Synthesis and One-Dimensional Self-Assembly of Acicular Nickel Nanocrystallites under Magnetic Fields

Article abstract of DOI:10.1021/jp0361172

Polycrystalline nickel wires with an average length of 10 μm and diameter of about 200 nm were prepared at 70°C by a hydrothermal process with a 0.25 T magnetic field applied. Studies show that the magnetic field induced one-dimensional assembly of acicular Ni nanocrystallites with dimensions of 200 nm in length and 10-30 nm in diameter, leading to the formation of the polycrystalline nickel wires. Magnetic measurements show saturation magnetization (Ms) of the sample prepared in a 0.25 T external magnetic field is higher than that of the sample synthesized without an external magnetic field applied. It is suggested that the one-dimensional self-assembly of acicular nickel nanocrystallites with their magnetic easy axes [111] aligned along the magnetic line of force results in the improvement of the magnetic properties.

Full text of DOI:10.1021/jp0361172

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3996
J. Phys. Chem. B 2004, 108, 3996-3999
Synthesis and One-Dimensional Self-Assembly of Acicular Nickel Nanocrystallites under
Magnetic Fields
Helin Niu, Qianwang Chen,* Min Ning, Yaoshun Jia, and Xiaojuan Wang
The Structure Research Laboratory and Department of Materials Science & Engineering,
UniVersity of Science & Technology of China, Hefei 230026, China
ReceiVed: July 20, 2003; In Final Form: NoVember 5, 2003
Polycrystalline nickel wires with an average length of 10 µm and diameter of about 200 nm were prepared
at 70 °C by a hydrothermal process with a 0.25 T magnetic field applied. Studies show that the magnetic
field induced one-dimensional assembly of acicular Ni nanocrystallites with dimensions of 200 nm in length
and 10-30 nm in diameter, leading to the formation of the polycrystalline nickel wires. Magnetic measurements
show saturation magnetization (Ms) of the sample prepared in a 0.25 T external magnetic field is higher than
that of the sample synthesized without an external magnetic field applied. It is suggested that the
one-dimensional self-assembly of acicular nickel nanocrystallites with their magnetic easy axes [111] aligned
along the magnetic line of force results in the improvement of the magnetic properties.
Introduction
H2O, 0.8 g of PEG (M ) 10000), 0.3 g of CTAB, and 45 mL
of distilled water were mixed in a flask at room temperature. A
grass-green color was observed in the solution after strongly
stirring for half an hour. A half-hour ultrasound treatment was
carried out to ensure that Ni2+ ions were dispersed homoge-
neously in the solution. Then 5 mL of 80 wt % hydrazine
hydrate solution was added dropwise into the solution, and the
color changed from grass green to navy blue after strongly
stirring for several minutes; the main possibility is the formation
of a coordination compound of Ni[N2H4]6. The solution was
transferred into two Teflon-lined stainless steel autoclaves with
60 mL capacity (one without a magnet attached, and another
with a magnet made of NdFeB under the Teflon vessel; the
strength of the magnetic field on the inner surface of the Teflon
vessel is 0.25 T at room temperature), respectively. Both of the
autoclaves were closed tightly to perform hydrothermal pro-
cesses at 70 °C for 3 h. After the reaction was completed, the
