Se Nanocomposites via

Mar 1, 2010 - Novel uniform straw-like Fe3O4/Se one-dimensional (1D) nanoplank bundles have been successfully prepared by a facile and green ...
0 downloads 0 Views 1MB Size
Download PDF
4846

J. Phys. Chem. C 2010, 114, 4846–4851

Novel Bifuncitonal One-Dimensional Fe3O4/Se Nanocomposites via Facile Green Synthesis Wensheng Lu,† Yuhua Shen,*,†,‡ Anjian Xie,*,†,‡ Xiuzhen Zhang,† and Wengui Chang† School of Chemistry and Chemical Engineering, Anhui UniVersity, Hefei 230039, P. R. China, and State Key Laboratory of Coordination Chemistry, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed: NoVember 21, 2009; ReVised Manuscript ReceiVed: January 28, 2010

Novel uniform straw-like Fe3O4/Se one-dimensional (1D) nanoplank bundles have been successfully prepared by a facile and green glucose-reducing method at gentle temperature for the first time. The width, thickness, and length of as-prepared Fe3O4/Se nanoplanks can be observed at the range of 300-400 nm, 50 nm, and 6-8 µm, respectively. Fe3O4 nanoparticles may act as seeds and catalytic agents for the formation of the bifunctional nanocomposites. The products bear both fluorescent and superparamagnetic properties. More importantly, quantum size effect in optical properties, which is reflected by a marked blue shift of the band gap and direct transitions relative to the values of bulk t-Se, can be observed. The mechanism of formation can be described as the linear aggregation and growth of Fe3O4/Se nanospheres and the self-assembly of 1D Fe3O4/Se nanoplanks. The strawlike Fe3O4/Se can be potentially converted to a series of bifunctional nanocomposites (e.g., Fe3O4/Ag2Se, Fe3O4/Bi2Se3, and Fe3O4/CdSe) with special morphology via the sacrificing template method. This method might provide a new, mild, and economical concept for the synthesis and construction of nanowires, nanobelts, nanorods, and some other kinds of 1D nanostructures. Moreover, this novel bifunctional nanocomposites will open up a potential and broad application in microelectronics, biology, and medicine. Introduction Synthesis of bifunctional or multifunctional nanostructures (e.g., core/shell nanostructures1,2 and exchange-coupled magnetic nanocomposites3,4) has been rapidly developed due to their advantageous properties, viz., enhanced optical, magnetic, and catalytic properties in contrast with their individual singlecomponent materials.5–9 Compared to two- (2D) or threedimensional (3D) nanostructures, one-dimensional (1D) nanocomposites are also very important thanks to their potential applications in constructing nanoscale electronic and optoelectronic devices via some approaches, e.g., vapor-liquid-solid growth,10,11 surfactant-assisted technique,12,13 and hard templatelimited approach.14,15 However, investigations on multifunctional 1D nanocomposities have been very limited until now. Therefore, it is necessary to synthesize novel multifunctional 1D nanostructured materials so as to explore their applications in chemo- or biosensing. As is well-known, selenium has been successfully used in photocells, solar cells, rectifiers, photographic exposure meters, and xerography due to its photoelectrical and semiconductor properties.16 In addition, selenium also has a high reactivity toward other chemicals to convert itself into a series of functional materials such as Ag2Se, Bi2Se3, Se@Ag2Se, and [email protected]–20 Selenium exhibits a remarkable structural diversity in the bulk with trigonal (t-Se, helical chains), monoclinic (m-Se, Se8 rings), rhombohedral (r-Se, Se6 rings), and amorphous forms (R-Se, disordered chains), and t-Se is the most stable phase of all Se allotropes. 1D selenium nanostructures have been fabricated through a few approaches, such as laser ablation,21 solution-phase approach,22 vapor-phase * To whom correspondence should be addressed. E-mail: s_yuhua@ 163.com (Y.S.); [email protected] (A.X.). Tel.: +86-551-5108090. Fax: +86551-5108702 . † Anhui University. ‡ Nanjing University.

