Lightweight and high performance microwave absorbing heteroatom

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Lightweight and high performance microwave absorbing heteroatom doped carbon derived from chicken featherfibers Sandeep Kumar Singh, Hari Prakash, M J Akhtar, and Kamal K Kar ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00183 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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ACS Sustainable Chemistry & Engineering

Lightweight and high performance microwave absorbing heteroatom doped carbon derived from chicken featherfibers Sandeep Kumar Singh†, Hari Prakash†, M. J. Akhtar†,‡ and Kamal K. Kar †,§,* †

Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India

‡

Department of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur, India

§

Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering and

Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur-208016, India *Corresponding Author (Email: [email protected], Phone: +91-512-2597687, Fax: +91-5122597408) KEYWORDS: Chicken featherfiber. Heteroatom doped carbon, Porous structure, Pyrolysis, Microwave absorption, Complex permittivity

ABSTRACT

The doped material is an innovation in developing the lightweight microwave absorbing material. Herein, heteroatom doped carbon is synthesized by pyrolysis of chicken featherfibers (CFFs) in the temperature range of 400 to 1400 °C. The synthesis method exhibits that poultry 1 ACS Paragon Plus Environment

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waste is more nature-friendly as no external hazardous dopants are used during pyrolysis, and has much lower cost. The morphology and structural characteristics have been studied via SEM, AFM, TEM, XRD, Raman, and XPS. The density of surface chemical states, defects, roughness and structural property are found to vary significantly with pyrolysis temperature. The electromagnetic properties of CFF/epoxy composites have been studied in the frequency range of 8.2 to 12.4 GHz (X band). In addition, the correlations between pyrolysis temperature and absorption properties are established. High absorption properties in the temperature of ≥800 °C are attributed to the large fraction of heteroatoms, defects, surface roughness and high porosity. In addition, the CFF pyrolyzed at 1400 °C is further activated with potassium hydroxide that results in numerous porous morphologies with large surfaces. This optimized porous CFF illustrates substantial absorption efficiency corresponding to the absorber thickness of 1.68 mm and RL of -44.6 dB (99.99% microwave absorption), which exhibits broad -10 dB (90 % absorption) bandwidth that shares 52.9 % of the entire X band frequency width. The strong microwave absorption originates from defect polarization, electric/dipolar polarization, interfacial polarization and 3D porous structure. The porous 3D architecture improves the impedance matching and can generate multiple reflections and scattering of electromagnetic waves, which attenuate microwave waves largely. This work suggests that the heteroatom doped carbon derived from CFF is a potential candidate to design a lightweight and efficient microwave absorber.

INTRODUCTION In the recent years, the importance of electromagnetic (EM) wave technology has significantly increased due to its potential application in military, telecommunication, satellite

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communication, electronic devices and stealth technology.1-5 However, the electromagnetic radiations are becoming a major problem for normal operation of electronic devices and to protect human health.2 Therefore, much efforts have been emphasized on the design and development of high performance EM wave absorbing materials. For commercial applications, the necessity of strong microwave absorption with thin thickness and light weight are the basic parameters.5-6 The lightweight is a novel way to make the absorber more efficient and costeffective for the strategic utility. Recently, carbonaceous materials such as graphene, carbon nanotubes (CNTs), carbon black and porous carbon have become important materials as advanced microwave absorbers due to their high dielectric constant and losses with low specific weights.7-10 Among them, CNTs have become the hotspot microwave absorbing material owing to its unique electrical, mechanical and thermal properties. However, the data reported in recent literatures shows that the CNT based absorber is a costly material because it exhibits poor EM wave absorption with negligible -10 dB (90% absorption) bandwidth, which is not less than -10.5 dB (90.1% absorption).9,

11-12

Moreover, several investigations have engrossed on engaging

CNTs with magnetic/dielectric composite structures to further enhance the microwave absorption efficiency including wide effective bandwidth, which results additional weight and rises the cost of absorber .9,

13-14

Therefore, exploration of low cast alternative carbonaceous materials with

strong absorption properties still remain as a major challenge to the scientific community. Recently the usefulness of biomass-derived carbon (BDC) has been successfully verified in the fields like batteries, supercapacitors, wastewater treatments, fuel cells and solid-state solar cells.15-20 The wide applications of this material are due to the naturally occurring doped carbon with porous structure. In addition, the simple fabrication process and low synthesis cost are the foremost advantages of using BDC. The preparation of BDC from biomass requires pyrolysis of 3 ACS Paragon Plus Environment


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biomass at elevated temperatures. The presence of nitrogen and phosphorous in biomass helps naturally to attain doped carbon. In addition, some other precursors are also used to get doped carbon.15-16 But, the precursors used are environmentally toxic. Therefore, for safeguarding the environment, researchers are currently emphasizing on the naturally occurring precursors. Recently, porous carbon structure has been used as microwave absorber because it backs the interface polarization, multiple reflection and scattering loss that eventually amplifies the microwave absorption properties.10 The synthesis of porous biomass carbon is preferentially attained through potassium hydroxide (KOH) activation because the activation method is simple and highly efficient.20 Chicken feathers are low cost and copious store on earth with huge prospective as raw materials in several technological applications, which is known as byproduct from poultry industries. A recent study on poultry sector in USA reveals that 50.4 billion pounds of chickens are produced annually and it is anticipated that if feathers share only 5% of weight in chicken as a result approximately 3 billion pounds of chicken feathers are created in a year by the USA only.21 Formerly used methods for the disposal of chicken feathers including landfill, ocean dumping, and disposal on agricultural land have become much less acceptable as they are potential threat to human health and environment. Lately, Sharma and Kar have explored an application of BDC by pyrolysis of the chicken feather fibers (CFFs) in the temperature range of 400 to 1000 °C to enhance the proton exchange membrane fuel cell activity.16 Similarly, Benjwal et al have successfully demonstrated the utility of BDC for adsorption of organic water pollutants.17 The soft part of chicken feather (barbules), which is known as CFFs and composed of highly rich keratin protein, is a polymer containing C, N, and O elements.15-16 Therefore, when pyrolysed it rearranges its bonding configurations to produce heteroatom (N, O) doped carbons. For example, 4 ACS Paragon Plus Environment

