Microporous Co@C Nanoparticles Prepared by Dealloying CoAl@C

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Microporous Co@C Nanoparticles Prepared by Dealloying CoAl@C Precursors: Achieving Strong Wideband Microwave Absorption via Controlling Carbon Shell Thickness Da Li, Haoyan Liao, Hiroaki Kikuchi, and Tong Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13538 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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ACS Applied Materials & Interfaces

Microporous Co@C Nanoparticles Prepared by Dealloying CoAl@C Precursors: Achieving Strong Wideband Microwave Absorption via Controlling Carbon Shell Thickness

Da Li,a Haoyan Liao,a Hiroaki Kikuchi,b and Tong Liua*

a.

Key Laboratory of Aerospace Materials and Performance (Ministry of Education)

School of Materials Science and Engineering, Beihang University, No.37 Xueyuan Road, Beijing 100191, P. R. China. *

Corresponding author: [email protected]

b.

Faculty of Engineering, Iwate University, Ueda, Morioka, 020-8551, Japan.

Abstract The excellent magnetic features make Co-based materials a promising candidate as high-performance microwave absorbers. However, it is still a significant challenge for Co-based

absorbers

to

possess

high-intensity

and

broadband

absorption

simultaneously owing to the lack of dielectric loss and impedance matching. Herein, microporous Co@C nanoparticles (NPs) with carbon shell thickness of 1.8-4.9 nm have been successfully synthesized by dealloying CoAl@C precursors. All the samples exhibit high microwave absorption performance. The microporous Co@C sample possessing a carbon shell of 1.8 nm exhibit the highest absorption intensity among these samples with a minimum reflection loss (RL) of -141.1 dB, whose 1

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absorption bandwidth for RL ≤ -10 dB is 7.3 GHz. As the thickness of the carbon shell increases, the absorption bandwidth of the NPs becomes wider. For the sample with the carbon shell thickness of 4.9 nm, the absorption bandwidth for RL ≤ -10 dB reaches a record high of 13.2 GHz. The outstanding microwave attenuation properties are attributed to the dielectric loss of the carbon shell, the magnetic loss of the Co core, and the cooperation of the core-shell structure and microporous morphology. The strong wideband microwave absorption of the carbon-coated microporous Co NPs highlights their potential applications in microwave absorbing systems. KEYWORDS: Co@C nanoparticles, dealloying, core-shell structure, microporous morphology, wideband microwave absorption

1. INTRODUCTION Recently, massive efforts have been dedicated to exploring high-performance microwave absorbers with broadband absorption and strong reflection reduction used in the fields of healthcare, radiation protection and electronic safety. As an important magnetic material, Co has become a promising microwave absorber owing to its large anisotropy field, high saturation magnetization and high-frequency level Snoek' limit,1 which offer Co-based materials excellent microwave absorption properties at high frequency.2,3 Over the past several decades, various Co microwave absorbers with different architectures such as nanochains4, nanobelts5, snowflakes6 and microflowers7 have been extensively studied. Although these Co-based materials can make high magnetic loss contribution to the electromagnetic (EM) wave absorption, it 2


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is still difficult for them to reach wide absorption bandwidth due to the lack of dielectric loss contribution, which greatly impedes their practical application. To achieve outstanding absorption performance, a variety of strategies have been devoted to integrating two or more kinds of materials to tune the EM parameters and adjust the impedance matching characteristics. One efficient way is to combine magnetic and dielectric materials by forming core-shell structure which can produce significant synergetic effects through heterogeneous EM couplings. Therefore, different dielectric materials were coated on the magnetic absorbers to further enhance the absorption capabilities, such as ZnO,8 TiO2,9 SnO2,10 PANI11 and so on. Recently, carbon-based materials with lightweight, high dielectric loss and thermal stability properties have attracted extensive research.12-15 The combination of carbon and Co is expected to improve the impedance matching of the absorber to achieve a wider absorption

bandwidth.

Lü

and

co-workers

synthesized

carbon-coated

Co

nanocomposites by the thermal decomposition of metal-organic framework (MOF) materials.16 The Co@C nanocomposite obtained at 500 °C displays the effective absorption bandwidth (RL≤-10 dB) of 5.8 GHz, but the highest absorption intensity value is just -35.3 dB. In order to address this limitation, Qi and co-workers tried to couple different kinds of carbonaceous materials with Co NPs to design hybrids with enhanced

interfacial

and

effects.17

synergistic

According

to

this

idea,

heteronanostructured Co@carbon nanotubes-graphene (Co@CNTs-G) ternary hybrids were fabricated by decomposing acetylene at different temperatures. The hybrids obtained at 400 °C cover a wide absorption frequency range (RL≤-10 dB) from 9-16 3