growth,23 electrochemical synthesis,24 photothermally assisted solution phase,25 ultrasonic,26 hydrothermal, or solvothermal methods,27 and micelle-mediated synthesis.28 Developing a straightforward and mild method for low-dimensional Se nanostructures is still important and challengeable. Meanwhile, magnetic nanoparticles with properties of superparamagnetism in addition to their low toxicity and biocompatibility have been extensively studied because of their potential technological applications, including magnetic storage, magnetic ink printing, microwave absorption, biosensors, bioseparation, in vivo drug delivery, immunomagnetic array, magnetic resonance imaging contrast agents, hyperthermia treatment of cancer, and so forth.29–35 Bifunctional nanocomposites, e.g., FePtCdS heterodimer nanoparticles,36 γ-Fe2O3/MS (M ) Zn, Cd, or Hg) heterojunctions,37 FePt-ZnS nanosponges,38 Fe2O3 bead-CdSe/ZnS quantum dot nanocomposites,39 and Co@CdSe core-shell nanocomposites,40 have been widely investigated because of their combined fluorescent and magnetic properties. Herein, we have demonstrated a facile and green route for the preparation of novel uniform straw-like bifunctional Fe3O4/ Se 1D nanoplank bundles using R-D-glucose as the reductant without any additional cross-linking agent and dispersant in aqueous solution for the first time. Using as-prepared Fe3O4 nanoparticles as seeds, SeO32- (Se(IV)) anions adsorbed onto the surface of Fe3O4 nanoparticles are reduced to Se(0) by R-Dglucose at 80 °C and ambient pressure. Two attractive features of Fe3O4/Se bifunctional nanocomposites, i.e., fluorescence and superparamagnetism, could greatly enhance their potential and broad applications in microelectronics, biology, and medicine. Interestingly, we have first found that Fe3O4 nanoparticles may also have the effect of catalyst in the reaction that SeO32(Se(IV)) anions are reduced to Se(0) by R-D-glucose, for in the control experiment SeO32- (Se(IV)) anions cannot be reduced to Se(0) by R-D-glucose without addition of Fe3O4 nanopar-

10.1021/jp911073k  2010 American Chemical Society Published on Web 03/01/2010

1D Fe3O4/Se Nanocomposites via Facile Green Synthesis

J. Phys. Chem. C, Vol. 114, No. 11, 2010 4847

ticles under the same reaction conditions. No similar reported work exists in the literature to the best of our knowledge. Experimental Section Materials. All chemicals were analytical reagent grade and used without further purification, and all solutions were prepared with doubly distilled water. Ferric chloride hexahydrate (FeCl3 · 6H2O), R-D-glucose, NH3 · H2O (25%, w/w), and SeO2 were all purchased from Shanghai Reagent Co., China. Preparation of Fe3O4 Magnetic Nanoparticles. Fe3O4 magnetic nanoparticles were synthesized without any additional stabilizer and dispersant according to the following procedure. First, 5 mL of 0.05 M glucose solution was added into 5 mL of aqueous solution containing 0.3 M Fe3+ under mechanical stirring for a period of time at 80 °C. Second, the mixture solution was added dropwise into 50 mL of 1 M NH3 · H2O solution at a constant of pH 10 under vigorous mechanical stirring (2000 rpm) for 30 min at 60 °C. The color of the suspension turned into black immediately. The suspension was cooled to room temperature, and the precipitated powders were separated from the suspension and washed three times with doubly distilled water by applying an external magnetic field. The supernatant solution was removed from the precipitate after decantation. Preparation of Bifunctional 1D Fe3O4/Se Nanocomposites. The preparation is very easy and simple without any additional cross-linking agent and dispersant in aqueous solution. In a typical procedure, the Fe3O4 nanoparticles obtained above and 1.5 mmol of SeO2 were added into 40 mL of doubly distilled water under magnetic stirring for 10 min at room temperature so that the Fe3O4 nanoparticles were capable of adsorbing SeO32- anions onto their surface.41 Then 10 mL of aqueous solution containing 4.5 mmol of glucose was added dropwise into the suspension above under magnetic stirring at 80 °C, and the reaction was allowed to proceed for 24 h with constant magnetic stirring. The color of the suspension was turned from black into ochreous. After cooling to room temperature, the ochreous product was collected. The products were separated, and washed three times with doubly distilled water and several times with ethanol by applying an external magnetic field. Finally, the products obtained were dried in a vacuum oven at 60 °C for 12 h, and stored in a stoppered bottle for further use. Characterization. The prepared samples were characterized as follows. Powder X-ray diffraction (XRD) was performed on a MAP18XAHF instrument with an X-ray diffractometer using Cu KR radiation (λ ) 1.54056 Å) at a scan rate of 4.0000 deg/ min to determine the crystalline phase, and the accelerating voltage and applied current were 40 kV and 30 mA, respectively. The Raman scattering spectrum was collected at ambient temperature on a Spex 1403 Raman spectrometer with an argon ion laser at an excitation wavelength of 514.5 nm. X-ray photoelectron spectrometry (XPS) analysis was carried out on an ESCALAB MKII instrument at a pressure greater than 10-6 Pa. The general scan C1s, O1s, Fe2p, and Se3d core level spectra were recorded with Mg KR radiation as the exciting source (photon energy ) 1253.6 eV). Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and highresolution TEM (HRTEM) were obtained on JEM model 100SX and 2010 electron microscopes (Japan Electron Co.) and operated at accelerating voltages of 80 and 200 kV, respectively. Field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectrometer (EDS) analysis were taken on an FE-SEM (Hitachi S-4800). UV-visible (UV-vis) and fluorescence spectra were recorded on a Perkin-Elmer Lambda

