Decorating carbon nanotubes with nanostructured nickel particles via chemical methods

Article abstract of DOI:10.1016/j.cplett.2006.09.039

An alternative method for decorating carbon nanotubes (CNTs) with Ni nanoparticles is described. In this work our purpose is to establish a novel and low cost alternative route for coating the outer walls of multiwalled CNTs with metals such as Ni. Using nanotubes during the dissociation of nickel nitrate upon dehydration, it is possible to obtain nanostructured nickel oxide. After reduction of this metal oxide in H₂ nickel nanoparticles are obtained and randomly anchored to the surface of nanotubes. Comparative experiments were carried out using pure and nitrogen-containing CNTs. A detailed characterization of the products is also presented.

Full text of DOI:10.1016/j.cplett.2006.09.039

Chemical Physics Letters 431 (2006) 104–109
www.elsevier.com/locate/cplett
Decorating carbon nanotubes with nanostructured nickel particles
via chemical methods
a,
a
b
b
c
c
*
´
P. Ayala , F.L. Freire Jr. , L. Gu , David J. Smith , I.G. Solorzano , D.W. Macedo ,
e
d
d
d
J.B. Vander Sande , H. Terrones , J. Rodriguez-Manzo , M. Terrones
a
Departamento de Fı´sica, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Caixa Postal 38071, 22453-970 Rio de Janeiro, RJ, Brazil
b
Center for Solid State Science, Az State University, Tempe, AZ 85287, USA
Departamento de Ciencia dos Materiais e Metalurgia, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Caixa Postal 38071,
c
22453-970 Rio de Janeiro, RJ, Brazil
Advanced Materials Department, IPICYT, Camino a la Presa San Jose´ 2055, 78216, San Luis Potosı´, Mexico
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
d
e
Received 24 April 2006; in final form 11 September 2006
Available online 20 September 2006
Abstract
An alternative method for decorating carbon nanotubes (CNTs) with Ni nanoparticles is described. In this work our purpose is to
establish a novel and low cost alternative route for coating the outer walls of multiwalled CNTs with metals such as Ni. Using nanotubes
during the dissociation of nickel nitrate upon dehydration, it is possible to obtain nanostructured nickel oxide. After reduction of this
metal oxide in H2 nickel nanoparticles are obtained and randomly anchored to the surface of nanotubes. Comparative experiments were
carried out using pure and nitrogen-containing CNTs. A detailed characterization of the products is also presented.
Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction
Regarding the production of metal–nanotube compos-
ites, some attempts have been carried out. However, this
is not a trivial problem because controlling the wetting
properties of nanotubes becomes a crucial issue that could
ensure strong interactions between the CNT surfaces and
the metal matrix. As stated by Dujardin et al. [8], materials
with high surface tension, such as metals, do not spontane-
ously wet CNTs. Therefore, alternative ways of processing
metal–nanotube composites need to be explored in depth
[9,10]. Most published work to date has focused on the
coating and filling of carbon nanotubes with metals. How-
ever, the difficulty of obtaining a uniform nanotube coating
imposes limitations. This uniformity is desirable and we
envisage that it should be possible to form homogenous
nanowires by coating the outer tube walls uniformly with
different metals. Several publications have shown that,
depending on the metal, the uniformity of the coating var-
ies drastically depending on the metal coating desired. In
this respect, some experimental work was reported on the
decoration of CNTs with Ni and other metals [9–13], using
Over the last decade, various publications have empha-
sized the outstanding physicochemical characteristics of
Carbon Nanotubes (CNTs) [1]. The study of these nano-
structures opened up an important field of research with
enormous potential for developing new technologies [2].
Concerning mechanical properties, carbon nanotubes
could exhibit high strength-to-weight ratio; their individual
Young’s modulus has been measured to reach about
ꢀ1 TPa [3–5]. In fact, the excellent elasticity and flexibility
of CNTs make them attractive as alternative reinforcing fil-
ler in composite materials. Numerous authors have
reported the incorporation of these tubular structures in
diverse matrices, although the most reproducible and suc-
cessful advances have been achieved when fabricating cera-
mic and polymer composites [6,7].
*
Corresponding author. Fax: +55 21 3527 1040.
E-mail address: paola@vdg.fis.puc-rio.br (P. Ayala).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.09.039

