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FeO Nanoparticles Embedded Hollow Mesoporous Carbon Nanofibers and Polydimethylsiloxane Based Nanocomposites as Efficient Microwave Absorber Bablu Mordina, Rudra Kumar, Rajesh Kumar Tiwari, Dipak Kumar Setua, and Ashutosh Sharma J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 23, 2017

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The Journal of Physical Chemistry

Fe3O4

Nanoparticles

Embedded

Hollow

Mesoporous

Carbon

Nanofibers

and

Polydimethylsiloxane Based Nanocomposites as Efficient Microwave Absorber Bablu Mordina1, 2, Rudra Kumar2, Rajesh Kumar Tiwari1, Dipak Kumar Setua1,*, Ashutosh Sharma2,* 1

Defence Materials and Stores Research and Development Establishment, Kanpur-208013, India

2

Department of Chemical Engineering, Indian Institute of Technology Kanpur-208016, India

Abstract Combined effect of both hollow, mesoporous structure of carbon nanofiber and Fe3O4 nanoparticles on the microwave absorption properties of polydimethylsiloxane nanocomposites have been investigated. Nanofibers with above characteristics were prepared via coelectrospinning the solutions of polyacrylonitrile/FeCl3 and polymethylmethacrylate followed by stabilization and carbonization at elevated temperatures. Carbonized nanofibers contained Fe3O4 nanoparticles with average crystallite size of ~12.3-14.6 nm and exhibited surface area of 126.4377.7 m2/g. Catalytic graphitization surrounding Fe3O4 nanoparticles was seen in high resolution transmission electron microscopy and also supported by a decrease in intensity ratio of D to G bands in Raman spectra. Microwave absorption properties of nanocomposites were investigated in a vector network analyzer using a coaxial waveguide in the frequency range of 2-18 GHz and found to be dependent on thickness, filler loading and Fe3O4 content of the nanofibers. At an absorber thickness of 7.5 mm with 25 wt % carbon nanofibers (consisting 5 wt% Fe3O4), the absorption bandwidth was found to be maximum of 4.33 GHz with reflection loss of -25 dB. However, corresponding bandwidth was increased to 4.51 GHz with reflection loss of -44 dB for nanocomposite with 25 wt % carbon nanofibers (but containing 7.5 wt% Fe3O4) at only 5.5 mm absorber thickness. *

Corresponding

authors.

Email:

[email protected]

[email protected] (Ashutosh Sharma)

ACS Paragon Plus Environment

(Dipak

Kumar

Setua),


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1. Introduction In recent years there is extensive growth of microwave technologies on superior microwave absorbing materials (MAMs), pollution control of the harmful electromagnetic radiations on human being, use of microwave absorbing materials in Defence (Aircraft stealth, military shielding in apparels/gadgets, etc.) and electronic devices (Laptops, Cellphones, Computers, etc.).1-4 Development of efficient, light weight MAMs are essential particularly in space and Defence application to facilitate optimization of aircraft aerodynamic profile as well to increase the payloads mass of launcher at relatively low launching cost. Nano-materials based MAMs possess superior characteristics over traditional MAMs owing to their light weight and strong microwave absorption capability over wider waveband at low thickness and are suitable coating materials for aircraft. Wide varieties of nanomaterials, such as carbon nanotube in combination with non-conducting polymer, nano magnetic metals and alloys have been utilized as nano MAMs.5-8 The main constituents that govern the absorption characteristics of MAMs are commonly categorized into three types: dielectric, resistive and magnetic losses.7, 9,

10

Among

these, the magnetic loss type of absorbers is able to form thin-layered MAMs with strong microwave absorption capabilities owing to presence of both magnetic and dielectric loss components.11-13 Carbon materials and polymer nanocomposites containing high aspect ratio conducting fillers emerge promising for several engineering and microwave absorption application because of their easy processing, design flexibility, low density and high conductivity at lower filler loading.14-18 Nanocomposites consisting only carbon nanomaterials are not able to show electromagnetic performances equivalent to those of magnetic ferrites and magnetic alloys. Microwave absorption properties of the composites can be greatly enhanced by utilizing carbon nanotube (CNT) coated or filled with active magnetic filler (e.g., Fe, Ni, FeCoNi, FeNi, and FeCo).19-23 CNT containing BaFe12O19, Fe3O4, Ni17S18, CoFe2O4, and Fe7S8 have been observed to show excellent microwave absorption characteristics.24–27 Improved microwave absorption capability of carbon nanotube and magnetic hybrid nanomaterials is basically due to electrical conductivity of carbon materials which favors the flow of eddy current generated by the magnetic field and hence efficient absorption of electromagnetic wave can be achieved.28


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Among different carbon based materials, CNT based nanocomposites have extensively been studied for microwave absorption2, 19-23, 25, 27 and electromagnetic interference (EMI) shielding.2933

Other carbonaceous materials e.g., carbon nanofiber,34, 35 carbon black34, 36 and graphene37-39

based nanocomposites are also reported. Metal coated carbon fiber composites showed better EMI shielding properties compared to carbon fiber based composites.40,

