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RESEARCH PAPER
Liquid-phase syntheses of cobalt ferrite nanoparticles
Katalin Sinko • Enik}o Manek •
Aniko Meiszterics • Karoly Havancsak •
Ulla Vainio • Herwig Peterlik
Received: 3 October 2011 / Accepted: 28 April 2012
� Springer Science+Business Media B.V. 2012
Abstract The aim of the present study was to
synthesize cobalt-ferrite (CoFe2O4) nanoparticles
using various liquid phase methods; sol–gel route,
co-precipitation process, and microemulsion tech-
nique. The effects of experimental parameters on the
particle size, size distribution, morphology, and
chemical composition have been studied. The anions
of precursors (chloride and nitrate), the solvents
(water, n-propanol, ethanol, and benzyl alcohol), the
precipitating agent (ammonia, sodium carbonate, and
oxalic acid), the surfactants (polydimethylsiloxane,
ethyl acetate, citric acid, cethyltrimethylammonium
bromide, and sodium dodecil sulfate), their concen-
trations, and heat treatments were varied in the
experiments. The smallest particles (around 40 nm)
with narrow polydispersity and spherical shape could
be achieved by a simple, fast sol–gel technique in the
medium of propanol and ethyl acetate. The size
characterization methods have also been investigated.
Small-angle X-ray scattering (SAXS), dynamic light
scattering (DLS), and scanning electron microscopy
(SEM) provide the comparison of methods. The SAXS
data correspond with the sizes detected by SEM and
differ from DLS data. The crystalline phases, mor-
phology, and chemical composition of the particles
with different shapes have been analyzed by X-ray
diffraction, SEM, and energy dispersive X-ray
spectrometer.
Keywords Cobalt ferrite nanoparticles �Sol–gel method � Co-precipitation � DLS � SEM �SAXS
Introduction
Recently, nanostructured transition metal oxides have
attracted a lot of attention because of their outstanding
properties and various applications. The properties
(e.g., magnetic, optic, catalytic, and electronic) of
nanomaterials depend strongly on their microstruc-
tural features such as morphology, crystallite size, and
porosity. Magnetite nanoparticles may behave as
single magnets when the domain size is as large as
the particle. Cobalt ferrite (CoFe2O4) is a well-known
hard magnetic material with high coercivity and
K. Sinko (&) � E. Manek � A. Meiszterics
Institute of Chemistry, L. Eotvos University, Budapest
1117, Hungary
e-mail: [email protected]
K. Havancsak
Institute of Physic, L. Eotvos University, Budapest 1117,
Hungary
U. Vainio
DESY, 22607 Hamburg, Germany
H. Peterlik
Faculty of Physics, University of Vienna, Vienna 1090,
Austria
123
J Nanopart Res (2012) 14:894
DOI 10.1007/s11051-012-0894-5
moderate magnetization. The magnetic properties of
the ferrites, MFe2O4, are accompanied with the cation
configuration of the spinel lattice. CoFe2O4 can be
usually characterized by an inverse spinel structure.
CoFe2O4 nanoparticles have a broad prospect of
applications, e.g., in electronic devices, ferrofluids,
and high-density information storage (Laurent et al.
2008; Sun and Zeng 2004). The magnetite nanopar-
ticles could have many applications in the medical
diagnostics and therapy; targeted drug delivery (Jain
et al. 2005; Chourpa et al. 2005); magnetic resonance
imaging (MRI) as contrast agents (Bulte 2006; Burtea
et al. 2005; Boutry et al. 2006); and tissue repair and
cell separation (Gupta and Gupta 2005). Such mag-
netic nanoparticles can bind to drugs, proteins,
enzymes, antibodies, or nucleotides and can be
directed to an organ, tissue, or tumor using an external
magnetic field (Chastellain et al. 2004). In contrast to
the metal nanoparticles, the nano ferrites are very
stable in different chemical environments, which
provide the ferrites with great importance (Laurent
et al. 2008) in the biomedical research. All these
biomedical applications require that the nanoparticles
have high magnetization values, a size smaller than
100 nm, and a narrow particle size distribution.
Many different synthesis techniques give access to
nanomaterials with a well-defined crystallite size. The
liquid-phase syntheses offer a good technique and
control for tailoring the structures, the compositions,
and the morphological features of nanomaterials. The
liquid-phase routes include the co-precipitation, the
hydrolytic as well as the nonhydrolytic sol–gel
processes, the hydrothermal or solvothermal methods,
the template synthesis, and microemulsion-based
processes.
In the sol–gel synthesis, soluble precursor mole-
cules are hydrolyzed and condensed to form a disper-
sion of nano-sized particles. In the preparation of
cobalt ferrite powders, inorganic cobalt and ferric salts
are subjected to hydrolysis and condensation generally
in ethanol at 50–80 �C (Silva et al. 2005; Lee et al.
1998; Meron et al. 2005). Further heat treatments are
needed to develop the final crystalline state. The drying
process and the heat treatment of wet sol solutions have
a strong effect on the surface area, pore volume,
crystallinity, particle structure, and corresponding
electrochemical properties (Laurent et al. 2008; Ennas
et al. 1998; Brinker and Sherrer 1990). Modifying
agents are often applied in the sol–gel technique; e.g.,
citric acid (Liu and Zhang 2009) or polyvinyl alcohol
(Pramanik et al. 2004). Some experiments on the sol–
gel synthesis performed in benzyl alcohol have been
recently reported (Murray and Agan 2000; Pinna and
Niederberger 2008; Bilecka et al. 2008). According to
the published results, additional surfactant is not
needed in the preparation of metal oxides in benzyl
alcohol. The benzyl alcohol is acting as solvent, ligand,
and reactant in the synthesis.
