenzhong Yang,* and Yunfeng Lu*
Nanoporous carbon films and membranes are of importance for gas separations, membrane reactors, ultrafiltrations,
sensors, fuel cells, and other applications.[1±5] Current synthesis methods such as chemical vapor deposition,[2] pulsed-laser
deposition,[3] spray coating,[6] and ultrasonic deposition[7±9]
techniques often result in microporous carbon thin films
or membranes with average pore diameters of less than
1 nm.[4,6±10] Although microporous carbon thin films and
membranes are effective for gas separation[6±9] and other
applications, the small pore diameters may limit other applications that involve larger molecules. Incorporation of mesoporosity into nanoporous carbon thin films and membranes is
therefore paramount in order to allow the transport of larger
molecules and to enhance internal diffusion of the molecules.[1,10] Foley and co-workers[10] pioneered the synthesis of
mesoporous carbon films by carbonizing the blends of poly
(ethylene glycol) (PEG) and poly(furfuryl alcohol) (PFA).
Removal of the PEG during the carbonization process resulted in mesoporous carbon thin films. This method allows
the ready and efficient synthesis of mesoporous carbon thin
films at low cost; however, it is still a challenge to achieve porous carbon thin films with high porosity, controllable pore
size, and adequate pore accessibility.[10] This communication
reports the synthesis of mesoporous carbon thin films by a
rapid sol±gel spin-coating process, using low-cost sucrose and
tetraethyl orthosilicate (TEOS) as precursors. As illustrated
in Scheme 1, continuous sucrose/silica nanocomposite thin
films are firstly formed by spin-coating homogenous sucrose±
silicate±water solutions that are prepared by reacting TEOS
in acidic sucrose-containing solutions. Carbonization converts
the sucrose/silica thin films into carbon/silica nanocomposite
Scheme 1. Proposed nanoporous carbon formation mechanism.
thin films. Subsequent removal of the silica network using HF
results in mesoporous carbon thin films with a uniform, interconnective pore network. Although nanoporous carbons have
been synthesized using inorganic templates (e.g., zeolite and
clays,[11±13] surfactant-templated mesoporous silica,[14±18] silica
colloidal particles,[19±21] and sol±gel derivate silica frameworks[22±24]), to the best of our knowledge, this is the first report of the synthesis of continuous mesoporous carbon thin
films through a direct and rapid organic/inorganic self-assembly and carbonization process.
Figure 1 shows the representative scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of
the mesoporous carbon films after the removal of silica template, indicating the formation of continuous, smooth, and
±
[*] Prof. Y. Lu, Dr. J. Pang, X. Li, D. Wang, Z. Wu, Prof. V. T. John
Department of Chemical and Biomolecular Engineering
Tulane University
New Orleans, LA 70118 (USA)
E-mail: [email protected]
Prof. Z. Yang
State Key Laboratory of Polymer Physics and Chemistry
Institute of Chemistry
The Chinese Academy of Sciences
Beijing 100080 (P.R. China)
E-mail: [email protected]
[**] The authors gratefully acknowledge the financial support of this
work by NASA (Grant No. NAG-1-02070 and NCC-3-946), Office of
Naval Research, Louisiana Board of Regents (Grant No.
LEQSF(2001-04)-RD-B-09), National Science Foundation (Grant
No. NSF-DMR-0124765 and CAREER award), and NSF of China
(Grant No. 50325313 and 20128004). Supporting Information is
available online from Wiley Interscience or from the authors.
884
2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
b
Figure 1. Top-view SEM image (a) and AFM image (b) of the mesoporous carbon films.
DOI: 10.1002/adma.200306602
Adv. Mater. 2004, 16, No. 11, June 4
900
(c)
450
300
150
0
(b)
3 -1
600
-1
750
dV/dD (cm g nm )
3 -1
Volume Adsorbed (cm g , STP)
of the porosity is contributed by the mesoporous networks.
