Materials Science and Engineering B 158 (2009) 7–12
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Materials Science and Engineering B
journal homepage: www.elsevier.com/locate/mseb
Synthesis and magnetic studies of flower-like nickel nanocones
Ambily Mathew a , N. Munichandraiah b , G. Mohan Rao a,∗
a
b
Department of Instrumentation, Indian Institute of Science, Bangalore 560012, India
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history:
Received 19 June 2008
Received in revised form
18 November 2008
Accepted 22 December 2008
Keywords:
Nickel nanocone
Magnetic properties
Morphology evolution
a b s t r a c t
Flower-like nickel nanocone structures are synthesized by a simple chemical reduction method using
hydrazine hydrate as the reducing agent. The structure, morphology and magnetic properties of as synthesized products are studied by X-ray diffraction (XRD), field emission scanning electron microscopy
(FESEM), transmission electron microscopy (TEM) and SQUID magnetometer. The morphology evolution is studied by varying the reaction temperature and concentration of nickel chloride keeping other
conditions unchanged.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
In recent years controlling the morphology of nanostructures
has been the subject of many studies due to the shape effect
of these particles on their properties [1]. So far, various studies
have been attempted to grow 2D and 3D organized structures by
manipulation of individual units, which is a crucial step towards
utilizing their magnetic, optical, catalytic and electronic properties [2–7]. For example, flower-like cobalt nanocrystals by a
complex precursor reaction route [8], noble metal dendrites via
a simple mixed surfactant route [9], controlled synthesis of copper nanostructures under a direct current electric field treatment
[10] and the synthesis of indium hollow spheres and nanotubes
by a simple template-free solvothermal process [11] have been
reported.
Being an important ferromagnetic material, anisotropic Ni
nanoparticles are expected to exhibit interesting magnetic properties [12]. Hence Ni nanostructures have potential applications
in magnetic sensors and memory devices [13]. Many groups have
synthesized nickel with different morphologies such as nanoparticles, nanodots, nanowires, nanorods, nanocones and nanofibres by a
variety of methods like hydrothermal reduction, electrodeposition
and template-based methods [14–17]. Recently, Cauliflower-like Ni
structures via chemical solution method [18], flower-like structures with petals composed of Ni nanotips [19] and hexagonal Ni
nanoplatelets via hydrothermal method [20] have been reported.
With an exception of cauliflower like structure, the flower like Ni
structures exhibit enhanced magnetic properties compared to bulk
∗ Corresponding author. Tel.: +91 80 22932349.
E-mail address: [email protected] (G. Mohan Rao).
0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.mseb.2008.12.032
nickel. So great attention has been given to grow flower like 3D
structures by assembling individual units, expecting an enhancement in the magnetic properties.
In the present paper, we report the synthesis of hierarchical
flower-like Ni nanocones via a simple chemical reduction method
without using any template or surfactant. The morphology of Ni
nanostructures can be readily tuned by adjusting the experimental
parameters. Since this method is simple and controllable, it can be
used for commercial applications.
2. Experimental
All chemicals used in this experiment were of analytical grade
and used without further purification. In a typical procedure for
the preparation of flower like nickel nanocones 2.37 g of NiCl2 ·6H2 O
was dissolved in 25 ml of ethanol to obtain a green transparent solution. Then this solution was added drop wise to 25 ml hydrazine
hydrate (N2 H4 ·H2 O) solution under stirring to form a purple colored solution. The pH of the solution was adjusted to 12 by adding
4 M NaOH and heated to 60 ◦ C. In about 10 min, the color of solution started turning black indicating the formation of Ni particles.
The stirring was continued till the solution became clear and all the
particles were attached to the surface of the Teflon covered magnetic bar. The particles were washed with de-ionized water three
times and dried at 60 ◦ C in air for 1 h. Samples were also prepared by
varying the concentration of NiCl2 ·6H2 O and the reaction temperature, and the details of preparation conditions are given in Table 1.
Samples numbered A–I correspond to different synthesis conditions listed in Table 1. All the experiments were repeated to ensure
the reproducibility of microstructures.
The phase purity of the products was characterized by X-ray
diffraction (XRD, PHILIPS ANALYTICAL XPERT PRO) using Cu K␣
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A. Mathew et al. / Materials Science and Engineering B 158 (2009) 7–12
Table 1
Preparation conditions for different Ni samples. Volume of hydrazine hydrate: 25 ml;
concentration of NaOH: 4 M; pH: ∼12.
