Reduced graphene oxide decorated with in-situ growing ZnO nanocrystals: Facile synthesis and enhanced microwave absorption properties

Reduced graphene oxide decorated with in-situ growing ZnO nanocrystals: Facile synthesis and enhanced microwave absorption properties

Accepted Manuscript Reduced graphene oxide decorated with in-situ growing ZnO nanocrystals: Facile synthesis and enhanced microwave absorption propert...

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Accepted Manuscript Reduced graphene oxide decorated with in-situ growing ZnO nanocrystals: Facile synthesis and enhanced microwave absorption properties Wei Feng, Yaming Wang, Junchen Chen, Lei Wang, Lixin Guo, Jiahu Ouyang, Dechang Jia, Yu Zhou PII:

S0008-6223(16)30539-5

DOI:

10.1016/j.carbon.2016.06.084

Reference:

CARBON 11108

To appear in:

Carbon

Received Date: 9 May 2016 Revised Date:

17 June 2016

Accepted Date: 22 June 2016

Please cite this article as: W. Feng, Y. Wang, J. Chen, L. Wang, L. Guo, J. Ouyang, D. Jia, Y. Zhou, Reduced graphene oxide decorated with in-situ growing ZnO nanocrystals: Facile synthesis and enhanced microwave absorption properties, Carbon (2016), doi: 10.1016/j.carbon.2016.06.084. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Reduced graphene oxide decorated with in-situ growing ZnO nanocrystals: Facile synthesis and enhanced microwave absorption properties Wei Feng, Yaming Wang*, Junchen Chen, Lei Wang, Lixin Guo, Jiahu Ouyang, Dechang Jia, Yu

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Zhou Department of Materials Science and Engineering, Harbin Institute of Technology

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NO.92 Xidazhi Street, Harbin, Heilongjiang Province, P.R.China

Abstract: Reduced graphene oxide (RGO) was decorated with well dispersed ZnO nanocrystals with

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narrow grain size distribution by a facile in-situ growth strategy. Zinc ions were absorbed on graphene oxide (GO) sheets through interaction with functional groups in absolute ethanol. After adding hydroxyl ions, the precursor of ZnO formed on GO sheets and crystallized after thermal treatment during which the GO was reduced. The RGO/ZnO nanocrystals composites were mixed

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with wax with different mass fraction (5, 10, 15, 20 wt.%) to examine the dielectric and microwave absorption properties. The sample loaded with 15 wt.% composites exhibits the most prominent

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microwave absorption properties, with strong absorption (maximum reflection loss of -54.2 dB),

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broad effective absorption bandwidth (6.7 GHz) and small thickness (2.4 mm). The microwave absorption mechanism of the composites mainly based on the conduction loss and dielectric loss was discussed. The RGO/ZnO nanocrystals composites in this study are very promising as strong absorption and light weight microwave absorber.

1. Introduction: Electromagnetic interference especially gigahertz electromagnetic waves arising from the extensive

* Corresponding author. Tel.: +86-451-86402040-8403. E-mail addresses: [email protected] Fax: +86-451-86414291

ACCEPTED MANUSCRIPT utilization of electronic device and communication facilities has promoted the demand of novel high efficient and light weight microwave wave absorption materials[1-3]. In the past decades, magnetic loss materials, such as ferrite[4], carbonyl iron[5], and conductive loss materials such as carbon

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materials[6-8], conduction polymers[9, 10], have been widely used as microwave absorber. Compared with magnetic materials, conductive loss materials have lower density, higher complex

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permittivity values, and better corrosion resistance [11, 12]. Besides, many conductive loss materials such as carbon nanotubes (CNT), carbon fibers (CNF) and graphene have potential of application at

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elevated temperature considering their increasing complex permittivity with growing temperature and thermo stability[13-15].

