Carbon Hollow Microspheres with a Designable Mesoporous Shell for

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Carbon Hollow Microspheres with a Designable Mesoporous Shell for High-Performance Electromagnetic Wave Absorption Hailong Xu, Xiaowei Yin,* Meng Zhu, Meikang Han, Zexin Hou, Xinliang Li, Litong Zhang, and Laifei Cheng Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an 710072, China S Supporting Information *

ABSTRACT: In this work, mesoporous carbon hollow microspheres (PCHMs) with designable mesoporous shell and interior void are constructed by a facile in situ stöber templating approach and a pyrolysis-etching process. The PCHMs are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectra, Raman spectroscopy, and nitrogen adsorption and desorption system. A uniform mesoporous shell (pore size 4.7 nm) with a thickness of 55 nm and a cavity size of 345 nm is realized. The composite of paraffin mixed with 20 wt % PCHMs exhibits a minimum reflection coefficient (RCmin) of −84 dB at 8.2 GHz with a sample thickness of 3.9 mm and an effective absorption bandwidth (EAB) of 4.8 GHz below −10 dB (>90% electromagnetic wave is attenuated). Moreover, the composite of phenolic resin mixed with 20 wt % PCHMs exhibits an ultrawide EAB of 8 GHz below −10 dB with a thinner thickness of 2.15 mm. Such excellent electromagnetic wave absorption properties are ascribed to the large carbon−air interface in the mesoporous shell and interior void, which is favorable for the matching of characteristic impedance as compared with carbon hollow microspheres and carbon solid microspheres. Considering the excellent performance of PCHMs, we believe the as-fabricated PCHMs can be promising candidates as highly effective microwave absorbers, and the design philosophy can be extended to other spherical absorbers. KEYWORDS: carbon hollow microspheres, mesoporous shell, carbon-air interface, dielectric properties, electromagnetic wave absorption

1. INTRODUCTION Electronic devices are being made smaller and growing at an exponential rate every day, particularly used in the chemical corrosive environment. Lightweight electromagnetic (EM) wave absorbers with chemical stability that can prevent EM pollution are highly desirable.1,2 Over the past decade, considerable efforts have been made to prepare efficient EM wave-absorbing materials. The primary features of the absorbers are high absorption capacity and broad absorption bandwidth.3 Previously, choosing a proper material is crucial to the final performance of the absorbers. Early researches mainly focused on dielectric loss materials including graphite,4 porous carbon,5 carbon coils,6,7 carbon fibers,8 and magnetic loss materials such as ferrite.9,10 To further strengthen the EM wave absorption capability, new materials have been explored to construct efficient EM wave absorbers. One-dimensional (1D) carbon nanotubes (CNTs),11−13 two-dimensional (2D) graphene,14−20 Mxenes,2,21 and MoS222 are considered to be ideal candidates. However, it is difficult to satisfy the impedance match condition for unilateral dielectric or magnetic materials. In addition, composites contain dielectric and magnetic materials such as Fe3O4@ZnO sphere, CoNi@SiO2@TiO2 sphere, CoFe2O4/ graphene oxide, CoNi/graphite layer, Fe3O4@C, and Fe3O4@ SiO2@NiO23−29 have exhibited higher EM wave absorption © 2017 American Chemical Society

capability. Nevertheless, for certain practical application, the features of lightweight and anticausticity are indispensable.30−33 Unfortunately, the high pristine density and poor resistance to chemical corrosion of the above composites greatly limit their applications. To get an ideal absorber, rational design on nanostructure should be performed.34 Recent progress indicates that porous or hollow nanostructure may contribute to the higher EM wave absorption capability due to their interfacial polarization and multireflection.47 Meanwhile, this special structure is benefit for its lower density.35 As a result, hollow MnO2 spheres, hollow glass spheres coated Ni−Zn ferrite, hollow ZnO spheres, hollow CdSe nanospheres, yolk−shell Ni@SnO2, and flowerlike CuS hollow spheres are being developed.15,36−41 Compared with these metal oxides, carbon material with lower pristine density and better resistance to chemical corrosion can be an ideal candidate. Actually, porous carbon,42−44 hollow carbon spheres,30,45 and yolk shell carbon spheres46 have been investigated. However, investigation of these absorbers has mostly concentrated on the enhanced EM wave absorption Received: December 9, 2016 Accepted: January 20, 2017 Published: January 20, 2017 6332

