Applied Surface Science 241 (2005) 218–222 www.elsevier.com/locate/apsusc
Size-controlled synthesis of nickel nanoparticles Y. Houa,b, H. Kondohb, T. Ohtab,*, S. Gaoa a
State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Abstract A facile reduction approach with nickel acetylacetonate, Ni(acac)2, and sodium borohydride or superhydride leads to monodisperse nickel nanoparticles in the presence of hexadecylamine (HDA) and trioctylphosphine oxide (TOPO). The combination of HDA and TOPO used in the conventional synthesis of semiconductor nanocrystals also provides better control over particle growth in the metal nanoparticle synthesis. The size of Ni nanoparticles can be readily tuned from 3 to 11 nm, depending on the ratio of HDA to TOPO in the reaction system. As-synthesized Ni nanoparticles have a cubic structure as characterized by power X-ray diffraction (XRD), selected-area electron diffraction (SAED). Transmission electron microscopy (TEM) images show that Ni nanoparticles have narrow size distribution. SQUID magnetometry was also used in the characterization of Ni nanoparticles. The synthetic procedure can be extended to the preparation of high quality metal or alloy nanoparticles. # 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Y; 75.50.K; 61.46 Keywords: Synthesis; Nickel; Nanoparticles; Magnetism
1. Introduction With increase interest in fabricating nanodevices with nanosized blocks, much attention has been focused on exploiting a general route to control size and morphology of nanoscale materials [1,2]. In recent years, nanoscale magnetic materials have * Corresponding author. Tel.: +81 3 5841 4331; fax: +81 3 3812 1896. E-mail address:
[email protected] (T. Ohta),
[email protected] (S. Gao).
attracted much interest due to the potential application in magnetic recording technology [3,4]. A flexible synthetic route is indispensable to exploit magnetic storage materials. So far, a number of physical and chemical routes have also been applied to produce nanoscale magnetic materials, including mechanical grinding, sonochemistry, organometallic precursor pyrolysis, metal melt reduction in micelle phase, and electrochemical deposition, etc. [5]. However, the size distribution of the products is not ideal. Recent developments of the organometallic route to produce high quality semiconductor nanocrystals included the
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.09.045
Y. Hou et al. / Applied Surface Science 241 (2005) 218–222
combination of trioctylphosphine oxide (TOPO)-TOP, pure hexadecylamine (HDA) or HDA–TOPO-TOP as the stabilizing agents [6]. Some combinations of TOPO and oleic acid [1], oleic acid and oleylamine [2], ACA and TOP [4] have also been applied to produce magnetic nanocrystals via the decomposition of organometallic precursors. However, the organometallic precursors are volatile and thermally unstable, gradually releasing toxic CO and metal at ambient temperature. The simple reduction route should be considered to produce monodisperse magnetic metal nanoparticles [7,8]. Reduction synthesis of Co, Ni nanocrystals in the presence of the alkylphosphine oxide-alkylphosphine combination was reported recently [9]. Compared with other magnetic metals such as Co, FePt, etc., relatively few works have been done on the fabrication of monodisperse Ni nanoparticles. Previously, we prepared monodisperse Ni nanoparticles in monosurfactant (HDA) system without other solvents [10]. Considering the growing demand of metal nanocrystals, we tried to improve the TOPO–oleic acid by introducing HDA into the metal colloidal synthesis system. Herein we describe the size-controlled synthesis of nickel nanoparticles through the modification of surfactants, such as HDA–TOPO–oleic acid. The solution reduction of Ni(acac)2 (acac = acetylacetonate) by sodium borohydride or superhydride in dichlorobenzene was employed to produce nickel nanoparticles. Surfactants were used to control the growth of nanocrystals and coat the nanoparticles to prevent them from further oxidation and aggregation.
2. Materials and methods HDA (90%) and TOPO (99%) were purchased from Aldrich, Ni(acac)2, sodium borohydride and oleic acid were analytic grade reagents. All of them were used without further purification. The synthesis was conducted using a standard airless technology. In a typical procedure, 0.2 g of Ni(acac)2 was dissolved in 5 ml of o-dichlorobenzene at 100 8C, and quickly injected into the mixture including 40 ml of dichlorobenzene, 0.5–1.5 g of TOPO, designed amount of HDA, and 0.15 g of sodium borohydride at 120–160 8C during vigorously stirring.
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The resulting mixture was heated to 180 8C and kept at this temperature for 30 min under Ar atmosphere. During this process, the color change from light yellow to dark was observed, indicating the formation of nickel particles. The resulting solution was allowed to cool to room temperature. Ni nanoparticles were extracted from the solution upon adding ethanol. The as-prepared products were redispersed in hexane or octane. Particle size and morphology were studied using a Hitachi 800 transmission electron microscope (TEM). TEM samples were prepared by dropping the hexane dispersion of nanoparticles on a carbon-coated copper grid. Powder X-ray diffraction (XRD) patterns of the samples were recorded with a Rigaku D/max 2000 X-ray diffractometer equipped with a Cu Ka radiation source (l = 0.15418 nm).
3. Results and discussion It is well known that strong intermolecular forces, such as van der Waals attraction, p–p interaction, etc., contribute to the aggregation of nanoparticles. As for magnetic nanoparticles, magnetic dipole–dipole interaction makes this kind of attraction stronger. Thus it is a challenge to obtain monodisperse magnetic nanoparticles dispersion. Different ligands, such as polymer and surfactants, have been used to modify the surface of nanoparticles for stabilization and to control the particle growth. In our experiments, a mixture of TOPO and/or HDA was used to control the particle size, stabilize nanoparticle dispersion and limit further oxidation on the particle surface. Oleic acid is an excellent stabilizing agent. But when it is employed alone, it binds so tightly to particle surfaces that the particle growth is impeded. TOPO can control the growth rate of nanoparticles because of the coordination with metal. If it is used alone, however, it cannot prevent nanoparticles from growing aggregates. The combination of TOPO–oleic acid proves the steadily growth of the particles [4], but was not very effective to produce smaller size nickel nanoparticles even if the concentration of TOPO was increased to 2.4 g in the present system. The long chain alkylamine such as hexadecylamine has stronger ligand ability with metal on the surface of nanometals than TOPO. By introducing HDA, we obtained nearly monodisperse smaller nickel nanoparticles.