The co-precipitation is a simple and rapid tech-
nique. This method provides several possibilities to
modify the particle size, surface, and shape. However,
the control of the particle size and distribution is
difficult. A commonly used procedure for preparing
ferrite particles has been the co-precipitation of M2?
and Fe3? ions by a base, usually NaOH or NH3, in an
aqueous solution (Zhang et al. 1998; Neveu et al.
2002; Olsson and Salazar-Alvarez 2005; Chinnasamy
et al. 2003). There are few examples for the applica-
tion of hexamethylene tetramin (Liu et al. 2008) or
tetraalkyl ammonium hydroxides (Paike et al. 2007;
Gyergyek and Makovec 2010) as a precipitating agent.
Nanoparticles can also be formed on liquid–liquid
interfaces. Among the chemical methods, the micro-
emulsion process involving reverse micelles has been
demonstrated as a versatile method for obtaining a
wide variety of nanocrystalline oxides (Pillai and Shah
1996; Ahn et al. 2001; Moumen and Pileni 1996; Han
et al. 2004). For example, cobalt ferrite nanoparticles
could be prepared by microemulsion method from a
mixture of Co(II) and Fe(III) dodecylsulfates treated
with an aqueous solution of methylamine (Moumen
and Pileni 1996). Monodisperse CoFe2O4 nanocrys-
tals have been synthesized using normal and reverse
micelle microemulsion methods and by combining a
non-hydrolytic process and seed-mediated growth
(Han et al. 2004).
The hydrothermal technique has been commonly
used to prepare ferrite nanoparticles (Li and Xu 2010;
Rebolledo et al. 2008; Komarneni et al. 1998; Bilecka
and Niederberger 2010). Most of these preparations
involve a combination of co-precipitation and hydro-
thermal synthesis (Rebolledo et al. 2008). An innova-
tion to the hydrothermal method is the application of
microwaves during the hydrothermal synthesis (Ko-
marneni et al. 1998; Bilecka and Niederberger 2010).
The aim of the present study was to prepare
ferromagnetic cobalt ferrite (CoFe2O4) nanoparticles
by different liquid-phase syntheses; sol–gel, co-
Page 2 of 14 J Nanopart Res (2012) 14:894
123
precipitation, and microemulsion techniques com-
bined with thermal decomposition. We have moni-
tored the effect of the synthesis route, the type of
precursors and solvents, the chemical compositions,
the concentration of the initial materials, the applica-
tion of surfactants, and the heat treatments on the size,
and the chemical and crystalline compositions of the
particles. The research study also concentrated on a
comparison of the various size characterization meth-
ods. The particle sizes have been determined by
dynamic light scattering (DLS), scanning electron
microscope (SEM), and small angle X-ray scattering
(SAXS). The identification and characterization of the
phases and the morphology of products have been
performed by X-ray diffraction (XRD) and scanning
electron microscope (SEM). The processes and the
weight loss occurred by heat treatments have been
recorded by thermal analysis in a controlled
atmosphere.
Experimental methods
Preparation methods
In the experiments of the sol–gel method, CoCl2�6H2O
(Aldrich, a.r.) and Co(NO3)2�6H2O (Aldrich, a.r.)
were provided as cobalt precursor, FeCl3�6H2O
(Aldrich, a.r.) and Fe(NO3)3�9H2O (Aldrich, a.r.) as
iron precursor. The Co2? and Fe3? ions were allowed
to hydrolyze for 1–2 h at 50 �C in first step and for
2–6 h at 85 �C in the second step in ethanol or
1-propanol solutions with (citric acid, PDMS, or ethyl
acetate) or without any surfactant. The chemical
compositions of the preparation experiments are
summarized in Table 1. By partial evaporation of the
solvent, a precipitate formed. The sol–gel technique
produced mixed basic Co- and Fe-containing precip-
itates. The precipitates were centrifuged and dried at
80 �C. The heat treatment was carried out at different
temperatures under oxidative atmosphere to obtain
cobalt ferrite particles. The initial Co- and Fe-
containing solutions were treated either in a common
system or separately. In order to avoid the usual
problem of ferrite preparations, e.g., the formation of
hematite, the alcoholic solution of ferric salts was
slowly added with a dropping rate of 0.5 cm3 min-1
into the alcoholic solution of cobalt(II) salts at 80 �C.
After 5-h reflux of mixture at 80 �C, a gelatinous
precipitate formed. The average yield was very low
(5–6 %) in the case of chloride precursor and the use
of nitrate salts yielded 45–60 % of the theoretical mass
(Table 1).
In the experiments of the co-precipitation method,
cobalt and ferric chloride were the initial materials.
The aqueous solution of precipitating agents (sodium
carbonate, oxalic acid, and ammonia) was dropped
into the aqueous solution of precursors. Poly-
dimethylsiloxane (PDMS, Aldrich, 550 g mol-1,
5600 g mol-1); polyethylene glycol tert-octylphenyl
ether (Triton X-100, Aldrich); sodium dodecyl sulfate
(NaDS, Merck); sodium dodecylbenzene sulfonate
(NaDBS, Aldrich); and tetradodecylammonium bro-
mide (TDAB, Aldrich) were used as surfactants in the
concentration of 0–10 w/w%. Precipitates formed
directly during the addition of precipitating agents.