DOI: 10.1021/acsami.6b15826 ACS Appl. Mater. Interfaces 2017, 9, 6332−6341

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ACS Applied Materials & Interfaces

Figure 1. A schematic illustration of the synthesis of PCHMs, CHMs, and CSMs. was used for all experiments. All chemicals were used without any further purification. 2.2. Preparation of PCHMs, CHMs, and CSMs. The synthetic procedure was based on the method of Chengzhong Yu.48 In a typical synthesis of PCHMs, 2.88 mL of TEOS was added to the solution containing 55 mL of ethanol, 7 mL of H2O, and 2 mL of NH3·H2O (25 wt %) under stirring at room temperature. After 30 min, 0.4 g of resorcinol and 0.56 mL of formaldehyde (37%) were added to the solution, and then the system was kept stirring for 18 h. After that the precipitates were separated by centrifugation, washed with water and ethanol three times, and dried at 60 °C overnight. PCHMs were obtained after carbonization at 650 °C under pure argon for 6 h and removal of silica by etching with hydrofluoric acid (HF, 25 wt %). For the preparation of CHMs, the interval of the addition of resorcinol and formaldehyde was delayed from 30 to 120 min, while the other conditions were kept unchanged. To prepare CSMs, the same recipe was used without TEOS and etching process. 2.3. Characterization. The morphology and nanostructure of the samples were investigated by scanning electron microscopy (SEM; s4700, Hitachi, Japan) and transmission electron microscopy (TEM; G20, FEI-Tecnai, USA). Raman spectra were obtained on a Renishaw Ramascope (inVia, Renishaw, U.K.) equipped with a He−Ne laser (λ = 514 nm). X-ray photoelectron spectra (XPS) were measured using an X-ray photoelectron spectrometer (K-Alpha, Thermo Scientific, USA). The surface structures were characterized by FT-IR (iN10MX, Nicolet, USA). Nitrogen sorption and desorption isotherms were obtained using a Micromeritics ASAP 2020 system. Samples were normally degassed under vacuum at 180 °C for 6 h before analysis. Nitrogen adsorption isotherms were measured at 77 K. The pore size distributions were calculated using the Barrett−Joyner−Halenda (BJH) method. To investigate the EM wave absorption properties of the obtained absorbers, paraffin was selected as the matrix material. A sample containing 20 wt % of obtained microspheres was pressed into a ring with an outer diameter of 7 mm and an inner diameter of 3 mm for EM measurement. To avoid aggregation, the spheres and paraffin were

performance; the mechanism analysis of the porous or hollow structure on enhancing reflection loss have been rarely reported. Therefore, the structure that contains a rationally designed mesoporous shell with interior cavity is required to investigate the porous/hollow function. In this work, the mesoporous carbon hollow microspheres (PCHMs), carbon hollow microspheres (CHMs), and carbon solid microspheres (CSMs) were successfully prepared by the in situ generated silica primary particles as template and followed by a pyrolysis-etching process.48,49 The mechanism of the porous/hollow structure affecting their EM wave absorption properties were investigated based on their structures and EM wave absorption properties. Compared with regular porous carbon,44 the PCHMs present some advantages. On one hand, the interior cavity can induce multiple reflection and scattering under alternated electromagnetic field.39 On the other hand, each hollow microsphere is composed of mesoporous shell and interior cavity, contributing dielectric loss together and achieving well-matched characteristic impedance with the matrix surrounding.46 Moreover, the mesoporous shell was introduced into the hollow structure, which can further meet the requirement of a lightweight characteristic compared with regular hollow carbon sphere.35 Our findings show that the rational design of mesoporous shell with hollow structure is effective in EM wave absorption enhancement, and this route can be extended to other spherical EM wave absorbers.

