Hollow N-Doped Carbon Polyhedron Containing CoNi Alloy

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Surfaces, Interfaces, and Applications

Hollow N-Doped Carbon Polyhedron Containing CoNi Alloy Nanoparticles Embedded within Few-Layer N-Doped Graphene as High-Performance Electromagnetic Wave Absorbing Material Xiao Zhang, Feng Yan, Shen Zhang, Haoran Yuan, Chunling Zhu, Xitian Zhang, and Yujin Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07107 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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ACS Applied Materials & Interfaces

Hollow N-Doped Carbon Polyhedron Containing CoNi Alloy Nanoparticles Embedded within Few-Layer N-Doped Graphene as High-Performance Electromagnetic Wave Absorbing Material Xiao Zhang,†,#,‡ Feng Yan,† Shen Zhang,† Haoran Yuan,† Chunling Zhu,*,‡ Xitian Zhang,# Yujin Chen*,† † Key Laboratory of In-Fiber Integrated Optics, Ministry of Education, and College of Science, Harbin Engineering University, Harbin 150001, China ‡ College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China # Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, and School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China.

KEYWORDS: CoNi alloy nanoparticles, hollow structure, N-doped porous carbon polyhedron, N-doped graphene, electromagnetic wave absorption

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ABSTRACT: Magnetic metal nanostructures have exhibited good electromagnetic wave (EMW) absorption properties. However, the surface of the nanostructures is easily oxidized upon exposure to air, leading to the bad stability of the EMW absorption properties. We use metal-organic framework structure as template to fabricate hollow N-doped carbon polyhedron containing CoNi alloy nanoparticles embedded within N-doped graphene (CoNi@NG-NCPs). The atomic ratio of Co/Ni can be tuned from 1: 0.54 to 1: 0.91 in the hollow CoNi@NG-NCPs. Experimental results demonstrate that the EMW absorption properties of the CoNi@NG-NCPs can be improved through the Ni introduction, and increased with an increase of the Ni content. Typically, the minimal reflection loss of the optimal CoNi@NG-NCP can reach –24.03 dB and the effective absorption bandwidth (reflection loss below –10 dB) is as large as 4.32 GHz at the thickness of 2.5 mm. Furthermore, our CoNi@NG-NCPs exhibit favorably comparable or superior the EMW absorption properties to other magnetic absorbers. In addition, because the CoNi alloy nanoparticles are coated with N-doped graphene layers their surface oxidation behavior can be efficiently limited. The mechanism of the enhanced the EMW absorption property is relevant to the enhanced dielectric loss and better impedance matching characteristic caused by the Ni incorporation.


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1. INTRODUCTION Electromagnetic wave (EMW) absorbing material has attracted wide attention due to electromagnetic interference and pollution induced by the widely usage of the electric devices. Owing to the small size effect, the electromagnetic wave absorbing materials based on the nanostructures exhibited greatly enhanced absorption ability toward EMW in comparison to their bulk counterparts.1 Among the nanostructures, some soft-magnetic metals and alloys, including hexagonal Fe microflakes,2 sword-like cobalt particles,3 FeCo nanoplates,4 amorphous CoxFe10-x alloy,5 FeCo flower-like structure,6 Fe100-xNix nano powders,7 FeNi particles,8 FeNi alloy nanoparticles,9 and Fe20Ni80 submicron fiber,10 have shown fascinating EMW absorption performance. However, the magnetic materials have obvious shortcomings such as corrosion and oxidization susceptibilities and poor dispersity, limiting their practical applications. Confining the magnetic metal and alloys within oxides or carbonaceous materials, forming core-shelled or yolk shelled structures, could limit the oxidization behavior to some degree.11-15 At the same time, some coating materials themselves have dielectric loss characters to some degree, facilitating the enhancement of the magnetic metal and alloys in EMW absorption properties. For example, Zhu et al. reported that CoO@Co yolk-shell nanoparticles on graphene sheets had a minimal reflection loss value of –51.1 dB toward EMW at a frequency of 11.30 GHz as the absorber thickness was 2.6 mm.16 Zhao et al. fabricated Ni@SnO2 yolk−shelled nanoparticles, showing a minimal reflection loss value of –50.2 dB toward EMW at a frequency of 17.40 GHz as absorber thickness was 1.5 mm.17 Yang et al. found that Fe@air@Co core-shelled nanoparticles exhibited distinct EMW


