ACS Appl.Mater.Interfaces,2016,8(9),6101– Fe3O4-GCs Composites Microwave Absorption Properties

Facile Synthesis of Fe3O4/GCs Composites and Their Enhanced Microwave Absorption Properties

Xian Jian,*,?,?Biao Wu,?Yufeng Wei,?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,Chengdu611731,China

?Institute for Superconducting and Electronic Materials,Australian Institute for Innovative Materials,University of Wollongong, North Wollongong2500,Australia

*Supporting Information

experimentally measured electromagnetic properties and simulation studies

of eddy current,natural and exchange resonance,as well as

RL value of?32dB at8.76GHz along with the absorption

10dB.The better performance of the composite based on the

around the composites,and the polarization of free carriers in3D

microwave more e?ectively.These results o?er an e?ective way

research in electromagnetic shielding and microwave absorption.

deposition,Fe3O4,microwave absorption,hydrothermal

1.INTRODUCTION

Nowadays,with the rapid development of communication devices and extensive use of electronic devices,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.However,the present development is not su?cient to overcome the issues raised by the growing usage of communication devices,and further research is required to provide the healthy environment to humankind.Generally,the microwave-absorbing materials can be classi?ed into two categories on the basis of their microwave loss mechanism, likewise dielectric loss and magnetic loss.1,4,5The loss mechanism of dielectric loss materials such as carbon nanotubes(CNTs),carbon nano?bers(CNFs),and graphene is mainly based on the polarization.Although the carbon-based materials possess the advantages of low density,superior mechanical and electronic properties,and great thermal stability,their microwave absorption capability is not good enough,limiting their applications.6?9For example,the graphene had a minimum RL value of?6.9dB at7.0GHz without any e?ective bandwidth that is higher than the required ?10dB;however,the CNT arrays have a minimum RL value of

?24.6at9.0GHz with a bandwidth of2.1from8.2to10.3 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 magnetic metal, most absorbers consisting of a 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?17Among 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,

Received:January13,2016

Accepted:February18,2016

Published:February18,2016

and combination of both dielectric and magnetic loss mechanisms.Recently,researchers have found that by combining the two di ?erent 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 multiwalled carbon nanotubes (MWCNTs)containing Fe particles possessed improved microwave absorption ability as compared to that of the individual components.18Furthermore,it is found that the higher absorption capabilities of the carbon ?metal composites are because of the polarization free carriers in the carbon component and the charge transfer at the metal/carbon interface.19Though most of the research has been concentrated on developing one-dimensional (1D)and 2D carbon materials like CNTs,20CNFs,21and graphene,22it is notable that 3D graphene capsules (GCs)will be more bene ?cial to enhance the microwave absorption because of their special structure that resulted in an adjustable dielectric and magnetic proper-ties.21,23,24Recently,yolk ?shell C@C microspheres have been developed to observe the e ?ect 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.25It is well-known that the electromagnetic absorption is based on the combination of enhanced perform-ance and the e ?ective absorption range (2?18GHz);thus,for practical applications,to reduce the e ?ective RL value less than ?10dB 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 a unique hybrid structure of Fe 3O 4/GCs via catalytic chemical vapor deposition (CCVD)and hydrothermal process as schematically shown in Figure 1.The resulting composite has a sandwiched structure where Fe 3O 4nanoparticles are attached both on the inner and outer sides of the carbon wall of GCs,which brings an exclusive 3D morphology for the composite.Thus,the carbon backbone provides multiple advantages such as it connects the particles and brings e ?ective synergism among the di ?erent components of the composite along with introduction of an additional loss mechanism,i.e.,dielectric loss to the magnetic loss mechanism of the 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 absorption.The resulting composite exhibited a minimum RL value of ?32dB at 8.76GHz as well as lower RL values than ?10dB in the absorption bandwidth range from 5.4to 17GHz.Further dielectric loss analyzed by using Debye dielectric relaxation model and free electronic theory as well as magnetic loss analysis performed by using eddy current e ?ects,natural resonance,and exchange resonance have proven that Fe 3O 4particles and GCs have a strong synergistic e ?ect that brings the 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 de ?ne the special capsule structure.To be precise,200mg of

