Functionalized Carbon Nanofibers Enabling Stable and Flexible

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Functional Inorganic Materials and Devices

Functionalized Carbon Nanofibers Enabling Stable and Flexible Absorbers with Effective Microwave Response at Low Thickness Bin Quan, Xiaohui Liang, Xin Zhang, Guoyue Xu, Guangbin Ji, and Youwei Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16088 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 10, 2018

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ACS Applied Materials & Interfaces

Functionalized Carbon Nanofibers Enabling Stable and Flexible Absorbers with Effective Microwave Response at Low Thickness Bin Quana, Xiaohui Lianga, Xin Zhanga, Guoyue Xua, Guangbin Ji*a, Youwei Dub a College

of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, P. R. China.

b Laboratory

of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China.

Key words: Carbon nanofiber; Co3O4; low thickness; device; microwave absorption *Corresponding Author: Prof. Dr. Guangbin Ji. Tel: +86-25-52112902; Fax: +86-25-52112626 E-mail: [email protected]

ABSTRACT ACS Paragon Plus Environment

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Lots of work has been done to develop microwave absorbing materials (MAM) utilized as flexible electronic devices and communication instruments. Conventional developed powder MAM are often limited in practical applications due to the bad stability and poor durability, which is out of the scope for exploiting flexible and long-term microwave absorbers. To overcome such limitations, a facile and binder-free technique from Co-based Zeolitic Imidazolate Framework (ZIF-67, a member of metalorganic frameworks) coated carbon fiber precursor is developed for the in-situ horizontal growth of Co3O4 nanoparticles, which embedded nitrogen-doped carbon array (triangular nanoplates) on the surface of carbon fibers in the carbon paper (NC-Co3O4/CP) as low-thickness microwave absorbing materials. The maximum reflection loss (RL) values reaches -16.12 dB and -34.34 dB when the thickness is 1.1 mm and 1.5 mm, respectively. As the thickness increasing, the absorbing performance at low frequency performs well (RL < -20 dB). The hierarchical architecture is facilely originated from a metal-organic framework precursor. In view of the simple preparation technique, NC-Co3O4/CP exhibit huge potential in large-scale production of portable microwave absorbing electronic devices with strong microwave response at low thickness.

1. INTRODUCTION ACS Paragon Plus Environment

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Currently, high-performance electromagnetic (EM) absorbing materials act as a crucial role in mitigating or controlling EM radiation pollution, which seriously affect the routine use of sensitive electronic apparatus and human health.1-3 In principle, unlike EM interference shielding materials with intense surface reflection for preventing EM wave penetrating,4-6 EM wave absorption technique requires candidates with ideal impedance matching, allowing EM energy to be consumed in the absorbing medium.7-9 In addition, mechanical properties such as flexible, lightweight, durable, robust, etc.,10-12 functional characters (thin thickness, weak surface reflection) should all be simultaneously fulfilled as far as possible.13-15 In the past decades, dielectric/magnetic powder absorbers have been intensively explored.16-19 Among the alternative candidates, carbonaceous substances, especially graphene, carbon nanotubes and carbon fiber, have demonstrated huge potentials on EM wave absorbing due to their high electrical conductivity, large surface area, electrical and mechanical properties.20-21 Interestingly, the weakness in impedance matching for carbonaceous materials could also be made up by integrating carbon materials with other ingredients like metal oxide, semiconductors and perovskites.22, 23 For example, Zhu el al. combine reduced graphene oxide with spherical carbonyl iron and got the improved impedance matching ability when comparing to pure rGO.24 Although the great progress had been achieved, unfortunately, most reported powder absorbers still require subsequent treatment to form a solid dense bulk by adding polymeric binders or conductive additives, which suffers from weakening absorbing performance and poor stability, thus, these composites can not meet the criteria for achieving highefficiency microwave absorbing films or devices. It would be more promising to directly grow nanoarrays or nanoparticles on conductive substrates with better electrical/mechanical contact as binderfree microwave absorbing devices.25-27 Whereas, the construction of as-mentioned films or devices is still a major challenge. Here, Co3O4 was chosen as another ingredient due to its controllable morphology, facile fabrication means, and proper EM parameter to match with carbon fiber.28-30 A facile combined carbonizationoxidation procedure was developed to obtain the Co3O4 nanoparticles embedded nitrogen-doped carbon array on carbon paper (NC-Co3O4/CP). The carbonization-oxidation process from ZIF-67 coated carbon ACS Paragon Plus Environment

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fiber precursor results in a uniform dispersion of Co3O4 nanoparticles in N-doped triangular carbon array. Furthermore, the obtained NC-Co3O4/CP can be directly utilized as a flexible integrated microwave absorbing device without any binder or additives, which make it more attractive in this field. More importantly, the excellent EM wave absorbing ability at very low thickness of 1.1 mm and 1.5 mm can be reached.

