CoxFe3-xO4 ternary nanocomposites: Controllable synthesis and their excellent microwave absorption capabilities

CoxFe3-xO4 ternary nanocomposites: Controllable synthesis and their excellent microwave absorption capabilities

Journal of Alloys and Compounds 813 (2020) 151996 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:/...

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Journal of Alloys and Compounds 813 (2020) 151996

Contents lists available at ScienceDirect

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

Graphene oxide/carbon nanotubes/CoxFe3-xO4 ternary nanocomposites: Controllable synthesis and their excellent microwave absorption capabilities Mei Wu a, Xiaosi Qi a, b, c, *, Ren Xie a, Zhongchen Bai a, Shuijie Qin a, Wei Zhong c, **, Chaoyong Deng b a College of Physics, Guizhou Province Key Laboratory for Photoelectrics Technology and Application, Guizhou University, Guiyang City, 550025, People's Republic of China b Key Laboratory of Electronic Composites of Guizhou Province, Guizhou University, Guiyang City, 550025, People's Republic of China c Nanjing National Laboratory of Microstructures and Jiangsu Provincial Laboratory for NanoTechnology, Nanjing University, Nanjing, 210093, 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 18 June 2019 Received in revised form 12 August 2019 Accepted 23 August 2019 Available online 30 August 2019

By regulating the addition of iron source, different magnetic contents and Co:Fe molar ratios of graphene oxide/carbon nanotubes/CoxFe3-xO4 (GO/CNTs/CoxFe3-xO4) (x ¼ 0.6, 0.43 and 0.375) ternary nanocomposites (NCs) could be selectively synthesized by one-step hydrothermal method. The FE-SEM and TEM investigations indicated that the paper-like GO, CNTs and CoxFe3-xO4 nanoparticles were well bound together to form the good three dimensional hierarchical networks, which could effectively provide more interface contacts. Among these obtained samples, the as-prepared GO/CNTs/CoxFe3-xO4 (x ¼ 0.375) ternary NCs displayed superior electromagnetic wave (EMW) absorption capabilities in terms of the optimal reflection loss value of 71.3 dB and the absorption bandwidth of 2.52 GHz. And the enhanced EMW absorption capabilities of GO/CNTs/CoxFe3-xO4 (x ¼ 0.375) ternary NCs were analyzed in details. Therefore, a simple route was proposed to produce three dimensional hierarchical networks of GO/CNTs based NCs for simultaneous constructing more interfacial polarizations, dielectric and magnetic loss mechanisms, which was demonstrated to effectively optimize the EMW absorption performances and could be applied to design high performance EMW absorbers. © 2019 Elsevier B.V. All rights reserved.

Keywords: Graphene oxide/carbon nanotubes/CoxFe3xO 4 Hierarchical network Interfacial polarizations Dielectric and magnetic loss mechanisms Microwave absorption performances

1. Introduction In the past decades, high performance microwave absorbers (HPMAs) with the features such as light weight, strong absorption ability, wide absorption frequency and high stability have been received intensively attentions owing to the expanded electromagnetic interference problem and their promising application prospects in the fields of military and civil technology [1e3]. The traditional microwave absorption materials (MAMs) with the single composition such as magnetic nanoparticles (NPs) [4], conducting

* Corresponding author. Nanjing National Laboratory of Microstructures and Jiangsu Provincial Laboratory for NanoTechnology, Nanjing University, Nanjing, 210093, People's Republic of China. ** Corresponding author. E-mail addresses: [email protected], [email protected] (X. Qi), [email protected] cn (W. Zhong). https://doi.org/10.1016/j.jallcom.2019.151996 0925-8388/© 2019 Elsevier B.V. All rights reserved.

polymers [5], carbon-based materials [6,7] and metallic oxide [8] have many associated limitations so that they cannot meet these requirements. Therefore, different strategies have been proposed to fabricate the ideal HPMAs. For example, Ji and co-workers designed and synthesized different categories of film-like metal oxide/graphene (G) nanocomposites (NCs) to significantly enhance the interface polarization effect, which achieved a broad effective frequency bandwidth (7.0 GHz) at a thin coating layer [9]. In order to reduce and provide more active sites for reflection and scattering of electromagnetic wave (EMW), Che and Zhang et al. respectively constructed the yolk-shell structured [email protected]@TiO2 and [email protected] NCs, which were found to exhibit significantly enhanced EMW absorption abilities with the low minimum reflection loss (RLmin) values and effective absorption bandwidths [10,11]. Chen and coworkers developed a strategy for coupling hollow Fe3O4eFe NPs with G sheets to design HPMAs. The obtained NCs displayed very excellent EMW absorption performances, which were mainly

