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Synthesis of lightweight and flexible composite aerogel of mesoporous iron oxide threaded by carbon nanotubes for microwave absorption
Journal of Alloys and Compounds 697 (2017) 138e146
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Synthesis of lightweight and flexible composite aerogel of mesoporous iron oxide threaded by carbon nanotubes for microwave absorption Xilai Jia a, Jie Wang b, Xiao Zhu b, Tihong Wang b, Fan Yang b, Wenjun Dong a, Ge Wang a, *, Haitao Yang c, **, Fei Wei d a
Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Changping, 102249, Beijing, China c Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China d Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 November 2016 Received in revised form 28 November 2016 Accepted 30 November 2016 Available online 1 December 2016
Inorganic composites with mechanical flexibility are desired in synthesis of advanced materials for multifunctional applications. Here, we demonstrated the synthesis of a lightweight and flexible composite aerogel composed of carbon nanotubes (CNTs) and mesoporous iron oxide (Fe3O4) using an in-situ growth method. CNTs serve as frameworks for the growth of magnetic ferrite nanoparticles; while the growth of the magnetic nanoparticles crosslinks CNTs into an integrated structure. Therefore, mesoporous Fe3O4 particles are threaded by CNTs, leading to formation of the monolithic magnetic foam with a high particle loading of more than 88 wt%. This magnetic foam is highly porous, effective for microwave absorption. Based on the rational combination of Fe3O4 and CNTs, the composite material can offer excellent microwave absorbing performance, particularly in the low-frequency range of 3e5 GHz. Moreover, the porous composite aerogel can be compacted into a flexible magnetic nanopaper. Because of the structure properties, the composite may be used in many other applications such as energy storage, catalyst, and biochemistry. © 2016 Elsevier B.V. All rights reserved.
Keywords: Carbon nanotube Mesoporous iron oxide Microwave absorption Composite materials
1. Introduction With the development of information technology, electromagnetic (EM) devices are widely used in civil and military fields. With respect to healthcare and safety concerns, high-performance materials for electromagnetic wave absorption (EA) or shielding have been investigated for the device applications [1e6]. In addition, the EA materials with lightweight properties will be much favored in the fields of aerospace, aviation, and fast-growing smart electronics [7e10]. To make EA materials, one of the classical methods is to simply mix a small fraction of EA nanomaterials with a polymer matrix [11e14]. When increasing the nanomaterial content,
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Wang), [email protected]. cn (H. Yang). http://dx.doi.org/10.1016/j.jallcom.2016.11.421 0925-8388/© 2016 Elsevier B.V. All rights reserved.
however, the problems of aggregation occur, which may lead to complicated processing and limited mechanical properties of EA materials. Moreover, the EA materials made from this method are mostly very cumbersome, converse to the lightweight requirements. Nanocarbon materials, particularly for the developments of carbon nanotubes (CNTs) and graphene nanosheets [15], are attractive for the use in applications requiring multifunctional features, due to the physicochemical and lightweight properties. For example, CNT networks [16] and graphene foams [17,18] can be engineered into high-performance EA materials with broadband features. However, the performance of pure nanocarbons may be limited. On the other hand, making composite materials of EA nanomaterials and CNTs or graphene are expected to harvest improved microwave absorption performance [19e25], considering the rational combination of their properties. Significant efforts on the synthesis techniques have been developed to prepare
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such nanocomposites for microwave absorption. For example, simple mechanical mixing is one practical method to make the composites [26]. However, the produced composite materials from simple mixing usually show poor structure stability, which may hinder their applications. Other techniques such as in-situ growth methods [27] and hydrothermal methods [28] are also proposed to make the composites with enhanced structure integrity. However, the combination of inorganic nanoparticles with nanocarbons always leads to brittle powders, and cannot form integrated structures with flexibility. Only very few literature of flexible inorganic composite structure are reported [29,30], with a reasonable loading of nanopartilces. The challenge, therefore, is to rationally combine CNTs with EA materials at high concentrations, yet not compromise the mechanical properties of the composites. The produced composites are also expected to achieve lightweight properties. Herein, we propose an effective approach using CNTs as the frameworks for the growth of magnetic particles, thereby forming a lightweight and flexible composite aerogel with high particle loading (more than 88 wt%). This composite aerogel is highly porous, and featured with a threaded structure. That is, mesoporous Fe3O4 particles are threaded by CNTs and crosslinked into monolithic porous foams. Based on the rational combination of these two, increment of both magnetic and dielectric losses is attained. Also, the hierarchically structured composites induce multiple reflection and scattering. Those factors contribute to enhance the microwave absorption of the composite aerogel. In addition, the structure of this composite aerogel is durable, offering wide potential applications. 2. Experimental 2.1. Chemical reagents Iron acetylacetonate (C15H21FeO6), 1-methyl-2-pyrrolidinone (NMP) (aladdin industrial corporation), C2H5OH (Beijing Chemical Works). All chemicals used were analytical grade. CNTs were produced from a chemical vapor deposition (CVD) based mass production method. They were purified and mildly treated for use [31]. 2.2. Synthesis of CNT/Fe3O4 aerogels CNT/Fe3O4 composites were synthesized by a one-pot solvothermal method. In a typical synthesis for 88 wt% CNT/Fe3O4 composite aerogel, 25 mg of CNTs and 100 mL of 1-methyl-2-pyrrolidone (NMP) were added into a homogenizer and stirred vigorously for dispersion for 1 h. Subsequently, the homogeneous solution was condensed to 40 ml by filtering, followed by washing with ethanol several times. Then the solution was transferred into a three-neck flask and gradually heated to 60 C in an oil bath under stirring with vacuum for 6 h. After that, the solution was transferred into a 50 ml Teflon-sealed autoclave, in which iron acetylacetonate was added slowly with protection of inert gas under stirring. The Teflon-sealed autoclave was washed by alkaline solution in advance. Next, the mixture was treated at 180 C for 12 h. Eventually, the products were freeze-dried overnight to get the dry aerogel. Other composite aerogels (70 wt% and 95 wt%) were synthesized based on the same procedures with proper weight of precursors. 2.3. Characterization SEM experiments were conducted on a Quanta 200F FE-SEM. TEM experiments were conducted on a FEI F20 instrument operated at 120 kV. X-ray diffraction was conducted on a BRUKER D8 ADVANCE X-ray diffractometer using Cu-Ka radiation (l ¼ 1.54 Å).
