Enhanced Electromagnetic Microwave Absorption Property of Peapod

Oct 31, 2018 - Investigating lightweight electromagnetic microwave absorption materials is still urgent because of the issue related to the electromag...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 40078−40087

Enhanced Electromagnetic Microwave Absorption Property of Peapod-like MnO@carbon Nanowires Yongli Duan,† Zhihua Xiao,† Xiaoya Yan, Zhenfei Gao, Yushu Tang, Liqiang Hou, Qi Li, Guoqing Ning, and Yongfeng Li* State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, Beijing Changping 102249, China

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ABSTRACT: Investigating lightweight electromagnetic microwave absorption materials is still urgent because of the issue related to the electromagnetic pollution or military defense. Our findings indicate that core−shell MnO@carbon nanowires (MnO@C NWs) achieve substantially enhanced microwave absorption, suggesting the suitable impedance matching induced by the synergetic effect between MnO and carbon. Furthermore, the peapod-like MnO@C NWs with internal void space can be facially synthesized by partial etching of core−shell MnO@C NWs. The peapod-like MnO@C NWs with internal voids/cavities exhibit dramatically enhanced electromagnetic microwave absorption property when the carbon content is about 64 wt %, a minimum reflection loss (RL) of −55 dB at 10 wt % loading was observed at 13.6 GHz, and the bandwidth of RL less than −10 dB (90% absorption) covers 6.2 GHz at the thickness of 2 mm. The excellent electromagnetic microwave absorption performance is superior to the most of MnOx/C composites in the literatures, which probably benefits from the dielectric polarization among conductive network structure between MnO and carbon, as well as the multiple reflection and absorption induced by internal void space. Our work is expected to pave an effective way to extend the electromagnetic microwave absorption performance of MnO/C composites through partial etching to create a void space. KEYWORDS: electromagnetic microwave absorption, peapod, carbon coated, MnO, nanowires

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INTRODUCTION Exploration of efficient electromagnetic microwave absorption materials (EMMs) is one of attractive research fields facing the electromagnetic wave issue such as radiation, electronic safety, and military defence.1−4 As is well known, the electromagnetic microwave energy is converted into thermal energy with the incident waves exposure to a lossy dispersive material due to the interaction between the electromagnetic field and the material’s electronic structure.5 Thus, ideal EMMs can make the electromagnetic waves permeate into its interior as much as possible and enhance the dissipation of the electromagnetic microwave energy. During the last decades, carbonaceous materials have played a significant role in EMMs due to their facial accessibility, low density, abundant morphologies, and tunable conductivity.6−10 Among the widely investigated carbon-based EMMs, great effort have been taken to reveal the relationship between the electromagnetic microwave absorption (EMA) performance and novel structure, such as hollow,11 flower-like,12 and yolkshell13 structure, which have noticeably enhanced the EMA performance due to the shape anisotropy and size distribution. Even though the carbon materials with different morphology possess high dielectric loss, the single dielectric loss shows some limitation to broaden the higher efficient EMA performance. Metal oxide decorated by carbon materials © 2018 American Chemical Society

could effectively enhance the microwave absorption ability due to the tunable electromagnetic properties and synergetic loss mechanism. Typically, several carbon-coated core−shell nanocomposites such as Fe3O4/C,14,15 FeCo@C,16,17 CoNi@ C,18 and MnOx@C19,20 have been compounded with the aim to improve the electromagnetic microwave attenuation. The manganese oxides have been widely investigated as lightweight EMMs for enhancing the electromagnetic wave attenuation of carbon composites materials owing to its low density, effective permittivity, and environment friendliness.21 Bora and co-workers found that coating MnO2 nanorods with poly(vinyl butyral) (PVB) nanocomposites can improve the dielectric permittivity compared with MnO2 nanosphere because the nanorods in the PVB matrix increase the heterogeneity, which results in interfacial polarization.22 Reduced graphene oxide (RGO)/Mn3O4 nanocomposites have demonstrated excellent electromagnetic interference shielding performance owing to the enhanced electrical conductivity induced by unique two-dimensional nanostructure of RGO.23 The enhanced microwave absorption properties of composite carbonyl iron/MnO2 are attributed to the Received: July 9, 2018 Accepted: October 31, 2018 Published: October 31, 2018 40078

