Reduced Graphene Oxide/Fe3O4/Polyaniline Ternary Composites as a Superior Microwave Absorber in the Shielding of Electromagnetic Pollution.

The present work is focused on fabrication of reduced graphene oxide/iron(II/III) oxide/polyaniline (RGO/Fe3O4/PANI) ternary composite by a hydrothermal method, its characterization, and application in the development of a high microwave absorbing shielding material. The RGO/Fe3O4/PANI composite showed dramatic enhancement of dielectric loss and magnetic loss compared to Fe3O4/PANI and RGO/Fe3O4 binary composites. This is ascribed to the embedment of more heterostructure phases. As a result, RGO/Fe3O4/PANI showed remarkably high SET (∼28 dB) through the absorption dominant mechanism. Our findings also showed maximum R L of Fe3O4/PANI, RGO/Fe3O4, and RGO/Fe3O4/PANI in the range of 2-8 GHz corresponding to -25 to -35, -40 to -46, and approximately -64 dB, respectively. This is in all probability due to the good impedance matching between permittivity/permeability and dielectric/magnetic losses.

Introduction

The interference of electromagnetic wave (EM) is an undesirable manifestation of radiation originating from different electronic instruments/appliances at different frequencies.[1−8] As a result, their performance is effected in terms of speed and secrecy resulting in the loss of data storage, revenue, efforts, and time.[9,10] In addition, it also effects human health leading to leukemia, miscarriages, brain cancer, and, in some cases, even death.[11,12] Therefore, a considerable amount of research has been focused on developing suitable electromagnetic interference (EMI) shielding materials for suppression of such electromagnetic pollution either by reflection, absorption, or multiple reflections.

In view of this, metals, such as Fe, Co, and Ni, have been used in controlling electromagnetic pollution due to their reflection ability, conductivity, and very shallow skin depth.[12] However, their application is restricted due to the processing difficulties, heavy weight, poor flexibility, and environmental degradation. Several carbonaceous materials, such as carbon black, graphite, reduced graphene oxide, carbon nanotube (CNT), and carbon nanofiber (CNF), have also been investigated in EMI shielding applications.[3,13,14] Among them, carbon black/graphite exhibit poor dispersability and high percolation threshold leading to their poor EMI shielding performance.[15,16] In contrast, other carbonaceous materials explicitly are not preferred in EMI shielding due to their cost, purification, and prolonged functionalization steps. Alternatively, combinations of several other materials, such as Fe3O4,[17−21] Fe2O3,[22,23] NiFe2O4,[24] CoFe2O4,[25−27] ZnO,[28] SiO2,[29,30] TiO2,[31] BaTiO3,[32] polyaniline (PANI), polypyrrole (PPy),[23,33] etc., in the form of binary composites have also been investigated as an EMI shielding material. In recent years, reduced graphene oxide (RGO) attracted considerable attention due to their unique properties.[34] However, RGO sheets alone are not favorable in absorbing EM wave due to its poor impedance matching mechanism.[35,36] Therefore, several magnetic materials, such as Fe3O4,[20] NiFe2O4,[24] CoFe2O4,[25,26] Ni,[37] and carbonyl iron[38] have been compounded with graphene in order to improve the impedance matching. Among these, several works are reported on Fe3O4 introduced in graphene due to its large magnetic anisotropy, low toxicity, and high chemical stability although its high weight and poor dispersibility hinder its practical application as a microwave absorber.[39]

Recently, ternary and quaternary nanocomposites have been receiving more attention due to their excellent microwave absorption properties. In this context, conducting polymers have been invariably employed in fabrication of ternary composites due to its tunable conductivity, adaptable permittivity or permeability, low density, low cost, easy synthesis, and good environmental stability.[33,40] The enhancement in EMI SE of ternary composites could be attributed to the dipole polarization, electronic spin, and charge polarization of multi-interfaces generated in the ternary system. Further, the presence of graphene in these ternary composites could also account for the flow of eddy current induced by the magnetic field imparted by the magnetic component and accounts for the absorption of EM radiation.[41] In addition, complex permittivity/permeability and impedance matching also play a key role.[42,43]

