The fabrication and magnetic properties of Ni fibers synthesized under external magnetic fields

Article abstract of DOI:10.1002/ejic.200800200

One-dimensional (1D) nanocrystallites Ni fibers with different lengths were fabricated by the reduction of Ni²⁺ ions by hydrazine hydrate in the presence of external magnetic fields. The effect of the reaction conditions and magnetic field intensity on the microstructures and magnetic properties of the Ni fibers were systematically investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and measurement of hysteresis loops at room temperature. It was found that both the intensity of the external magnetic field and the concentration of the nickel salt solution played key roles in governing the microstructures and magnetic properties of the Ni fibers. Namely, the mean length of the Ni fibers increased markedly with increasing Ni²⁺ ion concentration and intensity of the external magnetic field as well. Moreover, the Ni fiber samples prepared under external magnetic fields had higher squareness and coercivity values than those synthesized in the absence of the external magnetic field. Therefore, a relatively high intensity of the external magnetic field and concentration of the Ni²⁺ ions was selected for the preparation of the desired Ni fibers with improved microstructures and magnetic properties. The present approach has the advantages of having a fast reaction rate and low cost and might be promising for the effective control of the shape and magnetic properties of magnetic materials and for large-scale production as well. The resulting Ni fibers might be potential catalysts, magnetic storage materials, and conductive fillers for shielding of electromagnetic interference (EMI). Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800200

FULL PAPER
DOI: 10.1002/ejic.200800200
The Fabrication and Magnetic Properties of Ni Fibers Synthesized Under
External Magnetic Fields
[a]
[a]
[b]
[a]
[a]
Chunhong Gong, Laigui Yu, Yuping Duan, Juntao Tian, Zhishen Wu, and
Zhijun Zhang*[
a]
Keywords: Nickel fibers / Nanocrystallites / Chemical reduction / Magnetic fields / Magnetic properties
One-dimensional (1D) nanocrystallites Ni fibers with dif-
ferent lengths were fabricated by the reduction of Ni2 ions
by hydrazine hydrate in the presence of external magnetic
fields. The effect of the reaction conditions and magnetic
field intensity on the microstructures and magnetic proper-
ties of the Ni fibers were systematically investigated by scan-
ning electron microscopy (SEM), X-ray diffraction (XRD),
and measurement of hysteresis loops at room temperature. It
was found that both the intensity of the external magnetic
field and the concentration of the nickel salt solution played
key roles in governing the microstructures and magnetic
properties of the Ni fibers. Namely, the mean length of the
magnetic fields had higher squareness and coercivity values
than those synthesized in the absence of the external mag-
netic field. Therefore, a relatively high intensity of the exter-
nal magnetic field and concentration of the Ni2 ions was se-
lected for the preparation of the desired Ni fibers with im-
proved microstructures and magnetic properties. The present
approach has the advantages of having a fast reaction rate
and low cost and might be promising for the effective control
of the shape and magnetic properties of magnetic materials
and for large-scale production as well. The resulting Ni fibers
might be potential catalysts, magnetic storage materials, and
conductive fillers for shielding of electromagnetic inter-
ference (EMI).
+
+
2
+
Ni fibers increased markedly with increasing Ni ion con-
centration and intensity of the external magnetic field as
well. Moreover, the Ni fiber samples prepared under external
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
growth can be controlled almost exclusively in the direction
normal to the substrate surface. However, template-based
synthetic routes have drawbacks in that the introduction of
the templates or other additives inevitably adds to the com-
plexity in the synthetic procedures and in the purification
of the final products as well. Therefore, it is imperative to
develop simple template-free methods for the fabrication of
One dimensional (1D) magnetic materials have recently
been attracting extensive attention because of their funda-
mental interest and technological applications.[1–7] Many
methods requiring the use of structure-directing templates
have successfully been tried for the fabrication of the 1D
magnetic materials. To date, a variety of metal nanowire
arrays has been produced by electrodeposition with the use
1D magnetic materials. These materials are more valuable
in terms of the expected advantages they present, such as
relatively lower cost, higher product purity, and better feasi-
bility for large-scale production.
