Sandwich-structured [email protected]@carbon nanocomposites with enhanced electromagnetic absorption properties

Sandwich-structured [email protected]@carbon nanocomposites with enhanced electromagnetic absorption properties

Materials Letters 144 (2015) 26–29 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet San...

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Materials Letters 144 (2015) 26–29

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Sandwich-structured [email protected]@carbon nanocomposites with enhanced electromagnetic absorption properties Ying Huang a,b,n, Lei Wang a,b, Xu Sun a,b a b

Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi'an, P.R China Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, Northwestern Polytechnical University, Xi'an, P.R China

art ic l e i nf o

a b s t r a c t

Article history: Received 10 October 2014 Accepted 5 January 2015 Available online 12 January 2015

Sandwich-structured [email protected]@carbon nanocomposites were prepared by a rational route. Transmission electron microscopy measurements show that an amorphous carbon layer is covered on the surface of [email protected] and the sandwich-structured [email protected]@carbon is formed, and the TGA results indicate that the carbon content is 42.4 wt%. Compared with [email protected], the asprepared [email protected]@carbon nanocomposites exhibit enhanced microwave absorption properties in terms of both the maximum reflection loss value and the absorption bandwidth. The maximum reflection loss of [email protected]@carbon is  30.1 dB at 14.8 GHz with a thickness of only 1.8 mm, and the absorption bandwidths with a reflection loss below 10 dB ranges from 12.1 to 17.5 GHz. & 2015 Elsevier B.V. All rights reserved.

Keywords: Carbon materials Nanocomposites Microwave absorption

1. Introduction In recent years, microwave absorption materials have been drawn much attention owing to increasing electromagnetic interfaces problems. To date, the ideal electromagnetic absorbers are requested to have not only strong absorption and wide absorption frequency range, but also low density and good thermal stability [1]. Graphene, a new crystalline form of a two-dimensional sp2 bond carbon sheet, possesses an excellent thermal conductivity (5000 W m  1 K  1) [2], high specified surfaces area (1000 m2 g  1) [3] and excellent electronic conductivity (6000 S cm  1) [4]. These properties make graphene or graphene-based materials very promising to meet the requirement for the ideal electromagnetic absorbers. However, its high conductivity may degrade the microwave absorption ability, and the microwave absorption property of pure graphene is very poor [5]. Therefore, how to design and prepare good electromagnetic absorbing materials based on graphene still remains a challenge. Recently, it has been found that graphene-based heteronanostructures exhibit enhanced microwave absorption properties due to the presence of the different kinds of functional materials and the formation of heterojunctions at the interface. The formed heterointerface has played an important role in the enhanced absorption properties [6–10]. Herein, sandwich-structured [email protected]@carbon

n Corresponding author at: Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi'an, P.R China Tel.: þ 86 2988431636; fax: þ86 2988492724. E-mail address: [email protected] (Y. Huang).

http://dx.doi.org/10.1016/j.matlet.2015.01.015 0167-577X/& 2015 Elsevier B.V. All rights reserved.

nanocomposites were fabricated and their electromagnetic properties were investigated. The results show that the sandwich-structure exhibits enhanced EM absorption in the terms of both the maximum reflection loss value and the absorption bandwidth. The maximum reflection loss value can reach -30.1 dB at 14.8 GHz with a thickness of only 1.8 mm, and the absorption bandwidths with the reflection loss lower than -10 dB are 5.4 GHz.

2. Experimental All of the chemicals and reagents were purchased from Sinopharm Chemical Reagent Co., Ltd, China and used as received. Deionized water was used for all experiments. Preparation of sandwich-structured [email protected]@carbon nanocomposites: Graphene oxide (GO) was synthesized busing natural graphite flakes according to the literature method [11]. The preparation of [email protected] was carried out by the reduction reaction between FeCl3 and diethylene glycol (DEG) in the presence of GO [12]. Sandwich-structured [email protected]@carbon was synthesized by coating amorphous carbon onto [email protected] through a hydrothermal route using glucose as the carbon source [13]. Briefly, [email protected] (200 mg) was dispersed in 120 mL deionized water and sonicated for 30 min. Then, 0.378 g glucose was added to the solution and stirred for 30 min at room temperature. The solution was transferred to an autoclave and heated to 180 1C for 10 h. The product was centrifuged and washed

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Fig. 1. XRD patterns of GO, [email protected] and [email protected]@carbon (a); TGA curves of [email protected] and [email protected]@carbon (b), XPS spectra of C1s of [email protected] (c) and [email protected]@carbon (d). Inset: XPS spectra of C1s of GO (c).