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nitrogen-doped carbon exists typically in four bonding configurations i.e., pyridinic nitrogen (pyridinic-N), pyrrolic nitrogen (pyrrolic-N), graphitic nitrogen (graphitic-N), and oxide nitrogen (oxide-N). During pyrolysis pyridinic-N and pyrrolic-N rearrange their bonding configurations with loss of C atom, and are considered as structural defects.17, 22 Although, BDC obtained from CFFs has many applications, but it has never been explored in microwave absorption applications. It has disordered structure and different chemical compositions, which are beneficial for microwave absorption efficiency. Because doped atoms and structural defects increase the density of polarization centers.23-24 Again, further activation of BDC results numerous pores, which amplify its apparent surface area and also create numerous solid-air interfaces. Theses solid-air interfaces give interfacial polarization under oscillating EM wave. In addition, numerous pores generate multi-reflection and diffraction of EM wave, which again attenuate the microwave power. Therefore, it is anticipated that the BDC from CFFs has a great potential to become high performance lightweight EM wave absorber. Herein, an attempt has been made to develop a low cost, thin, lightweight and effective microwave absorber by pyrolysis of CFFs in the temperature range of 400 to 1400 °C. Notable, no hazardous precursors are used during pyrolysis of CFFs. The importance has been given to understand the effect of structural and surface chemical state of the CFF on the microwave absorption properties. The roughness, structural, surface chemical state, defects densities and porous morphologies of BDC are correlated with the absorption performances, which are found to increase with the pyrolysis temperature, and enhances the microwave absorption properties significantly. The KOH activated BDC exhibits strong microwave performance and the reflection loss reached -44.6 dB (99.99% EM wave absorption) with 2.2 GHz of -10 dB (90%



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absorption) bandwidth at 1.68 mm of thickness. Therefore, the CFF has a huge potential for lightweight and strong microwave absorber applications. EXPERIMENTAL Synthesis procedures Thermal degradation of CFFs: The thermo-gravimetric analysis (TGA) is used to determine a pyrolysis profile of CFFs as shown in Figure S1. The heat-treatment of CFFs reveals that the initial weight loss (below 90 ºC) is primarily due to the removal of moisture and other volatile impurities. Below 220 ºC, CFFs do not exhibit much degradation. From the temperature derivative of weight loss (dW/dT), it is clear that the onset of degradation starts above 220 ºC, while the maximum rate of weight loss can be seen around 300 °C. Significant weight loss (~70 %) in the range of 220 to 400 ºC shows a pyrolysis process zone. The aim of holding the CFF at 220 ºC for 1 h is to increase the cross-linking of polymer chains. Pyrolysis temperature is selected above the maximum weight loss rate and the temperature is kept constant for 0.5 h at 400 ºC to assure the complete removal of volatiles. Similarly, for pyrolysis of CFFs at high temperatures i.e., 600, 800, 950, 1200 and 1400 ºC; we have chosen 1 h of holding time to ensure full pyrolysis of CFFs. The samples are designated as CFF-600, CFF-800, CFF-950, CFF-1200 and CFF-1400 corresponding to the pyrolysis temperatures of 600, 800, 950, 1200 and 1400oC, respectively. In addition to this, the activation of CFF-1400 is further executed using elsewhere reported process.20 Initially, as prepared CFF-1400 was mixed with KOH (KOH was purchased from M/S Loba Chemie Pvt. Ltd, India) in a weight ratio of 1:3.5 and activated in nitrogen environment at 900 °C for 1 h. In the next step, the activation reaction debris were eliminated by keeping the specimen in 1 mol/L HCl (HCl was received from M/S Qualigens Fine Chemicals,


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India) solution for 24 h. Finally, samples were washed many times with deionized water and subsequently dried overnight. The resulted activated CFF-1400 was denoted as PCFF-1400. Pyrolysis process: Chicken feathers were bought from the poultry shop. The rachis and barbs were carefully removed from feathers. The barbs, soft fibrous part of the chicken feather were collected (Figure 1). In the process of deriving carbon, CFFs were washed many times under laboratory conditions. The washed CFFs were again rinsed repeatedly with deionized water for 1 h and kept into an oven for drying at 60 ºC for 2 h. The cleaned CFFs were placed inside a horizontal quartz tube and purged with N2 for 0.5 h. Pyrolysis was done by heating the CFFs to the desired temperature at a heating rate of 3 ºC min-1 in N2 (90 mL min-1) and then holding it for 1 h at that temperature. CFFs were pyrolyzed at various temperatures i.e. 400, 600, 800, 950, 1200 and 1400 ºC. Further, samples were cooled to room temperature in presence of constant N2 (90 mL min-1) flow. After cooling, pyrolyzed samples were crushed to fine powder (Figure 1) for further characterizations.