Among those conductive loss materials, reduced graphene oxide (RGO) has specific high surface areas and carrier mobilities coupled with abundant defects and hydroxyl, epoxy, and carboxyl groups,

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which make it very promising to be light weight and high efficient microwave wave absorber[3, 14, 16]. However, due to the agglomeration effects and poor impedance matching of pristine RGO sheets,

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the microwave absorption efficiency of RGO is still limited. The most promising approach with

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increasing concerns to resolve the problems is decorating RGO with other microwave absorber. Therefore, Fe3O4[17, 18], Fe2O3[19, 20], Co3O4[21, 22], polyaniline[23, 24] and so on have been successively utilized to decorate RGO for improving its microwave absorption properties. The decorated absorbers can vary loss mechanisms and improving impedance matching of RGO. Another approach is constructing unique structure of RGO such as ultralight foam [25, 26]. The intricate reticulated structures consisting of entangled RGO sheets enables long-range electromagnetic induced currents, shortens the impedance gap and results in multiple reflection of

ACCEPTED MANUSCRIPT incident microwave. ZnO, as an important wide band gap semiconductor, has widely applications in optic, optoelectronics and sensors[27, 28]. Recently, it was proved to be an ideal candidate for microwave

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absorbption application on account of its light weight, semiconductive properties and large scale synthesis that can be easily realized[29, 30]. Because of the unique properties of RGO and ZnO as

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well as the synergistic effect between them, their composites have been investigated for many uses, such as photocatalyst[31-33], photoluminescence[34], supercapacitor[35], sensors[36, 37] and so on.

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Lately, their electromagnetic wave absorption properties have also been reported. Wu et al. mixed freeze-dried RGO aerogel and commercial ZnO nanoparticle in distill water with high speed stirring, then investigated the dielectric and microwave absorption properties of the hybrid[38]. Han et al. synthesized RGO-wrapped ZnO hollow spheres using surfactant-assisted solution mixing method,

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which showed enhanced microwave absorption properties[39]. A composite fabricated by mixing RGO and tetrapod-like ZnO exhibited good microwave absorption properties according to Han’s

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research[11]. However, all the composites mentioned above are prepared by solution mixing method

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thus the uniformity and dispersity of the hybrids are limited. It is known that interfacial polarization plays an important role in microwave attenuation, which can be enhanced by increasing the interface between different dielectric in absorber [18, 39]. The uniformity and dispersity of composites are beneficial to increasing of the interface of heterostructure. Meanwhile, the electromagnetic wave impedance matching of RGO can be modulated by doping ZnO on the surface of RGO, which would reduce the reflection of electromagnetic wave. Therefore, the microwave absorption properties of ZnO/RGO composites can be further improved by modifying their microstructures.

ACCEPTED MANUSCRIPT This study expands the interface in ZnO/ RGO heterostructure by decreasing the grain size of the ZnO nanocrystals and avoiding the aggregation of ZnO nanocrystals to improve the interfacial polarization of RGO/ZnO composites through an in-situ growth strategy. In the process, ZnO

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precursor formed on graphene oxide (GO) which had numerous functional groups as reactive sites. Then the ZnO precursor and GO hybrids were annealed in air. After annealing, ZnO precursor

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formed into nanocrystals with grain diameters around 10nm dispersed on RGO uniformly. Meanwhile, the well dispersed ZnO nanocrystals improved the impedance matching of RGO, which

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reduced the reflection of microwave. The experimental results show that the composites has prominent microwave absoprtion properties with small thickness (2.4 mm), low filler loading (15 wt.%), strong absorption (-54.2 dB reflection loss maximum) and a broad effective absorption bandwidth (6.7 GHz). This method is facile and low cost, which can be realized easily in large scale,

2. Experimental

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2.1 Synthesis of composites

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making the resulted composites an ideal candidate as microwave absorber.

