Mechanistic insight into the nature of dopant in graphene derivative

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C: Physical Processes in Nanomaterials and Nanostructures

Mechanistic insight into the nature of dopant in graphene derivative influencing EMI shielding properties in hybrid polymer nanocomposites Kumari Sushmita, Aishwarya V. Menon, Shubham Sharma, Ashutosh C. Abhyankar, Giridhar Madras, and Suryasarathi Bose J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10999 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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The Journal of Physical Chemistry

Mechanistic Insight into the Nature of Dopant in Graphene Derivative Influencing

EMI

Shielding

Properties

in

Hybrid

Polymer

Nanocomposites Kumari Sushmitaa, Aishwarya V. Menona, Shubham Sharmab, Ashutosh C. Abhyankarb, Giridhar Madrasc, Suryasarathi Bosed* a

Centre for Nanoscience and Engineering, Indian Institute of Science, Bangalore- 560012, India

b

Department of Materials Engineering, Defence Institute of Advanced Technology (DU), Pune- 411025,

India c

Department of Chemical Engineering, Indian Institute of Science, Bangalore- 560012, India

d Department

of Materials Engineering, Indian Institute of Science, Bangalore- 560012, India

*Corresponding author: [email protected] Abstract The recent surge in the usage of electronics has led to a new kind of problem; electromagnetic interference (EMI) which necessitates finding alternate materials that offer ease of processing, design flexibility, lightweight, ease of embedding and integrating with the existing systems in place as shields to protect the precise electronic circuitry. Herein, lightweight Polycarbonate (PC) based nanocomposites using doped graphene derivatives and multiwalled carbon nanotubes (MWCNT) has been explored for effective shielding of EM radiation in X- and Ku- band. To get a mechanistic insight as to how the dopant in graphene derivate influence the EM shielding properties, two dopants have been explored here; ferrimagnetic (ferrite, Fe3O4) and the other one as paramagnetic (Gadolinium oxide, Gd2O3). The doped graphene derivatives when composited with PC and MWCNTs resulted in materials that can shield the incoming EM radiation through magnetic and dielectric losses. This strategy of doping improves the state of dispersion of these dopants in the nanocomposites besides enhancing the shielding effectiveness. The PC based nanocomposites illustrated a total shielding effectiveness (SET) of -28 dB and -33 dB at 18 GHz for a given concentration of Gd2O3 and Fe3O4 hybrid respectively. A closer look into the mechanism of shielding reveals that irrespective of the dopant, various losses (magnetic and 1 ACS Paragon Plus Environment

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dielectric) decide the shielding effectiveness in polymeric nanocomposites facilitated by the multiple internal reflections. Taken together, this study brings in new insight as to how the losses contribute towards effective shielding rather than the choice of the dopant and will help guide researchers working in this area from both industrial as well as academic perspective.

1. Introduction Electromagnetic interference occurs when electromagnetic (EM) signals are unintentionally transmitted from an emitter to another susceptible element by radiation and/or conduction. With the increasing rate in device development and the growing demand of high-speed wireless devices, it has become imperative to suppress unwanted EM radiations which can otherwise lead to malfunctioning of electronic devices. EMI shielding has a wide range of applications in the field of flexible electronics, satellite and aircraft communication, automotive sector and military stealth technology. Stealth technology requires techniques to be used with personnel, aircraft, ships, submarines, missiles and satellites to make them less visible or invisible to radar and other detection methods. Thus with such wide variety of applications, it becomes a necessity to develop broad band shield that works in GHz frequency range 1. The properties that are inevitable to any EMI shielding include- presence of magnetic and electric dipoles and moderate electrical conductivity. Metals (both magnetic and conducting) and ferrites are the traditional choices available for EMI shielding. Metals are good reflectors of EM waves because they have large number of free charge carriers 2. The electric field inside a metal is zero and thus the incoming EM waves are reflected back to the surrounding which can further damage the performance of the nearby devices. Moreover, metals are mostly high density materials, susceptible to corrosion, less flexible and cumbersome to handle 3. Again, though magnetic metals like Fe, Ni and Co as well as ferrites are EM wave absorber, because of Snoek’s limit they fail to respond fully for excitations in GHz frequency range 4. To design high performance EM shield, various dielectric, magnetic and conducting particles have been investigated in literature. The widely used nanoparticles includes copper, silver, iron, cobalt, nickel, magnetic alloys, ferrites, barium titanate, carbon derivatives 1,3,5-15, etc. In the light of the above facts, it is apparent that the prime requisite for an EM shield is to have low reflection and high absorption losses and it is challenging to obtain such material, especially in GHz frequency range. 2 ACS Paragon Plus Environment

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Carbon based materials like carbon black, carbon fibers (CF), carbon nanotubes (CNT), graphene sheets (GS), graphene nanoplates (GNP) and reduced graphene oxides (rGO) have been explored as conducting fillers for EMI shielding applications especially in the GHz frequency range

1,16.

These materials possess free electrons which can reflect EM waves. It can

further help in attenuating EM waves by multiple reflections which eventually leads to the suppression of these waves. CNTs exhibit superior electrical and mechanical properties as compared to many conventional materials. They have low density, high elastic modulus and twice the strength as compared to CFs 9. CNTs have exceptional electrical conductivity and its high aspect ratio results in lower percolation threshold. CNTs also help in better interaction with the host polymer due to its high surface area. These carbonaceous materials are especially noteworthy since they can be easily functionalized with wide variety of lossy nanoparticles (magnetic particles and/or dielectric materials) to yield hybrid and multifunctional nanoparticles. To be used for practical applications, these nanoparticles must be incorporated in wax or any suitable polymer matrix. They should possess high mechanical strength, alongside being lightweight and cost effective. Recent research has focused on both conducting and insulating polymers as matrix material. These include polyaniline, polypyrrole, polyamine, polyvinylidene fluoride, polyurethane, polycarbonate, etc

13-14.

Polycarbonate, is an engineering polymer noted for its flammability

resistance, toughness in thin sections, high impact resistance and high service temperature which makes PC attractive for various applications 17. It is widely used in electronics (for e.g.- cellular phone back covers) and automotive sector and therefore has attracted recent attention in the field of EMI shielding as well. Arjmand et al.

8

prepared 5 wt% MWCNT/ PC composite by melt

mixing 15 wt% MWCNT/ PC masterbatch to obtain EMI SE of ca.-24 dB for a shield with thickness 1.85 mm in X-band. Pande et al.

