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Facile Synthesis of Fe3O4/GCs Composites and their Enhanced Microwave Absorption Properties Xian Jian, Biao Wu, Yufeng Wei, Shixue Dou, Xiaolin Wang, Weidong He, and Nasir Mahmood ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00388 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on February 23, 2016

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ACS Applied Materials & Interfaces

Facile Synthesis of Fe3O4/GCs Composites and their Enhanced Microwave Absorption Properties Xian Jian†,⊥,ζ,*, Biao Wu†,ζ, Yufeng Wei⊥,ζ, Shi Xue Dou⊥, Xiaolin Wang⊥, Weidong He†,*and Nasir Mahmood⊥,* †

School of Energy Science and Engineering, State key Laboratory of Electronic Thin Films and

Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 611731, China. ⊥

Institute for Superconducting and Electronic Materials, Australian Institute for Innovative

Materials, University of Wollongong, North Wollongong, 2500, Australia.

KEYWORDS: graphene capsules; catalytic chemical vapor deposition; Fe3O4; microwave absorption; hydrothermal.

ABSTRACT: Graphene has good stability and adjustable dielectric properties along with tunable morphologies, hence can be used to design novel and high-performance functional materials. Here, we have reported a facile synthesis method of nanoscale Fe3O4/graphene capsules (GCs) composites using the combination of catalytic chemical vapor deposition (CCVD) and hydro-thermal process. The resulted composite has the advantage of unique morphology that offers better synergism among the Fe3O4 particles as well as particles and GCs. The microwave-absorbing characteristics of developed composites were investigated through 1 Environment ACS Paragon Plus


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experimentally measured electromagnetic properties and simulation studies based on the transmission line theory, explained on the basis of eddy current, natural and exchange resonance as well as dielectric relaxation processes. The composites bear minimum RL value of -32 dB at 8.76 GHz along with the absorption bandwidth range from 5.4 to 17 GHz for RL lower than -10 dB. The better performance of the composite based on the reasonable impedance characteristic, existence of interfaces around the composites and the polarization of free carriers in 3D GCs that make the as-prepared composites capable of absorbing microwave more effectively. These results offer an effective way to design highperformance functional materials to facilitate the research in electromagnetic shielding and microwave absorption.

1. INTRODUCTION Nowadays, with the rapid development of communication devices and extensive use of electronic devices, the electromagnetic interference (EMI) pollution has become a serious problem.1-3

To resolve the issues associated with the EMI pollution, considerable

attention has been devoted to develop high-performance microwave absorption materials with low density, low thickness strong absorption over a broad frequency and high thermal stability. But the present development is not sufficient to overcome the issues raised by the growing usage of communication devices and further research is required to provide the healthy environment to the humankind. Generally, the microwave absorbing materials can be classified into two categories based on their microwave loss mechanism likewise dielectric loss and magnetic loss.1,

4, 5

The loss mechanism of dielectric loss

materials such as carbon nanotubes (CNTs), carbon nanofibers (CNFs) and graphene is mainly based on the polarization. Although, the carbon-based materials possess the

2 Environment ACS Paragon Plus

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advantages of low density, superior mechanical and electronic properties and great thermal stability, but their microwave absorption capability is not good enough that limits their applications.6-9 For example, the graphene had a minimum RL value of −6.9 dB at 7.0 GHz without any effective bandwidth that is higher than the required -10dB; however, the CNT arrays have a minimum RL value of −24.6 at 9.0 GHz with a bandwidth of 2.1 from 8.2 to 10.3 GHz but its short bandwidth limits the applications.6, 10 Similarly, to realize the microwave loss of materials like Fe3O4, Fe and Co where the working mechanism is magnetic loss mainly depends on the magnetic properties of these materials. However, despite the high density of magmatic metal, most absorbers consisting of single component are incapable of producing high dielectric and magnetic loss simultaneously, thus limiting their applications.11, 12 Thus, constructing composites with high dielectric and magnetic loss has attracted a lot of attention.13-17 Among them, the composites composed of magnetic nanoparticles and carbon materials have been the focus of major research due to many advantages such as low density, high thermal and chemical stability, tunable dielectric and magnetic properties, combination of both dielectric and magnetic loss mechanisms. Recently researchers have found that by combining the two different syntheses such as dielectric loss based materials with magnetic loss one can bring a rational design for high microwave absorption. Likewise, Che et al. reported that mutli-walled carbon nanotubes (MWCNT) containing Fe particles possessed improved microwave absorption ability than the individual components.18 Furthermore, it is found that the higher absorption capabilities of the carbon-metal composites are because of the polarization free carriers in carbon component and the charge transfer at metal/carbon interface.19 Though most of the