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All chemicals were of analytical grade and used directly without further purification. Graphene oxide (GO) was prepared from natural graphite powder by modified Hummers method[40] with the details shown in supporting information. 20 mg freeze-dried graphene oxide was dissolved in absolute ethanol using ultrasonication for 60 min. 220 mg ZnAc2·2H2O was dissolved in 50ml absolute ethanol in an ultrasonic bath. Then the GO dispersion was dropped into the ZnAc2 solution. The mixture was sonicated for 10min. After that 42mg LiOH·2H2O was added into the mixture followed by 10 min sonication. 100ml hexane was added into the above mixture with stirring. The resulting

ACCEPTED MANUSCRIPT solution was left undisturbed for 12 h to ensure the precursor of ZnO nucleates and grows on GO sheets. All of the procedures were undertaken in ice-bath. After 12 h, the resulting precipitation was separated from the supernatant by centrifugation, and then washed by distilled water several times to

thermally treated at 400°C in a flowing N2 for 2 h.

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2.2 Characterization

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remove the unwanted ionic species (Li+, Ac-). Finally, the precipitation was freeze-dried and

The morphologies of the composites were examined by scanning electron microscopy (SEM;

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Nanolab600i, Helios, 20kV, U.S.A.) and transmission electron microscopy (TEM; TecnaiF2F30, FEI, 300kV, U.S.A.). X-ray diffraction tests were carried on X-ray diffractomer (XRD, Empyrean, PANalytical, Netherlands) using Cu Kα (λ=1.54 Å) radiation to examine the phase composition of the composites. X-ray photoelectron spectra were measured using an X-ray photoelectron

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spectrometer (XPS, K-Alpha, Thermo Scientific, USA), in order to determine the functional groups on the composites. Raman spectra were obtained on a Confocal Raman Microscope (Confocal

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Raman Microscope, inVia, Renishaw, U.K.) equipped with a He-Ne laser (λ=532nm). Thermogravimetric Analysis was performed using a Thermal Gravimetric Analyzer (TGA, STA

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449C, NETZSCH, USA) in a temperature range of 25-800°C with heating rate of 10°C min-1 in air. The photoluminescence spectra were obtained at excitation wavelength of 360 nm and 380 nm on a fluorescence spectrophotometer (F-4600, HITACHI, Japan). The composites were mixed with molten paraffin and compressed to standard rings with outer diameter of 7mm, inner diameter of 3mm and thickness of 3mm. And the samples contained different proportion of ZnO/RGO composites (5 wt.%, 10 wt.%, 15 wt.% and 20 wt.%). The complex

ACCEPTED MANUSCRIPT permittivity (εr) of the samples was measured by a waveguide method using a vector network analyzer (VNA, N5230A, Agilent, U.S.A.) at 2-18 GHz band. Microwave absorption properties were calculated according to the transmission line theory.

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3. Results and discussion 3.1 Microstructures characterization

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The morphologies of the RGO/ZnO composites were examined by SEM and TEM. As displayed in Figure 1(a), the composites before thermal treatment show typical wrinkles of graphene oxide. After

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thermal treatment, the composites display layered structures as shown in Figure 1 (b). The ZnO nanocrystals cannot be discerned in SEM images due to being restricted to the resolution of microscopy. Figure 1 (c) and (d) are the TEM images of the composites and its magnification,

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respectively. Many mono-dispersed nanoparticles with narrow diameter distribution in the range of 6-12nm, can be clearly discerned on transparent RGO sheet. Figure 1(e) shows a high-resolution TEM (HRTEM) inserted a fast Fourier transform (FFT) pattern. Two groups of parallel fringes with

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spacing values of 0.28nm and 0.26 nm illustrate (100) and (002) crystalline plane of ZnO phase

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respectively. The selected area electron diffraction (SAED) pattern of the composites in Figure 1 (f) displays the coexistence of diffraction spots of RGO and diffraction rings of ZnO nanocrystals. The diffraction spots of RGO reveal the good crystalline of RGO. And the pattern index of the diffraction rings as shown in Figure 1 (f) manifest the existence ZnO nanocrystals. Figure 2 shows the XRD patterns of the RGO/ZnO composites before and after thermal treatment. On the XRD pattern of the composites before thermal treatment, the diffraction peak appears at 2θ=10.5° corresponding to the (001) plane of GO[35]. The peak is intense and sharp demonstrating the complete oxidation of

