Formation of porous carbon materials with in situ generated NaF nanotemplate

Article abstract of DOI:10.1021/jp0618835

Porous carbon materials with pore sizes from 3 to 200 nm were synthesized by reacting hexafluorobenzene with Na liquid at 623 K. NaF crystals, a byproduct formed in the reaction, acted as nanotemplate to assist the pore formation. By employing hexafluorobenzene to react with Na incorporated within the channels (diameter 200 nm) of anodized aluminum oxide (AAO) membranes at 323-623 K, the carbon material can be fabricated into aligned porous nanotube arrays (ca. 250 nm in diameter, ca. 20 nm in wall thickness, ca. 0.06 mm in length, and ca. 3-90 nm in pore diameter). These materials were characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray energy dispersive spectroscopy, electron diffraction, thermal gravimetric analysis, and nitrogen physical adsorption experiments.

Full text of DOI:10.1021/jp0618835

11818
J. Phys. Chem. B 2006, 110, 11818-11822
Formation of Porous Carbon Materials with in Situ Generated NaF Nanotemplate
Chih-Hao Huang,† Yu-Hsu Chang,† Hsiao-Wan Wang,‡ Soofin Cheng,‡ Chi-Young Lee,§ and
Hsin-Tien Chiu*,†
Department of Applied Chemistry, National Chiao Tung UniVersity,
Hsinchu, Taiwan, 30050, Republic of China, Department of Chemistry, National Taiwan UniVersity,
Taipei, Taiwan, 10673, Republic of China, and Materials Science Center, National Tsing Hua UniVersity,
Hsinchu, Taiwan, 30043, Republic of China
ReceiVed: March 27, 2006; In Final Form: April 27, 2006
Porous carbon materials with pore sizes from 3 to 200 nm were synthesized by reacting hexafluorobenzene
with Na liquid at 623 K. NaF crystals, a byproduct formed in the reaction, acted as nanotemplate to assist the
pore formation. By employing hexafluorobenzene to react with Na incorporated within the channels (diameter
200 nm) of anodized aluminum oxide (AAO) membranes at 323-623 K, the carbon material can be fabricated
into aligned porous nanotube arrays (ca. 250 nm in diameter, ca. 20 nm in wall thickness, ca. 0.06 mm in
length, and ca. 3-90 nm in pore diameter). These materials were characterized by X-ray diffraction, scanning
and transmission electron microscopy, X-ray energy dispersive spectroscopy, electron diffraction, thermal
gravimetric analysis, and nitrogen physical adsorption experiments.
Introduction
Porous materials provide excellent opportunities in numerous
technological applications.1,2 Among different types of porous
materials, porous carbon (p-C) has high potential as the com-
ponent in catalysis, adsorption, sensing, and fuel-cell systems.3-6
Many types of carbon materials with different pore sizes have
been fabricated through replica approaches employing various
zeolites or silica as the templates and casts.7-12 Another fre-
quently used method employs defluorination of PTFE, poly(tetra-
fluoroethylene), by alkali metals.13-15 In the present work, we
wish to report an efficient new synthesis of p-C by reacting the
vapor of C6F6 with Na metal. The growth is an interesting exam-
ple of a phase separation assisted self-templating process. More-
Figure 1. XRD of an as-formed raw product prepared at 623 K.
over, template-assisted growth of carbon nanotubes (CNT) has
bubbled with the assistance of a constant flow of Ar (5 sccm)
been demonstrated to be an efficient method to produce aligned
at 323 and 623 K for 4 h to generate black products. The as-pre-
arrays of CNT.16-23 By coupling the reaction between C6F6 and
pared products were further washed in refluxing distilled water
Na with a reactive-template strategy reported before,24 which
(300 mL) overnight. Then, the AAO was removed by immersion
employs anodic aluminum oxide (AAO) membrane filled with
in 48% HF at room temperature for 9 h. Finally, the products were
Na metal as an active cast, arrays of porous CNT (p-CNT) can
filtered, rinsed with distilled water, and dried at 373 K in air.
be fabricated.
Characterization of p-C Materials. A scanning electron
Experimental Section
microscope (JEOL JSM-6330F at 15 kV and HITACHI S-4000
at 25 kV) and a high-resolution transmission electron microscope
(JEOL JEM-4000 at 400 kV) were used to observe the sample
morphology. X-ray energy dispersive spectroscopy was used
to confirm the element composition of the samples. Crystallinity
of the samples was investigated by an X-ray diffractometer
(BRUKER AXS D8 ADVANCE, Cu KR radiation, 40 kV and
40 mA). A physical adsorption instrument (Micromeritics ASAP
2010, at 77 K with N2 gas) was used to investigate surface area
and pore size distribution of the samples. Thermal gravimetric
analysis (TGA) was carried out on a Pyris Diamond TG/DTA
instrument (heating rate of 5 deg/min from room temperature
to 900 °C, air flow rate at 100 mL/min).
Synthesis of p-C. Na metal was prepared by pyrolyzing NaH
(0.15 g, 6.3 mmol, Aldrich) under Ar atmosphere on a silica boat
inside a tube furnace at 623 K for 1 h. To the as-formed Na
metal at 623 K, C6F6 (99%, Aldrich) vaporized at 298 K was in-
troduced by bubbling under a constant flow of Ar (5 sccm) for
4 h. The as-prepared black product was further washed in reflux-
ing distilled water (300 mL) overnight. The solid product was
collected, rinsed with distilled water, and dried at 373 K in air.
Synthesis of p-CNT. Na@AAO was prepared by pyrolyzing
NaH (0.15 g, 6.3 mmol, Aldrich) on AAO (Whatman Anodisc
13, pore diameter of 200 nm, thickness of 60 µm) at 623 K for
1 h under an Ar atmosphere.24 The as-prepared Na@AAO was
reacted with C6F6 (99%, Aldrich), maintained at 298 K, and
Results
* Address correspondence to this author. E-mail: htchiu@cc.nctu.edu.tw.
† National Chiao Tung University.
Preparation and Characterization of p-C. In general, via
a Wurtz-type coupling reaction,25 the vapor of C6F6 was reacted
with Na metal, prepared by pyrolyzing NaH under Ar, at 623
‡ National Taiwan University.
§ National Tsing Hua University.
10.1021/jp0618835 CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/27/2006

