MS (M = Zn, Cd, Hg, Pb

12728

J. Phys. Chem. C 2008, 112, 12728–12735

Generalized and Facile Synthesis of Fe3O4/MS (M ) Zn, Cd, Hg, Pb, Co, and Ni) Nanocomposites Xiaowang Liu,*,† Qiyan Hu,‡ Xiaojun Zhang,† Zhen Fang,† and Qiang Wang† College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal UniVersity, Wuhu 241000, China, and Department of Pharmacy, Wannan Medical College, Wuhu 241002, China ReceiVed: April 22, 2008; ReVised Manuscript ReceiVed: June 8, 2008

This study presents a novel method for the preparation of Fe3O4/MS (M ) Zn, Cd, Hg, Pb, Co, and Ni) nanocomposites via a sonochemical aqueous route. This method is mild, convenient, cheap, and efficient. A creditable synthetic mechanism for the formation of the nanocomposites has been proposed. It can be found that the concentration of TAA plays an important role in the controlling of the size and density of the nanoparticles immobilized on the surface of the magnetic microspheres. Fe3O4/ZnS and Fe3O4/CdS nanocomposites display fluorescence and exhibit ferromagnetic behavior at room temperature. Photocatalytic activity studies demonstrate that the former ones have highly photocatalytic activity toward the photodegradation of eosin Y in aqueous solution. The catalytic efficiency only decreased by 5% when cyclic time was up to 15 cycles, which means that as-prepared Fe3O4/ZnS nanocomposites have promising applications not only in biomedical fields but also in organic synthesis or wastewater treatment as recyclable photocatalysts. Furthermore, this method can be extended to the synthesis of other magnetic nanocomposites. 1. Introduction In recent years, people have achieved a great deal of success in size- and shape- controllable synthesis of inorganic nanocrystals, which have great potential in both fundamental research and technical applications.1 In some cases, the size, shape, and composition of the nanocrystals can be tuned freely, which means that nanocrystals with desirable properties can be obtained.2 However, those nancrystals usually supply people with only one function, for example, semiconducting metal sulfide nanocrystals are widely used as fluorescence probes for visualizing biological processes in vitro or in vivo,3 magnetic nanocrystals frequently serve as magnetic resonance image (MRI) agents for the diagnosis of many diseases,4 and noble metal nanoparticles are usually exploited as catalysts in organic synthesis or fuel cell.5 One common structure containing several different functionalities required by practical applications is difficult to achieved, especially the nanocrystals that only have one component. Thus, design and synthesis of nanostructures with sophisticated and multifunctional properties is the natural step in the development of nanostructures. Recently, several groups have achieved delightful progress in multifunctional nanostructure synthesis, which can be mainly sorted as following several kinds: The first kind is magneticsemiconductor nanocomposites, which not only can be used as fluorescence probes but also as imaging agents in the biological field. For their useful applications, they have attracted more and more attention and some papers have reviewed the progress in this area.6 Taking advantage of lattice mismatching and selective annealing, Xu and co-workers have synthesized FePt-CdS heterodimer nanoparticles in one pot that exhibited both superparamagnetism and fluorescence. In a follow-up work, they * Corresponding author. Phone: +86-553-3869303. Fax: +86-5533869303. E-mail: [email protected]. † Anhui Normal University. ‡ Wannan Medical College.

synthesized FePt-ZnS spongelike nanostructures in a similar way. Very recently, FePt-CdSe core-shell nanocrystals, FePt-CdSe nanosponges, and Fe3O4-CdSe heterodimers have also been synthesized by them.7 Other nanocomposites of this kind, such as γ-Fe2O3/MS (M ) Zn, Cd, or Hg) heterojunctions, Fe2O3-CdSe/ZnS nanocomposites, Co@CdSe core-shell nanostructure, Fe3O4-PbS hybrid nanoparticles, and silica-coated Fe2O3-CdSe heterodimers, have emerged.8 The second kind is magnetic-metallic nanocomposites which evolved as useful nanoscale building blocks for nanoelectronic, biological, and catalytic applications. CoPt3-Au heterodimer nanocrystals, Fe3O4-Au dumbbell-like bifunctional nanoparticles, and Fe3O4-Ag, FePt-Au, and FePt-Ag heterodimer nanocrystals were produced.9 The third kind is semiconductor-metallic nanocomposites; it has been proved that the photoinduced charge carriers in them are trapped by the noble metal, which can promote the interfacial charge-transfer process and enhance their photocatalytic activities. CdSe-Au heterostructures, PbSe-Au hybrid nanocrystals, PbS-Aun (n ) 1, 4, n) nanostructures, and ZnO-Ag and Se-Ag nanocomposites were advanced.10 The last kind is semiconductor-semiconductor nanocomposites containing PbSe-CdS heterodimer nanocrystals, PbSe-CdSe heterostructures, CdS/CdSe nanorod heterostructures, CdTe-CdSe multiple-branched rods, CdSe-CdTe nanobarbells, and so on.11 However, most nanostructures involved above were synthesized through nonaqueous routes, in which such corrosive, toxic, expensive agents as oleic acid, oleylamine, trioctylphosphine oxide, and trioctylphosphine were used frequently. Those molecules that could absorb on the surface of the nanocrystals result in the poor water solubility of the products before surface modification. Furthermore, a standard airless technique and high temperature are necessary. Except for nonaqueous-phase routes, layer-by-layer and surface-functionalized techniques are useful methods for nanocomposite synthesis. Noble metal-, semiconductor-, and magnetic nanoparticle-modified SiO2 or PS (Polystyrene) microspheres, and silica microspheres both containing