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research has been concentrated on developing one dimensional (1D) and 2D carbon materials like CNTs,20 CNFs21 and graphene,22 it is notable that 3D graphene capsules (GCs) will be more beneficial to enhanced the microwave absorption because of their special structure that resulted in an adjustable dielectric and magnetic property.

21, 23, 24

Recently, yolk-shell C@C microsphere have been developed to observe the effect of such a structure on the microwave absorption and it is found that this structure brings an enhanced value, but the complicated synthesis process is one hurdle here.25 It is wellknown that the electromagnetic absorption is based on the combination of enhanced performance and the effective absorption range (2-18 GHz), thus for practical applications, to reduce the effective RL value less than -10 dB with the relatively low frequency remains a challenge. Thus, a rational design for carbon/metal/oxide composites considering their morphology, structure and composition that can construct a strong synergism among the two components for better properties is highly required. Here, we have developed unique hybrid structure of Fe3O4/GCs via catalytic chemical vapor deposition (CCVD) and hydrothermal process as schematically shown in Figure 1. The resulted composite having sandwiched structure where Fe3O4 nanoparticles are attached both on inner and outer side of the carbon wall of GCs, which brings an exclusive 3D morphology for composite. Thus, carbon backbone provides multiple advantages such as it connects the particles and brings effective synergism among the different components of composite along with introducing an additional loss mechanism i.e. dielectric loss to the magnetic loss mechanism of magnetic component of the composite. Furthermore, this additional loss mechanism introduced by the GCs provides a reasonable impedance characteristic to the composite for improved microwave


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absorption. The resulted composite exhibited minimum RL value of -32 dB at 8.76 GHz as well as shows lower RL values than -10dB in the absorption bandwidth range from 5.4 to 17 GHz. Further dielectric loss analysed by using Debye dielectric relaxation model and free electronic theory as well as magnetic loss analysis done by using eddy current effects, natural resonance and exchange resonance have proved that Fe3O4 particles and GCs have strong synergistic effect that brings aforementioned excellent results. Thus, we believed that as-synthesized hybrid structure is a potential candidate for the microwave shielding and will open an avenue for the development of functional materials. 2. EXPERIMENTAL METHODS Synthesis of Graphene capsules: the GCs were synthesized by CCVD through catalytic decomposition of highly pure acetylene without any carrier gas using ZnO nanoparticles as catalysts as well as substrate to define the special capsule structure. To be precise, 200 mg ZnO nanoparticles were dispersed on a quartz boat located inside a quartz tube and heated up to 700 °C under vacuum. As the temperature was stabilized for about 30 min, C2H2 was introduced into quartz tube for 30 min at a flow rate of 50 mL/min and then reaction system was cooled down to room temperature. After completion of reaction the products were collected from the quartz boat and washed with nitric acid for 24 h in order to remove the ZnO nanoparticles. Finally, the pure GCs were obtained after 5 times washing with copious amount of water and ethanol repeatedly and dried at 60 °C for 4 hours in vacuum oven. Graphene capsules/Fe3O4 composites: the composite was prepared using 2.7 g of FeCl3·6H2O and 5 g sodium acetate (NaAc) by dissolving in 70 mL and 30 mL of ethylene glycol to form transparent solution, respectively. The GCs were immersed in the solution of FeCl3,



