Carbon 42 (2004) 3177–3182
www.elsevier.com/locate/carbon
Preparation of carbon-encapsulated iron nanoparticles by
co-carbonization of aromatic heavy oil and ferrocene
Junping Huo, Huaihe Song *, Xiaohong Chen
Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education,
Beijing University of Chemical Technology, 100029 Beijing, PR China
Received 29 January 2004; accepted 4 August 2004
Abstract
Carbon-encapsulated iron nanoparticles with uniform diameters have been synthesized on a large scale by co-carbonization of an
aromatic heavy oil and ferrocene at 480 °C under autogenous pressure. The morphologies and structural features of the iron/carbon
composites were investigated using TEM, HREM and XRD measurements. It was found that, by increasing the amount of ferrocene added from 2 wt.% to 45 wt.%, the size of the nanoparticles increased from 15 nm to 50 nm and the morphologies of the resulting products changed from spherical-type to iron-filled carbon nanorods when the ferrocene loading was higher than 30 wt.%. The
iron particles pyrolyzed from ferrocene exist mainly in the form of a-Fe and small amounts of Fe3C were also formed when the
ferrocene content was higher than 20 wt.%. The formation mechanism of carbon-encapsulated iron nanoparticles is discussed briefly.
This novel and simple approach constitutes a more practical method to prepare carbon-encapsulated metal nanoparticles than those
reported to date.
Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: B. Carbonization, Heat treatment, Pyrolysis; D. Reactivity, Microstructure
1. Introduction
Since the first observation of carbon-encapsulated
LaC2 in 1993 [1,2], carbon-encapsulated metal nanoparticles (CEMNs) have attracted a great deal of research
interest all over the world, and many results in terms of
preparation, growth mechanism and properties have
been reported over the past ten years. Magnetic CEMNs
might have important applications in areas such as highdensity magnetic data storage, magnetic inks, ferrofluids
[3,4], magnetic toners for xerography and contrast agents
in magnetic resonance imaging [5]. The role of the carbon
layer is to isolate the particles magnetically from each
other, thus avoiding the problems caused by interactions
*
Corresponding author. Tel.: +86 10 64434916; fax: +86 10
64437587.
E-mail address: songhh@mail.buct.edu.cn (H. Song).
0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2004.08.007
between closely compacted magnetic units, and to enhance the oxidation resistance of the bare metal nanoparticles. Moreover, the carbon coatings can endow
these magnetic particles with biocompatibility and stability in many organic and inorganic media. These combined attributes make CEMNs interesting candidates
for many bioengineering applications including drug
delivery, biosensors and magnetic hyperthermia [6,7].
Various techniques have been developed for synthesizing CEMNs, such as standard carbon arc techniques
[5], tungsten arc techniques [8], magnetron and ion-beam
co-sputtering [9], RF plasma torch techniques [10], and
modified arc deposition techniques [11,12]. However,
these methods are apparently not suitable for producing
CEMNs in large quantities. Harris et al. [13,14] reported
a simple method for the synthesis of CEMNs by impregnating non-graphitizing carbon with a salt of the metal to
be encapsulated and further heat treatment in the range
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J. Huo et al. / Carbon 42 (2004) 3177–3182
1800–2500 °C. However at present the yield of filled carbon nanoparticles produced by this method is rather low
[15]. Several catalytic methods have been tried [16,17],
which possess the advantages of low cost, simplicity,
and the ready availability of raw materials. However,
so far catalytic methods for the synthesis of CEMNs
have not been very successful because of problems
regarding the separation of CEMNs from other materials, especially the by-product carbon nanotubes (CNTs),
as well the relatively low yields [18,19].
In our previous work, a novel and simple method
for the synthesis of carbon-encapsulated iron carbide
nanoparticles involving co-carbonization of 1,2,4,5tetramethylbenzene and ferrocene under autogenous
pressure was proposed [20]. In this article, an aromatic
heavy oil was chosen as carbon source and ferrocene
as metal source in order to synthesize carbon-encapsulated iron nanoparticles via co-carbonization at temperatures about 480 °C under pressure. The effects of
ferrocene content on the yield, morphology and structure of the resulting CEMNs were investigated by transmission electron microscopy (TEM), high-resolution
transmission electron microscopy (HREM) and X-ray
diffraction (XRD) measurements. The formation mechanism of CEMNs is also discussed.
duced into a 200 ml autoclave. After replacing the air
in the vessel three times by flowing pure N2, the reaction
vessel was sealed and was then heated with a rate of
3 °C/min up to 480 °C. The system was maintained at
480 °C for 10 min under autogenous pressure with the
final pressure in the range 6–15 MPa according to the
ferrocene content, and was then cooled down to room
temperature. The resulting product was repeatedly extracted (at least three times) with pyridine at 110 °C until
the filtrate became colorless in order to completely remove the unreacted YD and small molecular species.