10.1021/jp8035617 CCC: $40.75  2008 American Chemical Society Published on Web 07/24/2008

Generalized and Facile Synthesis of Fe3O4/MS Nanocomposites

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12729

Figure 1. (a) SEM image, (b) TEM image, (c) HRTEM image of magnetic microspheres, and (d) EDAX spectrum of magnetic microspheres. The inset in panel b is the selected-area electron diffraction pattern.

maghemite nanoparticles and CdSe/CdZnS core/shell quantum dots were reported.12 However, surface modification of the core parts was needed in their synthesis procedure. Herein, we report a mild, convenient, cheap, green, and efficient method for the preparation of Fe3O4/MS (M ) Zn, Cd, Hg, Pb, Co, and Ni) nanocomposites under ambient air condition via an aqueousphase method. Moreover, the surface of core parts, Fe3O4 microspheres, need not be altered. On the basis of experiments, a credible mechanism for the formation of the nanocomposite has been proposed. 2. Experimental Section Chemicals. FeCl3 · 6H2O, ethylene glycol, polyethylene glycol 4000, NaAc · 3H2O, Zn(Ac)2 · 2H2O, CdCl2 · 2.5H2O, HgCl2 · 2H2O, Pb(Ac)2 · 3H2O, CoCl2 · 6H2O, NiCl2 · 6H2O, ethanol, and thioacetamide (TAA) were purchased from Shanghai Chemical Reagents Company. All of the reagents are analytical grade except for polyethylene glycol 4000, which is chemical pure. Monodisperse Fe3O4 Microspheres Synthesis. Monodisperse Fe3O4 microspheres were synthesized by the method reported by Li with a tiny modification.13 FeCl3 · 6H2O (5 mmol) was dissolved in ethylene glycol (40 mL) to form a clear solution, and 1.0 g of polyethylene glycol 4000 and 3.6 g of NaAc · 3H2O was added subsequently. This mixture was then transferred into a Teflonlined autoclave and heated at 180 °C for 18 h. The products were collected and rinsed with water and ethanol with the help of an external magnetic force. Finally, the microspheres were stocked in distilled water for the following reactions. Fe3O4/MS (M ) Zn, Cd, Hg, Pb, Ni, and Co) Nanocomposites Synthesis. In a typical synthesis, 7 mL of the above stock solution was injected into a tube and dispersed with the help of ultrasound radiation for 20 min. Three millimoles of thioacetamide was added to the same tube and the reaction was radiated for another 10 min. Then, 1.5 mmol of Zn(Ac)2 solution (3 mL) was introduced into the mixture. Subsequently, the resulting solution was irradiated by 40 kHz ultrasonic waves at 80% output power in the air for 30 min. During the reaction, temperature was controlled at about 45 °C. The precipitates were collected and washed with distilled water and absolute ethanol several times and then dried in a vacuum at 60 °C overnight. The amount of TAA used in the reaction was tuned from 1.5 to 4.8 mmol to obtain different samples. Other nanocomposites were synthesized similarly. As Ksp of transition metal sulfides varies greatly, the mole ratio of the reagents and the temperature used in the reaction were changed accordingly:. 1.5 mmol of CdCl2 · 2.5H2O, 1.0 mmol of CoCl2 · 6H2O, 1.0 mmol of NiCl2 · 6H2O, 0.3 mmol of Pb (Ac)2 · 3H2O, and 0.3 mmol of

Figure 2. XRD patterns of as-prepared Fe3O4 microspheres and Fe3O4/ MS (M ) Zn, Cd, Pb, Hg, Co, and Ni) nanocomposites.

HgCl2 · 2H2O corresponded to 3, 3, 4, 2, and 1.5 mmol of TAA. It is important to note that a drop of TEA and higher temperature were necessary during the synthesis of Fe3O4/CoS and Fe3O4/ NiS nanocomposites. To determine the structure and composition of Fe3O4/ZnS, Fe3O4/CoS, and Fe3O4/NiS nanocomposites, they were annealed in a quartz tube furnace under Ar atmosphere at 200 °C for 10 h. Characterization. The samples were characterized by X-ray diffraction (XRD; XRD-6000), selected-area electron diffraction (SAED), scanning electron microscopy (SEM; Hitachi S-4800), and transmission electron microscopy (TEM; JEOL-2010 with an energy dispersive X-ray (EDX) system). Room temperature UV/vis absorption and photoluminescence measurements of Fe3O4/MS (M) Zn and Cd) nanocomposites were recorded with a U-3010 UV/vis spectrophotometer (220V, 50 Hz) and fluorometry (F-4500). They were excited by a Xe arc lamp with a wavelength of 290 and 320 nm at room temperature, respectively. IR spectra were obtained on a Vectortm 22 FTIR spectrometer (Bruke, Germany). Magnetic properties of the magnetic microspheres and nanocomposites were investigated by using a vibrating sample magnetometer with an applied field between -10 000 and 10 000 Oe at room temperature. 3. Results and Discussion Fe3O4 Microspheres Synthesis. The SEM image of the microspheres is shown in Figure 1a, from which nearly mondisperse spherical nanoparticles with a diameter of about 200 nm are observed. It is worth noting that the magnetic microspheres are composed of smaller particles and their surface is not smooth. The TEM image (Figure 1b) further confirms the above results and indicates that most of the microspheres

12730 J. Phys. Chem. C, Vol. 112, No. 33, 2008

Liu et al.

