J. Phys. Chem. C 2009, 113, 14119–14125
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One-Dimensional CdS/r-Fe2O3 and CdS/Fe3O4 Heterostructures: Epitaxial and Nonepitaxial Growth and Photocatalytic Activity Le Wang,† Hongwei Wei,† Yingju Fan,†,‡ Xin Gu,† and Jinhua Zhan*,† School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, P. R. China and School of Chemistry and Chemical Engineering, Jinan UniVersity, Jinan 250020, P. R. China ReceiVed: March 30, 2009; ReVised Manuscript ReceiVed: June 28, 2009
One-dimensional (1D) heterostructures of uniform CdS nanowires separately decorated with hematite (RFe2O3) nanoparticles or magnetite (Fe3O4) microspheres were successfully synthesized via a two-step solvothermal deposition method. Each CdS nanowire had a uniform diameter of 40-50 nm and a length ranging from several to several tens of micrometers. Quasicubic R-Fe2O3 nanoparticles, with edge lengths up to about 30 nm, and Fe3O4 microspheres, with diameters of about 200 nm, anchored on nanowires free from any surface pretreatment form 1D dimer-type CdS/R-Fe2O3 semiconductor heterostructures or CdS/Fe3O4 semiconductor magnetic functionally assembled heterostructures. It was also found that R-Fe2O3 nanoparticles with a smooth surface were well-crystallized, and Fe3O4 microspheres with a relatively rough surface showed a polycrystalline nature. The relationship between the crystal structures and effects of lattice mismatch on the formation of heterojunctions were systematically investigated. The magnetic and optical properties and photocatalytic activities of the as-obtained heterostructures were separately investigated. The 1D CdS/Fe3O4 nanostructures displayed ferromagnetic properties of the Fe3O4 microspheres, and the photoluminescence behaviors of CdS nanowires were conserved in both heterostructures. The enhanced photocatalytic activity under visible light (λ > 420 nm) was observed in 1D CdS/R-Fe2O3 heterostructures due to fast charge separation. 1. Introduction One-dimensional (1D) heterostructures are of particular interest because of their fascinating properties and potential applications in the field of nanoscale science.1 Considerable research efforts have been recently directed on the shape and compositional control of various heterostructures such as nanowires with superlattice structures,2 coaxial nanocables,3 biaxial or sandwichlike triaxial nanowires,4 and anisotropic (e.g., dimer-type and hierarchical composite materials) heterostructures.5 Established examples have demonstrated that the optical, electronic, magnetic, or chemical properties of these chemically distinct heterostructures have largely been enhanced or modified. The development of new heterostructures still remains a challenging subject because the key point of this work is to understand and control the nucleation of one phase on the surface of the other phase. Wurtzite CdS, a direct band gap semiconductor, is one of the first discovered semiconductors that has promising applications in various fields.6 In particular, CdS has a relatively narrow band gap energy of 2.42 eV (300 K), corresponding well to the spectrum of sunlight and, therefore, is regarded as one of the most attractive visible lightdriven photocatalysts. On account of this, various CdS nanomaterials especially 1D nanostructures, have been fabricated through various routes.7 Moreover, compositional control has been applied to CdS-included heterostructures, attempting to improve the efficiency connected with photogenerated electrons and holes or incorporate new properties.8 However, there was a few reports concerning the effects of the structural correlation on the formation of CdS-based heterostructures, which is * To whom correspondence should be addressed. E-mail: jhzhan@ sdu.edu.cn. † Shandong University. ‡ Jinan University.
Figure 1. (a) XRD pattern of a CdS nanowire sample before (bottom) and after (top) R-Fe2O3 growth. TEM images of (b) CdS nanowires and (c) nanowires loaded with R-Fe2O3 nanocrystals.
essential for the design and preparation of complicated nanocomposites.9
10.1021/jp902866b CCC: $40.75 2009 American Chemical Society Published on Web 07/15/2009
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Figure 2. EDS analysis spectra recorded from the heterojunction part of a single CdS/R-Fe2O3 nanostructure.