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ascribed to the well-matched characteristic impedance and numerous interfaces existed in the as-prepared NCs [12]. Cao et al. proposed a simple strategy of small NiFe2O4 NPs on reduced graphene oxide (RGO), which could tune the EMW attenuation by their synergistic effect between dielectric loss and magnetic loss. The RLmin value of as-prepared binary NCs could reach 42 dB with a broad bandwidth (RL <¼ 10 dB) of 5.3 GHz [13]. In general, there are two mainly ways to improve EMW absorption performance of MAMs, namely the impedance matching and EMW attenuation characteristic [14e16]. It is well known that the excellent impedance matching property implies that the incident EMW can preferably enter the interior of MAMs and subsequently be attenuated through dielectric and/or magnetic loss mechanisms. Therefore, HPMAs must simultaneously possess the outstanding characteristics of impedance matching and EMW attenuation. And constructing NCs with both magnetic and dielectric materials is considered as the most promising way to develop HPMAs [17e20]. As the representative candidates for HPMAs, carbon-based NCs especially carbon nanotubes (CNTs)- and G-based NCs have attracted increasing attentions owing to their unique characteristics such as the large specific surface area, high thermal conductivity, low density, an ideal substrate for the deposition of other functional materials, etc [21e26]. Therefore, different categories of CNTs- and G-based MAMs has been designed [27,28]. For example, CoFe2O4/RGO [29], NiFe2O4/G [30], FeCo/G [31], [email protected] [32], Fe/Co/Ni/CNTs [33], which were demonstrated to display very outstanding EMW absorption properties. Generally, most of the previous works mainly focused on the investigations of CNTs-based or G-based NCs and their reported RLmin values are usually higher than 50 dB [34e36]. The report about CNTs/G-based NCs is very exiguous [37e39]. In this work, in order to improve the RLmin values of carbonbased NCs, graphene oxide/CNTs/CoxFe3-xO4 (GO/CNTs/CoxFe3-xO4) (x ¼ 0.6, 0.43 and 0.375) ternary NCs were elaborately designed and fabricated by one-step hydrothermal method. By regulating the addition of iron source, different magnetic contents and Co:Fe molar ratios of GO/CNTs/CoxFe3-xO4 ternary NCs could be selectively synthesized, which was conducive to regulate their impedance matching and EMW attenuation characteristic.

2. Experimental 2.1. Chemicals and materials All the used chemical regents were analytically pure and used without further purification. Among these initial reactants, carboxylated CNTs were obtained from by Jiangsu XFNANO Materials Tech. Co., Ltd. Cobalt (II) acetate tetrahydrate (CH3COOCo$4H2O), ferric chloride hexahydrate (FeCl3$6H2O), ethylene glycol (EG) and urea (CH4N2O) were purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD.

2.2. Synthesis of GO/CNTs/CoxFe3-xO4 ternary NCs Firstly, GO was produced by the modified Hummers' method [40]. As depicted in Fig. 1, 80 mg GO was ultrasonically dispersed in 40 mL deionized water for 30 min at room temperature (RT) to obtain suspension liquid. After that, 40 mg carboxylated CNTs was added into the GO solution aqueous suspension under ultrasonic processing for ca. 30 min to form GO/CNTs substrate solution. Secondly, as listed in Table 1, different qualities (1.08, 1.62 and 1.89 g) of FeCl3$6H2O and 0.249 g CH3COOCo$4H2O were respectively dissolved in 30 mL EG for the production of GO/CNTs/CoxFe3xO4 ternary NCs with different moral ratios of Co and Fe. Afterwards, the formed three kinds of Co2þ and Fe3þ solutions was respectively mixed with the above GO/CNTs substrate solution and stirred for 1 h. Finally, 0.9 g urea was added into the mixed three solutions, and the obtained solutions were transferred to 100 mL Teflon-lined stainless steel autoclave for hydrothermal reaction at 200  C for 24 h. After being cooling to RT, the products were separated by centrifugation, washed with distilled water and absolute ethanol, and freeze-dried. For easy description, as presented in Table 1, the as-synthesized GO/CNTs/CoxFe3-xO4 (x ¼ 0.6, 0.43 and 0.375) ternary NCs were denoted as GCCFO-1, GCCFO-2 and GCCFO3, respectively.

