Synthesis of nickel and nickel oxide nanoparticles via heat-treatment of simple octanoate precursor

Article abstract of DOI:10.1016/j.jallcom.2010.01.063

Nickel oxide nanoparticles have been synthesized via heat-treatment of a new precursor nickel octanoate Ni(octa)₂; (octa = octanoate); in the range of 400-900 °C. The effect of calcination temperature on particle size has been investigated. When the calcinations were conducted in air from 400 to 900 °C, the precursor Ni(octa)₂ was first changed to Ni, then to NiO. When calcined in argon atmosphere, Ni(octa)₂ was directly converted into pure Ni at as low as 500 °C. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) spectroscopy. The synthesized NiO nanoparticles have quasi-spherical particles with size of about 24 nm. The optical absorption band gap of NiO nanoparticles is 3.41 eV.

Full text of DOI:10.1016/j.jallcom.2010.01.063

Journal of Alloys and Compounds 494 (2010) 410–414
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis of nickel and nickel oxide nanoparticles via heat-treatment of simple
octanoate precursor
Masoud Salavati-Niasaria,b,∗, Fatemeh Davara,∗, Zeinab Fereshteha
a Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran
b Department of Inorganic Chemistry, Faculty of Chemistry, 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:
Nickel oxide nanoparticles have been synthesized via heat-treatment of a new precursor nickel octanoate
Ni(octa)2; (octa = octanoate); in the range of 400–900 ◦C. The effect of calcination temperature on particle
size has been investigated. When the calcinations were conducted in air from 400 to 900 ◦C, the precursor
Ni(octa)2 was first changed to Ni, then to NiO. When calcined in argon atmosphere, Ni(octa)2 was directly
converted into pure Ni at as low as 500 ◦C. The products were characterized by X-ray diffraction (XRD),
scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform
infrared (FT-IR) spectroscopy. The synthesized NiO nanoparticles have quasi-spherical particles with
size of about 24 nm. The optical absorption band gap of NiO nanoparticles is 3.41 eV.
Received 15 November 2009
Received in revised form 11 January 2010
Accepted 12 January 2010
Available online 21 January 2010
Keywords:
Nanostructures
NiO
Chemical synthesis
Magnetic properties
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
A great many methods have been employed to synthesize NiO
nanoparticles such as evaporation [10], electrodeposition [11],
Recently, nanomaterials have attracted extensive interest
because of their unique properties in various fields in compari-
son with their bulk counterparts [1]. A reduction in particle size to
nanometer scale results in various interesting properties compared
with their bulk properties. Having a large surface area, metal oxide
nanomaterials show great advantages over conventional materials
in many applications. Nickel oxide is an antiferromagnetic mate-
rial that has a wide range of technological applications at the
nanoscale.
For example, NiO nanoparticles have been found to be promis-
ing as an electrode material for lithium ion batteries [2]. Nanosized
NiO has demonstrated excellence properties such as catalytic [3],
magnetic [4], electrochromic [5], optical and electrochemical prop-
erties [3]. Furthermore, nickel oxides can be used as a transparent
p-type semiconducting layer [6] and are being studied for applica-
tions in smart windows, electrochemical supercapacitors [7] and
dye-sensitized photocathodes [8]. For technological applications, a
detailed understanding of the size, morphology and interparticle
interactions of nanoscale NiO could be important [9].
thermal decomposition [12] and sol–gel techniques [13].
Recently Shah reported synthesis of 50–60 nm nickel oxide
nanoparticles after 12 h using sonication and hydrothermal meth-
ods [14]. More recently the NiO nanoparticles were synthesized
using two different microemulsion approaches by Palanisamy and
Raichur. It was reported that Ni(OH)2 particles were first formed,
then calcined at 600 ◦C to obtain 47 nm nickel oxide NiO parti-
cles [15]. Song and Gao synthesized NiO nanorods consisting of
numerous NiO nanoparticles by thermal decomposition of rod-like
nickel oxide precursor achieved by microwave-assisted method
[16]. Rodríguez-Llamazares et al. synthesized nickel/nickel oxides
nanoparticles by a method based on ligand displacement of bis(1,5-
cyclooctadiene)-nickel(0), zerovalent organometallic precursor
and simultaneous formation of a thiourea inclusion compound [17].
Nickel oxide nanoparticles have been synthesized via decomposi-
tion of nickel oxalte NiC2O4 at 450 ◦C [2]. Thermal decomposition
method has some advantages such as simple process, low cost and
easiness to obtain high purity products hence it is quite promising
and facile route for industrial applications.
In our group, we have been interested for a few years in the syn-
thesis of metal, metal oxide and metal sulfide nanoparticles, using
new inorganic precursor, taking profit of the tools of organometallic
chemistry [18–22]. A major interest at the moment is in the devel-
opment of organometallic or inorganic compound for preparation
of nanoparticles [23–25]. Using of the novel compound can be use-
ful and open a new way for preparing nanomaterials to control
nanocrystal size, shape and distribution size.
∗
Corresponding authors at: Institute of Nano Science and Nano Technology, Uni-
versity of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran.
Tel.: +98 361 5555333; fax: +98 361 5552935.
E-mail addresses: salavati@kashanu.ac.ir (M. Salavati-Niasari),
davar@kashanu.ac.ir (F. Davar).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.01.063

