A simple approach for the synthesis of Co3O4 nanocrystals

Article abstract of DOI:10.1134/S0020168511040017

By using Co²⁺ and Co³⁺ salts, and freshly extracted ovalbumin, Co₃O₄ nanocrystals have been synthesized successfully. The pH of the solution was self-regulated for the hydrolysis of metal ions as the ovalbumin-water mixture was highly basic. Water soluble ovalbumin proteins served as a perfect matrix for entrapment of Co²⁺ and Co³⁺ ions thus forming a gel. Upon heat treatment, the dried gel precursor decomposed into nanocrystalline Co₃O₄. The crystallite size obtained by XRD line profile fitting was 45 ± 8 nm and particle size estimated from the SEM was in the range 20 nm-2 μm. EPR results show a very good fit to literature reports for nanocrystals in the size range of 8-17 nm. Even though the overall particle size is quite large and its distribution is quite wide EPR results confirm nanocrystalline nature of the particles obtained. Presented route is simple, cost effective, and environmentally friendly.

Full text of DOI:10.1134/S0020168511040017

ISSN 0020ꢀ1685, Inorganic Materials, 2011, Vol. 47, No. 4, pp. 426–430. © Pleiades Publishing, Ltd., 2011.
A Simple Approach for the Synthesis of Co3O4 Nanocrystals1
T. O. Ahmadova, Z. Durmusa, A. Baykala, and H. Kavasb
a Department of Chemistry, Fatih University, B. Çekmece, 34500 Istanbul, Turkey
b Department of Physics, Fatih University, B. Çekmece, 34500 Istanbul, Turkey
eꢀmail: hbaykal@fatih.edu.tr
Received May 22, 2009
Abstract—By using Co2+ and Co3+ salts, and freshly extracted ovalbumin, Co3O4 nanocrystals have been
synthesized successfully. The pH of the solution was selfꢀregulated for the hydrolysis of metal ions as the ovalꢀ
buminꢀwater mixture was highly basic. Water soluble ovalbumin proteins served as a perfect matrix for entrapꢀ
ment of Co2+ and Co3+ ions thus forming a gel. Upon heat treatment, the dried gel precursor decomposed
into nanocrystalline Co3O4. The crystallite size obtained by XRD line profile fitting was 45
8 nm and parꢀ
ticle size estimated from the SEM was in the range 20 nm–2 . EPR results show a very good fit to literature
µ
m
reports for nanocrystals in the size range of 8–17 nm. Even though the overall particle size is quite large and
its distribution is quite wide EPR results confirm nanocrystalline nature of the particles obtained. Presented
route is simple, cost effective, and environmentally friendly.
DOI: 10.1134/S0020168511040017
1
INTRODUCTION
solution, egg white has been used with metal ions in
solution, egg white has been used as a binder cum gel
for shaping material, particularly bulk and porous
ceramics [19–21]. The use of egg white simplifies the
process and would provide another alternative process
for the simple and economical synthesis of nanocrysꢀ
talline ceramic particles [22]. Most recently, it was
used as a matrix for entrapment of aluminum ions to
Among transition metal oxides, cobalt oxide
(Co3O4) is known as promising materials due to its
extensive applications in lithiumꢀion batteries, gas
sensing, data storage, catalysis and electrochromic
devices [1–5]. Co3O4 is very attractive in the applicaꢀ
tion of oxidation reactions because of the presence of
mobile oxygen, i.e., unsupported cobalt oxide is an
active catalyst in air pollution control for the abateꢀ
ment of CO [6], NOx [7] and organic pollutants from
effluent streams [8]. Co3O4 is an important magnetic
generate a gelled precursor which resulted in –aluꢀ
α
mina particles with crystallite size of 15–80 nm [23].
Kumari et al. [24] and Maensiri et al. [22] used eggꢀ
white as a water soluble polymeric matrix to produce
nanocrystalline LiMn2O4 and NiFe2O4 nanocrystals
respectively. Recently, Durmus et al. synthesized
Mn3O4 nanocrystals by using ovalbumin [25].
We hereby report on an innovative green chemistry
approach adapted for the synthesis of Co3O4 nanocrysꢀ
tals via calcination of a precursor obtained by gelling
and hydrolyzing Co2+ and Co3+ ions in eggꢀwhite proꢀ
tein matrix. To the best of our knowledge, this study is
the first report on the synthesis of Co3O4 nanocrystals
using eggꢀwhite, ovalbumin.
p
ꢀtype semiconductor. It belongs to the normal spinel
crystal structure based on a cubic close packing array
of oxide ions, in which Co(II) ions occupy the tetraheꢀ
dral 8a sites and Co(III) ions occupy the octahedral
16d sites [9]. Much effort has been made to prepare
Co3O4 nanocrystals with various morphologies [10, 11]
using several techniques including sol–gel [12, 13],
reduction/oxidation route [14], thermal decomposition
[15], metalꢀorganic chemical vapor deposition
(MOCVD) [16], and chemical spray pyrolysis [17].
However, all of these methods need relatively high
reaction temperatures and are therefore inconvenient
EXPERIMENTAL
for the bulk production of nanocrystalline Co3O4
.
Co(ac)2
⋅
4H2O and Co(acac)3 4H2O and freshly
⋅
A new simplified route to prepare nanocrystalline
ceramic nanocrystals is proposed. Egg white proteins
are well known for their gelling, foaming and emulsiꢀ
fying characteristics, binding adhesion in addition to
their high nutrition quality [18]. Due to its solubility in
water and its ability to associate with metal ions in
extracted egg white (ovalbumin) were used as the startꢀ
ing materials. 40 ml of egg white was first mixed with
40 ml deionized water with vigorous stirring at room
temperature until a homogeneous solution was
obtained. Subsequently, 0.5 g of Co(ac)2
0.72 g of Co(acac)3 4H2O (a mole ratio corresponding
to the nominal composition of Co2+ : Co3+ ratio of 1 : 2)
⋅
4H2O and
⋅
1
The article is published in the original.
426

