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Synthesis of Ni Nanochains with Various Sizes: The Magnetic and Catalytic Properties Wei Zhou,† Lin He,‡ Rong Cheng,§ Lin Guo,*,† Chinping Chen,*,‡ and Jianlong Wang*,§ School of Chemistry and EnVironment Science, Beijing UniVersity of Aeronautics and Astronautics, Beijing 100191, China, Department of Physics, Peking UniVersity, Beijing 100871, China, and Laboratory of EnVironmental Technology, INET, Tsinghua UniVersity, Beijing 100084, China ReceiVed: July 2, 2009; ReVised Manuscript ReceiVed: August 22, 2009
A series of pure Ni thin chains with particle diameters ranging from 15 to 80 nm were successfully synthesized by a wet chemical method. Magnetization measurements for the Ni chains reveal that the saturation magnetization increases and the coercivity decreases with increasing particle diameter. The magnetization reversal mechanism of both the samples with a diameter of 30 and 50 nm can be described by the model of “chain of spheres”. Using the fanning mode, the calculated coercivities and the remnant ratios agree well with the experimental results. Surprisingly, the coercivity is greatly enhanced, reaching as high as 790 Oe at T ) 5 K for the Ni chains with a diameter of ∼15 nm, which are composed of single-crystal particles. Meanwhile, in the degradation of pentachlorophenol (PCP) solutions with Fe0 nanoparticles as reducing agents, Ni nanochains with a diameter of ∼80 nm were added, and the results indicate that the sample could serve as a good catalyst in dechlorination systems. Introduction Intensive research on nickel nanomaterials is inspired by their magnetic and catalytic properties.1-3 It is well-known that the size and the morphology of nanomaterials significantly influence their physical and chemical properties.4 At present, a simple wet chemical route has attracted much attention due to the high quantity of product, ease of chemical control, and naturally occurring self-assembly process. Using this method, Ni nanomaterials with various morphologies have been synthesized, including rods, belts, triangular plates, chains, and other structures.5-8 The magnetic properties of Ni nanomaterials depend on their shape and size. For example, compared with bulk Ni (∼0.7 Oe), the coercivity of the nanobelts is increased to 640 Oe, whereas that of the triangular plates only to 27 Oe at room temperature.6,7 It is also believed that the coercivity depends strongly on the anisotropy of the samples, especially the shape anisotropy.7,9,10 However, systematic studies on the magnetic properties influenced by their size effect of onedimensional (1D) nanostructure are still rarely reported.11,12 Meanwhile, Ni nanomaterials are widely used in many fields as catalysts. They could be used for hydrogenation and dehydrogenation of some organic matter,13-15 for the Suzuki coupling reaction,16 and for the preparation of carbon nanotubes17 as well as other nanomaterials.18 Previously, we have fabricated and simply studied the magnetic properties of chain samples with a diameter of 150-250 nm.8 Later, the magnetic properties of other three samples with an average diameter of 50, 75, and 150 nm were studied.12 The present work, as a continuation in our ongoing research, reports the synthesis of four pure Ni chains with decreased diameters (∼80, ∼50, ∼30, and ∼15 nm) using a wet chemical solution. The magnetic properties, varying with * To whom correspondence should be addressed. E-mail: guolin@ buaa.edu.cn (L.G.),
[email protected] (C.C.),
[email protected] (J.W.). † Beijing University of Aeronautics and Astronautics. ‡ Peking University. § Tsinghua University.
