Controlled Synthesis and Morphology-Dependent Electromagnetic

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Controlled Synthesis and Morphology-Dependent Electromagnetic Properties of Hierarchical Cobalt Assemblies Chao Wang,† Xijiang Han,*,† Xiaolin Zhang,† Surong Hu,† Tao Zhang,† Jinyu Wang,† Yunchen Du,† Xiaohong Wang,‡ and Ping Xu*,† Chemistry Laboratory Center, Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China and Beijing Institute of Aeronautical Materials, Beijing 100095, China ReceiVed: June 2, 2010; ReVised Manuscript ReceiVed: July 23, 2010

Hierarchical cobalt assemblies such as spheres, flowers with dendritic petals, and flowers with sharp petals are successfully synthesized via a facile liquid-phase reduction method by simply adjusting the reaction conditions. The morphology evolution process and transformation mechanism from spheres to dendrites and finally to flowers have been systematically investigated. It is determined that coercivity Hc depends more on sample size than on shape anisotropy, while saturation magnetization Ms is greatly affected by pinned surface magnetic moment. Even at a thinner thickness, as-synthesized cobalt samples exhibit stronger microwave absorbing ability compared with reported cobalt in the same frequency band. Especially, the cobalt flowers with dendritic petals exhibit the strongest absorption in middle frequency because incident wave and reflected wave are totally canceled at matching thickness. The architectural design of material morphologies is critical for improving properties toward future application. 1. Introduction Large-scale controllable synthesis of hierarchical building components with specific morphology and novel properties are now attracting much attention of researchers not only for its role in deeply understanding the mechanism of self-assembly but also for its promising applications in functional materials.1-6 As a typical magnetic material, metal cobalt has been extensively studied because of its morphology-dependent magnetic and electronic properties and potential applications in high-density magnetic recording, magnetic separation, and catalysis.7-9 Therefore, it is of great interest for chemists and material scientists to develop several approaches for the controllable preparation of different-shaped cobalt. Among which, snowflowerlike cobalt was obtained by common liquid-phase reduction with hydrazine hydrate;10 cobalt dendrite and sphere were synthesized via hydrothermal process.11-13 However, the reaction is always accomplished at high temperature and high pressure with long time via hydrothermal route,12,14-17 which consumes a lot of energy and time. If we can develop a hierarchical cobalt structure that combines above three different morphologies such as flower, dendrite, and sphere without high temperature and high pressure within short time, it may bring a new physics and chemistry insight due to the novel artificially designed structure and improved property in promising application. In addition, the electromagnetic (EM) materials with low reflection and high absorption have received extensive attention due to their promising application as antielectromagnetic interference coatings, microwave darkrooms, and self-concealing technology in industry, commerce, and military affairs. Majority of the microwave absorbing materials are composed of magnetic powders such as ferrite,18 nickel,19 alloys,20,21 and other dielectric loss materials such as carbon materials,22,23 conducting poly* To whom correspondence should be addressed. Tel: +86-45186413702. Fax: +86-451-86418750. E-mail: (X.H.) [email protected]; (P.X.) [email protected]. † Harbin Institute of Technology. ‡ Beijing Institute of Aeronautical Materials.

mers.24 However, most of them only exhibit absorbing peaks at high (12-18 GHz) frequency, it is still a challenge to develop a simple and reliable synthetic method for hierarchical selfassembled architectures that own strong microwave absorption in low (2-6 GHz) or middle (6-12 GHz) frequencies. Furthermore, as a kind of typical magnetic material, only Kato et al. and Cao et al. measured the electromagnetic properties of cobalt with weak microwave absorption during certain frequency bands,25,26 which lacked further analysis of morphology-dependent electromagnetic absorption. Therefore, it is valuable to explore a kind of microwave absorber that shows strong absorption in low or middle frequency during 2-18 GHz and to investigate the relationship between morphology and electromagnetic properties. In our previous work, we reported the synthesis of differentshaped nickel and discussed their morphology-dependent electromagnetic wave absorbing properties.27 As a sequence, here we exhibit the facile synthesis of cobalt hierarchical structures via changing the amount of reducing agent (hydrazine hydrate), NaOH and surfactant (PVP). The synthesis of hierarchical cobalt flowers with dendritic petals that also consist of compacted spheres may provide a new insight on crystal growth and morphology-dependent electromagnetic absorption properties. The as-synthesized samples with strong microwave absorption at thin thickness demonstrate their potential in industry, commerce, and military affairs. 2. Experimental Section 2.1. Materials. All chemicals used in this experiment were analytical grade and used without further purification. CoCl2 · 6H2O, PVP K30, N2H4 · H2O 80%, and ethylene glycol (EG) were purchased from Guangfu Chemical Co. Ltd. (Tianjin, China), and NaOH was purchased from Dalu Chemical Co. Ltd. (Tianjin, China). 2.2. Synthesis. Cobalt with different morphologies were prepared by adding 0.35 g CoCl2 · 6H2O and 0.5 g PVP into 22.5 mL EG at 85 °C in a three-necked flask. After vigorous

