Preparation and characterization of NiO nanoparticles from thermal decomposition of the [Ni(en)3](NO3)2 complex: A facile and low-temperature route

Article abstract of DOI:10.1016/j.poly.2010.12.044

NiO nanoparticles with an average size of 15 nm were easily prepared via the thermal decomposition of the tris(ethylenediamine)Ni(II) nitrate complex [Ni(en)₃](NO₃)₂ as a new precursor at low temperature, and the nanoparticles were characterized by thermal analysis (TGA/DTA), X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), UV-Vis spectroscopy, BET specific surface area measurement, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and magnetic measurements. The magnetic measurements confirm that the product shows a ferromagnetic behavior at room temperature, which may be ascribed to a size confinement effect. The NiO nanoparticles prepared by this method could be an appropriate photocatalytic material due to a strong absorption band at 325 nm. This method is simple, fast, safe, low-cost and also suitable for industrial production of high purity NiO nanoparticles for applied purposes.

Full text of DOI:10.1016/j.poly.2010.12.044

Polyhedron 30 (2011) 971–975
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Preparation and characterization of NiO nanoparticles from thermal
decomposition of the [Ni(en)3](NO3)2 complex: A facile and low-temperature route
⇑
Saeid Farhadi , Zeinab Roostaei-Zaniyani
Department of Chemistry, Lorestan University, Khoramabad 68135-465, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
NiO nanoparticles with an average size of 15 nm were easily prepared via the thermal decomposition of
the tris(ethylenediamine)Ni(II) nitrate complex [Ni(en)3](NO3)2 as a new precursor at low temperature,
and the nanoparticles were characterized by thermal analysis (TGA/DTA), X-ray diffraction (XRD), Fou-
rier-transformed infrared spectroscopy (FT-IR), UV–Vis spectroscopy, BET specific surface area measure-
ment, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission
electron microscopy (TEM) and magnetic measurements. The magnetic measurements confirm that
the product shows a ferromagnetic behavior at room temperature, which may be ascribed to a size con-
finement effect. The NiO nanoparticles prepared by this method could be an appropriate photocatalytic
material due to a strong absorption band at 325 nm. This method is simple, fast, safe, low-cost and also
suitable for industrial production of high purity NiO nanoparticles for applied purposes.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 7 December 2010
Accepted 27 December 2010
Available online 9 January 2011
Keywords:
NiO nanoparticles
Tris(ethylenediamine)Ni(II)
Metal oxides
Thermal decomposition
Ferromagnetic behavior
1. Introduction
ticles are still limited to the laboratory scale due to some unre-
solved problems, such as special conditions, tedious procedures,
Nanomaterials exhibit optical, catalytic, electronic and mag-
netic properties that are significantly different from those of the
corresponding bulk materials [1–3]. Among these materials, transi-
tion metal oxides have attracted much attention due to their out-
standing electrical, magnetic and catalytic properties [4]. Nickel
oxide (NiO) is one of the most important transition metal oxides
due to its applications in diverse fields, including the fabrication
of p–n heterojunctions [5], catalysis [6,7], electrochromic films
[8,9], fuel cell electrodes [10], gas sensors [11,12], battery cathodes
[13,14], magnetic materials [15,16], photovoltaic devices [17],
electrochemical capacitors [18] and smart windows [19]. Most of
these applications require particles with a small size and a narrow
size distribution. Because of the volume effect, the quantum size
effect and the surface effect, NiO nanoparticles are expected to pos-
sess many improved properties and even more attractive applica-
tions than those of bulk-sized NiO particles.
There are many methods for the preparation of NiO nanoparti-
cles, such as the coprecipitation method [20–22], microemulsion
method [23,24], ultrasonic radiation [25], hydrothermal synthesis
[26,27], anodic arc plasma method [28], microwave irradiation
[29], sol–gel method [30–32], low pressure spray pyrolysis
[33,34] amongst others. However, to the best of our knowledge,
most of the reported techniques for the synthesis of NiO nanopar-
complex apparatus, need for expensive agents or special equip-
ment, low-yield and high-cost. From a practical viewpoint, it is vi-
tal to develop a way to manufacture high-quality nanoparticles at a
high throughput with low cost.
The solid-state thermal decomposition of complexes is one of
the simplest and lowest-cost techniques for preparing pure and
nanosized transition metal oxides with relatively high specific sur-
face area at low temperature [35]. Because of the high degree of
homogenization of the precursor, much lower temperatures are
sufficient for the reaction to occur. This method exhibits many
advantages; no need for solvent, surfactant and complex appara-
tus, high yield, low energy consumption and simple reaction tech-
nology. In this context, various simple and mixed metal oxides
nanoparticles have been prepared by solid-state thermal decompo-
sition of their corresponding complexes [36–39].
In this work, which is a continuation of our studies on the prep-
aration of nanosized transition metal oxides via the decomposition
of complexes [40,41], we wish to report our results on the thermal
decomposition of the tris(ethylenediamine)Ni(II) nitrate complex
[Ni(en)3](NO3)2, which leads to the formation of pure NiO nanopar-
ticles at low temperature. The product was identified by X-ray
diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-
IR), UV–Vis spectroscopy, BET specific surface area measurement,
scanning electron microscopy (SEM), energy-dispersive X-ray
spectroscopy (EDX), transmission electron microscopy (TEM), ther-
mal analysis (TGA/DTA) and magnetic measurements.
⇑
Corresponding author. Tel.: +98 6612202782; fax: +98 6616200088.
E-mail address: sfarhad2001@yahoo.com (S. Farhadi).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.12.044

