Simple and low-temperature synthesis of NiO nanoparticles through solid-state thermal decomposition of the hexa(ammine)Ni(II) nitrate, [Ni(NH 3)6](NO3)2, complex

Article abstract of DOI:10.1016/j.poly.2011.01.028

NiO nanoparticles with an average size of about 12 nm were easily prepared via the thermal decomposition of hexa(ammine)Ni(II) nitrate complex, [Ni(NH ₃)₆](NO₃)₂, at low temperature of 250 °C. The product was 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 measurement. The magnetic measurement revealed a small hysteresis loop at room temperature, confirming a superparamagnetic (weak ferromagnetic) nature of the synthesized NiO nanoparticles. Indeed, the NiO nanoparticles prepared by this method could be an appropriate semiconductor material due to the optical band gap of 3.35 eV which shows a red shift in comparison with the previous reports. 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.2011.01.028

Polyhedron 30 (2011) 1244–1249
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Simple and low-temperature synthesis of NiO nanoparticles through solid-state
thermal decomposition of the hexa(ammine)Ni(II) nitrate,
[
Ni(NH ) ](NO ) , complex
3
6
3 2
⇑
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 about 12 nm were easily prepared via the thermal decompo-
sition of hexa(ammine)Ni(II) nitrate complex, [Ni(NH ) ](NO ) , at low temperature of 250 °C. The prod-
3 6 3 2
Received 30 December 2010
Accepted 28 January 2011
Available online 4 March 2011
uct was 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 micros-
copy (TEM), and magnetic measurement. The magnetic measurement revealed a small hysteresis loop
at room temperature, confirming a superparamagnetic (weak ferromagnetic) nature of the synthesized
NiO nanoparticles. Indeed, the NiO nanoparticles prepared by this method could be an appropriate semi-
conductor material due to the optical band gap of 3.35 eV which shows a red shift in comparison with the
previous reports. This method is simple, fast, safe, low-cost and also suitable for industrial production of
high purity NiO nanoparticles for applied purposes.
Keywords:
NiO
Nanoparticles
Hexa(ammine)Ni(II)
Complex
Thermal decomposition
Superparamagnetic
Ó 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
Among transition metal oxides, nickel oxide (NiO) has been re-
the method of solid-state thermal decomposition of complexes is
one of the simplest and lowest-cost techniques for preparing pure
transition metal oxides with relatively high specific surface area at
low temperature [52–57]. This method not only avoided usage of
the template and complex apparatus, but also exhibited affectivity
in the shape control of the target product. By selecting a proper
precursor, coupled with a rational calcining procedure, products
with unique size and shapes could be obtained. This method also
has potential advantages, including operational simplicity, high
purity and high yield of product, no need for solvent, low energy
consumption and no special equipment required.
ceived considerable attention due to their wide range of applica-
tions in various fields, such as catalysis [1–3], electrochromic
films [4–6], fuel cell electrodes [7] and gas sensors [8–10], battery
cathodes [11–15] and magnetic materials [16–19], photovoltaic
devices [20]. Furthermore, NiO is being studied for applications
in smart windows [21], electrochemical supercapacitors [22] and
dye-sensitized photocathodes [23]. Because of the quantum size
and surface effects, NiO nanoparticles exhibit optical, catalytic,
electronic, and magnetic properties that are significantly different
than those of bulk-sized NiO particles [24,25]. For example, NiO
nanostructures are p-type semiconductors with particular
magnetic behaviors such as superparamagnetic, superantiferro-
magnetic, and ferromagnetic order depending on the particle size,
particle shape, and synthesis route, whereas bulk-sized NiO is an
In this work, we wish to report on the thermal decomposition of
hexa(ammine)Ni(II) nitrate complex, [Ni(NH ) ](NO ) , which re-
3 6 3 2
sulted in the facile synthesis of pure NiO nanoparticles at low tem-
perature of 250 °C. 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), thermal analysis
(TG/DTA), and magnetic measurement.
antiferromagnetic insulator with a Neel temperature (T
50 °C [26,27].
