Fabrication of monodispersed nickel flower-like architectures via a solvent-thermal process and analysis of their magnetic and electromagnetic properties

Article abstract of DOI:10.1016/j.jssc.2011.09.022

Monodispersed Ni flower-like architectures with size of 12 μm were synthesized through a facile solvent-thermal process in 1,2-propanediol solution in the presence of polyethylene glycol (PEG) and sodium alkali for electromagnetic absorption application. The Ni architectures are composed of nanoflakes, which assemble to form three dimensional flower-like structure, and the thickness of nanoflakes is about 1040 nm. A possible formation mechanism for Ni flower-like architectures was proposed and it was confirmed by the control experiments. The Ni architectures exhibited a saturation magnetization (M _s) of 47.7 emu/g and a large coercivity (H_cj) of 332.3 Oe. The epoxy resin composites with 20 vol% Ni sample provided good electromagnetic wave absorption performance (reflection loss <-20 dB) in the range of 2.86.3 GHz over absorber thickness of 2.65.0 mm.

Full text of DOI:10.1016/j.jssc.2011.09.022

Journal of Solid State Chemistry 184 (2011) 2994–3001
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Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Fabrication of monodispersed nickel flower-like architectures via a
solvent-thermal process and analysis of their magnetic and
electromagnetic properties
Jing Kong a, Wei Liu b,n, Fenglong Wang a, Xinzhen Wang a, Liqiang Luan b, Jiurong Liu a,nn, Yuan Wang a,
Zijun Zhang a, Masahiro Itoh c, Ken-ichi Machida c
a Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education and School of Materials Science and Engineering, Shandong University,
Jinan 250061, People’s Republic of China
b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China
c Center for Advanced Science and Innovation, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Monodispersed Ni flower-like architectures with size of 1–2 mm were synthesized through a facile
Received 2 June 2011
solvent-thermal process in 1,2-propanediol solution in the presence of polyethylene glycol (PEG) and
sodium alkali for electromagnetic absorption application. The Ni architectures are composed of
nanoflakes, which assemble to form three dimensional flower-like structure, and the thickness of
nanoflakes is about 10–40 nm. A possible formation mechanism for Ni flower-like architectures was
proposed and it was confirmed by the control experiments. The Ni architectures exhibited a saturation
magnetization (Ms) of 47.7 emu/g and a large coercivity (Hcj) of 332.3 Oe. The epoxy resin composites
with 20 vol% Ni sample provided good electromagnetic wave absorption performance (reflection loss
o ꢀ20 dB) in the range of 2.8–6.3 GHz over absorber thickness of 2.6–5.0 mm.
Received in revised form
8 September 2011
Accepted 14 September 2011
Available online 17 September 2011
Keywords:
Nickel flower-like architecture
Solvent-thermal
Nanoflake
& 2011 Elsevier Inc. All rights reserved.
Anisotropic field
Electromagnetic wave absorption
1. Introduction
ripening in a facile hydrothermal condition [12]. Liu et al. synthe-
sized flower-like Ni–Fe alloy nanostructures with an anisotropic
The recent researches in nanotechnology have witnessed an
increasing trend toward fabrication of nanoscale inorganic mate-
rials with one-, two-, and three-dimensional (3D) structures,
owing to their shape-property correlation that has been revealed
in various fields [1–6]. Novel 3D nano/micro-structures con-
structed by low-dimensional nanoscale building blocks, such as
nanoparticles, nanocones, nanorods, nanoribbons, etc., have
attracted extensive attentions, because complex structures with
anisotropic assembly are believed to exhibit unusual properties
and may have potential applications as future nano/micro-devices
[7–10]. As a ‘‘bottom-up’’ synthesis approach, the solution-based
procedures have been proved effective in simple fabrication of
self-assembled 3D architectures because of their potential to
allow for precise control over the morphology and size of
growth of nanorods along their easy magnetic axis under kinetic
control [13]. Narayanaswamy et al. also reported that ZnO, MnO
or CoO nanocrystals assembled to form flower-like structures in a
common crystallographic orientation [14]. The stable 3D flower-
like structures can offer potential applications for the develop-
ment of high performance electronic, optoelectronic, and sensing
devices.