GHz and exhibit a stronger absorption intensity of -65.6 dB. Nevertheless, the synthesis process of the hybrids is relatively complicated and the minimum RL value needs to be improved further. Hence, it remains a scientific and technical challenge to develop an efficient strategy to further promote the absorption intensities of the composites composed of carbon and Co materials. Recently, porous architectures with high specific surface area have been found conducive to high microwave absorption intensity due to their interfacial polarization and multireflection.3,18,19 Porous iron particles display stronger absorption intensity than nonporous particles owing to the multipolarization, multiple dispersion and interference of EM waves in pores.19 Microporous Co@CoO nanoparticles exhibit a lower minimum reflection coefficient of −90.2 dB than the nonporous counterpart.3 Therefore, combining core-shell structure and porous architecture is expected to be an effective method to obtain ideal microwave absorber with both wide absorption bandwidth and strong EM wave absorbency. In our previous work, we synthesized carbon-coated microporous Co@CoO NPs through a two-step process by chemical dealloying first and coating carbon on these NPs by chemical vapor deposition (CVD) subsequently.20 However, the open micropores of the Co core are blocked by the deposited carbon and it is also difficult to achieve an even thickness of the carbon shell. Thus, it is still a significant challenge to construct core-shell structure together with porous architecture effectively. The arc plasma method is convenient and efficient for preparing in-situ carbon-encapsulated materials. Previously, the core of these core-shell products, such 4


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as Fe@C,21 Co@C22 and Ni@C, 23 is usually composed of one element. In this work, we synthesized in-situ carbon-coated binary Co-Al NPs by utilizing the arc discharge method, in which methane was used as the carbon source. By controlling methane volume in the plasma reaction chamber, the thickness of the carbon shell was accurately adjusted. After the chemical dealloying process, a series of microporous Co@C NPs with different carbon shell thickness were obtained. The NPs simultaneously possessed an outer carbon shell from 1.8 to 4.9 nm and inner micropores in the range of 0.5-1.2 nm. As we expected, these samples with core-shell structure and microporous morphology exhibited enhanced microwave attenuation properties. The microporous Co@C sample with a carbon shell of 1.8 nm has the minimum RL value of -141.1 dB at 13.3 GHz among these samples, whose effective absorption bandwidth is 7.3 GHz. With the increase in the thickness of carbon shell, the absorption bandwidth of the NPs becomes wider. For the sample with the carbon shell thickness of 4.9 nm, the absorption bandwidth for RL ≤ -10 dB reaches a record high of 13.2 GHz (4.8-18 GHz). The rational designed microporous Co@C NPs may provide new thoughts for synthesizing novel EM wave absorbing materials with both strong attenuation and wide absorption bandwidth.

2. EXPERIMENTAL SECTION 2.1 Preparation of the microporous Co@C NPs: The appliance to perform the arc plasma reaction has been described elsewhere.24 After arc melting Al and Co (the purities ≥99.9 wt%), the prepared Co-31.4 wt% Al bulk ingot was put into the reactor 5



chamber containing a mixed argon hydrogen atmosphere (the volume ratio of argon to hydrogen is 1:1) with a total pressure of 0.09 MPa. Next, a required volume of methane was added into the reaction chamber as the carbon source. The reaction current was selected to be 200 A. Methane was decomposed to provide the necessary carbon under a high temperature. The generated CoAl@C NPs were referred to as CA@C-50, CA@C-150 and CA@C-500, for the methane volume of 50, 150 and 500 mL, respectively. The carbon-coated microporous Co samples were then prepared by dealloying the CoAl@C precursors with an aqueous sodium hydroxide solution (20 wt%) at room temperature for 30 min with continuous agitation. The products were collected by magnet and washed with deionized water repeatedly. When the pH value of the washing water reached 7, these NPs were then immersed into ethanol and naturally dried in air. The final microporous Co@C samples prepared by dealloying the precursors CA@C-50, CA@C-150 and CA@C-500 were denoted as S50, S150 and S500, respectively. 2.2 Characterization: The structures of these NPs were investigated by X-ray diffraction (XRD) using a Rigaku X-ray diffractometer with monochromatic Cu Kα radiation. The morphology, composition and size distribution of the NPs were observed by transmission electron microscopy (TEM), high resolution-TEM (HR-TEM) and energy dispersive spectrometry (EDS) using JEOL JSM-2100F. The graphitization degree of the carbon shell was gauged using Lab RAM HR800 Raman spectrometer. The N2 adsorption-desorption isotherm of these porous NPs was measured by the Quantachrome Autosorb-IQ2 type volumetric gas analyzer at 77 K. 6


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To remove the absorbed impurities, the samples were activated by being evacuated in vacuum at 393 K for 60 min. The specific surface area (SBET) of the samples was calculated by the Brunauer-Emmett-Teller (BET) equation from the nitrogen adsorption data, and the pore size distribution was evaluated by the Saito-Foley (SF) method. The MS and HC values of the NPs were recorded by a LakeShore-7307 type vibration sample magnetometer (VSM) with an applied field up to 10 kOe. The cylindrical toroidal samples (7 mm for the outer diameter, 3 mm for the inner diameter and 2 mm for the thickness) were prepared by mixing the produced samples with paraffin wax (PW) at a weight ratio of 1:1 for the electromagnetic measurement. The relative permittivity (εr) and permeability (µr) in the frequency range from 2 to 18 GHz were measured by using a coaxial method with the HP-8722ES vector network analyzer. The microwave absorption properties were analyzed based on the transmission line theory.

