Template-free synthesis of Co nanoporous structures and their electromagnetic wave absorption properties

Template-free synthesis of Co nanoporous structures and their electromagnetic wave absorption properties

Materials Letters 78 (2012) 69–71 Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mat...

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Materials Letters 78 (2012) 69–71

Contents lists available at SciVerse ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Template-free synthesis of Co nanoporous structures and their electromagnetic wave absorption properties Jing Kong a, Fenglong Wang a, Xinzhen Wan a, Jiurong Liu a,⁎, Masahiro Itoh b, Ken-ichi Machida b a b

Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China Center for Advanced Science and Innovation, Osaka University, Osaka 565-0871, Japan

a r t i c l e

i n f o

Article history: Received 14 January 2012 Accepted 9 March 2012 Available online 17 March 2012 Keywords: Cobalt porous particles Magnetic materials Microstructure Electromagnetic wave absorption

a b s t r a c t We report a template-free hydrogen reduction approach to prepare sponge-like cobalt nanoporous structures employing Co3O4 as precursors, which were obtained by thermal-decomposing of CoCO3 intermediates prepared by a facile solvent-thermal route. A three-step formation mechanism for the Co nanoporous structures was proposed. The epoxy resin composites with 65 wt.% rhomb-like and rod-like samples showed efficient electromagnetic wave absorption characteristics (RL b −20 dB) in the ranges of 12.8–18 GHz and 11.2–18 GHz over absorber thicknesses of 1.05–1.5 mm and 1.0–1.6 mm, respectively. It is believed that the porous metallic magnets would gain wide applications as more efficient electromagnetic wave absorbers. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction In the family of nanomaterials with various morphologies, nanoporous materials with high specific surface area have been attracting intensive attention since their unique physical properties different from the solid counterparts, which make them critically important in technological applications [1–4]. In recent years, nanostructured magnets with different architectures were synthesized and have shown applications in high-density magnetic recording, medical diagnosis, microwave absorption, etc. [5–7], while nanoporous magnets are rarely reported. As important ferromagnetic materials, Co nano/ micro-structures have drawn extensive research interests for electromagnetic (EM) wave absorption applications. EM wave absorption properties of Co nano/micro-structures such as flower-like Co spheres, hollow Co nanochains, Co nanoplatelets etc. have been investigated, and the EM absorption characteristics, i.e., frequency, thickness, and absorbing band-width are strongly related to Co microstructures [8–10]. However, to the best of our knowledge, no investigation in exploring Co porous nano/micro-structures as EM absorption materials has been presented. Gu et al. synthesized mesoporous NiFe2O4 with enhanced EM absorption properties by using mesoporous silica as a hard template, in which the porous structure played a significant role [11]. The above results suggest that Co porous nano/micro-structures might be good EM wave absorbers due to their porous networks, low density, and large surface area.

⁎ Corresponding author. Tel.: + 86 531 88392036; fax: + 86 531 88392315. E-mail address: [email protected] (J. Liu).

Template-based approach is usually used to generate porous structures, but this approach is somewhat complicated. Other methods such as microwave-assisted synthesis [12], reaction-limited aggregation of nanoparticles [13], chemical/electrochemical de-alloying from a binary or multi-component alloy [14,15], are also employed to generate selected porous structures. The above synthesis routes usually suffer from low yield, and thus it is still desirable to find new efficient routes to largescale synthesis of porous metallic materials. In this study, we report a template-free hydrogen reduction approach to prepare Co nanoporous structures and the electromagnetic wave absorption properties were investigated in detail. 2. Experimental The CoCO3 intermediates were prepared by a facile solventthermal route, which was developed on the base of the previous report [16]. 1 g polyvinyl-pyrrolidone (PVP) was dissolved in 40 ml diethylene glycol (DEG) or the mixed solution of 38 ml DEG and 2 ml deionized water, followed by the addition of 0.05 M Co(CH3COO)2·4H2O under magnetic stirring. After they were completely dissolved, 1.8 or 0.3 g urea was added. Then the clear transparent solution was sealed in a teflon-lined autoclave and maintained at 200 °C for 24 h. The pink CoCO3 products were obtained. In air, the CoCO3 intermediates were converted to Co3O4 precursors by thermal decomposing at 400 °C for 2 h. In order to get the final Co products, the Co3O4 precursors were treated at 300 °C for 2 h under H2 flux with a ramping rate of 2 °C min − 1 in a quartz tube furnace. The microstructures of the products were examined using a JSM-6700F field emission scanning electron microscope (FESEM). X-ray diffraction (XRD) patterns were obtained by a Rigaku

0167-577X/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.03.026

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Fig. 1. (a) Schematic for the formation of Co nanoporous structures, and the XRD patterns of CoCO3 intermediates (b), Co3O4 precursors (c), and Co products (d).

Fig. 2. SEM images of as-prepared Co nanoporous structures with rhomb-like (a, c), and rod-like (g, h) morphologies.