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Fig. 1. XRD patterns for the samples prepared in different amounts of: (a) 1.5 g of TOPO, (b) 1.5 g of TOPO + 4.0 g of HDA and (c) 4.0 g of HDA.
To study the influence of HDA on the size of Ni nanocrystals, we kept the total amount of the solvent, Ni(acac)2, sodium borohydride and oleic acid constant. In the case of 1.5 g TOPO, high crystalline nickel particles were obtained. As shown in Fig. 1a, the XRD pattern shows the feature of cubic nickel structures, whose size is calculated to be about 11 nm
by using Scherrer’s formula. To obtain smaller Ni nanoparticles, HDA was introduced into the above solution. In the case of molar ratio 1:4 for TOPO to HDA, the XRD pattern of the product shows broadening of the peaks which correspond to the size of 5 nm by Scherrer’s formula. While in the case of only HDA, the X-ray reflections (2 0 0) and (2 2 0) in the sample (c) are further broadened and attenuated by the stacking faults along c-axis. The size of this sample was calculated to be 3 nm by Scherrer’s formula. The size and morphology of the Ni nanoparticles were characterized by TEM. As shown in Fig. 2, the average size of Ni nanoparticles corresponding to the samples in Fig. 1 is 11, 5 and 3.7 nm, respectively, which is in good agreement with the calculated values by Scherrer’s formula. The size distributions of nickel nanoparticles are shown in Fig. 2d–f. The appearance of some darker particles results from an enhanced diffraction contrast due to their orientation with respect to the electron beam. The selected-area electron diffraction (SAED) patterns in the inset reveals that the samples are crystalline or semicrystalline. With increase of HDA, the size of nickel
Fig. 2. TEM images of samples with various sizes (a) 11 nm, (b) 5 nm, (c) 3 nm, respectively. The size distributions of the samples are shown in the bottom of corresponding TEM image, respectively (d)–(f). The insets show the selected diffraction patterns of the samples (samples (a)–(c) from Fig. 1).
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nanocrystals decreases. It is worthwhile noting that the introduction of HDA strongly decreases the size of nanoparticles to 3.7 nm and narrows the size distribution of nanoparticles, as described above. The formation of Ni nanoparticles from the redox reaction in the present system is a very complicated process. The process begins with rapid nucleation and growth of nuclei into smaller clusters. HDA and TOPO were used as the capping agents to tune the growth of nanoclusters. It is generally accepted that HDA and TOPO reversibly coordinate with surface metals into the ‘‘intermediate’’, followed by the limit growth of metal nuclei [11]. In contrast to the linear structure of HDA, TOPO has bulky end groups, which probably protect the surface of nuclei from coating with surfactants, leading to high surface free energy. This is why the particles grow to larger sizes and crystallization easily occurs in contrast to experiments with fatty acids [4]. The capping agents allow the particles to be dissolved in non-polar solvent and also prevent agglomeration and oxidation of the particles. TOPO–oleic acid has been applied to obtain Co nanorods, which may be attributed to the metastable hcp structure, through dynamic control process recently [1]. In fact, we prepared anisotropic Ni nanocrystals by selecting the type of surfactants in the solvothermal reduction environment [12]. In the case of TOPO and/or HDA, however, we could not observe anisotropic structures of Ni nanocrystals. There are several possible factors to hinder the formation of a low dimensional structure. First, Ni nanocrystals have a strong tendency to form particles with fcc structure which is the most stable for bulk Ni. Second, it is necessary to use suitable stabilizers for controlling the facet growth to obtain the low dimensional structures. The combination of TOPO and/or HDA seems difficult to control the growth of one certain facet of Ni nanocrystals. The reaction condition such as reaction temperature, time, solvent and the potential between reaction agents might also have significant effects on the shape of nanocrystals. Considering the above factors, preparation of anisotropic nanostructures should be challenged, including Ni, CoPt, etc. [12]. Preliminary magnetic measurements were performed on Ni nanoparticles with use of a superconducting quantum interference device (SQUID).
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The temperature dependence of the magnetization was measured by using zero-field cooling (ZFC) and fieldcooling (FC) procedures. The blocking temperature of Ni nanoparticles with the size of 3, 11 nm was found to be 12 K and 80 K, respectively. The coercivity of the 3 nm nanoparticles at 5 K is about 200 Oe, whereas the coercivity at 300 K is nearly negligible, corresponding to the superparamagnetism [18]. More detailed magnetic studies are currently under way.
4. Conclusion We have developed a facile and chemical reduction route to synthesize size-controlled nickel nanoparticles. The surfactants including HDA and/or TOPO were used to control the particle size. In particular, the introduction of HDA is an effective approach to obtain smaller monodisperse metal nanoparticles. This approach provides useful information for the synthesis of other metal nanoparticles.
Acknowledgements This work was in part supported by the State Key Project for Fundamental Research of China (G1998061305), the National Science Fund for Distinguished Young Scholars of China (20125104), the Chinese Postdoctoral Science Fund, Japan Society for the Promotion of Science (JSPS) and the 21st Century COE Program from the Ministry of Education, Culture, Sports, Science and Technology. We thank for the helpful discussion with Dr. Yunhui Huang in Tokyo Institute of Technology.
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