2. EXPERIMENTAL SECTION 2.1 Synthesis of A-CP@ZIF-67 Purchased carbon paper (CP) with the size of 3 ×3 × 0.02 cm3 (carbon fiber diameter: cat. 8~9 μm) treated with nitric acid (2.0 M, 100 mL) at 40 °C for 5 h, which is denoted as A-CP. Then, 50 mL of 2methylimidazole aqueous solution (0.4 M) was added into 50 mL of aqueous solution containing Co(NO3)2·6H2O (25 × 10

−3

M) with continuous stirring, after which the as-obtained A-CP was

immersed into the mixture solution for 6 h. Finally, ZIF-67 coated A-CP (A-CP@ZIF-67) was obtained by cleaning with water and drying in vacuum oven overnight. 2.2. Synthesis of NC-Co3O4/CP The as-obtained A-CP@ZIF-67 was annealed in air at 350 °C for 2 h with ramp rate of 1 °C min

−1.

After cooling down to room temperature, Co3O4 nanoparticles embedded nitrogen-doped carbon array on carbon paper (NC-Co3O4/CP) were obtained. 2.3. Synthesis of TA-CP For comparison, A-CP was also annealed at the same condition as that of NC-Co3O4/CP, the A-CP underwent heat treatment was denoted as TA-CP. 2.4. Measurement The as-prepared sample was characterized by X-ray diffractomer (XRD), Raman spectroscopy, Scanning electron microscope (SEM), BET, Transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) and so on. In detail, the structure, composition and chemical states were characterized by Bruker D8 Advanced X-ray diffractometer (Cu Kα radiation) with scanning range ACS Paragon Plus Environment

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from 10 to 90 °; X-ray photoelectron spectroscopy technique with an Al Kα X-ray source at 150 W; and Raman spectrometer (Renishaw InVia). Morphology was determined by scanning electron microscopy (HITACHI S-4800) and transmission electron microscopy (JEOL, JSM-2010) equipped with energydispersive X-ray spectrometer (EDS). To measure the electromagnetic parameter of as-prepared sample, wax was chosen as the matrix materials. Mashed NC-Co3O4/CP with mass ratio of 40 wt% was mixed with paraffin and then compacted into toroidal-shaped samples (Φin: 3.04 mm, Φout: 7.00 nm). Electromagnetic parameters were detected using Agilent PNA N5244A vector network analyze in the frequency range from 2 to 18 GHz. The calculation method of reflection loss (RL) value is based the transmission line theory as following:

RL  20 log ( Z in  Z 0 ) /( Z in  Z 0 )

Z in  Z 0

r 2fd tanh( j r r ) r c

(1)

(2)

3. RESULTS AND DISCUSION

Figure 1. Fabrication schematic of TA-CP and NC-Co3O4/CP. ACS Paragon Plus Environment

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The schematic fabrication procedures of TA-CP and NC-Co3O4/CP are shown in Figure 1. First, the purchased CP was cut into the size of 3 ×3 × 0.02 cm3, followed by nitric acid washing. The obtained A-CP was then transferred into the solution mixed with 2-methylimidazole and cobalt nitrate at room temperature. After stirring and ageing, ZIF-67 was directly grown onto the A-CP surface. During the subsequent heat treatment, ZIF-67 changed to Co3O4 nanoparticles which embedded nitrogen-doped carbon arrays, indicating the successful fabrication of NC-Co3O4/CP. As comparison, TA-CP was also prepared by directly calcining A-CP at the same condition as that of NC-Co3O4/CP.

