Magnetic (γ-Fe2O3@SiO2)n@TiO2 Functional Hybrid Nanoparticles

Feb 18, 2009 - An easy chemical route for the synthesis of (γ-Fe2O3@SiO2)n@TiO2 functional hybrid nanoparticles is reported in this paper. The hybrid...
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J. Phys. Chem. C 2009, 113, 4008–4011

Magnetic (γ-Fe2O3@SiO2)n@TiO2 Functional Hybrid Nanoparticles with Actived Photocatalytic Ability Chengxiang Wang, Longwei Yin,* Luyuan Zhang, Le Kang, Xianfen Wang, and Rui Gao Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong UniVersity, Jinan, 250061, China ReceiVed: NoVember 7, 2008; ReVised Manuscript ReceiVed: December 18, 2008

An easy chemical route for the synthesis of (γ-Fe2O3@SiO2)n@TiO2 functional hybrid nanoparticles is reported in this paper. The hybrid nanoparticles are composed of spherical nanoparticles about 100 nm in diameter, with several γ-Fe2O3 fine particles about 15 nm in diameter as cores distributing within the titania matrix and silica as coatings and barrier layers between the magnetic cores and titania shells. The (γ-Fe2O3@SiO2)n@TiO2 hybrid nanoparticles show good magnetic response and got together within 20 s in a magnetic field. Photodegradation examination of the magnetic hybrid nanoparticles was carried out in methylene blue solutions illuminated under UV light in a photochemical reactor. It is shown that the (γ-Fe2O3@SiO2)n@TiO2 functional hybrid nanoparticles are photocatalytically actived. About 80% of methylene blue decomposed in 80 min in the presence of magnetic hybrid nanoparticles under illumination of UV light. The synthesized magnetic hybrid nanoparticles display high photocatalytic efficiency and will find potential applications for cleaning polluted water with the help of magnetic separation. 1. Introduction Magnetic composites have been investigated widely for their excellent function and applications. The excellent separability of the magnetic composites has been applied extensively in a variety of fields, especially in biotechnology.1-4 They can be used for moderate separation and purification of bioactive substances, for example, cell separation,1 immobilization of enzymes,2 supported catalyst,3 drugs delivery,4 etc. The typical structure of the magnetic composites is the core/shell one, with the inorganic magnetic nanoparticles (iron, nickel, cobalt, rare earth, or their oxides) as the cores and the organics or inorganics as the shells. Among the magnetic materials, magnetic iron oxide is preferred because it is nontoxic. The inherent nature of magnetic materials is magnetic reunion, which is detrimental to the magnetic separation. Superparamagnetic particles can completely eliminate the magnetic reunion. It has been reported that the magnetic properties of γ-Fe2O3 particles change with the diameter size decreasing, and display superparamagnetic characteristics as the diameter of the particles reaches a critical value. The ideal diameter of the magnetic γ-Fe2O3 cores with the superparamagnetic feature in magnetic composites for the applications of separation is less than 30 nm. Titania materials are widely used in various research fields for their unique properties, such as wide band gap, nontoxic, high refractive index, etc. They are used in the fields of photocatalysts,5-9 solar cells,10-14 gas sensors,15-17 electrochromic devices,18,19 and lithium ion battery.20-22 For the photocatalytic application, the efficiency of titania has been reported to be high enough. However, in practical applications, separation and recovery of titania is difficult to control and deal with, remaining an issue to be solved in the field of photocatalytic and biotechnological applications related to titania functional materials. The incorporation of magnetic iron oxide into titania provides a way to solve this problem. However, after introducing * Author to whom correspondence should be addressed. Phone: +86531-88396970. E-mail: [email protected].

