Synthesis and characterization of CoCrxFe2-xO4 nanoparticles

Article abstract of DOI:10.1016/j.jallcom.2009.06.087

Spinel-phase nanocrystalline CoCr_xFe_2-xO₄ powders (0.0 ≤ x ≤ 2.0) were synthesized by citrate-gel precursor method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmissio

Full text of DOI:10.1016/j.jallcom.2009.06.087

Journal of Alloys and Compounds 485 (2009) 798–801
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of CoCrxFe2−xO4 nanoparticles
P.P. Hankarea,∗, U.B. Sankpala,∗, R.P. Patila, I.S. Mullab, P.D. Lokhandec, N.S. Gajbhiyed
a Department of Chemistry, Shivaji University, Kolhapur - 416 004, India
b National Chemical Laboratory, Pune - 411 008, India
c Department of Chemistry, University of Pune, Pune - 411 007, India
d Dr. Harisingh Gour University, Sagar - 470 003, M.P., India
a r t i c l e i n f o
a b s t r a c t
Article history:
Spinel-phase nanocrystalline CoCrxFe2−xO4 powders (0.0 ≤ x ≤ 2.0) were synthesized by citrate-gel pre-
cursor method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy
(SEM), transmission electron microscopy (TEM) and infra-red spectroscopy (FT-IR). The XRD and selected-
area electron diffraction (SAED) patterns indicated that the synthesized nanoparticles have single-phase
spinelstructurewithoutthepresenceofanyotherimpurities. Thechangesinelectricalandmagneticprop-
erties of different stiochiometric compositions have also been investigated. It is seen that with increase
in chromium contents, the measured magnetic hysteresis loops become narrow with decreasing trend
in saturation magnetization. Electrical resistivity indicated that all the compounds are semiconducting
Received 1 February 2009
Received in revised form 5 June 2009
Accepted 12 June 2009
Available online 21 June 2009
Keywords:
X-ray diffraction
Sintering
Transmission electron microscopy
Magnetic measurements
nature.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
characteristics, uniform grains and narrow particle size, thereby
influencing structural, electrical and magnetic properties of fer-
A lot of works has been reported by many investigators on
spinels ferrites using various methods of preparations [1–3].
Nanocrystalline spinel ferrites with general formula AB2O4 are very
important materials due to their interesting electric and magnetic
properties. These properties are varying with their nature of ions,
charge distribution and site preference energy among tetrahedral
and octahedral sites. These compounds are technologically impor-
tant and have been used in many applications, including magnetic
recording media, microwave devices, catalysis, sensors, and pig-
ments [4,5].
Recently, synthesis of nanocrystalline ferrospinels is one of the
interesting fields of material science in material processing and
technology. These nanoscale particles have few different interesting
properties as compared to the bulk properties. The various process-
ing techniques, which are used for the synthesis of spinel ferrite
powders, include microwave refluxing, hydrothermal, sol–gel,
co-precipitation and spray pyrolysis [6–10]. In the present inves-
tigation, we have employed citrate-gel autocombustion method to
synthesize Cr substituted cobalt ferrite nanoparticles. It is a sim-
ple process, which offers a significant saving of time and energy
consumption over traditional methods, and requires less sintering
temperature. This method employed to reach improved powder
rospinels.
2. Experimental
Nanocyrstalline powders of CoCrxFe2−xO4 (0.0 ≤ x ≤ 2.0) were synthesized by
citrate-gel autocombustion method. The A.R. grade citric acid (C5H8O7·6H2O), cobalt
nitrate (Co(NO3)2·6H2O), chromium nitrate (Cr (NO3)3·9H2O) and ferric citrate
(C6H5 FeO7·3H2O) (≥99%) were used as raw materials. An aqueous solution of cit-
ric acid was added to cobaltous nitrate and chromium nitrate separately until a
clear transparent solution of metal–citrate complex was formed. The above mixed
citrate–metal complex was then slowly added to iron (III) citrate solution with a
constant stirring. The precursor mixture was evaporated slowly to dryness to obtain
floppy mass. All the dried precursors were sintered at 700 ◦C for 4 h in air, at the
heating rate of 150 ◦C/h. The sintered ferrite powders were characterized by using
X-ray diffractometer (PW 1710 Philips) with Cu K␣ radiation (1.5406 Å).
X-ray density was calculated from full width at half maximum peak using,
311, plane. The FT-IR spectra (PerkinElmer—USA) of all sintered compositions were
recorded in the range of 400–800 cm−1
.
The powders were pelletized using 2% polyvinyl alcohol as a binder. The surface
morphology was studied by scanning electron microscopic studies (JEOL JSM 6360),
while, nanocrystalline nature was verified by transmission electron microscopy
(Philips CM 20). The D.C. resistivity measurements were carried out using two probe
method from room temperature to 500 ◦C. Magnetic hysteresis loops were traced at
room temperature for different composition.
3. Results and discussion
3.1. X-ray analysis
∗
Corresponding authors. Tel.: +91 231 2609381; fax: +91 231 2691533.
E-mail addresses: p hankarep@rediffmail.com (P.P. Hankare),
Fig. 1 shows the X-ray diffraction pattern of the sintered powder
of different compositions. The XRD patterns clearly indicate that the
sankpalumes@yahoo.co.in (U.B. Sankpal).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.06.087