Iron oxides (R-, γ-Fe2O3, and Fe3O4) have been extensively studied in diverse fields, including catalysis,10 environmental protection,11 sensors,12 magnetic storage media,13 and clinical diagnosis and treatment.14 Recently, the synthesis of semiconductor iron oxide nanocomposites has achieved much progress.15 The results showed that the incorporation of magnetic Fe3O4, γ-Fe2O3, or R-Fe2O3 into the semiconductor makes it possible to enable the exploration of novel functionality combined with the inherent properties of semiconductor nanostructures. In this paper, we try to use preformed CdS nanowires as 1D nanoscale substrates for the growth of hematite (R-Fe2O3, Eg ) 2.1 eV) or magnetite (Fe3O4) nanoparticles via a solutionphase method, while no surface pretreatments were needed to introduce new surface functional groups or additional covalent and/or noncovalent interconnectivity in our experiments. The resulting 1D nanostructures of CdS nanowires decorated with hematite or magnetite were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and energy dispersive spectrometry (EDS) techniques. A detailed crystallographic relationship was investigated. Epitaxial and nonepitaxial growth processes were proposed on the basis of the experimental results. Magnetic and optical properties and photocatalytic activity under visible light (λ > 420 nm) of the as-prepared hybrid nanostructures were also demonstrated. 2. Experiment Section Sample Preparation. Preparation of CdS Nanowire. Uniform CdS nanowires were grown through a modified method.16 In a typical process, 1.124 g of Cd(S2CNEt2)2, prepared by precipitation from a stoichiometric mixture of NaS2CNEt2 and CdCl2 in water, was added to a Teflon-lined stainless steel autoclave with a capacity of 55 mL. Then, the autoclave was filled with 40 mL of ethylenediamine to about 70% of the total volume. The autoclave was maintained at 180 °C for 24 h and then allowed to cool to room temperature. A yellowish precipitate was collected and washed with absolute ethanol and distilled water to remove residue of organic solvents. The final products were dried in a vacuum at 70 °C for 6 h. Hematite r-Fe2O3 Nanoparticles Growth on CdS Nanowires. As a general procedure, 0.03 g of CdS nanowires were well-dispersed in 45 mL N,N-dimethylformamide (DMF) under sonication and then 0.16 g of Fe(NO3)3 · 9H2O and 0.30 g og poly(vinylpyrrolidone) (PVP, Mw ) 30 000) were added in sequence. The resulting mixture was loaded into a 55 mL Telfon-lined autoclave and maintained at 180 °C for 30 h. After
Figure 3. (a) Magnified TEM image of CdS/R-Fe2O3 heterostructure and HRTEM images of the marked regions in a panels b and c. Inset in panel b is the FFT result of the region marked with a circle.
the reaction was completed, the autoclave was cooled to room temperature naturally, and the resulting solid products were collected, washed with absolute ethanol and distilled water twice, and then dried in a vacuum at 70 °C for 6 h. Magnetite Fe3O4 Microspheres Growth on CdS Nanowires. A similar two-step solvothermal approach was used to grow 1D CdS/Fe3O4 nanocomposites. A total of 0.03 g of CdS nanowires were well-dispersed in 45 mL of ethylene glycol (EG) under sonication, and then 0.68 g of FeCl3 · 6H2O, 0.50 g of polyethylene glycol (PEG), and 1.8 g of CH3COONa were added in sequence. After stirring for 0.5 h at room temperature, the resulting mixture was loaded into autoclave and maintained at 200 °C for 16 h. The resulting solid products were collected, washed with absolute ethanol and distilled water 3 times, and then dried in a vacuum at 70 °C for 6 h. Sample Characterization. The crystal structure of the product was determined from the X-ray diffractometer (Bruker D8) with a graphite monochromator and Cu KR radiation (λ ) 1.5418 Å) in the range of 20-80° at room temperature, while the tube voltage and electric current were held at 40 kV and 20 mA. The morphology and microstructure of the products were determined by TEM (JEM-100CXII) with an accelerating voltage of 80 kV and high-resolution TEM (HR-TEM, JEOL2100) with an accelerating voltage of 200 kV equipped with an energy-dispersive X-ray spectrometer (EDS). X-ray photoelectron spectra (XPS) were measured with X-ray photoelectron spectroscopy XPS (ESCALAB 250). Magnetization measurements of the nanocomposites were performed with a Micromag 2900 instrument at room temperature under ambient atmosphere. The nanocomposite powders were pressed into thin slices. Room photoluminescence (PL) was performed on a Tianjin Gangdong WGY-10 spectrofluorimeter.
