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Hierarchically Structured Carbon Synthesis of Carbon Nanofibers Nested inside or Immobilized onto Modified Activated Carbon.

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Communications
Carbon Nanoparticles
DOI: 10.1002/anie.200500685
Hierarchically Structured Carbon: Synthesis of
Carbon Nanofibers Nested inside or Immobilized
onto Modified Activated Carbon
Dang Sheng Su,* Xiaowei Chen, Gisela Weinberg,
Achim Klein-Hofmann, Olaf Timpe, Sharifah Bee
Abd. Hamid, and Robert Schl%gl
The rapidly expanding knowledge about the physical and
chemical properties of carbon nanotubes and nanofibers
(CNTs and CNFs) has led to a range of potential applications
in nanotechnology[1–5] including field-effect transistors, onedimensional quantum wires, field emitters, and hydrogenstorage devices. Most applications of CNTs/CNFs either use
individual objects (in nanoelectronics) or embed the fibers
into a host matrix. The outer surface of CNTs/CNFs can be
tailored to display the desired chemical properties ranging
from metallic (graphite layers parallel to the tube or fiber
axis) and to acidic–basic (all prism faces are exposed along the
fiber axis and saturated with hydroxy groups).[6, 7] The
selective chemisorption of undesired species from drinking
water (Mn, As, Hg, Fe), the modification of the properties of
polymers (heat conduction, electrical conduction, mechanical
reenforcement), and the use of CNTs/CNFs as catalysts or
catalyst supports are only some of the applications of
chemical nanotechnology.[8–19] These applications require,
however, large amounts of CNTs and CNFs with well-defined
surface-chemical and mechanical properties. In addition, the
nanocarbon particles must be formed into larger objects to
optimize the hydrodynamic properties and allow effective
contact with reacting matrices. Loose CNTs/CNFs are
unsuitable as their suprastructural properties cannot be
controlled, and operations of compaction can destroy or at
least inhibit the access of the reactant medium to the
nanostructures. On the molecular level it is potentially
useful to grow dendritic structures of CNTs/CNFs onto each
other to maximize the surface area exposed and to provide
molecular roughness for entanglement of, for instance,
polymer strands.
A hierarchical organization of the nanocarbon units on
itself to form small objects and on a robust carrier structure on
larger dimensions is therefore highly desirable. For chemical
applications it is essential that only one chemical element is
[*] Dr. D. S. Su, Dr. X. Chen, G. Weinberg, A. Klein-Hofmann,
Dr. O. Timpe, Prof. R. Schl,gl
Department of Inorganic Chemistry
Fritz Haber Institute of the Max Planck Society
Faradayweg 4–6, 14195 Berlin (Germany)
Fax: (+ 49) 30-8413-4401
E-mail: dangsheng@fhi-berlin.mpg.de
Prof. S. B. A. Hamid
Combinatorial Technology and Catalysis Research Center
University of Malaya
50603 Kuala Lumpur (Malaysia)
5488
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angewandte
Chemie
used for the construction and that the combination of
nanocarbon with the non-carbon support structures is avoided
as the resulting discontinuities in transport and in chemical
properties would lead to deterioration of the overall compound properties and chemical instability.
An ideal substrate is activated carbon from natural
sources. Such bioinorganic polymers containing typically
lignins and silicates are strong, well-structured in several
dimensions, accessible for chemical modifications, and available in large amounts.[20] Natural precursors to activated
carbons[21] utilize the biological cell structure to preset the
disposition of the macropore network, and they benefit from
inorganic additives as anchoring sites for the catalyst and
therefore for the nanocarbon. It is essential that the chemical
interaction between the nanocarbon and the activated carbon
carrier is strong and inert to preserve the hierarchical
structure during extended operation.
The present paper describes ways to modify the activated
carbon obtained from the biowaste of palm oil production to
serve as the support for the growth of nanocarbon. We
obtained a family of immobilized CNFs in various modifications. Nanostructured carbon can be obtained on the outer
surface of the activated carbon yielding a useful geometry for
binder–filler applications. Alternatively, burning large pores
into the bulk structure of the activated carbon and subsequently filling the pores with carbon nanostructures leads to
hierarchically structured carbon materials with nested CNFs
inside the activated carbon which are highly suitable for
sorption and catalytic applications.