ACCEPTED MANUSCRIPT graphite. No diffraction peaks belong to ZnO or Zn(OH)2 suggesting that the precursor of ZnO aggregating on GO may be amorphous. As for the pattern of the composites after thermal treatment, the peak of GO disappears and the broad peak at 2θ=24.2° corresponding to RGO proves the

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reduction of GO. The diffraction peaks at 2θ=31.77°, 34.42°, and 36.25° correspond to (100), (002) and (101) plane, which can be assigned to the hexagonal wurtzite structure of ZnO (P63mc, JCPDS

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card No.36-1451). The average size of the ZnO nanocrystals was estimated by Debye-Scherrer’s formula based on the diffraction peak corresponding to the (110), (002) and (101) plane. According

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with the observed result of TEM.

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to the results calculated by JADE 6.0, the average crystal size of ZnO is 7.2nm, which is consistent

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Figure 1.SEM images of the RGO/ZnO composites before (a) and after (b) thermal treatment; TEM image of the composites (c) and the magnified image (d); A HRTEM image of the ZnO nanocrystal on RGO with inserted FFT pattern (e); SAED pattern of the composites (f)

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Figure 2 XRD pattern of the RGO/ZnO composites before (a) and after (b) thermal treatment

Figure 3 shows Raman spectrum of GO prepared by modified Hummer’s method and the RGO/ZnO composites. The Raman spectrums of GO and the RGO/ZnO composites look similar

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with two distinct peaks. The peak at ~1354 cm-1 named as D band suggest the presence of local defects and disorder. The peak at ~1591 cm-1 called as G band originates from the in-plane vibration of sp2 carbon atoms[41]. The ratio of the intensity of D band and G band (ID/IG) is a measure of

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disorder degree and average size of the sp2 domains in graphite materials[42]. Compared with GO,

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the ID/IG value of the composites increases from 0.85 to 0.92, revealing the increased degree of defects in the RGO/ZnO composites compared to GO. It may be attributed to the interfacial interaction between ZnO and RGO, which may benefit to the microwave absorption properties of the composites [11, 39].

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Figure 3.Raman spectra of graphene oxide and the RGO/ZnO composites

XPS survey scan spectras of the RGO/ZnO composites before and after thermal reduction are shown in Figure S1 in supporting information.They both display the XPS peaks of carbon, oxygen

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and zinc. In order to manifest the reduction of GO after thermal treatment, the high-resolution of C1s XPS spectra was fitting in Figure.4 (a) and (b). Curve fitting of the XPS spectra was performed using

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Gauss-Lorentrzian peak shape after carrying out a Shirley background correction. The fitting peak

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with binding energy of 284.6eV is assigned to C-C bonds, which remain unchanged after thermal treatment. The fitting peaks of samples before thermal treatment with binding energy of 286.4, 287.4 and 288.8eV can be attributed to hydroxyl (C-OH), carbonyl (C=O) and carboxyl (O=C-OH) functional groups, respectively[37, 42]. Epoxide groups (C-O-C) existing in most structural models have a C1s binding energy similar to C-OH[43]. The peaks of C-OH, C=O and O=C-OH drop significantly after thermal treatment suggesting the effective reduction of GO in the composites. The percentage areas for different functional groups aquired from XPS deconvolution analysis are shown

ACCEPTED MANUSCRIPT in Table 1. After annealing, the value of C-C bonds increases from 55.73% to 67.80%, and significant reduction from 25.68% to 13.08% was observed for the value of C-OH group, indicating

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the removal of oxygen functional groups on RGO after annealing in400°C for 2h.

