Controlled synthesis and phase transformation of ferrous nanowires inside carbon nanotubes

Article abstract of DOI:10.1016/S0009-2614(03)00808-X

Carbon nanostructures filled with Fe were synthesized. It was found that both the shape and the phase of the carbon nanocapsulate may be controlled during the growth, and controlled ferrous amorphous-crystalline phase transformation may be realized via electron beam irradiation. Since an electron beam may be focused into a nanoscale probe, this method may in principle be used for fabricating amorphous-crystalline heterostructures inside carbon nanotubes.

Full text of DOI:10.1016/S0009-2614(03)00808-X

Chemical Physics Letters 375 (2003) 59–64
www.elsevier.com/locate/cplett
Controlled synthesis and phase transformation of
ferrous nanowires inside carbon nanotubes
a
R. Che , L.-M. Peng
b,a,
b
, Q. Chen , X.F. Duan , B.S. Zou , Z.N. Gu
a
a
c
*
a
Beijing Laboratory of Electron Microscopy, Institute of Physics and Center for Condensed Matter Physics,
Chinese Academy of Sciences, P.O. Box 6034, Beijing 100080, China
b
Department of Electronics, Peking University Beijing 100871, China
Department of Chemistry, Peking University, Beijing 100871, China
c
Received 11 February 2003; in final form 9 May 2003
Abstract
Carbon nanostructures filled with Fe were synthesized. It was found that both the shape and the phase of the carbon
nanocapsulate may be controlled during the growth, and controlled ferrous amorphous–crystalline phase transfor-
mation may be realized via electron beam irradiation. Since an electron beam may be focused into a nanoscale probe,
this method may in principle be used for fabricating amorphous–crystalline heterostructures inside carbon nanotubes.
Ó 2003 Elsevier Science B.V. All rights reserved.
1
. Introduction
control. In this Letter, we show that, by a simple
multi-step catalytic decomposition of methane gas,
either carbon nanocages or carbon nanotubes may
be filled with continuous ferrous metal with high
filling efficiency, and the phase of the filled ferrous
metal may be readily controlled via varying cool-
ing rate. While the ferrous metal filled inside the
carbon nanocage or nanotube is amorphous for a
high cooling rate, it becomes crystalline a-Fe if a
lower cooling rate is used. For amorphous ferrous
encapsulates we may also transform (via a spa-
tially controllable fashion) them into crystalline
a-Fe using a nanoscale electron probe. This
method may in principle be used for fabricating
amorphous–crystalline ferrous heterostructures
inside carbon nanotubes, and therefore modifying
the magnetic properties of the confined ferrous
nanowires.
Carbon nanotubes filled with fullerenes, nano-
particles and metallic nanowires are important
materials since they may potentially be used in
nanoelectronics devices, magnetic recording me-
dia, and biological sensors [1–4]. Many elements
such as chromium, iron, silver, gold, lead, copper
and bismuth have been successfully filled into the
hollow interior of carbon nanotubes using various
methods, including thermolysis of ferrocene, py-
rolysis of organic compounds and arc discharge
methods [5–19]. However the shape and phase of
the resulting nanoencapsulates are not easy to
*
Corresponding author. Fax: +86-10-62751762.
E-mail addresses: plm@ele.pku.edu.cn, lmpeng@blem.ac.cn
L.-M. Peng).
(
0
009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00808-X