41

Recently hollow or

porous materials have been found promising for high performance MAMs because of effective microwave absorption by multiple reflections inside the porous structures, high permeability and moderate permittivity at high frequency due to presence of large number of pores.42-45 Since dielectric loss component is more significant for microwave absorption compared to the magnetic loss component, porous dielectric materials such as carbon nanofiber with embedded magnetic particles e.g., Fe3O4 nanoparticles are expected to give superior microwave absorption. Moreover, Fe3O4 nanoparticles increase the graphitization degree as well as conductivity of the nanofibers due to catalytic graphitization effect which ultimately assist in efficient microwave absorption.46 In this study, we for the first time investigate the mutual effect of both hollow and mesoporous carbon nanofibers as well as magnetic nanoparticles (Fe3O4 nanoparticles) on the microwave absorption properties of polydimethylsiloxane nanocomposites. Hollow mesoporous carbon nanofibers with embedded Fe3O4 nanoparticles (CNF@Fe3O4) were prepared by coelectrospinning of polyacrylonitrile/ FeCl3 (Fe3O4 precursor) and polymethylmethacrylate polymer solutions followed by stabilization and carbonization at 900°C temperature. CNF@Fe3O4 was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Brunauer–Emmett–Teller (BET) surface area, vibrating sample magnetometer (VSM), field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM) for structural, porosity, magnetic and morphological properties. Polydimethylsiloxane (PDMS) nanocomposites containing varied amount of CNF@ Fe3O4 were fabricated by casting technique. Coaxial nanocomposites samples were tested in vector network analyzer (VNA) for microwave absorption properties in the frequency range 2-18 GHz. Nanocomposites were further characterized by VSM and FESEM to correlate the observed microwave absorption properties with the magnetic and morphological properties.



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2. Experimental 2.1 Fabrication of hollow magnetic carbon nanofibers Initially, 8 wt % polyacrylonitrile (PAN) (Sigma Aldrich, MW=150000) and 15 wt% polymethylmethacrylate (PMMA) (Sigma Aldrich, MW =120000) polymer solutions were prepared separately in dimethylformamide (DMF) solvent. 5, 7.5 wt % of ferric chloride (FeCl3) salt was mixed with PAN solution as precursor of Fe3O4. Both the polymer solutions were loaded in two separate plastic syringes connected to a common nozzle by means of silicone rubber pipes. The nozzle with two annular channels was connected to a single needle. During electrospinning, PMMA and PAN/FeCl3 solutions were pumped through core and shell side channels, respectively. The distance between needle and rotating drum collector (speed 5000 r.p.m) was maintained at 10 cm. The flow rates of core and shell side solutions were 5 and 8 µL/min. High voltage of 15 kV was applied between needle and collector which led to formation of Taylor cone and subsequent stretching of polymer solutions to form composite nanofibers. The nanofibers were collected on an aluminium foil wrapped over the drum collector. The nanofibers were stabilized for one hour at 250 °C in air atmosphere, and then carbonized at 900 °C for another one hour under nitrogen atmosphere in a tubular furnace. The heating rate during the carbonization process was 5 °C/min. Carbonization at 900 °C completely removed the PMMA by thermal degradation and the gaseous products generated escaped follow a tortuous paths to form mesopores at the wall of the nanofibers. PAN formed outer carbon shell with embedded Fe3O4 nanoparticles at both inside and outside wall of the fibers and these hollow magnetic nanofibers is designated as CNF@5 Fe3O4 and [email protected] Fe3O4, respectively depending on FeCl3 concentration. 2.2 Fabrication of polydimetylsiloxane/CNF@ Fe3O4 nanocomposites Polydimethylsiloxane nanocomposite samples containing 10 and 25 wt% of either CNF@5 Fe3O4 or [email protected] Fe3O4 were prepared by solution casting technique. These nanocomposites were designated as PDMS-10 CNF@5 Fe3O4, PDMS-10 [email protected] Fe3O4, PDMS-25 CNF@5 Fe3O4 and PDMS-25 [email protected] Fe3O4. Typically in a 50 ml beaker, the required quantity of CNF@5 Fe3O4 or [email protected] Fe3O4 was dispersed in 5 ml acetone for 10 minutes in a bath Ultrasonicator (Enertech Electronics Pvt. Ltd., India). Sylgard 184 (Dow Corning, USA) was


