Synthesis and characterization of cobalt oxide nanoparticles by thermal treatment process

Article abstract of DOI:10.1016/j.ica.2009.07.023

The simple preparation of Co₃O₄ nanoparticles from a solid organometallic molecular precursor N-N′-bis(salicylaldehyde)-1,2-phenylenediimino cobalt(II); Co(salophen) has been achieved via two simple steps: firstly, the Co(salophen) p

Full text of DOI:10.1016/j.ica.2009.07.023

Inorganica Chimica Acta 362 (2009) 4937–4942
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and characterization of cobalt oxide nanoparticles
by thermal treatment process
c
a
Masoud Salavati-Niasari a,b, , Afsaneh Khansari , Fatemeh Davar
*
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
c Department of Chemistry, Faculty of Science, University of Guilan, Rasht, P.O. Box 413354-1914, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
The simple preparation of Co3O4 nanoparticles from a solid organometallic molecular precursor N-N0-
bis(salicylaldehyde)-1,2-phenylenediimino cobalt(II); Co(salophen) has been achieved via two simple
steps: firstly, the Co(salophen) precursor was precipitated from the reaction of cobalt(II) acetate and
N-N0-bis(salicylaldehyde)-1,2-phenylenediimino; H2salophen; in propanol under nitrogen condition;
then, cubic phase Co3O4 nanoparticles with the size of mostly 30–50 nm could be produced by thermal
treatment of the Co(salophen) in air at 773 K for 5 h. The as-synthesized products were characterized by
powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron
microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and scanning electronic microscopy (SEM).
These results confirm that the resulting oxide was pure single-crystalline Co3O4 nanoparticles. The opti-
cal property test indicates that the absorption peak of the nanoparticles shifts towards short wavelength,
and the blue shift phenomenon might be ascribed to the quantum effect. The hysteresis loops of the
obtained samples reveal the ferromagnetic behaviors the enhanced coercivity (Hc) and decreased satura-
tion magnetization (Ms) in contrast to their respective bulk materials.
Article history:
Received 7 April 2009
Received in revised form 18 July 2009
Accepted 23 July 2009
Available online 27 July 2009
Keywords:
Co3O4
Thermal treatment synthesis
Nanomaterials
Magnetic materials
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
mendous efforts have been directed in recent years to the synthesis
and investigation of properties of Co3O4 nanostructures.
Metal oxides can adopt a large variety of structural geometries
with an electronic structure that may exhibit metallic, semicon-
ductor, or insulator characteristics, endowing them with diverse
chemical and physical properties. Therefore, metal oxides are the
most important functional materials used for chemical and biolog-
ical sensing and transduction. Moreover, their unique and tunable
physical properties have made themselves excellent candidates for
electronic and optoelectronic applications. Nanostructured metal
oxides have been actively studied due to both scientific interests
and potential applications [1,2].
Co3O4 is an important antiferromagnetic p-type semiconductor
with excellent properties such as gas-sensing, catalytic and elec-
trochemical properties, and has been studied widely for applica-
tions in solid-state sensors, electrochromic devices and
heterogeneous catalysts as well as lithium batteries [3–5]. When
reduced down to the nanometer scale, Co3O4 were found to have
interesting magnetic, optical, field emission and electrochemical
properties that are attractive in device applications [6–8]. Thus tre-
So far there are several approaches available for the synthesis of
Co3O4 nanostructures, e.g. the nanocasting method for producing
Co3O4 nanowires[9], thesurfactanttemplated approachforfabricat-
ing Co3O4 nanoboxes [10], the mechanochemical reaction method
for the synthesis of Co3O4 nanoparticles [9], the thermal decomposi-
tion and oxidation route for the growth of Co3O4 nanorods [11], etc.
Besides, it has been reported that long time calcining of cobalt nano-
wires at 873 K in air oxidized them into Co3O4 nanotubes, and heat-
ing a cobalt foil for 12 h in air at 623 K led to the growth of Co3O4
nanowalls [12]. These methods are complicated and/or require long
time thermal treatment. New methodologies are therefore in great
demand for the synthesis and fabrication of Co3O4 nanostructures.
