Synthetic Metals 196 (2014) 125–130
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Synthesis and enhanced electromagnetic absorption properties of polypyrrole–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 on graphene nanosheet Yan Wang, Ying Huang ∗ , Juan Ding Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Key laboratory of Space Applied Physics and Chemistry, Ministry of Education, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 12 May 2014 Received in revised form 24 July 2014 Accepted 27 July 2014 Keywords: BaFe12 O19 Ni0.8 Zn0.2 Fe2 O4 Polypyrrole Graphene Electromagnetic properties
a b s t r a c t The polypyrrole(PPy)–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 was produced by an in situ polymerization, and then PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene composites were prepared by a deoxidation technique. The structures, morphology and electromagnetic properties of the samples were characterized by various instruments. Results show that PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 nanoparticles are dispersed on the graphene sheets. The saturation magnetization of the composites decrease from 33.93 to 26.93 emu/g with increasing the contents of graphene. However, the conductivities of the composites increase from 0.33 to 1.32 S/cm. Measurement of electromagnetic parameters indicates the reﬂection loss of the composites with 10% graphene is below −10 dB at 7.8–11.6 GHz and its maximum loss value is –25.5 dB at 9.8 GHz. The bandwidth corresponding to the reﬂection loss below –10 dB is 3.8 GHz. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Microwave absorbing materials have attracted much attention for the development needs of modern military and the environmental pollution of microwave irradiation [1–3]. A number of microwave absorbing materials have been reported such as ferrites, conducting polymers, carbon nanotubes and graphene [4–7], etc. The spinel ferrites and hexagonal ferrites are well known as traditional microwave absorbers with advantages of high saturation magnetization and magnetic loss, but their single absorbing mechanism (magnetic loss) leads to their low microwave absorbing properties and narrow frequency band. In addition, high density and low dielectric loss of ferrite absorbents also restrict their wide applications as microwave absorbers [8,9]. In order to improve their microwave absorbing properties as well as widen their frequency bands, the dielectric loss ﬁllers such as graphene and conducting polymers are always added into the system. It can be predicted that the microwave absorbing properties of the composites may be better than those of single components due to the synergistic effect between dielectric loss and magnetic loss . As we know, polypyrrole (PPy) have emerged as a new kind of materials in the last few years due to their high electrical conductivities, easy to be prepared and excellent environmental stability [11–14]. Graphene, as a two-dimensional planar sheets composed by sp2 -bonded
carbon atoms, has received great attention due to its unique electrical, mechanical, thermal and optical properties. Graphene has been used in nanoelectronic devices, energy storage devices, sensors, electromagnetic interference (EMI) and supercapacitors [15–17]. In particular, the composites of graphene and polymers can improve the mechanical and electrical properties of the polymers and can be used as new microwave absorbing materials with wide absorption bandwidth, thin thickness and light weight. In recent years, although researchers have focused their attention on the synthesis of PPy–ferrite [18–20], PPy–graphene [21,22] and ferrite–graphene [23,24], the ternary composites (PPy–ferrite–graphene) were seldom reported. In this paper, PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 was deposited on the surface of graphene sheets. The composites can obtain excellent electrical conductivities and electromagnetic properties, which signiﬁcantly increase the complex permittivity of the composite and provide the material with interesting microwave absorbing properties. The nanostructures and morphologies of the composites were characterized by XRD, TEM, FTIR and Raman. Properties such as thermal stability, conductivity, magnetism and microwave absorbing were also investigated. 2. Experimental 2.1. Preparation of nanocrystalline BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 ferrite
∗ Corresponding author. Tel.: +86 29 88431636. E-mail address: [email protected]
(Y. Huang). http://dx.doi.org/10.1016/j.synthmet.2014.07.027 0379-6779/© 2014 Elsevier B.V. All rights reserved.
The BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 nanocomposite ferrite was prepared by the sol–gel process. In the solution, according to the
Y. Wang et al. / Synthetic Metals 196 (2014) 125–130
stoichiometry, 15.333 g citric acid was dissolved in 200 mL deionized water. Then 10.336 g ferric nitrate, 0.3908 g nickel nitrate, 0.1 g zinc nitrate and 0.5 g barium nitrate were added into the citric acid solution and stirred magnetically under room temperature. Glycol (8.135 mL) and quadrol were added to the above solution to adjust pH to 7. The mixed solvent was evaporated in a magnetic blender at 75 ◦ C to remove surplus water until a viscous liquid was obtained. The gel was then dried in an oven at 130 ◦ C for about 24 h. Finally, the gel was put in a mufﬂe furnace to presinter at 400 ◦ C for 3 h and was subsequently calcined at 1100 ◦ C for 2 h to form the nanocomposite ferrite.
2.2. Preparation of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 ferrite PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 ferrite was prepared by in situ polymerization in aqueous solution. In a typical procedure, according to the stoichiometry, 1 g BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 ferrite nanoparticles and 0.5 mL pyrrole monomer were added into 200 mL aqueous solution and sonicated for 30 min. Then 4.55 g FeCl3 ·6H2 O (the molar ratio of FeCl3 ·6H2 O to pyrrole was 2.33:1) dissolved in 100 mL aqueous solution was added drop by drop to the mixture with constant stirring. The polymerization was carried out at 0 ◦ C for 12 h. The composites obtained were ﬁltered, washed and dried under vacuum at 60 ◦ C for 24 h.
2.3. Preparation of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene nanocomposites In a typical step, PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 (1.5 g) were dispersed in N,N-dimethylformamide (DMF) by ultrasonication for 2 h. And then GO (5%,10%,15%, and 20%) dissolved in DMF was added drop by drop to the PPy-BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 with mechanical stirring for 1 h. After that, hydrazine was added to the mixture at 95 ◦ C for 12 h. The resulting precipitate was ﬁltrated, washed with distilled water and ethanol repeatedly and dried under vacuum at 60 ◦ C for about 24 h.
2.4. Characterization The crystal structure of the PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 – graphene nanocomposites were analyzed by X-ray diffraction (XRD) patterns (German Bruker D8 advance diffractometer with ˚ in the 2 scanning range from 10◦ Cu-K␣ radiation ( = 1.5418 A) to 80◦ ). Transmission electron microscopy (TEM, American FEI F30 G2 ) was employed to analyze the morphology. The Raman spectra of the composites were obtained by using an inVia Laser-Raman spectrometer (Renishaw Co, England) with a 514 nm radiation. The thermal stabilities of the composites were analyzed by using a thermogravimetric analysis (TGA) (Model Q50, TA, USA) from room temperature to 800 ◦ C in air atmosphere, with a heating rate of 20 ◦ C/min. The magnetic properties were measured by using a vibrating sample magnetometer (VSM, Lake Shore7307) with a maximal applied ﬁeld of 13500 Oe. The conductivities of the composites were measured by four point probe method using a SX1944 four-point probe instrument (Baishen Science Co. China). The electromagnetic parameters of them were analyzed by using a HP8720ES vector network analyzer and the samples were pressed to be toroidal samples with OD 7 mm, ID 3.04 mm and height about 3 mm. The mass ratio of parafﬁn to PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene nanocomposites is 7:3.
Fig. 1. XRD pattern of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene composites.
3. Results and discussion 3.1. XRD analysis The crystal structures of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 – graphene can be characterized by XRD analysis and the results are shown in Fig. 1. Some diffraction peaks at 30.44◦ , 32.29◦ , 34.31◦ , 35.81◦ , 37.18◦ , 40.54◦ , 42.56◦ , 55.16◦ , 57.16◦ and 63.21◦ can be seen, which correspond to the (0 0 8), (1 0 7), (1 1 4), (1 0 8), (2 0 3), (2 0 5), (0 0 4), (2 1 7), (3 3 3) and (2 2 0) planes of BaFe12 O19 (ﬁle number: PDF#27-1029) and Ni0.8 Zn0.2 Fe2 O4 (ﬁle number: PDF#52-0277), respectively. The results indicate that BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 nanocomposite has a crystal structure. However, for the low contents of PPy and graphene, the diffraction peaks of PPy and graphene are not obvious due to the low contents of PPy and graphene. Besides, the strong diffraction peaks of BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 may also cover the diffraction peaks of PPy and graphene. So, some other testing instruments were used to analyze the relation between graphene, PPy and BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 . 3.2. Structural analysis Raman spectroscopy is widely used to obtain the structural information about disordering in sp2 carbon materials. Results are shown in Fig. 2. The Raman spectroscopy of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene exists two prominent peaks at about 1580 and 1350 cm−1 , which correspond to the G and D peaks of graphene, respectively. The G band represents the ﬁrst-order scattering of the E2g vibrational mode while the D band attributes to the reduction in size of the in-plane C sp2 atoms .