Numerous new techniques have been developed for the prepa-
N
) of
2
ration of NiO nanoparticle, such as ultrasonic radiation, pyrolysis
by microwave, hydrothermal synthesis, sol–gel method, precipita-
tion–calcination method, carbonyl method, laser chemical method,
mechanochemical processing, microemulsion method, flame spray
pyrolysis, solid-state method, and so forth [28–51]. Among them,
2
. Experimental
.1. Preparation of [Ni(NH
A solution of Ni(NO
2
3
)
6
](NO
3 2
) precursor
3
)
2
ꢀ6H O (25 mmol) in 10 mL water was
2
⇑
Corresponding author. Tel.: +98 6612202782; fax: +98 6616200088.
E-mail address: sfarhad2001@yahoo.com (S. Farhadi).
placed in an ice-water bath and treated with a slight stoichiometric
excess (6 mL) of concentrated ammonia solution. The resultant
0
277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.01.028

S. Farhadi, Z. Roostaei-Zaniyani / Polyhedron 30 (2011) 1244–1249
1245
3 6 3 2
violet crystals of [Ni(NH ) ](NO ) was completely precipitated by
slow addition of 15 mL of cold ethanol. After standing for several
hours in the cold, the crystals are filtered on a Buchner funnel,
washed with alcohol, ether, and dried in open air at 50 °C. The
complex was characterized by FT-IR, thermal analysis, and elemen-
3 6 3 2
tal analyses. Anal. Calc. for [Ni(NH ) ](NO ) : Ni, 20.62; H, 6.32; N,
3
9.34. Found: Ni, 20.56; H, 6.41; N, 39.42%.
2.2. Preparation of NiO nanoparticles
The [Ni(NH ](NO complex was decomposed at various tem-
3
)
6
3 2
)
peratures for 1 h in ambient air. The temperatures for the decom-
position of the complex in the range of 200–400 °C were selected
from the results of TGA–DTA analysis. The decomposition products
were collected for characterization.
2.3. 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 Schimadzu system FT-IR 160 spectro-
photometer using KBr pellets. The elemental analysis of the start-
ing complex was performed using a Carlo Erba 1106 instrument.
The thermal decomposition behavior of the precursor complex
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 of 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
3 6 3 2
Fig. 1. TG and DTA (inset) curves of the [Ni(NH ) ](NO ) complex.
The de-ammination process resulted in the formation of the tetra-
and diamine complexes, in succession, according to the following
reactions:
2
5 min. The powder morphology was observed by a scanning elec-
tron microscope (SEM, Philips XL30) equipped with a link energy-
dispersive X-ray (EDX) analyzer. The particle size was determined
by a transmission electron microscope (TEM, Philips CM10) at an
accelerating voltage of 80 kV. The powders were ultrasonicated
in ethanol and a drop of the suspension was dried on a carbon-
coated microgride for the TEM measurements. The magnetic prop-
erties of NiO nanoparticles were measured with a vibrating sample
magnetometer (VSM) (BHV-55, Riken, Japan) at room temperature.
The specific surface area of the product was measured by the BET
½
NiðNH
3
Þ ꢂðNO
4
3
Þ ! ½NiðNH
2
3
Þ ꢂðNO
2
3
Þ þ 2NH
3
3
step 1
step 2
ð1Þ
ð2Þ
6
2
4
2
½NiðNH Þ ꢂðNO Þ ! ½NiðNH Þ ꢂðNO Þ þ 2NH
3
3
3
3
2
The composition of the [Ni(NH
3
)
2
](NO
3
)
2
complex, the final
product of the de-ammination process (steps 1 and 2), was con-
firmed by the elemental analysis, (Anal. Calc.: Ni, 27.11; H, 2.82;
N, 25.93. Found: Ni, 27.09; H, 2.77; N, 25.84%) and the FT-IR spec-
trum (see Fig. 2b below). The endothermic de-ammination process
is immediately followed by a big exothermic peak at about 235 °C.