In view of this, attenuation of EM waves have also been achieved in fabricating ternary composites derived from different permutation combination of conducting polymers, graphene, and Fe3O4, such as, graphene/Fe3O4 incorporated polyaniline (PANI),[44] PANI nanorod/Fe3O4 microspheres on graphene nanosheets,[45] graphene/polypyrrole (PPY)/Fe3O4,[46] N-doped graphene@PANI nanorod arrays hierarchical structures modified by Fe3O4 nanoclusters,[47] and graphene/PPY/Fe3O4.[48] In addition, RGO-based ternary composites comprising Fe3O4 and PANI (or PPY) have also been studied in electromagnetic shielding interference applications, such as PANI-coated Fe3O4/reduced graphene,[49] RGO-magnetic porous nanospheres-PANI,[50] RGO/PPY nanotube/Fe3O4 aerogel,[51] RGO-PANI-Co3O4,[52] PPY-RGO-Co3O4,[53] RGO/Fe3O4/PANI,[54] γ-Fe2O3/(SiO2)–SO3H/PPY core/shell/shell microspheres[55] for the application of electromagnetic interference shielding.

In most cases, binary composite RGO/M3O4 (M: Fe, Co) has been fabricated using graphene oxide and FeCl3/FeCl2 as precursors in the presence of the reducing agent.[44−46,49,50,54] However, this introduces more defects in RGO due to the presence of metal ions by damaging its sp2 network.[56] As a result, a low dielectric loss as a consequence of the decrease in the conductivity could result in the poor absorption/reflection loss in the ternary RGO-Fe3O4 composite.[57] Motivated by this, we fabricated an RGO/Fe3O4 binary composite at room temperature using a previously prepared water-soluble reduced graphene oxide derived from graphene oxide and sodium dodecyl benzene sulfonate (SDBS) and coprecipitation of Fe3O4 in the presence of an ammonia solution. Then, prepared RGO/Fe3O4 is used for in situ polymerization of aniline. It is anticipated that better conjugation in water soluble RGO could account for the increase in the conductivity and for enhanced electromagnetic shielding performance. Subsequently, the RGO/Fe3O4/PANI composite has been characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron micrograph (FESEM), high-resolution transmission electron microscopy (HRTEM), and Raman study. Finally, electromagnetic interference shielding performance of RGO/Fe3O4/PANI has been evaluated and compared with RGO/Fe3O4 and Fe3O4/PANI binary composites. Our findings showed RGO/Fe3O4/PANI exhibiting maximum reflection loss (RLmax) of about −64 dB corresponding to the sample thickness of 0.45 mm over the entire frequency range (2−8 GHz).

Results and Discussion

Figure shows a schematic diagram of the formation of the RGO/Fe3O4/PANI composite. According to this, reduction of graphene oxide (GO) was prepared by the modified Hummer’s method by hydrazine hydrate in the presence of SDBS to form water-soluble reduced graphene oxide. This is ascribed to the interaction between benzene moiety of SDBS and RGO through π–π stacking and the ionic sulfonate group of SDBS with water molecules.[58,59] Subsequent addition of aqueous solution of FeSO4·7H2O and FeCl3·6H2O to this water-soluble reduced graphene oxide solution results in electrostatic interaction of Fe2+ and Fe3+ ions with the remaining functionalized groups in RGO and newly incorporated ones due to SDBS.[54] Further, dropwise addition of ammonia (25%) to this solution forms Fe3O4 nanoparticle anchored on the RGO surface forming an RGO/Fe3O4 composite. Finally, RGO/Fe3O4 dispersed in 1 (N) HCl upon subjecting to in situ polymerization of aniline at 0–5 °C forms an RGO/Fe3O4/PANI ternary composite.

Figure 1

Schematic presentation for the synthesis of the RGO/Fe3O4/PANI ternary composite.

