The synthesis and magnetic properties of monodispersed single-domain nickel nanospheres and highly globular nanostructures of NicoreNiO shell

Article abstract of DOI:10.1016/j.jmmm.2005.06.009

Monodispersed single-domain nickel nanospheres have been successfully prepared by the microwave-assisted (MW) polyol method. TEM and AFM analysis reveal that the nickel nanospheres have a size distribution in the 100-180 nm range. Highly globular nanostructures of Ni_coreNiO_shell are achieved by oxidizing the nickel nanospheres in a flow of H ₂O₂ and the structure is studied by means of TEM, XPS, and EDAX. The single-domain magnetic structure is confirmed by MFM measurements and the ferromagnetic properties are demonstrated by VSM and SQUID studies. Surprisingly, no shift in the magnetization loop is observed despite the ferromagnet/antiferromagnet interface. This is attributed to the relatively large volume of our nanoparticles.

Full text of DOI:10.1016/j.jmmm.2005.06.009

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 301 (2006) 13–21
www.elsevier.com/locate/jmmm
The synthesis and magnetic properties of monodispersed
single-domain nickel nanospheres and highly globular
nanostructures of NicoreNiOshell
Xiansong Liua,b, Udi Meridorc, Peng Zhaoa, Gongbao Songa,
Ã
Aviad Frydmanc, Aharon Gedankena,
aDepartment of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan University Center for Advanced Materials
and Nanotechnology, Bar-Ilan University, Ramat-Gan 52900, Israel
bAnhui Key Laboratory of Information Materials & Devices, School of Physics & Materials Science, Anhui University,
Hefei 230093, PR China
cThe Physics Department and the Minerva Center, Bar-Ilan University, Ramat Gan 52900, Israel
Received 20 January 2005; received in revised form 30 May 2005
Available online 7 July 2005
Abstract
Monodispersed single-domain nickel nanospheres have been successfully prepared by the microwave-assisted (MW)
polyol method. TEM and AFM analysis reveal that the nickel nanospheres have a size distribution in the 100–180 nm
range. Highly globular nanostructures of NicoreNiOshell are achieved by oxidizing the nickel nanospheres in a flow of
H2O2 and the structure is studied by means of TEM, XPS, and EDAX. The single-domain magnetic structure is
confirmed by MFM measurements and the ferromagnetic properties are demonstrated by VSM and SQUID studies.
Surprisingly, no shift in the magnetization loop is observed despite the ferromagnet/antiferromagnet interface. This is
attributed to the relatively large volume of our nanoparticles.
r 2005 Elsevier B.V. All rights reserved.
Keywords: Core-shell structures; Polyol reaction; Magnetic properties
1. Introduction
structures, resulting in unique physical properties
[1–6]. Metal magnetic nanoparticles, especially those
having a spherical core–shell structure, have received
a considerable amount of attention in recent years
[7–12]. This is due to the structure– property
relations that have led to important applications,
such as materials for ultra-high-density data storage,
magnetic nanodevices, and medicine [13–19]. Nickel
has been used for wide applications in areas such as
The synthesis and characterization of nano-
structured materials is aimed at designing and
arranging nanomaterials in complex functional
Ã
Corresponding author. Tel.:+972 35318315;
fax: +972 3531250.
E-mail address: gedanken@mail.biu.ac.il (A. Gedanken).
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2005.06.009