Figure 2. SEM (a, b, c) and TEM (d, e) images of NC-Co3O4/CP. Corresponding SEM-EDS and TEMEDS mapping of elements C (g, l), N (h, m), Co (i, n), O (j, o) for the selected image (f, k); digital images of A-CP (p), TA-CP (q), A-CP@ZIF-67 (r) and NC-Co3O4/CP (s). As can be seen from Figure 2a-c, the carbon fibers are uniformly bundled-up by triangular nanoplate arrays. As shown in Figure 2d and 2e, clear nanoparticles distribution can be found in the ACS Paragon Plus Environment

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nanplates. For the purpose of preliminary demonstration of the observed nanoparticles, SEM energy dispersed X-ray spectroscopy (SEM-EDS) elemental mapping was primarily employed. As seen from Figure 2f-j, carbon element is dispersed in the main body of fiber; the dispersion of Co element basically matches with that of O, indicating the acquisition of Co3O4 nanoparticles. In addition, sporadic N element dispersion indicates the doping of nitrogen into carbon arrays. Likewise, TEM energy dispersed X-ray spectroscopy (TEM-EDS) elemental mapping of one nanoplate was also carried out. The structure of Co3O4 nanoparticles embedded nitrogen-doped carbon arrays grown onto the carbon fiber surface can be proved by the TEM mapping once again. Differently, the nitrogen distribution is clear, which is derived from the relatively more elemental information gather by the testing equipment. Moreover, the real digital graphs of A-CP, TA-CP, A-CP@ZIF-67 and NC-Co3O4/CP are also presented in Figure 2p-s. It can be found that the carbon paper exhibits different degree of shrink after acid washing, clearly, all the treated-carbon paper maintains flexible features. Specifically, the TA-CP becomes thinner than A-CP, and the sample turns into purple after coating ZIF-67. After the final heat treatment, the obtained NC-Co3O4/CP seems more pyknotic when compared to that of A-CP, TA-CP, A-CP@ZIF-67.

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Figure 3. XRD patterns from 10° to 90° (a), magnified XRD patterns from 7° to 24° (b) and 32° to 70° (c) of figure a, Raman spectra (d) for A-CP, TA-CP, A-CP@ZIF-67 and NC-Co3O4/CP, respectively. XPS survey scan (e) and high-resolution C 1s (f), Co 2p (g), N 1s (h) spectra of NC-Co3O4/CP. X-ray diffraction of A-CP, TA-CP, A-CP@ZIF-67 and NC-Co3O4/CP were detected firstly. Due to the high diffraction intensity of (002) peak derived from the main body of carbon fiber, other diffraction peaks can hardly be observed from the overall spectra from 10° to 90° (Figure 3a). Hence, the magnified patterns from 7° to 24° and 32° to 70° were also detected. Obvious diffraction peaks of ZIF-67 indicate its successful coating onto CP surface (Figure 3b). After heat treatment at low temperature in air, fiber rods were preserved but ZIF-67 was transformed into Co3O4 nanoparticles embedded nitrogen-doped carbon arrays, which is demonstrated by the patterns in Figure 3c, where the (101) and (004) diffraction peaks of fiber were maintained and the diffraction peaks (220), (311), (511) and (440) of Co3O4 emerged.31 The Co3O4 diffraction peaks prove the composition of as-mentioned sample once more. Graphitization degree was detected by Raman spectra, shown in Figure 3d. TA-CP exhibits much enhanced graphitization degree (higher ratio of IG/ID) compared to that of A-CP, which is originated from the removal of oxygen-groups during the thermal treatment process.32 However, for A-CP@ZIF67 and NC-Co3O4/CP, the related D band and G band about graphitized characters cannot be found, which may come from the fact that the surface coating of ZIF-67 or carbon arrays hinder the Raman signal monitoring.33 The result reveals that the obtained carbon arrays are amorphous, and there are two different states of carbon in the sample NC-Co3O4/CP. On the flip side, it also reveals that the compactness of coating layers is good. Nitrogen adsorption–desorption was employed to explore the specific surface area (Figure S1a-d) and porosity (Figure S1e-h) of as-prepared samples. The Brunauer– Emmett–Teller surface areas (SBET), Langmuir surface areas (SLangmuir), total pore volumes (Vpore) and pore size (Vsize) are summarized in Table S1. The carbon paper is typical nanoporous material with SBET, SLangmuir, Vpore, Vsize of 4.22 m2 g-1, 5.89 m2 g-1, 0.0045 cm3 g-1 and 4.34 nm, respectively. Similar, related parameters of TA-CP vary a little. However, when coating ZIF-67 onto its surface, the specific surface areas increase a lot and pore sizes decrease to micropore scale (1.84 nm). As continuous heat treatment, ACS Paragon Plus Environment

