# Synthesis and Characterization of Magnetic NanosizedFe3O4/MnO2 Composite Particles

Authors: Zhang Shu; Shulin Wang
Journal: Journal of Nanomaterials
 (2009)

---

## Abstract

Using the preparedFe3O4 particles of 10 nm–25 nm as magnetic core, we synthesized Fe3O4/MnO2 composite particles with MnO2 as the shell by homogeneous precipitation. Their structure and morphology were characterized by X-ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), transmission electronic microscopy (TEM), Fourier transform infrared spectra (FTIR), and vibration-sample magnetometer (VSM). We show that with urea as precipitant transparent and uniform MnO2 coating of ca.3 nm thick on Fe3O4, particles can be obtained. The composite particles have better dispersivity than the starting materials, and exhibit super-paramagnetic properties and better chemical adsorption ability with saturated magnetization of 33.5 emu/g. Decoloration experiment of methyl orange solution with Fe3O4/MnO2 composite suggested that the highest decoloration rate was 94.33% when the pH of methyl orange solution was 1.3 and the contact time was 50 minutes. So this kind of Fe3O4/MnO2 composite particle not only has super-paramagnetic property, but also good ability of chemical adsorption.


---

## Body

## 1. Introduction

In recent years,  nanocomposite materials consisting of core and shell have been
attracting great interest and attention of  researchers because of their
potential application in catalysis, environmental protection, and especially
biomedical use. Ikram ul Haq and Egon Matijevic studied the preparation and
properties of manganese compounds on hematite in 1997 [1] with three manganese compounds: manganese (II) 2,
4-pentanedionate (MP); manganese (II) methoxide (MMO);
and manganese (II) sulfate/urea (MSU) solution under different experimental conditions.In this paper, we present an approach to synthesize and characterizeFe3O4/MnO2 composite particle by homogeneous precipitation
with Fe3O4 as the magnetic core and MnO2 as
the shell, where the Fe3O4 nanoparticles were prepared by
coprecipitation method, and the Fe3O4/MnO2 composite core-shell structure was synthesized via hydrolyzation of MnSO4 with precipitant. The structures and properties of the materials are discussed in some detail.


## 2. Materials and Methods

### 2.1. Preparation ofFe3O4 Nanoparticles

Fe3O4 nanoparticles were prepared by chemical coprecipitation as follows: at first, 2 mol/L NaOH solution and 5% polyethylene glycol (PEG) solution were prepared. 3.24 g of FeCl3 and 2.39 g of FeCl2⋅4H2O were dissolved in 100 mL of distilled water, respectively. Then we mixed solutions
of FeCl3, FeCl2⋅4H2O, and 5% PEG together and dispersed them by ultrasonic stirring for 10 minutes. Then the mixture was heated to 75C°, followed by the addition of 2 mol/L NaOH solution until the pH value of the mixture reached about 11.5. The mixing and stirring lasted for 2 hours
at 60C°, and then aged at 80C° for 30 minutes. The resulting particles were
purified repeatedly by magnetic field separation, washed several times with
distilled water or ethanol, and dried at 80C°.


### 2.2. Synthesis ofFe3O4/MnO2 Composite by Homogeneous Precipitation

0.5 gFe3O4 particles were dispersed by ultrasonic
stirring in 100 mL 5% PEG for 30 minutes, and were mixed with 100 mL MnSO4 solution of certain concentration, and followed by addition of NH3⋅H2O or urea solution. Table 1 shows the experimental conditions. The reaction lasted for 12 hours at 60C°. At the end of
the period, the solids were separated by repeated magnetic separation and
washed with distilled water for three times. The products were dried at 80C°  and calcined at 280°C for 3 hours.Table 1
Reaction condition for synthesis ofFe3O4/MnO2 composite.
SampleMnSO4/mol⋅L−1V(precipitant)/mLV(PEG)/mLFe3O4/g(a)0.07215(amine)*1000.5(b)0.07210(amine)*1000.5(c)0.03610(amine)*1000.5(d)0.03610(1 mol/L urea)1000.5*Mass fraction of amine
is 25%.The as-prepared magneticFe3O4 nanoparticles and Fe3O4/MnO2 composite
particles were characterized by D/max-γ-B X-ray diffractometer (XRD) with CuKα radiation, Thermo ESCALAB
250 X-ray photoelectron
spectroscopy (XPS) with AlKα radiation, and Nicolet 200SXV Fourier transform
infrared (FTIR). The morphology of the particle was studied with JEM-2100F transmission electron microscopy (TEM). Magnetic
properties of Fe3O4 and Fe3O4/MnO2 composite
particles were investigated by Model-155 vibrating sample magnetometer (VSM).