Fig. 2. Raman spectra of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene composites.
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Fig. 5. EDX of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene composites. Fig. 3. FTIR spectra of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene composites.
Additionally, there are two small peaks at 1040 and 970 cm−1 , which are the characteristic peaks of PPy . To further conﬁrm the above interactions exist, and had taken place between graphene, PPy and BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 , the FTIR spectrogram of the sample was investigated and the results are shown in Fig. 3. Peaks at 1560 and 3431 cm−1 are associated with the C C and N H stretching vibration in pyrrole ring. The peaks at 2921 and 2853 cm−1 are designated as the asymmetric and symmetric stretching vibrations of CH2 . The peak at 1397 cm−1 may be attributed to a small amount of O H deformation (no reduction) in graphene. Strong peaks at 1190 cm−1 , 1034 cm−1 and 905 cm−1 are attributed to C H in plane vibration, C H deformation vibration and C H out-of-plane vibration . Furthermore, the peaks at 440 cm−1 and 582 cm−1 can be found, which are the typical stretching vibration between metal ions and oxygen in the ferrites .
The morphologies of the PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 – graphene composites were further investigated by TEM measurements. Fig. 4a is a low-resolution TEM image of the ternary nanocomposites. It can be seen that PPy–BaFe12 O19 / Ni0.8 Zn0.2 Fe2 O4 nanoparticles are deposited on the surface of graphene sheets and between the layers of graphene sheets. Magniﬁed TEM images (Fig. 4b–d) show that the ferrite nanoparticles distributed in PPy matrixes and are homogenously enwrapped by PPy coating. The HRTEM image (Fig. 4e) clearly shows sets of the lattice fringes, indicating the crystal nature of the nanoparticles. The lattice spaces are 0.263 nm and 0.161 nm, respectively, corresponding to (1 1 4) the plane of BaFe12 O19 and the (3 3 3) plane of Ni0.8 Zn0.2 Fe2 O4 . Therefore, in terms of the TEM, Raman and FTIR analyses, we can draw a conclusion that the composites are composed of polycrystalline ferrite particles, PPy and graphene, which has not been reported before. Elements and their contents of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 – graphene composites were measured by EDX analysis. From Fig. 5,
Fig. 4. TEM images of the samples, (a): Low-resolution TEM image, (b–d): magniﬁed TEM images, (e): HRTEM image of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene composites.
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Table 1 The content of various elements (PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene composites). Element
C(K) O(K) Fe(K) Ni(K) Zn(K) Ba(K)
43.65 18.58 30.66 1.37 0.81 4.93
64.21 22.85 11.79 0.49 0.26 0.41
Fig. 7. Hysteresis loops of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene (x wt%) composites, (a):0 (b):5 (c):10 (d):15 (e):20. Table 2 Electrical conductivities of the composites prepared with different graphene contents. Graphene content (%) Conductivity (S/cm)
Fig. 6. TGA curves of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene (x wt%) composites, (a):0 (b):5 (c):10 (d):15 (e):20.
it can be seen that the samples are composed of C, Ba, Ni, Zn, Fe and O. Table 1 shows the atom content of the samples. The mass content of Ba, Ni, Zn, Fe and O in the composites are 4.93%, 1.37%, 0.81%, 30.66% and 18.58%, respectively, while it is 43.65% for the C element. The abundant C element may come from the PPy and graphene.