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absorption properties with a minimal reflection loss value of –42.75 dB and effective bandwidth of 4.10 GHz at a frequency of –10 dB.18 Huang et al. prepared the core−shell nanoparticles of CoNi@SiO2@rGO–PANI quaternary composite and found that the minimal reflection loss of the composite was approximately –43.0 dB at a frequency of 15.4 GHz.19 Feng and co-workers designed Co20Ni80@TiO2 core−shelled materials with a minimal reflection loss of –25.0 dB at a frequency of 3.8 GHz.20 The results mentioned above demonstrated that confining the magnetic metal and alloys within foreign materials provided a new way for the rational design of high-performance EMW absorbing materials. Recently, some porous or hollow nanostructures have shown enhanced EMW absorption properties compared to their corresponding counterparts.21-30 On one hand, these porous/hollow nanostructures have more free spaces than that of non-porous counterparts, leading to their better impedance matching characteristics. On the other hand, the porous/hollow nanostructures have more edged atoms with unsaturated bands, which can serve as polarized centers and thus increase the dielectric loss characteristics. However, it remains a huge challenge to fabricate porous/hollow nanostructures composed of confined magnetic metal and alloys within foreign materials. Metal−organic frameworks (MOFs) have recently attracted great attention due to their unique structures. In addition, MOFs can be used as starting materials to fabricate porous magnetic metal containing carbonaceous materials for applications in EMW absorption field. Recently, some porous carbon-based absorbers have been fabricated using MOFs as templates,31-34 but their EMW absorption properties are still unsatisfactory.


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Herein, we use zeolitic imidazolate frameworks-67 (ZIF-67) as solid template to fabricate hollow N-doped carbon polyhedron containing CoNi alloy nanoparticles embedded within N-doped graphene. For convenience, the obtained samples were denoted as CoNi@NG-NCPs. Applied as absorbers for attenuating EMW energy, the CoNi@NG-NCPs have the following advantages, illustrated in Figure 1: i) the CoNi alloy nanoparticles are coated with N-doped graphene layers, efficiently inhibiting the surface oxidation behavior of the magnetic metals; ii) the introduction of Ni improves the impedance matching characteristic of the CoNi@NG-NCPs; iii) the hollow feature affords more edged atoms with unsaturated bands to CoNi@NG-NCPs, resulting in larger dielectric loss; iv) multiple interfaces exist in the CoNi@NG-NCPs and thus the interfacial polarizations enhance the EMW absorption performance of the CoNi@NG-NCPs. As a result, the CoNi@NG-NCPs exhibit excellent EMW absorption property, including strong attenuation ability toward EMW and wide absorption bandwidth, superior to Co@NG-NCP and other magnetic nanostructures reported previously.

2. EXPERIMETNAL SECTION The fabrication of the CoNi@NG-NCPs included three steps, schematically illustrated in Figure 1. ZIF-67 was first synthesized through a previously reported method.35 Then the ZIF-67@CoNi layered double hydroxides (ZIF-67@CoNi LDH) were obtained through dispersing ZIF-67 into nickel nitrate ethanol solution at room temperature under stirring condition for different times.36 The obtained samples with stirring time for 30, 60, 90 and 120 min were denoted as ZIF-67@CoNi LDH-30, ZIF-67@CoNi LDH-60, ZIF-67@CoNi


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LDH-90

and

ZIF-67@CoNi

LDH-120,

respectively.

Finally,

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CoNi@NG-NCP-30,

CoNi@NG-NCP-60, CoNi@NG-NCP-90 and CoNi@NG-NCP-120 were synthesized after heating ZIF-67@CoNi LDH-30, ZIF-67@CoNi LDH-60, ZIF-67@CoNi LDH-90 and ZIF-67@CoNi LDH-120 at 800oC for 2 h under Ar flow, respectively. For comparison, Co@NG-NCP was fabricated after heating ZIF-67 at 800oC for 2 h under Ar flow. The detailed fabrication processes and the structural characterizations of samples, and electromagnetic parameter measurements were described in Supporting Information.

Figure 1 Schematic illustration of the formation process of CoNi@NG-NCPs and their advantages for the EMW absorption.


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Figure 2 SEM and TEM images of (a,f) ZIF-67, (b,g) ZIF-67@CoNi LDH-30, (c,h) ZIF-67@CoNi LDH-60, (d,i) ZIF-67@CoNi LDH-90 and (e,j) ZIF-67@CoNi LDH-120. Scale bars in the images: 500 nm.