### 2.3. Decoloration Experiment of Methyl Orange Solution withFe3O4/MnO2 Composite Particles

20 mg/L methyl orange solution was prepared as the simulated
dye wastewater.  Theλmax of methyl orange solution was 465 nm. PH value of the
methyl orange solution was regulated to the set value with NaOH and NH3⋅H2O.
50 mg
Fe3O4/MnO2 particles were dispersed by ultrasonic stirring in 50 mL methyl orange solution per time with
different pH values for 30 minutes, and absorbance at wavelength of 465 nm was determined by UV-1700
visible spectrophotometer. By calculating the decoloration rate of
methyl orange, the
appropriate pH value for decoloration was found.50 mgFe3O4/MnO2 particles were
dispersed by ultrasonic stirring in 50 mL methyl orange solution per time with the appropriate pH value for a different contact time (t), and the absorbance at wavelength of 465 nm was also determined. By calculating
the discoloration rate of methyl orange, the appropriate contact time value was found too.


## 2.1. Preparation ofFe3O4 Nanoparticles

Fe3O4 nanoparticles were prepared by chemical coprecipitation as follows: at first, 2 mol/L NaOH solution and 5% polyethylene glycol (PEG) solution were prepared. 3.24 g of FeCl3 and 2.39 g of FeCl2⋅4H2O were dissolved in 100 mL of distilled water, respectively. Then we mixed solutions
of FeCl3, FeCl2⋅4H2O, and 5% PEG together and dispersed them by ultrasonic stirring for 10 minutes. Then the mixture was heated to 75C°, followed by the addition of 2 mol/L NaOH solution until the pH value of the mixture reached about 11.5. The mixing and stirring lasted for 2 hours
at 60C°, and then aged at 80C° for 30 minutes. The resulting particles were
purified repeatedly by magnetic field separation, washed several times with
distilled water or ethanol, and dried at 80C°.


## 2.2. Synthesis ofFe3O4/MnO2 Composite by Homogeneous Precipitation

0.5 gFe3O4 particles were dispersed by ultrasonic
stirring in 100 mL 5% PEG for 30 minutes, and were mixed with 100 mL MnSO4 solution of certain concentration, and followed by addition of NH3⋅H2O or urea solution. Table 1 shows the experimental conditions. The reaction lasted for 12 hours at 60C°. At the end of
the period, the solids were separated by repeated magnetic separation and
washed with distilled water for three times. The products were dried at 80C°  and calcined at 280°C for 3 hours.Table 1
Reaction condition for synthesis ofFe3O4/MnO2 composite.
SampleMnSO4/mol⋅L−1V(precipitant)/mLV(PEG)/mLFe3O4/g(a)0.07215(amine)*1000.5(b)0.07210(amine)*1000.5(c)0.03610(amine)*1000.5(d)0.03610(1 mol/L urea)1000.5*Mass fraction of amine
is 25%.The as-prepared magneticFe3O4 nanoparticles and Fe3O4/MnO2 composite
particles were characterized by D/max-γ-B X-ray diffractometer (XRD) with CuKα radiation, Thermo ESCALAB
250 X-ray photoelectron
spectroscopy (XPS) with AlKα radiation, and Nicolet 200SXV Fourier transform
infrared (FTIR). The morphology of the particle was studied with JEM-2100F transmission electron microscopy (TEM). Magnetic
properties of Fe3O4 and Fe3O4/MnO2 composite
particles were investigated by Model-155 vibrating sample magnetometer (VSM).