3.4. TGA analysis The effect of the graphene content on the thermal stability of the composites was investigated by TGA in the air and the results are shown in Fig. 6. As can be seen from Fig. 6, PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 composite shows a twostep weight loss process. The ﬁrst small fraction of weight loss from room temperature to about 100 ◦ C is mainly due to the evaporation of the absorbed water and HCl . The second step of weight loss occurs in the range of 200–500 ◦ C, which can be attributed to the degradation of PPy chain. In the case of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 composites, weight loss has a constant value at 500 ◦ C. However, after addition of the graphene, an enhancement in thermal stability has been observed. Weight loss has kept stable at 560 ◦ C as compared to 500 ◦ C for the PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 composites which demonstrate the improved thermal stability. The enhancement in thermal stability of the composites can be ascribed to the restriction imposed by the graphene nano sheets on the mobility and thermal vibration of PPy chains at the graphene–PPy interface , which further delay the degradation of the PPy chains. At the same time, different amounts of residue are caused by different contents of BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 . With the increasing of graphene content, the residues decrease from 58.34% to 54.86, which is consistent well with the mass of BaFe12 O19 –Ni0.8 Zn0.2 Fe2 O4 . All the above observations have revealed that the PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene nanocomposites have been synthesized.
3.5. Magnetic and electrical properties The magnetic properties of the as-prepared nanocomposite were measured by VSM. Fig. 7 shows the magnetic hysteresis curves of the samples measured at room temperature. The introduction of graphene can decrease the saturation magnetization (Ms) of the composites. With the increasing graphene contents, the Ms value decrease from 33.93 to 26.93 emu/g. The remnant magnetization (Mr) value decrease from 16.95 to 12.09 emu/g. The decrease of the saturation magnetization can be attributed to the presence of nonmagnetic graphene. The conductivities of the samples were measured by four point probe method. Results are shown in Table 2. The conductivity of graphene is 28.57 S/cm, which is higher than that of PPyBaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 . The introduction of graphene makes the conductivity of the composites increase. With the increasing graphene contents, the conductivities increase from 0.33 to 1.32 S/cm. The increased conductivity may be attributed to the – stacking between the graphene layers with high aspect ratio, large speciﬁc surface area and PPy chains . 3.6. Microwave absorption properties The electromagnetic parameters such as complex permittivity (ε* = ε −jε ), complex permeability (* = −j ), dielectric loss (tanıε=ε”/ε’) and magnetic loss (tanım = ”/’) of the composites were measured between 2 GHz and 18 GHz. Results are shown in Fig. 8(A–F). The real part of the complex permittivities (ε’) for PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 is lower than those of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene. With the increase of graphene contents, the real part of the complex permittivities (ε’) for PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene increase while the imaginary part of the complex permittivities (ε”) for PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 are higher than those of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene between 2 GHz and 10 GHz. The ε’ and ε” values of the samples ﬁrst decrease, and then increase with the increasing of the frequency. Two strong absorption peaks appear at 12 GHz and 16 GHz, which may be due to the increase in bound charges present in the material and the increasing bound charges create high displacement current in the material . The PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 has better dielectric loss
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Fig. 8. Electromagnetic parameters(ε* = ε − jε , * = − j ) of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene (x wt%) composites, (a):0 (b):5 (c):10 (d):15 (e):20.
(tan␦) than PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene. The real parts of the complex permeabilities (’) for all the samples are almost equal while the imaginary part of complex permeabilities (”) for PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 are lower than those of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene. The reason may be that the introduction of graphene increases magnetic anisotropy ﬁeld between graphene layers, PPy and ferrite, which results in increasing of the natural resonance of the composites. The ’ values appear two absorption peaks at 12 GHz and 14 GHz. The ” values decrease from 2 GHz to 12 GHz, and a strong absorption peak appears at 14 GHz, which is attributed to the magnetic resonance and magnetic domain-wall motion of the composites . The magnetic loss (tanım) of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 is poorer than that of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene. The microwave absorbing properties of the materials can be deﬁned by the reﬂection loss. Generally, the reﬂection loss (RL) of the electromagnetic waves is relative to the complex permittivity, complex permeability and the thickness of the samples. Excellent RL is originated from efﬁcient complementarities between complex permittivity and complex permeability in materials. Based on the measured data of the electromagnetic parameters, the microwave absorbing properties of the obtained samples can be calculated by the following equation .