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followed by ultrasonication for 5 min. The GCs and Fe3+ are in molar radio of 1:5 (GF15), 1:3 (GF13), 1:1(GF11), 3:1(GF31) and 5:1(GF51), respectively. Later on, the NaAc solution was dropped into the above suspension slowly with continues magnetic stirring. The mixture was then sealed in a Teflon-lined stainless steel autoclave and aged at 200 °C for 24 hours. After natural cooled down to room temperature, the as-synthesized products were centrifuged and washed with distilled water and absolute ethanol repeatedly for three times and finally dried in vacuum oven at 60 °C for 4 hours. Characterization: the morphological and structural features of pure GCs, Fe3O4 nanoparticles and their composites were characterized using a field emission scanning electron microscope (FE-SEM, Fei, Inspect-F) with an accelerating voltage of 20.0 kV, transmission electron microscopy (TEM, Fei-F200) at an accelerating voltage of 200 kV, and x-ray diffraction spectrometer (XRD, Panalytical X'Pert PRO diffractometer with Nifiltered, the Netherlands). To evaluate the microwave absorption properties of original GCs and Fe3O4/GCs composites, their mixtures with paraffin were pressed into toroidal shaped samples of 7.0 mm outer diameter and 3.4 mm inner diameter, respectively. The electromagnetic parameters of the samples with 30 wt.% Fe3O4 or GC-related materials were measured at 2–18 GHz with an AV3618 network analyzer. The reflection losses R (dB) of the composites were calculated according to the transmission line theory, using the measured data of relative complex permeability and permittivity.


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Figure 1. The schematic presentation for the preparation route of GCs/Fe3O4 composites. 3. RESULTS AND DISCUSSION The synthesis process of Fe3O4/GCs composites is depicted in Figure 1, where ZnO nanoparticles were used as structure defining substrate as well as catalyst to decompose the carbon source in order to prepare GCs. The nitric acid was used to completely remove the ZnO nanoparticles to obtained pure GCs as host for magnetic nanoparticles. The Fe3O4 nanoparticles were decorates on the inner and outer surfaces of GCs by utilizing the hydrothermal method.26 Thus, as-synthesized hybrid has a unique morphology as the nanoparticles of Fe3O4 are decorated both inner and outer side of the GCs wall that will provide the strong synergism among the nanoparticles as well as nanoparticles and GCs for better



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microwave absorption through dual loss mechanism e.g. dielectric loss and magnetic loss. Further the Fe3O4 nanoparticles are connected through the GCs backbone which will reduce the inter-particle resistance and bring a strong correlation among the particles. The structural features of both GCs and GCs/Fe3O4 composites were performed using XRD analysis. The XRD pattern of pristine GCs is of typical quasi-graphene structure, which shows two characteristics peaks around 26° and 41° according to the standard card No. 00-001-0640 as shown in Figure 2. Moreover, no peaks were detected for ZnO which indicate the high purity of as-synthesized GCs after acid treatment. However, the addition of the Fe3O4 nanoparticles to the GCs results in very strong and sharp diffraction peaks correspond to the Fe3O4 according to the standard card No. 01-075-0033, suggesting their high crystallinity. Interestingly, the diffraction peaks for graphene are not appeared in case of Fe3O4/GCs composites because of the highly strong and sharp reflection from of Fe3O4 and slightly lower crystallinity of GCs.

Figure 2. The XRD patterns of GCs, Fe3O4 and Fe3O4/GCs composites (GF11, GF13 and GF15).


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The morphological and microstructures of the GCs and GCs/Fe3O4 composites were carried out using SEM, TEM and HRTEM studies as shown in Figure 3 and S1. From Figure 3a&b, it is notable that the as-synthesized GCs exhibit a variety of morphologies including rod, triangular prism and cubic based on the morphological diversity of ZnO nanoparticle used as substrate and catalysts. Thus, from these results it can be concluded that the morphology of the GCs can easily be tuned using the developed method, simply by selecting the required shape of catalyst nanoparticles which will define the morphology of the GCs. From the TEM images it is obvious that the GCs are completely hollow and the catalyst particles are completely removed by the acid treatment, which assures the high purity of the as-synthesized GCs for further use. It is found that the sizes of GCs are within the range of 200-500 nm with an average size of ~250 nm. To develop the functional materials, GCs were decorated with the Fe3O4 nanoparticles through hydrothermal reaction as shown in Figure 3c and S1. From Figure S1 it is clear that after decorating the nanoparticles the GCs maintain their original shape and 3D characteristics for the GCs/Fe3O4 composites. The high magnification FESEM studies have further revealed that Fe3O4 particles are closely attached to GCs surface (Figure S1(c)), that made the GC surface relatively rough. The rough surface along with the strong attachment of particle to GCs are witnessed that in-situ growth of Fe3O4 nanoparticles on GCs is an effective approach to develop better synergistic effect among the components, not by simple mixing of two components. Figure 3c shows the existence of the Fe3O4 nanoparticles on the surface of GCs, indicating that Fe3O4 nanoparticles have successfully grown on the outer and inner surfaces of GCs, additional some nanoparticles are also present in the core of the GCs. It can be speculated that some Fe3+ ion migrate to the inner part of the GCs to that were