The sharp exothermic peak can be explained by the explosive
method using N
196 °C on a Surface Area Analyzer (Micromeritics ASAP 2010). Be-
fore each measurement, the sample was degassed at 150 °C for 2 h.
2
adsorption–desorption isotherm carried out at
ꢁ
3 2 3 2
decomposition of [Ni(NH ) ](NO ) complex via an intramolecular
redox process taking place between the reductants (NH
3
ligands)
3
. Results and discussion
ꢁ
and the oxidants (NO
3
). The explosive redox reaction resulted in
the solid NiO and gaseous products i.e. N
according to the following reaction:
2
, NO, N
2
O and H
2
O,
3 6 3 2
Fig. 1 shows TG/DTA curves recorded for [Ni(NH ) ](NO ) at a
constant heating rate of 10 °C min in the temperature range of
5–600 °C. The TG curve shows that the decomposition of complex
proceeds in three main stages. The first stage occurred at 155 °C,
shows 12.25% weight-loss which is consistent with the theoretical
ꢁ1
2
½
NiðNH
3
Þ2ꢂðNO
3
Þ2 ! NiO þ ðNO þ N
2
O þ N
2
Þ þ H
2
O
step 3
ð3Þ
The FT-IR spectra of the complex samples decomposed at differ-
3
value of 11.9% caused by the loss of two mole of NH per mole of
complex. On further heating to 180 °C in second stage, the residue
complex shows about 12% weight-losses which related to liberate
ent temperatures are shown in Fig. 2. For the starting complex as
shown in Fig. 2a, the characteristic stretching bands of NH and
are observed at about 3250–3500 and 1350 cm , respectively
[58]. The intensity of these bands decreases when the complex is
heated at 200 °C (Fig. 2b), so that the spectrum in this temperature
3
ꢁ1
two other NH
observed at ca. 235 °C, followed by the gradual weight loss up to
50 °C. Above 250 °C, the weight remained constant, confirming
3
ligands. In third stage, an extensive weight loss is
NO
3
2
the complete decomposition of the complex. The weight loss of
all steps to be about 74% which is consistent with the theoretical
value (73.76%) calculated for the formation of NiO from the com-
3 2 3 2
can be related to the diamine [Ni(NH ) ](NO ) complex in consis-
tent with the TG–DTA data described above. As can be seen in
Fig. 2c, all bands of the complex were disappeared with increasing
ꢁ1
plex. The DTA curve for the [Ni(NH
3
)
6
](NO
3
)
2
complex as shown
temperature to 250 °C and a strong band at around 440 cm is ob-
in the inset of Fig. 1, gave three characteristic peaks in consistent
with TG data. The two small endothermic peaks at 155 and
served which assigned to the Ni–O stretching of the octahedral
NiO
6
groups in the face center cubic NiO structure [59]. At this
ꢁ1
1
3
80 °C can be explained by stepwise freeing four NH molecules.
temperature, the bands appeared at about 3550 and 1650 cm

1
246
S. Farhadi, Z. Roostaei-Zaniyani / Polyhedron 30 (2011) 1244–1249
Fig. 2. FT-IR spectra of the [Ni(NH
3 6
) ](NO
)
3 2
complex decomposed at various temperatures.
are related to the stretching and bending vibrations of the water
molecules absorbed by the sample or KBr. As shown in Fig. 2b–f,
in the FT-IR spectrum of the samples decomposed in the range of
acteristic morphology of the complex is observed, indicating the
complete decomposition into the extremely fine spherical
particles.
3
00–400 °C, only the strong band related to the Ni–O stretching
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. As shown in Fig. 5, the uniform NiO particles have
sphere shapes with weak agglomeration. The particle sizes possess
a narrow distribution in a range from 8 to 16 nm, and the mean
particle diameter is about 12 nm. Actually, the mean particle size
determined by TEM is very close to the average particle size calcu-
lated by Debye–Scherer formula from the XRD pattern.