2. EXPERIMENTAL SECTION 2.1. Chemical Reagents. Resorcinol, formaldehyde (37%), tetraethyl orthosilicate (TEOS), absolute ethanol (EtOH), hydrofluoric acid (HF, 25 wt %), and concentrated ammonia aqueous solution (NH3·H2O, 25%) were of analytical grade. Deionized water 6333


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Figure 2. SEM and TEM images and structure schematic diagrams of CSMs (a, d, g), CHMs (b, e, h), and PCHMs (c, f, i). dispersed into hexane solution by ultrasonication. To prepare the functional composites with 20 wt % PCHMs dispersed homogeneously in phenolic resin, the phenolic resin was dissolved in ethanol; then, the PCHMs were added into the above solution, the mixture was stirred until the ethanol evaporated completely at room temperature, and finally the black residue was molded into a ring with an outer diameter of 7 mm and an inner diameter of 3 mm for EM measurement. The relative complex permittivity and the values of scattering parameters (S-parameters) of the prepared samples were measured by coaxial method using a vector network analyzer (N5234A; Agilent, USA) in the frequency range of 2−18 GHz.

The SEM and TEM images are shown in Figure 2. Figure 2a−c shows the low-magnification SEM images, which reveal the uniform spherical morphology. The as-prepared CSMs and CHMs have smooth surface, while the surface of PCHMs is rough, indicating that PCHMs may have a mesoporous shell. The hollow morphology and the radial pore channels are evidenced by the TEM image (see Figure 2f). The structure of PCHMs displays a uniform mesoporous shell with a thickness of 55 nm and a cavity size of 345 nm (see Figure 2f). A highresolution TEM of PCHMs (see Figure S1 in the Supporting Information) indicates they have a low graphitization degree, as no obvious graphene-like nanosheets appeared. For CHMs, the hollow structure with a dense shell size of 26 nm and a cavity size of 407 nm are displayed in Figure 2e. There is some silica residue in the cavity for the reason that the shell of CHMs is solid, and it is difficult to remove all the silica core under the same etching conditions. Figure 2d shows the solid spherical morphology of CSMs. The dynamic light scattering size distribution of PCHMs, CHMs, and CSMs (see Figure S2 in the Supporting Information) show that they have narrow size distribution with an average particle size of 455, 459, and 463 nm. N2 sorption analysis was utilized to investigate the structural information on the microspheres. As observed in Figure 3a, PCHMs give a IV-type isotherm with a long and narrow loop at relative pressure (P/P0) from 0.4 to 1.0, which is the characteristic of mesoporous material according to the IUPAC classification. The surface area and pore volume are

3. RESULTS AND DISCUSSION 3.1. Microstructures of PCHMs, CHMs, and CSMs. PCHMs, CHMs, and CSMs are prepared by varying the initial raw agents and adjusting the addition time of resorcinol and formaldehyde ethanol solution to the stöber system, followed by a certain temperature pyrolysis and HF etching to remove the silica template. The preparation route is illustrated in Figure 1. Detailed studies have revealed that the stöber particles are formed via the aggregation of silica primary particles with a size of 2−3 nm.50 These primary particles are mata-stable and exist with a short lifespan before being consumed. The primary particles will be consumed and transform into silica core particles after 2 h.48,49 The mesoporous shell could be prepared by controlling the addition time of the resorcinol and formaldehyde ethanol solution. When the addition time is over 2 h, all of the primary particles have transformed into silica cores, and the CHMs with a solid shell are prepared. 6334


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Figure 3. N2 adsorption−desorption isotherm (a), pore size distribution (b), Raman spectra (c), and XPS spectra in the C 1s region (d) of PCHMs, CHMs, and CSMs.