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Nitrogen sorption isotherms were measured at 77 K with a Micromeritics JW-BK222 analyzer. Thermogravimetric analysis was conducted on a STA7200 (HITACHI) instrument at a ramping rate of 10 C min1 under an O2 flow. Raman spectra were performed on a RenishawinVia confocal Raman microscope system with He-Ne laser excitation at 633 nm. X-ray photoelectron spectroscopy (XPS) of the nanocomposite was analyzed by a Thermo Fisher KAlpha spectrometer with a PHI 3057 spectrometer using Mg-K Xrays at 1253.6 eV. The magnetic properties were carried out on a superconducting quantum interference device (SQUID) magnetometer at 300 K. The charge/discharge measurements of the composite electrodes were carried out on LAND CT2000 battery tester. 2.4. EM absorption measurement EM absorption measurement of the samples were prepared by mixing 30 wt%various composite foams and 70 wt% paraffin. The mixtures were then pressed into toroidal-shaped samples (Fout ¼ 7.00 mm and Fin ¼ 3.04 mm). The complex permittivity and permeability values were measured in the 2e18 GHz range with the coaxial wire method by an Agilent N5224A vector network analyzer. 3. Results and discussion Scheme 1 outlines the synthesis route. We started with preparation of CNT dispersion by high-speed fluid shearing, into which was then added the precursor (iron acetylacetonate, C15H21FeO6) of Fe3O4; an in-situ solvent-thermal reaction leaded to the growth of mesoporous Fe3O4 microparticles within CNT networks; after freeze-drying to remove the solvent, a monolithic aerogel of CNT/ Fe3O4nanocomposite with a threaded network structure was obtained. The chemical composition of the composite can be easily tuned by changing the concentration of the precursors of Fe3O4, forming composite aerogels with different particle loading. In our work, apart from the optimized 88 wt% composite aerogel, 70 wt% and 95 wt% composite aerogels were also prepared for comparison. It needs to be point out that CNTs act as frameworks for the growth of Fe3O4; while the growth of those particles crosslinks the dispersed CNTs into integrated structure in this process. The composite foam was lightweight and flexible, thus could be compressed into stiff yet flexible nanopapers. Note that most composites composed of CNTs and metal oxides only present formation of powder composites with no mechanical properties [32e35]. By comparison, a lightweight, porous, yet flexible magnetic CNT/ Fe3O4nanocompsotie aerogel was designed and synthesized in this work. Fig. 1(a) displays the morphology of as-prepared foam containing 88 wt% Fe3O4. It is highly porous and lightweight, with a bulk density of 12 mg cm3. This value of bulk density is much higher than that of typical silica aerogel [36], and even comparable to that of reported CNT sponge [37]. Due to the porous network structure, it can provide large deformation of the dry foam. Thus, compaction of the composite aerogel creates stiff magnetic nanopaper films because of the excellent mechanical properties of the robust structure (Fig. 1(b)). The porosity of the resulting dried nanocomposite was controlled from 99.8% (upon removal of solvent during freeze-drying) in the aerogel to 95.7% (upon compaction of 1 MPa) in the nanopaper. Fig. 1(c) demonstrates the high flexibility of the composite nanopaper; repeated bending causes no apparent damage of the composite. The dry and flexible magnetic aerogel is resistant to water (Fig. 1(d)). Even in vigorous stirring, the composite materials maintain the integrated structure, suggesting its robustness. This observation is unexpected, because inorganic
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Scheme 1. Schematic of synthesis of lightweight and compressible composite aerogel and flexible nanopaper. (a) A mixture dispersion containing CNT and precursors (C15H21FeO6, the red molecule) of Fe3O4, (b) the formation of mesoporous Fe3O4microparticles threaded by CNTs after in-situ solvent-thermal reaction (step I), (c) the monolithic aerogel of CNT/ Fe3O4nanocomposite after freeze-drying (step II), and (d) the flexible nanopaper after compression (III).
aerogels are typically brittle. Our findings suggest that the robust aerogels may be used in many other applications such as absorption of pollution. Fig. 1(eeg) show that the composite aerogel can easily uptake gasoline of more than 20 times its own weight, an indication of high porosity and uptake capacity. Moreover, the aerogel after absorbing gasoline still maintains the integrated structure, and can be attracted by a magnet. This further confirms the robust structure of the composite. The observed flexibility can be explained by the strong and highly crosslinked structure, as confirmed by the micrographs of freeze-dried composites. Fig. 2(a) shows the SEM image of the composite aerogel. It displays uniform porous structure, with no CNT aggregations and structure defects observed throughout the whole aerogel. In more close observation, the TEM image reveals that the magnetic particles are threaded by CNTs (Fig. 2(b)), particularly around the crosslinks. On the other hand, a geometric confinement of CNT networks can enhance the growth of magnetic particles and prevent their agglomerations [38]. It is quietly deferent from the structure where surfaces of CNTs are coated by oxide particles [32]. Note that the high-magnification TEM image reveals that as-formed Fe3O4 particles have a porous morphology (Fig. 2(c)). Their porous structure of CNT/Fe3O4nanocomposites was further confirmed using nitrogen adsorption measurement (Fig. 2(d)). The Brunauer-Emmett-Teller (BET) surface area of the composite is 125 m2 g1. The nanocomposite exhibited hierarchically structured pores with an averaged pore size of several nanometers (inset of Fig. 2(d)). As reported in literature [39,40], the high surface areas and porous structure are effective for microwave absorption. Fig. 3(a) shows the XRD pattern of the composite aerogel. It exhibits typical diffraction peaks of Fe3O4 phase (JCPDS card No. 653107), suggesting a successful transformation of the precursors into
Fe3O4 in CNT dispersion. Although the typical peaks of CNTs in the XRD are not obvious, Raman spectrum (Fig. 3(b)) of the nanocomposites detect carbon peaks in the composite, with characteristic disorder-induced D and graphitic G bands at 1331 and 1600 cm1 respectively. Also, the Raman peaks of Fe3O4 component of the composite are observed at 226, 295, 411, and 655 cm1. The results suggest Fe3O4 and CNTs coexist in the nanocomposites. XPS measurements are further performed to determine the composition of the CNT/Fe3O4 nanocomposites. The XPS survey spectrum of C1s, O1s and Fe2p peaks is shown in Fig. 3(c). The typical C1s peak was observed at 285 eV; while the characteristic peaks of Fe3O4 were observed at 711.4 eV and 724.8 eV, corresponds to the Fe 2p3/2 and Fe 2p1/2, respectively [41,42]. All those results suggest that Fe3O4 was successfully prepared in the CNT dispersion. Very importantly, this synthesis strategy can be extended to synthesis of other metal-oxide composites or other compounds [43,44], which will offer huge multifunctional composites. The mass percentage of the composite was determined by thermogravimetric analysis (Fig. 3(d)). 12% of the weight loss before 520 C is mostly assigned to the oxidation of CNTs, while the left 88 wt% is ascribed to Fe3O4, suggesting a high density of nanoparticles. The decomposed temperature of the composite is more than 250 C, which may extend the composite for high-temperature microwave absorption [45]. Fig. 4(a) shows the composite foams can be attracted by a magnet easily, indicating the good magnetic properties. This magnetic aerogels are desired in multifunctional applications such as electronic actuators [46]. Fig. 4(b) shows the hysteresis loops of the composites with different loading of Fe3O4 particles measured at 300 K. We can see that all as-prepared composites exhibit the same S-like curve. The saturation magnetization (Ms) values increase with the increase of Fe3O4 loading. The Ms of 95, 88, and 70 wt% CNT/Fe3O4 is 63, 57, and 48 emu g1 respectively, which are all
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Fig. 1. Photographs of CNT/Fe3O4nanocomposite aerogel (a) before and (b) after compression. (c) Photograph of the composite nanopaper under bending. (d) Photograph of nanocomposite aerogel floating on water, showing that it is hydrophobic. (eeg) photographs of the nanocomposite aerogel showing that the composite aerogel could absorb dyed gasoline.