In addition to being utilized as catalyst, magnetic recording
media, medical diagnosis, and electrode [15–17], nickel nano/
micro-structures as electromagnetic (EM) wave absorption mate-
rials have drawn extensive research attention with the ever-
accelerated development of wireless communications and high
frequency circuit devices. Recently, different nickel based nano/
micro-structures such as nanospheres, urchinlike chains,
branched nanowires, polyaniline coated Ni, and carbon coated
Ni nanocapsules have been synthesized and their EM wave
absorption has also been investigated in GHz range [18–25]. The
experimental results indicated that the EM absorption character-
istics, i.e., frequency, thickness, and absorbing band-width are
strongly related to Ni micro-structures. Wang et al. synthesized
nickel nanomaterials with different morphologies including
superfine nanoparticles, rings, smooth chains, and urchinlike
architectures [11]. Self-assembled 3D flower-like hierarchical
b-
Ni(OH)2 hollow architectures composed of nanosheets were
synthesized by Zhu et al. via the stepwise growth and Ostwald
n Corresponding author.
nn Corresponding author. Fax: þ86 531 88392315.
E-mail addresses: jrliu@sdu.edu.cn (W. Liu), weiliu@sdu.edu.cn (J. Liu).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.09.022

J. Kong et al. / Journal of Solid State Chemistry 184 (2011) 2994–3001
2995
chains by adjusting the reaction conditions [20]. Urchinlike nickel
chains showed more effective EM absorption in contrast with
other structures mainly due to the large aspect ratio of the
aciculaes on the nickel chain surface. The incident EM wave will
be multi-scattered in the presence of aciculaes resulting in
repeated absorption just as the wall in an EM wave darkroom
consisting of cone-shaped absorbers. EM absorption properties of
aloe-like nickel flowers assembled on hollow glass spheres have
also been investigated [26], but the material only gave weak
absorption (RL4 ꢀ20 dB) in GHz range because the addition of
non-magnetic component decreased the saturation magnetiza-
tion (Ms) of the composites thus affecting its EM absorption. The
above results suggest that pure nickel 3D flower-like nano/micro-
structures will probably provide more efficient EM wave absorp-
tion due to their high Ms, large aspect ratio of flake-like petal and
large surface area. To the best of our knowledge, few contribu-
tions have been presented exploring Ni flower-like nano/micro-
structures composed of nanoflakes as EM absorption materials.
Wet chemical procedures exploring hydrazine as reducing
agent have been widely investigated to synthesize Ni flower-like
structures assembling from nanocones, swordlike petals, or col-
umns made up of aligned nanoplates [27–30]. In the above works,
the oriented growth of Ni building blocks intensively relies on the
mediated surfactant or structure-directing agent. However, the
assembled nanostructures are not monodispersed or have broad
size distribution, which will affect their practical applications. In
surfactant-free synthesis method, flower-like Ni nanostructures
could be obtained under a rotating magnetic field [31], but this
approach usually suffers from high cost and complicated opera-
tion. Bai et al. also fabricated Ni flower-like clusters composed of
nanoflakes employing glycol as both solvent and reducing agent
[32], however, the flower-like structures with broad size distribu-
tion are not apparent. More recently, Guan’s group also synthe-
sized Ni flower-like nanostructure through a polyol reduction
process without any surfactant. The introduction of water to the
polyol system plays a key role in the formation of the flower-like
nanostructure although the exact reaction mechanism is not
clear. In this synthesis process, the addition amount of water
needs to be precisely controlled and long reaction time (12 h) is
necessary [33].
stirring. The green solution became purple emulsion with the
addition of 2 M sodium alkali. After magnetic stirring for 1 h, the
purple emulsion was transferred into a 60 mL Teflon-lined auto-
clave, and subsequently sealed and heated at 443 K for 3 h in an
oven. The black product was centrifuged and washed with
deionized water and ethanol for five times, respectively, and then
dried under vacuum overnight for further characterization. To
investigate the formation mechanism of the flower-like architec-
tures, we carried out a series of experiments by varying the
reaction time and the addition amount of sodium alkali and PEG.
