CdS Nanocomposites and

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CRYSTAL GROWTH & DESIGN

Preparation and Characterization of Fe3O4/CdS Nanocomposites and Their Use as Recyclable Photocatalysts Xiaowang Liu,* Zhen Fang, Xiaojun Zhang, Wei Zhang, Xianwen Wei, and Baoyou Geng* College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal UniVersity, Wuhu 241000, P.R. China

2009 VOL. 9, NO. 1 197–202

ReceiVed February 26, 2008; ReVised Manuscript ReceiVed August 8, 2008

ABSTRACT: This paper presents a novel method for the preparation of Fe3O4/CdS nanocomposites via a sonochemical route in an aqueous solution. The products were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray analysis, UV/vis absorption, fluorescence spectrum, and vibrating sample magnetometer. Compared with the similar nanostructures synthesized by other methods, it is found to be mild, convenient, inexpensive, green, and efficient. These Fe3O4/CdS nanocomposites display fluorescence and exhibit excellent magnetic properties at room temperature. Photocatalytic activity studies confirm that as-prepared nanocomposites have highly photocatalytic activity toward the photodegradation of methyl orange in aqueous solution. Furthermore, the photodecomposition rate decreases only slightly after 12 cycles of the photocatalysis experiment. Thus, these Fe3O4/CdS nanocomposites can be served as effective and convenient recyclable photocatalysts. Introduction Recently, size- and shape-controllable synthesis of monodisperse inorganic nanocrystals has attracted a great deal of attention, owing to their unique size- and shape-dependent properties, as well as their great potential in technique applications.1 However, those nanocrystals commonly supply people with only one function; for example, semiconducting metal sulfide nanocrystals are widely used as fluorescence probes for visualizing biological processes in vitro and in vivo, the magnetic nanocrystals are frequently served as magnetic resonance image (MRI) agents for the diagnosis of many diseases, and noble metal nanoparticles are usually exploited as catalysts in organic synthesis or fuel cells.2 Multifunctional nanostructures are necessary in practical applications, for example, reusable catalysts. Silicon nanowires and carbon nanotubes have been utilized as carriers in recoverable catalyst to make the separation process more convenient.3 However, centrifugation or filtration is needed in such separation procedures, which may result in the loss of the catalysts. Immobilizing catalytic parts onto the surface of magnetic nano- or microparticles is an effective way to solve the above problem, as they can be separated easily from the solution with the help of an external magnet. Recently, some groups have achieved delightful success in this area; for example, Pd nanoparticle immobilized Fe3O4 and NiFe2O4 nanoparticles have been used for hydrogenation reactions and Suzuki and Heck reactions as facile recoverable catalysts.4 It is well-known that semiconductor nanoparticles can be utilized as photocatalysts in organic and photodecomposition reactions.5 Hereby, synthesis of magnetic nanocrystal-semiconductor nanocomposites is interesting, as these nanostructures may be served as effective recoverable photocatalysts in various organic syntheses or photodegradation reactions. Moreover, from the viewpoint of biological applications, they can be served as not only fluorescence probes but also imaging agents. In this article, we used Fe3O4 microspheres as cores and synthesized Fe3O4/ CdS nanocomposites, which display fluorescence and exhibit excellent magnetic properties at room temperature and show highly photocatalytic activity toward the decomposition of methyl orange in aqueous solution. Renewable photocatalytic * To whom correspondence should be addressed. E-mail: xwliu601@ yahoo.com.cn; [email protected].