3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the CoAl@C NPs prepared with different volumes of methane. All the samples display similar diffraction peaks to the Co-Al NPs prepared by arc plasma reaction without containing methane.3 The weak and broad hump at about 22° can be attributed to the amorphous carbon,25 indicating the low graphitization degree of carbon in the samples. As reported, a comparatively low graphitization degree is favorable for microwave absorbers to give rise to low complex permittivity and improve the impedance matching.26,27 It is worth noting that 7



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the XRD pattern of CA@C-500 displays four specific weak peaks at 31.7, 35.8, 40.1 and 55.1°. For example, in the region of 52-60°, it can be seen clearly that the characteristic peak at 55.1° is peculiar to the CA@C-500 NPs. These specific peaks can be indexed as Al4C3 (JCPDS 35-0799). A14C3 is usually fabricated by milling Al and C and applied as strengthening dispersoid in aluminum-matrix composites.28,29 In this work, carbon atoms are provided by the pyrolysis of methane during arc plasma according to the reaction (1): CH4 → C+2H2

(1)

When carbon concentration is large enough during arc plasma process with 500 mL methane, the evaporated Al atoms will react with carbon atoms to form Al4C3 in terms of the following reaction (2): 4Al+3C → Al4C3

(2)

For the cases with 50 and 150 mL methane, we suggest the carbon concentration is relatively low so that the reaction (2) cannot occur. Al4C3 is not the target product which could be regarded as the impurities in the system. It is well known that, at room temperature, aluminum carbide can hydrolyze to yield methane according to reaction (3):29 Al4C3 + 12H2O → 4Al(OH)3 + 3CH4

(3)

Considering Al(OH)3 is a non-magnetic material, it can be easily separated from the final magnetic NPs by magnet. Therefore, the subsequent chemical dealloying process is not only expected to create porous architecture, but also expected to be an effective way to remove the aluminum carbide in CA@C-500 NPs. 8


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The detailed information about the morphology and microstructure of the CA@C NPs were further investigated by TEM and HR-TEM. As displayed in Figure 2a-b, the CA@C-50 NPs with a spherical shape has a smooth carbon shell of 1.9 nm. The lattice fringes with a spacing of 0.205 nm and 0.202 nm correspond to the (16 2 0) and (110) crystal planes of Al13Co4 (JCPDS 50-0694) and CoAl (JCPDS 44-1115), respectively, which are in good accordance with the XRD results. Meanwhile, no lattice fringes can be detected in the carbon shell region, implying that the shell of these CA@C-50 NPs belongs to amorphous carbon. In the process of arc plasma, the high cooling rate provides the possibility of forming carbon with amorphous state. The morphology and microstructure of the CA@C-150 and CA@C-500 NPs are similar to those of the CA@C-50 NPs, shown in Figure 2c-f. The average sizes of these as-prepared NPs are 51 nm for CA@C-50, 48 nm for CA@C-150 and 45 nm for CA@C-500 (Figure S1). From the HR-TEM images (Figure 2b, 2d and 2f), one can see that the thickness of carbon shell increases with the methane volume. These results indicate that with proper control of the volume of methane, the thickness of carbon shell can be successfully tuned at the nanoscale. In addition, as the volume of methane increases, the thickness of the carbon layer coated on the Co-Al core remains uniform whereas the thickness of the adherence regions between particles becomes thicker and much less uniform, which resembles other carbon-encapsulated materials.30,31 Note that some tiny, scattered particles (2-8 nm) are distributed around the CA@C-500 nanoparticle (Figure 2f). The lattice fringe with a spacing of 0.274 nm corresponds to the (0 0 9) crystal plane of Al4C3, which can be clearly observed by the 9



HR-TEM image in Figure 2g. This phenomenon agrees well with the XRD results. Note that when the methane volume increases to 1000 mL (Figure S2), a large quantity of Al4C3 is generated. Therefore, it can be deduced that once the methane volume is larger than 500 mL, increasing methane volume does not lead to effective increase of carbon shell thickness. At the mean time, the yield of the desired dealloying precursor Al13Co4 phase also decreases due to the consumption of Al for the formation of Al4C3. As shown in Figure 3a, the strongest peak at 44.4° can be ascribed to fcc-Co (JCPDS 15-0806). Based on the Scherrer formula, the crystallite sizes of Co cores in S50, S150 and S500 are evaluated to be about 3, 5 and 6 nm, respectively. Note that, unlike the dealloyed Co NPs3 and the carbon-deposited microporous Co NPs,20 which contain a certain amount of CoO, no obvious diffraction peaks of CoO can be discerned in Figure 3a, indicating that the carbon shell is able to offer these samples outstanding anti-oxidation performance. Additionally, for S500, the diffraction peaks of Al4C3 also vanish, indicating that the aluminum carbide hydrolyzed during the dealloying process. More interestingly, one can see from Figure 3b that the S50 NPs float on the surface of sodium hydroxide solution after dealloying treatment, similar for the S150 and S500 NPs, which can be easily attracted by a magnet. This phenomenon illustrates that the dealloyed products possess a very low density and good magnetic feature. The bulk densities of S50, S150 and S500 are measured to be 0.881, 0.897 and 0.964 g cm-3, respectively. More detailed structural information of the carbon shell was further investigated by 10