the diameter of the samples, are systematically studied and analyzed. Meanwhile, the sample with a diameter of 80 nm is selected to serve as a catalyst in the degradation of pentachlorophenol (PCP) solutions. Experimental Section In a typical procedure, all of the reagents were analytical grade and used without further purification. Quantities of 0.119 g of NiCl2 · 6H2O (5 mmol/L) and 0.333 g of polyvinyl pyrrolidone (PVP; average MW, 58 000; K29-32) were dissolved in 100 mL of the solvent ethylene glycol (EG) at room temperature, yielding a light green transparent solution. During the process of dropping the hydrazine monohydrate liquid (80%) by 0.25 mL to the mixture, the solution first turned blue and then slowly into light purple. Afterward, the solution was heated to 196 °C in an oil bath. It was refluxed for 3 h under vigorous magnetic stirring at that temperature. Subsequently, the product was washed with ethanol and deionized water several times. At last, Ni nanochains with grain sizes of ∼15 nm (sample A) were obtained by centrifugation and drying in an oven at 80 °C. Sample B (∼30 nm), C (∼50 nm), and D (∼80 nm) were achieved by changing the concentration of nickel salt amd the quantity of PVP and hydrazine monohydrate (detailed descriptions in the Supporting Information). The structures and components of the as-prepared products were characterized by X-ray powder diffraction (XRD) using a Rigaku Dmax2200 X-ray diffractometer with Cu KR radiation (λ ) 1.5416 Å). The XRD specimens were prepared by means of flattening the powder on the small slides. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) investigations were carried out by a JEOL JEM-2100F microscope. The as-grown samples were dispersed in ethanol and dropped onto a copper grid supported by carbon films. Field emission scanning electron microscopy (FE-SEM) was carried out by a field emission scanning electron microanalyzer (Hitachi S-4800, 5 kV) with the samples prepared in the thick suspension dropping on the silicon slice. Magnetic properties of the nanochains in a powder collection were measured using a
10.1021/jp906234n CCC: $40.75 2009 American Chemical Society Published on Web 09/15/2009
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Figure 1. (a) TEM image of Ni chains (sample A) composed of particles with a diameter of ∼15 nm. (b) HRTEM image showing a single-crystal structure corresponding to the part marked in (a).
Quantum Design superconducting quantum interference device (SQUID) magnetometer. Results and Discussion Figure 1a shows the bright field TEM image of sample A. It is obvious that the chains are composed of distinct nanoparticles with an average diameter of 15 nm. Shown in Figure 1b, the HRTEM image was taken from the marked region in Figure 1a to further reveal the structure. In Figure 1b, it is apparent that the particles are single crystals showing the planes of (111)Ni with the spacing of 0.203 nm. From our observation, the HRTEM results of other particles also show a single-crystal structure (Figure S1 in the Supporting Information). Figure 2 shows TEM images of samples B (30 nm) and C (50 nm). The uniform nanochains aggregate to form irregularly cross-linked networks. Figure 2a shows a TEM image for sample B, and the inset shows a HRTEM image corresponding to the marked particle. It reveals the planes of (111)Ni with a spacing of 0.203 nm, showing a single-crystal nature. In Figure 2b, the HRTEM image clearly shows a polycrystalline structure for a different particle with sample B. It could be concluded that the chains (sample B) are composed of single-crystal particles mixing with polycrystalline particles. Figure 2c shows the TEM image for sample C, and the inserted selected area electron diffraction (SAED) pattern shows a polycrystalline structure. The spacings of 0.203, 0.176, 0.124, 0.106, and 0.081 nm are nicely indexed to the fcc Ni with the (111), (200), (220), (311), and (331) planes, respectively. Sample D has a similar morphology and structure with sample C, and SEM images of sample C and D are shown in Figure S2 (Supporting Information). According to the morphology and structure characterizations, it is clear that the Ni chains are grown with their shape approaching wirelike, and the particles forming the chains vary from single-crystal (samples A) to polycrystal (samples B and C) with increasing particle size. The single component of Ni for the four samples was confirmed by the XRD patterns in Figure 3. All of the diffraction peaks can be assigned to the planes of (111), (200), and (220) of a Ni face-centered cubic (fcc) structure (JCPDS no. 04-0850). The diffraction peaks become broadened as the particle size decreases, from pattern d for sample D (80 nm) to pattern a for sample A (15 nm). Figure 4a shows the M-H data measured at T ) 5 K for the four samples A, B, C, and D. The mass for the powder samples is 0.66 mg for sample A, 0.90 mg for sample B, 0.16 mg for sample C, and 0.73 mg for sample D. Obviously, the saturation magnetization decreases as the grain diameter decreases, and it is 48.5 emu/g (0.52 µB/Ni), 46.4 emu/g (0.50 µB/Ni), 44 emu/g