11

reported an EMI SE of -43 dB for a shield of

thickness 2 mm with 20 wt% loading of MWCNT in X-band. Maiti et al. 7 reported an EMI SE value of ca.-23.1 dB for a 5.6 mm shield prepared using solution blending of PC in presence of 2 wt% MWCNT and 70 wt% of commercial PC beads in the frequency range of 8.2-12.4 GHz. In another work 6, they reported melt mixing of PC with 4 wt% MWCNT /GNP (in the ratio 2:3) to obtain an EMI SE value of ca.-21.6 dB.

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Amongst the ferrites, Fe3O4 nanoparticles have been extensively studied as promising microwave absorbers due to their satisfactory magnetic properties and strong spin polarization at room temperature 18. Fe3O4 grafted on graphene oxide (rGO-Fe3O4) based nanocomposites have been reported by different research groups but it is observed that rGO-Fe3O4 alone do not result in high SE due to the low intrinsic conductivity of rGO as well as low magnetic losses in Fe3O4 in the microwave region 18. However, the synergistic effect of MWCNT and rGO- Fe3O4 has been reported to exhibit high SE even at low filler loading and shield thickness

13,15.

On the other

hand, Gd2O3 is a material with large band gap, high resistivity and high relative permittivity 19-22. It has been reported as one of the potential candidates as contrast agent for application in magnetic resonance imaging

19,23-24.

Moreover, Gd2O3 serve as the host material and can easily

be doped with other rare earth ions (such as Eu3+, Ce3+, Tb3+, Yb3+ and Er3+) 19. Lanthanide iondoped Gd(OH)3 and Gd2O3 have also been studied for applications in displays and fluorescence imaging

23.

Especially, cubic Gd2O3:Eu3+ has wide applications in X-ray scintillator, high

definition projection televisions, flat panel displays and photoelectronic apparatus, owing to its good luminescent properties when doped with rare-earth ions (Eu3+, Tb3+) 25-26. Though Gd itself is toxic in its free form but in its bound form its toxicity is comparatively reduced. Moreover, within acceptable limits, its heavily used as MRI contrast agent also 27-28. As per our knowledge, there exist limited literature on Gd based material for EMI shielding application and particularly Gd2O3 has not yet been explored 29. Most conducting fillers show promising results as potential EM shielding candidates at all frequencies (mostly by reflection). To improve upon the overall absorption performance, often conducting fillers are either used in tandem or conjugated (doped) with magnetic or dielectric nanoparticles, notwithstanding the fact that most magnetically (Fe and its ferrites) and dielectrically strong (including BaTiO3) materials fail to retain their properties at high operational frequencies (X and Ku band) owing to low response time at these frequencies. These results begin to suggest that at high frequencies, magnetically or dielectrically active materials do not necessarily shield. In fact, the most important determinant will be the losses incurred by the nanoparticles while interacting with the incoming EM wave at these frequencies. In the light of the above-mentioned facts, it is therefore necessary to discuss if the choice of dopant (with high permeability and permittivity) is even a prerequisite to achieve high shielding effectiveness in the X and Ku band. 4 ACS Paragon Plus Environment

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In the present work, PC serves as the matrix material owing to its growing demand in electrical and electronic industry

17.

Lossy nanoparticles (Fe3O4 and Gd2O3) grafted on rGO sheet along

with MWCNT in the PC matrix has been used to maximize the EMI shielding. CNTs in PC matrix serve a dual purpose of making PC conducting and improving upon the mechanical properties of the composite material. The lossy nanoparticles are grafted on rGO sheet to avoid the aggregation of nanoparticles and the restacking of the exfoliated graphene sheets by hindering the inter-particle van der Waals’ interaction

10.

The novelty of this work lies in the

comparison of paramagnetic Gd2O3 and ferrimagnetic Fe3O4 particles for EMI shielding applications. Using two different dopants, we have tried to understand the role of dopants in the enhancement of the shielding effectiveness (SET). The nearly similar value of SET obtained using these two model dopants suggests that it is worthwhile to delve deeper into the loss mechanisms. Thus, we have further discussed the probable mechanisms that can explain the various losses involved in PC nanocomposites.

2. Experimental Section 2.1 Materials: PC (Lexan-143R) with melt flow index 11 g/10 min was acquired from Sabic. Pristine MWCNT, NC7000 (length 1.5 μm and diameter 9.5 nm) was procured from Nanocyl SA (Belgium). Graphene oxide (GO) powder, BTGOX was procured from BT Corp. Gadolinium (III) nitrate hexahydrate (Gd (NO3)3.6H2O) was obtained from Alfa Aesar. Ferric chloride hexahydrate (FeCl3.6H2O) LR 98% was obtained from Thomas Baker. Tetrahydrofuran (C4H8O), Hydrazine hydrate (H4N2.H2O) (99%), Potassium hydroxide (KOH) pellets, Ethylene glycol (C2H6O2), and Urea (NH2CONH2) were procured from SDFCL. 2.2 Synthesis of rGO-Fe3O4 nanoparticles Solvothermal route was employed for the synthesis of rGO-Fe3O4 nanoparticles (scheme 1a and c)

30.

In a typical synthesis, 200 mg GO was dispersed in 60 ml ethylene glycol and probe

sonicated for 20 min to remove the primary agglomeration. It was later bath sonicated for 45 min to remove the secondary agglomeration. The GO dispersion was then added to the aqueous solution of FeCl3.6H2O, prepared by adding 500 mg of FeCl3.6H2O and 1g of urea in 20 ml of 5 ACS Paragon Plus Environment


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Ethylene glycol. The reaction mixture was well stirred and transferred into a 100 ml Teflon lined stainless steel autoclave. Hydrazine hydrate (3 ml) was immediately added and the autoclave was sealed manually. The reaction was then carried out in a pre-heated oven at 180 °C for 10 h. The autoclave was then allowed to cool down to room temperature and the black powder of rGOFe3O4 was separated using magnetic decantation. It was washed several times with DI water and ethanol before it was dried and stored for further usage. 2.3 Synthesis of rGO-Gd2O3 nanoparticles rGO-Gd2O3 nanoparticles were prepared using solvothermal route followed by calcination of nanoparticles in Ar environment (scheme 1b and c). In a typical synthesis, 150 mg GO was dispersed in 15 ml Ethylene glycol by probe sonication, followed by bath sonication. The GO dispersion was then added to the aqueous solution of Gd (NO3)3.6H2O, obtained by dissolving 250 mg of Gd (NO3)3.6H2O in 10.5 ml of ethylene glycol. KOH solution (4.5 ml, 1 M) was added till pH~12 was obtained. The reaction mixture was well stirred and transferred into a 60 ml Teflon lined stainless steel autoclave and was sealed manually. The reaction was then carried out in a pre-heated oven at 200 °C for 4 h. The autoclave was then allowed to cool down to room temperature and the nanoparticles obtained were washed several times with DI water and Ethanol. It was then dried in hot air oven and calcined at 700 °C for 4 h in Ar to obtain crystalline Gd2O3 nanoparticles on rGO sheet 19. Inert environment was chosen for calcination to avoid the oxidation of rGO.

