Magnetic-field-assisted synthesis of Ni nanostructures: Selective control of particle shape

Article abstract of DOI:10.1016/j.cplett.2009.06.085

Metallic nickel particles have been selectively synthesized via chemical reduction using a static magnetic field. Our results demonstrated that both reduction temperature and magnetic-field strength were the crucial factors in determining the final partic

Full text of DOI:10.1016/j.cplett.2009.06.085

Chemical Physics Letters 477 (2009) 184–188
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Magnetic-field-assisted synthesis of Ni nanostructures: Selective control
of particle shape
a
b,*
Hailong Hu , Katsuyasu Sugawara
a
Venture Business Laboratory, Akita University, Akita 010-8502, Japan
Faculty of Engineering and Resource Science, Akita University, Akita 010-8502, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Metallic nickel particles have been selectively synthesized via chemical reduction using a static magnetic
field. Our results demonstrated that both reduction temperature and magnetic-field strength were the
crucial factors in determining the final particle shapes. Upon subjecting a 0.02 T magnetic field, the par-
ticles change from sea urchin-like, through spherical, to one-dimensional (1D) in shape with decreasing
the reduction temperature. In the synthesis of 1D structure, sphere-chains with fluctuant diameter are
assembled under weaker magnetic field while relatively smooth nanowires are formed under stronger
one. Thermal analysis of the particles was also investigated.
Received 20 January 2009
In final form 26 June 2009
Available online 1 July 2009
Ó 2009 Published by Elsevier B.V.
1
. Introduction
Ultrafine nanometer-sized nickel particles have been studied
addition, this method facilitates the decrease in critical radius of
nuclei, which is of advantage to grow small sized particles.
Recently, magnetic interaction has attracted increasing attention
in the assembly and alignment of ferromagnetic NPs since the
superparamagnetism–ferromagnetism transition was observed by
Puntes et al. [13]. A magnetic field could be employed to orient 1D
ferromagnetic nanostructures [14–16]. More interestingly, mag-
netic-field-assisted hydrothermal process was proven to be an
effective way for directing the growth of several 1D magnetic mate-
rials when a parallel magnetic field was applied during the synthe-
extensively over the past two decades due to their potential appli-
cations in optical, electronic, catalytic, and magnetic materials [1].
Most physical and chemical properties of these nanoparticles
(
NPs) depend on their size and shape. The shape control of nano-
structures has therefore become a new and interesting research
field, and considerable efforts have been devoted to prepare nano-
scaled structures with special morphologies [2]. To date, it is still
challenging to develop general synthesis strategies that enable us
to systematically tailor the morphology and the structure of the
Ni NPs to fine-tune their properties. As a well-established approach,
wet chemical process has superior capability for controlling compo-
sition, size and shape of the NPs and is widely used to synthesize
monodisperse nanostructures. Take for example the case of Ni
nanostructures, which are usually synthesized via the reduction
of Ni ions by hydrazine. In aqueous-solution synthesis, however,
the as-synthesized particles tend to be oxidized owing to their high
surface reactivity. Moreover, the reaction process is rather complex
and parameters such as the pH, concentration of metal ions, and
reduction temperature need to be carefully controlled. To avoid
these drawbacks, polyol has accordingly been used instead of aque-
ous medium. With these methods, many metallic NPs such as Ag,
Fe, Co, Pt, Ni, Pd, Ru, Te, and FePt, have been successfully prepared
3 4 x
sis. Though various 1D structures such as Co, Fe O and FeS nano-/
microwires have been successfully obtained using this method
[17–19], there have been few reported systemically regarding the
crucial technological parameters, aside from verifying the feasibility
of 1D structure assembly. In this work, Ni particles have been
synthesized via a magnetic-field-assisted non-aqueous wet chemi-
cal process. The shape evolvement of the particles was studied in
view of the reduction temperature and magnetic-field strength,
respectively.
2
+
2
. Experimental
All chemicals were of reagent grade and used without further
purification. In the case of Ni NPs synthesis, the reducing ability
of polyol, ethylene glycol (EG), is insufficient to reduce the Ni ions.
[
3–11]. In these processes, the poly-alcohol is used as both solvent
Thus hydrazine monohydrate (N
2
H
4
ÁH
2
O) is employed as a reducing
and reducing agent. The polyol itself can also act as protective agent
to avoid particle agglomeration and restrain the growth [12]. In
agent. Two 6 Â 3 Â 1 cm permanent magnets were fixed face-to-
face to generate a static magnetic field, its strength being controlled
by adjusting the interval between these two magnets. In the central
region of the two magnets, where the reduction of Ni was carried
out, the magnetic field was a nearly parallel field. In a typical
2
+
*
Corresponding author. Fax: +81 18 889 2750.
