Synthesis and microwave absorption enhancement property of core–shell [email protected] reduced graphene oxide nanosheets

Synthesis and microwave absorption enhancement property of core–shell [email protected] reduced graphene oxide nanosheets

Materials Letters 157 (2015) 285–289 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet S...

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Materials Letters 157 (2015) 285–289

Contents lists available at ScienceDirect

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

Synthesis and microwave absorption enhancement property of core–shell [email protected] reduced graphene oxide nanosheets Xiao Ding, Ying Huang n, Meng Zong Department of Applied Chemistry and the Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, School of Science, Northwestern Polytechnical University, Xi'an 710072, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 25 February 2015 Received in revised form 28 April 2015 Accepted 1 May 2015 Available online 28 May 2015

[email protected] reduced graphene oxide ([email protected]@rGO) nanocomposites were prepared by combining liquid-phase reduction reaction with hydrothermal reaction. TEM results illustrate that the [email protected] nanoparticles (NPs) are of core–shell structure with an average diameter of about 50 nm. XPS analysis shows the reduction of GO happens in the hydrothermal process. The maximum reflection loss (RL) is  49.4 dB at 8.64 GHz with an optimal thickness of 3.8 mm and the bandwidth (RL o  10 dB) is 3.38 GHz ranges from 6.72 to 10.1 GHz. The microwave absorption mechanism is mainly attributed to the dielectric loss. It is believed that the [email protected]@rGO nanocomposites can serve as an excellent microwave absorbent and can be widely used in practice. & 2015 Elsevier B.V. All rights reserved.

Keywords: [email protected]@rGO Core–shell Carbon materials Nanoparticles Microwave absorption

1. Introduction In recent decades, electromagnetic (EM) absorption materials with a low density, strong absorption and high resistivity in a wide frequency range have been receiving increasing attention owing to the expanded EM interference problems. Metallic magnetic materials, such as Fe–Co and Fe–Ni, are the most promising materials in practical application among the candidates due to high saturation magnetization, small coercive forces, low magnetostriction and high resistivity. But they are easy to oxidize and corrode, researchers generally used anti-oxidation materials (C [1–5], SiO2 [6,7], metal oxides [8–12] or polymers [13]) covering on the surface. However, these researches focus mostly on two-dimensional nanocomposites and only a few studies of three-dimensional materials are reported. In this work, [email protected] NPs with core–shell structure were prepared by the Stöber method [14] and loaded on rGO by hydrothermal route without using any reducing agent.

dispersed in a mixture of ethanol and water by ultrasonication, ammonia (3 mL) and TEOS (0.2 mL) were added dropwise, respectively. The solution was stirred for 12 h at room temperature (RT). The obtained [email protected] NPs were washed with water by magnetic decantation and dispersed in GO solution (1 mg/ml, 50 ml), then transferred into a 100 ml Teflon-lined stainless steel autoclave and kept in an oven at 180 °C for 18 h. After cooling down to RT, the final products were washed with water and freeze dried. 2.2. Characterization

2.1. Preparation of [email protected]@rGO nanocomposites

The obtained product was characterized by X-ray diffraction (XRD, Rigaku, CuKα), Raman (Renishaw Company), X-Ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermofisher Co), transmission electron microscopy (TEM, Tecnai F30 G2, FEI, USA), and vibrating sample magnetometer (VSM, Riken Denshi, BHV525). EM parameters were analyzed using a vector network analyzer (NA, HP8720ES) in the range of 2–18 GHz. The measured samples were prepared by uniformly mixing 10 wt% of the sample with a paraffin matrix and were pressed into toroidal shaped samples (φout: 7.00 mm; φin: 3.04 mm).

FeNi3 NPs were prepared using liquid-phase reduction reaction with some modification [15]. As prepared FeNi3 NPs were

3. Results and discussion

2. Experimental

n

Corresponding author. E-mail address: [email protected] (Y. Huang).

http://dx.doi.org/10.1016/j.matlet.2015.05.130 0167-577X/& 2015 Elsevier B.V. All rights reserved.

The formation mechanism of the synthesis process for the [email protected]@rGO can be explained as Fig. 1. As-prepared graphite

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Fig. 1. Illustration of the synthesis procedure of [email protected]@rGO nanocomposites.

oxide by the Hummers' method [16] was dispersed in water to form GO. The FeNi3 NPs were synthesized by liquid-phase reduction reaction and [email protected] NPs were prepared through the TEOS hydrolysis process in the presence of ammonia. The [email protected]@rGO nanocomposites were obtained by the one-step hydrothermal method due to the ionization groups on the edge and surface of nanosheets. Fig. 2a shows the XRD patterns of prepared samples. The characteristic diffraction peak of GO appears at 2θ¼9.38° corresponding to d-spacing of 0.94 nm between the layers of GO, which is due to the formation of the oxygen functionalities groups. For [email protected], the peaks at 2θ ¼44.3°, 51.5°, and 75.8° show agreement with those of the JCPDS (no. 38-0409) data with cubic phase, no other peaks can be detected indicating that the SiO2 in [email protected] is amorphous. For [email protected]@rGO, a broaden peak of GO shifts to about 25° indicating the successful reduction of GO formation of graphitic structures. Raman spectra of samples are shown in Fig. 2b. It is clear that there are two high peaks centered at 1352 cm  1 (D band) and 1578 cm  1 (G band) for GO. The D band is related to the defect or disorder in the graphitic structure [17] and the ratio (ID/IG) is a measurement of disorder in structure. The ID/IG is 0.83 for GO and increase to 0.99 for [email protected]@rGO; an increase in the ratio is assigned to the restoration of the sp2 network upon reduction [18] by the hydrothermal process. The magnetic properties of samples are measured with VSM at RT. The hysteresis loops (Fig. 2c) show S-like, which are typical soft magnetic materials. The saturation magnetization (Ms), coercivity (Hc), and remnant magnetization (Mr) of FeNi3 are 96.8 emu/g, 131.2 Oe, and 10.1 emu/g, respectively. The Ms value of [email protected] (75.8 emu/g) is lower due to the amorphous SiO2 layer is covered on the NPs. For [email protected]@rGO, the decrease of Ms can be attributed to the existence of rGO [19]. Surface composition analysis

