Well-Aligned CoPt Hollow Nanochains Synthesized in Water at Room

Article pubs.acs.org/JPCC

Well-Aligned CoPt Hollow Nanochains Synthesized in Water at Room Temperature Qian Sun,† Shouguo Wang,*,‡ and Rongming Wang*,† †

Key Laboratory of Micro-nano Measurement-Manipulation and Physics (Ministry of Education) and Department of Physics, Beijing University of Aeronautics and Astronautics, Beijing 100191, China ‡ State Key Laboratory of Magnetism, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: Synthesis and manipulation of advanced bimetallic nanomaterials via a green and low-cost wet chemical route are of great importance for the industrialization potential. Materials design integrating the synthesis of nanomaterials through an environmentally benign route with a simple manipulation method is a challenge. The CoPt hollow nanochains have been successfully synthesized in aqueous solution with shell thickness of about 5 nm and tunable length from 300 nm to 2 μm. The as-prepared CoPt hollow nanochains can be easily aligned by the external magnetic fields and can be attached onto substrates, such as silicon wafer. The synthesis strategy is characterized by room temperature reaction (300 K), low cost, and utilization of facile reagents (water as solvent). Growth kinetics investigation shows the magnetostatic interactions between Co clusters together with the spontaneous galvanic replacement between Co clusters and Pt ions are indispensable for the formation of aligned hollow nanochains. Magnetic measurements indicate that the shape anisotropy of 1D aligned nanochains plays a dominant role on the good controllable behavior.

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INTRODUCTION In materials science, great interest has been attracted to bimetallic nanomaterials because this new type of materials exhibit novel properties distinct from those of pure element nanostructures or the corresponding bulk matters.1−5 Enhancements in magnetic and/or catalytic properties benefitting from the synergistic effect upon alloying were observed in Pt-based bimetallic nanomaterials including FePt, CoPt, RhPt, and so on.6−9 Particularly, CoPt nanomaterials show alluring and fertile behavior such as good electrical, catalytic, and magnetic properties together with high stability.10−15 Moreover, the large crystalline anisotropy provides the possibility of assembling shape-symmetric units into their asymmetric counterparts, such as one-dimensional (1D) CoPt nanochains/wires.16 The shape anisotropy mainly due to a large aspect ratio allows CoPt 1D nanostructures to be well aligned, providing a direct and effective way to manipulate and assemble them along certain orientations by the external magnetic fields. These integrated features of CoPt 1D nanostructures guarantee their practical applications in magnetic devices. To obtain CoPt 1D nanostructures, electrochemical deposition has been widely adapted with utilization of anodic aluminum oxides as templates.17−20 This method is well-known for its production of high-quality CoPt nanowires but crippled to conduct the fabrication of nanostructures with more complex building blocks such as hollow spheres. It is these building blocks that significantly promote the catalytic properties for © 2012 American Chemical Society

their intrinsic characterizations, such as specific surfaces and more mass transportation paths.21−26 On the other hand, wet chemical routes assisted by magnetic-field-induced assembly give more freedom to fabricate 1D nanostructures with versatile building blocks.27,28 Therefore, by applying magnetic fields as manipulating method, 1D nanostructures can be further assembled onto specified substrates which is vital for the practical application in magnetic devices. The attachment of CoPt hollow nanochains on the electrode by tactfully applying magnetic fields was reported.29 However, the curl-like hollow CoPt nanochains were overlapped and dispersed without any orientations on the substrate. Well-aligned CoPt hollow nanochains can improve the distribution uniformity and increase exposure surface, enhancing catalytic properties. In addition, the research on the magnetic interaction between hollow CoPt spheres as well as their contribution to the magnetic anisotropy originating from the shape anisotropy are also of importance to understand the magneto-synergetic assembly of 1D nanostructures. Unfortunately, few reports on the high-quality well-aligned CoPt 1D hollow nanostructures have been given. Herein, we report an efficient and environmentally benign strategy to synthesize the well-aligned CoPt hollow nanochains. Received: October 22, 2011 Revised: February 19, 2012 Published: February 21, 2012 5352

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Figure 1. (a) EDS spectrum of as-prepared products with atom ratio of Co to Pt of 30:70. (b) Powder XRD patterns with various compositions. (c) XPS spectrum of Pt 4f for as-prepared samples, vertical line for pure Pt.

interference device magnetometer (Quantum Design). Through the distribution droplets method, 0.25 mg CoPt hollow nanochains was dispersed in 2 mL of ethanol solution and then put onto silicon wafer, which was aligned by external magnetic fields.

