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Thermodynamic Phase Formation of Morphology and Size Controlled Ni Nanochains by Temperature and Magnetic Field Pengwei Li,† Weimeng Chen,‡ Wei Liu,† Zi’an Li,§ Yimin Cui,† Anping Huang,† Rongming Wang,*,† and Chinping Chen*,‡ Key Laboratory of Micro-nano Measurement-Manipulation and Physics (Ministry of Education), Department of Physics, Beijing UniVersity of Aeronautics and Astronautics, Beijing 100191, P. R. China, Department of Physics, Peking UniVersity, Beijing 100871, P. R. China, and Fakultät für Physik and Center for Nanointegration (CeNIDE), UniVersity Duisburg-Essen, Germany ReceiVed: December 25, 2009; ReVised Manuscript ReceiVed: March 24, 2010
A series of highly uniform one-dimensional Ni nanochains, with diameters ranging from 20 to 200 nm, have been synthesized by a facile, template-free, wet chemical method at diverse temperature and magnetic field. Our results indicate that the crystallite size, diameter and length of the prepared nanochains depend critically on the reaction temperature. The characteristic thermodynamic cohesive energies, ∆ED and ∆EL, are obtained for the formation of the Ni nanochains. In addition, the chain length also depends on the applied field because of the Zeeman energy of the magnetic Ni nanoparticles. For the morphology control, an external field is required in order to obtain axially aligned rather than dendritic nanochains. The size-dependent magnetic properties are studied systematically. The saturation magnetization is shown to reduce inversely with the chain diameter, attributable to a core-shell structure. The thickness of the shell which encloses the ferromagnetic Ni core is determined to be about 2.3-3.4 nm. Introduction Single-chain magnets with quasi-one-dimensional (quasi-1D) morphology are of great interest because of their unique physical properties and wide application potentials in high-density data storage and quantum-computing.1 In the past few years, tremendous efforts have been devoted to exploring innovative and effective strategies for fabricating 1D magnetic nanostructure,2-4 such as anodic aluminum oxide (AAO) templating,5,6 electrochemical deposition,7 hydrothermal/solvothermal method,8,9 and surfactant assistance.10 Compared to traditional methods, magnetic-field-induced synthesis seems to be easier in the assembly of aligned magnetic nanostructures.9,11-13 For instance, Cheng et al. prepared highly linear chains of Co nanoparticles in the magnetic field.14 Wang and co-workers reported that magnetic γ-Fe2O3 nanotubes can be produced with the magnetic field applied perpendicular and parallel to the AAO membrane plane.5 Though various 1D structures such as Ni, Co, and Fe2O3 nano/microwires have been successfully synthesized by this method,14-17 template-free magnetic-field-induced synthesis of Ni nanochains with diameters less than 100 nm have so far rarely been reported. More importantly, the thermodynamic processes for the phase formation of Ni nanochains with the length, diameter, and morphology controlled by the reaction temperature and applied field have not been studied systematically. The dynamic phase formation of morphology and size controlled mechanism is therefore of great interest.18,19 Here we demonstrate a facile, template-free, wet chemical method to synthesize Ni nanochains in a magnetic field. The effects of the applied field and the reaction temperature on the * To whom correspondence should be addressed. E-mail: rmwang@ buaa.edu.cn;
[email protected]. † Beijing University of Aeronautics and Astronautics. ‡ Peking University. § University Duisburg-Essen.