Previous studies have shown that the magnetic materials
can be manipulated by properly adjusting the magnetic
field, and field-assisted assembly can be utilized to obtain
[
8–11]
of templates.
tube arrays of Fe, Co, and Ni by electrodeposition on poly-
For example, Cao et al. prepared nano-
[
8]
aniline membranes. Ge et al. used the electrodeposition
on polycarbonate membranes to prepare Co nanowire ar-
[9]
rays with perpendicular anisotropy. And diblock copoly-
mer[ or anodized alumina (AAO) templates[11] was also
used to yield nanowire arrays of excellent quality. As a sim-
ple and versatile method, the template-based electrochemi-
cal synthesis has been taken as one of the most efficient
methods in controlling the growth of nanowires because the
10]
1D structures through the induced dipolar effect. For exam-
ple, Niu et al. and Wang et al. fabricated polycrystalline Co,
Ni, and single crystalline Fe O4 wires using the self-as-
3
sembly of nanocrystallites under external magnetic
[12–14]
fields.
Zhang et al. prepared chainlike and ringlike Ni–
Zn ferrite nanostructures under an external magnetic field,
[
[
a] Key Laboratory of Special Functional Materials of Ministry of by carefully controlling the thickness of the stabilizing rea-
Education, Henan University,
[15]
gent on the surfaces of the ferrite particles. Athanassiou
Kaifeng 475004, People’s Republic of China
b] School of Materials Engineering, Dalian University of Technol- et al. used a combination of magnetic fields and atmosphere
ogy,
pressure reducing flame to explore the possibility for pre-
Dalian 116023, People’s Republic of China
paring metallic nanowires at high production rates.[ Nat-
16]
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
884
urally, 1D nickel nanostructures, as a type of important fer-
2
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2884–2891

Ni Fibers Synthesized Under External Magnetic Fields
romagnetic materials, have been attracting intensive interest 3), any further increase in the concentration of N H ·H O
2
4
2
owing to their potential applications in magnetic sensors seemed to have little effect on the reduction duration, poss-
and memory devices.[
13,17]
Because of the shape- and size- ibly because of the reduction kinetics.
[22,23]
Figure 1(A–D)
dependent properties, one of the main challenges in this shows the SEM morphologies of the Ni fibers synthesized
area is how to precisely control the shape and crystal struc- from reaction systems with varied molar ratios of N H /
2
4
2+
tures of the 1D nanostructured Ni products, which is criti- Ni and a fixed concentration of NaOH (0.04 ). In gene-
cal to the tailoring of the physical/chemical properties in a ral, the Ni fibers prepared from the reaction systems with
[
18–20]
controllable way.
Unfortunately, previous studies on a molar ratio from 3 to 32 had a few differences both in the
1D nickel nanostructures mainly concentrated on the mi- SEM morphologies and the reaction times, and therefore a
crostructures at a certain external magnetic field in a solvo- higher molar ratio, for instance 16 and 32, would be benefi-
thermal or hydrothermal system, which is less competitive cial for the formation of uniform and smooth nanocrys-
in terms of the facile and simple operation for the fabrica- tallite fibers (Figure 1C and D and Supporting Information
tion of the magnetic fibers with tunable morphology.
In the present study, therefore, we demonstrate a direct
approach for the preparation of Ni fibers under normal
pressure in the absence of any inorganic or organic tem-
plates, which serves to help easily avoid the introduction of
impurities into the final product. The dependence of the
morphology of the Ni fibers on the concentration of nickel
chloride (NiCl ·6H O), hydrazine monohydrate (N H ·
Figure S1).
2
2
2
4
H O), and sodium hydroxide (NaOH) has been investi-
2
gated. The effect of the external magnetic field on the mor-
phology of the products was investigated within the mag-
netic field intensity range 0–0.35 T. Moreover, the magnetic
properties of the nanocrystallite Ni fibers were evaluated.
The main objectives of the present research are to analyze
2
+
and optimize the parameters, such as N H /Ni molar ra-
2
4
2
+
tio, NaOH concentration, Ni ion concentration, and in-
tensity of the applied magnetic field, so as to provide clues
to factors that control and tune the growth of the Ni fiber
nanostructures in engineering.
Figure 1. SEM micrographs of the samples obtained at 60 °C from
2
+
the reaction systems with different N
2 4
H /Ni molar ratios: (A) 3,
(
B) 6, (C) 16, and (D) 32.
