Direct synthesis of carbon nanotubes decorated with size-controllable Fe nanoparticles encapsulated by graphitic layers

Abstract

A simple method has been developed for direct synthesis of magnetic multi-walled carbon nanotubes (MWCNTs) homogeneously decorated with size-controllable Fe nanoparticles (Fe-NPs) encapsulated by graphitic layers on the MWCNT surface by pyrolysis of ferrocene. These composites have similar C/Fe atomic ratio of ∼10 and exhibit sufficiently high saturation magnetization for magnetic separation in a liquid phase. Moreover, with 0, ∼1, ∼2 wt% sulfur as growth promoter, the size of Fe-NPs can be controlled with an average diameter of ∼5, ∼22 and ∼42 nm, respectively. When compared to time-consuming wet-chemical methods, the simplicity of this method should allow easy large-scale production of these magnetically functionalized MWCNTs, which can be used as catalyst supports with high stability for effective magnetic separation in liquid-phase reactions, especially under acid/basic conditions.

Full text

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Direct synthesis of carbon nanotubes decorated with size-
controllable Fe nanoparticles encapsulated by graphitic layers
Qingfeng Liu, Wencai Ren, Zhi-Gang Chen, Bilu Liu, Bing Yu, Feng Li,
Hongtao Cong, Hui-Ming Cheng*
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China
A R T I C L E I N F O
Article history:
Received 20 February 2008
Accepted 2 June 2008
Available online 12 June 2008
A B S T R A C T
A simple method has been developed for direct synthesis of magnetic multi-walled carbon
nanotubes (MWCNTs) homogeneously decorated with size-controllable Fe nanoparticles
(Fe-NPs) encapsulated by graphitic layers on the MWCNT surface by pyrolysis of ferrocene.
These composites have similar C/Fe atomic ratio of ?10 and exhibit sufficiently high satu-
ration magnetization for magnetic separation in a liquid phase. Moreover, with 0, ?1,
?2 wt% sulfur as growth promoter, the size of Fe-NPs can be controlled with an average
diameter of ?5, ?22 and ?42 nm, respectively. When compared to time-consuming wet-
chemical methods, the simplicity of this method should allow easy large-scale production
of these magnetically functionalized MWCNTs, which can be used as catalyst supports
with high stability for effective magnetic separation in liquid-phase reactions, especially
under acid/basic conditions.
? 2008 Elsevier Ltd. All rights reserved.
1.Introduction
It is known that a smaller catalyst particle means a higher
activity and experiences no significant attrition (no reduction
in particle size) in liquid-phase reactions [1]. Consequently,
both the activity and the stability of solid catalysts suspended
in a liquid phase can benefit greatly from the utilization of
catalyst nanoparticles (NPs). Moreover, catalyst supports are
usually used in catalytic processes in order to achieve high
dispersion, maximum utilization, as well as to avoid catalyst
agglomeration and consequently realize a high catalysis per-
formance [2]. Considering that with solid catalysts suspended
within liquids the transport within the liquid to the surface of
catalyst supports is proportional to 1/D2(D, the diameter of
catalyst supports) [3], it is attractive to utilize small catalyst
supports to avoid transport limitations, and thus to raise
the rate of the liquid-phase catalytic reactions. A very impor-
tant issue in a liquid catalytic process is that the separation of
these supports with nano-sized catalysts from the reaction
products is almost impossible by conventional means such
as centrifugation and filtration, since they can easily lead to
the blocking of filters and valves [4,5]. The efficient separation
of suspended magnetic supports of nanocatalysts from the li-
quid products by using an external magnetic field offers a
solution to this problem. For example, small magnetic sup-
ports are used to host enzymes or other biological materials
that can only function under narrow or restricted reaction
conditions [6–9]. However, most supports are chemically
unstable under the conditions such as acidic, basic, corrosive,
oxidizing, reducing environments or elevated temperatures,
which are commonly encountered in catalytic fine-chemical
synthesis. Recently, carbon nanotubes (CNTs) as nano-carbon
catalyst support have drawn a growing interest due to their
specific characteristics such as large surface area to volume
ratio, good adsorption, unique structure, and high thermal
and chemical stability [10–12]. As a result, CNTs decorated
with magnetic NPs may provide an ideal candidate as catalyst
support for effective magnetic separation in a liquid phase.