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 30min,C 2H 2was introduced into

a quartz tube for 30min at a ?ow rate of 50mL/min,and then the 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 24h in order to remove the ZnO

nanoparticles.Finally,the pure GCs were obtained after 5washes with a copious amount of water and ethanol repeatedly and a 4h period of drying at 60°C in a vacuum oven.Graphene Capsules/Fe 3O 4Composites.The composite was prepared

using 2.7g of FeCl 3·6H 2O and 5g of

sodium acetate (NaAc)by dissolving in 70and 30mL of ethylene glycol to form a transparent solution,respectively.The GCs were immersed in the solution of

FeCl 3,followed

by ultrasonication for 5min.The GCs and Fe 3+

are in

a molar radio of 1:5(GF15),1:3(GF13),1:1(GF11),3:1(GF31),and 5:1(GF51),https://www.360docs.net/doc/c117814283.html,ter,the NaAc solution was dropped into the above suspension slowly with continuous magnetic stirring.The

mixture was then sealed in a Te ?on-lined stainless steel autoclave and

aged at 200°C for 24h.After naturally cooling down to room temperature,the as-synthesized products were centrifuged and washed

with distilled water and absolute ethanol repeatedly three times and were ?nally dried in a vacuum oven at 60°C for 4h.Characterization.The morphological and structural features of

pure GCs,Fe 3O 4

nanoparticles,and their composites were

characterized using a ?eld emission scanning electron microscope

(FE-SEM,Fei,Inspect-F)with an accelerating voltage of 20.0kV,

transmission electron

microscopy

(TEM,Fei-F200)at an accelerating voltage of

200kV,and X-ray di ?raction spectrometer (XRD,

Panalytical X ’Pert PRO di ?ractometer with Ni-?ltered,The Nether-lands).To

evaluate the microwave absorption properties of original

GCs and Fe 3O 4/GC composites,their mixtures with para ?n were

pressed into toroidal shaped samples of 7.0mm outer diameter and 3.4mm

inner diameter,respectively.The electromagnetic parameters of

the samples with 30wt %Fe 3O

4or GC-related materials were measured at 2?18GHz with an AV3618network analyzer.The

re ?ection

losses R (dB)

of the composites

were calculated according to

the transmission line theory,using the measured data of relative complex permeability and permittivity.3.RESULTS AND DISCUSSION

The synthesis process of Fe 3O 4/GC composites is depicted in Figure 1,where ZnO nanoparticles were used as structure de ?ning 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 obtain pure GCs as host for magnetic nanoparticles.The Fe 3O 4nanoparticles were decorating the inner and outer surfaces of GCs by utilizing the hydrothermal method.26Thus,the as-synthesized

hybrid

Figure 1.Schematic presentation for the preparation route of GC/Fe 3O 4composites.

