Nickel nanofibers and nanowires: Elaboration by reduction in polyol medium assisted by external magnetic field

Article abstract of DOI:10.1016/j.ssc.2010.12.004

Nickel nanowires, with diameter 250 nm and a length of several microns, were prepared by the polyol process (chemical reduction) while an external magnetic field of 1.4 T has been applied during preparation. This combination has allowed the elaboration of Ni nanowires with a yield of over 90%. X-ray diffraction (XRD) showed that these nanowires crystallize with the face-centered-cubic structure. Magnetic static measurements showed the effect on the nanoparticles' morphology of the external magnetic field applied during the synthesis. They also allowed studying the effect of the external magnetic field on the magnetic properties of nanowires as a function of their orientation. When nanowires are aligned parallel with magnetic field, the hysteresis loop obtained is very open with a coercivity field (Hc) value of 385 Oe and a high remanence to saturation ratio Mr/Ms of 0.85.

Full text of DOI:10.1016/j.ssc.2010.12.004

Solid State Communications 151 (2011) 284–288
Contents lists available at ScienceDirect
Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Nickel nanofibers and nanowires: Elaboration by reduction in polyol medium
assisted by external magnetic field
Y. Soumarea, A. Dakhlaoui-Omrania, F. Schoensteina, S. Merconea, G. Viaub, N. Jouinia,∗
a Laboratoire des Propriétés Mécaniques et Thermodynamiques des Matériaux, LPMTM, CNRS UPR 9001 Université Paris XIII, 99 Avenue J.B. Clément, 93430 Villetaneuse, France
b Laboratoire de Physique et Chimie des Nano-Objets, INSA de Toulouse, 135 Avenue de Rangueil, 31077 Toulouse cedex 4, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Nickel nanowires, with diameter 250 nm and a length of several microns, were prepared by the
polyol process (chemical reduction) while an external magnetic field of 1.4 T has been applied during
preparation. This combination has allowed the elaboration of Ni nanowires with a yield of over 90%.
X-ray diffraction (XRD) showed that these nanowires crystallize with the face-centered-cubic structure.
Magnetic static measurements showed the effect on the nanoparticles’ morphology of the external
magnetic field applied during the synthesis. They also allowed studying the effect of the external magnetic
field on the magnetic properties of nanowires as a function of their orientation. When nanowires are
aligned parallel with magnetic field, the hysteresis loop obtained is very open with a coercivity field (Hc)
value of 385 Oe and a high remanence to saturation ratio Mr/Ms of 0.85.
Received 21 September 2010
Received in revised form
1 December 2010
Accepted 6 December 2010
by J.F. Sadoc
Available online 10 December 2010
Keywords:
A. Nickel nanowires
B. Soft chemistry
B. Magnetic field
D. Magnetic properties
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
physico-chemical properties. In addition, using a strong reducing
agent such as hydrazine or hydrogen makes manipulation very
Up to now two main chemical methods have been developed
to synthesize anisotropic ferromagnetic nanoparticles. The first
method consists of conducting chemical or electrochemical
reduction in the pores of inorganic or organic templates such
as alumina, polycarbonate membranes [1–3], mesoporous silica
(5–40 nm in diameter) [4] or carbon nanotubes [5].
The second method consists of reduction in solution without
the presence of a template agent. This method is more difficult to
conduct because of the high surface tension of metals, which leads
generally to spherical particles. Among these approaches, one can
find the organometallic route which consists of the reduction by
hydrogen gas of organometallic complexes in the presence of a
surfactant [6]. Another recent approach is to use a strong reducing
agent such as hydrazine in aqueous or polyols media with the
application of an external magnetic field. This method allows to
obtain a coalescence of anisotropic nanoparticles bonded to each
other forming aligned chains [7,8]
difficult and dangerous especially for a possible application to the
industrial scale.
A survey of the literature shows that the polyol process presents
an alternative to partly overcome these difficulties. This process
belongs to the soft chemistry methods. It takes benefit from
the properties of polyol as solvent, complexing and reducing
agents to produce a great variety of inorganic materials (oxides,
metals, layered hydroxysalts) [9,10]. Furthermore it provides high
yield and can easily be scaled-up to produce large quantities
of nanoparticles. More, polyols allow forming highly crystalline
particles [11,12]. Recently, it has been possible by adjusting the
speed of nucleation and growth to produce Co or Co–Ni nanowires
in a polyol medium [13]. However this method cannot be extended
to the synthesis of pure nickel nanorods or nanowires. Indeed
only isotropic particles or platelets of nickel have been prepared
according to this process [14].
This work reports on a simple and friendly environmental
approach for the fabrication of Ni nanofibers and nanowires in the
absence of any template, substrate, surfactant, or strong reducing
agent such as hydrazine. Nickel nanofibers and nanowires were
prepared by combining chemical reduction by a polyol along
with the application of an external magnetic field. We show
that parallel assemblies of these nano-objects present enhanced
magnetic properties in comparison with nickel nanoparticles and
nickel bulk material.
All these techniques can produce anisotropic nanoparticles
with quite remarkable features. However, it necessitates the use
of a matrix (mineral, organic) and surface-active agents which
must be removed subsequently resulting in additional study on the
∗
Corresponding author.
E-mail address: jouini@univ-paris13.fr (N. Jouini).
0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2010.12.004