P. Ayala et al. / Chemical Physics Letters 431 (2006) 104–109
105
Table 1
electron-beam deposition of metallic layers with different
Nanotube growth conditions and morphologies
Solvent Reaction Concentration of Overall morphology
temperature (°C) Fe(C5H5)2 (wt%)
thicknesses.
Theoretical as well as experimental investigations have
also been carried out in order to explain the interactions
of metal layers with graphitic surfaces, indicating that met-
als usually exhibit non-covalent interactions with hexago-
nal layered carbon. Although it is not yet certain, it is
generally accepted that van der Waals interactions are
responsible for this apparent interface adhesion in metal–
CNTs, rather than chemical bonds [14].
This Letter aims to establish a novel and alternative
route for coating the outer walls of multiwalled CNTs with
metals such as Ni. In this context, we should emphasize
that it has been proved that Ni is able to establish enhanced
binding interaction with highly curved carbon layers [15].
We believe that these interactions could be enhanced if
structural nanotube surface defects or oxide layers are
present.
The experimental method we report is based on the
work of Brocchi et al. [16], who suggested that some com-
pounds such as nickel nitrate could be easily dissociated
into nanoscale particles (in this case leading to the forma-
tion of the metal oxide). In this Letter, we provide evidence
that such chemical transformations occur in the presence of
CNTs, consequently leading to Ni decorated tubes, after
nickel oxide reduction in the presence of hydrogen. In
particular, we report a simple and inexpensive way of dec-
orating different types of carbon nanotubes with nano-
structured nickel using wet chemical routes in ambient
conditions. We envisage that this material could be used
as a matrix precursor to fabricate homogenous metal–
nanotube composites with enhanced elastic properties.
Scanning electron microscopy (SEM) was used for a
general morphology study of the resulting material. A care-
ful characterization was performed using high resolution
electron microscopy (HREM) and electron diffraction tech-
niques. Chemical analysis was carried out using energy-dis-
persive X-ray spectroscopy (EDX) and electron-energy loss
spectroscopy (EELS) in conjunction with scanning trans-
mission electron microscopy (STEM).
C7H8
C7H9N 800–850
800
1.5–2%
1.5–2%
Long MWCNTs
N-doped MWCNTs
exhibiting bamboo-like
morphologies
in N-containing carbon nanotubes that usually exhibit a
bamboo-like morphology [18], as previously reported by
Terrones et al. [19]. All nanotube materials were collected
by scratching the quartz tube walls.
The powder samples were dispersed in isopropanol, and
a few droplets of the suspension were transferred onto a
holey carbon grid for TEM and HREM observation and
analysis. For this purpose a JEOL 2010 and a JEOL-
4000EX Transmission Electron Microscopes were used.
The latter instrument, operating at 400 keV, was employed
for a detailed structural characterization of the tubes as
well as the material described in the next section.
2.2. Formation of nanostructured nickel and decoration of
nanotubes
Ni oxide is obtained by the dehydration of the metal
nitrate in solution with deionized water. Preparation of
the nickel oxide is carried out through the reaction of
99% pure nickel nitrate Ni(NO3)2 dissolved in H2O, and
dissociating it at 500 °C according to the following
reaction:
2Ni(NO3)2ðsÞ ! 2NiO + 4NO2ðgÞ + O2ðgÞ
ðsÞ
For the nanotube decoration, we carried out the same
Ni(NO3)2 dissociation but this time in the presence of
our synthesized nanotubes. The NiO + nanotubes synthe-
sis is carried out by homogeneously dispersing nickel ni-
trate together with 1%wt of nanotube powder (pure
CNTs and CNx were used in separate experiments) in
deionized water. Mild sonication is initially required in
order to avoid nanotube agglomeration caused by their
low solubility in water.
2. Experimental
nanotubes 2NiOðsÞ þ 4NO2ðgÞ þ O2ðgÞ
2.1. Nanotube synthesis and characterization
2NiðNO3Þ2ðsÞ
!
The solution was then dehydrated at ꢀ500 °C and later an-
nealed in ambient conditions at the same temperature.
After obtaining the nanotube–metal oxide product, an
additional step is required to reduce the oxide to metallic
nickel. For this purpose it is possible to propose the follow-
ing reaction when hydrogen is present in a reducing
atmosphere.
Experiments were carried out using pure multi-walled
CNTs and N-doped multi-walled CNTs synthesized by
the spray pyrolysis method reported by Kamalakaran
et al. [17]. This method is able to produce high yields of
nanotubes (g/h), and involves the atomization of a solution
released from a glass liquid container through a quartz
tube preheated at 800–850 °C in an Ar atmosphere. In
our experiments, Ferrocene (Fe(C5H5)2(s)) was dissolved
in Toluene (C7H8) or Benzylamine (C7H9N). These two
solutions were then pyrolyzed in order to obtain two differ-
ent types of tubular structures as summarized in Table 1.
The experiments using benzylamine as the solvent resulted
NiO+H2ðgÞ!Ni+H2O
We used two different reducing atmospheres at 650 °C: ex-
tra high purity 99% H2 and a mixture of Ar 95% with H2
5%. The samples were placed into a quartz tube and heated

