J. Phys. Chem. C 2008, 112, 6613-6619
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Nickel Chains Assembled by Hollow Microspheres and Their Magnetic Properties Ning Wang,† Xia Cao,† Desheng Kong,‡ Weimeng Chen,‡ Lin Guo,*,† and Chinping Chen*,‡ School of Materials Science and Engineering, Beijing UniVersity of Aeronautics and Astronautics, Beijing 100083, China, and Department of Physics, Peking UniVersity, Beijing 100871, China ReceiVed: NoVember 13, 2007; In Final Form: February 1, 2008
Nickel nanochains assembled with submicrometer-sized hollow spheres were synthesized through a mild polyol reduction method by using PVP as the soft template. The resulting fiberlike superstructures were characterized by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. A self-assembly mechanism of PVP-assisted bubble templating is responsible for the formation of hierarchical superstructures. It was found that PVP concentration plays a key role in determining both the morphology and the further self-assembly process of nickel hollow fibers. This first synthesis of such hierarchical structures implies a simple and inexpensive way to prepare transition metal superstructures on a large scale. Magnetic measurements by SQUID were also carried out to explore their magnetic properties; the micromagnetic simulation result by OOMMF code showed good agreement with the characters of our samples.
Introduction The direct fabrication and assembly of very small structures into controlled nanopatterns is not only of scientific interest but also of down-to-earth practicality with such benefits as cost and energy savings over traditional lithographic techniques.1-5 In the past several years, much attention has been focused on the fabrication of nano- and microscale hollow spheres because of their potential applications in catalysts, artificial cells, coatings, and especially in delivery vehicle systems for the inks and dyes.6-9 Metal nanoparticles with a hollow structure exhibit a range of interesting properties superior to their solid counterparts due to their controlled release of drugs, cosmetics, low density, high specific surface, and large surface permeability without much sacrifice of mechanical/thermal stability.10-13 In addition, 1D assemblies of magnetic nanoparticles are attracting more and more attention recently due to their technological application and their new challenging magnetic transport properties; for example, using the magnetic dipoles inherently associated with magnetic nanoparticles to form 1D assemblies of hollow nanocrystals of semiconductors has been explored.14 Furthermore, the nanoscale Kirkendall effect has been applied to a magnetically pre-assembled nanostructure to form 1D assemblies of hollow nanocrystals of nickel chalcogenides at elevated temperatures.15 Nickel is one of the most used elements, and its applications cover almost all sizes and applications in science and technology. Some of its uses today and novel applications in the future would benefit greatly from nickel materials designed down to nano sizes, such as in catalysis,16,17 electrodes,18,19 chemically protective coatings,20-22 soft magnetic material,23-25 and lowtemperature super plastic materials.26,27 The various synthesis procedures, which involve several treatment steps, were used to prepare metal particles.28,29 Hollow * Corresponding authors. Fax: +86 10 82338162. E-mail: guolin@ buaa.edu.cn and
[email protected]. † Beijing University of Aeronautics and Astronautics. ‡ Peking University.
