Nanoparticles Ni and NiO: Synthesis, characterization and magnetic properties

Article abstract of DOI:10.1016/j.jallcom.2008.09.121

The present investigation reports, the novel synthesis of nanoparticles Ni and NiO using thermal decomposition and their physicochemical characterization. The nanoparticles Ni powder have been prepared using [bis(2-hydroxyacetophenato)nickel(II)] as precu

Full text of DOI:10.1016/j.jallcom.2008.09.121

Journal of Alloys and Compounds 476 (2009) 797–801
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Nanoparticles Ni and NiO: Synthesis, characterization and
magnetic properties
Fatemeh Davara, Zeinab Fereshteha, Masoud Salavati-Niasaria,b,∗
a Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran
b Department of Chemistry, Faculty of Science, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The present investigation reports, the novel synthesis of nanoparticles Ni and NiO using thermal decom-
position and their physicochemical characterization. The nanoparticles Ni powder have been prepared
using [bis(2-hydroxyacetophenato)nickel(II)] as precursor. Transmission electron microscopy (TEM) anal-
ysis was demonstrated nanoparticles Ni with an average diameter of about 14–22 nm. The products were
characterized by X-ray diffraction (XRD), TEM, high-resolution transmission electron microscopy (HRTEM)
and Fourier transform infrared (FT-IR) spectroscopy. The magnetic property of Ni and NiO was studied
with vibrating sample magnetometer (VSM).
Received 19 July 2008
Received in revised form
15 September 2008
Accepted 20 September 2008
Available online 7 November 2008
Keywords:
Nanoparticles
Ni
© 2008 Elsevier B.V. All rights reserved.
Inorganic materials
Characterization methods
Chemical synthesis
1. Introduction
ily oxidized. To obtain pure nickel nanocrystals, many methods
were performed in organic media to avoid the formation of oxide
Nanostructured materials have been extensively explored for
the fundamental scientific and technological interests in accessing
new classes of functional materials with unprecedented proper-
ties and applications [1–3]. Magnetic nanoparticles are being used
widely in conducting paints [4], rechargeable batteries [5,6], chemi-
cal catalysts [7], optoelectronics [8], magnetic recording media [9],
ferro-fluids [10], magnetic resonance imaging contrast enhance-
ment and drug delivery [11–13], and so on.
Nickel is one of the transition metals that exhibit magnetism
as bulk material and other interesting properties and applications,
such as in hydrogen storage and catalysis. Methods to produce Ni
nanoparticles can be found in the literature and involve electro-
chemical reduction [14], chemical reduction [15,16], and sol–gel
[17]. Recently, the use of organometallic precursors has been proven
interesting since it leads to particles with controlled size, surface
coordination, and crystallinity that are monodisperse to 1 atomic
shell [18].
or hydroxide. Organometallic precursors, such as Ni(CO)4 and
Ni(Cp)2, have been used for the synthesis of nickel nanoparticles
[19], and Ni(COD)2 was also used to synthesize nickel parti-
cles by spontaneous decomposition in CH2Cl2 in the presence
of poly(vinylpyrrolidone) [20,21]. Nickel oxide nanoparticles have
been synthesized via thermal decomposition NiC2O4 at 450 ◦C [22].
Zhang et al. [23] synthesized Ni nanaocrystals with diameters in the
range of 20–60 nm through decomposition of nickel acetylacetone
in oleylamine.
Among various techniques developed for the synthesis of nickel
and nickel oxide nanoparticles, thermal decomposition is a novel
method to produce stable monodispersed [24] and it is a rapidly
developing research area. As compared to conventional method,
it is much faster, cleaner and economical. However, an improve-
ment in the thermal reduction process should be made in preparing
nickel nanoparticles with controllable size and shape in order to
extend the application areas and satisfy the needs of fundamental
research.
Among the transition metal magnetic nanoparticles, nickel
nanocrystals are difficult to be prepared because they are eas-
The current synthetic procedure is a modified version of the
method developed by Hyeon group for the synthesis of various
nanoparticles of metals and oxides, which employ the thermal
decomposition of metal–surfactant complexes in hot surfactant
solution [25]. Recently, our groups reported the synthesis of metal
oxide nanoparticles via thermal decomposition [26,27]. A major
interest at the moment is in the development of organometallic
∗
Corresponding author at: Institute of Nano Science and Nano Technology,
University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran.
Tel.: +98 361 5555333; fax: +98 361 5552935.
E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.09.121