Fe3O4 as the cores of the Fe3O4@TiO2 hybrid nanostructures, photodissolution may happen in the hybrid nanoparticles.21,22 This will decrease the photocatalytic efficiency of TiO2 materials. In the Fe3O4@TiO2 core/shell structures, it is necessary to coat nontoxic and hydrophilic inorganic or organic surfaces onto the magnetic core sections. In the present paper, silicon oxides, which are nontoxic and hydrophilic, are used to act as coatings and barrier layers between the magnetic cores and titania shells. (γ-Fe2O3@ SiO2)n@TiO2 hybrid nanostructures are synthesized via a twostage route. The Fe3O4@SiO2 composite nanoparticles were first prepared. Then the silica-coated Fe3O4 particles were wrapped by titania in a hydrolyzation process of tetrabutyl titanate. After calcinations, Fe3O4 transformed into the γ-Fe2O3 phase. Photodegradation examination of the magnetic hybrid nanoparticles was carried out in methylene blue solutions illuminated under UV light in a photochemical reactor. It is shown that the synthesized (γ-Fe2O3@SiO2)n@TiO2 composites display good magnetic response with photocatalytic activity. 2. Experiment Section 2.1. Preparation of (γ-Fe2O3@SiO2)n@TiO2 Hybrid Nanoparticles. The Fe3O4 nanocrystals were prepared according to the previously reported method.23 A 0.3 g sample of magnetite particles was redispersed in 100 mL of deioned water under sonication for 10 min. The undispersed magnetic particles were removed and the suspension above was transferred to another beaker. Ammonia aqueous solution (2.0 mL, 25 wt %) and tetraethyl orthosilicate (TEOS, 0.5 mL) were added in sequence to the suspension. After the solution was stirred at room temperature for 6 h, the Fe3O4@SiO2 particles were separated by magnetic separation and washed with ethanol and water three times, then dried at room temperature. The Fe3O4@SiO2 particles were resuspended in the mixture of 100 mL of hexane and 0.1 mL of deioned water, followed by the addition of tetrabutyl titanate (4.0 mL) under sonication for 1 h. The mixture was transferred to Teflon-lined autoclave

10.1021/jp809835a CCC: $40.75  2009 American Chemical Society Published on Web 02/18/2009

(γ-Fe2O3@SiO2)n@TiO2 Functional Hybrid Nanoparticles

Figure 1. XRD patterns of the hybrid nanoparticles (a) calcined at 550 °C for 2 h and (b) without calcinations.

and kept at 100 °C for 2 h. The precipitate was collected and washed with hexane two or three times, then dried at room temperature. The powder was then calcined at 550 °C for 2 h. 2.2. Materials Characterization. Samples of the as-prepared products were characterized by X-ray powder diffraction (XRD) with a Rigaku D/max-kA diffractometer with Cu KR radiation (60 kV, 40 mA). The microstrucutres of the hybrid nanoparticles were observed in an H-800 transmission electron microscope (TEM) operating at an accelerating voltage of 150 kV. Photodegradation experiments of methylene blue were carried out in a photochemical reactor. The UV spectrum and concentration of methylene blue were obtained in a TU-1901 double-beam UV-visible spectrophotometer. 2.3. Photodegradation of Methylene Blue. First, 150 mL of methylene blue solution in a concentration of 25 mg/L was prepared. Then, the solution was divided into approximately three parts: 0.10 g of as-prepared (γ-Fe2O3@SiO2)n@TiO2 magnetic microspheres and pure titania powder (commercial powder, Tianjin Kermel Chemical Reagent Co., Ltd.) were weighed and added to two parts of the above methylene blue solutions, respectively. All three parts of the methylene blue solutions were illuminated under UV light in the photochemical reactor. The solutions were fetched at 20-min intervals for each solution and centrifuged. Then, time-depended absorbance changes of the transparent solution after centrifugation at the wavelength between 500 and 800 nm were obtained. According to the Lambert-Beer law,24 the concentration of methylene blue solutions can be obtained from the absorbance at a wavelength of 660 nm; time-depended degradation rates of methylene blue were also obtained. 3. Results and Discussion Figure 1 depicts XRD patterns of the hybrid nanoparticles before and after calcinations. It is shown that the products are mainly composed of titania and iron oxide phases. Before calcinations the titania and Fe3O4 prepared via the sol-gel route are almost amorphous, as shown in Figure 1b. After calcinations at 550 °C for 2 h, the amorphous TiO2 transformed into the anatase phase (PDF no. 21-1272) and Fe3O4 transformed into the γ-Fe2O3 (PDF no. 39-1346) phase, which was another form of magnetic iron oxides, as shown in Figure 1a. In the XRD pattern in Figure 1b, the peaks of (105), (204) planes of titania

