N-doped reduced graphene oxide aerogels for high-performance microwave absorption

Chemical Engineering Journal 388 (2020) 124317

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CoFe2O4/N-doped reduced graphene oxide aerogels for high-performance microwave absorption Xiangyu Wanga, Yukai Lua, Tao Zhua, Shucheng Changa, Wei Wanga,b, a b

T

⁎

Department of Physics and Electronics, School of Mathematics and Physics, Beijing University of Chemical Technology, Beijing 100029, China Beijing Key Laboratory of Environmentally Harmful Chemical Analysis, Beijing University of Chemical Technology, Beijing 100029, China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

Embedding CoFe O nanoparticle into • N-doped rGO aerogels is designed. is −60.4 dB at 14.4 GHz with a • RL thickness 2.1 mm and a filler loading 2

4

min

20 wt%.

absorption bandwidth can be • Effective up to 6.48 GHz (11.44–17.92 GHz) at 2.2 mm.

only 3 mm, effective absorption • Atbandwidth can completely cover Xband.

aerogels exhibit typical ferro• N-rGO magnetic behavior.

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanoparticle N-doped Reduce graphene oxide aerogels Porous structure Microwave absorption

Nowadays, to solve electromagnetic radiation issues, an urgent demand is to develop high-performance microwave wave absorption materials with lightweight, broad bandwidth and strong absorbing capacity. In this work, by embedding CoFe2O4 (CFO) nanoparticle into N-doped reduced graphene oxide (N-rGO) aerogels, a unique CFO/N-rGO aerogel microwave wave absorber with a 3D porous architecture is synthesized via a facile solvothermal method and lyophilization technique. Impressively, the as-synthesized N-rGO aerogels exhibit typical ferromagnetic behavior. The electromagnetic parameters of CFO/N-rGO aerogels can be immensely improved by tuning the additive amount of CoFe2O4 nanoparticle. An optimal microwave absorption performance is achieved when the mass ratio of graphene oxide (GO) to CoFe2O4 is 1:2. Here, a strong reflection loss (RL) reaches −60.4 dB at 14.4 GHz with a thickness of 2.1 mm and a low filler loading ratio of 20 wt%. Further, the effective absorption bandwidth (RL < −10 dB) can be up to 6.48 GHz (11.44–17.92 GHz) with a thickness of 2.2 mm. Notably, at only 3 mm, its effective absorption bandwidth can completely cover X-band. The superior microwave wave absorption performance of as-synthesized CFO/N-rGO aerogels is attributed to the specific surface morphology and porous structures, leading to the multiple scattering and reflecting, interfacial polarization and optimal impedance matching. This work not only provides a deep insight on tuning microwave absorption performance of ferrite/N-rGO aerogels, but also offers a new route to design high-performance magnetic/dielectric absorbers.

⁎ Corresponding author at: Department of Physics and Electronics, School of Mathematics and Physics, Beijing University of Chemical Technology, Beijing 100029, China. E-mail address: [email protected] (W. Wang).

https://doi.org/10.1016/j.cej.2020.124317 Received 19 November 2019; Received in revised form 23 January 2020; Accepted 2 February 2020 Available online 04 February 2020 1385-8947/ © 2020 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 388 (2020) 124317

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1. Introduction

doped graphene should be much more suitable to be used as microwave absorbers. For instance, Liu et al. designed N-doped graphene foams absorbers with high porosity, exhibiting highly efficient electromagnetic wave absorption performance [23]. Zhou et al. reported 3D Fe2O3 nanoparticles-embedded carbon nanotubes/N-doped graphene absorbers, where the maximum reflection loss of the composites achieves −45.8 dB at 9.32 GHz with a matching thickness of 3 mm and the effective absorption bandwidth reaches 14.5 GHz (3.5–18.0 GHz) [24]. Wang et al. fabricated N-doped graphene@polyaniline nanorod arrays hierarchical structures with enhanced electromagnetic absorption properties, where the electromagnetic absorption performance is ascribed to the unique hierarchical structure and N-doping in host graphene [25]. Further, theoretical calculation has pointed out that introducing a robust long-range magnetic order into diamagnetic graphene by nitrogen doping is feasible, where the N-defect complexes in graphene are responsible for the observed ferromagnetism [26]. However, developing ferromagnetic graphene with higher coercivity and various nitrogen configurations (pyrrolic, pyridinic and graphitic) is still a key challenge in the fields of nanotechnologies, magnetism and spintronics [27]. Now, in this work, we design N-doped reduced graphene oxide (NrGO) aerogels with CoFe2O4 (CFO) nanoparticles embedding into the graphene matrix to form a 3D hierarchical porous architecture via a facile solvothermal method. Notably, the N-rGO aerogels display typically ferromagnetic behavior. Then, combing with magnetic CFO nanopartciles, the electromagnetic parameters of the CFO/N-rGO composites can be effectively tuned. Compared with pure N-rGO aerogels, the as-synthesized 3D porous CFO/N-rGO composites achieve a minimum RL of −60.4 dB at 14.4 GHz with a low filler loading ratio of 20 wt% and a thin matching thickness of 2.1 mm. Meanwhile, a wide effective absorption bandwidth reaches up to 6.48 GHz from 11.44 to 17.92 GHz with a thickness of 2.2 mm. These indicate that the assynthesized CFO/N-rGO aerogels are expected to be a promising candidate for lightweight and high-efficiency microwave absorbers.