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F. Davar et al. / Journal of Alloys and Compounds 476 (2009) 797–801
Scheme 1. Schematic diagram illustrating the formation of NiO nanoparticles.
or inorganic compound for preparation of nanoparticles. Using
of the novel compound can be useful and open a new way for
preparing nanomaterials to control nanocrystal size, shape and dis-
tribution size. We synthesized Ni nanoparticles from the thermal
decomposition of new precursor nickel 2-hydroxyacetophenato in
oleylamine (Scheme 1). In this process, oleylamine was used as both
the medium and the stabilizing reagent. To the best our knowl-
edge, this is the first report on the synthesis of Cu nanoparticles
from [Ni(aceto)2] (aceto = 2-hydroxyacetophenato). The aim of the
present work is to prepare nanocrystals Ni using thermal reduction
method and its physicochemical characterization.
easily be re-dispersed in nonpolar organic solvents, such as hexane or toluene and
oxidized in air (Scheme 1).
3. Results and discussion
The XRD pattern of the fresh nickel nanoparticles, shown in
Fig. 1a, shows the face-centered cubic (fcc) structure of nickel. The
peaks are extremely broad. Three characteristic peaks at 44.58◦,
51.9◦, and 76.54◦ for fcc nickel (JCPDS, no. 03–1051) marked by their
2. Experimental
2.1. Materials
The precursor complex, [bis(2-hydoxyacetophenato)nickel(II)], was prepared
according to the procedure described previously [28]. Oleylamine, triphenylphos-
phine (TPP), toluene, hexane, and ethanol were purchased from Aldrich and used as
received.
2.2. Characterization
XRD patterns were recorded by a Rigaku D-max C III, X-ray diffractometer
using Ni-filtered Cu K␣ radiation. Elemental analyses were obtained from Carlo
ERBA Model EA 1108 analyzer. Transmission electron microscopy (TEM) images and
electronic diffraction (ED) pattern were obtained on a Philips EM208 transmission
electron microscope with an accelerating voltage of 200 kV. The high-resolution
transmission electron microscopy (HRTEM) image and energy dispersive X-ray
(EDX) were recorded on a JEOL-2010 TEM at an acceleration voltage of 200 kV. The
compositional analysis was done by energy dispersive X-ray (EDX, Kevex, Delta Class
I). Fourier transform infrared (FT-IR) spectra were recorded on Shimadzu Varian
4300 spectrophotometer in KBr pellets. The electronic spectra of the complexes were
taken on a Shimadzu UV–vis scanning spectrometer (Model 2101 PC). The magnetic
measurement was carried out in a vibrating sample magnetometer (VSM) (BHV-55,
Riken, Japan) at room temperature.
2.3. Synthesis of Ni nanoparticles
In this synthesis, Ni nanoparticles were prepared by the thermal reduction of
[Ni(aceto)2]–oleylamine complex as precursor. First, the [Ni(aceto)2]–oleylamine
complex was prepared by reaction 0.5 g of [Ni(aceto)2] and 5 mL of oleylamine. The
mixed solution was placed in a 50 mL three-neck distillation flask and heated up to
150 ◦C for 60 min under an argon atmosphere. The resulting metal-complex solution
was injected into 5 g of triphenylphosphine, TPP, at 245 ◦C. As the thermal reduction
proceeded, the green solution turned to black, indicating that colloidal products
were generated. The black solution was aged at 235 ◦C for 40 min, and was then
cooled to room temperature. The black powders were precipitated by adding excess
ethanol to the solution. The final products were washed with ethanol for at least
three times to remove impurities, if any, and dried at 100 ◦C. This product could
Fig. 1. XRD patterns of the (a) Ni and (b) NiO nanoparticles.

F. Davar et al. / Journal of Alloys and Compounds 476 (2009) 797–801
799
indices (1 1 1), (2 0 0) and (2 2 2) were observed. This revealed that
the as-synthesized nanocrystals were pure nickel.
The crystallite sizes of the as-synthesized nickel, Dc, was calcu-
lated from the major diffraction peaks of the base of (1 1 1) using
the Scherrer formula (Eq. (1)):
Kꢀ
Dc
=
(1)
ˇ cos ꢁ
where K is a constant (ca. 0.9) [26]; ꢀ is the X-ray wavelength
used in XRD (1.5418 Å); ꢁ is the Bragg angle; ˇ is the pure diffrac-
tion broadening of a peak at half-height, that is, broadening due
to the crystallite dimensions. The diameter of the nanoparticles
calculated by the Scherrer formula is 14 nm. The XRD pattern
after exposing the nanoparticles to air for than 48 h showed a
NiO structure (Fig. 1b), demonstrating that the initially synthe-
sized metallic-nickel nanoparticles were readily oxidized to NiO
nanoparticles. The X-ray photoelectron spectroscopy (XPS) spec-
trum of the nanoparticles after air exposure revealed the NiO
electronic structure rather than the metallic-nickel structure, con-
firming the XRD data.
FT-IR spectroscopy is a useful tool to understand the functional
group of any organic molecule (Fig. 2a–c). The presence of oley-
lamine group on Ni nanoparticles (Fig. 2b) is indicated by the N–H
wagging mode from 650 to 900 cm−1; NH2 bending modes at 909,
964 and 993 cm−1; and NH2 scissor mode at 1568 cm−1 [29]. The
spectrum shown in Fig. 2b also reveals the characteristic peak of
the C–N stretch at 1042 cm−1 which suggests that the C–N bonds
in amine groups, and therefore oleylamine ligands, remain intact,
capping the Ni nanoparticles. The peak at 1468 cm−1 is associ-
ated with the C–H bending mode and the two peaks at 2865 and
2921 cm−1 represent the C–H stretching modes of the oleylamine
carbon chain. The presence of various N–H peaks suggests that
amines are bound to the surface of the Ni nanoparticles. Based
on the FT-IR data, we deduce the following reaction mechanism.
Upon injection of [Ni(aceto)2] and oleylamine into hot TPP solution,
the Ni–O bond is cleaved by the oleylamine group and Ni is subse-
quently reduced. The weak peaks at 3435 cm−1 has been assigned
to the EtOH used in the separation step. The presence of various
N–H peaks suggests that amines are bound to the surface of the Ni
nanoparticles. There was no evidence of free precursor, nickel 2-
Fig. 3. (a and b) TEM images of sample (the inset shows the ED pattern) and (c)
HRTEM image of the sample.
hydroxyacetophenato, in the sample, because stretch vibration of
C
O (ꢂC O) and C–O (ꢂC–O) disappeared. So the oleylamine serves
as the capping ligand that controls growth. Fig. 2c shows that the
surface was oxidized because the sample was exposed to air and
some water was adsorbed probably on the external surface of the
samples during handling to record the spectra. Two new peaks
were shown at 445 and 490 cm−1 in spectrum (Fig. 2c). These peaks
were undoubtedly assigned to Ni–O stretching as was reported at
literature [30].
Transmission electron microscopic (TEM) photographs of the
product have been given in Fig. 3a. The sizes of nanocrystals
obtained from the XRD diffraction patterns are in close agreement
with the TEM studies which show sizes of 14–22 nm (Fig. 3a and b).
These sizes are nearly spherical. We may consider that when the
Fig. 2. FT-IR spectra of (a) oleylamine (b) Ni nanoparticles and (c) NiO nanoparticles.