resulting black precipitates were separated and washed with
alcohol and distilled water several times, respectively, in order
to remove the remaining PEG and CTAB. The as-synthesized
products were dried in a vacuum oven at 40 °C. The samples
obtained were characterized by X-ray powder diffraction (XRD)
using an 18 kW advanced X-ray diffractometer with Cu KR
radiation (λ ) 1.54056 nm) and high-resolution transmission
electron microscopy (HRTEM) (JEOL-2010). The TEM images
and selected area electron diffraction pattern (SAED) were taken
with a HITICHAI H-800 microscope. The scanning electron
microscopy (SEM) was also taken on a HITICHAI X-650
microscope. Magnetic hysteresis loops were measured using a
vibrating sample magnetometer (VSM, BHV-55). For magne-
tization measurements, the powder was pressed strongly and
fixed in a small cylindrical plastic box.
Nanoscale magnetic materials have attracted intensive interest
because of their potential applications including high-density
magnetic recording, magnetic sensors, and addressing some
basic issues about magnetic phenomena in low-dimensional
systems.1-4 Magnetic metal materials, such as Fe,5,6 Co,7-9 and
Ni,10,11 have been studied for many years. Various approaches
have been developed to prepare nanoscale magnetic metal
materials, including metal carbonyl pyrolysis,12,13 water-in-oil
microemulsion,14 hydrogen arc plasma,15γ-ray irradiation,16
borohydride reduction of metal salts,17 and templated synthesis.18
However, little attention has been paid to the effect of a magnetic
field on the nucleation and growth process of magnetic materials
and on the self-assembly behavior of magnetic nanocrystallites.
It has been indeed found that the magnetic field can significantly
influence the movement of magnetic particles.19-21 It is,
therefore, significant to study the growth and self-assembly
behavior of magnetic nanocrystallites under an external magnetic
field. In a recent letter, we reported the results of magnetic-
field-induced self-assembly of spherical Co particles.22 Co
polycrystalline wires with an average length of 2 mm and
diameter of 13 µm were formed by the self-assembly of Co
nanocrystallites (15 nm in average size) under the induction of
a 0.25 T external magnetic field. The wires were nearly parallel,
likely due to their axes were all parallel to the magnetic line of
force. Because of Co nanocrystallites without shape anisotropy,
it is hard to understand whether a magnetic easy axis oriented
to the magnetic line of force determines the assembly behavior
of magnetic particles or not. In this paper, we further report the
self-assembly behavior of acicular nickel nanocrystallites with
great shape anisotropy under magnetic induction.
Experimental Section
Results and Discussion
A controlled hydrothermal synthesis route was developed to
prepare acicular nickel nanocrystallites at low temperatures. The
chemical reaction for the synthesis can be expressed as Scheme
1. The standard Gibbs free energy change ∆G° of reaction 2
implies a very strong tendency to progress toward the right side.
In a typical experiment, all the reagents are analytical grade
and are used as received. Quantities of 0.5 g of Ni(CH3CO2)2‚
* Corresponding Author: Tel: +86-551-3607292. Fax: +86-551-
3631760. E-mail: cqw@ustc.edu.cn.
10.1021/jp0361172 CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/05/2004