Figure 3. (a) SEM image, (b) TEM image, (c) HRTEM image of as-prepared Fe3O4/ZnS nanocomposites, and (d) EDXA spectrum of the nanocomposites. The inset of panel a shows the elemental mapping of Fe (green dots), Zn (purple dots), and S (red dots).

Figure 4. SEM images of Fe3O4/ZnS nanocomposites obtained with different amounts of TAA: (a) 1.5, (b) 4, (c) 4.5, and (d) 4.8 mmol. The insets of panels a-d show the elemental mapping of Fe (green dots), Zn (purple dots), and S (red dots).

have porous structures. More detailed information on the magnetic microspheres was acquired by using HRTEM. The HRTEM image (Figure 1c) shows a selected area of the microsphere, demonstrating the high crystallinity and singlecrystalline nature of the particles, which supported the conclusion obtained from the selected-area electron diffraction (SAED) pattern (inset of Figure 1b). The measured spacing of the lattice planes is about 0.30 nm, which agrees well with the separation of the (112) plane of orthorhombic phase Fe3O4. The composition of the microspheres was measured by using energydispersive X-ray analysis (EDXA), which is shown in Figure 1d. Analysis of the EDXA spectrum illustrates that the microspheres contain Fe and O, and the atomic ratio Fe/O is near 3:4. The phase purity of the magnetic microspheres prepared in the first step was investigated by using XRD, as shown in Figure 2. The XRD pattern of the material proves its crystalline nature and the peaks match well with standard Fe3O4 reflections (JCPDS card No. 75-1609), which is consistent with Li’s result.13 Fe3O4/ZnS nanocomposites Synthesis. We employed the synthesis of Fe3O4/ZnS nanocomposites as a model system for studying the preparation of Fe3O4/MS (M ) Zn, Cd, Hg, Pb,

Figure 5. (a) IR spectra of Fe3O4 microspheres obtained before and after addition of TAA. (b) SEM image of the Fe3O4 microspheres obtained after the addition of TAA.

Co, and Ni) with our method. The SEM image (Figure 3a) of as-prepared Fe3O4/ZnS nanocomposites shows that a large scale of ZnS nanoparticles is immobilized onto the surface of the magnetic microspheres, which is substantially different from the bare microsphere as we observed from Figure 1a. A typical TEM image of the nanocomposites is shown in Figure 3b, from which many ZnS nanoparticles are presented on the surface of the microsphere. The HRTEM image (Figure 3c) demonstrates


J. Phys. Chem. C, Vol. 112, No. 33, 2008 12731

Figure 6. Schematic illustration of the formation of the nanocomposites in the whole synthetic process.

that the diameter of the nanoparticle is about 25 nm and is formed by smaller nanoparticles with a diameter of about several nanometers. This image also implies that the nanoparticles assembled onto the surface of the magnetic microspheres are noncrystalline. There are two main reasons for this: one is the power and temperature used in our reaction system is low, and the other is the duration of the reaction is short. An X-ray power diffraction experiment was carried out to determine the structure and composition of the nanocomposites after being annealed under inert atmosphere. As shown in Figure 2, all the peaks in the pattern can be readily indexed to a pure hexagonal phase ZnS (JCPDS card No. 80-0007) and orthorhombic phase Fe3O4. The EDXA spectrum of the nanocomposites is showed in Figure 3d. The result obtained from it indicates that the atomic ratio of Fe/O/Zn/S is about 30/40/3/3, illustrating that the nanoparticle formed on the surface of the magnetic spheres is ZnS. Elemental mapping of Fe, Zn, and S (inset of Figure 3a) also reveals a multisegmented feature of the nanocomposites. Green, purple, and red dots represent element Fe, Zn, and S, respectively. From this inset, we can see that these elements are well-proportioned, indicating that the density of the ZnS nanoparticle is quite high and the diameter of the nanoparticles is rather small. On the basis of the SEM images shown in Figure 4, we see that the size and density of the ZnS nanoparticle immobilized on the surface of the magnetic microspheres are influenced by the concentration of TAA. With a low concentration of TAA, 1.5 mmol, the ZnS nanoparticles attached onto the surface of the microspheres are limited (Figure 4a). When 3 mmol of TAA was used, the density of the ZnS nanoparticle was enhanced (Figure 3a). As a higher concentration of TAA is used in the reaction, a larger number of ZnS nuclei will be formed. The more nuclei, the more energy the system has. To reduce their surface energy, those nuclei will attach to the surface of magnetic spheres quickly. Then the classical Ostwarld ripening process will take place and lead to the formation of ZnS nanoparticle by assimilation of the nuclei. Compared with the results obtained from the above two samples, we can see that the diameter of the ZnS nanoparticles is without obvious alteration. However, the density of ZnS nanoparticles in the latter sample is enhanced greatly. So, we can see that the concentration of TAA plays an important role in enhancing the density of ZnS nanoparticles. If the concentration of TAA increased further, nanocomposites with a bigger ZnS nanoparticle were produced. For example, when 4 mmol of TAA was used, the diameter of the ZnS nanoparticles is about 40 nm (Figure 4b). ZnS nanoparticles with a diameter of about 60 nm corresponded to 4.5 mmol of TAA (Figure 4c). However, when 4.8 mmol of

