Ni0.4Zn0.4Co0.2Fe2O4 nanocomposites

Ni0.4Zn0.4Co0.2Fe2O4 nanocomposites

Author’s Accepted Manuscript Fabrication and microwave absorption of reduced graphene oxide/Ni0.4Zn0.4Co0.2Fe2O4 nanocomposites Peijiang Liu, Zhengjun...

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Author’s Accepted Manuscript Fabrication and microwave absorption of reduced graphene oxide/Ni0.4Zn0.4Co0.2Fe2O4 nanocomposites Peijiang Liu, Zhengjun Yao, Jintang Zhou www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)30158-4 http://dx.doi.org/10.1016/j.ceramint.2016.03.026 CERI12397

To appear in: Ceramics International Received date: 15 January 2016 Revised date: 24 February 2016 Accepted date: 3 March 2016 Cite this article as: Peijiang Liu, Zhengjun Yao and Jintang Zhou, Fabrication and microwave absorption of reduced graphene oxide/Ni0.4Zn0.4Co0.2Fe2O n a n o c o m p o s i t e s , Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.03.026 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 galley proof before it is published in its final citable 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.

Fabrication and microwave absorption of reduced graphene oxide/Ni 0.4Zn0.4Co0.2Fe2O4 nanocomposites Peijiang Liua, b, Zhengjun Yaoa, b,*, Jintang Zhoua, b a

College of Materials and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing

211100, Jiangsu, People’s Republic of China b

Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing

211100, Jiangsu, People’s Republic of China *

Correspondence to: Zhengjun Yao (E-mail: [email protected] Tel: +86 152 5181 6557)

Abstract Reduced graphene oxide/Ni0.4Zn0.4Co0.2Fe2O3 (rGO/NZCF) nanocomposites were synthesized via facile hydrothermal reactions of rGO and metal nitrate. Transmission electron microscopy and scanning electron microscopy revealed that uniform NZCF particles with diameters of approximately 26.6 nm were homogeneously distributed on the rGO nanosheets. The electromagnetic properties and microwave absorption properties were determined of nanocomposites prepared with different rGO-to-NZCF weight ratios (mrGO:mNZCF). When the value of mrGO:mNZCF is 1:10, the rGO/NZCF nanocomposite with a thickness of 3 mm achieved a maximum reflection loss of –57.6 dB at 10.1 GHz and an effective adsorption frequency bandwidth of 4.2 GHz between 8.2 GHz and 12.4 GHz, which almost covers the whole X band. Therefore, the obtained composites are potential candidates for applications in microwave absorption. Keywords: Reduced graphene oxide, In situ reaction, Nanocomposites, Microwave absorption property

1.

Introduction Recently, electronic products and their applications have developed rapidly. Most electrical

devices like televisions, computers, mobile phones, global positioning systems, and household electronical appliances generate electromagnetic interference (EMI), which is a kind of electronic pollution that can harm other electronic devices and humans [1-3] . To overcome the problems caused by electromagnetic interference, great efforts have been made to develop materials that absorb electromagnetic waves in the gigahertz (GHz) range [4]. These materials are widely used in industrial electronics, commercial equipment, scientific electronic devices, and military weapons. Ideal wave absorption materials should be thin and have low densities, strong absorptions, wide absorption frequency bands, and good thermal stabilities [5-7]. Previously, ferrites were widely used as conventional electromagnetic wave absorption materials, because of their low costs, excellent magnetic losses, good stabilities, and low skin effects [8-11]. However, the traditional ferrites have some limitations. For example, they often have high eddy current losses at high-frequency, poor flexibilities, high densities, and narrow wave absorption bands, all of which restrict their utility as microwave absorbers [6, 12]. Ultimately, the design and production of excellent electromagnetic wave absorption materials based on ferrite remains a challenge. In the past few decades, intensive research efforts have focused on combining ferrites with graphite or one of its exotic forms such as expanded graphite, reduced graphene oxide (rGO), graphene, carbon fibers, and carbon nanotubes, because these materials are light weight, flexible, corrosion resistant, and environmentally friendly [13-20]. Of these carbon materials, rGO, a two-dimensional (2D) single layer of carbon, has been particularly effective as a new microwave absorption material because it possesses not only the intrinsic properties of graphite but also unique characteristics like low cost,