Among the numerous methods developed for synthesizing me-
tal oxides ultrafine powders, the organometallic molecular precur-
sor way has been regarded as one of the most convenient and
practical techniques, because it not only enables to avoid special
instruments and complicated processes and severe preparation
conditions, but also provides good control over purity, composi-
tion, homogeneity, phase and microstructure of the resultant prod-
ucts [13–17]. By choosing a proper organometallic molecular
precursor, coupled with a rational calcining procedure [13–17]
or other thermolysis processes (for example, high pressure
hydrothermal treatment), tiny and crystalline products could be
* Corresponding author. Address: 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).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.07.023

4938
M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 4937–4942
obtained under the conditions significantly milder than those em-
ployed in conventional solid-state synthesis, usually in the absence
of any other template or surfactant.
amine solution (0.01 mol) in 25 ml methanol. The contents were
refluxed for 3 h and a bright yellow precipitate of symmetrical
Schiff-base ligand (H2salophen) was obtained. The yellow precipi-
tate was separated by filtration, washed and dried in vacuum. It
was then recrystallized from methanol to yield H2salophen (92%).
Elemental and spectroscopic analysis of neat complex confirmed
the molecular composition of ligand.
In our group, we have been interested for a few years in the syn-
thesis of metal, metal oxide, magnetic nanoparticles and metal sul-
fides using new inorganic precursor, via decomposition methods,
taking profit of the tools of organometallic chemistry [18–22].
For instance, we could prepare Co3O4 nanorods by decomposition
of Co(oxalate) [23], while quasi-spherical nanoparticles with aver-
age size about 15–25 nm were synthesized by decomposition of
[bis(2-hydroxyacetophenato)cobalt(II)] [22]. We found that the
starting materials could be very important to synthesis of nanom-
aterials with different size and morphology, so, we decided to de-
velop our research. A major interest at the moment is in the
development of organometallic or inorganic compound for prepa-
ration of nanoparticles. Using of the novel compound can be useful
and open a new way for preparing nanomaterials to control nano-
crystal size, shape and distribution size.
Here, Co3O4 nanoparticles were synthesized of an easily ob-
tained solid organometallic molecular precursor; N-N0-bis(salicyl-
aldehyde)-1,2-phenylenediimino cobalt(II), Co(salophen); by
thermal treatment method. There are many advantages in it such
as: saving time and energy, free organic solvent, using cheap avail-
able chemicals, and the reaction temperature is easy to control. The
obtained Co3O4 were characterized by X-ray diffraction (XRD), Fou-
rier transform infrared spectroscopy (FT-IR), X-ray photoelectron
spectroscopy (XPS), transmission electron microscopy (TEM) and
scanning electronic microscopy (SEM). Magnetic properties of the
prepared sample were investigated by VSM.
2.4. Preparation of Co(salophen) [25]
The flask containing a stirred suspension of cobalt(II) acetate
tetrahydrate (3.96 g, 0.016 mol) in propanol (100 ml) was purged
with nitrogen, and then warmed to 323 K under a nitrogen atmo-
sphere. N-N0-bis(salicylaldehyde)-1,2-phenylenediimino cobalt(II)
(0.016 mol) was added in one portion, and the resulting black sus-
pension was then stirred and heated under reflux under a nitrogen
atmosphere for 8 h. Then the mixture was cooled and filtered un-
der reduced pressure. The collected solid was washed with dieth-
ylether and dried in air to give pink crystalline Co(salophen)
2. Experimental
2.1. Materials
All the chemical reagents used in our experiments were of ana-
lytical grade and were used as received without further purifica-
tion. Cobalt(II) acetate, salicylaldehyde and o-phenylenediamine
were obtained from Merck Co.
2.2. Characterization
Thermogravimetric analysis (TGA) were carried out using a
thermal gravimetric analysis instrument (Shimadzu TGA-50H)
Fig. 1. XRD patterns of (a) the precursor Co(salophen) and (b) Co3O4 nanoparticles
obtained by the thermal treatment of the precursor at 773 K for 5 h.
with a flow rate of 20.0 ml minꢀ1 and a heating rate of 10 °C minꢀ1
.