2.2. Characterizaion
X-ray diffraction (XRD) pattern was obtained by a Rigaku
Dmax-rc X-ray diffractometer with Ni filter CuK
a radiation
(V¼40 kV, I¼50 mA) at a scanning rate of 41/min. The micro-
structures of the resultant products were examined using a JSM-
6700F field emission scanning electron microscope (FE-SEM) at an
accelerating voltage of 20 kV and electric current of 1.0 ꢂ 10ꢀ10 A,
and a Hitachi H-800 transmission electron microscope (TEM) at
an accelerating voltage of 150 kV. To measure the relative
permeability (mr) and permittivity (er) values, epoxy resin com-
posites were prepared by homogeneously mixing epoxy resin
with 20 vol% Ni flower-like powders and compressing into
cylindrical-shaped compacts. After cured at 333 K for 60 min,
these compacts were cut into toroidal-shaped samples with outer
diameter of 7.00 mm and inner diameter of 3.04 mm. The mea-
surements were carried out with a vector network analyzer
(Agilent Technologies E8363A) in the range of 0.05–18.05 GHz.
The frequency dependence of reflection loss (RL) was calculated
from the measured mr and er at the given frequency and absorber
thickness.
3. Results and discussion
3.1. Structure and morphology
The phase and purity of the as-synthesized nickel flower-like
architectures was investigated by XRD. The characteristic peaks of
XRD pattern (Fig. 1(a)) could match well with face-centered cubic
(fcc) nickel (JCPDS 04-0850), and no peak of any other phase is
detected indicating that pure nickel flower-like architectures have
been prepared in this solvent-thermal process. In addition, we
found that the peak intensity of (111) face is much stronger than
the normal level. The FE-SEM and TEM images provide further
insight into the structure and morphology of the as-synthesized
nickel architectures. From the panoramic SEM image (Fig. 1(b)),
Herein, we report a novel facile solvent-thermal approach to
fabricate monodispersed flower-like Ni architectures, in which
1,2-propanediol was employed as solvent and reducing agent
with the assistance of sodium alkali. Through controlling the
growth rate, 2D building blocks (nanoflakes) were obtained due
to the oriented growth of Ni crystallites along crystallographically
specific direction. A possible formation mechanism was proposed
based on a series of control experiments. The magnetic and EM
wave absorption properties of the monodispersed flower-like Ni
architectures were investigated.
Ni formed flower-like architectures with size of 1.0–2.0 mm. Close
inspection reveals that the nickel architectures are built of
nanoflakes, which intersect with each other in the center to form
the three dimensional flower-like structures (Fig. 1(c)), and the
thickness of nanoflake is ca. 10–40 nm. The prepared architec-
tures are very stable, and even two hours of ultrasonication
cannot break them into discrete flakes, indicating that the
flower-like architectures are integrative. The TEM image
(Fig. 1(d)) shows that the obtained flower-like structures have a
solid interior core with nanoflakes growing on it. By aligning the
electron beam perpendicular to one nanoflake, the selected area
electron diffraction (SAED) pattern (inset of Fig. 1(d)) reveals the
single crystalline nature of the nanoflake. In the SAED pattern, six-
fold symmetry near the center could be indexed to {220} diffrac-
tions of fcc nickel, which suggests that the electron beam was
along the [111] zone axis of the fcc crystal structure [29]. The
highly oriented diffraction spots indicate that the surface of two-
dimensional (2D) flake is the (1 1 1) plane, which is in good
2. Experimental details
2.1. Synthesis of monodispersed Ni flower-like architectures
Tetrahydrated nickel acetate (Ni(CH3COO)2 ꢁ 4H2O), polyethy-
lene glycol (PEG), 1,2-propanediol, sodium chloride (NaCl), and
sodium alkali (NaOH) were purchased from Sinopharm Chemical
Reagent Co., Ltd. All reactants were of analytical grade and used as
received without any further purification.
The monodispersed Ni flower-like architectures were synthe-
sized by a facile one-pot solvent-thermal reduction process.
Typically, 1 g polyethylene glycol (PEG) was dissolved in 40 mL
1,2-propanediol, followed by the addition of 2.5 g tetrahydrated
nickel acetate (Ni(CH3COO)2 ꢁ 4H2O) and 1 g NaCl under magnetic

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J. Kong et al. / Journal of Solid State Chemistry 184 (2011) 2994–3001
Fig. 1. XRD pattern (a), SEM (b, c), and TEM (d) images of the as-obtained Ni flower-like architectures. The inset of (d) is SAED pattern of an individual nanoflake.