Owing to the wide usage of wireless communication and electric equipment in our daily life, electromagnetic radiation has become a serious global environmental pollution problem, which is severely threatening the human health and gravely hampering the normal operation of some electric devices [1–4]. To address this problem, lots of continual efforts have been paid to develop high-performance microwave absorbing materials [5–7]. To meet the rising demand of practical application, the microwave absorbers should have the characteristics of strong absorption capacity, broad absorption bandwidth, thin matching thickness, light weight and good thermal stability [8,9]. In terms of electromagnetic energy conversion theory, the reflection and attenuation features are determined by the appropriate matching between the dielectric loss and the magnetic loss. That is, the complementarity of magnetic and dielectric properties can effectively tune the electromagnetic parameters, leading to the improved microwave absorption performance. Then, due to single loss mechanism, the practical application of traditional microwave absorbing materials, such as carbonaceous material, metals and their oxides, conducting polymers, metal sulfides, has been immensely hindered. Apparently, the exploition of magnetic/dielectric composites is becoming a feasible and promising method to gain high-performance microwave absorbers [10–12]. Most notably, due to the excellent microwave absorption property, high thermal stability and corrosion resistance, the absorber obtained by coupling graphene sheets with magnetic materials has received more and more attention in recent years [13]. For example, Wang et al. synthesized hierarchical core-shell NiFe2O4@MnO2 composite microspheres decorated graphene nanosheet, where the strongest reflection loss is −47.4 dB at 7.4 GHz with the matching thickness of 3 mm [14]. Sun et al. designed a Fe3O4-rGO nanocomposite wherein Fe3O4 is stably distributed within the reduced graphene oxide nanosheets, and the samples have the maximum reflection loss of −49.53 dB at 6.32 GHz for a thickness of 3.4 mm [15]. However, the graphene sheets usually suffer from the serious aggregation problem due to the strong π-π interaction [10]. Recently, establishing a three-dimensional (3D) graphene network from individual graphene sheets, named as 3D graphene aerogel, has been regarded as an effective thought to overcome the aggregation problem. Meanwhile, formed by self-assembly of graphene layers, 3D graphene aerogels retain the intrinstic properties of graphene. Besides, in comparison with graphene sheet, 3D graphene aerogels exhibit much higher specific surface area, lower bulk density and larger conductivity [16–18]. Additionally, the 3D architecture, made of porous graphene network, provides large internal free space and enhances the multiple reflection of microwave entering the 3D architecture with a closed cell wall, which predicts that 3D graphene aerogel is an ideal candidate for microwave absorbers [3,19]. Further, designing graphene aerogelsbased composites combined with magnetic materials to achieve excellent microwave absorption performance has been a hot topic in this field. Qin et al. prepared graphene aerogel with metallic-CNTs absorbing on the surface of GO flakes and magnetic Fe3O4 nanoparticles concentrating on the edge of GO flakes, where the maximum RL value reaches −49 dB and the effective absorption bandwidth covers the C, X and Ku bands [20]. Zhao et al. synthesized amorphous carbon nanotube networks on a 3D graphene aerogel/BaFe12O19 nanocomposite, where the as-prepared material with 3D network structure can be used as a good absorber for microwave absorption [21]. On the other hand, implanting heteroatoms (N, F, S, P, etc.) into graphene can effectively introduce much more structure defects and generate additional polarization relaxation, which is also a promising approach to enhance the microwave performance of graphene. Here, owing to the similar molecular weight and atomic size to carbon atoms, nitrogen is the preferential doping atom, which is beneficial to induce the disordered carbon structure, improve the conductivity of graphene and accelerate the transportation of electrons [22]. Therefore, nitrogen-

2. Experimental 2.1. Materials Fe(NO3)3·9H2O, Co(NO3)2·6H2O and NaBH4 were utilized to synthesize CoFe2O4. Graphite powder (325 mesh), Phosphoric acid (H3PO4), concentrated sulfuric acid (H2SO4), hydrochloric acid (HCl), hydrogen peroxide (H2O2) and potassium permanganate (KMnO4) were used to prepare Graphene oxide (GO). GO was synthesized according to a modified Hummer’s method [28]. Ethylene glycol (EG) was used as solvent to dissolve CoFe2O4. Urea is a reducing agent and the nitrogen source. All the chemicals, purchased from Sinopharm Chemical Reagent Co., Ltd. China, were of analytical grade without any further treatment. And all the experiments will use the deionized water. 2.2. Synthesis of CoFe2O4 nanoparticles CoFe2O4 were prepared via a hydrothermal method. In the synthetic procedure, 1.7616 g Fe(NO3)3·9H2O and 0.582 g Co(NO3)2·6H2O were dispersed into 30 mL deionized water by ultrasonically treating for 1 h. After, 1.368 g NaBH4 was dissolved in 20 mL deionized water by ultrasonically treating for 0.5 h. Subsequently, 20 mL NaBH4 solution was added slowly into the previous mixture. After ultrasonically treating 1 h, the resulting mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave, and heated at 200 °C for 2 h. Then, the solution was cooled to room temperature. Finally, the as-prepared CoFe2O4 nanoparticle was washed with water, and dried at 60 °C for 12 h. 2.3. Synthesis of CFO/N-rGO aerogels The synthetic route is illustrated in Scheme 1. CFO/N-rGO aerogels 2

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Scheme 1. Schematic illustration of the synthetic route CFO/N-rGO aerogels.

Fig. 1. (a) XRD patterns of samples CNGA-1, CNGA-2, CNGA-3, CoFe2O4 and NGA. (b) Raman spectra of GO, NGA and CNGA-1, CNGA-2, CNGA-3.

2.4. Characterization

(CNGA) were synthesized via a facile solvothermal method. Typically, 30 mg GO and 300 mg urea were dissolved into 6 mL deionized water by ultrasonically treating for 1 h to produce a homogeneous solution (solution A).Then, a certain amount of CoFe2O4 was dispersed in 4 mL EG, and, after ultrasonically treating for 30 min, the as-prepared solution was dispersed in solution A with ultrasonic treatment for 1 h. The obtained mixture was transferred to a 50 mL Teflon-lined stainless-steel autoclave and heated at 200 °C for 2 h to form hydrogels. Next, the asprepared hydrogels were soaked in an ethanol solution for 10 h to remove impurities. Then, the final products were freeze-dried for 36 h, where the as-synthesized CFO/N-rGO aerogel (CNGA) samples were labelled as CNGA-1, CNGA-2, CNGA-3 with the additive amount 30 mg, 60 mg, 90 mg of CoFe2O4 particles in the composites, respectively. Here, the pure N-doped rGO aerogels without CoFe2O4 were also prepared and named as NGA.