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Raman spectra. As shown in Figure 3c, all the three samples display two distinguishable peaks assigned to D-bands (1350 cm−1) and G-bands (1590 cm−1), which are in good agreement with the previous literatures.32-34 The intensity ratio of D-band to G-band (ID/IG) is usually employed to determine the defects of graphitic materials. In general, a higher ID/IG ratio indicates a lower degree of graphitization in carbon.18 The ID/IG values of S50, S150 and S500 are 1.12, 1.24 and 1.58, respectively, much higher than that of other reported carbon-encapsulated materials (less than 1.0),35,36 implying that the carbon shell in the dealloyed NPs contains lots of defects. The specific surface areas and pore structures of the dealloyed samples were measured by the nitrogen adsorption-desorption isotherms at 77K. Based on the International Union of Pure and Applied Chemistry (IUPAC), all the three samples exhibit typical type H3 hysteresis loop (Figure 3d-f) which is the feature of materials with slit-like pores. The inset of Figure 3d-f displays the pore size distribution curves of S50, S150 and S500 calculated by the SF method. It can be seen that all the three samples display dominant pore size regions in the range of 0.5-1.2 nm. These results indicate that the microporous architecture has been successfully obtained by dealloying the CA@C NPs. As listed in Table 1, the determined BET surface areas (SBET) of S50, S150 and S500 are 64.5, 59.1 and 47.2 m2 g−1, respectively. The specific surface areas of the samples are larger than that of the carbon-deposited microporous Co@CoO NPs prepared by the CVD method,20 proving that the procedure of producing the CoAl@C NPs first and conducting the chemical dealloying subsequently is a more effective way to integrate the core-shell 11



architecture and microporous topography. Nanoscale particles with large specific surface area have more chemical bonds suspended on the surface, leading to high chemical activity which is favorable to the improvement of interfacial polarization37 and the final microwave absorbing performance. Figure 4 shows the TEM and HR-TEM images of the dealloyed NPs. The dealloyed samples also possess spherical shape with a number of tiny pores disperse uniformly in each nanoparticle (Figure 4a, 4c, 4e). The mean sizes of the dealloyed NPs (49 nm for S50, 45 nm for S150 and 44 nm for S500) decrease from their corresponding precursors due to the elimination of aluminum atoms in the Co-Al cores (Figure S1). Clearly, the contrast in the HR-TEM images (Figure 3b, 3d, 3f) confirms the core-shell structure of these dealloyed NPs. It is noteworthy that the dispersed small particles around the CA@C-500 nanoparticle (Figure 2f) disappeared in Figure 4f, which further corroborates the XRD results shown in Figure 3a. The thickness of the carbon shell does not change apparently after dealloying process, but it becomes rough owing to the volume shrinkage of the core during the chemical dealloying. The measured interlayer spacing of 0.203 nm (Figure 4b, 4d) and 0.205 nm (Figure 4f) correspond to the (111) plane of fcc-Co. The corresponding FFT images are shown in the inset of the HR-TEM images and the diffraction spots of {111} and {200} are marked with circles. All the results mentioned above indicate that the core-shell structure and porous architecture have been combined successfully in these dealloyed NPs. Based on the above analysis, the synthetic process for the microporous Co@C NPs 12


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is illustrated in Scheme 1. Under the effect of the plasma, Co and Al atoms are evaporated from the melted Co-Al ingot and C atoms are generated from the pyrolysis of methane. Then Co, Al and C atoms collide and aggregate into the Co-Al-C clusters. When the methane volume is 500 mL, some of Al atoms aggregate with the surplus C atoms to form Al-C clusters. With the decrease of the temperature, the Co-Al-C clusters grow into the CoAl@C NPs (During the consolidation process, carbon atoms will precipitate from the Co-Al alloy because according to the Co-C and Al-C phase diagram carbon can hardly dissolve in Co and Al at low temperature, especially near room temperature. Consequently, the metal atoms crystallize into nanocrystalline particle and the carbon atoms form the amorphous carbon layer surrounding the metal core during the rapid cooling process.38) and the Al-C clusters grow into the Al4C3 NPs. Finally, after the chemical dealloying process, the microporous Co@C NPs are obtained (the Al4C3 NPs were hydrolyzed during the dealloying process). For the cases of the methane volume of 50 and 150 mL, the corresponding microporous Co@C NPs are created in the similar way. The magnetic hysteresis loops of S50, S150 and S500 measured at 300 K are shown in Figure 5. Based on these M-H curves, the saturation magnetization (Ms) and coercivity (Hc) of S50, S150 and S500 were listed in Table 1. The Ms values of S50, S150 and S500 are 78.4, 74.5 and 64.9 emu g-1, respectively. With the increase of carbon shell thickness, the Ms values exhibit a decline tendency. The Hc values of S50, S150 and S500 shown in the inset of Figure 5 are 470.0, 432.8 and 397.3 Oe, respectively, significantly greater than that of the bulk Co (10 Oe)16 and microsize Co 13



