Synthesis , Characterization and Microwave Dielectric Properties of Flower-like Co ( OH ) 2 / C Nanocomposites

Flower-like Co(OH) 2 /C nanocomposites have been synthesized by a facile hydrothermal method. Flower-like Co(OH) 2 /C nanocomposites were self-assembled by the nanosheets with the thickness distribution of 50-80 nm and abundant flocculent carbon structures. With the help of transmission electron microscopy and energy dispersive spectrometer, nanosheets were observed to have core/ shell structure, in which Co(OH) 2 worked as cores and amorphous C as shells. When the as-prepared products were heated for 6 h at 180 °C, the Co(OH) 2 cores were amorphous. The heating time increased to 10 h, the Co(OH) 2 cores became crystalline. The formation mechanism and the self-assembly evolution process were proposed. The microwave dielectric properties of Co(OH) 2 /C nanocomposites were investigated in the frequency range of 0.03-18 GHz. Compared with the amorphous Co(OH) 2 /C nanocomposites, crystalline nanocomposites had the better conductivity.


Introduction
Nanostructured materials have received much attention because of their novel electronic, optical, magnetic and catalytic properties.In recent decades, much attention has been paid to the core-shell structured nanoparticles, namely 'nanocapsules' [1][2][3][4][5] .Up to now, with the development of local electronic devices, microwave communication, and the rising pollution of electromagnetic (EM) interference, materials with EMwave absorption in a wide frequency range, with strong absorption, low density, and low cost are more and more desirable.Among these nanocomposites, nanocapsules with magnetic nanoparticles as cores and dielectric shells are of great interest due to the synergetic effect between magnetic and dielectric losses.Recently, the EM-wave absorption properties of non-magnetic material-based nanocomposites have been driving considerable theoretical and experimental investigation.Wang et al synthesized the carbon coated Sn nanorods by arc discharge method and reported that reflection loss (RL) exceeding -20 dB could be realized in any interval within the 2-18 GHz range by choosing an appropriate thickness of the absorbent layer between 1.5 and 9 mm [6] .Zhang et al prepared the carboncoated Cu nanocapsules and investigated the permittivity in the GHz range 7 .In our previous work, carbon-coated Ti nanocapsules and carbon-coated VC nanocapsules were synthesized by arc discharge method and investigated their EM properties in detail 8,9 .
Carbon can be used as EM interference shielding and EM absorption materials, due to their good electrical, low density, thermal stability and excellent mechanical properties.Carbon has been widely used as the outer shell in the nanocapsules as microwave absorbers [10][11][12] .Compared with nanoparticles without core/shell structure, carbon shell can dramatically improve their EM absorption properties.Recently, transition metal hydrates have attracted great attentions due to their interesting physical and chemical properties.Among them, Co(OH) 2 is widely investigated as electrode material, catalytic material and magnetic cooling materia [13][14][15][16][17][18] .There are few reports on the EM properties of carbon-coated Co(OH) 2 nanocomposites.
In this work, we synthesized flower-like Co(OH) 2 /C composites self-assembled by Co(OH) 2 /C core/shell structured nanocapsules through a hydrothermal method.The phase composition and microstructure of Co(OH) 2 /C composites were investigated by means of X-ray diffraction, transmission electron microscopy and energy dispersive spectrometer.The dielectric properties of Co(OH) 2 /C composites also were investigated in the frequency range of 0.03-18 GHz.

Co(OH) 2 nanosheets
All reagents were analytical grade and used without further purification.The Co(OH) 2 nanosheets were synthesized by a hydrothermal method.In a typical process, 0.6g Cobalt-nitrate hexahydrate (Co(NO 3 ) 2 •6H 2 O) was dissolved in 50ml deionized water.The solution turned to be deep green immediately after 2ml ethanol and 20ml diethanolamine were added.The resultant solution was magnetic stirred for 15 min and subsequently transferred into a 100 ml teflon-lined stainless steel autoclave.After heating at 180 °C for 6 h, the tank was naturally cooled down to room temperature.The product was washed by deionized water and ethanol for three times, and finally dried in a vacuum oven at 80 °C for 2 h.

Flower-like Co(OH) 2 /C nanocomposites
The above black products were dissolved in 100 ml distilled water with 4 g glucose.The mixture was automatically stirred for 15 min and then was transferred into a 150 ml autoclave.The autoclave was sealed and put into a furnace with the temperature of 180 °C.After heating for 6 h, the autoclave was cooled naturally to room temperature.The product was washed with distilled water and ethanol several times to remove impurities before the characterizations.For the preparation of crystalline samples, the heating time increased to 10 h under the same experimental conditions.