of GO and [email protected]@rGO was characterized by XPS. The survey scan shows that [email protected]@rGO is composed of C, Fe, Ni, and Si elements (Fig. 2e). Fig. 2f shows the C 1s spectra, peaks centered at 284.6, 286.6, 287.6, and 289.6 eV corresponding to the C–C/C ¼ C, C–O, C ¼O and O–C ¼O groups, respectively. Compared with C 1s spectra of GO (Fig. 2d), the oxygen content of [email protected]@rGO decreases sharply and further indicates an excellent reduction of GO. Fe 2p spectrum (Fig. 2g) located at 712.0 eV and 724.7 eV are attributed to the characteristic doublets of Fe 2p3/2 and Fe 2p1/2, respectively. Fig. 3h shows that the core level binding energy at 856.4 eV and 874.2 eV are assigned to the Ni 2p3/2 and Ni 2p1/2, respectively. Si 2p spectrum in which the peak located at 102.7 eV is shown in Fig. 3i. The results illustrate that the [email protected]@rGO nanocomposites are formed. Fig. 3(a–c) shows the TEM images with different magnifications. It is clear that the [email protected] NPs with a size of about 50 nm are well distributed and decorated on GO nanosheets. No nanoparticles are found outside the nanosheets. The selected rectangular area with red borders in Fig. 3a is enlarged and shown in Fig. 3b; it is further demonstrated that the [email protected] NPs are of core–shell structure with fuscous centers and light-colored sides. The enlarged selected area of Fig. 3b is observed and shown in Fig. 3c. The thickness of rGO nanosheets, which is calculated by measuring a wrinkle on nanosheets (marked with red arrow), is 0.67 nm, indicating that the GO is two layers in the nanocomposites. The HRTEM image in Fig. 3d shows the crystallite structure with periodic fringe spacing of 0.20 nm agree well with the (111) plane of the cubic FeNi3 NPs. The SEM image shows the thickness of SiO2 shell is about 5–7 nm. SAED pattern (Fig. 3e) shows the (111), (200), (220), and (222) planes of cubic FeNi3, suggesting the crystalline nature of these NPs. The EDX results (Fig. 3f) confirm the presence of C, O, Fe, Ni and Si elements.

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Fig. 2. (a) XRD patterns of GO and [email protected], [email protected]@rGO; (b) Raman spectra of GO and [email protected]@rGO; (c) Magnetization curves of FeNi3 and [email protected], [email protected]@rGO at RT (the inset is from  500 to 500 Oe); XPS spectra: C 1s spectra of GO (d), survey (e), C 1s (f), Fe 2p (g), Ni 2p (h) and Si 2p (i) of [email protected]@rGO.

The RL at a given frequency range is calculated by the following equations [20]:

Z in − 1 Z in + 1

(1)

μ r /εr tan h [j (2πfd/c ) ] εr μ r

(2)

R L (dB) = 20 log

Z in =

where Zin is the input impedance, ƒ is the frequency, d is the thickness and c is the velocity of EM waves in vacuum. Fig. 4a and b shows the RL of the [email protected] NPs and [email protected]@rGO with different thicknesses, respectively. For [email protected], the maximum RL is about  3.91 dB at 8.64 GHz. The [email protected]@rGO nanocomposites exhibit the maximum RL of  49.4 dB at 8.64 GHz with the thickness of 3.8 mm and the bandwidth corresponding to the RL below  10 dB is 3.38 GHz (6.72 to 10.1 GHz). It is clear that [email protected]@rGO nanocomposites exhibit enhanced EM absorption properties. The real part (ε′, μ′) and the imaginary part (ε″, μ″) represent the storage and dissipation of EM energy, respectively

[21]. The lower ε″ indicates the higher electric resistivity [22]. All values of the real part are higher than those of imaginary part (Fig. 4c and d), implying the tangent values being less than 1.0. The tan δE values are greater than tan δM in 3–18 GHz, suggesting that the dielectric loss makes a major contribution to EM loss. For [email protected]@rGO, the interface of two media having different dielectric constants leads to interfacial polarization [23]. In addition, the GO forms a conductive network and the existence of residual defects and groups in rGO can act as polarized centers [24], which are beneficial to absorbing EM energy.

4. Conclusion In summary, [email protected] nanocomposites with core–shell structure were successfully synthesized by liquid-phase reduction reaction followed coating SiO2 shell, and then loaded on rGO nanosheets through the hydrothermal process without reducing

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Fig. 3. TEM (a–c), HRTEM (d) images (inset is SEM image of [email protected]), SAED (e) and EDX patterns of [email protected]@rGO. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Fig. 4. Reflection loss curves of [email protected] (a) and [email protected]@rGO (b); frequency dependence of the complex permittivity (εr ¼ ε′ jε″), permeability (μr ¼ μ′  jμ″) (c) and the loss tangent (d) of [email protected]@rGO.

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agent. The paraffin composite composed of 10 wt% [email protected] @rGO exhibits excellent microwave absorption properties. The maximum RL is 49.4 dB at 8.64 GHz with the thickness of 3.8 mm.

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