Zhou et al. (0.47 µB/Ni), and 38 emu/g (0.41 µB/Ni) for samples A, B, C, and D, respectively. This is consistent with our previous result, in which the reduction of the saturation magnetization is ascribed to the presence of a spin glass surface layer and, perhaps, also attributed to the surface encapsulation effect of the PVP.12 The open hysteresis loops in the low field region are shown in Figure 4b. The coercivity, HC, at T ) 5 K is determined to be 414 Oe (80 nm), 555 Oe (50 nm), 570 Oe (30 nm), and 790 Oe (15 nm). There is a slight enhancement in the coercivity, compared with the Ni nanochains reported before.12 For example, at T ) 5 K, HC for sample D (80 nm) is 414 Oe, whereas it is 305 Oe for the sample with a diameter of 75 nm.12 On the other hand, the coercivity of the sample decreases with increasing diameter, and such variation with the grain size is predictable. A similar result has also been observed with Ni nanowires by Zheng et al.11 They have reported that the coercivity of Ni nanowires decreases with increasing diameter from ∼18 to ∼22 nm. The remnant ratios are determined to be MR/MS ) 0.38, 0.48, 0.45, and 0.58 for samples D, C, B, and A, respectively, in which MR is the remanence and MS is the saturation magnetization determined at T ) 5 K. By considering the coherence length of Ni, ∼25 nm, it is interesting to find that the magnetization reversal mechanism of both samples B and C can be described by the model of “chain of spheres”.19 For a chain consisting of single-domained nanoparticles, HC predicted for the fanning mode at T ) 0 K is expressed as HC,n ) (πms/6) · (6Kn - 4Ln), where ms is the saturation magnetization per unit volume. The expression in the latter parentheses accounts for the pairwise dipolar interaction between each pair of magnetic spheres in the chain based on the assumption of the fanning mode, and the index, n, is for the number of spheres with the chain.19 In this model, the thermal activation is not accounted for. The calculated result is only for the ground-state property. Therefore, it is adequate to compare the calculated value with the experimental data at low temperature, for example, at T ) 5 K. By using the experimental values of the saturation magnetization, MS ) 44 and 46.4 emu/g for samples B (30 nm) and C (50 nm), the density of bulk Ni, ∼8.9 g/cm3, and assuming n ) 8, we obtain the values of HC as 510 and 520 Oe, respectively. It is noted that the coercivity calculated with n ∼ ∞ and n ) 12 are about 21% and 8%, respectively, larger than the one calculated with n ) 8. By taking into account the correction factor for the randomly oriented effect for a powder sample, the estimated value of the coercivity is H’C ∼ 1.1 HC ∼ 570 and 560 Oe. This agrees well with our experimental result, ∼570 and 555 Oe for 30 and 50 nm samples, respectively. It indicates that the magnetization reversal of the samples is dominated by the shape anisotropy arising from the quasi-one-dimensional structure.20 The remnant ratios, MR/MS, at T ) 5 K for samples B and C are 0.45 and 0.48, respectively. These are also in agreement within 10% with the prediction by the fanning model, ∼0.5, for the randomly oriented particles. It is surprising that the magnetization reversal for sample C (50 nm) agrees well with the description of the fanning mode, although 50 nm is much larger than the coherence length of Ni, ∼25 nm. On the other hand, the coercivity, ∼414 Oe, and the remnant ratio, ∼0.38, for sample D (80 nm) are much smaller than the values predicted by the model of fanning mode, which are HC ∼ 600 Oe and MR/MS ∼ 0.5. The much reduced coercivity and remnant ratio from the calculated values are expected for sample D (80 nm) since the sample size, 80 nm, is much larger than the coherence length of Ni, ∼25 nm. In this case, the magnetization reversal by nucleation rotation rather than coherence rotation is expected. In addition, the coercivity
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Figure 2. (a) TEM image of sample B with a diameter of ∼30 nm. The inset shows a single-crystal structure of the marked particle. (b) Another HRTEM image of sample B showing a polycrystalline structure. (c) TEM image of sample C with a diameter of ∼50 nm. The SAED pattern shows a polycrystalline structure.
Figure 3. XRD patterns of the as-synthesized nanochains with different diameters: (a) sample A (∼15 nm), (b) sample B (∼30 nm), (c) sample C (∼50 nm), and (d) sample D (∼80 nm).
