J. Phys. Chem. C 2007, 111, 601-605
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Novel Hierarchical Nanostructures of Nickel: Self-Assembly of Hexagonal Nanoplatelets Xiaomin Ni, Qingbiao Zhao, Dongen Zhang, Xiaojun Zhang, and Huagui Zheng* Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026 People’s Republic of China ReceiVed: September 7, 2006; In Final Form: October 21, 2006
Novel hierarchical nanostructures of nickel, which were assembled by well-aligned hexagonal nanoplatelets, were fabricated through a simple solution method. The process involved the reduction of nickel dimethylglyoximate with hydrazine under controlled conditions. Based on a series of contrast experiments, the formation mechanism of the flowers was proposed, which could be ascribed to the cooperative effect of the complexant of dimethylglyoxime, a proper reaction rate, and inherent magnetic interactions. The present work provided an example for the synthesis of magnetic assembly nanostructures through properly selection of the reaction conditions, which were independent of any surfactants or external magnetic field.
I. Introduction Assembling nanosized materials into organized and designed structures has been of great interest for materials scientists for the assemblies usually exhibit novel properties different from bulk or discrete counterparts and show potential applications as nanodevices.1 Among the diverse assembled nanostructures, the magnetic assemblies are paid particular attention for their unique properties and uses in electronic, optoelectronic, magnetic, and biomedical fields.2 Generally, there are mainly two routes for assembling magnetic nanoparticles into desired architecture. One is making use of the molecular interactions between surfactant- or polymer-modified particles, including van der Waals forces, π-π interactions, electrostatic forces, and hydrogen bonding, which organize the building blocks together into a specific geometry configuration.3 For example, uniform spherical assembly of Fe3O4, aggregates of γ-Fe2O3, films of FePt nanoparticles and ribbons built by cobalt nanorods were all created via this method.4 In these assembled structures, the content of surfactants or polymers was relatively high and sometimes the assemblies collapsed when the surfactants or polymers were removed or dissolved in the solvent, which resulted in difficulties with their potential applications as nanodevices.5 Another effective route for assembling magnetic nanoparticles is by inducement of external magnetic field. Typically, the application of a magnetic field to a suspension of magnetic particles produces one-dimensional (1D) chains of particles with their magnetic dipoles aligned head-to-tail, parallel to the magnetic field, such as CoPt3 microwires and elongated clusters of Fe3O4, nickel nanochains, etc.6 But these assemblies usually required an external magnetic field to remain stable, and when the external magnetic field was removed from the system, the alignments destabilized.7 As a result, complicated synthetic equipments were needed to keep the external magnetic field unchanged. Currently, it is still challenging to explore simple routes for multidimensional interconnection of magnetic nanoparticles into assembly structures. Nanosized particles of nickel show diverse applications as catalysts, magnetic recording media, and medical diagnosis and * Corresponding author. Telephone: +86-551-3606144. Fax: +86-5513601600. E-mail:
[email protected].
conducting materials.8 In past decades, many different-shaped nickel nanocrystals had been fabricated with respect to the dependence of properties and applications on morphology. For example, nanotubes, hollow spheres, nanobelts, nanorods, nanoprisms, microwires aggregated by acicular particles, and chains consisting of spherical particles had been created by various methods.9 However, to date, little work has been seen for the creation of three-dimensional (3D) assembly nanostructures of nickel independent of any molecular interaction or external forces. Previously, we have shown the fabrication of hexagonalshaped nickel platelets by reducing Ni(dmg)2 (nickel dimethylglyoximate) with hydrazine.10 The present work, as a continued part of our research endeavor in this area, reports our new findings in the shape control of face-centered-cubic (fcc) Ni nanostructures. By properly adjusting the reaction conditions, a novel 3D flowerlike assembly structure was achieved. In the absence of any surfactants or external magnetic forces, we realized the oriented growth of nickel nanocrystals into hexagonal platelets, which was always difficult for high symmetrical fcc phase metals,11 and their synchronous assembly into regularly arranged structures. II. Experimental Section All chemicals were analytical grade and used without purification. A typical experiment was as following. A 12 mL aliquot of saturated ethanol solution of dimethylglyoxime was added dropwise into 28 mL of aqueous solution of NiCl2‚6H2O (0.01 M) to give a mass of red floccus. Then the desired NaOH solution and 2.0 mL of hydrated hydrazine (80%) was added. The whole mixture was transferred into a 40 mL autoclave, sealed, and maintained at 110 °C for 12 h. The reaction we employed could be formulated as
2Ni(dmg)2 + N2H4 + 4OH- 98 2Ni + N2 + 4H2O + 4dmgAfter the heating treatment was over, black powder that settled as sediment at the bottom of the clave was collected, rinsed with distilled water and ethanol by magnetic decantation several times, and finally vacuum-dried at 50 °C.