Figure 1. Digital photographs of reaction solution: (a) before the reaction; (b) after the reaction for 24 h at 80 °C; (c) the products are attracted to the wall of the vial when a magnet is present; (d) the control experiment without the addition of the Fe3O4 nanoparticles for 24 h at 80 °C.

900 UV/vis/NIR spectrometer and a Perkin-Elmer luminescence spectrometer (LS55), respectively. The thermogravimetric (TGA) analysis and differential scanning calorimetric analysis (DSC) were carried out on a TGA/DSC apparatus (Pyris-1, PerkinElmer) at a heating rate of 10 °C/min in flowing high purity nitrogen gas with 20 mL/min. The magnetization of the samples was measured with a Quantum Design superconducting quantum interference device (SQUID) magnetometer (MPMS-XL-7) at 300 K. For zero-field-cooled (ZFC) and field-cooled (FC) experiments, the sample was cooled, and a constant field was applied during the warm scan. Results and Discussion During an attempt to synthesize bifunctional Fe3O4/Se 1D nanoplank bundles by self-assembly pattern, it has been found that Fe3O4 nanoparticles may play the dual role of seeds and catalytic agents for the formation of the bifunctional nanocomposites. First, SeO32- (Se(IV)) anions can be adsorbed onto the surface of Fe3O4 nanoparticles41 and reduced to Se(0) by R-Dglucose at 80 °C and ambient pressure; subsequently, the selenium atoms directly deposited onto the surface of Fe3O4 nanoparticles. Second, Fe3O4 nanoparticles may act as catalyst in the reduction reaction of SeO32- (Se(IV)) to Se(0) atoms in that SeO32- (Se(IV)) anions cannot be reduced to Se(0) by R-Dglucose in the control experiment without addition of Fe3O4 nanoparticles under the same reaction condition (shown in Figure 1). The crystalline structure of the products was characterized by XRD. Figure 2a shows XRD patterns of the as-prepared Fe3O4 nanoparticles (Figure 2a(i)) and the Fe3O4/Se composites (Figure 2a(ii)). As shown in Figure 2a(i), the diffraction peaks at 2θ ) 30.4°, 35.5°, 43.2°, 57.2°, and 62.9° can be well indexed to the (220), (311), (400), (511), and (440) planes of the inverse cubic spinel structure of Fe3O4 (JCPDS card no. 75-1610), respectively, according to the reflection peak positions and relative intensities, which confirms the formation of the Fe3O4 nanoparticles. Meanwhile, all of the Bragg reflection peaks are relatively broad because of the extremely small dimensions of the Fe3O4 nanoparticles (Figure 2a(i)). The average size of the Fe3O4 nanoparticles obtained from the Scherrer equation is about 12.3 nm. Figure 2a(ii) contains two sets of the diffraction peaks. One set of peaks at 2θ ) 10.6°, 13.0°, 19.5°, 21.0°, 24.9°, and 29.0° can be well indexed to (100), (101), (102), (111), (112), and (210) planes of the hexagonal phase of selenium (JCPDS card no. 01-0848), respectively, with the lattice parameters of a ) 4.34 Å and c ) 4.95 Å. In comparison with the standard pattern, the intensity of the (101) diffraction peak is enhanced, indicating that the Fe3O4/Se composites tend to preferentially