When proceeded for 90 min, the weak diffraction peaks assigned
to fcc Ni phase appeared, indicating that the reduction of Ni(OH)2
intermediate began to occur. At the same time, a few small
nanoparticles with size less than 400 nm were observed on the
SEM image (Fig. 3(b)). Increasing the reaction time to 100 min,
the intensity of the diffraction peaks of fcc Ni increased and a lot
of nanoparticles less than 1 mm with smooth surface formed as
shown in Fig. 3(c) and (d). When the reaction was continued for
110 min, plenty of spheres with relatively rough surface full of
thin slices could be observed (Fig. 3(e) and (f)), suggesting that
the flower-like structure began to form at the current time.
Further prolonging the reaction time to 130 min, the intensity
of diffraction peaks of fcc Ni (Fig. 2) increased drastically indicat-
ing that most of Ni(OH)2 intermediate has been reduced to Ni,
meanwhile, the surface became more rough with the growth of
flower-like structure (Fig. 3(g) and (h)). When the reaction was
maintained for 180 min, all peaks of XRD pattern could be
assigned to fcc Ni and more homogeneous well-defined flower-
like architectures with smooth nanoflakes were prepared (Fig. 1).
Therefore, the growth of flower-like architectures involves the
evolution processes containing nucleation of crystallites, aggrega-
tion of initial crystallites, growth of slices, and re-crystallization
of primary building units.
Fig. 2. XRD patterns of samples reacted at 443 K for different times.
agreement with the rather strong diffraction peak of (1 1 1) in the
XRD pattern.
3.2. Formation mechanism
On the basis of the investigation and analysis above, the
possible formation mechanism of nickel flower-like architectures
was proposed in Fig. 4. Firstly, Ni(OH)2 with poor crystallization
was produced through the reaction among the raw materials.
With the progress of reaction, the original Ni(OH)2 intermediate
nucleated to form nanoparticles, and the tiny particles agglom-
erate into larger spheres to reduce the surface energy. In this
solvent-thermal synthesis, 1,2-propanediol is used as solvent and
reducing agent for Ni2þ. It initially dehydrates to produce
propionaldehyde and then reacts with Ni2þ generated from
Ni(OH)2 intermediate to form primary Ni atoms [34,35]. Follow-
ing the aggregation of nanoparticles into spheres, the number of
free Ni2þ ions released from Ni(OH)2 in solution reduced a lot;
thereafter, relatively small thin slices grew slowly since the Ni
atoms will preferentially adsorb and bond to the active sites of
In order to gain insight into the formation mechanism of the
flower-like nickel architectures, time-dependent experiments
were performed. The morphology and phase of products were
studied by SEM, TEM, and XRD measurements. After 80 min, a
green suspension was obtained, and the XRD pattern (Fig. 2)
presents the characteristic of poorly crystallized Ni(OH)2 without
any shaped particle (Fig. 3(a)). Generally, Ni(OH)2 has two
different phases:
metastable phase, can be spontaneously transformed to
in strong alkaline condition [33]. The diffraction peaks could be
mainly indexed to -Ni(OH)2 after 80 min reaction, while a weak
diffraction peak (2
¼231) is assigned to the (006) plane of
phase, which disappeared with increasing the reaction time.
a
phase and
b
phase.
a
phase, which is a
b
phase
b
y
a

J. Kong et al. / Journal of Solid State Chemistry 184 (2011) 2994–3001
2997
Fig. 3. SEM and TEM images of samples reacted at 443 K for times of the following: (a) 80 min, (b) 90 min, (c, d) 100 min, (e, f) 110 min, and (g, h) 130 min.
Fig. 4. Schematic for the formation mechanism of Ni flower-like architectures.

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J. Kong et al. / Journal of Solid State Chemistry 184 (2011) 2994–3001
small protuberances (shown in Fig. 3(e) and (f)), which have
relatively higher free energy on the sphere surface [28]. Crystal
faces with high surface energy exhibit the fastest growth rate and
are minimized or even disappear in the final morphology [36].
Wulff’s rule [37] illustrates the equilibrium morphology of a
crystal as being given by the minimum surface energy of all
exposed crystal faces and this thermodynamic model can be used
to explain the oriented growth of slices in this synthesis process.