Formation of Porous Carbon Materials
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11819
Figure 2. Images of p-C prepared at 623 K.: (A) low and (B) high magnification SEM and (C) low- magnification and (D) high-resolution TEM.
Figure 3. TEM images of phase separated p-C and NaF: (A) low magnification image showing NaF crystals pointed out by the arrow and (B)
high-resolution image of NaF nanocrystals selected from the circled area in part A.
K for 1 h to offer a dark solid product. As described in the next
section, the reaction can be performed at a temperature as low
as 323 K. As shown in Figure 1, X-ray diffraction (XRD) of
the solid produced at 623 K displays a diffraction pattern that
is composed of a broad (002) reflection from graphite (JCPDS
23-0064) and (200), (220), and (222) reflections from NaF
(JCPDS 36-1455). No other significant structural information
was observed. After the sample was washed with distilled water,
the salt was removed and the black solid product was collected.
Figure 2A is a typical scanning electron microscopic (SEM)
image of the isolated material. In the enlarged image, Figure
2B, morphology of the material is shown to have innumerable
pores with diameters less than 200 nm. X-ray energy dispersive
spectroscopy (EDX) confirms that the solid is composed of
carbon while the concentrations of sodium and fluorine are
below the detection limits. A low-magnification transmission
electron microscopic (TEM) image in Figure 2C also shows
the porous nature of the solid product, with pore diameters less
than 200 nm. In a high-resolution transmission electron micro-
scopic (HRTEM) image (Figure 2D) of the sample, the presence
of fringes spaced ca. 0.35 nm apart is observed. Since the fringes
are not well ordered, we suggest that the edges of the p-C have
a short-range ordering of graphite texture. The thickness of the
p-C wall, deduced from the image, is 3-15 nm. Occasionally,
NaF nanoparticles can be observed inside p-C by TEM. For
example, at point indicated by the arrow in Figure 3A, the
presence of a NaF crystal ca. 80 nm in size is identified by
EDX. In addition, from the circled area in Figure 3A, several
minute nanocrystals with sizes of 2-4 nm are observed by
HRTEM, as shown in Figure 3B. The lattice fringes are spaced
0.23 nm apart, which coincides well with the {200} interplanar
distance of NaF (JCPDS 36-1455). This confirms that the
nanosized NaF crystals acted as the templates for the formation
of the porous structure.
Preparation and Characterization of p-CNT. Previously,
we demonstrated that by pyrolyzing NaH on top of AAO
membranes, the as-formed Na flowed into the channels of AAO
and produced reactive templates Na@AAO.24 By reacting
Na@AAO with C6Cl6, amorphous carbon nanotubes were
fabricated inside the AAO cast. On the basis of the product
morphology, we concluded that metallic Na existed as nanotubes
inside the AAO channels of Na@AAO. We extend the strategy
in this study. By employing C6F6 to react with Na@AAO at
323 and 623 K, black products were insolated. After the AAO
template and the salt were removed, the products were
investigated by electron microscopy. A low-magnification SEM
image of the product prepared at 323 K after the AAO cast
was removed is shown in Figure 4A. It displays that the sample