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mixed thoroughly with these magnetic nanofibers and finally poured inside a self-designed cylindrical glass mould. The mixture was cured at room temperature for 48 hours and the nanocomposite samples (outer diameter 7 mm with a center hole of diameter 3 mm and thicknesses between 3.93 mm to 7.3 mm) were removed by breaking the glass mould. 3. Characterization techniques Crystal structures of CNF@5 Fe3O4 and [email protected] Fe3O4 were investigated by wide angle X-ray diffraction (X’Pert PRO, PAN-alytical, Netherland) using Cu Kα radiation (wave length 1.54 Å). Raman spectrophotometer (Model: Alpha, Witec, Germany) with He-Ne laser source (wavelength 514 nm) was used in characterization of these materials. X-ray photoelectron spectroscopy (XPS) analysis was performed in PHI5000 Versa Probe II Scanning XPS Microprobe, ULVAC-PHI Inc., Japan. Morphological analyses of the nanofibers were carried out by FESEM (Quanta 200, Zeiss, Germany) fitted with energy dispersive X-ray spectroscopy (EDX) (Oxford elemental system) and in TEM (Model FEIG2 T2, Tecnai, US). Magnetic properties of the nanofibers and the nanocomposite samples were recorded in VSM (Model 3472-70, GMW Magnet System, San Carlos, US). Temperature dependent magnetic properties of the nanofibers were measured in the temperature range from 5 to 350 K in Magnetic Properties Measurement System (Model No. CXL179CE, Quantum Design Inc., USA) at two different heating rates i.e., 2 K/min in temperature range between 5 to 200 K and 5 K/min between 200 to 350 K. A small field of 100 Oe was applied for measurement of zero field cooled (ZFC) and field cooled (FC) curves. Average pore size and BET surface area were calculated by studying nitrogen adsorption / desorption isotherm recorded in multiple-point Brunauer– Emmett–Teller (BET) analyser (Autosorb iQ, Quantachrome Instrument) by Barrett–Joyner– Halenda (BJH) method and density functional theory (DFT) plus software respectively. Microwave absorption properties were evaluated in VNA instrument (Model no. E8364B, PNA Series Network Analyzer, Frequency range 10 MHz- 50 GHz, Agilent Technology) using a coaxial wave guide transmission line. The instrument measures the scattering parameters (S11 and S21) and computes the value of complex permittivity (εr', εr") and permeability (µr', µr"). The value of εr', εr", µr' and µr" were then used to calculate normalized input impedance (Z) at the airnanocomposite inter-phase based on Naito and Suetake model.47 According to this model Z is given by Eqn. 1.



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Z= Z0 (µr/εr)1/2tanh[(j2π/c)(µrεr)1/2fd]

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(1)

µr and εr are represented by Eqn. 2 and Eqn. 3 as follows µr = µr'-jµr"

(2)

εr = εr'-jεr"

(3)

Where, εr and µr represent relative complex permittivity and permeability of the nanocomposite. Z0 is wave impedance. f is frequency of microwave in free space. c and d are velocity of light and thickness of the nanocomposite sample. The microwave absorption was calculated from the following equation (Eqn. 4). RL= -20log10 [|(Z-Z0)/(Z+Z0)|]

(4)

Where, RL is reflection loss in decibels (dB). Maximum absorption of microwave or minimum reflection loss takes place when impedance of free space and nanocomposite sample is matched. For perfectly impedance matching condition Z=Z0=377Ω. This condition is fulfilled at a specific matching thickness (tm) and a matching frequency (fm). 4. Results and Discussion 4.1 Structural characterization 4.1.1 Wide angle X-ray diffraction Figure 1 exhibits the WAXRD of CNF@5 Fe3O4 and [email protected] Fe3O4 nanofibers. For CNF@5 Fe3O4, the most intense peak arises at 2θ value 26.04° which can be attributed to the (002) plane of carbon and the other at 44.78° is designated to (010) graphitic peak of carbon.46 XRD peaks at 2θ values 30.25°, 35.60°, 43.20°, 56.94° and 62.81° can be designated to the diffraction at (220), (311), (400), (511) and (440) planes of Fe3O4, respectively. In case of [email protected] Fe3O4, there are additional peaks along with the above. The peaks at 2θ values 37.8°, 49.29°, 54.4° and 71.0° can be accounted for diffraction at (222), (331), (422), and (620) planes of Fe3O4, respectively. Average crystallite size of Fe3O4 nanoparticles determined by Scherrer equation, considering the diffraction at (311) plane, are ~14.6 nm and ~12.3 nm for CNF@5 Fe3O4 and [email protected] Fe3O4, respectively. Figures S1 (a) and (b) represent the EDX spectra of CNF@5 Fe3O4 and [email protected]


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Fe3O4. Presence of Fe, O and C was confirmed in these nanofibers by EDX analysis. Figures S2 (a), (b) and (c) represent the elemental mapping of C, O and Fe in CNF@5 Fe3O4 while Figures S2 (d), (e) and (f) represent the elemental mapping of corresponding elements in [email protected] Fe3O4. It is observed that C, O and Fe are distributed throughout the nanofibers (see supporting information).

Figure 1: WAXRD of nanofibers Strain in the Fe3O4 nanoparticles can be determined from the XRD pattern using Eqn. 5.48 β cos θ/λ = 1/ ε + η sin θ/λ

(5)

where, β indicates full angular line width in radian at half maximum intensity, ε is effective particle size in nanometer, η represents the amount of strain in the lattice, θ is Bragg’s angle in degree and λ is wavelength of X-ray in nanometer. Figure 2 represents the βcosθ/λ versus sinθ/λ plots for Fe3O4 nanoparticles in CNF@5 Fe3O4 and [email protected]. It is observed from the Eqn. 5 that lattice strain η can be obtained from the slope of the βcosθ/λ versus sinθ/λ plot. The values of lattice strain calculated for CNF@5 Fe3O4 and [email protected] are 22.7×10-3 and 25.8×10-3, respectively.