Figure 4 High-resolution of C1s spectra of the RGO/ZnO composites before (a) and after (b) thermal treatment

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Table 1 Percentage areas of different functional groups before and after thermal treatment. ( %)

Before annealing

After annealing

55.73

67.80

C-OH

25.68

13.08

C=O

7.17

6.28

11.24

12.84

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C-C

O-C=O

TGA was performed to examine the mass ratio of ZnO nanocrystals to RGO in composites as shown in Figure 5. The slow decrease of the TG curve before 350°C may be attributed to the removal of residual functional groups on RGO. The dramatical decrease of the curve at about 400°C is due to the decomposition of RGO. After 500°C, the TG curve keep stable revealing the complete removal of RGO. The estimation based on the TG curve indicates that the mass ratio of ZnO to RGO is 55.4:

ACCEPTED MANUSCRIPT 45.6. The mass fraction value of ZnO is less than the calculated value after the added Zn2+ transforms to ZnO completely. It may be ascribed to the limited anchor sites for ZnO precursor to form on GO. The residual Zn2+ was removed in the procedure of centrifugation and washing.

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Assisted by the characterization results above, the formation process can be explained as illustrated in Figure 6. After ZnAc2 was dissolved in GO dispersion, Zn2+ was absorbed on GO sheets through ion exchange with H+ from carboxyl group or coordinate interactions of the hydroxyl

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group[35]. Then the added OH- reacted with zinc ions and after aging for 12 h the precursor of ZnO

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formed on the GO sheets. After washed by distilled water to remove the unwanted ions, the hybrids were annealed to reduce the GO sheet and crystallize the ZnO precursor. Consequently, the functional groups act as anchor sites to enable the formation of ZnO nanocrystals on the surface and

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edges of GO sheets.

Figure 5 TG curves of the RGO/ZnO composites

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Figure 6 A schematic illustration of the formation process of ZnO nanocrystals /RGO composites

3.2 Dielectric and microwave absorption properties

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Figure 7 (a) and (b) show frequency dependence of the real (ε’) and imaginary (ε’’) parts of the relative complex permittivity of the RGO/ZnO dispersed in paraffin with different mass fraction in

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2-18GHz. As shown in figure.7 (a), the value of ε’ of the sample loaded with 5 wt.% RGO/ZnO composites remains almost unchanged as the frequency increases. For the samples loaded with 10

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~20 wt.% composites, the values of ε’ show a declining tendency with increasing frequency, besides a slight rising at ~12GHz of the samples loading 15 wt.% and 20 wt.% composites. Meanwhile, with the increasing mass fraction of the composites, the value of ε’ increases significantly, except for the inconspicuous difference between 10 wt.% and 15 wt.% loading samples. The values of ε’’ also increase with a rise in the mass fraction of composites in the samples. ε' and ε” stands for the polarizability and dielectric loss of a material which mainly depend on dipolar polarization and interfacial polarization at microwave frequency[17]. Thus the dipoles and interfaces in the samples

ACCEPTED MANUSCRIPT increase as the mass percentage enhances resulting to the growth of ε’ and ε”. Except for the 5 wt.% loading sample, the values of ε’’ of other samples respectively have a peak in the range of 8 to 10 GHz. As shown in figure 7 (c), the values of the tangent loss of the samples loaded with 10~20 wt.%

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composites have a peak in the range 10 to 12 GHz. Due to the absence of magnetic substances in the composites, the real and imaginary part of complex permeability of the samples are almost constant

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as shown in Figure S2 in supporting information.