## 2.3. Decoloration Experiment of Methyl Orange Solution withFe3O4/MnO2 Composite Particles

20 mg/L methyl orange solution was prepared as the simulated
dye wastewater.  Theλmax of methyl orange solution was 465 nm. PH value of the
methyl orange solution was regulated to the set value with NaOH and NH3⋅H2O.
50 mg
Fe3O4/MnO2 particles were dispersed by ultrasonic stirring in 50 mL methyl orange solution per time with
different pH values for 30 minutes, and absorbance at wavelength of 465 nm was determined by UV-1700
visible spectrophotometer. By calculating the decoloration rate of
methyl orange, the
appropriate pH value for decoloration was found.50 mgFe3O4/MnO2 particles were
dispersed by ultrasonic stirring in 50 mL methyl orange solution per time with the appropriate pH value for a different contact time (t), and the absorbance at wavelength of 465 nm was also determined. By calculating
the discoloration rate of methyl orange, the appropriate contact time value was found too.


## 3. Results and Discussion

### 3.1. Structure and Composition of The Materials

Figure1
illustrates the X-ray diffraction patterns of the as-manufactured two samples
mentioned above. While the lower pattern represents the synthesized standard Fe3O4 particles, differential peaks appeared at 2θ=27.5° and 2θ=40.3° in the upper line, which are just the evidence of the existence of MnO2 in the Fe3O4/MnO2 composite. Calculating the grain size according to
Scherrer formula, we obtained 18.76 nm for the Fe3O4 particles.Figure 1
XRD patterns ofFe3O4 and Fe3O4/MnO2 composite particles.Figure2
shows the XPS spectrum of Mn and Fe in Fe3O4/MnO2 composite
particles.  Binding energy of Fe2p3/2 is 711.1 eV and  binding energy
of Fe2p1/2 is 724.6 eV which correspond to the XPS spectrum of Fe3O4,
and  that for
Mn2p3/2
and
Mn2p1/2
are, respectively, 642.2 eV and 653.8 eV. According to the Handbook of X-ray photoelectron spectroscopy [2],
the Mn in the composite particles exists as Mn4+. These are consistent
with the XRD in Figure 1, respectively.XPS spectrum of Mn (a) and Fe
(b) inFe3O4/MnO2 composite particles.
(a)
Mn inFe3O4/MnO2(b)
Fe inFe3O4/MnO2


### 3.2. Morphology and Shaping Mechanism ofFe3O4 Nanoparticles

TEM images of
theFe3O4 particles are shown in Figure 3. Two different kinds of morphology
can be seen some of particles are spherical with diameter of about 10 nm, and
the others are square with the diameter of ca.25 nm, it is comparable to that
from the XRD pattern in Figure 1.Figure 3
TEM image ofFe3O4 nanoparticles.The
reason for the two different particle shapes lies in the different roles the PEG played in the reaction
process. At the beginning of the reaction, the amount of PEG was relatively more
than that of theFe(OH)3
seed formed in solution because less NaOH
was added, part of PEG acted as templates. Certain surface of the Fe(OH)3 crystal stopped to grow because of PEG absorbing on it, and in the calcine
process the morphology of Fe(OH)3 remained, and finally the square shape of the Fe3O4 particles was formed, this was also observed elsewhere. As more and more NaOH
dropped in the solution, the nuclei of Fe(OH)3 increased
correspondingly, and the effect of dense coating of PEG dominated rather than selective
absorption, stopping the Fe(OH)3 particles to grow. Finally, small spherical Fe3O4 nanoparticles were released by the decomposition of Fe(OH)3 [3, 4].
Figure 4 shows the schematic shaping mechanism of Fe3O4 nanoparticles.Figure 4
Schematic sketch of the
formation ofFe3O4 with different morphologies.


