Template-free synthesis of magnetic chains self-assembled from urchin-like hierarchical Ni nanostructures

Article abstract of DOI:10.1002/ejic.200900882

Magnetic chains self-assembled from urchin-like hierarchical Ni nanostructures have been successfully synthesized by a simple hydrothermal, method requiring 4 h at 115 °C without any template or surfactant. The individual urchin-like hierarchical nanostructures have an average diameter of 2-4 μm and are composed of well-aligned sword-like nanopetals growing radially from the surfaces of the spherical particles. The products were characterized, by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission elec-tron microscopy (TEM), energy-dispersive X-ray spectroscopy analysis (EDX), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric and differential scanning calorimetric analysis (TGA-DSC). A rational, formation mechanism of magnetic chains was proposed. Magnetic hysteresis measurements revealed that the as-synthesized Ni chain-like architectures display ferromagnetic behavior with saturation magnetization, remanent magnetization, and coercivity values of 52.58 emu/ g, 5.82 emu/g, and 211.67 Oe, respectively.

Full text of DOI:10.1002/ejic.200900882

FULL PAPER
DOI: 10.1002/ejic.200900882
Template-Free Synthesis of Magnetic Chains Self-Assembled from Urchin-Like
Hierarchical Ni Nanostructures
Lu-Ping Zhu,*[a] Gui-Hong Liao,[b] Wei-Dong Zhang,[b] Yang Yang,[b] Ling-Ling Wang,[a]
and Hong-Yong Xie[a]
Keywords: Nanostructures / Hydrothermal method / Ferromagnetism / Self-assembly / Nickel
Magnetic chains self-assembled from urchin-like hierarchical tron microscopy (TEM), energy-dispersive X-ray spectroscopy
Ni nanostructures have been successfully synthesized by a
simple hydrothermal method requiring 4 h at 115 °C without
analysis (EDX), Fourier transform infrared (FTIR) spectroscopy,
thermogravimetric and differential scanning calorimetric
any template or surfactant. The individual urchin-like hierar- analysis (TGA-DSC). A rational formation mechanism of mag-
chical nanostructures have an average diameter of 2–4 µm and netic chains was proposed. Magnetic hysteresis measurements
are composed of well-aligned sword-like nanopetals growing revealed that the as-synthesized Ni chain-like architectures
radially from the surfaces of the spherical particles. The prod-
display ferromagnetic behavior with saturation magnetization,
ucts were characterized by means of X-ray powder diffraction remanent magnetization, and coercivity values of 52.58 emu/
(XRD), scanning electron microscopy (SEM), transmission elec- g, 5.82 emu/g, and 211.67 Oe, respectively.
drug delivery because of their dimensions and morpho-
Introduction
logies, the synthesis of nickel hierarchical architectures is
It is believed that the properties of nanomaterials
desirable. Zheng et al. synthesized nickel nanocrystals with
strongly depend on their size, shape, and dimensionality.[1]
In recent years, there has been great interest in the synthesis
distinct flower shapes by reduction of the mixed complexes
2+
of Ni(N2H4)3 and Ni(dmg)2 (nickel dimethylglyoximate)
in alkaline solution with hydrazine hydrate.[18] Fu and co-
workers prepared Ni dendritic nanostructures by a hydro-
thermal reduction approach in the presence of the surfac-
tant CTAB.[19] Very recently, the self-assembly of Ni nano-
petals into dandelion/urchin-like three-dimensional nano-
structures by a solvo-/hydrothermal route was reported.[20]
However, to the best of our knowledge, the synthesis of Ni
chain-like architectures consisting of urchin-like hierarchi-
cal nanostructures has not been reported until now.