10.1021/jp1050386  2010 American Chemical Society Published on Web 08/13/2010

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Figure 1. XRD patterns of cobalt with different morphologies.

stirring for 10 min, 0.4 g NaOH was added and 10 min later, 2-12 mL N2H4 · H2O was injected, then the reaction was maintained for another 30 min. In fact, after the reaction was maintained for about 15-20 min, the solution became clear and some black products emerged. To make sure the reaction was completed, we prolonged the reaction time to 30 min. After the reaction, the products were collected by centrifugation at 3000 rpm for 10 min and then washed with water six times and with ethanol three times to eliminate PVP. Then the products were dried in a vacuum oven at 60 °C for 12 h. 2.3. Characterization. The morphologies of the samples were characterized by scanning electron microscopy (SEM, FEI SIRION). The crystallite structure of the prepared samples were determined using an XRD-6000 X-ray diffractometer (Shimadzu) with a Cu KR radiation source (λ ) 1.5405 Å, 40.0 kV, 30.0 mA). The magnetic properties were carried out by a vibrating sample magnetometer (VSM, Lake Shore 7307). The relative permeability and permittivity during 2-18 GHz were determined via HP-5783E vector network analyzer for the calculation of reflection loss. A sample containing 60 wt % of obtained products with wax as the binder was pressed into a ring with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 2 mm for microwave measurement. 3. Results and Discussion

Figure 2. SEM images of the cobalt synthesized at varying amount of N2H4 · H2O (80%). (A) Big sphere is surrounded by small spheres, 2; (B) rudiment of dendrite coexist with sphere, 2.5; (C) dendrite coexist with sphere, 3; (D,E) flower coexist with dendrite, 5; (F,G:) flower consists of dendritic petals, 8; (H,I) flower consists of sharp petals, 12 mL. The top-right insets in panels A, B, C, G, and I are the schematic illustration of cobalt structures. The bottom-left insets in panels G and I show the bottoms of flowerlike cobalt are flat.

X-ray diffraction (XRD) patterns are used to determine the chemical composition and crystal structure of the resulting samples. As shown in Figure 1, the characteristic peaks of the as-prepared products at 2θ ) 41.64, 44.49, 47.28, and 76.03° can be well indexed to the (100), (002), (101), and (110) planes of hexagonal-phase cobalt (JCPDS #05-0727). No characteristic peaks due to the impurities of cobalt oxides or hydroxides are detected, indicating that the as-prepared products are pure hexagonal-close-packed (hcp) cobalt. Notably, the relative intensities of the peaks corresponding to the (002)/(100) and (002)/(101) planes for all three different-shaped products are significantly higher than the standard values, indicating the preferred growth orientation is along the [001] direction and independent of morphology. 3.1. Morphology. Typical SEM images of the as-prepared products with different morphologies via varying the amount of N2H4 · H2O are presented in Figure 2. Figure 2A indicates that the obtained product consists of a large number of Co spheres with the sizes about 0.5-2 µm when only 2 mL

N2H4 · H2O is added. A big sphere is usually surrounded by some small spheres due to the static magnetic attraction. When the amount of N2H4 · H2O is increased to 2.5 mL, though surrounded spheres still exist, some spheres are compacted and linearly linked together to form chains because of magnetic dipole-dipole attraction, which constitute the rudiment of subsequent dendrites sharing the same core. With 3 mL N2H4 · H2O, perfect dendrites with leaves consisting of compressed spheres are obtained and no regular spheres can be found in the dendrites. Interestingly, some leaves connected to the main branch also act as secondary branches to be connected by smaller leaves (tertiary branches, see the red lines in Figure 2C). Moreover, the second branches arranged in the same profile of main branch are nearly parallel to each other and emerge at 60° angles with respect to the main branch. The same phenomena are also found in the tertiary branches, which is similar to the reported Ag, FeNi3 dendrites.28-30 Further increasing the amount of N2H4 · H2O to 5 mL, flowerlike Co structure composed of dendritic petal is produced, which appears as a result of oriented attachment and self-assembly as