### 3.3. Morphology and Shaping Mechanism ofFe3O4/MnO2 Composite Particles

Figures5(a)
and 5(d) present the TEM images of Fe3O4/MnO2 composite particles prepared under different conditions shown in Table 1. Figures
5(a) and 5(b) suggest that by the same concentration of MnSO4, the
volumes of NH3⋅H2O added could affect the
thickness of MnO2 coating on the Fe3O4 particles. Though the volume of NH3⋅H2O was reduced to 10 mL (Table 1), a lot of dissociative MnO2 still  existed (Figure 5(b)(b)),
so we have to reduce further the concentration of MnSO4 solution.
When the concentration of MnSO4 solution was halved, the coating
effect was much better as was expected (Figure 5(c)).TEM images ofFe3O4/MnO2 composite particles prepared under different reaction
conditions in Table 1.
(a)(b)(c)(d)To improve
the coating, we changed the precipitant fromNH3⋅H2O to
urea (1 mol/L). As shown in Figure 5(d), the composite particles look uniformly
with darker Fe3O4 core and transparent MnO2 shell, and the coating thickness is
ca.3 nm, a demonstration, that the conditions selected are rational.It is very
interesting to show the TEM image of large area in Figure6, from which we may
claim that the Fe3O4/MnO2 core-shell
nanoparticles prepared have much better dispersive behavior than the starting materials.Figure 6
TEM images ofFe3O4/MnO2 composite particles in large area.


### 3.4. Property for Potential Applications

Figure7 shows
the hysteresis curves of the Fe3O4 nanoparticles and the
Fe3O4/MnO2 composite particles. The
two patterns resemble each other, and also the Fe3O4/MnO2 composite particles have super-paramagnetic properties. With
the decreasing of the magnetic field, the magnetization decreased and reached zero at H=0, no residual magnetization remained. This prevents particles from
aggregation, and the particles can be redispersed rapidly when magnetic field is
removed. The saturation magnetization of Fe3O4 nanoparticles is 68.1 emu/g, and that for the Fe3O4/MnO2 composite particles measured 33.5 emu/g.Figure 7
Magnetic hysteresis loop ofFe3O4,
Fe3O4/MnO2 composite particles.The IR spectra of theFe3O4 and the Fe3O4/MnO2 composite particles are shown in Figure 8. The characteristic absorption peak of Fe3O4 lies in 588.13 cm−1,
while that in the Fe3O4/MnO2 composite particles moved to 606 cm−1, resulting in a blue shift
of ca.18 cm−1.
It may be that the Fe3O4 particles in the composite
were monodispersive in relation to the starting materials. It is significant
that there appeared a
new peak at 535 cm−1 in the response of the composite
particles, implying that there may be some new bond generated between the
core and the shell.Figure 8
IR spectrum of samplesFe3O4, Fe3O4/MnO2 composite particles.The absorption peaks
near 3432cm−1 are flexible vibrating peaks of hydroxy on the
surface of composite; it is wider and
stronger than that of Fe3O4. It means that the composite
particles have more hydroxys than Fe3O4, which in turn could
make MnO2 more active. Thus, the composite particles may
have better ability of chemical adsorption, which can be used for dyestuff
treatment.Then the decoloration of methyl orange withFe3O4/MnO2 was studied, and the influence of initial solution pH value and contact time on the decoloration
was investigated. Figures 9(a) and 9(b) suggest that pH value is the key factor
influencing decoloration efficiency. The lower pH value and adequate contact
time (t) are favorable for the methyl orange solution
decoloration. With the same contact time (t=30 minutes), the highest
decoloration rate was 91.4% when the pH=1.3, and it can reach the highest
decoloration rate 94.33% when the contact time was 50 minutes. So this kind of Fe3O4/MnO2 composite particle
not only has super-paramagnetic property, but also has good ability of chemical adsorption
which may find wide applications in the field of dyestuff adsorption.Effect of pH value (a) and contact time (b) on
decoloration rate.
(a)(b)