morphology and size control are explored. In particular, characteristic cohesive energies, such as ∆ED for the particle diameter and ∆EL for the chain length, are determined. The bending ratio, which is a parameter defined to characterize the morphology of the nanochains, is also analyzed to study the dependence of morphology on the applied field. Furthermore, size dependent magnetic properties of Ni nanochains, with diameter ranging from 20 to 200 nm, have been investigated in detail. Experimental Section All chemicals used in this experiment were analytical grade and used without further purification. In a typical process, 40 mL of ethylene glycol (EG) was first put into a three-necked flask and heated up to a definite reaction temperature (TR) by a heating tape. Then 0.4 mL of hydrated hydrazine (50%) was added and maintained for 5 min. An external magnetic field (Happ) up to 5 kOe was applied, and 20 mL of NiCl2 · 6H2O (5 × 10-4 mol/L, EG solution) was added into the solution stirred by flowing the N2 gas at 20 mL/min. After refluxing for an hour, black products were filtrated, and then rinsed with ethanol and deionized water for 5-6 times. The final products were then preserved in ethanol for further investigation. By varying TR and Happ, the diameter and the morphology of the sample can be thus adjusted. The crystal structures, morphologies, and chemical compositions of the as-prepared products were studied using X-ray diffraction (XRD, X’Pert Pro MPD system, Cu KR), scanning electron microscopy (SEM, Hitachi S-4800), and transmission electron microscopy (TEM, JEOL 2100F with field emission gun and accelerating voltage of 200 kV). Magnetic properties of the as-synthesized nanochains were measured using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design). For the XRD and magnetic
10.1021/jp912168h 2010 American Chemical Society Published on Web 04/05/2010
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Figure 1. XRD patterns of the Ni nanochains with diameters from 20 to 60 nm.
measurements, powder samples were used. The SEM micrographs were taken with the nanochains dispersed over the silicon substrates. The specimen 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. Characterizations The characterizations are performed and presented for the samples with diameters of 20-60 nm. Figure 1a shows the XRD patterns for the crystal phase of the samples with diameter ranging from 20 to 60 nm, which were prepared with conditions of TR ) 90-210 °C and Happ ) 5 kOe. The peak positions match those of the standard metallic face-centered cubic (fcc) Ni (JCPDF card no. 04-0850, a ) 0.3521 nm). With sample diameter decreases, all of the three stamped peaks are broadening in the XRD pattern, mainly attributed to the smaller crystallite size of the samples. The average crystallite size of typical chains with the diameter of 20 nm is calculated to be ∼8 nm by the Scherrer’s equation. It indicates a polycrystalline nature of the nanochains. The crystallite size for the other samples is also calculated to be in the range of 10-20 nm, showing dependence on the reaction temperature. This result suggests a possible mechanism of the burst nucleation and growth process for the primary particles.20 Figure 2 shows the SEM images of the Ni nanochains prepared at different conditions. The SEM images for the samples prepared at TR in steps of 30 °C from 90 to 210 °C with Happ ) 0 Oe are shown in Figure 2a-e, while those prepared at the same temperature with Happ ) 5 kOe are shown in Figure 2f-j. With the applied field of 5 kOe, the nanochains appear to be highly axially aligned in comparison with those prepared without the applied field (Happ ) 0 Oe) at the same temperature. Obviously, the size of the Ni nanoparticles is dependent on the reaction temperature. The higher the reaction temperature is, the smaller in size the nanoparticles are. Figure 3a shows a typical morphology of the sample synthesized at conditions of Happ ) 5 kOe and TR ) 150 °C. The products are uniform and axially aligned. The diameter and the length of the as-synthesized nanochains are analyzed from the SEM images by extraction from about 100 randomly selected individual nanochains. Statistical diameter distribution of the nanochains (lower left inset) is estimated to be 33 ( 3 nm with a deviation of about 10%. The length is also estimated to be about 1 µm. A high magnification bright-field TEM image (upper right inset) further reveals the chain-like structure. Analytical TEM investigations provide further insight into the nanostructures. Figure 3b shows a high-resolution TEM (HRTEM) image of a representative nanoparticle in the nanochains. The interplanar spacing of 0.203 nm matches the d spacing
Figure 2. SEM images for Ni nanochains synthesized at the temperature of TR ) 90, 120, 150, 180, and 210 °C, respectively. (a-e) without applied magnetic field and (f-j) with magnetic field intensity of Happ) 5 kOe.