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exhibited in Figure 2D. And some dendrites still exist in Figure 2E, indicating that self-assembly is still underway. Figure 2F,G demonstrates that the sample is composed entirely of wellassembled three-dimensional (3D) flowerlike Co structures that are formed by an ensemble of dendritic petals when 8 mL N2H4 · H2O is applied. Additionally, the wirelike dendrite is composed of linearly linked compressed cobalt spheres. However, as shown in Figure 2H,I, excessive N2H4 · H2O (12 mL) would only produce sharp petals, with no dendritic petals visible. Regardless of the morphologies of the flowers, the bottoms are both flat as exhibited in bottom-left insets in Figure 2G,I. On the basis of above experimental observations, it is possible to investigate the formation mechanism of hierarchical cobalt. It is reported that the crystal morphology and size are dependent on the competition between nucleation and growth that can be adjusted through controlling the reaction rate.31,32 The fractal growth of dendritic structures is usually associated with the diffusion-limited aggregation (DLA) and nucleation-limited aggregation (NLA) process.14,33 For the varied amount of N2H4 · H2O, the kinetics of nucleation and growth of reduced cobalt atoms can be well regulated and, further efficient control of the morphology and structure of final products can be achieved by the interaction between the stochastic diffusive force and directive force. With high concentration of N2H4 · H2O, the reaction rate is faster, resulting in the nonequilibrium system and the promotion of diffusion-limited growth, which is beneficial to the formation of dendrite.28 From another point of view, PVP is a widely used shape modifier and structure director.34,35 Because of the interaction between surfactant PVP molecules and Co particles by van der Waals forces, surface defect and active crystal surface are generated on early formed Co nuclei, which could serve as heterogeneous nucleation sites and result in preferential oriented growth. Anisotropy has been proved to be essential to produce stable tip behavior and repeated side branching of fractals.36 In addition, the oscillation of metal ion concentration during vigorous stirring in solution is another important factor to stable fractals formation. This oscillation is responsible for the periodic hesitation of fractals growth and the side-branching phenomenon.14 At a higher amount of reducing agent N2H4 · H2O, the nonequilibrium system and preferential growth are much more enhanced, and cobalt flowers with dendritic petals are prepared. It is documented that a large reduction rate is helpful to the formation of flowerlike Co microcrystal.37 However, excessive N2H4 · H2O would lead to too high reduction rate of Co2+, and cobalt spheres are closely compacted and linearly fused, resulting in dendritic petals replaced by sharp petals. Hence, a cobalt flower with sharp petals is formed. This may indicate that DLA and NLA processes, which are responsible for the generation of dendrites, will not completely work at a high concentration of metal ion. Similar phenomenon has been reported when using PEG-2000 as surfactant to effectively control the anisotropy growth of nanocrystals in the preparation of Fe3O4 dendrite.14 In addition, these magnetic cobalt nuclei will generate a magnetic field that can induce newly reduced atoms to be arranged along magnetic force line.38 It is reported that the magnetic anisotropic energy (MAE) is the lowest when magnetizing along the magnetic easy axis. The [001] direction is the magnetic easy axis of hexagonal cobalt crystal.39 Therefore, the [001] direction grows faster than other directions, which is verified by the XRD analysis in Figure 1. Though magnetic cobalt spheres are linearly linked, it is well-known that the intensity of magnetic field in both ends of the pole magnet is

Wang et al.

Figure 3. SEM images of cobalt with (A) no PVP added; (B) NaOH increased to 1.2 g, while other experimental parameters keeping the same as the preparation of the sample as described in Figure 2F.

