Fabrication of nickel ferrite microspheres decorated multi-walled carbon nanotubes hybrid composites with enhanced electromagnetic wave absorption properties

Fabrication of nickel ferrite microspheres decorated multi-walled carbon nanotubes hybrid composites with enhanced electromagnetic wave absorption properties

Journal of Alloys and Compounds 784 (2019) 422e430 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:...

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Journal of Alloys and Compounds 784 (2019) 422e430

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Fabrication of nickel ferrite microspheres decorated multi-walled carbon nanotubes hybrid composites with enhanced electromagnetic wave absorption properties Jiabin Zhang, Ruiwen Shu*, Changlian Guo, Ruirui Sun, Yanan Chen, Jia Yuan School of Chemical Engineering, Anhui University of Science and Technology, Huainan, 232001, People's Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 October 2018 Received in revised form 31 December 2018 Accepted 6 January 2019 Available online 8 January 2019

Herein, we fabricated the multi-walled carbon nanotubes/nickel ferrite (MWCNTs/NiFe2O4) hybrid composites by a facile one-pot solvothermal strategy. The structure, composition, micromorphology and electromagnetic parameters of the as-prepared hybrid nanocomposites were investigated by various analysis techniques. Experimental results revealed that the three-dimensional (3D) conductive networks consisting of NiFe2O4 microspheres twined MWCNTs were formed in situ in the hybrid composites. Moreover, the additive amounts of MWCNTs had a significant influence on the electromagnetic wave (EMW) absorption properties. Remarkably, the minimum reflection loss (RLmin) reached 42.3 dB with a thin thickness of 1.2 mm and effective absorption bandwidth (EAB, RL less than 10 dB) was 3.8 GHz. Furthermore, the possible EMW absorption mechanism was proposed. Therefore, it could be concluded that the hybridization of magnetic NiFe2O4 microspheres with dielectric MWCNTs could be an effective strategy to enhance the EMW absorption performance of NiFe2O4. Our results could be helpful for designing and developing novel MWCNTs-based hybrid composites as the high-performance EMW absorbers. © 2019 Elsevier B.V. All rights reserved.

Keywords: Hybrid composites Solvothermal Electromagnetic wave absorption Multi-walled carbon nanotubes Nickel ferrite

1. Introduction Electromagnetic interference and pollution problems are increasingly serious originated from the extensive use of electronic equipment and devices, which lead to the electromagnetic wave (EMW) absorbing materials gaining significant attentions in the field of functional materials [1e3]. However, it is still a great challenge for developing novel EMW absorbing materials that simultaneously satisfy the following requirements, such as broad frequency, strong absorption, light weight and thin thickness [4e8]. As an important kind of spinel ferrites, magnetic nickel ferrite (NiFe2O4) has been used as the EMW absorbers due to its remarkable properties, such as good chemical stability, outstanding magnetic loss and low cost [2,9e11]. However, its shortcomings like inferior impedance matching, sole magnetic loss absorption mechanism and narrow absorption bandwidth, which limit the practical applications of NiFe2O4 in the field of EMW absorption [2]. Multi-walled carbon nanotubes (MWCNTs) have been regarded

* Corresponding author. E-mail address: [email protected] (R. Shu). https://doi.org/10.1016/j.jallcom.2019.01.073 0925-8388/© 2019 Elsevier B.V. All rights reserved.

as promising candidates for EMW absorption owing to their uniquely tubular structure, low density, high aspect ratio and good electrical conductivity [12e17]. Generally, the two electromagnetic principles of maximum attenuation and impedance matching should be carefully regulated for designing ideal EMW absorbers. Recent investigations revealed that the complexation of MWCNTs with magnetic spinel ferrites for fabricating the MWCNTs-based hybrid composites could be a useful way to further enhance the EMW absorption performance of MWCNTs [12,13,18e22]. For instance, Chen et al. prepared the three-dimensional (3D) Fe3O4MWCNTs hybrid composites through a simple co-precipitation route and found that the minimum reflection loss (RLmin) reached 52.8 dB and effective absorption bandwidth (EAB, RL less than 10 dB) was 3.0 GHz with a coating thickness of 6.8 mm [20]. Liu et al. synthesized Zn ferrite/MWCNTs composites by a one-pot hydrothermal method. The obtained composites exhibited the RLmin of 42.6 dB with a thickness of 1.5 mm [18]. In our previous work, we fabricated the MWCNTs/ZnFe2O4 hybrid composites and further investigated the effects of microspheres size and MWCNTs length on the EMW absorption properties [12]. Results revealed that the hybrid composites exhibited the RLmin of 55.5 dB and