between (111) planes of fcc Ni. Nanocrystallites with the size of 10-20 nm are also found with grain boundaries clearly visible, consistent with the XRD result. The selected area electron diffraction (SAED) pattern (inset) also confirms that the nanochains consist of purely metallic fcc Ni nanoparticles. Thermodynamic Phase Formation The effects of TR and Happ on the length, diameter, and morphology of the nanochains are investigated systematically by synthesizing the samples at different reaction temperature from 90 to 210 °C (363 to 483 K in absolute temperature scale) and at different magnetic field from 0 to 5 kOe. The relevant geometric parameters such as the diameter, length and bending ratio of the samples are determined by the statistical average values estimated from the SEM images. To present the results, TR is expressed in the absolute temperature units, K, instead of °C for convenience in physical analysis. The diameter of nanochains vs reaction temperature is depicted in Figure 4a. The average diameter of the nanochains (D) in fixed Happ decreases exponentially with increasing TR. The exponential dependence is better revealed in the inset by the linear relation between ln D and 1/TR in constant Happ, and is described by the expression, ln D ) a/TR + b, where a and b are fitting parameters. Alternatively, the particle diameter is
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J. Phys. Chem. C, Vol. 114, No. 17, 2010 7723 field (Happ), as shown in Figure 4d. It can be described by the equation L(TR,Happ) ) L0(TR) + δL(TR)(1 - e-RHapp), which shows the dependence on the Boltzmann distribution factor, e-RHapp, with the ratio of Zeeman energy over thermal energy characterized by RHapp. By using the experimental values of L0(TR) at constant TR and Happ ) 0, the two parameters, δL(TR) and R, are determined by the fitting analysis and listed in Table 2. Interestingly, the cohesive energy for the chain formation (∆EL) depends also on the applied field besides on the reaction temperature (inset of Figure 4d). It indicates that the Zeeman energy of the magnetic nanoparticles in the applied field contributes significantly to the chain formation in competition with the thermal activation energy. The Zeeman energy is also a major factor in tailoring the morphology of the nanochains. As shown in Figure 2, the morphologies of the nanochains with and without the applied field are markedly different. For a quantitative description on the morphology of the chain structure, a bending-ratio (R) is defined as the highest chordal height at the flexure versus the length of the nanochain (hChord/L), and is determined for each sample from the SEM images. With a fixed applied field, R is found to increase linearly with TR as shown in Figure 5a. While at a constant reaction temperature, R decreases with Happ, as shown in Figure 5b. The power law dependence is shown in the inset, and is described as R ) R0Hγ(TR), where R0 ) β1T + β2 is determined from the analysis by Happ ) 0 Oe (Figure 5a), and the parameter, Hγ(TR), is obtained from the fitting analysis on γ(TR), as tabulated in Table 2. It reveals that the morphology of the nanochains is controlled by the competition between the axially alignment effect of the applied magnetic field and the random orientation effect of the thermal activation. Magnetization Measurements
Figure 3. Typical morphology and structure of Ni nanochains synthesized by the magnetic field induced method. (a) SEM micrograph and bright field TEM image (upper right inset) of the well aligned Ni nanochains with diameter distribution of 33 ( 3 nm (lower left inset) and (b) HRTEM image and SAED pattern (inset) of the sample in (a).