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Figure 1. The schematic for the synthesis of heteroatom-doped carbon derived from CFF. Fabrication of composite: BDC synthesized from CFF of 30 wt.% was mixed with epoxy and prepared in a rectangular shaped composite with dimension of 22.86 mm X 10.16 mm X 2.0 mm. In addition, another sample with the same dimension was prepared using PCFF with 30 wt.% of loading in epoxy to highlight the role of the pore structure of microwave absorption properties. The pressure of 0.2 MPa at room temperature was given to avoid any bubble formation in the sample. Finally, prepared samples were post cured in an oven at 70 ºC for 4 h in order to have a flat rectangular sheet. Characterizations Scanning electron microscope (SEM; Zeiss EVO MA-15) and atomic force microscope (AFM; Agilent 5500, using SPIP 6.6.5 software) were used to investigate the surface morphology. The microstructural characteristics were examined using transmission electron microscope (TEM (HR-TEM), FEI Titan G2 60 -300). The Raman spectra were collected from Raman microscope (Horiba Jobin Yvon LabRAM HR) equipped with a He-Ne laser source (λ = 632.7 nm). X-ray diffraction (XRD) spectra were captured by the Panalytical X'Pert for ultra-fast X-ray diffraction on bulk and powder samples using line detector in the 2θ range of 10 to 60o with Cu kα radiation using the wavelength of 1.5418 Å, scan rate of 2° min-1 and step size of 0.01°. In addition, chemical states of the synthesized CFFs were characterized by the X-ray photoelectron spectroscopy (XPS) using a PHI 5000 (Versa Prob II, FEI Inc.) spectrometer equipped with Al Kα (1486.6 eV) monochromatic source. Similarly, TGA was performed using Perkin-Elmer Diamond thermogravimetric analyzer to study the thermal behavior of as received CFFs. Finally, the microwave absorption properties of CFF/epoxy composites were studied with Agilent vector


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network analyses (VNA) (E8364B) with the WR-90 (X band calibration kit) having sample dimensions of 22.86 mm X 10.16 mm X 2.0 mm in the frequency range of 8.2 to 12.4 GHz. RESULTS AND DISCUSSION Figure 2a represents the XRD patterns of CFFs and shows its structural changes with increasing pyrolysis temperature. The broad diffraction peak around 2θ of 26.0° represents the (002) reflection plane (JCPDS 41-1487) of graphitic carbon. It confirms the presence of amorphous carbon with significant crystallinity.16 In addition, a spike noticed at broad diffraction peak around 2θ of 26.7° indicates a graphitic order. The peak height increases with increasing temperature, which discloses the strong graphitization of CFFs at high pyrolysis temperatures. No significant changes are found for 400 and 600 °C. But, samples with ≥ 800 °C of pyrolysis temperature show additional diffraction peaks centered at angles of 44.0 and 51.0° with (101) and (102) reflection planes of graphite (JCPDS 41-1487), respectively and signifies the high graphitization of respective CFFs.



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Figure 2. (a) XRD spectra and (b) XPS survey spectra of CFF powder sample pyrolyzed at different temperatures. Again, TEM, selected area electron diffraction (SAED) and high resolution-TEM (HRTEM) patterns of CFFs synthesized at 400 and 1400 °C were recorded to scrutinize their morphology and crystalline nature with respect to temperature as shown in Figure 3a-d. From inset of Figure 3a, the SAED observation of CFFs synthesized at 400 °C (CFF-400) indicates crystalline nature that has coarse structure and some amorphous behavior, whereas SAED of CFFs synthesized at 1400 °C (CFF-1400) indicates continuous ring as presented in the inset of Figure 3b; suggesting more crystalline nature. The HRTEM image reveals that CFF-400 exhibits carbon nano fiber morphology with a well-defined interplanar spacing of 0.39 nm that assigns to be (002) plane of graphite as shown in Figure 3c. Moreover, the HRTEM image of CFF-1400 appears to be more crystalline compared to CFF-400 with the absence of carbon nanofiber morphology that appears due to the removal of impurities at higher temperature as shown in Figure 3d. In the CFF-1400, a slight variation with 0.35 nm of interplanar spacing (d002) is observed along with an appearance of 0.21 nm of d101 spacing. These information from TEM analysis also supports the XRD results, where (002) peak intensity of CFF-1400 is found significantly higher than CFF-400 including appearance of additional (101) peak. However, it is difficult to detect N and O elements in TEM and HRTEM as they possess sizes comparable to C. Nevertheless, the presence of these elements is further studied by XPS and variation of defects is examined by Raman analysis.


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Figure 3. TEM, SAED (inset) and HRTEM micrographs of (a and c) CFF-400 and (b and d) CFF-1400. Typical Raman spectra of CFFs pyrolyzed at different temperatures are shown in Figure 4a, and represent partially resolved peaks centered around 1357 and 1590 cm-1 that matches well with the characteristics of amorphous carbon. Again, to investigate the structural order with pyrolysis temperature, the Raman spectra are deconvoluted by least square fitting using Voigt distribution as shown in Figure 4b with peak positions at 1210, 1362, 1541 and 1610 cm-1 corresponding to the sp3 carbon, D, G and D´ bands of the graphitic carbon, respectively. The Raman parameters, G band assigns to the phonon mode of E2g symmetry at Brillouin zone center of sp2 graphitic carbon network, whereas, D band endorses the defects and lattice 11 ACS Paragon Plus Environment


distortions in C- network.17,

25

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Again, the deconvoluted broad D and G bands signify low

crystallinity that is well supported by XRD analysis. Further, intensity ratio (ID/IG) and integrated peak area ratio (AD/AG) show a significant variation with pyrolysis temperature as shown in Figure 4c. On increasing temperature, both ID/IG and AD/AG values decrease from 1.60-1.05 and 1.62-1.02, respectively, which suggest high graphitic order and low structural disorder of CFFs with increasing temperature. Interestingly, it is well proven that the intensity ratio (ID/IG) is inversely proportional to distance between the defects (La), which is a measure of interdefect nano-crystallite size and can be calculated using relationship (1)25-26

I  L d (nm) = (2.4 × 10 -10 ) × λ 4  D   IG 

-1

(1)

Similarly, defects density (nd) can be determined by the expression (2)27

n d (cm -2 ) =

(1.8 ± 0.5) × 1022  I D    λ4  IG 

(2)

Where λ is the excitation wavelength and ID/IG is intensity ratio corresponding to D and G bands, respectively. From Figure 4d it is obvious that interdefect distance increases and defect density decreases with increasing pyrolysis temperature. The CFF-1400 appears to be highly graphitic carbon with fewer defects. Notably, the N, O at% decreases in response to the increasing pyrolysis temperature as supported by XPS analysis. This suggests that nano-crystallite size increases remarkably with the decrease of N and O shares in pyrolyzed CFFs, which is due to the low defect density. This can be understood on the fact that with elevated pyrolysis temperature, the local atoms in the carbon network rearranges their bonding configurations due to the boosted


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atomic diffusion, which ultimately enhances the graphitic share with a reduced amount of defects.

