SiC Nanowires Foam

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Flexible and Thermostable Graphene/SiC Nanowires Foam Composites with Tunable Electromagnetic Wave Absorption Properties Meikang Han, Xiaowei Yin, Zexin Hou, Changqing Song, Xinliang Li, Litong Zhang, and Laifei Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00951 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 23, 2017

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ACS Applied Materials & Interfaces

Flexible and Thermostable Graphene/SiC Nanowires Foam Composites with Tunable Electromagnetic Wave Absorption Properties Meikang Han, Xiaowei Yin*, Zexin Hou, Changqing Song, Xinliang Li, Litong Zhang, and Laifei Cheng Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, 710072, China ABSTRACT Three-dimensional (3D) flexible foams consisting of reduced graphene oxides (rGO) and in-situ grown SiC nanowires (NWs) were prepared using freeze-drying and carbothermal reduction process. By means of incorporating SiC nanowires into rGO foams, both the thermostability and electromagnetic absorption of the composites were improved. It was demonstrated that rGO/SiC NWs foams were thermostable beyond ~630 °C (90% weight retention in air atmosphere). As expected, rGO/SiC NWs foams in the poly (dimethylsiloxane) matrix achieved effective absorption in the entire X-band (8.2-12.4 GHz) with a thinner thickness (3 mm), when compared with those of the pure rGO foams. It is revealed that SiC nanowires with abundant stacking faults, twinning interfaces and bridged junctions play an important role in the enhanced electromagnetic absorption performance, besides the contribution of interconnected graphene networks. Hierarchical rGO/SiC NWs foams not only are efficient absorbers in the critical environments, but also can be applied in photocatalytic and thermal management fields. KEYWORDS: graphene, SiC, electromagnetic absorption, flexible, thermostable

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INTRODUCTION The explosive development of wireless communication equipment and electronic devices has given rise to serious electromagnetic (EM) pollution, justifying the urgent need to expand new EM absorbing materials to meet complex environmental demands.1-7

Carbon-based

materials

gained

extensive

favor

to

achieve

high-performance EM wave attenuation, owing to their light weight, strong dielectric loss and high electronic conductivity. Among the current studies, combining carbon materials (rGO, CNTs, carbon fibers, colloidal carbon, etc.) and magnetic materials (magnetic metals, ferrites and magnetic oxides) is considered to be an efficient route to broaden the effective bandwidth of EM absorbers, such as rGO/FexOy,8-10 rGO/CuS,11 rGO/CoNi,12 CNTs/Fe3O4,13 CNTs/Ni/MoS2,14 carbon fiber/Fe,15 and core-shell structured Fe3O4/C, FeSn2/Sn/C and Ni/C.16-19 It is attributed to the complementation of preferred absorbing frequency ranges between carbon materials and magnetic materials, as well as the optimization of impedance matching with free space. However, the high density, easy oxidation and ferromagnetic characteristic of magnetic materials limit their practical applications. As a result, carbon-based absorbing materials with light weight and wide bandwidth need to be further explored. Recently, 3D graphene (or reduced graphene oxide) foams, which have ultralow density, interconnected conductive network and good compressibility, have attracted much interest in EM absorption.20-24 Typically, Zhang et al. reported excellent EM absorbing properties of macroscopic rGO foams whose qualified absorption


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bandwidth reached 50.5 GHz.25 The reticulated structure was demonstrated to make great contribution on the enhanced EM absorption, because the cell walls of rGO foams arose currents accumulation, leading to EM wave dissipation in the form of thermal energy. However, the porous structure with ultrahigh porosity also resulted in the increase of the absorber thickness. Additionally, the pure rGO foams possess a weak anti-oxidation ability. SiC nanowires which present high strength, good thermal stability and chemical resistivity, are a promising candidate as reinforcement applied in harsh environments.26, 27 More importantly, SiC nanowires with stacking faults exhibit good EM wave absorption performance. The charges separation along the heterointerface of stacking faults leads to strong dielectric resonance under the altering EM fields.28, 29 It also has been identified that SiC nanowires can enhance EM wave absorption of graphene-based ceramics by the construction of hierarchical networks in our previous work.30 Therefore, incorporating SiC nanowires into rGO foams could be an effective approach to enhance the EM absorbing capability and thermal stability at the same time. Herein, we report a feasible strategy based on freeze-drying and carbothermal reduction processes for the synthesis of hierarchical rGO/SiC NWs foams. 3D graphene oxide (GO) foams were obtained by one-step freeze-drying process, then SiC nanowires were embedded in the foams using silicon and silicon carbide powders by a heating process. It is demonstrated that the thermal stability was improved obviously, owing to the dispersive distribution of 1D SiC nanowires in the 3D