metal nanostructures are often prepared by templating against existing entities, such as silica beads and30 polymer beads,31 and the seed-mediated method.32 Various nickel nanostructures such as hollow nanospheres33,34 as well as Ni nano-ring and hollow-sphere arrays were prepared via chemical reduction or electrodeposition methods.35 However, to the best of our knowledge, 1D assembly of Ni hollow microspheres into such a chainlike structure has not yet been reported up till now. In this paper, we present a well-known chemical route of polyol reduction for the first synthesis of the chainlike nanostructure consisting of Ni hollow microspheres in an ethylene glycol (EG) solution. These fibers and spherical superstructures were found to form through the aggregation of small Ni nanoparticles. The novelty of this work is characterized by a one-pot procedure which combines the formation of nanoparticle precursor, self-assembly, microsphere shaping, and chain formation under mild solution conditions. In addition, the unique magnetic properties of the novel chainlike nanostructure are also reported. Experimental Section Synthesis. All chemicals (purchased from Beijing Chemical Co., Ltd) used in this experiment were analytical grade and were used without further purification. The growth of the pure nickel hollow-particle chain structure was carried out in a solutionphase system. First, 0.27 g of NiCl2‚6H2O and 1 g of PVP (MW 30 000) were dissolved in 75 mL of ethylene glycol (EG) by intensive stirring for 2 h. A mixture of 1.25 mL of hydrazine monohydrate (50% vol A.R.) and 5 mL of ethanol was then introduced drop by drop to the well-stirred mixture at room temperature by simultaneous vigorous agitation. The mixture was subsequently heated to the boiling point of EG for refluxing (∼ 470 K). After refluxing for 48 h, the dark precipitate was collected by centrifugation, and it was then rinsed 6 times with absolute ethanol. Subsequently, the products were then dried in vacuum at 333 K for further characterization. Characterization. The X-ray powder diffraction (XRD) pattern of the as-prepared products was collected by a Rigaku
10.1021/jp710850n CCC: $40.75 © 2008 American Chemical Society Published on Web 04/08/2008
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Figure 1. XRD pattern of the as-synthesized nickel chain network. All of the peaks can be indexed to pure phase face-centered-cubic nickel.
X-ray diffractometer (Rigaku Goniometer PMG-A2, CN2155D2, wavelength ) 0.15147 nm) with Cu KR radiation. Transmission electron microscopy (TEM) and scanning electron microscopy images were obtained by employing a JEOL JEM-2100F transmission electron microscope and a Hitachi S4800 cold field emission scanning electron microscope (CFE-SEM). Magnetization measurements of the products were performed on a superconducting quantum interference device (SQUID) magnetometer (Quantum Design). The temperature-dependent magnetization curves were measured by the zero-field-cooling and field-cooling (ZFC and FC) modes from 5 to 300 K. M-H measurements was performed at T ) 5 K. Results and Discussion The XRD spectrum for the micro particles is shown in Figure 1. All of the peaks match well with Bragg reflections of the standard face-centered cubic structure (space group: Fm3m (225), a ) 0.3514 nm, JCPDF # 03-1051), the three sharp peaks at 44.5, 51.8, and 76.4 degrees can be assigned to their characteristic (111), (200), and (222) indices. Relatively broadened diffraction peaks indicate that the nickel crystals constituting hollow spheres are in small sizes. The particle size was calculated using the Debye-Scherer formula, t ) 0.89λ/(β cos θB), where λ is the X-ray wavelength (1.5406 Å), θB is the Bragg diffraction angle, and β is the peak width at half-maximum. The XRD peak of (111) in Figure 1 gives the nickel particle diameters of 17.5 nm. These estimates agree well with the value obtained from the TEM analysis. Scanning electron microscopy (SEM) images of the Ni products are shown in Figure 2. It can be clearly seen that the Ni product consists of parallel aligned (ordered) microspheres (Figure 2A) and the spheres have uniform sizes with a diameter ∼1000-2000 nm. These spheres are in close contact with each other, forming branched necklace-like chains with a length of tens of micrometers. Close examination can disclose that the building blocks of these superstructures have a hollow nature, and broken sites of the hollow spheres can be seen everywhere within the fibers. In the high-magnification SEM image (Figure 2B), broken sites of some nickel spheres within the fibers and spherical superstructures were displayed and their hollow interiors were exposed, which provides direct evidence that the Ni microspheres serving as building blocks of the chains have a hollow structure with a shell thickness of about 100 nm. Transmission electron microscopy (TEM) was also used to further study the fine structures and the hollow nature of the Ni microspheres. As evidenced by the SEM results, the strong contrast between the pale edges and the dark center within the