E-mail address: katsu@ipc.akita-u.ac.jp (K. Sugawara).
0
009-2614/$ - see front matter Ó 2009 Published by Elsevier B.V.
doi:10.1016/j.cplett.2009.06.085

H. Hu, K. Sugawara / Chemical Physics Letters 477 (2009) 184–188
185
synthesis, the starting solution was prepared by dissolving
.25 mmol nickel sulfate hexahydrate (NiSO O) in 40 ml of
Á6H
than those (about 2.5
l
m) in aqueous solution. By adjusting the
1
4
2
pH value of EG solution, uniform urchin-like Ni particles with aver-
age size in a range of 250–500 nm could be produced at 150 °C. As
observed from the SEM image, the sea urchin-like particles were of
fat branches, which was different from the sharp ones grown in
aqueous solution. In polyol medium, the diffusion coefficient for
reduced Ni is lessened. And the difference of solidification rate be-
tween the tip and the side of a branch is not large enough to grow
sharp dendrites.
EG (98%) in a beaker. 0.02 mol of NaOH was added to control the
pH of the solution and the obtained precursor was stirred vigor-
ously for 30 min to form a transparent green solution. Afterward,
the beaker was moved into the center of magnetic field: the
magnetic-field strengths on the inner surface of the beaker near
magnets were fixed at 0.02, 0.01 and 0.005 T, respectively. The solu-
tion was heated to the set temperature followed by pouring-in of
3
ml N
2
H
4
ÁH
2
O (98%). After reduction, the black products was
With the decrease in reduction temperature, the reduction pro-
ceeded quiescently at 100–120 °C. A representative SEM micro-
graph of the Ni particles obtained at 110 °C is shown in Fig. 1b.
The produced particles are spherical in shape with average size
filtered and washed repeatedly with distilled water and methanol.
Subsequently, the sample was dried in an oven at 100 °C for 1 h.
The phase structure was identified by X-ray diffraction (XRD,
Rigaku Geigerflex) using Cu K
chromator. The particle size and morphology analyses were per-
formed using field emission (FE-) scanning electron microscopy
a
radiation with a graphite mono-
of ꢀ0.3
lm. From the SEM image, it can be seen that these particles
are monodisperse with narrow size distribution and most of them
were of rough surfaces.
(
SEM, Hitachi S-4500, with accelerating voltage of 30 kV). Thermo-
1D nickel nanowires were obtained when reduced at tempera-
ture lower than 90 °C. Fig. 1c shows a typical SEM image of Ni nano-
wires synthesized at 70 °C. The Ni nanowires show smooth surfaces
and good uniformity. The mean diameter of these nanowires was
gravimetry/differential thermal analysis (TG/DTA, Bruker AXS
TG-DTA2000SA) of the samples were conducted at a heating rate
of 5 °C min under dry air.
À1
about 150 nm; their length was in the range of 3–10 lm. No curly
nanowire was observed in the SEM image, suggesting that it is a fac-
ile method to synthesize straight nanowires. Comparably, only
those with spherical shapes were obtained at 70 °C in the absence
of magnetic field while keeping other parameters the same (not
shown here). This reveals that the magnetic field plays a crucial role
in determining the morphologies of the products. The nanowires
are likely formed due to the tight aggregation of Ni nanospheres
drawn together by magnetic interattraction force. It is important
to note that under a 0.02T magnetic field, 1D Ni nanowires can be
obtained only at relatively low temperature.
In general, the average size of the final particles decreased with
decreasing reduction temperature, except for synthesized at 60 °C.
Apparently, an elevated reduction temperature was quite helpful
in accelerating the reaction rate. It is believed that no further
nucleation will occur once the number of nuclei is large enough
to lower the concentration of reduced atoms [22]. At high temper-
ature, the nuclei generated at the nucleation step can efficiently
consume the reduced Ni atoms by particle growth. In this regard,
a large amount of Ni species reduced at the later period of reaction
are directly involved in particle growth rather than form new nu-
clei. Accordingly, the total number of nuclei generated throughout
the reaction decreases with increasing reduction temperature, and
large sized particles are formed. Inversely, only a few nuclei are
generated at the nucleation step in the rather slow reductions
3
. Results and discussion
Table 1 summaries the experimental observations and particle
characteristics at different technological parameters. After reduc-
tion by hydrazine, the initial green solution turned colorless for
all samples and the black Ni particles were drawn to the magnets.