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architecture. Furthermore, rGO/SiC NWs foams were infiltrated into the dilute poly (dimethylsiloxane) for the investigation of dielectric and EM wave absorption properties. Compared with the pure rGO foams, rGO/SiC NWs foams significantly decrease the absorbing thickness, and maintain the effective absorption in the whole X-band. The contributions of stacking faults, junction boundaries and porous networks on the enhanced EM absorption were discussed. EXPERIMENTAL SECTION Preparation of GO and rGO foams. In a typical synthesis, GO powders (0.1 g; Nanjing XF Nano Materials Tech Co., China) were dispersed in deionized water (100 mL), and then sonicated for 10 h to obtain stable GO suspension (1 mg/mL). After that, the dispersion was poured into the mold followed by freezing process (-50 °C). Subsequently, the frozen mixture was lyophilized for 18 h to fix the stable GO foam. The rGO foam was obtained by annealing the as-prepared GO foam at 1300 °C for 2 h under a flowing Ar atmosphere. Preparation of rGO/SiC NWs foams. Firstly, the silicon powder (>99.3 purity, Jinan Yinfeng Silicon Products Co. Ltd., China) and silica powder (the mole ratio is 1:1) were mixed by ball milling for 24 h. After that, the mixed powders were dispersed uniformly into an alumina crucible, and the as-synthesized GO foams were set on the powders. The crucible was placed in furnace, and then heated to 1300 °C under a flowing Ar atmosphere at a rate of 5 °C min-1 for 60, 120 and 180 min, respectively. The rGO/SiC NWs foams were obtained after a vapor-solid reaction process. The as-obtained samples were designated as S-60, S-120 and S-180, respectively, which


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are corresponding to the reacted time. Characterization. The morphology of the foams was observed using scanning electron microscopy (SEM; S-4700, Hitachi, 10 kV, Japan) and transmission electron microscopy (TEM; F-30, FEI-Tecnai, 300 kV, USA). During the preparation of TEM specimens, a tiny as-prepared rGO/SiC NWs foam was put into absolute alcohol, and then sonicated for 30 min. The supernatant was dropped in holey support film for TEM observation. The thermostability of rGO and rGO/SiC NWs foams was measured by Thermogravimetry analysis (TGA; STA449F3, Netzsch, Germany) from 30 to 800 °C under air atmosphere. X-ray diffraction (XRD) patterns of the hybrids were recorded on an X′ Pert Pro system (Bruker, D8 Advance, Germany) with Cu Kα (λ = 1.54 Å) radiation. Raman spectra were obtained on a confocal Raman microspectrometer equipped with a He-Ne laser (inVia, Renishaw, Gloucester-shire, U.K.). The surface chemical information of rGO, SiC NWs and rGO/SiC NWs was measured on an X-ray photoelectron spectrometer (XPS; K-Alpha, Thermo Scientific, USA). The pure SiC nanowires specimen for XPS measurement was prepared by annealing the as-prepared rGO/SiC NWs foams at 800 °C under air atmosphere. The as-prepared rGO foam and rGO/SiC NWs foams (S-60, S-120 and S-180) were dipped into the dilute poly (dimethylsiloxane) (PDMS; Sylgard 184, Dow Corning, USA) for dielectric measurement. The foams were filled with PDMS by vacuum impregnation for 15 min, followed by a curing process at 80 °C for 4 h. The effective permittivity of the as-prepared samples (22.86×10.16×2 mm) was measured by the vector network analyzer (VNA; MS4644A, Anritsu, Japan) in the frequency



