Synthesis of well-dispersed CoFe2O4 nanoparticles via PVA-assisted low-temperature solid state process

Article abstract of DOI:10.1016/j.jallcom.2009.04.047

A facile PVA-assisted low-temperature solid state process using CoCl₂·6H₂O, FeCl₃·6H₂O, and NH₄HCO₃ as precursors and using PVA chains as a medium to synthesize well-dispersed CoFe₂

Full text of DOI:10.1016/j.jallcom.2009.04.047

Journal of Alloys and Compounds 482 (2009) 508–511
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis of well-dispersed CoFe O nanoparticles via PVA-assisted
2
4
low-temperature solid state process
Runhua Qina,b, Fengsheng Li , Li Liu , Wei Jiang
a,∗
a
a
a
National Special Superfine Powder Engineering Research Center, Nanjing University of Science & Technology,
Xiaolingwei 200, Nanjing 210094, PR China
Department of Physics, North University of China, Taiyuan 030051, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 March 2009
Received in revised form 13 April 2009
Accepted 14 April 2009
Available online 21 April 2009
A facile PVA-assisted low-temperature solid state process using CoCl ·6H O, FeCl ·6H O, and NH HCO
2
2
3
2
4
3
as precursors and using PVA chains as a medium to synthesize well-dispersed CoFe2O4 nanoparticles was
developed. The effects of the amount of added PVA and calcined temperature on the characteristics of
the products were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and
BET surface area analysis. Results showed that the facile addition of PVA chains in the reactant mixture
resulted in the formation of well-dispersed CoFe2O4 nanoparticles, and the increase in specific surface
Keywords:
2
2
area from 14.82 m /g to 66.92 m /g.
Nanostructures
X-ray diffraction
Nanofabrications
Surfaces and interfaces
©
2009 Elsevier B.V. All rights reserved.
1
. Introduction
conventional low-temperature solid state reaction generally exited
in agglomeration. In order to resolve the above problem, in this
paper, we reported, for the first time, a facile PVA-assisted low-
Spinel ferrite nanoparticles have been intensively investigated
in recent years because of their remarkable optical, electrical, and
magnetic properties [1–4]. As an important spinel ferrite, cobalt fer-
rite (CoFe O ) nanoparticles are candidate materials for a variety
temperature solid state synthesis of CoFe O4 nanoparticles using
2
CoCl ·6H O, FeCl ·6H O, and NH HCO as the reactants. Our aims
2
2
3
2
4
3
are to develop a simple synthetic method of good dispersibility
2
4
of applications, for example, high-density magnetic recording tech-
nology, ferrofluid technology, bio-medical technology, magnetic
targeted delivery, and so on [5–9].
and uniform crystallinity CoFe O4 nanoparticles.
2
2.
Experimental
Various synthetic methods, such as sol–gel [10], microemulsion
[
11], chemical co-precipitation [12], hydrothermal method [13],
In a typical procedure, analytically pure CoCl2·6H2O, FeCl3·6H2O, NH4HCO3, and
microwave synthesis [14], ceramic method [15], and so on, have
been developed to synthesize CoFe O4 nanoparticles.
PVA were mixed in an agate mortar at a molar ratio of 1:2:10:0.5 and ground uni-
formly for about 50 min. The reaction started readily during the mixing process, and
accompanied by release of heat and gas. The gas evolution helped in limiting the
inter-particle contact and hence, the resultant products consisted of fine and loosen
particulates. After spontaneous air-drying, the products were thoroughly washed
with deionized water for several times to remove ammonium chloride and excess
NH HCO . Then, the products were collected by centrifugation and the precipitate
2
In comparison to the above-outlined techniques, low-
temperature solid state process requires neither complex apparatus
and sophisticated techniques nor solvent or solution. So, it is a
convenient, environment-friendly, low-cost, time-saving, and low-
energy consumption process [16–18]. So far, CoFe O nanoparticles
have been synthesized by a low-temperature solid state process
using various salts, such as sulphate, acetate, nitrate, and chloride
salts. It has been noted that when chloride salts were used during
low-temperature solid state process, the least particle size could
be obtained [19]. However, the nanoparticles were synthesized by
4
3
◦
was dried in an oven at 70 C for 2 h. Finally, CoFe2O4 nanoparticles were obtained
by calcining in muffle at 700 C for 2 h.
2
4
◦
The as-prepared product was characterized by X-ray diffraction on a Bruker
Advance D8 X-ray diffractometer with Cu K␣1 radiation (ꢀ = 0.154056 nm) in the
◦ ◦ ◦
ꢁ range from 20 to 70 , by a step of 0.02 . The working voltage was 40 kV and the
2
current was 40 mA. The crystallite size DXRD was calculated from line broadening
of the (3 1 1) XRD peak by Scherrer’s formula, where the Scherrer constant (particle
shape factor) was taken as 0.9. Transmission electron microscopy (TEM) was carried
out with a JEOL TEM-200CX microscope. The specific surface area and the adsorption
isotherm curve of the powders were measured with a COULTER SA 3100 analyzer
using the multipoint Brunauer, Emmett, and Teller (BET) adsorption. The particle
sizes were estimated from the formula: DBET = 6/ꢂSBET. The degree of agglomeration
of the obtained particles was valuated via DBET/DXRD.
∗ Corresponding author. Tel.: +86 25 84315942; fax: +86 25 84315942.
E-mail address: qinrunh@126.com (F. Li).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.04.047