Figure 4. (a) Representative Raman spectra of CFFs pyrolyzed with temperature range of 400 1400 °C. (b) Voigt fits partially deconvoluted Raman spectra of CFF-400 along with sp3 carbon, D, G and D´ bands of graphitic carbon. (c) Variation of Raman parameters ID/IG and integrated peak area ratio (AD/AG) with pyrolysis temperature. (d) Variations of distance between defects (Ld) and defects density (nd) for CFFs with pyrolysis temperature. In addition, Figure 5a-c shows AFM images of as-synthesized CFFs pyrolyzed at temperatures of 400, 800 and 1400 °C. Results recommend substantial improvement of surface roughness with increasing pyrolysis temperature. Importantly, the roughness parameter, Sz (ISO 25178) is found ~ 30 nm for CFF-400, ~ 80 nm for CFF-800 and ~ 180 nm for CFF-1400. This



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results from irregular flange formation and exhibited more evolution of volatile by-products with increase in pyrolysis temperature. The surface roughness of CFFs may be a vital parameter for determining the electromagnetic properties, which depends on pyrolysis temperatures.

Figure 5. AFM images of (a) CFF-400, (b) CFF-800 and (c) CFF-1400. The SEM images of crushed CFFs pyrolyzed with temperatures of 400, 800 and 1400 °C are shown in Figure S2a-c, respectively; reveals the irregular morphology of samples. The irregular particles can be seen in Figure S2a, and at high magnification, few porosities with fragments that adhered to it surface are noticed. This is due to the strong cross-linking of polymers. Similarly, irregular particles with the formation of less porosity and overlapping layers are observed for samples with 800 °C, which is attributed to the protein degradation i.e., removal of volatile byproducts at higher synthesis temperature. In addition, irregular particles become gleaming at 1400 °C and with few more porosity can be seen in Figure S2c, which is possible due to the increase in crystallinity and removal of more volatile chemical elements during pyrolysis. This increase in porosity with temperature is advantageous in microwave absorption applications as it facilitates the larger surfaces to interact more with the microwaves. Moreover, the SEM


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micrographs of CFF-1400 and PCFF-1400 are presented in Figure 6a-d. A noticeable transformation of the surface topography between CFF-1400 and PCFF-1400 is observed. Without activation, some natural pores of size ~2-5 µm can be in CFFs. After activation, numerous pores are created in the surface of PCFF as shown in Figure 6c-d. Nevertheless, it is evident from the images that the external surface of the activated sample is full of voids i.e. exhibits a huge number of solid-vacant interfaces.

Figure 6.

SEM images (a)-(b) of CFF-1400 and (c)-(d) of PCFF-1400 at different

magnifications. The surface elemental compositions of CFFs at different temperatures were investigated via XPS analysis. Figure 2b shows full range XPS spectra of CFFs synthesized at 400, 800 and 1400 °C. The result discloses the presence of C, N, and O heteroatoms on their surface. The strong peak at 284.4 eV indicates that chief component is carbon in all the samples. It is observed that the O 1s peak intensity increases and then decreases with increase in pyrolysis temperature. However, N 15 ACS Paragon Plus Environment


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1s peak intensity decreases with increase in pyrolysis temperature. The atomic fraction (at.%) of C determined by XPS analysis is found to be 80.0, 82.1 and 85.9 at% at 400, 800 and 1400 °C, respectively. Similarly, N at% are 12.4, 6.0 and 4.9 % for 400, 800 and 1400 °C temperatures, respectively. However, the atomic fraction for O is 7.6, 11.4 and 10.3 at% for corresponding CFFs.

Figure 7. XPS spectra of heteroatom doped carbon derived from CFFs with pyrolysis temperatures of 400, 800 and 1400 °C ((a) C 1s, (b) N 1s and (c) O peaks fitted with various components). Figure 7a-c represents the high-resolution XPS spectra corresponding to C 1s, N 1s and O 1s peaks of CFFs pyrolyzed at 400, 800 and 1400 °C, respectively. The Gaussian-Lorentzian 16 ACS Paragon Plus Environment

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distribution is utilized with Shirley type background correction for fitting the high-resolution XPS signals. The C 1s peak is deconvoluted in four components and centered at ~ 283, 284, 286 and 288 eV that corresponds to Cα (H-C), C-C (CC, sp2), C-C (CC, sp3)/C-N (CN, sp2) and NC=O bonds, respectively as shown in Figure 7a. It is noticeable that peak at 283 eV assigned to Cα chemical state diminishes with increasing temperature and at higher temperature i.e. 1400 °C its signature disappeared, which shows low temperature partial decomposition of keratin helix. Additionally, the CC, sp2 component of C 1s spectra increases significantly with increasing pyrolysis temperature whereas, the large contribution of CC, sp3/CN, sp2 component exists at high temperature suggesting the presence of increased C-C (graphitic carbon) and C-N bonds at high pyrolysis temperature. Moreover, the N 1s peak shifts towards higher binding energy with increasing synthesis temperature; signifies the appearance of fresh C-N bonds in samples as shown in Figure 7b. The N 1s peak corresponding to 400 °C comprises four deconvoluted components at 396.67, 397.5, 398.4 and 399.1 eV that have assigned to HC-N=C, pyridinic (N connected to two C), HC-NH-C and pyrrolic (N of a pentagon ring connected to two C), respectively. The peak intensity associated with HC-N=C and HC-NH-C components decreases and new graphitic peak (N bonded with three sp2 carbon) is appeared at 800 °C. Similarly, the fractions of graphitic-N, pyrrolic-N, and pyridinic-N components are maximum and a new pyridine-N-oxide is emerged around 403 eV at 1400 °C as shown in Figure 7b. Whereas the occurrence of pyrrolic nitrogen disrupts the keratin protein structure of feather fibers. Similarly, pyridinic and graphitic nitrogen also denote divacancy defects. So the presence of N atoms in CFFs is witnessing the atom arrangement defects such as vacancies, and bonding disorders that generate a structure with large defects.



