J. Phys. Chem. C 2007, 111, 12663-12668
12663
Controlled Synthesis of 3D and 1D Nickel Nanostructures Using an External Magnetic Field Assisted Solution-Phase Approach Genqiang Zhang, Tao Zhang, Xiaoli Lu, Wei Wang, Jifeng Qu, and Xiaoguang Li* Hefei National Laboratory for Physical Sciences at Microscale, Department of Physics, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China ReceiVed: April 21, 2007; In Final Form: June 26, 2007
Nickel three-dimensional (3D) urchin-like particles and one-dimensional (1D) chainlike nanowires are selectively synthesized by an in situ magnetic-field-directed solution-phase method at ambient conditions. The results indicate that the morphology of the products strongly depends on the external magnetic-field distribution. The urchin-like particle is composed of a spheric core connected by spear-shaped branches under the nonuniform magnetic field, while the chainlike nanowire is formed through 1D alignment of spheres with the aid of parallel magnetic-field distribution during the reaction. The branch lengths and the whole size of the urchin-like particles can be tailored by adjusting the pH value and reaction temperature, respectively. The magnetic properties of the urchin-like particles are studied in detail as well and the results reveal that the saturated magnetization and coercivity are strongly related to the corresponding microstructure. This work provides a simple and effective strategy to modulate the morphology of the magnetic materials through the external magnetic-field force.
1. Introduction Ferromagnetic metal and metal oxide nanostructures with controllable morphology and dimension are of great importance because of both their size- and shape-dependent properties and potential applications in diverse areas, including magnetic recording media, sensors, ferrofluids, and catalysts.1,2 Thus, considerable efforts have been devoted to exploit possible methodologies to fabricate well-defined nanostructures of these kinds of materials and remarkable progress has been achieved.3-9 For example, sphere- and rod-shaped cobalt nanocrystals were obtained by decomposing Co2(CO)8 in a binary surfactants mixture.3,4 Nickel hollow spheres,5 nanobelts,6 and nanorods7 have been synthesized using surfactant-assisted hydrothermal processes and refluxing method. Also, the synthesis of various metal oxides, such as Fe2O3 and CoFe2O4 nanostructures, has been studied in detail.8-11 However, most of the present strategies involved need rigorous conditions such as high pressure, high temperature, or metallorganic precursors and surfactants in order to effectively tailor the size and shape of products. In this case, much work has been done in order to search more facile and friendly morphology-directing agents. Recently, magnetic-field force has been adopted to assemble magnetic particles into one-dimensional (1D) chains,12-14 twodimensional (2D) lattices, and nanorods into three-dimensional (3D) microstructures by a two-step process.15-17 In addition, magnetic-field-assisted hydrothermal process has been proved to be an effective way for directing the growth of several 1D magnetic materials, such as Co and FeSx nano-/microwires.18-20 It is believed that 1D magnetic nanostructures could be obtained when the parallel magnetic field is applied during the synthesis.12-14 It is due to these previous works12-14 that we expect to know if it is possible to tailor the morphology of the magnetic materials by tuning magnetic-field distribution, which * To whom correspondence should be addressed. Telephone/fax: +86551-3603408. E-mail:
[email protected].
