Controllable Fabricating DielectricâDielectric SiC@C CoreâShell

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Controllable Fabricating Dielectric-Dielectric SiC@C Core-Shell Nanowires for High-Performance Electromagnetic Wave Attenuation Caiyun Liang, and Zhijiang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13063 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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ACS Applied Materials & Interfaces

Controllable Fabricating Dielectric-Dielectric SiC@C Core-Shell Nanowires for High-Performance Electromagnetic Wave Attenuation

Caiyun Liang and Zhijiang Wang *

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China

KEYWORDS: SiC, nanowires, dielectric property, core-shell structure, electromagnetic wave absorption

ACS Paragon Plus Environment


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ABSTRACT: Heterostructured dielectric-dielectric nanowires of SiC core and carbon shell (SiC@C) with high-performance electromagnetic wave absorption were synthesized by combining an interfacial in situ polymer encapsulation and carbonization process. This approach overcomes the shortcomings of previous reported methods to prepare carbon shell that both carbon shell and free carbon particles are formed simultaneously. In our developed approach, the core of SiC nanowires are firstly positively charged. Then, the negative resorcinol-formaldehyde polymers as the carbon source are anchored on SiC nanowires under the attraction of electrostatic force, which well suppresses the nucleation of free carbon particles. The thickness of the carbon shell could be modulated from 4 to 20 nm by simply adjusting the moral ratio of resorcinol to SiC nanowires. The resulting SiC@C core-shell nanostructures without free carbon particles offer synergism among the SiC nanowires and the carbon shells, generating multiple dipolar polarization, surfaced polarization and associated relaxations, which endows SiC@C hybrid nanowires with a minimum reflection loss (RL) value of –50 dB at the frequency of 12 GHz and an effective absorption bandwidth of 8 GHz with RL value under –10 dB at the optimized state. Our results demonstrate that SiC@C hybrid nanowires are promising candidates for electromagnetic wave absorption applications.


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1. Introduction Combining of different components into heterostructures will lead to orbital reconstructions and electronic transfer at the interfaces.1,2 The successfully engineering interfaces have endowed numerous materials with fascinating properties, offering the opportunity to extend their potential applications in environmental protection3, catalyst4 and energy conversion5. Especially, the controllable fabrication of composites with multiple interfaces is desired in the applications that address electromagnetic wave pollution. Because the composites with synergistic effect from difference components can activate interfacial polarization and dipolar relaxation to attenuate electromagnetic (EM) waves, protecting biological systems and instruments from the destructive effect of EM waves. Generally, EM absorbers mainly include magnetic materials and dielectric materials. The magnetic materials with high permeability depend on their magnetic properties to absorb EM waves, such as nickel6, iron7, and their related based composites8, 9. The dielectric materials with high dielectric constant rely on electronic polarization, molecular polarization and surface polarization to attenuate EM waves, such as silicon carbide (SiC)10-13, multi-walled carbon nanotubes (MWCNT)14, graphene15 and zinc oxide16. Among them, SiC has triggered numerous interests due to its advantages in relative low density, chemical inertness and outstanding thermal stability.17-20 Carbon materials are also found to be potential EM absorbers owing to their excellent electrical property.21 To enhance the EM absorption ability, lots of high-performance magnetic-dielectric hybrids, such as, MWCNT/Fe3O422 SiC-Co23, SiC-Fe3O4 24, SiC-Ni 25, are fabricated. The effective complementarity between dielectric loss and magnetic loss endows these hybrids with outstanding EM attenuation ability. However, some magnetic-resistance situation highly desires the dielectric-based absorbers with high-efficient EM absorption properties and broad absorption bandwidth to fulfill the EM attenuation.