Material characterization
The as-prepared sample was characterized by x-ray diffraction equipped with monochromatized Cu-Ka radiation (XRD, Bruker D8) and scanning electron microscopy (SEM, JEOL-6300 F) equipped with energy dispersive spectrometer (EDS) and Transmission electron microscopy (TEM).High-Resolution TEM (HRTEM) images were obtained on a JEOL JEM-2010 TEM.An EDS attached to the TEM was used to analyze the composition of the products.The Co(OH) 2 /C nanocomposite-paraffin composite was prepared by uniformly mixing Co(OH) 2 nanocomposites with paraffin and pressing them into cylinder-shaped compacts.Then the compact was cut into toroidal shape with 7.00 mm outer diameter and 3.04 mm inner diameter.The EM parameters are measured for Co(OH) 2 /C microsphere-paraffin composite containing 40 wt% Co(OH) 2 /C, using an Agilent N5244A network analyzer.Coaxial method was used to determine the EM parameters of the toroidal samples in a frequency range of 0.03-18 GHz with a transverse EM mode.The VNA was calibrated for the full two-port measurement of reflection and transmission at each port.The complex permittivity e r ( r ε = ε − ε ′ ′′, where ε′ and ε′′ are the real and imaginary parts of e r , respectively) was calculated from S-parameters tested by the VNA, using the simulation program of Reflection/ Transmission Nicolson-Ross model 18 .

Results and Discussion
Figure 1 shows the XRD patterns of products with heating time of 6 and 10 h.In Figure 1a, the broad diffraction peak at 20-25° arises from the amorphous carbon.It is worthy noted that there are no peaks for Co(OH) 2 , indicating that the Co(OH) 2 is not crystalline at heating time of 6 h.When the heating time reaches to 10 h, main peaks of Co(OH) 2 are observed at 32.48°, 37.90°, 57.92°, 61.52° and 67.94°, corresponding to the reflections of (100), (101), (102), (111) and (200) of Co(OH) 2 , respectively.All the observed peaks can be matched with the Co(OH) 2 (JCPDS no.34-0443).The average grain size of Co(OH) 2 is calculated to be about 30 nm from the full width at half maximum of the Co(OH) 2 (101) Bragg scatting peak intensity using Debye-Scherrer's formula θ , where l is the X-ray wavelength, d(2q) is the line broadening at half the maximum intensity in radians, and q is the Bragg scattering angle.
Figure 2 shows different magnified SEM images of the products with heating time of 6 and 10 h, respectively.All images clearly indicate that the presence of threedimensional (3D) flower-like structures.As shown in Figure 2a, the as-synthesized Co(OH) 2 /C sample presents a uniform flower-like microspheres with a averaged diameter of 3 µm.Furthermore, the magnified SEM images (Figure 2b) demonstrate that 3D flower-like microspheres are self-assembled by lots of nanosheets building blocks with the thickness ranging between 40 and 60 nm.In addition, abundant flocculent structures present in the 3D flower-like Co(OH) 2 /C microspheres, which are amorphous carbon proved by EDS. Figure 2c shows the SEM image of Co(OH) 2 /C samples with heating time of 10 h.The samples exhibit the uniform flower-like microsphere with a averaged diameter of 4 µm.In Figure 2d, the nanosheets have the thickness ranging between 60 and 80 nm.After being heated for 10 h, the size of microspheres and nanosheets and the amount of flocculent structures evidently increase, since the increasing time is helpful for the growth of microspheres and nanosheets and the decomposition of glucose.It is worthwhile noting that the as-obtained flowerlike Co(OH) 2 /C superstructures cannot be destroyed and broken into the individual Co(OH) 2 /C nanosheets even after subjecting long-time ultrasonication.In order to clearly demonstrate their microstructures, the TEM images of 3D flower-like Co(OH) 2 /C microspheres with different magnifications are shown in Figure 4. Figure 4a clearly shows the amorphous nature of nanosheet structure.Figure 4b demonstrates the element composition of nanosheet structure, in which Cu element is from the Cu grids.As shown in Figure 4c, the clear core-shell structure exists in the present nanosheet structure with heating time of 6 h, in which the amorphous shell has a thickness of 5 nm.The shell material is determined to be carbon from the EDS result (Figure 4d) and the experimental process.Figure 4e exhibits the microstructure of Co(OH) 2 /C nanocomposites with heating time of 10 h, in which the nanocomposites also have the core-shell structure with a shell of 6 nm.The results are consistent with the SEM results, indicating the heating time has an important influence on the growth and the crystalline of Co(OH) 2 /C nanocomposites.In addition, the HRTEM image in Figure 4e clearly shows that the d-spacing of 0.24 nm in the core corresponds to the lattice plane {101} of Co(OH) 2 , indicating Co(OH) 2 is crystalline in the nanocomposites with heating tine of 10 h.
As shown in Figure 5, the real part (ε′) and imaginary part (ε′′) of relative permittivity for the Co(OH) 2 /C-paraffin composite with 40 wt%.Co(OH) 2 /C nanocomposites are  plotted as a function of frequency at 0.03-18 GHz.ε′ is relative to the polarization and ε′′ implies the dielectric loss in the particles.It can be seen that ε′ exhibits the similar variation tendency of decreasing with the frequency increasing from 0.03 to 18 GHz which is due to increased lagging behind of the dipole-polarization response with respect to the electric-field change at higher frequencies 22 .The frequency dependence of ε′′ in Figure 5b can be explained by the following equation according to the free electron theory 23 : , where d is the electrical conductivity, f is frequency and e 0 is the dielectric constant in vacuum.The conductivity of Co(OH) 2 /C nanocomposites originates from its free electrons.Except for the natural frequency at around 14 GHz, the ε′′ of Co(OH) 2 /C nanocomposites with heating time of 10 h is higher than that of nanocomposites with heating time of 6 h, indicating that the crystalline state of Co(OH) 2 has better conductivity than the amorphous one.As we known, the ideal microwave absorbers with thin layer thickness should process the ability of strong absorption at the broader frequency band.The natural frequency at around 14 GHz indicates the enhanced dielectric loss in a limited frequency range.The dielectric loss is sensitive to the absorbing frequency, which becomes the challenge for high technological applications 24 .In addition, it can be found that ε′ and ε′′ for Co(OH) 2 /C nanocomposites exhibit a significant and similar fluctuation, over the 0.03-18 GHz range, ascribed to the displacement current lag at the core/shell interface, similar to the Fe/C and Fe/ZnO nanocapsules 25,26 .