Results and Discussion
Similarly, the Ni fibers obtained from the reaction sys-
Hydrazine hydrate, as a potent reducing agent in alkali tems with different concentrations of NaOH when the
2+
solution, has been successfully used for the syntheses of a N H /Ni molar ratio was fixed at 32 were also analyzed
2
4
variety of nanoscale metal materials because it has a low by means of SEM, in order to shed light on the relationship
concentration of impurities and a low cost.[ A series of between the morphology of the target products and the
nanocrystallite Ni fibers was prepared from reaction sys- concentration of NaOH in the reaction system under an
tems containing a fixed concentration of nickel ions external magnetic field. To our surprise, an optimal amount
21]
(
0.01 ) but varied concentrations of N H ·H O and of NaOH was imperative for the formation of Ni fibers at
2 4 2
NaOH in the presence of a 0.3-T external magnetic field at a temperature of 60 °C. Namely, no black products were
0 °C, with the aim of revealing the effects of the reducing formed in the absence of NaOH until the temperature was
6
agent and alkaline on the morphology of the products.
increased to 80 °C, which is consistent with that reported in
[
22]
According to Equation (1) (see Experimental Section), literature.
It was supposed that in the reaction systems
2
+
the molar ratio of N H /Ni theoretically needs to be kept containing hydrazine hydrate as a kind of alkali, trace
2
4
2
+
as 1:2 for the reduction of the dissolved Ni ions. However, amounts of NaOH do not solely act as a simple alkaline
the reaction system should be kept still and away from any agent but may also act as a catalyst for the formation of Ni
source of agitation to avoid any possible disturbance that nanoparticles. Figure 2 gives the SEM images of the Ni fi-
may affect the magnetic line of force for the self-assembly bers obtained at 60 °C from the reaction systems with dif-
of the Ni nanocrystallites. This implies that the diffusion of ferent concentrations of NaOH (0.04 , 0.02 , and 0.01 )
the reactant ions is confined and a high concentration of and the image of the product prepared from the same reac-
the reducing agent is needed to complete the reaction. As tion system at 80 °C but without NaOH for comparison
evident in our screening experiments, it was impossible to (Figure 2D, reaction duration 60 min). It can be seen that
obtain any desired products until the molar ratio for N H / the morphology of the Ni fibers is highly dependent on the
2
4
2
+
Ni increased to 3. Moreover, once the concentration of concentration of NaOH in the reaction solution. Namely,
2+
N H ·H O reached a sufficiently high level (N H /Ni
2
Ն
sample A has almost the same shape as sample B, and these
4
2
2
4
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C. Gong, L. Yu, Y. Duan, J. Tian, Z. Wu, Z. Zhang
FULL PAPER
two samples seem to be more homogeneous and smooth Figure 3 shows the SEM micrographs of the nanocrystallite
than samples C and D. This could be related to the alka- Ni fibers prepared from the reaction systems containing
line-dependent reducing power of hydrazine.[ When the 0.003–0.20  of NiCl ·6H O. It can be seen that the average
23]
2
2
2+
2+
concentration of NaOH is sufficiently high (NaOH/Ni
), some free OH ions may remain in the liquid phase and concentration, while the average diameter remains about
Ն
length of the Ni fibers increase with increasing Ni ion
–
2
adequate reduction of the nickel ions might be effectively 250 nm. The average length of the Ni fibers prepared at
achieved with hydrazine hydrate (Figure 2A and B). The re- nickel chloride concentrations of 0.003, 0.01, 0.02, and
ducing capability of hydrazine hydrate is limited and the 0.05  are about 20, 30, 50, and 100 µm, respectively (Fig-
reaction rate is reduced at a relatively weaker alkalinity; Ni ure 3A–D).
fibers with a larger roughness and clustered structure were
therefore obtained. In contrast, if the molar ratio of NaOH/
2
+
Ni is too high, the by-product Ni(OH)2 is generated.
Thus, the optimal concentration of NaOH in the reaction
system is suggested to be in the range 0.02–0.04 .