DOI: 10.1021/acsami.8b11395 ACS Appl. Mater. Interfaces 2018, 10, 40078−40087

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the synthesis process of core−shell MnO@C NWs and peapod-like MnO@C NWs. different mass ratios (1:1, 1:2, 1:3) at 700 °C for 60 min under Ar atmosphere. The obtained core−shell MnO@C NWs were denoted as MnO@C-1, MnO@C-2, and MnO@C-3, respectively. Synthesis of the Peapod-like MnO@C Nanowires. Solution A was obtained by mixing the hydrochloric acid (36 wt %) with stilled water at a volume ratio of 1:15. Subsequently, a series of MnO@C-1, MnO@C-2, and MnO@C-3 composites (1 g) were respectively added into 150 mL of solution A accompanied by ultrasonic agitation for 30 min. Then, the above-obtained solution was transferred into a round-bottom flask and refluxed for 30 min at 100 °C. After cooling to room temperature, the as-prepared peapod-like MnO@C NWs were denoted as MnO@C-1a, MnO@C-2a, and MnO@C-3a, respectively. Characterization. The X-ray diffraction (XRD) patterns were carried on Bruker D8-advance. The surface morphology and the structure of MnO@C nanowires were observed by FEI Quanta 200F scanning electron microscopy (SEM) and FEI F20 transmission electron microscopy (TEM). Specific surface areas and pore size distribution of the samples were obtained by N2 adsorption− desorption isotherms at 77 K (micromeritics ASAP 2020 instrument) by using Brunauer−Emmett−Teller (BET) method and Barret− Joyner−Halenda (BJH) method, respectively. Raman spectroscopy was performed on a Renishaw InVia Reflex (at 532 nm). The X-ray photoelectron spectroscopy (XPS) carried out by a Thermo Fisher KAIpha spectrometer was used to analyze the elemental composition of the MnO@C NWs. Thermogravimetric analysis (TGA) was carried out on a TGA instrument STA-7200 at a heating rate of 5 °C min−1 in air. The direct current conductivity of the samples was characterized by a four-probe resistivity tester. Electromagnetic Measurements. The electromagnetic parameters of the sample were measured by an Agilent PNA N5224A vector network analyzer in the frequency range of 2−18 GHz at room temperature. The core−shell MnO@C NWs were uniformly dispersed in paraffin wax with 30 wt %. In addition, 10 wt % peapod-like MnO@C NWs mixed with paraffin because the lowdensity composites are hard to shape when the samples are overloading. (The weight percentage of the samples has been explored to achieve an optimal performance as shown in Figure S1.) And then, the samples mixed with paraffin were pressed into a coaxial cylinder with 7.0 mm outer diameter and 3.04 mm inner diameter. Based on the transmit line theory, the electromagnetic microwave absorption performance can be reflected by the reflection loss,28 which loss can be calculated by the following formula

coexistence of dielectric loss, magnetic loss, and electromagnetic match.24 Interestingly, core−shell MnO@C composites have been widely invested as high-capacity and long-life anode material for lithium-ion battery,25−27 whereas they remain rarely regarded as efficient EMMs. Therefore, it is urgent to investigate the feasibility of core−shell MnO@C for the application of EMMs. Hence, by combining low-density high dielectric of the carbon and better dielectric property of MnOx, we have facially integrated the core−shell carbon coated MnO nanowires (MnO@C NWs) by the pyrolysis of triphenylphosphine (TPE) and the carbon layers were in situ deposited on the surface of MnO NWs, which was the first time used as the electromagnetic microwave absorbers. The carbon content can be adjusted by controlling the mass of the TPE, which offer us an opportunity to explore the core−shell structure and synergetic function between carbon and MnO upon adsorption performance at a detail level. Notably, our work reveals that the specific carbon content coated on MnO NWs also plays an important role in determining the EMA performance. Furthermore, the MnO@C NWs have been partially etched by boiling HCl, during which partial MnO was etched and numerous voids/cavities were produced. As expected, the peapod-like MnO@C NWs possess large surface area, suitable impedance matching with relative complex permittivity, and substantially enhanced EMA performance, with the minimum reflection loss (RLmin) of −55 dB at 13.6 GHz when the thickness is 2.1 mm; also, effective EMA bandwidth (RL < −10 dB) can cover 6.2 GHz when the thickness is only 2 mm at 10 wt % loading. Such an outstanding EMA performance is probably due to the good impedance matching, multiple interfacial polarization, and multiple reflection and absorption arising from the peapod-like MnO@C NWs with numerous internal space void. The rationally designed peapod-like MnO@C NWs may provide a new way that can be extended to other lightweight EMMs.