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Figure 4. (a) Simulated ED patterns separately correspond to the CdS [0001] zone axis or the R-Fe2O3 [001] zone axis. (b) Registration between crystal faces at the CdS(101j0)/R-Fe2O3(43j0) interface.
Figure 5. Schematic illustration and structural model of the formation of 1D dimer-type CdS/R-Fe2O3 or CdS/Fe3O4 heterostructures.
Photocatalytic Decomposition of Methylene Blue (MB). To evaluate the photocatalytic activity of the synthesized 1D nanocomposites, we carried out the degradation of MB in a jacketed quartz reactor filled with 50 mL of the test solution in the presence of the catalyst (50 mg) by using a 300 W Xe lamp with a cutoff filter (λ > 420 nm) as the light source. Prior to illumination, the suspension was stirred for 30 min in the dark to favor the adsorption of the pollutant onto the catalyst surface, followed by determination of the concentration of the pollutants as the initial concentration C0. The remaining concentration C of pollutants in the suspension at given intervals of irradiation was measured on a TU-1901 UV-vis spectrophotometer. 3. Results and Discussion Characterization and Epitaxial Growth of 1D CdS/rFe2O3 Semiconductor Heterostructures. Brown-red products are prepared through a simple two-step solvothermal method using a mixture of preformed CdS nanowires and Fe(NO3)3 · 9H2O as the source materials. The XRD patterns show neat CdS nanowires (bottom) and 1D CdS/R-Fe2O3 heterostructures (top) in Figure 1a. Those marked with “#” can be indexed to hexagonal wurtzite CdS (JCPDS card 41-1049), while additional peaks marked with “*” correspond well to rhombohedral hematite R-Fe2O3 (JCPDS 33-0664). No other crystalline impurities are detected. As seen in a low-magnification TEM image (Figure 1b), numerous preformed CdS nanowires with diameters of about 45 nm and lengths up to several tens of micrometers are uniformly distributed on the TEM copper grid. In Figure 1c, a representative TEM image of as-synthesized 1D
CdS/R-Fe2O3 heterostructures shows that the quasicubic R-Fe2O3 nanoparticles, whose edge lengths are up to about 30 nm, are attached to the CdS nanowires. Some isolated R-Fe2O3 nanoparticles or slight aggregates on the grid are observed in our TEM observations, which may attribute to intense sonication or intrinsic correlation of their structures. The composition was further determined and confirmed by EDS analysis. The EDS image (Figure 2) of the heterojunction part indicates that the nanocomposite is mainly composed of Cd, S, Fe, and O, with an approximate stoichiometry of CdS and Fe2O3 (C and Cu signals come from a TEM grid). A typical magnified TEM image of a CdS/R-Fe2O3 heterostructure (Figure 3a) reveals that the R-Fe2O3 nanoparticles can grow on multiple nucleation sites provided by an individual CdS nanowire. The high-resolution TEM (HRTEM) images (Figure 3b,c) from Figure 3a show both components are single crystalline with distinguished and coherent interfaces, indicating the formation of chemical bonds between them. The marked interplanar spacings of 0.34 nm corresponds well to that of the (0002) lattice planes of wurtzite CdS, while lattice fringes with interplanar spacing of 0.27 or 0.25 nm separately corresponding to the {104} and (110) planes of hematite R-Fe2O3 are observed. The fast Fourier transform (FFT) pattern (inset of Figure 3b) also confirms rhombohedral R-Fe2O3 along the [44j1j] zone axis. As implied by the above-mentioned HRTEM images and FFT analysis, the angle between the (0002) plane of CdS and (110) plane of R-Fe2O3 is ≈ 166°, while the (0002) plane of CdS is perpendicular to the interface between them. Furthermore, no legible electron diffraction (ED) patterns of a heterojunction part can be obtained, which is probably attributed to the interference of neighboring heterostructures and the difficulty of modulating proper orientation of the incident electron beam (i.e., [0001] zone axis of h-CdS).17 However, the structural homogeneity and possibility of preferential crystal growth on multiple nucleation sites around the c axis of CdS nanowire can be illuminated by the structural model. Figure 4a depicts the simulated ED patterns correspond to the CdS [0001] zone axis and R-Fe2O3 [001] zone axis, which reveals an epitaxial relationship may be established, i.e., (101j0)CdS//(43j0)R-Fe2O3, which is a small lattice mismatch (∼1.2%) between d101j0(CdS) plane and 3d43j0(R-Fe2O3). The theoretically calculated angle between the (43j0) and (110) lattice planes
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Figure 6. (a) XRD pattern of CdS nanowires (bottom) and 1D CdS/Fe3O4 nanocomposites (top). (b) TEM image of 1D CdS/Fe3O4 nanocomposites.