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The result from XPS measurement gave additional information regarding presence of O functionalities and their connection with C and N atoms on CFFs. Figure 7c shows that O 1s spectra of CFFs pyrolyzed at 400, 800 and 1400 °C, respectively. For 400 °C, the deconvoluted components of O 1s peak centered at ~ 530, 531 and 532 eV are responsible for to NC=O, OC=O, and OCO bonds as can be seen in the Figure 7c.16-17 It is observed that the fraction of NC=O component decreases with increasing pyrolysis temperature. This results in shift of whole O 1s peak towards higher binding energies. Overall, the variation of C, N and O elements with pyrolysis temperature are shown in Table 1. Table 1: Variations of the fractional composition of C, O, and N surface species for the CFFs pyrolyzed with different temperature and their overall elemental compositions attained from the XPS investigation Elements

CFF-400

CFF-800

CFF-1400

Total N (at%)

12.4

6.00

4.90

N-pyridinic

0.19

0.16

0.44

N-pyrrolic

0.22

0.24

0.26

N-graphitic

-

0.18

0.22

HC-NH-C

0.27

0.26

-

HC-N=C

0.32

0.21

-

N-oxide

-

-

0.08

Total C (at%)

80.0

82.1

85.9

Cα

0.37

0.04

-

C sp2

0.45

0.48

0.51

C sp3

0.11

0.38

0.40

N-C=O

0.04

0.13

0.10


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Total O (at%)

7.6

11.4

10.3

N-C=O

0.64

0.17

-

O-C-O

0.10

0.48

0.65

O-C=O

0.26

0.35

0.36

From Table 1 it is obvious that CFFs with pyrolysis temperature ≥800 °C has higher quantities of graphitic C sp2, C sp3, pyridinic, pyrrolic and graphitic nitrogen and O containing functional groups whereas CFFs with temperature ≤ 800 °C sustains the lowest values. However, CFF-1400 exhibits larger surface species than the others. Again among several species of N, it is expected that the pyridinic, pyrrolic and graphitic nitrogen would be most effective for the microwave absorption as it forms more compartments and disorder for nitrogen doped carbon. These defects may act as polarization center that gives rise the absorption properties. Similarly, amounts of graphitic carbon would be the most decisive parameter for absorption properties due to the abundance of electrons. Therefore, it is expected that the presence of different atoms and functional groups on the CFFs may play an active role to enhance the microwave absorption properties. After structural and morphological studies of CFFs, microwave absorption properties of CFF/epoxy and PCFF-1400/epoxy composites are studied as a function of pyrolysis temperature. Figure 8a-b represents the frequency dependent real (ε’) and imaginary (ε’’) parts of complex permittivity of CFFs (temperature: 400-1400 °C)/epoxy and PCFF-1400/epoxy composites. It is well accepted that real part of permittivity is related to the amount of polarization in the composite that basically originate from different polarizations, and imaginary part of permittivity is associated with dielectric losses.28 It can be seen from the plots that both real and imaginary 19 ACS Paragon Plus Environment


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part of complex permittivity increase with increasing the pyrolysis temperature. From Figure 8a, the real permittivity values are found to be around 3.3, 4.4, 5.3, 10.9, 12.9, 14.8, 15.4 and 25.1 at 8.2 GHz for samples with synthesis temperatures of 400, 600, 800, 950, 1200, 1400 °C and PCFF-1400, respectively. Similarly, the imaginary permittivity reaches to 0.2, 0.7, 0.99, 1, 2, 1.8, 3.4 and 6.5 at 8.2 GHz for respective samples as can be seen in Figure 8b. The ε' and ε'' of CFF/epoxy composites with pyrolysis temperatures of 400 and 600 °C are small, which means that composites exhibit weak electrical responses (low fraction of graphitic carbon).

The

enhanced permittivity of the composites pyrolyzed at temperature of ≥ 800 °C reflects the combined effect of carbon (CC, sp2 graphitic) that induces electrical polarization and interfacial polarization due to the presence of different heteroatoms (N, O), nitrogen generated defects and N, O containing groups that act as polarization centres.21 In addition, high porosity and high roughness also contribute effectively to enhance the interfacial polarization. The defects and functional groups easily polarize under the oscillating EM fields. Therefore, the abundance of N, O, C heteroatoms, functional groups, high roughness and high porosity on the CFFs surfaces with the effect of synthesis temperature; enhances multiple polarizations. The large the fraction of heteroatoms, defects, and functional groups, the greater could be the permittivity. Therefore, CFFs/epoxy composites show an ability to induce more polarizations that could have the great effect on the microwave absorption performance. In comparison, the real permittivity of PCFF1400 is substantially larger than CFF-1400. Innovation, the KOH activation method that results porous structure can increase the permittivity further. The porous structure contains solid-vacant interfaces, which enhances the interfacial polarization. In addition, defects in the form of pore morphology act as polarization center and facilitates multiple reflection and scattering of microwaves that contribute largely to amplify the permittivity values. In the same way, PCFF-


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1400 has higher imaginary permittivity than CFF-1400. This implies that PCFF-1400 has better dielectric losses due to the availabilities of pores and solid-vacant interfaces.