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(generally < 100 Oe). This may be ascribed to the ultrafine grain sizes of these dealloyed NPs. Actually, a higher Hc value is favorable to shift the optimal RL peak to higher frequency region.39 Therefore, among these samples, the relatively low Hc value of S500 will be beneficial to improve its microwave absorption performance in lower frequency range. To evaluate the microwave absorption properties, we calculated the reflection values of the three samples based on the transmission line theory by the following two equations:40 = / tanh2/√

(1)

RLdB = 20 log| − / + |

(2)

where Zin is the input impedance of the absorber, Z0 is the impedance of free space, c is the velocity of the electromagnetic wave in free space, f is the frequency of the microwaves, and d is the thickness of the absorber. The calculated reflection losses of the three samples at different matching thickness in the range of 2-18 GHz are presented in Figure 6. For S50, the minimum RL (RLmin) value is as low as -141.1 dB at 13.3 GHz at a sample thickness of only 1.2 mm (Figure 6a-b), which is the highest RL value among the reported microwave absorbers. At the same sample thickness, the effective absorption bandwidth (RL ≤ -10 dB) reaches 7.3 GHz (10.7-18.0 GHz), see Figure 6b. For S150, the effective bandwidth of S150 increases to 8.1 GHz (9.9-18 GHz), see Figure 6d. From Figure 6f, it can be observed that the effective absorption bandwidth of S500 reaches up to 13.2 GHz (4.8-18 GHz) at a thickness of 2.7 mm, which almost covers the whole C, X and Ku band and is much wider than those of 14


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S50 and S150 and other reported materials. Compared with the Co@CoO@C NPs prepared by the CVD method,20 S500 exhibits both stronger absorption intensity and wider absorption bandwidth, demonstrating that the unique core-shell structure and microporous morphology generated in the Co@C NPs by dealloying CoAl@C NPs are more effective to achieve high microwave absorption performance. Table 2 comprehensively summarizes the microwave absorption performance of S50, S150 and S500. It is clearly that the absorption bandwidth becomes wider with the increase in the thickness of carbon shell. Moreover, the Co@C NPs with the thinnest shell thickness of 1.8 nm possess the strongest absorption intensity. These phenomena suggest that the microwave absorption performance of these microporous Co@C NPs can be easily tuned by controlling the thickness of carbon shell. Among the three samples, S500 with the widest absorption band of 13.2 GHz and strong absorption intensity of -111.5 dB is desirable for the application as a wideband absorber. Once the strongest absorption intensity in the range of X and Ku band is emphasized, S50 becomes the most suitable absorber. Thus we can flexibly choose the optimum microporous Co@C NPs for different absorbing applications. The optimal absorption performance of S500 and different Co-based materials and other core-shell absorbers are summarized in Figure 7. As shown in Figure 7a, many Co-based materials, such as Co@TiO2,41 Co/graphene,42 hollow Co spheres,2 CNFs-Co,43 Co ternary hybrids,24 Porous CNTs/Co composite,44 and sword-like Co,45 all exhibit strong absorption intensities from -40 to -70 dB, whereas their effective absorption bandwidths are often less than 7 GHz. After being coated by carbon, 15



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Co-based materials such as Co@graphite22 and Co@C NPs27 show an improved absorption bandwidth of about 11 GHz, but their absorption intensities are not enhanced. On the other hand, various types of core-shell absorbers, such as ZnO@Ni,46

Fe3O4@CuSiO3,47

Ni@Ag,48

Ni@SiO2,10

Co20Ni80@TiO249

and

PPy@PANI50 show satisfactory absorption bandwidth of more than 4 GHz (RL≤-10 dB), see Figure 7b. Recently, core-shell materials with multiple layers gradually become the hot point in EM absorbing research owing to their outstanding attenuation performance. For example, Fe@C@BaTiO351 displays an absorption bandwidth wider than 7 GHz, and FeSn2@Sn@C52 and CoNi@SiO2@TiO253 exhibit enhanced absorption bandwidth of 10 GHz whereas their absorption intensities need to be further enhanced. By combining core-shell and microporous structure, S500 has the strongest absorption intensity (RLmin value of -111.5 dB) and the widest absorption bandwidth (13.2 GHz for RL≤-10 dB) among these Co-based and other core-shell absorbers. This proves that S500 is highly promising for practical applications as high performance microwave absorbers. The thought of integrating core-shell structure and microporous architecture can be extended to the design of other EM wave absorbers. To further comprehend the possible microwave absorption mechanism of these microporous Co@C samples, the electromagnetic parameters of S50, S150 and S500 were measured between 2-18 GHz. Figure 8a displays the changes in the real part (ε') and the imaginary part (ε'') of the complex permittivity of all samples versus frequency. For S50, the real part (ε') stays at about 24 between 2-12 GHz, and then increases slowly to 26.3 at 15 GHz and decreases sharply to 21.2 at 18 GHz. Besides, 16