Figure 9. Luminescence spectra of CdS nanowires, 1D CdS/R-Fe2O3 and CdS/Fe3O4 nanocomposites. Figure 7. EDS analysis spectra recorded from the heterojunction part of a single CdS/Fe3O4 nanocomposite.
Figure 8. (a) Magnified TEM image of a single CdS/Fe3O4 heterostructure. (b) HRTEM images of the rectangular regions in panel a. (c) SAED pattern of the circular region in panel a.
TABLE 1: Measured Lattice Spacing, d,a Standard Atomic Spacing for Fe3O4, and Respective hkl Indices rings d (Å) Fe3O4 hkl
1
2
3
4
5
6
7
2.97 2.97 220
2.54 2.53 311
1.63 1.62 511
1.48 1.48 440
1.27 1.26 622
1.09 1.09 731
0.99 0.99 660
a
Lattice spacing and atomic spacing are based on rings in Figure 8c, and hkl indices are from the PDF database.
of R-Fe2O3 is 76.1° coincide with the measured angle between the (110) plane of R-Fe2O3 and interface in panels b and c of Figure 3. Also, the registration between crystal faces at the CdS(101j0)/ R-Fe2O3(43j0) interface is illustrated to ascertain the epitaxial
Figure 10. (a) Hysteresis loop. (b) Well-dispersed ethanol solutions of 1D CdS/Fe3O4 nanocomposites and (c) after applying a magnet to solution in panel b.
relationship in Figure 4b. As shown in the diagram, the CdS(101j0)planes and R-Fe2O3(43j0) planes are arranged with different symmetry (yellow and brown spheres denote S and Fe ions. respectively). Whereas, the 2D unit cell lattice parameters can be defined as aR-Fe2O3 ) 6.868 Å and bR-Fe2O3 ) 18.131 Å for the Fe3+ ions on the (43j0) plane of R-Fe2O3. The corresponding 2D lattice parameters for S2- anions on the (101j0)
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Figure 11. Absorption spectral changes of a MB aqueous solution (15 mg/L) degraded by (a) 1D dimer-type CdS/R-Fe2O3 or (b) CdS/Fe3O4 heterostructures with irradiation time t: 0, 1, 2, 3, 4, 5, 6, and 7 h. (c) Visible light photodegradation of MB under different conditions. (d) Scheme depicting charge separation on the surface of CdS.
plane of wurtzite CdS are 6.719 and 4.14 Å. When the two planes are brought together, the new unit cell contains one original 2D unit cell of Fe3+ and four of S2- along the b direction, and an effective lattice mismatch is defined as |(bR-Fe2O3 - 4bCdS)/bR-Fe2O3| ) 8.6%. Similarly, the mismatch along a is |(aR-Fe2O3 - aCdS)/aR-Fe2O3| ) 2.17%.9a Recent studies have shown that hexagonal nanostructures are enclosed with (011j0), (101j0), and (11j00) facets, which have equal chemical activity (Figure 5, left).18 As a result, R-Fe2O3 can deposit on multiple nucleation sites around the c axis of CdS nanowire generating nanocylinders (Figure 5, top right). More interestingly, we detect in Figure 2a that even some R-Fe2O3 nanoparticles grown on the same facets of CdS have nearly identical shapes and orientations. Characterization and Nonepitaxial Growth of 1D CdS/ Fe3O4 Semiconductor Magnetic Functionally Assembled Heterostructures. A similar two-step solvothermal approach was also used to grow 1D CdS/Fe3O4 heterostructures using FeCl3 · 6H2O instead. The resulting dark brown product was identified to be a composite material composed of wurtzite CdS and cubic magnetite Fe3O4 (JCPDS 19-0629) by XRD (Figure 6a). A low-magnification TEM image (Figure 6b) reveals that the as-grown products display heterodimer structures, in which Fe3O4 microspheres with diameters of about 200 nm anchor on preformed CdS nanowires. EDS analysis recorded from the heterojunction part as shown in Figure 7 and the inset confirms an Cd/S ratio of 1:1, while the Fe/O ratio is 3:4. The existence of Fe3O4 was further determined by XPS because of its high sensitivity to Fe2+ and Fe3+ cations. As shown in Figure S1 of the Supporting Information, the levels of Fe2p3/2 and Fe2p1/2 are 710.45 and 724.05 eV, respectively. The peaks broaden and shift to high binding energy for Fe3O4 due to the appearance of Fe2+(2p3/2) and Fe2+(2p1/2), which has already been reported and verified in the literature.19 A magnified TEM image shows that instead of the smooth surface of the R-Fe2O3 nanoparticles Fe3O4 microspheres resulting from the assembly of small nanoparticles have