Figure 8. (a) Frequency dependent (a) real part, (b) imaginary part of permittivity and (c) loss tangent of CFF (400-1400 °C) and PCFF-1400 mixed epoxy composites. Loss tangent quantifies the energy losses i.e., a dissipative factor as it converts EM energy into heat and is typically expressed as the ratio of imaginary permittivity (ε″) to real permittivity (ε′).29 Figure 8c represents frequency dependent loss tangent of composites containing PCFF1400 and CFFs pyrolyzed at different temperatures. A significant increment in loss tangent is observed with temperature, representing strong EM energy losses of such composites. The loss tangent of PCFF-1400/epoxy composite lies slightly lower than CFF-1400 in the frequency range of 9.5-10.5 GHz. However, the loss tangent exhibits an indistinctive distribution in the measured



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frequency, which may be attributed to the variation of surface heteroatoms, defects, and functional groups. It is well documented in the recent literatures that various polarizations and the related relaxation processes can enhance the microwave absorption performances. And these relaxation process typically validated by the Cole-Cole semicircle plots. According to the Debye relaxation theory, permittivity can be written as (eq. 3)28 2

(ε ' − ε ∞ ) + (ε '' ) 2 = ( ε s − ε ∞ ) 2

(3)

where εs and ε∞ are the static dielectric constant and dielectric constant at infinity, respectively. Therefore, the curves of ε´´ vs ε´ would be a single semicircle, which signifies a Cole-Cole plot.30 Each semicircle represents one Debye relaxation process that has a decisive effect on the microwave absorption properties of the dielectric material. The Cole-Cole plots of all samples are shown in Figure 9a-g. Meanwhile, the number of semicircles decreases and clear segment of semicircle increases with increasing temperature. It should be especially noted that three ColeCole semicircles emerged in CFF-1400 together with one big semicircle as shown in Figure 9f. This suggests that few relaxation processes disappear at high-temperature pyrolysis but the remaining are dominating strongly. This behavior appeared due to the high percentage of graphitic (CC, sp2; as confirmed in XRD analysis) carbon that contains a huge number of delocalized electron that is responsible for dielectric relaxation. It should further observed that the cole-cole semicircles exhibited in Figure 9 are quite irregular, suggesting that in addition to Debye relaxation, the other multi-relaxation mechanisms that may also occurred in the CFF/epoxy composites such as defect, electric dipole and interfacial polarizations. As discussed in XPS and Raman analysis, when CFFs are pyrolysed they rearrange their bonding 22 ACS Paragon Plus Environment

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configurations to produce heteroatom (N, O) doped carbons with many nitrogen and oxygencontaining groups. The nitrogen-doped carbon exists typically in four bonding configurations i.e. pyridinic nitrogen, pyrrolic nitrogen, graphitic nitrogen, and oxide nitrogen. However, pyridinicN and pyrrolic-N rearrange bonding configurations with loss of C atom are considered as structural defects, which act as polarization centers. Moreover, CFF also contains numerous nitrogen and oxygen containing functional groups i.e., (O/N)-C or (O/N)=C chemical bonds on CFFs surfaces that show different capability to attract electron between carbon-nitrogen and carbon-oxygen atoms in presence of fast oscillating EM waves, which resulting as electric dipole polarization. And, under the changing EM field; the lag of electron movement gives rise the additional polarization relaxation. In addition to this, the interfacial polarization (the Maxwell– Wagner effect)3 that arose between the epoxy and CFFs due to the gathering of charges at the boundaries creates cluster of large dipoles in the system, which enhances microwave losses greatly. Finally, the PCFF-1400 exhibits six cole-cole semicircles as shown in Figure 9g, which confirms the occurrence of multi relaxation processes. As confirmed in SEM analysis of PCFF1400, it exhibits unique porous structures with plentiful of solid- vacant interfaces. These interfaces are contributed additional interface polarizations (the Maxwell–Wagner effect) in such sample that eventually enhance the microwave attenuation properties. To sum up, all these relaxation processes contribute significantly to enhance the reflection losses of the CFFs.



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Figure 9. Cole-Cole plots of (a) CFF-400, (b) CFF-600, (c) CFF-800, (d) CFF-950, (e) CFF1200, (f) CFF-1400 °C and (g) PCFF-1400. The microwave absorption capability of CFF/epoxy composites in terms of reflection loss can be calculated by the following eq 4 and 529 RL = 20log

1/ 2

µ  Z in =  r   εr 

( Z in − Z 0 )

(Z

in

(4)

+ Z0 )

 j 2π ft tanh   c µ r ε r

  

(5)


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where Zin is the input impedance, Z0 is the impedance of free space, f is the frequency, t is the sample thickness and c is the velocity of light in free space. It is well known that the impedance matching condition, Zin=Z0 has to be satisfied in order to gain maximum absorption of incoming EM waves. This indicates that for dielectric material (µ r = 1), if the real and imaginary permittivity is out of balance then most of the incoming EM will be reflected off from the composite surface that gives rise the poor absorption.31 Furthermore, RL value less than -10 dB exhibits more than 90% absorption, which is a demanding requirement from the microwave absorbers in practical applications. Figure 10a exhibits the frequency dependent reflection loss curves of CFF/epoxy composites with various pyrolysis temperature when the composite thickness is 2.0 mm. It can be seen from the plots that with increasing pyrolysis temperature, the value of minimum RL increases and absorption peak is shifted towards lower side in the measured frequency range. The CFF/epoxy composites pyrolyzed at 400 and 600 °C attain very weak absorptions i.e., -0.6 and -1.1 dB, respectively. However, composites with pyrolyzing temperatures ≥800 °C exhibit strong absorption and significantly cross the magical -10 dB performance level, which is also attributed to the increased porosity and roughness as observed in SEM and AFM images, respectively19. The minimum reflection losses for CFF/epoxy composites pyrolyzed at 800, 950, 1200 and 1400 °C reach -10.6 (91.29% absorption), -13.8 (95.83% absorption), -14.2 (96.19% absorption )and 20.1 dB (99.02% absorption) with 11.9, 11.8, 10.9 and 10.2 GHz of matching frequency, respectively (Figure 10b). In addition, the results portrayed at high temperature and shown in Figure 10b state that higher synthesis temperature has a wider -10 dB bandwidth, and the bandwidth and RL intensity together perceptibly increases in well balance manner with increasing pyrolysis temperature. Notably, it is also observed that EM waves are efficiently 25 ACS Paragon Plus Environment