Wang et al. TEM image is also evidence for its hollow feature (see Figure 3, left). To image the interior architecture of the hollow chains and the crystal structure of hollow spheres, the sample was investigated by high-magnification TEM after being processed by sonication for 30 min. It can be seen clearly that the hollow sphere consists of orderly arranged microspheres situated in a parallel manner (Figure 2A), and the spheres have uniform sizes with a diameter of ∼1000 nm. Moreover, the Ni hollow spheres are connected either by penetrating their hollow interiors or by sharing their shells. The corresponding HRTEM images taken from the outside of the shell of the contact region of two contiguous nickel hollow microparticles indicate that the hollow microspheres are polycrystalline though with local area crystallinity (about 10 nm). High-resolution TEM (HRTEM) also enabled the viewing of lattice fringes, confirming local area crystallinity of the walls of the spheres. The vague boundaries among particles at the surface implicate a growth mechanism of particle attachment. As shown in Figure 3b, the lattice fringe spacings are measured to be 0.20 nm, matching the d value for cubic nickel (111) planes (0.202 nm). Superstructures that spontaneously form from nanoparticle assembly might provide materials with new properties. Because this is the first synthesis of nickel hollow chains with a hierarchy consisting of ordered and self-assembled microsized nickel hollow spheres, the property investigation might be relevant and interesting. Magnetization of the sample was investigated in a powder collection ∼0.78 mg by a SQUID magnetometer. Magnetic properties were characterized by recording both temperature dependence of magnetization, zero-field-cooling (ZFC) and field-cooling (FC), and magnetic field dependence of magnetization. In the ZFC mode, the sample was cooled from room temperature to 5 K. Then a magnetic field of 90 Oe was applied, and the magnetization of the sample was measured during the warming process. In the FC mode, the sample was initially cooled from room temperature to 5 K under an applied magnetic field of 10 kOe, and then a subsequent magnetization measurement was recorded from 5 to 380 K under a constant magnetic field of 90 Oe. As shown in Figure 4, MFC(T) and MZFC(T) curves separate from each other with the temperature going up to 380 K, indicating the presence of an anisotropy barrier. The behavior of M(T) curves reveals the main feature of thermal activation effect against the barrier, that is, MZFC(T) increases and MFC(T) decreases while the temperature increases.36,37 The peak at about 12 K in the MZFC(T) curve is identified as the freezing temperature, TF, which reveals the defreezing of the frozen moment in the surface layer of particles. This behavior has been observed with the chains of Ni nanoparticles using PVP as the surface modifier.37 Hysteresis loops were measured at 5 and 300 K, respectively. As can be seen from Figure 5, the magnetic saturation is reached with the external field exceeding 5 kOe. It shows a value of saturation magnetization of about 64.7 emu/g and coercivity about 45 Oe for the loop at 5 K, whereas for the loop at 300 K, the values are 56.9 emu/g for the saturation magnetization and 31 Oe for the coercivity. In order to better understand the magnetic reversal in the sample, 3D micromagnetic simulation is performed for the ground-state property at T ) 0 K based on the Object Oriented Micro Magnetic Framework (OOMMF) package from NIST.38 The parameters used in the simulation are Ms ) 480 × 103 A m-1 for the saturation magnetization and A ) 10 × 10-12 J m-1 for the exchange stiffness with the bulk nickel.39 The
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Figure 2. SEM image of the as-synthesized Ni microsphere chain network. The scale bars of (A) and (B) are 10 and 1 µm, respectively.
Figure 3. TEM and HRTEM image of hollow microsphere self-assembled chainlike nickel structure (left) and a lattice-resolved HRTEM image taken from the outer shell of the joint region (within the white frame) of two contiguous nickel hollow microspheres (right). The scale bars in (A) and (B) are 200 and 3 nm, respectively.
Figure 4. Zero-field-cooling (ZFC) and field-cooling (FC) curves between 5 and 380 K, measured in an applied field of 90 Oe.