A SIMPLE APPROACH FOR THE SYNTHESIS OF Co3O4 NANOCRYSTALS
427
1
2
10
20
30
40
, deg
50
60
70
2
θ
Fig. 1. Xꢀray powder diffraction pattern (
gel precursor at 800 for 3 h.
1) and line profile fitting (2) for the nanocrystals obtained after calcination of the dried
°
C
were added slowly to the egg white solution with vigorꢀ Co3O4 spinel oxide with ICDD card no of 42ꢀ1467. No
ous stirring for 2 h to obtain a wellꢀdissolved solution. secondary phases or impurities were observed (Fig. 1).
Throughout the whole process described above, no pH Chemical compositions of samples were confirmed by
adjustment was made as the pH was measured around AAS. Based on this composition then we attempted to
~9.6. Then, the mixed solution was evaporated by calculate the lattice constants for the investigated sysꢀ
heating in an oven at 80°C for 3 h until a dried precurꢀ tems. The sharpness of XRD peaks reveal high crystalꢀ
sor was obtained, crushed into powder and then was linity of the nanocrystals. The mean size of the crystalꢀ
calcinated at 800
°
C
for 3 h.
lites was estimated from the powder diffraction pattern
by performing line profile fitting using the Eq. (1)
described in Wejrzanowski et al. [26] and Pielaszek [27].
The line profile, shown in Fig. 1, was fitted for 8 peaks
111, 220, 311, 222, 400, 422, 511, 440 and the average
Fourier transform infrared (FTIR) spectra of the
samples were recorded in transmission mode with a
Perkin Elmer BX FTꢀIR infrared spectrometer with
KBr (4000–400 cm–1). XRD analysis was conducted
on a Huber JSODEBYEFLEX 1001 difractometer
crystallite size (DXRD) was calculated as 45
8 nm.
Taking into account the cubic symmetry of the
Co3O4, the lattice constant for each peak of each samꢀ
ple was calculated by using the formula:
operated at 40 kV using Cu
K radiation. Atomic absorpꢀ
α
tion spectroscopy (AAS) (Varian SpectrAAꢀ200) was
used to analyze the content of metal ions by dissolving
a known weight of powder and analyzing it for the
metallic components in the composition. SEM analyꢀ
sis was performed, in order to investigate the microꢀ
structure of the sample using FEI XL40 Sirion FEG
Digital Scanning Microscope. A conventional Xꢀband
a
=
dhkl
( + + l
h2 k2 2)1/2,
where h, k, and l are miller indices for the same crysꢀ
talꢀlattice planes. To determine the exact lattice
parameter for each composition, Nelson–Riley
(f = 9.8 GHz) Bruker EMX model EPR spectrometer
method was used. The Nelson–Riley function
given as
F(θ) is
employing an ac magnetic field (100 kHz) modulation
technique was used to record the first derivative
absorption signal. The thermal stability was deterꢀ
mined by thermogravimetric analysis (TGA, Perkin
Elmer Instruments model, STA 6000, with heating
F(θ
) = 1/2[(cos2
The values of lattice constant a0 of all the peaks for
each sample was plotted against (not given). Then
θ/sin
θ
) + cos2
θ/θ].
F( )
θ
rate of 10 C/min in the temperature range of 30–
°
using a least square fit method exact lattice constant a0
was determined. The point where the least square fit
1000°C
under nitrogen flow).
straight line cuts the
actual lattice parameter of the composition which is
obtained as 0 = 8.083 Å.
The formation of the spinel Co3O4 structure was
der and revealed that the only phase observed was further supported by FTꢀIR spectrum presented in
yꢀaxis (where
F(θ) = 0) is the
RESULTS AND DISCUSSION
a
4
XRD analysis was performed on the obtained powꢀ
INORGANIC MATERIALS Vol. 47
No. 4 2011

