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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