governed by the Arrhenius law,20,21 D ) D0 exp (∆ED/TR), where the cohesive energy (∆ED) is determined by the fitting analysis to be in the range of 1.4 to 1.8 × 103 K (inset of Figure 4b), and the high temperature limit of particle diameter thus obtained is, D0 ∼ 0.5 to 1.0 nm. The parameters, ∆ED and D0, determined for six different Happ are summarized in Table 1. There is no systematic dependence of the particle diameter D, hence, ∆ED, on the applied field. Only random fluctuation effect is observed, as depicted in Figure 4b. The above results suggest that the nucleation and growing processes of the nanoparticles are mainly controlled by the thermal activation effect. The average length (L) of the nanochains is also found to decrease exponentially with TR at a constant magnetic field, as shown in Figure 4c. The solid curves in the figure are the fitting results. The inset reveals the linear variation of ln L with 1/TR at different Happ. From the fitting analysis by the function, L ) L0 exp(∆EL/TR), the cohesive energy (∆EL) for the formation of the nanochains is obtained to be in the range of 2.6 to 3.0 × 103 K (inset of Figure 4d). The high temperature limit of the chain length, L0, is also determined from the fitting analysis to be about 0.6-0.8 nm. The constants, ∆EL and L0, are tabulated in Table 1, too. In contrast to the result that the particle diameter depends only on TR, the chain length (L) also increases with the applied
The field-dependent magnetizations, M(H), of the well-aligned nickel nanochains with diameters of 20, 25, 33, 45, 60, 110, and 200 nm were measured at T ) 5 and 300 K and shown in Figure 6, panels a and b, respectively. It is noted that the results of MS for 110 and 200 nm are also included in the analysis, while the other characterization results for these two are not presented. The saturation magnetization, MS, are derived to be 54.3 (200 nm), 51.4 (110 nm), 41.3 (60 nm), 38.1 (45 nm), 32.9 (33 nm), 29 (25 nm), and 18.3 emu/g (20 nm) at T ) 5 K. It reveals that Ms decreases with the diameter of the nanochains. The Ms for the nanochains of 200 nm is only slightly reduces from the bulk value (57 emu/g). For 60 nm, it further decreases to ∼72% of the bulk value, while it decreases sharply to only ∼32% for the nanochains of 20 nm in diameter. The suppression of saturation magnetization is commonly observed with magnetic nanoparticles of various materials, such as Ni nanochains,22,23 MnFe2O4 nanoparticles,24 iron nanoparticles,25 etc. Specifically, an experiment was performed to study the surface magnetic states of three samples of Ni nanochains with a core-shell structure, which were synthesized by using 3 different surfactants.23 It clearly reveals the effects of surface magnetism on the saturation magnetization. The same property is also observed at T ) 300 K, however, with a more reduced magnetization expected at a higher temperature. The values of HC are determined as 465 (200 nm), 325 (110 nm), 464 (60 nm), 301 (45 nm), 337 (33 nm), 401 (25 nm), and 590 Oe (20 nm) at T ) 5 K, close to those determined for the other reported Ni nanochains.22 For Ni, the coherence length is Lcoh ∼ 25 nm. One would expect a superparamagnetic (SPM) behavior for nanoparticles (NPs) with D < 25 nm at a temperature higher than the blocking temperature. However, for a quasi-1D structure
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Figure 4. (a,b) Variation of chain diameter vs reaction temperature TR and applied field Happ and (c,d) variation of chain length vs TR and Happ.
TABLE 1: Cohesive Energies for the Formation of Nanoparticles and Nanochains (∆ED and ∆EL) and the Corresponding High Temperature Limits of Particle Diameter and Chain Length (D0 and L0) in Different Applied Fields, Happ Happ (k Oe)
∆ED (103 K)
D0 (nm)
∆EL (103 K)
L0 (nm)
0.00 0.06 0.25 1.00 2.00 5.00
1.77 1.58 1.61 1.59 1.67 1.49
0.51 0.80 0.62 0.62 0.68 0.95
2.64 2.70 2.86 2.90 2.90 2.91
0.77 0.85 0.59 0.71 0.73 0.82
TABLE 2: Parameters Characterizing the Effects of Applied Field on Chain Length and Morphology TR (K)
R (10-3 Oe-1)
δL (nm)
γ (TR)
363 393 423 453 483
1.23 1.40 1.57 1.39 1.31
1390 672 315 220 150
-0.53 -0.27 -0.20 -0.17 -0.13
such as the nanochain (NC), the magnetic shape anisotropy becomes important for soft ferromagnets with the demagnetization factor roughly equal to 0.2.22 This increases the blocking potential considerably for the NC from that for the NP. Further on this size dependent magnetic behavior, we draw the saturation magnetization MS as a function of the inverse diameter 1/D (Figure 6c). It exhibits a linear behavior. This property can be interpreted by a magnetic core-shell structure for the nanochains with a vanishing magnetism of the shell layer. With a shell layer of the same thickness, ∆R, the saturation magnetization for NPs, is MSNP ) (R - ∆R)3/R3MSbulk ) [1 3(∆R/R) + 3(∆R/R)2 - (∆R/R)3]MSbulk. By first order approximation, it becomes MSNP ∼ [1 - 3(∆R/R)]MSbulk ∼ [1 -
Figure 5. Dependence of bending ratio on TR and Happ.