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attenuated by CFF-1400 as it touch -20 dB mark, which represents 99.0 % of EM wave absorption. This means that the pyrolysis temperature has significantly enhanced impedance matching, which results in the excellent absorption properties. Hence, the aforementioned findings are in good agreement with the discussions of the electromagnetic characteristics and relaxation processes, which support that the epoxy composite with heteroatom doped CFFs is advantageous for strong microwave absorption.

Figure 10. (a) Reflection loss curves against frequency and (b) variations of minimum reflection loss, central frequency and -10 dB bandwidth of CFF/epoxy composites with different synthesis temperature. As noticed in earlier section, the contents of most of the surface species increase with increasing pyrolysis temperature. However, few surface elements are disappeared at higher pyrolysis temperature. Therefore, a correlation between minimum reflection loss (Min. RL) and wt. fraction of C, N, O, functional groups with pyrolysis temperature has been studied to investigate the absorption efficiency of the samples. Figure 11a depicts the significant variations of the Min. RL with temperature-sensitive N-component (pyridinic, pyrrolic and graphitic), C-component (C sp2 and C sp3) and O-component (O-C-O and O-C=O) of the CFF/epoxy composites. In case of 26 ACS Paragon Plus Environment

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vigorous involvement of a given surface elements in the microwave adsorption process, an increasing correlation between the minimum reflection loss and the weight fraction of the elements is anticipated. As shown in Figure 11a, with increasing compositions of N-pyrrolic, Ngraphitic, graphitic C sp2, C sp2 and O-species O-C-O and O-C=O, the min. RL increases linearly whereas, N-pyridinic shows complex behavior means min. RL increases with decreasing/increasing content of pyridinic nitrogen, suggesting that each surface element is exactly paired with min. RL. This can be understood by the fact that pyridinic and pyrrolic form defects that act as defect polarization relaxation whereas, graphitic carbon and nitrogen give rise the electric polarization relaxation. Similarly, oxygen act as a polarization center and O functional groups may contribute to functional group electric dipole polarization relaxation. However, from Figure 11b it is evident that no significant correlation is found between the min. RL and C-component N-C=O and presence of Cα exhibit poor absorption performance. Further, no substantial effect of N components i.e., HC-NH-C and HC-N=C on min. RL is found whereas, N-oxide has shown good response for absorption as shown in Figure 11b. In addition, N-C=O is ineffective to show the significant influence of min. RL (Figure 11b). Overall, the surface elements are responsible for min. RL that hugely depends on the pyrolysis temperature.



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Figure 11. Variation of min. RL with (a) N-pyridinic, N-pyrrolic, N-graphitic, C sp2, C sp3, O species O-C-O and O-C=O and (b) NHC-NH-C, NHC-N=C, oxide, CN-C=O, Cα and ON-C=O surface species of CFF/epoxy composites with pyrolysis temperatures of 400, 800 and 1400 °C. The 3D representation of RL for CFF-1400/epoxy and PCFF-1400/epoxy composites along with min. RL at best-matched thickness is shown in Figure 12a-d to highlight the role of porous structure.32 CFF-1400 with 2 mm of thickness shows – 20.1 dB (99.02% absorption) with negligible BW as depicted in Figure 12a (blue region) and Figure 12b. Whereas, PCFF1400/epoxy shows outstanding min. RL value of -44.6 dB (99.99 % absorption) at 1.68 mm of thickness with 2.2 GHz of -10 dB (more than 90% absorption) BW and 0.57 GHz of -20 dB (more than 99% absorption) BW as illustrated in Figure 12c (blue region) and Figure 12d. The numerous pore structures are responsible for the further enchantment in the microwave absorption properties of PCFF-1400. The porous morphology is an extremely advantageous parameter for multiple reflection and scattering of EM waves. The multiple reflection and scattering prolong the path of EM wave and hence increase the absorption performances.10 In addition, the porous structure creates several solid-vacant interfaces, which induce interfacial polarization relaxation loss under oscillating EM waves that too eventually improve the absorption efficiency greatly. To sum up, irrespective of strong microwave absorption properties; the synthesis method of PCFF-1400 is simple and require low running cost, which is beneficial for batch production


Page 29 of 42 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


Figure 12. (a) 3D illustration of RL for CFF-1400/epoxy composite, (b) The min. RL with matching thickness for CFF-1400/epoxy, (c) 3D image of RL for PCFF/epoxy composite and (d) min. RL with matching thickness for PCFF-1400/epoxy. The recently reported microwave absorption properties of some traditional absorbing materials have been listed in Table 2.11, 12, 33-42 Notably, even with excellent electrical properties, CNTs exhibit poor absorption performances that are no less than -10.5 (91.08 % EM wave absorption).11 However, some other research groups have reported good microwave absorption from CNT-dielectric or CNT-magnetic hybrid structured based absorbers that were too no less than -26.5 dB.12, 33-35 Most recently, new form of carbon material with RL value of -42.04 dB has been reported.36 However, the filler loadings are 40 wt.% with 2.5 mm of thickness and have narrow -10, -20 dB BWs. Lately, Wu et al38 have synthesized hierarchical NiCo2O4–CoNiO2 using facile hydrothermal approach and explored its utility as microwave absorber, which shows



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RL value of -42.13 dB with 3.92 GHz of wide -10 dB BW at 1.55 mm of thickness. Although 10 dB BW of hierarchical NiCo2O4–CoNiO2 is larger than our proposed CFF material, but the filler loading is much higher than that of CFF. Therefore, it is worthy to say that CFFs show high microwave absorption, including wide operational bandwidth, thin thickness and low filler ratio as compared to famous reported microwave absorbers. Table 2. Comparison of microwave absorption properties itemized in recent papers.