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EXPERIMENTAL SECTION

Synthesis of MnO Nanowires. The MnO nanowires have been prepared by a hydrothermal method and subsequent calcination process. The steps are as follows: commercial MnO2 (2 g), urea (1 g), and sodium dodecyl benzene sulfonate (2 g) were dissolved in 70 mL deionized water with constant electromagnetic stirring for 30 min. Then, the solution was transferred into a 80 mL teflonlined autoclave and maintained at 150 °C for 12 h. After cooling to room temperature naturally, the attained brown material was centrifuged and washed for several times with deionized water and ethanol. Subsequently, the brown material was dried in an oven at 80 °C for 12 h. The MnO nanowires were prepared by heating the as-synthesized MnOOH whiskers in 700 °C for 2 h at the rate of 5 °C min−1 under the Ar atmosphere. Synthesis of Core−Shell MnO@C Nanowires. The carboncoated MnO NWs were prepared by heat treatment with TPE with

RL = 20 log

Zin − Z0 Zin + Z0

Zin = (μr /εr)1/2 tanh {j(2πfd /c)(με )1/2 } r r where μr is the relative permeability, εr is the relative permittivity, f is the microwave frequency, c is the velocity of light, d is the thickness of the samples, Z0 is the free-space impedance, and Zin presents the input impedance. 40079

DOI: 10.1021/acsami.8b11395 ACS Appl. Mater. Interfaces 2018, 10, 40078−40087

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) XRD pattern of the MnO NWs, MnO@C-1/1a, MnO@C-2/2a, and MnO@C-3/3a and (b−d) SEM images of MnO NWs, MnO@ C-2, and MnO@C-2a.

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RESULTS AND DISCUSSION The nanoscale MnO NWs have been synthesized by the hydrothermal method and subsequent calcination process. The preparation process of the core−shell MnO@C NWs and peapod-like MnO@C NWs is illustrated in Figure 1. First, the core−shell structure of carbon-coated MnO nanowires were successfully synthesized by the pyrolysis of TPE, and the carbon atoms were in situ deposited on the pore channels and surface of MnO NWs. Meanwhile, the encapsulated MnO particles in the carbon layers will shrink due to the long-time calcination at 700 °C. To some extent, the calcination process generates little space between the two adjacent MnO particles. The carbon content of MnO@C NWs can be changed by adjusting the carbon source mass ratio. The as-prepared core− shell MnO@C NWs were partially etched with an acidic solution by the condensation return equipment. During the process, the low concentration of acidic solution was homogeneously permeated into the pore channels of the core−shell MnO@C NWs and reacted with the partial MnO, which leads to the generation of a larger void space between the two adjacent MnO particles. In addition, a limited reaction between the HCl and the MnO particles by a short-time (30 min) refluxing process, which also contributed to the formation of the peapod-like MnO@C composites. The XRD analysis confirmed the formation of MnO, as shown in Figure 2a. For pure MnO, MnO@C-1/1a, MnO@C2, and MnO@C-3, five diffraction peaks at 2θ values of 34.91, 40.547, 58.722, 70.176, and 73.703° can be indexed to the diffraction of (111), (200), (220), (311), and (222) planes of standard cubic MnO (JCPDS no. 07-0230), indicating the formation of MnO.25,29 However, the diffraction peak of carbon for these samples cannot be observed in the XRD pattern because the content of carbon is too low. Apparently,