1097.7 m2·g−1 and 1.41 m3·g−1 respectively, which are much higher than those of CHMs (619.9 m2·g−1 and 0.06 m3·g−1) and CSMs (237.6 m2·g−1 and 0.28 m3·g−1; see table S1 in the Supporting Information). The BJH pore size distribution of PCHMs calculated from the adsorption branch reveals a pore size of 4.7 nm (see Figure 3b). The above results indicate that the PCHMs are actually containing an uniform mesoporous shell and an interior cavity. Furthermore, the bonding state of carbon atoms will greatly affect the EM wave absorption performance.51 Thus, Raman spectra are necessary to discern the difference between the prepared spheres. As shown in Figure 3c, they display quite similar spectra with two distinguishable peaks assigned to D-bands (1340 cm−1) and G-bands (1590 cm−1), and more interestingly, their ratios of Dbands and G-bands (ID/IG) are almost identical, implying that they have quite similar bonding state of carbon atoms (degree of graphitization).52 All results mentioned above indicate that the structure of PCHMs does not work for their crystallinity and graphitization degree, as they pyrolyzed at the same temperature. Further details of the chemical structure of the three kinds of microspheres were characterized by XPS (see Figure 3d, as well as Figure S3a in the Supporting Information). The same elements (C, O) with nearly the same concentrations are detected from the survey spectra of the samples (see Table S2 in the Supporting Information); otherwise, there is silica element due to the residue SiO2 in the interior cavity of CHMs. As shown in Figure 3d, the C 1s spectra of the three samples exhibit three predominant peaks at 284.6 (284.59 eV), 286.0 (286.1 eV), and 287.5 eV, which correspond to the C−C, C− O, CO, respectively.53−55 The above results are consistent

with the O 1s spectra (see Figure S3b in the Supporting Information), and the O 1s spectra are fitted by components corresponding to the −CO, −C−O, −O−H bonds, which are located at 534.4, 533.2, and 531.7 eV.56,57 The Fourier transform infrared (FTIR) spectroscopy was used to confirm that PCHMs have oxygen functional groups such as hydroxyl and carbonyl groups (see Figure S4 in the Supporting Information). For CHMs, a peak of 530.7 eV appears corresponding to SiO2 for the reason for the residue SiO2.58 From Table S2, the O atoms concentration does not show sharp differences of all the three samples (a concentration of 0.19% of Si due to the residue SiO2, which is consistent with the TEM results from Figure 2e). In fact, all of them have residual oxygen functional groups such as hydroxyl and carbonyl groups, and these functional groups will contribute to the EM wave absorption act as polarized centers.63 3.2. Dielectric and Electromagnetic Wave-Absorption Properties. EM absorption properties of a dielectric absorber are highly associated with its complex permittivity, where the real parts of complex permittivity (ε′) represent the storage capability of electric energy, and the imaginary parts (ε″) stand for the loss capability of electric energy.25 The EM wave absorption of PCHMs, CHMs, and CSMs were studied by a coaxial method in the frequency range of 2−18 GHz. As shown in Figure 4, the real part (ε′), the imaginary part (ε″), and the loss tangent (tan δ = ε″/ε′) of PCHMs, CHMs, and CSMs in a wax matrix with a loading of 20 wt % were studied. Both CHMs and CSMs have low ε′ and ε″ values, which make a minor contribution to EM wave dissipation. While for PCHMs, the complex permittivity has a significant increase, where the real part (ε′) decreases from 8 at 2.0 GHz to 5.5 at 11.6 GHz and 6335


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Figure 5. Modulus of Zin − 1 of various samples with a thickness of 3.9 mm in the frequency range of 2−18 GHz. Figure 4. Permittivity (real (ε′) and imaginary (ε ″)) and the loss tangent (tan δ) vs frequency of PCHMs, CHMs, and CSMs.