relatively rough surfaces (Figure 8a).20 Figure 8b shows the HRTEM image of the marked area in Figure 8a. The marked interplanar spacings of 0.34 nm corresponds well to that of the (0002) lattice planes of wurtzite CdS. The lattice fringes with calculated interplanar spacings of 0.297 and 0.25 nm correspond to the (220) and (311) planes, respectively, typical for cubic Fe3O4 structures. When the whole particle was taken as the selected area for ED measurement, the diffraction pattern exhibited a remarkable polycrystalline feature shown in Figure 8c. Table 1 displays the measured lattice spacing based on the rings in the diffraction pattern and compares them to the theoretical lattice spacing for bulk Fe3O4 along with their respective hkl indices from the PDF database. The growth of Fe3O4 microspheres onto CdS nanowires may be attributed to a nonepitaxial process. Nonepitaxial growth was also observed for other composite systems, established examples comprise various associations of semiconductor nanostructures with semiconductor analogues,21 noble metals,22 transition metals, or magnetic nanomaterials.23 S2- on the surface of CdS nanowires could bond to Fe3+ (or Fe2+ after heating) strongly and appear to aid in the attaching process of the Fe3O4 nanoparticles on multiple nucleation sites of CdS nanowires. With the reaction proceeding (Figure 5, middle), the particle assembly was carried out in the solution to form larger microsperes, which is related to the aggregation-driven shapetransformation process to increase the size of the nanocrystal (Figure 5, bottom right).24 Optical and Magnetic Properties. Luminescence spectra of CdS nanowires, 1D CdS/R-Fe2O3 and CdS/Fe3O4 nanocomposites, are shown in Figure 9, basically exhibiting the fluorescence behavior of CdS with an obvious absorption shoulder around 520 nm (excitation at 300 nm). A slight blue shift for CdS/RFe2O3 and CdS/Fe3O4 nanocomposites is observed, which could be attributed to a change in surface state of the CdS nanowires due to immobilization.15c It should be noted that the emission maximum of nanocomposites is slightly asymmetric because of the joints between the iron oxides and CdS segments.15a,25
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Field-dependent magnetization measured at 298 K displays hysteretic behavior for 1D CdS/Fe3O4 nanocomposites, giving Ms, Mr, and Hc values of 0.056 emu/g, 0.009 emu/g, and 55.5 Oe, respectively, while the magnetic property is negligible for CdS/R-Fe2O3 heterostructures (Figure 10a). As indicated by the M-H curves, the ferromagnetic properties of Fe3O4 in the hybrid nanocomposites are conserved, which can be explained by the cooperative effects of the two types of particle interactions: magnetic dipole and exchange interactions.26 Simultaneously, the saturation magnetization was reduced largely in the presence of amorphous and nonmagnetic or weakly magnetic interfaces compared to that of the Fe3O4 nanocrystals reported in the literature.20,27 The decrease in Ms of the magnetic segments might also be attributed to the decrease in