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J. Phys. Chem. C 2007, 111, 6630-6633
Improved Magnetic Anisotropy of Monodispersed Triangular Nickel Nanoplates Yonghua Leng,† Yan Li,† Xingguo Li,*,†,‡ and Seiki Takahashi§ Beijing National Laboratory for Molecular Sciences (BNLMS), (The State Key Laboratory of Rare Earth Materials Chemistry and Applications), College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China, College of Engineering, Peking UniVersity, Beijing, 100871, China, and Department of Materials Science and Engineering, Faculty of Engineering, Iwate UniVersity, 4-3-5 Ueda, Morioka, 020-8551, Japan ReceiVed: NoVember 17, 2006; In Final Form: March 7, 2007
Monodispersed triangular Ni nanoplates were successfully synthesized based on a facile thermal decomposition method. The Ni nanoplates have an average edge length of 15.4 nm, and their thickness is about 6 nm. Owing to high anisotropic structures, the Ni nanoplates exhibit typical ferromagnetic behaviors at room temperature. The blocking temperatures of the Ni nanoplates are over 400 and 226 K when the applied field is 100 and 500 Oe, respectively. Compared with bulk Ni, the Ni nanoplates exhibit significant increase in magnetic anisotropy due to the presence of shape anisotropy and the reduction in particle size. These nickel nanoplates are expected to bring new opportunities in application of magnetic storage and catalysis.
Introduction The synthesis of nanoparticles has attracted intensive attention not only for fundamental scientific interest but also for their technological applications. Monodisperse nanoparticles with controlled sizes or shapes are of key importance, because their properties depend strongly on the particle size and shape.1-3 Recently, synthesis of two-dimensional (2D) nanoparticles with large surface area and high aspect ratio (the edge length over the thickness) has received considerable attention. For metal nanocrystals with face-centered cubic (fcc) crystal structures, the {111} face has the lowest surface energy ((110)>(100)>(111)),4,5 resulting in their possibility to nucleate and grow into nanoplates with their surface bounded by {111} faces under kinetic control. There have been reports on successful synthesis of noble metal nanocrystals, such as Au,6-9 Ag,10-12 and Pd13,14 nanoplates with triangular and/or hexagonal shapes, which mainly involve the reduction of salt compounds using weak reducing agents to controll the reduction kinetics. However, in terms of transition metal nanocrystals, which also have fcc crystal structure such as Co and Ni metal, synthesis of 2D plate structures are poorly explored. The reason may be attributed to the strong reducing agents required to reduce Co and Ni to their elemental forms, which makes the reaction kinetics hard to control. The final product will have no choice but to take the thermodynamically favored shapes, i.e., spherical nanoparticles. Though there have been reports related to the preparation of 2D cobalt nanodisks15,16 by thermal decomposition of Co2(CO)8, however, the cobalt nanodisks have a hexagonal closely packed crystal structure, which has a crystal asymmetry. The fabrication of 2D transition metal nanoplates (fcc) with well-defined shapes and in high yield, by a simple and low-cost route, remains a challenge. For fcc Ni metal nanocrystals with no preferential crystal growth direction, 2D plates structure could not be obtained by * Corresponding author. Phone: +86-10-6276-5930. Fax: +86-10-62765930. E-mail:
[email protected]. † College of Chemistry and Molecular Engineering, Peking University. ‡ College of Engineering, Peking University. § Iwate University.