The pyridine insoluble fraction (PI) was regarded as
the desired product. The carbonization yield was calculated by PI weight over the total weight of YD and ferrocene used.
2.3. Characterization
Samples for TEM and HREM observation were prepared by dispersing the products in ethanol for 15 min
with an ultrasonic bath and then a few drops of the
resulting suspension were placed on a copper grid.
XRD patterns of PI were recorded on a Rigaku D/
max-2500B2+/PCX system using Cu-Ka radiation
(k = 1.5406Å) over the range of 5–100° (2h) at room
temperature.
2. Experimental
3. Results and discussion
2.1. Raw materials
A Chinese aromatic heavy oil (a heavy fraction of
light diesel oil from the fluid catalytic cracking of petroleum naphtha, hereinafter abbreviated as YD), purchased from Yanshan Petrochemical Company, was
chosen as the carbon source. It consists mainly of 2and 3-ring aromatic compounds with alkyl substituents
and a few heteroatoms. Table 1 shows the analytical
data for YD. Ferrocene (analytically pure grade) was
chosen as the metal source.
2.2. Preparation
Mixtures of YD and ferrocene in different proportions (from 2 wt.% to 45 wt.% ferrocene) were intro-
Fig. 1 shows the effect of ferrocene addition on the PI
yield of products. It can be observed that, without ferrocene addition, YD pyrolysed at 480 °C affords polyaromatic hydrocarbons (PAHs) which can be completely
dissolved in pyridine. With the increase of ferrocene
content from 2 wt.% to 45 wt.%, the PI yield increased
from 2 wt.% to about 25 wt.%, indicating that ferrocene
loading promotes the generation of heavy PAHs and the
development of carbon. Many workers [21,22] have reported the accelerating effect of iron in petroleum pitch
and ascribed it to the catalysis of iron clusters derived
from ferrocene decomposition.
Fig. 2 shows low-magnification TEM images of the
products prepared with different ferrocene loadings. It
can be seen that dark iron species with different diame-
Table 1
Analytical data for YD
MWa
Elemental analysis (wt.%)
C
90.48
a
H
N
H/C
8.80
0.3
1.16
c
210
Compounds content (wt.%)b
Boiling point (°C)
Sat.
F1
F2
F3
F4
10
3
31
51
3
275–354
Average molecular weight measured by the VPO method.
b
Obtained by HPLC, Sat.—C16–C24 alkyl compounds; F1—C8–C15 alkyl benzene; F2—C1–C4 alkyl biphenyl and C1–C6 alkyl naphthalenes;
F3—C1–C4 alkyl anthracenes, phenanthrenes, and fluorenes etc.; F4—alkyl carbazole and alkyl dibenzofuran homologues, phenols and aromatic
ketones.
c
Atomic ratio.
J. Huo et al. / Carbon 42 (2004) 3177–3182
30
Yield of PI /wt%
25
20
15
10
5
0
0
10
20
30
40
50
Content of ferrocene /wt%
Fig. 1. The effect of ferrocene content on the yield of PI at a reaction
temperature of 480 °C for 10 min.
ters and various morphologies were formed. When the
ferrocene content was low (from 2 wt.% to 25 wt.%),
most of particles were spherical with diameters in the
range 15–45 nm (Fig. 2a–d), and a few particles showed
elliptical or irregular shapes. The sizes of the iron particles tended to become larger when the ferrocene content
was increased in the range 2–25 wt.%, which can be ascribed to the aggregation of iron particles formed via
pyrolysis with increasing amounts of added ferrocene.