Figure 4. Magnetic hysteresis (M-H) loops measured at room temperature for cobalt samples with different morphologies.

stronger than other position. That is why dendrite and flower share the same core. To verify the role of PVP in the reaction as deduced above, we do not add any PVP to prepare a sample as described in Figure 2F while keeping other parameters identical. Only discrete cobalt spheres are synthesized and no flowerlike cobalt exists at the absence of structure-directing agent PVP, as shown in Figure 3A. Furthermore, we find too much NaOH is also harmful to the formation of flowerlike structures and results in atrophic flower, which is an analog of a sphere as shown in Figure 3B. NaOH affects the reaction kinetics through tuning the dissolution-deposition equation of Co(OH)2, because soluble CoCl2 · 6H2O will first react with NaOH to form Co(OH)2 precipitate (Ksp ) 1.09 × 10-15). The trend for Co(OH)2 to release Co2+ decreases if the concentration of OH- is relatively high, leading to the thermodynamic change of nucleation and growth velocities, which is beneficial for the formation of spherical structure.40 3.2. Magnetic Property. We choose three typical morphologies, sphere (Figure 2A), flower with dendritic petals (Figure 2F), and flower with sharp petals (Figure 2H) of cobalt to characterize the magnetic properties at room temperature. The magnetic hysteresis (M-H) loops are exhibited in Figure 4 and magnetic parameters such as coercivity (Hc), saturation magnetization (Ms), remanent magnetization (Mr) are listed in Table 1. According to the spherical reversal magnetization model, Hc is inversely proportional to the diameter.41 The diameter of sphere is obviously smaller than that of flower, that is why it owns larger Hc. From another point of view, coercivity is dependent on shape anisotropy,42 thus flower with dendritic petals possesses a little larger Hc compared with flower with

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TABLE 1: Magnetic Data of Obtained Cobalt with Different Morphologies at Room Temperature cobalt structures Hc (Oe) Ms (emu/g) Mr (emu/g)

sphere

flower with dendritic petals

flower with sharp petals

51.62 169.52 2.37

45.18 158.21 2.95

43.44 167.46 2.78

sharp petals. Hence, we can conclude that Hc depends more on size than on shape anisotropy. It is reported that the Ms of microsphere is larger than that of dendrite for cobalt due to the difference of the pinned surface magnetic moments in overall magnetization, and Hc and Ms for bulk cobalt are a few tens of Oe and 168 emu/g, respectively.43 Similarly, in our reaction system, flower with dendritic petals shows the smallest Ms, and because the synthesized products are all in micrometer scale (close to the size of bulk Co), the magnetic parameters such as Hc and Ms are very close to those of bulk cobalt. 3.3. Electromagnetic Properties. Generally, the real parts of relative complex permittivity and permeability symbolize the storage ability of electromagnetic energy, and the imaginary parts represent the loss ability. Figure 5 reveals that both ε′ and µ′ of different samples decrease with the increase of frequency and exhibit evident frequency dispersion effect, which is in favor of the electromagnetic wave absorption. It is interestingly found that broad peaks centered at 5.3 GHz are present in all three samples in Figure 5D, indicating the occurrence of natural resonance in the prepared cobalt samples. According to the measured data of permittivity and permeability, the reflection loss (RL) usually can be evaluated by the following equation44

RL ) 20 log|(Zin - Z0)/(Zin + Z0)|

(1)

where Z0 is the impedance of free space, and Zin is the input characteristic impedance, which can be expressed as

Zin ) Z0√(µr /εr)tanh{j(2πfd/c)√µrεr}

(2)

Figure 5. Electromagnetic parameters of three cobalt samples in the frequency range of 2-18 GHz: (A) real and (B) imaginary parts of relative complex permittivity; (C) real and (D) imaginary parts of the relative complex permeability.

Figure 6. Reflection loss (RL) of different cobalt samples in the frequency range of 2-18 GHz with a sample thickness of 2 mm.

where c is the velocity of light and d is the thickness of an absorber. The reflection loss (microwave absorption) abilities of three samples are shown in Figure 6. It is reported that cobalt nanoparticles exhibit reflection loss about -5 dB in 2-5 GHz at the thickness of 3.16 mm, and cobalt nanochains exhibit a reflection loss peak (-11 dB, 17.2 GHz) in 2-18 GHz at the thickness of 2.5 mm.25,26 While at thinner thickness of 2 mm as shown in Figure 6, sphere (-7 dB, 5 GHz) and flower with sharp petals (-11.6 dB, 5 GHz) can reach stronger reflection loss than the reported cobalt nanoparticles in 2-5 GHz, while flower with sharp petals (-12.3 dB, 5.4 GHz) and flower with dendritic petals (-13.6 dB, 9 GHz) can also reach stronger reflection loss compared with the reported cobalt nanochains in 2-18 GHz. Cobalt is universally considered as magnetic loss material, which can be verified by the fact that reflection loss peaks of cobalt sphere and flower with sharp petals emerge at 5-6 GHz that just covers the magnetic natural resonance frequency, indicating that the reflection loss mainly comes from magnetic natural resonance loss. As we know, the positive µ′′ is often used to manifest energy loss for magnetic materials, while a negative µ′′ value means the magnetic energy is radiated out without absorption. Hence, flower with sharp petals exhibits stronger reflection loss compared with sphere as shown in Figure 6 due to its higher µ′′. Though cobalt flower with dendritic petals possesses the lowest µ′′ and becomes negative from 14.5 GHz, its electromagnetic reflection loss is the strongest. Actually, apart from the dielectric loss and magnetic loss, the electromagnetic wave may also be absorbed via “geometrical effect”. It means if the thickness of absorber satisfies the equation