X-ray power diffraction (XRD) was measured to investigate the crystal structure and crystallinity of the samples. Raman spectra were recorded using a Jobin-Yvon spectrometer (LabRAM ARAMIS) equipped with a CCD detector and an Ar ion laser (λ = 532 nm). The morphologies of the aerogels were analyzed by transmission electron microscope (TEM) and scanning electron microscopy (SEM) (Hitachi S4700). The elemental composition was analyzed by Energy Dispersive Spectrometer (EDS) mapping. X-ray photoelectron spectroscopy (XPS) was obtained using a Thermo Fisher Scientific ESCALAB 250, where twin anode Al Kα X-ray was used as the excitation source. The surface properties were measured by Brunner-Emmett-Teller surface area analysis (ASAP 2460, Micromeritics instrument Co. USA). Thermal gravimetric analyzer analysis was carried out by using TA Q50 system under nitrogen atmosphere at a heating rate of 10 °C/min. The magnetic performance was measured by a physical property measurement system 3

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is presented in Fig. 2(d), where three peaks located at 398.2, 399.5, 401.3 eV correspond to pyridinic N, pyrrolic N and graphitic N, respectively [28,37]. Here, the pyridinic N bonding configuration represents N]C bonds at the defects or edges in the N-rGO aerogels, while the pyrrolic N bonding corresponds to the bond of the tertiary N atoms (in the form of N-(C)3 or H-N-(C)2) in a five-membered ring structure. Also, the graphitic N bonding configuration is related to the bond of sp2 hybridized N atoms with three sp2 hybridized C neighbors in the N-rGO lattice [38]. As to Fe 2p spectrum in Fig. 2(e), two major peaks located at 711.1 and 724.8 eV are in correspondence to Fe 2p3/2 and Fe 2p1/2, respectively. Also, the appearance of a satellite peak at 718.9 eV confirms the existence of Fe3+ species in CoFe2O4. Further, the peaks of Fe 2p3/2 at 713.0 eV and Fe 2p1/2 at 724.8 eV are attributed to the contribution of Fe3+ ions in tetrahedral sites, while the peaks at 711.0 and 718.9 eV are assigned to Fe3+ ions in octahedral sites in spinel ferrites [39]. At the meantime, owing to the occurrence of oxygen vacancies, a part of Fe3+ should be reduced to Fe2+, which will induce the conductivity of CoFe2O4 because of the electron hopping between Fe3+ and Fe2+ in equivalent lattice sites [40]. The Co 2p spectrum in Fig. 2(f) can be resolved into five peaks at 780.6, 782.6, 786.8, 796.6, 803.0 eV, where two main peaks at 780.6 and 796.6 eV are ascribed to Co 2p3/2 and Co 2p1/2, respectively. Additionally, these satellite peaks belonging to Co 2p3/2 and Co 2p1/2 with high binding energy are located at 782.6, 786.8 and 803.0 eV, respectively, due to the spin orbital splitting, which also manifests the presence of Co2+ and Co3+ in CFO/N-rGO aerogels [41,42]. To reveal the functional groups in the samples, FTIR spectra of assynthesized samples GO, NGA, CFO and the representative composite CNGA-2 are presented in Fig. 3(a). Herein, five major characteristic peaks located at 1056, 1222, 1625, 1721 and 3421 cm−1 can be observed from the GO curve, which are assigned to CeO, CeOeC, C]C, C]O and OeH bonds, respectively [5,43]. This indicates the presence of plentiful oxygen-containing functional groups at the edges and basal plane of the nanosheets. As to NGA, the peak intensity related to the oxygen functional groups reduces immensely, suggesting the removal of some oxygen-containing functional groups in GO and the successful reduction of GO. Meanwhile, the peaks at 3414 and 1350 cm−1 can be attributed to OeH stretching vibration of H2O and CeN stretching vibration, which further points out the existence of N atoms in N-rGO aerogels. Seen from the curve of CFO, two characteristic adsorption peaks of spinel ferrites appear at around 460 and 578 cm−1, corresponding to the vibration of octahedral metal-oxygen bonds and tetrahedral metal-oxygen bonds, respectively [44]. Then, in sample CNGA-2, the specific peaks belonging to CoFe2O4 and N-rGA can be clearly observed, indicating the formation of CFO/N-rGO composites. Fig. 3(b) and (c) describe the magnetic performance of N-rGO aerogels at different temperatures, and, Fig. 3(d) plots the hysteresis loops of samples CFO, CNGA-1, 2, 3 at room temperature. Here, seen from Fig. 3(b), the as-synthesized N-rGO aerogels show typical ferromagnetic behavior at 5, 10 and 50 K, where the corresponding saturation magnetization Ms is 0.46, 0.34 and 0.11 emu/g under the external magnetic field up to 60 kOe. According to Curie-Weiss law, it is obvious that magnetization of N-rGO decrease with the increase in temperatures. Moreover, from the enlarged hysteresis loops in Fig. S1(a), high coercivities Hc of N-rGO aerogels at 5, 10 and 50 K are 887, 1332 and 1025 Oe, respectively. Next, continuing to increase the temperature, distinctly, magnetization further decrease, as shown in Fig. 3(c). Impressively, at 100 K, N-rGO aerogels still display robust ferromagnetism, while, at 200 and 300 K, N-rGO aerogels exhibit diamagnetic characteristics. Actually, it is noted that an obvious ferromagnetism is superimposed onto large diamagnetism of N-rGO aerogels at 200 and 300 K [45]. The enlarged hysteresis loops of N-rGO at 100, 200 and 300 K in Fig. S1(b) confirm the high Hc values of 605, 205 and 103 Oe, respectively. It has been demonstrated that N doping into graphene can usually generate various defects, structural disorders, dangling bonds or carbon