The suspensions were stirred for 2 h at room temper-
ature with or without a surfactant. The particles were
separated by centrifugation. The dried precipitates
were subjected to heat treatment at various tempera-
tures. The average yield was 66 % of the theoretical
mass by carbonate precipitator and 49 % by oxalate or
ammonia agents.
The system of water-in-oil (W/O) microemulsion
method consists of sec. buthylalcohol as an oil phase,
CTAB, NaDS, and PDMS as surfactans and an
aqueous phase of cobalt and ferric salts. Aqueous
solution was prepared by dissolving stoichiometric
amounts of cobalt and ferric chloride in deionized
water. Sodium carbonate was taken as a precipitating
agent. The precipitating agent was separately dis-
solved in water. The aqueous solution of precursors
and after that the aqueous solution of precipitating
agent were dropped into the surfactant-containing oil
phase during intensive mixing. The microemulsion
synthesis of the nanoparticles could be carried out with
yield of 20–30 %.
Characterization methods
Dynamic light scattering (DLS) measurements were
performed by means of a DLS equipment (Brookha-
ven) consisting of a BI-200SM goniometer and a BI-
9000AT digital correlator. An argon-ion laser (Omni-
chrome, model 543AP) operating at 488-nm wave-
length and emitting vertically polarized light was used
as the light source. The signal analyzer was used in
real-time ‘‘multi tau’’ mode. In this mode, the time
J Nanopart Res (2012) 14:894 Page 3 of 14
123
axis was logarithmically spaced over an appropriate
time interval and the correlator used 218 time
channels. The pinhole size was 100 lm. The particles
were generally dispersed in ethanol for DLS measure-
ments instead of water to avoid the aggregation of the
particles in water. The number-weighted particle size
distribution was detected by DLS.
The particle size and morphology were studied by a
FEI Quanta 3D FEG SEM. The SEM images were
prepared by the Everhart–Thornley secondary electron
detector (ETD), its ultimate resolution is 1–2 nm.
Since the conductance of the particles investigated is
high enough to remove the electric charge accumu-
lated on the surface, the SEM images were performed
in high vacuum without any coverage on the specimen
surface. For the best SEM visibility, the particles were
deposited on a HOPG (graphite) substrate surface.
SEM combined with energy disperse X-ray spectros-
copy (EDX) is mainly applied for spatially resolved
chemical analysis of bulk samples.
SAXS experiments were conducted on several
instruments. The laboratory equipment was operated
Table 1 Chemical compositions in preparation experiments
Methods Molar ratios Yield w/w%
Co-precursor Fe-precursor Solvent Precipitator Additives
Sol gel Nitratea
1
Nitrateb
1
n-propanol
70
– Ethyl acetate 80 50–60
Sol gel Nitratea
1
Nitrateb
1
Ethanol
70
– Ethyl acetate 10–100 45–55
Sol gel Nitratea
1
Nitrateb1 Ethanol
70
– Citric acid 0.1–1.0 10–15
Sol gel Nitratea
1
Nitrateb
1
Ethanol
70
– PDMS 550 0.1–5 10–25
Sol gel Nitratea
1
Nitrateb
1
Benzyl alcohol
18–280
– – 1–5
Sol gel Chloridec
1
Chlorided
1
n-propanol
70
– Ethyl acetate 80 5–6
Sol gel Chloridec
1
Chlorided
1
Benzyl alcohol
18–280
– – 1-5
Co-precipitation Chloridec
1
Chlorided
1
Water 40–200 Na2CO3 2.5–3 PDMS 550 0–10e 60–70
Co-precipitation Chloridec
1
Chlorided
1
Water 80 Na2CO3 2.5–3 PDMS 5600 0–10e 50–70
Co-precipitation Chloridec 1 Chlorided 1 Water 50 (COO)2 2.5–3 PDMS 550 0–10e 40–50
Co-precipitation Chloridec
1
Chlorided 1 Water 50 (COO)2 2.5–3 PDMS 5600 0-10e 40–50
Co-precipitation Chloridec 1 Chlorided
1
Water 80 NH3 5–7 PDMS 5600 0–10e 40–60
Microemulsion Chloridec 1 Chlorided 1 Water 40–100 sec-
butanol 100
Na2CO3 2.5–3 PDMS 5600 5, 10e 25–30
Microemulsion Chloridec
1
Chlorided
1
Water 40–100 sec-
butanol 100
Na2CO3 2.5–3 CTAB 5, 10e 25–30
Microemulsion Chloridec
1
Chlorided
1
Water 40–100 sec-
butanol 100
Na2CO3 2.5–3 NaDS 5, 10e 20–30
a Co(NO3)2�6H2O; bFe(NO3)3�9H2O; cCoCl2�6 H2O; dFeCl3�6H2O; ew/w%
Page 4 of 14 J Nanopart Res (2012) 14:894
123
with a 5.4 kW rotating anode X-ray generator
(Nanostar from Bruker AXS, Karlsruhe), a pinhole
camera with variable sample to detector distance
(25–108 cm), and a 2D position sensitive detector
(Bruker AXS). The gels were covered in vacuum
tight foil. The 2D spectra were corrected for
parasitic pinhole scattering, as well as for the foil
scattering. Simultaneous small-angle and wide-angle
X-ray scattering experiments (SAXS and WAXS)
were also recorded on the JUSIFA beamline of
HASYLAB at DESY in Hamburg (8 keV photon
energy; 925-, and 3625-mm sample-to-detector dis-
tances). The SAXS intensities were fitted with a
form factor from spheres with a Gaussian size
distribution. In the case of the small particles, the fit
can be slightly improved by an additional structure
factor using the local monodisperse approximation.