has a unique morphology as the nanoparticles of Fe 3O 4are decorating both the inner and outer sides of the GC walls that will provide the strong synergism among the nanoparticles as well as nanoparticles and GCs for better microwave absorption through dual loss mechanism,e.g.,dielectric loss and magnetic loss.Further,the Fe 3O 4nanoparticles are connected through the GC backbone which will reduce the interparticle resistance and bring a strong correlation among the particles.The structural features of both GCs and GC/Fe 3O 4composites were performed using XRD analysis.The XRD pattern of pristine GCs is that of a typical quasigraphene structure,which shows two characteristic peaks around 26°and 41°according to the standard card no.00-001-0640as shown in Figure 2.Moreover,no peaks were detected for ZnO which indicates the high purity of as-synthesized GCs after acid treatment.However,the addition of the Fe 3O 4nanoparticles to the GC results in very strong and sharp di ?raction peaks corresponding to Fe 3O 4according to the standard card no.01-075-0033,suggesting their high crystallinity.Interestingly,the di ?raction peaks for graphene are not appearing in the case of Fe 3O 4/GC composites because of the highly strong and sharp re ?ections from Fe 3O 4and slightly lower crystallinity of GCs.The morphological and microstructures of the GCs and GC/Fe 3O 4composites were carried out using SEM,TEM,and HRTEM studies as shown in Figure 3and Figure 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 the catalyst nanoparticles which will de ?ne 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 ensures the high purity of the as-synthesized GCs for further use.It is found that the sizes of GCs are within the range 200?500nm with an average size of ～250nm.To develop the functional materials,GCs were decorated with the Fe 3O 4nanoparticles through hydrothermal reaction as shown in Figure 3c and Figure S1.From Figure S1it is clear that after decorating the nanoparticles the GCs maintain their original shape and 3D characteristics for the GC/Fe 3O 4composites.The high-magni ?cation FESEM studies have further revealed that Fe 3O 4particles are closely attached to the GCs ’surface (Figure S1c ),that made the GC surface relatively rough.The rough surface along with the strong attachment of particle to GCs are evidence that in situ growth of Fe 3O 4nanoparticles on GCs is an e ?ective approach to develop better synergistic e ?ect among the components,not by simple mixing of two components.Figure 3c shows the existence of the Fe 3O 4nanoparticles on the surface of GCs,indicating that Fe 3O 4nanoparticles have successfully grown on the outer and inner surfaces of GCs;additionally,some nanoparticles are also present in the core of the GCs.It can be speculated that some Fe 3+ions migrate to the inner part of the GCs to that were converted to Fe

3O 4under high pressure and temperature.The

as-prepared GCs/Fe 3O 4composites have a uniform grain

diameter distribution of Fe 3O 4nanoparticles with an average size of about 28nm.Furthermore,from the high-magni ?cation TEM image it is clearly that GCs have uniform wall with thickness of about 10nm.The composites with di ?erent ratios of GCs and Fe were prepared to adjust their microwave absorption ability.The microstructure of the composite was analyzed by evaluation of both the walls of the GCs and crystal structure of the nanoparticles.Figure 3d depicts the crystal structure of the Fe 3O 4nanoparticles;the lattice spacing of 0.48nm is found that corresponds to the plane (111)of Fe 3O 4

according to standard card no.01-075-0033.27Similarly,the lattice spacing of 0.34nm is observed for GC walls that corresponds to the (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 proven that the Fe 3O 4nanoparticles have high crystallinity corresponding to the bright electron di ?raction spots (Figure 3f).Besides this,some di ?used

electron di ?raction rings are found that might correspond to relatively low crystalline GCs in the composite structure.Thus,these results are well-matched with the XRD studies that show Fe 3O 4nanoparticles have decorated the GCs inner and outer surface for better synergistic e ?ects to improve the functionality of individual components.

In general,high-performance microwave absorption usually

comes from e ?cient complementarities between complex

permittivity and permeability of materials.Single-type dielectric loss or magnetic loss in materials results in weak electro-magnetic impedance matching.GCs mainly possess the dielectric loss for microwave absorption,whereas Fe 3O 4is a typical magnetic material.Then,Fe

3O 4/GCs composites have the potential to exhibit excellent EM absorption properties.Most of the related literature studies cover wax loading in the range 70?90%,and it found that the amount of wax used for measurement has a great e ?ect on the EM properties.Shibing Ni et al.reported the in ?uence of wax loading and concluded that the complex permittivity and permeability of the composite of para ?n wax and Fe

3O 4increased linearly with the increase in the volume fraction of Fe

3O 4.28Interestingly,the calculated re ?ection loss in the

wax/Fe 3O 4composite reaches a maximum value of ?21.2dB 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 Fe 3O 4,pristine

GCs,and their composites in a frequency range 2?18GHz are measured,as shown in Figure 4.The dielectric loss of the original Fe 3O 4is negligible,and magnetic loss dominates

the loss mechanisms.In addition,it can be found that two peaks appear at 9.3and 16.5GHz for ε″,and dielectric loss factor for GF11therefore exhibits a signi ?cant

similar

Figure 2.XRD patterns of GCs,Fe 3O 4,and Fe 3O 4/GCs composites (GF11,GF13,and GF15).