The concept behind the production of these hierarchically
structured carbon materials is illustrated in Scheme 1. On the
macroscopic level, bulk activated carbon aggregates are
transformed into a macroporous scaffold by selectively
removing the “soft” parts of the biopolymer. This is achieved
by catalytic burning during or after carbonization. The
catalyst particles are then deliberately embedded in the
carbon matrix by transforming the oxidic form of the catalyst
used for creating porosity into a metallic form. The metal
Scheme 1. The synthesis of hierarchically structured carbon materials
by modifying activated carbon with nanocarbon.
Angew. Chem. Int. Ed. 2005, 44, 5488 –5492
particles gasify the support locally under a reducing atomosphere. This form is then used for growing nanocarbon by
chemical vapor decomposition (CVD) of organic molecules
in one or several generations; if needed additional catalyst is
embedded. The resulting immobilized nanocarbon-activated
carbon composite (NAC) can then be assembled into larger
solid aggregates by growing nanocarbon on the outside of the
activated carbon grains to bind NAC particles firmly to each
other. The transport of matrix media can be well controlled by
changing the density of the nanostructures and by varying the
filling factor of the porous carbon support. A final stage of
chemical surface modification that may be necessary is not
shown in Scheme 1. The preferential location of catalyst
deposition inside or outside of the activated carbon scaffold
can be varied by changing the deposition conditions of the
catalytic precursor from ion exchange to homogeneous
precipitation.
By varying the unit operations—1) impregnation with the
catalyst, 2) calcinations of the support carbon, 3) reduction of
the catalyst and growth of nanocarbon—a family of nested or
immobilized CNTs/CNFs can be obtained. We use the CVD
method for growing the CNTs/CNFs as it has several
advantages over electric-arc discharge and laser ablation.
For example, the CVD method leads not only to single-walled
or multiwalled CNTs but also to aligned CNTs when different
hydrocarbons are used. Carbon growth on inner surfaces is
possible, large amounts of CNTs/CNFs can be synthesized at
moderate temperatures and with low cost, and structure can
be controlled by regulating the growth parameters. The CVD
method is the most promising method for the large-scale
synthesis of CNTs/CNFs.[1, 22–26] A suitable choice for the
catalyst system is iron, as it catalyzes both the oxidation of
carbon and nanocarbon growth.
A typical carbonized precursor obtained from palm
kernel shell exhibits a BET surface area of 1081 m2 g 1; the
pore volume is 0.365 cm3 g 1. A scanning electron micrograph
(SEM) of this activated carbon is displayed in Figure 1 a. To
prepare the host for the designed hierarchical structure, the
as-obtained AC was oxidized under mild conditions at 400 8C
in air for 4 h. The activated carbon suffered a weight loss of
about 5.2 %. Figure 1 b shows a SEM image of activated
carbon after the mild oxidation. Oxidation at this temperature
removes also carbon debris from the surface and cleans the
pores of activated carbon. Furthermore the oxidation
increases the pose size as is apparent by comparison of the
electron micrographs in Figure 1 a and 1 b.
The SEM image in Figure 1 c shows the iron catalyst
particles observed on the surface of the AC catalyst after
impregnation, oxidation, and reduction of the catalyst (see
the Experimental Section). The specific BET surface area of
the Fe/AC catalyst is 1490 m2 g 1, and the pore volume
increases to 0.551 cm3 g 1. The plot of BJH pore-size distribution in Figure 2 shows that the volume of mesopores in the
Fe/AC catalyst is substantially greater than that of the parent
material. In parallel to the formation of mesopores, the
micropores are also enlarged as evident in the BJH plot at a
pore size of about 2 nm (Figure 2). After catalytic decomposition of a mixture of C2H4 and H2 at 700 8C on the Fe/AC
catalyst, carbon nanofibers form and cover the activated
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
The yield of carbon nanofibers on an Fe/AC host reaches
ca. 105 g of C per g of Fe. The growth mechanism is the same
as that known for the CVD preparation of CNTs/CNFs, which
consists of the dissolution, diffusion, and precipitation of
carbon atoms on the catalyst particle.[27] Secondary electron
and backscattered electron images of carbon nanofibers on
the activated carbon are presented in Figures 3 a and 3 b,
Figure 1. SEM images of a) activated carbon (AC); b) activated carbon
calcined at 400 8C; c) 1 wt % Fe/activated carbon, the inset is a
enlarged image of the catalyst particles; d) carbon nanofibers on activated carbon; e) a cross section of CNFs on the outer surface of activated carbon; f) a single CNF showing the rough surface morphology.
Scale bars: 2 mm (a–c), 20 mm (d), 10 mm (e), and 500 nm (f).