Fig. 6 shows the EDX analysis of the product. Ni and O signals
come from the NiO nanoparticles and the atomic percentages of
Ni and O are 50.44% and 49.56%, respectively. Furthermore, the ra-
tio of Ni and O is about 1:0.98 consistent with the theoretical value
of NiO. The Si and Au signals come from the coating material of the
instrument.
was observed without any change.
3 6
XRD patterns of the decomposition products of the [Ni(NH ) ](-
) complex at various temperatures for 1 h are shown in Fig. 3.
3 2
NO
As shown in Fig. 3a, the XRD pattern of the sample decomposed at
50 °C reveals only the diffraction peaks attributable to NiO with
2
face-centered cubic phase at 2h = 37.40°, 43.45°, 62.95°, 75.40°,
and 79.45°, which can be perfectly related to (1 1 1), (2 0 0),
(
2 2 0), (3 1 1), and (2 2 2) crystal planes, respectively (JCPDS Card
No. 73-1523). This finding confirms that at 250 °C the complex was
decomposed completely to nickel oxide. No peaks of impurity were
found in the XRD pattern, indicating that the nanocrystalline NiO
obtained via this synthesis method consists of ultrapure phase. It
can be seen from Fig. 3a that the diffraction peaks are markedly
broadened due to the small size effect of the particles. The average
size of the NiO particles was estimated to be about 12.5 nm by the
Debye–Scherrer equation [60]: DXRD = 0.9k/(bcosh) where DXRD is
BET surface area measurements were also made on the NiO
nanoparticles obtained from the decomposition of complex at
2
250 °C. The specific surface area of the product was 73.75 m /g.
the average crystalline size, k is the wavelength of Cu K
a, b is the
Assuming the NiO nanoparticles are almost spherical, as confirmed
full width at half maximum (FWHM) of the diffraction peak and
h is the Bragg’s angle. As we can see in Fig. 3b and c, no new phase
is observed when the decomposition temperature increases to 300,
by TEM, the surface area can be used to estimate the particle size
according to DBET = 6000/(
q
ꢃ SBET) equation, where DBET is the
is the theoretical density
diameter of a spherical particle (in nm),
q
3
3
50, and then 400 °C, but the width of the NiO peaks decreases
of NiO (6.67 g/cm ), and SBET is the specific surface area of NiO
2
with an increase in the decomposition temperature because of
crystallite growth.
powder in m /g. The particle size calculated from the surface area
data is about 12.25 nm, which is in good agreement with the XRD
and TEM results.
3 6 3 2
Fig. 4 shows the SEM images of [Ni(NH ) ](NO ) complex and
its decomposition product at 250 °C. The SEM micrographs of the
starting complex at two different scales are presented in Fig. 4a
and b. It is obvious that the starting complex powder was made
of very large needle crystals in different sizes. The SEM images of
the product in Fig. 4c and d clearly shows that the shape and size
of particles are quite different from that of the precursor complex.
It can be seen that the product was formed from extremely fine
semi-spherical particles which were loosely aggregated. No char-
As shown in Fig. 7, the optical properties of the NiO nanoparti-
cles were investigated by UV–Vis spectroscopy. In order to a com-
parison, UV–Vis spectra of the starting [Ni(NH ) ](NO ) complex
3 6 3 2
and a commercial bulk NiO powder in water solvent were also pre-
sented in Fig. 7. As expected, the aqueous solution of the starting
complex in Fig. 7a shows two weak d–d bands at about 385 and
665 nm (the third band shielded by CT band) while only a strong
band at 335 nm is observed on the UV–Vis spectrum of the NiO

S. Farhadi, Z. Roostaei-Zaniyani / Polyhedron 30 (2011) 1244–1249
1247
Fig. 5. TEM image of the NiO nanoparticles.
nanoparticles in Fig. 7b. It is clearly evident that UV–Vis spectrum
of the NiO nanoparticles is quit different from that of the starting
complex, confirming that the strong band appeared at 335 nm is
2
+
due to NiO nanoparticle not Ni . In addition, the UV–Vis spectrum
of a commercial bulk NiO powder (Fig. 7c) does not show any ob-
servable absorption band, further confirming that the strong band
appeared at 335 nm is characteristic of the NiO nanoparticles.
