Abstract
We report the fabrication of core-shell Fe3O4@SiO2@TiO2 microspheres through a wet-chemical approach. The Fe3O4@SiO2@TiO2 microspheres possess both ferromagnetic and photocatalytic properties. The TiO2 nanoparticles on the surfaces of microspheres can degrade organic dyes under the illumination of UV light. Furthermore, the microspheres are easily separated from the solution after the photocatalytic process due to the ferromagnetic Fe3O4 core. The photocatalysts can be recycled for further use with slightly lower photocatalytic efficiency.
1. Introduction
In recent years, an enormous research effort has been dedicated to the study of semiconductors in the area of photocatalysis. Among the semiconductor photocatalysts, TiO2 has attracted a lot of attention because of its high photocatalytic efficiency, high stability, and low cost [1–5]. For the application of TiO2 in the area of wastewater purification, TiO2 slurry reactor is the most common device because of its high specific surface area, efficient light absorption, and good dispersion [6–8]. However, the separation of TiO2 particles from treated water, especially from a large volume of water, is an expensive and time-consuming work, which limited the application of TiO2 slurry reactor in industrial application. To solve this problem, many methods including titania beads [9], TiO2-based thin film [10, 11], fiberglass loaded with titania [12], and encapsulated titania within a zeolite framework [13] have been developed to immobilize TiO2 in fixed beds. However, the photocatalytic activity of TiO2 is considerably reduced by these immobilization techniques because the effective surface area of photocatalysts is decreased.
Recently, magnetic core-shell composites have gained a great deal of attentions. The magnetic core has good magnetic responsibility and can be easily magnetized. Therefore, the magnetic composites can be conveniently separated and collected from water by applying an external magnetic field. Many studies have been carried out on the preparation and application of TiO2/magnetic composites [14–20]. In these studies, Fe3O4 and γ-Fe2O3 nanoparticles were applied as the core magnetic materials. However, Fe3O4 nanoparticles are easily oxidized, even at room temperature. The TiO2 shell in the composites was usually prepared by sol-gel methods, where heat treatment was essential. After heat treatment, the ferromagnetic γ-Fe2O3 may transform to paramagnetic α-Fe2O3.
In this work, through the medium of SiO2, TiO2 nanoparticles were successfully coated on Fe3O4 microspheres, and Fe3O4@SiO2@TiO2 microspheres with well-defined core-shell structures were obtained. The prepared Fe3O4@SiO2@TiO2 microspheres are multifunctional. The TiO2 nanoparticles are at the outmost of the core-shell microspheres, which can act as photocatalysts to decompose organic dyes in wastewater. The Fe3O4 core makes them very easy to be separated and recycled from water with the help of an external magnet. Furthermore, the Fe3O4 microspheres are stable at room temperature, and are unchanged after heat treatment under nitrogen atmosphere. Therefore, the Fe3O4@SiO2@TiO2 microspheres may serve as an ideal photocatalyst for the treatment of wastewater.
2. Experimental
All the chemical reagents were of analytical grade and were used without further purification.
2.1. Synthesis of Fe3O4 Microspheres
The procedure for the synthesis of Fe3O4 microspheres was carried out according to a previous report [21], summarized as follows: 1.35 g of iron chloride (III) hexahydrate, 1.0 g of polyethylene glycol (Mw = 4000), and 3.6 g of sodium acetate trihydrate were added into 40 mL ethylene glycol under constant magnetic stirring to form a clear solution. Then the solution was transferred into a Teflon-lined stainless steel autoclave with a capacity of 50 mL and heated at 180°C for 19 h. The products were collected and fully rinsed with deionized water and absolute ethanol with the help of an external magnet, and then dried under vacuum at 60°C for 2 h for further use.
2.2. Synthesis of Fe3O4@SiO2 Microspheres
The synthesis of Fe3O4@SiO2 microspheres was carried out according to a previous report [22]. Typically, 0.2 g of as-prepared Fe3O4 microspheres was dispersed into a mixture of 20 mL ethanol and 4 mL deionized water in a glass beaker, and then, under constant mechanical stirring, 1 mL of ammonia solution (25%) and 0.8 mL of tetraethyl orthosilicate (TEOS) were consecutively added. The mixture was further stirred for 3 h. The resultant products were collected and washed, and then dried under vacuum at 60°C for 2 h for further use.