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P. Ayala et al. / Chemical Physics Letters 431 (2006) 104–109
until attaining the reduction temperature in presence of Ar
flow. Subsequently, the reduction was carried out for 1 h in
a hydrogen flow of 100 sccm for the pure H2 and 150 sccm
for the Ar/H2 gas mixture.
The nickel oxide obtained in the presence of the tubes as
well as the Ni-decorated tubes were carefully characterized
by HREM. Annular dark field scanning transmission elec-
tron microscopy as well as energy dispersive X-ray spec-
troscopy elemental mapping were performed using a VG
HB603 dedicated STEM equipped with a large X-ray
detector operating at 250 kV. Energy-filtered imaging was
performed in an FEI Tecnai F20 equipped with a post-col-
umn Gatan imaging filter (GIF).
impurities were not usually evident and appeared in very
low concentrations in rare cases.
The material resulting from the pyrolysis experiments
with benzylamine:ferrocene mixtures (N-containing nano-
tubes) were also tubular structures with a similar range
of diameter distribution but exhibiting a characteristic
bamboo-like morphology (Fig. 2a). They also displayed
a certain amount of nitrogenated defects on their outer-
most surfaces, and occasionally an incomplete layer coat-
ing on the outer layers (Fig. 2b). The nitrogen is
presumably found either encapsulated inside the tubes or
incorporated within the walls in very low amounts (<2%
confirmed by EELS measurements), thus introducing
structural defects on the crystalline graphene walls. In
addition, these tubes are rather pure and contained almost
no foreign particles such as amorphous carbon or encap-
sulated metal particles.
Following the nickel nitrate decomposition, the result-
ing NiO material was characterized prior to the reduction
process. For the NiO synthesized in the presence of
MWCNTs as well as CNx nanotubes, it was not possible
to determine a nanotube concentration by means of elec-
tron microscopy, since TEM and STEM images do not
reveal clearly their presence in the intermediate product.
Identification of the carbon nanotubes by Analytical Elec-
tron Microscopy resulted unfeasible because the carbon
nature of the supporting holey film used for the microscopy
characterization combined with the strong signals from Ni
and O, blurred out the tubular carbon features in the bulky
NiO material (figure not shown here).
3. Results and discussion
In order to compare the surface reactivity and the metal
decorating ability, we have analyzed the differences when
using pure CNTs and N-doped CNTs. Thus, the morpho-
logical characterization of the nanotubes used as precursor
material is fundamental, as significant differences were
observed for the multiwalled nanotubes from toluene
solutions and the N-containing tubes produced with
benzylamine.
We noted that the pure carbon MWCNTs obtained
when pyrolyzing toluene:ferrocene solutions were heteroge-
neous regarding the tube diameter distribution but con-
sisted of nested carbon cylinders (Fig. 1a and b). The
encapsulation of Fe-metal inside the nanotube cores is a
common feature observed in our samples. In some cases
long Fe nanowires were witnessed, although the Fe mate-
rial was most commonly found as small agglomerates
inside the nanotube cores. The presence of straight and
parallel graphite fringes suggests a good crystallinity within
the produced tubes, prior to the decoration experiments
(see Fig. 1b). It is also noteworthy that amorphous carbon
The decorated nanotube structure is only revealed once
the reduction of the NiO–nanotube material in a hydrogen-
rich atmosphere. We observed different particle sizes
assembled on the tubes depending on reduction time, nano-
tube type, gas flow and reduction temperature. Thus, we
succeeded to obtain a uniform distribution of metallic Ni
Fig. 1. (a) Bright field low magnification image of pure MWCNTs showing the overall distribution and morphology prior to nanotube decoration
experiments and (b) high resolution image of a MWCNT.