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It has been proposed that hybrid of different dielectric materials can optimize their permittivity, which can also improve the EM absorption performance. Lu et. al. found that decorating MWCNT with ZnO nanocrystals can improve its dielectric loss and EM absorption performance.26 Han et. al. reported that the combination of interfaces and native defects of carbon in carbon/TiO2 composites is facilitated to achieve excellent EM wave absorption.27 Carbon as an important shell material for hybrid EM absorber has been studied. It is employed as a shell material on TiO2 28, Mg 29, Ni

30

or other materials to improve the

EM attenuation ability. A variety of synthetic routes including chemical vapor condensation31, arc plasma method

32

and hydrothermal/solvothermal processes33 have been employed to

prepare core-shell structured composites with carbon shells. However, these methods usually face the challenges of complicated procedure, separated nucleation of carbon particles and poor controllability of carbon shell. The lying of free carbon nanoparticles will largely weaken the function of carbon shell. So, a method that can well avoid the separated nucleation of carbon particles and achieve uniform carbon shells coating on the core material surface is highly desired. Herein, we report an in situ polymer encapsulation-carbonization method, which can well suppress the formation of free carbon nanoparticles and prefer to form carbon shell on the core materials. Taking the SiC nanowires as the core materials, well-constructed dielectric-dielectric SiC@C core-shell hybrids possessing high-performance EM absorption ability are prepared. The approach is very convenient and effective. The thickness of carbon shell could be modulated easily by changing the additional concentration of resorcinol– formaldehyde polymers. The EM wave absorption properties of core-shell structured materials can be tuned by controlling the thickness of carbon shell. The developed method can be generally applied to improve other single EM absorbers by coated with carbon shell. Compared with a physical mixture with the same carbon and SiC content, the SiC@C


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core-shell hybrids exhibit superior dielectric loss and high-efficiency EM absorption abilities. The dielectric properties and EM absorption performance of SiC@C core-shell nanowires are studied systematically. The mechanism for the enhanced EM absorption properties is also proposal.

2. Experimental section 2.1 Materials Silicon

dioxide

(SiO2),

Silicon

(Si)

powder,

hexadecyl

trimethyl

ammonium

bromide(CTAB), resorcinol and formaldehyde solution were obtained from Sinopharm Chemical Reagent Co., Ltd, China. MWCNTs were purchased from Shenzhen Nanotech Port Ltd. Co.

2.2 Preparation of SiC nanowires The fabrication of SiC nanowires has been reported in our previous study.11 A mixture of Si powder, SiO2 particles and MWCNT in a molar ratio 1:1:4 was ground for two hours. An alumina ceramic boat loaded with the above mixture was pushed into the central zone of a tube furnace and sintered under argon flow (10L h–1) at 1500 °C with a heating rate of 5 °C min–1.

The dwell time was set as 2 hours. In order to remove residual carbon, the sintered

products were calcined at the temperature of 700 °C for 8 hours in the air. Finally, green-white SiC nanowires were obtained.

2.3 Preparation of SiC@C core-shell hybrid nanowires The SiC@C core-shell hybrid nanowires were fabricated by in situ polymer encapsulation and subsequent carbonization process. Firstly, 0.1 g of SiC nanowires were dispersed in 30 mL of water with the addition of 0.1 mL of ammonia. Then, 1 mL of CTAB solution (0.01 mol/L) was added to the above suspension, ultrasonicated for 30 min. After adding 33 mg of resorcinol, the suspension was stirred for 30 min. Finally, 44 µL of formaldehyde solution (38



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wt%) was injected into the suspension to initiate the formation of resorcinol–formaldehyde (RF) polymers. This reaction was lasted for 24 h under mechanical stirring. Then, The as-obtained SiC@RF hybrid nanowires were centrifuged and washed with deionized water and ethanol at least six times. The products were dried in a vacuum at 60 °C. To obtain SiC@C hybrid nanowires, the SiC@RF hybrid nanowires were calcined in a furnace under N2 atmosphere at the temperature of 600 °C for 4 hours with temperature ramp of 5 °C min–1. The mole ratio of formaldehyde to resorcinol was set as 2:1. By changing the molar ratios of SiC to resorcinol, being 8:1, 4:1, 2:1 and 1:1, a series of SiC@C hybrid nanowires with different carbon shell thickness were obtained. The corresponding samples were marked as S1, S2, S3 and S4, respectively.

