Dispersing and coating of transition metals Co, Fe and Ni on carbon materials

Article abstract of DOI:10.1016/S0009-2614(02)01044-8

Interaction between transition metals Co, Fe and Ni and carbon materials, such as multi-walled carbon nanotubes (MWNTs), single-walled carbon nanotubes (SWNTs), activated carbon (AC) and layered graphite (LG), has been investigated at high temperatures. Complete wetting for AC, partial wetting for MWNTs, and almost no wetting for SWNTs and LG have been observed, respectively. It is found that the defects in the carbon materials play a key role in the interaction.

Full text of DOI:10.1016/S0009-2614(02)01044-8

13 August 2002
Chemical Physics Letters 362 (2002) 135–143
www.elsevier.com/locate/cplett
Dispersing and coating of transition metals Co, Fe and Ni
on carbon materials
a
Ziyi Zhong , Binghai Liu , Lianfeng Sun , Jun Ding ,
b
a
b
a,
a
Jianyi Lin *, Kuang Lee Tan
a
Department of Physics, Surface Science Laboratory, National University of Singapore, 10 Kent Ridge Crescent,
Singapore 119260
b
Department of Materials science, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
Received 7 August 2001; in final form 25 June 2002
Abstract
Interaction between transition metals Co, Fe and Ni and carbon materials, such as multi-walled carbon nanotubes
(MWNTs), single-walled carbon nanotubes (SWNTs), activated carbon (AC) and layered graphite (LG), has been
investigated at high temperatures. Complete wetting for AC, partial wetting for MWNTs, and almost no wetting for
SWNTs and LG have been observed, respectively. It is found that the defects in the carbon materials play a key role in
the interaction. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
elements into the empty channel of CNTs by arc-
discharge method [4] and wet chemistry method
[5–8], and a recently reported two-step catalytic
reaction that results in a high-yield synthesis of
GaN nanowires encapsulated inside CNTs [9],
coating of metals on the outer walls of CNTs
[10,11], and even directly using CNTs as template
to prepare wire-structured GaN [12] and carbide
nanorodes [13]. However, in some cases these
methods can only meet with limited success. For
examples, the conventional arc-discharge tech-
nique is successful only for the encapsulation of
rare earth metals, but to the transition metals such
as Fe, Co and Ni, it is not successful in producing
the completely filled nanotubes with macroscopic
quantities [14]. By wet chemistry method, the
abundance of completely filled structures is also
low and their production is difficult to control
Carbon nanotubes (CNTs), which were first
discovered by Iijima [1], have attracted intense
interest in recent years because of their novel one-
dimensional structure and expected electronic and
magnetic, mechanical and gas adsorption proper-
ties. To fabricate nanoscale devices based on car-
bon nanotubes with improved electronic
properties and functions, several important tech-
niques have been employed and developed. These
techniques include the designed doping of other
elements such as boron and potassium in the layer
graphitic structure of CNTs [2,3], filling of metallic
* Corresponding author. Fax: +65-874-6126.
E-mail address: phylinjy@nus.edu.sg (J. Lin).
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2614(02)01044-8

136
Z. Zhong et al. / Chemical Physics Letters 362 (2002) 135–143
[7,14,15]. To get good wetting effect, it usually
demands the chosen materials have a surface ten-
sion of less than 100–200 N=m. There are only
limited materials that can match the above wetting
criteria [16,17].
were prepared by decomposition of their nitrates
at 600 °C in air for 1.5 h, followed by reduction in
hydrogen at 600 °C for 1 h.
The prepared metals Fe, Co and Ni were
physically mixed with the MWNTs or other car-
bon materials, respectively, and heated in highly
purified argon in a quartz tube with a flow rate of
5–10 ml/min. The weight loss after heat treatment
was usually less than 5–8 wt%. The microstructure
and the morphology of the annealed products were
observed employing a JEM-100CX transmission
electron microscope. X-ray diffraction (XRD)
scans were obtained using a Bruker D8 Advance
diffract meter with a Cu target and a scanning step
of 0.05°/s for conventional measurements. Slow
scan mode was also used to measure precisely the
strongest X-ray diffractions of metallic Co, Fe and
Ni with a step size of 0.001°/s. In situ XRD mea-
surement for Fe–MWNTs system at different
temperatures was conducted in argon gas within a
HTK 16 high-temperature chamber. The heating
rate was 30 °C/min. Each XRD pattern was re-
corded after the chosen temperature had been
reached and stabilized for 30 min. All the samples
for XRD measurement were pressed into thin
flakes.