11820 J. Phys. Chem. B, Vol. 110, No. 24, 2006
Huang et al.
Figure 4. Images of p-CNT prepared at 323 K.: (A) low and (B) high magnification SEM and (C) low- and (D) high-magnification TEM.
Figure 5. Images of p-CNT prepared at 623 K: (A) low and (B) high magnification SEM and (C) low- and (D) high-resolution TEM.
is an aligned array of nanotubes with diameters of 200-250
nm. This is close to the channel diameter of the AAO template.
An EDS study confirms that the material is composed of carbon.
From other SEM images, the length of the nanotubes is
estimated to be 60 µm, which is close to the thickness of the
AAO membrane. Figure 4B shows an enlarged SEM image of
a p-CNT with undulate inner and outer surfaces. On the basis
of the results discussed in the previous section, the unevenness
of the surfaces is attributed to the presence of pores generated
by the byproduct NaF. A TEM image (Figure 4C) of a tube
also shows an average diameter of 220 nm, which is comparable
to the SEM observation. In Figure 4D, an HRTEM image shows
the detailed microstructure of a p-CNT. It reveals a wall
thickness of ca. 15-20 nm and numerous pores of several
nanometers inside the wall. Thus, we successfully fabricated,
via a simple process, a new type of p-CNT from this unique
self-templating and casting approach.
From SEM images and EDX studies, the sample prepared at
623 K is also identified to be an aligned array of p-CNT (lengths
of ca. 60 µm, diameters of ca. 200-300 nm, and pore sizes of
ca. 20-100 nm). Examples of low- and high-magnification
images are shown in Figure 5, panels A and B, respectively. In
Figure 5C, a TEM image of an individual p-CNT shows the
presence of innumerable pores of 15-90 nm in the wall, which
correlates well with the SEM observation. An HRTEM image
(Figure 5D), selected from the edge of the p-CNT shown in
Figure 5C, displays textured graphene layers. The overall pore
structure in p-CNT is nearly identical with that of p-C discussed