When the ferrocene content was high (from 30 wt.% to
45 wt.%), an interesting phenomenon was observed in
that, in addition to the iron nanoparticles, iron-filled
carbon nanorods with diameters 35–50 nm and lengths
several tens to several hundreds of nanometers were present (Fig. 2e and f) in the pyrolytic products. Moreover,
some particles in Fig. 2b have a gap between the iron
crystal and the carbon cage, a phenomenon which has
also been seen by a number of other groups [1,2,
23,24]. This may be related to the different activities of
each active spot of the surface of iron species which result from the structural anisotropy of imperfect sphericity. There were also some hollow carbon nanoparticles
formed, as can be seen in Fig. 2b.
Fig. 3 shows HREM micrographs of the co-carbonized products of YD and ferrocene with ferrocene contents of 13 wt.% and 40 wt.%, respectively. In Fig. 3a,
many spherical CEMNs with uniform diameters of 15–
30 nm can be observed, in which the outer carbon layers
with thickness of about 5 nm surround the crystal iron
nanoparticles, as shown in Fig. 3b. When the ferrocene
content was higher than 30 wt.%, in addition to some
small carbon-encapsulated iron nanoparticles with
diameters from 10 to 40 nm in the products, iron fullfilled carbon nanorods with diameters of 30–50 nm are
also plainly present in Fig. 3c, which can be attributed
to the coalescence of iron nanoparticles. Moreover, in
both Fig. 3b and c, an interface between iron cores
3179
and carbon shells is clearly seen, indicating that the outside carbon layer is generated as a result of catalysis by
inner iron nanoparticles. It is reasonable to believe that
the carbon on the outside of the metal particles (indicated by arrow A in Fig. 3b) is more ordered [22] compared with that generated from direct pyrolysis of YD
without the aid of any catalyst (indicated by arrow B
in Fig. 3a). The catalytic carbon is a semi-graphitic carbon, which readily develops into well-ordered graphitic
carbon via graphitization accelerated by the enclosing
iron nanoparticles when further heated at about
1000 °C [22,25].
Fig. 4 shows the XRD patterns of the samples containing different amounts of iron. The main peaks
appearing at 2h = 44.67°, 65.02°, and 82.33° can be ascribed to the (1 1 0), (2 0 0) and (2 1 1) reflections of a-Fe
[26], respectively, suggesting that the main metal core
encapsulated in the carbon shells was bcc a-Fe. The peak
at 26.02° can be attributed to the (0 0 2) reflection of carbon. The broad diffraction peak indicates that the carbon
has a disordered structure. In addition, small amounts of
an iron carbide (Fe3C) phase, with diffraction peaks at
37.64°, 40.76°, 42.82°, 43.56°, 44.10°, 44.92°, 45.74°,
49.10°, 54.33°, 58.20°, 78.38°, 86.02° [26] appeared in
samples when the ferrocene content was relatively high
(from 20 wt.% to 45 wt.%) (Fig. 4d–f).
The above observations are closely related to the formation mechanism of CEMNs. The formation of
CEMNs by this method is based on condensation and
polymerization in the liquid phase by the aid of catalysis
under pressure. It is well known that ferrocene decomposes into iron atoms when it is heated above 450 °C
and the iron atoms aggregate to form iron nanoparticles
or clusters, which show a very high catalytic activity.
When YD was heated together with ferrocene above
450 °C in an inert atmosphere, PAHs and even carbon
can be formed by condensation and polymerization catalyzed by pseudo-liquid (quasi-solid) iron clusters derived from ferrocene. If the surface of the iron clusters
possess more active spots and the activity of the nanoparticles is sufficiently high, catalytically grown polycyclic aromatic hydrocarbons can arrange epitaxially
encircling the iron clusters and gradually encapsulate
the clusters to become multi-layer carbon-encapsulated
iron nanoparticles [20]. With the increase in ferrocene
content relative to YD, more pseudo-liquid iron clusters
were formed via pyrolysis. Such nanoclusters contain a
very high percentage of surface atoms, creating a tremendous surface energy per atom [27]. One way to decrease the surface energy should be that, once the iron
clusters serve as a nucleation center for the carbon, an
excess of iron may continue to grow onto the nanoparticles, which may gradually result in the formation of
one dimensional long iron nanorods. At the same
time, different amounts of carbonaceous substances
were adsorbed and decomposed on the surface of the
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J. Huo et al. / Carbon 42 (2004) 3177–3182
Fig. 2. TEM images of various-shaped carbon-encapsulated iron nanomaterials heated at 480 °C in the presence of ferrocene contents of (a) 7.0 wt.%,
(b) 10.0 wt.%, (c) 13.0 wt.%, (d) 20.0 wt.%, (e) 30.0 wt.% and (f) 40.0 wt.%.
pseudo-liquid iron clusters. Subsequently, the carbon
atoms dissolve and diffuse into the interior nanoparticles
to form a metal-carbon solid solution. Nanotube growth
occurs when supersaturation leads to carbon precipitation into a tubular form along the cylindrical axis [28].