Figure 3. a, b) SEM images of immobilized CNFs with the Fe catalyst:
a) SE mode, b) BSE mode; light spots correspond to Fe particles;
c) TEM image of an immobilized CNF with a Fe particle on the tip;
d) high-resolution TEM image showing the herringbone structure of a
CNF. Scale bars: 5 mm (a, b).
Figure 2. Pore-size distribution (determined by BJH adsorption) of the
activated carbon, Fe/AC, and activated carbon with CNFs grown inside
and outside. For distinction the plot of NAC is offset by 0.02 units; the
plot of AD is offset 0.03 units.
carbon (as shown in Figure 1 d). They orient randomly and are
entangled. The diameters of the fibers are widely distributed
from 20 to 300 nm. The cross-section SEM image in Figure 1 e
confirms that the CNFs are grown on the outer surface of the
AC. The SEM image of a single CNF in Figure 1 f reveals the
considerable roughness of the outer surface of the CNFs. Such
a morphology is beneficial for the hydrodynamics of sorption
processes from the liquid phase and provides mechanical and
chemical anchoring sites when it is used as additive in polymer
materials.
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respectively. From a comparison of the contrasts, the iron
catalyst particles can be located as bright objects in the
backscattered electron micrograph. The growth of CNFs
follows the “tip-growing” model:[5, 28] most of the iron catalyst
particles are found on the tips of the CNFs (transmission
electron micrograph (TEM) in Figure 3 c), although in few
cases the iron particles are found not only at the tip but also in
the middle of carbon nanofiber.
The high-resolution TEM image (Figure 3 d) of the carbon
nanofibers reveals that the carbon nanofibers are of the
herringbone type, which explains the origin of the surface
roughness seen in SEM (Figure 1 f). Keeping in mind the
intended applications, we wanted to expose as many prismatic
faces as possible on the surface of the nanocarbon without
losing mechanical stability. The shape and the size of the
metal catalyst control the nanocarbon morphology.[29] Baker
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 5488 –5492
Angewandte
Chemie
et al. reported that nanofibers are obtained from large Fe
particles (> 20 nm), while nanotubes were formed from
smaller particles (< 20 nm).[30, 31] The high loading and the
activation conditions caused the catalyst particles to aggregate (see Figure 3 c) and form the desired rough surface of the
CNFs.
Figure 4 a presents a cross-section SEM image though an
AC particle with a large pore in it after the CNF growth. Hair
or microbrush structures are found inside the pore (cf. the
Figure 4. a) SEM image of cross section of CNFs on activated carbon,
b) enlarged image of a; c) TEM image of cross section of CNFs on
activated carbon; d) high-resolution image of a CNF in (c). Scale bars:
2 mm in a, 200 nm in b–c.
reaction atmosphere of their growth. Chemisorption by ion
exchange with OH groups should thus be negligible. On the
other hand, the nanostructured voids between the entangled
CNFs and their rough surfaces should support the agglomeration of condensable species and should thus allow the
filtering of difficult-to-adsorb species, for example, the oxo
anions of transition metals.
We tested this in a demonstration experiment comparing
the adsorption of noncondensable chromate in basic solution
and condensable heteropolymolybdate H3[PMo12O40] in solution. In Table 1 the adsorption capacities per unit mass and
unit surface area are listed. The activated carbon adsorbs
chromate by means of ion exchange with the acidic surface
hydroxy groups. After treatment and growth of the CNFs
these surface hydroxy groups are absent, depriving the
material of its adsorption function. On the other hand, the
polyoxomolybdate is poorly anchored to the hydroxy groups
owing to its unfavorable charge-to-surface ratio. Sites for
polycondensation of the anions into oxyhydroxide polymers
are created by changing the surface texture of the activated
carbon, and even better by introducing the CNF filling into
the pore system. The absorption capacity of the activated
carbon increases a factor of 5 when it is mildly oxidized at
400 8C, but by a factor of 27 when CNFs are grown on and
inside the activated carbon (Table 1).
In summary, we have prepared hierarchically structured
carbon with CNFs nested inside or immobilized onto
modified activated carbon by means of chemical vapor
decomposition of ethylene. The activated carbon is obtained
from the biowaste of palm oil production, pretreated by mild
Table 1: Adsorption of chromate and heteropolymolybdate in aqueous solution by activated carbon
SEM image Figure 1 b). The enlarged SEM
(AC), activated carbon calcined at 400 8C (AC-400), and nanocarbon-activated carbon composite (NAC).
image in Figure 4 b reveals that the dentritic
BET
Ads. [CrO4]2 (rel.)