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X. Liu et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 13–21
catalysis [20,21], fuel cell electrodes [22–24], and
magnetic storage media [25–27]. Recently, research
on core–shell nanostructures of nickel was focused
on metallic nickel nanoparticles supported on silica
balls for magnetic properties [28–31], nanoparticles
of carbon and palladium encapsulated with Ni
nanoparticles used in catalysis [32,33], and nick-
el–nickel oxide complex structures used for high-
density magnetic recording due to the magnetic
exchange coupling between the ferromagnetic (FM)
material (Ni), with the antiferromagnetic (AFMG)
material (NiO) [34–36]. The methods by which nickel
nanoparticles were fabricated are: ultrasound irra-
diation [37], evaporation technique [38], ultrasonic
spray pyrolysis [39], chemical reduction [40], electro-
chemical technique [41], and the polyol method [42].
In the current study, the microwave (MW)-
assisted polyol method is used to obtain mono-
dispersed nickel spheres with diameters ranging from
100 to 180 nm. Although the conventional polyol
method was previously used to prepare submicron
metallic nickel particles [43], the corresponding
reaction under MW radiation has never been
reported. Furthermore, in this work we report on
the formation of core–shell nanostructures of highly
globular NicoreNiOshell, which are synthesized from
the Ni nanospheres, and the magnetic properties of
the core–shell nanostructures are investigated. We
should also mention that, unlike the conventional
reaction where ethylene glycol was employed as the
polyol solvent, in the MW-assisted polyol reaction
triethylene glycol (TREG) was used as the solvent.
In the current process, the metallic salt was
dissolved in TREG and the product, glycol–metal
salt mixture, was then heated by MW irradiation.
Our modified MW oven included a distillation
system, and the reaction mixture was refluxed while
the metallic moieties precipitated out of the solution.
The solution was refluxed at 287 1C, the b.p. of
TREG, which is higher than that of ethylene glycol
(196 1C) used in the conventional polyol reaction.
oxide. The starting material was a solution of
nickel(II) acetate tetrahydrate (99.998%, Aldrich
Chemical Co.) in TREG (499%, Bio Lab Ltd.).
Following the previously described process [44,45],
a 100 ml glass flask was placed in a MW oven
(Spectra 900 W) and connected to a condenser.
Before the reaction, 50 ml of a solution of 0.125 M
Ni(Ac)2 in TREG was purged by argon for 20 min,
after which the MW oven was turned on at a
power level of 70% with a continued flow of the
gas. The reaction was stopped as soon as a black
suspension appeared, which was immediately
cooled in iced water. The resulting solid product
was washed thoroughly with ethanol and centri-
fuged. All these processes were repeated five times.
The nickel nanospheres product was dried under
vacuum and kept in a glove box.
In the second synthetic step, 100 mg of nickel
nanospheres, put in a porcelain boat, was intro-
duced into a tubular furnace. During the reaction,
oxygen at a flow rate of 100 cm3 minꢀ1 was
bubbled through a solution of hydrogen peroxide
(H2O2 of 30%, Bio Lab Ltd.). The reaction took
3 h at different temperatures: 300, 325, and 350 1C.
At the end of the reaction, the cell was cooled
down with continued purging of the gas.
2.2. Characterization
The powder X-ray diffraction (XRD) patterns
were collected on a Bruker AXS D* Advance
Powder X-ray diffractometer (using Cu-Ka radia-
tion l ¼ 0:15418 nm). The particle size, morphol-
ogy, and nature of the NiO/Ni particles were
studied by transmission electron microscopy
(TEM) employing a JEOL-JEM 100 SX micro-
scope working at 100 kV. Samples for the TEM
experiments were prepared by suspending the
dried sample in absolute ethanol in a sonication
bath for 20 min. In order to examine the dimen-
sions and mechanical stability of the nanospheres,
we employed a Digital Instruments 3100 dimen-
sion AFM. For the AFM measurements, the
particles were spin-coated on a Si substrate and
were scanned in the tapping mode. The AFM tips
used for the scan were TAP300 tips made by
NanoDevices and TAP300 made by BudgetSen-
sors. We used an MFM tip and scanned the
2. Experimental section
2.1. Preparation
A two-step approach had been employed in this
investigation for the preparation of nickel–nickel