FTIR Spectra

FTIR spectra of GO, RGO/SDBS, RGO/Fe3O4, Fe3O4/PANI, RGO/Fe3O4/PANI, and PANI are displayed in Figure . The spectra of GO shows the presence of peaks corresponding to the OH stretching vibrations of intercalated water and structural OH groups (3430 cm–1), C=O stretching vibrations originating from carboxylic acid/carbonyl moieties (1719 cm–1), skeletal vibrations from unoxidized graphitic domains (1620 cm–1), C–OH stretching vibration (1214 cm–1), and C–O–C stretching vibrations (1060 cm–1).[60] The spectra of RGO/SDBS and RGO/Fe3O4 shows significant reduction of the intensity of 1214 cm–1 (C–OH) and 3430 cm–1 (O–H) confirming the successful reduction of GO.[60] In addition, new peaks are also appeared at 2960, 2920, and 2846 cm–1 corresponding to the C–H stretching of the alkyl chain due to SDBS attached in RGO/SDBS. The peaks at 1028 and 828 cm–1 in RGO/SDBS correspond to S=O and C–S, respectively.[57] Further, FTIR spectra of RGO/Fe3O4 show the presence of a peak at 540 cm–1 due to Fe–O bonds in the crystalline lattice of Fe3O4.[61] In the case of Fe3O4/PANI and RGO/Fe3O4/PANI, the additional peaks corresponding to the C=N (1571 cm–1), C=C of benzenoid and quinoid rings (1486 and 1300 cm–1) of PANI, C–N (1237 cm–1) and C–H stretching vibration (873 cm–1) also appeared.[47] These findings confirmed the successful incorporation of PANI.[62] However, the absence of peaks at S=O (1028 cm–1), C–S (828 cm–1), and C–H stretching of the alkyl chain (2960, 2920, and 2846 cm–1) in the spectra of RGO/Fe3O4 and RGO/Fe3O4/PANI confirmed removal of SDBS. It is also noted that stretching vibrations of PANI of C–N–C (1270 cm–1) and C=C (1450 cm–1) have shifted due to possible interactions between PANI and RGO/Fe3O4 in RGO/Fe3O4/PANI composites to 1300 and 1486 cm–1, respectively.[63]

Figure 2

FTIR of GO, RGO/SDBS, RGO/Fe3O4, Fe3O4/PANI, RGO/Fe3O4/PANI, and PANI.

XRD

XRD patterns of GO, RGO, and RGO/SDBS are displayed in Figure a. The appearance of a diffraction peak at ∼9.4° is found to be typical of GO consisting several oxygen-containing functional groups.[5] The 002 plane of RGO/SDBS appeared at a lower angle (∼230) compared to RGO (∼24°), which is ascribed to the SDBS attached to RGO by the π–π stacking of the benzene ring in SDBS and RGO.[59]Figure b depicted presence of peaks corresponding to Fe3O4 located at ∼30.4° (d220 = 0.29 nm), 35.55° (d311 = 0.25 nm), 43.36° (d400 = 0.21 nm), 57.34° (d511 = 0.16 nm), and 62.73° (d440 = 0.15 nm) in RGO/Fe3O4, Fe3O4/PANI and RGO/Fe3O4/PANI.[64] However, the disappearance of the RGO diffraction peak (∼230) in RGO/Fe3O4 is attributed to the low crystallinity of RGO compared to Fe3O4 and the high degree of exfoliation of RGO.[45,63] The additional peaks of Fe3O4/PANI and RGO/Fe3O4/PANI at ∼15° (d011 = 0.59 nm), 20.4° (d020 = 0.44 nm), and 24.9° (d200 = 0.36 nm) show the presence of semi-crystalline peaks of PANI and its emeraldine salt.[47] Further, disappearance of the (002) reflection plane in RGO and merging of the planes with Fe3O4 and polyaniline demonstrated good interfacial interaction between RGO, Fe3O4, and PANI.[44,65]

Figure 3

XRD pattern of (a) GO, RGO, and RGO/SDBS and (b) RGO/Fe3O4, Fe3O4/PANI, and RGO/Fe3O4/PANI.