facile processability, large surface area, excellent mechanical properties, large aspect ratio and suitable dielectric loss. In addition, rGO can be synthesized from graphene oxide using chemical, thermal and hydrothermal methods. Some residual defects and functional groups remain on the surface of rGO. These features improve the impedance matching of the materials and enhance energy transitions from contiguous states to the Fermi level, while also allowing for relaxations of defect polarizations and of the functional groups’ electronic dipoles [21, 22]. Therefore, because of their excellent properties, rGO-based materials are very promising candidates for microwave absorption materials. In view of that, some efforts have been made to combine ferrites and rGO for the production of electromagnetic wave absorption materials. Qi and coworkers [23] obtained graphene–Fe3O4 composites with a maximum reflection loss of –40.36 dB at a thickness of 5.0 mm at 7 GHz. The effective absorption bandwidth of their composite was approximately 2 GHz. Yang et al. [24] investigated the microwave absorption properties of bowl-like Fe3O4 hollow spheres/reduced graphene oxide composites. Results showed that their as-prepared composites with a thickness of 2.0 mm exhibited the total microwave absorption of – 24 dB at 12.9 GHz, and the absorption bandwidth with reflection loss values less than –10 dB was up to 4.9 GHz (in the frequency range of 10.8–15.7 GHz). Wang and colleagues [21] prepared hierarchical [email protected] [email protected]@MnO2 nanosheet array composites, which exhibited a maximum absorption of –38.8 dB at 15 GHz with a thickness of only 1.8 mm and the effective absorption bandwidth of 5.4 GHz (in the frequency range of 12.3–17.7 GHz). According to these previous studies, lightweight materials with broad absorption bandwidths and excellent microwave absorption properties can be produced through the combination of rGO and ferrites. To further investigate the microwave absorption properties of rGO/ferrite composites, we designed reduced graphene oxide by low-temperature reduction method which could introduce more defect

polarization relaxation and electronic dipole relaxation, and then combined them with Ni0.4Zn0.4Co0.2Fe2O3 (NZCF) ferrite nanoparticles via an in situ reaction to form rGO/NZCF nanocomposites. The microstructures, morphologies and electromagnetic properties of the as-prepared composites were investigated. The microwave absorption performances of nanocomposites were also evaluated using transmission line theory. 2. Experimental section 2.1 Materials Nickel nitrate (Ni(NO3)2∙6H2O), zinc nitrate (Zn(NO3)2∙6H2O), ferric nitrate (Fe(NO3)3∙9H2O), and cobalt nitrate (Co(NO3)2∙6H2O) were commercially procured from Aladdin Chemical Reagent, China. Graphite oxide sheets used in present work were prepared from natural graphite powder (Sinopharm Chemical Reagent) according to the modified Hummers and Offeman’s method [25]. All of the chemicals and reagents were of analytical grade and used as received. The deionized water was produced in our laboratory and used for all experiments. 2.2 Preparation of reduced graphene oxide sheet To obtain graphene oxide (GO), graphite oxide powders were dispersed in ether, and after mildly ultrasonicated for 2 h, the solution was dried in a vacuum oven at room temperature over 1 day. Then the GO nanosheets were heated from room temperature to 450 oC in the tube furnace under vacuum, isothermal heating for 10 min, to obtain the rGO. 2.3 Preparation of reduced graphene oxide/Ni 0.4Zn0.4Co0.2Fe2O3 (rGO/NZCF) nanocomposites Preparation procedure of rGO/NZCF nanocomposite was as follows: firstly, rGO nanosheets were re-dispersed in 40 mL water to form rGO solution and ultrasonicated for 1.5 h, then the solution was cooled down to room temperature. Secondly, Ni(NO3)2∙6H2O, Zn(NO3)2∙6H2O, Fe(NO3)3∙9H2O, and