may contribute to the development of magnetic-field-assisted methods. In our work, besides the uniformly distributed parallel magnetic field, a nonuniformly distributed magnetic field is applied for the first time during the synthesis of nickel nanoparticles to investigate the influence on the microstructure of the products. Compared with 1D nanostructures under the parallel magnetic field, a quite different morphologys3D urchinlike particleswas obtained. The results illustrate that it is possible to obtain magnetic materials with controllable morphology using adjustable magnetic-field force as the shape-guiding agent. This work may provide an additional strategy for applying external magnetic field to tailor the morphology of magnetic materials. 2. Experimental Section All chemicals were of analytical grade and used without further purification. In a typical synthesis, 5 mmol Ni(NO3)2‚ 6H2O was dissolved in 40 mL of ethylene glycol to form a transparent green solution followed by the addition of 8 mL of distilled water containing different amounts of NaOH. The obtained precursor solution was magnetically stirred for about 30 min followed by the addition of 2 mL of hydrazine monohydrate solution (N2H4‚H2O, 85%). Then, the precursor solution was transferred to a 100 mL round-bottom flask followed by heating to the settled temperature, and the nickel nanocrystals were obtained by the reduction reaction21 under different kinds of magnetic-field distributions. For the 3D urchin-like particle synthesis, a single circular magnet was put under the flask to make the nonuniformly distributed magnetic field. For the 1D chainlike nanowire synthesis, an electric magnet was utilized to produce the parallel magnetic-field distribution. The magnetic-field intensity 1 cm above the circular magnet was about 600 Oe, where the round-bottom flask was placed. The intensity of the parallel magnetic field was also 600 Oe, which was controlled by the working electric current. After refluxing for 40 min at the settled temperatures, the black
10.1021/jp073075z CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007
12664 J. Phys. Chem. C, Vol. 111, No. 34, 2007
Zhang et al.
Figure 1. Integrated characterization of the urchin-like particles obtained at 120 °C for 40 min with NaOH concentration of 1 mol/L under the nonuniform magnetic-field distribution: (a) XRD pattern, (b) SEM image, (c) TEM image, (d and e) TEM images of a single urchin-like particle and an individual branch, (f and g) SAED pattern and the corresponding HRTEM image of the single branch in e.
product was collected and washed with distilled water and ethanol several times. The sample was then dried in vacuum at 60 °C for further characterization. The X-ray powder diffraction (XRD) pattern of the asprepared products was collected on a Philips X’pert diffractometer with Cu KR (KR ) 1.5418 Å) radiation. Field-emission scanning electron microscope (FE-SEM) measurements were carried out on a field-emission microscope (JEOL 6700). Electron microscopy analyses were performed on a Hitachi H-800 transmission electron microscope (TEM) and JEOL-2010 high-resolution TEM (HRTEM) at an acceleration voltage of 200 kV. The magnetization measurements of the as-synthesized samples were carried out on a superconducting quantum interference device (SQUID) (MPMS, Quantum Design). 3. Results and Discussion Figure 1 shows the integrated characterization of the urchinlike nickel particles obtained after reaction for 40 min with the aid of a single circular magnet under the flask during refluxing. The NaOH concentration and the reaction temperature are 1 mol/L and 120 °C, respectively. The characteristic peaks of a typical X-ray diffraction (XRD) pattern shown in Figure 1a can be steadily indexed to (111), (200), and (220) crystalline planes corresponding to the face-centered cubic (fcc) structure of nickel (JCPDS Card No. 04-0850), which indicates the single phase of the as-synthesized sample. The low-magnification scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, as shown in Figure 1b,c, indicate the highyield growth and good uniformity of the urchin-like particles. The inset in Figure 1b and the image in Figure 1d exhibit magnified views of the product, confirming that the urchinlike morphology consists of a spheric core attached with various spearlike branches. Figure 1e gives the morphology of an individual branch of the urchin-like particle. The corresponding selected area electron diffraction (SAED) pattern and highresolution TEM (HRTEM) image indicate the single crystalline nature of the branch. The SAED pattern can be steadily indexed to (111), (111h), and (220) facets, and the adjacent space of about 2.013 Å corresponds to the distance between two (111) crystalline planes. It should be noted that the single circular magnet used above makes the nonuniform magnetic-field distribution inside the
Figure 2. (a, b) SEM images; (c, d) TEM images; (e) SAED pattern of the 1D chainlike nanowires obtained at 120 °C for 40 min with NaOH concentration of 1 mol/L under the parallel magnetic-field distribution.