3. RESULTS AND DISCUSSION X-ray diffraction (XRD) pattern shows that the as-prepared ZIF-67 has similar crystalline structure to the previously reported ones (Figure S1, Supporting Information).37 There is not obvious difference in the diffraction peaks between ZIF-67 and ZIF-67@CoNi LDHs, implying small amount of CoNi LDH formed in ZIF-67@CoNi LDHs (Figure S1, Supporting Information). The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (Figure 2a,f) display that the ZIF-67 particles have a rhombic dodecahedron shape with smooth surfaces and their sizes are in range of 600 - 800 nm. After being stirred in the nickel nitrate ethanol solution, the surfaces of the ZIF-67 particles become relatively rough (Figure 2b-e). Interestingly, yolk-shell structures are formed gradually with the increase of the stirring time (Figure 2g-j). Furthermore, the void space between the core and shell is gradually extended (Figure 2g-j). The energy dispersive X-ray spectrometry (EDX) elemental mappings for the ZIF-67@CoNi LDH-90 indicate that Co elements


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distribute in both the core and shell regions, while Ni elements mainly distribute in the shell region (Figure S2, Supporting Information), suggesting that the core and shell is composed of ZIF-67 and CoNi LDH, respectively.38 The EDX patterns show that the atomic ratio of Co/Ni for ZIF-67@CoNi LDH-30, ZIF-67@CoNi LDH-60, ZIF-67@CoNi LDH-90 and ZIF-67@CoNi LDH-120 are 1: 0.60, 1: 0.72, 1: 0.86 and 1: 0.95, respectively (Figure S3, Supporting Information). The result means that the Ni content is increased with the increase of the stirring time. However, EDX data (not shown) indicates that the Ni content will not be increased if the stirring time is increased more than 120 min.

Figure 3 SEM and TEM images of annealed product (a,f) Co@NG-NCP, (b,g) CoNi@NG-NCP-30, (c,h) CoNi@NG-NCP-60, (d,i) CoNi@NG-NCP-90, and (e,j) CoNi@NG-NCP-120. Scale bars in the images: 500 nm. After ZIF-67 and ZIF-67@CoNi LDHs being heated at 800

o

C under Ar flow,

Co@NG-NCP and CoNi@NG-NCPs can be obtained, respectively. The diffraction peaks in the XRD pattern of Co@NG-NCP comes from graphitic carbon (JCPDs card no. 41-1487) and metal Co nanoparticles (JCPDs card no. 15-0806), as shown in Figure S4a (Supporting Information). SEM image (Figure 3a) displays that the Co@NG-NCP inherits the


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morphology of ZIF-67 well. TEM image (Figure 3f) shows that Co nanoparticles distribute uniformly in the NCP matrix, evidenced by EDX elemental mappings (Figure S5, Supporting Information). Figure S6a,b (Supporting Information) indicates that the size of Co nanoparticles is in range of 4–60 nm. High-resolution TEM image (Figure S6c-e, Supporting Information) reveals that the Co nanoparticles are encapsulated within about 7 layers of graphene shell. The well-resolved lattice fringes with lattice spacing of 0.207 nm in the Co core region can be clearly observed in the HRTEM image (Figure S6e), which corresponds to the (111) plane of metal Co nanoparticles. In the graphene shell region, the interlayer distance is around 0.342 nm, confirming that the metal Co nanoparticles are coated with the graphene layers. Selected area electron diffraction (SAED) (Figure S6f, Supporting Information) displays the diffraction rings from the (002) lattice plane of graphite carbon, and from (111), (200), (220) lattice planes of metal Co nanoparticles, further confirming the presence of the graphite carbon and metal Co in the [email protected]

Figure 4 Structural characterizations of CoNi@NG-NCP-90. (a) TEM image of


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CoNi@NG-NCP-90, (b,c) EDX Co and Ni mappings of CoNi@NG-NCP-90, (d) High-resolution TEM image of CoNi@NG-NCP-90, (e) HRTEM image of Co nanoparticles encapsulated in graphene shell, (f) SAED pattern of CoNi@NG-NCP-90. After the introduction of Ni, the surfaces of CoNi@NG-NCPs become relatively rough in comparison to Co@NG-NCP, but they keep polyhedron-like morphologies, as shown in Figure 3b-e. Furthermore, the interiors of the CoNi@NG-NCPs are almost transparent to electron beams, suggesting that they possess hollow structures (Figure 3g-h).38 The hollow feature can afford more edged atoms with unsaturated bands to CoNi@NG-NCPs, facilitating the enhancement of their dielectric loss and thus the improvement of their EMW absorption properties. EDX patterns (Figure S7, Supporting Information) indicate that the atomic ratios of