TAA was used, the unattached ZnS nanoparticles connected the attached ones and resulted in the aggregation of the nanocomposites, as shown in Figure 4d. The sphere-shape configuration of the core part is not clear. The density and the diameter of ZnS nanoparticles grown as the concentration of TAA increased are also confirmed by elemental mapping of those elements because the density of purple dots (Zn) and red dots enhanced gradually and the distribution of those dots was more and more asymmetric (insets in Figure 4). EDXA spectra indicate that the atomic ratio of Fe/Zn can be tuned from 15/1 to 17/11 (Supporting Information). Figure 5a shows the IR spectra of Fe3O4 microspheres obtained from the solution before and after addition of TAA, respectively. There is no obvious absorption between 700 and 3200 cm-1 of curve 1, demonstrating that the surface of Fe3O4 is bare. However, curve 2 exhibits some characteristic absorptions of TAA, such as, NsH, CsH, and CdS stretch vibrations around 3117, 2947, and 1165 cm-1 and NsH distorted stretch vibration around 1716 cm-1, which confirm that TAA molecules are absorbed on the surface of the microspheres. This result is also confirmed by Figure 5b, an organic layer was formed on the surface of the microspheres after the addition of TAA. On the basis of the above results, the mechanism for the formation of the nanocmposites was proposed as follows. First, as-prepared microspheres have excellent absorption activity because of their high surface-to-volume ratio and porous structure, which can provide ideal absorption sites for TAA molecules. Recent studies have shown that materials with such structure can be used as effective sorbents in water treatment.14 When TAA molecules dissolve in the solution, they will be absorbed on the surface of Fe3O4 forming a TAA layer. Then, with the assistance of ultrasonic irradiation and the inducement of metal ions, TAA hydrolyzes quickly as follows:

CH3CSNH2+2H2O ) CH3COONH4+H2Sv

(1)

Once the concentration of H2S in the solution reaches a certain value, it will react with metal ions and produce metal sulfide nuclei. The nuclei are attached onto the surface of the magnetic microspheres by using interstices or grains in/on the microspheres’ surface as active points, and then nanoparticles are formed. Those nanoparticles become bigger through the assimilation of other nuclei. When the surface of the microsphere is full of the nanoparticles, the excrescent nanoparticles will attach to the ZnS nanoparticles on the surface of the microspheres and cause the aggregation of the nanocomposites. This phenomenon will occur when the amount of TAA is 4.8 mmol.


Liu et al.

Figure 7. SEM images (a), TEM images (b), HRTEM images (c), and EDS spectra (d) of the nanocomposites: (1) Fe3O4/CdS; (2) Fe3O4/PbS; (3) Fe3O4/HgS; (4) Fe3O4/CoS; and (5) Fe3O4/NiS.

The schematic illustration of the whole formation process of the nanocomposites is shown in Figure 6. Other Fe3O4/MS (M ) Cd, Hg, Pb, Ni, and Co) Nanocomposite Syntheses. The formation of other nanocomposites was found to be very similar to that of Fe3O4/ZnS nanocom-

posites. XRD measurements (Figure 3) were used to prove the successful synthesis of those materials. It is important to note that XRD patterns of Fe3O4/CoS and Fe3O4/NiS only show the peaks of Fe3O4, which mean that the layer of CoS and NiS formed on the surface of magnetic microsphere is noncrystalline


Figure 8. UV/vis absorption and photoluminescence spectra of the nanocomposites: (a) Fe3O4/ZnS; (b) Fe3O4/CdS; and (c) magnetic hysteresis of Fe3O4 microspheres (black), Fe3O4/ZnS (red), and Fe3O4/ CdS nanocomposites (blue) at room temperature.

due to the low temperature, short reaction period, and larger Ksp. However, after being annealed under inert atmosphere, the noncrystalline structures are converted to the pure hexagonal phase CoS (JCPDS card No. 01-1279) and orthorhombic phase NiS (JCPDS card No. 02-1280). Representative SEM images of as-prepared nanocomposites shown in Figure 7a tell us that metal sulfides formed on the magnetic microspheres’ surface have two main morphologies. CdS, PbS, and HgS are nanoparticles; however, CoS and NiS have a layer shape. The reason for this is that a drop of triethanolamine (TEA) was added to the latter synthesis system. TEA can accelerate the hydrolyzation rate of TAA in water, otherwise there is no product. However, it is well-known that TEA has coordination ability with metal ions and can affect the morphology of the metal sulfide affirmatively.15 In addition, to accelerate the hydrolyzation of TAA, a high temperature, 90 °C, was also necessary. When a drop of TEA was added to the former synthesis systems, the morphologies of the products were without obvious change because the Ksp of CdS, PbS, or HgS is several orders of magnitude lower than that of CoS or NiS, which means that metal ions react with H2S, which comes from the hydrolyzation of TAA instantly, and thus, the coordination between metal ion and TEA has a poor effect on the morphology of the products. TEM images of Fe3O4/MS (M ) Cd, Pb, and Hg) nanocomposites (Figure 7b1,b2,b3) show that large numbers of nanoparticles are assembled onto the microspheres’ surface, which are similar to Fe3O4/ZnS nanocomposites. The mean