Zin − 1 is still larger than zero. For PCHMs, the modulus of Zin − 1 is closer to zero than CHMs and CSMs, which indicates the characteristic impedance of PCHMs is matched well, and the large carbon−air interface in mesoporous shell and interior void is the main reason. The above results indicate that the PCHMs should have an excellent EM wave-absorbing performance. For evaluating the EM absorbing performance, a comparison of calculated reflection coefficient (RC) in the frequency range of 2−18 GHz with various absorber thicknesses from 0 to 5.0 mm is shown in Figure 6. The minimum reflection coefficient (RCmin) of PCHMs is −84 dB at 8.2 GHz with a sample thickness of 3.9 mm (blue region of Figure 6a). While for CHMs, the RCmin value is −25 dB at 17.3 GHz with a sample thickness of 1.75 mm (blue region of Figure 6b). It is notable that the RCmin value of CSMs is only −8.5 dB at 9.5 GHz with a sample thickness of 4.3 mm (blue region of Figure 6c). The interior void of CHMs results in the higher reflection loss than CSMs, and the mesoporous shell of PCHMs plays a decisive role in its excellent EM wave attenuation performance. Figure 7a,b shows the calculated RC values of PCHMs (20 wt % mixed with paraffin) with different thickness, and the corresponding thickness is nλ/4 (where n = 1, 3, 5···). The reflection loss peak shifts from high frequency to low frequency with the thickness increasing. Figure 7c shows the minimum RC of the sample with 20 wt % PCHMs versus thicknesses. The reflection loss below −10 dB with the sample thickness from 1.65 to 5.0 mm. Especially, the reflection loss can reach −40 dB with the thickness from 3.5 to 5 mm. Effective absorption bandwidth (EAB) of the sample with 20 wt % PCHMs versus frequency at different thicknesses (mm) is shown in Figure 7d. The largest EAB is 4.8 GHz with a sample thickness of 2.25 mm, and almost all the C, X, and Ku band can be covered by adjusting the thickness. The EM wave absorption performance of carbon spheres can be adjusted by a rational design on the nanostructures. The associated mechanisms based on structure characteristics are proposed, as illustrated in Figure 8. First, the existing large solid−air interface in mesoporous shell and interior void improve the characteristic impendence matching of PCHMs compared with CHMs and CSMs15 (see Figure 5). Therefore, more EM wave can enter absorbers and dissipate by the following mechanisms. Second, according to the XPS and FTIR

then increases to 6.5 at 14.0 GHz and finally decreases to 5.2 at 18 GHz, the imaginary part (ε″) decreases from 3.8 at 2.0 GHz to 1.6 at 13.0 GHz and then increases to 2.2 at 15 GHz and finally keeps approximately constant near to 2.2. The difference of the complex permittivity in carbon materials is usually attributed to their bonding state of carbon atoms (the degree of graphitization) and microstructure. Raman spectra have proved that they have the quite similar degree of graphitization (see Figure 3c). So the microstructure of PCHMs should be responsible for the increase of their complex permittivity. The microstructure of PCHMs can be understood as a two-phase composite containing carbon (solid) and air (void).59 The complex permittivity of PCHMs presents additional increase compared with CHMs due to larger solid−air interface in the mesoporous shell and interior cavity. Furthermore, the tan δ value of PCHMs has a constant increase compared with CHMs and CSMs, implying the improved EM wave attenuation capability. The results confirm that the large solid−air interface in mesoporous shell have enhanced effect on the complex permittivity and loss tangent. For EM wave absorbers, both tangent loss and matched characteristic impedance are important to produce considerable reflection loss. The matched characteristic impedance can determine the transmission behavior of EM wave.44 More EM wave can enter absorbers and dissipate if the characteristic impedance is matched well. Otherwise EM wave will be reflected at the front surface of absorbers. However, it is difficult to evaluate the normalized characteristic impedance directly, as it is an imaginary number.60 The modulus of Zin − 1 can be used to investigate the reflection loss properties according to eqs 1 and 2 (see the additional Experimental Section in the Supporting Information). When the modulus of Zin − 1 is getting closer to zero, stronger reflection loss will be produced. As shown in Figure 5, the modulus of Zin − 1 of various samples with 20 wt % microspheres in the frequency range of 2−18 GHz were calculated. The thickness of the samples are 3.9 mm. For CSMs, the modulus of Zin − 1 is far from zero, representing the characteristic impedance is not matched well. The hollow structure of CHMs makes their characteristic impedance getting better, while the modulus of 6336


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Figure 6. Three-dimensional images of calculated theoretical reflection coefficient value of samples for (a) PCHMs, (b) CHMs, and (c) CSMs.

Figure 7. Minimum RC of the samples with 20 wt % PCHMs vs frequency at different thicknesses (a, b), minimum RC of the sample with 20 wt % PCHMs vs thickness (c), EAB of the sample with 20 wt % PCHMs vs frequency at different thicknesses (mm) (d).