M. Salavati-Niasari et al. / Journal of Alloys and Compounds 494 (2010) 410–414
411
Table 1
Reaction conditions and corresponding products.
Reaction temperature (◦C)
Reaction time (h)
Reaction atmosphere
Composition of the products (XRD result)
300
350
400
500–900
350
3
4
4
4
4
4
4
Air
Air
Air
Air
Ar
NiO + Ni(octa)2 (main)
NiO + Ni(octa)2 (minor)
NiO (main) + Ni (minor)
NiO
Ni (main) + Ni(octa)2 (minor)
Ni
Ni
400
500–800
Ar
Ar
Herein, we report a systematic synthesis of Ni and NiO nanopar-
ticles via the heat-treatment of Ni(octa)2 precursors, which were
prepared through a simple precipitation method. In this process,
pure NiO nanoparticles were synthesized without any surfactant
and solvent by dry, simple method. To the best of our knowledge,
this is the first report on the synthesis of NiO nanoparticles from
Ni(octa)2; (octa = octanoate). The aim of the present work is to pre-
pare nanocrystals of NiO using thermal decomposition method and
its physicochemical characterization.
3. Results and discussion
Fig. 1a–f shows the XRD patterns of the products prepared
through heating Ni(octa)2 in air at 400–900 ◦C for 4 h, respectively.
The diffraction peaks in pattern a can be indexed to cubic Ni and
cubic phase of NiO. The diffraction peaks in patterns b–f can be
indexed to pure NiO cubic phase. Fig. 1h is the XRD pattern of the
product resulted from heating Ni(octa)2 in Ar at 400 ◦C for 4 h. All
diffraction peaks can be indexed to pure cubic Ni. The peaks are
extremely broad. Three characteristic peaks at 44.3◦, 52.1◦, 76.6◦
for face-centered cubic (fcc) nickel (JCPDS, no. 01-1260) marked by
their indices (1 1 1), (2 0 0) and (2 2 0) were observed. This revealed
that the as-synthesized nanocrystals were pure nickel.
2. Experimental
2.1. Materials
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
All the chemical reagents used in our experiments were of analytical grade and
were used as received without further purification. Octanoic acid, nickel nitrate,
NaOH 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. Scanning electron microscopy (SEM) images were obtained
on Philips XL-30ESEM equipped with an energy dispersive X-ray spectroscopy.
Transmission electron microscopy (TEM) images were obtained on a Philips EM208
transmission electron microscope with an accelerating voltage of 100 kV. X-ray pho-
toelectron spectroscopy (XPS) of the as-prepared products was measured on an
ESCA-3000 electron spectrometer with nonmonochromatized Mg K␣ X-ray as the
excitation source. Fourier transform infrared (FT-IR) spectra were recorded on Shi-
madzu Varian 4300 spectrophotometer in KBr pellets. Mass spectrum (MS) was
measured with a JMS-700 (JEOL Ltd.) operated in a negative ion mode. The sample
for MS was mixed in diethanolamine as a matrix. The elemental analysis (carbon and
hydrogen) of the materials was obtained from Carlo ERBA Model EA 1108 analyzer.
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 nickel octanoate (Ni(octa)2)
Five gr of NaOH was dissolved into 5 mL deionized water and 10 mL absolute
ethanol then soda solution was introduced into 2.5 g of octanoic acid under con-
tinuous stirring for about 2 h. Then 15 mL deionized water was introduced into the
above solution and was put on ice bath and was added below solution (15 g NaCl
was dissolved into 100 mL deionized water), in the result, white precipitate was
formed after 10 min, this precipitate was filtrated and was washed by cool deion-
ized water then was dried. This white precipitate is sodium octanoate (Na(octa)).
Sodium octanoate (24 mmol) was completely dissolved in deionized water (50 mL).
Nickel(II) nitrate (12 mmol) in deionized water (10 mL) was added to the solution.
A deep green precipitate was formed after 2 h reflux. The precipitate was collected
by filtration and washed with deionized water. Then, the solid was reprecipitated
by adding MeOH. The precipitate was dried in oven in 50 ◦C for 5 h. Analytical data.
FAB-MS: m = z 344.7 [M−]; analytical calculations for C16H30O4Ni consist: C, 55.69;
H, 8.76; Ni, 17.01%. Found: C, 55.72; H, 8.79; Ni, 17.05%; IR (FT-TR method): 2922
(CH3), 2854 (CH2), 1589 (COO), 1470–1415 (CH3, CH2), 722 (OCO) cm−1
.
2.4. Preparation of Ni and NiO nanoparticles
The as-prepared Ni(octa)2 precursor was subsequently calcined in the temper-
ature range of 300–900 ◦C in a furnace for 4 h to get nickel oxide. In parallel, pure
Ni nanoparticles were also prepared through calcining Ni(octa)2 precursor under
flowing argon atmosphere at 350–800 ◦C for 4 h (see Table 1).
Fig. 1. XRD patterns of the as-prepared nanoparticles calcined at different temper-
atures.