In our previous work, we reported the synthesis of chain-
like ferromagnetic CoNi alloy particles, Co dendritic super-
structures and flower-like hierarchical microspheres using
wet-chemical methods.[3c,3d,21] Herein, we report the facile
synthesis of Ni chain-like architectures consisting of urchin-
like hierarchical nanostructures by an ethylenediamine
(EDA)-assisted self-assembly process. The chain-like archi-
tectures can be expected to bring about new opportunities
in research and application as catalysts and other related
micro- and nanodevices. We have proposed a possible for-
mation mechanism for the Ni chains self-assembled from
urchin-like hierarchical nanostructures.
of nanomaterials with various architectures, because of
their potential applications in optics, electronics, magnet-
ism, and biology.[2] Remarkable progress has been made in
the self-assembly of highly organized building blocks of
metals,[3] semiconductors,[4] copolymers,[5] organic–inor-
ganic hybrid materials,[6] and biomaterials[7] based on dif-
ferent driving mechanisms. However, since the self-assembly
process is considerably complex, it is still a big challenge to
develop simple and reliable synthetic methods for hierarchi-
cally self-assembled architectures with novel and interesting
morphologies, which would lead to novel properties of
micro-/nanomaterials.
As an important material displaying magnetic anisot-
ropy, nickel has also attracted much attention and has been
the focus of intense research. Various Ni micro/nanostruc-
tures, such as nanocrystals,[8] nanorods,[9] nanoneedles,[10]
nanochains,[11] nanobelts,[12] nanotubes,[13] nano-/submicro-
meter hollow spheres,[14] microwires,[15] nanofibers,[16] and
nanosheets[17] have been successfully synthesized by dif-
ferent methods. Since ordered micro/nanostructures have
potential application in catalysts, magnetic recording, and
[a] School of Urban Development and Environmental Engineer-
ing, Shanghai Second Polytechnic University,
Shanghai, 201209, China
Results and Discussion
E-mail: lpzhu@mail.ipc.ac.cn; lpzhu@eed.sspu.cn
[b] Technical Institute of Physics and Chemistry, Chinese Academy
of Sciences,
The phase structure and purity of the as-synthesized
samples were examined by XRD. The XRD pattern of the
Beijing 100190, China
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as-obtained products is shown in Figure 1. Three character- ent atmosphere; this is consistent with both the XRD and
istic peaks can be indexed as the (111), (200), and (220) the results reported in the literature.[12] The DSC curve
planes of the face-centered cubic (fcc) phase Ni, which is in shows an exothermic peak with a maximum located at ca.
agreement with reported data (JCPDS: No. 04-0850). No 585 °C. The temperature range of the exothermic peak in
characteristic peaks due to the impurities of nickel oxides the DSC curve fits well with that of the weight gain in the
or hydroxides were detected, indicating that the as-obtained TGA curve, corresponding to exothermic behavior during
product was pure. Considering the presence of hydrazine the oxidation of Ni to NiO.
hydrate and the release of N2 when the reaction was carried
out, it is not difficult to understand this.
Figure 1. XRD pattern of the as-synthesized Ni architectures
treated at 115 °C for 4 h.
The morphology and dimensions of the as-prepared
Figure 2. SEM images of the Ni chains self-assembled from urchin-
products were examined by SEM (Figure 2). Figure 2(a)
like hierarchical nanostructures synthesized by hydrothermal treat-
shows the panoramic morphology of the samples, which in-
ment at 115 °C for 4 h. (a) The low-magnification SEM image; the
dicates that the as-obtained products consist of large-scale
inset is the photograph of the as-prepared Ni chain-like architec-
chain-like structures composed of relatively uniform
spheres with an average diameter of 2–4 µm. These chain-
like assemblies with many branches range from several mi-
crons to dozens of microns in size and are present in high
quantity. In fact, it is hard for us to discern where the chains
tures in the mother liquor. (b) and (c) The high-magnification SEM
images. (d) The EDX spectrum; the inset in (d) shows the corre-
sponding TGA-DSC curve.