4848

J. Phys. Chem. C, Vol. 114, No. 11, 2010

Lu et al.

Figure 2. (a) XRD patterns of the as-prepared products: (i) the assynthesized Fe3O4 nanoparticles and (ii) the Fe3O4/Se nanocomposites obtained, and (b) Raman scattering spectrum of the as-prepared Fe3O4/ Se nanocomposites.

grow along the [101] direction. The other set can correspond to the Fe3O4 phase, in which the low intensity of the Fe3O4 reflection peaks at the (311) and (440) planes could be observed. On the basis of the fact that the other diffraction peaks from Fe3O4 are not observed in Figure 2a(ii), Fe3O4 could be well covered by Se, which indicates the formation of Fe3O4/Se composites. Raman scattering spectrum of the as-prepared Fe3O4/Se in Figure 2b shows that the intensive resonance peak at 236.6 cm-1 is attributed to the vibration of the Se helical chain, further confirming the trigonal phase of Se in Fe3O4/Se products. Both the XRD and Raman scattering results confirm that the Fe3O4/Se composites consisting of well-crystallized spinel Fe3O4 and t-Se can be obtained by using glucose as reductant in aqueous solution at 80 °C. Furthermore, chemical composition of the as-prepared composites can be determined with EDS analysis. As seen from the EDS spectrum (Figure 3a), Fe, O, and Se signals also confirm that the products consist of Fe3O4 and Se, which is agreement with the XRD result (Figure 2a(ii)). The surface composition can be further proven through XPS. It has been found that the strong peak at 56.0 eV (Figure 3b) corresponds to Se3d binding energy. In addition, the levels of Fe2p3/2 and Fe2p1/2 are 711.2 and 724.7 eV (Figure 3c), respectively, which suggest that the sample indeed contains Fe3O4 components. To sum up, Fe3O4/t-Se nanocomposites can be yielded in the present case.

Figure 3. EDS spectrum (a) and XPS spectra [(b) Se3d level; (c) Fe2p level)] of the as-prepared Fe3O4/Se nanocomposites.

The TEM image in Figure 4a shows that the average size of as-synthesized Fe3O4 magnetite nanoparticles is ca. 12.5 nm with narrow size distribution, which is consistent with the size (12.3 nm) obtained from the XRD of the same sample. Polycrystalline structure of Fe3O4 nanoparticles can be observed from electron diffraction pattern (Figure 1a inset), which is consistent with the XRD result (Figure 2a(i)). The HRTEM image in Figure 4b indicates that the sample is similar to well dispersed Fe3O4 nanoparticles, which are possibly ascribable to gluconic acid (the oxidative product of glucose) playing the role of stabilizer and dispering agent. The inset in Figure 4b shows that the nanoparticles are structurally uniform with a lattice fringe spacing ca. 0.25 nm, which corresponds well with the values of 0.253 nm of the (311) lattice plane of the cubic Fe3O4 (JCPDS card no. 75-1610). The morphology and microstructure of the Fe3O4/Se products were investigated using FE-SEM. Figure 4c and Figure 4d show that a large number of 1D nanoplank bundles appear as a sheaf of straw tied at the center of bundles so as to form the strawlike Fe3O4/Se 1D nanocomposites. Moreover, as observed from Figure 4e,f, each as-prepared strawlike Fe3O4/Se is composed of numerous, well-defined, and closely packed 1D nanoplanks with widths of 300-400 nm, thickness of 50 nm, and lengths up to 6-8 µm.

