Advances in Materials Science and EngineeringVolume 2012 (2012), Article ID 409379, 8 pagesdoi:10.1155/2012/409379

Research Article

Preparation and Photocatalytic Activity of Magnetic Fe₃O₄/SiO₂/TiO₂ Composites

Rijing Wang,1,2 Xiaohong Wang,1,2 Xiaoguang Xi,1,2 Ruanbing Hu,1,2 and Guohua Jiang1,2

¹Department of Materials Engineering, College of Materials and Textile, Zhejiang Sci-Tech University, Hangzhou 310018, China²Key Laboratory of Advanced Textile Materials and Manufacturing Technology (ATMT) of Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China

Received 29 October 2011; Accepted 8 December 2011

Academic Editor: Qiang Yang

Copyright © 2012 Rijing 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.

A simple sol-gel method was used to prepare magnetic Fe₃O₄/SiO₂/TiO₂ composites with core-shell structure. Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) have been applied to investigate the structure and morphology of the resultant composites. The obtained composites showed excellent magnetism and higher photodegradation ability than pure TiO₂. The photocatalytic mechanism was also discussed. The magnetic composites should be extended to various potential applications, such as photodegradation, catalysis, separation, and purification processes.

1. Introduction

Currently, there has been great interest in the preparation of core-shell micro- and nanoparticles for their widespread potential applications in catalysis, chromatography separation, drug delivery, chemical reactors, and protection of environmentally sensitive materials [1–4]. Heterogeneous photocatalysis using semiconducting oxide catalysts is an effective way to purify wastewater or gas. TiO₂-based semiconductors have attracted considerable attention due to their high efficiency, good stability, availability, and nontoxicity [5–9]. In recent years, in order to enhance the photocatalytic activity, great efforts have been made to prepare ideal structure of TiO₂-based semiconductors [10–12].

Magnetic separation provides a very convenient approach for removing and recycling magnetic composites by applying an added magnetic field. The incorporation of Fe₃O₄ magnetic particles into TiO₂ matrix may block the aggregation of nanoparticles during renewal and can increase the durability of the catalysts [13, 14]. Moreover, such catalysts have a high surface area and well-defined pore size, which enhance their photocatalytic activity [15]. However, magnetic nanoparticles would inescapably encounter an hindrance when applied in practice due to the fact that a photocatalytic reaction is conducted in a suspension. It is not allowed to use magneton to agitate the mixed solutions. Therefore, in the experiment, Ar gas is purged so as to make the magnetic particles suspend in the methylene blue (MB) solution.

Many efforts have been made in the development of the design and preparation of magnetic core-shell microspheres. Ye et al. and Yu et al. reported the magnetic material/SiO₂/TiO₂ composites with core-shell-shell structure [16, 17]. Their methods involve superparamagnetic Fe₃O₄ and γ-Fe₂O₃ with an inner layer of SiO₂ and outer layer of TiO₂ [18, 19]. Their resultant samples exhibit superior photodegradation ability and can be easily recycled by applying an external magnetic field. Zhou et al. reported the preparation of core-shell structure of Fe₃O₄/SiO₂ nanospheres via a modified inverse emulsion process [20]. Many methods have been applied to prepare composites with core-shell structure. However, it remains a great challenge to explore a feasible, easily controllable, and repeatable method for the preparation of core-shell nanostructure composites.

Nanosize TiO₂ exhibits superior photocatalytic activity compared to the common TiO₂ because of its rough surface and larger pore volumes. Herein, we directly use the Fe₃O₄ nanoparticles (a mean diameter of approximately 20 nm) as the core of the composites, then a inner layer of SiO₂ and outer layer of TiO₂ were coated via a simple sol-gel process and a rapid combustion process. Silica coating is a necessary step to prepare core-shell structure because the hydrophobic Fe₃O₄ nanoparticles cannot be easily encapsulated in a TiO₂ shell [21]. Moreover, SiO₂ can be selectively etched onto the hollow magnetic/TiO₂ microspheres, allowing enough spaces for uploading some materials that can absorb polluted gas or wastewater. In the photocatalytic experiment, the pH of solution effect on the photocatalytic activity of catalyst was investigated and peroxide solution (30%) was added to enhance the catalyst activity through promoting the formation of hydroxyl radical and the reduced rate of interfacial electron transfer. This may open up new possibilities to synthesize core-shell structure for other composites and extend their applications.

2. Experimental Section

2.1. Materials and Reagents

Nanoiron (II, III) oxide (spherical, diameter 20 nm, 99.5%) was purchased from Shanghai Chemical Co., Ltd., China. Ammonia solution and isopropyl alcohol were obtained from Hangzhou High-Crystal Fine Chemical Co., Ltd., China. Tetraethoxysilane (TEOS) and tetrabutyltitanate (TBOT) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd., China. Anhydrous ethanol was obtained from Hangzhou Changzheng Chemical Reagent Co., Ltd., China. Pure TiO₂ was purchased from Degussa Co. Ltd. All reagents were used without further purification. Deionized water was used in all experiments.