The chain-like structure of the products was further ex-
end, because they are almost intertwined with each other. amined by TEM and HRTEM studies. Figure 3(a) shows a
The SEM image at a higher magnification shown in Fig- representative TEM image of the as-synthesized nickel sam-
ure 2(b) displays the detailed structure of the chain-like ples. It can be seen that the chain-like architectures consist
architectures assembled from urchin-like hierarchical nano- of urchin-like hierarchical nanostructures, agreeing well
structures. A partially magnified view of the samples indi- with the SEM observations. Figure 3(b) is the enlarged
cates that the urchin-like hierarchical nanostructures com- TEM image of the edge area of an individual Ni urchin-
prise dozens of sword-like petals radiating from the center like hierarchical nanostructure [from Figure 3(a)]. Sword-
[see Figure 2(c)]. The petals with sharp ends and protruding like petals radiating from the spherical particles can be seen
surface ridges have sizes in the range 200–400 nm in dia- clearly, which is consistent with the SEM results shown in
meter, 100–200 nm in thickness, and 0.5–1.2 µm in length. Figure 2. Figure 3(c) is the corresponding selected area elec-
It is noteworthy that the chains as well as the urchin-like tron diffraction (SAED) pattern of a sword-like petal [Fig-
hierarchical nanostructures show considerable stability and ure 3(b)]. The SAED pattern exhibits a regular and clear
they maintain their structure even after 30 min of ultra- diffraction spot array, which illustrates the single crystal na-
sonication. Figure 2(d) shows a representative EDX spec- ture of the sword-like petal. Figure 3(d) is a HRTEM image
trum of Ni chain-like architectures, which indicates that the of the part of a typical sword-like petal enclosed in a box
sample is essentially pure Ni. In order to confirm this result, in Figure 3(b). The lattice interplanar spacing is measured
the products were investigated with TGA-DSC measure- to be ca. 0.20 nm, which corresponds to the (111) plane of
ments. As shown in the inset in Figure 2(d), the as-synthe- the fcc Ni metal. Both the HRTEM image and the SAED
sized nickel products began to be oxidized (weight gain) at pattern indicate that the sword-like petal of Ni metal grows
around 250 °C and were fully oxidized at around 700 °C. in the [011] direction, which agrees well with the XRD re-
The final weight gain was around 27.0%, which was almost sult. In addition, a kind of outer shell of a different material
consistent with the theoretical weight gain (27.3%) for the in this petal can be observed from Figure 3(d). It is well
perfect conversion of pure Ni to NiO. Both the EDX and known that EDA can be employed as a capping agent and
TGA analyses demonstrate that a pure metallic Ni product shape controller in the synthesis of nanoparticles.[22–29]
could be obtained under the current synthesis conditions Therefore, the material forming this outer shell may be the
and the Ni chain-like architectures were very stable in ambi- EDA that has not been removed by washing.
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Template-Free Synthesis of Magnetic Chains
crease gradually with the increase in reaction time. Accord-
ing to the Scherrer formula:[30] D = kλ/(βcosθ) (where D is
the average dimension of particles, K is the Scherrer con-
stant, and β is the FWHM of a reflection), the above-men-
tioned result indicates that the average grain diameter of
the as-obtained particles increases gradually, which is con-
sistent with the SEM results shown in Figure 4.
Figure 3. (a) and (b) TEM images of the as-synthesized Ni architec-
tures treated at 115 °C for 4 h. (c) SAED pattern of one petal taken
from the urchin-like Ni hierarchical nanostructures. (d) HRTEM
image for the area enclosed in a box in (b).
Figure 4. SEM images of the as-synthesized Ni samples collected
at different intervals after the appearance of the precipitate: (a)
0.5 h, (b) 1 h, (c) 2 h, and (d) 4 h.
To investigate the growth process of the Ni chain-like
architectures in detail, we studied the structure and mor-
phology evolution of the product with SEM and XRD at
different growth stages of the hydrothermal process. As
shown in Figure 4(a), in the early stage, spherical nickel
particles with diameters of about 1–2 µm emerged as the
initial product. The surfaces of the as-synthesized nickel
particles are quite smooth. After a reaction time of 1 h,
short sword-like petals began to grow out of these spherical
particles, and some particles developed into chain-like
architectures assembled from deficient urchin-like nano-
structures, as shown in Figure 4(b), indicating that self-as-
sembly was still under way. After a reaction time of 2 h,
most of the product had evolved into chain-like architec-
tures assembled from urchin-like nanostructures at an early
stage of development, as shown in Figure 4(c). The length
of the sword-like petals is about 500 nm. After a reaction
time of 4 h, all of the products had evolved into chain-like
architectures assembled from fully developed urchin-like
Figure 5. XRD patterns of the as-prepared samples at various
growth stages of the hydrothermal process: (a) 0.5 h, (b) 1 h, and
(c) 2 h.