972
S. Farhadi, Z. Roostaei-Zaniyani / Polyhedron 30 (2011) 971–975
powder morphology was observed by a scanning electron micro-
2. Experimental
2.1. Preparation of materials
scope (SEM, Philips XL30) equipped with a link energy-dispersive
X-ray (EDX) analyzer. The particle size was determined by a trans-
mission electron microscope (TEM, Philips CM10,) at an accelerat-
ing voltage of 80 kV. The powders were ultrasonicated in ethanol
and a drop of the suspension was dried on a carbon-coated micro-
grid for the TEM measurements. The magnetic properties of the NiO
nanoparticles were measured with a vibrating sample magnetome-
ter (VSM) (BHV-55, Riken, Japan) at room temperature. The specific
surface area of the product was measured by the BET method using
a N2 adsorption–desorption isotherm carried out at ꢁ196 °C on a
Surface Area Analyzer (Micromeritics ASAP 2010). Before each
measurement, the sample was degassed at 200 °C for 2 h.
Although the [Ni(en)3](NO3)2 complex is not commercially
available, the synthesis is trivial and requires no specialized appa-
ratus. An aqueous solution of commercial Ni(NO3)2ꢀ6H2O is treated
with a slight stoichiometric excess of ethylenediamine (en) and the
resultant deep-purple [Ni(en)3](NO3)2 was precipitated by slow
addition of alcohol. After standing for several hours in the cold,
the crystals were filtered on a Buchner funnel, washed with alcohol,
ether, and dried in the open air at 50 °C. The complex was charac-
terized by FT-IR, elemental analyses (C, H, N) and thermal analysis.
Anal. Calc. for [Ni(en)3](NO3)2: C, 19.85; H, 6.67; N, 30.82. Found: C,
20.04; H, 6.72; N, 30.80%.
In order to prepare NiO nanoparticles, the [Ni(en)3](NO3)2 com-
plex was decomposed at various temperatures for 1 h in ambient
air. The temperatures for the decomposition of the complex in
the range of 200–400 °C were selected from the results of the
TGA-DTA analysis. The decomposition products were collected
for characterization.
3. Results and discussion
In order to obtain a better recognition of the thermal decompo-
sition process of [Ni(en)3](NO3)2, thermal analysis was done on this
complex. Fig. 1 shows the TG and DTA curves recorded for [Ni(en)3]
(NO3)2 at a constant heating rate of 10 °C minꢁ1 in the temperature
range 25–600 °C. The TG curve shows that the decomposition of the
complex proceeds in two main stages. The first stage occurs at
2.2. Characterization techniques
The XRD patterns were recorded by a Rigaku D-max C III, X-ray
diffractometer using Ni-filtered Cu K
a radiation (k = 1.5406 Å) to
determine the phases present in the decomposed samples. Infrared
spectra were recorded on a Shimadzu system FT-IR 160 spectropho-
tometer using KBr pellets. The elemental analysis (C, H, N) of the
starting complex was performed using a Carlo Erba 1106 instru-
ment. The thermal decomposition behavior of the precursor com-
plex was studied by a Netzsch STA 409 PC/PG thermal analyzer at
a heating rate of 10 °C/min in air. The optical absorption spectrum
was recorded on a Shimadzu 1650PC UV–Vis spectrophotometer
with the wavelength range 300–700 nm at room temperature.
The sample for UV–Vis studies was well dispersed in distilled water
to form a homogeneous suspension by sonication for 25 min. The
Fig. 2. XRD patterns of the [Ni(en)3](NO3)2 complex decomposed at selected
Fig. 1. Thermal analysis (TGA/DTA) of the [Ni(en)3](NO3)2 complex.
temperatures.