lower than that of bare Fe3O4 (67 emu g1 at 300 K), but are higher than that of CNTs (10 emu g1 at 300 K). According to effective medium theory [47], the Ms of CNT/Fe3O4 is decreased due to the introduction of CNTs, thereby making Ms values become smaller as decreasing the content of Fe3O4. Based on the structure features, we investigate the electromagnetic properties of CNT/Fe3O4 composites. Fig. 5(aef) show the frequency dependence of electromagnetic parameters of the composites with different Fe3O4 nanoparticle loading. The real permittivity (ε0 ) and the real permeability (m0 ) represent the ability to store electric and magnetic energy, while the imaginary permittivity (ε00 ) and the imaginary permeability (m00 ) represent the dissipation of electric and magnetic energy [30,48]. As shown in Fig. 5(a), the ε0 values of 88 wt% CNT/Fe3O4 and 95 wt% CNT/Fe3O4 show negligible fluctuations. The ranges of their ε0 are 14 to 11 and 7 to 6 respectively. But the ε0 value of 70 wt% CNT/Fe3O4 ranges from 37 to 18, which is significantly larger than that of 88 and 95 wt % CNT/Fe3O4nanocomposites. It can be seen that the real
permittivity (ε0 ) decreases when increasing the frequency or increasing the content of Fe3O4. Fig. 5(b) shows the frequency dependence of ε00 . ε00 values of 95 wt% CNT/Fe3O4nanocomposite are very close to zero, indicating very poor dielectric loss. With the decreasing of Fe3O4 loading, the value of ε00 is obviously enhanced. The range of ε00 for 70 wt% CNT/Fe3O4 and 88 wt% CNT/Fe3O4 is 20 to 10 and 3 to 5, respectively. The ε0 decreases with increasing frequency can be explained by Debye theory equation and the decrease of ε0 and ε00 with increasing of Fe3O4 loading can be illustrated rationally according to the effective medium theory. Fig. 5(ced) show m0 and m00 of all CNT/Fe3O4 composites in the range of 2e18 GHz. 70 wt% CNT/Fe3O4 shows the slowly decreased m0 values from 1.10 at 2 GHz to 1.02 at 12 GHz, and then the values abruptly increase to 1.2 at 18 GHz with two peaks at 14.8 GHz and 17.6 GHz. Also, the m00 values keep the same trend. But for 88 wt% CNT/Fe3O4 and 95 wt% CNT/Fe3O4, the m0 and m00 values exhibit a decreasing trend with increasing frequency. This may be attributed to crystallinity and size of Fe3O4 [27]. Meanwhile, we can see that
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Fig. 2. (a) SEM and (b, c) TEM images of CNT/Fe3O4nanocomposites, showing the crosslined networks. (d) N2 sorption isotherms and BJH pore size distribution (inset) of CNT/ Fe3O4nanocomposite foams.
Fig. 3. (a) XRD patterns of CNT/Fe3O4nanocomposites compared with bare Fe3O4. (b) Raman spectra of CNT/Fe3O4, Fe3O4, and CNTs. (c) XPS spectrum and Fe2p spectrum (inset) of CNT/Fe3O4nanocomposites. (d) TGA curve of CNT/Fe3O4nanocomposites with a loading of 88 wt%.
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Fig. 4. (a) CNT/Fe3O4nanocomposite foam adsorbed onto a magnet. (b) The magnetization curves of CNT/Fe3O4nanocomposite compared with that of bare Fe3O4 and CNTs. Inset is the expanded low field magnetization curves.
Fig. 5. Frequency dependence of (a) real part and (b) imaginary part of relative complex permittivity, (c) real part and (d) imaginary part of relative complex permeability, (e) dielectric loss tangent, and (f) magnetic loss tangent of CNT/Fe3O4 nanocomposite with the iron oxide loading of 70, 88, and 95 wt% respectively.