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the NC-Co3O4/CP exhibits nanoporous characters. The results show that the Co3O4 nanoparticles embedded nitrogen-doped carbon arrays are also non-porous. As shown in Figure 3e, the XPS survey scan demonstrates the presence of C, Co and O elements. The high-resolution XPS spectrum of C in Figure 3f shows one main peak about C-C/C=C (carbon fiber) and some C-O/C=O bonds (decomposition of ZIF-67).34 The high-resolution XPS spectrum of Co 2p in Figure 3g shows two major peaks with binding energy of 780.22 and 795.43 eV, corresponding to the typical Co 2p 3/2 and Co 2p 1/2 orbitals of Co3O4 phase. Besides, binding energy difference between Co 2p 1/2 and Co 2p 3/2 peaks (spin-orbit splitting) is 15.21 eV, which also matches with that of pure Co3O4 phase reported in literatures.35 In addition, N 1s spectra was also deconvoluted into four peaks (Figure 3h), corresponding to the common nitrogen species: pyridinic-N, pyrollic-N, quaternary-N, and oxidized-N, 36 among which pyrrolic N occupies the largest proportion.


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Figure 4. Dielectric parameter (real permittivity, a; imaginary permittivity, b; dielectric loss, c ); Magnetic parameters (real permeability, d; imaginary permeability, e; magnetic loss, f) of TA-CP and NC-Co3O4/CP; Cole-Cole curves of TA-CP (g) and NC-Co3O4/CP (h). EM parameters-related analysis was plotted to evaluate the microwave absorption mechanisms of NC-Co3O4/CP. TA-CP and NC-Co3O4/CP exhibit typical frequency dispersion phenomena at 2-18 GHz (Figure 4a), which is in favor of the dissipation of EM wave. Due to the introduction of low-permittivity NC-Co3O4, the ε′ value of NC-Co3O4/CP decreases much compared to that of TA-CP. Similar trend can be found for the imaginary permittivity in Figure 4b, except for a broad resonance peak at around 14 ACS Paragon Plus Environment

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GHz for NC-Co3O4/CP, which makes for the attenuation of microwave. The variation of dielectric loss (ε′′/ε′) values is in accordance with that of ε′′ values, strong EM wave absorption ability of NCCo3O4/CP at around 14 GHz is confirmed once more (Figure 4c). Magnetic parameters were also explored in Figure 4d-f. Co3O4 particles do not exhibit any magnetic feature due to the nanoscale size as well as the package structure by carbon arrays. The maximum μ′ and μ′′ value of NC-Co3O4/CP is no more than 1.25 and 0.10. In addition, there appear some negative μ′′ value and magnetic loss value for NC-Co3O4/CP, which goes against the absorbing of EM wave.37 In spite of the magnetic loss capacity is relatively low, the NC-Co3O4/CP exhibits good dielectric-magnetic synergistic effect, where both the dielectric materials and magnetic stuff act important roles in the enhanced EM wave absorption. In view of the vital function of dielectric fiber/carbon in microwave loss, further research about dielectric loss mechanisms are done by Cole-Cole curves, as shown in Figure 4g and 4h. Generally, one semicircle stands for one dielectric relaxation process. Obviously, NC-Co3O4/CP possesses more semicircles than that of TA-CP, which is originated from the dielectric polarizations from amorphous carbon (AC) arrays, Co3O4 nanoparticles, carbon fiber, paraffin and so on. The small semicircle radian of TA-CP indicates its weak polarization capacity compared to that of NC-Co3O4/CP. Although the semicircles outlines of NC-Co3O4/CP are clearer, their morphologies are not regular. It reveals that other polarization processes act on the EM wave absorbing, such as the MWS effect.38 We can see that there are several interfaces in the composites, like the AC/Co3O4, Co3O4/CP, AC/CP, AC/wax, CP/wax, and Co3O4/wax. All these interfaces contribute to the enhanced EM wave absorbing performance based on the interfacial polarization.39 Impedance difference between air and absorbers were also evaluated according the input impedance matching, calculated based on equation 2. Closer to 1 is, better impedance matching is.40 The Zin values at 1-5 mm for both TA-CP and NC-Co3O4/CP were detected. As shown in Figure S2a and S2b, the Zin values of NC-Co3O4/CP is more close to 1 compared to TACP, indicating that more microwave can be introduce into the interior of absorber. In consideration of that permeability value is often much lower than permittivity value, dielectric–magnetic coupling in the NC-Co3O4/CP composites can effectively shorten the impedance gap between air and absorber.33 Magnetic loss mechanisms were also detected by related magnetic analysis. There are three main loss ACS Paragon Plus Environment