the wide diffraction peaks appearing at 22.7 and 44.2° in MnO@C-2a and MnO@C-3a are indexed to the characteristic planes (002) and (101) of graphitic carbons, respectively. The result can be further confirmed by the reported literature.30 However, no obvious characterization diffraction peaks of MnO can be detected in MnO@C-2a and MnO@C-3a, probably because MnO is partially etched and the remaining MnO is embedded deep in carbon layers, which can be verified by the TEM images. The morphology and structure of the samples have been characterized by the SEM and TEM. After calcination, porous MnO NWs with a smooth surface can be observed in Figure 2b. The rough surface of the nanowires has been regard as the carbon layers (Figure 2c), which can be ascribed to the in situ pyrolysis and deposition of TPE. MnO@C-1 and MnO@C-3 show the same morphology with different carbon content coated on the surface of the MnO nanowires (Figure S2). The nanowire structures of MnO@C-2a were well kept (Figure 2d) when it was partially etched. The TEM images of MnO@C-2 and MnO@C-2a with different magnifications are shown in Figure 3. Clearly, Figure 3a,b displays that MnO NWs are well confined in the carbon shell, which can effectively improve the electrical conductivity. After partial etching, MnO was partially removed and a lot of voids were generated between the carbon layers, as shown in Figure 3c,d. Meanwhile, many peapod-like MnO@C NWs are crossed and twisted with each other. The high-resolution TEM (HRTEM) image in Figure 3e taken from the edge of a nanopeapod indicates that the lattice space of the sample is 0.26 nm, consistent with the distance of (111) plane of the cubic MnO.25 The inset of Figure 3e shows the corresponding fast Fourier transform (FFT) pattern, which can be assigned to the cubic structure. The surface-scanning element mapping (Figure 3f) clearly indicates the distribution of Mn, C, and O elements. Along the nanopeapod, Mn and O 40080

DOI: 10.1021/acsami.8b11395 ACS Appl. Mater. Interfaces 2018, 10, 40078−40087

Research Article

ACS Applied Materials & Interfaces

edges, and lattice distortion, whereas the G band is caused by the sp2-hybridized structure, which is associated with the crystalline structure.33 It is evident that the signal of carbon is weak in MnO@C-2 because the carbon content is much lower than the MnO content. The low ID/IG intensity ratio of 0.62 and 0.96 for MnO@C-2 and MnO@C-2a, respectively, indicates that there exist many edges in the graphite structure and large number of defects. The carbon content of all the MnO@C NWs has been measured by TGA in Figure S3. From room temperature to 150 °C, the weight loss is attributed to the evaporation of the water absorbed on the surface of porous mesopores. However, the weight changes above 200 °C depend on the oxidation of MnO to Mn2O3 and combustion of carbon. The carbon contents of 3, 6.4, and 13.2% were estimated from the TGA curves of MnO@C-1, MnO@C-2, and MnO@C-3, respectively. After MnO was partially removed, the carbon contents are sharply increased in peapod-like MnO@C NWs, as shown in Figure S3. The element analysis of MnO@C-2a has been carried out by XPS to analyze the information on the surface electronic state and the composition of the samples. As shown in Figure 4d, the peaks of Mn (2s, 2p1/2, 2p3/2, and 3p) and O (1s) in the survey of the XPS spectrum can be assigned to MnO. The Mn2+ 2p3/2 and 2p1/2 peaks at 641.2 and 653 eV, respectively, are consistent with the characteristics of MnO.34,35 The C 1s spectrum can be deconvoluted into three peaks at 284.7, 286.2, and 289.0 eV (Figure 4e). The peak at 284.7 eV corresponds to the C−C bonds (Figure 4f). Meanwhile, the weaker peaks at 286.2 and 289.0 eV are attributed to some residual oxygenbond C atoms.36 The EMA performance of MnO@C NWs has been investigated by the vector network analyzer. Generally, the EMA mechanism depends on the relative complex permittivity (ε′, ε″) and permeability (μ′, μ″). The real part of the permittivity and the imaginary part of the permittivity are related to the dielectric properties of the materials, whereas the real part of the permeability and the imaginary part of the permeability are associated to magnetic loss. As shown in Figure S4, the values of μ″ and μ′ are nearly constant at 0 and 1, respectively, indicating that the microwave absorption enhancement of all the MnO@C NWs is primarily from the dielectric loss.37,38 It is well known that ε′ and ε″ represent the storage ability of electric and loss capacity of electric, respectively. However, for all the MnO@C NWs samples, both ε′ and ε″ increase with increase in carbon content, demonstrating that dielectric properties are dependent on the conductivity (Table S2).39−42 Similarly, both real permittivity and imaginary permittivity of the MnO@C NWs present a declining trend during the frequency range from 2 to 18 GHz, which is ascribed to the relaxation effect of the hybrids. As shown in Figure 5a,b, both ε′ and ε″ of the core−shell MnO@ C NWs increase with increase in carbon content, which means both performance of storing electric energy and transforming electromagnetic energy to heat energy are enhanced. Particularly, both ε′ and ε″ of MnO@C-3a are dramatically higher than that of other samples, as shown in Figure 5c,d, which is consistent with the results of other studies for carbon nanotubes43−45 because the carbon content is about 95 wt %, as shown in Figure S3b. And, the MnO@C-3a possesses the percolation threshold resonant effect property.46 Namely, the carbon layers construct the conductivity network, leading to accelerated electron hopping and migrating.47 However, the higher conductivity could result in more reflection due to the