Figure 2. SEM images of nanostructured Ni fibers obtained at
60 °C from the reaction systems with different NaOH concentra-
tions: (A) 0.04 , (B) 0.02 , and (C) 0.01 ; (D) shows the image
of the product obtained at 80 °C in the absence of NaOH (reaction
duration 60 min).
Figure 3. SEM images of the Ni fibers obtained at 60 °C from the
reaction systems with different concentrations of Ni ions: (A)
2+
0.003 , (B) 0.010 , (C) 0.020 , (D) 0.050 , and (E) 0.200 .
The insets present higher magnification images.
In order to investigate the effect of the concentration of
the Ni ions on the morphology of the resulting Ni fibers,
2
+
the reduction was performed in a field of 0.3 T at 60 °C at
As a type of classical ferromagnetic materials, Ni crystals
2
+
a fixed N H /Ni molar ratio of 32 and NaOH concentra- have an inherent magnetism and can produce a local mag-
2
4
2
+
2+
tion of 0.040  but with varied concentrations of the Ni
ions. It could be worth pointing out that at a Ni ion con- creased, the locally induced magnetic field is enhanced too.
centration of 0.20 , a molar ratio of 32 for N H /Ni
netic field. When the concentration of the Ni ions is in-
2
+
2
+
However, the orientation of each nanocrystallite Ni domain
2
4
might be too high in terms of the alkalinity of the N H4 is random unless an external magnetic field is applied. The
2
solution and its influence on the volume and component of local magnetic field is induced in the direction of the exter-
the solvent. We did a series of preliminary experiments in nal magnetic field and strongly enhances the magnetic field
this respect and found that the nanocrystallite Ni fibers syn- when the intensity of the external magnetic field is suffi-
thesized from the reaction systems containing 0.01  ciently high, which results in a higher magnetic field density.
2
+
NiCl ·6H O and N H /Ni with varied molar ratios from This helps to increase the induced dipole moments and pro-
2
2
2
4
3
to 32 had few differences in their morphology, which indi- mote the self-assembly process and the orientation of the
cates that the concentration of hydrazine is not a key factor Ni nanocrystallites. Subsequently, more Ni particles and fi-
in governing the morphology of the Ni fibers. Bearing in bers are assembled together, which results in longer fibers
mind our previous finding that Ni2+ ions with a concentra- with increasing Ni ion concentration. Moreover, when the
2+
concentration of the Ni2 ions is sufficiently high (up to
+
tion of 0.20  to 0.80  can be effectively reduced by N H
2
4
2
+
at a N H /Ni molar ratio from 6 to 16 in ethylene glycol 0.05 ), because of the reason already mentioned, the corre-
2
4
solvent and under agitation, we chose a molar ratio for sponding Ni fibers attract each other in the direction of the
2
+
2+
N H /Ni of 16 and a Ni ion concentration of 0.20 . magnetic force and experience an enhanced assembling and
2
4
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Ni Fibers Synthesized Under External Magnetic Fields
aggregating effect, which results in bundlelike self-as- low concentration (not higher than 0.05 ) of Ni2+ ions re-
sembled arrays of Ni fibers with parallel orientation (Fig- mains a limitation.
ure 3D and E).
Figure 4 shows the XRD patterns of two typical Ni fi-
2
+
When the Ni ion concentration in the reaction system bers prepared from reaction systems containing 0.05  and
reached 0.20 , a mixture of black product and blue mate- 0.01  Ni2 ions (the reaction conditions for these two sam-
rial was obtained at 60 °C even after the reaction period ples are the same as that used for the corresponding sam-
was extended to more than 4 h, which implies that the re- ples shown in Figure 3). Three characteristic peaks for Ni
ducing reaction could not reach completion at this high (2θ = 44.5°, 51.8°, and 76.4°), corresponding to Miller in-
+
2
+
level of Ni ion concentration. The as-synthesized mixture dices (111), (200), and (220), are observed for both samples,
of solid products was characterized by XRD, to evaluate which indicates that they are composed of pure fcc Ni and
[
13]
the structure of the blue material. From the XRD pattern do not contain impurities (JCPDS 01-1260). At the same
see the Supporting Information Figure S2), in addition to time, the peaks for both samples show obvious signs of
the three characteristic peaks assigned to the Miller indices broadening, which corresponds to the nanocrystallite struc-
111), (200), and (220) of Ni (2θ = 44.5°, 51.8°, and 76.4°), tures. The average crystalline sizes of the nanocrystallites
(
(
there are indeed some peaks of unknown materials, which were calculated from the half-width of the peaks of (111),
cannot be identified as existing nickel compounds such as by using the Scherrer formula d = 0.89λ/βcosθ, to be about
nickel chloride, nickel chloride hydrate, nickel hydroxide, 12 nm (Ni2 ion concentration 0.01 ) and 20 nm (Ni ion
and nickel oxide et al.. This indicates that the resulting concentration 0.05 ), where λ is the X-ray wavelength, β is
products are composed of fcc Ni and some unknown nickel the peak width at half-maximum of the corresponding
+
2+
compound.