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converted to Fe3O4 under high pressure and temperature. The as-prepared GCs/Fe3O4 composites have a uniform grain diameter distribution of Fe3O4 nanoparticles with an average size of about 28 nm. Furthermore, from the High magnification TEM image it is clearly that GCs have uniform wall with thickness of about 10 nm. The composites with different ratios of GCs and Fe were prepared to adjust their microwave absorption ability. The microstructure of the composite was analyzed by evaluation both the walls of the GCs and crystal structure of the nanoparticles. The Figure 3d depicts the crystal structure of the Fe3O4 nanoparticles, the lattice spacing of 0.48 nm is found that corresponds to the plane (111) of Fe3O4 according to the standard card No. 01-075-0033.27 Similarly, the lattice spacing of 0.34 nm is observed for GCs walls that corresponds to (002) plane of graphite in accordance with the standard XRD card No. 00-001-0640, shown in Figure 3e. Furthermore, the corresponding fast Fourier transform (FFT) studies have further proved that the Fe3O4 nanoparticles have high crystallinity corresponding to the bright electron diffraction spots (Figure 3f). Beside this some diffused electron diffraction rings are found that might be corresponds to relatively low crystalline GCs in the composite structure. Thus, these results are well-matched with the XRD studies that Fe3O4 nanoparticles have been decorated on the GCs inner and outer surface for better synergistic effects to improve the functionality of individual components.


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Figure 3. The TEM images of (a, b) GCs and (c) GCs/Fe3O4 composites of GF11. The HRTEM images of (d) Fe3O4 nanoparticles and (e) GCs walls. (f) FFT pattern for GF11 composites.



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In general, high-performance microwave absorption usually comes from efficient complementarities between complex permittivity and permeability of materials. Singletyped dielectric loss or magnetic loss in materials results in a weak electromagnetic impedance matching. GCs mainly possess the dielectric loss for microwave absorption, whereas Fe3O4 is a typical magnetic material. Then Fe3O4/GCs composites have the potential to exhibit excellent EM absorption properties. Most of the related literatures cover the wax loading in the range of 70%-90% and it found that the amount of wax used for measurement has great effect on the EM properties. Shibing Ni et al. reported the influence of wax loading and concluded that the complex permittivity and permeability of the composite of paraffin wax and Fe3O4 increased linearly with the increase in the volume fraction of Fe3O4.28 Interestingly, the calculated reflection loss in the wax/Fe3O4 composite reaches a maximum value of −21.2 dB with 70% volume fraction of wax. Thus, we have carried out the measurement for the EM property at the 70% mass percentage of GF composite, which is required for practical applications. To investigate the microwave absorption properties of composites, the real part and imaginary part of relative complex permittivity and permeability for the Fe3O4, pristine GCs and their composites in a frequency range of 2-18 GHz are measured, as shown in Figure 4. The dielectric loss of the original Fe3O4 is negligible and magnetic loss dominates the loss mechanisms. In addition, it can be found that two peaks appear at 9.3 GHz and 16.5 GHz for ε′′ and dielectric loss factor for GF11 therefore exhibits a significant similar fluctuation in the range of 2–18 GHz. From the dielectric curves, the GF11 has the stronger fluctuation than pure GCs, GF13, GF15. The resonance peak amplitudes of GF31 and GF51 become smaller than other composites; however, the peak also shifts when adjusting the ratio of