Figure 5. Hysteresis loops measured at 5 and 300 K.
magnetocrystalline anisotropy is not accounted for since the effective anisotropy tends to average out for polycrystalline samples.40 The default damping constant R ) 0.5 is used.40 For the numerical modeling, discrete calculation cells of cubic shape
Figure 6. Simulated hysteresis loops.
are selected with the volume size of (20 nm)3. For a 1 µm microsphere with a shell thickness of 100 nm, the simulated hysteresis loop is normalized to the experimental saturation magnetization and shown in Figure 6. The coercivity of the simulated loop is then determined as 42 Oe, slightly smaller than the measured coercivity of 45 Oe at 5 K in our experiment by about 7%. The overall shape of the simulated hysteresis curve also agrees well with the experimental measurement. The consistency between the theoretical simulation and the experiment results indicates that the interparticle interaction within the chains of Ni hollow microspheres is indeed negligible. The present sample is polycrystalline consisting of Ni nanocrystallites about 10 nm in size. However, the coercivity, about 42 Oe at T ) 5 K, is greatly enhanced from the bulk value of 0.7 Oe.41 Since the nanocrystallite size is smaller than the coherence length of Ni, about 25 nm,42,43 a single crystallite is qualified as a Stoner-Wohlfarth particle in the region of magnetization reversal by coherent rotation. The coercivity scales with d6, in which d is the crystallite size.44,45 It is usually
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much larger for such a single crystallite than the bulk value. The present hollow spheres of Ni are polycrystalline formed of these nanocrystallites. Although the anisotropy is expected to become lower due to the randomly oriented nature of these crystallites, we do not expect it to average out completely to give the coercivity of the bulk. On the other hand, the coercivity of the present sample, 42 Oe at T ) 5 K, is smaller than the value of 392 Oe measured at 5 K for the Ni nanochains formed by 50 nm solid nickel particles.37 This is consistent with the fact that as the particle size exceeds the coherence length, the coercivity reduces with D-1, in which D is the size of polycrystalline particle.44,45 In the present experiment, the shell thickness of the hollow spheres is about 100 nm, far beyond the coherence length of Ni. The coercivity is therefore expected to be smaller than that of the 50 nm particle consisting of only a few nanocrystallites. To investigate the magnetization reversal of an isolated hollow microsphere, which serves as a building block of the chainlike superstructures, the interaction of the interfacing spheres must be taken into consideration. But it is well-known that the interparticle interaction is considered as a lower-order effect, especially for such a polycrystalline sample, thus here this effect is intentionally omitted for convenience even though the chains were formed by sharing part of their shells. In the simulation, the magnetization reversal is modeled by the complex multidomain mode. The close agreement of the calculated coercivity with the experimental value supports the point mentioned above that the observed coercivity is mainly attributed to the polycrystalline nature of the hollow sphere. As far as the growth mechanism is concerned, the effects of reducer and capping agents must be taken into account. Series of experiments were carried out to further investigate the possible route of formation of such novel superstructures. First, it is interesting to find that N2H4 concentration has a remarkable effect on the morphology, structure, and phase of nickel crystals, as shown in Figure 7. When the molar ratio between N2H4 and nickel precursor was less than 6, no precipitation can be obtained. And if the molar ratio between N2H4 and nickel precursor was greater than 14, nickel chains with less regular morphology can be found (Figure 7A). This phenomenon may be attributed to the stronger reduction ability of N2H4 at higher pH value. Generally, it is expected that the reducing strength of hydrazine with higher concentration is stronger; in this case the reduction reaction occurred more facilely and the reaction rate will be much higher. Nickel nanoparticles will then grow faster and form larger particles. When substituting N2H4 with NaOH (equal molar ratio), nanorods with an average size of 300 nm were formed (Figure 7C) under typical experimental conditions. The XRD pattern in Figure 7D displays several peaks corresponding well to the hexagonal structure of NiO (JCPDS Card Number 22-1189). The formation of NiO may be ascribed to the dehydration of Ni(OH)2, which may take place according to the following reaction:
Ni2+ + 2OH - f Ni(OH)2 f NiO + H2O
(1)
Furthermore, surfactant-templating routes have been well reported in the past several years for the preparation of hollow structured materials because surfactants could form diverse assemblies in solution and be used as templates. In this scheme, the shape of such a chainlike superstructure is mainly determined by the molar ratio (R) between PVP and nickel precursors. To examine whether PVP had exerted an influence on the formation of the nickel hollow spheres, a controlled experiment in the absence of PVP was conducted while other conditions remained unchanged. SEM images in Figure 7 reveal that, although the
nickel spheres could be formed even without PVP, the sizes of nickel spheres (Figure 7E) are much less irregular than those in typical synthesis conditions (Figure 2); the spheres are of a solid nature and have no assembly into chainlike superstructures, which suggests that PVP did have both the templating function for hollow sphere formation and obvious contribution to shape control of chainlike hollow nickel superstructures. With the increase of R, however, through fine comparison, it was found that the self-assembly of nickel chains became very uniform with the help of PVP, leading to nickel hollow spheres formed here looking more tight and regular than spheres that resulted from lower R values. Surely the existence of PVP is beneficial for the nickel chains self-assembling in a more ordered and tight manner to form regular hollow sphere superstructures, where PVP molecules probably form both spherelike template and interparticle bilayers inducing the nickel spheres forming and gluing together in a 1D manner during the self-assembly process. In fact, such similar self-assemblies assisted by organic molecules such as micromolecules and surfactants as organic connectors to link inorganic building blocks to produce nanoparticle-based superstructures have been reported.46 Hence, with respect to the formation of nickel chains self-assembled by hollow spheres in this work, the crystallinity, uniform shape, and size of nickel hollow spheres may afford structure match and spatial proximity to realize effective self-assembly induced by PVP molecules. It must be noted that the particle aggregates and growth at the intermediate stage are generally attributed to the traditional colloidal theory, the Ostwald ripening process, and the sizes of the nanoparticle observed within the walls of the spheres developed while the refluxing process continued. In addition, the reaction environment promotes the surface domains on neighboring nanoparticles to match up driven by magnetic dipole-dipole attraction, as reported previously.47,48 Meanwhile, the superstructure of hollow spheres can be assembled into chains to satisfy the geometric criterion of a low energy.47,48 Under the conditions of our experiments, the following reactions are thought to occur in solution:
Ni2+ + 3N2H4 f [Ni(N2H4)3]2+
(2)
[Ni(N2H4)3]2+ + N2H4 f NiV + 4NH3v + N2v + 1/2H2v + 2H+ (3) At the beginning of the process, Ni2+ in the solution reacts with hydrazine to form a dark blue complex, [Ni(N2H4)3]Cl2, which is very stable in ambience as observed previously.49,50 At the boiling point of the ethylene glycol (∼470 K), the excessive hydrazine acted as a reducing agent and converted [Ni(N2H4)3]2+ to small Ni nanoparticles. These nanoparticles have a tendency to aggregate, resulting from either their high surface energy or the magnetic effects, and at the same time a large amount of gas microbubbles of NH3, N2, and H2 generated in the reaction provide the aggregation centers. Here, as elucidated above, the microbubbles could be interfacially reinforced by the viscosity of ethylene glycol and PVP adsorption. Driven by the minimization of interfacial energy, small Ni nanoparticles may aggregate around the gas-liquid interface, and finally hollow Ni microspheres form. Again, the selfassembly of hollow spheres to chains is accompanied with the magnetic dipole-dipole attraction between them, which leads to the assembly to a chain structure. On the basis of the present results, a plausible proposition involving bubble-templating coupled with a particle self-
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Figure 7. Products synthesized by changing the following experimental parameters: (A) R ) 16; (B) XRD patterns of product A; (C) substituting N2H4 with NaOH; (D) XRD pattern of the as-prepared sample C; (E) products without PVP; (F) R ) 2; (G) R ) 4; (H) R ) 6. The scale bars for Figure 7A, C, E, F, G, and H are 50, 4, 10, 5, 50, and 10 µm, respectively.