J. Phys. Chem. C, Vol. 113, No. 10, 2009 4009 overlap with those of (422), (440) planes of γ-Fe2O3. The peaks of silicon oxides are not observed, because of the amount of them is small and they are mainly amorphous. As is known, the photocatalytic ability of amorphous titania was much lower than that of crystalline ones, and the calcinated (γ-Fe3O4@ SiO2)n@TiO2 crystalline nanostructured products should display good photocatalytic ability. The microstructures of the synthesized products are further investigated via H-800 type transmission electron microscope. The synthesized products are composed of (γ-Fe2O3@SiO2)n@ TiO2 hybrid nanoparticles with γ-Fe2O3@SiO2 as the cores dispersing in the matrix of titania. Figure 2 shows the TEM images of the hybrid nanoparticles. As can be seen from Figure 2a, the products are composed of approximately spherical particles about 100 nm in diameter. The structural nature and details of the nanoparticles can be seen in Figure 2b. The mean sizes of the titania matrix and magnetic γ-Fe2O3 core particles are about 100 nm and 15 nm, respectively. The crystal structure and phase components of the synthesized products are analyzed by using a selected electron diffraction pattern. Figure 2c shows a typical electron diffraction pattern of the hybrid nanoparticles. After calcination, titania has an anatase structure. The (100), (110), (200), and (120) diffraction rings correspond to that of the anatase titania, and (200) and (111) diffraction rings correspond to that of γ-Fe2O3. The ED pattern results are in good agreement with the XRD results. Figure 3 illustrates the formation process of the (γ-Fe2O3@ SiO2)n@TiO2 hybrid nanoparticles. Water cannot dissolve in hexane. When the Fe3O4@SiO2 particles and water were added to hexane, they stayed under hexane and cannot disperse. Ti4+ is a hydrophilic group, while the -OC4H9 group can dissolve in hexane, so tetrabutyl titanate can be used as a dispersant for the above system. First, the small water drops are dispersed in hexane with the tetrabutyl titanate molecules around them, with or without Fe3O4@SiO2 particles distributed in them, as shown in the first step in Figure 3. After hydrolization of tetrabutyl titanate, small sol particles form with or without Fe3O4@SiO2 cores, as demonstrated at the second step shown in Figure 3. After the hydrothermal process, the small sol particles of titania assemble into large particles with Fe3O4@SiO2 dispersing in them, as illustrated at the last step shown in Figure 3. After calcinations, amorphous Fe3O4 transferred into γ-Fe2O3 with antispinel structure. Figure 4 shows the separation-redispersion process of (γ-Fe2O3@SiO2)n@TiO2 hybrid nanoparticles in a magnetic field with an intensity of 1300 Oe, displaying a good magnetic response. The magnetic particles can gather quickly in 20 s in the magnetic field of a magnet without residues left in the solution. This will prevent loss of materials and cost savings. The materials coming together can be redispersed in the solution easily by a slight shake as shown in Figure 4. They can be separated easily in spite of their own magnetic properties. This is also important in the industrial application of water cleaning. To examine the photocatalytic ability of the hybrid magnetic composites, the photodegradation of methylene blue was carried out. Under normal conditions, methylene blue is very stable. Absorption spectra can be used to describe the concentration changes of methylene blue in the solutions. The methylene blue displays an absorption peak at the wavelength of about 660 nm in the absorption spectra, as depicted in Figure 5. The photodegradation of the methylene blue in the as-prepared solution was illuminated by UV light in the presence of (γ-Fe2O3@ SiO2)n@TiO2 hybrid nanoparticles. Figure 5 shows the evolution of absorption spectra of methylene blue after photodegradation.

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Wang et al.

Figure 2. (a and b) Low- and high-magnification TEM images of (γ-Fe2O3@SiO2)n@TiO2 nanospheres after calcinations; (c) electron diffraction pattern of the hybrid materials.

Figure 3. Schematic diagram for the formation of (γ-Fe2O3@SiO2)n@TiO2 nanospheres.