0.08

(a) CNF@5Fe3O4

0.07

β cosθ/λ

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[email protected]

0.06

0.05 2.0

2.4

2.8

3.2

3.6

sinθ/λ Figure 2. βcosθ/λ versus sinθ/λ plot for Fe3O4 nanoparticles 4.1.2 X-ray photoelectron spectroscopy Figure 3(a) indicates the survey spectra of [email protected] Fe3O4. It shows XPS peak for C1s, O1s and Fe2p indicating the occurrence of these elements in synthesized carbon nanofiber. Figure 3(b) represents the high resolution deconvoluted spectra of C1s. It shows sharp peak at 284.45 eV due to C-C bond of the fiber and a weaker peak at 286.2 eV due to oxygen (present in Fe3O4) bonded to C of the nanofiber.49 The C1s peak at 284.45 eV is used as reference peak for calibration and binding energy calculation. Figure 3(c) exhibits the deconvoluted spectra of O1s resolved into three peaks, the low binding energy peak at 530 eV can be assigned to O2- contribution of Fe3O4, peaks at 531.15 and 532.28 eV due to C-O and C-H, O-H bonds, respectively.50 High resolution spectra of Fe2p can be deconvoluted into four main peaks with binding energy of 710.39, 711.26, 723.38 and 725 eV corresponding to Fe2+2p3/2, Fe3+2p3/2, Fe2+2p1/2 and Fe3+2p1/2, respectively as shown in Figure 3(d).50, 51 Apart from these, two peaks arise at 713.53 and 718.76 eV and can be attributed to the surface and satellite peak of Fe3+2p3/2. It has been reported in the literature that if the satellite peak of Fe2p3/2 arises at 6 eV higher value than the main peak then oxidation state of Fe will be +2 whereas if the difference is 8 eV then oxidation state will be


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+3.16,52 In our case the satellite peak at 718.76 eV comes 7.5 eV higher value than the main peak at 7.11.26 eV, indicating presence of both +3 and +2 oxidation states and hence confirms the formation of Fe3O4 particles within the hollow nanofibers. From the total integration area of the deconvoluted peaks of Fe3+2p and Fe2+2p we have calculated the percentage of Fe3+ and Fe2+ ions and the values are 66.6 and 33.4%, respectively. Fe3O4 consists of two sublattices viz. tetrahedral sublattice (A site) and octahedral sublattice (B site) and have the structural formula [Fe3+]A [Fe3+, Fe2+]B O4. Half of the Fe3+ ions are situated at A sublattice and other half at the B sublattice and all the Fe2+ ions are present at the B sublattice. Hence we can say that 33.4% Fe2+ ions occupy the octahedral crystal site whereas 33.3% of Fe3+ ions occupy the tetrahedral crystal site and other half (i.e., remaining 33.3% Fe3+ ions) occupy the octahedral crystal site.

Figure 3: XPS of nanofiber (a) survey spectra; high resolution spectra (b) C1s, (c) O1s and (d) Fe2p



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4.1.3 Raman spectroscopy Raman spectra of nanofibers are shown in Figure 4. CNF@5 Fe3O4 shows sharp peak (graphitized or G peak) at 1589 cm-1and a disordered or D peak at 1345 cm-1 having intensity ratio of D-band to G-band (ID/IG) ~1.55. Whereas Raman spectra of [email protected] Fe3O4 exhibits peaks at 1581 cm-1 (G peak) and 1345 cm-1 (D peak) with ID/IG ~0.946. The decrease in intensity ratio in later is due to higher concentration of Fe3O4 causing catalytic graphitization effect in the nanofiber.46 Apart from these both the nanofibers show peaks at 215, 298, 408, 491 and 587 cm-1 characteristics of Fe3O4.

Figure 4: Raman spectra of the nanofibers 4.2 Surface area and pore size distribution Porosity of the nanofibers is determined by BET surface area, total pore volume and average pore diameter. Figures S3 (a), (b) and (c), (d) represent the adsorption-desorption isotherm and pore size distribution curves (PSD) for CNF@5 Fe3O4 and [email protected] Fe3O4, respectively. The