1D Fe3O4/Se Nanocomposites via Facile Green Synthesis

J. Phys. Chem. C, Vol. 114, No. 11, 2010 4849

Figure 4. TEM image (a) and HRTEM image (b) of the as-synthesized Fe3O4 nanoparticles. FE-SEM images (c,d,e) and TEM image (f) of the as-prepared strawlike Fe3O4/Se 1D nanoplank bundles.

Figure 6. (a) ZFC and FC curves of the as-prepared Fe3O4/Se samples measured with the field of 100 Oe, and (b) the magnetic hysteresis loop at 300 K. The inset panel b is a magnified picture of panel b at low field at 300 K.

Figure 5. TGA (a) and DSC (b) curves for the Fe3O4/Se samples in the temperature range 55-1000 °C.

Figure 5 shows the results of simultaneous thermal analysis (TGA and DSC) on the Fe3O4/Se samples in the range of 55-1000 °C. As seen from Figure 5a, a significant mass loss can be observed between 290 and 430 °C, which can be ascribed to desorption of the Se species from the surface of Fe3O4 nanoparticles. When the temperature is elevated to 700 °C, the mass loss could be increased to maximum value (68.2 wt %). On the other hand, the DSC curve (Figure 5b) indicates an endothermic peak at 214 °C, which is close to the melting point of bulk t-Se (∼217 °C), but far from that of m-Se (∼175 °C) and R-Se (∼70 °C). Apparently, this result also supports the idea that the products indeed contain t-Se. The magnetic properties of the same sample were measured by SQUID. ZFC and FC magnetization data measured with a field of 100 Oe in the range of 2-300 K are shown in Figure 6a. Accompanied with increase of the temperature in the ZFC measurement, more and more particles could easily reorient their magnetization with the external field, the total magnetization increased, and it reached the maximum at the blocking tem-

perature (shown in Figure 6a). In the FC measurement, the magnetization monotonically decreased when the temperature increased. Both ZFC and FC magnetization data exhibit the same trend above the blocking temperature; that is, the magnetization decreases with increasing temperature; however, the data significantly diverge if the temperature is below the blocking temperature (Figure 6a). Such behavior is characteristic of superparamagnetism. The Fe3O4 in the as-prepared Fe3O4/Se samples exhibit superparamagnetism and ferromagnetism above and below the blocking temperature, respectively. Figure 6b shows the magnetization versus the magnetic field at 300 K by cycling the field between -10 K Oe and 10 K Oe. The magnetic hysteresis loop (Figure 6b) shows a saturation magnetization of 10.2 emu/g, and the absence of coercivity and remanence indicates that the as-prepared Fe3O4/Se products have superparamagnetic properties at room temperature. The larger magnetization of the as-prepared Fe3O4/Se nanocomposites should facilitate biosensors, bioseparation applications, and so forth. To understand the growth mechanism of the strawlike Fe3O4/ Se 1D nanoplank bundles, the intermediate products at various stages of the reaction are performed for further analysis. Parts a-d of Figure 7 show the TEM images of the products obtained after reaction for 2, 5, 7, and 12 h, respectively. These images display the evolution of Fe3O4/Se from nanospheres into 1D nanoplanks and the strawlike morphology by prolonging the

4850

J. Phys. Chem. C, Vol. 114, No. 11, 2010

Lu et al.

Figure 7. Time-dependent morphology evolution of the Fe3O4/Se nanocomposites grown at 80 °C: (a) 2 h, (b) 5 h, (c) 7 h, (d) 12 h.

