J Porous Mater (2009) 16:315–319
DOI 10.1007/s10934-008-9202-2
Direct synthesis of ordered mesoporous polymer/carbon
nanofilaments with controlled mesostructures
Rong Kou Æ Qingyuan Hu Æ Donghai Wang Æ
Vijay T. John Æ Zhenzhong Yang Æ Yunfeng Lu
Published online: 11 April 2008
Ó Springer Science+Business Media, LLC 2008
Abstract One-dimensional mesoporous polymer/carbon
nanofilaments with controlled mesostructures have been
prepared by an infiltration process of phenolic oligomers/
surfactant into anodized alumina membranes followed by
carbonization. Transmission electron microscopy (TEM),
nitrogen sorption and X-ray diffraction (XRD) investigations show that as-prepared polymer nanofilaments possess
ordered mesoporous structure tunable from circular hexagonal to cubic and concentric lamellar mesostructures.
After carbonization, carbon nanofilaments with corresponding circular hexagonal, cubic and concentric lamellar
mesoporous structure are obtained.
Keywords One-dimensional Mesoporous Polymer Carbon Nanofilaments
1 Introduction
One-dimensional carbon nanostructures with high surface
area may potentially enhance the device performance in
energy storage and catalysis due to its low dimension and
R. Kou Q. Hu D. Wang V. T. John
Department of Chemical & Biomolecular Engineering,
Tulane University, New Orleans, LA 70118, USA
Z. Yang (&)
State Key Laboratory of Polymer Physics and Chemistry,
Institute of Chemistry, Chinese Academy of Science,
Beijing 100080, P.R. China
e-mail: [email protected]
Y. Lu (&)
Department of Chemical & Biomolecular Engineering,
UCLA, Los Angeles, CA 90095, USA
e-mail: [email protected]
high surface area [1, 2]. Templated synthesis using anodized
alumina membranes (AAM) as templates is widely used to
synthesize one-dimensional nanostructures due to its simplicity and wide applicability of a large variety of materials
[3, 4], which has been used for fabrication of mesoporous
carbon nanofilaments [5, 6]. Previous reports on fabrication
of carbon nanofilaments are based on replication process
from mesoporous silica nanofilaments which involves
infiltration of carbon precursor, carbonization, and removal
of silica template [5, 6]. Recently two groups reported a
simple approach to synthesize one-dimensional carbon
nanostructures by confined assembly of carbon precursor
and structural directing agents within AAM [7, 8]. This
approach avoids tedious replication processes and silica
template removal steps [5, 6, 9, 10]. However, the reported
synthesis approach has not shown flexibility to control
mesostructure and therefore controlled mesostructure in
mesoporous carbon filaments has not been reported. Here we
report a direct synthesis of one-dimensional polymer/carbon
nanofilaments containing tunable ordered mesoporous
structure by a facile infiltration process of phenolic oligomer/surfactant gel solution into AAM followed by direct
carbonization. The resultant mesostructure in polymer/carbon nanofilaments is dependent on that of the feeding gel
used for infiltration. The mesostructure of polymer/carbon
nanofilaments were tunable from concentric hexagonal to
cubic and concentric lamellar mesostructures.
2 Experimental
The experimental procedure used to synthesize the oligomers is similar to that reported previously [11, 12]. Briefly,
0.30 g phenol, 0.06 g 20% NaOH, and 0.53 g formalin
(37%) were reacted at 70 °C for 1 h. After neutralization to
123
316
3 Results and discussion
As synthesized polymer nanofilaments show lengths up to tens
of micrometers and an average diameter of 200 nm (Fig. 1),
which is consistent with the pore structure of the AAO
membranes. Mesostructure of the polymer nanofilaments can
be readily tuned by adjusting surfactant concentration or by
using Pluronic surfactants with different EO-block lengths.