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experimentally determined values of the surface area, total pore volume and average pore diameter are 126.43 m2/g, 0.205 cc/g, 3.78 nm and 377.7 m2/g, 0.316 cc/g, 3.81 nm, for CNF@5 Fe3O4 and [email protected] Fe3O4, respectively. Figures S3 (a) and (c) reveals type IV nature of adsorption isotherm of the nanofibers. Both isotherm show large hysteresis loops indicating presence of pores in the range of 2-10 nm (See supporting information). 4.3 Morphological analysis 4.3.1 Field Emission Scanning Electron Microscopy (FESEM) Figures 5 (a), (b), (c) and Figures 5 (d), (e), (f) represent the low and high magnification FESEM images and histogram of fiber diameter distribution of CNF@5 Fe3O4 and [email protected] Fe3O4, respectively. Both the nanofibers exhibit a hollow structure. Fiber diameter is increased from 513 nm to 585 nm and distribution pattern widened when FeCl3 concentration is increased from 5 to 7.5 wt%. Figures 5 (d) and (e) also exhibit an increased density of nanoparticles with rise in FeCl3 concentration. Figures S4 (a) and (b) represent FESEM images of the PDMS nanocomposites filled with CNF@5 Fe3O4. Nanofibers are observed to be homogeneously dispersed within the polymer matrix and there is formation of interconnected nanofibers. [email protected] Fe3O4 based PDMS nanocomposites also show similar features as that of CNF@5 Fe3O4 based one. Increasing the fiber concentration to 25 wt% in case of PDMS-25CNF@5 Fe3O4 (Figure S4 (b)) or PDMS-25 [email protected] Fe3O4 (Figure S4 (d)) improved fiber to fiber connectivity resulted in enhancement of conductivity and microwave absorption properties of these composites compared to composite with 10 wt% filler (see supporting information) .



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Figure 5: (a) Low magnification, (b) high magnification FESEM image, (c) fiber diameter distribution histogram of CNF@5 Fe3O4; (d) low magnification, (e) high magnification FESEM image, (f) fiber diameter distribution histogram of [email protected] Fe3O4

4.3.2 Transmission Electron Microscopy (TEM) Figures 6 (a) and (b) represent the TEM images of CNF@5 Fe3O4 and [email protected] Fe3O4 before carbonization at 900°C. The nanofibers show presence of two polymeric phases. The dark central core is made of PMMA and lighter shell consists of polyacrylonitrile. The thickness of the central core as well as the outer shell is uniform throughout the length of the nanofibers which lead to the formation of hollow CNF with perfectly cylindrical hollow structure on carbonization,


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as shown in Figures 6 (c), (d) and (e), (f) with embedded Fe3O4 nanoparticles of variable shapes and particle size between few nanometer to hundreds of nanometer.

Figure 6: TEM images of raw electrospun nanofibers before carbonization (a) CNF@5 Fe3O4, (b) [email protected] Fe3O4; after carbonization (c), (d) those of CNF@5 Fe3O4 and (d), (e) [email protected] Fe3O4 Figures 7 (a) and (d) represent the HRTEM images of CNF@5 Fe3O4 and [email protected] Fe3O4. It is observed that the nanofibers are mostly amorphous and due to catalytic graphitization by Fe3O4 there is formation of graphitic carbon layers surrounding the Fe3O4 nanoparticles [Figures 7 (b) and (e)]. As expected, the extent of graphitization is higher in case of [email protected] Fe3O4 than CNF@5 Fe3O4. Higher graphitic content of [email protected] Fe3O4 is further supported by the decreased ID/IG ratio in its Raman spectra, described above. Figures 7 (c) and (f) represent the selected area electron diffraction (SAED) pattern of CNF@5 Fe3O4 and [email protected] Fe3O4. In both cases, the nanofibers exhibited diffused ring pattern which reveals that within the nanofibers randomly oriented nano polycrystal of Fe3O4 having body centered cubic (bcc) phase structure were resulted due to carbonization.



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Figure 7: (a), (b) HRTEM images, (c) SAED pattern of CNF@5 Fe3O4; (d), (e) HRTEM images, (f) SAED pattern of [email protected] Fe3O4

4.4 Magnetic Properties Figures 8 (a) and (b) represent the specific magnetization curves of CNF@5 Fe3O4 and [email protected] Fe3O4, respectively. It is observed that CNF@5 Fe3O4 exhibits magnetic saturation while [email protected] Fe3O4 shows unsaturation in the applied field range between +17500 to -17500 Oe. CNF@5 Fe3O4 nanofiber shows ferromagnetic characteristic with hysteresis ferromagnetism (large hysteresis loop) in the field range between +5400 to -5400 Oe while [email protected] Fe3O4 shows S-shape nature (super paramagnetic and ferromagnetic nature) with magnetic hysteresis (small hysteresis loop) in the field rage between +5500 to -5500 Oe. Hence these different magnetic characteristics of the same magnetic phase (i.e. Fe3O4) could be the possible reason for the different nature of the magnetization curves of CNF@5 Fe3O4 and [email protected] Fe3O4. Both the nanofibers have comparable coercivity (Hc) however, [email protected] Fe3O4 shows higher saturation