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15
particles, once for their topography and then 40nm
above their surface in order to study the magnetiza-
tion, and to observe whether the particles are single
domain. The MFM tips used for the scanning
process were MESP Veeco nanoprobe tips. X-ray
photoelectron spectroscopy (XPS) was recorded
using an AXIS, HIS 165, ULTRA (Kratos
Analytical). The Ar+ sputtering was carried out
for 15min at an energy of 4 keV and a current
density of 0.5 mA/cm2. Under these conditions the
(4–850) data. The product is therefore identified as
FCC crystalline nickel.
Fig. 2(a) presents a magnified TEM image of a
single ꢁ180 nm nickel sphere before it undergoes
oxidation by H2O2. Fig. 2(b), which depicts a
number of Ni spheres after oxidation, shows the
existence of an outer shell having a brighter image
than the dark inner core. In addition, we detect some
small particles on the surface of the globular Ni. This
also confirms the formation of the nickel–nickel
oxide core–shell structure. The core–shell nano-
sphere particles (Fig. 2(b)) have a morphology
similar to that of the nickel balls (Fig. 2(a)), and
the NiO–extended Ni sphere particles have a
uniform and remarkably thin NiO layer with
30 nm of the shell thickness on their outer surface.
The thickness of the NiO outer layer depends on
the reaction time, the reaction temperature, and
the amount of H2O2 flowing through the solution.
In an attempt to control the thickness of the NiO
layer, we have varied the reaction temperature
while keeping all other parameters constant. The
reaction time was fixed at 3 h and the flow rate of
H2O2 at 100 cm3 minꢀ1. As could be expected, we
˚
estimated surface removal depth was 10–15 A. The
elemental composition of the material was analyzed
by energy-dispersive X-ray analysis (EDAX, JEOL-
JSM 840 scanning electron microscope).
Magnetic measurements at room temperature
were conducted using an Oxford Instrument
vibrating sample magnetometer (VSM). A total
of 20–25 mg of nickel nanoparticles were inserted
into the gelatin capsule and plugged with cotton to
prevent dispersal of the powder. Temperature-
resolved magnetic measurements were conducted
using a Quantum Design MPMS SQUID magnet-
ometer. Specifically, measurements were per-
formed by cooling a sample in zero field down to
4 K, when a magnetic field of 20 kOe was applied.
After that, the sample was slowly warmed up to a
high temperature (typically 300 K) in steps of a few
K per minute with stabilization at each tempera-
ture level and subsequent measurement of the
magnetic moment (ZFC). To induce the maximal
unidirectional anisotropy at Ni/NiO interface, a
magnetic field (10 kOe) should be applied during
cooling from 550 to 300 K by using the above-
mentioned VSM. Then, the sample was moved to
the SQUID hood and cooled down to 4 K with a
magnetic field of 20 kOe while measuring the
magnetic moment (FC) at the corresponding
temperature point of ZFC.
* : NiO
(c)
*
(b)
*
*
*
*
(a)
3. Results and discussion
3.1. Characterization and morphology
30
40
50
60
70
80
2θ (degrees)
Fig. 1(a) presents the XRD patterns of the
precipitated powder. The position of the diffrac-
tion peaks matches closely the reported JCPDS
Fig. 1. X-ray powder diffraction pattern of the samples: (a) Ni,
(b) and (c) Ni–NiO prepared at 300 and 350 1C, respectively.

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X. Liu et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 13–21
without a noticeable change in their overall shape.
The picture shown in Fig. 3 is representative of a
large number of nanospherical particles that have
been scanned and measured by the AFM and
MFM techniques.
Fig. 4 shows four scans of MFM images of two
types of particles. The first is an image of an
Fig. 2. TEM of (a) a nickel sphere, (b) highly spherical
NicoreNiOshell nanostructures.
have found that as the reaction temperature
increased, a thicker NiO shell was obtained. A
NiO shell thickness of about 30 nm was obtained
for a reaction temperature of 350 1C (Fig. 2(b)).
Simultaneously, we observed an increase in the
intensity of the XRD peaks of nickel oxide
(Figs. 1(b) and (c)) and a smooth uniform NiO
shell was observed for all oxidation temperatures.
The morphological studies were performed by
AFM measurements. Fig. 3 shows a typical AFM
micrograph of Ni spheres, demonstrating that the
average size of the particles is below 180 nm in
diameter and 180 nm in height. In order to test
their mechanical stability, we tried to move them
using AFM manipulation in a non-contact mode.
The spheres maintained their original geometry
Fig. 3. 2D (a) and 3D (b) AFM images of monodispersed
nickel nanospheres. The heights mentioned in the text were
taken from the 2D image.