(PPMS, Quantum Design, VSM). The electromagnetic parameters were measured using a vector network analyzer (VNA, Agilent E8362B) in the frequency from 2 to 18 GHz by coaxial-line method, where the assynthesized aerogels were mixed with 20 wt% paraffin and pressed to be toroidal with the outer and inner diameters of 7 mm and 3.04 mm, respectively. Then, in terms of the transmission line theory, microwave absorbing performance of as-synthesized samples can be assessed. 3. Results and discussion To reveal the crystal structure and crystallinity, the XRD patterns of all the samples are shown in Fig. 1(a). From the XRD pattern, two diffraction peaks of the pure N-doped rGO aerogels appear at 25.5° and 44.0°, corresponding to the (0 0 2) and (1 0 0) crystal planes of rGO. Here, the typical peaks of GO cannot be observed, which means the successful reduction of GO to rGO because of the addition of reducing agent urea during the hydrothermal process. As to as-synthesized CoFe2O4, the diffraction peaks located at 18.3°, 30.1°, 35.4°, 37.2°, 43.5°, 53.9°, 57.0°, and 62.7° can be assigned to the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes of cobalt ferrites with a face-centered cubic spinel structure, respectively [29,30]. In comparison with rGO and CoFe2O4 nanoparticles, the CFO/ N-rGO composites (CNGA-1, 2, 3) show all the characteristic peaks of cubic spinel CoFe2O4 without any other impurity diffraction peaks, suggesting the high purity of as-synthesized composites. Whereas, owing to the weak crystallinity and scattering power, the diffraction peaks belonging to rGO can barely be observed in the CNGA composites [31,32]. Further, the structural information of as-synthesized GO, NGA and CNGA is examined by Raman measurement, as shown in Fig. 1(b). Here, two characteristic peaks located at 1345 and 1592 cm−1 are attributed to the disordered carbons with the vibrations of sp3 defects or lattice distortion in graphene sheets (D band) and the in-plane vibration of sp2 hybridized carbon atoms (G band), respectively. Then, the disorder and defects of the carbon atoms can be evaluated by the intensity ratio of the D band and G band (ID/IG). Seen from Fig. 1(b), the ID/IG value of pure GO is 0.92, which is lower than that (1.10) of NGA. It indicates that the reduction of GO and incorporation of N atoms in graphene lattice generate the increase in disorder and defects of carbon atoms, owing to the destruction of in-plane vibration of sp2 carbon domains and the formation of defects or lattice distortion. Of course, these defects and N atoms are advantageous to the electronic polarization, and then enhance the dielectric loss of graphene aerogels [23]. In addition, the ID/IG values of samples CNGA-1, 2, 3 are 1.08, 1.06 and 1.05, suggesting that the embedding of CFO leads to the slight change in defects and structural disorders in N-rGO aerogels [33]. The elemental composition and surface chemical status of the representative sample CNGA-2 is characterized by X-ray photoelectron spectroscopy. On the basis of XPS, the N, Fe and Co content in CNGA-2 is 4.44, 10.88 and 4.90 at.%, respectively, where the atom ratio of Fe to Co is very close to the expected value 2. The wide scan XPS spectrum in Fig. 2(a) confirms the existence of C, O, N, Fe and Co elements in the CFO/N-rGO aerogels. The high-resolution XPS spectra for C 1s, O 1s, N 1s, Fe 2p and Co 2p are presented in Fig. 2(b)–(f), respectively. In Fig. 2(b), five fitting peaks at 284.7, 285.4, 286.2, 288.1, 289.3 eV can be observed in the C 1s spectrum, assigned to CeC/C]C, CeN, CeO, C]O and OeC]O bonds, respectively [34]. Here, the CeN bonds originate from either graphitic or pyridinic N bonding configuration, indicating the occurrence of N doping behavior in rGO lattice [35]. Meanwhile, in comparison with CeC/C]C bond, the much lower intensity of CeO, C]O and OeC]O bonds implies the removal of some oxygen-containing groups and the apparent reduction of GO during the formation of CFO/N-rGO aerogels [36]. The O 1s spectrum in Fig. 2(c) shows two main peaks with the binding energies at about 530.4 and 531.8 eV, indexed to the oxygen vacanies of CoFe2O4 and oxygen-based functional groups (eOH and eCOOH), respectively [6,8]. To further investigate the profiles of doping N, the high resolution N 1s spectrum 4

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Fig. 2. XPS spectra of sample CNGA-2: (a) survey scan, (b) C 1s, (c) O 1s, (d) N 1s, (e) Fe 2p, (f) Co 2p.

size distributions are plotted in Fig. S2. Then, the pore volumes and the average pore diameters of CNGA-1–3 are obtained as 0.156 cm3/g, 0.171 cm3/g, 0.208 cm3/g, 12.74 nm, 12.50 nm and 15.44 nm, respectively. Actually, the larger specific surface area and higher porosity can provide a rich interface for multiple reflection and scattering of electromagnetic wave inside the absorber, which is helpful to improve its microwave absorption performance. To further determine the relative composition of CNGA, thermogravimetric analyses (TGA) of samples CNGA-1, CNGA-2, CNGA-3 are performed under a N2 atmosphere. Fig. 5 plots the TGA curves of the three samples in the temperature range from 20 to 850 °C. At below 100 °C, a slight weight loss (about 2%) can be found owing to the evaporation of H2O. Then, from 100 to 400 °C, about 5–8% weight loss is due to the elimination of the labile oxygen-containing functional groups. Next, a dramatic weight loss takes place from 400 to 850 °C because of the removal of the more stable oxygen-containing functional groups and the decomposition of the graphene skeletons. Finally, due to the high thermal stability of CFO ferrite, the residual weights of samples CNGA-1–3 are mainly assigned to the CFO in the composites. Here, the relative weight of CFO in CNGA-1–3 are 68.89, 76.75, 83.60 wt%, respectively, which confirms that the component of CFO in the different composites. The morphologies of the as-synthesized samples N-rGO aerogels, CNGA-1, CNGA-2 and CNGA-3 are characterized by SEM, as shown in Fig. 6. The SEM image of pure N-rGO aerogels is depicted in Fig. 6(a), where a 3D interconnected architecture with a highly porous network can be observed. From the enlarged SEM image presented in Fig. 6(e), the 3D crisscross porous structure with wrinkled sheets of ultra-thin graphene is confirmed. Then, in the composites CNGA-1, CNGA-2 and CNGA-3, it can be seen from Fig. 6(b–d) that abundant CoFe2O4 nanoparticles with a regular spherical shape tightly stick to the surface of rGO sheets. Further, from the enlarged SEM images in Fig. 6(f–g), with increasing the CoFe2O4 contents in the CFO/N-rGO composites, more