2.2. Preparation of Fe₃O₄/SiO₂ Nanospheres

The Fe₃O₄ nanoparticles (0.25 g) were ultrasonicated for 1 h to make them uniformly disperse in anhydrous ethanol (40 mL). Concentrated ammonium hydroxide (4.5 mL) was diluted to the above solution, and TEOS (0.8 mL) was quickly added under vigorous stirring. The solution was left to stir for 12 h. The product was collected by centrifugation and washed with anhydrous ethanol three times.

2.3. Preparation of Pomegranate-Like Fe₃O₄/SiO₂/TiO₂ Composite Microspheres

The resultant Fe₃O₄/SiO₂ nanocomposites were redispersed in anhydrous ethanol (40 mL). Subsequently, a proper amount of TBOT (1 mL) dissolved in isopropyl alcohol (8 mL) was introduced to the system dropwise, followed by heating the solution at about 70°C. The whole process was under vigorous stirring. After 12 h, the red brown precipitates were washed with deionized water and ethanol five times and dried in a vacuum oven at 60°C for 8 h. Finally, the products were calcined in air at 500°C for 2 h.

2.4. Characterization

X-ray diffraction (XRD) patterns were analyzed by X-ray diffractometer using Cu Kα radiation source at 35 kV, with a scan rate of 0.1° 2θ s⁻¹ in the 2θ range of 10–80°. The morphology and microstructure of products were characterized by ULTRA-55 field-emission scanning electron microscopy (FE-SEM) and JSM-2100 transmission electron microscopy (TEM) equipped with an energy dispersive X-ray spectrum (EDS, Inca Energy-200) at an accelerating voltage of 200 kV. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 5700 spectrophotometer using KBr pellets for samples.

2.5. Photocatalytic Activity Evaluation

The photocatalytic activity of Fe₃O₄/SiO₂/TiO₂ composites was investigated by the photodegradation of MB aqueous solution at ambient temperature. The photodegradation experiments were carried out in a closed box, of which UV radiation source is 100 W high-pressure mercury lamp, its wavelength range is 290−450 nm, and the peak intensity is 365 nm (Model OCRS-I, Kaifeng Hxsei Science Instrument Factory, China). No pure oxygen was supplied because it has enough oxygen for oxidation photodegradation under continuously stirring in atmosphere in previous experiment [22, 23]. The initial MB concentration () was 30 mg/L, the photocatalyst concentration was 0.5 g/L, and the pH of the solution was adjusted to 2, 7, and 10 with hydrochloric acid and sodium hydroxide solution. Before switching on the mercury, we passed Ar gas into the mixture solution of MB and photocatalyst in a dark condition for 30 min to achieve adsorption balance. The concentration of MB () was analyzed through JASCO V-570 UV/Vis/NiR spectrophotometer at  nm. The concentration of MB () can be obtained by the following formula (where is a constant): Acetic acid (5 vol%, 5 mL) was also used to be photodegraded with Fe₃O₄/SiO₂/TiO₂ composites (20 mg) and pure TiO₂ using the aforementioned method. The amount of evolved CO₂ was determined by a gas chromatograph (GC, Agilent 6890).

3. Results and Discussion

The formation of Fe₃O₄/SiO₂/TiO₂ composites can be divided into two steps (Scheme 1). The first step is the hydrolyzation of TEOS on the Fe₃O₄ nanoparticles by the classical Stöber method; the silica layer can be easily coated on the surface of Fe₃O₄ nanoparticles. Some Fe₃O₄/SiO₂ nanoparticles would be aggregated, and the hydrolyzation of TBOT on the Fe₃O₄/SiO₂ composites resulted in the formation of pomegranate-like structure.

Scheme 1: The schematic process for preparing Fe₃O₄/SiO₂/TiO₂ composites.

The Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/TiO₂ composites were characterized by XRD. As shown in Figure 1(A), all peaks can be indexed as the magnetite phase of Fe₃O4 (JCPDS 19-0629). No characteristic peaks of SiO₂ were detected, indicating that the SiO₂ was amorphous. In contrast to Figure 1(A), the successful TiO₂ coating was confirmed by the presence of the new peaks (Figure 1(B)). The reduction of Fe₃O₄ peaks also confirmed the successful TiO₂ coating. The as-prepared samples displayed good crystallinity; and the TiO₂ peaks can be ascribed to the (101), (004), (200), (105), and (211) planes of anatase phase (JCPDS 21-1272). According to the Scherrer equation, where is the average crystallite size, the factor 0.89 is characteristic of spherical objects, is the X-ray wavelength, and are the full width at half-maximum and diffraction angle of an observed peak, respectively. The primary crystallite size calculated from the (101) peak of the XRD pattern was about 12.64 nm.