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Figure 8. Schematic illustration of EM wave absorption for PCHMs.

Figure 9. (a) Optimal EM wave absorption performance of different form carbon materials as the function of RCmin and EAB, (b) optimal EM wave absorption performance of spheres made with different materials as the function of RCmin and EAB.

Figure 10. Calculated theoretical RC value of phenolic resin containing 20 wt % PCHMs (a) three-dimensional image, (b) two-dimensional projection image.

spectra in Figure 3d and Figure S4 (see Figure S4 in the Supporting Information), there are residual oxygen functional groups such as hydroxyl and carbonyl groups after pyrolysis of microspheres; furthermore, disorder carbon and defects exist in the mesoporous carbon shell for the low degree crystallization (see Figure 3c and Figure S1 in the Supporting Information). These defects and functional groups act as polarized centers.15,63 Third, the EM wave undergoes multiple reflection and scattering in the interior void and mesoporous shell,23,62,69 which increases the propagation path in the samples to get further attenuation, and the ratio of absorption/reflection is increased. Fourthly, the large surface area of PCHMs gives rise to interfacial polarization (called as Maxwell−Wagner effect) and the associated relaxation.18,61

Optimal EM absorption performance of different carbon materials as the function of minimum RC and EAB is shown in Figure 9a. The special mesoporous shell and interior void make this structure highly competitive to the conventional structural materials. In fact, all carbon materials have an effective EAB larger than 3 GHz. For carbon materials without intricate structure design, such as pure carbon,51 porous carbon,44 amorphous carbon nanotubes,12 and carbon nanocoils,7 all have an effective absorption, and recent outstanding graphene@ CNTs11 show well absorption performance. Unsurprisingly, rational design on structures such as yolk shell C/C spheres,46 hollow carbon spheres,30 carbon spheres,45 and our PCHMs show stronger absorption and larger EAB. Figure 9b shows the spherical particles made with different materials as the function 6338


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ACS Applied Materials & Interfaces of RCmin and EAB. For hollow glass spheres,37 Fe3O4@C,64 HCM-microspindles,36 their pristine high densities limit their practical applications. Other metal oxide (ZnO spheres,15 MnO2 microspheres,35 Ni@SnO2 core−shell sphere,65 porous Co/CoO sphere66) and metal sulfide (CuS hollow sphere,38 ZnS sphere67) all show good EM wave absorption performance. Unfortunately, their chemical instability is fatal when used in corrosive chemical environment. Our rationally designed PCHMs achieve lower RCmin (−84 dB) and wider EAB (4.8 GHz), indicating much more excellent EM wave absorption performance. PCHMs combine the advantage of carbon materials including chemical stability, adjustable properties (such as crystallization degree controlled by pyrolysis temperature), and the tunable sphere structure with mesoporous shell and interior void will be an ideal candidate for EM wave absorption. The EM absorbing properties of PCHMs can be tuned by changing the porosity and the interior void. Moreover, the idea of rational structural design can be extended to other spherical absorbers. The above data display that the as-prepared PCHMs are a kind of excellent EM wave absorbers. On the basis of the above work, we constructed a functional composite with phenolic resin containing 20 wt % PCHMs and investigated its EM wave absorption capability. The real part (ε′) and the imaginary part (ε″) of permittivity used to calculate the reflection coefficient are shown in Figure S5a (see the Supporting Information). On the basis of the generalized transmission line theory and metal back-panel model, the calculated reflection coefficient is shown in Figure 10 (also see Figure S5b,c in the Supporting Information). The RCmin of PCHMs is −60 dB at 9.7 GHz with the thickness of 2.5 mm (blue region of Figure 10a,b, also see Figure S5b in the Supporting Information); it is just a little milder than −84 GHz, while the thickness is almost half thinner than 3.9 mm compared with the sample mixed with paraffin. Moreover, the EAB is increased to 8 GHz (from 10 to 18 GHz) with a composite thickness of 2.15 mm (see Figure 10b and the Figure S5b in the Supporting Information), compared with 4.8 GHz of PCHMs mixed with paraffin with a sample thickness of 2.25 mm. According to the quarter wavelength matching mechanism, the peak frequency of the reflection coefficient will move to low-frequency region after the relative permittivity increase (see eq 3 in the Supporting Information). As the permittivity of phenolic resin is larger than paraffin, the prepared phenolic composites containing 20 wt % PCHMs have a higher relative permittivity, so the peak frequency of the reflection coefficient decreases, and the emerged double peak absorption leads to a board EAB up to 8 GHz for a certain thickness of 2.15 mm. Obviously, the rational design of absorbers and reasonable choice of matrix that lead to a double peak absorption are beneficial to broaden effective absorption bandwidth.68