Raman Analyses

Raman analyses of RGO, RGO/Fe3O4, Fe3O4/PANI, and RGO/Fe3O4/PANI have been carried out, and the corresponding findings are displayed in Figure . RGO shows the presence of two intrinsic peaks appearing at ∼1345 and 1578 cm–1 due to D and G bands, respectively. They are correspondingly manifested from the first order scattering of E2g mode signifying vibration of sp2 carbon and defects in RGO.[66,67] It is also noted that the G band is blue shifted in RGO/Fe3O4 by 7 cm–1 with respect to RGO. This could be attributed to the charge transfer between RGO and Fe3O4.[67] However, the peak position of the D band (1345 cm–1) remains unaltered in RGO/Fe3O4 with respect to RGO. Raman spectra of RGO/Fe3O4/PANI shows no shifting of the D band, though the G band is blue shifted due to the π–π interaction between PANI and RGO.[53,68,69] Further, the appearance of the peaks corresponding to the in-plane C–H bending of the quinoid ring (1168 cm–1), in-plane C–H bending of benzenoid ring (1250 cm–1), C–C stretching of quinoid ring (1397 cm–1), and C=C (1496 cm–1) in the spectra of RGO/Fe3O4/PANI confirmed the successful incorporation of PANI in the composites.[70] The increase in the ID/IG ratio of RGO from 1.75 to 1.82 in RGO/Fe3O4 suggests a disordered graphitic crystal structure of RGO owing to the interaction between RGO and Fe3O4.[44] Sahoo et al.[71] also observed a higher intensity ratio (ID/IG) of GO-Fe3O4-APTES than pure GO ascribing to the defects generated by the interaction between Fe3O4-APTES and GO.[72,73] It is anticipated that Fe3O4 nanoparticles generated could adhere to defects present in RGO more strongly. As a consequence, enhanced exfoliation effect could account for the smaller size of graphitie crystal as well as their number to be larger.[18,50]

Figure 4

Raman spectra of RGO, RGO/Fe3O4, and RGO/Fe3O4/PANI.

FESEM and HRTEM

To contemplate the morphology and structure of binary and ternary composites, FESEM and HRTEM analyses are shown in Figure and Figure , respectively. It is clearly seen that FESEM of spherical-shaped tiny grains of Fe3O4 exists as agglomerates due to its intrinsic magnetic properties. On the contrary, the agglomeration tendency of Fe3O4 is significantly diminished in RGO/Fe3O4 and demonstrates uniform dispersion on the RGO surface (Figure b). This fact is also established by HRTEM showing the formation of the uniformly distributed Fe3O4 nanoparticles anchored on the surface of wrinkled RGO sheets (Figure a). To comprehend the dispersion of Fe3O4 in Fe3O4/PANI and RGO/Fe3O4/PANI composites, HRTEM analysis is employed. HRTEM image of Fe3O4/PANI indicates the agglomeration of Fe3O4 nanoparticles in PANI (Figure b). On the contrary, HRTEM of the ternary composite RGO/Fe3O4/PANI shows very well-dispersed spherical Fe3O4, which has been densely coated by PANI (Figure c). This clearly establishes the importance of RGO sheets in the dispersion and distribution of the Fe3O4 nanoparticle in the ternary composite. However, FESEM of the ternary composite, RGO/Fe3O4/PANI, shows (Figure d) nanorod-like polyaniline coating on RGO/Fe3O4. Energy dispersive X-ray (EDX) of FESEM (Figure S1) indicated the presence of nitrogen in Fe3O4/PANI (57.4%) and RGO/Fe3O4/ PANI (59.33%). Further, EDX analysis of RGO/Fe3O4, Fe3O4/PANI, and RGO/Fe3O4/PANI showed the presence of 39.5, 6.67, and 4% of Fe respectively. All these findings clearly established successful coating of PANI in Fe3O4/PANI and RGO/Fe3O4/PANI.[39,63] The selected area diffraction pattern (SAED) of RGO/Fe3O4/PANI composite shown in Figure exhibits the presence of well-defined diffraction rings corresponding to the (220), (311), (400), (511), and (440) planes of Fe3O4, which is in agreement with the XRD findings discussed earlier. The SAED pattern in Figure also indicates RGO and PANI existing as an amorphous phase in the ternary composite.[41]

Figure 5

FESEM of (a) Fe3O4, (b) RGO/Fe3O4, (c) Fe3O4/PANI, and (d) RGO/Fe3O4/PANI.

Figure 6

HRTEM of (a) RGO/Fe3O4, (b) Fe3O4/PANI, and (c) RGO/Fe3O4/PANI.

Figure 7

SAED pattern of RGO/Fe3O4/PANI.