activity of those nanocomposites has also been examined and indicated that the catalytic activity decreases slightly after 12 cycles of test. It is worth noting that study on the renewable photocatalytic activity of nanostructures is rarely performed.6 Presumably the reason for this is twofold. First, those nanostructures are generally not stable and easily coalesce into larger ones owing to their extremely large surface-to-volume ratio; that is, they are less readily recovered for multiple uses. Second, regular operations used for recovering nanostructures, such as centrifugation and filtration, may cause the loss of them, which influences the results in the following reactions absolutely. Based on the above reasons, as-prepared Fe3O4/CdS nanocomposites have the quality required by recoverable photocatalysts. Experimental Section Chemicals and Apparatus. FeCl3 · 6H2O, ethylene glycol, polyethylene glycol 4000, NaAc · 3H2O, CdCl2 · 2.5H2O, thioacetamide (TAA), and methyl orange (MO) were purchased from Shanghai Chemical Reagents Company. All of the reagents are analytical grade except polyethylene glycol 4000, which is chemically pure. X-ray diffraction (XRD; XRD-6000), transmission electron microscopy (TEM; JEOL2010 with an energy dispersive X-ray (EDX) system), selected-area electron diffraction (SAED), scanning electron microscopy (SEM; Hitachi S-4800), UV/vis absorption spectroscopy (U-3010), and fluorometry (F-4500) were applied to characterize the structure, size distribution, morphology, composition, optical response, and fluorescence of as-synthesized nanocomposites. Magnetic properties of asprepared Fe3O4 microspheres and nanocomposites were investigated using a vibrating sample magnetometer with an applied field between -10000 and 10000 Oe at room temperature. Synthesis of Magnetic Cores. Fe3O4 microspheres were prepared using the method previously reported by Li with tiny modification.10 The details are summarized as follows: FeCl3 · 6H2O (5 mmol), polyethylene glycol 4000 (1.0 g), and 3.6 g of NaAc · 3H2O were added to ethylene glycol (40 mL) in an orderly manner. This mixture was then transferred into a Teflon-lined autoclave and treated at 180 °C for 18 h. After collection and rinsing of the microspheres, the as-prepared microspheres were stocked in distilled water for the following reactions. Synthesis of Fe3O4/CdS Nanocomposites. The nanocomposites were synthesized via a sonochemical route in a simple ultrasonic bath. Typically, 7 mL of the above stock solution was first injected into a tube and dispersed with the help of ultrasound radiation for 20 min. Then 3 mmol of TAA was added. After TAA dissolved completely, 1.5 mmol of CdCl2 · 2.5H2O solution (3 mL) was introduced. Subse-

10.1021/cg800213w CCC: $40.75  2009 American Chemical Society Published on Web 11/20/2008

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Figure 1. (a) XRD patterns of Fe3O4 microspheres and as-prepared Fe3O4/CdS nanocomposites; (b) typical TEM image of the sample; (c) HRTEM image of a small region of a particle (inset: the SAED patterns of the sample); (d) typical SEM image of the products; and (e) EDXA spectrum taken from a particle. quently, the tube was exposed to ultrasound irradiation in the air for another 30 min. The sample was collected and washed under the assistance of magnetic force and dried in a vacuum at 60 °C for 12 h. Photocatalytic Activity Studies of the Fe3O4/CdS Nanocomposites. To investigate the photocatalytic activity of the Fe3O4/CdS nanocomposites, MO was chosen as the test molecule. In a typical procedure, 15 mg of the nanocomposites and 30 mL of 6 × 10-5 M MO mother solution were used. Three milliliters of the turbid solution was removed from the above solution to measure their absorption at regular intervals after the mother solution was irradiated with light produced from a 100-W mercury lamp.

Results and Discussion Characterization of Magnetic Cores. The crystal structure of the cores was characterized using XRD pattern and TEM images. The XRD pattern (Figure 1a) of the cores reveals the typical diffraction patterns of pure Fe3O4 (JCPDS card no.: 75-

1609), which is consistent with Li’s report.7 Figure 1b shows the typical low-magnification TEM image of as-prepared Fe3O4 microspheres, which indicates that the sample is composed of well-dispersed spherical particles with uniform size and shape. Most of the microspheres have hollow structures and the average size is about 200 nm. A representative high-resolution TEM image of a microsphere in Figure 1c shows that the lattice fringe spacing is about 0.3 nm, which corresponds to an interplanar distance of the (112) plane of orthorhombic Fe3O4, indicating the single-crystalline nature of the particles. This conclusion is consistent with the result obtained from SAED patterns (inset of Figure 1c). To further characterize the morphology of the magnetic cores, the sample was examined by field emission scanning microscopy (FESEM). A large scale of microspheres is clearly visible from the SEM image (Figure 1d). The diameter of the magnetic microspheres is about 200 nm, which is

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Figure 2. (a) and (b) SEM images of as-prepared Fe3O4/CdS nanocomposites with different magnifications; (c) and (d) high-magnification TEM image and HRTEM image of the nanocomposites; (e) EDXA spectrum of the nanocomposites.

consistent with the results obtained from TEM observation. EDXA spectrum of the nanocomposites is shown in Figure 1e, which illustrates that the atomic ratio of Fe/O in the sample is about 3/4. C and Cu were derived from the carbon-coated copper TEM grid. Characterization of the Fe3O4/CdS Nanocomposites. An X-ray power diffraction experiment was carried out to determine the structure and composition of the nanocomposites, which is shown in Figure 1a. XRD pattern of the nanocomposites shows that the nanocomposites contain not only orthorhombic phase Fe3O4 but also cubic phase CdS (JCPDS card no.: 10-0454). However, because of the low content of CdS, the pattern of CdS is rather weak. The morphology of the obtained Fe3O4/ CdS nanocomposites was first characterized by SEM. Figure 2a and 2b show the morphology of the nanocomposites with different magnifications, which demonstrate that a large amount of nanocomposites with uniform structure was produced. The striking feature of the morphology is that large numbers of nanoparticles are immobilized onto the microspheres’ surface,