Figure 3. TEM images of (a, b) MnO@C-2 and (c, d) MnO@C-2a. (e) HRTEM images of MnO@C-2a (the inset showing the corresponding FFT image). (f) Elemental mapping analysis for the MnO@C nanopeapod.

display gaps between two MnO nanoparticles, whereas carbon uniformly covers the whole nanopeapod, further confirming the formation of peapod-like MnO@C. The porous texture of MnO@C NWs have been investigated by N2 adsorption−desorption isothermal (Figure 4a) and Barrett−Joyner−Halenda (BJH) pore size distribution curves (Figure 4b). The BET specific surface area is about 26 cm2 g−1 and the pore volume is 0.17, 0.22, and 0.16 cm3 g−1 for MnO@ C-1, MnO@C-2, and MnO@C-3, respectively (Table S1). However, the pure MnO NWs exhibit a minimum surface area of 14.7 cm2 g−1 and a pore volume of 0.08 cm3 g−1, indicating the key role of carbon in the formation of mesopores. After partial etching, the surface areas of the samples are greatly increased, and the corresponding values are 74.993, 679.949, and 778.95 cm2 g−1, respectively (Table S1). Apparently, the acid treatment effectively increased the surface area and pore volume of MnO@C owing to the creation of numerous voids/ cavities. It is well known that the pores play a significant role in the EMA processes, which can improve the dielectric property and promote the interfacial polarization, giving rise to a relatively high reflection loss.31 Furthermore, the structure of carbon was analyzed by Raman spectroscopy, as shown in Figure 4c. The diffraction peaks of MnO at 352 and 640 cm−1 indicate the existence of MnO, for which the D and G bands are observed at 1354 and 1592 cm−1, respectively.32 The D band is attributed to defects, 40081

DOI: 10.1021/acsami.8b11395 ACS Appl. Mater. Interfaces 2018, 10, 40078−40087

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ACS Applied Materials & Interfaces

Figure 4. (a) Nitrogen adsorption−desorption isotherm diagram of MnO@C-2 and MnO@C-2a. (b) Pore size distribution of MnO@C-2a. (c) Raman shift of the MnO NWs, MnO@C-2 and MnO@C-2a. (d−f) XPS patterns of MnO@C-2a: (d) survey spectra, (e) Mn 2p spectrum, and (f) C 1s spectrum.