However, as the tendency for agglomeration is small
(described by a low value for the hard-sphere
volume factor), the structure factor was set to one
in all samples for an easier comparison of the data.
The WAXS curves were evaluated by means of the
standard PDF cards.
The XRD measurements were carried out by means
of a Philips (PW1130) X-ray generator set up with a
Guinier-chamber. The chamber has a diameter of
100 mm, and the patterns were recorded on FUJI
Imaging Plates (BAS MS2025). The XRD data were
collected over the 2h range of 9–90� with a step size
0.005�. The identification of phases was carried out by
comparing the diffraction patterns with the standard
PDF cards.
Thermogravimetric analysis (thermogravimetry—
TG; and differential thermal analysis—DTA) was
used to investigate the processes that occurred during
the heat treatment. TG and DTA curves were recorded
using Derivatograph-C System (MOM, Hungary)
under air or nitrogen flow at a heating rate of
6 �C min-1 on crushed bulk specimens from room
temperature to 1,000 �C.
Results
Particle size measurements
The particle sizes have been determined by various
techniques: DLS, SAXS, and SEM. For the exact
comparison of various methods, a sample having
nearly monodisperse size distribution was chosen. The
results, which are summarized in Table 2, show
considerable differences between different character-
ization techniques. The sizes (diameters) obtained by
SEM and SAXS are consistent with each other. The
size derived from DLS is two or three times larger.
SEM delivers direct images of the size and shape of
solid nanoparticles, and the photograph is taken under
vacuum. Dried powders have also been measured
under vacuum in the SAXS experiments. In DLS
technique, the particles are dispersed in a solvent. The
nanoparticles can be hydrated or solvated in polar
solutions. The difference in the sizes might be
attributed to the hydration/solvation shells. In order
to verify the influence of hydration/solvation shells on
the particle size, we measured the size of nanoparticles
by DLS in various solvents. The sizes obtained in
aqueous solution are significantly bigger than those
detected in ethanol solutions (Table 2). The dipole
moment of water is larger than that of ethanol resulting
in a stronger connection of solvent molecules to the
particle surfaces. Tobler et al. provide further expla-
nation for the size difference (2009). The highly
hydrous and open-structured particles (e.g., silica) can
collapse because of the dehydration and relaxation
processes under high vacuum (Tobler et al. 2009). In
the DLS measurements, surfactants were used to
hinder the aggregation of nanoparticles in their
aqueous dispersions. The usually applied ionic sur-
factants proved to be ineffective against aggregation
(Table 2).
The particle sizes identified by various methods are
listed in Table 3. The size means diameter. The
particles were synthesized by different routes and
Table 2 Particle size determination by various techniques
Method Average sizea
(nm)
Size-range
(nm)
SEM 40 30–60
SAXS 40 42–54
DLS in ethanol 86 53–143
DLS in water 120 85–135
DLS in CTABb 153 120–180
DLS in NaDSc 205 140–255
a Number-weighted average valuesb 10 mM aqueous solution of CTABc 10 mM aqueous solution of NaDS
J Nanopart Res (2012) 14:894 Page 5 of 14
123
dried at 80 �C for 2 h, and heat treated at 600 �C also
for 2 h. The sizes measured by DLS are generally two
or three times larger than those derived from SEM or
SAXS. For the correct explanation of these differ-
ences, an additional reason must be taken into account
apart from those detailed above. That is, by the
variation of precipitated particles, not only cobalt
ferrite is formed in the preparation, but even a small
amount of larger particles can also significantly
modify the average particle size and size distribution
in DLS method. SAXS technique is less susceptible to
the presence of larger aggregates, and SEM is capable
of distinguishing between the particle types. The
difference between the sizes depends on the hydro-
phobicity, the roughness of the particle surface, and
the particle shape. The amorphous shape (e.g., sol–gel
derived products) and the rough or porous surface
(e.g., sol–gel-derived and carbonate-co-precipitated
products) thicken the hydration/solvation layers.
The smallest cobalt ferrite nanoparticles could be
achieved by an uncomplicated, fast sol–gel method
starting from nitrate salts (40–41 nm, SEM) and by co-
precipitation with carbonate in the presence of 5 w/w%
PDMS (40–43 nm, SEM). The co-precipitation with
oxalate acid yielded slightly bigger (58 nm) particles.