Co/Ni

for

CoNi@NG-NCP-30,

CoNi@NG-NCP-60,

CoNi@NG-NCP-90,

CoNi@NG-NCP-120 are 1: 0.54, 1: 0.76, 1: 0.86 and 1: 0.91, respectively. There is little difference in the atomic ratio between CoNi@NG-NCPs and ZIF-67@CoNi LDHs, revealing that the annealing treatment at 800oC has a little effect on the atomic ratio of Co/Ni. XRD patterns shows that the CoNi@NG-NCPs have almost same diffraction peaks, indicating their similar crystalline components (Figure S4b-e, Supporting Information). Besides diffraction peaks of the graphitic carbon, the locations of other diffraction peaks lie between those of metal Co and Ni (JCPDs card no. 04-0850), implying that Co and Ni may integrate into alloys. Notably, in the XRD patterns of CoNi@NG-NCP-90 and CoNi@NG-NCP-120 (Figure S4d, e, Supporting Information), two small diffraction peaks at 41.97o and 47.05o comes from other type of carbon material (JCPDs card no. 50-1083). Due to the similar


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crystalline component and morphology, we select CoNi@NG-NCP-90 to further analyze the detailed structural characteristics of the NCPs. Figure 4a-c display the TEM and the corresponding EDX Co and Ni mappings. It can be clearly found that the spatial distributions of Co and Ni are overlapped very well, demonstrating the formation of alloy phase between Co and Ni in the CoNi@NG-NCPs. From Figure 3g-h and Figure 4a, it can be observed that some CoNi particles have large size, while others have relatively small size. The small CoNi particles have a diameter of 6 nm, and are encapsulated in 3–6 layers of graphene shell (Figure 4d), while the large particles with a diameter of around 50 nm are encapsulated in more than 15 layers of graphene shell (Figure S8, Supporting Information). Figure 4e shows HRTEM image of an individual CoNi alloy particle. The particle is coated with 3 layers of the graphene shell with the layer distance of 0.342 nm. The lattice distance of 0.204 nm labeled in the particle region is close to the standard lattice spacing of (111) lattice plane of CoNi alloy.40 The diffraction rings in the SAED pattern of CoNi@NG-NCP-90 (Figure 4f) correspond to (002) lattice plane of graphitic carbon, and (111), (200), (220) lattice planes of CoNi alloy particles, consistent with the XRD and TEM results. The surface composition and chemical valence states of the Co@NG-NCP and CoNi@NG-NCP-90 were further studied by X-ray photoelectron spectroscopy (XPS). The survey spectrum of Co@NG-NCP (Figure S9a, Supporting Information) demonstrates that the presence of Co, N, C and O elements in the surface of the Co@NG-NCP. In the Co 2p XPS spectra of the Co@NG-NCP (Figure S9b, Supporting Information), the main peaks at 777.9 and 792.9 eV can be assigned to Co 2p3/2 and Co 2p1/2 of metal Co species,


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respectively.41-42 The peaks at 780.0 and 792.9 eV with two satellites at approximately 784.7 and 803.5 eV indicate the existence of CoO species (Figure S9b, Supporting Information).43-44 The integrated area of the peaks relevant to CoO is greatly larger than those of the peaks relevant to metal Co, suggesting the high degree of the surface oxidation of metal Co in the Co@NG-NCP. In the C 1s XPS spectra (Figure S9c, Supporting Information), three peaks at 284.5, 285.5, and 288.2 eV can be assigned to carbon in the forms of C-C, C-N, and C-O, respectively.39-40 The N 1s spectra (Figure S9d, Supporting Information) can be deconvoluted into five peaks at 398.4, 399.1, 400.1, 400.9 and 402.3 eV, which are attributed to pyridinic-N, Co-N, graphitic-N, pyrrolic-N and Ni-N, respectively.45 The N content in the Co@NG-NCP is about 6.73 at% in terms of the XPS data. The survey spectrum of CoNi@NG-NCP-90 (Figure S10a, Supporting Information) indicates that the Co, Ni, N, C and O elements exist in the CoNi@NG-NCP-90. In comparison with Co@NG-NCP, the CoNi@NG-NCP-90 has additional XPS peaks at 779.0 and 794.0 eV (Figure S10b, Supporting Information), which can be assigned to Co species in the CoNi alloy. In the Ni 2p XPS spectra of CoNi@NG-NCP-90 (Figure S10c, Supporting Information), the peaks at 852.3 and 870.0 eV along with two satellites at approximately 859.0 and 874.9 eV can be assigned to the binding energies of metallic nickel, while the peaks at 853.6 and 870.9 eV can be assigned to Ni species in the CoNi alloy.46 Obviously, the integrated areas of XPS peaks for metal Co and Ni are greatly larger than those of XPS peaks for metal oxides. This result indicates that the introduction of Ni significantly improves antioxidation ability of the magnetic metal particles (Figure S10c, Supporting Information). From the C 1s XPS spectra