Fig. 2. TEM images of [email protected] (a) and [email protected]@carbon (b); AFM images of [email protected] and [email protected]@carbon (c).

with deionized water several times and dried in a vacuum oven at 60 1C to obtain [email protected]@carbon. Characterization: The obtained products were characterized by Xray diffraction (XRD, PANalytical, Holland), thermogravimetric analysis

(TGA/DTA92 Setaram II testing system), transmission electron microscopy (TEM, Philips Tecnai-12), field emission scanning-electron microscope (FESEM, Hitachi S-4800), X-Ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermofisher Co). The electromagnetic parameters were analyzed using a HP8753D vector network analyzer.

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The measured samples were prepared by uniformly mixing 25 wt% of the sample with a paraffin matrix and pressed into toroidal shaped samples (φout: 7.00 mm; φin: 3.04 mm).

3. Results and discussion Fig. 1(a) shows the XRD patterns of GO, [email protected] and [email protected]@carbon. For GO, the characteristic diffraction peak appears at around 2θ ¼ 9.81 corresponding to a d-spacing of 0.90 nm, which is due to the formation of the oxygen functional groups between the layers of GO. As seen in the XRD pattern of [email protected], the pattern displays obvious diffraction peaks of Fe3O4, and the peak positions and relative intensities match well with the standard XRD data for magnetite (JCPDS card, File No.190629). In addition, an additional small and broad diffraction peak around 231 corresponds to C(0 0 2) indicates the reduction of GO in the reaction process [12]. Compared with [email protected], no characteristic peaks of other materials can be detected in [email protected]@carbon, which indicates the carbon coating is amorphous. To obtain the carbon coating content, TGA was performed on [email protected] and [email protected]@carbon nanocomposites under air atmosphere at a heating rate of 10 1C min  1, as shown in Fig. 1(b). For [email protected], 3% mass loss around 120 1C and 30% mass loss around 450 1C are attributed to evaporation of absorbed water and decomposition of graphene, respectively. By subtracting the absorbed water, a weight ratio of graphene: Fe3O4 ¼1:2.6 is determined. The losses due to the absorbed water, decomposition of graphene and carbon for the [email protected]@carbon composite are 5 and 57 wt%, respectively. By taking into

account for both the absorbed water weight and the graphene to Fe3O4 weight ratio (1:2.6), a net carbon coating content is determined to be 42.4 wt%. Furthermore, the reduction of GO can be reflected by XPS characterization of the bonding configurations of carbon atom. Fig. 1(c) shows the XPS spectra of Cls spectrum of GO and [email protected] For GO (inset in Fig. 1(c)), it clearly displays a considerable degree of oxidation with four components that correspond to carbon atoms in different functional groups: C-C/C¼C (284.6 eV) in the aromatic rings, C-O (286.5 eV) of epoxy, C¼O (288.3 eV) and O-C¼O (289.1 eV) groups. Compared with GO, the oxygen content of [email protected] decreases rapidly, which indicates a reduction of GO. Fig. 1(d) shows XPS spectra of Cls spectrum of [email protected]@carbon. For [email protected]@carbon, the further decreases of the oxygen content demonstrates the further reduction of graphene in [email protected] nanocomposites and the formation of [email protected]@carbon nanocomposites with sandwich structure [13]. Fig. 2 shows the TEM images of [email protected] (a) and [email protected]@carbon (b), AFM images of [email protected] and [email protected]@carbon (c). As shown in Fig. 2(a), the thin and almost transparent graphene sheet is densely covered by narrowly distributed Fe3O4 nanoparticles with an average size of 5 nm, and neither big conglomerations of Fe3O4 nanoparticles nor large vacancies are observed on graphene sheet. The lattice fringe spacing (0.25 nm) displayed in HRTEM image (right in Fig. 2(a)) is consistent with the lattice spacing of (3 1 1) planes of cubic magnetite. Fig. 2(b) shows the TEM image of [email protected]@carbon. The darker and less transparent substrate indicates that a carbon layer covers [email protected] nanocomposites. The corresponding HRTEM image (the right in Fig. 2(b)) further confirms