2.3. Synthesis of Fe3O4@SiO2@TiO2 Microspheres
The synthesis of Fe3O4@SiO2@TiO2 microspheres was carried out according to following procedure. Firstly, 5 mL tetrabutyltitanate was dissolved in 35 mL ethanol to form a clear solution. Then, 0.1 g of Fe3O4@SiO2 microspheres as dispersed in the above solution with the aid of ultrasonication for 5 min. After that, 20 mL of a 1 : 5 (v/v) mixture of water and ethanol was added dropwise to the suspension of Fe3O4@SiO2 microspheres with constant mechanical stirring over a period of approximately 15 min. The suspension was further stirred for about 1 h. The products were collected and washed with ethanol for three times, then calcined under nitrogen atmosphere at 500°C for 3 h. The whole synthesis procedure is shown in Scheme 1.
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2.4. Photocatalytic Degradation of Methyl Orange
The photocatalytic degradation was conducted as the following procedure: 40 mg of Fe3O4@SiO2@TiO2 microspheres were dispersed into an aqueous solution of methyl orange (0.01 g L−1, 50 mL). The pH value of the methyl orange solution was adjusted by hydrochloric acid and sodium hydroxide solution. Before photocatalytic experiments, the solution was stirred in the dark to permit the adsorption/desorption equilibrium until the concentration of methyl orange solution was constant. After that, a 300 W column-like low-pressure mercury lamp was placed over the solution with a distance of 20 cm, and the solution was irradiated with the lamp under constant mechanical stirring and fan cooling. At intervals of 10 min, 3 mL of the solution was taken out and analyzed with a UV-vis absorption spectrometer.
2.5. Characterization
X-ray powder diffraction (XRD) patterns were recorded using a Shimadzu XRD-6000 X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 0.15406 nm). The field-emission scanning electron microscopy (FESEM) images were captured with a Hitachi S-4800 field-emission scanning electron microscope. Transmission electron microscopy (TEM) images were taken on a JEOL-2010 high-resolution transmission electron microscope. UV-vis absorption spectra were measured with a Shimadzu UV-3010 spectrometer.
3. Results and Discussion
The phase and composition of the as-obtained samples were examined by XRD. Figure 1 shows a typical XRD pattern of the Fe3O4@SiO2@TiO2 microspheres. All of the diffraction peaks in this pattern can be classified into two sets. The peaks that marked with “#” can be indexed to the orthorhombic phase of Fe3O4 (JCPDS card no. 75–1609), while the other peaks that marked with “*” can be indexed to the tetragonal phase of TiO2 (JCPDS card no. 21–1272). No diffraction peaks corresponding to SiO2 were observed because the SiO2 is amorphous.
The morphology and size of the as-prepared products were characterized by FESEM and TEM. Figure 2(a) displays an FESEM image of the Fe3O4 microspheres, in which many nearly monodisperse microspheres with diameters of about 400 nm can be seen. The surfaces of the Fe3O4 microspheres are rough. Figure 2(b) shows an FESEM image of the Fe3O4@SiO2 microspheres. The surfaces of the Fe3O4@SiO2 microspheres are smooth, which is distinctly different from the initial Fe3O4 microspheres that are shown in Figure 2(a). Such a difference can be attributed to the coating of SiO2 layer on Fe3O4. Figure 2(c) is an FESEM image of the Fe3O4@SiO2@TiO2 microspheres, from which we can see that many tiny TiO2 nanoparticles with diameters of 10–20 nm are adhered to the SiO2 layer. Figure 2(d) is a TEM image of the Fe3O4@SiO2@TiO2 microspheres, which shows many nanoparticles adhered to the microspheres, consistent with the FESEM observations.