10.1021/jp065832j CCC: $37.00 © 2007 American Chemical Society Published on Web 12/08/2006
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Figure 1. XRD pattern of the resulting sample obtained with 50 mM sodium hydroxide.
X-ray diffraction (XRD) patterns of the samples were recorded on a Philips X’pert diffractometer with Cu KR radiation (λ ) 1.5418 Å). The morphology and structure of the sample were studied with field emission scanning electron microscopy (FE-SEM, JEOL JSM-6300F) and transmission electron microscopy (TEM, Hitachi, H-800) with an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) images and the selected area electron diffraction (SAED) patterns were performed with a JEOL-2010 transmission electron microscope at an acceleration voltage of 200 kV. Infrared absorption (IR) spectrum analysis was performed with a Nicolet FT-IR-170 SX spectrometer at room temperature. M-H hysteresis loops were recorded on a vibrating sample magnetometer (BHV-55). III. Results and Discussion Structure and Morphology. Figure 1 shows the XRD pattern of the sample obtained by hydrothermally treating the mixtures of Ni(dmg)2 and hydrazine in the presence of 50 mM NaOH at 110 °C for 12 h, which could be easily indexed as fcc phase nickel (JCPDS 01-1260). No characteristic peaks due to the impurities of nickel oxides or hydroxides were detected, indicating that pure crystalline nickel was fabricated by the presented procedures. The morphology of as-prepared nickel was studied with FESEM, as shown in Figure 2. A panoramic image of Figure 2a demonstrated that the sample consisted of flowery spheres with a diameter of 0.5-1 µm. The magnified images of Figure 2b,c indicated that the flower exhibited hierachical structure, which comprised dozens of columnar petals radiating from the center. Close examination indicated that all the constituent columns were made up of well-aligned hexagonal platelets with the side length of 50-70 nm and the thickness of about 10 nm (Figure 2d). Such structural characters were also reflected in the following TEM images of Figure 3a,e, in which the stacked platelets were viewed edgewise. Thus-prepared superstructures were very stable, and even long-time ultrasonication of 2 h could not break them into discrete platelets, suggesting that the flowers were integrative, not made up of loosely aggregated hexagonal platelets through magnetic interactions. HRTEM and SAED gave more details of the platelet subunits. A typical isolated hexagonal platelet is displayed in Figure 3b, showing the side length of about 60 nm and the angle between the two adjacent edges of 120°. In the corresponding SAED pattern (Figure 3c), spots located farther were due to the (220) lattice planes of a fcc single crystal, while the six spots nearest the center spot could be indexed to 1/3(4h22) diffractions of fcc
Ni et al. nickel, as observed in thin Au or Ag crystals.12 The HRTEM image of the platelet in Figure 3d shows two-dimensional (2D) lattice with the spacing of 2.0 Å, which accorded with the separation between the 1/3(4h22) planes of fcc Ni. This was in good agreement with a fcc single crystal oriented along the (111) direction, indicating the platelet was a single crystalline structure with the top and bottom faces of a (111) plane (Figure 3d). Figure 3e was the HRTEM image recorded on the joint part of two neighboring platelets (as arrowed in Figure 3e). The little lattice misorientation between the bordering units indicated that the platelets were epitaxially attached and fused together.13 The observed spacing of lattice fringes was 2.04 Å, which was consistent with the separation of (111) planes