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Acicular Ni Nanocrystallites under Magnetic Fields
J. Phys. Chem. B, Vol. 108, No. 13, 2004 3997
while the AF sample contains highly oriented straight wires;
this indicates that a magnetic field can effectively affect the
aggregation of nickel nanocrystallites.
To further reveal the fine structure of the wires shown in
Figure 2b, TEM analysis was also carried out. Figure 3, parts
a and b, depict straight wires with an average length of 10 µm
and diameter of about 200 nm forming in the AF sample, while
zigzag wires formed in the ZF sample. To understand well the
effect of magnetic field on the linear alignment feature of
acicular nanoparticles, a postsynthesis magnetic induction
experiment was carried out on the ZF sample. It is observed
that after postsynthesis induction by a 0.25 T magnetic field,
the morphology of the particles in the ZF sample is still zigzag
wires (a1, inset of Figure 3a); no straight wires such as those
that appeared in the AF sample were observed, which further
reveals that the magnetic structure of acicular nanoparticles
could be strongly modified by a magnetic field during particle
formation. These acicular nanoparticles can be aligned to form
straight wires under a magnetic field. It is found the dipole
magnetic interactions of these nanoparticles in the wires are
quite strong. As a rule, samples must be sonicated continuously
for more than half an hour to prepare a sample for TEM
observation; however, straight wires have not been destroyed,
indicating that the polycrystalline wires are rather stable as a
result of strong magnetic interactions. Figure 3c shows a
magnified image of an individual polycrystalline wire. The
acicular nanoparticles are about 200 nm in length and 10-30
nm in diameter. The SAED pattern (c1, inset of Figure 3c) for
one typical wire also shows that it is polycrystalline. The
diffraction pattern can readily be indexed as (111), (200), (220),
(331) of the face-centered cubic Ni with a ) 3.517 Å. An
individual acicular nanocrystallite was characterized by HRTEM
analysis. The SAED pattern (Figure 3C (c2)) reveals the acicular
nanocrystallite to be single crystalline. An HRTEM image of a
nanocrystallite is shown in the inset of Figure 3C (c3). The
nanocrystallite is imaged to have nearly parallel lines, which
are nickel atomic planes separated by about 2.03 Å, correspond-
ing to the {111} planes of a face-centered cubic nickel crystal.
The growth orientation of an acicular nanocrystallite is along
the {111} planes, namely, the long axis of the nanocrystallite
is perpendicular to the [111] axis. It is known that [111] is the
magnetic easy axis of a cubic nickel crystal.26 The nanocrys-
tallites are not oriented growing along the magnetic easy axis
under magnetic induction, which indicates that the magnetic
field could not have an influence on the growth direction of Ni
nanocrystallites although a significant effect on their assembly
was observed.
Figure 1. XRD pattern of nickel nanocrystallites prepared with a 0.25
T magnetic field applied.
Figure 2. SEM micrographs of products prepared at 70 °C for 3 h (a)
without and (b) with a 0.25 T magnetic field applied.
SCHEME 1: The Chemical Reaction for the Synthesis of
Nickel Nanocrystallites
The XRD patterns of the products prepared with and without
an external magnetic field applied are similar. Figure 1 shows
an XRD pattern of the obtained product with a 0.25 T external
magnetic field applied. It can be well indexed with the
reflections of face-centered cubic Ni (PDF standard cards,
JCPDS 01-1260, space group Fm3m), without impurity peaks.
Furthermore, the broadening of the peaks is obvious, and the
crystalline size is about 25 nm, calculated by the Scherrer’s
equation from the full width at half-maximum (fwhm) of (111),
(200), and (220) reflections. The size distribution could fall into
the stable single domain range of magnetic particles; hence, each
crystallite might contain a single magnetic domain.19 As we
know, single-domain magnetic particles with uniaxial anisotropy
are fascinating for magnetic recording media.
Spherical particles are common due to the low surface energy
associated with the morphology.23 It is reported that CTAB
associated with the polymer chains could facilitate the formation
of rod-shaped micelles;24,25 in our experiments polymer (PEG)
and surfactant (CTAB) complexes were employed for controlled-
synthesis nickel nanocrystallites with shape anisotropy. Figure
2 shows the difference in morphology between the products
prepared with (AF sample) and without (ZF sample) an external
magnetic field applied during particle growth. The morphology
of the ZF sample appears disorderly, consisting of zigzag wires,
Regardless of whether or not an external magnetic field was
applied, the acicular nickel nanocrystallites were formed by
addition of PEG and CTAB. The acicular nickel nanocrystallites
will aggregate due to dipolar interaction, which is affected by
several factors such as velocity of nucleation, spacial hindrance,
dipolar interaction, and magnetic field alignment as well. It is
known that the magnetic anisotropy energy (MAE) is the lowest
when magnetizing along the magnetic easy axis. [111] is the
magnetic easy axis of a cubic nickel crystal. When an external
magnetic field is applied, magnetization makes the magnetic
easy [111] axes of all particles orientate along the magnetic
line of force, leading to acicular nanocrystallites to be arranged
regularly and finally the formation of straight wires. Because
nanocrystallites did not aggregate along other directions such
as long axes of acicular nanocrystallites, but the magnetic easy
axes, it is, therefore, suggested that the alignment of the
magnetic easy axis along the magnetic line of force determined