MnO@C-2a with 64 wt % (Figure S3) carbon content has been further fabricated after the partial etching of core−shell MnO@C-2 NWs, which also exhibits the optimized EMA performance at 10 wt % loading. The RLmin of MnO@C-2a reaches −55 dB at 13.6 GHz with a thickness of 2.1 mm, and the effective absorption bandwidth can cover 6.2 GHz with the thickness of 2 mm (Figure 6d). Compared with the core−shell MnO@C-2 NWs, increase in voids/cavities in the MnO@C NWs can shift the maximum attenuation toward a lower frequency and increase the maximum absorbing effect, and also can enlarge the absorbing frequency bandwidth. The excellent EMA performance is possibly attributed to the enhanced multiple interfacial polarizations, as well as multiple absorption and reflection induced by internal voids/cavities. After the comparison of the other MnOx compounds (Table S3) and carbon-based hybrids (Table S4), the peapod-like MnO@C-2a NWs show evident advantages of absorber loading, thickness, and effective absorption width. Therefore, the introduction of voids/cavities can increase both reflection loss and absorption bandwidth, especially reducing the mass fraction when mixed with paraffin. To further understand the reasons for the excellent EMA performance, the dielectric loss tangent and modulus of Zin − 1 of various samples have been investigated, as shown in Figure

shorter distance of the resonance multipoles with more carbon nanotubes. The RL has been calculated based on transmission line theory, and the results of different thickness (3 and 3.5 mm) for all MnO@C NWs are shown in Figure 6a,b. It is clear that MnO@C NWs with different thickness demonstrate different EMA performance because of the quarter wavelength interference enhancement.48 When the thickness is 3 mm, MnO@C-2 and MnO@C-2a perform the best with RLmin of nearly −40 dB at 8 and 8.48 GHz, respectively. When the thickness is 3.5 mm, MnO@C-2a outperforms with −51.5 dB at 7.1 GHz. The EMA ability of MnO@C-2 and MnO@C-2a at different thicknesses has been further evaluated (Figure 6c,d), and the best matching thickness is given for each sample. For the core−shell MnO@C NWs (30 wt % loading), MnO@ C-2 exhibits the best EMA ability, as shown in Figure 6c, with its RLmin of −47.5 dB at 12.2 GHz at a thickness of 2.1 mm. As is well known, because the carbon content can effect the conductivity, the lower conductivity will result in weak attenuation due to decreased dielectric loss. However, the higher conductivity may lead to a poor absorption capacity like metal due to the strong reflection toward incident waves on the first surface.12 Therefore, the suitable content of carbon and MnO can reach a well impedance matching. The peapod-like 40082

DOI: 10.1021/acsami.8b11395 ACS Appl. Mater. Interfaces 2018, 10, 40078−40087

Research Article

ACS Applied Materials & Interfaces

Figure 5. (a, b) Real and imaginary parts of the permittivity of core−shell MnO@C NWs with different MnO/C ratios. (c, d) Real and imaginary parts of the permittivity of peapod-like MnO@C NWs with different MnO/C ratios.

Figure 6. Reflection loss of MnO@C-1, MnO@C-2, MnO@C-3, MnO@C-1a, MnO@C-2a, and MnO@C-3a with thickness of (a) 3 and (b) 3.5 mm. Reflection loss of (c) MnO@C-2 and (d) MnO@C-2a with different thickness.

7. The dielectric loss tangent (tan δe = ε″/ε′) is calculated based on the complex permittivity, indicating the dielectric loss capacity of the EMMs. As shown in Figure 7a, MnO@C-2 owns the relative high dielectric loss tangent in the whole frequency, which means that the excellent dielectric loss capacity originates from conductivity loss and polarization loss. The enhanced EMA performance of MnO@C-2 is probably attributed to the interfacial polarization between the MnO and carbon layers. In addition, the suitable carbon content of

MnO@C contributes to the dielectric loss. For the peapod-like MnO@C NWs, the trends of tan δe are different, and the value of tan δe increases with increasing carbon content (Figure 7b). When the carbon content is 95%, there is a sharp increase in dielectric loss tangent. Such a phenomenon corresponds to a phase transition from an insulator to a conducting composite, during which a dramatic change in the electrical resistivity with a corresponding change in its electromagnetic characteristics can be observed.20,45 It is worth noting that too a high 40083

DOI: 10.1021/acsami.8b11395 ACS Appl. Mater. Interfaces 2018, 10, 40078−40087

Research Article

ACS Applied Materials & Interfaces

Figure 7. Dielectric loss tangent of (a) core−shell MnO@C NWs and (b) peapod-like MnO@C NWs with different MnO/C ratios. (c, d) Modulus of Zin − 1 of various samples with a thickness of 2.1 mm.