2.4 Characterizations The composition and phase of pure SiC nanowires and SiC@C core-shell hybrid nanowires were examinated by Rigaku D/max-γB X-ray diffraction (XRD) diffractometer using Cu Kα as radiation source. The morphological features and microstructure of the as-synthesized samples were observed by a field-emission scanning electron microscope (SEM nano600i), transmission electron microscopy (TEM, FEI Tecnai X200) and high-resolution transmission electron microscopy (HR-TEM, FEI Tecnai X200). The thermal properties of the samples were evaluated by thermogravimetric analysis (TGA, SDT Q600, TA Instruments) with a heating rate of 5 °C min–1 up to the temperature of 800 °C under air flow. Raman spectroscopy were acquired by a confocal Raman microspectrometer (Renishaw, In Via) with a 633 nm incident laser.

2.5 Measurement of EM parameters and absorption properties The real and imaginary part of permittivity and permeability of the as-obtained samples are evaluated by a Vector Network Analyzer (VNA, Agilent N5230A, USA) in 2–18 GHz. The preparation process of the measurement samples are as follow. First, 50 wt% of SiC


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nanowires or the obtained SiC@C core-shell nanowires with a paraffin wax matrix were milled at 70 °C. Then, they are pressing into a ring. In order to fix well with the test holder, the external diameter, inner diameter and thickness of the ring are fixed in 7, 3 and 3 mm, respectively. According to the transmission line theory34, EM absorption performance of samples can be evaluated using reflection loss (RL) value, which is determined from the measurement of real and imaginary part of permeability and permittivity based on equation (1) and (2): z in = z 0 ( µ r / ε r )1/ 2 tanh[ j (2π fd / c )( µ r / ε r ) 1/ 2 ]

RL = 20log10 |

zin − zo | zin + zo

(1)

(2)

Where Zin and Z0 refer to the input impedance of the material and the impedance of air, respectively; εr and µr represent the relative complex permeability and permittivity of the material, respectively; f is the applied frequency of the EM waves; d is the thickness of the material and c is the velocity of EM waves in the free space. The value of µr can be regarded as 1 due to the negligible magnetic loss of SiC and SiC@C materials.

3. Results and Discussion The synthetic procedure of SiC@C core-shell nanowires is shown in Figure 1. First, the SiC nanowires fabricated through solid state reaction are modified by a cationic surfactant of CTAB, which will render a positive charged surface. Then, the polymerization of resorcinol and formaldehyde is initiated to form RF polymer shells, which bear negative charges.35 Under electrostatic attraction, the RF polymer will in-situ nucleate and grow on the surface of the SiC nanowires. Finally, the SiC@RF core-shell hybrid nanowires were carbonization at 600 °C under an inert atmosphere of N2 to obtain core-shell SiC@C hybrid nanowires.



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Figure 1. Synthetic route of SiC@C core-shell nanowires.

The phase composition of the as-obtained samples was characterized by XRD, which is shown in Figure 2. The XRD pattern of pristine SiC nanowires is a cubic zincblended structure of 3C-SiC (JCPDS Card 75–0254), which shows five characteristic peaks at 35.5°, 41.4°, 60.0 °, 71.8 ° and 75.4°. The low intensity peaks (denoted as SF) located at 33.6° are ascribed to the stacking faults in SiC grains. After coating with carbon shells, broad and weak peak located at 23.5° are detected, implying that the carbon materials in these composites is amorphous state. At elevated concentrations of resorcinol, the diffraction peaks of the SiC become much lower. These results verify that changing the molar ratio between SiC nanowires and resorcinol can tune the thickness of carbon shell effectively.


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Figure 2. XRD pattern of SiC nanowires and SiC@C hybrid nanowires.

SEM morphologies of SiC nanowires and SiC@C core-shell nanowires are shown in Figure S2. It can be seen that both the SiC and SiC@C samples have wire-like morphology. The average diameter of samples is within 30-50 nm and their length is above 500 nm. TEM images of SiC and SiC@C samples are presented in Figure 3. The SiC nanowires have a worm-like structure whose diameter is around 30 nm (Figure 3a). Figure 3b-e are the TEM images of the as-prepared SiC@C core-shell hybrid nanowires. Notably, each sample has a core-shell structure without any free carbon particles. The average thickness of carbon shell for S1, S2, S3 and S4 are 4, 7, 12 and 20 nm, respectively. These results further indicate that the carbon shell thickness is increased with the addition amount of resorcinol and formaldehyde. HRTEM images of SiC@C nanowires are shown in Figure S3. It can be observed that SiC core has high crystallinity. The core material have an interplanar crystal spacing of 0.25 nm, which is conformed to (111) plane of 3C-SiC. A layer of carbon shell of disordered graphite-like structure on the SiC surface is clearly visible. The selected area electron diffraction pattern (SAED) of the SiC@C nanowires with well-defined rings is