Figure 8. Schematic depicting possible growth routes for the asprepared strawlike Fe3O4/Se 1D nanoplank bundles.

reaction time at 80 °C. The growth process could be involved with the linear aggregation and growth of Fe3O4/Se nanospheres and the self-assembly of 1D Fe3O4/Se nanoplanks. After reaction for 2 h, the TEM image (Figure 7a) reveals the formation of Fe3O4/Se spherical nanoparticles with diameters of 30-50 nm. The selenium in Fe3O4/Se spherical nanoparticles is R-Se (Supporting Information, Figure S). With an increase in time to 5 h, 1D nanoplanks with length of 1-4 µm can be formed (Figure 7b). All Se in Fe3O4/Se 1D nanoplanks has completely been converted into t-Se (Figure S). As observed from Figure 7c, when the reaction has proceeded for 7 h, 1D nanoplanks can grow continually along their longitudinal axis, and aggregate with lengths up to 3-6 µm. When the time is prolonged to 12 h, the strawlike Fe3O4/Se composites with lengths up to 6-8 µm and some 1D nanoplanks can finally be formed by selfassembly (Figure 7d). The sample obtained at 24 h (Figure 4c) indicates that strawlike Fe3O4/Se 1D nanocomposites have been produced on a large scale. According to the analysis of these TEM images, the possible growth mechanism can be described diagrammatically in Figure 8. However, further comprehensive investigations on the formation mechanism for the strawlike Fe3O4/Se 1D nanoplank bundles are still under way. It is well-known that t-Se is a p-type, extrinsic semiconductor with an indirect band gap of 1.6 eV.4 We have performed optical studies on the as-prepared Fe3O4/Se composites to evaluate the energy band structure and other potentially optical properties of the products. Figure 9 shows the UV-vis and fluorescence spectra of a series of Fe3O4/Se nanocomposites with different morphologies. Curve 1 in Figure 9a shows that Fe3O4/Se spherical nanoparticles obtained after the reaction for 2 h have

Figure 9. UV-vis (a) and fluorescence (b) spectra of a series of Fe3O4/ Se nanocomposites with different morphologies (curve 1: Fe3O4/Se spherical nanoparticles; curve 2: Fe3O4/Se 1D nanoplanks; curve 3: the strawlike Fe3O4/Se 1D nanocomposites). Plot c shows the band gap of the strawlike Fe3O4/Se 1D nanocomposites determined by plotting the absorption coefficient (R) against the excitation energy (hν, eV). All measurements were carried out at room temperature.

a absorption peak at 569 nm (2.18 eV), corresponding to the absorption of R-Se in Fe3O4/Se spherical nanoparticles. However, there are two peaks at 370 nm (3.35 eV) and 250 nm (4.96 eV) in curves 2 and 3 (Figure 9a), owing to the absorption of t-Se in the Fe3O4/Se 1D nanoplanks and the strawlike Fe3O4/Se gained after the reaction for 5 h and 24 h, respectively. Compared with curve 1, the blue shift of the absorption peak in curves 2 and 3 may result from the change of morphology and crystal transformation of selenium as previously reported.16,42 There is no peak shift in curves 2 and 3 at the reaction time of 5 and 24 h, which indicates that the peak change be mainly caused by crystal transformation of selenium. In the fluorescence spectra of Fe3O4/Se nanocomposites (Figure 9b), there is a weak peak at 694 nm under an excitation wavelength of 462 nm (the curve 1), resulting from the emission of R-Se in Fe3O4/Se