Figure 2 shows XRD patterns of polymer nanofilaments
before and after surfactant removal. Before surfactant
removal, the polymer nanofilaments prepared from P123,
phenol, formaldehyde at a molar ratio of 0.0135:1:2.05 show
an intense reflection peak at the d-spacing of 9.1 nm accompanied by the second peak at 4.7 nm (Fig. 2a). After
surfactant removal, XRD shows a reflection peak with a
decreased d-spacing of 7.8 nm (Fig. 2b). A higher P123
concentration (P123:phenol:formaldehyde = 0.0227:1:2.05)
results in polymer nanofilaments with similar XRD reflections
at 9.2 and 5.4 nm (Fig. 2e). After surfactant removal, broad
XRD peaks centered at 6.9 and 4.5 nm were obtained
(Fig. 2f). The use of F127 (containing longer EO blocks than
100
100
200
f
200
×10
Intensity / a.u.
pH 7 using HCl solution, water in the solution was removed
under vacuum. The obtained oligomers were mixed with
designed amount of Pluronic surfactant in ethanol. Surfactant used included P123 (EO20PO70EO20) and F127
(EO107PO70EO107). Viscous oligomer/surfactant composite
gels obtained after removing most of the ethanol were
infiltrated into AAO membranes with an average pore
diameter 200 nm (Whatman International Ltd.) at 60 °C.
The infiltrated AAO membranes were then heated at 100 °C
in air for 24 h to allow further polymerization. Surfactant
was removed by heating the infiltrated AAO membranes at
350 °C in nitrogen for 2.5 h with a heating rate of 1 °C/min.
The carbonization process was conducted by heating the
infiltrated AAO membranes in nitrogen at 900 °C for 5 h
with a heating rate of 1 °C/min. Products of nanofilaments
were obtained after removal of the AAO membranes using
5 M sodium hydroxide solution. The structure of the
nanofilaments was characterized using transmission electron microscope (TEM, JEOL 2011 FasTEM, 200 kV),
nitrogen sorption (Micromeritics ASAP 2010 at 77 K), and
X-ray diffraction (XRD, Simens D500, Cu-Ka, 40 kV)
techniques. N2 sorption isotherms of polymer nanofilaments
were measured by using polymer/alumina composite. The
sample weight used in the calculation included alumina
membrane which contributes more than 50% to the whole
weight. Since the difficulty to accurately calculate the exact
mass of the polymer nanofilaments, the nitrogen adsorption
isotherms and the corresponding pore size distributions for
the polymer nanofilaments should only be used to provide
qualitative information.
J Porous Mater (2009) 16:315–319
e
110
d
c
100
b
200
×10
0.5
1
1.5
2
a
2.5
3
2 Theta / degree
Fig. 1 SEM image of polymer nanofilaments prepared using AAM
with pore diameter of 200 nm
123
Fig. 2 XRD patterns of as-synthesized polymer nanofilaments using
P123 surfactant at low (a) and high concentration (e) and using F127
(c) as the structural directing agent, and of mesoporous polymer
nanofilaments (b, d, f) prepared by removing surfactant from (a), (c)
and (e) samples, respectively
J Porous Mater (2009) 16:315–319
317
P123) at a molar ratio of F127:phenol:formaldehyde =
0.00653:1:2.05 results in polymer nanofilaments with a broad
peak at 14.4 nm (Fig. 2c). After surfactant removal, the
d-spacing was slightly decreased to 13.1 nm (Fig. 2d).
In order to understand the mesostructure of the polymer
nanofilaments, we synthesized mesoporous polymer films
on glass following a similar synthesis and casting procedure. XRD patterns of these mesoporous polymers (Fig. 3)
indicate the formation of 2D hexagonal (p6m with a unit
cell parameter 9.8 nm), lamellar (interlayer distance of
13 nm before surfactant removal), and body-centered cubic
mesostructure (Im
3m with a unit cell parameter of 13 nm),
which is consistent with mesostructure reported previously
[11, 12]. We believe that mesostructure of the polymer
nanofilaments are similar to those of the films. The less
defined XRD patterns observed for the polymer nanofilaments are due to their much smaller ordered domains that
diffract much less X-ray.
TEM investigations further confirm the 2D hexagonal,
lamellar and the cubic mesostructure within polymer
nanofibers. Figure 4 shows TEM images of the mesoporous
polymer nanofilaments, revealing the formation of novel
ordered mesostructure in compliance with geometric constraint of the cylindrical pore. Consistent with the XRD of
the mesoporous polymer nanofilaments prepared using the
low P123 concentration (Fig. 2b), TEM shows a unique
circular hexagonal mesostructure (Fig. 4a). The hexagonally arranged mesopores are clearly observed at the edges
of the nanofilaments. The formation of such a circular
hexagonal mesostructure is due to the bending of hexagonal liquid crystalline tubes in adapting to the curvature of
AAO pore surface, which has been observed previously
when silicate and surfactant were assembled within AAO
membranes [13, 14].