0008-6223/$ - see front matter ? 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2008.06.021
* Corresponding author: Fax: +86 24 2390 3126.
E-mail address: cheng@imr.ac.cn (H.-M. Cheng).
C A R B O N 4 6 ( 20 0 8 ) 1 4 1 7–1 4 2 3
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon

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Generally, owing to the chemically stable and highly hydro-
phobic nature of the CNT surface, wet decoration methods are
commonly employed for the decoration of CNTs with mag-
netic NPs of elements or compounds [13–22]. By wet decora-
tion routes, CNTs can be decorated with magnetic NPs of
several to tens of nanometers, which easily give sufficiently
high saturation magnetization for magnetic separation. How-
ever, such CNTs are not suitable for magnetic support of nan-
ocatalysts in a liquid phase, because (i) the magnetic NPs are
just attached to the CNT surface, and thus can be easily peeled
off from the CNT surface during liquid-phase reactions, espe-
cially with violent stirring, and (ii) the magnetic NPs are fairly
reactive not only with acids but also with complexe agents,
which are often encountered in liquid-phase catalytic reac-
tions. Therefore, in order to further realize the application of
CNTs in heterogeneous catalysis, two structural characteris-
tics should be satisfied for magnetically functioned CNTs as
catalyst support. Firstly, strong linkage between the magnetic
NPs and the CNTs is required. Secondly, it is necessary to com-
pletely shield the inner magnetic core from external environ-
ments with ideal ‘‘safety spacer’’ such as graphitic shells that
are chemically inert and impermeable to most chemicals,
while the catalytically active part located on the CNT surface
can perform its function with no interference from the core.
This paper reports a simple method for direct synthesis of
multi-walled carbon nanotubes (MWCNTs) decorated with
magnetic Fe nanoparticles (Fe-NPs) encapsulated with gra-
phitic layers by pyrolysis of ferrocene in chemical vapor depo-
sition (CVD) growth process. By changing the experimental
conditions, the size of Fe-NPs can be controlled with an aver-
age diameter of ?5, ?22 and ?42 nm. These MWCNT/Fe-NP
composites exhibit sufficiently high saturation magnetization
for magnetic separation in a liquid phase. Moreover, chemical
linkage between the Fe-NPs and the MWCNTs is found to
tightly fasten Fe-NPs on the MWCNT surface. As a result,
the complete encapsulation of the magnetic core anchored
onto the surface of MWCNTs allows direct handling of intrin-
sically sensitive magnetic materials (Fe and Fe3C) in acid/ba-
sic solution or in air, and hence separation of these catalyst
supports in an external magnetic field can be achieved.
2.Experimental
The synthesis was performed in a quartz tube reactor (i.d.
40 mm) inside an electrical furnace. In a typical experiment,
?2000 sccm Ar flow was used as a carrier gas through the
quartz tube. When the temperature of the center of the fur-
nace reached 1100 ?C, ?100 mg ferrocene/sulfur mixture
(S = 0–3 wt%) was sublimated in the temperature range of
120–250 ?C, and then the MWCNT/Fe-NP composites (40–
70 mg) grew downstream in the region of 700–1000 ?C. After
the growth, the furnace was cooled naturally to room temper-
ature under the protection of Ar.
The samples were characterized by using scanning elec-
tron microscope (SEM, LEO SUPRA35 at 15 kV), transmission
electron microscope (TEM, JEOL2010 and TecnaiG2 F30 at
200 kV), thermogravimetric analyzer (NETZSCH STA 449C),
X-ray diffractometer (XRD, CuKaradiation), and superconduc-
ting quantum interference device (SQUID) magnetometer.