1D Fe3O4/Se Nanocomposites via Facile Green Synthesis spherical nanoparticles. A strong peak at 470 nm can be observed under an excitation wavelength of 265 nm from curves 2 and 3, respectively, originating from the emission of t-Se in Fe3O4/Se 1D nanoplanks and the strawlike Fe3O4/Se. In comparsion with curve 1, a significant blue shift, enhancement, and broadening of the emission peak can be seen in curves 2 and 3. The fluorescence intensity in curve 3 is also stronger than that in curve 2 because the emission peak of a semiconducting nanoparticle is strongly dependent on its size and shape.8 Through the discussion mentioned above, the as-prepared Fe3O4/ Se 1D nanoplank bundles could be used as fluorescent probes for visualizing biological processes in vitro. Figure 9c shows the dependence of the absorption coefficient (R) on photon energy, hν, with the absorption edge derived from the linear intercept (at R ) 0). The result reveals that the band gap (1.81 eV) of t-Se in the as-prepared Fe3O4/Se samples is larger than the value (1.6 eV) for the bulk material. The blue shifts of the band gap and direct transitions indicate that quantum confinement effects can be obtained in the products. Conclusion In summary, we have reported the preparation of novel uniform strawlike Fe3O4/Se 1D nanoplank bundles by a facile and green glucose-reducing method at gentle temperature in aqueous solution for the first time, in which the as-synthesized Fe3O4 nanoparticles play the dual role of seeds and catalytic agents. The as-prepared Fe3O4/Se products bear both fluorescent and superparamagnetic properties, and the optical absorption band gap and band energy of Se in Fe3O4/Se composites exhibit a marked blue shift compared with the bulk material due to quantum size effects. Further research on the preparation of other bifunctional nanocomposites (such as Fe3O4/Ag2Se, Fe3O4/ Bi2Se3, and Fe3O4/CdSe) and their potential application in microelectronics, biology, and medicine are underway. Acknowledgment. This work is supported by the National Science Foundation of China (20871001, 20671001, and 20731001), the Research Foundation for the Doctoral Program of Higher Education of China (20070357002), the Important Project of Anhui Provincial Education Department (ZD2007004-1 and KJ2008A024), and the Anhui Key Laboratory of Functional Material of Inorganic Chemistry. Supporting Information Available: XRD patterns of Fe3O4/ Se nanocomposites with different morphologies. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019. (2) Chen, M.; Kim, Y. N.; Lee, H. M.; Li, C.; Cho, S. O. J. Phys. Chem. C 2008, 112, 8870. (3) Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. Nature 2002, 420, 395. (4) Zeng, H.; Li, J.; Wang, Z. L.; Liu, J. P.; Sun, S. Nano Lett. 2004, 4, 187. (5) Camargo, P. H. C.; Xiong, Y.; Ji, L.; Zuo, J. M.; Xia, Y. J. Am. Chem. Soc. 2007, 129, 15452.