Apart from the reaction time, EDA, which is an impor-
nanostructures, as shown in Figures 4(d) and 2. It is notable tant ligand for metal ions and has been widely employed as
that besides chain-like architectures assembled from fully a shape controller and stabilizer in the synthesis of Fe3O4
developed urchin-like nanostructures, some microwires with hollow microspheres,[22] fine handkerchief-like Ni and NiCo
hierarchical nanostructures were also observed [inset in Fig- nanoalloys,[23] ZnSe flower-like nanoarchitectures,[24] hol-
ure 4(d)], which may be caused by the tight attachment of low CdS nanoboxes,[25] hexagonal, star-shaped and snow-
adjacent urchin-like nanostructures. This result indicates flake-shaped Cu structures,[26] dendrite-like SnS,[27] nano-
that it would not be possible to distinguish the boundary rods and flower-like ZnO structures,[28] and submicrometer-
between two urchin-like nanostructures after a certain reac- sized Cu and Ag crystallites,[29] has a significant effect on
tion time. The corresponding XRD patterns of the Ni sam- the formation, morphology, and structure of the as-ob-
ples obtained at different time intervals are shown in Fig- tained Ni products. We have systematically investigated the
ure 5. Notably, when the reaction time was prolonged, the effect of different amounts of EDA in the range 0–8 mL.
intensity of all the peaks belonging to the Ni fcc phase in- SEM images of the products prepared with different
creased, suggesting that the longer reaction time favors the amounts of EDA are shown in Figure 6. When no EDA is
crystallization of the Ni phase. The corresponding full added, the products take on an irregular congeries ac-
width at half-maximum (FWHM) of the reflections de- companied by a few hexagonal discs [Figure 6(a)]. With
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L.-P. Zhu et al.
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0.5 mL EDA, undeveloped urchin-like spheres as well as
irregular nanosheets are obtained [Figure 6(b)]. With
1.5 mL EDA, a high yield of uniform magnetic Ni chain-
like architectures assembled from urchin-like hierarchical
nanostructures can be obtained, as shown in Figures 2 and
3. When the amount of EDA was increased to 2.0 mL, some
separate urchin-like hierarchical nanostructures could also
be observed in addition to chain-like architectures [Fig-
ure 6(c)]. When the amount of EDA was increased to
4.0 mL or more, separate urchin-like hierarchical nanos-
tructures were obtained as the dominant product [see Fig-
ure 6(d)]. These results indicate that the amount of EDA
plays an important role in determining the morphology of
the products. In general, a relatively high amount of EDA
does not favor formation of chain-like architectures but
causes formation of dispersive urchin-like nanostructures.
We also tested effects of other synthetic parameters, such
as the amount of ethanol, on the morphology of the prod-
uct and found that ethanol is also an important factor in
determining the shape of the final product. For example,
when ethanol is replaced by deionized water while other
parameters are kept unchanged, neither chain-like architec-
tures nor urchin-like nanostructures can be obtained, only
Ni microrods and nanoparticles are observed, as shown in
Figure 6(e). On the basis of the literature[22–29] and the in-
Figure 6. SEM image of Ni particles obtained with various
vestigations described above, we believe that EDA plays two amounts of EDA: (a) 0 mL, (b) 0.5 mL, (c) 2.0 mL, (d) 4.0 mL, (e)
1.5 mL and without ethanol, and (f) FTIR spectrum of the Ni sam-
major roles in our system. On the one hand, EDA can coor-
ples hydrothermally treated in the presence of 1.5 mL EDA at
dinate with nickel ions to form [Ni(EDA)3]2+ complexes,
115 °C for 4 h and unwashed before measurement.
which can decrease the free Ni2+ concentration in solution
and result in the slow generation of Ni nanoparticles. Reac-
Ni2+ + 3EDA Ǟ [Ni(EDA)3]2+
(1)
tion velocity can be adjusted through the complexation
slow-release method, which can regulate the kinetics of nu-
cleation and growth of the products and further efficiently
control the morphology and structure of the final products.
