The effect of anion on the magnetic properties of nanocrystalline NiO synthesized by homogeneous precipitation

Article abstract of DOI:10.1016/j.jmmm.2009.03.039

The effect of using different anions (nitrate, chloride, sulfate, and acetate) during the precursor synthesis, by homogeneous precipitation, on the magnetic properties of the final product (nanocrystalline NiO), has been studied. The precursors and the ox

Full text of DOI:10.1016/j.jmmm.2009.03.039

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Journal of Magnetism and Magnetic Materials 321 (2009) 2556–2562
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Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
The effect of anion on the magnetic properties of nanocrystalline NiO
synthesized by homogeneous precipitation
Ã
V. Ranga Rao Pulimi, P. Jeevanandam
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of using different anions (nitrate, chloride, sulfate, and acetate) during the precursor
synthesis, by homogeneous precipitation, on the magnetic properties of the final product (nanocrystal-
line NiO), has been studied. The precursors and the oxide were characterized by various analytical
techniques including powder X-ray diffraction, FT-IR spectroscopy, thermal gravimetry (TGA), and
magnetic measurements. The synthesized NiO samples possess crystallite size in the range, ꢀ2ꢁ6 nm,
depending on the anion of the nickel salt. The nickel oxide nanoparticles exhibit superparamagnetic
behavior. Acetate and sulfate anions lead to NiO with higher saturation magnetization (ꢀ1.2ꢁ1.8 emu/
g), while chloride and nitrate anions lead to NiO nanoparticles with lower saturation magnetization
Received 15 December 2008
Received in revised form
4
February 2009
Available online 25 March 2009
Keywords:
Nanocrystalline oxide
Nickel oxide
Homogeneous precipitation
Magnetic measurement
(ꢀ0.1–0.4 emu/g) values. The observed magnetic behavior has been attributed to the size effect.
&
2009 Elsevier B.V. All rights reserved.
1.
Introduction
The magnetic properties in the nanoscale regime differ
2. Experimental methods
The synthesis of the precursor to NC-NiO was carried out by
homogeneous precipitation as follows [22,23]. In a 250 ml beaker,
about 5.2 mmol of a nickel salt (nickel nitrate, nickel chloride,
nickel sulfate, and nickel acetate) was dissolved in about 80 ml of
distilled water. To this, 0.083 mmol (5 g) of urea was added and
the contents were heated to 80 1C and kept for 4 h at this
temperature. During the reactions, green precipitates were
formed. The precipitates were filtered, washed with distilled
water many times followed with ethyl alcohol and then dried at
room temperature in air. The yield of the precursors was about
0.5 g. The precursors prepared using nickel nitrate, nickel chloride,
nickel sulfate, and nickel acetate will henceforth be referred as
precursor-1, precursor-2, precursor-3, and precursor-4, respec-
tively. The thermal gravimetric analysis (TGA) showed that the
green precursor powders, decompose at about 330ꢁ350 1C; the
observed overall weight loss percentage ranged from 28 to 36. So
the precursor powders were subjected to calcination at about
350 1C for 1 h inside a muffle furnace. Black powders were
obtained in all the cases which were analyzed by various
analytical techniques. The nickel oxide samples will be referred
to as NiO-1 to NiO-4.
considerably from those observed in bulk macro-crystalline
materials [1] and the magnetic behavior of nanoparticles has
been attributed to their size, shape, and surface effects [2–4]. The
interesting properties of magnetic nanoparticles include increased
magnetic moments [5], spin wave behavior [6], quantum tunnel-
ing [7], etc. The magnetic nanoparticles are of interest from the
point of view of fundamental research [8], and applications such
as magnetic recording [9], biomedicine [10], magnetic resonance
imaging and drug delivery [11], separation [12], sensors [13,14],
magnetic bar codes [15], etc. Among the magnetic nanoparticles,
antiferromagnetic nanoparticles (AFN) are promising candidates
for spin valves [16], magnetic random access memory [17], etc.
Among the AFN, NiO nanoparticles have been extensively studied
to understand the finite size and surface effects [18], spin glass
behavior [19], surface anisotropy [20], and magnetic anomalies
[
21]. In the present study, the effect of using different anions,
during the precursor synthesis, on the magnetic properties of
nanocrystalline NiO (the final product) was studied. To the best of
author’s knowledge, this effect has not been studied before.
Nanocrystalline NiO (NC-NiO) was synthesized by thermally
decomposing
a
-Ni(OH)
2
, a layered precursor, synthesized by
Powder X-ray diffraction (XRD) measurements were carried
homogeneous precipitation starting from different nickel salts.
out on a Bruker AXS D8-Advance powder X-ray diffractometer
˚
with CuꢁK
a
radiation (
l
¼ 1.5418 A) with a scan speed 21/min. FT-
IR spectroscopy measurements were done using a Thermo Nicolet
Nexus FT-IR spectrometer using KBr technique. TGA measure-
ments were carried out on a Perkin Elmer Thermal Analyzer (Pyris
Diamond) under nitrogen with a heating rate of 101/min in the
Ã
Corresponding author. Tel.: +911332 285444; fax: +911332286202.
E-mail address: jeevafcy@iitr.ernet.in (P. Jeevanandam).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.03.039