6(∆R/D)]MSbulk, and for nanowires (NWs), it is MSNW ∼ [1 2(∆R/R)]MSbulk ∼ [1 - 4(∆R/D)]MSbulk. For NCs, the formula is therefore expressed as, MSNC ∼ [1 - f(∆R/D)]MSbulk, with the geometric factor f falling in the range from 4 to 6. With (∆R/ R) ) 0.1, the error introduced by the higher order approximation
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Figure 6. Field dependent magnetization measured at (a) T ) 5 and (b) 300 K for Ni chains with diameter ranging from 20 to 200 nm. (c) Variation of saturation magnetization MS vs the inverse of nanochain diameter 1/D and (d) typical HRTEM image of the Ni nanochains with diameter of 20 nm.
is only about 3% for NPs and 1% for NWs, and with (∆R/R) ) 0.2, it becomes 12% and 4% respectively for NPs and NWs. All of the NCs in the present work, except for the 20 and 25 nm, satisfy the condition of (∆R/R) ∼ 0.2 or less, as will be revealed in the analysis presented next that ∆R is about 3 nm. The two straight lines are for the linear fittings to MS of the seven nanochains. The slope for the data at T ) 5 K gives the shell thickness of 2.3 to 3.4 nm, depending on whether these samples are more particle-like or more wire-like. For the samples of 20 and 25 nm, the shell thickness is calculated directly from the magnetization ratio over the bulk value by assuming the particle-like nature. For the 25 nm, the shell thickness is determined as 2.5 nm, which is smaller than the average thickness determined by the fitting shown in Figure 6c, and for the 20 nm, it is 3.2 nm. In order to further confirm the core-shell structure, investigation by HRTEM is performed (Figure 6d). There is an apparent surface layer of about 3-4 nm with an interplanar spacing of about 0.34 nm. It is assigned to a hexagonal carbon structure,26 possibly formed from the glycol during the synthesis process. The equivalent mass of this material to Ni accounts for only a few tenths of a nanometer. Furthermore, the carbonyl ligation easily results in an additional magnetic “dead” layer of Ni.27,28 Conclusions A series of Ni nanochains with diameters ranging from 20 to 200 nm have been controllably synthesized in the absence of any template. The thermodynamic driving forces for the phase formation of the Ni nanochains have been studied in detail by adjusting the applied field and reaction temperature. It is found that both the diameter and the length depend on the reaction temperature following the Arrhenius law. In addition, the length and morphology of the nanochains are controllable by the applied field. It is attributable to the dipolar interaction of the