which is substantially different from the bare Fe3O4 microspheres. To further investigate the surface structure of these nanocomposites, a high-magnification TEM image of one Fe3O4/ CdS nanostructure was obtained (Figure 2c). A cloudlike layer was formed. A more detailed examination of the layer by HRTEM image (Figure 2d) shows that the layer is built by smaller nanoparticles. Representatively, a nanocrystal with diameter of about 15 nm is attached to the Fe3O4 microspheres, and the interplanar distance of that nanoparticle is about 0.36 nm, revealing the crystalline nature of the nanocrystal; the interplanar distance of the Fe3O4 microsphere is about 0.41 nm and corresponds to its (002) plane. The inset of Figure 2d shows the SAED patterns of Fe3O4/CdS, from which both strong point patterns and weak ring pattern can be seen. The weak ring pattern can be indexed as the (111) plane of cubic CdS. The EDXA spectrum (Figure 2e) of the sample shows that the nanocomposites mainly contain four elements, and the atomic ratio of them (Fe/O/Cd/S) is about 30:40:1:1.

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Figure 3. (a) UV/vis absorption (curve 1) and photoluminescence spectra (curve 2) of the Fe3O4/CdS nanocomposites (excitation at 320 nm); (b) magnetic hysteresis loops of Fe3O4 microspheres (black) and Fe3O4/CdS nanocomposites (red) at room temperature; (c) the enlarged view of the central loop of Fe3O4 microspheres (black) and Fe3O4/CdS nanocomposites (red).

Optical and Magnetic Properties of the Fe3O4/CdS Nanocomposites. CdS, as one of the most important II-VI group semiconductors, has been extensively investigated and used in the development of sensitive and selective detection and biolabeling in biological fields due to its excellent emission property in recent decades.8 Absorption and emission spectra of Fe3O4/CdS nanocomposites are shown in Figure 3a. The absorption spectrum shows that the nanocomposites have an obvious absorption shoulder around 470 nm. Room temperature photoluminescence (excitation at 320nm) of the nanocomposites exhibits an emission maximum around 473 nm, which is slightly asymmetric because of the joints between magnetic spheres and CdS segments.9 Magnetic measurements, which were taken at room temperature, showed there is no significant change in the magnetic moment of Fe3O4 microspheres and Fe3O4/CdS nanocomposites. Figure 3b displays hysteresis loops of Fe3O4 microspheres and Fe3O4/CdS nanocomposites, which indicates they possess magnetic saturation (Ms) values of about 74 and 52 emu/g, respectively. The decrease in Ms may be attributed to the increased mass of the CdS nanoparticles. Figure 3c shows the enlarged view of the central loop of both samples, from which coercivity of them can be deduced, 47 Oe for Fe3O4 and 30 Oe for Fe3O4/CdS. Such excellent magnetic properties imply strong magnetic responsivity of the nanocomposites, which enable them to be separated easily. Photocatalytic Activity Studies of the Fe3O4/CdS Nanocomposites. Semiconductor nanocrystals, such as TiO2, ZnO, Cu2O, ZnS, and CdS, have been exploited as photocatalysts for the photoreductive dehalogenation of halogenated benzene derivatives, photocatalytic degradation of dye, and photocatalytic synthesis of organic compounds.10 Among them, CdS is an

important photocatalyst because of its excellent properties in that the band gap (2.3 eV) corresponds well with the spectrum of sunlight. For example, Torimoto and co-workers have synthesized azoxybenzene with high selectivity by visible light irradiation of a deaerated 2-propanol aqueous containing nitrobenzene and silica-coated CdS nanocomposites.11 Furthermore, the conduction band edge of it is more negative than the H2O/H2 redox potential, which means it can be utilized for hydrogen generation from water under light irradiation.12 However, most of the studies have showed the highly photocatalytic activity of the semiconductor nanostructures in the first cycle, and little attention has been paid to the investigation of the renewable photocatalytic properties of them in the following reaction. Herein, we have examined not only the photocatalytic activity of the nanocomposites toward the photodegradation of MO in aqueous solution in the first cycle but also the recyclable photocatalytic activity in the following 12 cycles of usage as the magnetic parts in the nanocomposites supply us with a simple way for separation. Furthermore, the concentration of Fe3O4/CdS nanocomposites on the change of the solution with UV irradiation has also been investigated. Photocatalytic activity of the nanocomposites was measured using the method reported previously by Wan and co-workers.13 Typically, 15 mg of the nanocomposites was added to 30 mL of 6 × 10-5 M MO mother solution, which was irradiated with light from a mercury lamp after a few minutes of ultrasonic irradiation. Three milliliters of turbid solution was taken at regular time intervals and separated by using an external magnet for UV/vis absorption measurements. Figure 4 shows the effect of irradiation duration on the intensity of the absorption of the removed solution at different intervals, from which we can see

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Figure 4. Absorption spectrum of a solution of MO (6.0 × 10-5 M, 30 mL) in the presence of Fe3O4/CdS nanocomposites (15 mg) under exposure to UV light (insets: the photographs of the resulting solution taken at regular time intervals).