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range 8.2-12.4 GHz (X-band). The reflection coefficient (RC) values of the composites are calculated from the measured permittivity (details in the supporting information, Figure S1). RESULTS AND DISCUSSION Figure 1 presents the typical morphology of as-synthesized pure rGO and rGO/SiC NWs foams. The pure rGO foam shows a randomly oriented 3D network which is assembled by reduced graphene oxide sheets (Figure 1a). After the heating process with Si and SiO2 powders, it can be seen clearly that SiC nanowires with a high aspect ratio were successfully incorporated into rGO foam in a random distribution (Figure 1b and c). Particularly, in-situ grown SiC nanowires in rGO/SiC NWs foam are joined, implying the construction of 3D network (Figure 1d). The corresponding macroscopic morphology of the samples also changed obviously, as shown in the bottom insets of Figure 1a and b. It can be observed that rGO/SiC NWs foam is ash black which is distinct from the atramentous rGO foam. The as-prepared rGO/SiC NWs foams maintain the good flexibility, as shown in the photograph of a twisted foam (Figure S1). The formation of SiC nanowires in the porous rGO framework is a typical Vapor-Solid (VS) reaction process via the heating treatment.31 The reactions on the it-situ growth of SiC nanowires are as follows: Si (s) + SiO2 (s) = 2SiO (v)

(1)

3SiO (v) +3C (s) = 2SiC (s) + SiO2 (l) + CO (v)

(2)

3SiO (v) + CO (v) = SiC (s) + 2SiO2 (l)

(3)


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Figure 1. SEM images of the pure rGO foam (a), rGO/SiC nanowires foam (b), rGO/SiC nanowires foam with a high magnification (c) and the typical junction of SiC nanowires (d). The insets show the corresponding digital photographs. Figure 2 shows the XRD patterns and Raman spectra of rGO and rGO/SiC NWs foams with different reaction time (S-60, S-120 and S-180). The presence of only a broad peak at around 26.5° is corresponding to carbon phase. The absence of the typical GO peak (~10°) is ascribed to the thermal reduction of GO foams at 1300 °C (Figure 2a).9 When silicon resources were introduced, the samples generated new diffraction peaks at 35.6°, 60° and 71.7°, respectively, which are assigned to the (111), (220) and (311) planes of β-SiC phase (JCPDS card no.29-1129).30 In the case of S-60, S-120 and S-180, the increasing peak intensity of SiC phase indicates the increasing growth of SiC nanowires with the increasing annealing time. As shown in Figure 2b, the peaks at about 1344 and 1582 cm-1 in all the samples are characteristic for the D and G bands of graphitic carbon. It is noteworthy that the sharp peak is detected in ~2665 cm-1 which is assigned to 2D band of carbon. This implies that the rGO sheets



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in the foams are with few layers.32 It is reasonable to conclude that 3D framework of the samples prevents the possible restack of graphene due to π-π interactions, which hinders the full utilization of 2D graphene surface for EM wave absorption.33 Significantly, all the rGO/SiC nanowires foams (S-60, S-120 and S-180) show a higher ratio of D and G band intensities (ID/IG) than the pure rGO foam. It indicates the increasing disorder degree of carbon which is ascribed to carbothermal reaction. This is of benefit to EM wave dissipation due to the enhanced defect polarization. There are few characteristic peaks of SiC nanowires can be detected, which is ascribed to the weak sensitivity of the SiC scattering in Raman measurement.