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(Figure S10d, Supporting Information), the CoNi@NG-NCP-90 has the same carbon composition as that of Co@NG-NCP, indicating that the introduction of Ni has little effect on the valence state of carbonaceous species. Besides pyridinic-N, Co-N, graphitic-N, pyrrolic-N and Ni-N, Ni-N species are detected in the Ni 2p XPS spectra of the CoNi@NG-NCP-90 (Figure S10e, Supporting Information).47 The N content in the CoNi@NG-NCP-90 is estimated to be about 4.86 at% in terms of the XPS data, slightly lower than that in the Co@NG-NCP. Figure S11 (Supporting Information) give the comparison of Raman spectra between Co@NG-NCP and CoNi@NG-NCP-90. The peaks centered at 1350.0 cm−1 and 1597.0 cm−1 correspond to D-band (disorder carbon) and G-band (ordered carbon). The intensity ratios of the D-band to G-band (ID/IG) for Co@NG-NCP and CoNi@NG-NCP-90 are 1.05 and 1.03, respectively, suggesting many defects in the Co@NG-NCP and [email protected], 48 Based on the results above, hollow N-doped carbon polyhedron, containing CoNi alloy nanoparticles embedded within NG layers, can be successfully synthesized by the present method. The NG layers are expected to efficiently limit the oxidation of the CoNi cores. To verify the possibility, we annealed the CoNi@NG-NCP-90 at 280 oC for 3 h at air atmosphere. XRD pattern (Figure S12) shows that only partial CoNi particles are oxidized into Co3O4 and NiO, while Co or Ni particles on the graphene substrates are totally oxidized into Co3O4 or NiO under the same oxidation conditions.49-50 Thus, the NG layers can efficiently prevent the CoNi alloy nanoparticles from serious oxidation.


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Figure 5 Three-dimensional RL data dependent on the frequency and the thickness of the absorbers for the Co@NG-NCP (a), the CoNi@NG-NCP-30 (b), the CoNi@NG-NCP-60 (c), the CoNi@NG-NCP-90 (d), and the CoNi@NG-NCP-120 (e). The magnetic properties of the Co@NG-NCP and CoNi@NG-NCPs were measured by a vibrating sample magnetometer at room temperature. The hysteresis loops in the M–H curves reveals the ferromagnetic properties of the samples (Figure S13, Supporting Information). All


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the samples have slightly difference in the saturation magnetization, coercivity, and retentivity indicates that the introduction of Ni has little effect on the magnetic property of the samples. The EMW absorption performance of an absorber is directly associated with its reflection loss (RL). The lower RL value means that the absorber has the better EMW absorption property. When the electromagnetic wave is vertically incident to the surface of the absorbing material based on a good conductor, the RL for the absorbers can be calculated according to the following equation,

Zin = Z 0

µr  j 2π fd  tan  × µr ε r  εr  c 

(1)

zin − z0 zin + z0

(2)

RL = 20 log

where, Zin is the input impedance of the absorber, Z0 is the impedance of free space,

ε r = ε '− jε " is the relative complex permittivity, µr = µ '− j µ " is the relative complex permeability, f is the frequency of microwaves, d is the thickness of the absorber and c is the velocity of electromagnetic waves in free space. In terms of Equations (1), (2), the RL curves for our absorbers at different thickness can be calculated, as shown in Figure 5a-e. Among the absorbers, the Co@NG-NCP exhibits attenuation performance toward EMW to some degree. The minimal RL (RL,min) values for the Co@NG-NCP at d ranging from 2.5 to 5.0 mm are below –10 dB. Especially, the RL,min can reach to –21.16 dB at d = 5.0 mm. After the introduction of Ni, the EMW absorption properties of our Co-based absorbers are improved significantly. The enhancement in EMW absorption properties includes the follow aspects. i) Co@NG-NCPs have more negative