r 2fd √ tanh(j r εr ) εr c
Zin − 1 Z +1
R(dB) = 20lg
where R (dB) denotes to the reﬂection loss in decibel unit, Zin is the normalized input impedance relating to the impedance in free space, f is the microwave frequency, d is the thickness of the absorb layer, c is the velocity of electromagnetic wave in vacuum, and εr and r are the complex relative permittivity and permeability, respectively. The calculated RL curves of the samples (the thickness of coating is 3.0 mm) with different graphene contents are shown in Fig. 9. From the graphs, we can see that with the increase of graphene contents, the microwave absorbing properties of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene ﬁrstly increase and then decrease. Comparing with the conductivities in Table 2, it can
Fig. 9. Reﬂection loss curves of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene (x wt%) composites, (a):0 (b):5 (c):10 (d):15 (e):20.
be found that too higher conductivity or too lower conductivity is not beneﬁcial to improving microwave absorbing properties and the maximum microwave absorbing demands an intermediate conductivity. Too high permittivity is harmful to the impedance matching and results in weak absorption . When the permittivity and permeability meet the impedance matching requirement, higher dielectric loss and magnetic loss imply better EM attenuation properties. When the mass ratio of graphene is 10%, the synergistic effect of the complex permittivity and complex permeability reaches the maximum. The microwave absorbing property of the composite with 10% graphene content is superior to that of other different graphene contents. The reﬂection loss of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene (10%) composite is below −10 dB at 7.8–11.6 GHz and its maximum loss value is –25.5 dB at 9.8 GHz. The bandwidth corresponding to the reﬂection loss below –10 dB is 3.8 GHz. The possible reason for this is that, in graphene/polymer nanocomposites, two-dimensional graphene has a higher speciﬁc surface area and can form larger composite interface, conductive network and dipole polarization between graphene and polymer interface. The dipole interacts with microwave ﬁeld and leads to the lattice vibrations . Meanwhile, the microwave losses are depleted in the form of heat. For the difference of the dielectric constants in the interface, the microwave produces scattering and multiple reﬂections. In general,
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the microwave absorbing of graphene/polymer nanocomposites originates from conduction loss, polarization functions, interface scattering and multiple scattering. The main absorbing mechanism is that graphene and PPy are dielectric loss absorbents, and BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 ferrite is a magnetic loss absorbent. When the mass content of graphene is 10%, the magnetic loss and dielectric loss can be matched well. The synthesized composite PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene can obtain better microwave absorbing properties. 4. Conclusion In this study, PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene nanocomposites were prepared by a deoxidation technique. The results of XRD, FTIR, Raman and TEM reveal PPy–BaFe12 O19 / Ni0.8 Zn0.2 Fe2 O4 nanoparticles dispersed on the surface of graphene sheets. The electrical conductivities of the composites increase with the increasing of graphene contents and the magnetic properties are in the contrary. The TGA analysis suggests that the introduction of graphene make the PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene nanocomposites have a better thermal stability than PPy/BaFe12 O19 –Ni0.8 Zn0.2 Fe2 O4 . Measurements of the reﬂection loss (RL) show that when the mass ratio of graphene is 10%, the reﬂection losses of the composites achieve the best and the electromagnetic parameters can be matched well. The reﬂection loss of graphene (10%) composites below –10 dB range from 7.8 to 11.6 GHz and its maximum loss value is –25.5 dB at 9.8 GHz. The bandwidth corresponding to the reﬂection loss below -–10 dB is 3.8 GHz. The microwave absorbing properties of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4 –graphene are superior to those of PPy–BaFe12 O19 /Ni0.8 Zn0.2 Fe2 O4. Acknowledgments This work was supported by the Spaceﬂight Foundation of the People’s Republic of China under Grant no. NBXT0002, the Spaceﬂight Innovation Foundation of China under Grant no. NBXW0001.
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