S. Farhadi, Z. Roostaei-Zaniyani / Polyhedron 30 (2011) 971–975
973
185 °C, showing a 16% weight-loss which is consistent with the the-
oretical value of 16.54% caused by the loss of one mole of ethylene-
diamine (en) per mole of complex. On further heating, the residue
[Ni(en)2](NO3)2 complex decomposed explosively at ca. 235 °C, fol-
lowed by a gradual weight loss up to 380 °C. Above 400 °C, the
weight remained constant, confirming the complete decomposition
of the complex. The weight loss of all the steps was about 80%,
which is consistent with the theoretical value (79.40%) calculated
for the formation of NiO from the complex. The DTA curve for the
[Ni(en)3](NO3)2 complex, as shown in the inset of Fig. 1, gave two
characteristic peaks, consistent with the TG data. The small endo-
thermic peak at 185 °C can be explained by freeing one en molecule,
resulting in the formation of the bis(ethylenediamine) complex,
according to the following reaction:
ged completely to nickel oxide. No characteristic XRD peaks arising
from other impurity phases are visible, indicating the preparation
of pure NiO by the present method. It can be seen from Fig. 2 that
the diffraction peaks are markedly broadened due to the small size
effect of the particles. Furthermore, the width of the NiO peaks de-
creases with an increase in the decomposition temperature up to
400 °C, because of crystallite growth. The average size of the NiO
particles was estimated to be about 15.5 nm by the Debye–Scher-
rer equation [42]: DXRD = 0.9k/(bcosh) where DXRD is the average
crystalline size, k is the wavelength of Cu Ka, b is the full width
at half maximum (FWHM) of the diffraction peak and h is the
Bragg’s angle.
The FT-IR spectra of the complex samples decomposed at differ-
ent temperatures are shown in Fig. 3. For the starting complex, the
characteristic stretching bands of NH2, CH2, NO3 are observed at
about 3250–3500, 2850 and 1350 cmꢁ1 [43]. As can be seen in
Fig. 3, all of these bands disappear when the complex is heated
up to 300 °C. At the same time, the bands attributable to carbonate
groups are observed in the 1300–1500 cmꢁ1 region for the samples
decomposed up to 300 °C [43]. The intensity of these bands de-
creases with increasing temperature, so that all of them disap-
peared at 400 °C. From these data, it can be concluded that the
carbonate groups are formed during the decomposition of the en
ligands of complex, and their contents decrease with the increase
of temperature. In the FT-IR of the product at 400 °C, the strong
band at 435 cmꢁ1 is assigned to the Ni–O stretching of the octahe-
dral NiO6 groups in the face center cubic structure [44]. It is noted
that the peaks at 1600 and 3600 cmꢁ1 in the FT-IR spectra of the
samples decomposed at 300 °C should be assigned to H2O absorbed
by the samples or KBr.
The morphology of [Ni(en)3](NO3)2 and its decomposition prod-
uct at 400 °C were investigated by SEM. Fig. 4a and b show the SEM
micrographs of the starting complex at two different scales. It is
obvious that the starting complex powder was made of large crys-
tals and grains with different shapes. The sizes of these grains are
in the range of 0.5–3.5 lm. The SEM micrographs of the product
powder in Fig. 4c and d clearly show that the shape and morphol-
ogy of the NiO are quite different from that of the precursor com-
½NiðenÞ3ꢂðNO3Þ2 ! ½NiðenÞ2ꢂðNO3Þ2 þ en
step I
ð1Þ
The big exothermic peak at about 235 °C is due to the explosive
decomposition of [Ni(en)2](NO3)2 complex via the redox process
taking place between the reducing agent (en ligands) and the oxi-
dizing agent (NO3ꢁ), leading to N2, NO, N2O, H2O, and CO2 gaseous
products, according to the following reaction:
½NiðenÞ2ꢂðNO3Þ2 ! NiO þ N2 þ N2O þ NO þ CO2 þ H2O
step II
ð2Þ
The XRD patterns of the complex and its decomposition prod-
ucts at various temperatures after 1 h are shown in Fig. 2. For the
sample decomposed at 250 °C, peaks attributable to the metallic
Ni phase (., JCPDS File No. 76-0147) and NiO (j, JCPDS Card No.
73-1523) are observed. At 300 °C, the characteristic peaks of cubic
NiO appeared together with a decrease in the intensity of the Ni
phase peaks. When the decomposition temperature increases to
350 °C, apparent diffractive peaks of nickel oxide appear with high
intensity, and the diffractive peaks of metallic nickel become
weakly evident. Finally, the XRD analysis for the sample decom-
posed at 400 °C, showed only the pattern corresponding to cubic
NiO, with lattice constants a = 4.1771 Å (JCPDS, File No. 73-1523).
This confirms that at this temperature the metallic nickel has chan-
Fig. 3. FT-IR spectra of the [Ni(en)3](NO3)2 complex decomposed at various temperatures.

