International Journal of PhotoenergyVolume 2012 (2012), Article ID 202519, 6 pagesdoi:10.1155/2012/202519

Research Article

Synthesis of Core-Shell @@ Microspheres and Their Application as Recyclable Photocatalysts

Zhenghua Wang, Ling Shen, and Shiyu Zhu

Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China

Received 14 January 2012; Revised 2 March 2012; Accepted 2 March 2012

Academic Editor: Xuxu Wang

Copyright © 2012 Zhenghua Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.

Scheme 1: The synthetic route to Fe₃O₄@SiO₂@TiO₂ core-shell microspheres.

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.

Figure 1: XRD pattern of the Fe₃O₄@SiO₂@TiO₂ microspheres.

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.

(a)

(b)

(c)

(d)

Figure 2: (a)–(c) FESEM images of the Fe₃O₄ microspheres, Fe₃O₄@SiO₂ microspheres, and Fe₃O₄@SiO₂@TiO₂ microspheres, (d) TEM image of the Fe₃O₄@SiO₂@TiO₂ microspheres.

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.

(a)

(b)

Figure 3: (a) Digital photograph of Fe₃O₄@SiO₂@TiO₂ microspheres dispersed in water, (b) digital photograph of Fe₃O₄@SiO₂@TiO₂ microspheres collected by an external magnet.

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.

Figure 4: Time-dependent absorption spectra of a methyl orange aqueous solution (0.01 g L⁻¹, pH = 3) in the presence of Fe₃O₄@SiO₂@TiO₂ composites under exposure to UV light.

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.

Figure 5: Degradation rate of methyl orange aqueous solution in the dark or in the presence of different photocatalyst under exposure to UV light.

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.

Figure 6: Degradation rate of methyl orange aqueous solution at different pH values in the presence of Fe₃O₄@SiO₂@TiO₂ composites under exposure to UV light.

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.

Figure 7: Relationship between the degradation efficiency of Fe₃O₄@SiO₂@TiO₂ photocatalyst and cyclic time.

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.

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