Formation of Ni chains induced by self-generated magnetic field

Article abstract of DOI:10.1016/j.materresbull.2008.04.010

Ultrafine chain-like Ni assemblies with a length of about 5 μm were successfully prepared by the reduction of Ni salts with hydrazine hydrate under normal pressure in the absence of any inorganic or organic templates. The resulting Ni chains were characte

Full text of DOI:10.1016/j.materresbull.2008.04.010

Materials Research Bulletin 44 (2009) 35–40
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Formation of Ni chains induced by self-generated magnetic field
*
Chunhong Gong, Juntao Tian, Tao Zhao, Zhishen Wu, Zhijun Zhang
Key Laboratory of Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475001, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Ultrafine chain-like Ni assemblies with a length of about 5
mm were successfully prepared by the
Received 15 September 2007
Received in revised form 28 March 2008
Accepted 17 April 2008
reduction of Ni salts with hydrazine hydrate under normal pressure in the absence of any inorganic or
organic templates. The resulting Ni chains were characterized by means of X-ray diffraction (XRD) and
scanning electron microscopy (SEM). Moreover, the growth process of the Ni chains was investigated for
the first time, using transmission electron microscopy (TEM) and XRD as well, which is valuable to study
the self-assembly mechanism of magnetic nanocrystallites. Results indicated that the novel chain-like Ni
assemblies were integrated from Ni microspheres with a diameter of about 250 nm. Magnetic hysteresis
Available online 24 April 2008
Keywords:
A. Metals
measurement revealed that the Ni chains showed ferromagnetic behavior with
a saturation
B. Chemical synthesis
C. Electron microscopy
D. Magnetic properties
magnetization of 15.07 emu/g and a coercivity of 115.1 Oe at room temperature. It was supposed
that the self-generated magnetic field could induce the formation of the Ni chains.
ß 2008 Elsevier Ltd. All rights reserved.
1. Introduction
synthetic routes are unfavorable in terms of the complicated
synthetic procedures with respect to the purification process
Recently, nanosized metal magnetic materials have been
extensively highlighted because of their expected intriguing
properties when compared with their bulk stuffs [1–6]. In particular,
anisotropic one-dimensional (1D) magnetic nanoscale materials
have attracted much attention because of their potential applica-
tions in magnetic sensors, memory devices, catalysts and galvano-
magnetic materials [7–9]. Taking nickel as an example, various
methods have been developed for the synthesis of nanosized nickel
with different shapes, and the morphology control of Ni has been an
active research theme [10–14]. Through those research experi-
ments, hollow microspheres of nickel [13,14], triagonal nickel
nanoparticles [15], and flower shaped nickel nanocrystals [16],
together with different 1D nickel nanostructures such as single-
crystal Ni nanobelts [17] and polycrystalline Ni wires [18] have been
successfully fabricated using various methods [19,20].
The fabrication of Ni nanowires mainly depends on a template
technique that involves electrochemical deposition or metal
organic chemical vapor deposition (CVD) of metals into the
nanopores of template materials [18,21–26] such as anodic
aluminum oxide (AAO) film [21,22], carbon nanotubes [23] and
porous silicon (PS) [12,26]. The template technique has been
successfully used, as one of versatile approaches, in the prepara-
tion of the arrays of some magnetic nanomaterials such as Fe, Co,
Ni and alloy nanowires [18]. However, the template-based
(removal of the templates) of the final products. Therefore, it is
imperative to develop template-free methods for the fabrication of
1D magnetic nanomaterials. The magnetic field-directed assembly
used for the fabrication of various 1D metal magnetic nanos-
tructures gives one of the examples of the template-free methods
[27–32]. As a kind of permanent magnetic materials, metallic
nickel has inherent magnetism. Thus we suppose that magnetic Ni
particles could magnetize those particulates nearby via dipolar
interaction, leading to the 1D self-assembly of the magnetic
nanocrystallites even in the absence of external magnetic fields. To
testify this point of view, in the present work we prepared ultrafine
Ni chains via a very simple reduction process—the reduction of
nickel chloride by hydrazine hydrate under normal pressure and in
the absence of any morphology-controlling media such as exterior
magnetic field, template materials, and surfactants or passivating
materials, aiming at simplifying the synthesis procedures and
avoiding introduction of impurities into the final products. The
resulting Ni chains were characterized by means of SEM and
XRD, and their magnetic properties were measured as well. The
formation process of the Ni chains in the presence of the self-
generated magnetic field was also analyzed using TEM and XRD.
This article deals with the relevant research results, and hopefully,
it would help to acquire new insights into the growth and self-
assembly mechanisms of nickel nanocrystallites and design of a
new general method for the preparation of other magnetic metal
nanostructures with a broad range of well-defined and controllable
morphologies, so as to fully exploit their peculiar properties and
unique applications.
* Corresponding author. Tel.: +86 378 3881358; fax: +86 378 3881358.
E-mail address: jwzhang@henu.edu.cn (Z. Zhang).
0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2008.04.010

