Synthesis and Characterization of Magnetic Nanosized Fe 3 O 4 / MnO 2 Composite Particles

Using the prepared Fe3O4 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.


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 characterize Fe 3 O 4 /MnO 2 composite particle by homogeneous precipitation with Fe 3 O 4 as the magnetic core and MnO 2 as the shell, where the Fe 3 O 4 nanoparticles were prepared by coprecipitation method, and the Fe 3 O 4 /MnO 2 composite core-shell structure was synthesized via hydrolyzation of MnSO 4 with precipitant.The structures and properties of the materials are discussed in some detail.4H 2 O were dissolved in 100 mL of distilled water, respectively.Then we mixed solutions of FeCl 3 , FeCl 2 •4H 2 O, and 5% PEG together and dispersed them by ultrasonic stirring for 10 minutes.Then the mixture was heated to 75 • C, 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 60 • C, and then aged at 80 • C for 30 minutes.The resulting particles were purified repeatedly by magnetic field separation, washed several times with distilled water or ethanol, and dried at 80 • C.    1 shows the experimental conditions.The reaction lasted for 12 hours at 60 • C. 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 80 50 mg Fe 3 O 4 /MnO 2 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. Figure 2 shows the XPS spectrum of Mn and Fe in Fe 3 O 4 /MnO 2 composite particles.Binding energy of Fe2p 3/2 is 711.1 eV and binding energy of Fe2p 1/2 is 724.6 eV which correspond to the XPS spectrum of Fe 3 O 4 , and that for Mn2p 3/2 and Mn2p 1/2 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 Mn 4+ .These are consistent with the XRD in Figure 1, respectively.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.

Structure and Composition of the Materials
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 the Fe(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 Fe 3 O 4 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 Fe 3 O 4 nanoparticles were released by the decomposition of Fe(OH) 3 [3,4].Figure 4 shows the schematic shaping mechanism of Fe 3 O 4 nanoparticles.1), a lot of dissociative MnO 2 stillexisted (Figure 5(b)), so we have to reduce further the concentration of MnSO 4 solution.When the concentration of MnSO 4 solution was halved, the coating effect was much better as was expected (Figure 5(c)).

Morphology and Shaping Mechanism of Fe
To improve the coating, wechanged the precipitant from NH 3 •H 2 O to urea (1 mol/L).As shown in Figure 5(d), the composite particles look uniformly with darker Fe 3 O 4 core and transparent MnO 2 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 Figure 6, from which we may claim that the Fe 3 O 4 /MnO 2 core-shell nanoparticles prepared have much better dispersive behavior than the starting materials.Fe 3 O 4 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.

Property for Potential Applications.
The absorption peaks near 3432 cm −1 are flexible vibrating peaks of hydroxy on the surface of composite; it is wider and stronger than that of Fe 3 O 4 .It means that the composite particles have more hydroxys than Fe 3 O 4 , which in turn could make MnO 2 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 with Fe 3 O 4 / MnO 2 was studied, and the influence of initial solution pH value and contact time on the decoloration was investigated.Figures 9(a

Conclusion
Using the prepared Fe 3 O 4 particles of 10 nm-25 nm as magnetic core, we synthesized Fe Figure 1 illustrates the X-ray diffraction patterns of the as-manufactured two samples mentioned above.While the lower pattern represents the synthesized standard Fe 3 O 4 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 MnO 2 in the Fe 3 O 4 /MnO 2 composite.Calculating the grain size according to Scherrer formula, we obtained 18.76 nm for the Fe 3 O 4 particles.

Figure 5 :
Figure 5: TEM images of Fe 3 O 4 /MnO 2 composite particles prepared under different reaction conditions in Table1.
Figure 5: TEM images of Fe 3 O 4 /MnO 2 composite particles prepared under different reaction conditions in Table1.
3 O 4 Nanoparticles.Fe 3 O 4 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 FeCl 3 and 2.39 g of FeCl 2 3 O 4 /MnO 2 Composite by Homogeneous Precipitation.0.5 g Fe 3 O 4 particles were dispersed by ultrasonic stirring in 100 mL 5% PEG for 30 minutes, and were mixed with 100 mL MnSO 4 solution of certain concentration, and followed by addition of NH 3 •H 2 O or urea solution.Table O 4 /MnO 2 Composite Particles.20 mg/L methyl orange solution was prepared as the simulated dye wastewater.The λ max of methyl orange solution was 465 nm.PH value of the methyl orange solution was regulated to the set value with NaOH and NH 3 •H 2 O. 50 mg Fe 3 O 4 /MnO 2 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.

Table 1 :
Reaction condition for synthesis of Fe 3 O 4 /MnO 2 composite..2. Morphology and Shaping Mechanism of Fe 3 O 4 Nanoparticles.TEM images of the Fe 3 O 4 particles are shown in Figure * Mass fraction of amine is 25%.3 3 O 4 /MnO 2 Composite Particles.Figures 5(a) and 5(d) present the TEM images of Fe 3 O 4 /MnO 2 composite particles prepared under different conditions shown in Table 1.Figures 5(a) and 5(b) suggest that by the same concentration of MnSO 4 , the volumes of NH 3 •H 2 O added could affect the thickness of MnO 2 coating on the Fe 3 O 4 particles.Though the volume of NH 3 •H 2 O was reduced to 10 mL (Table 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 Fe 3 O 4 /MnO 2 composite particle not only has superparamagnetic property, but also has good ability of chemical adsorption which may find wide applications in the field of dyestuff adsorption.
3 O 4 /MnO 2 composite particles with MnO 2 as the shell by homogeneous precipitation.The composite particles look uniformly with darker Fe 3 O 4 core and transparent MnO 2 shell, and the coating thickness is ca.3 nm.The as-synthesized Fe 3 O 4 /MnO 2 composite particles exhibit super-paramagnetic properties and have better dispersivity than the starting materials.The saturation magnetization of Fe 3 O 4 nanoparticles is 68.1 eum/g, and that for the Fe 3 O 4 /MnO 2 composite particles measured 33.5 eum/g.Decoloration experiment of methyl orange solution with Fe 3 O 4 /MnO 2 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 Fe 3 O 4 /MnO 2 composite particle not only has super-paramagnetic property but also has good ability of chemical adsorption.