# Homogeneous Precipitation Synthesis and   Magnetic  Properties of Cobalt Ferrite Nanoparticles

Authors: Zhigang Liu; Xiaodong Li; Yonghua Leng; Jingbao  Lian; Shaohong Liu; Zhimeng Xiu; Di Huo; Ji-Guang Li; Xudong Sun
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Magnetic nanoparticles (NPs) of cobalt ferrite have been synthesized via a homogeneous precipitation route using hexamethylenetetramine (HMT) as the precipitant. The particle size, crystal structure, and magnetic properties of the synthesized particles were investigated by X-ray diffraction, transmission electron microscopy, and vibrating sample magnetometer. The NPs are of cubic inverse spinel structure and nearly spherical shape. With the increase of oxidation time from 30 to 180 minutes in the reaction solution at90∘C, the average particle size increases from ~30 nm to ~45 nm. The as-synthesized NPs ~30 nm in size show higher Ms (61.5 emu/g) and moderate Hc (945 Oe) and Mr/Ms (0.45) value compared with the materials synthesized by coprecipitation method using NaOH as precipitate at high pH value.


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## Body

## 1. Introduction

Cobalt ferrite nanoparticles (NPs) with an inverse spinel structure are promising
materials for high-density recording applications because of their high
magnetocrystalline anisotropy, high coercivity, moderate saturation
magnetization, and high chemical and structural stability at higher
temperatures [1–3]. Recently, cobalt ferrite NPs have attracted great interest in
the biomedical field, especially in magnetic targeted drug delivery and in
magnetic fluid hyperthermia (MFH), due to their larger magnetic anisotropy and
larger magnetic moments than iron oxides [4]. Cobalt ferrite NPs have been
synthesized using various methods, such as coprecipitation in an aqueous
solution [1], hydrothermal synthesis [5], microemulsion method [6], sol-gel-like technique [7], and sonochemical
reaction method [8]. Among them coprecipitation of iron and cobalt ions in
alkaline medium (usually NaOH and NH3·H2O) has proved to
be a convenient and inexpensive synthesis route suitable to synthesize large
batches of materials with reasonable control of composition and particle size. However,
the local heterogeneous distribution of precipitant and metal ions during the reaction
may lead to the precipitation of NPs with a relatively broad size distribution.
To reduce the polydispersity of particle size, several groups have used
microemulsions or vesicles at room temperature. However, with these techniques
the crystallinity of the product is poor and the amount of material produced is
highly reduced [9]. In this paper, a new homogeneous precipitation route was
developed to synthesize cobalt ferrite NPs using hexamethylenetetramine (HMT)
as the precipitant. The magnetic properties of cobalt ferrite NPs as
synthesized by this method were also investigated in this paper.


## 2. Experimental Section

All the reagents used in this experiment, (CH2)6N4(HMT),
FeCl2·4H2O, and Co(NO3)2·6H2O,
were of analytical grade and were used without any further purification.
Required weights of FeCl2·4H2O, Co(NO3)2·6H2O,
and HMT were dissolved into distilled water to achieve a mixed aqueous solution.
The mixed aqueous solution was bubbled for 30 minutes by argon under magnetic
agitating at room temperature to reduce the dissolved oxygen. The concentration of Me2+ (Me2+ represents Fe2+ and Co2+) was kept at 0.0125 M, with mole ratio of Fe2+/Co2+ at 2 and HMT/Me2+ at 10 in the mixed solution. The
temperature of the mixed solution was elevated to 80±2°C by silicon oil bath
within 30 minutes and kept at the temperature for 1 hour to finish the coprecipitation
reaction. Sage green alloy precipitate (partially oxidized Fe(OH)2 by NO3− or residual dissolved O2 in solution) defined as precursory precipitate was obtained. Thereafter,
the reaction temperature was elevated to 90°C as quickly as possible and argon
protection was then removed. Without the argon protection the reaction solution
contacted with the air directly and concentration of dissolved O2 in solution increased. The obtained
precursory precipitate then oxidized rapidly into black cobalt ferrite NPs by NO3− in solution and dissolved O2 from atmosphere. The precursory precipitate
was oxidized for 5, 30, and 180 minutes, respectively, and then withdrawn from
the reaction vessel. The products were separated by decanting clear solution
using a 2.0 T permanent magnet to retain the NPs. All the samples were then
washed with distilled water for 4 times and ethanol for 2 times, and then dried
at 80°C in air for 10 hours. Production yield of this method with the oxidation
time of 30 minutes was measured for three times and an average of 98 wt% was
achieved. The small percentage of loss was induced during the decanting stages.Phase identification was performed by X-ray diffractometry (XRD, Mode PW3040/60,
Philips, Eindhoven, The Netherlands). Lattice constants were calculated based on
the XRD patterns using software package of X'Pert
HighScore Plus version 2.0 (PANanalytical B. V. Almelo, The Netherlands). The morphology and
the particle sizes of the samples were observed using transmission electron
microscopy (TEM, Model, 200CX, JEOL, Tokyo, Japan). The
magnetization loops for the samples were measured using a vibrating sample
magnetometer (VSM, JDM-13). The actual stoichiometric composition of the
obtained powders was determined by a Shimadzu ICPS-75000 inductively coupled
plasma atomic emission spectrometer (ICP-AES).