XRD peak, and θ is the Bragg diffraction angle.
The corresponding black product has an irregular micro-
structure, which is characterized by loosely connected
spherelike particulates mixed with a few irregular fibers
(
Figure 3E). This could be because of the confined diffusion
2
+
of the Ni ions at high concentration levels. Thus, it was
2
+
suggested to keep the concentration of the Ni ions in the
reaction system to below 0.05  so as to ensure the function
of the external magnetic field to tailor the morphology and
microstructure of the target product.
Nickel fibers could be suitable for electromagnetic shield-
ing as they are good conductors. A small diameter for the
metal fibers is preferred because of the skin effect, since the
electromagnetic radiation at high frequency interacts only
with the near surface region of a conductor. The depth at
which the fields are reduced by a factor of 1/e is called the
Figure 4. XRD patterns of the Ni fibers obtained at 60 °C from
2
+
skin depth (δ), and it decreases with increasing frequency the reaction systems with different concentrations of Ni ions: (A)
0.010  and (B) 0.050 .
and conductivity or permeability. The skin depth is depend-
ent on the type of the conductor metal and the frequency
of the fields applied to the conductor. As nickel with ferro-
magnetic nature has a skin depth (δ) of 0.47 µm and
Figure 5 shows the hysteresis loops of the same samples
shown in Figure 4. Obviously, the sample derived at a
higher Ni2 ion concentration (0.05 ) has an M value of
+
s
[
24]
0.33 µm at frequencies of 1 and 2 GHz, respectively,
it
can be rationally anticipated that the ultrafine Ni fibers
with a diameter size equal to δ or less would be particularly
attractive in the fabrication of the composites for electro-
magnetic interference (EMI) shielding. Up to now, few re-
ports have been available with composites based on nickel
fibers or nickel filaments – this might be largely due to the
difficulties in the fabrication technique. The fabrication of
submicron nickel filaments with a diameter of 0.4 µm by
nickel electroplating on carbon filaments with a diameter
of 0.1 µm by Chung et al. is one example.[ Unfortunately,
these nickel filaments had incomplete coverage and poor
adhesion to the substrate fibers. Taking into account the
significantly increased reaction rate, reduced cost, high yield
of Ni fibers, and the feasibility for large-scale preparation
of the present approach, we believe it would find promising
application in fabricating large quantities of Ni fibers as a
25]
Figure 5. Hysteresis loops of the Ni fibers obtained at 60 °C from
2+
the reaction systems with different concentrations of Ni ions: (A)
type of conductive fillers for EMI shielding, although the 0.010  and (B) 0.050 .
Eur. J. Inorg. Chem. 2008, 2884–2891
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C. Gong, L. Yu, Y. Duan, J. Tian, Z. Wu, Z. Zhang
FULL PAPER
3
1.66 emu/g and an H value of 197.57 Oe, which are signif- growth of the nanocrystallite nickel. In other words, the
c
icantly higher than those of the sample obtained at lower nanocrystallite Ni could easily nucleate and grow in the di-
2
+
Ni ion concentration (8.94 emu/g and 133.8 Oe, respec- rection of the magnetic force rather than randomly in the
2
+
tively). It could thus be concluded that the Ni ion concen- reaction vessel, which results in fibers and not chains. When
tration has an obvious effect on the morphology and mag- the intensity of the magnetic field is increased to 0.3 T (Fig-
netic properties of the nanocrystallite Ni fibers.