Apart from the above mentioned and potential
applications for CNTs, carbon supported cata-
lysts have been found to be efficient for many
catalytic reactions such as hydrogenation reac-
tions [18,19]. However, to date, we have found
that little attention has been paid to the interac-
tion of carbon materials such as CNTs with
transition metals, especially in high temperature
range, though it is a very important aspect in
fundamental research. In this study, four typical
carbon materials, namely MWNTs, SWNTs, AC
and GC, and three transition metals, Co, Ni and
Fe have been selected to investigate the interac-
tion between the metals and carbon materials.
The interaction has found to be determined by
the defects in the carbon materials.
2. Experimental
Nitrates of iron, cobalt, nickel and magnesium
were obtained from Aldrich, graphite powder and
activated carbon from Merck. All of the chemicals
were used as received.
3. Results and discussion
The preparation of MWNTs by decomposition
of methane over Ni/MgO catalysts at 700 °C was
described in literature [20,21]. The produced
MWNTs were treated with concentrated sulfuric
acid to dissolve metallic catalyst, rinsed with dis-
tilled water and dried at 300 °C for 4 h in air. The
preparation of SWNTs was described in our recent
Letter [22] using a Mo0:01Co0:05Mg0:94O catalyst
and a mixture reaction gas of H2 and CH4 with a
molar ratio of 1:4 and a total flow rate of 250 cm3/
min. The prepared SWNTs were first treated with
nitric acid and washed with distilled water, then
dried at 400 °C in air for 3 h to remove the
amorphous carbon. The content of the SWNTs in
the treated sample was evaluated to be 70–80%
according to a temperature-programmed oxida-
tion measurement. The impurities in the above-
treated SWNTs are residual metal Co, graphitic
particles and MWNTs. The metals Fe, Co and Ni
The XRD patterns for metal (Co–, Fe– and
Ni)–carbon systems have been measured before
and after the heat-treatment in Ar gas at 1000 °C
for 10 h. The interaction was shown to depend
very much on the type of the carbon materials.
MWNTs and activated carbon behave alike
whereas LG and SWNTs are very different. As
shown in Fig. 1, after heating there is only a slight
decrease in diffraction intensity for the Co(1 1 1)
diffraction in Co–LG and Co–SWNTs systems
(Figs. 1a–d, respectively), an obvious decrease in
intensity for the Co diffraction in Co–MWNTs
system (Figs. 1e and f), but an almost complete
vanish of the diffraction peak for Co in Co–AC
system. LG seems to be the most inert in the in-
teraction with the metals. Although not shown
here, very similar phenomena have been observed
for Ni and Fe.

Z. Zhong et al. / Chemical Physics Letters 362 (2002) 135–143
137
Fig. 1. The slow scans of the Co(1 1 1) diffraction for various
metal–carbon material systems before and after heat-treatment
at 1000 °C for 10 h in Ar gas: Co–LG system (a and b); Co–
SWNTs system (c and d); Co–MWNTs system (e and f); Co-AC
system (g and h).
Fig. 2. XRD patterns of Co–, Fe– and Ni–MWNTs systems
before and after heating at 1000 °C for 10 h in argon.
The changes in the diffraction intensity of the
metals after the heat-treatment of the metal/C
mixture directly reflect the strength of the inter-
action between the metals and the carbons, be-
cause the decrease in intensity is caused by the
dispersing of the metal particles and/or the trans-
formation of the metals to other phases (such as
metal carbides). Obviously, the interaction be-
tween the metals and various carbon materials
the formation of trace carbide Co2C. For Ni–
MWNTs system (two top patterns in Fig. 2), the
appearance of a new small diffraction peak at
2h ¼ 39:4°; and a hump near 2h ¼ 45° indicate the
formation of nickel carbide Ni3C referred to
ASTM card 72–1467. Actually we observed these
peaks in the samples heated at 750 °C for 10 h too,
showing the nickel carbide can be formed at lower
temperatures. For Fe–MWNTs system a huge
hump centered at about 2h ¼ 43° can also be ob-
served after heating at 1000 °C for 10 h (Fig. 2).