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the imaginary part (ε'') of S50 shows a slow increase from 2-15 GHz and exhibits a resonance peak between 15-18 GHz. As observed in Figure 8a, the permittivity curves of S150 have similar trend to that of S50 over the entire frequency range. For S500, the ε' value decreases from 19.6 to 9.5 in the range of 2-16 GHz and then stays constant up to 18 GHz, while the ε'' value increases from 1.9 to 6.7 between 2-10 GHz, and then exhibits a broad resonance peak around 12 GHz. With the increase of carbon shell thickness, the resonance peaks of the complex permittivity curves shift to low frequency, suggesting that a thicker carbon shell can induce enhanced polarization in lower frequency. In addition, considering that the three samples have similar Co core, it is rational to deduce that the numerical distinction of the complex permittivity curves is mainly caused by the carbon shell. For carbon materials, the differences of the ε' and ε'' values are generally resulted from the bonding state of carbon atoms which represents the graphitization degree.12 From the Raman data, it can be found that S500 exhibits a much higher ID/IG value than S50 and S150 (1.12 for S50, 1.24 for S150 and 1.58 for S500), indicating that more defects exist in the carbon shell. High density defects are not conducive to energy storage but are favorable to energy consumption. Thus, compared to S50 and S150, S500 exhibits lower ε' value but higher ε'' value. The dielectric loss properties of these microporous Co@C NPs are further evaluated by the dependence of the dielectric loss tangent (tan δε =ε''/ε') on the microwave frequency as displayed in Figure 8b. It is found that the tan δε value of S500 is much higher than those of S50 and S150, demonstrating that S500 with the thickest carbon shell offers the largest dielectric loss among the three 17



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samples. It is generally known that the dielectric loss mainly originates from the electron

polarization,

ion

polarization,

interfacial

polarization

and

dipole

polarization.52,54 Given that the electron polarization and ion polarization usually generate at THz and PHz, the resonance of permittivity in these three samples should dominantly derive from the interfacial polarization and dipole polarization.30 The heterogeneous interface between the carbon shell and the Co core induces the accumulation of the space charges at the interface,36 thereby leading to the interfacial polarization. In addition, the amorphous state of carbon shell and the micropores formed via chemical dealloying will give rise to many defects and dangling bonds, which can act as polarization centers under the EM field. These critical factors result in enhanced interfacial polarization in all samples. With the increase of frequency, the dipole polarization becomes the dominant factor and causes fluctuation of complex permittivity curves in high frequency range. The dipole relaxation is commonly described by the Debye relaxation equation as follows:35 & ' −

() *(+ ,

- + ' ′, = &

() /(+ , ,

-

(3)

where 0 and 1 are the static permittivity and relative dielectric permittivity at the high frequency limit, respectively. The semicircle in the curve of ε' versus ε'' is named as “Cole-Cole semicircle” which represents the Debye relaxation processes. From Figure S6a-c, it can be seen that the ε' versus ε'' curves of S50 and S150 both have three screwy Cole-Cole semicircles, whereas two distinct semicircles exist in the curves of S500. Note that the semicircle radiuses of S500 are much larger than those of S50 and S150, implying that a thicker carbon shell can induce enhanced Debye 18


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relaxation due to the better matching between ε' and ε'' values. In addition, based on Maxwell-Garnett (MG) theory, a porous material can be treated as a mixture of air and the host material,44 which also makes contributions to Debye relaxation owing to the modulation of the effective permittivity. Therefore, the microporous structure offers the possibility for a better impedance matching. Different from the real part of the complex permittivity, the µ' values of the three samples fluctuate between 0.9 and 1.3 in 2-18 GHz, as shown in Figure 8c. From 13-18 GHz, the µ' value gradually increases with the carbon content in these samples, which is also observed in the Fe3O4@C composites35 and Ni@C microspheres.55 This suggests that a thicker carbon shell is apt to inducing a larger µ' value in high frequency range. The µ'' curves of S50 and S150 almost overlap in 2-18 GHz, while S500 exhibits a higher µ'' value than S50 and S150 in the whole frequency range. Given that the imaginary part of the complex permeability manifests the magnetic loss of a material, S500 is expected to cause greater magnetic loss than the other two samples. Generally, the magnetic loss of an absorber can be ascribed to the magnetic hysteresis, domain wall resonance, eddy current loss and magnetic resonance (including natural resonance and exchange resonance).31,44 The magnetic hysteresis effect is insignificant under the weakly applied magnetization field. Besides, the domain wall resonance can also be ignored because this effect usually occurs in 1-100 MHz. Considering all the µ''(µ')-2f