2.4 Preparation of PC nanocomposite PC was blended with the nanoparticles at 260 °C and 60 rpm in Haake minilab-II mini-extruder for 20 min under nitrogen purging (shown in scheme 1d). The concentration of MWCNTs was varied from 1-3 wt % in PC matrix. Similarly, 5 wt% rGO-Fe3O4 and 5 wt% rGO-Gd2O3 was also incorporated along with 3 wt % MWCNTs using the above protocol. Although it has been reported that certain level of Gd2O3 is toxic however, in our case the surface analysis using EDS reveals very little/no exposure of Gd2O3 in the composites. Moreover this reduces its chances of leaching when discarded into landfill. Now considering the worst-case scenario, Gd2O3 has a lethal dosage (LD50) of more than 5 g/kg. In this context also, our 6 ACS Paragon Plus Environment

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material even after the loss of utility will leach less than 300 mg of Gd2O3 nanoparticles because the maximum concentration of rGO-Gd2O3 in the total composite is restricted to 300 mg. So, this composite will still be sustainable and environmentally friendly even if it is dumped in the landfill.

(a)

GO + Ethylene glycol

ultrasonication

Hydrazine hydrate

G Eth O + y gly le ne col

Washed and dried

FeCl3 .6H2 O + Urea + Ethylene glycol Kept in autoclave at 180 0 C, 10 h

(b)

GO + Ethylene glycol

rGO-Fe 3O4

ultrasonication G Eth O + y gly le ne col

KOH solution

Washed, dried and annealed at 700 0 C, 4h in Ar

Gd(NO3 )3 .6H2 O + Ethylene glycol

rGO-Gd2O3

Kept in autoclave at 200 0 C, 4 h



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COOH

(c)

dr hy OH COOH

al rm e oth

HOOC

COOH

HOOC

HO

COOH

rGO- Fe3O4

Iron salt

HO O

OH

O HOOC OH COOH

COOH

Graphene Oxide

+ hyd rot her ma l Gadolinium salt

COOH

COOH

HO

HOOC COOH

rGO- Gd2O3

(d) COOH HO

HOOC

rGO- Fe3O4

Melt mixing @ 260oC,

COOH

+

+

or

followed by compression molding

COOH COOH

MWCNT

HO

HOOC

PC

Doughnuts for EM shielding measurements

COOH

rGO- Gd2O3

Scheme 1: a) Synthesis procedure of rGO-Fe3O4, b) synthesis procedure of rGO-Gd2O3, c) representation of the synthesis mechanism of nanoparticles (d) Cartoon illustrating incorporation of nanoparticles in PC matrix.

3. Characterizations X-ray diffraction (XRD) analysis of the synthesized powder was carried out using XPERT Pro from PANalytical. Cu-Kα radiation source (λ = 1.5406 Å, 40kV and 30 mA) was used for analysis. FEI Technai G2 Spirit Biotwin was used to acquire transmission electron microscopy (TEM) images of various nanoparticles at 120kV. The surface morphology was analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) using Carl 8 ACS Paragon Plus Environment

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Zeiss Ultra 55 FE-SEM. The elemental composition of nanoparticles was analyzed by X-ray photoelectron spectroscopy (XPS) by AXIS ULTRA (Kratos Analytical instrument) using monochromatic (Al) source. The defect characterization of nanoparticles was carried out using a HORIBA LabRAM HR Raman spectrometer. The magnetic measurements of the samples were performed using 9T CFMS of Cryogenics UK with a sweeping rate of 0.1 T/min till magnetic field ± 5T. The room temperature AC conductivity were studied using Alpha-A Analyser (Novocontrol, Germany) in a broad range of frequency varying from 10-1 to 107 Hz. Uniformly polished compression-molded discs (10 mm diameter and 1 mm thickness) were used as specimens and electrical conductivity was measured across the thickness. EM shielding interference was studied by Anritsu MS4642A vector network analyzer (VNA) by coaxial cable method using Damascus MT-07 toroidal sample holder in the X and Ku frequency band (8-18 GHz). Prior to measurement, the setup was calibrated by full SOLT (short-open-load-through). Toroidal sample of thickness 5 mm were prepared by compression molding at 260 °C. S-parameters (S11, S12, S21, S22) obtained from VNA was used to determine the total shielding effectiveness and shielding effectiveness due to reflection and absorption. The electromagnetic parameters like complex permeability and permittivity was calculated from the obtained s-parameters using the Nicolson–Ross-Weir algorithm.

4. Results and Discussions 4.1 Characterization of the synthesized nanoparticles Figure 1a shows the XRD patterns of rGO-Fe3O4. The peaks at 2θ values of 30.23, 35.60, 43.20, 53.73, 57.31 and 62.67 can be assigned to the diffraction planes (220), (311), (400), (422), (511) and (440) respectively, which corresponds to the cubic inverse spinel structure of Fe3O4 31-32. The broad peak around 2θ at ca. 24.2° indicates the reduction of GO to rGO

30,33.

The XRD patterns

of rGO-Gd2O3 are shown in figure 1b. It can be observed that the sites and intensity of the diffraction peaks are consistent with the standard pattern for JCPDS Card No. (00-012 - 0797) and thus it confirms the cubic phase of Gd2O3. Meanwhile, the (002) peak of rGO shows the reduction in the functional groups as compared to GO.



20

30

40

50

60

70

80

10

20

2  (degree)

30

40

 (622)

 (440)

 (431)

 (222)

 rGO  Gd2O3

 (400)

 (002)  (211)

 (440)

 (422)  (511)

 (400)

 (220)

 (002)

10

(b)

Intensity (a.u.)

 rGO  Fe3O4

 (311)

(a)

Intensity (a.u.)

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

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50

60

70

80

2  (degree)

Figure 1: XRD pattern of (a) rGO-Fe3O4, (b) rGO-Gd2O3. Energy dispersive X-ray spectroscopy was carried out to further confirm the presence of various elements as seen in figure 2a and figure 2b. A rough estimation of various elements was obtained. In rGO-Fe3O4, the atomic % of C, O and Fe was found to be 43.67, 38.98 and 17.75 respectively. While in rGO-Gd2O3, the atomic % of C, O and Gd was estimated as 69.32, 22.30 and 8.38 respectively. Though the aim was to obtain nearly similar content of Fe and Gd, but this was the nearest that we could obtain after optimizing the synthesis protocol. Nevertheless, as one of the rationales was to understand as to how the choice of dopant influence the EMI shielding properties in hybrid nanocomposites, these two hybrid structures were further evaluated and is discussed in the subsequent sections.