edge termination, which can induce the localized magnetic moments and play a significant effect on the ferromagnetism of graphene [46]. Pyrrolic N greatly affects the formation of magnetic moments in N-rGO because of a net magnetic moment of 0.95 μB/N [38]. Additionally, pyrrolic N is the main defect in N-rGO while pyridinic N and graphitic N have less effects on the spin polarization. Usually, pyrrolic and pyridinic N are accompanied by defects or edges in the graphene sheets. Here, the localized magnetic moments resulting from pyrrolic N and defects can give rise to ferromagnetic response through indirect magnetic coupling effect defined as RKKY exchange interaction [47,48]. Further, graphitic nitrogen can offer itinerant π-electrons, acting as the source of delocalized magnetic moment. Therefore, the ferromagnetic order of N-rGO also originates from the direct exchange between the delocalized magnetic moment of the graphitic defects [26]. Likewise, the plentiful defects and structural disorders result in the high coercivity of the as-synthesized N-rGO aerogels. Moreover, owing to the diamagnetism of N-rGO, in comparison to CFO, the saturation magnetization Ms of the CFO/N-rGO composites declines by about 15.4% ~35.9%, where the Ms values for CFO, CNGA-1, CNGA-2 and CNGA-3 are 78, 50, 61 and 66 emu/g, respectively. Herein, with increasing the CFO content in the CFO/N-rGO composites, Ms values increase distinctly, while their coercivities change slightly. Fig. 4(a) gives the SBET values of the samples NGA, CNGA-1–3, where SBET of pure NGA is up to 214.1 m2/g. Meanwhile, the introduction of CFO nanoparticles gives rise to the decrease in SBET of the CFO/N-rGO composites. The BET specific surface areas of CNGA-1–3 are 67.11, 67.51, and 56.90 m2/g, respectively, where SBET of CNGA-2 is slightly higher. Further, the pore size distribution and pore volume are analyzed by the measurement of N2 adsorption-desorption isotherms. As shown in Fig. 4(b–d), the CNGA samples show typical IV N2 adsorption isotherm and H2-type hysteresis loop, which proves the existence of mesoporous structure. Correspondingly, the calculated pore volume and pore size are listed in Table S1. Moreover, their pore 5

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Fig. 3. (a) FTIR spectra of samples GO, NGA, CFO and CNGA-2; Hysteresis loops of N-rGO aerogels (b) at 5, 10 and 50 K under the applied magnetic fields up to 60 kOe and (c) at 100, 200 and 300 K under the applied magnetic fields up to 18 kOe; (d) Hysteresis loops of samples CFO, CNGA-1, CNGA-2, CNGA-3 at 300 K under the applied magnetic fields up to 18 kOe.

and more aggregated CoFe2O4 nanoparticles uniformly distribute in reticular hole formed by graphene nanosheets. Certainly, we expect that the unique 3D porous network architecture can help to enhance the microwave absorption ability by promoting the attenuation and scattering of microwave insider the network structure of the as-synthesized CFO/N-rGO composites. Further, the TEM images of samples N-rGO aerogels, CNGA-1, CNGA-2 and CNGA-3 are illustrated in Fig. 7(a)–(d), respectively. Apparently, from Fig. 7(a), the formation of ultra-thin N-rGO nanosheets is confirmed. In Fig. 7(c) and (d), the CoFe2O4 nanoparticles display uniform size distribution, regular sphere shape and small particle size, firmly adhering to the N-rGO nanosheets. From statistical histogram of the particle size in the insert of Fig. 7(d), the average diameter of the assynthesized CoFe2O4 nanoparticles is about 16.7 nm. Meanwhile, among the three images, sample CNGA-3 has the largest number of CFO nanoparticles, which is in good agreement with the conclusion obtained from the analyses on SEM images. Fig. 7(e) presents HRTEM image of sample CNGA-2, where the lattice spacing of CoFe2O4 is 0.252 nm, corresponding to the basal spacing of (3 1 1) lattice plane. The selected area electron diffraction (SEAD) of N-rGO aerogels in Fig. 7(f) displays well-defined diffraction spots, revealing the crystalline nature of N-rGO. Additionally, the elemental mappings in Fig. 7(g–l) indicate the distribution of C, O, N, Fe, and Co elements on the surface of sample CNGA-2, which further confirms the successful synthesis of CFO/N-rGO aerogels. Generally, in terms of transmission line theory, the microwave absorption performance, including effective absorption bandwidth and microwave absorption ability, of the samples N-rGO aerogels, CNGA-1, CNGA-2 and CNGA-3 can be evaluated by reflection loss (RL) values. On the basis of the metal back-panel model, the expression for calculating RL is defined as [37]

RL = 20 log

Zin = Z0

Zin − Z0 , Zin + Z0

μr 2πfd ⎞ tanh ⎡j ⎛ μ εr ⎤, ⎢ c ⎠ r ⎥ εr ⎝ ⎣ ⎦

(1)

(2)

where Zin, Z0, εr, μr, f, d and c denote the normalized input impedance of the absorbers, the impedance of free space, the relative complex permittivity, the relative complex permeability, the frequency of the microwave, the thickness of the absorbers and the velocity of the microwave in free space, respectively. Normally, lower RL value means higher microwave absorption capacity. Then, if the RL value is lower than −10 dB, the relevant width of the absorption frequency band is named as the effective absorption bandwidth, for, in this case, more than 90% electromagnetic energy can be effectively absorbed by the absorbers. That is, if meeting the above condition, the as-synthesized samples are the suitable absorbers in practical applications. Fig. 8(a)–(d) present the contour maps of the calculated RL values in the frequency range of 2–18 GHz at different thickness for samples NrGO aerogels, CNGA-1, CNGA-2 and CNGA-3, respectively, where the filler loading ratio is selected as 20 wt%. Also, their frequency dependence of RL curves and the corresponding 3D diagrams are shown in Fig. S3. As to pure NGA, the obtained minimum RL (RLmin) value is only −8.81 dB at 10 GHz with a thickness of 2 mm, while the RLmin of pristine CFO nanoparticles is higher than −10 dB. Next, after the combination of N-rGO with CFO, the microwave absorption performance is significantly improved. Here, the sample CNGA-1 possesses the RLmin of −15.61 dB at 13.12 GHz with the effective absorption bandwidth of 5.18 GHz from 10.96 GHz to 16.24 GHz. Especially, sample CNGA-2 exhibits the best microwave absorption performance, where the effective absorption bandwidth reaches up to 6.48 GHz 6

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Fig. 4. (a) The specific surface area of samples NGA, CNGA-1, CNGA-2, and CNGA-3; (b–d) N2 adsorption-desorption isotherms of samples CNGA-1, CNGA-2, and CNGA-3.