Conclusion
In summary, flower-like Co(OH) 2 /C nanocomposites were fabricated by a facile hydrothermal method.Flowerlike Co(OH) 2 /C nanocomposites were self-assembled by the nanosheets and abundant flocculent carbon.The size of nanocomposites and amount of flocculent carbon increased with the heating time.TEM results proved the Co(OH) 2 /C nanocompsoites have core/shell structure, in which Co(OH) 2 worked as cores and amorphous carbon as shells.The formation mechanism and the self-assembly evolution process were proposed.The microwave dielectric properties of Co(OH) 2 /C nanocomposites were investigated in the frequency range of 0.03-18 GHz.Compared with the amorphous Co(OH) 2 /C nanocomposites, the crystalline nanocomposite had the better conductivity.

Figure 1 .
Figure 1.XRD patterns of products with heating time of (a) 6 h and (b) 10 h.

Figure 3
Figure3presents the formation process of the flowerlike Co(OH) 2 /C nanocomposites.Such a process is consistent with so-called two-steps growth process, which involves a fast nucleation of amorphous primary particles followes by a slow aggregation and crystallization of primary particles19 .Zhong et al. and Wei et al. have reported similar progress in preparing 3-D flower-like iron oxide and NiCo alloy nanostructures, respectively20,21 .In our experiment, the process divides into two steps.First, the mixed solution of Co(NO 3 ) 2 •6H 2 O, ethanol, glycolamine and deionized water is heated to 180 °C, and the Co(OH) 2 starts to nucleate.The nuclei growth and primary nuclei are formed and the small particle aggregates into plumelike structure nanosheets.Afterwards, the nanosheets assemble into flower-like microspheres.In the process of Co(OH) 2 /C nanocomposites, the glucose decomposes into carbon particles and H 2 O at 180 °C.The small C particles is attached to the surface of the flower-like microspheres, and the Co(OH) 2 /C nanocomposites are finally formed.In order to clearly demonstrate their microstructures, the TEM images of 3D flower-like Co(OH) 2 /C microspheres with different magnifications are shown in Figure4.Figure4aclearly shows the amorphous nature of nanosheet structure.Figure4bdemonstrates the element composition of nanosheet structure, in which Cu element is from the Cu grids.As shown in Figure4c, the clear core-shell structure exists in the present nanosheet structure with heating time of 6 h, in which the amorphous shell has a thickness of 5 nm.The shell material is determined to be carbon from the EDS result (Figure4d) and the experimental process.Figure4e

Figure 2 .
Figure 2. (a) SEM image and (b) magnified SEM image of Co(OH) 2 /C nanocomposites with heating time of 6 h; (c) SEM image and (d) magnified SEM image of Co(OH) 2 /C nanocomposites with heating time of 10 h.

Figure 3 .
Figure 3. Schematic illustration of the morphology evolution process of the flower-like Co(OH) 2 /C nanocomposites.

Figure 4 .
Figure 4. (a) TEM image of Co(OH) 2 /C nanocomposites with heating time of 6 h, (b) the corresponding EDS pattern, (c) HRTEM image of core-shell structure and (d) the corresponding EDS pattern at the amorphous shell part; (e) TEM image of Co(OH) 2 /C nanocomposites with heating time of 10 h and (f) the corresponding HRTEM image of the crystalline Co(OH) 2 core.