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9.3 nm
5
nm
8
nm
6.7 nm
10
20
30
40
50
60
70
2
θ (Degrees)
Fig. 1. Powder XRD patterns for the precursors synthesized by homogeneous precipitation. (a), (b), (c), and (d) are the samples prepared from nickel nitrate, nickel chloride,
nickel sulfate, and nickel acetate, respectively. The crystallite size of the precursor is also indicated.
Table 1
Elemental analysis data of precursors obtained by CHNS and AAS analyses.
a
S. no
Sample
Percentage of elements
Total weight loss % from TGA
Stoichiometry
C
H
N
S
Ni
1
2
3
4
Ni(OH)
Ni(OH)
Ni(OH)
Ni(OH)
2
-1
-2
-3
-4
5.9 (5.4)
5.6 (5.8)
4.4 (4.5)
4.5 (4.9)
1.9 (2.0)
2.4 (1.8)
2.0 (1.9)
1.9 (1.8)
5.9 (5.8)
6.4 (6.2)
4.4 (4.5)
5.2 (5.2)
–
53.5 (52.4)
51.5 (51.5)
55.8 (55.6)
53.5 (54.5)
36 (34)
32 (34)
28 (29)
30 (30.5)
[Ni(OH)1.46](OCN)0.46 (CO
3
)
0.04(H
2
O)0.4
2
2
2
–
[Ni(OH)1.4](OCN)0.5 (CO
3
)
0.05(H O)0.2
2
0.5 (0.0)
–
[Ni(OH)1.6](OCN)0.34 (CO
[Ni(OH)1.52](OCN)0.4 (CO
3
)
0.03(H
0.04(H
2
O)0.2
3
)
2
O)0.22
a
2 2
Ni(OH) -1 to Ni(OH) -4 refer to precursors obtained from different nickel salts. The values in the parentheses indicate those calculated using the molecular formula of
the precursors given in the last column.
temperature range, 25–600 1C. Atomic absorption spectrometric
measurements were done using a Perkin Elmer atomic absorption
spectrometer (AAnalyst 800). The carbon, hydrogen, nitrogen, and
sulfur (CHNS) analysis was performed using a Vario EL III CHNSO
elemental analyzer. TEM measurements were performed using a
FEI Technai 2G electron microscope operating at 200 kV. The
sample powders were dispersed in ethanol, under sonication, and
TEM grids were prepared using a few drops of the dispersion
followed by drying in air.
atomic absorption spectrometry (AAS), and the percentage of
other elements, as determined by the CHNS analyzer along with
their stoichiometries are given in Table 1. The FT-IR studies on the
precursor powders indicate the presence of characteristic
2
ꢁ
ꢁ
vibrations due to CO
, OCN , Ni–OH, and H O. On changing the
3 2
anions during the precursor synthesis, the stoichiometries show
minor variations with respect to the amount of anions in the
interlayer space and intercalated water molecules (Table 1). The
stoichiometry of the precursor samples are close to those reported
in the literature [24–27]. The TEM images recorded for all the
precursor samples are shown in Fig. 2. The TEM images show
features characteristic of turbostratic phases such as aggregation
of thin nanosheet-like structures. This is in accordance with what
3.
Results and discussion
First, the precursors were characterized using an array of
2
has been usually observed for a-Ni(OH) [24–27]. The mean
experimental techniques. The NiO samples, obtained by the
thermal decomposition of the precursors, were then character-
ized. The powder XRD patterns recorded for all the precursors
prepared using different nickel salts are shown in Fig. 1. From the
length and mean thickness of the precursors from TEM
measurements (Fig. 2) are given in Table 2. The mean length of
the precursor particles increases from about 87 nm in precursor-A
to about 307 nm in precursor-D. The mean thickness of the
precursor particles also increases from about 6.5 nm in precursor-
A to about 18 nm in precursor-D. The average crystallite size
calculated using Debye–Scherrer formula using full-width at half-
maximum of the most intense reflections (0 0 3) are also given in
Table 2. The average crystallite size of the precursor particles from
XRD can be compared to the thickness of the precursor since
analysis of XRD data, the product formed in all cases is
a
-Ni(OH)
2
˚
(JCPDS file: 38-0715) with a interplanar distance of ꢀ7.3 A,
calculated using (0 0 3) reflections.
2
a-Ni(OH) is a layered
hydroxy deficient compound with anions in the interlamellar
space for charge compensation [24–27]. The asymmetric nature of
the (101) reflections in the XRD pattern indicate the turbostratic
nature of the phases. The percentage of nickel, as determined by
2
(0 0 3) is the stacking direction for a-Ni(OH) , the precursor. The