The reaction is performed in water at room temperature. This strategy can be characterized by the integration of magneticfield-induced-assembly and the galvanic replacement approach. Magnetic fields are applied to tune the length and to control the alignment of these 1D hollow nanochains. Our work shows that the magnetic fields play a key role on the formation of Co nanochains, while the galvanic replacement between the Co nanochains and Pt ions serves as interpretation for the formation of hollow interiors. The well-aligned CoPt nanochains show a ferromagnetic behavior at room temperature and the shape anisotropy is found to play a dominant role on the tunable behavior.

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RESULTS AND DISCUSSION Figure 1a shows the typical energy-dispersive spectroscopy of as-prepared products, indicating that the atom ratio of Co to Pt is nearly 30:70. X-ray diffraction (XRD) data from samples with various ratios of Co to Pt are presented in Figure 1b, which show a good match with the Bragg reflections of the disordered face-centered cubic (fcc) structure of CoPt.17,30 The systematic XRD data for different compositions provide evidence of the alloy formation. The XRD data are in good agreement with the results reported by Y. Wang et al.31 Figure 1c shows X-ray photoelectron spectrum (XPS), where 4f7/2 and 4f5/2 core levels of Pt locate at 71.8 and 75.1 eV, respectively. The positive shift of the 4f peak (about 0.4 eV compared to pure Pt) is attributed to the alloying of Pt with Co atoms.23 Therefore, it is reasonable to conclude that the as-prepared products are made up of alloyed CoPt with composition ratio of 30:70. The morphology and structure of the as-prepared products were further investigated by SEM, TEM, and high-resolution TEM (HRTEM). Figure 2a shows as-prepared CoPt nano-

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EXPERIMENTAL SECTION In a typical synthesis, CoCl2·6H2O (8.5 mg) and PVP (poly vinylpyrrolidone, 20 mg) were dissolved in 43 mL of water in a three-necked flask, sonicated for 15 min, and stirred by flowing N2 gas of 60 mL/min during the whole reaction process. External magnetic fields of 5 kOe were applied during the reaction process. A freshly prepared solution of NaBH4 (20 mg in 20 mL of water) was then added dropwisely. Immediately after NaBH4 had been added, K2PtCl6 (10 mL, 6 mM) was added. After 120 min, the product was collected by centrifugation and washed several times with H2O and ethanol. All reagents were analytic grade and used as received. The experiments were carried out at room temperature. To study the influence of magnetic fields on the morphology of CoPt products, different external magnetic fields of 0, 0.5, 2, and 8 kOe were applied. To explore the growth kinetics, timeevolution experiments (reaction times of 30 and 120 min) were carried out with magnetic fields of 0 and 0.5 kOe, respectively. Experiments with higher molar of Pt salt (up to 2-fold) were also performed with reaction time of 20 and 60 min by applying magnetic fields of 0.5 kOe. The crystal structure, morphology, and chemical composition of as-prepared products were characterized using X-ray diffraction (XRD, X’Pert Pro MPD system, Cu Kα), scanning electron microscopy (SEM, Hitachi S-4800), and transmission electron microscopy (TEM, JEOL 2100F with field emission gun and accelerating voltage of 200 kV) equipped with energydispersive spectroscopy (EDS, EDAX). For XRD measurements, powder samples were used. The SEM micrographs were taken with the nanochains dispersed over silicon substrates by applying magnetic fields of 8 kOe. The specimens for TEM investigation were prepared by dispersing the powder products in alcohol by ultrasonic treatment and then dropped onto a porous carbon film supported on a copper grid and dried in air. Magnetic measurements of aligned nanochains on silicon substrates were carried out using a superconducting quantum

Figure 2. (a) SEM image of as-prepared CoPt products reorientated by applying magnetic field of 8 kOe. (b) Bright-field TEM image of CoPt nanochains. (c) HRTEM image taken from the area marked with frame in (b), inset: SAED pattern.