But it is still not very clear if the new compound is
Fe3C (ASTM card 75-910), or Fe7C3 (ASTM card
75-1499, and [23,24]), or e0-carbide ðFe2:2CÞ, or just
a solid solution of carbon that has incorporated in
iron. Because the diffraction peaks of these com-
pounds usually overlap in this range, it is not easy
to discern them in the XRD pattern. To better
understand this interaction process for Fe–
MWNTs system, we carried out an in situ XRD
measurement by heating the Fe/MWNTmixture
(1:2 by weight) in Ar from room temperature to
1000 °C, and the XRD patterns are shown in
follows
the
order:
MWNTs > metals–SWNTs > metals–LG.
metals–AC ꢀ metals–
The metal–MWNTs systems have been selected
for further investigation. Fig. 2 shows the XRD
patterns for Co, Fe and Ni mixed with MWNTs
before and after heated at 1000 °C. From the two
Co patterns at the bottom of Fig. 2, we can see,
after the heat-treatment, a hump appears near the
strongest diffraction peak of the fcc cobalt at
2h ¼ 44:24°. At the same time, two new small
peaks (marked with an asterisk) are identified,
with the first one being centered at 2h ¼ 42:5°
ꢀ
corresponding to a d-spacing of 2.13 A, and the
second one at 2 ¼ 37:3° corresponding to a d-
ꢀ
spacing of 2.41 A, all of which can be ascribed to

138
Z. Zhong et al. / Chemical Physics Letters 362 (2002) 135–143
Fig. 3a. The small peak at 2h ¼ 35:3° (marked
with an asterisk) for the samples heated between
400–800 °C is ascribed to the formation of trace
Fe3O4 (ASTM card 72-2303). The Fe3O4 may
originate from the oxidation of Fe by trace of
oxygen in the system. Unfortunately from this in
situ XRD measurement we still do not observe any
other diffraction peaks of the iron carbides such as
Fe3C and Fe7C3 than the hump peak near
2h ¼ 45°. The missing of these diffractions ex-
cludes, at least, the possibility that the carbides
were formed as intermediates but decomposed at
higher temperatures.
It has been reported that Co, Fe and Ni usually
form non-stoichimetric and interstitial solid solu-
tions with carbon. Some metastable phases were
observed [25,26], but the formation of these tran-
sition carbides is still a disputable issue. It seems to
depend strongly on reaction conditions such as
precursors, atmosphere, particles size, and so on.
For example, nanosized nickel particles reacted
with some organic carbon sources to form Ni3C at
200 °C, but this carbide decomposed at tempera-
tures above 300 °C [27]. In other reports [28] Ni3C
could be formed at the front side of Ni–Cu catalyst
particles at 550 °C, or on Nickel electrode above
900 °C [29]. In our case, the Co2C and Ni3C were
observed by XRD measurement after high tem-
perature treatment, clearly indicating the involve-
ment of a covalent bonding process between
MWNTs and metal atoms. In the case of Fe–
MWNTs system, though we cannot determine
what kind of carbide phase was formed, the ob-
servation of a hump near 2h ¼ 43° should be con-
nected to a bonding process between the Fe
particles and the MWNTs, at least at the MWNTs–
Fe interface. As mentioned above, the incorpora-
tion of carbon into iron can result in the formation
of interstitial solid solution.
In addition to the carbide formation, the de-
crease of the diffraction intensity after high tem-
perature heating may also be caused by the
decrease in the size of metal particles. Figs. 2 and 3
clearly show that the dispersion extent of the metal
particles increases with the annealing temperature.
We may monitor this interaction process by simply
observing the change in intensity of the X-ray
diffraction of these metals. Fig. 3b shows the in-
tensity of the strongest X-ray diffraction of me-
tallic iron changes with the annealing temperature.