The metallic impurities were identified by Inductively Coupled
Plasma-Optical Emission Spectrometer (ICP-OES) Model Perkin–El-
mer PLASMA-1000. XRD patterns were recorded by a Rigaku D-
max C III, X-ray diffractometer using Ni-filtered Cu K
a radiation.
Scanning electron microscopy (SEM) images were obtained on Phi-
lips XL-30ESEM equipped with an energy dispersive X-ray spec-
trometer. Transmission electron microscopy (TEM) images were
obtained on a Philips EM208 transmission electron microscope
with an accelerating voltage of 100 kV. Fourier transform infrared
(FT-IR) spectra were recorded on Shimadzu Varian 4300 spectro-
photometer in KBr pellets. X-ray photoelectron spectrum (XPS)
was collected on an ESCALab MKII X-ray photoelectron spectrom-
eter, using nonmonochromatized Mg K
a X-ray as the excitation
source and C 1s (284.6 eV) as the reference line. The magnetic mea-
surement was carried out in a vibrating sample magnetometer
(VSM) (BHV-55, Riken, Japan) at room temperature.
2.3. Preparation of N-N0-bis(salicylaldehyde)-1,2-phenylenediimino
cobalt(II); H2salophen [24]
The stoichiometric amount of salicylaldehyde (0.02 mol) in dis-
solved methanol (25 ml) is added drop by drop to o-phenylenedi-
Fig. 2. TGA curve of the Co(salophen) complex.

M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 4937–4942
4939
773 K for 5 h, it was allowed to cool to the room temperature
naturally.
3. Results and discussion
Fig. 1a displays a typical XRD pattern of the precursor prepared
at reflux for 8 h. All the reflection peaks in this pattern could be
readily indexed to crystalline Co(salophen) (JCPDS Card file No.
74–1656). No obvious peaks of impurities were seen in this pat-
tern. The XRD pattern shown in Fig. 1b is corresponding to the
sample obtained by thermal decomposition and oxidization of this
compound at 773 K for 5 h. All of the reflection peaks could be
readily indexed to crystalline cubic phase Co3O4 with a lattice con-
stant of a = 8.065 Å, which is consistent with the standard value of
a = 8.065 Å (JCPDS Card file No. 74–1656). The crystallite sizes of
the as-synthesized nickel, Dc, was calculated from the major dif-
fraction peaks of the base of (1 1 1) using the Scherrer formula
(Eq. (1)) [26],
Kk
b cos h
Dc ¼
ð1Þ
Fig. 3. IR spectra of (a) Co(salophen) and (b) the product obtained on thermal
treatment of the precursor at 773 K for 5 h.
where K is a constant (ca. 0.9); k is the X-ray wavelength used in
XRD (1.5418 Å); h is the Bragg angle; b is the pure diffraction broad-
ening of a peak at half-height that is, broadening due to the crystal-
lite dimensions. The diameter of the nanoparticles calculated by the
Scherrer formula is 30 nm. No other peaks for impurities were
detected.
The TGA curve of the Co(salophen) is shown in Fig. 2. There is
only one weight loss step in the temperature range of 573–763 K.
The weight loss may be ascribed to the decomposition of Co(salo-
phen) with residues amounting to cobaltous oxides. The weight
loss was found to be ꢁ34% which corresponds to the loss of organic
compounds to form the cobalt oxide, the observed weight loss is
close to the theoretical value. Upon the calcination of the obtained
Co(salophen) at 773 K in air, all the cobalt salophen was converted
into Co3O4 nanoparticles. The XRD pattern of the calcined sample is
shown in Fig. 1b.
which was purified by recrystallization from chloroform. Elemen-
tal analysis indicates that complex is monomeric species formed
by coordination of 1 mol of cobalt ion and 1 mol salophen. The me-
tal chelate in this study is insoluble in water but soluble in most
organic solvents. Electrical conductivity measurements of the me-
tal complex give KM values of 20
X
ꢀ1 cmꢀ1 molꢀ1 and confirm that
they are no electrolyte. The chemical composition confirmed the
purity and stoichiometry of the neat complex.