ure 7E), the corresponding Ni fibers appear to be signifi-
A series of samples of nanocrystallite Ni fibers were pre- cantly elongated, and the average length of this type of Ni
2+
pared from the reaction systems with a fixed N H /Ni
fiber could be as much as 30 µm. As the intensity of the
2
4
molar ratio of 32, an NaOH concentration of 0.04 , a magnetic field is further increased to 0.35 T, the corre-
NiCl ·6H O concentration of 0.01 , and at a temperature sponding Ni fibers are further elongated and characterized
2
2
of 60 °C, but with a varied magnetic field intensity of 0.13, by a tangled morphology and a large aspect ratio, and the
0.20, 0.22, 0.30, and 0.35, so as to investigate the effect of length of this type of Ni fibers is estimated to be larger than
the intensity of the external magnetic field on the mor- 100 µm (Figure 7F). Therefore, it could be feasible to easily
phology and magnetic properties of the products. Figure 6 control the length of the Ni fibers by adjusting the intensity
shows the corresponding XRD patterns of various Ni fibers of the external magnetic field.
samples (coded as sample AF annotated with numeral sub-
scripts, which show the intensity of the magnetic field), as
well as the XRD pattern for the product obtained under the
same reaction conditions but in the absence of the external
magnetic field for comparison (coded as sample ZF). Three
characteristic peaks assigned to the Miller indices (111),
(200), and (220) of Ni (2θ = 44.5°, 51.8°, and 76.4°) are
also observed for this series of samples, which indicates that
the resulting products are composed of pure fcc Ni and had
no impurities. There are a few differences in the XRD pat-
terns of the ZF and AF samples, and the average crystalline
sizes of the samples were calculated, with the Scherrer for-
mula, to be 12, 11, 18, 14, 12, and 9 nm, respectively.
Figure 7. SEM micrographs of samples prepared under different
magnetic fields: (A) ZF, (B) AF0.13T, (C) AF0.20T, (D) AF0.22T, (E)
AF0.3T, and (F) AF0.35T; the insets present higher magnification
images.
Figure 6. XRD patterns of the ZF, AF0.13T, AF0.20T, AF0.22T
,
AF0.30T, and AF0.35T samples.
Because of the complex conditions involved in the
The SEM images of the ZF and AF samples are shown growth of the nanocrystallite Ni and the lack of in situ ob-
in Figure 7. It is apparent that the morphology of the prod- servation facility, it is difficult to describe precisely the
ucts depends on the intensity of the external magnetic field. growth process of the Ni fibers. It may be primarily in-
Namely, in the absence of the external magnetic field, short ferred, by combining the above-mentioned SEM and XRD
and unsmooth Ni chains are obtained, whose average dia- results, that whether or not an external magnetic field was
meter and length are about 250 nm and 5 µm, respectively. applied, the Ni nanocrystallites are first formed during the
In the presence of the magnetic field with an intensity of reduction process, followed by rapid growth in the absence
not higher than 0.22 T (Figure 7B–D), smooth Ni fibers are of any additional surfactants that control size and shape. At
obtained, and the average length and diameter of these Ni the same time, the primary Ni crystals could also aggregate
fibers are marginally different from those of the Ni chains together to form spherical particles to decrease the surface
prepared in the absence of the magnetic field. This means energy. When no external magnetic field is applied, the self-
that during the chemical reaction, the magnetic field ap- assembly process of the Ni crystals may also occur, which
plied has an obvious influence on the nucleation and results in squiggly Ni chains (see Figure 7A). However, the
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Ni Fibers Synthesized Under External Magnetic Fields
self-generated magnetic field of the Ni particles is so weak the total magnetic moment.[29–31] In addition, the ethylene
that the disturbing forces caused by the uneven temperature glycol as a solvent in the reaction system might form a pro-
of the solution and Brownian motion cannot be neglected. tective layer on the surface of the particles, which reduces
Namely, the dipole–dipole interaction having random ori- the total magnetic moment and hence the M values de-
s
[
18,32]
entations leads to squiggly Ni chains.
crease.