direct thermolysis of nickel organometallic precursors as decomposition of Co2(CO)8. Our group developed a method to synthesize fcc Ni nanosheets with both triangular and hexagonal shapes by decomposition of Ni(COOH)2 with the aid of iron species.17 In this paper, we report the synthesis of monodisperse fcc Ni nanoplates with triangular shape and uniform sizes by assisted thermolysis method. By controlling the amount of the stabilizers (oleic acid and oleyl amine), the injection rate of Fe(CO)5 solution, and the reaction time, products with different morphologies could be obtained and the formation mechanism are proposed. The magnetic properties of the metallic nanoplates are also investigated, indicating significant increase in magnetic anisotropy. Experimental Synthesis. Under proper reaction condition, Ni triangular nanoplates with uniform sizes could be synthesized. In a typical synthesis, a slurry containing Ni(COOH)2 (1.8 mmol), oleic acid (4 mmol), oleyl amine (16 mmol), and octadecene (10 mL) in a three-necked flask (100 mL) was heated to 373 K to remove water and oxygen, with vigorous magnetic stirring under vacuum for 30 min in a temperature-controlled electromantle. The solution was then heated in nitrogen to 473 K. Then 0.05 mL Fe(CO)5 dissolved in 2 mmol octadecene was slowly injected into the three-necked flask with a syringe in 3 min. The reaction mixture was kept at 473 K for 20 min. After the reaction system was cooled to room temperature, the product was centrifuged and washed with cyclohexane once and then with ethanol for three times to remove the solvent and excess surfactants. Characterization. Structural analysis was carried out by X-ray diffraction (XRD, Rigaku D/ MAX-2000) using monochromatic Cu KR radiation. Morphology and size distribution were studied by transmission electron microscopy (TEM, JEOL JEM-200CX). The magnetic properties were investigated by a superconductive quantum interference device (SQUID) with an applied field up to 10 kOe. Results and Discussion The morphology of the products was characterized by TEM and selected-area electron diffraction (SAED). TEM sample
10.1021/jp0676686 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/14/2007
Monodispersed Triangular Nickel Nanoplates
Figure 1. (a) TEM image of self-assembled triangular Ni nanoplates. (b) Electron diffraction pattern of Ni triangular nanoplates. (c) Size distribution of the Ni nanoplates with average edge length of 15.4 ( 1.6 nm.
Figure 2. XRD pattern of the as-synthesized Ni nanoplates.
was prepared by dropping a cyclohexane dispersion of the nanoplates onto carbon-coated copper grids, which indicates that the as-synthesized nanoparticles have triangular plate shapes and self-assemble into hexagonal-close-packed (hcp) structure, as shown in Figure 1a. The corresponding SAED pattern is shown in Figure 1b, which could be indexed as the fcc structure of metallic Ni. Constrained by the resolving power of the electron microscope, electron diffraction pattern of a single particle could not be obtained. Our previous study indicated that the exposed plate planes were assigned to be (111) planes, which was similar to the reported noble metal nanoplates. By counting 207 particles, the average edge length of the nanoplates was estimated to be about 15.4 ( 1.6 nm, as depicted in Figure 1c. The crystal structure of the product was further confirmed by XRD. From the large-angle XRD pattern shown in Figure 2, we could see that all the peaks well match the face-centeredcubic (fcc) Ni metal with lattice parameter of a ) 3.523 Å (JCPDS 7440-02-0). No trace of nickel oxides was observed, indicating their good oxidation resistance due to the protection of surfactants. The broadening of the peaks indicates the nanocrystalline nature of the as-synthesized product. The as-synthesized Ni nanoplates were redispersed in cyclohexane/toluene (v/v in 1:1) solution, by slowly evaporation of the mixed dispersion of the product, the nanoplates selfassembled into ribbonlike nanoarrays via face-to-face formation and standing on their edges. To verify the triangular nanoplates shape, tilting experiments have been performed, as shown in Figure 3. The standing nanoplates varied in thickness when tilting the sample holder, which could be observed more clearly from the particles in the black circles. With the increased tilting
J. Phys. Chem. C, Vol. 111, No. 18, 2007 6631
Figure 3. TEM images of the standing Ni nanoplates by tilting to confirm the self-assembled plate structure. Tilted TEM images obtained at (a) -10° tilt, (b) 0° tilt, (c) +10° tilt, and (d) +25° tilt. The inset in panel b is the enlarged image of the standing edges of several triangular Ni nanoplates.
Figure 4. TEM images of Ni nanoplates synthesized (a) by decreasing the amount of oleyl amine to be half and (b) by injecting Fe(CO)5 dissolved in 2 mmol oleyl amine and keeping the whole amount of oleyl amine to be 8 mmol.