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the m00 values keep negative in a specific range of frequencies, demonstrating there is a resonance phenomenon between m0 and m00 [49]. In order to explain the mechanism of EM absorption, we also calculated the dielectric loss tangent (tandE ¼ ε00 /ε0 ) and the magnetic loss tangent (tandM ¼ m00 /m0 ) based on electromagnetic parameters, as shown in Fig. 5(eef). 95 wt% CNT/Fe3O4 composites show relative low dielectric loss factor among the range of whole frequency because of its small ε00 values, indicating poor dielectric loss. With the decrease of Fe3O4, the dielectric loss factor was improved; that is the less Fe3O4 loading, the bigger of tandE values. However, for tandM, 88 wt% CNT/Fe3O4 and 95 wt% CNT/Fe3O4 have similar trend and values in the whole frequency range. They also have close value of tandE and tandM. But the difference between tandE and tandM of 70 wt% CNT/Fe3O4 is great. Therfore, 88 wt% CNT/Fe3O4 and 95 wt% CNT/Fe3O4 should have better impedance matching. To evaluate the EM absorption ability of these composites, the reflection loss (RL) of these composites was calculated using the EM parameters. According to transmission line theory [50,51],
Zin ¼
rffiffiffiffiffi mr 2pfd pffiffiffiffiffiffiffiffiffi tanh j mr εr c εr
Z 1 RLðdBÞ ¼ 20 log in Zin þ 1
(1)
(2)
where Zin is the input impedance of the absorber, mr and εr are respectively the relative complex permeability and permittivity, f is the frequency of microwave, d is the thickness of the absorber, and c is the velocity of light in free space. Fig. 6(aed) compare the reflection loss (RL) of 70 wt% CNT/ Fe3O4, 88 wt% CNT/Fe3O4 and 95 wt% CNT/Fe3O4with pure Fe3O4 at
2e18 GHz. Overall, the bare Fe3O4 shows limited EA performance than that of the composites (Fig. 6(a)), suggesting the effectiveness of composite strategy. We can see that the maximum RL of all samples obviously shift to lower frequency with increasing thickness for all composites. This can be attributed to the phenomena of quarter-wavelength attenuation, in which the absorption meets the phase match conditions [23]. Also, the composition of the composite is important to obtain the high capacity of microwave absorption [52]. In Fig. 6(c), 88 wt% CNT/Fe3O4 nanocomposite exhibits the best EM wave absorption property, with the maximum RL of 25 dB at 4 GHz when the thickness is 5 mm. Even when the thickness of the sample is decreased to 2 mm, the maximum RL still can reach 22 dB. The absorption bandwidths below 10 dB (90% of EM wave absorption) for the composite are shifted when increasing the thickness from 2 to 5 mm. The microwave absorption properties of this nanocomposite aerogel are compared to those of reported Fe3O4-based composites in Table 1 [53e56]. The nanocomposite materials obtained in this work show reasonable absorption properties. But for 70 wt% and 95 wt%CNT/Fe3O4 composites, they have limited capability for EM wave absorption. For 70 wt% CNT/Fe3O4 (Fig. 6(b)), the maximum RL is only 11 dB at 7 GHz with a thickness of 2 mm, while the maximum RL of 95 wt% CNT/Fe3O4 composite is only 10 dB at 12 GHz with a thickness of 3 mm (Fig. 6(d)). We know that 88 wt% CNT/Fe3O4 has similar values of dielectric loss and magnetic loss which can lead to the good impedance match. For 95 wt% CNT/Fe3O4, it has good impedance match, however, it has very poor dielectric loss. The difference between tandE and tandM of 70 wt% CNT/Fe3O4 is great. Therefore, 88 wt% CNT/Fe3O4 has best EM wave absorption ability, while 70 wt% CNT/Fe3O4and 95 wt% CNT/Fe3O4 only display limited absorption ability. The microwave absorbing performance could be explained by the following factors: magnetic loss, dielectric loss, and multiple
Fig. 6. Reflection loss of (a) pure Fe3O4 compared with that of CNT/Fe3O4 nanocomposites with various particle loading of (b) 70 wt%, (c) 88 wt%, and (d) 95 wt% at the thicknesses from 1 to 5 mm.
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Table 1 Structure and microwave absorption property comparison of as-prepared CNT/Fe3O4 nanocomposite with reported composite materials. Material
Structure properties
Minimum RL value (dB)
d (mm) (RL < 10 dB)
Frequency range (GHz) (RL < 10 dB)
Ref.
Fe3O4/RGO Fe3O4/graphene Fe3O4/graphene Fe3O4/C Fe3O4/SiO2/PVDF Fe3O4/C nanofiber Fe3O4/MWCNT Fe3O4/CNT
powder 3D foam powder powder flexible composite powder powder 3D compressible foam
27 27 32 46 28.6 46 42 25
2e4 3e5 2.5e4 2.9 2e5 5 2e5 2e5
4.8e13.6 5e15 5.4e17 10e17 3.3e17.6 4.5e11.8 3e11.4 2.4e7.9 and 9.5e12.3
[22] [28] [53] [54] [55] [56] [32] This work
Acknowledgements This work was supported by Science Foundation of China University of Petroleum, Beijing (2462015YQ0315), partially supported by National Natural Science Foundation of China (No. 51502347) and Science Foundation of University of Science and Technology Beijing (No. 06500045). References
Scheme 2. The possible microwave absorbing mechanism of CNT/Fe3O4 nanocomposite for enhanced performance.