Clearly the dispersion extent of the iron particles
almost increases linearly with the annealing tem-
perature. The dispersion process has been more
clearly observed by TEM measurement.
Fig. 3. (a) XRD patterns for the Fe–MWNTs system with a
weight ratio of Fe:MWNTS ¼ 1:2 obtained from an in situ
measurement; (b) the intensity of (1 1 1) diffraction of the me-
tallic iron in (a) vs. heating temperatures.

Z. Zhong et al. / Chemical Physics Letters 362 (2002) 135–143
139
Fig. 4a shows a TEM image of the metal Co
after reduction. It is seen the particles are in mi-
crometer scale and are crystallized very well. After
heated with MWNTs at 1000 °C for 10 h, most of
the cobalt particles were highly dispersed, and
some of them coated on the surface of MWNTs
(Fig. 4b). From the TEM observation we evalu-
ated that the content of the coated MWNTs is
about 10–15% out of the total MWNTs. The
coated Co particles are in the range of 30–50 nm,
and most of them are disconnected to each other.
Fig. 4c is a magnified image of Fig. 1b. From it we
can still discern the empty channel structure in
MWNTs. In our TEM measurements, we never
found any carbon tubes that are filled with cobalt
particles, indicating this annealing method only
leads to the dispersion as well as coating of Co
particles on the surface of MWNTs. Definitely this
is related to the fact that the annealing tempera-
tures are much lower than those of the melting
points of metals Co, Fe and Ni.
Actually in each sample after heat-treatment we
always observed three forms of cobalt particles,
namely the big Co particles that had not been
dispersed yet, the dispersed and the coated cobalt
particles. There are several ways to promote the
interaction between the cobalt particles and
MWNTs. We tried to increase the annealing time
Fig. 4. TEM images of (a) the metallic Co particles from reduction of Co3O4 at 600 °C; (b) the coated Co particles on MWNTs in the
Co–MWNTs system with a weight ratio of Co:MWNTs¼ 1:2 and after being heated at 1000 °C for 10 h in argon; (c) magnified of (b);
(d) the highly dispersed Co particles in the Co–MWNTs system with a weight ratio of Co:MWNTs¼ 1:2 and after being heated at 750
°C for 18 h in argon.

140
Z. Zhong et al. / Chemical Physics Letters 362 (2002) 135–143
or the annealing temperature, or both of them. It
was observed by TEM that after heating at 750 °C
for more than 20 h or at 1000 °C for 10 h, about
70–80% of Co particles had been dispersed.
Heating at lower temperatures or in shorter time
resulted in a very incomplete dispersion of Co
particles. It seems that heating temperature plays a
crucial role in the dispersion effect. When the
temperature was below 650 °C, heating even more
than 20 h still could not lead to a sufficient dis-
persion of cobalt. Also, the weight ratio between
the metal Co and MWNTs has influence on dis-
persing effect. When the weight ratio between the
metal cobalt and the MWNTs exceeded 1:2, there
were still a lot of Co particles in big size after
heating at 1000 °C for 10 h. Fig. 4d shows a TEM
image of the dispersed Co particles after being
heated in 750 °C for 18 h. Its magnified image (not
shown here) indicated the black Co particles are
uniformly distributed in carbon matrix. The exis-
tence of the carbon matrix is similar to our pre-
vious observation that the cobalt favors to be
encapsulated in carbon [21].
The coated Fe particles on the surface of the
MWNTs can form a continuous structure, which
is different from the Co. Fig. 5a shows a TEM
image of Fe coated on MWNTs that had been
heated at 1000 °C for 3 h. The size of Fe particles is
in the range of 5–15 nm. But we still can discern
the empty channel structure in MWNTs, indicat-
ing the coating is still disconnected. After anneal-
ing for 10 h at 1000 °C, we found the coated Fe
particles had been grown up in size and became
connected to each other. At the same time, the
Fig. 5. TEM images of the Fe–MWNTs system (weight ratio Fe:MWNTs ¼ 1:2) heated at 1000 °C in argon for (a) 3 h and (b) for 10 h;
(c) the Ni–MWNTs system (weight ratio Ni:MWNTs ¼ 1:2) heated at 1000 °C for 10 h in argon; (d) the Co–graphite mixture with a
weight ratio ¼ 1:2 and after being heated at 750 °C for 18 h in argon.