Figure 2. (a) XRD patterns and (b) Raman spectra of rGO and rGO/SiC nanowires foams (S-60, S-120 and S-180). The TEM analysis brings further insight into the microstructure of the as-prepared rGO/SiC NWs foam, as shown in Figure 3. Figure 3a presents an overall morphology of electron-beam transparent rGO layers and bridged SiC nanowires. The clear streaks coupled with spots indicate high-density stacking faults, while the concentric rings are assigned to carbon, as shown in the selected area electron diffraction (SAED) pattern (inset of Figure 3a). Figure 3b presents high-resolution TEM (HRTEM) image of the twisted SiC nanowire. The stacking faults are


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perpendicular to the growth direction of (111) plane (type II). This is ascribed that hexagonal 2H-SiC segments are embedded into the 3C-SiC.29 Another type of heterointerfaces occurs on the joins of SiC nanowires. Figure 3c shows a typical Y-shaped junction in the as-prepared SiC nanowire with obvious step-like streaked lines. Although the bicrystal nanowire has a common growth direction, the individual “side-branches” maintain their original crystallographic orientation along the (111) plane, resulting in the formation of interface in the single wires (Figure 3d). It is believed that these heterostructures play a crucial role in EM wave dissipation which will be discussed later.

Figure 3. (a) TEM image and SAED pattern (the bottom inset) of the as-prepared rGO/SiC nanowires hybrids; (b) HRTEM image of the in-situ grown SiC nanowires; (c) TEM and (d) corresponding HRTEM images of the bridged SiC nanowires. Figure 4 shows XPS spectra of C 1s core level of as-prepared rGO foam, pure



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SiC nanowires and rGO/SiC NWs foam composites. The C 1s spectrum of the pure rGO foam was fitted by three peaks at 284.6, 285.98 and 288.58 eV, which are corresponding to C-C, C-O and O-C=O bonds, respectively (Figure 4a).34,

35

The

residual oxygen groups are ascribed to the thermal reduction process at 1300 °C. For rGO/SiC NWs foam, a new peak can be fitted at 283.4 eV besides the same peaks as rGO foam (Figure 4b). It is assigned to C-Si bond which conforms the formation of SiC nanowires by carbothermal reduction reaction.36 For SiC nanowires which were obtained by annealing rGO/SiC NWs foam, C-C bond is absent, while C-Si bond can be fitted well (Figure 4c). The results identify the high purity of rGO/SiC nanowires foam owing to the catalyst-free VS process.

Figure 4. XPS spectra (C 1s) of the pure rGO foam (a), rGO/SiC nanowires foam (b) and pure SiC nanowires (c). To evaluate the thermostability of rGO/SiC NWs foams, all the foams were applied to TGA in air atmosphere. As shown in Figure 5, S-60, S-120 and S-180 undergo a weight loss of 74.4%, 58.7% and 43.6%, respectively. This indicates the content of SiC nanowires in the composites increases with the extension of reaction time. Remarkably, rGO/SiC NWs foams show a weight retention of 90% up to ~630 °C，while rGO foam has a rapid weight loss from ~525 °C. The results demonstrate that the incorporation of SiC nanowires enhances the thermostabilty of


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the composites owing to the anti-ablative ability of SiC nanowires.37 Considering the improved thermostability as well as the good chemical resistivity of both graphene and SiC nanowires, rGO/SiC NWs foams could be applied in the more critical environments.

Figure 5. TGA curves of rGO and rGO/SiC NWs foams (S-60, S-120 and S-180). The as-prepared rGO foam and rGO/SiC NWs foams were immersed into PDMS for dielectric measurement. PDMS whose imaginary permittivity in X-band is almost zero makes few contributions on dielectric loss.23 This indicates filling the foams with PDMS can reflect the actual absorption properties of the rGO/SiC NWs foams. In addition, the PDMS matrix maintains the flexible characteristic of the foams without destroying the 3D network, as shown in the digital photograph of the bending composite (Figure 6a). Figure 6b and c show the real and imaginary permittivity of rGO foam, S-60, S-120 and S-180. Both the real and imaginary parts of all the samples present a frequency-dependent behavior that the values decrease with the increasing frequency. This is ascribed to the hysteresis between the displacement