The ε’-ε” curves of samples loaded 15 wt.% and 20 wt.% composites in Figure 7 (d) both show

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several semicircles indicating that Debye dielectric polarization take great part in the mechanism of microwave absorption[30, 39]. According to Debye theory, the relative complex permittivity can be expressed by the following equation:



  =   − " =  + 

(Equation 1)

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In which, f, εs, ε∞, and τ are frequency, static permittivity, relative dielectric permittivity at the high

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frequency limit, and polarization relaxation time, respectively. Thus ε’ and ε” can be expressed as: 

 ε =  +   

  =

   

(Equation 2) (Equation 3)

 

According to equation (2) and (3),the relationship between ε’ and ε” can be described as:   −

    

+ " = 

    

(Equation 4)

Thus the plot of ε’ and ε” would be a semicircle, which is named as Cole-Cole semicircle. Based on the measured data of complex permittivity and permeability, the values of the microwave reflection loss (RL) of the samples loading different content of composites were calculated according to the transmission line theory[44]. The microwave absorption properties were

ACCEPTED MANUSCRIPT estimated based on the RL (dB), described as following equations:  = 20lg "

#$% 

"

(Equation 5)

#$% 

Zin is the input characteristic impedance at the interface between the absorber and air expressed as

*

&'( = ) + ,-.ℎ0 +

 1

√34 4 

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following expression: (Equation 6)

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Where µr and εr are the relative complex permittivity and permeability of absorber, f is the frequency of the electromagnetic wave, c is the velocity of light, and d is the thickness of the

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absorber.

Figure 8 (a) shows the calculated reflection loss (RL) curves in 2-18 GHz frequency range of the samples with different loading concentration from 5 wt.% to 20 wt.% with thickness of 2.4mm. It can be seen that the microwave properties of the samples show strong dependence upon mass

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fraction of the composites. With the increase of the mass fraction of composites, the minimum values of RL of the samples decrease significantly. When the loading concentration reaches to 15 wt.%, the

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minimum value of RL of the sample reaches to -54.2 dB at 15.2 GHz and its effective microwave

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absorption bandwidth is up to 6.7 GHz (from 11.4 GHz to 18GHz) for RL ≤ -10dB. However, when the loading concentration is more than 15 wt.%, the microwave absorption properties decrease. It is known that too high permittivity does harmful to the impedance matching and results in strong reflection of microwave[45]. The high loading concentration of RGO/ZnO composites deteriorates the impedance matching and hinders the microwave absorption of the sample. Therefore the optimal loading concentration of the composites is 15 wt.%.

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Figure 7.Frequency dependance of the real part (a) and imaginary part (b) of permittivity and the tangent dieletric loss (c) for RGO/ZnO composites dispersed in paraffin with different mass

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fraction; (d) ε’-ε” curves of 15wt% samples

The three dimensional and contour plots of RL of the sample loaded with 15 wt.% RGO/ZnO composites versus frequency and thickness is given in Figure 9. And the plots of the samples loaded with 5, 10, 20 wt.% RGO/ZnO composites are shown in Figure S3 in supporting information. The contours all show the optimal thickness of the samples which reduce along with the increase of frequency. The peak shift is attributed to quarter-wavelength attenuation, caused by the inverse phase angle of the reflection microwave from the upper and bottom surface of absorber[46]. These figures

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thickness of absorber.

Figure.8 Reflection loss calculated for the samples loading different content of RGO/ZnO composites

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with a thickness of 2.4mm

Figure.9 Three dimensional and contour plots of the reflection loss of the sample loaded with 15 wt% RGO/ZnO composites versus the frequency and thickness

Table 2 shows the typical conductive loss materials and their composites with their corresponding microwave absorption properties reported in recent literatures. Compared with most of them, the in-situ growing RGO/ZnO nanocrystals composites has more prominent microwave absorption

ACCEPTED MANUSCRIPT efficiency and lower loading concentration, which meet the demand of high efficient and lightweight microwave absorber. The excellent microwave absorption properties of the RGO/ZnO composites can be attributed to these reasons below. Firstly, the in-situ growing strategy avoids the

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agglomeration of ZnO nanocrystals, thus expanding the interface between ZnO and RGO, which gives rise to interfacial polarization usually happening in heterostructure (called as Maxwell-Wagner

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effect)[19, 47]. Secondly, the residual oxygen functional groups (such as hydroxyl, carboxyl and carbonyl) on the thermally reduced graphene, and the lattice defects in the ZnO nanocrystals after