60
R. Che et al. / Chemical Physics Letters 375 (2003) 59–64
2
. Experimental
cooling rates. The sample obtained with a high
cooling rate of about 350 °C/min will be referred
to be B type sample, while that with a slow
cooling rate of 0.5 °C/min will be referred to be C
type sample. All the so-prepared soot-like de-
posits were subsequently washed by HCl (2M)
aqueous solution and distilled water to remove
the remained oxide support materials.
Transmission electron microscopy (TEM)
samples were prepared by dispersing the soot
product via ultrasonic treatment in alcohol and
then dropping it onto a holey carbon grid
supported on a copper mesh. High resolution
transmission electron microscopy (HRTEM) and
electron energy loss spectroscopy (EELS) experi-
ments were carried out using a Philips 200 keV
field-emission gun TEM (CM200-FEG) equipped
with a Gatan Imaging Filter system. All TEM
results presented in this Letter were obtained using
200 keV primary beam energy.
The iron filled carbon nanotubes were syn-
thesized in a horizontal quartz tube by chemical
vapor deposition (CVD) method as in [18].
Al O –SiO heterogeneous support material was
2
3
2
first prepared by a sol–gel routine. 1.8 g of alu-
mina was suspended in 20 ml of ethanol with
vigorous stirring. 2.08 g of tetraelthyl orthosili-
cate (TEOS) was introduced into the solution,
and 20 ml of water was added to hydrolyze the
TEOS. After the solution was stirred for 20 min,
1
.2 g of Fe
lution. The nominal atomic ratio of Fe:Al:Si is
:3.8:6.5. The suspension solution was then he-
2
(SO
4
)
3
ꢀ5H
2
O was added into the so-
1
ated up to 100 °C under nitrogen gas ambience to
evaporate water and ethanol. The dried powder
was ground into fine particles and then calcined
in a horizontal quartz tube at 750 °C for 0.5 h
under Ar
reached 930 °C at a controlled heating rate
about 25 °C/min), methane (flow rate: 1000
2
gas ambience. When the temperature
(
sccm) replaced argon for 0.3 h. The sample made
up to this stage and then cooled down to room
temperature (at a quick cooling rate 350 °C/min)
3. Results
Shown in Figs. 1a and b are two typical low
magnification TEM images of sample A and
sample B, respectively. These images show that
almost all carbon nanocapsules are filled with
nanoparticles or nanowires. Comparing the two
TEM images we see that when the decomposition
time increases from 0.3 h (Fig. 1a, sample A) to
under Ar
2
ambience is called A type sample.
Other samples were made by reducing the input
flow of methane supply to 200 sccm and de-
creasing the furnace temperature to 900 °C in 0.6
h. The samples were then cooled down to room
temperature under Ar2 ambience with different
Fig. 1. Two low magnification TEM images showing carbon nanocapsules filled with (a) nanoparticles and (b) nanowires with high
filling efficiency. (a) and (b) correspond to sample A (0.3 h of decomposition time) and sample B (0.9 h of decomposition time), re-
2 3 2
spectively, produced by the decomposition of methane using Fe/Al O –SiO catalyst. Arrows in (b) point to nanowires formed inside
the cavity of carbon nanotubes.

R. Che et al. / Chemical Physics Letters 375 (2003) 59–64
61
0
.9 h (Fig. 1b, sample B), the main reaction
being irradiated for about 5 s by a focused elec-
tron beam. The two micro-diffraction patterns
marked with A are obtained from the crystalline
region A for two different crystal zone axes. A
careful analysis of the micro-diffraction patterns
reveals that these patterns are consistent with
what one would expect from the [0 0 1] and [1 1 1]
zone axes of a-Fe. Micro-diffraction pattern
marked with B was obtained from region B which
had not been illuminated by intense electron
beam, showing that this region remained as
amorphous.
product changes from carbon nanocages filled
with nanoparticles (Fig. 1a) to nanotubes (Fig. 1b)
filled with nanowires. Fig. 1b shows (see also
Figs. 2,3,5) that the filled nanowires are typically
about hundreds of nanometers in length. Al-
though the nanotubes are not fully filled in the
sense that there exist always some gaps along
tube axis , the filling leaves no space between the
filled nanowires and the outer carbon nanotubes,
i.e., the filled segments of the nanotubes are fully
filled.
Shown in Fig. 2 are a TEM image of a
nanowire and three micro-diffraction patterns
obtained from region A and region B as marked
in the figure using a 1.4 nm electron probe. Ini-
tially the nanowire was amorphous as evidenced
from the electron diffraction pattern taken region
B, but a crystalline phase started to develop after
Shown in Fig. 3 is a HRTEM image taken from
the same nanowire as shown in Fig. 2, but the
whole nanowire had been irradiated by the inci-
dent electron beam for a prolonged period of time.
This image shows that the nanowire has been
transformed into crystalline a-Fe. The carbon
shells to the top left of the figure correspond to the
multi-walls of the carbon nanotube with lattice
spacing of 0.34 nm. The lattice fringes making
about 80° with the shells of the carbon nanotube is
about 0.202 nm, corresponding to the (1 1 0)
crystal plane of a-Fe.
Fig. 2. Low magnification TEM image showing two phases of
iron within a carbon nanotube. Corresponding micro-diffrac-
tion patterns obtained from regions A and B (marked with
circles in the TEM image) show that region A is crystalline a-Fe
while phase B is amorphous iron. The two micro-diffraction
patterns marked with A are obtained for two crystal zone axes,
i.e., [0 0 1] and [11 1], of the a-Fe.
Fig. 3. High-resolution TEM image showing the same nano-
wire as shown in Fig. 2 after the nanowire was irradiated for
about 20 s by an intense electron beam. The nanowire has been
completely transformed into crystalline a-Fe.