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Figure 2: Energy dispersive spectroscopy of (a) rGO-Fe3O4 and (b) rGO-Gd2O3 The morphology of the as-prepared rGO-Fe3O4 and rGO-Gd2O3 was characterized using TEM as shown in figure 3a and 3b respectively. Here, we observe a two-dimensional (2-D) sheet with nanoparticles grafted on it. This 2-D sheet with wrinkles are the characteristic feature of rGO, while the nanoparticles grafted in figure 3 (a) is Fe3O4 (average size: 3-7 nm) and in figure 3 (b) is Gd2O3 (average size: 4-8 nm).

Figure 3: TEM images of (a) rGO-Fe3O4 and (b) rGO-Gd2O3 XPS study was carried out to confirm the reduction of GO and the grafting of nanoparticles on rGO sheet as shown in figure 4. The presence of various elements is depicted through the wide scan XPS spectrum as shown in figure S1. The quantification of the elements in terms of their 11 ACS Paragon Plus Environment


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atomic weight % as obtained from XPS is represented in table 1. In rGO-Fe3O4 and rGO-Gd2O3, additional oxygen is contributed due to the presence of O atoms in Fe3O4 and Gd2O3. The atomic wt% of Fe and Gd is in accordance to that observed in the EDS analysis. In both EDS and XPS, it is observed that the atomic wt % of Gd is less than that of Fe. Calculation for C/O was accordingly done to find out the effective contribution of C/O due to rGO in rGO-Fe3O4 and rGO-Gd2O3 which is also tabulated in table 1. While doing this calculation, we have ensured to take in account the oxygen contribution coming only from reduction of GO and not from Gd2O3 or Fe3O4. It is quite evident from the table that C/O ratio has increased in both rGO-Fe3O4 and rGO-Gd2O3 as compared to GO indicating that there is significant reduction in the oxygen based functional groups. This can be easily verified from the C 1s XPS spectra of GO, rGO-Fe3O4 and rGO-Gd2O3 as shown in figure 4a, 4b and 4c respectively. C 1s spectrum of GO contains C=C, C-C, C-OH, C-O-C and O-C=O at 283.9, 284.8, 285.4, 286.9 and 288.8 eV respectively

13,34.

However, the C 1s spectrum of rGO-Fe3O4 and rGO-Gd2O3 shows a reduction in the relative contribution of the components associated with oxygenated functional groups. The reduction is observed to be slightly more in rGO-Fe3O4 as compared to rGO-Gd2O3. The possible reason for lesser reduction of GO with Gd2O3 can be attributed to the less solvothermal processing time in case of Gd2O3 (4 h) as compared to Fe3O4 (10 h). In the Fe 2p spectrum (shown in figure 4d), peaks at 711.12 and 724.71 eV are attributed to the Fe 2p3/2 and Fe 2p1/2 respectively, confirming the presence of Fe3O4 34-35. These two major peaks can be deconvoluted by five peaks at 710.59, 711.71, 713.43, 724.77and 726.47 eV 35-37.The absence of the shakeup satellite peak at ~ 719 eV confirms that the compound is Fe3O4, rather than γ- Fe2O3

35.

Fe3O4 is a mixed-valence

compound where the ratio of Fe3+ to Fe2+ is supposed to be 2:1 38. The peak at 711.71 and 710.59 eV can be ascribed to Fe3+ and Fe2+ respectively. The ratio of Fe3+ to Fe2+ is calculated from the corresponding area under the peak at 711.71 and 710.59 eV respectively and its value turns out to be 1.87, which is fairly close to the expected value of 2

35.

The XPS spectrum of Gd 3d

(shown in figure 4e) shows two strong peaks at 1188.53 and 1220.13 eV due to the spin–orbit splitting corresponding to 3d5/2 and 3d3/2 respectively 39-40. The peak at 1188.53 eV can be further deconvoluted into peaks at 1186.67 and 1189.61 eV, which is in agreement with reports in the literature for Gd2O3 nanoparticles 41. Also, the structural changes in terms of defect concentration in the graphitic structure of GO before and after functionalization with Fe3O4 and Gd2O3 nanoparticles were studied using Raman spectroscopy (as shown in Figure S2). 12 ACS Paragon Plus Environment

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Table 1: Atomic wt% of various elements in GO, rGO-Fe3O4 and rGO-Gd2O3 and their corresponding C/O ratio. C

O

Fe

Gd

C/O

GO

77.92

22.08

-

-

3.52

rGO-Fe3O4

76.63

19.31

4.06

-

5.51

rGO-Gd2O3

79.53

18.43

-

2.04

5.17

(a)

C 1s

C-C

(b)

C 1s

C-OH C-O-C

O-C=O

Intensity (a.u.)

Intensity (a.u.)

C-C

C-OH C-O-C O-C=O

C=C

282

C=C

284

286

288

290

282

284

(c)

C 1s

(d)

288

Fe 2p3/2

290

Fe 2p

Intensity (a.u.)

C-C

286

Binding energy (eV)

Binding energy (eV)

Intensity (a.u.)

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


C-OH

Fe 2p1/2

C-O-C C=C

282

284

286

288

290

705

710

715

720

725

730

735

Binding energy (eV)

Binding energy (eV)



(e)

Gd 3d

Gd 3d5/2

Intensity (a.u.)

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

1180

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Gd 3d3/2

1190

1200

1210

1220

1230

Binding energy (eV)

Figure 4: XPS survey spectra of (a) C 1s spectra of GO, (b) C 1s spectra of rGO-Fe3O4, (c) C 1s spectra of rGO-Gd2O3, (d) Fe 2p spectra of rGO-Fe3O4 and (e) Gd 3d spectra of rGO-Gd2O3. 4.2 Morphology and dispersion of nanoparticles in the composites Figure 5 represents the SEM images of PC nanocomposites with 3 wt% MWCNT, 3wt% MWCNT/ 5 wt% rGO-Fe3O4 and 3wt% MWCNT/ 5 wt% rGO-Gd2O3. It must be noted that prior to acquiring the SEM micrographs, the samples were cryofractured to obtain a sharp interface. The hairy like structure of MWCNT is quite evident from the higher magnification images as seen in figure 5b, 5d and 5f which shows the dispersion of MWCNT in PC matrix. MWCNTs as compared to PC, appears brighter in the SEM micrographs due to the electron density difference. The nanoparticles of Fe3O4 and Gd2O3 cannot be differentiated from the rGO sheet (shown in figure 5 c, d, e and f) due to the lack of electron density difference but the TEM images shown in figure 3 (a) and 3(b) has already revealed the presence of nanoparticles on rGO sheet. Though the presence of rGO sheet in the matrix can be observed in the SEM images.


