Abstract
Using the prepared particles of 10 nm–25 nm as magnetic core, we synthesized / composite particles with 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 coating of ca.3 nm thick on , 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 / 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 / composite particle not only has super-paramagnetic property, but also good ability of chemical adsorption.
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 characterize composite particle by homogeneous precipitation with as the magnetic core and as the shell, where the nanoparticles were prepared by coprecipitation method, and the composite core-shell structure was synthesized via hydrolyzation of with precipitant. The structures and properties of the materials are discussed in some detail.
2. Materials and Methods
2.1. Preparation of Nanoparticles
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 and 2.39 g of were dissolved in 100 mL of distilled water, respectively. Then we mixed solutions of , , and 5% PEG together and dispersed them by ultrasonic stirring for 10 minutes. Then the mixture was heated to 75, 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, and then aged at 80 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.
2.2. Synthesis of Composite by Homogeneous Precipitation
0.5 g particles were dispersed by ultrasonic stirring in 100 mL 5% PEG for 30 minutes, and were mixed with 100 mL solution of certain concentration, and followed by addition of or urea solution. Table 1 shows the experimental conditions. The reaction lasted for 12 hours at 60. 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 and calcined at C for 3 hours.
The as-prepared magnetic nanoparticles and composite particles were characterized by D/max--B X-ray diffractometer (XRD) with radiation, Thermo ESCALAB 250 X-ray photoelectron spectroscopy (XPS) with 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 and composite particles were investigated by Model-155 vibrating sample magnetometer (VSM).
2.3. Decoloration Experiment of Methyl Orange Solution with Composite Particles
20 mg/L methyl orange solution was prepared as the simulated dye wastewater. The of methyl orange solution was 465 nm. PH value of the methyl orange solution was regulated to the set value with NaOH and . 50 mg 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 mg particles were dispersed by ultrasonic stirring in 50 mL methyl orange solution per time with the appropriate pH value for a different contact time (), 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
Figure 1 illustrates the X-ray diffraction patterns of the as-manufactured two samples mentioned above. While the lower pattern represents the synthesized standard particles, differential peaks appeared at and in the upper line, which are just the evidence of the existence of in the composite. Calculating the grain size according to Scherrer formula, we obtained 18.76 nm for the particles.
Figure 2 shows the XPS spectrum of Mn and Fe in composite particles. Binding energy of is 711.1 eV and binding energy of is 724.6 eV which correspond to the XPS spectrum of , and that for and 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 . These are consistent with the XRD in Figure 1, respectively.
| (a) Mn in |
| (b) Fe in |
3.2. Morphology and Shaping Mechanism of Nanoparticles
TEM images of the 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.
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 seed formed in solution because less NaOH was added, part of PEG acted as templates. Certain surface of the crystal stopped to grow because of PEG absorbing on it, and in the calcine process the morphology of remained, and finally the square shape of the particles was formed, this was also observed elsewhere. As more and more NaOH dropped in the solution, the nuclei of increased correspondingly, and the effect of dense coating of PEG dominated rather than selective absorption, stopping the particles to grow. Finally, small spherical nanoparticles were released by the decomposition of [3, 4]. Figure 4 shows the schematic shaping mechanism of nanoparticles.
3.3. Morphology and Shaping Mechanism of Composite Particles
Figures 5(a) and 5(d) present the TEM images of composite particles prepared under different conditions shown in Table 1. Figures 5(a) and 5(b) suggest that by the same concentration of , the volumes of added could affect the thickness of coating on the particles. Though the volume of was reduced to 10 mL (Table 1), a lot of dissociative still existed (Figure 5(b)(b)), so we have to reduce further the concentration of solution. When the concentration of solution was halved, the coating effect was much better as was expected (Figure 5(c)).
(a)
(b)
(c)
(d)
To improve the coating, we changed the precipitant from to urea (1 mol/L). As shown in Figure 5(d), the composite particles look uniformly with darker core and transparent 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 core-shell nanoparticles prepared have much better dispersive behavior than the starting materials.
3.4. Property for Potential Applications
Figure 7 shows the hysteresis curves of the nanoparticles and the composite particles. The two patterns resemble each other, and also the composite particles have super-paramagnetic properties. With the decreasing of the magnetic field, the magnetization decreased and reached zero at , 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 nanoparticles is 68.1 emu/g, and that for the composite particles measured 33.5 emu/g.
The IR spectra of the and the composite particles are shown in Figure 8. The characteristic absorption peak of lies in 588.13 , while that in the composite particles moved to 606 , resulting in a blue shift of ca.18 . It may be that the particles in the composite were monodispersive in relation to the starting materials. It is significant that there appeared a new peak at 535 in the response of the composite particles, implying that there may be some new bond generated between the core and the shell.
The absorption peaks near 3432 are flexible vibrating peaks of hydroxy on the surface of composite; it is wider and stronger than that of . It means that the composite particles have more hydroxys than , which in turn could make 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 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 ( minutes), the highest decoloration rate was 91.4% when the , and it can reach the highest decoloration rate 94.33% when the contact time was 50 minutes. So this kind of 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.
(a)
(b)
4. Conclusion
Using the prepared particles of 10 nm–25 nm as magnetic core, we synthesized composite particles with as the shell by homogeneous precipitation. The composite particles look uniformly with darker core and transparent shell, and the coating thickness is ca.3 nm. The as-synthesized composite particles exhibit super-paramagnetic properties and have better dispersivity than the starting materials. The saturation magnetization of nanoparticles is 68.1 eum/g, and that for the composite particles measured 33.5 eum/g. Decoloration experiment of methyl orange solution with 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 composite particle not only has super-paramagnetic property but also has good ability of chemical adsorption.