On the other hand, EDA can also serve as a shape modifier
and controller, which may bind to certain crystal faces of
the nickel particles through its –NH2 function. The FTIR
spectrum of the sample [Figure 6(f)] exhibits characteristic
absorptions at 1635.4 cm–1 (δNH) and 3473.3 cm–1 (νNH),
which are shifted to lower frequencies relative to those of
free EDA, denoting some interaction between metal and
ligand. The interaction between metal and ligand could
force the nanoparticles to be assembled. Such a role of
EDA has been reported in the literature.[22–29] Thus, the for-
mation of the Ni chain-like architectures consisting of ur-
2[Ni(EDA)3]2+ + N2H4 + 4OH– Ǟ
2Niȇ + 6EDA + N2Ȇ + 4H2O (2)
Figure 7. Schematic illustration of the formation process of the
chain-like Ni architectures assembled from urchin-like hierarchical
nanostructures.
In the beginning (step A), Ni2+ in solution reacts
chin-like hierarchical nanostructures in hydrothermal con- first with EDA to form a relatively stable complex,
ditions would result from nucleation and continuous as- [Ni(EDA)3]2+, because of its strong affinity to Ni2+ at room
sembly through an oriented-attachment mechanism[31] as- temperature. Afterwards, in basic solution, the complex is
sisted by EDA.
treated with N2H4 causing Ni nucleation and release of
On the basis of previous studies of nickel/cobalt nano- EDA by a hydrothermal process (step B). The freshly crys-
structures from different complexes[3c,11,12,18] and the above talline nanoparticles are unstable because of their high sur-
results of the present study, we believe that EDA and reac- face energy and tend to aggregate and form larger micro/
tion time play important roles in the formation of chain- mesospheres (step C); this process is driven by the minimi-
like hierarchical architectures. A possible formation process zation of interfacial energy. Accompanied by the formation
involving the nucleation-assembly-further growth mecha- of Ni micro/nanoparticles is the magnetic dipole–dipole at-
nism is schematically illustrated in Figure 7. The chemical traction between them. The larger Ni micro/nanoparticles
reaction in the process to obtain Ni chain-like hierarchical were then assembled into chain-like assemblies (step D) be-
architectures could be formulated as follows:
cause of the stronger anisotropic magnetic forces[11] and the
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Template-Free Synthesis of Magnetic Chains
effects of EDA. This process is possible only when the mag-
netization of the particle does not fluctuate during the time
of magnetic interaction in the suspension.[32] After the for-
mation of chain-like structures, the micro/mesospheres that
had assembled into the chain structures continued the ori-
ented attachment process by combining with the remaining
primary particles in the presence of the released EDA, fi-
nally forming the chain-like hierarchical architectures as-
sembled from urchin-like nanostructures (step E). In our
synthesis, EDA serves as ligand and shape inducer. The for-
mation of the [Ni(EDA)3]2+ complex sharply decreased the
free Ni2+ concentration in the solution, which resulted in a
relatively low reaction rate of Ni2+ ions with N2H4 and OH–
ions. A slow reaction rate caused the separation of nucle-
ation and growth steps, which is crucial for high-quality
crystal synthesis. The EDA decomposed from [Ni(EDA)3]2+
can act as quasi-surfactants/templates to induce the further
growth and assembly of nanoparticles into sword-like petals
on the micro/mesosphere surfaces. The sword-like petals
could organize so well that they exhibited the features of
single crystals, as shown in Figure 3(c) and (d). Our time-
dependent experiments support the nucleation-assembly-
further growth mechanism suggested above; it was deter-
mined that the assembly process occurs after the formation
of the chain-like architectures.