Figure 4. (a) M-H data measured at T ) 5 K for the four samples, sample A (15 nm), B (30 nm), C (50 nm), and D (80 nm). (b) Open hysteresis loops observed in the low field region. The coercivities are determined to be 790 Oe (sample A), 570 Oe (sample B), 555 Oe (sample C), and 414 Oe (sample D).
for sample A (15 nm) has a surprisingly large value, ∼790 Oe, which is much larger than that calculated by the fanning mode, ∼470 Oe. This value is also much larger than the result, ∼600 Oe, obtained at T ) 5 K for the Ni nanowires with a diameter ranging from 40 to 100 nm and a length up to 5 µm.21 It indicates that there is a fundamental difference in the magnetization reversal mechanism for the sample with the smallest diameter. Hence, it remains an interesting issue to understand the enhancement effect in coercivity with the present Ni nanochains with a diameter of ∼15 nm, or even smaller. From these samples, sample D (∼80 nm) is chosen to study its performance as a catalyst in the degradation of pentachlorophenol (PCP), which is toxic, carcinogenic, and difficult to biodegrade as one kind of important contaminants in groundwater. The experimental details for the catalysis experiments are found in the Supporting Information.
Figure 5. UV curves of PCP solutions treated by (a) none for 20 h, (b) Ni nanochains for 20 h, (c) Fe0 nanoparticles for 4 h, and (d) Fe0 nanoparticles and Ni nanochains for 4 h.
Figure 6. Removal efficiency of PCP solutions by Fe0 nanoparticles with and without Ni nanochains.
Figure 5 shows the differences of UV curves of PCP solutions treated by different chemical components. Peak intensities at 220, 250, and 320 nm, which are the specific adsorption spectrum of PCP, lowers obviously in (c) and (d) compared with (a), whereas there is no evident reduction in (b). It indicates that Ni nanochains alone do not show an appreciable effect on the degradation of PCP. Additionally, (d) achieves a better result in cutting down the level of PCP than (c) does. This implies that Ni nanochains serve as a catalyst for this reaction. Figure 6 gives the removal efficiency of PCP solutions. It is higher by 17.7% and 17.4% for Fe0 nanoparticles with Ni nanochains than without Ni nanochains for 2 and 4 h, respectively. Further experiments are needed to study the catalysis effect with Ni nanochains. The degradation of PCP is mainly accomplished by dechlorination, which is the key point to improve the biodegradability in the detoxification of polychlorinated organic compounds.
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Three possible pathways are proposed for dechlorination with Fe, including direct electron transfer from iron metal at the iron surface, reduction by Fe2+ from corrosion of iron, and catalyzed hydrogenolysis by the hydrogen formed from reduction of water.22-24 We believe that the first way is the main one, according to the following reaction +
Fe + RCl + H ) Fe 0
2+
-
+ RH + Cl
Meanwhile, some Fe nanomaterials react with water and produce hydrogen, which could be adsorbed by the added Ni catalyst.25 The aggregation of hydrogen will promote the degradation of PCP by the third way above. Several researchers used nanoscale Ni/Fe bimetals to degrade the organic contaminants and obtained similar results.26,27 However, we use separate Fe and Ni particles instead of Ni/Fe bimetallic particles. This is advantageous for more convenient storage as a catalyst in industrial applications. Conclusion In conclusion, Ni nanochains with various diameters, including 15, 30, 50, and 80 nm, were synthesized by a wet chemical route. According to the characterization by HRTEM, the Ni particles of ∼15 nm are single crystals. Magnetic measurements on the powder samples of Ni chains reveal that the saturation magnetization decreases and the coercivity increases with decreasing diameter. The coercivity and the remnant ratio of the samples with a diameter of 30 and 50 nm agree well with the calculated results by the fanning mode, according to the chain of spheres model. However, the coercivity is greatly enhanced to 790 Oe at low temperature, T ) 5 K, beyond the description of the chain of spheres model for the sample with the smallest diameter (15 nm). In addition, catalytic measurement indicates that the sample with a diameter of 80 nm enhances the removal efficiencies of PCP with Fe0 nanoparticles by ∼18%. The samples thus produced are helpful to understanding magnetic properties with application potentials as well as to the degradation of water pollution with PCP. Acknowledgment. This project was financially supported by the National Natural Science Foundation of China (20673009, 10874006, and 50725208), the Specialized Research Fund for the Doctoral Program of Higher Education (20060006005), and the Innovation Foundation of BUAA for Ph.D. Graduates as well as by the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2006CB932301).