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3998 J. Phys. Chem. B, Vol. 108, No. 13, 2004
Niu et al.
Figure 3. Representative TEM images of two samples obtained( a) without and (b) with a 0.25 T external magnetic field applied. Inset in (a)
shows an image of a sample postsynthesis magnetic alignment of the ZF sample under a 0.25 T magnetic field a1. (c) A magnified image of a wire
within (b), of which c1 and c2 are two SAED patterns taken on the surface of a wire and an acicular nanocrystallite, respectively, and c3 is its
HRTEM image.
Figure 5. The hysteresis loops measured at room temperature for three
samples obtained (a) with a 0.25 T and (b) without an external magnetic
field applied, (c) the hysteresis loop (line of dashes) of the nanorods
postsynthesis magnetic alignment under a 0.25 T magnetic field.
Figure 4. Sketch of the ideal process of assembly of acicular Ni
particles induced by an external magnetic field.
Ms for the AF sample compared to that of the bulk Ni. It might
arise from the systematic and accidental error due to the nature
of the nanowires being measured, inconstant environmental
conditions, limitations in instruments et al., but it is worth
emphasizing that the increase tendency of Ms for the AF sample
is obvious, when compared to that for both the AF sample and
nanoparticles fabricated by other group. The Ms value of ZF
sample prepared without an external magnetic field applied is
very consistent with that of the nanoscale Ni particles reported
in the literature (32 emu/g, ref 29). Moreover, the Ms value of
AF sample is almost two times as large as that of ZF sample
with the measurements for the two samples being carried out
in the same manner. We, therefore, suggest that the self-
assembly structure of acicular Ni nanocrystallites formed under
a magnetic field could significantly improve the magnetic
properties. The magnetic easy axis [111] of acicular Ni
nanocrystallites is aligned along the long axis of the polycrys-
talline wires oriented parallel to the magnetic force line of the
external magnetic field, which would improve the Ms. There-
fore, it is suggested that Ms for acicular Ni nanoparticles is
largely dependent on alignment of the easy axis, not only
dependent on some above-mentioned factors such as size of
crystallites and degree of surface oxidation. The coercivity Hc
for the two samples with and without magnetic induction is 51
and 45 Oe, respectively. Although the value for the AF sample
is higher than the corresponding value for the ZF sample, both
are much lower than that of the corresponding bulk material
(Hc ) 100 Oe),27 which indicates that magnetic nanocrystallites
have a tendency to be in a superparamagnetic state. As shown
in Figure 5, parts b and c, the Ms and Hc of the ZF sample
their self-assembly behavior and the morphology of aggregates
formed. The formation of straight wires through magnetic field
alignment is schematically shown in Figure 4. The morphology
of the particles formed under magnetic induction is straight wires
with acicular Ni nanocrystallites homogeneously distributed.
However, the actual aggregating process is very complicated,
determined by nucleation, spacial hindrance, magnetic attraction
of excessive finer particles, and other kinetic factors. When no
external magnetic field is applied, the orientation of each
magnetic domain is spontaneously random. The acicular nanoc-
rystallines may magnetize one another by dipolar interaction
in an arbitrary direction, which finally results in formation of
zigzag wires.
Figure 5 shows M-H hysteresis loops of the products
prepared with (a) and without (b) an external magnetic field
applied. The Ms for the sample formed with a 0.25 T and
without an external magnetic field applied are 59 and 33 emu/
g, respectively. The saturation value of bulk nickel is 55 emu/
g.27 Generally, the Ms for nanoscale magnetic materials is lower
than that for corresponding bulk material because the spin
disorder on the surface and surface oxidation would significantly
reduce the total magnetic moment.28 Accordingly, it is reason-
able to explain the decrease of Ms for the ZF sample prepared
without a magnetic field applied, but it is hard to understand
that the Ms for the AF sample prepared under a magnetic field
is higher than that of the ZF sample, even the corresponding
bulk Ni (7%). It is hard to understand the 7% difference of the

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Acicular Ni Nanocrystallites under Magnetic Fields
J. Phys. Chem. B, Vol. 108, No. 13, 2004 3999
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Conclusions
In conclusion, polycrystalline nickel nanowires with an
average length of 10 µm and diameter of about 200 nm were
formed through a low-temperature hydrothermal process by
magnetic induction. It is found that an external magnetic field
can make acicular Ni nanocrystallites one-dimensionally self-
assemble with their magnetic easy axes aligned along magnetic
line of force, leading to the formation of polycrystalline wires.
The as-prepared Ni wires possess higher Ms than that of the Ni
nanocrystallites formed without a magnetic field applied. The
magnetic easy axis [111] is aligned along the long axis of the
polycrystalline wires, which would improve Ms as the wires
oriented parallel to the line of the external magnetic field during
the magnetic measurements. It is suggested that this process
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