P.P. Hankare et al. / Journal of Alloys and Compounds 485 (2009) 798–801
799
3.2. SEM and TEM studies
Fig. 2(a) and (b) shows SEM images of CoFe2O4 and CoFeCrO4,
respectively. It is evident that microstructures are affected due
to the substitution of Cr3+ ions. The average grain size goes on
decreasing with increase in substitution of Cr3+ ions. The interac-
tion between grains plays an important role to developing grain size
at operating sintering temperature. Energy barrier for formation of
CoFe2O4 is higher than CoCr2O4, resulting in the formation of large
particles [16].
Figs. 2(c) and (d) shows the TEM images of CoFe2O4 and CoCr2O4.
The TEM images reveal formation of the nanosized sintered sam-
ples. A decreasing trend in particle size with increase in Cr content
was also observed from TEM images. The selected-area electron
diffraction patterns (SAED) for each composition are shown as inset.
The SAED patterns of these compositions show spotty ring patterns
without any additional diffraction spots and rings of second phase,
revealing their crystalline spinel structure. Occasional spots in SAED
pattern (Fig. 2e) may be due to the agglomerisation or coarse par-
ticles. Above TEM images show that, the average particle size in
nanometers are in good agreement with XRD data.
Fig. 1. X-ray diffraction pattern of CoCrxFe2−xO4 system (0.0 ≤ x ≤ 2.0), where (a)
x = 0.0, (b) x = 0.5, (c) x = 1.0, (d) x = 10.5 and (e) x = 2.0.
3.3. FT-IR study
The FT-IR spectra’s of Cr substituted cobalt ferrites show two
strong bands in the range of 400–650 cm−1. Waldron [17], White
and De Angelis [18] assigned the high frequency band at 650 cm−1
(ꢁ1) to symmetrical stretching vibrations of tetrahedral groups and
lower frequency band at 400 cm−1 (ꢁ2) due to vibrations of the octa-
hedral M–O groups. During study of the above system it is observed
that the bands (ꢁ1) and (ꢁ2) are slightly shifted towards higher fre-
quency with increase in concentration of Cr3+ ions as shown in
Table 2. Tarate and Preudhomme [19] have observed that in normal
ferrites, both the absorption bands depend on the nature of octa-
hedral cations and do not significantly depend upon the nature of
tetrahedral ions. The difference in the band positions is due to the
difference in the Fe3+–O distances for octahedral and tetrahedral
complexes.
all prepared compositions contain single-phase cubic structure. A
close observationof theXRDpatterns revealthat the diffraction pat-
tern peaks became broader with increase in the chromium content
x, which may be due to distribution of nanocrystallinity. The crystal-
lite sizes of all the compositions were calculated by using Scherer’s
formula measuring the FWHM of the most intense peak (3 1 1). The
results are as shown in Table 1. Further more, it is observed that,
crystallite size goes on decreasing with increase in Cr content. The
same trend in variation of crystallite size was observed in Al sub-
stituted nickel ferrites and Cr substituted magnesium ferrospinels
[11,12].
The values of lattice constant decreased from 8.389 to 8.325 Å
with increase in concentration of chromium ions (Table 1). The
behavior of lattice constant with chromium content is probably
due to the smaller ionic radius of Cr3+ (0.68 Å) ions as compared
to Fe3+(0.73 Å) [13].
3.4. D.C. electrical resistivity
The X-ray density was calculated using the relation:
D.C. resistivity of all the sintered compounds measured as a
function of temperature varied between 107 and 1010 ꢂ cm. The
plots of log ꢃ vs. 103/T (Fig. 3) showed nearly linear nature without
any beak obeying Wilson’s law, ꢃ = ꢃ0 exp ꢀE/KT. All the composi-
tions showed semiconducting nature. The activation energy (ꢀE)
values obtained from the plots varied between 0.98 and 1.10 eV
(Table 1).
Conduction in the ferrites takes place via hopping mechanism.
CoFe2O4 possessesinversespinelstructurewhileCoCr2O4 isnormal
(octahedral site preference energy of Co2+ → −10.5, Fe3+ → −13.3
and Cr3+ → 16.0 kcal/g at. wt.) In CoFe2O4 conduction mostly takes
place due to hoping of Fe3+–Fe2+ ions. As the concentration of Cr
3+ increases electron exchange become more difficult due to stable
3+ oxidation state of Cr3+ ions, thereby resulting an increase in the
resistivity [20].
8M
dX
=
Na3
where N = Avagadros number (6.023 × 1023 atom/mole);
M = molecular weight, and a = lattice constant.
From porosity values (Table 1), it can be observed that CoFe2O4
is sintered better as compared to chromium containing spinels. This
is in agreement with the observations of Jonker who has shown that
sintering in chromites is rather poor due to high melting point of
chromium oxide [14,15] and therefore, CoCr2O4 shows high value
of porosity.
Table 1
Data of lattice parameter (a), crystallite size (D), X-ray density (dx), porosity (P), and
activation energy (ꢀE) of CoCrxFe2−xO4 system.
Table 2
Composition (x) Lattice
Crystallite
size (nm)
X-ray Porosity
density
Activation
energy
(ꢀE) (eV)
FT-IR spectral bands of CoCrxFe2−xO4 system.
constant
(a) Å
(gm/cm3)
Composition (x)
Octahedral band (cm−1
)
Tetrahedral band (cm−1
)
0.0
0.5
1.0
1.5
2.0
8.389
8.372
8.364
8.356
8.325
48
44
40
39
33
5.28
5.27
5.24
5.21
5.19
1.1
2.1
2.3
2.5
2.7
0.98
1.02
1.04
1.08
1.10
0.0
0.5
1.0
1.5
2.0
443
456
479
502
518
581
598
614
632
641