62
R. Che et al. / Chemical Physics Letters 375 (2003) 59–64
The composition of the nanowire is analyzed by
two EELS spectra recorded from a region that
contains carbon nanotubes and materials encap-
sulated inside the carbon nanotubes. These EELS
spectra confirm that carbon and iron are present in
the sample, but not oxygen. The energy peaks at
the techniques of EELS and elemental mapping.
Shown in Fig. 4a is a low-magnification TEM
image of a carbon nanotube and encapsulated
nanowire, and in Figs. 4b–d are three elemental
maps showing distributions of carbon (Fig. 4b),
iron (Fig. 4c) and oxygen (Fig. 4d). These ele-
mental maps show clearly that the tube shell is
composed of carbon and the material filled within
the tube is composed of iron but not oxygen. These
results agree with our earlier conclusion drew from
the combined studies of HREM and electron dif-
fraction that the tube is filled with pure iron in-
stead of iron oxides. Shown in Figs. 4e and f are
2 3
708 and 721 eV correspond to the L and L ion-
ization edges of iron, respectively, and the energy
peak at 283.4 eV corresponds to the carbon K-
edge. The oxygen K-edge would appear at 531 eV,
but Fig. 4f does not show any sign of the corre-
sponding peak. In a study of metal filling using the
carbon arc discharge [20] it was found that the
encapsulated metal was associated with significant
amount of S, corresponding to the presence of
Fig. 4. (a) Low magnification TEM image and Elemental maps showing (b) carbon, (c) iron and (d) oxygen distributions for the filled
carbon nanotube shown in (a). The scale bar in (a) represents 20 nm. In (e) and (f) are EELS spectra showing (e) the C K-edge (f) the Fe
L2;3-edge for a Fe filled nanotube.