-1

curves of the three samples intensively fluctuate

in 2-18 GHz (Figure S7), we suppose the eddy current effect has been markedly suppressed. The carbon shell coated on the Co core can limit the average size of the 19



particles to the level less than the skin depth, thereby avoiding the eddy current phenomenon. Therefore, the magnetic loss of these samples mainly derives from the magnetic resonance. With respect to the µ'' curves shown in Figure 8c, the resonance peak of S500 at about 2.5 GHz can be assigned to the natural resonance. Based on the Aharoni theory,56 the resonance peaks of S50 and S150 at high frequency range of 17.5 GHz correspond to the exchange resonance. Additionally, the broad resonance of the samples (4-15GHz for S50 and S150, 7-18 GHz for S500) can be attributed to the multiple-magnetic-resonance, which is favorable to widen the absorption bandwidth of the samples. Similar phenomenon has been reported in Ni@C nanocapsules57 and CoNi@C NPs.58 The frequency dependences of magnetic loss tangents (tan δµ= µ''/µ') of the microporous Co@C NPs are shown in Figure 8d. The magnetic loss tangents of three samples exhibit similar trend to their frequency dependences of µ'', indicating that the magnetic loss does make contributions to the absorption performance. It is worth noting that, for S500, the dielectric loss tangent and the magnetic loss tangent have relative close numerical values (0.08-0.71 for tan δε and 0.15-0.60 for tan δµ) over the whole frequency range, indicating that both dielectric loss and magnetic loss play vital roles in the microwave attenuation abilities. Many carbon-coated microwave absorbers such as Fe3O4@C nanorods,33 Co@graphite NPs22 and Ni@C microspheres55 only had strong dielectric loss or large magnetic loss, which failed to obtain the effective complementary between the complex permittivity and permeability. Under the synergistic effect of the carbon shell with appropriate thickness and the microporous architecture, S500 accomplishes the optimal match 20


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between the electromagnetic parameters, and consequently achieves both strong absorption intensity and wide absorption bandwidth. To further investigate the EM wave attenuation properties of S500, the frequency dependence of microwave reflection losses at different absorber thicknesses were calculated based on equation (1) and (2). The absorption peaks display negative shifts to lower frequency region and the peak values first go up and then fall off when the sample thickness increases from 1.8 to 3.2 mm, see Figure 9a, demonstrating that the absorption performance can be facilely tuned by changing sample thickness. This phenomenon can be interpreted by the quarter-wavelength cancellation model which obeys the following equation:43 23 = n/443 | || |6

(n = 1, 3, 5, …)

(4)

where tm and fm represent for the matching thickness and frequency of the minimum RL values, respectively, and the |εr| and |µr| are the modulus of the measured complex permittivity and permeability at fm, respectively. Based on this model, the RL reaches the minimum value at a given frequency when the thickness of the absorber satisfies equation (4). Figure 9b reveals the curve of tm versus fm for S500, where the violet line represents the tm derived from quarter-wavelength cancellation model and the red dots are the matching thicknesses related to minimum RL values derived from transmission line theory. It is clear to see that the relationships between the matching thickness and the frequency with RL peaks derived from transmission line theory are in good accordance with the results based on quarter-wavelength cancellation model. As analyzed above, the dielectric carbon shell cooperated with the magnetic 21



microporous Co core endow S500 better impedance matching. In fact, if the value of relative input impedance (|Zin/Z0|) is close to 1, most of the microwave will enter the absorber and achieve no reflection at the air-absorber interface. Figure 9c describes the frequency dependence of |Zin/Z0| for S500. At the thickness of 2.7 and 2.8 mm, the value of |Zin/Z0| is almost equal to 1, indicating that the microwave can favourably propagate into the absorbing materials rather than being reflected. On one hand, the air in the microporous Co core of S500 is conducive to the propagation of the microwave, thereby increasing the contact area between the microwave and the absorber. On the other hand, the large specific surface area derived from microporous morphology can produce more activity sites for energy attenuation. Additionally, the multi-scattering among the NPs also greatly contributes to the energy attenuation. Consequently, the EM wave can be effectively converted into thermal energy or dissipated. Therefore, S500 exhibits the best absorption performance at the matching thickness which agrees well with the calculated results (Table 2). By effectively combining core-shell structure and microporous morphology, the microporous Co@C NPs with light weight, small matching thickness, strong absorption intensity and wide absorbing bandwidth are promising for the EM wave absorption applications.