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annealing in inert atmosphere, which act as polarized centers, improve the polarization resulting in higher permittivity. To prove the existence of defects in ZnO the photoluminescence spectras under different excitation wavelength of the pure ZnO nanocrystals are used, considering the fluorescence quenching of RGO and ZnO composites[34, 37]. The pure ZnO nanocrystals are fabricated using the

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same procedure of fabricating the ZnO/RGO composites except for adding GO dispersion. As shown in Figure S4 in supporting information, the broad peak from 500-680 nm does not shift its position

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when the excitation wavelength change from 360 to 380nm indicating the existence of defects in

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ZnO lattice. Thirdly, the conductive network constructed by RGO sheets contributes to the conduction loss, which enhances the microwave attenuation. Meanwhile, the uniformly dispersed ZnO nanocrystals improve the impedance matching of RGO, which reduces the reflection of microwave.

Table 2.The microwave absorption performance of different materials in recent literatures Absorber Absorber

Maximum

Optimum

RL<-10dB

Matrix

Reference content

RL (dB)

thickness(mm)

band width(Hz)

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20 wt.%

-38.0

1.5

4.2

Ref.16

Graphene foam

--

1.6 mg/cm3

-34.0

10

14.3

Ref.25

3D-RGO/Fe3O4

Paraffin

30 wt.%

-27.0

2

5.8

Ref.17

RGO/γ-Fe2O3

Paraffin

45 wt.%

-59.65

2.5

RGO/PANi

Paraffin

10 wt.%

-36.9

3.5

Paraffin

10 wt.%

-25.95

Paraffin

50 wt.%

Paraffin

15 wt.%

RGO/ZnO hollow

RGO/ZnO/Fe3O4

Carbon nanotubes/ZnO

Ref.19

5.3

Ref.23

6.4

Ref.38

2.2

3.3

Ref.39

-35.0

5.0

5.4

Ref.18

-20.7

2.5

4.0

Ref.15

Paraffin

30wt.%

-40.9

2.5

2.4

Ref.47

Paraffin

15wt.%

-54.2

2.4

6.7

This work

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Carbon

15 wt.%

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SiO2

3.0

-45.05

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spheres

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nanoparticle

2.5

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3D-RGO/ZnO

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RGO

nanotubes/Fe3O4/ZnO

RGO/ZnO

nanocrystals

4. Conclusions A ZnO nanocrystals /RGO composite was synthesized successfully by an in-situ growth strategy

ACCEPTED MANUSCRIPT followed with thermal treatment. The ZnO nanoctrystals /RGO composites were mixed with wax loading different mass fraction (5, 10, 15, 20 wt. %) to examine their microwave absorption properties. Both the real and imaginary part of permittivity of the hybrids elevate with the increasing

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mass fraction of the composites. However the sample with medial mass fraction of composites (15 wt. %) has the most excellent microwave absorption properties. The minimum reflection loss reaches

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-54.2 dB at 15.2 GHz and the effective absorption band width is up to 6.7 GHz with the sample thickness of just 2.4mm. The enhanced microwave properties of the composites are mainly attributed

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to the expanded interface polarization at RGO/ZnO interface, the improved impedance matching of RGO resulting from the adhering ZnO nanocrystals, the residual functional groups of the reduced graphene oxide and the defects of ZnO formed in inert atmosphere. This study provides an effective and facile method to produce strong absorption and light-weight absorber for electromagnetic

Acknowledgements

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interference shielding application.

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The partial supports from the NSFC grant nos. 51571077, 51371071 and 51321061, National Basic

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Science Research Program (2012CB933900), the Fundamental Research Funds for the Central Universities (HIT. BRETIII.201202) and the program for New Century Excellent Talents in University of China (NCET-08-0166) are gratefully acknowledged. The authors acknowledge Yang Haoyue and Nishitani Shoichi from The University of Tokyo for their valuable language revise.

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