prepared functional material based on phenolic resin containing of 20 wt % PCHMs, the EAB reached up to 8 GHz with a thinner thickness of 2.15 mm, and the RCmin still achieved −60 dB. The superiority of PCHMs can be manifested from a comparison to other carbon absorbers and spherical absorbers made with other materials reported. It can be found that the large carbon−air interface in mesoporous shell and interior void should be responsible for the significant enhancement of EM wave absorption. Considering the enhancement of this special structure on EM wave absorption, a new strategy for making these PCHMs without a template should be developed in the future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15826. High-resolution TEM and FTIR spectra of PCHMs; DLS size distribution, outer diameter size distribution histograms of PCHMs, CHMs, and CSMs; surface area, pore volume, pore size, and XPS spectra of PCHMs, CHMs, and CSMs; the permittivity and reflection coefficient for PCHMs mixed with phenolic resin (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 29 88494947. Fax: +86 29 88494620. E-mail: [email protected]. ORCID

Hailong Xu: 0000-0003-1223-1693 Xiaowei Yin: 0000-0001-8756-8624 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51332004, 51372204, and 51602258)

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REFERENCES

(1) Shahzad, F.; Alhabeb, M.; Hatter, C. B.; Anasori, B.; Man Hong, S.; Koo, C. M.; Gogotsi, Y. Electromagnetic Interference Shielding with 2D Transition Metal Carbides (Mxenes). Science 2016, 353, 1137−1140. (2) Han, M. K.; Yin, X. W.; Wu, H.; Hou, Z. X.; Song, C. Q.; Li, X. L.; Zhang, L. T.; Cheng, L. F. Ti3C2 Mxenes with Modified Surface for High Performance Electromagnetic Absorption and Shielding in the XBand. ACS Appl. Mater. Interfaces 2016, 8, 21011−21019. (3) Yin, X. W.; Cheng, L. F.; Zhang, L. T.; Travitzky, N.; Greil, P. Fibre-reinforced Multifunctional SiC Composite Materials. Int. Mater. Rev. 2016, 61, 1−56. (4) Fan, Y. Z.; Yang, H. B.; Li, M. H.; Zou, G. T. Evaluation of the Microwave Absorption Property of Flake Graphite. Mater. Chem. Phys. 2009, 115, 696−698. (5) Zhou, H.; Wang, J. C.; Zhuang, J. D.; Liu, Q. A Covalent Route for Efficient Surface Modification of Ordered Mesoporous Carbon as High Performance Microwave Absorbers. Nanoscale 2013, 5, 12502− 12511. (6) Motojima, S.; Hoshiya, S.; Hishikawa, Y. Electromagnetic Wave Absorption Properties of Carbon Microcoils/PMMA Composite Beads in W bands. Carbon 2003, 41, 2658−2660.

4. CONCLUSIONS The PCHMs, successfully synthesized via a polymerization, pyrolysis, and etching process, display uniform particle size and are actually composed of an interior void and a mesoporous shell with the pore size of 4.7 nm. The PCHMs in paraffin matrix exhibit enhanced dielectric properties and EM waveabsorbing performance in the frequency range of 2−18 GHz, where the bandwidth with a reflection loss below −10 dB covers a wide range from 5 to18 GHz with the thickness of 1.65−5.0 mm. The RCmin can reach up to −84 dB at 8.2 GHz with a sample thickness of 3.9 mm. More importantly, for the 6339


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