Dmax-rc X-ray diffractometer. For evaluation of microwave absorption properties, epoxy resin composites were prepared by homogeneously mixing epoxy resin with 65 wt.% Co powders and compressed into toroidal-shaped samples (ϕout: 7 mm, ϕin: 3 mm). The relative permeability (μr) and permittivity (εr) were measured by a vector network analyzer (Agilent Technologies E8363A) in the range of 0.05–18.05 GHz. 3. Results and discussion Fig. 1a illustrates the three-step formation process for the Co nanoporous particles. When the as-prepared CoCO3 intermediates were gradually heated, CO2 started to release from the surface of particles. With temperature increasing, the inner CoCO3 was decomposed quickly and thus the gathered gas pressure made a mass of CO2 release from inside. The CO2 gas ran through channels originating from structure defects or vacancies of oxygen and carbon atoms, and the channels (See Fig. S1 in Supplementary Material) were reserved from the assembly of inner particles for decreasing the surface energy. In the next stage, the reduction gas molecules could homogeneously go through the existing channels and reduced the Co3O4 ligaments to Co. By kinetically controlling the thermal reduction, the crystallization of Co nanoparticles can be induced to constitute a 3D sponge-like structure. The XRD patterns of products in every step of transformation process were revealed in Fig. 1b. All diffraction

Fig. 3. The relative permittivity εr (a) and permeability μr (b) curves plotted against frequency for the resin composites with 65 wt.% nanoporous Co powders of sample H1 and H2 in the 0.5–18.0 GHz.

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to evaluate the EM wave absorption properties [19]. The RL values of the resin composites less than −20 dB, implying 99% of EM wave absorption, were obtained in the 12.8–18 GHz range with absorber thicknesses of 1.05–1.5 mm for sample H1 as shown in Fig. 4a. A minimum RL value of −42 dB was observed at 17.3 GHz with a matching thickness of 1.05 mm. For sample H2, the RL values less than − 20 dB were recorded in the 11.2–18.0 GHz range with absorber thicknesses of 1.0–1.6 mm, and a minimum RL of −38.7 dB was obtained at 14.8 GHz with a thickness of 1.2 mm. Comparing with some other Co hierarchical structures [8–10], the Co nanoporous structures exhibited more efficient EM wave absorption in GHz range, which can be attributed to that the special porous network is vital for the EM wave absorption since porous materials have better impedance matching with free space than the corresponding solid materials due to their low effective permittivity [20], meanwhile porous particles are available for suppressing the eddy current loss of metallic magnets and maintaining high permeability at high frequency, which will induce more EM energy attenuation.

4. Conclusions

Fig. 4. Frequency dependences of the reflection loss (RL) for the resin composites with 65 wt.% nanoporous Co powders of sample H1 (a) and H2 (b) at different absorber thicknesses in the 4–18.0 GHz.

peaks of the final products could be indexed to hcp Co phase (JCPDS 5-727), indicating the high purity of the Co nanoporous structures. SEM images of the as-prepared Co nanoporous products are demonstrated in Fig. 2. Monodispersed rhomb-like nanoporous particles with a diameter of ca. 1 μm were obtained when CoCO3 were synthesized in solution of 40 ml DEG and 1.8 g urea (Fig. 2a). When 0.3 g urea was added into the mixed solution of DEG and deionized water and the other experimental parameters were kept, rod-like nanoporous particles were prepared (Fig. 2b). Close inspection reveals that the sponge-like porous network is actually built from particle-like ligaments and there are a lot of holes on the surface. If CoCO3 are directly reduced by hydrogen without the first decomposing process, obvious cracks can be observed (See Fig. S2 in Supplementary Material). Fig. 3 shows the frequency dependences of the relative permittivity and relative permeability for the two resin composites containing 65 wt.% Co nanoporous structures with rhomb-like and rod-like morphologies, which were denoted as H1 and H2. As it is shown in Fig. 3a, the real part (ε′) and imaginary part (ε″) of relative permittivity for H1 and H2 were low between 0.5 and 18.0 GHz, in which the relative permittivity (εr = εr′ − jεr″) showed less variation. For the metalinsulator composites, the shape of permittivity could be ascribed to polarization ability, which mainly arises from space charge polarization and dipolar polarization [17]. The enhanced dipolar polarization dominated in the metal-insulator composites with increasing frequency [18], resulting in a little fluctuation at 9.3 GHz (H1) and 10.4 GHz (H2) of complex permittivity. Fig. 3b shows the real part of relative permeability (μr′) declined gradually with frequency in the range of 0.5–18.0 GHz, whereas the imaginary part of relative permeability (μr″) exhibited a broad peak in the 6.0–18.0 GHz with a maximum point at 10.0 GHz (for H1) and 10.8 GHz (for H2). The reflection loss (RL) curves calculated from the measured εr and μr at the given frequency and absorber thickness were employed

In conclusion, monodispersed cobalt nanoporous structures have been synthesized by a novel hydrogen reduction approach employing Co3O4 as precursors. The epoxy resin composites with 65 wt.% of rhomb-like and rod-like Co nanoporous samples showed efficient EM wave absorption (RL b −20 dB) in ranges of 12.8–18 GHz and 11.2–18 GHz, respectively. It is proposed that the nanoporous cobalt exhibited more efficient EM wave absorption, mainly due to the 3D porous structure. Supplementary materials related to this article can be found online at doi:10.1016/j.matlet.2012.03.026.

Acknowledgements This work was supported by the grants from the Qi-Lu Young Scholar program, the Doctoral Program of Higher Education of China (20090131120032) and the New Century Excellent Talent Program (NCET-10-0545), State Education Ministry.

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