Figure 1: XRD pattern of Fe₃O₄/SiO₂ (A) and Fe₃O₄/SiO₂/TiO₂ composites (B).

FT-IR was used to characterize the composition and structure of the Fe₃O₄, Fe₃O₄/SiO₂, and Fe₃O₄/SiO₂/TiO₂ composites. As shown in Figure 2, the Fe₃O₄/SiO₂/TiO₂ composites possess more signals than Fe₃O₄. It has been reported that the signal at the wave number around 800 cm⁻¹ corresponds to the symmetric vibration of Si–O–Si, 1080 cm⁻¹ for asymmetric stretching vibration of Si–O–Si, and 940–960 cm⁻¹ for Si–O–Ti vibration and 500–900 cm⁻¹ originates from Ti–O–Ti [24, 25]. Silica presence as an amorphous phase was confirmed by combining the results of FT-IR and XRD. The presence of water is evidenced by the appearance of the bending mode at 1630 cm⁻¹ and the stretching mode at 3370 cm⁻¹. This surface hydroxylation is advantageous for the photocatalytic activity of Fe₃O₄/SiO₂/TiO₂ microspheres because it provides higher capacity for oxygen adsorption [26].

Figure 2: FT-IR spectra of Fe₃O₄ (A), Fe₃O₄/SiO₂ (B), and Fe₃O₄/SiO₂/TiO₂ composites (C).

The morphology and structure of the as-prepared samples were investigated by field emission scanning electron microscopy (FE-SEM). Figure 3(a) shows that the Fe₃O₄ nanoparticles are of a diameter of 20 ± 2 nm. They prefer to gather together due to their small size and magnetic. The grape-like Fe₃O₄/SiO₂ nanospheres with a diameter of 25 ± 2 nm are shown in Figure 3(b). This indicated that SiO₂ was successfully coated on the Fe₃O₄ nanoparticles. Figures 3(c) and 3(d) present typical FE-SEM images of as-prepared Fe₃O₄/SiO₂/TiO₂ composites. These images indicate that the surfaces of Fe₃O₄/SiO₂/TiO₂ microspheres are rough and porous, which favor enhancing the photocatalytic activity [27]. A large number of Fe₃O₄/SiO₂ nanospheres are encapsulated in the core of TiO₂ shell, forming the pomegranate-like structure, which can make the dyes better contact with the catalysts so as to achieve the purpose of degradation [28].

(a)

(b)

(c)

(d)

Figure 3: FE-SEM images of Fe₃O₄ (a) and Fe₃O₄/SiO₂ (b) Fe₃O₄/SiO₂/TiO₂ composite microspheres (c, d).

The morphology and structure of the resultant samples were further investigated by transmission electron microscopy (TEM) (Figure 4). As expected, the Fe₃O₄/SiO₂ nanospheres were composed of aggregated spherical particles with sizes around 25 nm. Moreover, a large number of nanoparticles are encapsulated in the SiO₂ layer (Figure 4(a)). In contrast to the morphology of Fe₃O₄/SiO₂ nanoparticles, Figure 4(b) clearly displays that TiO₂ was successfully coated on Fe₃O₄/SiO₂ nanoparticles. The EDS pattern (Figure 5) taken from this area shows the presence of only Fe, Si, Ti, and O elements.

(a)

(b)

Figure 4: TEM images of Fe₃O₄/SiO₂ (a) and Fe₃O₄/SiO₂/TiO₂ composite microspheres (b).

Figure 5: EDS pattern of Fe₃O₄/SiO₂/TiO₂ composites calcined at 500°C for 2 h.

Magnetic separation provides a very convenient approach for removing and recycling magnetic catalysts. The magnetism of Fe₃O₄/SiO₂/TiO₂ composites was confirmed by Figure 6, and the Fe₃O₄/SiO₂/TiO₂ composites were tested in water by placing a magnet near the glass bottle. The red brown particles can be attracted toward the magnet with time increasing, and the Fe₃O₄/SiO₂/TiO₂ almost totally attracted after 5 min, leaving a clear solution. So the as-prepared samples can be easily recycled after the achievement of photocatalytic process.

(a)

(b)

(c)

(d)

Figure 6: Digital illumination photographs of Fe₃O₄/SiO₂/TiO₂ composites separated from solution by applying an added magnet ((a) 0 min, (b) 1 min, (c) 2 min, and (d) 5 min).