3. Results and discussion
Depending on the growth conditions with 0, ?1, ?2 wt% sulfur
as growth promoter, three kinds of MWCNT/Fe-NP composites
are obtained and denoted as Si, Sii, and Siii, respectively. Fig. 1
shows the typical SEM images of these samples that consist of
abundant MWCNTs. The whole surface of these MWCNTs is
homogeneously decorated with uniform Fe-NPs, whose size
is well controlled by the content of sulfur in the starting mix-
ture. EDX analysis reveals that all samples have a similar C/Fe
atomic ratio of ?10, proximate to the fixed C/Fe ratio of ferro-
cene molecule, i.e. Fe(C5H5)2. This result indicates that carbon
and iron atoms from pyrolysis of ferrocene can be effectively
transformed into such nanostructured composites. Thermo-
gravimetric analysis (TGA) in air indicates that these samples
have similar oxidation resistance (Fig. 2). There is a slight
weight increase at around 500 ?C, which can be ascribed to
the oxidation of Fe-NPs. The MWCNTs themselves are oxi-
dized in a narrow temperature range of 520–620 ?C, with the
comparable residual metal oxide (44–46 wt %), which is in
agreement with EDX analysis.
It is found that sulfur exerts an important influence on the
size of Fe-NPs anchored on the MWCNT surface, because all
other experimental parameters are unchangeable. Fig. 3a is
the TEM image of Siproduced by pyrolysis of pure ferrocene.
The diameter of the Fe-NPs on the MWCNT surface measured
from TEM images is in the range of 4–21 nm, with a Gaussian
mean diameter of ?9.7 nm (Fig. 3b). When a quantity of
?1 wt% sulfur is used as growth promoter in the starting mix-
ture, smaller Fe-NPs are anchored on the MWCNT surface in a
uniform way (Fig. 3c). Fig. 3d represents the corresponding
diameter histogram of these Fe-NPs, revealing that the diam-
eter distribution is in a narrow range of 2–14 nm, with a
Gaussian mean diameter of ?5 nm. Interestingly, with ?2 wt
% sulfur in the starting mixture, the Fe-NPs with larger diam-
eters are anchored onto the MWCNT surface (Fig. 3e). The
diameters of these Fe-NPs are also in a larger range of 25–
65 nm, with a Gaussian mean diameter of ?41 nm (Fig. 3f).
Considering the comparable C/Fe atomic ratio of ?10 for these
MWCNT/Fe-NP composites, it is natural and comprehensible
that the particle density on the MWCNT surface decreases
with the increase of the size of Fe-NPs.
The formation mechanism of these MWCNT/Fe-NP com-
posites can be well explained, based on the vapor–liquid–so-
lid(VLS) model.Atelevated
molecules quickly decompose to Fe clusters, floating carbon
and hydrogen atoms. Due to the dissolution of carbon and
the small size, the melting point of Fe clusters is far below
the melting point of the bulk metal [23]. This suggests that
Fe clusters are in a liquid state during the decomposition
and suddenly contract to form liquid Fe-NPs because of sur-
face tension. As the reaction proceeds, some Fe-NPs gradu-
ally become larger and act as the nucleation seeds for the
growth of MWNTs when the carbon concentration inside
these catalyst particles exceeds supersaturation. Meanwhile,
due to the high ratio of Fe/C concentration in the reaction
system and the strong affinity between Fe and C, liquid Fe-
NPs are superfluous and tend to deposit on the sidewalls of
the newly-formed MWNTs. Addition of a small quantity
of sulfur can effectively enhance the formation of the
temperatures,ferrocene
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MWNT/Fe-NP composite, because S easily combines with C in
the vapor phase at elevated temperatures, forms S-rich car-
bon clusters, and hence assists the graphitizing nanotubes
[24]. In addition, sulfur has a dramatic effect on the activity
of Fe-NPs through surface reconstruction [25,26], and simul-
taneously increases the defects on the surface of the formed
nanotubes [27]. This facilitates the decoration of Fe-NPs on
the MWCNT surface, especially on the defects, which act as
nucleation sites for the anchoring of Fe-NPs. Moreover, the
local zones adsorbed with sulfur on the Fe-NP surface have
a high adhesion energy toward graphite [27,28], which can
strengthen strong linkage between Fe-NPs and the MWCNTs.
However, a large quantity of S has increased the size of
Fe-NPs in our experiments. The possible reason is that S
has a high affinity to metal atoms due to the strong bonding
between them, which is in favor to the aggregation of Fe clus-
ters and forms larger Fe-NPs [26].