Fig. 1. Schematic illustration for the synthesis process of GO/CNTs/CoxFe3-xO4 ternary NCs.

M. Wu et al. / Journal of Alloys and Compounds 813 (2020) 151996 Table 1 Summary sheet for the experimental conditions and their corresponding samples. FeCl3$6H2O (g)

Co:Fe moral ratios

x values for Co3-xFexO4

Samples

1.08 1.62 1.89

1:4 1:6 1:7

0.6 0.43 0.375

GCCFO-1 GCCFO-2 GCCFO-3

2.3. Characterization and measurement The phase characterizations of as-prepared NCs were examined on an X-ray powder diffractometer (XRD) using CuKa radiation (model D/Max-RA). Raman spectra were obtained using a JobinYvon Labram HR800 instrument with 514.5 nm Arþ laser excitation. Thermal gravity analyses (TGA) were performed into air using a Netzsch Sta 449F3 thermal analyzer. The morphology investigations were characterized by transmission electron microscope (TEM) (model Tecnai-G20), and field emission scanning electron microscope (FE-SEM) (model ZEISS SUPRA-40). The elemental state was studied by X-ray photoelectron spectroscopy (XPS) (PHI Quantera system). The magnetic properties were measured at 300 K using a Quantum Design MPMS SQUID magnetometer (Quantum Design MPMS-XL). And 30 wt% of the asprepared NCs were mixed with wax and pressed into toroidal shaped samples (Rout: 7.0 mm, Rin: 3.0 mm) for the measurement of electromagnetic properties (ε and m), which were measured by a vector network analyzer (Agilent E8363B) with the transmission/ reflection coaxial line method in the frequency region of 2e18 GHz.

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3. Results and discussion Fig. 2 presents the XRD patterns, Raman spectra, TGA curves and RT hysteresis loops of the obtained GO/CNTs/CoxFe3-xO4 ternary NCs. As shown in Fig. 2a, all the as-prepared samples exhibit the diffraction peaks located at 18.29, 30.08, 35.44, 43.06, 53.44, 56.97 and 62.59 , which can be assigned to (111), (220), (311), (400), (422), (511) and (440) crystal planes of cubic CoFe2O4 with the cell parameters a ¼ 8:391 Å (JCPDS No. 22e0186). Similar to GO/CNTFe3O4 NCs reported elsewhere [38], the characteristic diffraction peaks corresponding to GO and CNTs are not found, which should be related to the effective suppression of GO sheet restacking and uniform dispersion of CoxFe3-xO4 NPs on the surface of GO sheets and CNTs [37]. In order to prove the existence of carbon materials, the Raman investigation of as-prepared NCs was conducted. As displayed in Fig. 2b, one can observe that all the as-prepared samples exhibit three evident peaks, which can be indexed to the D (disordered carbon), G (high crystallinity graphitic layer) and 2D band (characteristic Raman peak of GO), respectively [40e42]. To quantify the content of CoxFe3-xO4 NPs in the as-prepared NCs, their thermoanalysis was carried out. As provided in Fig. 2c, all the obtained samples exhibit the evident mass loss processes, which can be attributed to the evaporation of adsorbed water, the thermal decomposition of GO and CNTs. And the residues assigned to CoxFe3-xO4 NPs for the as-prepared GCCFO-1, GCCFO-2 and GCCFO3 are ca. 80.99, 86.59 and 84.82 wt%, respectively. Fig. 2d gives the RT M  H curves of the obtained GO/CNTs/CoxFe3-xO4 ternary NCs. It can be found that all the as-prepared NCs exhibit RT ferromagnetic properties, which should be ascribed to the existence of magnetic

Fig. 2. (a) XRD patterns, (b) Raman spectra, (c) TGA curves, and (d) RT hysteresis loops (inset is the enlarged part close to the origin) of the as-prepared GO/CNTs/CoxFe3-xO4 ternary NCs.

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Fig. 4. XPS spectra of the as-prepared GO/CNTs/CoxFe3-xO4 ternary NCs.

Fig. 3. FE-SEM and TEM images of (a,b) GCCFO-1, (c,d) GCCFO-2 and (e,f) GCCFO-3.