## 3.1. Structure and Composition of The Materials

Figure1
illustrates the X-ray diffraction patterns of the as-manufactured two samples
mentioned above. While the lower pattern represents the synthesized standard Fe3O4 particles, differential peaks appeared at 2θ=27.5° and 2θ=40.3° in the upper line, which are just the evidence of the existence of MnO2 in the Fe3O4/MnO2 composite. Calculating the grain size according to
Scherrer formula, we obtained 18.76 nm for the Fe3O4 particles.Figure 1
XRD patterns ofFe3O4 and Fe3O4/MnO2 composite particles.Figure2
shows the XPS spectrum of Mn and Fe in Fe3O4/MnO2 composite
particles.  Binding energy of Fe2p3/2 is 711.1 eV and  binding energy
of Fe2p1/2 is 724.6 eV which correspond to the XPS spectrum of Fe3O4,
and  that for
Mn2p3/2
and
Mn2p1/2
are, respectively, 642.2 eV and 653.8 eV. According to the Handbook of X-ray photoelectron spectroscopy [2],
the Mn in the composite particles exists as Mn4+. These are consistent
with the XRD in Figure 1, respectively.XPS spectrum of Mn (a) and Fe
(b) inFe3O4/MnO2 composite particles.
(a)
Mn inFe3O4/MnO2(b)
Fe inFe3O4/MnO2


## 3.2. Morphology and Shaping Mechanism ofFe3O4 Nanoparticles

TEM images of
theFe3O4 particles are shown in Figure 3. Two different kinds of morphology
can be seen some of particles are spherical with diameter of about 10 nm, and
the others are square with the diameter of ca.25 nm, it is comparable to that
from the XRD pattern in Figure 1.Figure 3
TEM image ofFe3O4 nanoparticles.The
reason for the two different particle shapes lies in the different roles the PEG played in the reaction
process. At the beginning of the reaction, the amount of PEG was relatively more
than that of theFe(OH)3
seed formed in solution because less NaOH
was added, part of PEG acted as templates. Certain surface of the Fe(OH)3 crystal stopped to grow because of PEG absorbing on it, and in the calcine
process the morphology of Fe(OH)3 remained, and finally the square shape of the Fe3O4 particles was formed, this was also observed elsewhere. As more and more NaOH
dropped in the solution, the nuclei of Fe(OH)3 increased
correspondingly, and the effect of dense coating of PEG dominated rather than selective
absorption, stopping the Fe(OH)3 particles to grow. Finally, small spherical Fe3O4 nanoparticles were released by the decomposition of Fe(OH)3 [3, 4].
Figure 4 shows the schematic shaping mechanism of Fe3O4 nanoparticles.Figure 4
Schematic sketch of the
formation ofFe3O4 with different morphologies.


## 3.3. Morphology and Shaping Mechanism ofFe3O4/MnO2 Composite Particles

Figures5(a)
and 5(d) present the TEM images of Fe3O4/MnO2 composite particles prepared under different conditions shown in Table 1. Figures
5(a) and 5(b) suggest that by the same concentration of MnSO4, the
volumes of NH3⋅H2O added could affect the
thickness of MnO2 coating on the Fe3O4 particles. Though the volume of NH3⋅H2O was reduced to 10 mL (Table 1), a lot of dissociative MnO2 still  existed (Figure 5(b)(b)),
so we have to reduce further the concentration of MnSO4 solution.
When the concentration of MnSO4 solution was halved, the coating
effect was much better as was expected (Figure 5(c)).TEM images ofFe3O4/MnO2 composite particles prepared under different reaction
conditions in Table 1.
(a)(b)(c)(d)To improve
the coating, we changed the precipitant fromNH3⋅H2O to
urea (1 mol/L). As shown in Figure 5(d), the composite particles look uniformly
with darker Fe3O4 core and transparent MnO2 shell, and the coating thickness is
ca.3 nm, a demonstration, that the conditions selected are rational.It is very
interesting to show the TEM image of large area in Figure6, from which we may
claim that the Fe3O4/MnO2 core-shell
nanoparticles prepared have much better dispersive behavior than the starting materials.Figure 6
TEM images ofFe3O4/MnO2 composite particles in large area.