Z. Zhong et al. / Chemical Physics Letters 362 (2002) 135–143
141
empty channel cannot be seen again, indicating an
almost completely continuous coating was reached
(Fig. 5b). Near the coated MWNTs, a lot of highly
dispersed particles are also observed. It seems that
these particles are composed of a mixture of the
metallic particles and carbon pieces, or of a hybrid
structure. Clearly the heating and the interaction
between metal and MWNTs can result in damage
of some MWNTs.
Ni can also interact with MWNTs strongly. In
our experiments a poorer and discontinuous
coating on MWNTs was observed for nickel as
compared with the cobalt and iron. However, a
much higher extent of dispersion of nickel was
observed. Fig. 5c shows a TEM image of the Ni–
MWNTs system after being heated to 10 h at 1000
°C. Clearly the nickel particles or their hybrid
structure formed with carbon are very uniform in
size. Bilaniuk and Howe [30] once reported that
the nickel could wet the carbon very readily. In-
deed in our experiments, nickel is the most easily
dispersed metal compared to Fe and Co.
Co3O4, a-Fe2O3 and NiO were once used to
replace metallic Co, Fe and Ni as precursors to
mix and interact with the MWNTs at high tem-
peratures. After being heated at 1000 °C for 2–3 h,
XRD results confirmed these metallic oxides had
been reduced to elemental metals accompanying a
release of carbon dioxide or carbon monoxide.
Longer heating will result in a similar dispersion as
observed by using the metals as precursors.
Very interestingly from this study we observed
the dispersion of metals Co, Fe and Ni on
MWNTs after the heat-treatment. Usually heating
of metals or their oxides in high temperatures will
lead to a marked growth in particle size or ag-
gregation of particles. For example, heating of the
metallic Fe or its oxide particles coated on alumina
spheres at 400 °C in argon caused a marked
growth in particles size. When the temperature was
increased to 1000 °C in air, the iron oxide particles
would incorporate into the alumina substrate and
aggregate together. The aggregation force was so
strong even that the alumina spheres were broken
into small pieces [31]. In fact we once tried to in-
troduce trace amount of oxygen into the Co–
MWNTs system by stopping flowing of argon gas
in the quartz tube when heating the Co–MWNTs
system at 1000 °C. Consequently an obvious
growth in particle size of the cobalt and a partial
consumption of the MWNTs were found, so the
dispersion of these transition metals is due to their
interaction with MWNTs in an inert atmosphere.
In other words, the MWNTs can promote the
dispersion of the metal particles after heating
treatment and stabilize them very well.
In Fig. 5d it is seen the layer graphitic structure
and the black metal particles in big size are clearly
discerned. There is little Ni particles dispersed and
coated on graphite sheets after the heat-treatment.
Similar results were observed in the metal–SWNTs
system (the image is not shown here). Whereas in
metal-activated carbon system, almost all metal
particles were highly dispersed after heating at
1000 °C for 10 h.
It is known the ideal graphite consists of layers
of hexagonal arrays of carbon in an ABAB-planar
stacking arrangement, and the carbon nanotubes
can be regarded as rolled graphitic layers having
hexagon, pentagon, and heptagon carbon rings as
a result of its unique hybridization of sp1, sp2 and
sp3 bonding [32,33]. The pentagon and heptagon
rings are not as stable as the hexagon rings, and
can be regarded as defect sites. For SWNTs, due to
the relatively high formation temperature
ð>900 °CÞ, their defects are very limited. Con-
versely, the MWNTs possess more defects because
of their relative low formation temperature.
Compared to the above three carbon materials,
activated carbon is poorly crystallized and pos-
sesses a lot of structure defects and dangling
bonds. Thus, when exposed to the transition
metals at high temperatures, the metal atoms
should attack these defect sites much more easily
to form covalent bonding. Compared to AC and
MWNTs, the layered graphite and SWNTs are
almost defect free, and therefore they are much
less reactive toward transition metals Fe, Co and
Ni. So the defect numbers in the carbon materials
should determine the strength of the interaction.