36
C. Gong et al. / Materials Research Bulletin 44 (2009) 35–40
2. Experimental
All the reagents are analytical grade, and used without further
purification. In a typical process, firstly, 0.118 g of nickel chloride
(NiCl2ꢀ6H2O, Tianjin Kermel Chemical Co., Ltd., China) was
dissolved into 50 ml of ethylene glycol (EG, Tianjin Kermel
Chemical Co., Ltd., China) to give a green transparent solution,
allowing the homogeneous dispersion of Ni2+. Then 1 ml of
hydrazine hydrate solution (N2H4ꢀH2O, concentration 80 wt%,
Tianjin Kermel Chemical Co., Ltd., China) was added dropwise
into the reseda solution under continuous stirring, allowing the
generation of a lavender solution within several minutes. Thirdly,
2 ml of 1 M NaOH (Tianjin Deen Chemical Co., Ltd., China) solution
was added dropwise into the lavender solution, allowing the
generation of a sky-blue solution in several minutes. The sky-blue
solution was then, in the absence of stirring, heated to 60 8C and
held there for a few minutes, to allow the appearance of black
floccules and very quick change to black of the solution color.
Several minutes later, the solution became clear and colorless,
while the desired black fluffy solid product was floating atop the
reaction solution, indicating the formation of metallic Ni. The
chemical reaction for the synthesis can be expressed as:
Fig. 1. XRD patterns for the final Ni samples obtained at the end of the reaction
under stillness condition (A) and stirring condition (B).
microscope (TEM, accelerating voltage 200 kV) was performed
to record the microscopic images and selected area electron
diffraction (SAED) patterns of the samples obtained at different
reaction durations, aiming at revealing the details about the
growth process of the ultrafine Ni chains. The samples for the TEM
analysis were collected via centrifuging separation. After being
washed and dispersed with alcohol in the presence of ultrasonic
stirring, a few drops of the dispersed alcohol solution were
introduced onto a carbon-coated copper grid and allowed to
evaporate in air at room temperature. Then the samples were
ready for the TEM analysis.
2Ni2þ þ N2H4 þ 4OHꢁ ¼ 2Ni þ N2 " þ 4H2O
(1)
where Ni2+ ions were reduced into zero-valenced Ni by N2H4 and
the nitrogen gas produced could help to protect the nascent nickel
nanocrystallites from oxidation. In order to demonstrate the effect
of stirring on the formation of Ni products, similar experiment was
performed under the same conditions but the solution was stirred
mechanically. At the end of the reaction, the products were
collected with the assistance of magnetic separation and washed
with alcohol for several times to remove the ions and other
materials possibly remaining in the final product, and finally dried
in a vacuum at 40 8C for 24 h. The obtained products were
characterized using a Philips X0 Pert Pro X-ray diffractometer (XRD,
3. Results and discussion
Fig. 1 depicts the XRD patterns of two typical Ni samples
prepared from the reaction systems kept in stillness (Fig. 1A) and
under stirred condition (Fig. 1B). Three characteristic peaks for Ni
(2u = 44.58, 51.88, and 76.48), corresponding to Miller indices
(1 1 1), (2 0 0), and (2 2 0), were observed for both samples,
indicating that they were both composed of pure face-centered
Cu Ka radiation, l = 0.15418 nm) and a JEOL JSM-5600LV scanning
electron microscope (SEM, acceleration voltage 20 kV). The
magnetic hysteresis loop of the product at room temperature
was measured using a sample-vibrating magnetometer (VSM, Lake
Shore 7300, USA).
A JEOL JEM-2010 transmission electron
Fig. 2. SEM images of the final samples formed under different reaction conditions: (A) the overall morphology of the ultrafine Ni chains formed under stillness condition; (B)
and (C) the magnified SEM images of the Ni chains; (D) and (E) the SEM images of the samples obtained under stirring condition; (F) the SEM image of the Ni chains treated by
ultrasonic vibration for 10 min.