Y. Soumare et al. / Solid State Communications 151 (2011) 284–288
285
Fig. 2. TEM image of Ni nanoparticles prepared in the absence of an external
magnetic field.
Fig. 1. Typical XRD pattern of Ni powders prepared by reduction in polyol medium
of nickel acetate in the presence of a magnetic field of 1.4 T.
First, we conducted experiments in the absence of an applied
magnetic field. In this case the sodium hydroxide concentration
hardly influences the morphology and size of the particles. TEM
characterization of the produced powder (Fig. 2) shows spherical
and platelets like Ni nanoparticles with 30–50 nm in diameter
whatever the sodium hydroxide concentration.
2. Experiments
The experimental protocol adopted to elaborate the nickel
nanowires can be summarized as follows: Nickel acetate (Ni
concentration of 0.08 M) and sodium hydroxide (0.08–0.25 M)
were dissolved in 1, 2-butanediol (BEG). A small amount of
ruthenium chloride (RuCl3) is added. The Ru will be reduced
before and will serve as seeds during the growth of nanoparticles
(heterogeneous nucleation). According to previous works, the
Ru/Ni molar ratio was set equal to 2.5% [13,14]. In some of
our experiments we have tripled this ratio (7.5%) without any
significant change neither in the length nor in the diameter of
the obtained nanowires. The reactor containing the mixture is
placed in an oil bath preheated to 170 °C and a magnetic field
was immediately applied. After 1.30 h of reaction at 170 °C the
reduction was completed and the solution cooled down. The
magnetic powder is recovered by centrifugation, washed with
absolute ethanol and dried under vacuum. In order to preserve the
particles from oxidation they are kept in the polyol solution.
X-ray diffraction (XRD) patterns were recorded using an INEL
diffractometer with a cobalt anticathode (λ = 1.7809 Å) over the
range 10° < 2θ < 100°, with a step size of 0.04°. The transmission
electron microscopy images were taken by a microscope JEOL-
2011 TEM, operating at an accelerating voltage of 200 kV. A
standard superconducting quantum interference device (SQUID)
magnetometer was used to measure the magnetic properties at
140 K.
In a second step, we conducted experiments under the highest
applied field allowed by our equipment (1.4 T). In this case, the
morphology appears to be dependent on the amount of sodium
hydroxide. When this amount is low namely 0.08 M, the obtained
particles appear as micrometric fibers with 200 nm in diameter
and 3 µm in length. They are aggregated, not well defined and
growing from a metallic heart with 5 µm in diameter (Fig. 3(a)).
For the highest value of sodium hydroxide amount (0.25 M),
the anisotropic morphology is no longer observed. Indeed, TEM
observations reveal that the obtained particles in this case are
spherical with diameters in the nanometric range (Fig. 3(b)).
Finally an intermediate sodium hydroxide amount (0.15 M)
appears to be the more appropriate amount to obtain nickel
anisotropic nanoparticles. TEM micrographs (Fig. 4) show that the
powder consists of two morphology types. The major part of the
powder is formed of nanofibers with 250 nm in diameter and a
length varying in a large range from a few to ten micrometers
(Fig. 4(a)). It is interesting to note that these nanofibers are in fact
constituted of nanoparticles of 250 nm in diameter. As revealed by
TEM observations, these fibers most likely grew by the coalescence
of the isotropic nanoparticles (Fig. 4(a) insert). This morphology
and growth type has been recently reported for nickel elaborated
by reduction in aqueous solution under an applied field [15] and for
cobalt nanofibers [8]. Another morphology type is also observed. It
consists of nanorods with 100 nm in diameter and a few microns
to about 5 µ in length and showing a smooth surface (Fig. 4(b)). As
can be shown in the HRTEM image (Fig. 4(b) insert), the measured
spacing 0.25 nm is slightly higher than that corresponding to the
(111) planes of the face-centered cubic (fcc) structure of Ni. It is
close to that corresponding to the (010) planes of the hexagonal
close compacted (hcp) structure of Ni (JCPDS file No. 45-1027.).
This suggests that the nanorods belong to this latter structure
type. It should however be noted that the amount of nanorods
with a hcp structure remains very low (not exceeding 5%) since X-
ray diffraction analysis only reveals nickel with a fcc structure as
discussed above.
3. Results and discussion
3.1. Structural properties and morphology
According to the work of Soumare et al. [13b], the concentra-
tion of nickel salt was taken equal to 0.08 M and 1, 2 butanediol
was used as solvent and reducing agent. Indeed, it was demon-
strated that such conditions allowed the synthesis of anisotropic
nanoparticles of cobalt and cobalt–nickel Co80Ni20. Two main pa-
rameters have been varied in the present work: the strength of the
applied field and the concentration of the sodium hydroxide. The
concentration of sodium hydroxide has been varied in the range
0.08–0.25 M. The magnetic field was varied up to 1.4 T the highest
limit allowed by our equipment.
All the conducted experiments led to the formation of pure
nickel. The typical X-ray diffraction (XRD) pattern is shown in
Fig. 1. Indeed all XRD patterns can be indexed in the face-centred
cubic system with a cell parameter characteristic of pure nickel
compound: a = 3.524 Å (JCPDS file No. 04-0850).
In order to further investigate if obtaining the nickel nanowires
is related to the presence of the magnetic field, a sample has been
prepared under similar conditions (sodium hydroxide amount
equal to 0.15 M) but changing the value of the applied magnetic
field to 0.7 T. The nanowires are no longer observed, only less
structured and poorly defined nanofibers are obtained (Fig. 5). One