reflections [57e59]. As shown in Scheme 2, the magnetic loss is mainly caused by the natural resonance and the eddy current effect. The introduction of CNTs can enhance dielectric loss due to the electron polarization relaxation and interfacial polarization [60]. The enhanced performance of electromagnetic wave absorption of 88 wt% CNT/Fe3O4 composites is attributed to the rational combination of iron oxide and CNT networks, which leads to the increment of both magnetic and dielectric losses. In addition, mesoporous structure of Fe3O4 and the hierarchically structured properties of the composite aerogels could cause multiple reflections of incident microwaves, which may further enhance the microwave absorbing capacity. 4. Conclusions In summary, we have demonstrated the synthesis of a lightweight and flexible composite aerogel consisting of CNTs and mesoporous iron oxide using the in-situ growth method. CNTs acted as templates for the growth of magnetic oxide nanoparticles; while the growth of magnetic particles crosslinked CNTs into integrated porous structure. Therefore, mesoporous Fe3O4 particles were threaded by CNTs, leading to formation of magnetic foams with the particle loading of more than 88 wt%. Based on the rational combination of these two, the composite aerogels offered excellent microwave absorbing performance, and also displayed lightweight properties. A strong absorption peak centered at 3.8 GHz and a minimum reflection loss approaching-25 dB could be achieved on 88 wt% CNT/Fe3O4composite with a thickness of 5.0 mm. In the frequency range of 2.4e7.9 and 9.5e12.3 GHz, the absorption area was below 10 dB with thickness of 2e5 mm. In addition, the porous composite aerogel could be compacted into a flexible magnetic nanopaper. Because of the structure properties, we believe that the composite materials can be used in many other applications such as energy storage, sustainable catalyst, and biochemistry.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis of lightweight and flexible composite aerogel of mesoporous iron oxide threaded by carbon nanotubes for microwave absorption Xilai Jia a, Jie Wang b, Xiao Zhu b, Tihong Wang b, Fan Yang b, Wenjun Dong a, Ge Wang a, *, Haitao Yang c, **, Fei Wei d a
Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Changping, 102249, Beijing, China c Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China d Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 November 2016 Received in revised form 28 November 2016 Accepted 30 November 2016 Available online 1 December 2016
Inorganic composites with mechanical flexibility are desired in synthesis of advanced materials for multifunctional applications. Here, we demonstrated the synthesis of a lightweight and flexible composite aerogel composed of carbon nanotubes (CNTs) and mesoporous iron oxide (Fe3O4) using an in-situ growth method. CNTs serve as frameworks for the growth of magnetic ferrite nanoparticles; while the growth of the magnetic nanoparticles crosslinks CNTs into an integrated structure. Therefore, mesoporous Fe3O4 particles are threaded by CNTs, leading to formation of the monolithic magnetic foam with a high particle loading of more than 88 wt%. This magnetic foam is highly porous, effective for microwave absorption. Based on the rational combination of Fe3O4 and CNTs, the composite material can offer excellent microwave absorbing performance, particularly in the low-frequency range of 3e5 GHz. Moreover, the porous composite aerogel can be compacted into a flexible magnetic nanopaper. Because of the structure properties, the composite may be used in many other applications such as energy storage, catalyst, and biochemistry. © 2016 Elsevier B.V. All rights reserved.
Keywords: Carbon nanotube Mesoporous iron oxide Microwave absorption Composite materials
1. Introduction With the development of information technology, electromagnetic (EM) devices are widely used in civil and military fields. With respect to healthcare and safety concerns, high-performance materials for electromagnetic wave absorption (EA) or shielding have been investigated for the device applications [1e6]. In addition, the EA materials with lightweight properties will be much favored in the fields of aerospace, aviation, and fast-growing smart electronics [7e10]. To make EA materials, one of the classical methods is to simply mix a small fraction of EA nanomaterials with a polymer matrix [11e14]. When increasing the nanomaterial content,
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Wang), [email protected]. cn (H. Yang). http://dx.doi.org/10.1016/j.jallcom.2016.11.421 0925-8388/© 2016 Elsevier B.V. All rights reserved.
however, the problems of aggregation occur, which may lead to complicated processing and limited mechanical properties of EA materials. Moreover, the EA materials made from this method are mostly very cumbersome, converse to the lightweight requirements. Nanocarbon materials, particularly for the developments of carbon nanotubes (CNTs) and graphene nanosheets [15], are attractive for the use in applications requiring multifunctional features, due to the physicochemical and lightweight properties. For example, CNT networks [16] and graphene foams [17,18] can be engineered into high-performance EA materials with broadband features. However, the performance of pure nanocarbons may be limited. On the other hand, making composite materials of EA nanomaterials and CNTs or graphene are expected to harvest improved microwave absorption performance [19e25], considering the rational combination of their properties. Significant efforts on the synthesis techniques have been developed to prepare
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such nanocomposites for microwave absorption. For example, simple mechanical mixing is one practical method to make the composites [26]. However, the produced composite materials from simple mixing usually show poor structure stability, which may hinder their applications. Other techniques such as in-situ growth methods [27] and hydrothermal methods [28] are also proposed to make the composites with enhanced structure integrity. However, the combination of inorganic nanoparticles with nanocarbons always leads to brittle powders, and cannot form integrated structures with flexibility. Only very few literature of flexible inorganic composite structure are reported [29,30], with a reasonable loading of nanopartilces. The challenge, therefore, is to rationally combine CNTs with EA materials at high concentrations, yet not compromise the mechanical properties of the composites. The produced composites are also expected to achieve lightweight properties. Herein, we propose an effective approach using CNTs as the frameworks for the growth of magnetic particles, thereby forming a lightweight and flexible composite aerogel with high particle loading (more than 88 wt%). This composite aerogel is highly porous, and featured with a threaded structure. That is, mesoporous Fe3O4 particles are threaded by CNTs and crosslinked into monolithic porous foams. Based on the rational combination of these two, increment of both magnetic and dielectric losses is attained. Also, the hierarchically structured composites induce multiple reflection and scattering. Those factors contribute to enhance the microwave absorption of the composite aerogel. In addition, the structure of this composite aerogel is durable, offering wide potential applications. 2. Experimental 2.1. Chemical reagents Iron acetylacetonate (C15H21FeO6), 1-methyl-2-pyrrolidinone (NMP) (aladdin industrial corporation), C2H5OH (Beijing Chemical Works). All chemicals used were analytical grade. CNTs were produced from a chemical vapor deposition (CVD) based mass production method. They were purified and mildly treated for use [31]. 2.2. Synthesis of CNT/Fe3O4 aerogels CNT/Fe3O4 composites were synthesized by a one-pot solvothermal method. In a typical synthesis for 88 wt% CNT/Fe3O4 composite aerogel, 25 mg of CNTs and 100 mL of 1-methyl-2-pyrrolidone (NMP) were added into a homogenizer and stirred vigorously for dispersion for 1 h. Subsequently, the homogeneous solution was condensed to 40 ml by filtering, followed by washing with ethanol several times. Then the solution was transferred into a three-neck flask and gradually heated to 60 C in an oil bath under stirring with vacuum for 6 h. After that, the solution was transferred into a 50 ml Teflon-sealed autoclave, in which iron acetylacetonate was added slowly with protection of inert gas under stirring. The Teflon-sealed autoclave was washed by alkaline solution in advance. Next, the mixture was treated at 180 C for 12 h. Eventually, the products were freeze-dried overnight to get the dry aerogel. Other composite aerogels (70 wt% and 95 wt%) were synthesized based on the same procedures with proper weight of precursors. 2.3. Characterization SEM experiments were conducted on a Quanta 200F FE-SEM. TEM experiments were conducted on a FEI F20 instrument operated at 120 kV. X-ray diffraction was conducted on a BRUKER D8 ADVANCE X-ray diffractometer using Cu-Ka radiation (l ¼ 1.54 Å).