Among the surfactants, the PDMS of 550 or
5,600 g mol-1 proved to be the most effective in the
reduction of the size and size distribution in the series of
co-precipitation. The particle size in the function of
PDMS concentration shows a minimum (Fig. 1). The
smallest particles (122 nm, DLS) with less polydisper-
sity (47–157 nm, DLS) could be obtained in the solution
of 5.0 w/w% for both PDMS 550 and 5,600 g mol-1 in
the co-precipitation series with carbonate (Fig. 1). A
minimum can also be observed at 5.0 w/w% PDMS in
the size of particles prepared with oxalate co-precipi-
tation. In the series of sol–gel technique, the use of ethyl
acetate in large amount (about 80 mol ethyl acetate/
Co2?) produced the smallest particles (40–41 nm,
SEM). The application of microemulsion technique
yielded the widest size distribution (20–2200 nm, SEM)
because of the several species. The modifying agents
decreased consistently the size in the function of
surfactant concentration. For example, raising the
volume of surfactants from 5 to 10 w/w% the mean
size reduced by 190–221 nm in every case, from 347 to
141 nm by NaDS; from 392 to 171 nm by CTAB; from
447 to 250 nm by PDMS.
Shape and morphology of nanoparticles
The shape and the morphology of nanopowders were
controlled by SEM and XRD. The nanoparticles were
produced by various synthesis techniques, dried at
Table 3 Particle size and distribution of cobalt ferrite particlesa synthesized by different methods
Preparation technique DLSb SEM SAXS
Average
size (nm)
Size-range
(nm)
Average
size (nm)
Size-range
(nm)
Average
size (nm)
Mod. sol gel nitrates, ethyl acetate 86 53–143 40 30–60 40 ± 6
Sol gel nitrates, ethyl acetate 72 26–139 41 23–67 40 ± 4
Sol gel chlorides, ethyl acetate 100 80–168 58 45–98 –
Sol gel nitrates, citric acid 156 95–210 – – –
Sol gel nitrates, PDMS 5600 102 76–142 – – –
Co-precipitation carbonate 155 90–205 52 12–54 55 ± 10
Co-precipitation carbonate, 5 % PDMS 5600 122 47–157 43 26–79 40 ± 6
Co-precipitation carbonate, 5 % PDMS 550 139 50–166 40 22–68 –
Co-precipitation oxalate 141 86–209 58 24–140 –
Co-precipitation oxalate, 5 % PDMS 5600 89 53–135 79 44–118 –
Microemulsion carbonate, 5 % PDMS 5600 447 222–476 31, 1400 30–2500 –
Microemulsion carbonate, 5 % CTAB 392 264–579 27, 1550 20–2200 –
a The precipitates were heated at 600 �C under airb DLS measurements were carried out in ethanol.
–, No data
Page 6 of 14 J Nanopart Res (2012) 14:894
123
80 �C for 2 h and heat treated at 600 �C also for 2 h.
The sol–gel process yields spherical cobalt ferrite
nanoparticles from nitrate salts and ethyl acetate
(Figs. 2, 3). Without a slow addition of Fe(NO3)3
solution, a small amount of hematite is precipitated
from the common solution of nitrate salts and large
amount from chloride salts (Fig. 4). Octahedral crys-
tals of large size represent the hematite phase (Fig. 3).
XRD identifies only cobalt ferrite crystalline phase
using a slow addition of ferric nitrate solution (Fig. 4).
The cobalt ferrite phase can be readily detected in the
gels obtained by sol–gel technique and dried at 80 �C.
By other preparation routes, hematite (Fe2O3) always
forms over cobalt ferrite.
Co-precipitation with carbonate without any sur-
factant produces inhomogeneous particles; nanoparti-
cles with amorphous shape (cobalt ferrite, verified by
XRD and EDX), plate-like aggregates of 1.5–2 lm
(NaCl, XRD, and EDX), octahedral crystals with
average size of 320 nm (hematite, XRD, and EDX)
(Figs. 3, 5). The inhomogeneity and the size of
particles reduce by the effect of PDMS, the volume
of ferrite phase increases, and the particle shape is
cubic rather than amorphous (Figs. 3, 6). In the
product of the co-precipitation with oxalic acid,
nanoparticles with amorphous shape (cobalt ferrite,
XRD, and EDX) and octahedral crystals with average
size of 97 nm (hematite, EDX) can be revealed
(Figs. 3, 5). The size and its dispersion change slightly
by addition of a surfactant. The microemulsion
products consist of many types of particles; fine
nanoparticles with amorphous shape (cobalt ferrite,
XRD, and EDX), octahedral crystals with average size
of 1.55 lm (hematite, XRD, EDX), plate-like aggre-
gates of 1.0–2.2 lm (NaCl, XRD, and EDX), and rod-
like aggregates of 1.0–2.0 lm (iron oxide, and EDX).
Effect of heating process
The nanoparticles obtained by various synthetic routes
were dried at 80 �C for 2 h to evaporate the main part
of solvents. The nanopowders dried at 80 �C have
been investigated with thermoanalysis and XRD. The
processes of weight loss finish by \300 �C in the
samples of the sol–gel technique starting from nitrate
salts (by 260–285 �C) and the co-precipitation method
using oxalic acid agent (by 210 �C) (Figs. 7, 8). The
processes of weight loss continue until 500–700 �C in
the other samples (Figs. 7, 8). The weight loss in the
range from 25 to 100–150 �C is generally 5–7 %, and
it can be attributed to the evaporation of residual
solvents (e.g., water and n-propanol). The temperature
range of 100–190 �C belongs to the volatilization of
bonded water content (e.g., crystalline water, mole-
cules of hydration layers) in precipitates, its weight
loss changes between 5 and 20 %. The nitrate content,
the N-containing organic molecules derived from the
reaction of 1-propanol and nitrate ions, and the organic
molecules connected around the metal ions escape
between 180 and 280 �C in two or three steps. The
combustion of organic molecules is indicated by the
exothermic changes on the DTA curves. The decar-
bonization of carbonate precipitates occurs between
150 and 300 �C. The chloride ions may decompose
above 300–400 �C (Figs. 7, 8).