C. Gong et al. / Materials Research Bulletin 44 (2009) 35–40
37
cubic (fcc) Ni (PDF standard cards, JCPDS 01–1260, space group
Fm3m) and did not contain impurities [28]. Though nickel is easily
oxidized, peaks due to its oxide were not observed, which might be
due to the fact that the reaction was carried out at appropriate pH,
temperature and hydrazine concentration, and inactive N2 was
produced and acted as a kind of protective atmosphere.
The SEM morphologies of the Ni samples obtained under
stillness and stirred condition are shown in Fig. 2, where Fig. 2A is
the overall morphology of the chain-like Ni sample. It is seen that
all the Ni samples exhibit a squiggly chain-like configuration. The
magnified SEM image shown in Fig. 2B indicates that all of the Ni
chains are made up of spherical-like particles with a diameter of
Fig. 3. TEM images of the as-prepared samples taken out at different reaction time: (A) 0 s, (B) 10 s, (C) 20 s, (D) 1 min, (E) 1.5 min, and (F) 2 min. (A1) the electron diffraction
(ED) pattern of sample A, (F1) the electron diffraction (ED) pattern obtain from the edge of a single chain, (F2) the magnified image of the interlinks between the spherical
particles of a chain, and (F3) the high-resolution (HRTEM) image from the joint zone.