Figure 8. The possible mechanism of the electromagnetic microwave absorption in peapod-like MnO@C NWs. (a) Conductivity network and (b) multiple interfacial polarization.

permittivity might promote reflection rather than absorption, which has negative effect on impedance matching. The dielectric loss tangent of MnO@C-2a increases from 0.35 to 0.45 and then 0.45−0.4 in the frequency range from 2−11 and 11−18 GHz and higher than MnO@C-2. Clearly, the peapodlike MnO@C-2a exhibits a suitable stronger dielectric loss, promoting its excellent EMA capacity. Generally, the huge difference between the complex permittivity is detrimental to impedance matching and usually results in strong reflection when incident waves are exposed to the surface of EMMs. The modulus of Zin − 1 can be used to characterize impedance matching, and the waves can achieve zero reflection when the value is equal or close to zero. It is evident that the |Zin − 1| value of MnO@C-2/2a with a thickness of 2.1 mm is closer to zero, suggesting the best impedance matching compared with the other MnO@C NWs (Figure 7c,d). In this case, the incident waves permeate the

MnO@C-2/2a absorber with minimum reflection and then converted to heat by strong dielectric loss. The peapod-like MnO@C NWs with a high conductivity have a positive effect on the dielectric loss, which plays a dominant role in the electromagnetic microwave attenuation, as shown in Figure 8. As the peapod-like MnO@C NWs are twisted together, the carbon shell structure can construct a denser conductive network, which effectively increases the propagation of electromagnetic microwaves.49 The conductive network plays an important role in dielectric loss, in which numerous conductive paths are provided for electrons hopping and migration between MnO particles and the conjugated carbon layers (Figure 8a),50−52 which is in accordance with the Debye theory.53 The peapod-like MnO@C NWs generate abundant interfaces, including carbon layers−carbon layers, carbon layers−MnO particles, and carbon layers−paraffin (Figure 8b), which bring strong relaxation loss as a result of 40084

DOI: 10.1021/acsami.8b11395 ACS Appl. Mater. Interfaces 2018, 10, 40078−40087

Research Article

ACS Applied Materials & Interfaces interfacial polarization.54−58 The pores originated from the carbon layers and voids/cavities are produced during the partial etching process, which also can contribute to the strengthening of the interfacial polarization.59 For the peapodlike MnO@C-2a, the MnO particles are well confined in the carbon matrix, which contributes to multiple internal scattering and attenuation of electromagnetic microwaves.60 In addition, the peapod-like MnO@C NWs achieve outstanding electromagnetic microwave attenuation because of the well impedance matching. Therefore, the peapod-like MnO@C NWs can be an excellent candidate for EMMs.

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CONCLUSIONS In summary, the peapod-like MnO@C NWs with internal void space are successfully prepared by partial etching of the core− shell MnO@C NWs, and samples with different carbon coating content are achieved by controlling the amount of carbon source. The core−shell MnO@C-2 NWs exhibit high level of electromagnetic microwave absorption ability owing to its perfect impedance matching and interfacial polarization. Remarkably, the peapod-like MnO@C NWs show improved EMA performance with minimum RL of −55 dB at 13.6 GHz with a thickness of 2.1 mm, and the effective absorption bandwidth is about 6.2 GHz at a thickness of 2 mm with only 10 wt % loading. The excellent EMA ability is associated with the conductive network, multiple interfacial polarization, and internal void/cavities. More importantly, this work is expected to open up new avenues for developing MnOx combined with carbon materials as efficient electromagnetic microwave absorbers.

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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.8b11395.

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SEM, TGA, BET, and electromagnetic microwave absorption parameters of the products (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: yfl[email protected]. Tel: +86-10-8973-9028. ORCID

Zhenfei Gao: 0000-0002-6175-0986 Yongfeng Li: 0000-0003-0855-7949 Author Contributions †

Y.D. and Z.X. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank the National Natural Science Foundation of China (Nos 21776308 and 21576289), Science Foundation of China University of Petroleum, Beijing (No. C201603), and Thousand Talents Program.

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DOI: 10.1021/acsami.8b11395 ACS Appl. Mater. Interfaces 2018, 10, 40078−40087