?uctuation in the range 2?18GHz.From the dielectric curves,the GF11has the stronger ?uctuation than pure GCs,GF13,GF15.The resonance peak amplitudes of GF31and GF51become smaller than other composites;however,the peak

also

Figure 3.TEM images of (a,b)GCs and (c)GCs/Fe 3O 4composites of GF11.The HRTEM images of (d)Fe 3O 4nanoparticles and (e)GCs walls.(f)FFT pattern for GF11

composites.

Figure 4.Measured relative (a,b)complex permittivity,(d,e)complex permeability,and (c)dielectric and (f)magnetic loss values of pure Fe 3O 4

nanoparticles,pristine GCs,and Fe 3O 4/GC composites.

shifts when adjusting the ratio of GC to Fe 3O 4.It is found that the resonance peak is mainly raised from the pure GCs with hollow structure;therefore,an obvious resonance peak present in all the GC-dominant composites and pure GCs.Thus,the resonance peak can be enhanced by adjusting ratio of GCs to Fe 3O 4.Thus,the resonance peaks are probably associated with the interfaces between Fe 3O 4nanocrystal and hollow GCs,which is due to the displacement current lag caused by the interface.8,29The analysis is based on the Debye theory 30and free electron theory;31two 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 GC materials,while the latter comes from the existence of the large amount of interfaces among Fe 3O 4nanocrystals and the interfaces between Fe 3O 4nanocrystals and GC materials.The beautiful design of the Fe 3O 4nanocrystals on inner and outer surfaces of GCs results in a large number of interfaces that cause the interfacial polarization associated with relaxation that could also give rise to dielectric loss.32?34The second factor is the contribution of conductivity loss originating by the GCs,according to the free electron theory ε″≈1/2πε0ρf ,where ρis the electrical resistivity.Thus,the graphene layer of GCs greatly increased the conductivity of the composites,resulting in an enhanced conductance loss.To investigate the microwave absorption property of these materials,the re ?ection loss (RL)were calculated according to the transmission line theory.35μεπμε=??????????

??Z Z j fd c /tanh 2in 0r r r r (1)=?+RL Z Z Z Z 20log in 0in 0(2)Here,the following abbreviations apply:Z in is the input impedance of the absorber,Z 0is 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 2?18GHz for the composites,original GCs and Fe 3O

4

nanoparticles with a thickness of 3.5mm.The minimum RL values of the Fe 3O 4,GCs,and the Fe 3O 4/GC composites are

about ?5,?19,and ?32dB,respectively.To investigate the RL

performance of the products in detail,three-dimensional RL values of the products are shown in Figure 5b ?d.The minimum RL of the GF11composites is ?32dB with thickness of about 3.5mm at 8.76GHz,and the absorption bandwidth with RL lower than ?10dB is from 5.4to 17GHz for an absorber thickness of 2.5?4mm as shown in Figure 5d.Moreover,it can be seen that both the minimum RL value and the absorption bandwidth have enhanced a lot compared with

Fe 3O 4and GCs,demonstrating that this novel Fe 3O 4/GCs composite structure improves the microwave absorption property of GCs through multiple advantages and mechanisms.First,it is generally known that,to obtain an e ?ective microwave,e ?cient complementarities between the relative permittivity and permeability should be satis ?ed.36The loss mechanism of Fe 3O 4/GCs nanoparticles consists of both

magnetic and dielectric loss,and its impedance characteristic

has been signi ?cantly tuned compared with GCs and Fe 3O 4.

Second,

interfaces around the nanoparticles and the polar-ization of free carriers in graphene result an enhancement in the microwave absorption property of the composites.19The

charge transfer between GCs and Fe

3O 4also 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 semicircle,37has an important

in ?uence on the permittivity behavior of microwave absorption

materials.

According to the Debye dipolar relaxation,30,38,39the relative

complex permittivity (ε

r )can be expressed by the following

equation:

Figure 5.(a)Microwave RL curves of pure Fe 3O 4,GCs,and GF11(GC:Fe =1:1)composite in the frequency range 2?18GHz.The simulated

curves for electromagnetic wave loss of (b)Fe 3O 4,(c)pristine GCs,

and (d)GF11composite.