Ads. [PMo][c]
Ads. [CrO4]2
Ads. [PMo] (rel.)
structure results from entangled CNFs
[m2 g 1]
[mmol m 2][b]
[mmol g 1][a]
[mmol g 1]
[mmol m 2]
created during the CVD process. The
AC-400
1490
67
0.016
15.5
0.120
cross-section TEM image of the pore in
AC
1081
73
0.034
2.5
0.024
Figure 4 c proves convincingly that CNFs
NAC
305
55
–[d]
16.6
0.660
were grown on the inner wall of the pore.
blank
–
55
–
0[e]
–
Moreover, the distribution of diameters of
[a] Estimated from absorption at l = 420 nm. [b] Values are based on difference to blank. [c] [PMo] =
CNFs in the pores is quite narrow, in the
[PMo12O40]3 . [d] The concentration after the experiment is identical to that of the blank. [e] In all
range of 20–50 nm. In addition, the CNFs
solutions the absorption (l = 325 nm) after the experiment was enhanced; adsorption of [PMo] for blank
grown inside the pores are shorter than
is used as the reference.
those grown on the surface of the activated
carbon. This could be a result of the limited
oxidation, and impregnated with an iron catalyst. The method
transport of ethylene into the pores. The microstructure of the
is cheap and can be scaled up. The preliminary results of
CNFs in the pores is revealed by high-resolution TEM
adsorption experiments prove that the deliberate nanostruc(Figure 4 d).
turing of a carbon surface can result in novel adsorbant
The microscopic data were confirmed by the specific BET
properties. It is expected that the polymerization of metallic
surface area of activated carbon with CNFs inside which
polyanions can be optimized by changing the pore-size
decreases to 305 m2 g 1 and is much smaller than that of
distribution, adjusting roughness of the CNFs, and adding
activated carbon and of the Fe/AC catalyst. The pore volume
special surface functional groups.
decreases to 0.289 cm3 g 1. This is attributed to the partial
filling of the pores in the matrix by CNFs. The pore-size
distribution (Figure 2) changes drastically with a broad
distribution of nanometer-sized pores being attributed to
Experimental Section
the desired voids between the entangled CNF fillers.
The activated carbon used as the support was supplied by COMBIThe surface of the freshly prepared CNFs should be poor
CAT research center, University of Malaya (Malaysia). It was made
in acidic oxygen functional groups due to the reductive
from palm kernel shells, a waste product from palm oil production.
Angew. Chem. Int. Ed. 2005, 44, 5488 –5492
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5491
Communications
Beside carbon, it contains substantial amounts of silicate and traces of
iron as iron silicate after the activation. Palm kernel shells were
carbonized in N2 at 350 8C and activated in steam at 600 8C. The
support activated carbon was calcined at 400 8C for 4 h in air. The Fe/
AC precursor (1 wt % Fe) was prepared by impregnation with a 0.09 m
aqueous solution of Fe(NO3)3 and activated carbon calcined at 400 8C
with a loading of 1 % iron oxide onto dry carbon. A drying step at
60 8C overnight was followed by the autogeneous pore formation
through calcination at 500 8C in N2 for 4 h. The Fe-impregnated
sample was reduced with H2 at 700 8C for 4 h. The Fe/AC catalyst
(100 mg, 1 wt % Fe) was put into the vertical quartz reactor and
flushed with He overnight. The carbon nanofibers were grown by
treating the Fe/AC catalyst at 700 8C in flowing 60 % C2H4/H2
(125 mL min 1) at atmospheric pressure for 2 h. The obtained
sample was cooled to room temperature in a stream of He.
For the adsorption experiments, 10 mg of activated carbon or the
NAC sample were suspended in 1.5 mL of aqueous H3PMo12O40 or
K2CrO4 (1 mm). The suspensions were stirred for 1 h at room
temperature. The concentration of [PMo12O40]3 was measured by
photometry (l = 325 nm). The adsorption test was performed in an
Eppendorf cap (Volume 2.0 mL, material is PE). The pH was not
adjusted (with additional buffer); the pH calculated for heteropolymolybdate solution is around 3.0, for chromate solution around 8.5.
SEM investigations were performed on a Hitachi S-4000 FEG in
secondary-electron (SE) mode and backscattered-electron (BSE)
mode at 15-kV accelerating voltage. TEM studies were performed on
a Philips CM 200 LaB6 operating at 200 kV.
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Received: February 23, 2005
Published online: July 25, 2005
.
Keywords: activated carbon · adsorption · carbon nanofibers ·
hierarchical structure
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