2.5. Synthesis of Co3O4 nanoparticles
Black Co3O4 nanocrystals were produced by subjecting 0.01 mol
of the as-prepared Co(salophen); powders to heat treatment at a
relatively low temperature (773 K) in air. An average temperature
increase of 303 K every minute was selected before the tempera-
ture reached 773 K, and after keeping the thermal treatment at
The addition of H2salophen to the propanol solution of cobalt
acetate provokes the precipitation of a pink solid of cobalt Schiff-
base as shown by FT-IR spectrum in Fig. 3a. The broad band at
Fig. 4. EDX spectrum of the Co3O4 nanoparticles.

4940
M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 4937–4942
Fig. 5. (a) SEM, (b) TEM image of Co3O4 nanoparticles obtained at 773 K for 5 h, SEM images of obtained Co3O4 at (c) 873 K and (d) 973 K for 5 h.
Table 1
Different reaction conditions for synthesis of Co3O4 nanoparticles.
Reaction temperature (K)
Reaction time (h)
Reaction atmosphere
Composition of the products (XRD result)
673
773
873
973
673
773
873
973
5
5
5
5
7
7
7
7
air
air
air
air
air
air
air
air
Co(salophen) (main) + Co3O4(minor)
Co3O4 (30 nm)
Co3O4 (40 nm)
Co3O4 (48 nm)
Co(salophen) (main) + Co3O4(minor)
Co3O4 (40 nm)
Co3O4 (45 nm)
Co3O4 (55 nm)
3440.1 cmꢀ1 is assigned to both the
m
s(O–H) and
mas(O–H) of
To exploit the effect of process parameter on the synthesis of
the Co3O4, various thermal treatment temperatures and times
were investigated in the synthesis of Co3O4. As shown in Table 1,
the calcination time also had an impact on the size of nanoparti-
cles, the longer the calcination the larger size, as shown in Table
1. As shown in Fig. 5c and d, the increase of calcination tempera-
ture resulted in obvious size growth of the particles.
hydration water. The of the spectra of the Co(salophen) showed
evidence for the synthesis of the complex since two bands at
1607 cmꢀ1 (attributed to C@C) and at 1537 cmꢀ1 (attributed to
m
mC@N) which are characteristic peaks of the Co(salophen) complex
appeared the IR spectra of the precursor [27]. The spectra (Fig. 3b)
also contained two strong absorption bands at 668.6 and
579.0 cmꢀ1 which confirm the spinel structure of Co3O4. The for-
mer peak at 668.6 cmꢀ1 is attributed to the stretching vibration
mode of M–O in which M is Co+2 and is tetrahedrally coordinated.
The band at 581 cmꢀ1 can be assigned to the M–O in which M is
Co+3 and so coordinates octahedrally [28,29]. Very weak absorp-
tions due to C–H and C@N stretching vibrations indicate that the
Co3O4 nanoparticles may be very poorly capped by the surfactant.
This result further confirmed the formation of Co3O4 by the above-
mentioned thermal treatment.
Chemical purity and stoichiometry of the Co3O4 was tested by
EDX. The strong peaks related to Co and O is found in the spectrum
Fig. 4. EDX spectrometry for nanoparticles shown oxygen that con-
firm sample was Co3O4. The Cu peaks are due to the 300 mesh cop-
per grid.
The morphology of the Co3O4 nanoparticles is examined by SEM
(Fig. 5a). From the micrograph, it was observed that the nanopar-
ticles were semi-spherical. The TEM image (Fig. 5b) shows the
presence of dense agglomerates. The particles have a spherical
shape, and their distribution, likewise, is not uniformed (30–
50 nm).
Fig. 6. High-resolution XPS core spectrum for Co 2p in the Co3O4 nanoparticles.

M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 4937–4942
4941
2
Fig. 7. (a) Optical absorption spectrum and (b) (
a
h
m
)
ꢁ h
m
curve for the Co3O4 nanoparticles.