CoxFe3-xO4 NPs. And their corresponding values of saturation magnetization (Ms) are 56.8, 68.3 and 68.1 emu/g, respectively. By comparison, the as-prepared GCCFO-2 has the largest Ms value, which is related to its highest content of CoxFe3-xO4 NPs in the NCs. The morphologies of as-prepared NCs were studied by TEM and FE-SEM, respectively. As shown in Fig. 3, similar to the previously reported CNT/[email protected] and RGO/CNTs/Fe3O4 NCs [43,44], it can be clearly observed that the as-prepared GCCFO-1 and GCCFO-2 (Fig. 3a and c) are composed of GO nanosheets, CNTs and CoxFe3xO4 NPs. The paper-like GO and CNTs are well bound together to form the good three dimensional (3D) hierarchical networks. CNTs are embedded into the inter-layers and exposed on the surface of GO nanosheets, and CoxFe3-xO4 NPs with a relatively uniform size are tightly anchored on the surface of GO and CNTs (Fig. 3b and d). Compared with GCCFO-1, the FE-SEM and TEM investigations of GCCFO-2 demonstrate that a larger number of CoxFe3-xO4 NPs attached on GO and CNTs are obviously seen. Moreover, compared to the binary NCs such as CoFe2O4/RGO, Co3O4/RGO and NiFe2O4/G [45e47], the formed 3D hierarchical networks of GO/CNTs/CoxFe3xO4 ternary NCs can provide more interface contacts among GO, CNTs and CoxFe3-xO4 NPs. Fig. 3e and f shows the FE-SEM and TEM images of GCCFO-3. Same to the obtained GCCFO-1 and GCCFO-2, the sample of GCCFO-3 also consists of paper-like GO, CNTs and CoxFe3-xO4 NPs. The obtained CoxFe3-xO4 NPs are well attached on the surface of GO and CNTs. The microstructure studies verify that the obtained GO nanosheets, CNTs and CoxFe3-xO4 NPs are well connected with each other to establish the good 3D hierarchical networks. Generally, the obtained results well confirm that the asprepared GCCFO-1, GCCFO-2 and GCCFO-3 are GO/CNTs/CoxFe3-xO4 ternary NCs, which exhibit the morphology of 3D hierarchical networks. In order to confirm the obtained samples, XPS spectra were conducted to determine their compositions. As displayed in Fig. 4, all the obtained samples exhibit four evident peaks with a binding

energy of 285, 530, 711 and 780 eV, which can be assigned to C 1s, O 1s, Fe 2p and Co 2p. In order to confirm the chemical valence states of Co and Fe, Fig. 5 gives the high resolution XPS spectra of Co 2p and Fe 2p. As indicated in Fig. 5a, the Co 2p spectra of the obtained samples show four peaks. The peaks centred at 780.9 and 796.6 eV can be ascribed to Co 2p3/2 and Co 2p1/2. And their corresponding satellite peaks at 786.8 and 803.3 eV indicate the existence of Co2þ ions in the as-prepared samples [48]. Similar to the Co 2p spectra, there are four peaks located at 718.8, 710.9, 724.8 and 733.4 eV can be observed clearly over the Fe 2p spectra (Fig. 5b). The Fe 2p3/2 and Fe 2p1/2 peaks are located at 710.9 and 724.8 eV. The peaks at 718. 8 and 733.4 eV are assigned to the satellite peaks of Fe 2p3/2 and Fe 2p1/2, which reveals the oxidation state of Fe3þ in the obtained samples [49]. Based on the obtained XPS results, it can be found that large quantities of CoxFe3-xO4 NPs exist in the as-prepared GO/ CNTs/CoxFe3-xO4 ternary NCs. 00 Fig. 6 provides the complex permittivity ðε ¼ ε0  jε Þ and 00 0 complex permeability ðm ¼ m  jm Þ of the as-prepared samples. The ε0 values for the GCCFO-1, GCCFO-2 and GCCFO-3 (Fig. 6a) are in the range of 4.85e5.47, 4.34e5.80 and 4.52e5.35, respectively. One can find that the obtained samples exhibit decreased ε0 values with the increasing frequency. On the basis of Debye theory [50], the reduction of ε0 is mainly attributed to the increase of angular fre00 quency (u). As displayed in Fig. 6b, the ε values of the obtained ternary NCs are between 0.47 and 0.63, 0.33e1.01 and 0.61e0.87. 00 Fig. 6c and d presents the m0 and m values of the obtained samples. Compared to GCCFO-1 and GCCFO-3, it can be observed that the 00 obtained GCCFO-2 exhibits the large m0 and m values, which should be related to the high content of CoxFe3-xO4 NPs in the NCs. Similar 00 to the recently reported NiFe2O4-RGO [13], the obtained m displays three evident peaks located at ~4 GHz, ~11 GHz and ~16 GHz, which are attributed to the natural resonance and exchange resonance, respectively [51]. In order to understand the EMW absorption performances, based on the measured EM parameters and the transmission line theory [52,53], the values of reflection loss (RL), impedance matching (Z), dielectric and magnetic loss tangent (tandE and tandM ), and attenuation constant (a) were calculated by the following equations:

Zin ¼ Z0

rffiffiffi  pffiffiffiffiffi 2pfd mε m tanh j ε c

(1)

M. Wu et al. / Journal of Alloys and Compounds 813 (2020) 151996

Fig. 5. High resolution XPS spectra of (a) Co 2p and (b) Fe 2p for the obtained GO/CNTs/CoxFe3-xO4 ternary NCs.

Fig. 6. EM parameters of the obtained GO/CNTs/CoxFe3-xO4 ternary NCs: (a,b) real and imaginary parts of permittivity, (c,d) real and imaginary parts of permeability.

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  Z  Z0   RL ¼ 20 log in Zin þ Z0 

(2)

  Z  Z ¼  in  Z

(3)

0

00

tandE ¼

00

ε m ; tandM ¼ 0 ε0 m

(4)

pffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 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

(5)

where f is the frequency of EMW, d is the thickness of absorber, c is the velocity of light, Z0 is the impedance of air and Zin is the input impedance of absorber. As shown in Fig. 7, the optimal RL values for GCCFO-1, GCCFO-2 and GCCFO-3 are ca. 23.5 dB at 17.16 GHz with the matching thickness of 9.87 mm, 63.0 dB at 17.96 GHz with the matching thickness of 5.87 mm and 71.3 dB at 13.32 GHz with the matching thickness of 7.92 mm, respectively. And as displayed in Fig. 8, the frequency bandwidth (fb) with RL < 10 dB for GCCFO-1, GCCFO-2 and GCCFO-3 are ca. 1.76, 1.64 and 2.52 GHz, respectively. Generally speaking, as shown in Fig. 9, by regulating the CoxFe3-xO4 content, the as-prepared GO/CNTs/CoxFe3-xO4 ternary NCs can exhibit excellent EMW absorption abilities compared to these similar NCs reported elsewhere [37,44,54e62]. According to the previously reported mechanisms for the interpretation of the good EMW absorption capabilities [2,12], similar to the recently reported N-doped G foams [63], the excellent EMW properties of the asprepared GO/CNTs/CoxFe3-xO4 ternary NCs are well interpreted by the geometrical effect. And this model is strongly dependent on the 1/4 wavelength equation [64]:

tm ¼ nlm 4

ðn ¼ 1; 3; 5/Þ

(6)

=

 ffi, tm and fm are the matching thickness and where lm ¼ c f pffiffiffiffiffiffiffiffiffiffi m jmjjεj peak frequency, respectively. As presented in Fig. 10, it can be clearly seen that the matching thicknesses obtained from their RL curves are completely in accorded with their theoretical curves of t sim m , which can be plotted on basis of formula (6). The good agreement between the experimental and theoretical values implies that a good EMW attenuation. Based on the obtained results, it can be seen that the asprepared GCCFO-3 displays evidently enhanced EMW absorption properties compared to GCCFO-1 and GCCFO-2. In order to understand their difference in properties, as reported previously results [65,66], the values ofZtandE ,tandM and a for GCCFO-1, GCCFO-2 and GCCFO-3 were achieved according to equations (3)e(5). Fig. 11