2.1, 2.2 and 3.0 mm. Here, it can be observed that the minimum RL of −60.42 dB appears at 2.1 mm and the largest effective absorption bandwidth of 6.48 GHz from 11.44 to 17.92 GHz with the RLmin of −32.36 dB emerges at 2.2 mm. In particular, it is noted that, at 3 mm, the effective absorption bandwidth can cover X-band, where the RLmin of −38.11 dB is attained. Meanwhile, the RLmin value and effective absorption bandwidth of sample CNGA-2 are compared with those of previously reported graphene-based microwave absorbers, as shown in Fig. 8(g) and Table 1, which suggests that the as-synthesized sample CNGA-2 can be taken as an ideal and promising microwave absorber with strong absorption capacity and superior effective absorption bandwidth at a thin thickness as well as a lower filler loading of 20 wt %. To further reveal the essential microwave absorption mechanism, the frequency dependence of the relative complex permittivity (εr = ε′ − jε″), the relative complex permeability (μr = μ′ − jμ″), the dielectric loss tangent tanδε (tanδε = ε″/ε′) and the magnetic loss tangent tanδμ (tanδε = μ″/μ′) of the absorbers are investigated in Fig. 9. Basically, in terms of electromagnetic field theory, the storage ability of electric and magnetic energy can be characterized by the real parts of permittivity (ε′) and permeability (μ′), while the dissipation or loss of electromagnetic energy is associated with the imaginary parts of permittivity (ε″) and permeability (μ″), respectively [9,39]. Accordingly, the dielectric and magnetic loss factors tanδε and tanδμ are usually used to assess the power loss in the absorber in relation to the stored power. Fig. 9(a) and (b) give the values of ε′ and ε″ of the as-synthesized samples in the frequency range of 2–18 GHz. Owing to the weak dielectric property, the ε′ and ε″ of bare CFO can be neglected. Otherwise, pure N-rGO aerogels own the largest values of ε′ and ε″ due to excellent dielectric performance. Meanwhile, in the test frequency range, both ε′

Fig. 5. TG curves of samples CNGA-1, CNGA-2 and CNGA-3.

(11.44–17.92 GHz) at a thin thickness 2.2 mm. What’s more, at 2.1 mm, the minimum RL of −60.42 dB is obtained at 14.4 GHz with a broad effective absorption bandwidth of 5.92 GHz (12.08–18 GHz). As to CNGA-3, the RLmin is −15.29 dB at 18 GHz with a thickness of 2 mm, and the effective absorption bandwidth achieves 4.56 GHz (11.92–16.48 GHz) at 2.5 mm. Fig. 8(e) gives the calculated RLmin value and the obtained effective absorption bandwidth for as-synthesized samples at a certain thickness, where it is distinctly found that sample CNGA-2 gains the optimal microwave absorption properties. Further, Fig. 8(f) presents the frequency dependence of RL for sample CNGA-2 at 7

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Fig. 6. SEM images of samples (a) pure N-rGO aerogels, (b) CNGA-1, (c) CNGA-2, (d) CNGA-3 and (e), (f) the corresponding partial enlarged images.

and ε″ values decrease gradually several small fluctuations, attributed to complicated multiple electric polarization relaxation process [5]. Here, the ε′ values decline from 24.8 to 9.6 for NGA, from 15.2 to 7.4 for CNGA-1, from 10.6 to 5.6 for CNGA-2 and from 7.6 to 4.8 for CNGA3, respectively. Apparently, seen from the curves of samples CNGA-1, CNGA-2 and CNGA-3, both ε′ and ε″ intensely depends on the content of CoFe2O4 in the composites. That is, increasing the mass ratio of CoFe2O4 can significantly reduce the ε′ and ε″ values in CFO/N-rGO aerogel composites. Further, the frequency dependence of the real part and imaginary part of complex permeability is presented in Fig. 9(c) and (d). Here, except for pure CFO nanoparticle and N-rGO aerogels, μ′ and μ″ with small values (μ′ → 1–1.1, μ″ → 0–0.1) change slightly in the frequency range of 2–18 GHz. Meanwhile, from Fig. 9(e) and (f), the values of the calculated dielectric loss tangent tanδε of the as-synthesized samples, apart from bare CFO, are much higher than those of magnetic loss tangent tanδμ, which implies that the dielectric loss of the absorbers is the main influence factor on absorbing electromagnetic

wave. Normally, dielectric loss of the absorbers mainly consists of conduction loss and polarization loss. According to free electron theory, ε″ ≈ σ/2πε0f, where σ is the electrical conductivity, ε0 is the dielectric constant in vacuum [6]. So, high electrical conductivity will lead to high ε″ value. As mentioned above, abundant CFO nanoparticles turbidly dispersed among N-rGO sheets to form an effective 3D conductive network structure, causing more physical contacts among conductive graphene sheets and CFO nanoparticles. Then, the presence of 3D conductive network structure with high porosity helps to increase the conduction loss of the CFO/N-rGO composites. Further, more active sites in the 3D structure can be provided to form many conductive systems, where the conductive units generate a large number of resistance-inductance-capacitance couple circuits. Here, the induced currents in the circuits can be attenuated by the resistance, and the corresponding electromagnetic energy will be gradually transformed into thermal energy [55]. Also, owing to the unique porous structure 8

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Fig. 7. TEM images of the samples (a) N-rGO aerogels, (b) CNGA-1, (c) CNGA-2, (d) CNGA-3; (e) HRTEM image of CNGA-2; (f) Selected-area electron diffraction (SAED) pattern of N-rGO aerogels; (g)–(l) elemental mapping on the distribution of the C, O, N, Fe, and Co elements for sample CNGA-2.

beneficial to enhance the polarization loss towards the incident microwave [12]. Now, according to Debye relaxation theory, Cole-Cole semicircles are used to describe the relaxation process, where a single semicircle represents one Debye relaxation process. Here, the semicircles can be plotted in terms of the equation on relationship between ε′ and ε″, expressed as

and winkles of N-rGO, microwave entering into the 3D structure will be rapidly trapped by inner space, giving rise to multiple reflection and diffuse scattering for longer periods [56]. As mentioned above, N-rGO aerogels contains of abundant remnant groups and defects, which can induce the defect polarization relaxation and electronic dipole relaxation on microwave penetrating into the CFO/N-rGO composites. Additionally, because of the existence of vast interfaces between CFO nanoparticles and N-rGO aerogels, the charges accumulated at the interfaces will result in multiple interfacial polarization. These are