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C. Gong et al. / Materials Research Bulletin 44 (2009) 35–40
led to a chain-like structure [33]. Since the present reaction could
be easily conducted in an open system, a series of samples were
collected at different reaction stages and analyzed by TEM, aiming
at acquiring more insights into the formation process of the Ni
chains. Because the reaction progressed very quickly (Once the
black floccules appeared in the solution, the reaction would be
finished in several minutes), the sampling process had to be timely
and expeditious. Namely, once the target samples were taken out
from the reaction solution, they were immediately put into liquid
nitrogen so as to ensure timely interruption of the reaction. When
the black floccules appeared, the first sample was extracted out;
then five more samples were extracted at an interval of 10 s, 20 s,
1 min, 1.5 min, and 2 min, respectively. The corresponding TEM
images of the totally six samples are labeled as A–F, respectively,
in Fig. 3.
At the initial reaction process, only floccules appeared in the
reactant solution and the floccules in this case were amorphous
(see Fig. 3A and A1 insert). With the proceeding of the reaction for
different durations, some short chain-like products emerged
quickly (see Fig. 3B and C). The XRD pattern of sample C (see
Fig. 4. XRD pattern of the product obtained at a reaction time of 20 s.
about 250 nm, and the particles are closely contacted with each
other to form squiggly chains with a length of about 5 m. The
Fig. 4) shows a broad amorphous peak centered at 2
u = 248 and a
m
small diffraction peak at 2 = 44.58, which can be assigned to Ni
u
high-magnification images of several Ni chains shown in Fig. 2C
reveal a clear and well-defined chain structure consisting of Ni
microparticles with quite smooth surface. Whereas, the products
obtained under stirred solution seem to be scrappy and exhibit
dendrite-like configuration (Fig. 2D and E). Moreover, to investi-
gate the strength of the chains, the final chain-like products
(Fig. 2A) were ultrasonically vibrated at a power of 100 W (KQ-218,
Kunshan Ultrasonic Instruments Co., Ltd., China) for over 10 min
and then were observed with SEM. Obviously, the morphologies of
the Ni chains kept almost unchanged after ultrasonic vibration
(Fig. 2F), implying that the Ni particles were inclined to grow
together rather than merely aggregate one another.
The formation of the Ni chains should be dependent on the
intrinsic growth behavior of the Ni crystal and the external
experimental conditions such as reactant concentration, tempera-
ture, type of surfactants etc. Thus, the mechanism for the growth of
one-dimensional Ni chains is very complicated and has yet not
been well revealed [32]. Fu and co-workers reported the
preparation of chain-like CoNi alloy assemblies by a surfactant-
assisted hydrothermal synthetic route and assumed that the
surfactants cetyltrimethylamonium bromide (CTAB) or cetyltri-
methylammonium chloride (CTAC) could act as a template, which
(1 1 1). In combination with the TEM image of sample C (see
Fig. 3C), it could be concluded that, at a reaction duration of 20 s,
the corresponding products were composed of amorphous
flocculent compound and crystalline nickel chains. With further
increase in the reaction duration up to 1 min and 1.5 min, more and
more chains with larger lengths were produced, and meanwhile
the proportion of the floccules decreased (see Fig. 3D and E). As the
reaction duration was extended to 2 min, only chain-like product
was obtained (see Fig. 3F). The SAED pattern of a single chain of
sample F is displayed in Fig. 3F1, where the four fringe patterns can
be indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1) of pure face-centered
cubic (fcc) nickel, indicating that the nickel chains in this case had
polycrystalline structure. Similarly, as shown in Fig. 3F2, the
magnified image at the joint between the spherical particles of a
chain indicated that the Ni chains consisted of spherical particles
with a size of 100–200 nm. A high-resolution TEM image from the
joint of two spherical particles is shown in Fig. 3F3, where the
(1 1 1) planes, with an inter-planar spacing of about 0.2 nm, are
marked with a series of parallel lines. It is seen that the (1 1 1)
planes have different orientations in different domains, which is
one of the characteristics of polycrystalline structures. Combining
Fig. 3F2 and Fig. 3F3, it could be inferred that the spherical Ni
Fig. 5. Schematic illustration of the formation of the ultrafine Ni chains assemblies: (A) the formation of the intermediate amorphous-phase floccules; (B) the re-dissolving of
the intermediate phase and the nucleation; (C) the aggregation of the nickel primary nuclei and the generation of the short Ni chain-like assembles; (D) longer Ni chains
formed.