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(Ms) and remnant (Mr) magnetization values compared to those of CNF@5 Fe3O4. Hysteresis loop (M-H loop) indicates that saturation magnetization and the initial magnetic permeability are different for CNF@5 Fe3O4 and [email protected] Fe3O4. [email protected] Fe3O4 with smaller crystallite size responds to higher magnetic field whereas CNF@5 Fe3O4 with larger crystallite size responds to relatively lower magnetic field. Similar observation has also been reported for Fe3O4 nanoparticles (with variable crystallite size) in the published literature.53 Therefore, the distribution in particle size ultimately determines the collective magnetic properties of the nanofibers. The magnetic moment of any material depends on the number of magnetic domains per unit volume. Rise in Fe3O4 density, therefore, increases magnetic domain concentration and [email protected] Fe3O4 experiences higher magnetic field induced dipole moment resulting in a higher saturation magnetization compared to CNF@5 Fe3O4. Further, it is well established that Fe3O4 has cubic inverse spinel structure where oxygen anions form a face centered cubic closed packed arrangement and iron cations are situated at the interstitial tetrahedral and octahedral sites. The electron hopping can take place between Fe2+ and Fe3+ ions in the octahedral sites at room temperature imparting half metallic characteristics in the Fe3O4.54 The magnetic moment of the unit cell originates only from Fe2+ ions (contains four unpaired electrons) with a magnetic moment of 4µB. In the literature it has been observed from the electron microprobe analyzer (EPMA) analysis that, with decreasing particle size of the Fe3O4 relative oxygen content of the sample also decreases, which subsequently, leads to the lowering of the valance state of the cations. Therefore +2 oxidation state becomes more favourable as compared to the +3 oxidation state. Since the resultant magnetic moment of Fe3O4 is due to the divalent Fe2+ ions, hence decrease in particle size could lead to the increased magnetization value in the Fe3O4.54 Mr/Ms values of CNF@5 Fe3O4 and [email protected] Fe3O4 are ~0.233 and ~0.117 respectively. This indicates that both the nanofibers contain non-interacting Fe3O4 nanoparticles with uniaxial single domain and arbitrarily aligned on easy axis.55 Moreover, CNF@5 Fe3O4 shows approximately two times higher Mr/Ms value compared to [email protected] Fe3O4 due to larger crystallite size (~14.6 nm) in case of former compared to the later (~12.3 nm). Figures 8 (c) and (d) represent the magnetic moment versus temperature plot of CNF@5 Fe3O4 and [email protected] Fe3O4. Both nanofibers exhibit continuously increasing magnetic moment with the rise of temperature. At 350 K temperature, zero field cooled (ZFC) and field cooled (FC) curves come almost close to each other which indicates that blocking temperature of nanofibers



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are higher than 350 K and above this temperature both nanofibers show superparamagnetic behavior.55, 56 Figures S5 (a), (b), (c) and (d) exhibit VSM curves of PDMS-10 CNF@5 Fe3O4, PDMS-10 [email protected] Fe3O4, PDMS-25 CNF@5 Fe3O4 and PDMS-25 [email protected] Fe3O4 nanocomposites, respectively at two different sample orientations [viz. 0 and 90° with respect to the applied magnetic field]. The nanocomposites exhibit similar hysteresis behaviour in the applied field at ±5500, ±5000, ±5350, and ±5000 Oe, respectively (see supporting information). Table 1 show that the nanocomposites show greater saturation and remnant magnetization with 0° orientation of the sample in applied field than 90°. However, a reverse trend is observed for the coercivity values. Anisotropic coefficient (measured at 17.5 kOe applied magnetic field), calculated from the ratio of saturation magnetization values at 0° and 90° sample orientations, were 1.057, 1.044, 1.035 and 1.031 for PDMS-10 CNF@5 Fe3O4, PDMS-10 [email protected] Fe3O4, PDMS-25 CNF@5 Fe3O4 and PDMS-25 [email protected] Fe3O4, respectively indicating similar anisotropic characteristics of the nanocomposites. Moreover, anisotropic coefficient values decrease with increasing nanofiber loading indicate transformation of anisotropic to isotropic characteristic at higher filler loading. Table 1: Magnetic properties of virgin nanofibers and their composites Hc (Oe)

Sample Name 0°

90°

Mr (emu/g )

Ms(emu/g)

0°

0°

90°

90°

Anisotropic coefficient at 17.5 kOe

CNF@5 Fe3O4

601.06

-

1.60

-

6.87

-

-

[email protected] Fe3O4

669.14

-

2.83

-

24.23

-

-

PDMS-10 CNF@5 Fe3O4

710.60

717.44

0.09

0.08

0.37

0.35

1.057

PDMS-10 [email protected] Fe3O4

698.01

698.60

0.21

0.20

1.91

1.83

1.044

PDMS-25 CNF@5 Fe3O4

435.37

438.24

0.21

0.20

1.18

1.14

1.035

PDMS-25 [email protected] Fe3O4

663.80

667.43

0.38

0.37

3.31

3.21

1.031


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Figure 8: VSM of (a) CNF@5 Fe3O4 and (b) [email protected] Fe3O4; Moment versus temperature plot of (c) CNF@5 Fe3O4 and (d) [email protected] Fe3O4