angle, the nanoarrays containing rodlike nanoparticles (Figure 3b) evolved to triangular nanoplates (Figure 3d). It indicates that the observed rods are actually the standing edges of the triangular nanoplates. The inset in Figure 3b indicates that the average edge length of the as-synthesized Ni nanoplates is about 6.0 nm, which is evaluated from their standing edges. The product morphologies and sizes strongly depend on the amount of oleyl amine. When decreasing the amount of oleyl amine from 16 to 8 mmol, it was found that the product consisted of both triangular and hexagonal Ni nanoplates with enlarged edge lengths and a broader size distribution (Figure 4a). Even keeping the whole amount of oleyl amine to be 8 mmol, but 2 mmol of oleyl amine was injected with Fe(CO)5, the product morphologies were also quite different. Except the triangular and hexagonal Ni nanoplates, a small percentage of nickel nanobelts (denotes as 1) and nanorods (denoted as 2) also existed in the products (Figure 4b). The formation mechanism will be discussed below. Only triangular nanoplates are obtained when their edge lengths are small enough. However, with increasing edge lengths, there exist other morphologies in the products, such as many hexagonal nanoplates, small percentage of nanobelts and nanorods. It is unbelievable that all the morphologies with various shapes and sizes originate from the same nucleation and growth manner. On the basis of these experimental results, we propose that the formation mechanism should be as follows: (i) small triangular nanoplates as the initial nuclei, where most will continue to grow within the {111} planes, along
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Figure 5. Schemes of the growth mechanism of Ni nanoparticles (a) large triangular nanoplates formed by atomic attachment, (b) large triangular nanoplates, and (c) large hexagonal nanoplates formed by connection.
Figure 6. ZFC and FC curves of the as-prepared Ni nanoplates at an applied field of (a) 100 Oe and (b) 500 Oe.
Leng et al. majority of the domains were still blocked at that temperature. It suggests that the blocking temperature (TB) of the nanoplates is higher than 400 K, which could be ascribed to their high anisotropic structure. Even at an applied field of 500 Oe, the value of TB is still as high as 226 K, which is even higher than the Ni nanoparticles at an applied field of 100 Oe.19 The magnetic anisotropy constant K is calculated using the formula: K ) 25kBTBV-1,20 where kB is the Boltzman constant, TB is the measured blocking temperature, and V is the volume of a single particle which was determined from TEM measurements. The calculated value of K is about 12.6 × 105 ergcm-3, which is much higher than that of bulk Ni (5 × 104 ergcm-3).21 Considering that our sample has high anisotropic plate structure, the shape anisotropy should be the dominant reason for the increase in magnetic anisotropy constant K. In addition, the reduction in particle size22 should also result in some improvement of anisotropy. The hysteresis loops for the as-synthesized Ni nanoplates measured at 4.2 and 300 K show typical ferromagnetic behaviors with coercive force and magnetic remanence (Figure 7). As the temperature decreases from 300 to 4.2 K, the saturation magnetization (Ms) increases a little from 33.9 to 37.6 emu/g, which is lower than the bulk one (55.0 emu/g)23 due to their small sizes. However, the remanence ratio (Mr/Ms) shows strong dependence on the measuring temperature, which decreases from 0.41 for 4.2 K to 0.06 for 300 K. This implies that the thermal energy for demagnetizing becomes dominant at 300 K for the Ni nanoplates. The thermal energy effects decrease greatly at 4.2 K; however, the shape anisotropy energy and the interparticle dipole-dipole interaction play a significant role in determining the magnetic properties. Owing to their smaller edge length and lower aspect ratio compared with the previous reported large Ni nanosheets, the coercive force at 300 K for the as-synthesized Ni nanoplates is much lower (27 vs 180 Oe).17 In addition, the strong interparticle interaction should also be responsible for the decrease of coercive force. This value increases to be 273 Oe at 4.2 K, which exhibits ferromagnetic behavior. Conclusions
Figure 7. Hysteresis loop of the as-synthesized Ni nanoplates at 4.2 and 300 K. (The insets depict the details of the hysteresis loop at low fields. No correction was made for the weight of the organic surfactants.)