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F. Davar et al. / Journal of Alloys and Compounds 476 (2009) 797–801
(HRTEM) image of the nanoparticles revealed the highly crystalline
nature of the nanoparticles. The lattice fringes clearly observed and
the spacing of the two neighbouring plane is about 0.12 nm, which is
consist with the interplanar separation of the (2 0 0) plane in cubic
(fcc) Ni. The HRTEM image of a single Ni nanoparticles showed that
the nanoparticles were highly crystalline (Fig. 3c).
Fig. 4 is the UV–vis spectrum of the as-synthesized NiO nanopar-
ticles dispersed in ethanol. A strong absorption in the UV region is
observed at wavelengths about 362 nm. The strong absorption in
the UV region is attributed to band gap absorption in NiO [31]. It
is well known that optical band gap (Eg) can be calculated on the
basis of the optical absorption spectrum by the following equation:
(Ahꢃ)n = B(hꢃ − Eg)
where hꢃ is the photo energy, A is absorbance, B is a constant relative
to the material and n is either two for direct transition or 1/2 for an
indirect transition [32]. Hence, the optical band gap for the absorp-
tion peak can be obtained by extrapolating the linear portion of the
(Ahꢃ)n − hꢃ curve to zero. The band gap of the NiO particles is about
3.42 eV, which is similar to the value (3.55 eV) reported by Boschloo
and Hagfeldt [33]. No linear relation was found for n = 1/2, sug-
gesting that the as-prepared NiO nanoparticles are semiconducting
with direct transition at this energy [31].
The magnetic property of Ni nanoparticles has been measured
(Fig. 5a). The saturation magnetization (Ms) for the nanospheres
samples is 54.2 emu g−1 (1 emu g−1 = 1 A m2 kg−1) which is close to
that of bulk Ni (ca. 55 Oe (1 Oe = 79.6 A m−2)). Coercivity field (Hc) of
Ni nanoparticles is about (19.4 Oe). Compared with the correspond-
ing value for bulk Ni (0.7 Oe), the coercivities of the Ni nanoparticles
clearly show significant enhancement. This enhancement can likely
be attributed to the reduced particle sizes of obtained product [34]
which change the magnetization reversal mechanism. This demon-
strates that the magnetic properties of the Ni nanoparticles are
greatly influenced by their microstructure. As shown in Fig. 5a, the
hysteresis loop is symmetric with respect to zero magnetic fields,
indicating that there is no exchange biasing effect, which is usually
caused by the presence of NiO [35,36]. Fig. 5b shows the hystere-
sis loops of NiO nanoparticles at 300 K. It can be seen that NiO
nanoparticles present a superparamagnetic behavior although NiO
bulk material is antiferromagnetic [37].
Fig. 4. UV–vis absorption spectrum of the NiO nanoparticles.
reaction was carried out at 245 ◦C, most organic molecules were
decomposed. Since only a few oleylamine molecules were adsorbed
on Ni nanoparticles. The ED pattern (Fig. 3b) indicates that the
Ni nanoparticles are single-crystalline. This high-resolution TEM
4. Conclusion
Nanoparticles of Ni and NiO with average particle
sizes of 14–22 nm from thermal decomposition of [bis(2-
hydroxyacetophenato)nickel(II)] as precursor have been
synthesized. The XRD pattern of the fresh nickel nanoparticles,
showed the fcc structure of nickel. The optical absorption band gap
of the nickel oxide nanoparticles was estimated to be 3.42 eV.
Acknowledgement
Authors are grateful to council of University of Kashan for pro-
viding financial support to undertake this work.
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