4.5 Microwave absorption properties It is well known that real component of both permittivity (ε') and permeability (µ') indicates the capability of materials to store electromagnetic (EM) wave energy, whereas their imaginary components (ε" and µ") denote the capability of materials towards dissipating EM radiations. Figures S6 (a) and (b) show the real and imaginary permittivity versus frequency plots of PDMS10 CNF@5 Fe3O4, PDMS-25CNF@5 Fe3O4 and PDMS-10 [email protected] Fe3O4, [email protected] Fe3O4, respectively, while Figures S6 (c) and (d) represent the corresponding real and imaginary permeability versus frequency plots of these materials (see supporting information). It is observed from the figures that for PDMS-10 CNF@5 Fe3O4 and [email protected] Fe3O4 imaginary permeability assumes negative value in some frequency regions. Although the reason behind this observation is unclear but can be attributed to the radiation of magnetic energy from the composites. Moreover, dissipation of electromagnetic wave energy in



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the mesopores as well as hollow cylindrical structure of the nanofibers is also responsible for the negative value of the µr″. Single-crystalline Fe3O4 hollow nanospheres based paraffin wax composites also exhibited similar characteristics.57 In this study, µr″ showed a resonance peak at ~4 GHz and assumed negative value up to -0.03 in the frequency range of 17.2-18GHz owing to the dissipation of electromagnetic wave energy in the cavity of the hollow nanospheres. Similarly µ" value of -0.41 has been reported in the frequency range 12.96–16.56 GHz for porous Fe3O4/SnO2 core/shell nanorod based wax composites.57 Negative value of εr" in some frequency ranges, may be because of formation of continuous conductive network of CNFs inside the polydimethylsiloxane polymer matrix. A long-range connectivity is generated among the CNFs due to the percolation. Under this condition when electric field is applied, large numbers of charge carriers are accumulated at the interfaces between conductive CNFs and nonconductive polymer matrix leading to the strong interfacial polarization.58 Moreover, owing to larger aspect ratio of the nanofiber, more field lines spread out in the dielectric medium for the field polarization parallel to the nanofiber. Hence large plasmon resonance may be expected arising from the cylindrical rod like structure of the nanofiber and results in a negative permittivity.59 Similar negative value for the imaginary part of permittivity has also been reported in conductive polymer based nanocomposite such as polyaniline-tungsten oxide and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) doped polyaniline.59,60 Same group, in another study reported negative value for real part of permittivity in case of carbon nanofibers filled elastomer nanocomposite and polypyrrole-tungsten oxide nanocomposite.58,61 At 10 wt % nanofiber loading i.e., PDMS-10 CNF@5 Fe3O4 and PDMS-10 [email protected] Fe3O4, the nanocomposites show relaxation phenomena or dielectric resonance in the frequency range 12.317.4 GHz and 11-16.5 GHz, respectively. Whereas, with increasing fiber concentration these were observed in the region 5.5-10 GHz and 13-17.2 GHz in PDMS-25CNF@5 Fe3O4 and 6-9.6 GHz and 12.5-17.3 GHz in [email protected] Fe3O4. Occurrence of an additional resonance range with lower frequency with increasing nanofiber loading can be attributed to matching frequency of electron hopping of electromagnetic wave between Fe2+ and Fe3+ ions and this type of resonance is responsible for high dielectric loss of the material.62 Fe3O4 has ferrite like crystal structure in which all the Fe2+ ions are situated at the octahedral crystal sites, whereas in case of Fe3+ half of them are situated at the tetrahedral and the other half are present on the octahedral sites.63 In ferrite materials, negative and positive ions of unlike valences are connected by the


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varying bond lengths which generate dielectric moments of different strengths and lead to dipolar polarization. Moreover, as observed from the SAED pattern of the hollow nanofibers [see Figures 7 (c) and (f)], Fe3O4 nanoparticles are polycrystalline in nature. Therefore Fe3O4 nanoparticles may contain both low resistive and high resistive grain boundaries separated to each other which lead to heterogeneity in the crystal structure as well as interfacial polarization.64 Therefore, both dipolar and interfacial polarization contribute to the dielectric constant of the nanofibers. With increasing Fe3O4 loading graphitized layers (low resistive layers) are formed within the amorphous carbon matrixes (high resistive layers) which also produce interfacial polarization and contribute to the rise of dielectric constant (ε'). Moreover, graphitization enhances electrical conductivity of the nanofibers and thus favors the dielectric loss e.g., higher dielectric constant and dielectric loss of [email protected] Fe3O4 and [email protected] Fe3O4 compared to PDMS-10CNF@5 Fe3O4 and PDMS-25CNF@5 Fe3O4 as shown in Figures S6 (a) and (b). To check the actual effect of graphitization, electrical conductivity of the nanocomposites is calculated using Eqn.6 σ = 2πf εr"ε0