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M. Salavati-Niasari et al. / Journal of Alloys and Compounds 494 (2010) 410–414
Fig. 2. Evolution of crystallite size of NiO powders calcined at different tempera-
tures.
Fig. 4. FT-IR spectra of NiO nanoparticles calcined at 500 ◦C in air for 4 h.
the Scherrer formula (Eq. (1)),
FT-IR spectroscopy is a useful tool to understand the functional
group of any organic molecule (Fig. 4). Metal oxides; NiO; generally
give absorption bands below 1000 cm−1 arising from inter-atomic
vibrations. The peak at 443 cm−1 in spectrum, showing Ni–O bond,
gave clear evidence about the presence of the crystalline NiO [27].
The board peak at 3433 cm−1 in spectra is representative of H2O.
Also, the H–OH bending vibration appears at 1635 cm−1. There
was no evidence of free precursor, nickel octanoate, in the sample,
because stretch vibration of C O (ꢂC O) and C–O (ꢂC–O) disap-
peared. About prepared samples in Ar atmosphere the absorption
peak below 1000 cm−1 were not observed. So we can say that FT-IR
spectra agree with XRD patterns, which shows that obtained sam-
ples under Ar atmosphere are pure metallic nickel and not oxidized.
But it is clear that the surface of these samples is oxidized because
the sample was exposed to air. After being oxidized under air, new
peak is appeared at about 450 cm−1, that this peak was undoubtedly
assigned to Ni–O stretching as was reported at literature.
Further evidence for the purity and composition of the products
was obtained by X-ray photoelectron spectra (XPS). Fig. 5 shows
the XPS of the prepared NiO nanoparticles at 500 ◦C in air for 4 h.
The binding energies of Ni 2p1/2, Ni 2p3/2 and O 1s, provided a fairly
complete picture of the sample powder. The Ni 2p3/2 XPS peak that
appears at 855.4 eV coincides with the findings for NiO. The O 1s
peak at 530.5 eV is attributed to the (O−2) in the NiO.
Kꢀ
Dc
=
(1)
ˇ cosꢁ
where K is a constant (ca. 0.9) [26], ꢀ is the X-ray wavelength used
in XRD (1.5418 Å), ꢁ is the Bragg angle and ˇ is the pure diffraction
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 24 nm.
The evolution of the crystal size (Dc) of the NiO prepared at dif-
ferent calcination temperatures is shown in Fig. 2. The data show
that the growth in crystallite size of nickel oxide has a strong rela-
tionship with the sintering temperature. The crystallite size NiO
increase linearly until the calcination temperature reaches 700 ◦C.
The size of NiO also increases linearly from 24 nm (400 ◦C) to 29 nm
(700 ◦C). After 700 ◦C, the crystallite size of nickel oxide significantly
increases to 36 nm at 800 ◦C and at higher calcination temperatures
(>800 ◦C) shows no significant change in crystallite size, which is
36 nm.
The surface morphological study of the nickel oxide nanoparti-
cles was carried out using SEM image. Fig. 3a shows the SEM image
of the prepared NiO at 500 ◦C in air for 4 h. From the SEM analyses,
one can conclude the formation of nanoparticles spherical structure
(Fig. 3a). The transmission electron microscopic (TEM) photographs
of the product has been given in Fig. 3b. The size of nanoparticles
obtained from the XRD diffraction patterns are in close agreement
with the TEM studies which show sizes of 20–30 nm.
Fig. 6 is the UV–vis spectrum of the as-synthesized NiO nanopar-
ticles dispersed in ethanol. A strong absorption in the UV region is
Fig. 3. SEM and TEM images of the prepared NiO at 500 ◦C in air for 4 h.