The inset shows the pore size distributions of the mesoporous
carbon and silica, which center at about 2.4 nm and 2.0 nm,
respectively. Transmission electron microscopy (TEM) studies
(see Fig. 3) of the mesoporous carbon films suggest the mesoporous carbon contains a disordered but uniform-sized mesoporous structure.[23,28±30]
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crack-free thin films. Cross-sectional SEM (see Supporting
Information) studies indicate an average film thickness of
1 lm. The SEM and AFM studies confirm that as-spun mesoporous carbon thin films with dimensions larger than
15 mm ” 25 mm are smooth and crack-free.
Figure 2 shows the N2 adsorption/desorption isotherms of
a) the carbon/silica nanocomposite, b) mesoporous silica prepared by removing carbon from the carbon/silica nanocom-
(d) (e)
1.50
1.25
1.00
0.75
0.50
0.25
0.00
1
2
3
4
5
Pore Diameter (nm)
Figure 3. A representative TEM image of the mesoporous carbon film.
(a)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Relative Pressure (P/P0)
Figure 2. N2 sorption isotherms of a) the carbon/silica nanocomposite,
b) porous silica produced by the removal of carbon, and c) porous carbon produced by the removal of silica (adsorption: solid symbols; desorption: open symbols). Inset: Pore size distributions of the mesoporous silica (d) and carbon (e), calculated using density functional theory
(DFT) (Software from Micromeritics).
posite, and c) mesoporous carbon prepared by removing silica
from the carbon/silica nanocomposite. The carbon/silica nanocomposite shows typical non-porous isotherms with non-detectable nitrogen adsorption, indicating that the carbon/silica
nanocomposite is dense to nitrogen at 77 K. Removal of the
carbon or silica from the dense nanocomposites respectively
results in mesoporous silica and mesoporous carbon. Both the
mesoporous silica (b) and mesoporous carbon (c) exhibit
isotherms with appreciable amounts of nitrogen uptake. The
absences of adsorption/desorption hysteresis and the flat
isotherms at relative pressure above 0.4 are consistent with a
narrow pore size distribution without inter-particle macroporosity.[25,26] The mesoporous carbon exhibits a high specific surface area of 2603 m2 g±1 and a specific pore volume of
1.39 cm3 g±1, while the porous silica shows a specific surface
area of 460 m2 g±1 and a specific pore volume of 0.21 cm3 g±1.
The high surface area and porosity of the mesoporous carbon,
which may be contributed to by both micropores and mesopores, are comparable to those of the porous carbons synthesized using zeolite[13,27] or silica xerogel[26] as templates. According to the Barrett±Joyner±Halenda (BJH) analysis,[23,24]
the surface area and pore volume contributed by the mesopores with pore diameter from 2.0 nm to 50 nm are
1472 m2 g±1 and 0.99 cm3 g±1, respectively, indicating that 70 %
Adv. Mater. 2004, 16, No. 11, June 4
The combination of the nitrogen sorption and TEM results
clearly suggests that the mesoporous carbon thin films contain
uniform, three-dimensional, interconnected pore channels. As
shown in Scheme 1, the formation mechanism may involve
co-assembly of sucrose and silicate into the bicontinuous mesostructured composites, which are subsequently converted
into the bicontinuous carbon/silica nanocomposites. As shown
in the Figure 2, the carbon/silica nanocomposite (before the
removal of silica) is dense to nitrogen at 77 K. Previous
study[24] also suggested that direct carbonization of sucrose
without silica template leads to the formation of non-porous
carbon. Subsequent removal of the silica or carbon from these
dense nanocomposites results in mesoporous carbon or mesoporous silica, whose porosities are templated by the silica or
carbon networks, respectively. The interfacial area between
the carbon and silica networks in the carbon/silica nanocomposite can be estimated as 676 m2 g±1 using the specific surface
area of the mesoporous carbon (2603 m2 g±1) and the carbon
content in the composites (26 %, obtained from the thermalgravimetric analysis (TGA)). Similarly, a lower interfacial
area of 340 m2 g±1 is obtained using the specific surface area
and content of the silica. The lower interfacial area may be
attributed to the silica sintering process, in particular, the preferred sintering of the micropores at high temperature. The
characteristic dimension of the carbon network (pore-wall
thickness of the mesoporous carbon) should be similar to the
pore size of the mesoporous silica, which is around 2.0 nm.