The sol–gel derived precipitate dried at 80 �C
proved to be amorphous (XRD) basic nitrate/chloride-
containing salts (TA). The precipitate obtained by a
slow addition of ferric nitrate solution has a much
lower nitrate content than that of precipitate produced
by a regular sol–gel route (DTA) and includes already
some cobalt ferrite ordering (XRD). The small basic
chloride-containing residue consists mostly of
CoCl2�2H2O (XRD). The products of co-precipitation
with sodium carbonate are amorphous (XRD) basic
carbonate salts (TA). By oxalic acid, CoC2O4�2H2O
precipitates (XRD). The samples produced by micro-
emulsion technique contain many compounds proved
Par
ticle
siz
e (n
m)
PDMS (w/w%)
carbonate 550
carbonate 5600
oxalate 5600
Fig. 1 Particle sizes of carbonate and oxalate precipitates
versus concentration of PDMS (550 and 5600 g mol-1)
J Nanopart Res (2012) 14:894 Page 7 of 14
123
Fig. 2 SEM image of the
nanopowders synthesized by
modified sol–gel method
from nitrate salts
Fig. 3 SEM images of the nanopowders prepared by various
techniques. The samples were prepared by 1 modified sol–gel
method from nitrate salts; 2 sol–gel method from nitrate salts; 3sol–gel method from chloride salts; 4 microemulsion; 5 co-
precipitation with carbonate and 5 % PDMS of 5,600 g mol-1;
6 co-precipitation with carbonate; 7 co-precipitation with
oxalate and 5 % PDMS of 5,600 g mol-1; 8 co-precipitation
with oxalate. The samples were heated at 600 �C. With
exceptions (4, 6), the magnification is 9100,000
Page 8 of 14 J Nanopart Res (2012) 14:894
123
by TA (Fig. 8) and SEM (Fig. 3). The main compo-
nent is an amorphous cobalt carbonate salt with less
OH groups (XRD).
The effect of heat treatment was investigated by in
situ, small and wide angle X-ray scattering (SAXS,
WAXS). Figs. 9 and 10 represent these measurements;
the SAXS (Fig. 9) and WAXS (Fig. 10) investigations
were carried out on the best sample obtained by
modified sol–gel method, i.e., using a slow addition of
the ferric nitrate solution. The SAXS curves indicate
particle sizes of \15 nm in the temperature of
20–400�C (Fig. 9). The size of particles grows signif-
icantly up to 40–44 nm above 400 �C. Above 600 �C,
a further growth can be observed (82 nm). The particle
sizes at C800 �C cannot be detected by SAXS because
the size is too large ([100 nm) for SAXS range. A
significant change can also be monitored by WAXS
between 400 and 500 �C (Fig. 10). WAXS identifies
only some ferrite ordering in the samples heated at
B400 �C. The real crystalline cobalt ferrite phase may
be detected in the nanopowders heat treated at
C500 �C. Thus, the crystallization results in a dramatic
change in the particle size above 400 �C.
The co-precipitated nanopowders are also very fine
(8–10 nm, SAXS) after a drying at 80 �C. The particle
sizes grow continuously with the temperature of heat
treatment. The sizes obtained by carbonate precipita-
tion in the presence of PDMS are around 20 nm at
400�C; &30 nm at 500 �C; and 40–45 nm at 600 �C.
WAXS as well as SAXS indicate a slow structural
transformation between 300 and 400 �C. WAXS
identifies already crystalline phases (NaCl, cobalt
ferrite, and hematite) in the sample of 400 �C;
however, the well-developed crystals of cobalt ferrite
appear only at 600–700 �C.
Discussion
Sol–gel method
A new, simple, fast way of sol–gel method has been
developed for preparation of cobalt ferrite nanoparti-
cles. The new sol–gel route has nitrate salts reacted in
Fig. 4 X-ray diffraction patterns of sol–gel derived nanopow-
ders heated at 600 �C. The samples were prepared by 1 modified
sol–gel method from nitrate salts; 2 sol–gel method from nitrate
salts; 3 sol–gel method from chloride salts
Fig. 5 X-ray diffraction patterns of nanopowders prepared by
co-precipitation and heated at 600 �C. The samples were
precipitated by 4 carbonate without any surfactant; 5 carbonate
and 5 % PDMS of 5,600 g mol-1; 6 oxalate and 5 % PDMS of
5,600 g mol-1; 7 carbonate using microemulsion technique
J Nanopart Res (2012) 14:894 Page 9 of 14
123
1-propanol in the presence of ethyl acetate. The nitrate
salts proved to be a more efficient precursor for the sol–
gel technique than chloride. The application of chlo-
ride precursors yields very small amount of particles:
5–6 % of theoretical mass. In the case of nitrate salts,
the average yield is 45–60 %. In the solution of
nitrates, the condensation reactions are more intensive.
The hydrolysis of metal ions produces OH groups
which make the condensation possible. A part of nitrate
content escapes as nitrous gases during the reactions
increasing the pH that also supports the condensation.