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shown in Figure S4. The diffraction rings from inside to outside are indexed to (111), (220) and (311) planes of 3C-SiC. The diffused background in SAED pattern is caused by inelastic scattering from the amorphous carbon, indicating that the amorphous nature of carbon shell. By using the as-obtained SiC nanowires as core materials, uniform carbon shell are formed on those nanowires by interfacial in situ encapsulation of RF polymer and followed carbonation process. According to the TEM images, combining the mass density of carbon and SiC, the weight percentage of carbon shell in the S1, S2, S3 and S4 hybrids are as 8.6, 14.4, 23.2 and 35.1 wt %, respectively. Thermogravimetric (TG) analysis was further carried out to confirm that the carbon content on SiC@C hybrid nanowires could be changed by controlling the additional concentration of RF polymers. Figure 3f is the TG curves of SiC nanowires and SiC@C core-shell nanowires, which were recorded under a flow of air. It can be observed that SiC nanowires remain unchanged in low temperature range of 25–500 °C and a weak weight increase region in high temperature range of 550–800 °C, which is due to slight oxidation of SiC in the air. In the cases of SiC@C core-shell hybrid nanowires, the weight losses appear at 450–600 °C, which are due to the combustion of carbon components. The weight percentage of carbon are determined to be 8.6, 14.6, 23.7 and 37.4 wt% for S1, S2, S3 and S4, respectively. These results further demonstrate that the content of carbon component in SiC@C core-shell nanowires is increased with the elevated additional concentration of resorcinol. The actual carbon content measured by TG is quite closed to the result estimated by carbon shell thickness shown in the TEM images, confirming that our developed method can effectively restrain the nucleation of free carbon particles.


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Figure 3. TEM images of samples: (a) SiC nanowires, (b) S1, (c) S2, (d) S3, and (e) S4. (f) TG curves of SiC nanowires and SiC@C hybrid nanowires.

Apart from the carbon content, the graphitization degree in the composites is another important factor that will affect the EM parameters and EM wave absorption abilities of carbon-containing materials. Figure 4 shows typical Raman spectra of SiC@C hybrid nanowires with different carbon content. They all possess two major peaks within the wavelength range of 1000–2000 cm–1. The peaks centered at about 1345 cm–1 and 1587 cm–1 are assigned to D band and G band, respectively. The intensity ratio of the D band to the G band (ID/IG) for all SiC@C hybrid nanowires with different carbon thickness are almost



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identical, indicating that the graphitization degree of carbon materials in all the SiC@C hybrid nanowires are almost the same. According to previous reported literatures36, 37, the D band represent a breathing mode of A1g symmetry producing phonons near the K zone boundary. Its signal is strong in disorder graphite materials but restrained in perfect graphite materials. The G band signal derives from in-plane vibration of sp2 carbon atoms and refer to the E2g mode at the Brillouin zone center, which can be generated by all sp2 sites. The value of ID/IG is used to evaluate the amounts of the defects formed within the samples. The enhanced graphitization degree will lead to the decreased ID/IG value due to the transition from amorphous state of carbon to crystallographic graphite. Very interestingly, the ID/IG value of SiC@C core-shell nanowires, being about 0.84, are higher than other carbon-containing composites38, 39. Hence the utilization of RF polymers as carbon precursor can promote the formation of carbon shells with low graphitization degree after carbonizing, which is consistent with the XRD results. In addition, the amorphous nature of carbon with plenty of defects can serve as effective polarization centers under electromagnetic field.40

Figure 4. Raman spectra of SiC@C hybrid nanowires with different carbon shell thickness.