εεεεεε

ωτ=′+″=+?+∞∞i i 1r s 0(3)Here,τ0,εs ,and ε∞are the relaxation time,the static dielectric constant,and the dielectric constant at in ?nite frequency,respectively.From eq 3,it can be deduced as given below.εεεεωτ′=+?+∞∞

1()s 02(4)εωτεεωτ″=?+∞()1()0s 02(5)According to eqs 4and 5,the relationship between ε′and ε″

can be further deduced

εεεεεε′?++″=?∞∞????????????2()2s 2

2

s 2

(6)

Thus,the plot of ε′versus ε″is a single semicircle,which is usually de ?ned as a Cole ?Cole semicircle,and each semicircle

corresponds to one Debye relaxation process.Plots of ε″versus

ε′for Fe 3O 4,GCs,and their composites are shown in Figure 6,where both large and small Cole ?Cole semicircles were found.

For Fe 3O 4,a relatively large smooth semicircle accompanied by

a tiny semicircle is found in Figure 6a,and this large semicircle

becomes more irregular.Small semicircles appear for the

GF15

Figure 6.Typical Cole ?Cole semicircles (ε″versus ε′)for (a)Fe 3O 4,(b)GF15,(c)GF13,(d)GF11,(e)GF31,(f)GF15,and (g)GCs in the frequency range 2?18GHz.

and GF13samples as shown in Figure6b,c,respectively. Interestingly,four small semicircles and a whole circle consisting of two large semicircles are found for GF11sample as presented in Figure6d,which consequently indicates the existence of several dielectric relaxation processes.However, only small semicircles are found for GF31and GF51samples as shown in Figure6e,f.Similarly,the middle-scale semicircle and small ones exist for the pure GCs sample,which may suggest that the dielectric relaxation processes happen in three discontinuous ways,shown in Figure6g.15The 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 Fe3O4and GCs,there are single or discontinuous Debye relaxation processes,while for the Fe3O4/GCs composite there are several continuous Debye relaxation processes.The Debye relaxation processes can be

enhanced by adjusting the molar ratio of Fe3O4to GCs.The existence of interfaces in this heterogeneous composite gives rise to the interfacial polarization or the Maxwell?Wagner e?ect.40It becomes easy to happen in the3D GCs with relatively high conductivity due to the accumulation of charges at the interfaces and the formation of large dipoles on Fe3O4 nanoparticles.

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 the eddy current e?ect,along with natural and exchange resonance in the microwave frequency band.15The eddy current loss can be expressed by the equationμ″=2πμ0(μ′)2σd2f/3,whereμ0is the permeability of vacuum,d is the thickness,andσis 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 ofμ″(μ′)?2f?1should be constant in case the magnetic loss only originates from the eddy current loss. However,in the present case the values ofμ″(μ′)?2f?1are 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 natural-resonance equation as follows:π=

f rH

2

r a(7)