of fcc nickel, further proving that the top/bottom faces of the platelet were (111) planes. Thus, the six side planes of the platelet could be decided as (110) planes. The assembly manner of the platelet building blocks is schematically demonstrated in Scheme 1. Effects of Reaction Conditions. It was found that the concentration of sodium alkali was an important factor that affected the product morphology. SEM images in Figure 4 show the typical morphologies of three samples obtained at different alkali concentrations with the other reaction conditions constant. At a lower NaOH concentration of 40 mM, the product appeared as spherical crystallites, which were assembled by nanoparticles with a diameter of 10-20 nm (Figure 4a). When the alkali concentration was increased to 75 mM, new ball-like crystallites with the surface covered by big hexagonal plates with the side length of about 100-200 nm emerged. Different from previous flowers, the outermost platelet of each column of the football particles had a much bigger size than that of the inner ones (Figure 4b). With the NaOH concentration was further improved to 0.1 M, the product became a mixture of hexagonal platelets, nanorods, and spherical crystallites (Figure 4c). At an optimized concentration of 50-60 mM, the flowery hierarchical structure could be repeatedly produced in a yield above 90%. It was known that a basic medium could improve the reducing power of hydrazine. Consequently, by increasing the alkali concentration, the crystal growth rate was increased correspondingly. The observed shape variation with alkali concentration clearly showed that a suitable alkali concentration could effectively control the redox rate, and thus kinetically controll the growth rate of the nickel platelets and their spontaneous assembly into flowers. In addition to the alkali concentration, temperature also influenced the crystal shape significantly. It was found that welldefined flowery nickel crystallites could only be fabricated at a proper range of 100-140 °C. At the temperature lower than 100 °C, the reaction could be completed. While higher temperature above 140 °C resulted in pricky or spherical particles (Supporting Information, Figure S1), possibly for a faster reaction rate. The evolution of the flowery assembly over time was investigated with FE-SEM studies. After the initial mixture was hydrothermally treated for 3 h, a small amount of black particles was magnetically decanted, indicating the production of metallic nickel at a relatively slow rate. The particles showed spherical morphology with a prickly surface, in which many small thorns protruded from the particle surfaces (Figure 5a). After heating for 7 h, flowery structures with the radial column petals aligned by hexagonal platelets already could be clearly observed (Figure 5b), but the columnar units were relatively short, only with a length of about 150 nm. When the heating time was prolonged to 11 h, the red floccules had disappeared and the supernatant solution had become colorless, indicating that the reaction had
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Figure 2. SEM images of the resulting sample by reducing Ni(dmg)2 with hydrazine in 50 mM sodium hydroxide: (a) a panoramic image; (b-d) magnified images of a.
Figure 3. TEM and HRTEM images of the resulting sample with 50 mM sodium hydroxide: (a) TEM image of an individual flower; (b) TEM image of a hexagonal platelet; (c) SAED pattern recorded on the platelet; (d) HRTEM image of the platelet; (e) TEM image of one columnar petal of the flower; (f) HRTEM image of the joint part of two neighboring platelet subunits.