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RL,min values at d ranging from 2.0 to 5.0 mm than Co@NG-NCP. For example, the RL,min for CoNi@NG-NCP-30 is –30.5 dB at d = 5.0 mm; the RL,min for CoNi@NG-NCP-60 is –39.33 dB at d = 4.0 mm; the RL,min for CoNi@NG-NCP-90 is –45.73 dB at d = 3.0 mm; the RL,min for CoNi@NG-NCP-120 is –47.79 dB at d = 3.0 mm. The more negative RL,min value indicates that the attenuation abilities of CoNi@NG-NCPs toward EMW are increased. ii) The required thickness (dm) for the RL,min is decreased with an increase of the Ni content in our absorbers. Typically, dm for Co@NG-NCP is 5.0 mm, while it is decreased to 3.0 mm for CoNi@NG-NCP-90 and CoNi@NG-NCP-120. In addition, even at d = 2.0 mm, Co@NG-NCPs have the RL,min values below -10.0 dB, while the Co@NG-NCP is -9.35 dB (Table S1, Supporting Information). The decreased dm means that a thinner absorber film is needed for the practical applications of the absorbers. iii) The effective bandwidths at RL below –10 dB (EAB10) at d ranging from 2.5 to 5.0 mm, and below –20 dB (EAB20) at d ranging from 2.0 to 5.0 mm are significantly extended, as shown in Figure 6a,b. Typically, EAB10 for the Co@NG-NCP at d = 2.5 mm is 2.42 GHz, smaller than those of CoNi@NG-NCP-30 (2.99 GHz), CoNi@NG-NCP-60 (4.18 GHz), CoNi@NG-NCP-90 (4.32 GHz), and CoNi@NG-NCP-120 (4.54 GHz), respectively. The RL,min for Co@NG-NCP is below –20 dB only at d = 5.0 mm, while the d value can be extended to 3.5–5.0 mm for CoNi@NG-NCP-30,

3.0–5.0

mm

for

CoNi@NG-NCP-60,

2.5–5.0

mm

for

CoNi@NG-NCP-90, and 2.0–5.0 mm for CoNi@NG-NCP-120, respectively. The extended EAB values facilitate the practical applications of the CoNi@NG-NCPs. Besides the reflection loss, the attenuation constant (α) was also an evaluation criterion for the EMW


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absorption property of an absorber.

α=

2 µ ' ε 'π f c

µ "ε " µ "ε " 2 ε " 2 µ " 2 −1 + ( ) + ( ) + ( ) +1 µ 'ε ' µ 'ε ' ε' µ'

(3)

Figure S14 (Supporting Information) displays α–f curves plotted according to Equation 3 for the Co@NG-NCP and CoNi@NG-NCPs. It clearly found that the α values are in range of 42.2–226.9, and increase with an increase of Ni content. Overall, in terms of RL,min, dm, α and EAB data, the EMW properties of the CoNi@NG-NCPs are gradually improved with the increase of the Ni content. Thus, the EMW absorption properties of our absorbers are increased in the order: Co@NG-NCP < CoNi@NG-NCP-30 < CoNi@NG-NCP-60 < CoNi@NG-NCP-90 < CoNi@NG-NCP-120.

Figure 6 (a) Comparison of EAB10 among our Co-based absorbers. (b) Comparison of EAB20 among our Co-based absorbers. (c) Comparison of impedance matching characteristics


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among our Co-based absorbers. The enhanced EMW absorption properties of CoNi@NG-NCPs can be explained by the following factors. First, an absorber with strong attenuation ability toward EWM should possess a better impedance matching characteristic, i.e., Zin is required to be close to Z0 as soon as possible. The impedance matching characteristic can be estimated by the equation Z = |Zin/Z0|. The closer the Z is to 1, the better impedance matching characteristic the absorber has. The calculated Z values for our Co-based absorbers are given in Figure S15 (Supporting Information) and the typical data is shown in Figure 6c. It can be found that the CoNi@NG-NCPs have better impedance matching characteristics than Co@NG-NCP. For example, all the Z values for the CoNi@NG-NCPs are larger than 1.26 at d of 2.0 – 5.0 mm; while the Z values are close to 1.0 for CoNi@NG-NCP-90 at d of 2.5 – 5.0 mm. Furthermore, the impedance matching characteristic becomes better with the increase of the Ni content, consistent with the order of their EMW absorption properties. Thus, the better impedance matching characteristic induced by the Ni introduction is attributed to the enhanced EMW absorption properties of CoNi@NG-NCPs. Second, the EMW absorption property of an absorber is directly relative to its dielectric loss and magnetic loss. Figure 7 gives the information about dielectric loss and magnetic loss of the Co@NG-NCP and CoNi@NG-NCPs. It can be found that the real and imaginary parts of the relative complex permittivity for the absorbers are almost increased with the Ni content (Figure 7a,b). Especially, the dielectric loss tangents (tan δ e ) of the CoNi@NG-NCPs are greatly larger than that of the Co@NG-NCP at the same frequency (Figure 7c). For example, the tangents