μ

=||

H K H M

4/3

a10s(8) Here,the following abbreviations apply:r is the gyromagnetic ratio,H a is the anisotropic energy,|K1|is the anisotropic coe?cient,and M s is saturation magnetization.As shown in Figure7,the plots ofμ″(μ′)?2f?1versus frequency for the GCs, Fe3O4,and their composites di?er from each other.For pure Fe3O4,the value ofμ″(μ′)?2f?1stays constant at8?12.8GHz originating from the eddy current loss only as it drops and then recovers in the range12.8?17.2GHz due to the natural response that might be contributed by the small size e?ect.In contrast,the GCs exhibit the natural resonance in the low-frequency range2?8GHz and remained constant over the left range up to18GHz.It is worth noting that,by combining the eddy current loss bearing Fe3O4nanoparticles and GCs with magnetic loss mechanism through natural resonance mainly; the composite comes up with a 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 Fe3O4gives rise to the increase in anisotropic energy of the composite due to the surface anisotropic?eld by the reduced size e?ect.40On the other hand,as the M s 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 that of the pure Fe3O4.Thus,it is well-known that the higher anisotropic energy is very helpful for the improved microwave absorption properties.40Furthermore,the exchange resonance can also be found in～4.9and6.5GHz for GC-based samples GF31and GF51.The natural response peaks shift to the16.3 GHz for the Fe3O4-based samples such as GF13and GF15. Some?uctuations happen for the GF11sample in11?17GHz, which may be caused from the exchange resonance between GCs and Fe3O4nanocrystals.Interestingly,the negative imaginary permeability of some samples such as GF11is found between2and18GHz,which indicates that magnetic energy is radiated from these samples due to the motion of charges among the3D hollow GCs.The motion of charges in an electromagnetic?eld produces an alternating electric?eld and induces a magnetic?eld according to the Maxwell equations.37Furthermore,a series of experiments were conducted to investigate the e?ect of molar ratio of Fe3O4 and GCs on the microwave absorption property of the composites,since a high percentage of Fe3O4or GCs would lead to relatively poor microwave absorption because of the imbalance between the dielectric and magnetic loss.Figure8 shows the re?ection loss of GF51,GF31,GF13,and GF15, having the minimum values of?27,?31,?16,and?12dB, respectively.It can be seen that,with the increasing percentage of Fe3O4,the microwave absorption behaviors of GF51and GF31perform better than GF13and GF15.Importantly,when the molar radio of Fe and GCs is1:1,the microwave absorption property is the 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 GF11and GF31shows 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 range8?12GHz is observed for all the samples due to their unique structural features,further small size,and3D hollow structure which contributes a lot toward the enhance-ment of magnetic loss;the forms of natural and exchange resonance as well as the magnetic loss can be tuned by adjusting the molar ratio of constituents for the composite, which proves that our composite is a versatile material for microwave

absorption.

Figure7.Plots ofμ″(μ′)?2f?1vs frequency for the samples of Fe3O4, GF15,GF13,GF11,GF31,GF51,and GCs.

■CONCLUSIONS In summary,we have developed a novel Fe 3O 4/GCs composite with an enhanced microwave absorption property using a combination of CCVD and facile hydrothermal methods.The introduction of Fe 3O 4nanoparticles around GCs not only signi ?cantly improves magnetic loss in the form of eddy current e ?ects,and natural and exchange resonance for developed composite,but also produces the consequent dielectric relaxation processes,leading to a rational impedance character-istic compared with pure GCs and Fe 3O 4.Moreover,the microwave absorption property of the composites could be tuned by varying the percentage of Fe 3O 4loadings.Better RL of ?32dB,broad e ?ective bandwidth 5.4?17GHz for RL lower than ?10dB,low density,and good chemical stability of Fe 3O 4/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 ?elds such as photocatalysis and energy storage devices.In addition,the developed methods open up an avenue to develop new functional materials.■ASSOCIATED CONTENT *Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acsami.6b00388.SEM images,complex permittivity,and permeability of pure Fe 3O 4and Fe 3O 4/GCs composites (PDF )■AUTHOR INFORMATION Corresponding Authors

*E-mail:jianxian@https://www.360docs.net/doc/c117814283.html,

.

*E-mail:weidong.he@https://www.360docs.net/doc/c117814283.html, .*E-mail:nasirm38@https://www.360docs.net/doc/c117814283.html, .Author Contributions

X.J.,B.W.,Y.W.,and W.H.designed the experiment,carried out the synthesis of materials,and performed all the characteristic and properties;they also wrote the manuscript.X.W.and N.M.provided guidance in experiments and in writing the manu-script.All the authors discussed the results.X.J.,B.W.,and Y.W.contributed equally.

Notes The authors declare no competing ?nancial interest.■ACKNOWLEDGMENTS This work was ?nancially supported by the National Natural Science Foundation of China (Grant No.51402040),the Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201411),China Postdoctoral Science Foundation (2015M582539),and the National Hi-

Tech Research and Development Program (863Program)of

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