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Nitrogen sorption isotherms were measured at 77 K with a Micromeritics JW-BK222 analyzer. Thermogravimetric analysis was conducted on a STA7200 (HITACHI) instrument at a ramping rate of 10 C min1 under an O2 flow. Raman spectra were performed on a RenishawinVia confocal Raman microscope system with He-Ne laser excitation at 633 nm. X-ray photoelectron spectroscopy (XPS) of the nanocomposite was analyzed by a Thermo Fisher KAlpha spectrometer with a PHI 3057 spectrometer using Mg-K Xrays at 1253.6 eV. The magnetic properties were carried out on a superconducting quantum interference device (SQUID) magnetometer at 300 K. The charge/discharge measurements of the composite electrodes were carried out on LAND CT2000 battery tester. 2.4. EM absorption measurement EM absorption measurement of the samples were prepared by mixing 30 wt%various composite foams and 70 wt% paraffin. The mixtures were then pressed into toroidal-shaped samples (Fout ¼ 7.00 mm and Fin ¼ 3.04 mm). The complex permittivity and permeability values were measured in the 2e18 GHz range with the coaxial wire method by an Agilent N5224A vector network analyzer. 3. Results and discussion Scheme 1 outlines the synthesis route. We started with preparation of CNT dispersion by high-speed fluid shearing, into which was then added the precursor (iron acetylacetonate, C15H21FeO6) of Fe3O4; an in-situ solvent-thermal reaction leaded to the growth of mesoporous Fe3O4 microparticles within CNT networks; after freeze-drying to remove the solvent, a monolithic aerogel of CNT/ Fe3O4nanocomposite with a threaded network structure was obtained. The chemical composition of the composite can be easily tuned by changing the concentration of the precursors of Fe3O4, forming composite aerogels with different particle loading. In our work, apart from the optimized 88 wt% composite aerogel, 70 wt% and 95 wt% composite aerogels were also prepared for comparison. It needs to be point out that CNTs act as frameworks for the growth of Fe3O4; while the growth of those particles crosslinks the dispersed CNTs into integrated structure in this process. The composite foam was lightweight and flexible, thus could be compressed into stiff yet flexible nanopapers. Note that most composites composed of CNTs and metal oxides only present formation of powder composites with no mechanical properties [32e35]. By comparison, a lightweight, porous, yet flexible magnetic CNT/ Fe3O4nanocompsotie aerogel was designed and synthesized in this work. Fig. 1(a) displays the morphology of as-prepared foam containing 88 wt% Fe3O4. It is highly porous and lightweight, with a bulk density of 12 mg cm3. This value of bulk density is much higher than that of typical silica aerogel [36], and even comparable to that of reported CNT sponge [37]. Due to the porous network structure, it can provide large deformation of the dry foam. Thus, compaction of the composite aerogel creates stiff magnetic nanopaper films because of the excellent mechanical properties of the robust structure (Fig. 1(b)). The porosity of the resulting dried nanocomposite was controlled from 99.8% (upon removal of solvent during freeze-drying) in the aerogel to 95.7% (upon compaction of 1 MPa) in the nanopaper. Fig. 1(c) demonstrates the high flexibility of the composite nanopaper; repeated bending causes no apparent damage of the composite. The dry and flexible magnetic aerogel is resistant to water (Fig. 1(d)). Even in vigorous stirring, the composite materials maintain the integrated structure, suggesting its robustness. This observation is unexpected, because inorganic
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Scheme 1. Schematic of synthesis of lightweight and compressible composite aerogel and flexible nanopaper. (a) A mixture dispersion containing CNT and precursors (C15H21FeO6, the red molecule) of Fe3O4, (b) the formation of mesoporous Fe3O4microparticles threaded by CNTs after in-situ solvent-thermal reaction (step I), (c) the monolithic aerogel of CNT/ Fe3O4nanocomposite after freeze-drying (step II), and (d) the flexible nanopaper after compression (III).
aerogels are typically brittle. Our findings suggest that the robust aerogels may be used in many other applications such as absorption of pollution. Fig. 1(eeg) show that the composite aerogel can easily uptake gasoline of more than 20 times its own weight, an indication of high porosity and uptake capacity. Moreover, the aerogel after absorbing gasoline still maintains the integrated structure, and can be attracted by a magnet. This further confirms the robust structure of the composite. The observed flexibility can be explained by the strong and highly crosslinked structure, as confirmed by the micrographs of freeze-dried composites. Fig. 2(a) shows the SEM image of the composite aerogel. It displays uniform porous structure, with no CNT aggregations and structure defects observed throughout the whole aerogel. In more close observation, the TEM image reveals that the magnetic particles are threaded by CNTs (Fig. 2(b)), particularly around the crosslinks. On the other hand, a geometric confinement of CNT networks can enhance the growth of magnetic particles and prevent their agglomerations [38]. It is quietly deferent from the structure where surfaces of CNTs are coated by oxide particles [32]. Note that the high-magnification TEM image reveals that as-formed Fe3O4 particles have a porous morphology (Fig. 2(c)). Their porous structure of CNT/Fe3O4nanocomposites was further confirmed using nitrogen adsorption measurement (Fig. 2(d)). The Brunauer-Emmett-Teller (BET) surface area of the composite is 125 m2 g1. The nanocomposite exhibited hierarchically structured pores with an averaged pore size of several nanometers (inset of Fig. 2(d)). As reported in literature [39,40], the high surface areas and porous structure are effective for microwave absorption. Fig. 3(a) shows the XRD pattern of the composite aerogel. It exhibits typical diffraction peaks of Fe3O4 phase (JCPDS card No. 653107), suggesting a successful transformation of the precursors into
Fe3O4 in CNT dispersion. Although the typical peaks of CNTs in the XRD are not obvious, Raman spectrum (Fig. 3(b)) of the nanocomposites detect carbon peaks in the composite, with characteristic disorder-induced D and graphitic G bands at 1331 and 1600 cm1 respectively. Also, the Raman peaks of Fe3O4 component of the composite are observed at 226, 295, 411, and 655 cm1. The results suggest Fe3O4 and CNTs coexist in the nanocomposites. XPS measurements are further performed to determine the composition of the CNT/Fe3O4 nanocomposites. The XPS survey spectrum of C1s, O1s and Fe2p peaks is shown in Fig. 3(c). The typical C1s peak was observed at 285 eV; while the characteristic peaks of Fe3O4 were observed at 711.4 eV and 724.8 eV, corresponds to the Fe 2p3/2 and Fe 2p1/2, respectively [41,42]. All those results suggest that Fe3O4 was successfully prepared in the CNT dispersion. Very importantly, this synthesis strategy can be extended to synthesis of other metal-oxide composites or other compounds [43,44], which will offer huge multifunctional composites. The mass percentage of the composite was determined by thermogravimetric analysis (Fig. 3(d)). 12% of the weight loss before 520 C is mostly assigned to the oxidation of CNTs, while the left 88 wt% is ascribed to Fe3O4, suggesting a high density of nanoparticles. The decomposed temperature of the composite is more than 250 C, which may extend the composite for high-temperature microwave absorption [45]. Fig. 4(a) shows the composite foams can be attracted by a magnet easily, indicating the good magnetic properties. This magnetic aerogels are desired in multifunctional applications such as electronic actuators [46]. Fig. 4(b) shows the hysteresis loops of the composites with different loading of Fe3O4 particles measured at 300 K. We can see that all as-prepared composites exhibit the same S-like curve. The saturation magnetization (Ms) values increase with the increase of Fe3O4 loading. The Ms of 95, 88, and 70 wt% CNT/Fe3O4 is 63, 57, and 48 emu g1 respectively, which are all
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Fig. 1. Photographs of CNT/Fe3O4nanocomposite aerogel (a) before and (b) after compression. (c) Photograph of the composite nanopaper under bending. (d) Photograph of nanocomposite aerogel floating on water, showing that it is hydrophobic. (eeg) photographs of the nanocomposite aerogel showing that the composite aerogel could absorb dyed gasoline.