The more the defect number, the stronger the in-
teraction should be.
We can further understand why the discon-
nected coating on the MWNTs is firstly formed at
the early stage of reaction, probably because there
is a distribution of these defect sites on the outer

142
Z. Zhong et al. / Chemical Physics Letters 362 (2002) 135–143
Acknowledgements
walls of MWNTs. With the increase of annealing
time or temperature more and more metal atoms
can combine with the carbon atoms, which, in
turn, will cause damage to the structure of the
MWNTs and produce more and more defects in
the wall. Therefore we can observe the dispersion
and coating of metal particles on MWNTs.
Eventually the growth and connection of coated
particles may develop into a continuous coating as
observed in Fe–MWNTs system.
This work is supported by National Science and
Technology Board of Singapore (GR6773). The
authors would like to thank Madam Loy in Bio-
logical Department in National University of
Singapore for her assistance in TEM measure-
ments.
Zhang et al. [11] once coated a series of metals
on SWNTs by electron beam deposition method.
Contrary to our experiments for the metal–
MWNTs systems, they observed a continuous
coating for Ni and a disconnected coating for Fe.
But in our study, almost no coating was observed
by TEM on SWNTs. As analyzed above, it is be-
cause the heating promoted coating or interaction
is strongly dependent on the structure, density and
distribution of the defects in the carbon materials.
In summary, the wetting of the carbon materi-
als by the transition metals is determined by the
number and structure of the defects in the carbon
materials. It is found the graphite sheets and
SWNTs are difficult to be wetted, whereas the
MWNTs and activated carbon are much easily to
be wetted. It also demonstrated that simple an-
nealing of the mixture of transition metals Co, Fe
and Ni with MWNTs could result in their disper-
sion and coating on MWNTs. For Co– and Ni–
MWNTs system, a disconnected coating was
obtained, and for Fe–MWNTs system, extension
of annealing time can change the disconnected
coating into a continuous coating. Trace amount
of Co2C and Ni3C were also confirmed for Co–
and Ni–MWNTs system, respectively, after being
heated at 1000 °C.
References
[1] S. Iijima, Nature 354 (1991) 56.
[2] D.L. Carroll, Ph. Redlich, X. Blase, J.C. Chalier, S.
ꢁ
Curran, P.M. Ajayan, S. Roth, M. Ruhle, Phys. Rev. Lett.
€
81 (1998) 2332.
[3] C. Zhou, J. Kong, E. Yenilmez, H. Dai, Science 290 (2000)
1552.
[4] C. Guerret-Piecourt, Y.L. Bouar, A. Loiseau, H. Pascard,
Nature 372 (1994) 761.
[5] C.H. Kiang, J.S. Choi, T.T. Tran, A.D. Bacher, J. Phys.
Chem. B 103 (1999) 7449.
[6] B.K. Pradhan, T. Kyotani, A. Tomita, J. Chem. Soc.,
Chem. Commun. (1999) 1317.
[7] D. Ugarate, A. Chatelain, W.A. de Heer, Science 274
(1996) 1879.
[8] J. Sloan, M.C. Novotny, S.R. Baily, G. Brown, C. Xu,
V.C. Williams, S. Friedrichs, E. Flahaut, R.L. Callender,
A.P.E. York, K.S. Coleman, M.L.H. Green, R.E. Dumin-
Borkowski, J.L. Hutchison, Chem. Phys. Lett. 329 (2000)
61.
[9] C.C. Chen, C.C. Yeh, C.H. Liang, C.C. Lee, C.H. Chen,
M.Y. Yu, H.L. Liu, L.C. Chen, Y.S. Lin, K.J. Ma, K.H.
Chen, J. Phys. Chem. Solids 62 (2001) 1577.
[10] P. Chen, X. Wu, J. Lin, K.L. Tan, J. Phys. Chem. B 103
(1999) 4559.
[11] Y. Zhang, N.W. Franklin, R.J. Chen, H. Dai, Chem. Phys.