Similarly, the characteristic dimension of the silica network
(pore wall thickness of the mesoporous silica) should be similar to the pore size of the mesoporous carbon, which is around
2.4 nm.
In conclusion, continuous mesoporous carbon thin films
have been synthesized for the first time through direct car-
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
885
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bonization of sucrose/silica nanocomposite films and subsequent removal of the silica to create a mesoporous carbon
network. The mesoporous carbon contains an average pore
wall thickness of around 2.0 nm and pore diameter of around
2.4 nm. This technique provides a simple and efficient method
to synthesize continuous, high surface area, and high pore volume mesoporous carbon thin films with uniform and interconnected pore channels. Such continuous mesoporous thin films
are of importance for sensors, separations, membrane reactors, fuel cells, and other applications.
Experimental
Synthesis: For a typical synthesis, a solution of 2.073 g (0.01 mol)
TEOS (Aldrich), 1.807 g (0.10 mol) H2O, and 0.213 g (0.0002 mol
HCl) 1 N HCl was reacted at 60 C for 6 h. 0.608 g sucrose (Sigma)
was then added to achieve a homogenous precursor solution. The
sucrose/silica nanocomposite films were prepared via spin-coating of
the solution on silicon wafers at 2000 rpm. To determine the composition and porosity of the carbon/silica nanocomposites and mesoporous carbon, a large amount of samples were prepared by casting the
precursor solutions in plastic Petri dishes and allowing the rapid evaporation of the solvent. As-synthesized thin films were carbonized at
900 C for 4 h under nitrogen atmosphere to achieve black, shiny carbon/silica nanocomposites. Mesoporous carbon thin films were
achieved by removing the silica using 1 wt.-% dilute HF solution. The
complete removal of silica from the carbon/silica nanocomposites was
confirmed by TGA and X-ray energy dispersive spectroscopy (EDS)
analyses. For comparison, mesoporous silica was also prepared by calcinating the carbon/silica nanocomposites in oxygen at 600 C for
10 h.
Characterization: The morphology and structure of the thin films
was characterized using scanning electron microscopy (SEM, JEOL
JSM-5410, operated using 20 kV voltage), atomic force microscopy
(AFM, Molecular Imaging PicoScan 5, operated using the MAC
mode), and transmission electron microscopy (TEM, JEOL 2010, operated at 120 kV voltage). The porosity of the mesoporous carbon
was measured by a nitrogen sorption technique at 77 K (Micromeritics, ASAP 2010). The samples were degassed at 200 C and below
1.33 Pa for several hours prior to the measurement. Specific surface
areas were determined using the Brunauer±Emmett±Teller (BET)
equation in the P/P0 range of 0.06 ~ 0.20. Pore volumes were determined using the amount of nitrogen uptake at the P/P0 of 0.975. The
surface area and pore volume of the pores with pore diameters from
2.0 to 50 nm were analyzed using the BJH method and the adsorption
isotherms. Compositions of the nanocomposites and mesoporous
carbon were determined by TGA (TA Hi-Res TGA 2950) and EDS
(Oxford Link ISIS 6498 spectrometer). A 80 mL min±1 oxygen flow
and a heating rate of 5 C min±1 were used in the TGA experiments.
Received: December 10, 2003
Final version: March 10, 2004
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Ordered Mesoporous Carbon Hollow
Spheres Nanocast Using Mesoporous
Silica via Chemical Vapor Deposition**
By Yongde Xia and Robert Mokaya*
Mesoporous carbons with well-ordered pore channels are
currently the focus of intense research interest in the field of
nanostructured materials.[1] Recently, various forms of wellordered mesoporous silicas (and aluminosilicates) have been
used as sacrificial templates for the preparation of mesoporous carbons.[2±8] The synthesis procedure basically involves
three steps: introduction of the carbon source (precursor) into
±
[*] Dr. R. Mokaya, Dr. Y. D. Xia
School of Chemistry
University of Nottingham
University Park, Nottingham NG7 2RD (UK)
E-mail: [email protected]
[**] The authors are grateful to the EPSRC for financial support.
DOI: 10.1002/adma.200306448
Adv. Mater. 2004, 16, No. 11, June 4