Interestingly, the sample derived under an ex-
When a magnetic field is applied, as a result of enhanced ternal magnetic field of 0.35 T has a large M value of
s
self-assembly in the direction of the magnetic force under 30.2 emu/g, which implies that it could be feasible to melio-
the attraction of the external magnetic field, Ni fibers with rate the magnetic properties of desired magnetic materials
a linear structure can be harvested. The fiberlike structures by applying a strong enough external magnetic field during
emerge as a result of the combined influence of thermal the synthesis.
kinetics and magnetic dipole interactions. Only when en-
dowed with adequate magnetization energy can the par-
ticles overcome Brownian motion and align in the direction
of the magnetic force, which favors the creation of smooth
and straight Ni fibers (otherwise the fibers would be squig-
gly-shaped), as shown in Figure 7B. If a stronger (e.g. B =
0.35 T) external magnetic field is applied, the orientation of
the Ni nanoparticles is further promoted, which results in
some parallel self-assembled arrays of Ni fibers. Therefore,
it is feasible to control the magnetic dipole interactions and
hence the length of the Ni fibers by properly adjusting the
intensity of the magnetic field applied.
Naturally, the detailed formation process of ferromag-
netic fibers is much more complicated than that qualita-
tively discussed above, because many other factors such as
experimental methods, reaction conditions, and reactant
types might also affect the aspect ratio of the fibers in the
magnetically directed agglomeration process. For example,
Figure 8. Room-temperature hysteresis loops for the ZF sample
and the AF0.13T, AF0.20T, AF0.22T, AF0.30T and AF0.35T samples.
On the other hand, the ZF sample and all the AF sam-
Sun et al. prepared nickel wires with lengths of up to several ples have a higher coercivity, H , than the bulk nickel
c
[
28]
hundreds of microns in the presence of magnetic fields, (100 Oe), which may be attributed to their special nano-
using a solvothermal synthesis route.[ They used a mixed structure, i.e. the reduced size and shape anisotropy of 1D
17]
solvent of distilled water and ethanol solution and obtained Ni assemblies.[
33,34]
In addition, all the AF samples have a
obviously longer Ni wires in the presence of external mag- significantly increased coercivity (133.8–147.5 Oe) than the
netic fields with a relatively lower intensity (0.10 T and ZF sample (115.1 Oe), which is consistent with the observa-
0.25 T) than in the presence of magnetic fields with a higher tion that all the AF samples have larger values for the squa-
intensity (0.40 T), which is just contrary to our finding here reness (M /M , 0.36–0.40) than the ZF sample (0.26). The
r
s
(
Figure 7). This might be attributed to the use of different increased H and M /M values of the Ni fibers prepared
c r s
types of solvents, which may have a thermodynamic influ- under an external magnetic field relative to that of the sam-
ence on the nucleation velocity and the subsequent growth ple in the absence of the external magnetic field may largely
of the crystals.
Needless to say, it is absolutely necessary to evaluate the under the attractive force of the external magnetic
be attributed to the oriented growth of Ni nanocrystallites
[
6,35,36]
magnetic properties of the Ni fibers before they are used as field,
magnetic recording media, sensors, and other devices.
which might be well utilized to improve the co-
ercivity of materials as is expected. However, the reasons
Figure 8 shows the magnetic properties of the Ni fibers ob- leading to the differences among the M /M and H values
[
26,27]
r
s
c
tained at different strengths of the external magnetic field. of various AF samples remain unknown at this stage.
The magnetization vs. magnetic field (M–H) loops for the
It could be interesting to note that the samples obtained
samples were obtained to determine magnetic parameters at a Ni2 ion concentration of 0.05  and a magnetic field
+
such as saturation magnetization (M ), coercivity (H ), and intensity of 0.30 T have very similar morphologies and mag-
s
c
squareness M /M (a ratio of the remanent magnetization netic properties to those obtained at a Ni2 ion concentra-
+
r
s
M to saturation magnetization M ). The ZF sample had tion of 0.01  and a magnetic field intensity of 0.35 T.
r
s
an M value of 15.07 emu/g, while the AF samples had M
Namely, the two groups of Ni fiber samples have a very
s
s
values of 11.41 (0.20 T), 11.31 (0.22 T), 8.94 (0.30 T), and high aspect ratio of over 500 and a large M value, which
s
3
0.20 (0.35 T) emu/g. All the samples had smaller M values shows their potential in information storage applications.