(6)

where, σ is the electrical conductivity of the material, f is the frequency, ε0 is the permittivity of the free space and the value is 8.85 x 10-12 F m-1. The electrical conductivity of PDMS-10CNF@5 Fe3O4, [email protected] Fe3O4 and PDMS25CNF@5 Fe3O4, [email protected] Fe3O4 nanocomposites measured at 2 GHz are 1.32×10-4, 9.65×10-4 and 9.21×10-4, 10.2 ×10-4 S/cm, respectively. It is observed that, electrical conductivity of the nanocomposites increases with increasing nanofiber loading as well Fe3O4 loading in the nanocomposite. The electrical conductivity increases significantly due to the percolation in CNF loading and this is the possible reason for negative permittivity of the nanocomposites as discussed above. Dielectric loss (ε") depends on the nature and number of different ionic species involved in the relaxation process. Moreover, nanostructured magnetic materials were reported to create more unsaturated bond on the hollow carbon fiber surface indicating the existence of large number of dipoles which are responsible for their enhanced dielectric loss.63



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Figures 9 (a), (b), (c), (d) represent reflection loss versus frequency plots of PDMS-10 CNF@5 Fe3O4, PDMS-25CNF@5 Fe3O4, PDMS-10 [email protected] Fe3O4 and [email protected] Fe3O4, respectively. It is observed from the Figures 9 (a), (b) that PDMS-10 CNF@5 Fe3O4 and PDMS25CNF@5 Fe3O4 nanocomposites exhibit highest reflection loss of -24.3 and -25 dB at 15.77 and 15.42 GHz, respectively at 7.5 mm sample thickness. The bandwidth of absorption peaks having < - 10 and < - 20 dB reflection losses are 0.81, 2.10 GHz and 0.46, 0.64 GHz, respectively as shown in Table 2. Similarly Figures 9 (c), (d) exhibit that PDMS-10 [email protected] Fe3O4 and [email protected] Fe3O4 nanocomposites can reach highest reflection loss of - 23.6 and - 44 dB at 13.83 and 15.75 GHz, respectively at 6.0 mm and 5.5 mm sample thickness. The bandwidth of absorption peaks having < - 10 and < - 20 dB reflection losses are 2.19, 2.26 GHz and 0.54, 0.68 GHz, respectively as depicted in Table 3. Larger bandwidth and greater microwave absorption capabilities of the PDMS [email protected] Fe3O4 nanocomposites compared to PDMS-CNF@5 Fe3O4 nanocomposites can be attributed to the higher Fe3O4 nanoparticle content which also increases the graphitization degree and leads to higher conductivity and dielectric loss in these composites. Therefore, [email protected] Fe3O4 nanocomposites exhibit enhanced microwave absorption characteristics compared to the PDMS-CNF@5 Fe3O4 owing to combined effect of increased dielectric, magnetic resonance and eddy current losses mechanism. Dielectric loss of the nanocomposites is governed by different electronic processes viz., electronic displacement, space charge and interfacial polarization. Eddy current loss is dependent on the electrical conductivity as well diameter of the Fe3O4 nanoparticles.65 Incorporation of Fe3O4 nanoparticles within the wall of hollow nanofibers generates the defects which are beneficial in absorbing microwave energy.66 Moreover, hollow structures of the nanofibers create the defects within the nanocomposites and hence assist in efficient absorption of microwave energy. Apart from the conductivity and defects of carbon nanofibers, microwave absorption properties largely depend on the conductivity of the nanocomposites. Nanocomposites show good conductivity when electrical percolation is achieved by the incorporation of nanofibers. The major parameters affecting the electrical percolation of nanocomposites are molecular weight, crystallinity, surface tension and polarity of the polymer. Percolation threshold is reached at higher filler loading if the surface tension of the polymer is high. Higher the surface tension of the polymer, lower will be the polymer-filler interfacial tension and hence polymer can easily wet the nanofillers. Good wetting leads to better dispersion of the nanofillers within the polymer matrix which ultimately


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increases the filler loading to reach the percolation threshold. Likewise percolation threshold is also greatly affected by the polarity of the polymer. More polar the polymer, more favorable is the polymer-filler interactions which ultimately give rise to well dispersion of nanofiller within the polymer matrix and hence increases the percolation threshold of the composites.37 By designing the hollow nanofibers percolation threshold is achieved at much lower nanofiller loading due to larger volume coverage capability and lower density of the hollow nanofibers compared to the solid carbon fibers. Polymer- filler interfacial tension is increased significantly owing to the presence of entrapped air within the hollow nanofibers which also favors in achieving percolation threshold at much lower filler loading. Therefore, conductivity of the nanocomposites in this study is considered to be significant enough for efficient microwave absorption by the dielectric loss mechanism. Moreover, microwave energy is absorbed by multiple reflections within the defects of the hollow nanofibers embedded within the nanocomposites and results in enhanced microwave absorption.

Figure 9: Reflection loss versus frequency plot of (a) PDMS-10 CNF@5 Fe3O4 (b) PDMS25CNF@5 Fe3O4 (c) PDMS-10 [email protected] Fe3O4 (d) [email protected] Fe3O4



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Table 2: Microwave absorption parameters for PDMS-CNF@5 Fe3O4 nanocomposites Sample

Sample

Fe3O4 Nanoparticles Embedded Hollow ... - ACS Publications

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