Figure 6. Photodegradation of MO (30 mL, 6 × 10-5 M) under UV irradiation with different amounts of the nanocomposites: (1) 10 mg; (2) 15 mg; (3) 20 mg; (4) 25 mg; (5) 30 mg.

Figure 5. Photodegradation of MO (30 mL, 6 × 10-5 M) under different conditions: (1) pure CdS nanoparticle (15 mg) with diameter of 20 nm in the dark; (2) with commercial TiO2 nanoparticles (P25, 15 mg) in the dark; (3) with as-prepared Fe3O4/CdS nanocomposites (15 mg) in the dark; (4) without catalyst; (5) with as-prepared Fe3O4/CdS nanocomposites (15 mg) with UV irradiation; (6) with 15 mg of commercial TiO2 nanoparticles with UV irradiation; (7) pure CdS nanoparticles (15 mg) with UV irradiation.

Figure 7. Behaviors of recycled Fe3O4/CdS nanocomposites in photodegradation of MO (30 mL, 6 × 10-5 M) in different cycles under UV irradiation (15 mg of Fe3O4/CdS was used in the first cycle).

that the intensity of the absorption peak decreases gradually and disappears completely after 50 min of irradiation. The black block between the bottles is a magnet in the inset of Figure 4. The black powder near the magnets in the bottles is the nanocomposites, which can be separated easily from solution by using external magnetic force, demonstrating the virtue of such nanocomposites used as recyclable catalysts. To compare the photocatalytic property of as-prepared Fe3O4/CdS nanocomposites with that of other materials, pure CdS nanoparticles with diameter of about 20 nm and commercial TiO2 nanoparticles (P25) were employed as contrastive catalysts. From curve 1 to curve 3 in Figure 5, we can see that pure MO absorption of all catalysts is negligible. Meanwhile, it can be found that with UV irradiation a slight decrease in the concentration of MO was detected in the absence of any catalysts (curve 4). Furthermore, this figure also shows that the photocatalytic activity of as-prepared Fe3O4/CdS nanocomposites is much weaker than that of pure CdS nanoparticles and commercial TiO2 nanoparticles mainly because of the low content of CdS loaded in the nanocomposites. The actual amount of CdS used in the photocatalytic experiment is about 1 mg using the results obtained from the EDXA spectrum. Figure 6 shows the concentration of the Fe3O4/CdS nanocomposites on the change of the solution with UV irradiation. Different amounts of Fe3O4/ CdS nanocomposites, such as 10, 15, 20, 25, and 30 mg, were

used. The results show that the photocatalytic ability of the nanocomposites increases gradually and this increase becomes weaker and weaker with enhancement of the mass of Fe3O4/ CdS added. As practical recyclable catalysts, highly catalytic activity in each cycle of usage is necessary. The renewable photocatalytic activity is also investigated. The result is presented in the same manner as shown in Figure 7, from which we can see that the photocatalytic activity of the nanocomposites decreases slightly after each cycle. However, 89% of MO is decomposed in the last cycle. In conclusion, results obtained from photocatalytic activity studies imply that as-prepared nanocomposites have highly photocatalytic activity in degradation of MO in aqueous solution and this activity only decreased slightly after 12 cycles of usage. Thus, as-prepared nanocomposites can be used as ideal photocatalysts in practical applications. Conclusions In summary, a novel method for the preparation of heterostructure nanocomposites of Fe3O4/CdS has been reported. This method is simple, inexpensive, green, and efficient compared with the former routes utilized to synthesize the similar nanostructures. The products exhibit fluorescence, display excellent magnetic properties at room temperature, and have highly photocatalytic degradation ability toward degradation of MO in aqueous solution. The renewable photocatalytic activity decreases slightly after each cycle of tests; however, 89% of MO is decomposed in the last usage, which means that

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as-prepared nanocomposites can be used as convenient recyclable photocatalysts. Acknowledgment. This work was supported by the Natural Science Foundation of the College of Anhui Province (No. KJ2008B168), the National Natural Science Foundation of China (No. 20671003 and 20701002), Science and Technological Fund of Anhui Province for Outstanding Youth (No. 08040106906), and the Education Department (No. 2006KJ006TD) of Anhui Province. We thank Dr. Guoxing Zhu (Nanjing University) for magnetic measurements.

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