Fig. 3. Complex permittivity (a), permeability (b), dielectric loss tangent and magnetic loss tangent (c) from 2 to 18 GHz for [email protected] and graphene[email protected]@carbon nanocomposites with 25 wt%.

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Fig. 4. The reflection loss of [email protected] (a) and [email protected]@carbon (b).

that amorphous carbon is covered on surfaces of [email protected] and the sandwich-structured [email protected]@carbon is formed. The above results are also supported by the corresponding AFM measurement. As shown in Fig. 2(c), the average thickness of [email protected] and is about 7.3 nm and 12.1 nm, respectively. The increase in thickness of [email protected]@C further indicates that sandwich-structured [email protected]@carbon is formed. Fig. 3 shows the complex permittivity (εr ¼ ε‘-jε“) (a), the complex permeability (mr ¼m‘-jm“) (b) and the loss tangent (tanσε ¼ ε“/ε‘, tanσm ¼m“/m‘) (c) of [email protected] and [email protected]@carbon. As shown in Fig. 3(a) and (b), both ε0 and ε″ values of [email protected]@carbon are much higher than that of [email protected], while the μ0 and μ″ for the two nanocomposites exhibit negligible difference. As shown in Fig. 3(c), the dielectric loss is higher than the magnetic loss for both composites, and the dielectric loss of [email protected]@carbon is much higher than [email protected], indicating that dielectric loss makes a major contribution to the electromagnetic loss and [email protected]@carbon nanocomposites exhibit enhanced dielectric losses. Calculation for the microwave absorption of the composites was carried out based on the experimentally determinate complex permittivity and permeability. The reflection loss (RL) can be calculated as   RLðdBÞ ¼ 20log ðZ in  1Þ=ðZ in þ 1ÞÞ ð1Þ While the normalized input impedance (Zin) was calculated by qffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi Z in ¼ μr =εr tanhðjð2π f d=cÞ μr εr Þ ð2Þ where f is the microwave frequency, d is the thickness of the absorb layer, c is the velocity of electromagnetic wave in vacuum, and εr and μr are the complex permittivity and permeability, respectively. As shown in Fig. 4, [email protected] nanocomposites exhibit the maximum RL of -11.7 dB at the optimal sample thickness of 2.0 mm and the RL values under  10 dB absorption frequency range from 15.4 to 16.5 GHz, while the maximum RL value of [email protected]@carbon nanocomposites is 30.1 dB at 14.8 GHz with the thickness of only 1.8 mm and the bandwidth corresponding to the reflection loss below -10 dB is 5.4 GHz (from 12.1 to 17.5 GHz). It is clearly that [email protected]@carbon nanocomposites display enhanced microwave absorption properties in terms of both the maximum RL value and the absorption

bandwidth. The enhanced absorption properties can be attributed to the multi-interfaces and triple junctions ([email protected], [email protected], [email protected]) in the [email protected]@carbon nanocomposites, which are advantageous for electromagnetic attenuation due to the existing interfacial polarization [14].

4. Conclusion In conclusion, the sandwich-structured [email protected]@carbon nanocomposites were fabricated. The nanocomposites exhibit significantly enhanced microwave absorption properties in comparison with [email protected] due to the unique sandwich structures. The maximum RL value of [email protected]@carbon nanocomposites is  30.1 dB at 14.8 GHz with the thickness of only 1.8 mm and the bandwidth corresponding to the reflection loss below  10 dB is 5.4 GHz. It is believed that such nanocomposites will find their wide applications in microwave absorbing area.

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