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approaches in magnetic loss processes: hysteresis loss, eddy current loss and residual loss.19 In view of that Co3O4 is a not typical ferromagnet, hysteresis loss can be firstly ignored. Eddy current loss is also vital to magnetic attenuation,41 which is derived from the variation of magnetic flux density at alternating EM field and related to conductivity (σ) and thickness (d) of absorber.

Co   ' ' (  ' ) 2 f 1  2 0 d 2

(3)

As can be seen from equation 3, eddy current C0 should be a constant with the varying frequency if the magnetic loss only originates from eddy current. Obviously (See Figure S3), C0 does not exhibit any direct relationship with frequency, and it fluctuates dramatically as the varied frequency. The result demonstrates that C0 is not the dominating factor at the measured frequency range. Residual loss is caused by the relaxation behaviors during magnetization process, which is mainly consist of dimension resonance, exchange resonance, domain wall resonance and nature resonance. In general, domain wall resonance exists in low frequency scope (< 2 GHz), hence it can be excluded in this work. Dimension resonance was also explored according to the half wavelength formula as following:

d  n / 2n  1,2,3,... (4) If the resonance peak comes from dimension resonance, absorber thickness would be equal to an integral multiple of half wavelength. The wavelength of microwave penetrating NC-Co3O4/CP was calculated by the following formula:19

  c/ 2 f  '  ' (5) As can be seen from Figure S4, there is only one situation that the absorber thickness is equal to the integral multiple of half wavelength, that is, 3.05 mm. However, we cannot find any sign from the curves in Figure S5, indicating the dimension resonance may be not act on the key role in microwave absorption when the thickness is 3.05 mm, where the frequency of maximum RL value (5.9 GHz) is far away from the calculated frequency around 14 GHz. Hence, dimension resonance can also be excluded.


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Generally, nature resonance and exchange resonance occur in the measured frequency scope (2-18 GHz).42 In view of that exchange resonance often occurs at relatively high frequency, the resonance peak in μ′′ curves may be caused by nature resonance.

(b)

-5

Reflection loss/dB

Reflection loss/dB

(a) 0 -10 -15

1.1 mm 1.2 mm 1.3 mm 1.4 mm 1.5 mm

-20 -25 -30

1.5 mm -34.34 dB

1.1 mm -16.12 dB

-35 2

4

6

8

10

12

14

16

0

-10 -20

1.6 mm 1.7 mm 1.8 mm 1.9 mm 2.0 mm

-30

18

2

4

6

(d)

0 -5

-10 -15 -20

2.1 mm 2.2 mm 2.3 mm 2.4 mm 2.5 mm

-25

2.3 mm -41.38 dB

-30 -35

7.32 GHz

8

10

12

14

16

18

Freq./GHz

Reflection loss/dB

Reflection loss/dB

(c)

1.6 mm -41.27 dB

-40

Freq./GHz

-40

0

-10

2.6 mm 2.7 mm 2.8 mm 2.9 mm 3.0 mm

3.0 mm -29.21 dB 5.96 GHz

-20

-30 2

4

6

8

10

12

14

16

18

2

4

6

8

10

12

14

16

18

Freq./GHz

Freq./GHz

Figure 5. Frequency dependence of RL values at various thickness range (1.0-1.5 mm, a; 1.5-2.0 mm, b; 2.0-2.5 mm, c; 2.5-3.0 mm, d) for NC-Co3O4/CP.