R. Che et al. / Chemical Physics Letters 375 (2003) 59–64
63
stable metal sulphides. The S L2;3-edge would ap-
pear at about 165 eV, but our EELS spectrum
shown in Fig. 4e does not show any detectable
signal associated with this element.
The main chemical reactions involved in our
synthesis of ferrous nanostructures inside the car-
bon nanotubes are as follows
growth of the nanowire inside the nanotube began
almost simultaneously. As the nanotube grown
longer and longer, iron was socked into the core
volume of the nanotube through capillary action.
In the second stage of decomposition, we set the
cooling rate (from 930 to 900 °C in about 0.6 h) so
slow that there was enough time for carbon na-
notubes to grow longer and for the ferrous metal
to accomplish the filling. The methane flow rate
used in the second decomposition stage (200 sccm)
was also much less than that used in the first stage
(1000 sccm) to protect the ferrous catalyst from
being poisoned.
We now consider the effect of cooling rate on
the phase of the ferrous material filled inside the
carbon nanotubes. Shown in Fig. 5a is a X-ray
diffraction (XRD) profile obtained from sample B.
This sample was synthesized using a quick cooling
rate of about 350 °C/min during the final stage of
sample preparation. The XRD profile shows a well
defined diffraction peak at about 26°, which cor-
responds to the graphite (0 0 2) layer spacing, but
otherwise the profile is featureless indicating that
the filled ferrous metal inside the carbon nanotube
is amorphous. Shown in Fig. 5b is another XRD
profile obtained from sample C which was syn-
thesized using a much slower cooling rate of about
0.5 °C/min. Apart from the graphite (0 0 2) peak,
2
Fe
2
ðSO
4
Þ3 ðsÞ ) 2Fe
2
O
3
ðsÞ þ 6SO
ðgÞ
2
ðgÞ
þ 3O
2
ð1Þ
ð2Þ
Fe
2
O
3
ðsÞ þ CH
4
ðgÞ ) 2Fe ðsÞ þ CO ðgÞ
þ 2H O ðgÞ
2
In reaction (1) Fe (SO ) was firstly decomposed
2
4 3
into Fe O at 750 °C. Reaction (2) began when
2
3
the methane gas flow into the quartz tube and
reduced Fe into Fe around the surface of the
Al –SiO support material. Fe not only acts as
2
O
3
2
O
3
2
the catalyst used for the growth of carbon na-
notubes but also as the feedstock of ferrous ma-
terials which were filled into the core volume of
the carbon nanotubes. The main differences be-
tween the above process and the conventional
CVD method [18] are (a) the mole ratio of Fe to
Al in our reactant is about four times more than
that used in [18] to provide sufficient amount of
iron as catalyst and filling feedstock. (b) A two-
step decomposition procedure is employed to re-
alize control over the shape of the final nanoen-
capsulates, i.e., using insufficient decomposition
time for A type sample (carbon nanocages filled
with ferrous nanoparticles) and sufficiently long
decomposition time for B type sample (long car-
bon nanotubes filled with ferrous nanowires). (c)
Different cooling rates for controlling the phase
of the ferrous metal inside carbon nanotubes, i.e.,
using a slow cooling rate (0.5 °C/min) for crys-
talline a-Fe and a high cooling rate (350 °C/min)
for amorphous Fe.
The high filling efficiency achieved in this study
results mainly from the high mole ratio of Fe to Al
and Si in our catalyst and the two-step decompo-
sition procedure used. Large amount of iron con-
gregated around the root of the carbon nanotube
and the surface of the support material. When
the methane started to decompose, the growth of
the nanotube out of the catalyst surface and the
Fig. 5. XRD profiles obtained from (a) a B type sample and (b)
a C type sample, showing that a slow cooling rate (0.5 °C/min)
results in crystalline ferrous nanowires (sample C) and a quick
cooling rate (350 °C/min) results in amorphous ferrous nano-
wires (sample B).