H) [34]. Cobalt oxide nanoparticles are made of small crystalline
domains. Each crystalline domain is characterized by its own mag-
netic moment oriented randomly. The total magnetic moment of
the nanoparticles is the sum of these magnetic domains coupled
by dipolar interactions. As a result, a low value of Ms is obtained.
The magnetic properties of nanomaterials have been believed to
be highly dependent on the sample shape, crystallinity, magnetiza-
tion direction, and so on.
In our thermal treatment synthesis of Co3O4 powders, the
source materials used were only Co(salophen). Like Schiff-base li-
gand, salophen may also take the role of electron transfer, display-
ing moderate intensity reductive property under the high
temperature condition. Especially, when salophen was firstly che-
lated with Co(II) to form a relatively stable coordination com-
pound, Co(salophen), the short distances between cobalt and
oxygen or nitrogen atoms in this solid molecular precursor were
favorable for electron to transfer. Furthermore, the thermal system
usually afforded a great driving force for the self-oxidation–reduc-
tion reaction processes to initiate. Thus, self oxidation-reduction
reactions of the Co(salophen) should take place to form Co3O4 un-
der the current thermal condition of 773 K for 5 h.
Fig. 8. Magnetization vs. applied field at 300 K for Co3O4 nanoparticles.
The purity and composition of the obtained samples were fur-
ther investigated by XPS measurements. The Co 2p core spectrum
is shown in Fig. 6. The two strongest peaks at 780.35 and 795.65 eV
correspond to Co 2p3/2 and Co 2p1/2, respectively, characteristic of
the Co3O4 phase [30]. (Co 2p1/2ꢀCo 2p3/2) energy separation is
approximately 15.3 eV. All of the result confirms the formation of
the pure Co3O4 nanoparticles.
4. Conclusion
The obtained Vis-NIR spectra for samples synthesized at 500 °C
are reported in Fig. 7a. The I band (k = 1510 nm) was attributed to
crystal field 4A2(F) ? 4T1(F) transitions in the Co3O4 structure [31].
The II signal (k = 1261 nm) was assigned to an ‘‘intervalence”
charge-transfer Co(II) M Co(III), representing an internal oxida-
tion–reduction process [32]. There are two absorption peaks (III,
k = 719 nm and IV, k < 500 nm) being obviously found in Fig. 7a,
which indicating ligand–metal charge-transfer events O(II) ? -
Co(III) and O(II) ? Co(II), respectively [9]. Co3O4 is P-type semicon-
ductor and its optical band gap can be obtained by the following
equation (Eq. (2)):
Adopting the self-prepared Co(salophen) as precursor, Co3O4
nanoparticles have been synthesized by thermal treatment meth-
od. The proposed methods to Co3O4 nanoparticles were simple,
mild and cheap, which makes them very suitable for scale-up pro-
duction. Furthermore, it is well expected that such techniques
would be extended to prepare many other important semiconduct-
ing metal oxides ultrafine powders, because many other metal ions
such as Zn(II), Cd(II), Ni(II), Cu(II), Mn(II), etc. can also easily form
solid metal salophen compounds, which are inclined to decompose
into metal oxides upon thermal treatment. And the optical absorp-
tion properties of the Co3O4 nanoparticles were investigated. These
results indicate that the nanoparticles are semiconducting with di-
rect transitions at 1.53 and 2.02 eV. The optical property test indi-
cates that the absorption peak of the nanoparticles shifts towards
short wavelength. And the blue shift phenomenon might be as-
cribed to the quantum effect.
n
ð
ahm
Þ ¼ Bðh
m
ꢀ EgÞ
ð2Þ
where h
m
is the photo energy,
a
is the absorption coefficient, B is a
constant relative to the material, and n is either 2 for a direct tran-
sition or 1/2 for an indirect transition. The (
a
h
m
)2 ꢁ h
m curve for the
products is shown in Fig. 7b, the band gaps of as-obtained Co3O4 are
2.02 and 1.53 eV, revealing obvious red shift of absorption peaks in
comparison with the previous report [33].
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