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current and the build-up potential when the frequency increases.38 Meanwhile, the permittivities of rGO/SiC NWs foams are higher than those of rGO foam, and the values increase with the increasing annealing time. The average value of the real part from 8.2 to 12.4 GHz increases from 4.47 to 9.68, while that of the imaginary part changes from 2.63 to 8.17. The results demonstrate the effective permittivity significantly increases with the increasing concentration of SiC nanowires. It is noted that the pure SiC nanowires in EM transparent matrix have to be with high mass ratio (generally >30 wt.%) to improve dielectric loss.28, 29, 39 It is not unreasonable that the well-dispersed SiC nanowires networks constructed by 3D architecture of rGO foam have a positive effect on the enhanced permittivity in a lower ratio of SiC nanowires. The frequency-dependent fluctuations of the real and imaginary permittivity (Figure 6b and c) indicate the typical characteristic of the nonlinear resonant behavior arising from polarizations relaxation. Furthermore, Cole-Cole plots (ε"-ε' curves) of S-60 and S-120 show more conspicuous semicircles than that of rGO foam, identifying additional polarization relaxation processes (details in supporting information, Figure S3).8 This is attributed to the generated SiC nanowires with abundant heterojunctions and stacking faults in rGO foam.29,

40

Interestingly, the corresponding tangent loss

(tanδ = ε"/ε') of the samples exhibits a nonlinear change which is distinct from the increase of the permittivity, as shown in Figure 6c. Remarkably, all the values of rGO foam, S-60 and S-120 are around 0.58. As is well known, both reduced graphene oxides and SiC nanowires make contribution on dielectric loss. The generation of SiC nanowires results in the loss of graphene as well during carbothermal reduction


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process. It is believed that this is related to the nonlinear change of tangent loss.

Figure 6. (a) Photograph of flexible rGO/SiC nanowires (S-120) in PDMS matrix; The real part (b), imaginary part (c) and tangent loss (d) vs frequency for the samples of rGO foam and rGO/SiC nanowires foams (S-60, S-120 and S-180). The RC values of rGO foam, S-60, S-120 and S-180 in PDMS matrix were calculated based on the measured effective permittivity in X-band, as show in Figure 7a-d. It can be seen that the rGO foam composite has exhibited high-performance EM wave absorbing capability. The minimum RC (RCmin) reaches -40.7 dB at 10.9 GHz, when the thickness of the composite is 3.5 mm. Moreover, the effective absorbing bandwidth covers the whole X-band with a thickness of 3.7 and 3.9 mm. For S-60, its effective absorbing bandwidth can also cover the entire X-band with a thickness of 3.6 mm. It is noteworthy that the RC values are still lower than -10 dB with a minimum value of -19.6 dB in the whole X-band, when the concertation of SiC



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nanowires further increases (S-120). Remarkably, the corresponding thickness decreases to 3 mm. Nevertheless, there is no effective absorption in any thickness for S-800, because its excessive dielectric loss results in the poor impedance matching with free space. In addition, as the thickness increases, the RCmin peaks of all the composites shift to the lower frequency, which is explained by the law of quarter-wavelength attenuation.41

Figure 7. RC values calculated for the samples (a) rGO foam, (b) S-60, (c) S-120 and (d) S-180 at different thickness. It is significant that both rGO foam and rGO/SiC NWs foams (S-60 and S-120) can achieve effective absorption in the whole X-band, reflecting the optimization criteria of the complex permittivity for effective EM wave absorption. To understand EM dissipation dependence on the permittivity clearly, here we illustrate a plot on the


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real part versus the imaginary part at 10 GHz when the RC values is lower than -10 dB with the different sample thickness. As shown in Figure 8, both the real and imaginary parts increase if RCmin value is obtained at a thinner thickness. However, the corresponding tangent loss decreases linearly, as marked with the dashed line in Figure 8. It is meaningless to obtain the thinner absorption materials by improving dielectric loss without the consideration of impedance matching. Apparently, all the permittivities of rGO foam, S-60 and S-120 are around the core zone with the corresponding thickness, except for that of S-180 whose imaginary part is too high. It reveals that the key factor for the enhanced EM wave absorption is to achieve the synergy between dielectric loss and the interfacial impedance gap.