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bandwidth of 3.6 GHz with a thickness of 1.5 mm [12]. However, it is still hard to develop the high-performance EMW absorbers of MWCNTs-based hybrid composites by a facile strategy with a thin coating thickness less than 1.5 mm. Herein, we reported a facile route to fabricate the 3D net-like multi-walled carbon nanotubes/nickel ferrite (MWCNTs/NiFe2O4) hybrid composites and further explored the effect of the additive amounts of MWCNTs on their EMW absorption performance. Besides, the possible EMW absorption mechanism was proposed. Therefore, our results could contribute to the designing and developing novel MWCNTs-based high-efficiency EMW absorbers. 2. Experimental section The detailed preparation procedures and characterization of MWCNTs/NiFe2O4 hybrid composites were described in the electronic supplementary materials. 3. Results and discussion 3.1. Formation process of MWCNTs/NiFe2O4 hybrid composites Fig. 1 shows a schematic illustration of the fabrication procedure of MWCNTs/NiFe2O4 hybrid composites. First, the pristine MWCNTs were treated by concentrated nitric acid reflux process [12]. Thus, the surface of acid-treated MWCNTs carried numerous oxygencontaining functional groups (eCOOH and eOH groups, etc.) and defects [12]. Then, the Fe3þ and Ni2þ in the dispersions of MWCNTs could be attached to the surface of MWCNTs by electrostatic interactions [12]. After the solvothermal reaction at 200  C for 12 h, the NiFe2O4 microspheres were deposited in situ on the surface of MWCNTs. 3.2. Structural analysis Fig. 2 depicts the Fourier transform infrared (FT-IR) spectra of the samples of S1, S2 and S3. The peaks appearing at 3346, 1633, 1393, and 1040 cm1 are assigned to the stretching vibration of OeH, C]C stretching vibrations from the aromatic zooms, CeOH stretching vibration from the carboxyl group, CeO vibrations from the alkoxy groups, respectively [23]. Besides, the characteristic peak at 593 cm1 can be ascribed to the lattice absorption of NiFe2O4 [23]. As shown in Fig. 3, the X-ray diffraction (XRD) patterns of the three samples are in accordance with the standard profile of spinel NiFe2O4 (JCPDS No. 10-0325) [2,9,23,24]. The diffraction peaks at 2q ¼ 30.2, 35.6, 43.3, 57.2 and 63.1 correspond to the (220), (311), (400), (511) and (440) crystal planes of NiFe2O4, respectively. Furthermore, no diffraction peaks of MWCNTs appear in the hybrid composites of S2 and S3, which may be ascribed to the low content of MWCNTs [14,25]. The degree of graphitization in carbonaceous materials can be identified by Raman spectroscopy [12]. As shown in Fig. 4, the Raman scattering peaks at around 1330, 1580 and 2650 cm1 can be assigned to the D band (the first-order vibration of sp3 bond), G band (the in-plane vibration of sp2 bond) and 2D band (the second order vibration of sp3 bond), respectively [12,25,26]. ID/IG, which defined as the intensity ratio of D to G band is generally used to characterize the disorder degree [12,25,26]. It can be seen that the value of ID/IG increases from 1.54 (S2) to 1.62 (S3), and ID/IG of S2 and S3 are less than that of MWCNTs (1.64) [12]. Therefore, the ID/IG increases with the increase of the additive amounts of MWCNTs. Notably, the Raman scattering peaks appearing in the low wavenumbers range (100e800 cm1), which are in good accordance with the previous reports on NiFe2O4 particles [2,23,24]. Besides,