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Fig. 2. TEM images for the precursors prepared from different nickel salts by homogeneous precipitation method: (a) nickel nitrate, (b) nickel chloride, (c) nickel sulfate,
and (d) nickel acetate.
Table 2
Summary of particle/crystallite size of precursors and NiO samples.
a
Mean length from TEM (nm)
Mean thickness from TEM (nm)
Average crystallite size (nm)
Precursor
A
B
C
D
87.279.9
92.7718.3
6.571.7
8.372.1
6.7
8
181.77113.6
306.97170.8
11.7710.6
17.9713.9
5
9.3
NiO-samples
NiO-1
228.37148.9
169.8770.1
274.57186.3
117.3736.7
5.672.1
8.471.3
9.6710.5
3.671.6
4
NiO-2
6.3
1.9
2.3
NiO-3
NiO-4
a
From XRD measurements.
observed mean thickness values from TEM images are slightly
larger compared to that obtained from the XRD data and this is
attributed to agglomeration.
precursor-2 possess relatively higher crystallite sizes (3.8 and
6.3 nm, respectively) compared to NiO obtained from precursor-3
and precursor-4 (1.9 and 2.3 nm, respectively). The size of
nanocrystalline NiO prepared in the present study is comparable
to that obtained by chemical methods such as sol–gel process
(3–22 nm) and reverse micro-emulsion method (ꢀ8 nm) but
smaller than that obtained by techniques such as spray pyrolysis
(14–30 nm), and microwave method (5–35 nm). The observed
TEM images for the nanocrystalline NiO samples are shown in
Figs. 4 and 5. The TEM images indicate the sheet-like nature of the
nanocrystalline NiO. The nanosheet morphology of the precursors
After calcination of the precursors at 350 1C for 1 h, the XRD
patterns were recorded and they are shown in Fig. 3. By
comparing the XRD patterns with the JCPDS Data, it was found
that the product is nickel oxide (JCPDS file no: 73ꢁ1519). All the
diffraction peaks are broad indicating the nanocrystalline nature
of the material. The crystallite size of NiO was calculated using
Debye–Scherrer formula using (2 0 0) reflections, and it is in the
range, ꢀ2–6 nm. The NiO samples obtained from precursor-1 and

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2559
2.3 nm
1.9 nm
6.3 nm
3.8 nm
10
20
30
40
50
60
70
2
θ (Degrees)
Fig. 3. Powder XRD patterns of nanocrystalline NiO samples obtained by thermal decomposition of the precursors. (a), (b), (c), and (d) are the NiO samples prepared from
precursor-1, to precursor-4, respectively.
Fig. 4. TEM images of nanocrystalline NiO obtained by calcination of the precursors, (a), and (b). a1, and b1 refer to the corresponding SAED patterns.

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Fig. 5. TEM images of nanocrystalline NiO obtained by calcination of the precursors, (c), and (d). c1, and d1 refer to the corresponding SAED patterns.
(
see Fig. 2) is retained even after calcination. The selected area
magnetization; smaller the crystallite size, greater will be the
surface spins with low coordination and broken exchange bonds,
and greater will be the net magnetization. The NiO samples,
prepared in the present study, possess smaller crystallite size
(ꢀ2–6 nm). This would lead to increased surface uncompensated
spins leading to M vs. H curves expected for superparamagnetic
nanoparticles (Fig. 6).
electron diffraction (SAED) patterns (Figs. 4 and 5) indicate the
crystalline nature of the samples. The discrete diffraction spots in
the pattern indicate highly oriented nature of nanocrystalline NiO.
This is observed for the NC-NiO obtained from precursor-2, and
precursor-4. In the remaining cases (i.e. NC-NiO’s from precursor-
1
and precursor-3), the observed diffraction rings indicate
polycrystalline nature of the samples. After calcination of the
precursors, the mean length of the NiO particles from TEM
measurements (Table 2) has increased from about 117 nm to about
In general, the NiO samples with smaller crystallite size, NiO-3,
and NiO-4 with crystallite sizes 1.9 and 2.3 nm, respectively, show
higher saturation magnetization at 10 kOe (ꢀ1–1.8 emu/g). How-
ever, the NiO samples with larger crystallite size, NiO-1 (3.8 nm),
and NiO-2 (6.3 nm) possess lower values of saturation magnetiza-
tion at the same applied field (ꢀ0.1–0.4 emu/g). When the
nanoparticles are small, it is not easy to distinguish the surface
and core effects; the surface/interface effects become more
prominent as particle size decreases. Nanoparticles of NiO with
smaller crystallite size (NiO-3 and NiO-4) possess a higher
fraction of surface uncompensated spins as compared to nano-
particles with bigger crystallite size (NiO-1 and NiO-2) and thus
possess higher saturation magnetization values. The observed
magnetic behavior is in agreement with that reported for NiO
nanoparticles. For example, Richardson et al. [31] have reported
that NiO with less than 100 nm exhibits superparamagnetism.
Khadar et al. have [32] reported that NiO with particle size 2–3 nm
exhibits superparamagnetic behavior while NC-NiO with particle
size 13–18 nm exhibits superantiferromagnetism. Rao et al. have
reported that NiO with average particle size 3–24 nm exhibits
superparamagnetism [33].
275 nm except in the case of precursor-D where a decrease is
observed. The mean thickness of the NiO particles varies from
about 3.6 nm (NiO-4) to about 9.6 nm (NiO-3) and is relatively
lower when compared to the thickness of the precursor particles
which varies from 6.5 to 18 nm.
The magnetization versus applied magnetic field (MꢁH) curves
for nanocrystalline NiO samples obtained from different precur-
sors are shown in Fig. 6. All the NiO samples show
superparamagnetic behavior. Bulk NiO is an antiferromagnet
with a Neel temperature of 523 K and a two sub-lattice model
has been used to explain its magnetic properties [28]. In the
nanoscale size region, the two sub-lattice model fails. The
nanoparticles are proposed to have multiple sub-lattice
magnetic structure, e.g. consisting of 4-, 6- or 8-sub-lattices
[
29]. Nanocrystalline NiO is an example, where surface effects can
dominate the net magnetic behavior. The nanoparticles possess
uncompensated spins due to a higher fraction of surface atoms
present on the surface of nanoparticles [30] leading to the net