lower than that of bare Fe3O4 (67 emu g1 at 300 K), but are higher than that of CNTs (10 emu g1 at 300 K). According to effective medium theory [47], the Ms of CNT/Fe3O4 is decreased due to the introduction of CNTs, thereby making Ms values become smaller as decreasing the content of Fe3O4. Based on the structure features, we investigate the electromagnetic properties of CNT/Fe3O4 composites. Fig. 5(aef) show the frequency dependence of electromagnetic parameters of the composites with different Fe3O4 nanoparticle loading. The real permittivity (ε0 ) and the real permeability (m0 ) represent the ability to store electric and magnetic energy, while the imaginary permittivity (ε00 ) and the imaginary permeability (m00 ) represent the dissipation of electric and magnetic energy [30,48]. As shown in Fig. 5(a), the ε0 values of 88 wt% CNT/Fe3O4 and 95 wt% CNT/Fe3O4 show negligible fluctuations. The ranges of their ε0 are 14 to 11 and 7 to 6 respectively. But the ε0 value of 70 wt% CNT/Fe3O4 ranges from 37 to 18, which is significantly larger than that of 88 and 95 wt % CNT/Fe3O4nanocomposites. It can be seen that the real
permittivity (ε0 ) decreases when increasing the frequency or increasing the content of Fe3O4. Fig. 5(b) shows the frequency dependence of ε00 . ε00 values of 95 wt% CNT/Fe3O4nanocomposite are very close to zero, indicating very poor dielectric loss. With the decreasing of Fe3O4 loading, the value of ε00 is obviously enhanced. The range of ε00 for 70 wt% CNT/Fe3O4 and 88 wt% CNT/Fe3O4 is 20 to 10 and 3 to 5, respectively. The ε0 decreases with increasing frequency can be explained by Debye theory equation and the decrease of ε0 and ε00 with increasing of Fe3O4 loading can be illustrated rationally according to the effective medium theory. Fig. 5(ced) show m0 and m00 of all CNT/Fe3O4 composites in the range of 2e18 GHz. 70 wt% CNT/Fe3O4 shows the slowly decreased m0 values from 1.10 at 2 GHz to 1.02 at 12 GHz, and then the values abruptly increase to 1.2 at 18 GHz with two peaks at 14.8 GHz and 17.6 GHz. Also, the m00 values keep the same trend. But for 88 wt% CNT/Fe3O4 and 95 wt% CNT/Fe3O4, the m0 and m00 values exhibit a decreasing trend with increasing frequency. This may be attributed to crystallinity and size of Fe3O4 [27]. Meanwhile, we can see that
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Fig. 2. (a) SEM and (b, c) TEM images of CNT/Fe3O4nanocomposites, showing the crosslined networks. (d) N2 sorption isotherms and BJH pore size distribution (inset) of CNT/ Fe3O4nanocomposite foams.
Fig. 3. (a) XRD patterns of CNT/Fe3O4nanocomposites compared with bare Fe3O4. (b) Raman spectra of CNT/Fe3O4, Fe3O4, and CNTs. (c) XPS spectrum and Fe2p spectrum (inset) of CNT/Fe3O4nanocomposites. (d) TGA curve of CNT/Fe3O4nanocomposites with a loading of 88 wt%.
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Fig. 4. (a) CNT/Fe3O4nanocomposite foam adsorbed onto a magnet. (b) The magnetization curves of CNT/Fe3O4nanocomposite compared with that of bare Fe3O4 and CNTs. Inset is the expanded low field magnetization curves.
Fig. 5. Frequency dependence of (a) real part and (b) imaginary part of relative complex permittivity, (c) real part and (d) imaginary part of relative complex permeability, (e) dielectric loss tangent, and (f) magnetic loss tangent of CNT/Fe3O4 nanocomposite with the iron oxide loading of 70, 88, and 95 wt% respectively.