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Y. Soumare et al. / Solid State Communications 151 (2011) 284–288
a
b
5 µm
10 nm
Fig. 3. TEM image of Ni nanoparticles obtained in 1, 2-butanediol in the presence of a magnetic field of 1.4 T: (a) in 0.08 M of NaOH solution; (b) in 0.25 M of NaOH solution.
a
b
0.2 µm
2 µm
2 µm
Fig. 4. TEM image of Ni nanowires obtained in 0.15 M NaOH solution in 1, 2-butanediol in the presence of a magnetic field of 1.4 T: (a) diameter of 250 nm (insert: coalescent
mechanism growth); (b) diameter of 100 nm (insert: HRTEM image).
Synthesis of Ni nanowires, also passes through a fine control of
the basicity of the solution.
Basicity can control the balance between the metallic ionic
species in solution and the unreduced intermediate phase [13,14].
Decreasing the basicity promotes the formation of metal ions in
solution and therefore the speed of nucleation and rapid growth
and thus not well controlled morphology. Increasing the amount of
sodium hydroxide causes the displacement of equilibrium towards
the formation of the intermediate phase namely hydroxyacetate
or alkoxide of the metal ion. This decreases the amount of metal
ions in solution and thus allows the nucleation and growth to
proceed slowly leading to the formation of platelets and isotropic
nanoparticles.
0.5 µm
A good compromise to obtain nanowires consists of an interme-
diate amount of sodium hydroxide. Therefore, a concentration of
sodium hydroxide, and applying a high external magnetic field, are
two essential conditions for obtaining anisotropic Ni nanowires.
Fig. 5. TEM image of Ni nanowires obtained in 0.15 M NaOH solution in 1,
2-butanediol in the presence of a magnetic field of 0.7 T.
3.2. Magnetic properties
can note that contrary to sample prepared under a higher applied
field (1.4 T), here the applied field (0.7 T) is insufficient to induce
the coalescence of the nanoparticles, and the fiber appears to be
formed by nanoparticles weakly bonded to each other. We also see
that the interface between particles is very low; this is in favor of
bonding via weak magnetic interactions between particles.
From these results, it can be concluded that the external applied
magnetic field is the key factor which controls the morphology
of Ni nanoparticles, the anisotropic character depending on the
strength of the field. Indeed under a low magnetic field, isotropic
particles have been obtained while a strong magnetic field favors
the anisotropic morphologies namely nanofiber and nanowire.
Static magnetic properties have been measured on isotropic
nanoparticles of Ni (50 nm) obtained by synthesis in the absence of
an applied magnetic field (Fig. 6(b)) and on Ni nanowires (Figs. 6(a)
and 7) obtained by synthesis under the application of an external
field of 1.4 T. Both isotropic and anisotropic samples present a fer-
romagnetic behavior at low temperatures (behavior at T = 140 K
is shown in Figs. 6 and 7). Their saturation magnetization (Ms)
is qualitatively the same as that of Ni bulk alloy (55 emu/g) in
the case of nanowires (50 emu/g) and slightly lower for isotropic
nanoparticles. This can be due either to the superficial oxidation of
the nanoparticles or to the presence of remaining organic species
adsorbed on their surfaces as it has been previously reported on