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GC to Fe3O4. It is found that the resonance peak is mainly raised from the pure GCs with hollow structure, therefore an obvious resonance peak resent in all the GCs-dominant composites and pure GCs. Thus, the resonance peak can be enhanced by adjusting ratio of GCs to Fe3O4. Thus, the resonance peaks are probably associated with the interfaces between Fe3O4 nanocrystal and hollow GCs, which is due to the displacement current lag caused by the interface.8, 29 The analysis based on the Debye theory30 and free electron theory31, two important factors are proposed to be accounting for dielectric loss. One factor is the combined loss of the dipole polarizations and interfacial polarizations. The former probably originates from defects in GCs materials, while the latter comes from the existence of the large amount of interfaces among Fe3O4 nanocrystals and the interfaces between Fe3O4 nanocrystals and GCs materials. The beautiful designing of the Fe3O4 nanocrystals on inner and outer surfaces of GCs results in large number of interfaces that cause the interfacial polarization associated with relaxation could also give rise to dielectric loss.32-34 The second factor is the contribution of conductivity loss originating by the GCs, aaccording to the free electron theory ε′′ ≈ 1/2πε0ρf, where ρ is the electrical resistivity. Thus, the graphene layer of GCs greatly increased the conductivity of the composites, resulting in an enhanced conductance loss.



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Figure 4. Measured relative (a, b) complex permittivity, (d, e) complex permeability and (c) dielectric and (f) magnetic loss values of pure Fe3O4 nanoparticles, pristine GCs and Fe3O4/GCs composites. To investigate the microwave absorption property of these materials, the reflection loss (RL) were calculated according to the transmission line theory.35 Z Z μ /ε tanhj

√μ ε

" %"

RL 20log "#$ '"& #$

&

(1) (2)

where Zin is the input impedance of the absorber, Z0 is the impedance of free space, µr is the relative complex permeability, εr is the complex permittivity, f is the frequency of microwaves, d is the thickness of the absorber and c is the velocity of light. Figure 5a shows a comparison of calculated RL curves in the frequency range of 2-18 GHz for the composites, original GCs and Fe3O4 nanoparticles with a thickness of 3.5 mm. The minimum RL of the Fe3O4, GCs and the Fe3O4/GCs composites are about -5 dB, -19 dB and -32 dB, respectively. To investigate the RL performance of the products in detail,


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three-dimensional RL values of the products are shown in Figure 5b-d. The minimum RL of the GF11 composites is -32 dB with thickness of about 3.5 mm at 8.76 GHz and the absorption bandwidths with RL lower than -10 dB is from 5.4 to 17 GHz for an absorber thickness of 2.5-4 mm as shown in Figure 5d. Moreover, it can be seen that both minimum RL value and the absorption bandwidth have enhanced a lot compared with Fe3O4 and GCs, demonstrating that this novel Fe3O4/GCs composite structure improve the microwave absorption property of GCs through multiple advantages and mechanisms. Firstly, it is generally known that to obtain an effective microwave, efficient complementarities between the relative permittivity and permeability should be satisfied.36 The loss mechanism of Fe3O4/GCs nanoparticles consists of both magnetic and dielectric loss and its impedance characteristic has been significantly tuned compared with GCs and Fe3O4. Secondly, interfaces around the nanoparticles and the polarization of free carriers in graphene result an enhancement in the microwave absorption property of the composites.19 The charge transfer between GCs and Fe3O4 also occurs with little hindrance. Therefore, the interfacial polarization and associated relaxation should contribute to the enhanced EM absorption properties. Conventionally the relaxation process, which can be described by a Cole–Cole semicircle37, has an important influence on the permittivity behavior of microwave absorption materials.



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Figure 5. (a) The microwave RL curves of pure Fe3O4, GCs and GF11 (GC:Fe=1:1) composite in the frequency range of 2-18 GHz. The simulated curves for electromagnetic wave loss of (b) Fe3O4, (c) pristine GCs and (d) GF11 composite. According to the Debye dipolar relaxation30, 38, 39, the relative complex permittivity (εr) can be expressed by the following equation: , %,

. ε ε( ) iε(( ε+ ) /'012

&

(3)

where τ0, εs, and ε∞ are the relaxation time, the static dielectric constant, and the dielectric constant at infinite frequency, respectively. From Eq. (3), it can be deduced as given below.


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, %,

ε( ε+ ) - . 3 /'12

(4)

&

ε((

12& ,- %,.

(5)

/'12& 3

According to Eqs. (4) and (5), the relationship between ε' and ε" can be further deduced, ε( −

,- ',.

) ε((

,- %,.