64
R. Che et al. / Chemical Physics Letters 375 (2003) 59–64
three additional diffraction peaks are observed
which may be indexed as the (1 1 0), (2 0 0) and
Acknowledgements
(
2 1 1) peaks of crystalline a-Fe. The phase of the
This work was supported by the Ministry of
Science and Technology (Grant No. 001CB
6 10502), National Science Foundation of China
(Grant No. 90206021) and Peking University.
ferrous nanowires encapsulated inside the carbon
nanotubes may therefore be controlled to be either
amorphous or crystalline, depending on the cool-
ing rate used during the final stage of the sample
preparation. In a separate paper [21], we will show
that different phases of the ferrous nanowire will
exhibit very different response to the external
electromagnetic fields and waves. Controlled syn-
thesis on the phase of the ferrous nanowires inside
carbon nanotubes and therefore their properties is
realized to a certain extent.
References
[
[
[
[
[
1] R.S. Lee, H. Kim, J.E. Fisher, A. Thess, R.E. Smalley,
Nature 388 (1997) 255.
2] E. Dujardin, T.W. Ebbesen, H. Hiura, K. Tanigaki,
Science 265 (1994) 1850.
3] F. Okuyama, T. Hayashi, Y. Fujimoto, J. Appl. Phys. 84
(
1998) 1626.
4] R.S. Ruoff, D.C. Lorents, B. Chan, R. Malhotra, S.
Subramoney, Science 259 (1993) 346.
4
. Conclusions
5] N. Grobert, W.K. Hsu, Y.Q. Zhu, J.P. Hare, H.W. Kroto,
D.R.M. Walton, M. Terrones, H. Terrones, Appl. Phys.
Lett. 75 (1999) 3363.
In summary controlled synthesis of carbon ca-
ges filled with ferrous nanoparticles or carbon
nanotubes filled with continuous nanowires have
been realized by using CVD method, and the se-
lection on the shape or aspect ratio of the encap-
sulate is achieved via a multi-step decomposition
procedure. Our procedure may in principle be used
for inserting magnetic materials such as Fe, Co, Ni
or elements with high surface tension into the
cores of carbon nanocapsules. For Fe, we shown
that the phase of the inserted ferrous metal may be
further controlled by changing cooling rate during
the last stage of the sample preparation. For
samples obtained with a high cooling rate the in-
serted ferrous metal is amorphous, but the amor-
phous ferrous metal can be transformed via a
spatially controllable fashion using a focused
electron beam into crystalline a-Fe. Although in
our case the phase transformation from amor-
phous to crystalline a-Fe is realized via electron
beam irradiation, we expect that similar irradia-
tion by high energy X-ray or laser beam would
produce the same results. Since amorphous and
a-Fe have different magnetic properties, our results
suggest that it is possible to construct confined
magnetic heterostructures within a single carbon
nanotube.
[6] D. Ugarte, A. Chatelain, W.A. Deheer, Science 274 (1996)
897.
1
7] A.A. Setlur, J.M. Lauerhaas, J.Y. Dai, R.P.H. Chang,
Appl. Phys. Lett 69 (1996) 345.
[
[
[
8] Ching-hwa Kiang, Carbon 38 (2000) 1699.
9] C. Guerret-Picourt, Y. Le Bouer, A. Loiseau, H. Pascard,
Nature 372 (1994) 761.
[
[
[
[
[
[
10] P.M. Ajayan, C. Colliex, J.M. Lambert, P. Bernier, M.
Tence, O. Stephan, Phys. Rev. Lett. 72 (1994) 1722.
11] S.W. Liu, J.J. Zhu, Y. Mastai, I. Felner, A. Gedanken,
Chem. Mater. 12 (2000) 2205.
12] Y.K. Chen, M.L.H. Green, S.C. Tsang, Chem. Commun.
2489 (1996).
13] F. Banhart, N. Grobert, M. Terrones, J.C. Charlier, P.M.
Ajayan, International J. Mod. Phys 15 (2001) 4037.
14] C.N.R. Rao, R. Sen, B.C. Satishkumar, A. Govindaraj,
J. Chem. Soc. Chem. Commun. 15 (1998) 1525.
15] M. Terrones, N. Grobert, J.P. Zhang, H. Terrones, J.
Olivares, J.P. Hare, H.W. Kroto, Chem. Phys. Lett. 285
(
1998) 299.
[
[
16] R.S. Lee, H. Kim, J.E. Fisher, A. Thess, R.E. Smalley,
Nature 388 (1997) 255.
17] F. Okuyama, T. Hayashi, Y. Fujimoto, J. Appl. Phys. 84
(
1998) 1626.
[18] A.M. Cassell, J.A. Raymakers, J. Kong, H. Dai, J. Phys.
Chem. B 103 (1999) 6484.
[
19] N. Demoney, O. Stephan, N. Brun, C. Colliex, A. Loiseau,
H. Pascard, Synth. Met. 103 (1999) 2380.
[
20] N. Demoncy, O. Stephan, N. Burn, C. Colliex, A. Loiseau,
H. Pascard, Eur. Phys. J. 4 (1998) 147.
[21] R.C. Che, L.-M. Peng, et al., in preparation.