the 〈110〉 direction by Ni atomic attachment, forming large triangular nanoplates (Figure 5a); (ii) some of the small nanoplates with the same size will be connected along the {110} lateral planes, leading to formation of large triangular and hexagonal nanoplates (see Figure 5b,c), which is similar to the formation of Au nanosheets.18 The presence of small percentage of nanobelts provides the direct evidence to indicate the connection mechanism. We measured the magnetic properties of the Ni nanoplates with an average edge length of 15.4 nm by recording the temperature-dependent magnetization curves (M-T curves) and the hysteresis loops (M-H curves), as illustrated in Figure 6 and Figure 7. At an applied field of 100 Oe, the zero-fieldcooled (ZFC) curve deviates from the field-cooled (FC) curve at the highest temperature measured (400 K), implying that the
Under proper reaction condition, monodisperse nickel nanoplates with triangular shape are synthesized by an assisted thermolysis method. The nanoplates have an average edge length of 15.4 nm and thickness of 6 nm. Though the nanoplates exhibit lower saturation magnetization than the bulk nickel metal, they show typical ferromagnetic properties at room-temperature rather than superparamagnetism as other nanocrystals. The significant increase in magnetic anisotropy should be ascribed to their 2 D plate structures and the reduction in particle size. Changing reaction parameters, large triangular and hexagonal nanoplates could be obtained by Ni atomic attachment within the {111} planes. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20221101, 10335040 and 20671004) and MOST of China (No. 2006AA02Z130). References and Notes (1) . Park, J.; Lee, E.; Hwang, N. M.; Kang, M. S.; Kim, S. C.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kini, J. Y.; Park, J. H.; Hyeon, T. Angew. Chem., Int. Ed. 2005, 44, 2872-2877. (2) Millstone, J. E.; Metraux, G. S.; Mirkin, C. A. AdV. Funct. Mater. 2006, 16, 1209-1214. (3) Tao, A.; Sinsermsuksakul, P.; Yang, P. D. Angew. Chem., Int. Ed. 2006, 45, 4597-4601. (4) Kan, C. X.; Zhu, X. G.; Wang, G. H. J. Phys. Chem. B 2006, 110, 4651-4656.
Monodispersed Triangular Nickel Nanoplates (5) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153-1175. (6) Kan, C. X.; Wang, G. H.; Zhu, X. G.; Li, C. C.; Cao, B. Q. Appl. Phys. Lett. 2006, 88. (7) Chu, H. C.; Kuo, C. H.; Huang, M. H. Inorg. Chem. 2006, 45, 808-813. (8) Zhou, M.; Chen, S. H.; Zhao, S. Y.; Ma, H. F. Chem. Lett. 2005, 34, 1670-1671. (9) Ah, C. S.; Yun, Y. J.; Park, H. J.; Kim, W. J.; Ha, D. H.; Yun, W. S. Chem. Mater. 2005, 17, 5558-5561. (10) Washio, I.; Xiong, Y. J.; Yin, Y. D.; Xia, Y. N. AdV. Mater. 2006, 18, 1745-+. (11) Bonacina, L.; Callegari, A.; Bonati, C.; van Mourik, F.; Chergui, M. Nano Lett. 2006, 6, 7-10. (12) He, Y.; Shi, G. Q. J. Phys. Chem. B 2005, 109, 17503-17511. (13) Bradley, J. S.; Tesche, B.; Busser, W.; Masse, M.; Reetz, R. T. J. Am. Chem. Soc. 2000, 122, 4631-4636. (14) Xiong, Y. J.; McLellan, J. M.; Chen, J. Y.; Yin, Y. D.; Li, Z. Y.; Xia, Y. N. J. Am. Chem. Soc. 2005, 127, 17118-17127.
J. Phys. Chem. C, Vol. 111, No. 18, 2007 6633 (15) Gao, Y.; Bao, Y. P.; Pakhomov, A. B.; Shindo, D.; Krishnan, K. M. Phys. ReV. Lett. 2006, 96. (16) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874-12880. (17) Leng, Y. H.; Zhang, Y. H.; Liu, T.; Suzuki, M.; Li, X. G. Nanotechnology 2006, 17, 1797-1800. (18) Li, C. C.; Cai, W. P.; Cao, B. Q.; Sun, F. Q.; Li, Y.; Kan, C. X.; Zhang, L. D. AdV. Funct. Mater. 2006, 16, 83-90. (19) Jeon, Y.; Lee, G. H.; Park, J.; Kim, B.; Chang, Y. M. J. Phys. Chem. B 2005, 109, 12257-12260. (20) Cullity, B. D. Introduction to Magnetic Materials; Addison-Wiley: Massachusetts, 1972. (21) Bonder, M. J.; Kirkpatrick, E. M.; Martin, T.; Kim, S. J.; Rieke, R. D.; Leslie-Pelecky, D. L. J. Magn. Magn. Mater. 2000, 222, 70-78. (22) Park, J.; Lee, E.; Hwang, N. M.; Kang, M. S.; Kim, S. C.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kini, J. Y.; Park, J. H.; Hyeon, T. International Edition- Angew. Chem. 2005, 44, 2872-2877. (23) Sun, X. C.; Dong, X. L. Mater. Res. Bull. 2002, 37, 991-1004.