Co(NO3)2∙6H2O were dissolved in rGO solution and stirred for a period time. 25% NH3∙H2O was added to the mixture drop by drop until the pH of solution was 11. After that, the solution was transferred to a 100 mL Teflon-lined autoclave and maintained for 12 h at 180 oC. After cooling to ambient temperature, the dark rGO/NZCF composites were collected, washed with deionized water, and dried at 80 oC in air for 12 h. The detailed schematic diagram of the preparation of rGO/NZCF nanoparticles is illustrated in Fig. 1. In addition, the molar ratios of Ni2+:Zn2+:Co2+:Fe3+ in NZCF particles are 0.4:0.4:0.2:2, which gives a composition of Ni0.4Zn0.4Co0.2Fe2O4. The nanocomposites are denoted as rGO/NZCF-10, rGO/NZCF-15, rGO/NZCF-20 and rGO/NZCF-30 which represent the weight ratios of rGO to NZCF of 1:10, 1:15, 1:20 and 1:30. Pristine Ni0.4Zn0.4Co0.2Fe2O4 nanoparticles were also synthesized in a similar method without the addition of rGO. 2.4 Characterization The chemical structures of rGO nanosheets, NZCF particles and rGO/NZCF nanocomposites were characterized by Fourier transform infrared spectra (FT-IR) using a spectrometer (Bruker Vertor 33) in the range of 4000-500 cm-1 with the samples pre-pressed with highly purified KBr into pellets. A high-resolution X-ray photoelectron spectrometer (XPS) (Thermo ESCALAB 250Xi, USA) was employed to investigate the presence of surficial elements of rGO with Al Kα radiation of 1486.6 eV. The crystalline phase structures of as-prepared samples were analyzed by X-ray powder diffraction (XRD) using a Bruker-D8 DISCOVER X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm, US) in the scattering range (2θ) of 10-80o at an accelerated voltage of 40 kV. The morphology and the sized of pre-synthesized samples were examined by a QUANTA 200 scanning electron microscope (SEM). Transmission electron microscopy (TEM) images were taken on a Tecnai 12 microscope instrument. Samples for TEM tests were prepared by ultrasonically suspending the powders in water

and placing the suspension on a Cu grid. For the measurements of electromagnetic parameters, the samples were prepared by homogeneously mixing the composites with paraffin (the weight ratio of the as-prepared powder was about 40 wt%), and then the mixture was pressed into a toroid with an inner diameter of 3.0 mm, outer diameter of 7.0 mm and height of about 2.5 mm. After that, the samples were inserted into a copper holder and connected between the waveguide flanges of instrument. The complex relative dielectric permittivity and magnetic permeability were obtained by Agilent PNA N5224A vector network analyzer in the range of 2–18 GHz using the coaxial-line method. 3. Result and discussion 3.1 Structural characterization of samples In this paper, the rGO nanosheets were obtained by normal thermal reduction process of graphene oxide sheets. It is well known that the surface of GO is covered mainly with oxygen–containing groups such as epoxy, carboxyl, carboxyl and hydroxyl groups [26]. These oxygen–containing groups destroy the big π conjugations above graphene nanosheets, leading to attenuation of the electronic property and increase of dielectric constant [26, 27]. Thus, it’s very important to detect the amount of oxygen– containing groups on the surface of rGO. In our paper, X–ray photoelectron spectroscopy (XPS) was applied to study the surficial elemental composition of rGO. As shown in Fig. 2a, the signals of C 1s and O 1s in XPS survey spectrum appear at binding energy of 286 and 531 eV, respectively. The intensity of C 1s is much higher than that of O 1s, which indicates that the rGO was prepared successfully by thermal reduction method. Fig. 2b shows the raw data and deconvoluted XPS spectrum of rGO in the C 1s region. It can be seen that approximately 4 oxygen-containing groups exist on the surface of rGO. The intensities of C–OH, C–O–C, C=O and O–C=O groups are much lower than that of C–C and C=C groups. The XPS results demonstrate that thermal reduction method can’t eliminate