⎛ε′ − ⎝ 9

εs + ε∞ 2 ε − ε∞ 2 ⎞ + (ε′ ′)2 = ⎛ s ⎞ , 2 ⎠ ⎝ 2 ⎠

(3)

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Fig. 8. (a–d) Contour maps of the calculated RL values for samples NGA, CNGA-1, CNGA-2 and CNGA-3 at different thicknesses; (e) the minimum RL and effective absorption bandwidth at a certain thickness for as-synthesized samples; (f) RL of sample CNGA-2 at 2.1, 2.2 and 3.0 mm in the frequency range of 2–18 GHz; (g) comparison of RLmin and effective absorption bandwidth of some previously reported graphene-based microwave absorbing materials.

where εs is the static dielectric constant and ε∞ is the dielectric constant at the infinite frequency. Fig. 10(a–c) present the ε′-ε″ curves of samples CNGA-1, CNGA-2 and CNGA-3, respectively, where each sample possess more than one semicircle. This phenomenon implies the appearance of

multiple Debye relaxation process in the as-synthesized CFO/N-rGO aerogels. Whereas, it is noted that the numbers of semicircles increase with increasing the CFO content in the composites. That is, in comparison to sample CNGA-1, more semicircles can be observed in 10

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Fig. 8. (continued)

the frequency of natural resonance, K = μ0MsHc/2 [8]. Consequently, owing to the weak Hc and high Ms of as-synthesized CFO/N-rGO aerogels, the fr of natural resonance will shift to a low frequency. It has been widely accepted that microwave absorption performance of the absorber is affected by the synergistic effects between magnetic loss and dielectric loss, and another two parameters are of crucial importance: impedance matching and attenuation constant [59]. Impedance matching generally reflects the boundary condition at the interface between free space and microwave absorbers. If the characteristic impedance of the absorbers is equal or close to that of free space, microwave can easily enter into the interior of the absorbers with the zero reflection at the front surface of the absorbers. Of course, once the impedance mismatch takes place, a majority of the incident microwave will be reflected off the front surface of the absorbers or pass through it without any dissipation [57]. Thus, in this case, even though owning higher dielectric and magnetic loss, little or no loss will be induced. Basically, the matching characteristic impedance is heavily related to the values of the complex permittivity and complex permeability. That is, a good impedance matching requires the optimal match between the complex permittivity and complex permeability. Now, to evaluate the impedance matching degree, a delta-function method is put forward as follows.

Table 1 Comparison of the RLmin and effective absorption bandwidth (EAB) of some previously reported graphene-based microwave absorbers. Sample

Filler loading (wt%)

RLmin (dB)

EAB (GHz)

Thickness (mm)

Refs.

FeNi3/N-GN GA@Ni rGO/MnFe2O4 CoFe2O4/rGO Fe3O4@LAS/rGO RGO/MWCNTs/ NiFe2O4 RGO/CoFe2O4/ ZnS rGO/CoFe2O4 NGF CNGA

50 <5 70 50 50 50

−57.2 −52.3 −47.5 −57.7 −65.0 −50.2

3.4 6.5 5.2 5.8 4.0 5.0

1.45 2.6 1.7 2.8 2.1 1.4

[37] [49] [50] [51] [52] [53]

50

−43.2

5.5

1.8/2.0

[31]

23 5 20

−50.0 −53.9 −60.4

6.16 4.56 6.48

2.3/2.0 3.5/2 2.1/2.2

[54] [23] this work

samples CNGA-2 and CNGA-3, which indicates that the polarization coming from the interface, defect or chemical bonds has a little contribution to the dielectric loss in sample CFGA-1. Actually, owing to the addition of a small amount of CFO nanoparticles, sample CNGA-1 should have higher conductivity and better conductive interconnection, which enables the conduction loss to be the main source of the dielectric loss [57]. More irregular semicircles appearing in samples CNGA-2 and CNGA-3 manifest the occurrence of more interfaces, defects and chemical bonds, and also confirm that the dielectric loss is mainly attributed to the polarization loss in the above two samples. Moreover, a long tail of the curves for samples CNGA-1 and CNGA-2 can be clearly observed, which is related to the conduction loss [58]. Generally, magnetic loss mainly result from natural resonance, exchange resonance and eddy current loss in the microwave frequency bands [9]. Here, the low-frequency resonance is mainly assigned to the natural resonance, which the exchange resonance takes place at higher frequencies [8]. If the eddy current loss is the dominated factor for the magnetic loss, the value of C0 (C0 = μ″(μ′)−2f-−1) should be a constant. Fig. 10(d) plots the C0-f curves of sample CNGA-1, CNGA-2, and CNGA3. Here, in 1–5 GHz, the noticeable resonance peaks can be detected, which portends that the magnetic loss mechanism is natural resonance. Then, in the frequency range of 5–18 GHz, C0 is nearly unchanged, however, some weak resonance peaks are also observed. These suggest that both eddy current loss and exchange resonance have a vital effect on the magnetic loss in the CFO/N-rGO composites at higher frequencies. In addition, the frequency peak position of magnetic resonance can be determined by the equation 2πfr = 4γ|K|/(3μ0Ms), where μ0 is the vacuum permeability, γ is the gyromagnetic ratio, fr is

|Δ| = |sinh2 (Kfd) − M|,

K=

M=

4π μ′ε′ sin

(4)

δε + δμ 2

c cos δε cos δμ

,

(5)

4ε′ cos δε μ′ cos δμ (μ′ cos δε − ε′ cos δμ )2 + ⎡tan ⎣

(

δμ 2

−

δε 2

2

) ⎤⎦ (μ′ cos δ + ε′ cos δ ) ε

μ

. 2

(6) here, it is found that the parameters K and M are determined by the values of the complex permittivity and complex permeability. An ideal impedance matching can achieve when the required delta value |Δ| is equal to or lower than 0.4 [60]. Fig. 11(a–d) present the calculated delta value maps for samples NGA, CNGA-1–3 with the thickness of 1–5 mm in the frequency range of 2–18 GHz, where the coverage area (|Δ| ≤ 0.4) of the as-synthesized samples are obviously different. However, in the selected f and d range, all the calculated |Δ| values for pure CFO nanoparticles are higher than 0.4, which forecasts its weak impedance matching degree. Then, seen from the delta value maps in Fig. 11(a–d), the coverage areas for samples NGA, CNGA-1–3 are 3.91%, 33.6%, 34.9% and 16.0%, respectively. This indicates that sample CNGA-2 possesses the highest impedance matching degree, which illustrates that the electromagnetic energy from the incident microwave, entering into the absorbers with 11