Figure 8. The calculated permittivity with different thickness corresponding to RC < -10 dB at 10 GHz. For the high-performance EM absorption of rGO/SiC NWs foams, a schematic to clarify the potential absorbing mechanisms is proposed, as shown in Figure 9. First of all, the 3D hierarchical network increases EM wave propagation distance, resulting in



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EM wave dissipation in the form of heat energy under the alternating EM field.16, 42 Meanwhile, the high surface-to-volume ratio of 2D rGO sheets and 1D SiC nanowires gives rise to the accumulation of charges at the interfaces, leading interfacial electron polarization.43-45 Secondly, the hopping of electrons occurs in the bridged rGO and SiC nanowires, which attributes to conductive loss.46, 47 More importantly, besides defect polarizations from residual groups in rGO,4,

34

the multiple polarizations

processes arising from the high-density stacking faults and the twinning interfaces in SiC nanowires play an important role in the enhanced EM dissipation of rGO/SiC nanowires foams, which is well supported by Cole-Cole curves. In addition, considering the wire-like shape of the 1D SiC nanowires in rGO foam, they can act as “micro-antenna” to receive EM wave.48, 49

Figure 9. A schematic for the EM wave absorption mechanisms of rGO/SiC nanowires foam composites. Here we summarized EM absorbing properties (X-band) of C foam-based composites reported in open literatures, as shown in Table 1. The thickness and the effective absorption bandwidth are two critical factors for a practical EM absorption material. Apparently, the incorporation of SiC nanowires into G foams efficiently


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decreases the thickness, and maintains broadband EM wave absorption. Particularly, G/SiC nanowires foams possess a reinforced thermal stability, when compared with the employment of magnetic materials in rGO foams. Therefore, rGO/SiC NWs foams could be a competitive EM absorbing absorber applied in harsh environments. Table 1. The typical C foam-based composites and their EM wave absorbing properties in X-band. Optimum thickness (mm)

Effective bandwidth in X-band (GHz)

Filler

Matrix

RC (dB)

C foam C foam/Fe C foam/G rGO foam rGO foam rGO foam/CNTs rGO foam/Fe3O4 rGO foam/Fe2O3 rGO foam rGO/SiC NWs foam

PPy PANI paraffin PDMS paraffin paraffin PDMS

~-17 ~-10 ~-25 ~-22 ~-33 -55 ~-23 ~-26 -35

1.2 2.5 2.5 10 2.75 3 4 3.7-3.9

~2 0.2 ~3.4 ~2.5 4.2 3.5 ~3.5 4.2 4.2

This work

PDMS

-19.6

3

4.2

This work

Ref. 50 24 51 52 25 23 53 54

CONCLUSIONS In summary, 3D flexible rGO/SiC nanowires foams with hierarchical architecture were successfully synthesized via a freeze-drying process and carbothermal reduction. In-situ grown SiC nanowires are randomly embedded in the interconnected rGO network, resulting in the enhanced thermostability and electromagnetic wave absorption capability of the composites. The thermostable temperature of rGO/SiC nanowires foams is approximately 100 °C higher than that of rGO foam. Meanwhile, rGO/SiC nanowires foams composites achieve the effective absorption in the whole X-band, and the thickness reduces to 3 mm. Incorporation of SiC nanowires into rGO



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foams demonstrates a facile and effective approach to design desirable EM absorbing materials applied in hash environments.

ASSOCIATED CONTENT Supporting Information The calculation model of reflection coefficient; Photograph of a twisted rGO/SiC nanowires foam; Cole-Cole plots of rGO foam and rGO/SiC nanowires foams (S-60 and S-120); This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Xiaowei Yin); Tel.: +86 29 88494947; Fax: +86 29 88494620. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (NSFC; Grant No. 51332004, 51372204 and 51602258), Excellent Doctorate Foundation of Northwestern Polytechnical University, and the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (Grant No. CX201604) REFERENCES


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Table of Contents Graphic


SiC Nanowires Foam

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