Synthesis of polygonized carbon nanotubes utilizing inhomogeneous catalyst activity of nonspherical Fe3O4 nanoparticles

Article abstract of DOI:10.1021/jp062690k

The synthesis of novel carbon nanotubes (CNTs) with polygonal cross sections by heating a powder mixture of ferrocene, oxalic acid, and the alkali metal potassium at mediate temperatures (480-500 °C) is reported. This kind of special polygonized CNTs has two distinctive characters: first, ribbonlike polygonized CNTs have diameters between 60 and 200 nm, and the lengths as long as several microns; second, the edge of polygonized CNTs is well-graphitized, the wall of which is amorphous. On the basis of evidence that the formation of polygonized CNTs appears to be strongly determined by inhomogeneous catalytic activity of nonspherical Fe₃O₄ nanoparticles, we propose the possible growth model.

Full text of DOI:10.1021/jp062690k

16404
J. Phys. Chem. B 2006, 110, 16404-16407
Synthesis of Polygonized Carbon Nanotubes Utilizing Inhomogeneous Catalyst Activity of
Nonspherical Fe3O4 Nanoparticles
Wen Qian, Jiafu Chen, Liusuo Wu, Fangyu Cao, and Qianwang Chen*
Hefei National Laboratory for Physical Sciences at Microscale and Department of Materials Science &
Engineering, UniVersity of Science & Technology of China, Hefei 230026, PRC
ReceiVed: May 2, 2006; In Final Form: June 26, 2006
The synthesis of novel carbon nanotubes (CNTs) with polygonal cross sections by heating a powder mixture
of ferrocene, oxalic acid, and the alkali metal potassium at mediate temperatures (480-500 °C) is reported.
This kind of special polygonized CNTs has two distinctive characters: first, ribbonlike polygonized CNTs
have diameters between 60 and 200 nm, and the lengths as long as several microns; second, the edge of
polygonized CNTs is well-graphitized, the wall of which is amorphous. On the basis of evidence that the
formation of polygonized CNTs appears to be strongly determined by inhomogeneous catalytic activity of
nonspherical Fe
O
3 4
nanoparticles, we propose the possible growth model.
1. Introduction
2. Experimental Section
1
The discovery of carbon nanotubes (CNTs) has stimulated
Ferrocene (98%), oxalic acid (99.95%), ethanol, hydrochloric
acid (analytically pure), and metallic potassium (98%, chemi-
cally pure) were all used as purchased. The typical reaction
begins by heating a mixture of 5.00 g of H2C2O4, 0.50 g of
Fe(C5H5)2, and 3.75 g of metallic K in a stainless steel autoclave
with a capacity of 15 mL at a temperature of 480-500 °C for
10 h, and then allowing it to cool to room temperature naturally.
The as-prepared products were gray and spongy, were washed
with ethanol and distilled water; it was designated as Sample 0
(S0). A part of the S0 sample was then treated with 4.0 mol/L
HCl aqueous solution, the precipitate was collected by use of a
microfilter, it was then washed with distilled water several times
until the pH value was about 6, and it was finally dried in an
oven at 110 °C; it was designated as Sample 1 (S1).
intense interest in the fields of synthesis and technological
applications due to their fascinating electronic, mechanical,
and chemical properties.2 Up to now, various shapes of CNTs,
such as straight, curved, helical, zigzag, and bamboo-shaped,
,3
4
-6
have been prepared from various carbon resources,
all of
them are one-dimensional cylinderlike carbon materials. Conven-
tionally, CNTs are generally believed to be perfect rolled
graphene sheets, because CNTs with circular cross sections are
assumed to minimize the strain energy of free, isolated nano-
tubes.
7,8
Several kinds of semiconductor nanomaterials, such as ZnO,
9
10
11
Sb2S3/Sb2Se3, GaN, and carbon nanocoils, can form polyg-
onal nanoprisms, but the formation of polygonized CNTs is very
difficult. Recently, several theoretical analyses have shown that
polygonization of the single-walled CNTs (SWNTs) may be
induced by flattening the tubes against each other in micro-
The XRD pattern was recorded in the 2θ range of 10-70°
by using a Rigaku (Japan) D/max-γA X-ray diffractometer
equipped with graphite monochromatized Cu KR radiation
(λ ) 1.54187 Å). The morphology and structure of CNTs were
observed by a Hitachi model H-800 transmission electron
microscope (TEM) with electron diffraction and a JEOL-2010
high-resolution transmission electron microscope (HRTEM) with
an energy dispersive X-ray detector, using an accelerating
voltage of 200 kV. For TEM and HRTEM observations, the
products were separated in ethanol by ultrasonic dispersion and
then transferred onto carbon-coated copper grids. The Raman
spectrum was taken on a LABRAM-HR Confocal Laser Micro-
12,13
bundles as the inter-tube van der Waals interaction increased.
Furthermore, the polygonized CNTs have ever been observed
14,15
under high pressure,
or synthesized through CO2 laser
ablation.16
*
SWNTs with different cross sections exhibit different σ -
π hybridization which occur locally at the polygonized edges,
and consequently different electronic properties, such as metal/
*
1
7,18
19
insulator character.
Tamura et al. reported that a type of
large paramagnetic persistent current is revealed in polygonal
CNTs through calculation of the persistent current-induced
magnetic moment. In general, the transformation of the cross
section induces modifications of the electronic band structure,
leading to modulation of the transport properties, which could
have potential applications in nanoelectronics.15
+
Raman spectrometer using an Ar laser with 514.5 nm at room
temperature. The thermogravimetric analysis (TGA) was re-
corded on a Shimadzu TGA-50H (heating rate: 10 °C/min, N2
floating rate: 30 mL/min).
In this paper, we report the synthesis of a novel structure of
CNTs with polygonal cross sections, which are not in the form
of bundles, but which exist separately. The unique microstruc-
ture characteristics and possible growth model for this kind of
polygonized CNTs are also discussed.
3
. Results and Discussion
The XRD pattern of the as-prepared sample (S0) is shown
in Figure 1. The reflection indexed with 111, 220, 311, 222, 400,
422, 511, 440 (labeled as b) can correspond to face-centered
cubic Fe3O4 (PDF standard cards, JCPDS 89-0688). The
reflection peaks indexed with 210, 002, 201, 220, 031, 131,
*
Corresponding author. Fax: +86-551-3631760. E-mail: cqw@ustc.edu.cn.
1
0.1021/jp062690k CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/28/2006