SCHEME 1: Schematic Illustration of the Assembly Manner of the Flower
been completed. Products obtained at this stage were dominated by the hierarchical flowers with much longer columnar petals of about 250-400 nm (seen in Figure 5c). The results indicated that formation of the complex 3D assembly went through a process in which reduction, crystal growth, and self-assembly co-occurred. Formation Mechanism. Assembly of magnetic particles was a complex process which was affected by several factors, including the velocity of the crystal growth, spatial hindrance, dipolar interactions, and other kinetic factors.9f On the basis of the above experimental results, we considered that formation of our novel flowery assemblies was a cooperative effect of
the complexant of dimethylglyoxime, a proper reaction rate, and inherent magnetic interactions. Here, dmgH acted as both the complexant and structure-directing agent. First, dmgH molecules coordinated with Ni2+ ions and formed the relatively stable precursor of Ni(dmg)2 (with an instability constant of 4.5 × 10-24), which greatly decreased the free Ni2+ ions concentration in solution.14 Such a low Ni2+ ions concentration resulted in a relatively slow reaction rate, which facilitated the oriented growth of nickel crystals.9e Second, dmgH directed the oriented growth of the nickel nanocrystals. When Ni(dmg)2 precursor was reduced to metallic nickel, the released dmg- molecules selectively absorbed on the (111) planes of the fcc nickel crystals. Thus the growth rates of different crystalline planes were kinetically controlled, in which the (111) planes were passivated, while the (110) planes were possibly unaffected. Under the confinement of dmg-, the nickel crystals were directed to develop into hexagonal plates with the top/bottom faces of (111) planes. On the other hand, the inherent magnetic interaction may drive the assembly of the hexagonal platelets into the special assembly architecture. It was known that (111) was the easy axis of fcc phase nickel crystals.15 Such an alignment manner with the magnetic easy axis oriented same favored to reach a low magnetic anisotropy energy, which was believed to play a role in the formation of the flowers.9f A possible formation process of the flowery assembly is illustrated in Scheme 2. At the initial reaction stage, nickel nuclei were
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Figure 4. SEM images of the product obtained at different alkali concentrations: (a) 40 mM, (b) 75 mM, and (c) 0.1 M.
Figure 5. SEM observations of the nickel flowers at different reaction stages: (a) 2, (b) 7, and (c) 11 h.
SCHEME 2: Schematic Illustration of the Possible Formation Process of the Nickel Flowers
produced and spontaneously aggregated into spherical particles to low surface energy. Then under the confinement of dmgmolecules, hexagonal platelets protruded from the surface of the spheres. With the reaction progressing, more nickel platelets were reduced and the newly produced platelets would spontaneously attach on the formerly formed one with their easy axis oriented the same to reach a low magnetic anisotropy energy. This process was repeated, and the columnar petals grew longer until all the Ni(dmg)2 precursors were consumed, which finally resulted in flowery nickel nanostructures. During the whole reaction process, a suitable reaction rate was crucial for the growth of the hexagonal platelets and their spontaneous assembly. Only by properly monitoring the reaction conditions, such as the alkali concentration and temperature, could the formation of hexagonal platelets and their subsequent oriented organization be realized. More detailed investigation on the assembly process is still needed. Magnetic Properties. Magnetic properties of the assembly were investigated at room temperature, which displayed coercivity (Hc), saturation magnetization (Ms), and remanent magnetization (Mr) values of ca. 154.2 Oe, 55.2 emu g-1, and 9.0 emu g-1, respectively (Figure 6). Compared with those of the bulk nickel (100 Oe, 55 emu g-1, and 2.7 emu g-1), the Hc value was enhanced, possibly for its nanosized structure.16 But this value was lower than that of the one-dimensional (1D) nickel nanobelts or nanorods with high anisotropy.9c,e Such a decrease could be ascribed to its special 3D assembled structure, which as an integral crystallite possessing low anisotropy. When subjected to an external magnetic field, those isolated 1D nanocrystals could all be aligned in the same direction, while only one column of our flower could be arranged along the direction of the external magnetic field; thus a relatively lower Hc was exhibited.17
Figure 6. M-H hysteresis loop of the flowery nickel assembly at room temperature.
IV. Conclusions In summary, we have found a simple and mild surfactantfree method for the synthesis of a new type of 3D nickel nanostructures in the absence of any external magnetic field. During the reaction process, properly monitoring the experimental conditions was crucial for the formation of the assembly structure. It was believed that three key points contributed to the configuration of the complex architectures: the special complexant of dmgH, a proper reaction rate, and inherent magnetic dipole interactions. Such a synthetic method presented an example for the preparation of magnetic assembly nanostructures by properly controlling the reaction conditions, which were independent of any surfactants or external magnetic field. And the novel assembly nanostructures of nickel are expected to show applications in catalysis, conduction, electron, and other fields. Acknowledgment. The authors were grateful for the financial support from the State Key Lab of Fire Science (Grant HZ2005-KF11) and the National Natural Science Fund of China (Grant 20571068). Supporting Information Available: Figure SI-1, showing SEM images of the samples obtained at higher temperatures above 140 °C for samples obtained at (a) 160 and (b) 180 °C
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