all the oxygen-containing groups on graphene oxide. In some sense, small amounts of residual oxygen-containing groups can promote the mutual compatibility between rGO nanosheet and ferrite, and increase the defect polarization in nanocomposite, which may bring some advantages for microwave absorption properties.[9, 27] Fig. 3 displays the FT-IR spectra of rGO, pure NZCF, and rGO/NZCF nanocomposites with different compositions in the wavenumber range of 400–4000 cm-1. In the spectra of rGO, there exist a few peaks of oxygen-containing functional groups at around 1710, 1389 and 1210 cm-1, which can be assigned to C=O stretching, O–H deformation and C–O epoxy, respectively. However, the intensities of these oxygen-containing groups are very small, indicating that most of the oxygen-containing groups have been removed during thermal reduction process. In addition, the broad band in the range of 3300– 3600 cm-1 corresponds to most of absorber water molecules and few stretching vibration of O–H. The band at 1620 cm-1 belongs to the skeletal vibrations of unoxidized graphitic domains or the retaining sp2 carbon character of C–C [28]. In the spectra of pure NZCF particles, bands at around 540–600 cm-1 and 400–460 cm-1 are assigned to metallic lattice vibrations that involve oxygen and cations at octahedral and tetrahedral positions, respectively, and the band at 1630 cm-1 is attributed to the O–H bending vibration. For rGO/NZCF-10, rGO/NZCF-15 and rGO/NZCF-20, the bands are quite similar to these of pure NZCF, and most of the bands related to the oxygen-containing groups in rGO vanish in FT-IR spectra. Moreover, there is blue-shifting of bands at 1630 cm-1 compared with pure NZCF, confirming the existence of rGO in the nanocomposites [29]. Fig. 4 demonstrates the X-ray diffraction patterns of reduced graphene oxide, NZCF particle, and rGO/NZCF-10 nanocomposite. It is clear that the broad peak appeared at 2θ = 24.5o is ascribed the (002) reflection plane of graphene sheets, which indicates that most of the oxygen-containing groups

on graphene oxide are removed during reduction process, revealing the typical amorphous carbon nature of rGO. The peaks of the pure NZCF particles are located at 2θ = 18.50o, 30.22o, 35.54o, 43.18o, 53.60o, 57.13o, 62.68o and 74.13o, which can well correspond to the (111), (220), (331), (400), (422), (511), (440) and (533) planes, respectively, demonstrating a typical and single spinel structure with the characteristic of the Fd3m cubic groups. All the observed diffraction peaks of pure NZCF are well matched with the standard reference data (JCPDS, PDF no. 04–009–3215). After reaction with rGO, all peaks in rGO/NZCF-10 nanocomposite correspond to pure NZCF and no obvious characteristic peaks in relation to rGO can be detected, indicating that the intercalation of NZCF nanoparticles between the inter-galleries of the rGO sheets destroy the regular layer stacking of graphene [15]. In addition, the characteristic peaks of NZCF in the rGO/NZCF-10 nanocomposite move in the direction of decreasing 2θ compared with those of pure NZCF, which confirms that the rGO/NZCF composites are prepared due to some proper interactions between rGO and NZCF [16, 30]. As illustrated in Fig. 1, the NZCF ferrite particles which possess positive charges can be easily attached to the negatively charged rGO nanosheets through physical electrostatic adsorption [18, 26]. The average crystallite size (D) of Ni0.4Zn0.4Co0.2Fe2O4 has been calculated by using the Debye Scherrer formula based on the full width at half-maximum of the diffraction peak (311) and is estimated as 28.84 and 22.72 nm for pure NZCF and rGO/NZCF-10, respectively. The slight decrease in the average size of NZCF for rGO/NZCF-10 nanocomposites compared with pure NZCF suggests that the rGO nanosheets not only serve as a platform to support the growth of NZCF nanoparticles, but also prevent NZCF from forming into larger clusters [31]. In order to investigate the content of NZCF nanoparticles covered on rGO, thermogravimetric analysis of the as-synthesized samples were employed from 25 to 900 oC under an air atmosphere. As