428
AHMADOV et al.
phologies makes proper average size estimation rather
difficult, however, a broad distribution of particles
with sizes ranging from 20 nm up to several micromeꢀ
ters are present. Very smooth surfaces of polyhedra and
particles with flake morphology suggest a strong passiꢀ
vation of some crystalline faces by the protein matrix
thus resulting in anisotropic growth due to the inhibiꢀ
tion of growth in some crystal directions.
Thermal stability of the precursor powder and final
powder has been analyzed using TGA. Thermograms
for both samples are presented in Fig. 4. Calcined
powder shows no weight loss up to ~850 C after which
°
Co3O4
Precursor
it looses about 5% weight. This could be due to very
stable residual organometallic/organic compounds
that decompose at such elevated temperatures. Precuꢀ
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber, cm–1
sor powder shows an overall weight loss of ~90%
.
According to the differential thermogram, DTG,
there is fourꢀstep weight loss (indicated with arrows)
for the precursor powder, of mainly ovalbumine,
Fig. 2. FTIR spectrum of precursor and Co O nanocrysꢀ
tals obtained after calcination of the dried gel precursor at
800 for 3 h.
3
4
matrix the first one at <200
the most intense one at ~300
~850 . The observed thermogram perfectly matches
that of pure ovalbumin reported by Kumari et al. [24]
above 200 . Weight loss up to 200 is due to the
adsorbed water and boiling of organic residues
released from Co salts. Weight loss around ~240
°C
, a shoulder at ~240
°
C,
°
C
°
C
and the last one at
°C
Fig. 2. The intensive broadband at ~3450 cm–1 and the
less intensive band at ~1620 cm–1 are assigned to O–H
stretching vibration interacting through H bonds.
Traces of adsorbed or atmospheric CO2 are evidenced
by the very small absorption peak around 2340 cm–1
In the range of 1000–100 cm–1, two main metal–oxyꢀ
ν1 ν2 were
gen bands at ~662 cm–1 and ~570 cm–1
observed in the FTꢀIR spectra given in Fig. 2. These
two bands are usually assigned to vibration of ions in
the crystal lattices. The band at ~662 cm–1 correꢀ
sponds to intrinsic stretching vibrations of the metal at
the tetrahedral site (Co
~397 cm–1 is assigned to octahedralꢀmetal stretching
(Co O) [28–31].
°C
°C
°C
suggests that water molecules in albumin are strongly
bonded to the protein molecules and that it takes
higher energies to remove them. The considerable
.
(
)
( )
weight loss at ~300
protein matrix and the last one at ~850
°
C
is due to the decomposition of
is due to the
°C
decomposition of Co3O4 powder to CoO and oxygen
gas [32].
The room temperature EPR spectra of Co3O4 parꢀ
ticles show a broad single signal with nonsymetric
lineshape similar to those in study of Dutta et al. [33]
in which the nano and bulk Co3O4 are compared. The
O), whereas the band at
EPR parameters; resonance field
lineꢀwidth HPP factor and
as 3185 G, 2160 G, 2.18 and 1.02 from Fig. 5. The
–
HRes, peak to peak
SEM analysis was performed in order to investigate
the morphology of formed particles and understand
the influence of the protein matrix on the shape of forꢀ
mations. Figure 3 shows the SEM micrographs of
Co3O4 particles synthesized by eggꢀwhite at different
magnifications. Various particle morphologies includꢀ
ing octahedron, truncated octahedron, tetrahedron,
–Δ
,
g
A/B ratio are obtained
A/B
is the ratio of the height of the positive to the negative
peak of the EPR resonance and is nearly unity, indiꢀ
cating that the spins are free electron type. The
obtained EPR parameters and resonance Lorentzian
line shape obtained by fitting are closer to the outputs
flake like and spheroidal were observed. These morꢀ of Co3O4 with 17 nm crystallite size reported by Dutta
10
μ
m
2
μ
m
2 μm
Fig. 3. SEM micrographs of Co O nanocrystals at different magnifications obtained after calcination of the dried gel precursor
3
4
at 800°C for 3 h.
INORGANIC MATERIALS Vol. 47
No. 4
2011