chains, which are well aligned by applying magnetic field of 8 kOe. The average length is estimated to be about 1 μm. Parts b and c of Figure 2 shows bright-field TEM and HRTEM images of the typical CoPt nanochains to investigate the structure of the CoPt nanochains. Figure 2b demonstrates that the nanochains are assembled by hollow nanospheres with diameter about 50 nm and shell thickness of nearly 5 nm. HRTEM image was taken from the contact region of two 5353

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adjacent hollow nanospheres shown in Figure 2c. The elongated lattice fringes across the two hollow nanospheres without any interruption by apparent boundary indicates the growth of the shell along the axe of the nanochains shown in Figure 2b.32 Inset of Figure 2c presents a selected area electron diffraction (SAED) pattern taken from the area shown in Figure 2b demonstrates a typical polycrystalline fcc structure with lattice parameter a = 0.38 nm, in good agreement with the value obtained from XRD spectrum. Furthermore, the measured lattice fringes of 0.22 and 0.19 nm can be indexed to (111) and (200) planes of alloyed CoPt. The impact of magnetic fields on growth of the CoPt hollow nanochains was checked. CoPt nanospheres with diameter of 50 nm and curl-like nanochains with length less than 300 nm were obtained under magnetic fields of 0 and 0.5 kOe, respectively, shown in parts a and b of Figure 3. It was further

Figure 4. TEM images of the as-prepared CoPt products with reaction time (t) of 30 min (a) and 120 min (b) without magnetic fields (H); (c, d) with H = 0.5 kOe and t = 30 and 120 min, respectively; (e, f) TEM images of as-prepared CoPt products by increasing molar of Pt precursor 2-fold with H = 0.5 kOe and t = 20 and 60 min, respectively.

between Co and Pt ions can be the driving force for the formation of hollow interiors due to our synthesis process.36 Time evolution experiments were carried out here. When reaction time (t) is 30 min, only solid nanoparticles shown in Figure 4a and solid nanochains shown Figure 4c were observed, respectivley. When t is increased to 120 min, hollow spheres shown in Figure 4b and hollow nanochains shown in Figure 4d were observed, indicating that the galvanic replacement induces morphology evolution from solid to hollow structures. To clarify this mechanism, more experiments were carried out by increasing the molar of Pt precursor 2-fold in the quest for changing the rate of galvanic replacement and keeping molar of other precursors constant. For t = 20 min (shorter than 30 min aforementioned), hollow nanochains were obtained as well as solid ones shown in Figure 4e, indicating that the dissolution of the Co cores become faster with high concentration of Pt precursor. Generally, the rate of galvanic replacement can be affected by the concentration of salts. Increasing time to 60 min, hollow nanchains with excessive Pt nanoaprticles attached on the surface were obtained without any observation of solid nanochains shown in Figure 4f. Thereby, the galvanic replacement is considered as the main driving force for the formation of hollow cavity and its rate can be accelerated by increasing the concentration ratio. Practically, a bright-field TEM image of a particle could be quite different under different focusing conditions even changing its morphology from “solid” to “hollow”. Defocus

Figure 3. SEM images of the CoPt products synthesized under magnetic fields of 0, 0.5, 2, and 8 kOe, respectively.

confirmed as CoPt hollow spheres and hollow chains, shown in parts b and d of Figure 4. With increasing magnetic fields to 2 kOe, the hollow nanochains became much straighter with length up to 500 nm shown in Figure 3c. The experiments with magnetic fields up to 8 kOe were also performed. Figure 3d shows aligned hollow nanochains with length up to 2 μm. Above results indicate that the length of the CoPt hollow nanochains can be controlled by magnetic fields. Furthermore, long CoPt hollow nanochains (up to 2 μm) can be assembled parallel to each other shown by Figure 2a and Figure 3d, exhibiting a better alignment than short nanochains (300 and 500 nm) shown in parts b and c of Figure 3. The morphology evolution of the CoPt products under different magnetic fields proves that the magnetic-field-induced-assembly is critical for the formation of CoPt hollow nanochains with controlled length, where as-prepared CoPt hollow nanochains can be conrolled by magnetic fields. What is the mechanism of the formation of hollow nanochains? Recently, the galvanic replacement, a class of sacrificial template method, has been found to be an efficient way to synthesize single metallic or bimetallic nanomaterials with hollow interiors.33−35 The galvanic replacement reaction 5354