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Figure 3(a) shows a digital photograph of the Fe3O4@SiO2@TiO2 microspheres that were dispersed in deionized water with the assistance of ultrasonication. The microspheres can be easily dispersed in water, and this suspension can be kept constant for several minutes. When a magnet was placed aside, the black microspheres can be quickly collected in several seconds, leaving a clear solution, as shown in Figure 3(b). This result shows that the Fe3O4@SiO2@TiO2 microspheres can be easily separated and collected from water with the assistance of external magnetic force.
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The photocatalytic activities of the Fe3O4@SiO2@TiO2 microspheres were investigated by photodegradation of methyl orange in aqueous solution under the irradiation of UV light. Figure 4 shows the time-dependent UV-vis absorption spectra of methyl orange aqueous solution (0.01 g L−1, pH = 3) in the presence of Fe3O4@SiO2@TiO2 microspheres under exposure to UV light. The main absorption peak of methyl orange is centered at 506 nm. It is clear that the intensity of the main absorption peak of methyl orange gradually decreases with increasing time due to the concentration decreasing of methyl orange. This result shows that methyl orange is gradually degraded or changes into other structures under the irradiation of UV light in the presence of Fe3O4@SiO2@TiO2 microspheres.
As a comparison, the photodegradation of methyl orange under different conditions was measured, as shown in Figure 5. When the reaction was carried out in the dark, there is hardly any degradation of methyl orange occurred. When Fe3O4@SiO2@TiO2 microspheres were substituted by Fe3O4@SiO2 or Fe3O4 microspheres, the degradation rate of methyl orange is dramatically decreased. This result shows that the TiO2 nanoparticles in the Fe3O4@SiO2@TiO2 microspheres play key role in the photodegradation of methyl orange.
The pH value of solution is an important parameter in photocatalytic degradation reactions that are taking place on the surfaces of semiconductors. It dictates the surface charge properties of the photocatalyst and therefore the adsorption behavior of pollutants [23]. Here we examined the photocatalytic activities of the Fe3O4@SiO2@TiO2 microspheres at different pH conditions. The results are shown in Figure 6. It is obvious that at acidic conditions the photocatalytic efficiency of the Fe3O4@SiO2@TiO2 microspheres is much higher, and the photocatalytic efficiency decreases with the increase of pH value. At acidic conditions, the surface of TiO2 will be positively charged, so the negatively charged dye anion can be effectively adsorbed, and higher photocatalytic efficiency is obtained.
The Fe3O4@SiO2@TiO2 microspheres can be conveniently separated and collected from the treated water after photocatalytic reactions are completed; therefore, they can be recycled for further use. Here we studied the photocatalytic efficiency of the Fe3O4@SiO2@TiO2 microspheres for recyclable usage. Figure 7 shows the photocatalytic efficiency of the Fe3O4@SiO2@TiO2 microspheres after 6 cycles. In each cycle, the methyl orange solution (0.01 g L−1, pH = 3) was irradiated under UV light for 45 min. The results show that the photocatalytic activity of the Fe3O4@SiO2@TiO2 microspheres only decreased a little after 6 cycles of the photocatalysis experiments. The reason may be that some of the TiO2 nanoparticles are broken away from the Fe3O4@SiO2@TiO2 microspheres when they are stirred; as a result, the photocatalytic efficiency of the Fe3O4@SiO2@TiO2 microspheres decreases. However, the degradation rate of methyl orange can still reach 91% after 6 cycles of reuse. So the Fe3O4@SiO2@TiO2 microspheres can be applied as recyclable photocatalysts.
4. Conclusions
In summary, we have successfully prepared Fe3O4@SiO2@TiO2 microspheres with well-defined core-shell structures through a wet-chemical method. The Fe3O4@SiO2@TiO2 microspheres are multifunctional. The magnetic core makes the microspheres very easy to be separated from solution with the help of an external magnet, and the titania nanoparticles at the outside can act as photocatalyst to decompose organic dyes in waste water. Photocatalytic experiments show that the prepared Fe3O4@SiO2@TiO2 microspheres have good photocatalytic activities under the illumination of UV light and can be applied as recyclable photocatalysts.
Acknowledgment
Financial supports from the National Natural Science Foundation of China (20701001, 21171006) are gratefully acknowledged.