P. Ayala et al. / Chemical Physics Letters 431 (2006) 104–109
107
Fig. 2. (a) Low magnification TEM image and (b) HREM of a N-doped carbon nanotube exhibiting the characteristic features of the bamboo-like
structure.
nanoparticles attached along the surfaces of the tubes (pure
CNTs and N-containing CNTs). For both types of nano-
tubes, the lower hydrogen content in the carrier gas, and
longer reduction times resulted in the formation of Ni par-
ticles of uniform size (e.g. 4–15 nm). Both types of tubes
have given similar results regarding the adhesion character-
istics of Ni particles onto the superficial layers of the tubes,
as observed by the SEM and TEM characterization in Figs.
3–5. Pure multiwalled CNTs tend to keep their crystalline
structure, and exhibit mainly nickel particles ranging from
4 to 15 nm attached on their outer layer. However, for N-
doped tubes, which to some extent exhibit structural
defects presumably induced by the nitrogen incorporation,
we found less size dispersion of the Ni material attached on
the surface (exhibiting a higher yield between 8 and 14 nm).
The particles attached reveal higher average size when
compared to the particles attached on pure MWCNTs
(see Figs. 4d and 5d). Nevertheless, this does not seem to
influence the contact angle observed between the Ni parti-
cles and the tube surfaces.
Fig. 3. (a) Bright field STEM image showing Ni particles on N-doped CNTs; (b) X-ray elemental mapping showing that the attached particles are Nickel
and (c) high resolution TEM image of Ni particles attached to the outer walls of the CNx tubes.

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P. Ayala et al. / Chemical Physics Letters 431 (2006) 104–109
with high surface tension materials such as metals and tran-
a)
100nm
sition metals is not possible, we have now corroborated the
theoretical work carried out by Curtin and Sheldon [7] and
the experimental observations performed by Zang et al. [9],
which illustrate that the interaction of Ni with single-
walled CNTs is possible and could have certain ‘covalent’
bonding characteristics that make the tube decoration pos-
sible. We believe this might be the reason why the Ni par-
ticles get easily attached to the nanotube surfaces, even on
carbon tubes without doping that are expected to be chem-
ically inert and tend to disallow good metallic wetting while
coating the surface.
b)
CNx Nanotubes +
nanostructured Ni
Since we started from the production of the metal
oxide–nanotube mixture (by dissociating Ni(NO3)2 dis-
solved in H2O with both type of nanotubes at 500 °C), it
highly likely that numerous carboxyl groups appear on
the tube surface, thus promoting the formation of an oxide
layer on the tube interface; this being responsible for an
efficient metal anchoring. We performed HREM studies
(Figs. 3c and 5d), and noted that Ni tends to adhere
strongly to the nanotube surface. In this context, we cannot
rule out the possibility of having some nickel oxide left on
the tube–metal interface (see high contrast fringes at the
tube interface; Fig. 5d). This interface arises from the fact
that metal reacts with the oxygen and carbon, thus forming
a few layers of an oxide, which facilitates the adhesion of
metallic clusters. This explains why we did not observe
clear differences in the particle attachment yield on both
types of tubes. However, the size of the clusters varied
between the two types of tubes.
4
6
10
12
14
24
40
Particle size (nm)
Fig. 4. Characterization of N-doped nanotubes after decoration: (a) TEM
image of a representative coated tube with Ni clusters and (b) size
distribution of Ni nanostructured particles attached to the walls of N-
containing nanotubes.
In addition, as part of the grid preparation for TEM
observations, a mild sonication process was carried out.
We observed no evident particle detachment from the son-
icated tubes in contrast to the grids prepared just by press-
ing the nanotube processed material onto the holey carbon
film.
Another important fact to consider is whether the oxida-
tion state of Ni remains with +5 oxidizing state when the
nickel nitrate decomposes, which will somehow lead to
determine the nature of the atomic interaction. Although
it has been suggested that wetting of the nanotube surfaces
For pure multiwalled CNT samples, the synthesis pro-
cess gave origin to encapsulated iron found in the hollow
Fig. 5. Characterization of pure carbon MWCNTs after decoration: (a) Dark field SEM Micrograph showing long MWCNTs decorated with Ni particles.
The long tubular structures remain after the sonication process, (b) micrographs obtained using an energy filter (GIF) showing that it is possible to obtain
Ni particles decorating the outer walls of the carbon nanotubes, (c) size distribution of Ni nanostructured particles attached to the walls of N containing
nanotubes and (d) high resolution TEM image showing the possible formation of a nickel oxide layer between the tube and the nickel particle.