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immediately shown in Figure 6c. During the period of galvanic displacement shown in Figure 6d, free Co ions are librated and diffused outward the nanochains and the core becomes “dissolved”. PVP molecular surrounding the nanochains can cap the free Co ions and the existence of excess NaBH4 will promise the coreduction chemical process. The galvanic displacement reaction and the coreduction processes continue until the sacrificed templates of Co nanochains are consumed completely. Finally, nanochains with hollow cavity and CoPt alloy shell” are formed as shown in Figure 6e. Another question is whether this strategy can be extended to other systems, such as NiPt. The experiments by substituting CoCl2 precursor for NiCl2 were performed. Different from well-aligned CoPt nanochains, as-prepared NiPt hollow nanospheres are monodispersive shown in Figure 7 even under

will make a filled sphere demonstrate a fake core−shell contrast only if the object size exceeds the limits of the field of depth by focusing asynchronously. The field of depth of the JEOL 2100F microscope (θsemi ≈ 5 mrad at 100 μm condenser aperture) is estimated to be ∼200 nm which is much larger than the size of CoPt spheres (∼60 nm). Therefore, there should be no doubt on the hollow feature of CoPt spheres regardless of the focusing condition. Moreover, the high angle anular dark field (HAADF) STEM technique is immune to such a probolem caused by defocus. This technique was ultilized to confirm the “solid” and “hollow” nature of the CoPt nanostructures as demonstrated in parts a and b of Figure 5, respectively. This results coincide with our TEM observation in Figure 4, which makes the conclusion convinced.

Figure 7. (a) SEM image and (b) TEM image of the as-prepared NiPt alloys with the fields of 8 kOe.

Figure 5. HAADF STEM images of as-prepares CoPt solid (a) and hollow (b) nanoparticles shown in parts a and b of Figure 4, respectively.

magnetic fields up to 8 kOe (highest in the lab). It is known that the magnetic moment of Ni is about 1/3 of that of Co atoms, and thereby it is hard for Ni clusters to form chainlike structures. Based on the experimental results that CoPt nanochains are well formed under magnetic fields of 5 kOe, NiPt nanochains are estimated to be formed under magnetic fields of 15 kOe. In other words, this strategy is also suitable for the synthesis of other bimetallic 1D nanostructure with high enough magnetic fields. The magnetic measurements of the well-aligned nanochains on Si wafer shown in Figure 2 were performed by SQUID. Figure 8a shows the magnetization as a function of magnetic

On the basis of the above results, the growth mechanism of CoPt nanochains was discussed with three stages illustrated in Figure 6. At the first stage shown in Figure 6a, Co ions in the

Figure 6. Schematic formation of CoPt hollow nanochains. (a) Formation of Co particles in PVP solution. (b) Growth and formation of Co nanochains covered with PVP under magnetic fields. (c) Galvanic replacement between Co atoms and Pt ions. (d) Diffused Co ions capped by PVP molecular and reduced together with Pt ions. (e) CoPt hollow nanochains formation after the replacement.

Figure 8. (a) M−H loop at 10 and 300 K. Inset: FC and ZFC curves. (b) M−H loop at 10 K with magnetic fields parallel (red open squares) and perpendicular to nanochains (blue open dots), respectively.

solution are reduced by NaBH4 forming clusters in the solution. Magnetic Co nanoparticles grow up with the presence of PVP and then assemble along the direction of external magnetic fields due to field-driven alignment effect shown in Figure 6b. At the third stage shown in parts c−e of Figure 6, the PtCl62− ions added in the solution are absorbed around the surface of Co nanochains and are reduced through galvanic reaction

fields (M-H loop) at 10 and 300 K, where the inset presents field-cooling (FC) and zero-field cooling (ZFC) temperature dependent magnetization (M−T curve). The coercivity (Hc) is 180 Oe at 300 K and increases with decreasing temperature (480 Oe at 10 K). From the FC and ZFC M-T data, the Curie temperature of the hollow nanochains is estimated to be above 350 K, showing ferromagnetism at room temperature. The 5355