During precipitation, some carbon remaining in the solid solution may form carbides.
As is well known, two types of polymerization occur
in aromatic systems when foreign substances such as
metal compounds are present [22,25]: common pyrocondensation via free radicals generated from direct
pyrolysis of the aromatic hydrocarbons and catalytic
condensation aided by additives, of which only the latter
can produce CEMNs. In order to attain the desired
CEMNs in large quantities with high purity, suitable
feedstocks, including the carbon source and metal precursors, and the reaction conditions must be chosen in
order to avoid the direct formation of pyrolytic carbon.
J. Huo et al. / Carbon 42 (2004) 3177–3182
3181
Fig. 3. HREM images of carbon-encapsulated iron nanomaterials obtained from YD heated at 480 °C in the presence of ferrocene contents 13.0 wt.%
[(a) and (b)] and 40.0 wt.% (c).
Intensity / CPS
2
1:C
2 : Fe
3 : Fe3C
333 3 3 33 3
0
20
2
2
1
40
60
3 3
80
3
a
b
c
d
e
f
100
2 Theta (degree)
Fig. 4. XRD patterns of carbon-encapsulated iron nanomaterials
obtained from YD heated at 480 °C in the presence of ferrocene
contents of (a) 7.0 wt.%, (b) 10.0 wt.%, (c) 13.0 wt.%, (d) 20.0 wt.%,
(e) 30.0 wt.% and (f) 40.0 wt.%.
In this paper, the carbon source YD contains mainly 2–3
ring aromatic hydrocarbons with some alkyl side chains
(as can be seen in Table 1). When it was heated at 480 °C
for 10 min, without any additives, the pyrolytic products
can be completely dissolved in pyridine as indicated in
Fig. 1. However, under the same reaction conditions,
with ferrocene added to the YD system, carbon formation was accelerated by catalysis by iron nanoparticles
resulting from ferrocene decomposition affording the
PI fractions (CEMNs). CEMNs with high purity were
obtained by pyridine extraction of soluble pyrolytic
impurities. It is reasonable to believe that the carbon
layers encapsulating the iron nanoparticles were generated by catalytic condensation. If the temperature is
higher than 480 °C or the temperature is maintained at
480 °C for long periods, more pyrolytic carbon is obtained, which leads to a decrease in the purity of the
CEMNs. Thus, the reaction temperature and soaking
time must be carefully controlled in the preparation of
CEMNs via the co-carbonization of YD and ferrocene.
Simplicity, low cost, controllability and high yields
are the key features of this preparation method. It
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J. Huo et al. / Carbon 42 (2004) 3177–3182
provides a more effective method for the synthesis of
CEMNs on a large scale than previously reported.
[8]
4. Conclusions
[9]
Large quantities of carbon-encapsulated iron nanoparticles have been synthesized successfully by a new
method. The formation of CEMNs by this method is
based on condensation and polymerization in the liquid
phase with iron-based catalysis. The yield of CEMNs increased from 2 wt.% to about 25 wt.% with increasing of
ferrocene content from 2 wt.% to 45 wt.% in reactions at
480 °C for 10 min under autogenous pressure. TEM and
HREM show that the ferrocene content has a significant
influence on the morphology and size of the CEMNs.
With increasing ferrocene content, the shapes of the
CEMNs change from spherical to nanorods, and the
size of the CEMNs increases. XRD indicates that all
as-prepared nanoparticles were mainly carbon-encapsulated iron while small quantities of iron carbides appeared when the ferrocene content was higher than
20 wt.%, and that the resulting carbon was disordered.
The ability to prepare large quantities of CEMNs with
various sizes and shapes by this method should promote
practical applications of CEMNs in various fields.
Acknowledgments
This work was supported by the National Natural
Science Foundation of China (59802002), the National
High-Tech Research and Development Program
(2003AA302650) and the Foundation of the State Key
Laboratory for Heavy Oil Processing (200303).
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