P. Ayala et al. / Chemical Physics Letters 431 (2006) 104–109
109
core. In this case, this feature arises due to use of ferrocene
Acknowledgements
as catalyst during the nanotube synthesis. We also noted
that these types of undoped tubes get decorated with Ni
particles of similar diameters to those observed in the hol-
low cores. We did not notice any preferential adhesion on
sites where the iron was encapsulated from other regions of
the tube where the core was completely hollow. However,
the ability that nanotubes possess to host gases as well as
solid compounds, induces different structural properties.
Therefore, further experiments such as magnetic interac-
tion between filling and coating should be carried out in
order to determine the further properties in these metal–
carbon systems.
The authors acknowledge the financial support of the
Brazilian Agencies: CNPq through CIAM and CTEnergia
programs and FAPERJ, as well as the National Science
Foundation (US) Grant DMR-0303429. We gratefully
acknowledge the use of facilities within the John Cowley
Center for High Resolution Electron Microscopy at Ari-
zona State University (USA). This work was also spon-
sored by CONACYT-Mexico grants: 45762 (HT), 45772
(MT), 41464-Inter American Collaboration (MT), 42428-
Inter American Collaboration (HT), 2004-01-013/SA-
LUD-CONACYT(MT), PUE-2004-CO2-9 Fondo Mixto
de Puebla (MT) and Ph.D. Scholarship (JARM). P.A.
thanks T.Pichler from IFW-Dresden for fruitful
discussions.
4. Conclusions
We have demonstrated that CNTs (pure and N-doped)
could be decorated with Ni nanoparticles (ꢀ4–15 nm in
diameter differently for each case). The advantage of our
method lies in the fact that it represents a relatively simple
and low cost process that could yield bulk quantities of
metal-decorated nanotubes. The reduction of the oxide
particles occurs when passing H2 gas on the NiOx–nano-
tube material at 650 °C. In our systems, the Ni particles
are able to attach firmly to the nanotube surfaces due to
the presence of a thin oxide layer established between the
tube and the metal. Our materials could also provide valu-
able information related to the interactions between nano-
tubes and metal clusters, and could motivate further
theoretical research in order to elucidate the anchoring
mechanisms. Similar techniques could be extended to effi-
ciently coat other metal compound nanostructrures using
the method reported here. The electronic, thermal and
mechanical properties of compounds based on Ni coated
CNTs should be different when compared to composites
based on uncoated dispersed tubes. We believe that these
coated tubular materials could be used on a bulk scale to
fabricate Ni–CNT composites that could exhibit outstand-
ing mechanical, thermal and electrical properties. More-
over, we believe that modified methods could also yield
even metal coatings on nanotubes (templates), thus gener-
ating 1D nanowires of different metals with interesting
properties, taking advantage of the potential for industrial
production owing its low costs.
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