## 3. Results and Discussion

At elevated temperatures, HMT releases
precipitating ligand (OH−) homogeneously into the reaction system by
decomposing into formaldehyde and ammonia species [10]. Because of the similar
solubility of Fe(OH)2 and Co(OH)2, it is possible for
iron (Fe2+) and cobalt (Co2+) ions to coprecipitate by
reacting with precipitating ligand (OH−) to form hydroxide alloy
precipitate under argon protection. Cobalt ferrite NPs are obtained through the oxidation of hydroxide alloy precipitate in air atmosphere. The cubic CoFe2O4 (JCPDS PDF no. 077-0426), orthorhombic Fe(OH)3 (JCPDS PDF no. 046-1436), and tetragonal FeO(OH) (JCPDS PDF no. 075-1594) phases were detected by
XRD for the precipitate oxidized for 5 minutes
(Figure 1). After oxidation for 30 minutes, pure cubic CoFe2O4 was obtained, as confirmed by the XRD pattern, in which all the peaks were
indexed to the cubic CoFe2O4 (JCPDS PDF no. 077-0426). In addition, the
elemental analyses of these NPs using ICP-AES showed that the molar ratio of Fe3+/Co2+ is close to 2.0. The peaks of XRD patterns became sharper as the oxidation time
increased from 30 to 180 minutes, indicating that grain size of the CoFe2O4NPs increases with the increase of the oxidation time. The average crystallite size was estimated from the full width at half
maximum (FWHM) values of diffraction peaks (440) and (311) using the Scherrer formula, and the results are summarized in Table 1. The crystalline
sizes are 26 and 38 nm for the oxidation time of 30 and 180 minutes,
respectively. Lattice parameter (a) was calculated and listed in Table 1. The
constants are close to the value reported in the literature (JCPDS PDF no. 077-0426,
a= 8.4 000 Å).Table 1
Summaries of some properties of CoFe2O4 NPs derived from XRD, TEM,
and magnetic measurement.
SampleOxide time (min)Average crystallite size from XRD (nm)Particle from TEM (nm)Lattice parametera=b=c (Å)Ms (emu/g)Hc (Oe)Remanence ratio(Mr/Ms)a555508.41213.67480.44b3026308.41361.59450.45c18038458.40663.09440.40[1]33.5378.38121.312810.4947.4478.39229.511800.48Figure 1
XRD patterns of precursory precipitate oxidized for 5, 30, and 180 minutes.Figure2 shows the TEM photographs of the products oxidized for various times. Precursor oxidized for 5 minutes is mainly composed of nearly spherical CoFe2O4 NPs~50 nm in size, with tiny acicular particles around nearly spherical CoFe2O4 NPs. The platelet and tiny acicular particles in Figure 2(a) were ascribed to the
residual iron hydroxide phase. After oxidation for 30 minutes, the acicular and
platelet particles disappeared and only nearly spherical CoFe2O4 NPs~30 nm in size were observed, as shown in Figure 2(b). This result is in
agreement with the analysis of XRD patterns. These CoFe2O4 NPs show some extent of aggregation owing to the magnetic attraction. Quasispherical
instead of spherical CoFe2O4 NPs were formed likely owing
to the low solution pH value (about 7.5), which slowed down the nucleation and
growth rate of spinel phase [11]. It is well known that faster growth rate
usually favors the formation of spherical particles as a result of less selective
crystallographic growth direction. Further, increase the oxidation time to 180
minutes, the particle size of CoFe2O4 NPs increased to~45 nm and more cubic particles appeared (Figure 2(c)). The average
crystallite sizes from XRD are in reasonable agreement with those from direct
observation from TEM images.TEM photographs of the products oxidized for various times, (a) for 5 minutes,
(b) for 30 minutes, and (c) for 180 minutes.
(a)(b)(c)Figure3 shows the hysteresis loops at room temperature for the samples oxidized for 5, 30, and 180 minutes. All the samples exhibit hysteresis loops
typical of magnetic behaviors, indicating the presence of ordered magnetic
structure in the spinel system. The saturation
magnetization (Ms), coercivity (Hc), and remanence ratio (Mr/Ms)
are listed in Table 1. The Ms value of the sample oxidized for 5 minutes is 13.6 emu/g, significantly lower than that of the
sample oxidized for 30 minutes (61.5 emu/g). This result is reasonable
considering the existence of residual hydroxide phase. The Ms value
for the sample oxidized for 30 minutes is slight lower
than those reported for the bulk samples (>70 emu/g). This can be attributed
to the surface effects aroused by the distortion of the magnetic moments at the
surface of nanocrystallite [1]. Although particle size is increased
as the oxidation time increases from 30 to 180 minutes, the magnetic properties of Ms, Hc, and Mr/Ms show little variations. This result is reasonable considering the particle size increment is small and particles are
rather big so that the influence of the surface on the magnetic properties is low compared to
the contribution of the volume of the particles. Compared with the corresponding counterpart materials
synthesized by coprecipitation method using NaOH as precipitate at high pH
value, the values of Ms are higher and Hc and Mr/Ms are moderate, as shown in Table 1.Figure 3
Hysteresis loops of the products oxidized for 5, 30,
and 180 minutes at room temperature.


## 4. Conclusion

Cobalt ferrite NPs about 30 nm in size were synthesized by a new
homogeneous precipitation routes using HMT as precipitant at low pH
environment. This method for the synthesis of Cobalt ferrite NPs is simple, low
in reaction temperature, high yielding, and inexpensive, without using
expensive equipment and reactants.



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*Source: 921654-2009-02-16.xml*