0

1-2 mm

-10

39

-20

RL/dB

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-30

This work 8

-40

12

16 14

-60 -70

17

18

13

7

-50

20

2-4 mm

2

22

11

21 1

24

10

23

1

2

3

4

Thickness/mm

5


13


Figure 6. Comparison of thickness-related RL values of NC-Co3O4/CP (red stars) with those reported literature. The number inside panel represents the reference number.

RL/dB

(a) -50 -10 -15 -20 -25 5.0 mm 4.0 mm -30 2.0 mm 3.0 mm -35 2.5 mm

(b) 5 2 Thickness/mm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

6

8

10

1.1 mm 1.3 mm 1.5 mm

12

14

16

18

/4

4 3

/4

2 1

2

4

6

8

10

Freq./GHz

12

14

16

18

Figure 7. Frequency dependence of reflection loss curves of NC-Co3O4/CP (a). 2D contour of reflection loss curves and calculated thickness based on λ/4 and 3λ/4 models of NC-Co3O4/CP (b).

In consideration of the increasing demands on microwave absorbers, it has become a mainstream to pursue a microwave absorber with thin thickness (below 2.0 mm, even below 1.5 mm) and low frequency response (below 10 GHz, even below 5 GHz). As can be found, NC-Co3O4/CP (Figure 5a-d) exhibits distinct microwave absorbing advantages compared with TA-CP (Figure S6a-d), which is derived from the shortened impedance matching and multiple absorbing mechanisms. In detail, the maximum RL value reaches -16.12 dB at 1.1 mm, and when the thickness increases to 1.5 mm, its maximum RL value get to -34.34 dB (Figure 5a). With that, the RL value becomes -41.27 dB when its thickness is only 1.6 mm (Figure 5b). As the thickness continuously increases to 2.3 mm, the RL value of -41.38 dB reaches at a low response frequency of 7.32 GHz (Figure 5c). When the RL value is -29.21 dB at 3.0 mm, the response frequency is 5.96 GHz (Figure 5d). The thickness and response frequency are much lower than that of the previous reports under the similar RL values. The advantage on ACS Paragon Plus Environment

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thickness for NC-Co3O4/CP is further highlighted by comparing with those reported works (Figure 6, Table S2). Generally, a RL values below -10.0 dB means that 90% of incident microwave can be absorbed; when the RL value is lower than -20.0 dB, basically most of the EM wave in the frequency region can be effectively assumed. It means that the pursuance of extremely low RL value makes no sense to the enhancement of practical microwave absorbing performance. In consideration of the high requirements on civil and military application, design of low-thickness absorber has become the hot spot in microwave absorption field. As shown in Figure 6, thickness-related RL values of some typical work were done to make a comparison with NC-Co3O4/CP. Most of the reported work possesses effective microwave absorption at 2.0 - 4.0 mm, even above 4.0 mm. Obviously, NC-Co3O4/CP exhibits huge advantage on thickness, it can reach the effective absorption capacity at very thin thickness (1.1 mm, 1.5 mm and 1.6 mm). 3D RL curves as functions of frequency and sample thickness for NC-Co3O4/CP were also presented in Figure S7, which exhibits that the maximum RL value of -50.1 dB appears at 7.92 GHz. Apart from the as-described attenuation-type absorption, interference absorption is another important channel in microwave absorption process. It can be expressed by the following equation:43 tm 

nc 4f

r  r

n  1,3,5,... (6)

If the absorber thickness (tm) at the peak frequency (fm) meets the above equation, incident and reflected waves would be out of phase of 180°, giving rise to the disappearance of each other at air-absorber interfaces. As can be seen in Figure 7, all the layer thickness related to peaks is consistent with the calculated quarter wavelength (λ/4 and 3λ/4 models, Figure 7b, blue curves) of NC-Co3O4/CP. The results demonstrate that interference absorption/geometrical effect act as a significant role in determining the microwave absorption performance, especially the maximum absorption peak.