Thus, the NiO nanoparticles prepared by this method could be a
promising photocatalytic material. The strong absorption band is
attributed to the electronic transition from the valence band to
3 6 3 2
Fig. 3. XRD patterns of the [Ni(NH ) ](NO ) complex decomposed at selected
temperatures.
3 6 3 2
Fig. 4. SEM micrographs of the [Ni(NH ) ](NO ) complex (a and b) and the NiO nanoparticles (c and d) at different scales.

1
248
S. Farhadi, Z. Roostaei-Zaniyani / Polyhedron 30 (2011) 1244–1249
The inset of Fig. 7 shows the plot of (Ahm m for the NiO
)2 versus h
sample. By the extrapolation on the linear region of this curve,
the band gap was estimated to be about 3.35 eV, which shows
some red shift in comparison with the previous reports [61,62].
The room temperature magnetic behavior of the NiO nanoparti-
cles presented in Fig. 8a and b shows a small hysteresis loop. As
shown in Fig. 8b, the shape of the hysteresis loop is characteristic
of superparamagnetic (weak ferromagnetic) behavior although
bulk NiO is antiferromagnetic [53–55,62–67]. From Fig. 8b, the
c r
coercive field (H ) and the remanent magnetization (M ) are esti-
mated to be only 66 Oe and 0.002 emu/g. The maximum field ap-
plied, 7 kOe does not saturate the magnetization and the
magnetization at this applied field is 0.065 emu/g. The low coer-
cive field and remanent magnetization confirm that NiO nanopar-
ticles exhibit a little ferromagnetic property. The non-saturation of
the magnetization is characteristic of superparamagnetic (weak
ferromagnetic) ordering of the spins in the nanoparticles. Nickel
oxide nanoparticles are made of small magnetic domains. Each
magnetic domain is characterized by its own magnetic moment
oriented randomly. The total magnetic moment of the nanoparti-
cles is the sum of these magnetic domains coupled by dipolar
Fig. 6. EDX spectrum of the NiO nanoparticles.
the conduction band in the NiO semiconductor [53–56]. The direct
optical band gap (E ) of NiO nanoparticles can be calculated by the
) equation, where hv is the photon energy, A is
the absorption coefficient, B is a constant relative to the material.
s
interactions [63]. As a result, a low value of M is obtained. The
g
magnetic properties of nanomaterials have been believed to be
highly dependent on the sample shape, crystallinity, magnetization
direction, and so on.
2
(
Ahv) = B(hv ꢁ E
g
Fig. 7. (a) UV–Vis spectrum of aqueous solution of the [Ni(NH
spectrum of a commercial bulk NiO powder.
3 6
) ](NO
)
3 2
complex, (b) UV–Vis spectrum and (Ahv)2 ꢁ hv curve (inset) of the NiO nanoparticles, and (c) UV–Vis

S. Farhadi, Z. Roostaei-Zaniyani / Polyhedron 30 (2011) 1244–1249
1249
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particle size of 13 nm were successfully synthesized through ther-
mal decomposition of the [Ni(NH ](NO complex as a precursor
at 250 °C. From this complex, NiO is formed via the de-ammination
process and then explosive decomposition of residue [Ni(NH ]-
NO complex due to redox process taking place between the
reductants (NH
3
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6
3 2
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(
3 2
)
ꢁ
3
ligands) and the oxidants (NO
3
). By this method,
uniform and spherical NiO nanoparticles with weak agglomeration,
narrow size distribution and superparamagnetic (weak ferromag-
netic) behavior can be obtained. The optical absorption band gap
of the NiO nanoparticles is 3.35 eV. This method is simple, low-
cost, safe and suitable for industrial production of high purity
NiO nanoparticles for various applications. The synthetic route
can be further used for the synthesis of other transition metal
oxides.
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