precursor solution, which may play a critical role in determining the morphology of the product. We therefore performed the synthetic process under the parallel magnetic field produced by an electric magnet while keeping other conditions constant in order to investigate the effect of magnetic-field distribution on the morphology. Parts a and b of Figure 2 exhibit typical SEM images of the as-prepared product with NaOH concentration of 1 mol/L in different magnifications. As can be seen, only chainlike nanowires composed of spheric particles are obtained. The lengths and average diameters of the nanowires are up to several micrometers and about 100 nm, respectively. By carefully observing the surface of the nanowires from the TEM images shown in Figure 2c,d, no branches are found, indicating that the morphology of the product is strongly related to the magnetic-field distribution during the reaction. The SAED pattern shown in Figure 2e indicates the polycrystalline nature of the sample and can be readily assigned to (111), (200), (220), and (311) crystalline planes. The polycrystalline nature of the products reveals that the chainlike nanowires could consist of magnetized particle aggregation through the magnetic interaction. Products obtained with different pH values under the nonuniform magnetic field are also investigated in detail for
3D and 1D Ni Nanostructure Controlled Synthesis
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12665
Figure 3. (a-e) SEM images of nickel nanostructures obtained at 120 °C with different NaOH concentrations: (a) 0.2, (b) 0.4, (c) 1, (d) 1.6, and (e) 2.5 mol/L under the nonuniform magnetic-field distribution. The insets show the corresponding magnified view of the products. (f, g) Typical SEM images of the products obtained without the external magnetic field at 120 °C for 40 min with NaOH concentrations of 1 and 2.5 mol/L, respectively.
Figure 5. Schematically illustrated formation process of the 3D urchinlike particles and 1D chainlike nanowires under different kinds of magnetic-field (H) distributions: (a) radially distributed and (b) parallel magnetic-field distribution.
Figure 4. TEM images of products obtained at 120 °C for different reaction times: a) 1, (b) 5, (c) 10, and (d) 40 min with a NaOH concentration of 1 mol/L under the nonuniform magnetic-field distribution. The insets show the corresponding magnified views of the products.
shedding light on the relationship of the morphology, pH value, and external magnetic field. Parts a-e of Figure 3 give the SEM images of the products obtained at 120 °C for 40 min with the NaOH concentrations of 0.2, 0.4, 1, 1.6, and 2.5 mol/L. The urchin-like particles having branches with different lengths, as shown in Figure 3a-c, are obtained with the NaOH concentrations of 0.2, 0.4, and 1 mol/L. By investigating the magnified SEM images of these three samples shown in the insets of Figure 3a-c, it can be concluded that the branch lengths increase with the increasing NaOH concentrations. However, the urchin-like morphology starts to be
destroyed when the NaOH concentration reaches 1.6 mol/L see Figure 3d, exhibiting the mixture of studded spheres together with some urchin-like particles. The product further evolves into pure spheres with a relatively slick surface when the NaOH concentration increases to 2.5 mol/L, as shown in Figure 3e. The morphology evolution process mentioned above indicates that the pH value of the medium drastically influences the branch length of the urchin-like particles. However, the precondition for controlling the morphology through adjusting the pH value is the existence of the nonuniformly distributed magnetic field during the reaction, which may act as the shape-directing agent. We therefore investigate the products prepared without external magnetic field in order to examine its effect on the morphology. Typical SEM images of the as-prepared samples (Figure 3f,g) with NaOH concentrations of 1 and 2.5 mol/L show that only quasi-spheres with respective diameters ranging from 80 to 100 nm can be obtained. Products obtained with other NaOH concentrations show no significant differences except the size. Therefore, it can be regarded that the urchin-like morphology strongly depends on the nonuniform magnetic-field distribution, and the pH value can effectively control the branch lengths. To understand the growth process of the urchin-like particles, the time-dependent products are prepared under the nonuniform magnetic-field distribution. Parts a-d of Figure 4 show the typical TEM micrographs of the as-synthesized products after refluxing for 1, 5, 10, and 40 min at 120 °C with the NaOH
12666 J. Phys. Chem. C, Vol. 111, No. 34, 2007
Zhang et al.