shown in Fig. 5, pure NZCF particle, with super high thermal stability, exhibits almost no weight loss over whole temperature range. For pristine rGO, the weight loss is only about 2% up to 400 oC, because most of the oxygen-containing groups are removed during thermal reduction process, and drops dramatically with the increasing of temperature in the range of 450–570 oC, indicating that the carbon skeleton of rGO is subject to pyrolysis [32, 33]. The residual mass is about 4.6 wt%, demonstrating the complete combustion of rGO. In case of rGO/NZCF-10 nanocomposite, thermogram shows two major mass losses corresponding to the loss of different species. The first loss stage is at about 100 oC and due to loss of absorbed water and other volatiles. The second loss at 330–500 oC is mainly attributed to degradation of rGO. In addition, the residual mass of composite is 92.5 wt% at 550 oC, exhibiting considerably higher thermal stability than rGO because of the combination of Ni0.4Zn0.4Co0.2Fe2O3 nanoparticles and rGO. 3.2 Morphological characterization Covering of ferrites over the rGO was carried out using in situ and hydrothermal reactions. The detailed preparation process is schematically presented in Fig. 1. The rGO nanosheets were synthesized from graphene oxide, and then mixed with metal nitrates to form rGO/NZCF nanocomposites by hydrothermal reaction. During the process, the rGO act as an efficient template which possesses negative charges, and the positively charged ferrites can easily form nucleation on the surface of rGO via electrostatic interaction [18]. The microstructural morphologies of rGO and rGO/NZCF-10 composite were studied by SEM. Fig. 6a shows the thermal reduced graphene oxide sheets composed of folded and wrinkled nanosheets that appeared as some isolated lamellar structures and were randomly distributed in a disordered solid, which provides potential sites for ferrites to occupy because there exist enough interfaces between two layers [30]. When in situ synthesis of NZCF was carried out

by hydrothermal method, the formation of NZCF takes place on rGO sheets having homogenous distribution in bunches, as shown in Fig. 6b. These ferrite particles are not single but aggregated into bunches on the silk-like and thin sheets, indicating a strong surficial interaction between rGO and ferrite particles. The results of SEM are quite consistence with previous researches [4, 34]. In addition, the rGO planar size is found up to few micrometers, however the thickness of rGO sheets and particle size of NZCF can’t be possibly explored by SEM technique. Figure 7a and b exhibits the TEM images of pure NZCF particles and rGO/NZCF nanocomposites. As shown in Fig. 7a, large-scale Ni0.4Zn0.4Co0.2Fe2O3 nanoparticles with a relatively uniform size of about 15-35 nm were obtained, and they have a tendency to agglomerate to some extent due to the high surface energy of the ferrite particles and the powerful inherent magnetic interactions among them. When ferrite particles reacted with rGO, many thin nanosheets of rGO decorated by a large quantity of NZCF nanoparticles are detected (Fig. 7b), indicating that these sheets are easily exfoliated by sonication process during the TEM measurement, since the aggregation of rGO layers is found from SEM images (Fig. 6b). Furthermore, it is evident that the NZCF particles deposit on both sides of rGO’s sheets in an orderly, dense and even manner, and both the outlines of rGO and NZCF can be clearly observed, which suggests that NZCF particles have grown on the rGO sheets by an excellent interaction between rGO and NZCF and are uniformly distributed over the surface of rGO as compared to the agglomerated morphology structure of pure NZCF nanoparticles. The histograms of the nanoparticle size distribution are given in Fig. 7c and d. It can be seen that, for both pure NZCF and rGO/NZCF nanocomposite, the average sizes of NZCF are around 25.0 nm and 26.6 nm, respectively. The result is so close to the value calculated from XRD analysis. In addition, the sizes of NZCF for rGO/NZCF nanocomposite are fairly well distributed from 10 to 40 nm compared with pure NZCF