In Table 1, the reduction time represents the interval from the
addition of hydrazine to the absolute formation of colorless
solution.
The reduction of Ni ions was first carried out under a 0.02 T
magnetic field to investigate the effects of reduction temperature.
At temperatures above 150 °C, the solution intensely boiled with
copious evolution of gas bubbles and the reduction of Ni ions
was instantly completed. The final particles were sea urchin-like
in shape with an average particle size of ca. 0.5 lm, as shown in
Fig. 1a. In the solution synthesis, the Ni particle usually grows into
sphere due to the smallest specific surface energy. The growth rate
of the sphere depends upon its radius which is increasing with
time. As the particle grows large, the solidification rate decreases
[
20]. In the rapid synthesis, the original spherical growth will be
interrupted as the growth rate is too slow. To maintain the growth
rate, therefore, the solidification of the reduced Ni atoms then fol-
lows the dendritic growth mode, which leads to a faster growth
than spherical mode does. In our previous work, similar multi-
branched particles were observed in aqueous solution via a rapid
chemical reduction [21]. In comparison, the average size of the
particles synthesized in such a polyol medium is rather smaller
(
<60 °C). The amount of solute available for particle growth per
growing particle increases with the decrease in reduction temper-
ature. This reaction mechanism also leads to forming large nickel
Table 1
Particle shape and size vs. technological parameters.
Sample
Reduction temp.
°C)
Field strength
(T)
Reduction time
(min)
Particle shapea
Particle size
(lm)
b
(
#
#
#
1
2
3
170
150
120
0.02
0.02
0.02
<0.2
0.2
0.8
U
U
U
S
S
S
W
W
W
W
N
S
0.55
0.50
0.44
0.25
0.30
0.30
0.28
0.27
0.15
0.40
0.20
0.25
#
#
#
#
#
#
#
#
4
5
6
7
8
9
10
11
110
100
90
80
70
60
70
70
0.02
0.02
0.02
0.02
0.02
0.02
0.01
0.005
1
3
4
5
10
>180
20
20
a
U: sea urchin-like; S: spherical; W: nanowire; N: necklace-like.
The particle size reveals the average diameter for nanowires.
b

1
86
H. Hu, K. Sugawara / Chemical Physics Letters 477 (2009) 184–188
Fig. 1. SEM micrographs of the Ni particles synthesized in the presence of a 0.02 T magnetic field at (a) 150 °C (sample #2), (b) 110 °C (sample #4), and (c) 70 °C (sample #8).
(
d) A magnified image of a nanowire taken from the black square in (c).
Fig. 2. SEM micrographs of Ni particles synthesized at 70 °C under (a) 0.01 T and (b) 0.005 T magnetic field. The inset shows a magnified view.
particles. Taken together, the Ni particles obtained at 70 °C are of
the smallest average size.
implied the external magnetic field facilitated the reduction of Ni2+
to generate a larger amount of nuclei.
To evaluate the effect of magnetic-field strength, the synthesis
was carried out at 70 °C under weaker magnetic fields. As shown
in Fig. 2a, necklace-like Ni chains with fluctuant diameters were
obtained under a 0.01 T magnetic field. Obviously, 1D chains are
formed due to the alignment of Ni spheres. Upon subjecting a
Although magnetic-field-assisted hydrothermal route has been
used to construct a variety of magnetic particles, the assembly
mechanism of 1D structure has not been well understood. On the
basis of our results, the probably formation process for 1D Ni nano-
structures is schematically illustrated in Fig. 3, taking the case of
uniform-diametered nanowire for example. During the growth of
nanowires, magnetic-field strength and reaction temperature re-
main constant and the particle morphology changes with growth
time only. At the beginning of the growth, the total magnetic mo-
ment of an individual particle can be considered as zero in spite of
subjecting a magnetic field. These small particles grow into dis-
persed nanospheres, drifting randomly just as in the usual hydro-
0
.005 T magnetic field, only a fraction of Ni spheres showed a ten-
dency towards the formation of 1D array, suggesting that the driv-
ing-force induced by the magnetic field is too weak in this case.