Figure 6. Integrated characterizations of the products obtained at different temperatures for 40 min under the nonuniform magnetic-field distribution: SEM images of the samples synthesized at 90 °C with NaOH concentration of (a, b) 0.2 and (c, d) 1 mol/L; TEM images of the products obtained at 170 °C with NaOH concentrations of (e) 0.2 and (f) 1 mol/L, respectively. The insets show the corresponding TEM images in larger magnifications.
concentration of 1 mol/L. It can be seen that the freshly obtained nickel nanocrystals under the nonuniform external magnetic field may sharply assemble into quasi-spherical particles with diameters in the range of 50-90 nm at the reaction time of 1 min, as shown in Figure 4a. The size of the particles increases to 160-200 nm along with the prolonged reaction time to 5 min, and the morphology evolves to the mixture of studded spheres and urchin-like particles with very short branches (Figure 4b). When the reaction time reaches 10 min, the product develops into pure urchin-like particles and the branches get longer (Figure 4c) compared with that of particles shown in Figure 4b. The well-defined urchin-like particles with branch lengths up to ∼300 nm are obtained when the reaction time reaches 40 min, as shown in Figure 4d. On the basis of these results mentioned above, the plausible formation processes for both the 3D urchin-like particles and 1D chainlike nanowires are schematically illustrated in Figure 5A,B, respectively. The stepwise formation process could be described as follows: For the urchin-like particles, at the early stage, the small nickel nanocrystals, as illustrated in stage I of Figure 5A, obtained by the reduction of Ni2+ with hydrazine monohydrate, are magnetized, leading to the enhanced magnetic dipole interactions among these particles. Therefore, they aggregate isotropically into quasi-spheres with a studded surface in a short period, and the aggregation velocity is superior to that for producing nickel nanocrystals at this moment. After this stage, the subsequently produced nickel nanocrystals may gradually attach to the spheric particles instead of the sharp
aggregation of a large number of these nanocrystals due to the reduced nickel ions in the medium and the relatively faster aggregation velocity. The formation of the spearlike branches could be a seed-induced growth process, as shown in stages II and III of Figure 5a, and the branch lengths are dominated by the pH value of the medium. Due to the complex growth conditions involved, it is hard to precisely describe the growth process of the branches. Nevertheless, it still could be concluded that, in this magnetic-field-assisted solution-phase method, the formation of the urchin-like morphology experiences a twostep process: the initial spheric particles consisted of nickel nanocrystals through aggregation and the subsequently seedinduced anisotropic growth of 1D spearlike branches attached to the core with appropriate pH value in the medium. On the other hand, the formation process of the chainlike nanowires under the parallel magnetic field is roughly illustrated in Figure 5B as well. In this condition, the magnetized small nanocrystals at close range still have a strong tendency to isotropically aggregate into spheric particles due to the strong magnetic interactions. Meanwhile, the magnetic-field distribution of the magnetized particles could exhibit an anisotropic feature according with the external magnetic-field distribution, especially when their sizes get larger after initial aggregation, as illustrated in stage I of Figure 5B. This will result in the 1D alignment of the particles. The size of the urchin-like particles can be easily controlled by the reaction temperature. Parts a,b and c,d of Figure 6 give the SEM images of the products synthesized at 90 °C with the
3D and 1D Ni Nanostructure Controlled Synthesis
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12667 TABLE 1: Coercivity (HC), Saturated Magnetization (MS), and Diameter of the Urchin-like Particles with Different Branch Lengths at 2 K NaOH concn/(mol‚L-1)
MS/ (emu‚g-1)
HC/Oe
core diam/nm
branch length/nm
0.2 0.4 1
54.1 51.3 32.5
219.5 304.9 458.5
240-300 300-350 400-450