C. Gong et al. / Materials Research Bulletin 44 (2009) 35–40
39
particles did not merely attract one another but did grow together;
in consistency with the corresponding SEM analytical results (see
Fig. 2F).
The magnetic properties of the chain-like Ni products were
investigated using a sample-vibrating magnetometer. Plot of
magnetization vs. applied magnetic field for the chain-like Ni
samples obtained at the end of the reaction is shown in Fig. 6,
where the hysteresis loop shows obvious ferromagnetic char-
acteristics at room temperature. The specific saturation magne-
tization (Ms) and the coercivity (Hc) values for the Ni chains were
calculated to be 15.07 emu/g and 115.1 Oe, respectively. Namely,
the as-prepared chain-like Ni samples obtained in the presence of
self-generated magnetic field had lower saturation magnetization
and higher coercivity than the bulk Ni sample at room temperature
(Ms = 54–55 emu/g, Hc = 100 Oe) [36,37].
It is well known that the crystallite size, structures and shapes
of magnetic materials have effects on the magnetic properties of
the products [32,38–40]. Generally, the Ms for nanoscale magnetic
materials is lower than that for bulk material because the spin
disorder on the surface significantly reduces the total magnetic
moment [10], and the shape of the hysteresis loop is strongly
affected not only by the specific surface area of the particles but
also by the magnetic anisotropy [33], including magnetocrystalline
anisotropy, shape anisotropy, stress anisotropy, induced aniso-
tropy and exchange anisotropy [41]. This could be cited to well
explain the decrease of Ms for the Ni nanostructures in the present
work. Namely, the synthesized representative chain-like Ni
assemblies had an average aspect ratio of about 20, implying
obvious shape anisotropy and hence increased coercivity as well.
Therefore, the higher coercivity of the as-prepared chain-like Ni
samples as compared with bulk Ni sample might be attributed to
their special nanostructure characterized by reduced size and
shape anisotropy, which could prevent them from oriental
magnetizing other than along their easy magnetic axes, leading
to a higher coercivity [10,29].
Based on the above-mentioned experimental results, a sche-
matic illustration of the formation mechanism of the as-
synthesized chain-like Ni assemblies is shown in Fig. 5, where
the Ni (II) species are involved in an equilibrium between the
solution and the unreduced intermediate phase. Initially, an
unknown intermediate phase—amorphous floccules were gener-
ated in the reactant solution at the beginning of the reaction, which
might act as Ni source materials and could sharply decrease the
concentration of free Ni2+ in the solution, resulting in a relatively
slow rate of generation of Ni atoms (see Figs. 3A and 5A). Then the
intermediate solid phase could be re-dissolved with the increase of
the reaction time, and two-valence Ni, during rising of its
concentration above the saturation concentration, was reduced
to zero-valence Ni [34,35], accompanied by the collision of initially
reduced metal atoms and their nucleating (see Fig. 5B). Along with
the reaction, the small Ni primary nuclei acted as seeds to allow
more metal atoms to be reduced and absorbed on the nuclei, and
the reduced Ni atoms were inclined to aggregate into larger
particles for the sake of decreasing surface energy. As a kind of
permanent magnetic materials, each nickel particle with a big
enough size has an inherent self-generated magnetic field. Driven
by the self-generated magnetic field, the Ni particles would prefer
aggregating along the magnetic force lines. Consequently, the
desired ultrafine Ni chain-like assembles would be generated at
properly extended reaction durations (see Figs. 3A–E and 5C). As
time prolonged, these chains transformed into longer ones and
meanwhile the proportion of the amorphous floccules decreased
until vanished (see Fig. 3F and 5D). However, since the self-
generated magnetic field of the Ni particle is very weak, the
disturbing forces caused by the uneven temperature in the solution
and Brownian motion should not be neglected. Actually, the
formation of the final squiggly chained target products in the
4. Conclusions
present research was
a
compromise between the oriented
In summary, a novel and facile route for producing ultrafine Ni
chains via a mild reduction process in the absence of any exterior
morphology-controlling techniques and/or devices has been
reported. The present method could be superior to other types of
synthetic routes in terms of the mild reaction conditions such as
normal pressure and lowered temperature, and might be extended
to the synthesis of other types of magnetic chains. The self-assembly
mechanisms of the Ni nanocrystallites induced by self-generated
magnetic field were investigated for the first time, which could help
to better understand the formation process of the chain-like Ni
assemblies and to improve the efficiencies of various devices based
on single particles or their composites as well [33,42]. It was found
that in the absence of exterior magnetic field, the one-dimensional
self-assembly of nickel nanocrystallites could be induced by their
magnetic attraction. The morphologies and microstructures of the
resulting chain-like ultrafine Ni particles were highly dependent on
the reaction duration. The chain-like Ni nanostructures showed
ferromagnetic characteristics and had saturation magnetization
(Ms) and coercivity (Hc) of 15.07 emu/g and 115.1 Oe, respectively,
showing lower saturation magnetization and higher coercivity than
bulk Ni sample at room temperature.
magnetic force and the random disturbing. This process had
better be handled only when the magnetization of the particles in
the suspension did not fluctuate during the magnetic interaction.
In other words, when the reaction system was stirred, the
oriented growth of the magnetic nanocrystallites would be
destroyed partially and the particles could contact with each
other in any directions, resulting in branched and short dendrite-
like configuration.
Acknowledgments
The authors have benefited a lot from the helpful discussion
with Prof. Zhen-sheng Jin and Si-xin Wu in our laboratory. The
financial support from the Ministry of Science and Technology of
China (Grant No. 2007CB607606, in the name of ‘‘973’’ Plan) is
acknowledged. Special thanks are due to Dr. Lai-gui Yu for help in
manuscript preparation.
Fig. 6. The hysteresis loop of the final chain-like Ni samples obtained at the end of
the reaction measured at room temperature.

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