R. Qin et al. / Journal of Alloys and Compounds 482 (2009) 508–511
509
Fig. 1. XRD patterns of the as-prepared CoFe2O4 nanoparticles in the absence of PVA
(
◦ ◦
1) 500 C and (2) 700 C.
Fig. 3. XRD patterns of the as-prepared CoFe2O4 nanoparticles at different
PVA addition amount (a) CoCl2·6H2O:FeCl3·6H2O:NH4HCO3:PVA = 1:2:10:1, (b)
◦
◦
CoCl2·6H2O:FeCl3·6H2O:NH4HCO3:PVA = 1:2:10:0.5. (1) 500 C and (2) 700 C.
3. Results and discussion
presence of PVA at two given temperatures. It is clear from Fig. 1
3.1. Effect analysis of PVA addition amount
that the diffraction peaks of Fe O and CoO phase are also observed
2
3
in addition to that of CoFe O4 phase. Further, it is evident that the
In our previous work, we found that the conventional low-
2
sample in Fig. 3 has a more complete crystalline structure com-
pared to the sample in Fig. 1, which suggests that the addition
of PVA can significantly improves the crystallinity and compo-
temperature solid state method often produced poor crystallinity
and agglomerated nanocrystallite, as shown in Figs. 1 and 2a.
The grinding did not effectively disperse reactants and restrain
agglomeration of resultants. Recently, we found that the intro-
duction of PVA into reactant mixture could effectively improve
crystallinity and prevent nanocrystallites from forming inseparable
three-dimensional network agglomerates. We denote the method
as PVA-assisted low-temperature solid state synthesis.
s
i
t
i
o
n
o
f
C
o
F
e
O
n
a
n
o
p
a
r
t
i
c
l
e
s
.
M
o
r
e
o
v
e
r
,
s
p
i
n
e
l
-
t
y
p
e
C
o
F
e
O
2
4
2
4
nanoparticles are formed when the resultant powder is calcined
◦
at a temperature higher than 400 C, and the crystallinity of the
product improves with increasing temperature. Here, the three-
dimensional network structure of PVA acts like a confined space
that allows the sufficient contact and reaction of the reactants and
In our present work, the amount of added PVA was varied
systematically to investigate the effect on characteristics of the as-
prepared nanoparticles. Figs. 1 and 3 respectively show the XRD
also limits the growth of the CoFe O4 nanoparticles, leading to the
2
formation of CoFe O4 nanoparticles of good crystallinity and high
2
dispersibility.
pattern of as-prepared CoFe O4 nanoparticles in the absence and
2
◦
Fig. 2. TEM micrograph for the CoFe2O4 nanoparticles at 700
C
with different PVA addition amount (a) in the absence of PVA (b) CoCl2·6H2O:
FeCl3·6H2O:NH4HCO3:PVA = 1:2:10:1 and (c) CoCl2·6H2O:FeCl3·6H2O:NH4HCO3:PVA = 1:2:10:0.5.