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Figure 5: SEM micrographs of (a) PC/3 wt% MWCNT (low magnification), (b) PC/3 wt% MWCNT (high magnification), (c) PC/3 wt% MWCNT/ 5 wt% rGO-Fe3O4 (low magnification), (d) PC/3 wt% MWCNT/ 5 wt% rGO-Fe3O4 (high magnification), (e) PC/3 wt% MWCNT / 5 wt% rGO-Gd2O3 (low magnification) and (f) PC/3 wt% MWCNT / 5 wt% rGO-Gd2O3 (high magnification)



Page 16 of 32

4.3 Electrical percolation and ac electrical conductivity of the composites The AC electrical conductivity of the composites was measured and recorded as a function of frequency at room temperature as shown in figure 6a. Neat PC is known to be insulating in nature but MWCNTs are known to have an electrical conductivity value close to 106-107 S/m

42

and thus as the content of MWCNT is enhanced, the electrical conductivity of the composite increases. It is evident that the conductivity of the nanocomposite scales with the MWCNT concentration and the percolation threshold is between 0.5 and 1 wt% as observed from figure 6a. This clearly indicates that the distance between two nanotubes decreases with increasing concentration of MWCNTs, thereby allowing the formation of a three-dimensional network like structure. This further helps in efficient charge transport through tunneling or hopping. The addition of 3 wt% MWCNTs manifested a strikingly enhanced electrical conductivity along with a frequency independent plateau at lower frequency. The electrical conductivity obtained with the addition of 3 wt% MWCNT is observed to be 2.4 x 10-4 S/cm at low frequencies. The addition of rGO-Fe3O4 has slightly increased the conductivity to 6.1 x 10-4 S/cm, whereas rGOGd2O3 has negligible impact on conductivity. rGO has lower intrinsic electrical conductivity as compared to MWCNT, as a result the contribution of rGO towards electrical conductivity of the composite material is negligible. In literature, Fe3O4 is reported to be semiconductor while Gd2O3 is reported as insulating in nature with a band gap of 5.4 eV 43. Thus, the insulating Gd2O3 nanoparticles plays no role in the enhancement of electrical conductivity, rather it hinders the MWCNT network. To get more insight into the mechanism of shielding, the universal power law fitting was carried out onto the frequency-dependent AC conductivity curve. Equation 1 displays the frequency dependent conductivity response, termed as Jonscher’s “Universal Dielectric Response (UDR)”. It explains the variation of AC Conductivity with frequency and is most often observed in complex systems consisting of multiple phases as in heterogenous or composite materials. In a typical UDR response, the bulk ac conductivity is frequency independent at low frequencies. This is a result of a percolated path of resistors across the network. Here, the ac conductivity of the capacitors is sufficiently low that current flows through the resistor percolation path. At higher frequencies the ac conductivity increases, following power law behavior due to the increasing conductivity of the capacitors. 16 ACS Paragon Plus Environment

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This analysis thus reveals the equivalent number of resistive and capacitive network explaining the charge transport mechanism in the nanocomposite 44. 𝜎′(𝜔) = 𝜎 (0) + 𝜎𝐴𝐶(𝜔) = 𝜎𝐷𝐶 + 𝐴𝜔𝑠

(1)

Here, the exponent “s”, ranges from 0 to 1 and is a function of both temperature and frequency. It gives an indication of the extent of charge transfer that is taking place through tunneling/hopping. By fitting the power law, “s” is observed to be in between 0.9 and 1, which suggests charge transport by tunneling/hopping mechanism 3. The value of ‘s’ is the representation of the capacitive and the resistive behavior of the composite. It decreases from 0.99 to 0.92 with an increase in the MWCNT concentration from 1 to 3 wt%. Neat PC shows a positive real permittivity value between 6.5 and 7 as observed in figure 6b. However, negative permittivity observed for the composite sample at lower frequencies is a unique characteristic property of metamaterials

45.

This unique negative permittivity has been

reported in literature for various nanocomposites, including polyaniline, polypyrrole, carbon nanofibers/elastomer nanocomposites, epoxy, graphene, and graphene nanocomposites 45-48. The negative permittivity observed in polymer nanocomposites is a function of the processing technique and compositional factors such as particle loading level, aspect ratio and microstructures of the nanofillers

46,48.

It is to be noted that for negligible dissipation and for

frequencies lower than the plasma frequency, the charges can move quickly to shield the interior of the medium from the electromagnetic radiation, resulting in negative permittivity 45. The 2-D graphene and graphene nanocomposites exhibits negative permittivity due to the unique electronic energy dispersions (also called surface plasmons) 45,47. The negative permittivity in PC composites can be attributed to the excellent interfacial interaction between PC, MWCNTs and rGO-based hybrid nanoparticles, resulting in the charge delocalization at the interface. This further suggests that strong interfacial polarization in the composite sample is one of the primary contributor in shielding via the absorption mechanism (as shown in figure 7c). Thus, we observe that the 3-D connectivity of nanofillers providing an internal conduction pathway not only decreases the resistivity of the composite but also results in strong interfacial polarization

46,48.

With this we can justify why even the bare MWCNTs in PC matrix shield primarily by



absorption (shown in figure 7c), rather than reflection. Here, it is worth noting that PC nanocomposite as a metamaterial is yet to be explored.

-3

10 -4 10 -5 10 -6 10 -7 10 -8 10 -9 10 -10 10 -11 10 -12 10 -13 10 -14 10 -15 10 -1 10

10-5

neat PC

10-6 10-7 10-8

 (S/cm)

 (S/cm)

(a)

10-9 10-10 10-11 10-12

PC+ 0.5 wt% MWCNT -13 10 PC+ 1 wt% MWCNT 10-14 PC+ 2 wt% MWCNT 100 101 102 103 104 PC+ 3 wt% MWCNT (Hz) PC+ 3 wt% MWCNT + 5 wt% rGO-Fe3OFrequency 4

105

106

107

6

10

PC+ 3 wt% MWCNT + 5 wt% rGO-Gd2O3 0

1

10

2

10

3

10

4

10

5

10

10

7

10

Frequency (Hz)

(b)

3

1.2x10

PC+ 0.5 wt% MWCNT PC+ 1 wt% MWCNT PC+ 2 wt% MWCNT PC+ 3 wt% MWCNT PC+ 3 wt% MWCNT + 5 wt% rGO-Fe3O4

3

1.0x10

2

8.0x10

2

6.0x10

PC+ 3 wt% MWCNT + 5 wt% rGO-Gd2O3

2

4.0x10

2

2.0x10

'