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In order to evaluate the EM absorption performance of SiC nanowires and SiC@C hybrid nanowires, the real part of permittivity (ε′), the imaginary part of permittivity (ε′′) and dielectric tangential loss (tanδe= ε′′/ε′) of samples were studied within 2–18 GHz. As sketched in Figure 5a, the ε′ values of SiC@C hybrid nanowires are higher that of SiC nanowires and show a declining tendency versus frequency. With the enhancement of carbon component, the ε′ value gradually increases. The ε′′ value of the SiC nanowires exhibits little change in the whole frequency range. After coating with carbon shell, many resonance peaks appear at around 3 GHz, 11 GHz and 14.5 GHz, respectively. Moreover, all the SiC@C core-shell nanowires exhibit certain fluctuations in the curves of ε′′ in 2–18 GHz. Similarly, the ε′′ values of SiC@C core-shell nanowires are also increased with the increasing carbon content, which should be attributed to the carbon shell covering on the surface of SiC nanowires. The energy loss caused by dielectric property can be further evaluated by dielectric tangent loss, which is shown in Figure 5c. The shapes and tendencies of the tan δe vs frequency are almost the same with those of the ε″ vs frequency. The variations in tan δe of SiC nanowires are relatively flat around 0.17. However, the tan δe of SiC@C core-shell nanowires exhibit many resonance peaks and the intensity of the peaks are increased with the increasing carbon content. According to Debye theory41, the dielectric loss is associated with dipole polarizations and interfacial polarizations. The former derives from the defects in amorphous carbon components, while the latter originates from the existence of multiple interfaces between carbon outer layers and SiC nanowires. The rational design of the SiC@C core-shell nanowires creates rich interfaces, which is facilitated for interfacial polarization and dielectric relaxation and thus enhance the dielectric loss. The dielectric resonance peaks of SiC@C nanowires should stem from their multi-interface core-shell structure and amorphous feature



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of the carbon shell. These peaks are beneficial to greatly attenuate EM waves.

Figure 5. Real part of permittivity (a), imaginary part of permittivity (b) and dielectric tangent loss (c) of SiC nanowires and SiC@C hybrids nanowires.

The 3D RL mapping plots of SiC nanowires, SiC@C hybrid nanowires varied with frequency and thickness are depicted in Figure 6. It can be observed that SiC nanowires exhibit poor EM absorption, whose minimum reflection loss (RLmin) is only –9 dB in 2–18 GHz with the thickness of absorber being 0.5–5 mm. A substantial enhancement of EM absorption performance is achieved after the SiC nanowires was coated with carbon shells. The RLmin for sample S1, S2, S3 and S4 are –16, –14, –50 and –20 dB, respectively. As summarized in Table S1, when the sample thickness is fixed in 2.8 mm, sample S3 with carbon shell thickness of 12 nm exhibits the optimal EM absorption properties in all the SiC@C core-shell nanowires. The RLmin reached as low as –50 dB at the frequency of 12 GHz and the effective absorption bandwidth with RL value under –10 dB is 6GHz (from 8.2 to 16.2 GHz). Compared with previously reported core-shell composites39, 42, 43, 23 shown in Table 1, SiC@C hybrid nanowires has superior EM absorption performance than the previous reported core-shell EM absorbers. The above EM absorption analysis indicates that the assembly of SiC@C core-shell nanowires constructed by carbon shells is beneficial to enhance the EM absorption performance. Furthermore, the EM absorption abilities of SiC@C hybrid nanowires can be easily modulated through changing the carbon shell thickness.


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Current SiC@C core-shell nanowires are promising EM absorption materials in practical applications. A control experiment is performed to verify the detrimental effect of free carbon particles on the EM absorption and to confirm that our developed method is effective to form carbon shell while suppressing free carbon formation. The free carbon particles are firstly prepared with the approach similar with SiC@C hybrid nanowires without SiC nanowires. Then, the prepared carbon particles are mixture with SiC nanowires or sample S1, which is denoted as SiC/C and S1/C. Both the SiC/C mixture and S1/C mixture containing the same amount of SiC nanowires (76.3 wt%) and carbon (23.7 wt%) with sample S3. The dielectric parameters and EM absorption ability of these three samples are compared. As presented in Figure S1, the ε′′ value of SiC/C mixture and S1/C mixture are very closed. being about 1.3. The most interesting phenomenon is that the ε′′ value of SiC/C mixture and S1/C mixture are much smaller and show little resonance peaks compared with sample S3. Obviously, the lying of free carbon particles annihilates the dielectric resonance peaks. Combining the results shown in Figure 5, it can be drawn that little free carbon nanoparticles containing in sample S3. Our developed method can well suppress the nucleation of free carbon particles.