In the current system, the crystalline growth of slices is prefer-
entially along the /110S or /100S direction, forming the top/
bottom faces of {111} planes with the lowest surface energy for Ni
fcc crystal structure [30]. The similar anisotropic growth to 2D
crystal nanosheets has also been observed in other fcc materials
[38,39]. Finally, the flower-like nickel architectures comprised of
sing-crystal flakes formed through oriented growth of slices at the
expense of adjacent smaller slices/nanoparticles and the meta-
stable crystallites in the sold interior core, which can be referred
to the Ostwald ripening process [40,41]. In the ripening process,
larger flakes with a relative small surface energy are thermo-
dynamically favored and the unstable tiny crystals dissolved to
provide the source material for the growth of flakes during the
dissolution and recrystallization process. Specifically, the dis-
solved nickel atoms may continuously attach and bond to the
surface of larger flakes, and spread the {111} planes in order to
achieve a minimum total free energy [42].
observed (Fig. 5(a)). With the NaOH concentration increasing to
1.5 M, the quantity of nanoflakes constituting one nickel flower
increased, nevertheless, the nanoparticles with diameter of ca.
200 nm still existed (Fig. 5(b)). When 4 M NaOH was added to the
solution, the Ni architectures (1.0–3.0 mm) with prickly surface
composed of short nanoplates with the thickness of 30–60 nm
were prepared as shown in Fig. 5(c). Our experimental results
confirmed that the increase of alkali concentration could improve
the reducing ability of 1,2-propanediol. The deposition rate is
accelerated with increasing NaOH concentration enhancing the
nucleation and crystal growth of Ni, which kinetically affects the
formation of Ni architectures, and thus the Ni morphology can be
controlled by varying NaOH concentration in the reaction. In this
study, the well-defined Ni flower-like architectures with smooth
flakes were obtained with the concentration of NaOH at 2 M.
PEG, a kind of nonionic surfactant, has been widely used to
facilitate the preparation of monodispersed nano/micro-particles
[45,46]. In order to investigate the effect of PEG on the final
morphology, the control experiment was performed in the
absence of PEG. Experimental results showed that monodispersed
Ni flower-like structures were not obtained in addition to a lot of
particles with a wide size distribution (Fig. 6). When the PEG
addition amount increased to 2 g, flower-like Ni architectures
with larger size of ca. 2–3 mm formed, and the quantity of
nanoflakes constituting one nickel flower increased (shown in
Fig. S2). It is believed that PEG plays an important role as the
surface modifier for the formation of flower-like structures. PEG
physically absorbed on the {111} planes of Ni subunits can inhibit
the growth of {111} planes resulting in the oriented growth of
nanoflakes during the reaction process. Xu et al. also reported the
3D NiS flower-like architectures assembled from the sheet-like
primary crystals with the assistance of PEG as the morphology-
directing reagent [47].
In this study, we also found that the morphology of final
product depends on the addition of NaCl. The messy clusters of
nanoflakes with no shaped feature were obtained without NaCl
addition (see Fig. S3), while the well-defined Ni flower-like
architectures with smooth flakes generated when 1 g NaCl was
added to the reaction. Previous investigations confirmed that the
3.3. Effects of reaction conditions
Usually, the final morphology of Ni structures prepared via wet
chemical procedures without template agent is correlated with
temperature, reaction time, reducing agent, surfactant, etc.
[27–29]. In this work, only a small amount of Ni(OH)2 was
reduced to Ni when the hydrothermal reaction temperature
decreased to 433 K after 3 h reaction. If the reaction temperature
is higher than 443 K, the size of Ni flower-like architectures will
increase with the temperature enhancement. Besides the time-
dependent morphology evolution in the formation of nickel
flower-like architectures, time prolonged experiments were also
carried out. When the reaction was maintained for 24 h, there
was no obvious change in the morphology of flower-like archi-
tectures in addition to a slight increase in the particle size
(ca. 2 mm) and a curling tendency of the nanoflakes (see Fig. S1).
The microstructure and size had no variation when the reaction
time was further increased to 48 h in comparison with the sample
produced after 24 h reaction (SEM images not shown). In addi-
tion, it was found that sodium alkali played an important role in
the formation of Ni morphology feature. SEM images in Fig. 5
show the typical morphologies of three samples obtained at
different sodium alkali concentration, whilst the other experi-
mental parameters were kept. If the NaOH concentration was less
than 0.8 M, pure metallic Ni could not be obtained suggesting that
NaOH acts as the assisting agent to enhance the reducing ability
of polyol [43,44]. When the addition quantity of NaOH was 1 M,
only two or three nanoflakes intersected with each other to form
the Ni flower-like structure and a lot of nanoparticles were
Fig. 6. SEM image of sample prepared at 443 K for 3 h with no PEG addition.