Magnetic measurements of the as-synthesized nickel
chain-like architectures were carried out at room tempera-
ture in an applied magnetic field sweeping from –10 to
10 kOe. The hysteresis loop of the nickel chain-like architec-
tures consisting of urchin-like hierarchical microspheres is
shown in Figure 8. The inset in Figure 8 shows the magni-
fied hysteresis loop at low applied field. The sample shows
hysteresis behavior, revealing that the nickel chain-like
architectures are ferromagnetic. The saturation magnetiza-
tion (Ms), remanent magnetization (Mr), and coercivity
(Hc) for the sample are 52.58 emu/g, 5.82 emu/g, and
211.67 Oe, respectively. Compared to the coercivity value of
bulk Ni (ca. 100 Oe)[33] and that of hollow Ni nanometer
spheres (ca. 102 Oe),[14b] dandelion-like (ca. 130 Oe),[20a] or
flower-like (ca. 173 Oe)[18] Ni nanostructures at room tem-
perature, the chain-like Ni architectures consisting of ur-
chin-like nanostructures exhibit an enhanced value. But this
value was lower than that of the one-dimensional nickel
nanorods (ca. 264 Oe)[10] or nanobelts (ca. 640 Oe)[10] with
high anisotropy. It is well known that the physical and
chemical properties of magnetic materials strongly depend
on the size, structures, and dimensions of the magnetic par-
ticles. The shape of the hysteresis loop may be strongly af-
fected not only by specific particle surface area but also by
the magnetic anisotropy.[34] An expression for the magne-
tostatic energy Ems of the magnetic materials in the shape
of a prolate spheroid with semi-major axis c and semi-
minor axes of equal length a has been given as follows:[35]
Figure 8. Hysteresis loop for the chain-like Ni architectures as-
sembled from urchin-like hierarchical nanostructures at room tem-
perature.
The shape-anisotropy constant Ks is given by
Ks = 1/2(Na – Nc)M2
(4)
As equation (4) shows, the strength of shape anisotropy
depends on the axial ratio c/a of the specimen. Symmetri-
cally shaped nanoparticles, such as spheres, do not have any
net shape anisotropy. However, the synthesized representa-
tive chain-like Ni architectures with an aspect ratio of about
10 on average would have shape anisotropy, which will in-
crease the coercivity. So, the different coercivity may be at-
tributed to the shape anisotropy of chain-like Ni architec-
tures, which prevent them from magnetizing in directions
other than along their easy magnetic axes,[35] hence leading
to the enhanced coercivity. As for the saturation magnetiza-
tion value of the sample, it is found to be little lower than
that of the bulk nickel (55 emu/g).[33] This may be due to the
spin disorder on the surface and the magnetic interaction
between the building blocks, which would reduce the total
magnetic moment.[36]
Conclusions
Magnetic metallic Ni chain-like architectures assembled
from urchin-like hierarchical nanostructures have been suc-
cessfully prepared by a simple hydrothermal reduction pro-
cess without any template or surfactant. The urchin-like
nanostructures are composed of dozens of well-aligned
metallic Ni sword-like nanopetals growing radially from the
surfaces of the spherical particles. A growth mechanism for
the formation of self-assembled chain-like architectures as-
sembled from urchin-like hierarchical nanostructures has
been proposed. A significantly enhanced magnetic coerciv-
ity has been observed for the magnetic Ni chain-like archi-
tectures compared to the bulk metal Ni. The present hydro-
thermal route can be easily controlled. Hence it can be ex-
tended to the preparation of complex hierarchical struc-
tures from other metals, alloys, or compounds.
Ems = 1/2M2Nc + 1/2(Na – Nc)M2sin2θ
(3)
Experimental Section
where M is the applied magnetic field, θ is the angle of the
applied magnetic field to c, Na and Nc are demagnetizing
coefficients along a and c, respectively.
All reagents were of analytic grade and used without further purifi-
cation. In a typical experiment, NiCl2·6H2O (0.72 g) was dissolved
in deionized water (30 mL) by intensive stirring for 2 h, and a
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Acknowledgments
This work is supported by the Overseas Outstanding Scholar Foun-
dation of the Chinese Academy of Sciences (Grant Nos.: 2005-1-3
and 2005-2-1), the Specialized Research Fund for Outstanding
Young Teachers in Shanghai Higher Education Institutions (Grant
No.: egd08013), and the Innovation Program of the Shanghai Mu-
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Received: September 7, 2009
Published Online: February 2, 2010
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