magnetic nanoparticles with the applied field. Systematic investigations reveal that the saturation magnetization of the Ni nanochains reduces inversely with the diameter. It is attributed to a magnetic core-shell structure of the nanochains with a much-reduced or vanishing magnetism of the surface layer. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 50671003, 50971011 and 10874006), Beijing Natural Science Foundation (No. 1102025), the National Basic Research Program of China (Nos. 2009CB939901 and 2010CB934601), the Program for New Century Excellent Talents in University (NCET-06-0175), and Research Fund for the Doctoral Program of Higher Education of China (20091102110038). References and Notes (1) Gatteschi, D.; Sessoli, R. Angew. Chem., Int. Ed. 2003, 42, 268. (2) Barakat, N. A. M.; Kim, B.; Kim, H. Y. J. Phys. Chem. C 2009, 113, 531. (3) Chueh, Y. L.; Lai, M. W.; Liang, J. Q.; Chou, L. J.; Wang, Z. L. AdV. Funct. Mater. 2006, 16, 2243. (4) 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. (5) Wang, J. H.; Ma, Y. W.; Watanabe, K. Chem. Mater. 2008, 20, 20. (6) Qin, J.; Nogues, J.; Mikhaylova, M.; Roig, A.; Munoz, J. S.; Muhammed, M. Chem. Mater. 2005, 17, 1829. (7) Li, D. D.; Thompson, R. S.; Bergmann, G.; Lu, J. G. AdV. Mater. 2008, 20, 4575. (8) Hu, M. J.; Lu, Y.; Zhang, S.; Guo, S. R.; Lin, B.; Zhang, M.; Yu, S. H. J. Am. Chem. Soc. 2008, 130, 11606. (9) He, Z. B.; Shu-Hong, Y.; Zhou, X. Y.; Li, X. G.; Qu, J. F. AdV. Funct. Mater. 2006, 16, 1105. (10) Zhang, J. G.; Chen, J.; Wang, Z. X. Mater. Lett. 2007, 61, 1629. (11) Niu, H. L.; Chen, Q. W.; Ning, M.; Jia, Y. S.; Wang, X. J. J. Phys. Chem. B 2004, 108, 3996. (12) Li, F. S.; Wang, Y.; Wang, T. J. Solid State Chem. 2007, 180, 1272.
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(13) Wang, J.; Wu, Y. J.; Zhu, Y. J.; Wang, P. Q. Mater. Lett. 2007, 61, 1522. (14) Cheng, G. J.; Romero, D.; Fraser, G. T.; Walker, A. R. H. Langmuir 2005, 21, 12055. (15) Zhang, F.; Wang, C. C. J. Phys. Chem. C 2008, 112, 15151. (16) Jia, B. P.; Gao, L. J. Phys. Chem. C 2008, 112, 666. (17) Zhang, X.; Liu, W. M. Mater. Res. Bull. 2008, 43, 2100. (18) Farkas, D.; Mohanty, S.; Monk, J. Phys. ReV. Lett. 2007, 98, 165502. (19) Sandoval, L.; Urbassek, H. M. Nano Lett. 2009, 9, 2290. (20) Park, J.; Privman, V.; Matijevic, E. J. Phys. Chem. B 2001, 105, 11630. (21) Palii, A. V.; Reu, O. S.; Ostrovsky, S. M.; Klokishner, S. I.; Tsukerblat, B. S.; Sun, Z. M.; Mao, J. G.; Prosvirin, A. V.; Zhao, H. H.; Dunbar, K. R. J. Am. Chem. Soc. 2008, 130, 14729.
Li et al. (22) He, L.; Zheng, W. Z.; Zhou, W.; Du, H. L.; Chen, C. P.; Guo, L. J. Phys-Condens. Mater. 2007, 19, 036216. (23) Chen, W. M.; Zhou, W.; He, L.; Chen, C. P.; Guo, L. J. PhysCondens. Mater. 2010, 22, 126003. (24) Liu, C.; Zhang, Z. J. Chem. Mater. 2001, 13, 2092. (25) Zhang, D.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Phys. ReV. B 1998, 58, 14167. (26) Felten, A.; Bittencourt, C.; Pireaux, J. J.; Reichelt, M.; Mayer, J.; Hernandez-Cruz, D.; Hitchcock, A. P. Nano Lett. 2007, 7, 2435. (27) Liebermann, L.; Clinton, J.; Edwards, D. M.; Mathon, J. Phys. ReV. Lett. 1970, 25, 232. (28) van Leeuwen, D. A.; van Ruitenbeek, J. M.; de Jongh, L. J.; Ceriotti, A.; Pacchioni, G.; Ha¨berlen, O. D.; Ro¨sch, N. Phys. ReV. Lett. 1994, 73, 1432.
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