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17
individual particle, while the second is a picture of
two connected nanospheres. Each pair of AFM
and MFM images was measured in the same
region. It should be noted that conclusions drawn
from the MFM pictures should be taken carefully
because of the effect of tip–sample interaction,
especially when dealing with low-coercivity parti-
cles. Nevertheless, Fig. 4(b) is consistent with the
notion that that the orientation of the magnetiza-
tion is identical for the whole particle, or, in other
words, the 100–180 nm nickel particles are single-
domain structures. Fig. 4(c) shows two magnetic
nanospherical particles. However, for the two
connected Ni spheres, we can distinctly observe
two domain structures possessing different mag-
netization regions.
The surface of the core–shell structure of the as-
prepared samples was also characterized by XPS.
The binding energy of the 2p3/2 core electron is
854.4 eV (Fig. 5) which is in good agreement with
the XPS data of Ni2+ ions [46]. The position of the
Ni0 2p3/2 peak is expected at 853.0 eV. The O 1 s
XPS spectrum (inset of Fig. 5) reveals a very broad
signal at around 529.0 eV. The 529.0 eV O (1 s)
peak is attributed to the oxygen atom bonded to
the divalent nickel. The XPS spectrum shows the
presence of Ni2+ and oxygen on the outer layer of
the globular particles, with atomic concentrations
of 44.5% and 55.5%, respectively. The stoichio-
metric ratio found from the area under the XPS
peak indicates a slight excess of oxygen, which we
attribute to water molecules adsorbed on the
surface of the particles [47].
field of 1.6 T, are depicted in Fig. 6. Typical FM
properties of the Ni nanospheres are observed, of
which the saturation magnetization MS and
coercivity HC are 44 kA m2 kgꢀ1 and 40 Oe,
respectively. Note that the curve is symmetric
around H ¼ 0.
Exchange bias is one of the phenomena asso-
ciated with the exchange anisotropy created at the
interface between an AFMG and FM material. It
occurs in a large variety of systems which are
composed of an AFMG in atomic contact with a
FM if the sample is grown, or after the system is
cooled, below the respective Neel and Curie
´
temperatures, TN (NiO, 525 K) and TC (Ni,
627 K), in an external cooling field H [48,49]. The
main manifestation of this effect is a shift in the
M– H loop so that the magnetization curve is not
symmetric around H ¼ 0. This result is expected in
our spheres since we have Ni (FM) cores
surrounded by NiO (AFMG).
Magnetization measurements as a function of
temperature were performed by zero-field-cooling
(ZFC) and field-cooling (FC) procedures at an
applied magnetic field H ¼ 20 kOe. MZFC was
measured while warming from 4 to 300 K. Before
MFC was recorded during the subsequent cooling,
a magnetic field (10 kOe) should be applied during
cooling from 550 to 300 K under VSM and 20 kOe
is used from 300 to 4 K in SQUID. Fig. 7
compares the ZFC and FC loops with SQUID
results from 4 to 300 K, showing no sign of the
exchange bias effect. The M–H loops are sym-
metric, showing no sign of a shift. Both ZFC and
FC magnetization curves rapidly decrease with the
increase of temperature and reveal no intersection
in the whole range shown in Inset (b) of Fig. 7. In
this respect our particles, for which the ZFC
magnetization curve coincides with the FC
magnetization, are similar to the system of 20 nm
iron particles embedded in PMMA [50]. This is in
contrast to other literature reports relating to
Ni/NiO particles with an average particle size
below 30 nm, which have demonstrated a non-
symmetrical magnetic hysteresis loop [38,51,52].
FM–AFMG exchange coupling may be under-
stood by assuming that the FM moments of the Ni
particles create an exchange field, m0Hex, acting on
the interface AFMG moments of the NiO layer.
The elemental composition of the core–shell
spheres was also analyzed by EDAX. A quantita-
tive measurement of the nickel spheres before and
after oxidation was 92.94% and 57.80% (atomic
percent), respectively. Although it is well known
that EDAX is not a good quantitative method for
light elements such as oxygen, the numbers
obtained, 7.06% and 42.20%, show the expected
trend of the successful oxidation of the particle’s
surface.
3.2. Magnetic properties
The results of the room temperature hysteresis
loop measured by VSM under an applied magnetic

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X. Liu et al. / Journal of Magnetism and Magnetic Materials 301 (2006) 13–21
Fig. 4. Magnetic domain structure of monodispersed nickel nanospheres observed using an MFM. (a) Topographic scan by AFM for
one sphere, (b) MFM image of the particle (a) showing a one-domain structure, (c) AFM image of two connected nanospheres, (d)
MFM image of (c) demonstrating two magnetic domains.