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

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0.0 4

-2x10

4

-3x10

4

-4x10

4

-5x10

4

-6x10

-1

10

0

10

1

10

2

10

3

10

4

10

5

10

6

10

7

10

Frequency (Hz)


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Figure 6: (a) AC electrical conductivity (b) Real permittivity of nanocomposites as a function of frequency. 4.4 Microwave shielding effectiveness of the developed composites EMI shielding is expressed in terms of total shielding effectiveness (SET). SET is defined in terms of the logarithm of the ratio of the incident power (PI) to the transmitted power (PT) through the shield material 16. 𝑃𝐼

𝑆𝐸𝑇 = 10 𝑙𝑜𝑔𝑃𝑇

(2)

SET represents the ability of the shield material to attenuate EM waves and it is expressed in units of dB. SET value of -10 dB and -20 dB corresponds to 90 and 99% attenuation of the incident radiation respectively, while the “negative sign” signifies that it is a loss. Shielding can occur via three mechanisms, namely reflection, absorption and multiple reflections (Scheme 2).

Scheme 2: Mechanism of shielding Therefore, SET is a summation of shielding by absorption (SEA), shielding by reflection (SER) and shielding by multiple internal reflections (SEMR) 49. 𝑆𝐸𝑇 = 𝑆𝐸𝐴 + 𝑆𝐸𝑅 + 𝑆𝐸𝑀𝑅

(3) 19 ACS Paragon Plus Environment


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𝑆𝐸𝑀𝑅 becomes insignificant when SET> 15 dB or when the shield thickness is greater than the skin depth and the 𝑆𝐸𝑇 can then be expressed as 16, (4)

𝑆𝐸𝑇 = 𝑆𝐸𝐴 + 𝑆𝐸𝑅

SEM is usually important for thin metals and at low frequency (approx. 20KHz) 50. A two port VNA measures the scattering parameters (S11, S12, S21 and S22), which is used to calculate 𝑆𝐸𝑇, 𝑆𝐸𝐴 and 𝑆𝐸𝑅 using the following expressions 49, 𝑆𝐸𝑇 = 10 𝑙𝑜𝑔10│𝑆

1 2 12│

= 10 𝑙𝑜𝑔10│𝑆

1 2 21│

1

𝑆𝐸𝑇 = 10 𝑙𝑜𝑔10(1 ― │𝑆

(6)

2 11│ )

(1 ― │𝑆11│2)

𝑆𝐸𝐴 = 10 𝑙𝑜𝑔10

│𝑆21│2

(5)

= 𝑆𝐸𝑇 ― 𝑆𝐸𝑅

(7)

Where, S11, S22 are the reflection coefficient and S12, S21 are the absorption coefficient. The magnitude of absorption is governed by the square root of the product of electrical conductivity and permeability of the shield material. It can also be enhanced by the increase in shield thickness and is expressed by the following equation 16,49, 𝑆𝐸𝐴 = ― 8.68𝑡

𝜔𝜎𝜇𝑟 2

(8)

Where, σ represents the total conductivity, ω is the angular frequency (ω= 2πf), μr corresponds to the relative permeability of the shield material and t is the thickness of the shield. The magnitude of reflection depends on the impedance mismatch between the incident EM wave and the shield material and can be estimated using the following relation 16,49, 𝜎

𝑆𝐸𝑅 = ― 10log 16𝜔𝜀0𝜇𝑟

(9)

Where, ε0 represents the dielectric constant in free space. Thus, the reflection of EM radiation is mainly governed by the ratio of electrical conductivity and permeability of the shield material. SE due to multiple reflections can be calculated using equation 10 16,49. 20 ACS Paragon Plus Environment

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―𝑆𝐸𝐴

𝑆𝐸𝑀𝑅 = 20 𝑙𝑜𝑔│(1 ― 10

10

)│

(10)

Thus, in the case of high absorption ability and thickness, SEMR becomes insignificant as mentioned in equation 4. This is because at higher frequencies, while travelling from one boundary to another the magnitude of EM wave becomes negligible due to the absorption. Moreover, it is to be noted that 𝑆𝐸𝐴, 𝑆𝐸𝑅 and 𝑆𝐸𝑀𝑅 are not dependent on the filler concentration. Let us now consider figure 7a, which shows the SET of the various samples as a function of frequency. As predicted from the ac conductivity results, the SET of the samples scales with the MWCNT content in the sample due to the increase in conductivity of the samples with increase in MWCNT content. With the incorporation of 5 wt% rGO-Fe3O4 and 5 wt% rGO-Gd2O3 along with 3 wt % MWCNTs, increase in SET value in contrast to PC composites with MWCNT is anticipated due to multiple internal reflections from the reduced rGO sheets. It should be noted that there is no significant increment in conductivity with addition of rGO based-nanoparticles as compared to bare MWCNTs. This is due to the lower inherent conductivity of rGO as compared to MWCNTs and moreover the connectivity of MWCNT was hindered especially by the insulating Gd2O3 nanoparticles. So, it is mainly the multiple internal reflection from the hybrids originating from the varied dielectric properties of the constituents which plays a crucial role. Now, considering the results obtained, it was interesting to note that PC/ 3 wt% MWCNT/ 5 wt% rGO-Fe3O4 and PC/ 3 wt% MWCNT/5 wt% rGO-Gd2O3 manifested in SET values of -28 dB and -33 dB respectively. With PC/ 3 wt% MWCNT/5 wt% rGO-Gd2O3 showing slightly higher SET compared to PC/ 3 wt% MWCNT/ 5 wt% rGO-Fe3O4 it was deemed important to examine the exact mechanism of shielding since both Fe3O4 and Gd2O3 have vastly different electromagnetic properties. To get a deeper insight into the exact mechanism of shielding, contribution of absorption and reflection to the shielding effectiveness was determined from the available s-parameters. Figure 7b depicts the SEA contribution of various composites as a function of frequency. Thus, to get a clearer picture of the contribution of absorption and reflection mechanism to SET, % contribution has been plotted in figure 7c. From the figure, the gradual increase in % absorption with increase in MWCNT content can be attributed to multiple scattering of the incoming EM wave as it travels through the entangled and well percolated MWCNT network. The incoming wave suffers 21 ACS Paragon Plus Environment