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(3) Zhou, S. H.; McIlwrath, K.; Jackson, G.; Eichhorn, B. J. Am. Chem. Soc. 2006, 128, 1780. (4) Lu, X. M.; Au, L.; McLellan, J.; Li, Z. Y.; Marquez, M.; Xia, Y. N. Nano Lett. 2007, 7, 1764. (5) Chen, J. Y.; Wiley, B.; McLellan, J.; Xiong, Y. J.; Li, Z. Y.; Xia, Y. N. Nano Lett. 2005, 5, 2058. (6) Park, K. W.; Choi, J. H.; Kwon, B. K.; Lee, S. A.; Sung, Y. E.; Ha, H. Y.; Hong, S. A.; Kim, H.; Wieckowski, A. J. Phys. Chem. B 2002, 106, 1869. (7) Wang, R. M.; Zhang, H. Z.; Farle, M.; Kisielowski, C. Nanoscale 2009, 1, 276. (8) Tao, F.; Grass, M. E.; Zhang, Y. W.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A. Science 2008, 322, 932. (9) Makarov, D.; Bermudez-Urena, E.; Schmidt, O. G.; Liscio, F.; Maret, M.; Brombacher, C.; Schulze, S.; Hietschold, M.; Albrecht, M. Appl. Phys. Lett. 2008, 93, 153112. (10) Alloyeau, D.; Ricolleau, C.; Mottet, C.; Oikawa, T.; Langlois, C.; Le Bouar, Y.; Braidy, N.; Loiseau, A. Nat. Mater. 2009, 8, 940. (11) Tsang, S. C.; Cailuo, N.; Oduro, W.; Kong, A. T. S.; Clifton, L.; Yu, K. M. K.; Thiebaut, B.; Cookson, J.; Bishop, P. ACS Nano 2008, 2, 2547. (12) Tian, Y.; Shen, C. M.; Li, C.; Shi, X. Z.; Huang, Y.; Gao, H. J. Nano Res. 2011, 4, 780. (13) Choi, J.; Shin, C. B.; Suh, D. J. Catal. Commun. 2008, 9, 880. (14) Chen, Q. S.; Sun, S. G.; Zhou, Z. Y.; Chen, Y. X.; Deng, S. B. Phys. Chem. Chem. Phys. 2008, 10, 3645. (15) Peng, Y.; Cullis, T.; Luxmoore, I.; Inkson, B. Nanotechnology 2011, 22, 245709. (16) Chen, H. M.; Hsin, C. F.; Chen, P. Y.; Liu, R. S.; Hu, S. F.; Huang, C. Y.; Lee, J. F.; Jang, L. Y. J. Am. Chem. Soc. 2009, 131, 15794. (17) Mallet, J.; Yu-Zhang, K.; Chien, C. L.; Eagleton, T. S.; Searson, P. C. Appl. Phys. Lett. 2004, 84, 3900. (18) Li, W. X.; Shen, T. H. J. Appl. Phys. 2005, 97, 10J706. (19) Shamaila, S.; Sharif, R.; Riaz, S.; Ma, M.; Khaleeq-ur-Rahman, M.; Han, X. F. J. Magn. Magn. Mater. 2008, 320, 1803. (20) Choi, J. R.; Oh, S. J.; Ju, H.; Cheon, J. Nano Lett. 2005, 5, 2179. (21) Sun, Q.; Ren, Z.; Wang, R. M.; Wang, N.; Cao, X. J. Mater. Chem. 2011, 21, 1925. (22) Zhou, X. W.; Chen, Q. S.; Zhou, Z. Y.; Sun, S. G. J. Nanosci. Nanotechnol. 2009, 9, 2392. (23) Chen, G.; Xia, D. G.; Nie, Z. R.; Wang, Z. Y.; Wang, L.; Zhang, L.; Zhang, J. J. Chem. Mater. 2007, 19, 1840. (24) Shen, G. Z.; Bando, Y.; Golberg, D. Int. J. Nanotechnol. 2007, 4, 730. (25) Shen, G. Z.; Bando, Y.; Ye, C. H.; Yuan, X. L.; Sekiguchi, T.; Golberg, D. Angew. Chem., Int. Ed. 2006, 45, 7568. (26) Fan, H. J.; Knez, M.; Scholz, R.; Nielsch, K.; Pippel, E.; Hesse, D.; Zacharias, M.; Gosele, U. Nat. Mater. 2006, 5, 627. (27) Li, P. W.; Cui, Y. M.; Behan, G.; Zhang, H. Z.; Wang, R. M. J. Phys. D: Appl. Phys. 2010, 43, 275002. (28) Huang, J.; Chen, W. M.; Zhao, W.; Li, Y. Q.; Li, X. G.; Chen, C. P. J. Phys. Chem. C 2009, 113, 12067. (29) Zhai, J. F.; Huang, M. H.; Zhai, Y. M.; Dong, S. J. J. Mater. Chem. 2008, 18, 923. (30) Tzitzios, V.; Niarchos, D.; Margariti, G.; Fidler, J.; Petridis, D. Nanotechnology 2005, 16, 287. (31) Wang, Y.; Yang, H. J. Am. Chem. Soc. 2005, 127, 5316. (32) Wang, R. M.; Liu, C. M.; Zhang, H. Z.; Chen, C. P.; Guo, L.; Xu, H. B.; Yang, S. H. Appl. Phys. Lett. 2004, 85, 2080. (33) Jin, Y. D.; Dong, S. J. J. Phys. Chem. B 2003, 107, 12902. (34) Chen, J. Y.; McLellan, J. M.; Siekkinen, A.; Xiong, Y. J.; Li, Z. Y.; Xia, Y. N. J. Am. Chem. Soc. 2006, 128, 14776. (35) Seo, D.; Song, H. J. Am. Chem. Soc. 2009, 131, 18210. (36) Vasquez, Y.; Sra, A. K.; Schaak, R. E. J. Am. Chem. Soc. 2005, 127, 12504. (37) Zhang, Z. T.; Blom, D. A.; Gai, Z.; Thompson, J. R.; Shen, J.; Dai, S. J. Am. Chem. Soc. 2003, 125, 7528.