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the intensity of Raman peaks of NiFe2O4 becomes weaker with the increase of the additive amounts of MWCNTs in the hybrid composites. 3.3. Morphological analysis Microscopic morphology and structure of the as-prepared hybrid composites were observed by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From Fig. 5(a) and (b), it is obvious that the NiFe2O4 particles present the spherical morphology with uneven size distribution (30e500 nm). Fig. 5(c) and (d) reveal that the NiFe2O4 microspheres are twined by the MWCNTs, which can further act as the bridges to crosslink the neighboring MWCNTs. Consequently, some locally 3D conductive networks are formed in situ in the hybrid composites, which are beneficial to the enhancement of conduction loss, and multiple reflections and scattering of incident EMWs [12,27]. As depicted in Fig. 5(e) and (f), the locally 3D conductive networks are also observed in the sample of S3. Compared the SEM images of the samples of S2 with S3, it can be observed that more MWCNTs appear in S3, which is in accordance with the feeding ratios in the experimental section. Besides, the EDS pattern of S2 clearly demonstrates that the existence of C, O, Fe and Ni elements (Fig. 5(g)). Fig. 6 displays the TEM observations of the samples of S1, S2 and S3. It can be observed that the NiFe2O4 particles in all samples exhibit the spherical morphology. Furthermore, the TEM images of hybrid composites of S2 and S3 (Fig. 6(c)(f)) reveal that most of the NiFe2O4 microspheres are well anchored on the surface of MWCNTs without large aggregation and almost no NiFe2O4 microspheres are dropped off from MWCNTs even under powerful ultrasound. These results demonstrated that the NiFe2O4 microspheres are tightly bonded on the surface of MWCNTs with strong interactions. Besides, the NiFe2O4 microspheres exhibit the rough surfaces, which may be originated from the self-assembly of small primary nanocrystals during the solvothermal process [12]. 3.4. Electromagnetic absorption properties Fig. 7 depicts the frequency dependence of reflection loss (RL) with different thicknesses for the three samples. From Fig. 7(a), it is clear that the RLmin of S1 (pure NiFe2O4) is larger than 10.0 dB, suggesting the poor EMW attenuation capacity. As shown in Fig. 7(b), after hybridization the NiFe2O4 microspheres with MWCNTs, the hybrid composite (S2) exhibits enhanced EMW absorption performance compared with S1. To be specific, the S2 presents the RLmin of 42.3 dB with a thin thickness of 1.2 mm and EAB of 3.8 GHz (11.8e15.6 GHz) with a thickness of 1.5 mm. However, with further increase of the additive amounts of MWCNTs, the S3 exhibits much weaker EMW absorption performance with the RLmin of only 11.3 dB (Fig. 7(c)) than S2. These results demonstrate that the additive amounts of MWCNTs have significant influence on the EMW absorption properties of the hybrid composites, and thus should be carefully controlled. Fig. 7(d) shows the 3D plots of reflection loss of S2. It can be seen that the RLmin corresponding to the maximum EMW absorption gradually appears at different frequencies, which is tunable by changing the coating thickness. Fig. 8 depicts the frequency-dependent electromagnetic parameters (ε0 , ε'', m0 , m'') and loss tangent. As shown in Fig. 8(a) and (b), both the ε0 and ε'' enhance with the increase of the additive amounts of MWCNTs originated from the introduction of conductive MWCNTs and further forming the 3D conductive networks. From Fig. 8(a), it is clear that the ε0 of S2 and S3 decreases as the frequency increases with slight fluctuations and the S3 shows much larger ε0 than that of S2. To be specific, the values of ε0 of S2 and S3 decrease from 21.0 to 13.0 and 33.6 to 12.5, respectively. However,

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Fig. 1. Schematic illustration of the synthesis process of MWCNTs/NiFe2O4 hybrid composites.

Fig. 2. FT-IR spectra of the samples of S1, S2 and S3.

Fig. 3. XRD patterns of the samples of S1, S2 and S3.

Fig. 4. Raman spectra of the samples of S1, S2 and S3.

the ε0 of S1 almost keeps constant around 5.0. As displayed in Fig. 8(b), the ε'' of S2 and S3 decreases with some fluctuations as the frequency increases, while that of S1 slightly decreases from 0.9 to 0.2. Furthermore, both S2 and S3 show obvious relaxation peaks of ε'' [24] in the frequency range of 8e16 GHz (marked by the dashed boxes in Fig. 8(b)). Fig. 8(c) depicts the frequency-dependent m'. For all the samples, the values of m0 are in the range of 0.8e1.4. Furthermore, it could be found that the values of m0 for the samples of S1eS3 increase at higher frequencies (8e17 GHz) and the S3 shows much larger m0 than that of S1 and S2. Recent investigations demonstrated that an external electromagnetic field could polarize the composites not only electrically but also magnetically, which formed electromagnetic coupling, resulting in the increasing of complex permeability at higher frequencies [28,29]. The similar phenomenon had been also reported by some researchers in their previous studies [28e30]. Therefore, we speculate that the increase of m0 at higher frequencies in the present study mainly originates from the enhanced electromagnetic coupling effect and this coupling effect enhances with the increase of the additive amounts of MWCNTs. From Fig. 8(d), it can be found that the m'' of all the

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Fig. 5. SEM images with different magnifications: (a)(b) of S1, (c)(d) of S2 and (e)(f) of S3; EDS pattern (g) of S2.