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Figure 8. Schematic illustration of microwave absorption mechanisms for NC-Co3O4/CP. Generally, the EM wave absorption performance arises from impedance matching characteristics, magnetic loss, dielectric loss and layer thickness, which are corresponding to their morphologies, structures, compositions and intrinsic natures. In this work, the enhanced microwave performance of NC-Co3O4/CP can be explained by the following points as illustrated in Figure 8. a. Shortened impedance matching gap. According to the impedance matching condition,

Z in  Z o  r /  r

, permeability value is often lower than that of permittivity. Higher the permeability is,

lower impedance difference is. The dielectric–magnetic coupling in the NC-Co3O4/CP composites can effectively shorten the impedance gap between air and absorber, bringing about more microwaves into the interior of absorber. b. Dielectric-magnetic synergistic effect. As has been analyzed above, the primary magnetic loss of Co3O4 in this work is residual loss (nature resonance dominates in all). In addition, dielectric loss is a ACS Paragon Plus Environment

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dominating part in microwave attenuation. As the intrinsic loss of dielectric loss, dipolar polarization is very important to EM wave absorption. In this work, Co3O4, amorphous carbon, even carbon fiber all can provide many dipoles at applied field, which is beneficial to the incident microwave. In addition, there are many interfaces in the composites, like the AC/Co3O4, AC/air, Co3O4/air, Co3O4/CP, AC/CP, AC/wax, CP/wax, and Co3O4/wax. Due to the differences in dielectric characters, multiple MWS effect would occur between the interfaces. Carbon fiber is a high-conductivity material. It not only offers nice conduction loss, but also provides abundant electrons to Co3O4 and amorphous carbon to accelerate their polarization such as interfacial polarization and dipolar polarization. c. Geometrical effect. Interference absorption is another important approach in microwave absorption, apart from the intrinsic attenuation derived from composition and structure. Evident effect of layer thickness on microwave absorption, especially the maximum absorption peak, can be demonstrated in this work, indicating that geometrical effect is another vital part in determining the microwave absorption ability properties.

4. CONCLUSIONS To overcome the drawback of the bad stability and poor durability for traditional powder microwave absorbing materials in practice, a facile and binder-free technique is developed for the in-situ horizontal growth of Co3O4 nanoparticles embedded nitrogen-doped carbon array on the surface of carbon fibers in the carbon paper (NC-Co3O4/CP) as low-thickness microwave absorbing materials. Its maximum RL values reaches -16.12 dB and -34.34 dB when the thickness is 1.1 mm and 1.5 mm, respectively. The hierarchical architecture is facilely originated from a metal-organic framework precursor. In view of the simple preparation technique, the NC-Co3O4/CP sample will prove valuable in various applications as low-thickness response microwave absorbers. ASSOCIATED CONTENT Supporting Information N2 adsorption/desorption isotherms and the pore size distribution, input impedance matching, eddy current loss, frequency dependence of the microwave transmitting in sample, reflection loss, frequency dependence of reflection loss at various thickness range, 3D RL curves. ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors approved the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial supports from the Aeronautics Science Foundation of China (No. 2017ZF52066), the National Nature Science Foundation of China (No. 11575085), the Qing Lan Project, Six talent peaks project in Jiangsu Province (No.XCL-035), the Outstanding Doctoral Dissertation in NUAA (No. BCXJ 18-07) are gratefully acknowledged. REFERENCES (1) Liu, W.; Li, H.; Zeng, Q.; Duan, H.; Guo, Y.; Liu, X.; Sun, C.; Liu, H. Fabrication of Ultralight Three-Dimensional Graphene Networks with Strong Electromagnetic Wave Absorption Properties. J. Mater. Chem. A 2015, 3, 3739-3747. (2) Zhang, Y.; Wang, X.; Cao, M. Confinedly Implanted NiFe2O4-rGO: Cluster Tailoring and Highly Tunable Electromagnetic Properties for Selective-Frequency Microwave Absorption. Nano Research 2018, 11, 1426-1436. (3) Cao, M.; Wang, X.; Cao, W.; Fang, X.; Wen, B.; Yuan, J. Thermally Driven Transport and Relaxation Switching Self-Powered Electromagnetic Energy Conversion. Small 2008, 14, 1800987. (4) Zeng, Z.; Lin, H.; Chen, M.; Li, W.; Zhou, L.; Xue, X.; Zhang, Z. Microstructure Design of Lightweight, Flexible, and High Electromagnetic Shielding Porous Multiwalled Carbon Nanotube/Polymer Composites. Small 2017, 13, 1701388. ACS Paragon Plus Environment

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Functionalized Carbon Nanofibers Enabling Stable and Flexible

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