particle, which may be due to the addition of rGO nanosheets. 3.3 Dynamic magnetic properties Fig. 8 shows the frequency dependence of the complex permittivities (εr = ε’–jε”) and complex permeabilities (μr = μ’–jμ”) of the paraffin resin composites in the frequency range from 2 GHz to 18 GHz. It is well known that the real parts of ε’ and μ’ represent the storage ability of electromagnetic energy, while the imaginary parts of ε” and μ” represent the inner dissipation of energy, which results from conduction, polarization, relaxation and resonance mechanisms [35]. As shown in Fig. 8a and b, the ε’ value of pure Ni0.4Zn0.4Co0.2Fe2O3 is nearly independent of the frequency. However, for the rGO, the ε’ values decrease considerably from 58.1 to 13.7 as the frequency increases, exhibiting an obvious dielectric dispersion. This dispersion may have been caused by the lags of polarization and leakage conductance at low frequencies and by the interfacial polarization and corresponding relaxation at high frequencies. The ε” values of NZCF and rGO follow a similar trend to that of ε’. In the case of rGO/NZCF nanocomposites, the ε’ values decrease gradually through the whole frequency range. Meanwhile, their ε” values decrease from 2 GHz to 9 GHz, keep constant from 9 GHz to 14 GHz, and then increase to a certain value after 14 GHz, indicating a resonant behavior [36]. In addition, it can be seen that both values of ε’ and ε” for all composites are larger than those of pure NZCF and smaller than those of rGO, which is due to rGO’s high conductivity. As we all know, a material’s dielectric behavior depends on its electronic, ionic, space charge, interfacial, and orientational polarizations [5, 36]. Since electronic polarization and ionic polarization usually arise in the range of THz and PHz [37], dielectric performances are believed to mainly originate from space charge polarization, orientational polarization, and interfacial polarization. For rGO/NZCF-10, covering the NZCF nanoparticles with more rGO sheets effectively increases its electrical conductivity and enhances its space charge

polarization. Meanwhile, more interfaces are introduced in the rGO/NZCF-10 nanocomposite than in other samples, causing the accumulation of more virtual charges at the interfaces between the materials, which have different dielectric constants, and creating additional interfacial polarization charges (Maxwell-Wagner polarization) [38, 39]. Based on the above discussion, the larger ε’ of the rGO/NZCF-10 nanocomposite is mainly caused by the introduction of rGO because of rGO’s intrinsic dielectric properties, and the ε” is attributed to the improved space charge and interfacial polarization. Fig. 8c and d show the real part (μ’) and imaginary part (μ”) of the samples’ relative permeabilities. For rGO nanosheets, the μ’ and μ” values remain constant in the range of 2–10 GHz and begin to dramatically fluctuate with frequency increase. However, in the case of pure NZCF and rGO/NZCF nanocomposites, it can be seen that all of them show a similar frequency dependence of the μ’, and the values of μ’ gradually decrease in the range of 2–9 GHz and keep constant with minor fluctuations in the 9–18 GHz range. The μ” component is usually used to explain magnetic losses. The μ” value of pure NZCF decrease with the increase of frequency throughout the entire frequency range. For the rGO/NZCF nanocomposites, the μ” values increase with the increasing frequency from 2 GHz to 4 GHz and exhibit broad resonance peak in the 4–7 GHz. It then decreases to a certain value with the frequency increasing to 18 GHz. The broad resonance peak in the 4 GHz to 7 GHz range can be explained by the combinatorial magnetic relaxation of domain-wall resonance movement and spin rotation in the ferrite composite [40]. A similar phenomenon has also been observed for NiCuZn ferrite/Ni/polymer functional composites [41]. Moreover, the μ” values of the pure NZCF particles are higher than those of the rGO/NZCF nanocomposites throughout the entire frequency range studied. This behavior is ascribed to the addition of the rGO nanosheets, which reduces the ability of the NZCF particles to store magnetic energy [42]. Notably, the μ” values of rGO/NZCF composites are negative

in the 14–18 GHz range, because of the electromagnetic field-induced eddy currents. Eddy currents will cause extra magnetic field, which can eliminate the external magnetic field, leading to a negative permeability [43]. 3.4 Reflection loss Microwave absorption properties of samples are represented by their reflection loss (RL), which is calculated using the relative complex permittivity and complex permeability at a given thickness according to transmit line theory [44] as the following equation:

RL (dB)  20log Zin  1/ Zin  1

Zin 



μr /εr tanh j 2πfd/c  εr μr



(1) (2)

where, Zin is the normalized input impedance of the absorber, d is the thickness of the absorbing layers, c is the velocity of the EM waves in free space and f is the frequency of the EM waves. According to Equations 1 and 2, the RL value of –10 dB is equivalent to a 90% attenuation of the EM waves. Accordingly, materials with RL values of less than –10 dB are regarded as suitable EM wave absorbers. The calculated reflection loss curves for all of the samples are shown in Fig. 9. As revealed in Fig. 9a, when the thickness of the absorber is 2.5 mm, the RL of pure rGO is expected to originate mainly from dielectric losses and reaches a maximum RL value of –3.3 dB. Compared with rGO, the RL performances of all rGO/NZCF nanocomposites in the range of 2–18 GHz frequency are enhanced to some extent. It can be found that the maximum RL of the rGO/NZCF-10 nanocomposite with an optimum weight ratio reaches –24.2 dB at 12.6 GHz, and the bandwidth corresponding to RL less than – 10 dB reaches 4.3 GHz (from 10.5 GHz to 14.8 GHz) for a thickness of 2.5 mm. Obviously, the combination ratio of NZCF nanoparticle and rGO sheet is critical to determine the absorption properties. With the increasing of rGO content, the maximum values of RL shift to lower frequencies

for all of the rGO/NZCF nanocomposites except rGO/NZCF-10, which is ascribed to the movement of the matching range of dielectric loss and magnetic loss [45]. To further demonstrate the influence of thickness and frequency on the rGO/NZCF-10 nanocomposite’s absorption properties, a three dimensional (3D) image map of its reflection loss is shown in Fig. 9b. It can be observed that for different thicknesses, the peak values of RL for rGO/NZCF-10 are all in the vicinity of –25 dB. The rGO/NZCF-10 shows the strongest absorption peak at 10.1 GHz with a maximum RL value of –57.6 dB with 3 mm in layer thickness, and the effective adsorption frequency bandwidth (RL < –10 dB) is from 8.2 to 12.4 GHz which covers nearly the whole X band. It is very clear that the composites made of NZCF nanoparticles with a certain amount of rGO nanosheets have the enhanced microwave attenuation properties. In addition, the absorption peaks tend to shift toward low frequency with the increase of absorber thickness. This result indicates that the microwave absorption ability of the rGO/NZCF-10 nanocomposite can be modified at different frequencies by adjusting its thickness. The enhanced absorption properties of the rGO/NZCF nanocomposites are explained by the following reasons. Firstly, incident microwaves are able to be effectively transmitted into the rGO/NZCF absorbers with maximum reflection, that is to say, the nanocomposites are closely impedance matched to free space [46]. Since the permeability value of the rGO nanosheets is much lower than its permittivity value, most incident microwaves are not efficiently transmitted into the absorber, and this will cause strong reflection and weak absorption. After the introduction of ferrite particles, the difference between the relative complex permittivities and the permeabilities of the samples becomes much smaller. Consequently, the microwave absorption performance of the rGO/NZCF composites is enhanced. Secondly, there exist a plenty of interfaces between the rGO nanosheets and the NZCF nanoparticles, leading to an increase in interfacial polarization and the

corresponding relaxation phenomena, which results in the enhancement of dielectric loss. Moreover, according to recent theoretical studies, charge transfer can occur through the interfaces between ferrite particles and graphene sheets due to their different functional theories, which leads to the introduction of free carriers into graphene [47]. In present work, the introduction of free carriers in the rGO/NZCF nanocomposites would vibrate with the stimuli of the microwaves and generate electric polarization in the rGO, which also make a great contribution to the increase of dielectric loss. Thirdly, the magnetic loss originating from the outstanding magnetic properties of the NZCF particles also plays a significant role in enhancing the microwave absorption. In general, the magnetic losses of ferrite mainly contain eddy current loss, hysteresis loss, domain wall resonance loss and natural resonance loss. In a weak applied electromagnetic field, hysteresis losses can be excluded [48]. Domain wall resonance cannot be a source of magnetic loss in our nanocomposite, because the diameter of NZCF ferrite is very small and can be considered as single domain. Therefore, the magnetic loss of nanocomposites is primarily ascribed to the eddy current effect or natural resonance. Eddy loss can be evaluated by C0 (C0 = μ”(μ’)-2f-1). If the magnetic losses of NZCF mainly results from the eddy current effect, then C0 value should be constant even when the frequency varies [32]. The C0 value versus measured frequency for the rGO/NZCF-10 are exhibited in Fig. 10. As can be seen, the value of C0 decreases drastically with increasing frequency, indicating that the eddy current effect is negligible. Therefore, the contributions to the magnetic losses in the nanocomposite are mainly originating from natural resonance. Fourthly, the lattice defects and functional groups in the rGO would lead to self-doping and further induce the generation of dipoles in the rGO, which plays a role in the imaginary permittivity [49]. Moreover, due to the formation of the rGO nanosheets under low temperature thermal reduced process, the rGO can possess more lattice defects and functional groups in the composites, which offers more opportunities