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S. Farhadi, Z. Roostaei-Zaniyani / Polyhedron 30 (2011) 971–975
Fig. 4. SEM micrographs of the [Ni(en)3](NO3)2 complex (a and b) and the NiO nanoparticles (c and d) at different scales.
plex. It can be seen that the product was formed from extremely
fine spherical particles which were loosely aggregated. No charac-
teristic morphology of the complex is observed, indicating com-
plete decomposition into the extremely fine spherical particles.
Fig. 5 shows the TEM image of the NiO product. The TEM sample
was prepared with the dispersion of powder in ethanol by ultra-
sonic vibration. It can be seen that the uniform NiO particles had
spherical shapes with weak agglomeration. As shown in Fig. 5,
the particle sizes possess a narrow distribution in a range from
10 to 20 nm, and the mean particle diameter is about 15 nm. Actu-
ally, the mean particle size determined by TEM is very close to the
average particle size calculated by the Debye–Scherrer formula
from the XRD pattern. Such consistence implies that the formed
nanoparticles are single-phase.
The EDX analysis of the prepared NiO nanoparticles confirmed
that the atomic percentages of Ni and O are 50.2% and 49.8%,
respectively. Accordingly, the atomic ratio of Ni and O is about
1:0.98, which is consistent with the theoretical value of NiO.
The specific surface area of the NiO nanoparticles obtained from
the decomposition of complex at 400 °C for 1 h was measured to be
84.5 m2/g by the BET method. The particle size was also calculated
from the data of specific surface area by the equation: DBET = 6000/
(q
ꢃ SBET), where DBET is the diameter of a spherical particle,
q is
the theoretical density of NiO in g/cm3 and SBET is the specific sur-
face area of NiO powder in m2/g. The particle size calculated from
the surface area, assuming spherical particles, is about 17 nm,
which is in good agreement with the XRD and TEM results de-
scribed above.
Fig. 6 represents the UV–Vis spectrum of the NiO nanoparticles,
with a strong absorption band in the UV–Vis region (kmax
=
325 nm), indicating that the NiO nanoparticles prepared by this
method could be a promising photocatalytic material. This absorp-
tion band is attributed to the electronic transition from the valence
band to the conduction band in the NiO semiconductor. The direct
optical band gap (Eg) of the NiO nanoparticles can be estimated by
Fig. 5. TEM image of the NiO nanoparticles.
Fig. 6. UV–Vis spectrum of the NiO nanoparticles.

S. Farhadi, Z. Roostaei-Zaniyani / Polyhedron 30 (2011) 971–975
975
Acknowledgments
The authors gratefully acknowledge the Lorestan University Re-
search Council and Iran Nanotechnology Initiative Council (INIC)
for their financial support.
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In summary, pure and nanosized NiO particles with an average
particle size of 15 nm were successfully synthesized through ther-
mal decomposition of the [Ni(en)3](NO3)2 complex as a precursor
at 400 °C. From this complex, NiO is formed via the explosive
decomposition of the [Ni(en)3](NO3)2 complex due to a redox pro-
cess taking place between the reductants (en ligands) and the oxi-
dants (NO3ꢁ). By this method, uniform and spherical NiO
nanoparticles with weak agglomeration, narrow size distribution
and ferromagnetic behavior can be obtained. The optical absorp-
tion band gap of the NiO nanoparticles is 3.4 eV, which shows a
red shift in comparison with the bulk sample. This method is sim-
ple, low-cost, safe and suitable for industrial production of high
purity NiO nanoparticles for various applications. We expect that
this method of precursor thermal decomposition can be extended
to synthesize nanoparticles of other kinds of metal oxides using
the corresponding precursors.
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