provides the obtained Z values of the as-prepared samples. It is well recognized that the MAMs will display a good impedance matching under the condition that the Z value closes to 1, which signifies that the EMW can well enter into MAMs and then be weakened [67,68]. By contrast, we can spot that the Z value is nearly equal to 1 only at the higher frequency range (>8 GHz) for the obtained samples, which is consistent with their obtained RL values. Moreover, the Z values of the as-prepared GCCFO-3 exhibit much closer to 1 than those of GCCFO-1 and GCCFO-2, which means that much more EMW can get into the interior of GCCFO-3. Fig. 12 gives the tandE ,tandM and a values of the as-prepared samples, which effectively reflects the penetrated EMW attenuation abilities of MAMs. Compared Fig. 12a with 12b, one can find that all the as-prepared samples exhibit larger values of tandE than tandM , which implying the EMW attenuation mainly originates from dielectric loss. Moreover, the as-prepared GCCFO-2 displays the highest tandE than tandM values, while their corresponding values for GCCFO-1 are the lowest. Fig. 12c shows their a values. It can be seen that the a values of samples are as follows: GCCFO2>GCCFO-3>GCCFO-1, which is same to the results of TGA, tandE and tandM . The comparison results indicate that the EMW attenuation capabilities of samples are closely related with the CoxFe3-xO4 content. It is well known that the outstanding EMW absorption capabilities should be the results of collaboration between EMW penetrated ability and attenuation capabilities of absorbers, which are determined by the values of ZtandE ,tandM and a, respectively. On basis of our obtained results, we can conclude that the enhanced EMW absorption performances of GCCFO-3 can be attributed to the good impedance matching and excellent collaboration among ZtandE ,tandM and a. On basis of the obtained results and related models [69,70], the excellent EMW absorption properties of 3D hierarchical networks of GO/CNTs/CoxFe3-xO4 ternary NCs can be explained by the following aspects: (1) the as-prepared 3D hierarchical networks of GO/CNTs/CoxFe3-xO4 ternary NCs present good impedance matching, which implies that the incident EM wave can well enter the interior (as shown in Fig. 11); (2) the formation of hierarchical networks will result in the formation of much more interfaces (as displayed in Fig. 3), which provide abundant interface polarizations and much more opportunities to the reflection of EMW. The formed interfaces can greatly improve the EMW absorption; (3) the obtained ternary NCs are composed of GO, CNTs and CoxFe3-xO4 (Fig. 2), which can simultaneously cause dielectric loss and magnetic loss. The double loss mechanism and their complementary effect are the key factor to enhance the EMW absorption capabilities; (4) the existence of residual defects and groups in the GO and CNTs can generate abundant defect dipoles and interface polarizations, which helps to the EMW attenuation.

Fig. 7. Three-dimensional RL curves of (a) GCCFO-1, (b) GCCFO-2 and (c) GCCFO-3.

M. Wu et al. / Journal of Alloys and Compounds 813 (2020) 151996

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Fig. 8. Typical RL curves of (a) GCCFO-1, (b) GCCFO-2 and (c) GCCFO-3, respectively.

4. Conclusions

Fig. 9. Comparison results of EMW absorption properties for the ferrite matrix NCs reported in recent representative papers.

In summary, we have elaborately designed and successfully synthesized GO/CNTs/CoxFe3-xO4 (x ¼ 0.6, 0.43 and 0.375) ternary NCs by one-step hydrothermal method. The FE-SEM and TEM investigations indicate that the paper-like GO, CNTs and CoxFe3-xO4 NPs are well bound together to form the good 3D hierarchical networks, which can effectively provide more interface contacts. By regulating the addition of iron source, different magnetic contents and Co:Fe molar ratios of GO/CNTs/CoxFe3-xO4 ternary NCs can be selectively synthesized, which is conducive to optimize the EM parameters. The obtained results present that the optimal RL values for the as-prepared samples are ca. 23.5, 63.0 and 71.3 dB, respectively. And their corresponding fb values are ca. 1.76, 1.64 and 2.52 GHz. The quarter-wavelength matching model can well explain the obtained excellent EMW absorption performances of the obtained samples. And the enhanced EMW absorption capabilities of GO/CNTs/CoxFe3-xO4 (x ¼ 0.375) ternary NCs are demonstrated to originate from the good impedance matching and the excellent collaboration among ZtandE ,tandM and a. Therefore, a simple route is proposed to synthesized different magnetic contents and Co:Fe molar ratios of GO/CNTs/CoxFe3-xO4 ternary NCs for simultaneous constructing more interfacial polarizations, dielectric and magnetic loss mechanisms, which can be effectively applied to accelerate the advancements of HPMAs.

Fig. 10. Comparison of the experimental and simulated matching thickness for (a) GCCFO-1, (b) GCCFO-2 and (c) GCCFO-3.

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Fig. 11. The impedance matching values for (a) GCCFO-1, (b) GCCFO-2 and (c) GCCFO-3, respectively.

Fig. 12. (a,b) loss tangent, and (c) attenuation loss of the obtained samples.

Acknowledgments [7]

This work was supported by the Platform of Science and Technology and Talent Team Plan of Guizhou province (2017e5610 and 2017e5788), the Major Research Project of innovative Group of Guizhou province (2018e013), the National Science Foundation of China (Grant Nos. 11604060 and 11964006), and the Foundation of the National Key Project for Basic Research (2012CB932304) for financial support.

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