As mentioned above, although a variety of magnetic NPs
ranging from several to tens of nanometers in size can be at-
tached on the MWCNT surface by wet chemical methods [13–
22], it is not easy for wet strategies to obtain such uniform
decoration of MWCNTs with tiny Fe-NPs such as Siiobtained
Fig. 1 – Typical SEM images of (a and b) Si, (c and d) Sii, and (e and f) Siii.
100 200300400
Temperature /
500600700
oC
800900 1000 1100
40
50
60
70
80
90
100
-12
-8
-4
0
4
Mass / %
Si
Sii
Siii
DTG
TG
DTG / (%/min)
Fig. 2 – TGA curves of Si, Siiand Siiiin the airflow.
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by our method. Fig. 4a is high-resolution TEM (HRTEM) image
of Sii, further revealing the MWCNTs decorated with ?5 nm
Fe-NPs, which are encapsulated with several graphitic layers.
The Fe-NPs are crystalline and have a lattice fringe spacing of
0.20 nm, related to the (110) plane of the a-Fe crystal
(JCPDS06-0696); the spacing of the lattice fringes of MWCNTs
is ?0.34 nm, which is close to that of the graphite (002)
planes. It can be seen that the highly ordered lattice fringes
of single-crystal metal are vanished in the area adjacent to
the Fe-MWCNT interface, which directly confirms strong
24681012 141618 2022
0
10
20
30
40
50
60
70
Gaussian fit
Mean diameter: d= 9.7 nm
Number of Fe-NPs
Diameter of Fe-NPs (nm)
245
Diameter of Fe-NPs (nm)
689 10 111213 14
0
5
10
15
20
25
30
35
Number of Fe-NPs
Gaussian fit
Mean diameter: d= 5.1 nm
20 253035
Diameter of Fe-NPs (nm)
4045 50 5560 65
0
20
40
60
80
Gaussian fit
Mean diameter: d = 41 nm
Number of Fe nanoparticles
bd
f
37
Fig. 3 – Typical TEM images and diameter distributions of the Fe-NPs of (a and b) Si, (c and d) Sii, and (e and f) Siii.
1420
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chemical linkage between Fe-NPs and the MWCNTs [29].
Obviously, the strong interaction between the Fe-NPs and
the MWCNT surface will be advantageous when this compos-
ite is used as magnetic supports for heterogeneous nanocata-
lysts in liquid-phase reactions, especially with violent
stirring. Fig. 4b shows the XRD pattern for Sii. The peaks at
44.7?, 65.0? and 82.3? can be identified as the (110), (200)
and (211) planes of crystalline body-centered cubic (bcc) a-
iron, respectively. A large fraction of orthorhombic cementite
Fe3C phase is also seen in the XRD pattern. The peak at about
26.2? is assigned to the (002) plane of hexagonal graphitic
structure of MWCNTs. This peak is symmetric and narrow,
indicating a relatively high crystalline dimension.
To evaluate the magnetic properties of Sii, SQUID magne-
tometer was used for magnetic characterization of this sam-
ple at room temperature, as shown in Fig. 5a. The smooth
S-shaped curve has a saturation magnetization (Ms) of
?60 emu/g. Considering the small size of Fe-NPs decorated
on the MWCNT surface, it is expected that a reduction of Ms
occurs, compared to bulk iron (Ms= 222 emu/g) [30]. Moreover,
the small coercivity of Siishould not result in the clustering of
this composite due to the mutual magnetic attraction, which
1020304050 60 70 80 90
(121)
+
(211)
(102)
Intensity / (a.u.)
2θ θ/ (°)
++
+
+
+
+
+
Graphite
α - Fe
+ Fe3C
++
(102)
+
+
++
+
+
(200)
(211)
(110)
(002)
b
Fig. 4 – (a) HRTEM image (taken from a section in the inset) and (b) XRD pattern of Sii.