M. Salavati-Niasari et al. / Journal of Alloys and Compounds 494 (2010) 410–414
413
Fig. 5. XPS spectra of the NiO nanoparticles.
Fig. 7. Magnetization versus applied at 300 K for (a) Ni and (b) NiO nanoparticles.
observed at wavelengths about 364 nm. The strong absorption in
the UV region is attributed to band gap absorption in NiO [28]. It
is well known that optical band gap (Eg) can be calculated on the
basis of the optical absorption spectrum by the following equation:
gap energies for NiO are in the range 3.4–3.8 eV [30], which is in
good agreement with our report. The value of band gap of the NiO
nanoparticles is about 3.41 eV, which is smaller than the value of
bulk material. It is well known that semiconductors with nanoscale
size show a blue shift in their spectra due to the quantum confine-
ment effects. However, the as-obtained samples have an Eg smaller
than the bulk one. This effect is likely due to the chemical defects
or vacancies present in the intergranular regions generating new
energy level to reduce the band gap energy [16].
(Ahꢃ)n = K(hꢃ − Eg)
where hꢃ is the photo energy, A is absorbance, K is a constant rela-
tive to the material and n is either two for direct transition or 1/2
for an indirect transition [29]. Hence, the optical band gap for the
absorption peak can be obtained by extrapolating the linear por-
tion of the (Ahꢃ)2–hꢃ curve to zero. The band gap is determined
by extrapolating the linear portion of the plot to the energy axis.
The band gap of the NiO particles is about 3.41 eV. Reported band
No linear relation was found for n = 1/2, suggesting that the
as-prepared NiO nanoparticles are semiconducting with direct
transition at this energy [28].
The magnetic property of Ni nanoparticles (obtained sample at
500 ◦C in argon atmosphere) has been measured (Fig. 7a). The sat-
uration magnetization (Ms) for the nanoparticles is 55.7 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 (90 Oe) [31]. Compared with the corresponding value for
bulk Ni, the coercivities of the Ni nanoparticles clearly show signif-
icant enhancement. This enhancement can probably be attributed
to the reduced particle sizes of obtained product [32] which change
the magnetization reversal mechanism. This demonstrates that the
magnetic properties of the Ni nanoparticles are greatly influenced
by their microstructure. The hysteresis loop of the Ni indicates a
ferromagnetic behavior. As shown in the Fig. 7a, 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 [33,34]. Fig. 7b shows the hysteresis loops of
NiO nanoparticles at 300 K. It can be seen that NiO nanoparticles
present a superparamagnetic behavior although NiO bulk material
is antiferromagnetic [35].
Fig. 6. UV–vis absorption spectrum and (Ahꢂ)2 vs. hꢂ curve (inset) for NiO nanopar-
ticles calcined at 500 ◦C.

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4. Conclusion
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Acknowledgements
Authors are grateful to the Council of Iran National Science Foun-
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financial support to undertake this work.
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