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2
1
0
1
NiO-4
NiO-3
NiO-2
NiO-1
-
-2
-
10000
-5000
0
5000
10000
Applied Magnetic field (Gauss)
Fig. 6. Magnetization versus applied magnetic field plots for nanocrystalline NiO samples prepared from four different precursors. NiO-1 to NiO-4 refers to nickel oxides
obtained from precursor-1 to precursor-4, respectively.
The present study is an attempt to study the influence of
anions, during the precursor synthesis by homogeneous precipi-
tation, on the magnetic properties of nanocrystalline NiO. Using
acetate and sulfate leads to smaller NiO nanoparticles (ꢀ2 nm)
with increased saturation magnetization values compared to
nitrate and chloride anions which produce relatively larger NiO
crystallites (ꢀ4–6 nm) with smaller saturation magnetization
values. Using different anions during the synthesis lead to
precursors with different dimensions (Table 2). For example, on
going from nitrate to chloride to sulfate, the mean thickness of the
precursor particles (from TEM measurements) increases from 6.5
to 8.3 to 18.0 nm, respectively. Similar trend is observed even after
nanocrystalline NiO was studied. The precursors and the oxide
samples were characterized by powder XRD, FT-IR, TGA, ele-
mental analysis, TEM, and magnetic measurements. The crystal-
lite size of NiO is in the range ꢀ2–6 nm; NC-NiO samples with
smaller crystallite size (ꢀ2 nm) were obtained using nickel
sulfate, and nickel acetate, while use of nickel nitrate or nickel
chloride lead to relatively bigger crystallite size (ꢀ4–6 nm).
Magnetic measurements on nanocrystalline NiO indicate super-
paramagnetic behavior for all the samples. NiO samples with
smaller crystallite size possess higher saturation magnetization
(ꢀ1.2–1.8 emu/g), compared to those with bigger crystallite size
that show lower saturation magnetization values (ꢀ0.1–0.4 emu/
g). The present method is one of the ways to produce very small
nanocrystallites of nickel oxide.
conversion of the precursors to nanocrystalline NiO, except in the
ꢁ
case of CH
3
COO . The anion affects the agglomeration/growth
kinetics during the homogeneous precipitation process leading to
precursor particles of different dimensions. The anions present in
the solution interact with the initially formed smaller colloidal
particles thus providing a barrier for further growth resulting in
smaller particles.
It has been reported in the literature that variation in heat
treatment temperature of the precursor materials can also lead to
variation in crystallite size of NiO nanoparticles. For example,
calcination of precursors such as nickel oxalate dihydrate [34],
nickel citrate [35], and nickel carbonate hydroxide [36] at
different temperatures can lead to variation in the crystallite size
of the NiO. Calcination of nickel oxalate dihydrate yields NiO
crystallites of size 4.1–22 nm (calcination temperature:
Acknowledgements
PJ is grateful to Department of Science and Technology,
Government of India for funding under the SERC Scheme (SR/S1/
PC-06/2007). Thanks are also due to Mr. Chamarthi Srikanth for
his help in analyzing the TEM images.
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