Fig. 5. SEM images of samples produced at 443 K for 3 h with different NaOH concentration (a) 1 M, (b) 1.5 M, and (c) 4 M.

J. Kong et al. / Journal of Solid State Chemistry 184 (2011) 2994–3001
2999
inorganic soluble salts as complexing agent added in solution can
inhibit the deposition of crystallites [48,49], and therefore, the
deposition rate of Ni decreased through the addition of NaCl in
favor of the growth of monodispered flower-like structures due to
the strong coordination of Clꢀ to Ni2þ in this reaction system.
3.4. Magnetic property
It is well known that the magnetic properties of nanomaterials
are affected by their size and morphology. Fig. 7 shows the typical
magnetization hysteresis (M–H) loops of nickel flower-like archi-
tectures measured at room temperature. The sample showed
saturation magnetization (Ms) of 47.7 emu/g, coercivity (Hcj) of
332.3 Oe, and remnant magnetization (Mr) of 15.4 emu/g. The
measured Ms value of nickel flower-like architectures is lower
than that of bulk Ni (Ms¼55.9 emu/g) [31]. The decrease of Ms is
probably attributed to the existence of non-magnetic or weak
magnetic Ni oxide shell due to the exposure of sample in air after
synthesis. Moreover, the flower-like architectures have large
surface area, which induced the enhancement of spin disorder
on the surface [20], might be another reason for the decrease of
Ms. However, the coercivity is much higher than that of the bulk
nickel (ca. 0.7 Oe) [31], mainly resulting from the enhanced shape
anisotropy of nickel nanoflakes [21]. As mentioned above, the
flower-like architectures are composed of nickel nanoflakes,
which were obtained by the anisotropic growth of Ni increasing
the aspect ratio (Fig. 1(c)). Generally, the magnetocrystalline
anisotropy and the shape anisotropy of materials contribute to
coercivity. In this work, the shape anisotropy increased due to the
high aspect ratio of Ni nanoflakes [50], which leads to a higher
coercivity comparing with other Ni nano/micro-structures
[20,26,51].
Fig. 8. Relative permittivity er (a) and permeability mr (b) curves plotted against
frequency for the resin composites with 20 vol% Ni flower-like architectures in the
0.05–10.0 GHz range.
Fig. 8(b) shows the frequency dependences of the relative
permeability for the resin composites containing 20 vol% Ni
flower-like powders. The real part of relative permeability (mr0
declined from 1.7 to 1.0 with frequency in the 0.5–10.0 GHz
range, while the imaginary part of relative permeability (mr00
)
3.5. EM wave absorption properties
)
Magnetic powders dispersed in nonconductive matrix, such as
resin or rubber, are favorable to EM wave absorption materials
since it is facile to prepare lightweight absorbers with a remark-
able flexibility in combining with various EM wave devices.
Fig. 8(a) shows that the real part (e0) and imaginary part (e00) of
relative permittivity for the resin composites with 20 vol% nickel
flower-like sample were low and almost independent of fre-
quency between 1.0 and 8.0 GHz, for which the relative permit-
increased from 0.1 to 0.49 over a range of 0.5–5.0 GHz, and then
decreased gradually in the higher frequency range. The mr00 curve
exhibited a peak in a broad frequency range (2.0–10.0 GHz) with a
maximum point at 4.8 GHz. The magnetic loss peak at 4.8 GHz
can be ascribed to the natural resonance of Ni flower-like
architectures [18,22]. Comparing with the fcc bulk Ni (several
tens of megahertz) [19], the natural resonance frequency shifted
to higher GHz range. According to the ferromagnetic resonance
theory [52], the natural resonance frequency lineally varying with
tivity (er¼
er0 ꢀjer00) showed less variation (er0 ¼ ꢃ16 and er00 ¼ ꢃ2).