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12733 diameter of the nanoparticles also can be obtained from these images: CdS is about 16 nm, PbS is about 20 nm, and HgS is about 12 nm. Figures 7b4 and 7b5 show the typical TEM images of Fe3O4/CoS and Fe3O4/NiS, from which we can confirm that the products have core-shell structure. The thickness of the layer is 25 nm for CoS and 80 nm for NiS. More detailed examinations of the nanocomposites by HRTEM show that CdS, PbS, and HgS nanoparticles have crystalline nature, and layers of CoS and NiS have amorphous structure, which is consistent with the XRD results. Figures 7c1, 7c2, and 7c3 show clear fringes of the nanoparticles and give lattice spacing of 0.36, 0.34, and 0.31 nm, which correspond to the lattice spacing for the (111) face of the cubic phase CdS (JCPDS card No. 100454), (111) plane of the cubic phase PbS (JCPDS card No. 78-1901), and (003) plane of the hexagonal phase HgS (JCPDS card No. 80-2192), respectively. To further confirm the composition of the samples, EDXA spectra were recorded for these nanocomposites. EDXA spectra show that all the nanocomposites have Fe, O, M (M ) Cd, Pb, Hg, Co, and Ni), and S, some samples also indicate Cu, which comes from the grid used for TEM observations. The atomic ratio of Fe/O in all samples is near 3:4, and M (M ) Cd, Pb, Hg, Co, and Ni)/S is about 1:1, which indicate that the metal sulfide nanoparticles were produced. Optical and Magnetic Properties Studies of Fe3O4/ZnS and Fe3O4/CdS Nanocomposites. As an important wide-band gap semiconductor with band gap energy (Eg) of 3.66 eV at room temperature, ZnS is among the oldest and probably the most important materials used as a phosphor host, which is also a very attractive candidate for applications in novel photonic crystal devices operating in the electromagnetic spectrum region from visible to near-IR.16 Figure 8a shows UV/vis absorption and photoluminescence spectra of Fe3O4/ZnS nanocomposites. The absorption peak of the nanocomposites around 261 nm displays an obvious blue-shift compared to the bulk ZnS (335 nm) because of the quantum confinement.17 The fluorescent spectrum of the samples exhibits an emission maximum at 344 nm (excitation at 290 nm), which is attributed to the band edge emission.18 CdS is another important group II-VI semiconductor with a direct band gap of 2.4 eV at room temperature, which have been investigated extensively during the past decades.19 Absorption and emission spectra of Fe3O4/CdS nanocomposites are shown in Figure 8b. The absorption spectrum shows that the nanocomposites have an obvious absorption shoulder around 470 nm. Room temperature photoluminescence (excitation at 320 nm) of the nanocomposites exhibits an emission maximum around 473 nm, which is slightly asymmetric because of the joints between magnetic spheres and CdS segments.7b Among all the metal sulfides mentioned above, CdS and ZnS nanoparticles have been used frequently as fluorescence probes in the biological field. So we only studied the magnetic properties of Fe3O4/ZnS and Fe3O4/CdS nanocomposites as models. Magnetic measurements were investigated with a VSM at room temperature in the applied magnetic field sweeping from -10 to 10 kOe. And the hysteresis loops of the Fe3O4 microspheres and the nanocomposites are shown in Figure 8c. It can be seen that the magnetic microspheres and the nanocomposites all show ferromagnetic behavior. The magnetic saturation value of Fe3O4 microspheres is about 74 emu/g, for Fe3O4/ZnS nanocomposites it is about 64 emu/g, and for Fe3O4/ CdS nanocomposites it is about 52 emu/g. Such parameters mean that as-prepared nanocomposites have strong magnetic responsivity and can be separated easily from the solution with the help of external magnet force.


Liu et al.

Figure 9. (a) Absorption spectrum of a solution of eosin Y (5.0 × 10-5 M, 30 mL) in the presence of Fe3O4/ZnS nanocomposites (18 mg) under exposure to UV light (inset: photopictures of the solution at different stages of the photodecomposition experiment). (b) Relationship between the decolorization efficiency and cyclic time.