Formation of Porous Carbon Materials
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11821
Figure 6. (A) Cross sectional SEM image of AAO after removal of p-CNT prepared at 623 K showing crystals in the channels and (B) EDS from
the circled area in part A.
above. A TGA result showed that a p-CNT sample, annealed
at 1173 K under vacuum followed by removing the AAO
template, was oxidized in air at 710 K. The sharp weight loss
suggests that the sample contained less ordered carbon struc-
ture.26 The process removed ca. 63% weight of the material in
the sample. The residue, which was stable up to 1173 K, is
assigned to unremoved NaF byproduct.
The following observations further support the role of the
NaF byproduct as an in situ generated nanotemplate for pore
formation. Figure 6A shows an enlarged cross-sectional SEM
view of an AAO membrane after p-CNT formed at 623 K was
removed from the cast. On the exposed channel surface, solid
islands with sizes of several tens of nanometers are observed.
An EDX study (Figure 6B) of an island, selected from the
circled area in Figure 6A, reveals the presence of Na and F.
Consequently, the islands are identified to be NaF crystals. The
crystal size and shape are in good agreement with the pore
structures observed in Figure 5B,C. Thus, we conclude that the
NaF nanocrystals and the AAO membrane acted as the cast to
influence the p-CNT structure cooperatively. The NaF worked
as the in situ generated nanotemplate, which assisted the pore
formation, while the AAO membrane performed the role of the
cast, which aided the tubular shape development.
BET Analysis of p-C Materials Prepared at 623 K. From
the SEM and TEM studies discussed above, the carbon materials
possessed pores with sizes ranging from several to several tens
of nanometers. Typical nitrogen adsorption-desorption iso-
therms of p-C and p-CNT prepared at 623 K are shown in Figure
7, panels A and B, respectively. In the region of middle and
high P/P0, the amount of N2 adsorption increases sharply. The
phenomenon, a steep increase in the isotherm at high pressures
(P/P0 f 1), confirms that large macropores are present in the
materials. In addition, large hystereses are observed in Figure
7. This is evidence for a network pore system with distinct
polydispersity in size.27,28 The observation is in good agreement
with the TEM images in Figures 2C and 5C. Estimated from
Figure 7A, the Brunauer-Emmett-Teller (BET) surface area
is 161 m2 g-1. By using Barret-Joyner-Halenda (BJH) method,
the smallest pore size in p-C can be calculated from the
desorption branch of the nitrogen isotherm. The reason the
desorption branches, instead of the adsorption ones, were used
is explained below. According to the Kelvin equation,27 the
desorption branch of the isotherms is usually more reasonable
to derive the diameters of cylindrical pores when the contact
angle between the adsorbate, which was nitrogen in this study,
and the pore wall is assumed to be zero. Nevertheless, the huge
hysteresis loops of the isotherms of p-C and p-CNT materials
indicate the pores could probably be described by an ink-bottle
model.27 The rapid increase in adsorption volume in the
Figure 7. Nitrogen adsorption-desorption isotherms and BJH pore
size distribution from the desorption branches (insets): (A) p-C (9:
adsorption branch; 0: desorption branch) and (B) p-CNT (b: adsorp-
tion branch; O: desorption branch).
adsorption branches at higher P/P0 was related to the large
diameter of the bottle shaped pores, while that in the desorption
branches at lower P/P0 was correlated to the smaller diameter
of the neck. Thus, the pores of this region are estimated to be
as small as 3-3.5 nm. This agrees with the mesopores (sizes
2-4 nm) observed in Figure 3B, which formed after the NaF
nanocrystals in the carbon matrix were removed. This is
comparable to a previous study in which removal of fluorides
from a carbon matrix by acid treatment was found to leave
empty sites as pores.29 The BET surface area estimated from
Figure 7B is 183 m2 g-1. The calculated pore size distribution
from the desorption branch, using the BJH method, also displays
that the pores exhibit a size limit at 3-3.5 nm. These

11822 J. Phys. Chem. B, Vol. 110, No. 24, 2006
Huang et al.
SCHEME 1: Preparations of p-C and p-CNT
material effectively by passing through the porous structure.34,35
This may offer a new type of low-density and high-strength
composite material. Investigation is in progress.
Acknowledgment. This work was supported by the National
Science Council of Taiwan, Republic of China.
Supporting Information Available: XRD pattern of as-
formed p-C raw product and TGA of p-CNT. This material is
available free of charge via the Internet at http://pubs.acs.org.
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For summary, a scheme for the formation of p-C materials
by reacting C6F6 with Na and Na@AAO is shown in Scheme
1. The process utilized the strong reducing capability of metallic
Na to remove F atoms from C6F6 and facilitated the solid carbon
formation via a Wurtz-type coupling reaction. As shown in the
TEM images in Figure 3, due to phase separation, the NaF
byproducts of various sizes are incorporated within the carbon
solid. This assists the formation of the macro- and mesoporous
structures. Another possible origin of the mesopores, as sug-
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graphene sheets shown in Figures 2D and 5D.30 The p-CNT
prepared from C6F6 in this study shows significant structural
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byproduct salts NaF and NaCl. This includes the melting point
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Conclusions
By employing C6F6 to react with Na, a unique type of p-C
material was formed. The self-generated NaF acted as the
nanotemplate to shape the carbon material into the observed
porous structure. By using the reactive template Na@AAO, the
p-C can be fabricated further into nanotubes with a structure
significantly different from the Iijima type CNT.32,33 The process
reported here is relatively low temperature and does not require
plasma, autoclave, and catalyst assistance. We anticipate these
new materials to be useful in many applications. For example,
a preliminary investigation showed that p-CNT retained essential
nanostructural features after being graphitized at 3073 K. We
expect polymer chains could entangle this unique carbon
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