d ) lλm /4(l ) 1, 3, 5, ...)

(3)

λm ) λ0 /(|µr ||εr |)1/2

(4)

where λm is the wavelength at certain frequency, |µr| and |εr|are the moduli of µr and εr, and λ0 is the wavelength in the free space, the incident and reflected waves in the absorber are out of phase 180° and resulting the reflected waves in the airabsorber interface are totally canceled,18 as shown in the lowerright inset in Figure 6. To illustrate the strong and wide reflection loss of cobalt flower with dendritic petals, we suppose the

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microwave absorbing ability is coming from the geometrical effect. At matching frequency fm ) 9 GHz, the εr ) 14.51 j4.81 and µr ) 1.14 - j0.28. Substituting these values into eq 4 with l ) 1, we get a matching thickness d ) 1.97 mm, which is exactly adjacent to thickness of 2 mm in measurement. Hence, the above calculation validates our supposition that the strong microwave absorbing ability of flower with dendritic petals arises from not only magnetic loss but also from geometrical effect. We can also find flowerlike structures, including flower with dendritic petals and flower with sharp petals, exhibit enhanced electromagnetic absorbing ability compared with cobalt sphere. Though flower’s petals are composed of linearly linked compacted spheres, the electromagnetic properties of flower and sphere are different. According to the morphology analysis, the conductive and compacted cobalt spheres are linearly fused to form a wire, which also acts as the petal of flower. Under electromagnetic field, the incident electromagnetic wave may penetrate the cobalt and the energy is induced into dissipative current along the wire that leads to energy attenuation by numerous conductive networks, resulting in strong absorption. A similar absorbing mechanism has been reported for flowerlike ZnO.45 This further proves that electromagnetic property would be significantly influenced by the detail morphology of the prepared samples. 4. Conclusion In summary, we successfully realize the evolution progress from cobalt spheres to the flowers with dendritic petals that also consist of compacted spheres and finally to flowers with sharp petals via adjusting the amount of reducing agent (hydrazine hydrate). DLA and NLA mechanisms, structure-directing agent PVP and magnetization along the magnetic easy axis are all responsible for the formation of hierarchical assemblies. Flower with dendritic petals owns higher Hc than flower with sharp petals due to the obvious shape anisotropy, while the sphere possesses the highest Hc because of its smallest size according to sphere reverse magnetic model. Different pinned surface magnetic moments would result in different Ms. In addition, even at thinner thickness within the same frequency band, cobalt sphere and flower with sharp petals possess enhanced microwave absorption compared with the reported cobalt nanoparticles, and two differently shaped flowers also show stronger absorption than cobalt nanochains. The novel hierarchical microstructure assemblies are important in enhancing microwave absorbing ability and widening their application in industry, commerce, and military affairs. Acknowledgment. This research is supported by the National Natural Science Foundation of China (No. 20776032). We thank S. Y. Shen (The Hong Kong University of Science and Technology, China) for the helpful discussion. References and Notes (1) Weiss, P. S. ACS Nano 2008, 2, 1085–1087. (2) Raula, M.; Rashid, M. H.; Paira, T. K.; Dinda, E.; Mandal, T. K. Langmuir 2010, 26, 8769–8782. (3) Janet, C. M.; Navaladian, S.; Viswanathan, B.; Varadarajan, T. K.; Viswanath, R. P. J. Phys. Chem. C 2010, 114, 2622–2632. (4) Chatterjee, T.; Jackson, A.; Krishnamoorti, R. J. Am. Chem. Soc. 2008, 130, 6934–6935. (5) Chen, H. J.; Kern, E.; Ziegler, C.; Eychmu¨ller, A. J. Phys. Chem. C 2009, 113, 19258–19262.

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