4. CONCLUSIONS Carbon-coated microporous Co NPs have been fabricated by combing the arc plasma method and chemical dealloying treatment. These NPs (44-49 nm) consist of outer carbon shell from 1.8 to 4.9 nm and inner micropores in the range of 0.5-1.2 nm. 22


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With the increase of the thickness of carbon shell, the Ms and Hc values of the samples display a downward trend. All the microporous Co@C samples exhibit enhanced microwave absorption performance. The NPs with a carbon shell of 1.8 nm have the highest absorption intensity among these samples with a minimum RL value of -141.1 dB at 13.3 GHz, whose effective absorption bandwidth is 7.3 GHz. As the thickness of the carbon shell increases, the absorption bandwidth of the samples becomes wider. The absorption bandwidth for RL ≤ -10 dB of the NPs with the carbon shell thickness of 4.9 nm reaches a record high of 13.2 GHz (4.8-18 GHz). The superior EM wave attenuation performance is ascribed to the dielectric loss caused by the carbon shell, the magnetic loss caused by the Co core and the cooperation between the core-shell structure and microporous morphology. The microporous Co@C NPs with the advantages of strong microwave absorbency, wide absorption bandwidth, thin matching thickness and low density may become a promising candidate for applications in the EM wave absorption field.

SUPPLEMENTARY INFORMATION The Supporting Information is available free of charge on the ACS Publications website. The particle size distributions of different samples; XRD pattern of CoAl@C NPs prepared with 1000 mL of methane; EDS spectrum of the S50, S150 and S500 NPs; Plots of the Cole–Cole semicircles and µ″(µ′)−2 f

−1

for the microporous Co@C NPs

supplied as Supporting Information. 23



ACKNOWLEDGEMENTS The authors acknowledge the support of this work by the Joint Fund of the National Natural Science Foundation of China and Baosteel Group Corporation (No. U1560106), the Aeronautical Science Foundation of China (No. 2016ZF51050) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry).

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32


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Table 1 Features of the microporous Co@C NPs Ala

Mean particle

Carbon shell

SBET

MS

Hc

[wt%]

size (nm)

(nm)

[m2 g-1]

[emu g-1]

[Oe]

S50

4.1

49

1.8

64.5

78.4

470.0

S150

4.5

45

2.9

59.1

74.5

432.8

S500

5.5

44

4.9

47.2

64.9

397.3

samples

a

The results are derived from EDS tests.

33



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Table 2 Microwave absorption properties of the microporous Co@C NPs Absorption bandwidth Optimal RL

Matching

RL≤

Thickness/

Sample

values/dB

thickness/mm

-10 dB/GHz

mm

S50

-141.1

1.2

7.3

1.2

S150

-89.6

1.3

8.0

1.3

S500

-111.5

2.8

13.2

2.7

34


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Figure Captions: Scheme 1. Schematic illustration of the synthesis procedure for the microporous Co@C NPs with different carbon shell thickness. Figure 1. XRD patterns of the CoAl@C NPs with different carbon contents. Figure 2. TEM and HR-TEM images of CA@C-50 (a, b), CA@C-150 (c, d) and CA@C-500 NPs (e, f); a HR-TRM image of the small particles around the CA@C-500 nanoparticle (g). Figure 3. XRD patterns of the three CA@C samples after chemical dealloying (a); Digital photographs of CA@C-50 NPs after dealloying treatment and attracted by a magnet (b); Raman spectra of the three dealloyed Co@C samples (c); Nitrogen adsorption-desorption isotherm of S50 (d), S150 (e) and S500 (f) and corresponding pore size distribution in the inset. Figure 4. TEM and HR-TEM images of the microporous Co@C samples: S50 (a, b), S150 (c, d) and S500 (e, f); the corresponding FFT images of S50, S150 and S500 in the inset of (b), (d) and (f). Figure 5. Magnetic hysteresis loops of the microporous Co@C samples: S50, S150 and S500; the inset shows the details of the magnetic hysteresis loops under the applied field of 1 kOe. Figure 6. Three-dimensional representations and two-dimensional projection images of the calculated theoretical reflection loss values of S50 (a, b), S150 (c, d) and S500 (e, f). Figure 7. Optimal microwave absorption performance of different Co-based materials 35



(a) and different core-shell absorbers (b). Figure 8. Frequency dependence of the real part and imaginary part of the complex permittivity (a) and the dielectric loss tangent (b) of the microporous Co@C NPs; Frequency dependence of the real part and imaginary part of the complex permeability (c) and magnetic loss tangents (d) of the microporous Co@C NPs. Figure 9. Frequency dependence of microwave reflection losses of the S500 sample (a); the relationship between thickness and peak frequency derived from quarter-wavelength cancellation model (b); the relationship between the relative input impedance (|Zin/Z0|) and the microwave frequency (c).

36


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Scheme 1

37



Figure 1

38


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Figure 2

39



Figure 3

40


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Figure 4

41



Figure 5

42


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Figure 6

43



Figure 7

44


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Figure 8

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Figure 9

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Table of contents

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Microporous Co@C Nanoparticles Prepared by Dealloying CoAl@C

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