The dispersive permittivity behavior can be attributed to the
decrease of electric conductivity and space-charge polarization
between Ni powders isolated by epoxy resin efficiently.
the anisotropy field can be expressed by the equation: 2
pfr¼rHeff
,
where r is the gyromagnetic ratio, and Heff is the effective
anisotropy field. Deng et al. [53] proposed using Hcj to replace
Heff considering that it is difficult to calculate Heff for magnetic
materials with nanostructures since magnetocrystalline aniso-
tropy, particle geometry, particle size, and magnetic interaction of
particles all contribute to Heff. In this study, the measured Hcj
(332.3 Oe) of Ni flower-like architectures is much higher than that
of the bulk nickel, therefore, the natural resonance frequency
shifted from several tens of megahertz to 4.8 GHz. The same
natural resonance frequency enhancements have also been
observed for cobalt nanoflakes, iron submicron cubes, and Ni
nanowires attributing to the increase of shape anisotropy
[21,50,54].
To further reveal the EM wave absorption properties, the
reflection loss (RL) curves were calculated from the measured
relative permeability and permittivity at the given frequency and
absorber thickness according to the following equations [55,56]:
Fig. 7. Hysteresis loop of the Ni flower-like architectures measured at room
1=2
1=2
Zin ¼ Z0ðmr
=er
Þ
tanhfjð2
pfd=cÞðmrer
Þ
g
ð1Þ
temperature.

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J. Kong et al. / Journal of Solid State Chemistry 184 (2011) 2994–3001
20 vol% Ni flower-like powders showed efficient EM wave absorp-
tion characteristics (RLo ꢀ20 dB) in the 2.8–6.3 GHz range.
Comparing with some other Ni hierarchical structures, the nickel
flower-like architectures exhibited more efficient EM wave
absorption in GHz mainly attributing to the flower-like structure
composed of nanoflakes leading to large anisotropic field and
repeated absorption of EM wave.
Acknowledgments
This work was supported by the grant for Qi-Lu Young Scholar
program and Independent Innovation Foundations of Shandong
University (2009JQ015, 2009TB010). The authors also acknowl-
edge the financial supports from National Natural Science Foun-
dation (21071090), the Research Fund for the Doctoral Program of
Higher Education of China (20090131120032), the Excellent
Young Scientist Fund of Shandong Province (BS2009CL040), the
New Century Excellent Talent Program (NCET-10-0545), and the
Returned Overseas Chinese Scholars, State Education Ministry.
Fig. 9. Frequency dependences of the reflection loss (RL) for the resin composites
with 20 vol% Ni flower-like architectures at different thicknesses in the
0.05–10.0 GHz range.
RL ¼ 20log9ðZin2Z0Þ = ðZin þZ0Þ9
ð2Þ
where f is the frequency of the electromagnetic wave, d is the
thickness of an absorber, c is the velocity of light, Z0 is the
impedance of free space, and Zin is the input impedance of
absorber.
Appendix A. Supporting materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2011.09.022.
The RL values of the resin composites less than ꢀ20 dB, which
is comparable to the 99% of EM wave absorption, were obtained
in the 2.8–6.3 GHz range with absorber thickness of 2.6–5.0 mm
as shown in Fig. 9. A minimum RL value of ꢀ48 dB was observed
at 3.9 GHz with a matching thickness of 3.8 mm. Comparing with
some other Ni hierarchical structures [18–22], the nickel flower-
like architectures exhibited more efficient EM wave absorption
(RLo ꢀ20 dB) in GHz range. The absorption characteristics can be
understood as follows: as mentioned previously [57], the EM
wave absorbing properties of magnetic materials, such as
frequency, thickness, and absorbing band-width, are mainly
determined by the frequency dependences of complex permittiv-
ity (er0–jer00) and permeability (mr0–jmr00). In this study, the complex
permittivity of the resin composites with 20 vol% Ni flower-like
architectures is low and almost independent of frequency indi-
cating that dielectric loss can be neglected, and thus the EM wave
absorbing properties strongly depend on the magnetic natural
resonance (mr00). The mr00 exhibited higher value than other nickel
nano/micro-structures [18–22], therefore, the Ni flower-like
architectures exhibited more efficient EM wave absorption, which
also suggested that there is a better electromagnetic matching of
the complex permittivity and complex permeability. In addition,
the 3D Ni flower-like architectures are composed of nanoflakes,
therefore, such morphology induced the multi-scattering and
repeated absorption of EM wave leading to more EM energy
attenuation, which might be another factor for the EM wave
absorption enhancement [20].
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