Synthesis of Polygonized Carbon Nanotubes
J. Phys. Chem. B, Vol. 110, No. 33, 2006 16405
Figure 1. X-ray diffraction pattern of the as-prepared sample (S0).
Figure 3. Typical Raman spectrum of the S1 sample.
Figure 2. X-ray diffraction pattern of the S1 sample after the treatment
with HCl aqueous solution.
221, (labeled as 1) can correspond to primitive orthorhombic
Fe3C (PDF standard cards, JCPDS 72-1110). The reflection
peaks indexed with 002 (labeled as 9) can correspond to
hexagonal graphite (PDF standard cards, JCPDS 41-1487).
Figure 2 shows the XRD pattern of the S1 sample. It shows
a strong peak at 2θ ) 26.3°, corresponding to the (002)
reflection of hexagonal graphite (PDF standard cards, JCPDS
Figure 4. TEM images of polygonized CNTs of the S1 sample
synthesized at 500 °C. (a) Polygonized CNTs with hexagonal cross
sections; (b) quadrigonal CNTs with an obvious edge between two sides,
a corresponding typical SAED pattern is also shown in (b); (c,d)
polygonized CNTs with quasi-hexagonal cross sections.
4
1-1487), and the other two weak peaks arose from (101) and
(
004) reflection. It is obvious that after the treatment with HCl
observations of nearly circular tubes. In Figure 4b, edges due
to the connecting of two sides of CNTs can be clearly observed,
revealing its characteristics of a quadrangle. The selected area
electron diffraction (SAED) pattern of the sample exhibits two
diffraction rings, corresponding to (002) and (101) reflections
of hexagonal graphite (Figure 4b, insert). In addition, Figure 4,
parts c and d, shows the polygonized CNTs with quasi-
hexagonal cross sections.
aqueous solution, the main product is hexagonal graphitic-like
carbon. The XRD of the S1 sample shows two relatively broad
diffraction peaks of (002) and (101), indicating the existence
of disordered and low graphitized structure.
A Raman spectrum is usually used to investigate the
vibrational properties of carbon structures, which also allows
us to draw further conclusions about the degree of crystalliza-
tion.20 The peak at ∼1596 cm-1 (G band) corresponding to an
2
E2g mode of graphite is related to the vibration of sp -bonded
Figure 5, parts a-c, shows TEM images of ribbonlike
polygonized CNTs of an S1 sample synthesized at 480 °C,
which have diameters between 60 and 200 nm and lengths as
long as several microns, indicating features of softness and
flexibility. Figure 5c depicts the magnified image of the boxed
area of Figure 5b, in which edges can be observed more clearly.
Figure 5d is the HRTEM image of the boxed area of Figure 5c,
in which it is found that the edge of polygonized CNTs is well-
crystallized but the wall is amorphous, features which are
different from those of other ordinary CNTs. The interlayer
spacing (0.35 nm) of the edge agrees well with the (002) lattice
distance in hexagonal graphite.
carbon atoms in a 2-dimensional hexagonal lattice, while the
-
1
peak at ∼1346 cm (D band) is related to the defects and
21
disorders in the hexagonal graphitic layers. In Figure 3, the
value of ID/IG is calculated to be 0.85, which means that the S1
sample has relative disorder and a low graphitization degree,
in agreement with the XRD pattern.
Figure 4 shows typical TEM images of polygonized CNTs
of the S1 sample synthesized at 500 °C. First, the diffraction
contrast of the TEM image (Figure 4a) proves they are
nanotubes. Then from the open section of nanotubes, we can
see a clear hexagonal outline, which is in contrast to the earlier
2
2