Lett. 331 (2000) 35.
[12] W. Han, S. Fan, Q. Li, Y. Hu, Science 277 (1997)
1287.
[13] H. Dai, E.W. Wong, Y.Z. Lu, S. Fan, C.M. Liber, Nature
375 (1995) 769.
As these metals and their carbides are good
magnetic materials [34], their highly dispersed and
stabilized states in high temperatures and inert at-
mosphere may be found with special applications in
industry. In addition, the coated CNTs may possess
improved electronic, magnetic and chemical prop-
erties, so this method can be applied to fabrication
of new nanodevices bases on CNTs. Finally, as the
CNTs can be used as the novel catalyst support [35],
we believe this study will be helpful to the devel-
opment of the new CNTs-based catalysts.
[14] S. Subramoney, Adv. Mater. 10 (1998) 15.
[15] Y.K. Chen, A. Chu, J. Cook, M.L.H. Green, P.J.E. Harris,
R. Heesom, M. Humphries, J. Sloan, S.C. Tsang, J.F.C.
Turner, J. Mater. Chem. 7 (1997) 545.
[16] H.J. Yuan, P. Xie, J. Xiang, D.P. Xu, J. Xu, S.S. Xie, S.Q.
Feng, Solid State Commun. 119 (2001) 573.
[17] P.M. Ajayan, O. Stephan, Ph. Redlich, C. Colliex, Nature
375 (1995) 564.
[18] C.P. Huu, N. Keller, L. Charbonniere, R. Ziessel, M.J.
Ledoux, Chem. Commun. (2000) 1871.
[19] D.J. Smith, R.M. Pisher, L.A. Freeman, J. Catal. 72 (1981)
51.

Z. Zhong et al. / Chemical Physics Letters 362 (2002) 135–143
143
[20] P. Chen, H.B. Zhang, G.D. Lin, Q. Hong, K.R. Tsai,
Carbon 35 (1997) 1495.
[28] V.V. Chesnokov, V.I. Zaikovskii, R.A. Buyanov, J. Mol.
Catal. A: Chem. 158 (2000) 267.
[21] Z.Y. Zhong, H. Chen, S. Tang, J. Ding, J. Lin, K.L. Tan,
Chem. Phys. Lett. 330 (2000) 41.
[29] V.P. Dymont, M.P. Samtsov, E.M. Nekrashevich, Tech.
Phys. 45 (2000) 905.
[22] S. Tang, Z.Y. Zhong, Z. Xiong, L. Sun, L. Liu, J. Lin, Z.
Shen, K.L. Tan, Chem. Phys. Lett. 350 (2001) 19.
[23] L.D. Mansker, Y. Jin, D.B. Bukur, A.K. Datye, Appl.
Catal. A 186 (1999) 277.
[30] M. Bilaniuk, J.M. Howe, Interface Sci.
317.
6 (1998)
[31] Z.Y. Zhong, T. Prozorov, I. Felner, A. Gedanken, J. Phys.
Chem. B 103 (1999) 947.
€
[24] A. Koniger, C. Hammerl, M. Zeitler, B. Rauschenbach,
Phys. Rev. B 55 (1997) 8143.
[32] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical
Properties of Carbon Nanotubes, Imperial Colleague
Press, 1998 (Chapter 1).
[25] A. Badzian, T. Badzian, Diamond Relat. Mater. 5 (1996)
93.
[33] R. Gao, Z.L. Wang, S. Fan, J. Phys. Chem. B 104 (2000)
1227.
[26] A. Cottrell, Chemical Bonding in Transition Metal Car-
bides, The University Press, Cambridge, 1995 (Chapters 1
and 2).
[34] X.Q. Zhao, B.X. Liu, Y. Liang, Z.Q. Hu, J. Magn. Magn.
Mater. 164 (1996) 401.
[27] P. Hooker, B.J. Tan, K.J. Klabunde, S. Suib, Chem.
Mater. 3 (1991) 847.
[35] B. Rajesh, K.R. Thampi, J.M. Bonard, B. Viswanathan,
J. Mater. Chem. 10 (2000) 1757.