The decomposition of nitrate ions depends on the
Fig. 6 SEM image of the
nanopowders precipitated
by carbonate and 5 %
PDMS of 5,600 g mol-1
Fig. 7 Thermoanalysis of sol–gel derived nanopowders dried
at 80 �C. The samples were prepared by 1 modified sol–gel
method from nitrate salts; 2 sol–gel method from nitrate salts; 3sol–gel method from chloride salts
Fig. 8 Thermoanalysis of nanopowders prepared by co-pre-
cipitation and dried at 80 �C. The samples were precipitated by
4 carbonate without any surfactant; 5 carbonate and 5 % PDMS
of 5,600 g mol-1; 6 oxalate and 5 % PDMS of 5,600 g mol-1;
7 carbonate using microemulsion technique
Page 10 of 14 J Nanopart Res (2012) 14:894
123
polarity of the medium. The lower the polarity, the
more intensive the decomposition. Thus, the results of
experiments in 1-propanol are more impressive than in
ethanol. The chloride ions can not escape during the
gelation at 80 �C.
The sol–gel method also needs any of the surfac-
tants to obtain nanoparticles. Citric acid, PDMS of 550
and 5,600 g mol-1 molecular weights, and ethyl
acetate were applied as surfactants in the sol–gel
procedures. The smallest particle size could be
achieved in the presence of ethyl acetate (Table 3).
Application of ethyl acetate in the concentration of
40 w/w% yielded spherical cobalt ferrite nanoparti-
cles of average diameter of 40 nm (SEM) with narrow
polydispersity (30–60 nm). If the common solution of
precursors is subjected to reaction and heating, then
hematite (Fe2O3) always forms. (See Fig. 4) The
formation of hematite can be avoided by a slow
addition of alcoholic solution of Fe(NO3)3 to the
solution of Co(NO3)3. The slow addition of ferric
nitrate solution results in the finest particles and the
lowest temperature for the reactions. The decomposi-
tion and the combustion of organic compounds and the
bonded nitrate content occur in one step between 175
and 260 �C. That proves that the nitrate content is less
bonded in the particles and escapes mostly as nitrous
gases during the gelation. The precipitate contains a
smaller amount of nitrate ions than the product made
by regular sol–gel route of a common precursor
solution. Treating a common precursor solution, the
processes of weight loss are carried out in three steps
until 285 �C. The gelation using a slow addition of
ferric nitrate solution produces an amorphous basic
nitrate-containing salt with some cobalt ferrite
Fig. 9 SAXS patterns of
the nanopowders
synthesized by modified
sol–gel method from nitrate
salts in the function of
temperature
Fig. 10 WAXS patterns of the nanopowders synthesized by
modified sol–gel method from nitrate salts in the function of
temperature
J Nanopart Res (2012) 14:894 Page 11 of 14
123
ordering and with average size of \10 nm at 80 �C.
The particle sizes increase significantly to 40–44 nm
(SAXS) above 400 �C accompanied by the appear-
ance of the crystalline ferrite phase.
The use of benzyl alcohol instead of aliphatic
alcohol resulted in only a small amount of inhomo-
geneous precipitates; however, the initial materials
(chloride and nitrate), the ratios of solvent/precursor
(18–280 molar ratios), and the reaction time (2–48 h)
were widely varied in the experiments.
Surfactant-assisted precipitation method
In the study on the surfactant-assisted precipitation
techniques, the precipitation agents, the surfactants,
their concentration, and the temperature of heat
treatment were varied. The precipitations with sodium
hydroxide or ammonia yield coarse and large particles
([100 nm) in aqueous solutions. Thus, the experi-
ments concentrated on the application of carbonate
and oxalate precipitators. The sizes of particles
obtained by oxalate precipitators are larger (58 nm)
and more polydisperse (24–140 nm, SEM) than that of
carbonate precipitates (12–54 nm, SEM) (Table 3).
The precipitator ratio has only a slight influence on the
size and distribution above 1 molar ratio of precipi-
tator/metal ion. Co-precipitation with carbonate with-
out any surfactant yields inhomogeneous particles;
cobalt ferrite nanoparticles (52 nm) with amorphous
shape, NaCl plate-like aggregates (1.5–2 lm), and
hematite octahedral crystals with average size of
320 nm (Figs. 3, 5). PDMS of 550 or 5,600 g mol-1
proved to be the most effective surfactant considering
the size and size distribution of the particles synthe-
sized with assistance of several surfactants (PDMS,
Triton X-100, NaDS, NaDBS, and TDAB) in the co-
precipitation series. The ionic surfactants hinder the
aggregation less than PDMS. The particle size
prepared with both precipitators (carbonate and oxa-
late) represents a minimum in the function of the
PDMS concentration (Fig. 1). The smallest cobalt
ferrite nanoparticles could be obtained by a carbonate
precipitator in the presence of 5 w/w% PDMS. By
PDMS, the volume of ferrite phase increases and the
amorphous particles assume a cubic shape (Figs. 3, 6).
The co-precipitation with sodium carbonate and
PDMS results in amorphous basic carbonate salts at
room temperature, from those cobalt ferrite and small
amount of hematite can be evolved at around 600 �C.
During the heat treatment, particles of 8–10 nm
(SAXS) grow up to 40–45 nm (SAXS). By oxalic
acid, CoC2O4�2H2O precipitates at room temperature,
which increases the inhomogeneity of the product. The
size and its dispersion change slightly by addition of a
surfactant.