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of CoNi@NG-NCP-90 are in range of 0.29–0.52 over 2–18 GHz, while they are in range of 0.14–0.40 for the Co@NG-NCP. Thus, the increased dielectric loss contributes to the enhanced EMW absorption properties of the CoNi@NG-NCPs. The real and imaginary parts of the Co@NG-NCP along with the magnetic loss tangents (tan δm) are in the middle of those ones of the CoNi@NG-NCPs, as shown in Figure 7d–f. Furthermore, the tan δm values are greatly smaller than the tan δ e values for all the absorbers. Thus, the magnetic loss is relative to the enhancement of the CoNi@NG-NCPs in EMW absorption properties. Notably, the real part of complex permeability for all the absorbers is lower than 1.0, which may be due to the cladding of nonmagnetic substance carbon layers.51 Based on the discussions above, the enhanced EMW absorption properties of the CoNi@NG-NCPs can be attributed to their better impedance matching characteristics and their increased dielectric loss.

Figure 7 (a) The real parts, (b) imaginary parts of the relative complex permittivities, and (c)


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the dielectric loss tangents of the Co@NG-NCP and CoNi@NG-NCPs. (d) The real parts, (e) imaginary parts of the relative complex permeabilities, and (f) the magnetic loss tangents of the Co@NG-NCP and CoNi@NG-NCPs. From the Figure 7, it can be found that the Co@NG-NCP and CoNi@NG-NCPs possess both dielectric loss and magnetic loss. In general, the dielectric loss originates from the polarizations induced by dipole, defect and interface, etc. Raman spectra (Figure S11, Supporting Information) indicate that the defects are existed in the Co@NG-NCP and CoNi@NG-NCPs, revealing that the polarizations induced by the defects contribute to their dielectric losses. The interfacial polarization is caused by the accumulation of free charge on the interface, and thus will lead to dielectric relaxation.52 There are many interfaces between Co/CoNi nanoparticles and the graphene shell, and thus interfacial polarizations contribute to the dielectric losses of the Co@NG-NCP and CoNi@NG-NCPs. The interfacial polarizations can be analyzed simply by the Debye theory. In terms of the Debye theory, the relationships between ε ' and ε " can be described by

[ε '− (ε s + ε ∞ ) / 2]2 + (ε ") 2 = [(ε s + ε ∞ ) / 2]2

(4)

where ε s and ε ∞ are the static permittivity and the relative dielectric permittivity at the high-frequency limit, respectively. The plots based on Equation 4 are also known as Cole–Cole curves, in which one single semicircle means one dielectric relaxation process. Figure S15f (Supporting Information) shows the Cole–Cole curves of the Co@NG-NCP and CoNi@NG-NCPs. It can be found that there is more than one semicircle in the curves for all the absorbers, providing an evidence for the contribution of the interfacial polarizations to


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their dielectric losses. In addition, the XPS measurements show that there are C-O and C-N species in the surfaces of the Co@NG-NCP and CoNi@NG-NCPs. Thus, the dipole polarizations also contribute to the dielectric losses of the Co@NG-NCP and CoNi@NG-NCPs to some degree. As for the magnetic loss, it is mainly relevant to eddy current effect, natural resonance, and exchange resonance at the EMW at 2–18 GHz. The relationships between the imaginary and real parts of relative complex permeability associated with the eddy current effect can be written as,53

µ ″ = 2πµ0 ( µ ′ ) σ d 2 f / 3 2

(5)

where µ0 and σ are the permeability of vacuum and the electric conductivity, respectively. the following equation can be obtained according to Equation 5,

C0 = 2πµ0σ d 2 / 3 = µ ″ ( µ ′ )

−2

f -1

(6)

In the case that the eddy current effect play a domain role in the magnetic loss, C0 is an constant over 2–18 GHz. However, the C0 changes obviously for all the absorbers over 2–18 GHz (Figure S16, Supporting Information), indicating the eddy current effect is not a key factor for the magnetic losses of the absorbers. From the Figure 7e, it can be found that there are two weak peaks centered at 6 and 12 GHz for all the absorbers. As previously reports, the former peak is attributed to natural resonance, while the latter comes from exchange resonance.54-55 In addition, TEM images display that the Co and CoNi nanoparticles in the Co@NG-NCP and CoNi@NG-NCPs are in range of 4–60 and 3–50 nm, respectively. Thus, multiple magnetic domain walls exist in the Co@NG-NCP and CoNi@NG-NCPs. The motion of the domain-walls under irradiation of EMW also contributes to the magnetic loss.