Wang et al. reported that between the weak-field produced zero-
dimensional (0D) particles and strong-field conduced 1D nano-
3 4
wires, there was an intergradation for Fe O synthesis in which
0
D particles and 1D uniform-diametered nanowires coexisted in
the products. By contrast, our results denote a different transitions
in particle morphology: (i) With decreasing the magnetic-field
strength, all the 1D structures change from uniform to fluctuant
in diameter (Fig. 2a). (ii) In the 0D and 1D coexistence, the 1D par-
ticles are sphere-chains, i.e. their diameter is not uniform. It also
suggested that the magnetic field in the solution was relatively
homogeneous, which could not result in weak-field products
B
thermal process. After the blocking time T , the magnetic
moment of a particle appears to be in the same direction and the
magnitude keeps increasing with time (stage II). Influenced by
the magnetic field, the moment tends to regulate its direction
along that of the external field. The particles with their magnetic
moments in other directions will be subject to a torque. As a result,
almost all the particles in the solution have a same magnetic mo-
ment direction (stage III). In addition, the interaction force be-
tween the dipoles becomes prominent. When the diffusing
particles are in a close range, they are draw to encounter by the
(
0D) and strong-field resultants (1D) simultaneously. In addition,
it was found that the average diameters of 1D nanostructures de-
creased a little with increasing the magnetic-field strength, which

H. Hu, K. Sugawara / Chemical Physics Letters 477 (2009) 184–188
187
Fig. 5. XRD patterns of the Ni particles: (a) sample #2, (b) sample #4, and (c)
sample #8.
Fig. 3. Schematic illustration of the formation of Ni nanowires.
SEM micrographs of the nanowires after magnetic alignment
magnetic dipole interattraction. And 1D necklace-like chain of par-
ticles with their dipoles aligned head-to-tail, parallel to the mag-
netic field is formed, as shown in stage IV. To decrease the
surface energy, the solidification of nickel at the later stage takes
place preferentially on the sphere–sphere interface to construct
uniform-diameter nanowires.
(H = 0.001 T). These nanowires under the magnetic field exhibit
aligned distribution in contrast to the randomly oriented 1D struc-
tures in Figs. 1 and 2.
Fig. 5 shows the XRD patterns of the particles synthesized at dif-
ferent reduction temperature. The characteristic peaks located at
4
(
4.5°, 51.8°, 76.4°, and 92.9° have been assigned to Ni(1 1 1),
2 0 0), (2 2 0), and (3 1 1) planes, respectively. All the diffraction
peaks of these samples match well with the Ni face-centered cubic
fcc) structure. By comparison of the intensities of the XRD peaks, it
can be deduced that the size of crystallites in the nanowires
Fig. 5c) are smallest, which is due to the small diameter. As shown
In the 1D assembly by dipole interaction, it is influenced by sev-
eral key parameters such as reaction temperature, magnetic field
intensity and also particle concentration. At high temperature,
the particles are in vigorous movement and the disturbances from
the solution may counteract the magnetic dipole interattraction.
This is the reason that no 1D structure can be obtained at temper-
atures >90 °C in this study. The magnetic field intensity is also
important since the blocking time will be greatly enhanced by
the magnetic field. A stronger magnetic field results in the earlier
assembly of 1D chains and the formation of smaller-diameter
nanowires, which is demonstrated in Table 1. Also, the stronger
field will induce larger magnetic moment, thus increasing the di-
pole interaction between the neighboring particles. This indicates
that the formation temperature of 1D structure can be raised pro-
vided a magnetic field with a large enough magnitude is used.
With a much strong field (0.25 T), 1D Co microwires were obtained
at 110 °C and Fe O nanowires at even 130 °C [17,18]. A high par-
3 4
ticle concentration means a large encounter probability, which
likely leads to the growth of long 1D structure.
Due to the same direction moments of the assembled spheres,
the final moment of the nanowire is along the diameter. And the
moment magnitude is greatly enhanced. Thus a weak magnetic
field can cause them preferential orientation. Fig. 4 shows the
(
(
in Fig. 1, the urchin-like particles consists of several branches,
whose size is smaller than that of the spherical particles. Thus crys-
tallites in such a structure are smaller in comparison with those in
Fig. 1b, which is in accord with the XRD results.