J. Phys. Chem. C, Vol. 114, No. 11, 2010 4851 (6) 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. (7) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Chem. Soc. ReV. 2006, 35, 1195. (8) Gao, J. H.; Zhang, B.; Gao, Y.; Pan, Y.; Zhang, X. X.; Xu, B. J. Am. Chem. Soc. 2007, 129, 11928. (9) Yang, L.; Shen, Y.; Xie, A.; Liang, J.; Zhu, J.; Chen, L. Eur. J. Inorg. Chem. 2007, 8, 1128. (10) Su, J.; Cui, G.; Gherasimova, M.; Tsukamoto, H.; Han, J.; Ciuparu, D.; Lim, S.; Pfefferle, L.; He, Y.; Nurmikko, A. V.; Broadbridge, C.; Lehman, A. Appl. Phys. Lett. 2005, 86, 013105. (11) Wu, Z. H.; Sun, M.; Mei, X. Y.; Ruda, H. E. Appl. Phys. Lett. 2004, 85, 657. (12) Vantomme, A.; Yuan, Z. Y.; Du, G. H.; Su, B. L. Langmuir 2005, 21, 1132. (13) Busbee, B. D.; Obare, S. O.; Murphy, C. J. AdV. Mater. 2003, 15, 414. (14) Ding, J. X.; Zapien, J. A.; Chen, W. W.; Lifshitz, Y.; Lee, S. T.; Meng, X. M. Appl. Phys. Lett. 2004, 85, 2361. (15) Guo, Y. G.; Li, C. J.; Wan, L. J.; Chen, D. M.; Wang, C. R.; Bai, C. L.; Wang, Y. G. AdV. Funct. Mater. 2003, 13, 626. (16) Gates, B.; Mayers, B.; Cattle, B.; Xia, Y. N. AdV. Funct. Mater. 2002, 12, 219. (17) Gates, B.; Wu, Y. Y.; Yin, Y. D.; Yang, P. D.; Xia, Y. N. J. Am. Chem. Soc. 2001, 123, 11500. (18) Jiang, X. C.; Mayers, B.; Herricks, T.; Xia, Y. N. AdV. Mater. 2003, 15, 1740. (19) Jeong, U.; Kim, J.; Li, Z.-Y.; Xia, Y. Nano Lett. 2005, 5, 937. (20) Jeong, U.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 3099. (21) Jiang, Z. Y.; Xie, Z. X.; Xie, S. Y.; Zhang, X. H.; Huang, R. B.; Zheng, L. S. Chem. Phys. Lett. 2003, 368, 425. (22) Chen, Z.; Shen, Y.; Xie, A.; Zhu, J.; Wu, Z.; Huang, F. Cryst. Growth Des. 2009, 9, 3. (23) Cao, X. B.; Xie, Y.; Zhang, S. Y.; Li, F. Q. AdV. Mater. 2004, 16, 649. (24) Zhang, S. Y.; Liu, Y.; Ma, X.; Chen, H. Y. J. Phys. Chem. B 2006, 110, 9041. (25) Zhang, B.; Dai, W.; Ye, X. C.; Zuo, F.; Xie, Y. Angew. Chem., Int. Ed. 2006, 45, 2571. (26) Li, X. M.; Li, Y.; Li, S. Q.; Zhou, W. W.; Chu, H. B.; Chen, W.; Li, I.; Tang, Z. K. Cryst. Growth Des. 2005, 5, 911. (27) Lu, Q. Y.; Gao, F.; Komarneni., S. Chem. Mater. 2006, 18, 159. (28) Ma, Y. R.; Qi, L. M.; Ma, J. M.; Cheng, H. M. AdV. Mater. 2004, 16, 1023. (29) Sophie, L.; Delphine, F.; Marc, P.; Alain, R.; Caroline, R.; Luce, V. E.; Robert, N. M. Chem. ReV. 2008, 108, 2064. (30) Sunderland, C. J.; Steiert, M.; Talmadge, J. E.; Derfus, A. M.; Barry, S. E. Drug DeV. Res. 2006, 67, 70. (31) Ito, A.; Shinkai, M.; Honda, H.; Kobayashi, T. J. Biosci. Bioeng. 2005, 100, 1. (32) Choi, H.; Choi, S. R.; Zhou, R.; Kung, H. F.; Chen, I.-W. Acad. Radiol. 2004, 11, 996. (33) Lin, S. P.; Brown, J. J. Magn. Reson. Imaging 2007, 25, 884. (34) Brahler, M.; Georgieva, R.; Buske, N.; Muller, A.; Muller, S.; Pinkernelle, J.; Teichgraber, U.; Voigt, A.; Baumler, H. Nano Lett. 2006, 6, 2505. (35) Yang, H. H.; Zhang, S. Q.; Chen, X. L.; Zhuang, Z. X.; Xu, J. G.; Wang, X. R. Anal. Chem. 2004, 76, 1316. (36) Gu, H. W.; Zheng, R. K.; Zhang, X. X.; Xu, B. J. Am. Chem. Soc. 2004, 126, 5664. (37) Kwon, K. W.; Shim, M. J. Am. Chem. Soc. 2005, 127, 10269. (38) Gu, H. W.; Zheng, R. K.; Liu, H.; Zhang, X. X.; Xu, B. Small 2005, 1, 402. (39) Wang, D. S.; He, J. B.; Rosenzweig, N.; Rosenzweig, Z. Nano Lett. 2004, 4, 409. (40) Kim, H.; Achermann, M.; Balet, L. P.; Hollingsworth, J. A.; Klimov, V. I. J. Am. Chem. Soc. 2005, 127, 544. (41) Loyo, R. L. D.; Nikitenko, S. I.; Scheinost, A. C.; Simonoff, M. EnViron. Sci. Technol. 2008, 42, 2451. (42) Mott, N. F.; Davis, E. A. Electronic Processes in Non-Crystalline Semiconductors; Clarendon Press: Oxford, 1979.

JP911073K