Photocatalytic Activity Studies of Fe3O4/ZnS Nanocomposites. ZnS has been used as a semiconductor-type photocatalyst for the photodegradation of halogenated benzene derivatives, water pollutants, dye, and toxic metal ions.20 Compared with the ZnS nanostructures synthesized by surfactant-assistant methods, the surface of as-prepared Fe3O4/ZnS nanocomposites prepared in this work is bare, and the loose small ZnS nanoparticles result in the high surface-to-volume ratio. These factors allow the Fe3O4/ZnS nanocomposites to exhibit excellent photocatalytic activity. Furthermore, a photocatalytic reaction is usually conducted in a suspension of semiconductor nanostructures, and therefore requires an additional separation step to remove the catalyst from the treated water. Removing such fine particles, especially nanoparticles, from a large volume of water involves further expense, which presents a major drawback to the application of the photocatalytic process for treating wastewaters.21 However, the nanocomposites synthesized in this work show ferromagnetic behavior at room temperature, which can solve the above problem easily. Thus, it is reasonable to expect that the as-prepared Fe3O4/ZnS nanocomposites may be used as ideal recyclable photocatalysts in practical applications. The photocatalytic activity of the nanocomposite was investigated according to a literature method.22 Characteristic absorption of eosin Y at 515 nm was chosen as the monitored parameter during the photocatalytic degradation process. In a typical procedure, 18 mg of as-prepared Fe3O4/ZnS nanocomposites was added to 30 mL of 5.0 × 10-5 M eosin Y solution. However, when using the EDXA spectrum result, the amount of ZnS added was about 2 mg. Figure a shows the absorption spectra of the eosin Y solution at different photodegradation stages after the addition of the nanocomposites. The intensity of the characteristic absorption of eosin Y was reduced gradually as the exposure duration increased and disappeared completely after 37 min. The bigger inset of Figure 9a shows the colorchange sequence, which corresponds to the absorption spectra mentioned above, from which colorless solution was obtained at last indicating that eosin Y was decomposed completely. Black blocks between the bottles in the insets of Figure 9a are magnets, which can pull the Fe3O4/ZnS nanocomposites from the solution expediently showing their unique merit in separation compared with the pure ZnS nanostructures. As a recyclable photocatalyst, another important factor, renewable photocatalytic activity was also investigated. To show the photocatalytic efficiency of the catalysts in each cycle, decolorization efficiency (%) was introduced, which has been calculated as follows:

decolorization efficiency (%) ) (C0 - C)/(C0 × 100%), where C0 is the initial concentration of dye and C is the concentration of dye after photoirradiation. The result is shown in Figure 9b, from which we can see that photocatalytic activity of the nanocomposites decreased slightly after 15 cycles of photocatalysis experiment. However, the decolorization efficiency was still up to 95% in the last cycle. In conclusion, as-prepared Fe3O4/ZnS nanocomposites have highly photocatalytic activity in degradation of eosin Y in aqueous solution and this ability decreases slightly when cyclic time is up to 15 cycles. Ferromagnetic behavior of the nanocomposites supplies us with a facile method for the separation of the nanocomposites from solution. On the basis of above discussion, as-prepared Fe3O4/ ZnS nanocomposites may serve as ideal recyclable photocatalysts in wastewater treatment or in organic synthesis. 4. Conclusions In summary, a novel method for the preparation of Fe3O4/ MS (M ) Zn, Cd, Hg, Pb, Co, and Ni) nanocomposites was reported. This method is simple, cheap, green, and efficient. The concentration of TAA plays an important role in controlling the size and density of the nanoparticles. The nanocomposites of Fe3O4/ZnS and Fe3O4/CdS display ferromagnetism and exhibit fluorescence at room temperature. The latter ones have highly photocatalytic activity toward the degradation of eosin Y in solution and this ability decreases slightly after 15 cycles of the photocatalysis experiments, which means that these nanocomposites can be used as recyclable photocatalsts in practical application. This method may be extended to synthesize other nanocomposites. Acknowledgment. This work was supported by the College Natural Science Foundation of Anhui Province (No. KJ2008B168 and No. KJ 2008B167), NSFC (No. 20701002), and the Education Department of Anhui Province (No. 2006KJ006TD). Supporting Information Available: EDXA spectra of Fe3O4/ZnS nanocomposites obtained with different amounts of TAA. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (b) Yin, Y. D.; Alivisatos, A. P. Nature 2005, 437, 664.