Page 22 of 32

a gradual decay in its inherent power as it scatters through the neighboring overlapping MWCNTs and this is manifested as absorption in the final data. Also, the strong interfacial polarization (as discussed in section 4.3) plays a major role in shielding of EM wave via the absorption mechanism. Further details of the mechanism of absorption by MWCNTs have been discussed in the next section. It was interesting to note that, the absorption contribution of PC/ 3 wt% MWCNT/5 wt% rGO-Gd2O3 (95 %) was slightly higher compared to PC/ 3 wt% MWCNT/ 5 wt% rGO-Fe3O4 (92 %). Fe3O4 is ferrimagnetic (shown in figure S3) in nature and leads to the magnetic losses in presence of an oscillating EM radiation. Gd2O3 is paramagnetic (shown in figure S3) but its dielectric properties help in the enhancement of shielding due to the dielectric losses. With merely 3 wt% MWCNT, we were able to enhance the shielding by the addition of magnetic and dielectric nanoparticles. This can be further justified in the section where we have tried to delve deeper into the exact mechanism using various electromagnetic parameters like permittivity and permeability which was derived from s-parameters using Nicholson-Ross-Weir algorithm. At high frequencies, EM waves penetrate only near the surface of the shield and the magnitude of the field decays exponentially with thickness. The thickness at which the magnitude of incident field drops to 1/e is termed as skin depth (δ) and can be estimated using the following relation and is expressed using the following equation 16,49, δ =

1

(11)

(𝜋𝑓𝜇𝜎)1/2

where μ=μ0μr and μ0 is the permeability of free space SE due to absorption can also be related to skin depth by the following equation 51, 𝑡

𝑆𝐸𝐴 = ― 8.68 𝛿

(12)

where, t is the thickness of the sample (in mm). Skin depth for various samples at 18 GHz is plotted in figure 7d. As the MWCNT content increases and by the inclusion of magnetic and dielectric particles, the skin depth decreases. This is primarily due to the increase in the absorption-based attenuation due to multiple reflections, magnetic and dielectric losses.


Page 23 of 32

(b)

0

(a)

-10

-10

-15

SE A (dB)

SET (dB)

-15 -20 -25 PC+ 1 wt% MWCNT PC+ 2 wt% MWCNT PC+ 3 wt% MWCNT PC+ 3 wt% MWCNT + 5 wt% rGO-Fe3O4

-30 -35 -40 -45

0 -5

-5

10

12

14

16

-25 PC+ 1 wt% MWCNT PC+ 2 wt% MWCNT PC+ 3 wt% MWCNT PC+ 3 wt% MWCNT + 5 wt% rGO-Fe3O4

-30 -35


8

-20

-40 -45

18


8

10

(c)

SEA

100

12

14

16

18

Frequency (GHz)

Frequency (GHz)

% Absoprtion and Reflection

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


(d)

SER

Skin depth (mm)

8

80

6 60

4 40

2

20

0 t% M

1w PC +

O4 O T -Gd 2 3 NT O-Fe 3 NT rGO WCN MWC MWC t% rG 5 wt% wt% wt% +5w + 2 3 T + T + N N PC PC MWC MWC 3 wt% 3 wt% PC + PC +

0 1 PC +

O O4 NT NT -Gd 2 3 CNT rGO O-Fe 3 MWC wt% MW MWC wt% wt% t% rG 2 5 wt% w 3 + + 5 + T C + P N PC NT MWC MWC 3 wt% 3 wt% PC + PC +

Figure 7: (a) SET vs frequency, (b) SEA vs frequency, (c) % absorption and reflection (d) Comparison of skin depth for various nanocomposites at 18 GHz

4.5 Comparative mechanistic study of the mechanism of shielding Mechanism of shielding is expressed in terms of electromagnetic parameters like complex permeability and complex permittivity. It is to be noted that permittivity is the ability of a material to be polarized by an electric field, while permeability is the ability of the material to support the formation of magnetic field lines within itself. And since the response of a material to alternating electric field and magnetic field is not instantaneous, it is expressed as a complex term. This is because material’s polarization and magnetization does not change instantaneously with alternating electric and magnetic field, but it involves a phase lag. Complex (relative) 23 ACS Paragon Plus Environment


Page 24 of 32

permittivity, εr and complex (relative) permeability, μr of a material is a function of frequency and is defined 3 as: 𝜀𝑟 = 𝜀′ ―𝑗𝜀′′, 𝜇𝑟 = 𝜇′ ―𝑗𝜇′′

(13)

where, 𝜀′ and 𝜇′ is the real part of complex permittivity and permeability respectively. It is a measure of how much energy from an external field is stored in a material (shown in figure S4 a and c). The imaginary part of complex relative permittivity (𝜀′′) and complex relative permeability (𝜇′′) is called the loss factor and is a measure of how dissipative or lossy a material is to an external field (shown in figure S4 b and d). There is a phase difference associated with the real part and the imaginary part of the complex component. The tangent of this phase angle is tan δ or loss tangent which is used to express the relative losses incurred by a material and it is defined as: tan δε =

𝜺′′ 𝜺′

,

tan δμ =

𝜇′′ 𝜇′

(14)

In figure 8a, we observe that PC/ 3 wt% MWCNT/5 wt% rGO-Gd2O3 showed maximum value of tan δε with respect to other samples at all frequencies. This suggest that dielectric loss is highest for this sample, as was expected owing to the dielectric properties of Gd2O3. As observed in figure 8b, PC/ 3 wt% MWCNT/ 5 wt% rGO-Fe3O4 showed a slight increase in the magnetic loss as compared to other samples. As Fe3O4 is a ferrimagnetic material, the nanocomposite exhibited slight increase in the value of tan δμ which confirms the role of magnetic losses in the shielding effectiveness. But when we consider the summation of tan δε and tan δμ (figure 8c), it is observed that both for PC/ 3 wt% MWCNT/ 5 wt% rGO-Fe3O4 and PC/ 3 wt% MWCNT/5 wt% rGO-Gd2O3 showed similar trend. This leads us to the conclusion that both ferrimagnetic Fe3O4 as well as dielectric and paramagnetic Gd2O3 cause relatively similar overall losses. It is also observed from figure 8c that as MWCNT content increases, summation of tan δε and tan δμ increases. This is because of the increase in charge transport network with the increase in MWCNT content as well as the loss in inherent power of the incoming network as the wave suffers multiple reflections amongst the overlapping MWCNTs. Also, in case of conducting or (semi conducting) materials like MWCNTs and rGO, the mobile charge carriers move back and forth through the materials under the influence of E field, creating an electric current. Any


Page 25 of 32

collision of these charge carriers with neighboring atoms or molecules leads to power dissipation in the form of heat.