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19
component is induced along the FM Ni moments
[54–56]. The preferred orientations of the particle
magnetization M are given by the delicate balance
between several anisotropy energies: the magneto-
crystalline bulk anisotropy, its surface and step
counterparts, and the shape anisotropy [57,58].
Considering that Hex (an exchange field) is
formally equivalent to an external field, the
coupling energy difference between two FM
moment directions [13] is given by
1800
1600
1400
1200
1000
800
200
100
525
53
0
535
Binding energy (eV)
DE ¼ ꢀ1=2 DwAFMG m0H2ex,
(1)
where DwAFMG is the associated difference in
susceptibility of the interface AFMG. The mini-
mum energy of NicoreNiOshell particles for coher-
ent rotation of the FM moments is
850
855
860
865
Binding energy (eV)
DEmin ¼ m0HcMSVFM
,
(2)
Fig. 5. XPS spectra of NicoreNiOshell spheres for nickel particles
oxidized at 300 1C. The inset is the O 1 s XPS spectrum.
where VFM is the FM Ni core volume and MS is
the saturation magnetization. According to our
MFM images, all the spheres are one single
domain. Since the spherical volume is proportional
to r3 (the spherical radius), one can expect a
significant difference in DEmin between our parti-
cles (of about 180 nm NicoreNiOshell spheres) and
previously reported particles (below 30 nm). For
Fe/Fe3O4, the coercive field first increased up to a
grain size of about 30 nm, which was comparable
to the FM exchange length, and decreased again
for larger grain sizes [59]. The ratio between the
volume of our nickel sphere to that of other works
is about 216, so the minimum energy, DEmin, for
coherent rotation of the FM moments of the larger
particles needs much more. In addition, it should
be noted that for small particles [60,61], the
thickness of the AFMG and FM are of the same
order, and hence the effect of the AFMG can be
expected to be much larger.
40
20
0
-20
-40
-15000 -10000 -5000
0
5000 10000 15000
Applied field (Oe)
Fig. 6. Hysteresis loop of the nickel nanospheres measured by
VSM at room temperature.
Early results of a room temperature magnetiza-
tion loop of 5 nm nickel particles embedded in
polyvinylpyrrolidone (PVP, average molecular
weight 40 000) showed an exchange bias of
2200 Oe. The oxidation of the outer nickel layer
is obtained by exposure of the particles to ambient
laboratory conditions. These results further sub-
stantiate our claim that the exchange bias depends
on the particle’s volume and it is reduced to zero
for large particles such as the particles in the
NiO is a collinear antiferromagnet and has a small
net magnetic moment due to an unequal number
of spins on the two sublattices [53]. For Nicore
NiOshell spheres at a compensated interface, the
number of AFMG moments from each sublattice
is the same. The resulting magnetization is zero,
and thus the coupling vanishes to a first order. To
a second order, the exchange field induces a
canting of the AFMG moments, and an FM

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30
20
H= 20 kOe
28
ZFC
FC
27
26
ZFC
FC
10
0
100
200
300
(b)
T (K)
0
5
0
-10
-20
-30
-5
(a)
-150
0
150
-20000
-10000
0
10000
20000
Applied field (Oe)
Fig. 7. Hysteresis loops at 4 K of NicoreNiOshell sphere particles. Note that the results for FC and ZFC are practically indistinguishable
and in both cases no shift is observed. Inset: (a) Zoom of the curve near the origin point for ZFC and FC. Note the symmetry around
H ¼ 0. (b) ZFC and FC magnetization curves as a function of temperature from 4 to 300 K.
current study. The investigation of the dependence
of the exchange bias on particle size is in progress.
is not enough to cause a shift in the M–H loop due
to the large FM core volume. Such spherical
particles could be very helpful in the study of
magnetically ordered structures and are expected
to provide insight into the understanding of
exchange-coupling anisotropy in core–shell parti-
cle structures.
4. Conclusions
We have presented a new method of fabricating
monodispersed nickel nanospheres and highly
globular nanostructures of NicoreNiOshell. These
monodispersed nickel spheres, in a size range
below 180 nm, are characterized by a magnetic
single domain structure. The present results
demonstrate that the magnetic coupling in a
NicoreNiOshell structure induces an anisotropy that
Acknowledgments
This work was supported by the Key Program
of the Education Department of Anhui Province
under Grant no. 2005KJ033ZD.

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21
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