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the m00 values keep negative in a specific range of frequencies, demonstrating there is a resonance phenomenon between m0 and m00 [49]. In order to explain the mechanism of EM absorption, we also calculated the dielectric loss tangent (tandE ¼ ε00 /ε0 ) and the magnetic loss tangent (tandM ¼ m00 /m0 ) based on electromagnetic parameters, as shown in Fig. 5(eef). 95 wt% CNT/Fe3O4 composites show relative low dielectric loss factor among the range of whole frequency because of its small ε00 values, indicating poor dielectric loss. With the decrease of Fe3O4, the dielectric loss factor was improved; that is the less Fe3O4 loading, the bigger of tandE values. However, for tandM, 88 wt% CNT/Fe3O4 and 95 wt% CNT/Fe3O4 have similar trend and values in the whole frequency range. They also have close value of tandE and tandM. But the difference between tandE and tandM of 70 wt% CNT/Fe3O4 is great. Therfore, 88 wt% CNT/Fe3O4 and 95 wt% CNT/Fe3O4 should have better impedance matching. To evaluate the EM absorption ability of these composites, the reflection loss (RL) of these composites was calculated using the EM parameters. According to transmission line theory [50,51],
Zin ¼
rffiffiffiffiffi mr 2pfd pffiffiffiffiffiffiffiffiffi tanh j mr εr c εr
Z 1 RLðdBÞ ¼ 20 log in Zin þ 1
(1)
(2)
where Zin is the input impedance of the absorber, mr and εr are respectively the relative complex permeability and permittivity, f is the frequency of microwave, d is the thickness of the absorber, and c is the velocity of light in free space. Fig. 6(aed) compare the reflection loss (RL) of 70 wt% CNT/ Fe3O4, 88 wt% CNT/Fe3O4 and 95 wt% CNT/Fe3O4with pure Fe3O4 at
2e18 GHz. Overall, the bare Fe3O4 shows limited EA performance than that of the composites (Fig. 6(a)), suggesting the effectiveness of composite strategy. We can see that the maximum RL of all samples obviously shift to lower frequency with increasing thickness for all composites. This can be attributed to the phenomena of quarter-wavelength attenuation, in which the absorption meets the phase match conditions [23]. Also, the composition of the composite is important to obtain the high capacity of microwave absorption [52]. In Fig. 6(c), 88 wt% CNT/Fe3O4 nanocomposite exhibits the best EM wave absorption property, with the maximum RL of 25 dB at 4 GHz when the thickness is 5 mm. Even when the thickness of the sample is decreased to 2 mm, the maximum RL still can reach 22 dB. The absorption bandwidths below 10 dB (90% of EM wave absorption) for the composite are shifted when increasing the thickness from 2 to 5 mm. The microwave absorption properties of this nanocomposite aerogel are compared to those of reported Fe3O4-based composites in Table 1 [53e56]. The nanocomposite materials obtained in this work show reasonable absorption properties. But for 70 wt% and 95 wt%CNT/Fe3O4 composites, they have limited capability for EM wave absorption. For 70 wt% CNT/Fe3O4 (Fig. 6(b)), the maximum RL is only 11 dB at 7 GHz with a thickness of 2 mm, while the maximum RL of 95 wt% CNT/Fe3O4 composite is only 10 dB at 12 GHz with a thickness of 3 mm (Fig. 6(d)). We know that 88 wt% CNT/Fe3O4 has similar values of dielectric loss and magnetic loss which can lead to the good impedance match. For 95 wt% CNT/Fe3O4, it has good impedance match, however, it has very poor dielectric loss. The difference between tandE and tandM of 70 wt% CNT/Fe3O4 is great. Therefore, 88 wt% CNT/Fe3O4 has best EM wave absorption ability, while 70 wt% CNT/Fe3O4and 95 wt% CNT/Fe3O4 only display limited absorption ability. The microwave absorbing performance could be explained by the following factors: magnetic loss, dielectric loss, and multiple
Fig. 6. Reflection loss of (a) pure Fe3O4 compared with that of CNT/Fe3O4 nanocomposites with various particle loading of (b) 70 wt%, (c) 88 wt%, and (d) 95 wt% at the thicknesses from 1 to 5 mm.
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145
Table 1 Structure and microwave absorption property comparison of as-prepared CNT/Fe3O4 nanocomposite with reported composite materials. Material
Structure properties
Minimum RL value (dB)
d (mm) (RL < 10 dB)
Frequency range (GHz) (RL < 10 dB)
Ref.
Fe3O4/RGO Fe3O4/graphene Fe3O4/graphene Fe3O4/C Fe3O4/SiO2/PVDF Fe3O4/C nanofiber Fe3O4/MWCNT Fe3O4/CNT
powder 3D foam powder powder flexible composite powder powder 3D compressible foam
27 27 32 46 28.6 46 42 25
2e4 3e5 2.5e4 2.9 2e5 5 2e5 2e5
4.8e13.6 5e15 5.4e17 10e17 3.3e17.6 4.5e11.8 3e11.4 2.4e7.9 and 9.5e12.3
[22] [28] [53] [54] [55] [56] [32] This work
Acknowledgements This work was supported by Science Foundation of China University of Petroleum, Beijing (2462015YQ0315), partially supported by National Natural Science Foundation of China (No. 51502347) and Science Foundation of University of Science and Technology Beijing (No. 06500045). References
Scheme 2. The possible microwave absorbing mechanism of CNT/Fe3O4 nanocomposite for enhanced performance.
reflections [57e59]. As shown in Scheme 2, the magnetic loss is mainly caused by the natural resonance and the eddy current effect. The introduction of CNTs can enhance dielectric loss due to the electron polarization relaxation and interfacial polarization [60]. The enhanced performance of electromagnetic wave absorption of 88 wt% CNT/Fe3O4 composites is attributed to the rational combination of iron oxide and CNT networks, which leads to the increment of both magnetic and dielectric losses. In addition, mesoporous structure of Fe3O4 and the hierarchically structured properties of the composite aerogels could cause multiple reflections of incident microwaves, which may further enhance the microwave absorbing capacity. 4. Conclusions In summary, we have demonstrated the synthesis of a lightweight and flexible composite aerogel consisting of CNTs and mesoporous iron oxide using the in-situ growth method. CNTs acted as templates for the growth of magnetic oxide nanoparticles; while the growth of magnetic particles crosslinked CNTs into integrated porous structure. Therefore, mesoporous Fe3O4 particles were threaded by CNTs, leading to formation of magnetic foams with the particle loading of more than 88 wt%. Based on the rational combination of these two, the composite aerogels offered excellent microwave absorbing performance, and also displayed lightweight properties. A strong absorption peak centered at 3.8 GHz and a minimum reflection loss approaching-25 dB could be achieved on 88 wt% CNT/Fe3O4composite with a thickness of 5.0 mm. In the frequency range of 2.4e7.9 and 9.5e12.3 GHz, the absorption area was below 10 dB with thickness of 2e5 mm. In addition, the porous composite aerogel could be compacted into a flexible magnetic nanopaper. Because of the structure properties, we believe that the composite materials can be used in many other applications such as energy storage, sustainable catalyst, and biochemistry.
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