saturation magnetization is estimated to be about 14.4 emu/g, where Ms = 37.5 emu/g for Co hollow nanochains.37 Interestingly, the blocking temperature (TB) is found to be around 140 K, much higher than that of well dispersed hollow CoPt nanoparticles.38 It is well-known that the TB is proportional to the anisotropy energy and interparticle interaction energy (Eint). The interaction between the CoPt nanocrystallines made up of shells may increase the value of Eint. The magnetic anisotropy was also investigated by applied magnetic fields parallel and perpendicular to nanochains during measurements. Figure 8b presents the M−H loop measured at 10 K with magnetic fields applied parallel (red open squares) and perpendicular (blue open dots), respectively. The magnetic anisotropy field (HA) is estimated to be 3 kOe, smaller than that of solid CoPt nanowires.18 It is known that the magnetic anisotropy of nanowires is determined by the shape anisotropy prior to the crystalline anisotropy. The CoPt hollow nanochains with big hollow interior and different crystalline orientated nanoparticles contribute a little but visible to the magnetic anisotropy comparing with solid nanoparticles or nanochains. Therefore, magnetic anisotropy of CoPt hollow chains is dominated by the shape anisotropy instead of the crystalline anisotropy.

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CONCLUSION In summary, we have successfully synthesized the well-aligned CoPt hollow nanochains through magnetic field-inducedassembly approach combining with sacrificial template synthesis route. The mechanism for the formation of the CoPt hollow nanochains was discussed, where the galvanic replacement plays a key role. Magnetic properties indicate that the shape anisotropy of 1D aligned nanochains plays a dominant role on their good controllable behavior. The assynthesized CoPt hollow chains can be well aligned by magnetic fields, which is quite promising for the future practical applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.G.W.); [email protected] (R.M.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (MOST 2009CB929203) and National Natural Science Foundation of China (NSFC 11174023, 50971011, and 50972163), Beijing Natural Science Foundation (1102025), Research Fund for the Doctoral Program of Higher Education of China (20091102110038), and the Innovation Foundation of BUAA for PhD Graduates and the Scholarship Award for Excellent Doctoral Student granted by Ministry of Education.

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REFERENCES

(1) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (2) Wang, R. M.; Dmitrieva, O.; Farle, M.; Dumpich, G.; Ye, H. Q.; Poppa, H.; Kilaas, R.; Kisielowski, C. Phys. Rev. Lett. 2008, 100, 017205. 5356

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(38) Le, T. L.; Tung, L. D.; Long, J.; Fernig, D. G.; Thanh, N. T. K. J. Mater. Chem. 2009, 19, 6023.

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