Sample
[NiCl2 ·6H2 O]
Temperature (◦ C)
Morphology
A
B
C
D
E
F
G
H
I
0.2 M
0.2 M
0.2 M
0.4 M
0.8 M
0.4 M
0.4 M
0.8 M
0.8 M
RT
60
90
60
60
RT
90
RT
90
Spherical
Spiky surface
Flower like
Flower like
Flower like
Irregular
Flower like
Aggregated spheres
Spherical
Fig. 2. (a and b) FESEM image of a typical flower like sample. EDAX spectrum taken
from the nanocones is shown in (b).
Fig. 1. XRD pattern of a typical flower like Ni nanocones.
(1.5406 Å) radiation. The morphology and elemental analysis of
the products were studied by field emission scanning electron
microscopy (FESEM, SIRION 200) energy dispersive X-ray analysis
(EDAX) and transmission electron microscopy (TEM, TECNAI F30).
Magnetic properties were measured using a SQUID (MPMS XL-5)
magnetometer.
3. Results and discussion
Fig. 1 shows the XRD pattern of sample A. The diffraction peaks
can be indexed as fcc Ni with lattice constant a = 0.3523 nm (JCPDS
File No.040850), indicating the crystalline nature. No characteristic
peaks of impurities such as nickel oxide or hydroxide are detected.
Also the XRD pattern shows the presence of only one phase. This
demonstrates a high purity of the as synthesized nickel powder.
Similar XRD patterns are obtained for Ni samples prepared by varying the concentration of NiCl2 and temperature.
Fig. 3. (a) TEM image of a typical flower like nanocone. Inset is the image of a single petal with cone shape. (b) HRTEM image taken from the same petal. (c) Magnified image
of the marked portion in (b). (d)SAED pattern.
A. Mathew et al. / Materials Science and Engineering B 158 (2009) 7–12
Fig. 2(a) shows the FESEM image of a flower-like nanocone (sample D). As shown in the figure the morphology is flower like with
petals made of nanocones, having diameter 50–160 nm at the root
and length of 100–200 nm from root to tip. The chemical composition (atomic percent) of flower like nickel nanocones determined
by EDAX analysis is shown in Fig. 2(b). In addition to the nickel
peak, an oxygen peak is also present which might be attributed to
the surface oxidation of nickel by oxygen because of high reactivity
of the elemental nickel.
The flower-like architecture was further characterized by TEM
equipped with selective area electron diffraction. Fig. 3(a) represents the typical TEM image of the as synthesized flower-like nickel
nanocones. The image of a single petal given in the inset shows the
cone shape of the petal with sharp edges. More details of the flowerlike structure were investigated by HRTEM (Fig. 3(b) and (c)) and
SAED (Fig. 3(d)). A typical petal was chosen as the object of investigation. Fig. 3c shows some magnified region of Fig. 3b, which clearly
shows the distance between the parallel fringes is about 0.2 nm, corresponding to the well recognized lattice spacing of {1 1 1} atomic
planes of Ni. The SAED pattern also confirms the presence of {1 1 1}
planes of Ni.
In our preliminary experiments, it was found that the formation of nickel nanaoparticle was not complete even after 6 h at
room temperature. However at 60 ◦ C it took only a few minutes to
precipitate nickel particles. This is because the reducing power of
hydrazine hydrate depends on temperature and it is reported that
it can reduce nickel from solution within 2 min at 60 ◦ C [21]. Also
previous studies have shown that highly alkaline medium is essential for the formation of anisotropic nickel nanostructures [22]. So
depending on the concentration of nickel chloride we have varied
the amount of sodium hydroxide to maintain the pH ≥ 12.
9
In order to shed light on the possible formation mechanism we
have prepared Ni at room temperature, 60 ◦ C and 90 ◦ C using 0.2 M
NiCl2 solution. At room temperature, we got Ni powder (sample A)
having spherical structure with diameter ranging from 150 nm to
300 nm which is shown in Fig. 4(a). As the temperature increased to
60 ◦ C, the spherical particles became bigger in diameter and their
surface seems to be spiky as shown in Fig. 4(b) (sample B). At a
temperature of 90 ◦ C we got Ni particles (sample C) having flowerlike morphology, within a few minutes, as shown in Fig. 4(c).
Also to see the effect of precursor concentration, we have varied
the concentration of NiCl2 by keeping the reaction temperature at
60 ◦ C (sample D and E) and compared with sample B. The results
are shown in Fig. 5(a)–(f). In the case of sample B we were not able
to observe any flower-like morphology. Instead it seems to be the
beginning stage of flower-like structures. However, the samples D
and E exhibit flower-like morphology (Fig. 5(c)–(f)) at 60 ◦ C itself.