Microemulsion technique
In the microemulsion preparation, sodium carbonate
serves as precipitating agent and sec. buthylalcohol as
an oil phase. The preparation conditions were similar
to the surfactant-assisted co-precipitation procedure to
compare the methods and to study the effect of the
microemulsion technique. The microemulsion tech-
nique resulted in a much more inhomogeneous
product than the co-precipitation. This route of the
microemulsion technique yielded the widest size
distribution (20–2200 nm, SEM) owing to the several
species. However, the cobalt ferrite amorphous parti-
cles are very fine—25–31 nm (SEM, the powders
treated with PDMS or CTAB surfactant and heated at
600 �C). The ‘‘nanocontainers,’’ i.e., the emulgated
droplets control the growth of amorphous ferrite
particles rather than that of crystalline species. The
components of the microemulsion-derived products
are cobalt ferrite fine nanoparticles with amorphous
shape, hematite octahedral crystals with average size
of 1.55 lm, NaCl plate-like aggregates of 1.0–2.2 lm,
and iron oxide rod-like aggregates of 1.0–2.0 lm. The
application of the microemulsion requires any mod-
ifying agent. The surfactants (CTAB, NaDS, and
PDMS) reduced significantly (on the average by about
200 nm) the particles size, especially in the presence
of a large amount agent.
Conclusions
In the present study, cobalt ferrite nanoparticles were
synthesized by various liquid-phase methods,
namely, by coprecipitation process, sol–gel route,
and microemulsion technique combined with thermal
decomposition. The cobalt ferrite nanoparticles can
be used as components of polymer nanocomposites
in medical diagnosis and targeted drug delivery. The
effects of experimental parameters on the particle
size, size distribution, morphology, and chemical
composition have been studied. The preparation
Page 12 of 14 J Nanopart Res (2012) 14:894
123
experiments were carried out by varying the param-
eters such as the anions of precursors (chloride and
nitrate), the solvents (water, n-propanol, ethanol, and
benzyl alcohol), the surfactants (polydimethylsilox-
ane, ethyl acetate, citric acid, cethyltrimethylammo-
nium bromide, and sodium dodecil sulfate), the
concentration of the surfactant (0–10 m/m%), the
precipitating agents (sodium carbonate and oxalic
acid), the temperatures of the hydrolysis (room
temperature, 50, and 80 �C), and the thermal treat-
ment (80–1,000 �C).
A new, simple, and fast way of sol–gel method has
been developed for preparation of cobalt ferrite
nanoparticles. The smallest particles (40 nm Ø,
SEM) and the best dispersion (30–60 nm) could be
achieved by this sol–gel route starting from nitrate
salts. The nitrate salts were reacted in the mixture of
1-propanol and ethyl acetate at 80 �C. The lower
polarities of propanol and ethyl acetate support the
decomposition of nitrate ions. The escape of nitrous
gases increases the pH, which promotes the hydrolysis
and condensation reactions of metal ions. In order to
avoid the usual problem of the ferrite synthesis, i.e.,
the formation of iron oxide (hematite), the iron
precursor must be slowly added to the excess of cobalt
solution during mixing at 80 �C. The fine precipitate
synthesized with slow addition of ferric nitrate solu-
tion in the presence of 1-propanol and ethyl acetate
contains significantly less nitrate ions than that
obtained by other surfactants in ethanol and requires
the lowest temperature for its reactions. The use of
chloride precursors in the sol–gel technique produces
inhomogeneous products (cobalt-ferrite and iron-
oxide), a very low yield (5–6 %), and some larger
sizes (58–80 nm, SEM).
In the surfactant-assisted precipitation techniques,
the basic precipitators (sodium hydroxide or ammo-
nia) produce coarse and large particles ([100 nm) in
aqueous solutions. The precipitates derived from
oxalate precipitation are inhomogeneous and polydis-
perse in nature (24–140 nm). The application of the
carbonate-precipitating agent yields a very fine ferrite
powder (40–43 nm) in the presence of PDMS of 550
or 5,600 g mol-1 used in 5.0 w/w%. The particle size
shows a minimum in the function of the PDMS
concentration. The particles prepared by carbonate
precipitator contain not only a cobalt ferrite phase but
a small amount of sodium chloride and iron oxides,
too. The co-precipitation carried out in a
microemulsion generates polydisperse and polymorph
particles; several shapes (spherical, octahedral, and
rod-like), sizes (from 27 nm to 1–2 lm), and different
chemical compositions (cobalt-ferrite, iron oxide,
sodium chloride, etc.).
The size characterization techniques (SAXS, SEM,
and DLS) have been compared. The SAXS data are
consistent with the sizes determined by SEM and
differ from the DLS data. The size derived from DLS
is two or three times larger. Dried powders are
measured under vacuum in the SAXS and SEM
experiments. The nanoparticles dispersed in a polar
solution can be hydrated or solvated in the DLS
technique and the hydration/solvation shells may
result in the difference in the sizes. The effect of the
polar solvent has been proved by DLS measurements
in aqueous and alcoholic solutions.
Acknowledgments This study has been supported I-04-009
EU in HASYLAB, DESY and OTKA NK 101704 funds. The
European Union and the European Social Fund have provided
financial support to the project under the grant agreement no.
TAMOP 4.2.1./B-09/KMR-2010-0003.
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