Supporting Information Available: Preparation of samples B, C, and D; other HRTEM images of sample A; SEM images of samples C and D; and experimental details for catalysis experiments. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Grancharov, S. G.; Zeng, H.; Sun, S. H.; Wang, S. X.; O’Brien, S.; Murray, C. B.; Kirtley, J. R.; Held, G. A. J. Phys. Chem. B 2005, 109, 13030. (2) Burton, J. D.; Sabirianov, R. F.; Jaswal, S. S.; Tsymbal, E. Y. Phys. ReV. Lett. 2006, 97, 077204. (3) Mahata, N.; Cunha, A. F.; Orfao, J. J. M.; Figueiredo, J. L. Catal. Commun. 2009, 10, 1203. (4) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Hu, J. T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (5) Cordente, N.; Respaud, M.; Senocq, F.; Casanove, M.-J.; Amiens, C.; Chaudret, B. Nano Lett. 2001, 1, 565. (6) Liu, Z.; Li, S.; Yang, Y.; Peng, S.; Hu, Z.; Qian, Y. AdV. Mater. 2003, 15, 1946. (7) Leng, Y.; Li, Y.; Li, X.; Takahashhi, S. J. Phys. Chem. C 2007, 111, 6630. (8) Liu, C. M.; Guo, L.; Wang, R. M.; Deng, Y.; Xu, H. B.; Yang, S. H. Chem. Commun. 2004, 2726. (9) An, Z.; Pan, S.; Zhang, J. J. Phys. Chem. C 2009, 113, 1346. (10) Ni, X.; Zhao, Q.; Zhang, D.; Zhang, X.; Zheng, H. J. Phys. Chem. C 2007, 111, 601. (11) Zheng, M.; Menon, L.; Zeng, H.; Liu, Y.; Bandyopadhyay, S.; Kirby, R. D.; Sellmyer, D. J. Phys. ReV. B 2000, 62, 12282. (12) He, L.; Zheng, W. Z.; Zhou, W.; Du, H. L.; Chen, C. P.; Guo, L. J. Phys.: Condens. Mater. 2007, 19, 036216. (13) Du, Y.; Chen, H.; Chen, R.; Xu, N. Appl. Catal., A 2004, 277, 259. (14) Wojcieszak, R.; Zielin˜ski, M.; Monteverdi, S.; Bettahar, M. M. J. Colloid Interface Sci. 2006, 299, 238. (15) Gao, J.; Guan, F.; Zhao, Y.; Yang, W.; Ma, Y. Mater. Chem. Phys. 2001, 71, 215. (16) Park, J.; Kang, E.; Son, S. U.; Park, H. M.; Lee, M. K.; Kim, J.; Kim, K. W.; Noh, H. J.; Park, J. H.; Bae, C. J.; Park, J. G.; Hyeon, T. AdV. Mater. 2005, 17, 429. (17) Hong, S. L.; Shin, Y. H.; Ihm, J. J. Appl. Phys. 2002, 141, 6142. (18) Tuan, H. Y.; Lee, D. C.; Hanrath, T.; Korgel, B. A. Nano Lett. 2005, 5, 681. (19) Jacobs, I. S.; Bean, C. P. Phys. ReV. 1955, 100, 1060. (20) Ely, T. O.; Amiens, C.; Chaudret, B. Chem. Mater. 1999, 11, 526. (21) Wernsdorfer, W.; Hasselbach, K.; Benoit, A.; Barbara, B.; Doudin, B.; Meier, J.; Ansermet, J.-Ph.; Mailly, D. Phys. ReV. B 1997, 55, 11552. (22) Schlimm, C.; Heitz, E. EnViron. Prog. 1996, 15, 38. (23) Yak, H. K.; Wnclawiak, B. W.; Cheng, I. F.; Doyle, J. G.; Wai, C. M. EnViron. Sci. Technol. 1999, 33, 1307. (24) Yak, H. K.; Lang, Q.; Wai, C. M. EnViron. Sci. Technol. 2000, 34, 2792. (25) Zhong, Z. Y.; Xiong, Z. T.; Sun, L. F.; Luo, Z. F.; Chen, P.; Wu, X.; Lin, J.; Tan, K. L. J. Phys. Chem. B 2002, 106, 9507. (26) Zhang, W. X.; Wang, C. B.; Lien, H. L. Catal. Today 1998, 40, 387. (27) Tee, Y.; Grulke, E.; Bhattacharyya, D. Ind. Eng. Chem. Res. 2005, 44, 7062.
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