Microwave absorber

Reinforcing-media

Thickness

Min. RL

(mm)

-(dB)

(wt.%)

-10 dB bandwidth

References

(GHz)

CNT/epoxy

CNT (15%)

3.0

10.50

0.6

11

CNT/T-ZnO/ epoxy

CNT (12%)

2.2

22.20

4.8

35

1.5

14.20

1.1

34

2.0

25.70

5.8

12

T-ZnO (8%) CNT@Fe@SiO2/

CNT@Fe@ SiO2

paraffin

(50%)

BaTiO3@CNT/

BaTiO3@CNT

paraffin

(40%)

CNT/paraffin

CNT (20%)

2.0

7.80

0

12

PVP@CNT/GNP/

PVP@CNT/

2.0

26.50

1.6

33

paraffin

GNP (10%)

Nano onion-like

NOC (40%)

2.5

42.00

0.54

36

Porous α-Fe2O3

3.5

25.00

5.2

37

carbons (NOC)/paraffin Porous α-Fe2O3 nanosphere/paraffin

nanosphere (30% )


Page 31 of 42 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


NiCo2O4–

NiCo2O4–CoNiO2

CoNiO2/paraffin

(50%)

γ-Fe2O3@C nanorod-

γ-Fe2O3@C

carbon sphere/paraffin

nanorod-carbon

1.55

42.13

3.92

38

--

8.11

--

39

sphere (70%) 3D CeO2/paraffin

3D CeO2

2.0

19.30

--

40

Urchin-like ZnO

Urchin-like ZnO

3.0

20.00

~ 1.0

41

hollow

hollow spheres

spheres/paraffin

(30%)

CFF/epoxy

CFF (30%)

2.0

20.10

2.9

In this work

PCFF/epoxy

PCFF (30%)

1.68

44.60

2.2

In this work

Furthermore, the electromagnetic wave attenuation inside the absorber is an important parameter for the microwave absorber, which can be characterized in terms of the attenuation constant. Hence, the attenuation constant α, which basically represents the attenuation capability of the designed material can be expressed asas43-45; α =

2π f c

(µ ´´ε ´´-µ ´ε ´) +

(µ ´´ε ´´-µ ´ε ´) 2 + (µ ´ε ´´-µ ´´ε ´) 2

(6 )

where f represents the frequency and c denotes the velocity of light. It is obvious from Figure 13 that most of the samples except pure epoxy composite show significant α value, which is the basic requirement for getting the excellent microwave absorption. The α values of PCFF-



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1400/epoxy composite is quite higher over the entire frequency range as compared to the simple CFF samples, which further approves its strong microwave absorption.

Figure 13. Attenuation constant α of CFF (400-1400 °C)/epoxy and PCFF/epoxy composites.

CONCLUSIONS In summary, the heteroatom-doped carbon are synthesized by the pyrolysis of CFF with temperature ranging from 400 to 1400 °C. The detailed morphological and structural information of the doped carbon are investigated using SEM, AFM, TEM, Raman, XRD, and XPS. The roughness, porosity, surface chemical states and its compositions are found to vary significantly with pyrolysis temperature. Moreover, the excellent absorption responses are due to the defect polarization, electric/dipolar polarization, interfacial polarization, and porous structure, which led to high in minimum reflection loss for samples pyrolyzed with temperature ≥ 800 °C. The highest synthesis temperature of 1400 °C contributes to absorb the microwave radiations effectively, which show substantial EM absorption efficiency at a frequency of 10.2 GHz, with 32 ACS Paragon Plus Environment

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RL reaching -20.1 dB and reflection loss below -10 dB up to 2.9 GHz with a thickness of 2.0 mm. The porous CFFs that fabricated by simple KOH activation method generates plentiful pores on the CFF-1400. The absorption efficiency of activated PCFF-1400 corresponding to the thickness of 1.68 mm reach -44.6 dB (99.999% microwave absorption), which exhibit broad -10 dB (90 % absorption) bandwidth that shares 52.9 % of the entire X band frequency width. The porous 3D structure helped to achieve the good matching and can create several multiple reflections and scattering of incoming electromagnetic waves, which attenuate microwave waves largely. 3D architecture assisted further enhancement in absorption properties, provides an approach to manufacture the lightweight and thin thickness absorber selectively. Therefore, the present study has opened a novel facile approach for the development of lightweight, low cost and high performance microwave absorber derived from CFFs. ASSOCIATED CONTENT Supporting Information TGA plot of raw CFF, and SEM images of CFF-400, CFF-800 and CFF1400. AUTHOR INFORMATION *Corresponding Author Email: [email protected], Phone: +91-512-2597687, Fax: +91-512-2597408 Author Contributions All authors have given approval to the final version of the manuscript. Funding Sources The authors declare no competing financial interest. 33 ACS Paragon Plus Environment


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SYNOPSIS In the present work, the sustainable-source heteroatom doped porous carbon has been synthesized by making pyrolysis of chicken featherfibers (CFF) with subsequent activation for strong microwave absorption applications. TOC



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Lightweight and high performance microwave absorbing heteroatom

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