(6)

Thus, the plot of ε' versus ε" is a single semicircle, which is usually defined as a Cole– Cole semicircle and each semicircle corresponds to one Debye relaxation process. Plots of ε" versus ε' for Fe3O4, GCs, and their composites are shown in Figure 6, where both large and small Cole–Cole semicircles were found. For Fe3O4, a relatively large smooth semicircle accompanied by a tiny semicircle is found in Figure 6a and this large semicircle becomes more irregular and small semicircles appear for the GF15 and GF13 samples as shown in Figure 6b and 6c, respectively. Interestingly, four small semicircles and a whole circle consisting of two large semicircles are found for GF11 sample as presented in Figure 6d, which consequently indicate the existence of several dielectric relaxation processes. However, only small semicircles are found for GF31 and GF51 samples as shown in Figure 6e and f. Similarly, the middle-scale semicircle and small ones exists for the pure GCs sample, which may suggest that the dielectric relaxation processes happen in three discontinues way, shown in Figure 6g.15 The mechanisms of the permittivity dispersion can be interpreted in terms of the Debye dielectric relaxation model (Cole–Cole model). For the individual component of pure Fe3O4 and GCs, there is single or discontinuous Debye relaxation processes, while for the Fe3O4/GCs composite several continuous Debye relaxation processes. The Debye relaxation processes can be enhanced by adjusting the molar ratio of Fe3O4 to GCs. The existence of interfaces in this heterogeneous composite gives rise to the interfacial polarization or the Maxwell–Wagner



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effect.40 It becomes easy to happen in the 3D GCs with relatively high conductivity due to the accumulation of charges at the interfaces and the formation of large dipoles on Fe3O4 nanoparticles.


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Figure 6. Typical Cole–Cole semicircles (ε" versus ε') for (a) Fe3O4, (b) GF15, (c) GF13, (d) GF11, (e) GF31, (f) GF15 and (g) GCs in the frequency range of 2–18 GHz.



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The magnetic loss is another important factor that contributes to EM wave attenuation in Fe3O4/GCs composites. It is reported that magnetic loss mainly comes from eddy current effect, natural and exchange resonance in the microwave frequency band.15 The eddy current loss can be expressed by the equation of µ”=2πµ0(µ’)2σd2f/3, where µ0 is the permeability of vacuum, d is the thickness and σ is the electrical conductivity of the composite, thus existence of the GCs will bring higher eddy current loss because of their high conductivity and might contribute positively to magnetic loss. The values of µ”(µ’)– 2 –1

f should be constant in case if the magnetic loss only originates from the eddy current

loss. However, in present case the values of µ”(µ’)–2f–1 are not constant which suggest that the magnetic loss does not only originate from eddy current loss. Furthermore, the natural resonance can be another source of the magnetic loss and can be described by the naturalresonance equation as follows: 2π67 89: 9: 4|=/ |/39? @A

(7) (8)

where r is the gyromagnetic ratio, Ha is the anisotropic energy, |K1| is the anisotropic coefficient and Ms is saturation magnetization. As shown in Figure 7, the plots of µ”(µ’)– 2 –1

f

vs. frequency for the GCs, Fe3O4 and their composites differ from each other. For

pure Fe3O4, the value of µ”(µ’)–2f–1 keeps constant at 8-12.8 GHz originating from the eddy current loss only as it drops and then recovers at the range of 12.8-17.2 GHz due to the natural response might be contributed by the small size effect. In contrast, the GCs exhibit the natural resonance in the low frequency range of 2-8 GHz and remained constant over the left range up to 18 GHz. It is worth noting that by combining the eddy current loss bearing Fe3O4 nanoparticles and GCs with magnetic loss mechanism through