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Based on the above results, we can conclude that the introduction of Ni can significantly improve the EMW absorption properties of the Co-based absorbers. Furthermore, compared to the other magnetic absorbers previously reported such as FeCo/ZnO composites,56 FeCo-coated carbon fiber,57 FeCo/C nanocapsules,58 FeCo/Al2O3 nanocapsules,59 and FeCo/graphene,60 our CoNi@NG-NCPs exhibit superior EMW absorption properties, as shown in Table S2. Remarkably, the addition amount of CoNi@NG-NCPs into the paraffin matrix is only 35 %, lower than the other magnetic absorbers previously reported. In addition, the CoNi nanoparticles encapsulated in graphene layers possess good antioxidation ability. Therefore, our CoNi@NG-NCPs have very promising applications in EMW absorption field.

4. CONCLUSIONS Hollow N-doped carbon polyhedron, containing CoNi alloy nanoparticles embedded within few-layer N-doped graphene, is successfully fabricated using ZIF-67 as solid template. We find that the introduction of Ni can significantly improve the EMW absorption properties of the N-doped carbon polyhedron containing CoNi alloy nanoparticles in terms of RL,min, dm,

α and EAB data. Experimental results demonstrate that the enhanced EMW absorption properties are mainly attributed to the enhanced dielectric losses and the better impedance matching characteristic due to the introduction of Ni. Remarkably, compared to the other magnetic absorbers previously reported, our CoNi@NG-NCPs exhibit comparable or superior EMW absorption properties. Furthermore, the addition amount of CoNi@NG-NCPs into the paraffin matrix is only 35 %, lower than the other magnetic absorbers previously reported. In addition, the introduction of Ni can efficiently inhibit the oxidation behavior of


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the magnetic metals. Therefore, our present strategy may open a new way for design and fabrication of high-performance magnetic EMW absorbers.

ASSOCIATED CONTENT

Supporting Information Available: Figure S1-Figure S16 and Table S1-Table S2. XRD patterns of the ZIF-67, the ZIF-67@CoNi LDH-90, the Co@NG-NCP, the CoNi@NG-NCPs and the CoNi@NG-NCP-90 after being heating at 280°C for 3 h at air atmosphere. TEM image and EDX elemental mappings of ZIF-67@CoNi LDH-90 and Co@NG-NCP. EDX pattern of the ZIF-67@CoNi LDHs and CoNi@NG-NCP-90s. HRTEM images and the corresponding SAED pattern of the Co@NG-NCP. The HRTEM image of the CoNi@NG-NCP-90. XPS spectra of the Co@NG-NCP and CoNi@NG-NCP-90s. Raman spectrum of the Co@NG-NCP and CoNi@NG-NCP-90s. Magnetization hysteresis loops of Co@NG-NCP and CoNi@NG-NCPs. The attenuation constant α–f curves of the Co@NG-NCP and CoNi@NG-NCPs. The modulus of Z–f curves of Co@NG-NCP and CoNi@NG-NCP-90s. The Cole-Cole semicircles of Co@NG-NCP and CoNi@NG-NCPs. The C0–f curves of Co@NG-NCP and CoNi@NG-NCPs. Detailed comparison of the EMW absorption property of the Co@NG-NCP to that of CoNi@NG-NCPs and Detail comparison of the EMW absorption property of the CoNi@NG-NCP-90 and CoNi@NG-NCP-120 to others. This Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT

This work is supported by the National Natural Science Foundation of China (Grant No. 51572051), the Natural Science Foundation of Heilongjiang Province (E2016023), the

Fundamental Research Funds for the Central Universities (HEUCF201708), and also the Open Project Program (PEBM 201703 and PEBM201704) of Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal University, and also the 111 project (B13015) of Ministry Education of China to the Harbin Engineering University. REFERENCES

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Table of Contents Graphic and Synopsis


1

Hollow N-Doped Carbon Polyhedron Containing CoNi Alloy

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