The flowers seem to be more agglomerated in the case of sample D.
From the above observation, it could be concluded that low temperature and low concentration of NiCl2 do not favor the formation of
flower-like Ni nanocones and the morphology of Ni nanostructures
is highly influenced by initial concentration of Ni2+ ions which in
turn depends on temperature.
To investigate the effect of precursor concentration at different temperatures we have conducted a series of experiments.
Fig. 6(a)–(d) shows the FESEM image of sample F, G, H and I. As
expected no flower like structures were formed at room temperature, whatever the precursor concentration may be. Though we got
flower like structures for sample G, the petals seem to be aggregated. Surprisingly we could not observe any flower like structure
for sample I. Instead we got pure spherical particles. This observation leads to the conclusion that for a given concentration there is a
Fig. 4. (a–c) FESEM images of samples A–C showing variation of morphology of Ni nanostructures with temperature.
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A. Mathew et al. / Materials Science and Engineering B 158 (2009) 7–12
Fig. 5. (a–f) FESEM images of sample, D, E showing the variation in morphology with concentration of nickel chloride under different magnification.
temperature limit for the formation of flower like structures. Above
this temperature the flowers aggregate to form spherical particles
possibly due to a faster reducing rate.
However in addition to concentration of NiCl2 and the reaction
temperature, we believe that various other factors like concentration of NaOH, the use of hydrazine hydrate as reducing agent and the
continuous magnetic stirring also influence the formation of flowerlike structures. In a previous study conducted by Yong Wang et al.
[24], it was confirmed that the use of hydrazine hydrate as reducing
agent is essential for the formation of hierarchical nanostructures.
So a further investigation is necessary to explain the factors responsible for the formation of these special nanostructures.
Based on the above results, we suggest that the formation of
flower-like structures consists of two steps: the formation of a solid
core and subsequent growth of nanoparticles on the surface in the
form of nanocones. At the early stage a certain number of nickel particles are produced and as the reduction proceeds the newly formed
particles spontaneously transfer to the surface of existing particles in all directions, which results in the formation of nanospheres
(sample A). The reaction rate is slowed down at this stage because
of the lesser concentration of nickel particles which leads to under
saturation around the existing particle and the continuous addition of nickel particles occurs on the circumferential edges since
these edges are having higher free energies [24] leading to the formation of spiky surface (sample B). These spikes act as seeds for
the growth of nanocones resulting in the formation of flower like
structures (sample C).
It is believed that the magnetic properties of nanomaterials are
closely related to size, morphology, crystallinity and composition.
Also it is known that the presence of shape anisotropy can significantly enhance the magnetic properties [20]. Plots of magnetization
versus magnetic field for samples A and D recorded at 300 K are
shown in Fig. 7(a) and (b), respectively. Both samples show hysterisis loop, revealing the ferromagnetic nature with coercivity (Hc ),
saturation magnetization (Ms ) and remnant magnetization values
136.30 Oe, 49.18 emu/g and 6.10 emu/g, respectively, for sample A;
and 213.73 Oe, 50.62 emu/g and 9.79 emu/g, respectively for sample D. Compared to spherical Ni particles, the magnetic properties
show enhancement in the case of flower-like structures. The saturation magnetization of sample D is very close to that of bulk Ni
(55 emu/g) [23]. Compared to the Hc value of bulk Ni (100 Oe) [25],
both the samples exhibit a much enhanced coercivity and the Hc
A. Mathew et al. / Materials Science and Engineering B 158 (2009) 7–12
11
Fig. 6. (a–d) FESEM images of sample F, G, H and I.
Fig. 7. Hysterisis loops of (a) sample A and (b) sample D. An enlargement of magnetic field from −2500 to 2500 for each figure is given in the inset.
value of flower-like structure is much higher compared to that of
spherical nanoparticles. This may be due to their special structure
having high shape anisotropy.
ture exhibits enhanced magnetic properties compared to spherical
nanostructures.
Acknowledgements
4. Conclusions
In summary, a simple facile method to synthesize flower-like
Ni nanocones by the reduction of nickel hydrazine complex in
presence of NaOH has been demonstrated. The possible formation
mechanism is suggested based on the experimental observations.
In addition, the effect of morphology on magnetic properties is also
investigated and it is found that nickel with flower like architec-
We would like to thank the Nano centre, Department of IPC, and
Department of Physics, Indian Institute of Science, Bangalore for
providing experimental facilities.
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