particle size and accompanied increase in surface area.27b Figure 10b displays the stability of ethanol solutions of Fe3O4 decorated 1D nanocomposites. It has been found that the nanocomposites can be easily dispersed in ethanol, and the dark brown solution can maintain stablilty for several months. As shown in Figure 10c, nanocomposites can be drawn to the sidewall from the solution by applying a magnet 5 cm away, eventually leaving a clear solution behind. Magnetic functionalizing of semiconductor nanostructures provides the possibility of aligning the nanowires exactly at the intended positions or orientations and then recycling them by an external magnetic force. Photocatalytic Activity. MB is commonly considered as a representative organic dye in textile effluents, which can easily be monitored by optical absorption spectroscopy.28 Herein, MB is chosen as model contaminants in the photocatalytic decomposition to evaluate the photocatalytic activities of the nanocomposites. Figure 11 separately shows the photocatalytic activities of a 1D dimer-type CdS/R-Fe2O3 semiconductor heterostructure and CdS/Fe3O4 semiconductor magnetic functionally assembled heterostructure evaluated by degradation of MB. The adsorption of MB at 664 nm almost disappeared, and the blue color of the solution completely vanished after the MB solution was irradiated for 7 h in the presence of a 1D CdS/RFe2O3 semiconductor heterostructure (99.9% decomposed). In the meantime, up to 62.22% of MB was degraded in the presence of a CdS/Fe3O4 semiconductor magnetic functionally assembled heterostructure, indicating the conserved activity of semiconductor CdS. As a comparison, the photodegradation with CdS nanowires (Figure 11c, curve 2), with commercial anatase TiO2 (curve 4) and photolysis in the absence of photocatalyst (curve 5) were also measured. The enhanced activity of the CdS/ R-Fe2O3 semiconductor heterostructure compared to that of the others may be due to a fast charge separation as a result of the difference in the positions of conduction bands between CdS and R-Fe2O3 (Figure 11d).29 4. Conclusions In summary, 1D dimer-type CdS/R-Fe2O3 and CdS/Fe3O4 heterostructures have been successfully fabricated by a mild solution-phase method. The relationship between the crystal structures and effects of lattice mismatch on the formation of 1D CdS/R-Fe2O3 heterojunctions were systematically investigated, while the growth of polycrystalline Fe3O4 microspheres onto CdS nanowires may be attributed to a nonepitaxial process. These results provide insights into heterostructural formation in large lattice mismatched systems and how different geometries may be synthesized. In addition, magnetic measurements showed that the magnetic properties of Fe3O4 microspheres were conserved after attachment onto the original CdS nanowires. The incorporation of magnetizm into the semiconductor makes
Wang et al. it possible for the CdS nanowires to be aligned exactly at the intended positions or in a certain direction and recycled by an external magnetic force. Also, PL behaviors and photocatalytic activities of both heterostructures were separately measured. Enhanced photocatalytic activity was observed in CdS/R-Fe2O3 heterostructures. These types of nanocomposites are likely to find potential applications in nanoscience because of the combination of ferromagnetic or catalytic domains with 1D semiconductor nanostructures in hybrid nanocomposites. Acknowledgment. Helpful discussions with Prof. Yitai Qian and financial support from National Natural Science Found of China (NSFC 20501014), Program for New Century Excellent Talents in University (NCET-06-0586), Key Project of Chinese Ministry of Education (109098, and National Basic Research Program of China (973 Program 2005CB623601, 2007CB936602) are gratefully acknowledged. Supporting Information Available: Fe2p XPS pattern of the 1D CdS/Fe3O4 heterostructure. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Chem. Soc. ReV. 2006, 35, 1195. (b) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Nature 2004, 430, 61. (c) Huang, Y.; Duan, X.; Wei, Q.; Cui, Y.; Lauhon, L. J.; Kim, K. H.; Lieber, C. M. Science 2001, 294, 1313. (2) (a) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (b) Solanki, R.; Huo, J.; Freeouf, J. L.; Miner, B. Appl. Phys. Lett. 2002, 81, 3864. (3) (a) Han, S.; Li, C.; Liu, Z. Q.; Lei, B.; Zhang, D. H.; Jin, W.; Liu, X. L.; Tang, T.; Zhou, C. W. Nano Lett. 2004, 4, 1241. (b) Lauhon, L. J.; Gudiksen, M. S.; Wang, D. L.; Lieber, C. M. Nature 2002, 420, 57. (c) Kong, X. Y.; Ding, Y.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 570. (4) (a) Hu, J.; Bando, Y.; Liu, Z.; Sekiguchi, T.; Golberg, D.; Zhan, J. J. Am. Chem. Soc. 2003, 125, 11306. (b) He, R. R.; Law, M.; Fan, R.; Kim, F.; Yang, P. D. Nano Lett. 2002, 2, 1109. (c) Zhan, J. H.; Bando, Y.; Hu, J. Q.; Sekiguchi, T.; Golberg, D. AdV. Mater. 2005, 17, 225. (5) (a) Zheng, Y.; Zheng, L.; Zhan, Y.; Lin, X.; Zheng, Q.; Wei, K. Inorg. Chem. 2007, 46, 6980. (b) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287. (c) Jung, Y.; Ko, D. K.; Agarwal, R. Nano Lett. 2007, 7, 264. (6) (a) Khan, M.; Bhardwaj, R.; Bhardwaj, C. Int. J. Hydrogen Energy 1988, 13, 7. (b) Agarwal, R.; Barrelet, C.; Lieber, C. M. Nano Lett. 2005, 5, 917. (c) Ma, R.; Dai, L.; Qin, G. Nano Lett. 2007, 7, 868. (d) Jie, J. S.; Zhang, W. J.; Jiang, Y.; Meng, X. M.; Li, Y. Q.; Lee, S. T. Nano Lett. 2006, 6, 1887. (e) Du, N.; Zhang, H.; Chen, B.; Chen, D.; Wu, J. B.; Yang, D. R. Nanotechnology 2007, 18, 115619. (7) (a) Chen, M.; Xie, Y.; Lu, J.; Xiong, Y.; Zhang, S.; Qian, Y.; Liu, X. J. Mater. Chem. 2002, 12, 748. (b) Barrelet, C. J.; Wu, Y.; Bell, D. C.; Lieber, C. M. J. Am. Chem. Soc. 2003, 125, 11498. (c) Ye, C.; Meng, G.; Wang, Y.; Jiang, Z.; Zhang, L. J. Phys. Chem. B. 2002, 106, 10338. (d) Wang, Y.; Meng, G.; Zhang, L.; Liang, C.; Zhang, J. Chem. Mater. 2002, 14, 1773. (8) (a) Chen, W. T.; Yang, T. T.; Hsu, Y. J. Chem. Mater. 2008, 20, 7204. (b) Datta, A.; Panda, S. K.; Chaudhuri, S. J. Phys. Chem. C 2007, 111, 17260. (c) Liu, X.; Fang, Z.; Zhang, X.; Zhang, W.; Wei, X. W.; Geng, B. Y. Cryst. Growth Des. 2009, 9, 197. (d) Shi, X. L.; Cao, M. S.; Yuan, J.; Zhao, Q. L.; Kang, Y. Q.; Fang, X. Y.; Chen, Y. J. Appl. Phys. Lett. 2008, 93, 183118. (9) (a) Kwon, K.; Shim, M. J. Am. Chem. Soc. 2005, 127, 10269. (b) McDaniel, H.; Shim, M. ACS Nano 2009, 3, 434. (10) (a) Zhang, J. L.; Wang, Y.; Ji, H.; Wei, Y. G.; Wu, N. Z.; Zuo, B. J.; Wang, Q. L. J. Catal. 2005, 229, 114. (b) Shekhah, O.; Ranke, W.; Schule, A.; Kolios, G.; Schlogl, R. Angew. Chem., Int. Ed. 2003, 42, 5760. (11) (a) Li, P.; Miser, D. E.; Rabiei, S.; Yadav, R. T.; Hajaligol, M. R. Appl. Catal., B 2003, 43, 151. (b) Wu, R. C.; Qu, J. H.; Chen, Y. S. Water Res. 2005, 39, 630. (12) Chen, J.; Xu, L. N.; Li, W. Y.; Gou, X. L. AdV. Mater. 2005, 17, 582. (13) Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. H. Nature. 2002, 420, 395. (14) Jordan, A.; Scholz, R.; Maier-Hauff, K.; Johannsen, M.; Wust, P.; Nadobny, J.; Schirra, H.; Schmidt, H.; Deger, S.; Loening, S.; Lanksch, W.; Felix, R. J. Magn. Magn. Mater. 2001, 225, 118.
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