800
P.P. Hankare et al. / Journal of Alloys and Compounds 485 (2009) 798–801
Fig. 2. SEM images of samples: (a) x = 0.0 and (b) x = 1.0. TEM images of samples: (c) x = 0.0, (d) x = 2.0 and (e) SAED pattern of CoFe2O4.
3.5. Magnetic properties
Magnetic hysteresis loops of the system CoCrxFe2−xO4 (where
x = 0.0, x = 1.0 and x = 2.0) were measured at room temperature using
magneta B–H loop tracer (Fig. 4). All these compositions show fer-
rimagnetic nature with fairly high value of coersive field (Hc) which
Fig. 4. Hysteresis loops for CoCrxFe2−xO4 system where (a) x = 0.0, (b) x = 1.0 and (c)
x = 2.0.
Fig. 3. Variation of D.C. electrical resistivity (ꢃ) with temperature (K) for
CoCrxFe2−xO4 system.

P.P. Hankare et al. / Journal of Alloys and Compounds 485 (2009) 798–801
801
Table 3
tization and remanent magnetization with increasing chromium
content occurs because the replacement Fe3+ ions by Cr3+ ions
weaken the sublattice interaction and lowers the magnetic moment
of unit cell. All the compositions to be ferrimagnetic in nature. It can
be concluded from these results that, citrate-gel technique is better
to achieve nanoscale particles in short duration and at low sintering
temperature.
Magnetic measurements for CoCrxFe2−xO4 system with x = 0.0, 1.0, and 2.0.
Composition (x)
Saturation
magnetiza-
tion (Ms)
emu/g
Observed
magnetic
moment
(ꢄB)
Remanent
magnetiza-
tion (Mr)
emu/g
Coersive field
(Hc) Oe
0.0
1.0
2.0
43.90
32.12
5.64
1.84
.32
0.64
16.04
3.78
1.87
537.34
211.04
102.27
Acknowledgement
may be due to the presence of anisotropy in these compounds. The
high value of Hc may also due to nanoscale particle size (about
33–48 nm) which is obtained because of citrate-gel technique was
used for synthesis. In spinels AB (JAB) interactions are strong as com-
pared to BB (JBB) interactions. The compound CoFe2O4 possesses
inverse spinel structure (Fe3+ [Co2+ Fe3+] O4) where intrasublattice
exchange magnetic interaction JAB is larger than JBB. The decrease in
value of saturation magnetization and magnetic moment (Table 3)
with increase in x is due to substitution of Cr3+ ions, which pos-
sesses much lower magnetic moment (3 ꢄB) as compared to Fe3+
(5 ꢄB).
Authors wish to thank UGC (India) for financial support through
the grant for Major Research Project [F. No. 32-289/2006 (SR)] and
UGC-SAP meritorious research fellowship.
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