A SIMPLE APPROACH FOR THE SYNTHESIS OF Co3O4 NANOCRYSTALS
429
CONCLUSIONS
100
80
60
(a)
A novel, environment friendly route utilizing
freshly extracted ovalbumin, and Co2+ and Co3+ salts,
has been successfully used for the synthesis of Co3O4
nanocrystals. Ovalbuminꢀwater mixture selfꢀregulated
the pH in the basic region for hydrolysis of metal ions,
which did not require any additional base for hydrolyꢀ
sis. A gel formed where water soluble ovalbumin proꢀ
teins served as a matrix for entrapment of Co2+ and
Co3+ ions. Upon heat treatment, the dried gel precurꢀ
sors decompose into nanocrystalline Co3O4. A crystalꢀ
40
20
0
lite size of 45
fitting and particle size estimated from the SEM is in
the range 20 nm–2 . This reveals polycrystalline
8 nm was obtained by XRD line profile
100
(b)
μm
nature of synthesized particles. EPR analysis of Co3O4
shows a broad single signal with nonsymetric lineꢀ
shape. EPR results show a very good fit to literature
reports for nanocrystals in the size range of 8–17 nm.
The ratio of the height of the positive to the negative
peak of the EPR resonance and is nearly unity, indiꢀ
cating that the spins are free electron type. Even
though the overall particle size distribution is quite
wide magnetic measurements confirm nanocrystalline
nature of the particles obtained. Presented synthetic
route is simple, cost effective, and environmentally
friendly that can be applied for the synthesis of various
oxide systems.
98
96
94
92
90
100 200 300 400 500 600 700 800 900 1000
t, °C
The authors are thankful to the Fatih University,
Research Project Foundation (Contract no:
P50020602) and Turkish Ministry of Industry and
Trade (Contract no: 00185.STZ.2007ꢀ2) for financial
support of this study. The fellowship from Knut and
Alice Wallenberg’s Foundation for Dr. M. S. Toprak is
thankfully acknowledged (no: UAW2004.0224).
Fig. 4. TGA thermograms and differenteial thermograms,
DTG, of (a) asꢀmade dried gel precursor; and (b) final
nanocrystals after calcination of dried gel precursor at
800°C for 3 h.
Experimental ESR
Lorentzian Fit
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