-15000 -10000-50000 5000 1000015000
-60
-40
-20
0
20
40
60
Magnetic Field (Oe)
-15000 -10000 -500005000 10000 15000
-60
-40
-20
0
20
40
60
Magnetic Field (Oe)
Magnetization (emu g-1)
Magnetization (emu g-1)
a
e
Fig. 5 – (a) Magnetization curve measured at room temperature for Sii. (b) Siidispersion in ethanol. (c) Response of Siito a
magnet. (d) Redispersion of Siiin ethanol. (e) Magnetization curve measured at room temperature for the HNO3-treated Sii
(inset, showing the TEM image of HNO3-treated Sii). (f) The HNO3-treated Siidispersion in ethanol. (g) Response of the HNO3-
treated Siito a magnet. (h) The HNO3-treated Siiredispersion in ethanol.
C A R B O N 4 6 ( 20 0 8 ) 1 4 1 7–14 2 3
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is proved by the following simple experiment to demonstrate
the potential of such composite as reusable catalyst support
for magnetic separation in a liquid phase. Fig. 5b shows Sii
well-dispersed in ethanol. The magnetic response of Siiis eas-
ily and quickly visualized when it is moved towards a small
magnet, and the clear solution after the magnetic separation
is shown in Fig. 5c. After the magnet was removed, only gen-
tle shaking could make the composite easily redisperse (Fig.
5d). Clearly, the magnetostatic attraction from the remanent
magnetic moment (the magnetic moment in the absence of
a magnetic field) is quite low, and does not lead to the cluster-
ing of MWCNTs. It is attributed to that the magnetic interac-
tion is effectively decreased by the relatively large mutual
distance of the fastened Fe-NPs on the MWCNT surface by
strong chemical linkage. Meanwhile, the graphitic shells of
the Fe-NPs and the MWCNTs act as non-magnetic separation,
which is also essential for the Fe-NPs to eliminate the dipolar
interaction between the neighboring ones [31].
As mentioned above, an important advancement is that
the inner magnetic core is completely shielded from external
environments with impermeable graphitic layers, conse-
quently ensuring long-term stability of the ferromagnetic
core. To validate this protection of graphitic layers on the
encapsulated Fe-NPs, Siiwas treated in concentrated HNO3
(?65 wt%) for 30 h. Magnetic characterization of the HNO3-
treated Siialso shows S-shaped magnetization curve with a
comparable small coercivity (Fig. 5e). It is observed that the
Fe-NPs are still anchored on the MWCNT surface after the
HNO3-treatment (the inset of Fig. 5e). In addition, a similar
magnetic response of the resulting sample in solution is ob-
served, as shown in Fig. 5f–h. Moreover, the HNO3-treated
composite shows a good dispersion in organic solvent or dis-
tilled water, because some functional groups such as –NO3
and –C=O were attached to the surface of MWCNTs during
the harsh acidic treatment. The high stability of this compos-
ite is significantly advantageous when it is used as magnetic
support of heterogeneous nanocatalysts in liquid-phase reac-
tions, especially under severe conditions such as acidic, cor-
rosive oroxidizingenvironments,
sulphided Co–Mo–K catalysts supported by Co-decorated
MWCNTs have demonstrated to display higher catalytic activ-
ity and selectivity for higher alcohol synthesis from syngas,
when compared to the reference catalyst supported by the
simple MWCNTs or conventional activated carbon [32].
etc. Forexample,
4.Conclusions
We have developed a simple method for synthesis of
MWCNTs decorated with size-controllable Fe-NPs from pyro-
lysis of ferrocene by controlling sulfur content in the starting
mixture. The Fe-NPs are encapsulated with graphitic layers
and tightly anchored on the MWCNT surface by chemical
linkages, and thus have high stability under acidic, corrosive
or oxidizing environments. The structural properties of these
composites make them not only exhibit sufficiently strong
magnetic response for ease of separation, but also effectively
prevent the clustering of MWCNTs. Furthermore, by pyrolysis
of metallocenes such as ferrocene, nickelocene, cobaltocene
or their mixtures, this technology is also expected to easily
prepare Fe/Ni/Co NPs decorating CNTs with tunable magnetic
properties, which can find various applications in nanoelec-
tronic devices, magnetic resonance imaging, and magnetic
data storage.
Acknowledgements
The work was supported by Ministry of Science and Technol-
ogy of China (No. 2006CB932701), National Science Founda-
tion of China (No. 90606008), and Chinese Academy of
Sciences (No. KJCX2-YW-M01).
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