510
R. Qin et al. / Journal of Alloys and Compounds 482 (2009) 508–511
3.2. TEM analysis
The size, shape and agglomeration state of the CoFe O nanopar-
2
4
ticles via a low-temperature solid state process are showed in Fig. 2.
Fig. 2a reveals that the CoFe O4 particles prepared in the absence
2
of PVA present the three-dimensional fractal formed by nanocrys-
tallite agglomeration. As shown in Fig. 2b and c, the introduction
of PVA chains breaks up the network structure of agglomerated
nanocrystallites and results in the formation of high dispersive
CoFe O nanoparticles. However, the dispersibility is not in propor-
2
4
tion to the amount of added PVA. It is evident that the size, shape
and agglomeration state of the as-prepared CoFe O4 nanoparticles
2
in Fig. 2c are better than that in Fig. 2b. Due to the agglomera-
tion inhibition of PVA during drying and sintering, no apparent
agglomeration between particles is observed.
3
.3. Reaction mechanism analysis
Fig.
4. XRD
pattern
of
as-prepared
CoFe2O4
nanoparticles
at
◦
◦
CoCl2·6H2O:FeCl3·6H2O:NH4HCO3:PVA = 1:2:10:0.5. (a) 400 C, (b) 500 C, (c)
Unlike reactions in solution, solid state reactions are carried
600 ◦C and (d) 700 ◦C.
out directly without solution. So they have a different reaction
mechanism from solution reactions. Considering that NH HCO3 is
4
by heating the diffusion couple to an elevated temperature. This is
why the solid state reaction often requires annealing of the diffusion
couple at an elevated temperature.
decomposed by a strong heat of reaction to produce CO , NH , and
2
3
H O, and based on the above experimental results, the mechanism
2
of the PVA-assisted low-temperature solid state reaction can be
expressed as follows:
3.4. Effect analysis of calcined temperature
2
CoCl ·6H O + 5NH HCO
2
2
4
3
→
→
Co (OH) CO + 4NH Cl + NH + 4CO + 8H O
The influences of different calcined temperatures on character-
2
2
3
4
3
2
2
istics of as-prepared CoFe O4 nanoparticles are shown in Fig. 4.
2
2CoO + 4NH Cl + NH + 5CO + 9H O(roomtemperature)
4
3
2
2
As is shown in Fig. 4, the crystallinity of CoFe O4 nanoparticles
2
(1)
improves with increasing temperature. When the sintering temper-
ature is elevated to 700 C, all the diffractive peaks are well indexed
◦
to spinel phase CoFe O4 (JCPDF card no. 22-1086), and no impu-
2
2
FeCl ·6H O + 6NH HCO
rities are detected. The sharp diffraction peaks manifest that the
3
2
4
3
as-synthesized CoFe O4 nanoparticles have high crystallinity.
2
→
→
2Fe(OH) + 6NH Cl + 6CO + 12H O
3
4
2
2
Fe O + 6NH Cl + 6CO + 15H O(roomtemperature)
(2)
(3)
2
3
4
2
2
3.5. BET analysis
The N2 adsorption study on CoFe O4 nanoparticles prepared at
2
◦
CoO + Fe O → CoFe O (400 C)
different PVA addition amount is shown in Fig. 5. The isotherms
generally belong to type II of the BET classification for all samples.
2
3
2
4
So the solid state reaction may be generalized and be expressed
as follows:
The volume adsorbed at same pressure of CoFe O4 nanoparticles
2
gradually increased with the introduction of PVA, which indicate
that the introduction of PVA has a great effect on the surface area
2
CoCl ·6H O + 4FeCl ·6H O + 17NH HCO + PVA
2
2
3
2
4
3
of CoFe O4 nanoparticles.
2
→
2CoFe O + 16NH Cl + 17CO + NH + 39H O + PVA (4)
2
4
4
2
3
2
Table 1 summarizes the effect of the amount of PVA and calcined
temperature on the properties of the CoFe O nanoparticles. For the
That is to say, when two solid phases are in contact with each
2
4
CoFe O4 nanoparticles prepared via low temperature solid state
other, a solid state reaction may occur in the presence of a thermo-
dynamic driving force. However, the thermodynamic driving force
is not a sufficient condition for the solid state reaction, and whether
the reaction would occur also depends on the reaction kinetics.
There is a kinetic barrier for the reaction, since it requires atomic
diffusion in solids and nucleation of new phases. To overcome the
kinetic barrier, the mobility of the atoms involved, i.e. their diffu-
sivities, must be sufficiently high. Atomic diffusivity is proportional
to temperature, and thus high atomic diffusivity can be achieved
2
process, the DBET is generally (except for sample 5) far larger than
DXRD, i.e. the degree of agglomeration is high, indicating that the
particles are composed of agglomeration crystallites. It is very sig-
nificant that the introduction of PVA into the low temperature solid
2
state process results in increase in surface area from 14.82 m /g
2
to 66.92 m /g and a decrease in degree of particle agglomeration
from 1.55 to 0.83, which can be also confirmed by the comparative
observation of TEM in Fig. 2a and c.
Table 1
The effects of PVA addition amount and calcined temperature on the properties of the CoFe2O4 nanoparticles prepared.
◦
2
Sample
Molar ratio
Calcined temperature ( C)
SBET (m /g)
DXRD (nm)
DBET (nm)
Degree of agglomeration (DBET/DXRD)
1
2
3
4
5
1:2:10:0
1:2:10:1
1:2:10:1
1:2:10:0.5
1:2:10:0.5
700
600
700
600
700
14.82
17.03
26.77
38.79
66.92
39.8
35.9
28.6
21.8
16.7
61.8
53.8
34.2
23.6
13.8
1.55
1.50
1.20
1.08
0.83

R. Qin et al. / Journal of Alloys and Compounds 482 (2009) 508–511
511
resulting in formation of the well-dispersed CoFe O4 nanoparti-
2
cles. A possible mechanism based on the experimental results is
proposed. Similarly, many other spinel ferrite nanoparticles can be
synthesized via PVA-assisted low-temperature solid state process.
Acknowledgment
The work described in this paper is supported by the National
Natural Science Foundation of China (Grant No. 50602024).
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