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Fig. 6. TEM images with different magnifications: (a)(b) of S1, (c)(d) of S2 and (e)(f) of S3.

samples are in the range of 0.4e0.7 and the S1 exhibits the largest m'' almost in the whole frequency range. Specifically, the S1 exhibits an obvious peak value of 0.64 at 3.7 GHz, which suggests the strong EMW attenuation ability. Furthermore, it should be noted that the m'' exhibits a slight decrease with the increase of the additive amounts of MWCNTs from the S1 to S3. This result may be caused by that the NiFe2O4 particles with uniformly spherical morphology (SEM images of Fig. 5 and TEM images of Fig. 6), which are hard to obviously improve the permeability of MWCNTs/NiFe2O4 hybrid composites [28,31]. Besides, there are obvious resonance peaks of m'' [24] for all the samples in the frequency range of 2e5 GHz (marked by the dashed boxes in Fig. 8(d)). In general, the electromagnetic attenuation losses include the magnetic loss and dielectric loss. The dielectric loss tangent (tande ¼ ε''/ε0 ) and magnetic loss tangent (tandm ¼ m''/m0 ) are widely used to evaluate the attenuation loss, which significantly affect the EMW absorption properties of absorbers [12,32]. As shown in Fig. 8(e), it is obvious that the tande enhances with the increase of the additive amounts of MWCNTs and the S3 exhibits the strongest dielectric loss capacity. Fig. 8(f) displays the frequency-dependent tandm. It is clear that the tandm for all the samples is in the range

of 0.3e0.6. Furthermore, all the samples exhibit obvious resonance peaks in tandm in the low-frequency range and the S1 exhibits the biggest magnetic loss peak with a value approximately equal to 0.53. In the microwave frequency range of 2e18 GHz, numerous investigations demonstrated that only the exchange resonance, eddy current loss and natural resonance contributed to the magnetic loss [12,32,33]. Because the NiFe2O4 in all the samples with average particle size much larger than the exchange length (~10 nm) [25,34], the exchange resonance could be excluded. Generally, the permeability can be described as follows [12,25,32,33]:

00

 0 2

m z2pm0 m

sd2 f =3

(1)

Herein s (S/cm) and m0 (H/m) are the conductivity and permeability in the vacuum, respectively. If the eddy current loss is the only mechanism for magnetic loss, the eddy current coefficient C0 (m''(m0 )2f1) should remain constant as the frequency changes [12,25,32,35]. From Fig. 9, it can be seen that the values of C0 of all the samples show obvious fluctuations in the high-frequency range

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Fig. 7. Frequency-dependent reflection loss with different thicknesses for the samples of (a) S1, (b) S2 and (c) S3; (d) 3D representation of reflection loss of S2.

of 8e17 GHz, which suggests the eddy current loss is not the dominated mechanism for magnetic loss. Besides, all the samples exhibit prominent peaks of C0 in the low-frequency range, suggesting the existence of significant natural resonance [25,34,36]. The two electromagnetic principles of maximum attenuation and impedance matching should be carefully regulated for designing an ideal EMW absorber. The impedance matching (Z) can be defined as follows [6,37e40]:

  Z  Z ¼  in  ¼ Z0

rffiffiffiffiffi     mr 2pfd pffiffiffiffiffiffiffiffiffi   mr εr   ε tanh j c r

(2)

If Z ¼ 1, the input impedance will be equal to that of the free space, which indicates the optimal impedance matching. Therefore, the value of Z equal or close to 1 is desired for an ideal EMW absorber [6,37e39]. The attenuation constant (a) determines the electromagnetic loss capacity, which can be defined as the following equation [12,25,41e44]:

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf 00 00 00 00 00 00  ðm ε  m0 ε0 Þ þ ðm ε  m0 ε0 Þ2 þ ðε0 m þ ε m0 Þ2 a¼ c (3) Fig. 10 depicts the frequency-dependent Z and a of the three samples. From Fig. 10(a), it can be seen that the Z values of S1 and S3 are far less than 1 in the whole frequency range, suggesting a bad impedance matching. The S2 shows larger Z values than that of S1 and S3, indicating the improved impedance matching. Notably, the S2 exhibits the optimal impedance matching (Z ¼ 1) at the frequency of 15.6 and 16.7 GHz. Therefore, most of the incident EMWs can enter into the sample of S2, and further be attenuated by the multiple electromagnetic loss mechanism. From Fig. 10(b), it is clear

that the S1 presents the weakest EMW attenuation capacity owing to the largest attenuation constant is only 81.4. However, the a values of S1 and S3 obviously enhance compared with S1. Furthermore, the S3 shows the largest a value (426.9), which indicates the strongest EMW attenuation capacity among the three samples. Taking the two electromagnetic principles into consideration, the S2 demonstrates the best EMW absorption properties among the three samples originated from the optimal impedance matching and moderate electromagnetic attenuation capacity. Based on the quarter-wavelength matching theory (l/4), the relationship between coating thickness (tm) and absorption peak frequency (fm) obeys the following equation [12,25,41,42]:

tm ¼

nl nc pffiffiffiffiffiffiffiffiffiffiffiffi ¼ 4 4fm jεr mr j

ðn ¼ 1; 3; 5; :::Þ

(4)

If tm and fm meet the equation, the phase cancellation effect will contribute to the EMW attenuation. From Fig. 11(a), it is obvious that the RL peaks of S2 shift to the lower frequency as the tm increases. Fig. 11(b) displays the relationship between tm and fm of S2. The pentagram stands for the exp experimental tm (denoted as texp m ). Remarkably, all the tm are well located at the l/4 curve, which suggests that the l/4 rule governs the relationship between tm and fm. Besides, the strongest RL peak of S2 (16.6 GHz and 1.2 mm) is in good accordance with the optimal impedance matching of Z equal to 1 (Fig. 11(c)). The possible EMW absorption mechanism of MWCNTs/NiFe2O4 hybrid composites could be described as the following aspects. Firstly, the surface of acid-treated MWCNTs carries the abundant defects and oxygen-containing functional groups, which can serve as the polar centers and introduce more dipole polarization, enhancing the EMW attenuation capacity [12]. Secondly, the multiply heterogeneous interfaces between NiFe2O4 and MWCNTs

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Fig. 8. Frequency dependence of (a) ε0 , (b) ε'', (c) m0 , (d) m'', (e) tande and (f) tandm for the samples of S1, S2 and S3.

can be regarded as the capacitor-like structure [12,22], which plays an important role in the EMW attenuation. Thirdly, according to the Cao's Electron-Hopping model [45e47], the NiFe2O4 microspheres can act as the bridges for the electron-hoping between two adjacent MWCNTs and thus attenuate the electromagnetic energies. Lastly, the locally 3D conductive networks in the hybrid composites, which are beneficial to the enhancement of conduction loss, multiple reflections and scattering [12,48]. Besides, the optimal impedance matching and moderate attenuation capacity are also helpful for enhancing the EMW attenuation. 4. Conclusions

Fig. 9. Frequency dependence of C0 for the samples of S1, S2 and S3.

In summary, we successfully synthesized the well-designed 3D net-like MWCNTs/NiFe2O4 hybrid composites. Results demonstrated that the hybridization of magnetic NiFe2O4 microspheres with dielectric MWCNTs could be an effective strategy to enhance the EMW absorption performance of NiFe2O4. Significantly, the

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Fig. 10. Frequency dependence of (a) impedance matching (Z) with a thickness of 1.2 mm and (b) attenuation constant (a) for the samples of S1, S2 and S3.

magnetic loss, balanced impedance matching and electromagnetic attenuation. Consequently, the obtained hybrid composites could be used as the high-efficiency EMW absorbers in the field of EMW absorption and shielding. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51507003), the Lift Engineering of Young Talents and Doctor's Start-up Research Foundation of Anhui University of Science and Technology (Grant No. ZY537), the Program of Innovation and Entrepreneurship for Undergraduates of Anhui Province (Grant No. 201710361261, 201710361280, 201710361283). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.01.073. References

Fig. 11. (a) Frequency-dependent reflection loss (RL) with different thicknesses, (b) simulations of the tm versus fm under the l/4 model and (c) impedance matching (Z) as a function of frequency with different thicknesses for the sample of S2.

superior EMW absorption performance of as-prepared hybrid composites could be facilely controlled by changing the additive amounts of MWCNTs and coating thicknesses. Furthermore, the possible EMW absorption mechanism was explored and could be ascribed to the multiple reflections and scattering in the 3D conductive networks, dipole polarization and interfacial polarization, synergistic effects of conduction loss, dielectric loss and

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