Figure 7 depicts the photocatalytic activity of Fe₃O₄/SiO₂/TiO₂ composites and shows the digital illumination photographs of MB aqueous solution under UV light irradiation at different pH values with H₂O₂ or without. In the experiment, we used the deionized water as the blank, comparing with H₂O₂ to ensure only a variable. Figure 7(a) shows that the rate of photodegradation of MB was higher at the neutral and alkaline environment. 78% photodegradation of MB solution (pH = 10) was observed after 5 min under UV irradiation. However, in the acidic condition, only 8.5% of MB has been photodegraded after 30 min. As shown in the digital illumination photographs of MB aqueous solution under UV light irradiation at different pH values. The color of MB solution fades gradually with irradiation time at pH = 7 and 10, but with no distinct change of the color at pH = 2. This is because the pH influenced the adsorption property of organic compounds and their dissociating state in solution. The surface charge properties of TiO₂ were also changed with the changes in pH value due to amphoteric behavior of semiconducting TiO₂ [29]. It is interesting to see from Figure 7(b) that photodegradation efficiency of MB at different pH values with H₂O₂ can reach 60% after 5 min under UV irradiation. H₂O₂ enhanced the photodegradation ability attributed to its electron acceptor behavior, which reacted with conduction band electrons to generate hydroxyl radicals. The point of zero charge (pzc) for titanium dioxide is at pH 6.5. The TiO₂ surface is positively charged in acidic solution and negatively charged in basic solution [30]. Since MB is a cationic dye, it is conceivable that, at higher pH value, its adsorption is favored on a negatively charged surface.

(a)

(b)

Figure 7: Photodegradation efficiency of Fe₃O₄/SiO₂/TiO₂ composite microspheres and digital illumination photographs at pH 2, 7, and 10 ((a) with 1 mL H₂O; (b) with 1 mL H₂O₂).

In order to demonstrate the photocatalytic activity of Fe₃O₄/SiO₂/TiO₂ composites, acetic acid was selected as a model organic acid. As shown in Figure 8, the amount of CO₂ evolution increases with irradiation time for pure TiO₂ and Fe₃O₄/SiO₂/TiO₂. However, in case of Fe₃O₄/SiO₂/TiO₂, the yield rate of CO₂ reaches about  μ mol h⁻¹ g⁻¹, which is higher than pure TiO₂ ( μ mol h⁻¹ g⁻¹) for 2 h. The higher CO₂ evolution rate for Fe₃O₄/SiO₂/TiO₂ sample may be attributed to rough and porous surface of Fe₃O₄/SiO₂/TiO₂ composites, which enhances the photocatalytic activity by facilitating the access to the reactive TiO₂.

Figure 8: Relationship between the amount of CO₂ evolved and irradiation time for photocatalytic degradation of acetic acid aqueous solution.

The schematic illustration of the charge transfer by Fe₃O₄/SiO₂/TiO₂ composites is summarized in Scheme 2. Photoexcitation of the Fe₃O₄/SiO₂/TiO₂ composites likely results in charge separation to form electrons and holes (3) [31]. Photo-generated electrons can directly react with the absorbed O₂ molecules to form active species (4). Photogenerated electrons also capture H₂O₂ molecules to produce ·OH and OH⁻ (5). The resultant can react with H₂O₂ to form ·OH and OH⁻ (6). The obtained OH⁻ can react with photogenerated holes to generate the hydroxyl free radical ·OH (7) [32]. Both and ·OH can photodegrade the MB solution:

Scheme 2: The scheme of photocatalytic mechanism of Fe₃O₄/SiO₂/TiO₂ composite microspheres.

H₂O₂ enhanced photodegradation ability attributed to its electron acceptor behavior, which reacted with conduction band electrons to generate hydroxyl radicals.

4. Conclusion

In summary, the magnetic Fe₃O₄/SiO₂/TiO₂ composites have been prepared by a simple sol-gel method. The morphology and structure of the resultant samples were characterized by FT-IR, XRD, FE-SEM, and TEM. The Fe₃O₄/SiO₂/TiO₂ composites show excellent magnetism and higher photocatalytic activity than pure TiO₂ attributed to rough and porous surface of Fe₃O₄/SiO₂/TiO₂ composites, which enhances the photocatalytic activity by facilitating the access to the reactive TiO₂. The photocatalytic mechanism was also discussed. The magnetic Fe₃O₄/SiO₂/TiO₂ composites should be extended to various potential applications, such as photocatalysis, separation, and purification processes.

Acknowledgments

This work was financially supported by the Qianjiang Talents Project of Zhejiang Province (2010R10023), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, the State Education Ministry (1001603-C), the Natural Science Foundation of Zhejiang Province (Y4100045, Y4080392, R210101054), Foundation of Zhejiang Province New Textile Research and Development Emphasised Laboratory (2009FZD001), the Key Bidding Project of Zhejiang Provincial Key Lab of Fiber Materials and Manufacturing Technology, Zhejiang Sci-Tech University (S2010002), and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT: 0654).

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