The oxidization properties of various shaped Ni nanostructures
were characterized with TG/DTA (Fig. 6). For all samples, the
weight loss occurred at 255 °C, which was likely attributed to the
evaporation of EG adsorbed on the particle surface. In Fig. 6a and
b, weight gain of 22.5% started at 275 °C and then stopped at
5
30 °C, implying that the nickel particles had been oxidized ther-
mally to form NiO powders. The weight gain of nanowires occurred
at ca. 295 °C. The Ni nanowires presented better oxidization stabil-
ity based on the shift in weight gain to high-temperature region. It
should be mentioned that the weight gain of nanowires (19.0%) is a
little smaller than those of nanospheres and nano-sea urchins. This
may be attributed to that the oxide layer formed on the surface of
Fig. 6. TG curves of the Ni particles: (a) sample #2, (b) sample #4, and (c) sample
Fig. 4. SEM image of field aligned Ni nanowires.
#8.

1
88
H. Hu, K. Sugawara / Chemical Physics Letters 477 (2009) 184–188
nanowires at the initial stage prohibits the penetration of oxygen
atoms. Eventually, the nanowires are partially oxidized. The good
thermal stability also suggests that the nanowires are tightly inte-
grated structures rather than loose aggregates of nanospheres.
References
[
[
1] D. Chen, C. Hsieh, J. Mater. Chem. 12 (2002) 2412.
2] C. Liu, L. Guo, R. Wang, Y. Deng, H. Xu, S. Yang, Chem. Commun. (2004)
2
726.
3] J. Piquenmal, G. Viau, P. Beaunier, F. Bozon-Verduraz, F. Fievet, Mater. Res. Bull.
8 (2003) 389.
[
3
4
. Conclusion
[
[
[
[
[
4] G. Viau, F. Fievet-Vincent, F. Fievet, Solid State Ionics 84 (1996) 259.
5] Y. Sun, Y. Yin, B.T. Mayers, T. Herricks, Y. Xia, Chem. Mater. 14 (2002) 4736.
6] L.K. Kurihara, G.M. Chow, P.E. Schoen, Nanostruct. Mater. 5 (1995) 607.
7] L.C. Varanda, M. Jafellici Jr., J. Am. Chem. Soc. 128 (2006) 11062.
8] C. Liu et al., J. Phys. Chem. B 108 (2004) 6121.
We have presented here a facile strategy for preparing Ni nano-
structures using a magnetic-field-assisted non-aqueous wet chem-
ical method. SEM observation reveals that under a given magnetic
field, Ni nano-sea urchins, nanospheres and nanowires can be
selectively synthesized by varying the reduction temperature.
Our results demonstrate that the effects of magnetic field can be
ignored at high temperature, in which dispersed NPs are produced.
On the other hand, 1D Ni nanowires with diameters in the range of
[9] S. Sun, C.B. Murray, D. Weller, L. Folbes, A. Moser, Science 287 (2000) 1989.
10] W. Zhu, W. Wang, H. Xu, L. Zhou, L. Zhang, J. Shi, Cryst. Growth Des. 6 (2006)
[
[
2804.
11] R. Harpeness, A. Gedaken, Langmuir 20 (2004) 3431.
[12] G. Couto, J. Klein, W. Schreiner, D. Mosca, A. Oliveira, A. Zarbin, J. Colloid
Interface Sci. 311 (2007) 461.
[
[
13] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115.
14] M. Tanase et al., Nano Lett. 1 (2001) 155.
1
50–400 nm are formed by the magnetic-field directed assembly
[15] J.C. Love, A.R. Urbach, M.G. Pretiss, G.M. Whitesides, J. Am. Chem. Soc. 125
(2003) 12696.
of nanospheres at low reduction temperature (<90 °C). Moreover,
a strong magnetic field leads to growing tight aggregated nano-
wires instead of necklace-like chains. The Ni NPs obtained have
been confirmed to be phase-pure crystalline nickel with fcc
structure on the basis of XRD characterization. Thermal analysis
indicates that Ni nanowires show superior oxidization stability
compared to other shapes.
[
[
[
16] L. Sun, K. Keshoju, H. Xing, Nanotechnology 19 (2008) 405603.
17] J. Wang, Q. Chen, C. Zeng, B. Hou, Adv. Mater. 16 (2004) 137.
18] H. Niu, Q. Chen, H. Zhu, Y. Lin, X. Zhang, J. Mater. Chem. 13 (2003) 1803.
[19] Z. He, S. Yu, X. Zhou, X. Li, J. Qu, Adv. Funct. Mater. 16 (2006) 1105.
[
[
[
20] J.S. Langer, Rev. Mod. Phys. 52 (1980) 1.
21] H. Hu, K. Sugawara, Mater. Lett. 62 (2008) 4339.
22] B.K. Park, S. Jeong, D. Kim, J. Moon, S. Lim, J. Kim, J. Colloid Interface Sci. 311
(2007) 417.