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Figure 6. EM absorption performance of SiC nanowires (a), S1(b), S2 (c), S3(d), S4 (e), physical mixture of SiC nanowires and free carbon (f), physical mixture of S1 and free carbon (g) and the reflection loss for sample S3 with different thickness (h).

Table 1. EM absorption properties of representative core-shell composites. Optimal RL value (dB)

weight ratio (%)

Fe3O4@C nanorods

–27.9

Fe3O4@C composites

Samples

Absorption bandwidth RL≤–10 dB (GHz)

Thickness (mm)

Ref.

55

4.0

2.0

Ref 39

–36.1

50

2.0

5.0

Ref 42

Co/MWCNT

–39.3

50

3.5

3.0

Ref 43

SiC/Co nanowires

–25

50

6.6

2.5

Ref 23

SiC@C nanowires

–50

50

8.0

2.8

This work


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The EM absorption performances of SiC/C mixture and S1/C mixture are also presented in Figure 6. It is very clear that the EM absorption performance of SiC/C mixture (RLmin= –12 dB) and S1/C mixture (RLmin= –18 dB) is inferior to that of sample S3 (RLmin= –50 dB), even though they contain the same amount of carbon and SiC. This result demonstrates that the SiC@C core-shell nanostructures without free carbon particles can achieve synergistic effects between SiC core and carbon shell, generating multiple dipolar polarization, surfaced polarization and associated relaxations to enhance EM absorption properties. The superior EM absorption abilities of SiC@C core-shell nanowires can be primarily derived from the following points: (i) The introduction of amorphous carbon layers contain many defects, which serve as effective polarization centers under electromagnetic field and lead to dipole polarization; (ii) The unique core-shell structure of SiC@C core-shell nanowires is facilitated for the interfacial polarization. By coating the SiC nanowires with carbon shell, numerous interfaces are formed at the conjunctions. Thus, the induced interfacial polarization and dipolar relaxation promoted by carbon coating are capable to improve the dielectric loss; (iii) The accumulated charges and collective interfacial polarization within the interfaces result in the transforming of EM energy to heat energy. 44

4. Conclusion In summary, we have fabricated SiC@C core-shell hybrid nanowires with a uniform carbon shell by in situ polymers encapsulation-carbonization approach. The carbon shell thickness can be tuned by merely adjusting the additional concentration of polymeric building block during encapsulation. The SiC@C dielectric-dielectric hybrid nanowires with 12 nm carbon shell thickness demonstrates the optimal EM absorption ability with RL value of –50 dB at the frequency of 12 GHz, which is 5 times higher than the SiC nanowires alone. An effective absorption bandwidth of 8 GHz with RL value under –10 dB can be achieved at optimized state. The introduction of carbon shell induces dipole polarization and interfacial



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polarization within the SiC@C core-shell hybrid nanowires, which is responsible to the enhanced EM absorption. Our results can provide a guideline for rational design and fabrication of high-efficiency core-shell structured materials with better chemical homogeneity and EM absorption performance.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Complex permittivity of SiC/C mixture and S1/C mixture; SEM morphologies of SiC and SiC@C hybrid nanowires; HRTEM images of SiC@C hybrid nanowires; SAED pattern of SiC@C core-shell nanowires; summary table of EM property for SiC and SiC@C core-shell nanowires Author Information *Corresponding author: Tel.&Fex: +86 451 86418409. E-mail address: [email protected] (Z. Wang) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work received the financial support from the National Natural Science Foundation of China (No. 51572062), Natural Science Foundation of Heilongjiang Province (No. B2015002) and Heilongjiang Postdoctoral Scientific Research Developmental Fund (No. LBH-Q16079).

REFERENCES (1) Hwang, H. Y.; Iwasa, Y.; Kawasaki, M.; Keimer, B.; Nagaosa, N.; Tokura, Y., Emergent


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Controllable Fabricating DielectricâDielectric SiC@C CoreâShell

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