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Formation of Ni/NiO Core/Shell Nanostructures and Their Attachment on Carbon
Nanotubes
Nitin Chopra,* Leslie Claypoole,
** Leonidas G. Bachas
***
* Department of Metallurgical and Materials Engineering, University of Alabama, Tuscaloosa, AL-35487,
nchopra@eng.ua.edu ** NSF-REU Fellow, Department of Chemistry, Fairmont State University, Fairmont, WV-26550.
*** Department of Chemistry, University of Kentucky, Lexington, KY-40508,
bachas-chem@uky.edu
ABSTRACT
A new approach combining chemical synthesis method
with microwave processing for the synthesis of Ni/NiO
core/shell nanoparticles is being reported here. A
systematic study of the synthetic parameters was done to
result in control of the size (6 – 40 nm) and, most
importantly, the shape and shell thickness of Ni/NiO
core/shell nanoparticles. The morphological evolution of
these core/shell nanoparticles was observed as a function of
the precursor salt concentration and reaction duration.
Microwave oxidation of synthesized Ni nanoparticles
resulted in Ni/NiO core/shell nanoparticles and hollow NiO
nanoparticles. In order to achieve high surface area loading
and minimal aggregation of these nanoparticles, they were
chemically functionalized onto multi-walled carbon
nanotubes.
Keywords: nickel, nickel oxide, core/shell nanoparticle,
chemical functionalization, carbon nanotubes
1 INTRODUCTION
Nanoparticles, due to their size-dependent properties,
are very promising for many new applications [1,2].
Among these, alloyed nanoparticles [3], a nanoparticle
coated with another nanoparticle [4], and core/shell
nanoparticles [5] are leading towards a new class of multi-
component and multi-functional nanoparticles. Most
interesting are core/shell nanoparticles, where a shell of
another material encapsulates the core nanoparticle. This
shell can serve multiple functions such as provide robust
surface passivation to the core nanoparticle, prevent
aggregation of the nanoparticles, and result in new
properties in the form of core/shell configurations [6].
There exists a variety of material choices including
metal/metal oxide core/shell nanoparticles. For instance,
metal/metal oxide core-shell nanoparticles, such as
Fe/Fe3O4, Zn/ZnO and Cu/Cu2O, where the core and the
shell originate from the same material, have shown some
potential applications in catalytic reactions, sensors, and
magnetic materials [7-9]. Nanostructured Ni and its oxides are of interest because of their surface chemistry, catalytic,
magnetic, and electronic properties [10-12]. The synthesis
of Ni/NiO core/shell nanoparticles has been previously
reported [10,13-16]. Physical methods to synthesize these
nanoparticles can be tedious and do not allow for different
well-controlled morphologies of these nanomaterials [16].
Similarly, synthesis using a wet-chemical method is
severely limited due to the choice of the precursor nickel
salt and difficulty with which Ni2+
can be converted into
metallic Ni through a liquid chemical process using
common reducing agents. In this regard, three major
synthetic issues that needed to be addressed for the Ni/NiO
core/shell nanoparticles are (1) facile synthesis of these
nanoparticles in large yields with controlled morphology,
(2) a well-controlled oxidation process resulting in oxide
shell with desired thickness, and (3) fundamentally
understanding the growth parameters.
In this systematic study we have successfully surpassed
the above mentioned difficulties by developing a new
approach to synthesize Ni/NiO core/shell nanoparticles and
have thoroughly investigated the involved growth
parameters. This approach combines wet-chemical
synthesis method with microwave irradiation process to
result in controlled morphology of these nanoparticles.
2 EXPERIMENTAL
Nickel(II) acetate tetrahydrate (8 x 10-4 – 20 x 10
-3 mol) and
7 mL of oleylamine were added together under an Ar gas
environment and heated at 80-90 °C for ~ 30 min. TOPO
(7.8 x 10-3 mol) and 2.2 x 10
-3 mol of TOP were further
added and slowly heated to at 240-250 °C for 30 - 480 min.
The reaction mixture was allowed to cool down to room
temperature and the products were washed and dried under
vacuum. The products were finally oxidized in a microwave
irradiation process for 30 - 180 min. The nanoparticles were
characterized using TEM and XRD.
3 RESULTS AND DISCUSSION
We report a new methodology that couples chemical
synthesis and microwave irradiation to achieve control of
the size (6 – 40 nm) and, most importantly, the shape and
shell thickness of Ni/NiO core/shell nanoparticles. The
XRD results further confirmed the presence of cubic NiO
and face centered cubic-Ni peaks. The size of the Ni nanoparticles was influenced by nickel salt/stabilizer ratio.
The shape of the nanoparticles was altered by varying the
NSTI-Nanotech 2009, www.nsti.org, ISBN 978-1-4398-1782-7 Vol. 1, 2009 187
reaction time, where longer reaction times resulted in
annealing effects and rupture of the stabilizer micelle
leading to different shapes of Ni/NiO core/shell
nanostructures (Figure 1). The chemically synthesized Ni
nanoparticles were oxidized to form Ni/NiO core/shell
nanoparticles in an oxygen microwave irradiation process.
The NiO shell thickness could be controlled (2-8 nm) by
varying the duration of the microwave treatment.
Furthermore, we report here, a one-step extended
microwave irradiation (3 h) process to form hollow NiO
nanostructures (Figure 2 A). These results offer a robust
technique to tailor the morphology of Ni/NiO core/shell
nanoparticles. In order to achieve high surface area loading
and minimal aggregation of these nanoparticles, they were
chemically functionalized onto multi-walled carbon
nanotubes. Towards this end, histidine-tagged Ni/NiO
core/shell nanoparticles were covalently linked to the
oxidized multi-walled carbon nanotubes (Figure 2 B) using
carbodiimide chemistry [17].
4 CONCLUSIONS
We have successfully combined wet-chemical synthesis
with microwave irradiation process to result in shape and
size control of the Ni/NiO core/shell nanoparticles. This
was achieved by varying the nickel salt/stabilizer ratio and
reaction duration. These nanoparticles were further loaded
onto carbon nanotubes via chemical functionalization
method. Morphological control of Ni/NiO core/shell
nanoparticles and formation of their heterostructures with
carbon nanotubes is critical for the development of
improved fuel cells, devices, and catalytic substrates.
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NSTI-Nanotech 2009, www.nsti.org, ISBN 978-1-4398-1782-7 Vol. 1, 2009 189
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