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(a)25

(c)

NGA CNGA-1 CNGA-2 CNGA-3 CFO

20

(e)

1.2 1.1

1.0 0.8

15 10

tan

1.0 0.9

5

NGA CNGA-1 CNGA-2 CNGA-3 CFO

1.2

NGA CNGA-1 CNGA-2 CNGA-3 CFO

0.6 0.4 0.2 0.0

0

0.8 2

4

6

8

10

12

14

Frequency(GHz)

16

18

2

4

6

8

10

12

14

Frequency(GHz)

16

18

2

4

6

8

10

12

14

Frequency(GHz)

16

18

Fig. 9. Frequency dependence of the real part (a) and imaginary part (b) of permittivity; real part (c) and imaginary part (d) of permeability; (e) dielectric loss tangent tanδε and (f) magnetic loss tangent tanδμ.

Fig. 10. Cole-Cole semicircle of samples (a) CNGA-1, (b) CNGA-2, (c) CNGA-3, and (d) the C0-f curves of samples CNGA-1, CNGA-2 and CNGA-3.

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Fig. 11. The calculated delta value maps of samples (a) NGA, (b) CNGA-1, (c) CNGA-2, (d) CNGA-3; (e) attenuation constant α of samples NGA, CNGA-1, CNGA-2, and CNGA-3.

little reflection, can be effectively converted to thermal energy. Therefore, as mentioned above, CNGA-2 exhibits optimal microwave adsorption performance. Besides, attenuation constant α, related to amplitude decay of incident electromagnetic wave, reflects the dissipation properties and overall attenuation capacity of the absorbers, which can be expressed as

α=

2 πf c

(μ′ ′ε′ ′ − μ′ε′) +

(μ′ ′ε′ ′ − μ′ε′)2 + (μ′ε′ ′ + μ′ ′ε′)2 .

(7)

The calculated attenuation constants of as-synthesized sample CNGA-1–3 are presented in Fig. 11(e), where α values of all the samples monotonously increase in the studied frequency range. Further, as for CNGA-1, α values change from 40.2 at 2 GHz to 315.0 at 18 GHz, and 13

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Fig. 12. Schematic illustration on microwave absorption mechanism of CFO/N-rGO aerogels.

as-prepared CFO/N-rGO composites exhibit superior microwave absorption capability with an optimal RLmin value of −60.4 dB at 2.1 mm when the filler loading is 20 wt%. Further, a broad effective absorption bandwidth of 6.48 GHz is obtained at 2.2 mm, and it can completely cover X-band at only 3 mm. Here, the unique porous structure, specific dielectric and magnetic properties, good impedance matching and strong attenuation capability give rise to the enhanced microwave absorption performance of the CFO/N-rGO absorbers. So, in this work, a new ferrite/N-rGO aerogel composite material with potential and promising application in the field of microwave absorption is designed, and further, this study affords another strategy to construct new graphenebased microwave absorbers with strong absorption intensity, broad absorption bandwidth, thin thickness and low filler loading.

correspondingly, the values of CNGA-2 and CNGA-3 increase from 22.0 to 230.5 and from 12.1 to 161.4, respectively. Especially, it is worthy to note that, at a certain frequency, α values strictly follow the order of CNGA-1 > CNGA-2 > CNGA-3, which is in good agreement with the variation tendency of complex permittivity in Fig. 9(a and b) and dielectric loss tangent in Fig. 9(e). This phenomenon manifests that the dissipation of electromagnetic energy is mainly attributed to the dielectric loss. Here, although CNGA-1 displays best attenuation capability among the three samples, the weak impedance matching leads to the relatively low reflection loss. Then, owing to the superior impedance matching degree and strong microwave attenuation ability, sample CNGA-2 exhibits optimal reflection loss characteristics. Next, according to the above analyses, the microwave absorption mechanism of CFO/N-rGO aerogels is schematically illustrated in Fig. 12. Firstly, 3D porous structure of graphene aerogels extents the propagation path of the incident microwave and generates multiple refection and scattering, which is beneficial to the electromagnetic attenuation [61]. Meanwhile, N-doping rGO aerogels offer a large surface area to stick CFO nanoparticles, then, abundant interface, remnant groups and defects are produced, which induces the interfacial polarization, dipole polarization and defect polarization [62]. These polarization relaxations are beneficial to enhance the dielectric loss. Further, the establishment of 3D conductive network can accelerate the hopping of the electrons accumulated at the interface between CFO and N-rGO, which helps to increase conduction loss of the CFO/N-rGO composites [63,64]. On the other hand, magnetic loss of the as-synthesized absorbers is mainly ascribed to natural resonance, eddy current loss and exchange resonance in the frequency range of 2–18 GHz.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We gratefully thank the financial support from National Natural Science Foundation of China (No. 11774020), Beijing Natural Science Foundation (No. 2172045), the Fundamental Research Funds for the Central Universities (No. XK1802-6) and the long-term subsidy mechanism from the Ministry of Finance and the Ministry of Education of PRC for Beijing University of Chemical Technology. Appendix A. Supplementary data

4. Conclusions

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2020.124317.

In summary, N-doped reduced graphene oxide aerogels combined with CoFe2O4 nanoparticles are successfully synthesized, where CFO nanoparticles tightly adhere to the surface of N-rGO aerogels and a 3D porous architecture is formed. Interestingly, N-rGO aerogels exhibit typical ferromagnetic behavior with high coercivity. Owing to the weak magnetic property of N-rGO, the combination of CFO nanoparticles with N-rGO leads to the decrease in saturation magnetization of the CFO/N-rGO composites. The effective tune on microwave absorption performance is achieved by changing the CFO content in the CNGA. The

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