Figure 8. Schematic illustration of a proposed mechanism for the formation of nickel chains.
assembly process can be given briefly, where N2H4 worked as not only as the reducer but also the source to afford the templates of gas bubbles. Actually, a gas bubble templating method has been shown to be feasible and facile to fabricate hollow spheres of inorganic materials.51,52 As illustrated in Figure 8, when appropriate amounts of N2H4 were present, the relatively low concentrations would lead to relatively low reaction rates. Driven
by reactions 2 and 3, a certain concentration of gas was generated, forming submicrometer-sized bubbles in the current situation, and simultaneously a large amount of nickel nuclei were produced in solution according to reaction 3, leading to a relatively high local concentration of nickel nuclei. Thus, it inevitably resulted in the nickel nuclei diffusing to the gasliquid interfaces between gas bubbles and liquid solution. On
6618 J. Phys. Chem. C, Vol. 112, No. 17, 2008 the other hand, PVP molecules tended to be enriched on the gas-liquid interfaces prompted by repulsion between their nonpolar ends and polar solvents because of their amphiphilic property. These two factors provide a good possibility for the self-assembly of nickel hollow spheres under direction of PVP molecules on the surfaces of gas bubbles. Subsequently, under continuous source feeding from solution during the growth process, the nickel nuclei grew into uniform hollow spheres with polycrystallinity, which gives much better structure recognition for self-assembly. Therefore, driven by energy minimization and magnetization dipole effects, those hollow spheres with uniform size and structure match self-assembled tightly in a parallel way to maximize the contact. This process was assisted by the surfactant PVP on the gas-liquid interfaces between gas bubbles and liquid solution, leading to the formation of nickel chains with a hierarchical hollow structure. Finally, further growth gives rise to the intact wall of the hollow spheres and their mutual sharing part, which ensures the hierarchical nickel chains driven by the self-assembly. Owing to the tight interactions between hollow spheres, the shape of hierarchical chains can be well preserved even after a series of ultrasonication treatment processes. In contrast, at suitable PVP and N2H4 concentration, the rates of gas generation and nickel nucleation and growth would be moderate on the basis of reactions 2 and 3, which thereby caused the formation of suitable amount of bubbles as well as nickel nuclei in a short time. In general, the crystallizing particles need time to select each other and follow the lowestenergy path; fast crystallization often leads to kinetically controlled products such as metastable crystal structures or those with even defects, probably resulting in the poor crystallinity of crystals. This principle was well evidenced by the XRD patterns of nickel crystals. In addition, nickel chains at high N2H4 concentration (molar ratio about 15) appeared a little irregular in shape and vague in outline, confirming the poor crystallinity as well. Apparently, these are unfavorable for the self-assembly process, because they gave a poor structure match and bad spatial proximity. As a result, the similar self-assembly behavior occurred at relatively low PVP concentration; however, the intermediate products were only unstable self-assemblies of solid spheres through weak contact of irregular spheres due to the reason given above. During the further growth of nickel assemblies into longer chains, the weak interactions between spheres got much worse as a consequence of increased mismatch and recognition adversity of the irregular larger-size spheres, and the superstructure would eventually be deformed into parts as detached solid units. On the basis of the above-mentioned time-dependent crystallinity and morphology evolution, a schematic illustration of the proposed particle attachment growth mechanism responsible for the formation of the nanodisks is listed in Figure 8. Conclusions Chains of Ni hollow microspheres were synthesized through the aggregation of small Ni nanoparticles via a mild solution chemical route. The novelty of this work is characterized by a one-pot procedure which combines formation of the nanoparticle precursor, self-assembly, and chain-shaping under mild solution conditions. This simple soft assembly strategy represents an attractive path to large-scale assembly of metallic hollow nanoor micro-sized spheres and the assembled superstructures. The magnetic properties have been investigated carefully both by experimental measurements and simulation. The blocking temperature of the sample is far above 380 K, indicating a very stable magnetic state at room temperature. The excellent stability
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