Consistent with the XRD studies (Fig. 2e, f), TEM of the
mesoporous polymer nanofilaments prepared using the high
100
d
110
200
Intensity / a.u.
300
×10
c
200
210
220
×5
b
100
200
110
210
×10
a
0.7
1.2
1.7
2.2
2.7
2 Theta / degree
Fig. 3 XRD pattern of bulk mesoporous polymer prepared by casting
precursor sol on the substrate with (a) 2D hexagonal (p6m), (b) cubic
(Im
3m) and lamellar mesostructure before (c) and after (d) calcination
Fig. 4 Representative TEM images of mesoporous polymer nanofilaments with (a) circular hexagonal, (b) concentric lamellar, and (c)
cubic mesoporous structures. Inset of (b) showing concentric lamellar
polymer nanofilaments with close ends. Inset of (c) showing a cubic
mesoporous polymer nanofilament at high magnification. Scale bar in
the inset is 50 nm
123
318
450
400
Hexagonal
structure
Cubic structure
350
Lamellar structure
(b)
300
250
200
150
100
Cubic structure
0.012
0.01
Lamellar structure
0.008
0.006
0.004
0
0
0.2
0.4
0.6
0.8
Relative Pressure
P123 concentration show a concentric lamellar mesostructure with inter-layer distances ranging from 13 to 16 nm
(Fig. 4b). The formation of such concentric lamellar mesostructure is due to compliance of a lamellar mesophase with
cylindrical pores that eliminates energetically unfavorable
edge effects [13, 14]. Distinct from thin films containing
lamellar mesostructure that collapses upon removing surfactant, the concentric lamellar structure is preserved after
surfactant removal, which is consistent with the XRD study
(Fig. 2f). Note that most of the concentric lamellar nanofilaments contain open ends; however, polymer nanofilaments with closed ends were also observed occasionally
(see the inset of Fig. 4b), which may be due to the templating
effect from the closed-end cylindrical alumina pores.
Figure 4c show TEM images of mesoporous polymer
nanofilaments prepared using F127 surfactant, revealing a
highly ordered cubic mesostructure. Ordered domains
viewed along [111] and [110] directions can be clearly
observed at the edge and center, respectively. The cell
parameter estimated from the TEM images is approximately 12.6 nm, which is consistent with the value (13 nm)
determined from XRD data. Previous research indicated
that Im
3m cubic mesostructured silica thin films templated
123
Hexagonal
structure
0.002
50
0
Fig. 6 TEM images
mesoporous carbon
nanofilaments with (a) circular
hexagonal, (b) cubic, and (c)
concentric lamellar
mesostructure
0.016
0.014
dV/dD (cm3/g-nm)
(a)
Volume Adsorbed (cm3/g STP)
Fig. 5 (a) Nitrogen adsorption/
desorption isotherms of
mesoporous polymer
nanofilaments with circular
hexagonal, cubic and concentric
lamellar mesostructure; (b) BJH
pore size distribution of the
polymer nanofilaments confined
within alumina pore channels
J Porous Mater (2009) 16:315–319
1
0
10
20
30
40
Pore Size (nm)
by F127 surfactant tend to align their (110) planes parallel
to vapor/liquid or liquid/solid interface [15]. The geometric
confinement imposed by the cylindrical pores directs the
orientation of (110) plane preferentially parallel to the pore
axis. The inset in Fig. 4c shows a high-magnification TEM
image of a cubic mesoporous polymer nanofilament, further
revealing the two distinctive mesostructure orientations.
While it took a large effort to synthesize enough amount
of polymer nanofilaments for nitrogen sorption studies,
nitrogen adsorption–desorption isotherms (Fig. 5a) of the
mesoporous polymer nanofilaments before removing AAO
membranes show type-IV isotherms with significant
adsorption–desorption hysteresis. The pore diameter of the
hexagonal, lamellar, and cubic polymer nanofilaments is
around 9, 5, and 11 nm, respectively, according to the BJH
model (Fig. 5b). The lamellar polymer nanofilaments show
a relatively low surface area and pore volume, which may
be due to partial structure collapse upon the surfactant
removal. Note the significant nitrogen uptake at a high
relative pressure ([0.7) indicates the presence of large
pores, which are the spaces between the polymer nanofilaments and the AAO pore walls created due to the shrinkage
of the polymer nanofilaments.