## 3.4. Property for Potential Applications

Figure7 shows
the hysteresis curves of the Fe3O4 nanoparticles and the
Fe3O4/MnO2 composite particles. The
two patterns resemble each other, and also the Fe3O4/MnO2 composite particles have super-paramagnetic properties. With
the decreasing of the magnetic field, the magnetization decreased and reached zero at H=0, no residual magnetization remained. This prevents particles from
aggregation, and the particles can be redispersed rapidly when magnetic field is
removed. The saturation magnetization of Fe3O4 nanoparticles is 68.1 emu/g, and that for the Fe3O4/MnO2 composite particles measured 33.5 emu/g.Figure 7
Magnetic hysteresis loop ofFe3O4,
Fe3O4/MnO2 composite particles.The IR spectra of theFe3O4 and the Fe3O4/MnO2 composite particles are shown in Figure 8. The characteristic absorption peak of Fe3O4 lies in 588.13 cm−1,
while that in the Fe3O4/MnO2 composite particles moved to 606 cm−1, resulting in a blue shift
of ca.18 cm−1.
It may be that the Fe3O4 particles in the composite
were monodispersive in relation to the starting materials. It is significant
that there appeared a
new peak at 535 cm−1 in the response of the composite
particles, implying that there may be some new bond generated between the
core and the shell.Figure 8
IR spectrum of samplesFe3O4, Fe3O4/MnO2 composite particles.The absorption peaks
near 3432cm−1 are flexible vibrating peaks of hydroxy on the
surface of composite; it is wider and
stronger than that of Fe3O4. It means that the composite
particles have more hydroxys than Fe3O4, which in turn could
make MnO2 more active. Thus, the composite particles may
have better ability of chemical adsorption, which can be used for dyestuff
treatment.Then the decoloration of methyl orange withFe3O4/MnO2 was studied, and the influence of initial solution pH value and contact time on the decoloration
was investigated. Figures 9(a) and 9(b) suggest that pH value is the key factor
influencing decoloration efficiency. The lower pH value and adequate contact
time (t) are favorable for the methyl orange solution
decoloration. With the same contact time (t=30 minutes), the highest
decoloration rate was 91.4% when the pH=1.3, and it can reach the highest
decoloration rate 94.33% when the contact time was 50 minutes. So this kind of Fe3O4/MnO2 composite particle
not only has super-paramagnetic property, but also has good ability of chemical adsorption
which may find wide applications in the field of dyestuff adsorption.Effect of pH value (a) and contact time (b) on
decoloration rate.
(a)(b)


## 4. Conclusion

Using the preparedFe3O4 particles of 10 nm–25 nm as magnetic
core, we synthesized Fe3O4/MnO2 composite particles with MnO2 as the shell by homogeneous
precipitation. The composite particles look uniformly with darker Fe3O4 core and transparent MnO2 shell, and the coating thickness is ca.3 nm. The as-synthesized Fe3O4/MnO2 composite particles
exhibit super-paramagnetic properties and have better dispersivity than the
starting materials. The saturation magnetization of Fe3O4 nanoparticles is 68.1 eum/g, and that for the Fe3O4/MnO2 composite particles measured 33.5 eum/g. Decoloration experiment of methyl orange solution with Fe3O4/MnO2 composite suggests that
the highest decoloration rate is 94.33% when the
pH value of methyl orange solution was 1.3
and the contact time was 50 minutes. So this kind of Fe3O4/MnO2 composite particle
not only has super-paramagnetic property but
also has good ability of chemical adsorption.



---

*Source: 340217-2008-12-10.xml*