Generalized and Facile Synthesis of Fe3O4/MS Nanocomposites (c) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121. (d) Song, Q.; Zhang, Z. J. J. Am. Chem. Soc. 2004, 126, 6164. (e) Cheon, J.; Kang, N. J.; Lee, S. M.; Lee, J. H.; Yoon, J. H.; Oh, S. J. J. Am. Chem. Soc. 2004, 126, 1950. (f) Jun, Y. W.; Choi, J. S.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414. (g) Lee, C. H.; Kim, M.; Kim, T.; Kim, A.; Paek, J.; Lee, J. W.; Choi, S. Y.; Kim, K.; Park, J. B.; Lee, K. J. Am. Chem. Soc. 2006, 128, 9326. (2) (a) Mikulec, F. V.; Kuno, M.; Bennati, M.; Hall, D. A.; Griffin, R. G.; Bawendi, M. G. J. Am. Chem. Soc. 2000, 122, 2532. (b) Zaitseva, N.; Dai, Z. R.; Leon, F. R.; Krol, D. J. Am. Chem. Soc. 2005, 127, 10221. (c) Du, H.; Chen, C.; Krishnan, R.; Krauss, T. D.; Harbold, J. M.; Wise, F. W.; Thomas, M. G.; Silcox, J. Nano Lett. 2002, 2, 1321. (d) Seo, W. S.; Jo, H. H.; Lee, K.; Park, J. T. AdV. Mater. 2003, 15, 795. (e) Tao, A.; Sinsermsuksakul, P.; Yang, P. Angew. Chem., Int. Ed. 2006, 45, 4597. (f) Li, Y.; Zhong, H.; Li, R.; Zhou, Y.; Yang, C.; Li, Y. AdV. Funct. Mater. 2006, 16, 1705. (3) (a) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 1289. (b) Alivisatos, P. Nat. Biotechnol. 2004, 22, 47. (c) Zhang, Y.; So, M. K.; Loening, A. M.; Yao, H. Q.; Gambhir, S. S.; Rao, J. H. Angew. Chem., Int. Ed. 2006, 45, 4936. (d) Yu, W. W.; Chang, E.; Falkner, J. C.; Zhang, J.; Al-Somali, A. M.; Sayes, C. M.; Johns, J.; Drezek, R.; Colvin, V. L. J. Am. Chem. Soc. 2007, 129, 2871. (4) (a) Hu, F.; Wei, L.; Zhou, Z.; Ran, Y.; Li, Z.; Gao, M. AdV. Mater. 2006, 18, 2553. (b) Jun, Y.-W.; Huh, Y.-M.; Choi, J.-S.; Lee, J.-H.; Song, H.-T.; Kim, S.-J.; Yoon, S.; Kim, K.-S.; Shin, J.-S.; Suh, J.-S.; Cheon, J. J. Am. Chem. Soc. 2005, 127, 5732. (c) Lee, H.; Lee, E.; Kim, D. K.; Jang, N. K.; Jeong, Y. Y.; Jon, S. J. Am. Chem. Soc. 2006, 128, 7383. (5) (a) Kidambi, S.; Dai, J.; Li, J.; Bruening, M. L. J. Am. Chem. Soc. 2004, 126, 2658. (b) Guin, D.; Baruwati, B.; Manorama, S. V. Org. Lett. 2007, 9, 1419. (c) Liang, H. P. H.; Zhang, M.; Hu, J. S.; Guo, Y. G.; Wan, L. J.; Bai, C. L. Angew. Chem., Int. Ed. 2004, 43, 1540. (6) (a) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Chem. Soc. ReV. 2006, 35, 1195. (b) Salgueirin˜o-Maceira, V.; Crrea-Duarte, M. A. AdV. Mater. 2007, 19, 4131. (7) (a) Gu, H. W.; Zheng, R. K.; Zhang, X. X.; Xu, B. J. Am. Chem. Soc. 2004, 126, 5664. (b) Gu, H. W.; Zheng, R. K.; Liu, H.; Zhang, X. X.; Xu, B. Small 2005, 1, 402. (c) Gao, J. H.; Zhang, B.; Gao, Y.; Pan, Y.; Zhang, X. X.; Xu, B. J. Am. Chem. Soc. 2007, 129, 11928. (d) Gao, J.; Zhang, W.; Huang, P.; Zhang, B.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2008, 130, 3710. (8) (a) Kwon, K. W.; Shim, M. J. Am. Chem. Soc. 2005, 127, 10269. (b) Wang, D. S.; He, J. B.; Rosenzweig, Z. Nano Lett. 2004, 4, 409. (c) Kim, H.; Achermann, M.; Balet, L. P.; Hollingsworth, J. A.; Klimov, V. I. J. Am. Chem. Soc. 2005, 127, 544. (d) Shi, W.; Zeng, H.; Sahoo, Y.; Ohulchanskyy, T. Y.; Ding, Y.; Wang, Z. L.; Swihart, M.; Prasad, P. N. Nano Lett. 2006, 6, 875. (e) Selvan, S. T.; Patra, P. K.; Ang, C. Y.; Ying, J. Y. Angew. Chem., Int. Ed. 2007, 46, 2448. (9) (a) Pellegrino, T.; Fiore, A.; Carlino, E.; Giannini, C.; Cozzoli, P. D.; Ciccarella, G.; Respaud, M.; Palmirotta, L.; Cingolani, R.; Manna, L. J. Am. Chem. Soc. 2006, 128, 6690. (b) Yu, H.; Chen, M.; Rice, P. M.; Wang, S. X.; White, R. L.; Sun, S. Nano Lett 2005, 5, 379. (c) Gu, H. W.; Yang, Z. M.; Gao, J. H.; Chang, C. K.; Xu, B. J. Am. Chem. Soc. 2005, 127, 34. (d) Choi, J. S.; Jun, Y. W.; Yeon, S. I.; Kim, H. C.; Shin, J. S.; Cheon, J. J. Am. Chem. Soc. 2006, 128, 15982.