16406 J. Phys. Chem. B, Vol. 110, No. 33, 2006
Qian et al.
Figure 5. TEM and HRETM images of polygonized CNTs of the S1
sample synthesized at 480 °C. (a-c) Ribbonlike polygonized CNTs as
long as several microns, and (c) is the magnified image of the boxed
area of (b); (d) HRTEM image of the boxed area of (c), showing the
different lattice structure between edge and wall of polygonized CNTs.
Figure 6. (a-b) TEM image of polygonal catalyst particle; (c) TEM
image of a catalyst particle surrounded by a hexagonal carbon nanotube;
(d) HRTEM image of polygonized CNTs, showing the different lattice
of structure between edge and wall.
It is well-known that ferrocene is a highly volatile organo-
metallic compound with excellent vaporizability. Up to now,
2
3,24
ferrocene has been widely used for the growth of CNTs.
In
the present work, ferrocene was reduced to atomic iron and
cyclopentadienyl group by metallic K (eq 1).24
+
-
Fe(C H ) + 2K f Fe + 2K + 2C H5
(1)
5
5 2
5
When heated over 100 °C, H2C2O4 is easily decomposed to
CO2, CO, and H2O (eq 2), and then CO and CO2 were
deoxidized to carbon by metallic K. Hata et al.25 had reported
that the activity and lifetime of the catalysts can be dramatically
enhanced by the addition of a controlled amount of water vapor
in the growth of SWNTs. Therefore, in our reaction system,
H2O might act as an effective mediator of catalysts.
Figure 7. Typical EDX spectrum of polygonal particles.
H C O f CO + CO + H O
(2)
polygonal particles might act as a catalyst for the formation of
CNT. These results indicated there are some kinds of relations
between the shape of the catalyst particles and polygonal
2
2
4
2
2
If the experiment was done using only ferrocene, it would
be decomposed to amorphous carbon, iron carbide, and organic
gas at 450 °C, in which iron carbide was encapsulated by
amorphous carbon. If the experiment was done by using oxalic
acid and metallic potassium, the products were mainly potassium
carbonate, graphene shells, and amorphous carbon. Combined
with above experimental truths, we could infer that the synthesis
of polygonized CNTs is the synergism of oxalic acid and
ferrocenesthe carbon source of oxalic acid is catalyzed by iron
originating from ferrocene.
The polygonal particles in the as-prepared products (S0) were
observed to be quadrigonal, pentagonal, and hexagonal by TEM,
as shown in Figure 6, parts a and b. The corresponding EDX
spectrum (Figure 7) shows Fe, Cu, C, and O peaks, in which
Fe and O signals originated from the polygonal particles and C
and Cu signals originated from carbon-coated copper grids,
respectively. The EDX spectrum analysis associated with the
XRD result of the S0 sample can confirm that the polygonal
particles are Fe3O4. The TEM image of Figure 6c shows a
representative hexagonal open end in the morphology of a
hexagonal CNT, which reveals a polygonal particle surrounded
by a hexagonal outer shell of carbon, indicating that the
2
6
sections of CNTs. Recently, El-Sayed et al. reported that
transition-metal nanoparticles are very attractive to use as
catalysts due to their high surface-to-volume ratio and their high
surface energy, which makes their surface atoms very active.
They also pointed out that during the early stages of reaction,
the catalytic activity is inhomogeneous, dependent on the shape
of the nanocatalyst used, and particularly dependent on the
fraction of surface atoms on corners and edges.
The edge of polygonized CNTs is well-graphitized, but the
wall is non- graphitized, as clearly shown in the HRTEM image
27
of Figure 6d. On the basis of Stiakaki’s computation s
chemisorption energy of corners is higher than that of edges of
catalyst particles, we put forward the following explanation. The
growth of the edge and wall of polygonized CNTs is controlled
by the corner and edge of the polygonal catalyst particle,
respectively. Compared with the edge, the corner of the catalyst
particle has better catalytic activity, and therefore it can induce
quicker deposition of carbon atoms and better crystallization
on the edge of polygonized CNTs.
Now, we would suggest the possible growth model of
polygonized CNTssa polygonal catalyst particle acts as a

Synthesis of Polygonized Carbon Nanotubes
J. Phys. Chem. B, Vol. 110, No. 33, 2006 16407
SCHEME 1: Schematic Illustration for the Growth
Model of Hexagonal CNTsa
of this kind of special CNTs is that the graphitization degree
of edges is better than that of walls, which can be explained
by the different catalytic activity between the corner and edge
of the polygonal catalyst particle. The yield of polygonized
carbon nanotubes was only 5-10% of the S1 sample, because
it is difficult to control the catalytic activity of Fe3O4 nano-
particles. Our experimental result represents the first step toward
synthesizing multiple- (or single-) wall CNTs with a polygonal
cross section on a large scale by using nonspherical catalyst
nanoparticles, which also serve as templates for the growth of
CNTs.
a
(A) Iron atoms were oxidized and agglomerated into hexagonal
Fe
3
O
4
particles; (B) forming initial carbon shell by the deposition of
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (20321101, 20125103, and
0206034).
carbon atoms, in which more atoms were inclined to aggregate on the
corner of catalyst tarticle; (C) keep growing upward along the side;
9
(D) finally forming hexagonal CNTs.
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