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natural resonance mainly, composite comes up with mixed mechanism comprising both eddy current loss and natural resonance phenomenon for magnetic loss in the composites. Thus, the natural resonance can be tuned by changing particle size and anisotropic energy according to the natural-resonance equation for the Fe3O4/GCs composites. On one hand, the smaller size of Fe3O4 gives rise to the increase in anisotropic energy of composite due to the surface anisotropic field by the reduced size effect.40 On other hand, as the Ms value of the Fe3O4/GCs composites should be lower than that of the pure Fe3O4, therefore, the composite will come up with higher anisotropic energy than the pure Fe3O4. Thus, it is well-known that the higher anisotropic energy is very helpful for the improved microwave absorption properties.40 Furthermore, the exchange resonance can also be found in ~4.9 GHz and 6.5 GHz for GCs-based samples GF31 and GF51. The natural response peaks shifts to the 16.3GHz for the Fe3O4-based samples such as GF13 and GF15. Some fluctuations happen for the GF11 sample in 11-17 GHz, which may be caused from the exchange resonance between GCs and Fe3O4 nanocrystals. Interestingly, the negative imaginary permeability of some samples such as GF11 is found between 2 and 18 GHz, which indicates that magnetic energy is radiated from these samples due to the motion of charges among the 3D hollow GCs. As the motion of charges in an electromagnetic field produce an alternating electric field and induces a magnetic field according to the Maxwell equations .37 Furthermore, a series of experiments were conducted to investigate the effect of molar ratio of Fe3O4 and GCs on the microwave absorption property of the composites, since a high percentage of Fe3O4 or GCs would lead to relatively poor microwave absorption because of the imbalance between the dielectric and magnetic loss. Figure 8 shows the reflection loss of GF51, GF31, GF13 and GF15, having the minimum values of -27dB, -



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31 dB, -16 dB and -12 dB, respectively. It can be seen that with the increasing percentage of Fe3O4, the microwave absorption behaviors of GF51 and GF31 perform better than GF13 and GF15. Importantly, when the molar radio of Fe and GCs is of 1:1, the microwave absorption property is best among all the samples, which is due to a suitable EM behavior. Therefore, by decorating the GCs with Fe3O4, the Fe3O4/GCs composite of GF11 and GF31 shows reduced imaginary permittivity and increased imaginary permeability compared to pristine GCs, which helps to improve the level of impedance matching. Thus, the loss in eddy current in the range of 8-12 GHz is observed for all the samples due to their unique structural features, further small size and 3D hollow structure contributes a lot in the enhancement of magnetic loss in the forms of natural and exchange resonance as well as the magnetic loss can be tuned by adjusting the molar ratio of constituents for composite, which proves our composite a versatile material for microwave absorption.

Figure 7. Plots of µ’’(µ’)–2f–1 vs. frequency for the samples of Fe3O4, GF15, GF13, GF11, GF31, GF51 and GCs. 22 Environment ACS Paragon Plus

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Figure 8. 3D representations of RL for (a) GF51, (b) GF31, (c) GF13 and (d) GF15 composites in the frequency range of 2-18 GHz. CONCLUSIONS In summary, we have developed a novel Fe3O4/GCs composite with enhanced microwave absorption property using a combination of CCVD and facile hydrothermal method. The introduction of Fe3O4 nanoparticles around GCs not only significantly improve magnetic loss in form of eddy current effects, natural and exchange resonance for developed composite, but also produces the consequent dielectric relaxation processes, leading to a rational impedance characteristic compared with pure GCs and Fe3O4. Moreover, the microwave absorption property of the composites could be tuned by varying the percentage of Fe3O4 loadings. Better RL of -32 dB, broad effective bandwidth 23 Environment ACS Paragon Plus


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5.4 to 17 GHz for RL lower than -10 dB, low density and good chemical stability of Fe3O4/GCs composites make it to be a promising candidate as a microwave absorber. Due to their better electrical, chemical and mechanical properties these materials can also be applied to other fields such as photocatalysis and energy storage devices as well as developed methods open up an avenue to develop new functional materials. ASSOCIATED CONTENT Supporting Information. The SEM images, complex permittivity and permeability of pure Fe3O4 and Fe3O4/GCs composites. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Xian Jian ([email protected]) * Weidong He ([email protected]) * Nasir Mahmood ([email protected] ) Author Contributions Xian Jian, Biao Wu, Yufeng Wei and Weidong He designed the experiment and carried out synthesis of materials and performed all the characteristic and properties, they also wrote the manuscript. Xiaolin Wang and Nasir Mahmood provided guidance in experiment and in writing manuscript. All the authors discussed the results. ζThese authors contributed equally. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 51402040), the Open Foundation of State Key Laboratory of Electronic 24 Environment ACS Paragon Plus

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