Y. Soumare et al. / Solid State Communications 151 (2011) 284–288
287
nanowires and nano-chains [17,18]. The high remanence confirms
a very good orientation of the nanowires in the frozen solution in
agreement with previous studies on Ni nanowires arrays grown
in porous alumina membranes [19]. The magnetization behavior
shows that the magnetic easy axis lies along the wires’ longest axis
due to the strong shape anisotropy. The coercivity of elongated
particles arrays depends on the magnetization reversal mecha-
nism that is related to several parameters like the coherence length
with respect to the wire diameter, the presence of crystalline
defects and also dipolar interaction. The maximum of coercivity on
Ni wires arrays has been observed on 18 nm-diameter nanowires
(i.e. Hc = 950 Oe) [19]. It has been demonstrated that the coer-
civity decreases when the nanowire diameter increases. It reaches
a value of 200 Oe for a nanowire of 50 nm in diameter. Our mea-
surements confirm such a decrease. However, despite their higher
diameter (200 nm) the Ni nanowires described in this paper
present a larger coercivity (385 Oe).
4. Conclusion
Fig. 6. Magnetization (SQUID) of: (a) Ni disoriented nanowires and (b) Ni
nanoparticles.
Ni nanowires have been obtained by a simple synthesis process
which does not require the use of host matrix, surfactant or
strong reducing agent such as hydrazine or hydrogen. This method
combines the effect of reduction and growth in solution of
nanoparticles by the polyol process with the application of an
external magnetic field during nucleation and growth processes.
It also allows the synthesis of large quantities of powder Ni
nanowires, with very high yields. The average diameter of the
nanowire is between 100 and 300 nm and the length is several
micrometers. Obtaining these nanowires is conditioned by a fine
control of the concentration of sodium and the value of the
magnetic field applied during synthesis. They crystallize in the
face-centered cubic (fcc) structure and exhibit enhanced magnetic
properties if they are aligned.
Acknowledgements
This work has been supported by the Region Ile-de-France
in the framework of C’Nano IdF. C’Nano IdF is the nanoscience
competence center of Paris Region, supported by CNRS, CEA, MESR
and Region Ile-de-France.
Fig. 7. Magnetization (SQUID) of the Ni nanowires: (a) nanowires aligned by an
external magnetic field and subsequently frozen.; (b) disoriented nanowires.
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