to induce dipole polarization and related relaxation loss [50-52]. In addition, many capacitor-like junctions are formed owing to the increasing mass ratio of the ultrathin rGO nanosheets. These junctions can improve the polarization in the electromagnetic field and further increase the relaxation, which significantly enhanced microwave attenuation of the rGO/NZCF composites. To further give a detailed demonstration of the microwave absorption mechanism as discussed above, a schematics is exhibited in Fig. 11. From all the above results, these rGO/NZCF composites could be used as a microwave absorbing materials. 4. Conclusion In summary, we reported the synthesis of rGO/NZCF nanocomposites with different rGO-to-NZCF weight ratios via a facile in situ reaction process using rGO and NZCF. The structural features of these nanocomposites were investigated using XRD, FT-IR, TGA and XPS. SEM and TEM images indicated that nanometer-sized NZCF particles were uniformly and firmly dispersed on the rGO nanosheets, which is beneficial to the microwave absorption properties. In addition, the rGO/NZCF-10 nanocomposite presented excellent electromagnetic wave absorption properties compared with those of the other samples. The maximum reflection loss of rGO/NZCF-10 nanocomposite was –57.6 dB at 10.1 GHz with 3 mm thickness, and the effective adsorption frequency bandwidth below –10 dB ranged from 8.2 to 12.4 GHz, covering nearly the entire X band. Thus, it is believed that the rGO/NZCF nanocomposites with low densities, small reflections, and broad bandwidths are believed to be promising candidates for microwave absorption applications. Acknowledgements This work was supported by A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Opening Project of Jiangsu Key Laboratory of

Advanced Structural Materials and Application Technology (ASMA201405) and the “Six Talent Peaks” (2015) Project of Jiangsu Province (YPC16005-PT). References [1] Z. Chen, C. Xu, C. Ma, W. Ren, H.-M. Cheng, Lightweight and Flexible Graphene Foam Composites for High-Performance Electromagnetic Interference Shielding, Adv Mater 25 (9) (2013) 1296-1300. [2]

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Figure Captions Figure 1. The schematic diagram of nanocomposite preparation process. Figure 2. (a) XPS survey scan and (b) C 1s spectrum of rGO. Figure 3. Comparison of FT-IR spectra of rGO, pure NZCF and rGO/NZCF nanocomposites with different weight ratios. Figure 4. XRD patterns for rGO, pure NZCF and rGO/NZCF-10 nanocomposite. Figure 5. Thermogravimetric plots of rGO, NZCF and rGO/NZCF-10 in an air atmosphere. Figure 6. SEM images of (a) pristine rGO sheets and (b) rGO/NZCF nanocomposites. Figure 7. TEM images of (a) pure NZCF nanoparticles and (b) rGO/NZCF nanocomposites, and particle size distributions of pure NZCF ferrites and rGO/NZCF nanocomposites. Figure 8. Frequency dependence on (a) real and (b) imaginary parts of the relative complex permittivity, and (c) real and (d) imaginary parts of the relative complex permeability of samples with a thickness of 2.5 mm. Figure 9. (a) Reflection loss curves of samples with a thickness of 2.5 mm in the range of 2–18GHz, and (b) three-dimensional presentation of the reflection loss of rGO/NZCF-10 nanocomposite in the frequency range of 2–18GHz with different thickness. Figure 10. The value of C0 for the rGO/NZCF-10 nanocomposite. Figure 11. Schematic presentation of possible microwave absorbing mechanisms in the rGO/NZCF composites.

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