J Porous Mater (2009) 16:315–319
Carbonization of the polymer nanofilaments converts
them into mesoporous carbon nanofilaments. Figure 6
shows TEM images of mesostructured carbon nanofilaments with (a) circular hexagonal, (b) cubic and (c)
concentric lamellar mesostructure, indicating that the
mesostructure can be preserved through carbonization
process. The average length of the mesostructured carbon
nanofilaments is around several hundred nanometers,
which is shorter than those of polymer nanofilaments
attributable to its fragile mechanical property. The mesostructures shown in Fig. 6 are similar to those shown in
Fig. 4. The cell parameter estimated from the TEM image
for circular hexagonal and cubic carbon nanofilaments are
slightly decreased compared with those of the polymer
nanofilaments due to framework shrinkage upon carbonization. The carbon nanofilaments show less ordered
concentric lamellar structure probably due to a higher
degree of structural collapse upon carbonization process.
319
Acknowledgements The work was partially funded 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-DMR0124765 and CAREER Award), and National Science Foundation of
China (Grant No. 50325313 and 20128004).
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
4 Conclusion
10.
In summary, we have demonstrated synthesis of onedimensional mesoporous polymer and carbon nanofilaments with controlled mesostructure via infiltration of
phenolic oligomer/surfactant gel solution into cylindrical
pores of AAMs. The mesostructure of polymer and carbon
nanofilaments is consistent with that of feeding gels (e.g.
hexagonal, cubic and lamellar) correspondingly. After
carbonization, the mesoporous structure within the novel
carbon nanofilaments are maintained and tunable from
circular hexagonal to cubic and concentric lamellar
mesostructures.
11.
12.
13.
14.
15.
M. Terrones, Annu. Rev. Mater. Res. 33, 419 (2003)
A. Soffer, J. Electroanal. Chem. 38, 25 (1972)
C.R. Martin, Science (Washington, D.C.) 266, 1961 (1994)
Z. Yang, Z. Niu, X. Cao, Z. Yang, Y. Lu, Z. Hu, C.C. Han,
Angew. Chem. Int. Ed. 42, 4201 (2003)
W.S. Chae, M.J. An, S.W. Lee, M.S. Son, K.H. Yoo, Y.R. Kim,
J. Phys. Chem. B 110, 6447 (2006)
D.J. Cott, N. Petkov, M.A. Morris, B. Platschek, T. Bein, J.D.
Holmes, J. Am. Chem. Soc. 128, 3920 (2006)
M.B. Zheng, J.M. Cao, X.F. Ke, G.B. Ji, Y.P. Chen, K. Shen,
J. Tao, Carbon 45, 1111 (2007)
K. Wang, W. Zhang, R. Phelan, M.A. Morris, J.D. Holmes,
J. Am. Chem. Soc. 129, 13388 (2007)
D.H. Wang, H.M. Luo, R. Kou, M.P. Gil, S.G. Xiao, V.O. Golub,
Z.Z. Yang, C.J. Brinker, Y.F. Lu, Angew. Chem. Int. Ed. 43,
6169 (2004)
D.H. Wang, W.L. Zhou, B.F. McCaughy, J.E. Hampsey, X.L. Ji,
Y.B. Jiang, H.F. Xu, J.K. Tang, R.H. Schmehl, C. O’Connor, C.J.
Brinker, Y.F. Lu, Adv. Mater. 15, 130 (2003)
S. Tanaka, N. Nishiyama, Y. Egashira, K. Ueyama, Chem.
Commun. 2125 (2005)
Y. Meng, D. Gu, F.Q. Zhang, Y.F. Shi, H.F. Yang, Z. Li, C.Z.
Yu, B. Tu, D.Y. Zhao, Angew. Chem. Int. Ed. 44, 7053 (2005)
D. Wang, R. Kou, Z. Yang, J. He, Z. Yang, Y. Lu, Chem.
Commun. (Cambridge, United Kingdom) 166 (2005)
Y. Wu, G. Cheng, K. Katsov, S.W. Sides, J. Wang, J. Tang, G.H.
Fredrickson, M. Moskovits, G.D. Stucky, Nat. Mater. 3, 816
(2004)
D.Y. Zhao, P.D. Yang, N. Melosh, Y.L. Feng, B.F. Chmelka, G.
Stucky, Adv. Mater. (Weinheim, Germany) 10, 1380 (1998)
123