Figure 5. Evolution of absorption spectra of methylene blue (25 mg/ L, 50 mL). Figure 4. Separation-redispersion process of (γ-Fe2O3@SiO2)n@TiO2 hybrid nanoparticles in a magnetic field with intensity of 1300 Oe.

With time increasing from 0 to 80 min, the intensity of the peaks of 660 nm decreased gradually, suggesting that the methylene blue was gradually photodegradated by the (γ-Fe2O3@SiO2)n@ TiO2 hybrid magnetic photocatalysts. Time-depended photodegradation of methylene blue is illustrated in Figure 6. It is shown that methylene blue decomposes in the presence of photocatalytic materials such as titania under illumination by UV light. For the Fe3O4@TiO2 hybrid composites without silica coating on the Fe3O4 cores, photo-

dissolution will happen under the illumination of UV light. This will influence the photocatalytic effect of titania. After coating magnetic cores with silica, the photocatalytic efficiency of the hybrid nanoparticles will be improved.22 From Figure 6, it is suggested that the photocatalytic ability of the as-prepared hybrid magnetic particles with γ-Fe2O3 cores coated by silicon oxides is close to that of pure titania, while the methylene blue does not decompose under the illumination of UV light in the absence of photocatalyst. The SiO2 layer can prevent photogenerated electrons transferring into the lower lying conduction band of the iron oxide core, which can almost eliminate photodissolution

(γ-Fe2O3@SiO2)n@TiO2 Functional Hybrid Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 10, 2009 4011 80% of methylene blue was decomposed in 80 min. The synthesized magnetic hybrid nanoparticles with high photocatalytic efficiency are very useful for cleaning polluted water with the help of magnetic separation. Acknowledgment. This work was supported by the National Science Foundation of China and Shandong Province (Nos. 50872071 and Y2007F03), Tai Shan Scholar Foundation of Shandong Province, and Gong Guan Foundation of Shandong Province (2008GG10003019). Supporting Information Available: TEM image of asprepared (γ-Fe2O3@SiO2)n@TiO2 nanoparticles and magnetic hysteresis loops. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 6. Time-depended photodegradation of methylene blue (25 mg/ L, 50 mL) under the illumination of UV light: (A) methylene blue only; (B) methylene blue with (γ-Fe2O3@SiO2)n@TiO2 hybrid nanoparticles; and (C) methylene blue with pure TiO2.

and improve photocatalytic ability.22 However, the interfaces between SiO2 and TiO2 can still capture a small portion of the photogenerated electrons and this will decrease the photocatalytic ability seriously. So the photocatalytic ability of (γFe2O3@SiO2)n@TiO2 hybrid nanoparticles was close to but lower than that of single phase TiO2. More importantly, the hybrid nanoparticles display superparamagnetic characteristics as the diameter of the γ-Fe2O3 is about 15 nm (Figures S1 and S2Supporting Information). The superparamagnetic characteristics of the hybrid nanoparticles can completely eliminate the magnetic reunion. The incorporation of magnetic iron oxide into titania provides a way to solve the embarrassments in practical applications, separation, and recovery of titania, which is traditionally difficult to control and deal with in the field of photocatalytic and biotechnological applications related to titania functional materials. 4. Conclusions We have synthesized magnetic (γ-Fe2O3@SiO2)n@TiO2 hybrid nanoparticles with γ-Fe2O3@SiO2 fine particles dispersing in the titania matrix. The hybrid nanoparticles display superparamagnetic characteristics as the diameter of the γ-Fe2O3 is about 15 nm. The superparamagnetic characteristics of the hybrid nanoparticles can completely eliminate the magnetic reunion. The incorporation of magnetic iron oxide into titania provides a way to solve the embarrassments in practical applications, separation, and recovery of titania, which are traditionally difficult to control and deal with in the field of photocatalytic and biotechnological applications related to titania functional materials. Photodegradation examination carried out in methylene blue solutions under UV light in a photochemical reactor suggests that the (γ-Fe2O3@SiO2)n@TiO2 functional hybrid nanoparticles display good photocatalytic ability. About

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