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12735 (10) (a) Mokari, T.; Sztrum, C. G.; Salant, A.; Rabani, E.; Banin, U. Nat. Mater. 2005, 4, 855. (b) Yang, J.; Elim, H. I.; Zhang, Q.; Lee, J. Y.; Ji, W. J. Am. Chem. Soc. 2006, 128, 11921. (c) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2004, 43, 4774. (d) Gao, X.; Yu, L.; Maccuspie, R. I.; Matsui, H. AdV. Mater. 2005, 17, 23. (e) Zheng, Y.; Zheng, L.; Zhan, Y.; Lin, X.; Zheng, Q.; Wei, K. Inorg. Chem. 2007, 46, 6980. (11) (a) Kudera, S.; Carbone, L.; Casula, M. F.; Cingolani, R.; Falqui, A.; Snoeck, E.; Parak, W. J.; Manna, L. Nano Lett. 2005, 5, 445. (b) Halpert, J. E.; Porter, V. J.; Zimmer, J. P.; Bawendi, M. G. J. Am. Chem. Soc. 2006, 128, 12590. (c) Zhong, H.; Zhou, Y.; Yang, Y.; Yang, C.; Li, Y. J. Phys. Chem. C 2007, 111, 6538. (d) Ouyang, L.; Maher, K. N.; Yu, C. L.; McCarty, J.; Park, H. J. Am. Chem. Soc. 2007, 129, 133. (12) (a) Kim, J.; Lee, J. E.; Lee, J.; Jang, Y.; Kim, S. W.; An, K.; Yu, J. H.; Hyeon, T. Angew. Chem., Int. Ed. 2006, 45, 4789. (b) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (c) Bao, J.; Chen, W.; Liu, T.; Zhu, Y.; Jin, P.; Wang, L.; Liu, J.; Wei, Y.; Li, Y. ACS Nano 2007, 1, 293. (d) Liu, N.; Prall, B. S.; Klimov, V. I. J. Am. Chem. Soc. 2006, 128, 15362. (e) Stoeva, S. I.; Huo, F.; Lee, J. S.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 127, 15362. (f) Cassagneau, T.; Caruso, F. AdV. Mater. 2002, 14, 732. (g) Hong, X.; Li, J.; Wang, M.; Xu, J.; Guo, W.; Li, J. H.; Bai, Y. B.; Li, T. Chem. Mater. 2004, 16, 4022. (h) Insin, N.; Tracy, J. B.; Lee, H.; Zimmer, J. P.; Westervelt, R. M.; Bawendi, M. G. ACS Nano 2008, 2, 197. (13) Deng, H.; Li, X. L.; Peng, Q.; Wang, X.; Chen, J. P.; Li, Y. D. Angew. Chem., Int. Ed. 2005, 44, 2782. (14) (a) Zhong, L.-S.; Hu, J.-S.; Liang, H.-P.; Cao, A.-M.; Song, W.G.; Wan, L.-J. AdV. Mater. 2006, 18, 2426. (b) Zhong, L.-S.; Hu, J.-S.; Cao, A.-M.; Liu, Q.; Song, W.-G.; Wan, L.-J. Chem. Mater. 2007, 19, 1648. (15) Wang, H.; Zhang, J.-R.; Zhao, X.-N.; Xu, S.; Zhu, J.-J. Mater. Lett. 2002, 55, 253. (16) (a) Park, W.; King, J. S.; Neff, C. W.; Liddell, C.; Summers, C. J. Phys. Status Solidi B 2002, 229, 949. (b) Yin, L.-W.; Bando, Y.; Zhan, J.H.; Li, M.-S.; Golberg, D. AdV. Mater. 2005, 17, 1972. (17) Li, Y.; Li, X.; Yang, C.; Li, Y. J. Phys. Chem. B 2004, 108, 16002. (18) Yu, J. H.; Joo, J.; Park, H. M.; Baik, S.-I.; Kim, Y. W.; Kim, S. C.; Hyeon, T. J. Am. Chem. Soc. 2005, 127, 5662. (19) (a) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (b) Jun, Y. W.; Lee, S. M.; Kang, N. J.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 5150. (c) Xu, D. S.; Xu, Y. J.; Chem, D. P.; Guo, L. G.; Gui, L. L.; Tang, Y. Q. AdV. Mater. 2000, 12, 520. (d) Warner, J. H.; Tilley, R. D. AdV. Mater. 2005, 17, 2997. (e) Barrelet, C. J.; Wu, Y.; Bell, D. C.; Lieber, C. M. J. Am. Chem. Soc. 2003, 125, 11498. (20) (a) Fujiwara, H.; Hosokawa, H.; Murakoshi, K.; Wada, Y.; Yanagida, S. Langmuir 1998, 14, 5154. (b) Shiragami, T.; Ankyu, H.; Fukami, S.; Pac, C. J.; Yanagida, S.; Mori, H.; Fujita, H. J. Chem. Soc., Faraday Trans. 1992, 88, 1055. (c) Kudo, A.; Sekizawa, M. Chem. Commun. 2000, 1371. (d) Zhao, Q.; Xie, Y.; Zhang, Z.; Bai, X. Cryst. Growth Des. 2007, 7, 153. (21) Beydoun, D.; Amal, R.; Low, G. K.-C.; Mcevoy, S. J. Phys. Chem. B 2000, 104, 4387. (22) Hu, J.-S.; Ren, L.-L.; Guo, Y.,-G.; Liang, H.-P.; Cao, A.-M.; Wan, L.-J.; Bai, C.-L. Angew. Chem., Int. Ed. 2005, 44, 1269.

JP8035617

MS (M = Zn, Cd, Hg, Pb

Recommend Documents