(b) 3.0

3.0

PC+ 1 wt% MWCNT PC+ 2 wt% MWCNT PC+ 3 wt% MWCNT PC+ 3 wt% MWCNT + 5 wt% rGO-Fe3O4

2.5

Tan  

2.0

2.0


1.5

1.0

0.5

0.5

0.0

0.0 10

12

14

16


1.5

1.0

8


2.5

Tan  

(a)

8

18

(c) 3.0

10

12

14

16

18

Frequency (GHz)

Frequency (GHz)

Tan    Tan  

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


2.5


2.0


1.5 1.0 0.5 0.0

8

10

12

14

16

18

Frequency (GHz)

Figure 8: (a) tan δε (b) tan δμ (c) total loss tangent as a function of frequency for various nanocomposites. Having measured and analyzed the different electromagnetic parameters responsible for various EMI shielding mechanisms, let us summarize the various losses contributing to the overall EMI shielding. The power loss owing to microwave heating involves two terms, one due to the electric field component and another due to the magnetic field component and it can be expressed mathematically by equation 15 2. 𝑃 = 𝜔( 𝜀0𝜀′′𝑒𝑓𝑓𝐸2𝑟𝑚𝑠 + 𝜇0𝜇′′𝑒𝑓𝑓𝐻2𝑟𝑚𝑠)

(15)



Page 26 of 32

Where, P represents the power density in the material, ω is the angular frequency of incident microwave, 𝜀0 is the permittivity of free space, 𝐸𝑟𝑚𝑠 represents the local value of the electric field strength, 𝜇0 is the permeability of vacuum, 𝐻𝑟𝑚𝑠 represents the local value of the magnetic field strength, 𝜀′′𝑒𝑓𝑓 and 𝜇′′𝑒𝑓𝑓 describes the effective dielectric loss factor and the imaginary part of effective magnetic permeability respectively. Considering the prominent losses in X- and Kuband, 𝜀′′𝑒𝑓𝑓 and 𝜇′′𝑒𝑓𝑓 can be expressed using equation 16 and 17 respectively2. 𝜎

(16)

𝜀′′𝑒𝑓𝑓 = 𝜀′′𝑑𝑖𝑝𝑜𝑙𝑎𝑟 + 𝜀′′𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑖𝑎𝑙 + 𝜔𝜀0 Where σ is the conductivity of the material 𝜇′′𝑒𝑓𝑓 = 𝜇′′𝑒𝑑𝑑𝑦 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 + 𝜇′′ℎ𝑦𝑠𝑡𝑒𝑟𝑖𝑠𝑖𝑠

(17)

The electric field component of the microwave is responsible for three types of losses, depending upon the material property. These include loss via the conduction mechanism, loss due to interfacial polarization and loss due to dipolar polarization 2. According to the literature, the electrical conductivity of MWCNT can be tuned from being a semiconductor to conductor depending upon the chirality

52.

While the electrical conductivity of rGO is decided by the

amount of reduction of the oxygen species but it generally falls in the semiconductor range, along with Fe3O4 53. In the presence of alternating electric field, the free charge carriers (for e.g.: electrons in MWCNTs and rGO), move back and forth through the material, creating an electric current. These induced currents cause heating in the sample due to the electrical resistance caused by the collision of electrons with neighboring molecules or atoms 2. In the case of MWCNT and PC, there is a mismatch in dielectric constant between these two materials and the electrons are unable to couple to the changes in the phase of electric field, resulting in energy dissipation in the form of heat. Gd2O3 is dielectric in nature and hence the primary loss is via the dipolar polarization effect. Electric dipole is sensitive to external electric field and attempts to align itself with the field by rotation. At high frequency, as in Ku- and X- band, the dipoles do not have sufficient time to respond to the oscillating electric field. This induces a phase lag and as the dipoles attempt to follow the field, they collide with each other, generating heat in the material. On the contrary, the principal mechanism governing losses due to magnetic field are eddy current loss and hysteresis loss 2. The eddy current loss is the loss due to joule heating in a material and is generated whenever there is a relative motion between a conducting material and 26 ACS Paragon Plus Environment

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external magnetic field 2. A changing magnetic flux induces an electric current in accordance to the Faraday’s law of electromagnetic induction. Moreover, Fe3O4 when subjected to an alternating magnetic field, the magnetic dipoles oscillate to align itself with the field. The rapid flipping of magnetic domains causes considerable friction and loss via heating in the material. Eddy current losses are induced in MWCNTs and rGO under the influence of the magnetic component of the incoming EM wave. In the case of PC/ 3 wt% MWCNT/ 5 wt% rGO-Fe3O4, the second term in equation 17 is significant while the first term in equation 16 is negligible. In case of PC/ 3 wt% MWCNT/ 5 wt% rGO-Gd2O3, the second term in equation 17 is negligible while the first term in equation 16 has considerable importance. These mechanisms justify the nearly similar response of SET in the two composite samples.

5. Conclusions In summary, EMI shielding efficiencies of graphene oxide functionalized with two types of strikingly different materials (magnetic Fe3O4 and dielectric and paramagnetic Gd2O3) was analyzed in the X and Ku band frequency. To enhance the shielding efficiency of these synthesized hybrid nanoparticles, 3 wt % MWCNTs were added. It was observed that MWCNTs worked in tandem with rGO-Fe3O4 and rGO-Gd2O3 contributing to SET of -28 dB and -33 dB respectively, which is comparable with the works reported in existing literature. Absorption was determined to be the major mechanism of shielding. It is interesting to note that even the bare MWCNTs in PC matrix exhibited absorption-based shielding primarily due to the 3-D network formation of nanofiller resulting in multiple reflections and strong interfacial polarization as depicted by negative permittivity values at lower frequencies. The different electromagnetic parameters like permeability and permittivity was analyzed and it was concluded that, magnetic losses are one of the contributing factors in rGO-Fe3O4 based nanocomposite, while in rGOGd2O3, dielectric losses have a significant role in total shielding. In addition to the dielectric and magnetic losses, eddy current losses (leading to power loss of incoming wave) arising out of the interaction between conducting MWCNTs and rGO and the magnetic component of incoming EM wave also contributes to the total shielding effectiveness. The acquired results provide a systematic insight into the mechanism of shielding in X and Ku band. It can be concluded that high permittivity and permeability materials need not necessarily provide optimum shielding 27 ACS Paragon Plus Environment


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efficacy rather lossy nanoparticles must be chosen. This is for the first time in literature that such a detailed comparative study has been conducted and we hope that these results will open new avenues for selection of microwave absorbing materials for suitable applications and also the challenge for choosing the right dopant remains an open question. Supporting information: Wide scan XPS spectrum; Raman analysis; room temperature magnetization plots and brief description of mechanism of shielding. Acknowledgement: The authors would like to thank DST India for the financial support and Ms. Madhu Renuka (Materials Engineering, IISc) for assisting us in the synthesis of nanomaterials.

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Mechanistic insight into the nature of dopant in graphene derivative

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