s
[28]
than bulk Ni (55 emu/g), possibly because of their small These fibers have a tendency to aggregate and form bundles,
sizes as evident by the XRD data (Figure 6). Generally, which leads to straight and well self-assembled arrays. In
nanoscale magnetic materials have lower M values than the summary, both the intensity of the magnetic field and the
s
2+
corresponding bulk materials, because the spin disorder on Ni ion concentration have a drastic influence on the
the surface and/or surface oxidation significantly reduces length and magnetic properties of the nanocrystallite Ni fi-
Eur. J. Inorg. Chem. 2008, 2884–2891
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2889

C. Gong, L. Yu, Y. Duan, J. Tian, Z. Wu, Z. Zhang
FULL PAPER
bers. It could be feasible to chemically synthesize novel chemical reaction for the synthesis of the nanocrystallite Ni fibers
can be expressed as below in Equation (1).
magnetic materials with improved properties by properly
applying an external magnetic field.
2
Ni2+ + N
2
H
4
+ 4OH– Ǟ 2Ni+ N
2
Ȇ + 4H
2
O
(1)
The Ni2+ ions were reduced into Ni and were immediately self-
assembled into fibers under the magnetic field, and the evolving
nitrogen aided in the protection of the nascent nickel nanocrystal-
Conclusions
An inexpensive and easy-to-perform route was applied
for the fabrication of well-oriented nanocrystallite Ni fibers lites against oxidation.
by chemical reduction at 60 °C in the presence of an exter-
The as-synthesized solid products were precipitated, separated,
washed with ethanol several times, and dried in a vacuum oven at
0 °C for about 24 h. The structures and shapes of the resulting
dried samples were characterized by means of X-ray diffraction
nal magnetic field. The effect of the magnetic field and reac-
tion parameters such as the nickel salt concentration, reduc-
ing agent concentration, and NaOH concentration of the
4
reaction solution on the microstructure and magnetic prop- (XRD, Philips X’ Pert Pro X-ray diffractometer; Cu–Kα radiation,
erties of the Ni fibers were investigated in detail. It was λ = 0.13418 nm) and scanning electron microscopy (SEM, JEOL
found that the concentration of the nickel salt solution and JSM-5600LV; acceleration voltage of 20 kV). The magnetic hystere-
sis loops of the products at room temperature were measured by
the intensity of the external magnetic field played key roles
using a vibrating sample magnetometer (VSM, Lake Shore 7300).
in tuning the length and magnetic properties of the Ni fi-
bers. It was found that a relatively high intensity of the mag- Supporting Information (see footnote on the first page of this arti-
2
+
netic field and concentration of Ni ions favor the genera- cle): SEM micrographs of the samples obtained at 60 °C from the
2
+
2 4
reaction systems with different N H /Ni molar ratios and XRD
tion of Ni fibers with increased length and magnetic proper-
ties. The present approach, with a fast reaction rate, low
cost, high yield, and feasibility for large-scale production
may be used for effectively controlling the shape and mag-
netic properties of magnetic materials and for large-scale
production of nanocrystallites. The resulting Ni fibers with
2
+
pattern of the as-synthesized mixture when the Ni ion concentra-
tion in the reaction system reached 0.20 .
Acknowledgments
different lengths and aspect ratios may have potential appli- The authors have benefited a lot from the helpful discussion with
cations as catalysts, magnetic storage materials, and con- Prof. Zhen-sheng Jin and Si-xin Wu in our laboratory. The finan-
cial support from the Ministry of Science and Technology of China
ductive fillers for EMI shielding.
(
Grant No. 2007CB607606, in the name of “973” Plan) is acknowl-
edged. Special thanks are due to Dr. Rong Sun at the Center of
Precision Engineering, Shenzhen Institute of Advanced Technol-
ogy, Chinese Academy of Sciences for the VSM characterization.
Experimental Section
All chemicals of analytical grade were commercially obtained and
used without further purification. In a typical process, an appropri-
ate amount of NiCl ·6H O (Tianjin Kermel Chemical Co., Ltd.,
2 2
China) was dissolved in 50 mL of ethylene glycol (EG, Tianjin Ker-
mel Chemical Co., Ltd., China) to form a transparent green solu-
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Eur. J. Inorg. Chem. 2008, 2884–2891
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2891