Thermogravimetric Analysis (TGA)

TGA of PANI, RGO/Fe3O4, Fe3O4/PANI, and RGO/Fe3O4/PANI is shown in Figure . All the samples except RGO/Fe3O4 show a significant mass loss up to 120 °C, which is attributed to the moisture present in samples. The significantly lower mass loss for RGO/Fe3O4 suggests the hydrophobic nature of RGO. The second weight loss from ∼200 to 320 °C of PANI is mainly attributed to the two main reasons. First is the liberation of HCl doped in PANI, and second is the degradation of the low molecular weight PANI polymer.[74] In the case of Fe3O4/PANI, the weight loss is much lower compared to pure PANI in the entire temperature range, which is around 7% compare to pure PANI. Furthermore, RGO/Fe3O4/PANI exhibits the lowest weight loss among all PANI composites, which is nearly 10% less compare to pure PANI in the entire temperature range, suggesting the enhancement of thermal stability of the ternary composite. This phenomenon is attributed to the restricted thermal motion of PANI molecules because of the strong interaction of PANI with Fe3O4 and RGO.[74]

Figure 8

TGA of pure PANI, RGO/Fe3O4, Fe3O4/PANI, and RGO/Fe3O4/PANI.

Magnetic Properties

Room temperature M–H curves of Fe3O4, RGO/Fe3O4, Fe3O4/PANI, and RGO/Fe3O4/PANI composites were recorded in the magnetic field ranging from −10,000 to +10,000 Oe and are displayed in Figure . The saturation magnetization (Ms), remanence (Mr), and coercivity (Hc) values of the corresponding samples inferred from the respective magnetic hysteresis loops are tabulated in Table . It is noted that the lower value of Hc in Fe3O4 (38 Oe) manifests weak ferromagnetism.[75] Fe3O4 lacks microwave absorbing capability due to the reduced crystal size and shape anisotropy.[76−78] In contrast, higher Hc values observed in RGO/Fe3O4 (43 Oe), Fe3O4/PANI (44 Oe), and RGO/Fe3O4/PANI (60 Oe) implied larger magnetic anisotropy. As a result, the possibility of high-frequency resonance in terms of the anisotropy constant, anisotropy energy, and resonance frequency cannot be ruled out.[41]Table also shows that Ms value of Fe3O4 (46.7 emu/g) decreased in presence of nonmagnetic RGO and PANI components in RGO/Fe3O4 (27.5 emu/g), Fe3O4/PANI (7.9 emu/g), and RGO/Fe3O4/PANI (1.01 emu/g).[76,77] Furthermore, the reduced Mr value of binary and ternary composites confirmed their super paramagnetic behavior similar to Fe3O4 (2.6 emu/g).[79] The higher value of Hc (60 Oe) and reduced Ms and Mr values of RGO/Fe3O4/PANI (Ms = 1.01 emu/g, Mr = 0.03 emu/g) could be related to the presence of Fe3O4. The significantly higher Hc value in RGO/Fe3O4/PANI (60 Oe) could be ascribed to the presence of a lesser percentage of magnetic grains. For that reason, less dipolar interactions among Fe3O4 nanoparticles could lead to a swift magnetization reversal process.[50] In addition, the super paramagnetic behavior in RGO/Fe3O4/PANI as evident from the negligible Mr values could facilitate superior absorption of electromagnetic radiation due to swift magnetization and demagnetization processes.[80]

Figure 9

Magnetic hysteresis loops of Fe3O4, RGO/Fe3O4, Fe3O4/PANI, and RGO/Fe3O4/PANI.

Table 1

Ms, Hc, and Mr Values of Fe3O4, RGO/Fe3O4, Fe3O4/PANI, and RGO/Fe3O4/PANI

sample	Ms (emu/g)	Mr (emu/g)	Hc (Oe)
Fe3O4	46.7	2.01	36
RGO/ Fe3O4	27.5	1.5	40
Fe3O4/PANI	7.9	0.75	44
RGO/Fe3O4/PANI	1.01	0.03	60

Complex Permittivity and Complex Permeability

Complex permittivity and complex permeability of a material are two key parameters that play an important role in understanding the EM wave absorption mechanism.[41] Generally, complex permittivity is associated with the impedance matching, and complex permeability is concerned with the electromagnetic wave attenuation in the interior of the absorber. Therefore, complex permittivity (ε = ε′ – jε″), and permeability (μ = μ′ – jμ″) of the samples have been measured, where ε′ and μ′ represent the storage capability of the electric and magnetic energy and the imaginary parts (ε″ and μ″) stands for the loss capability of the electric and magnetic energy.[41] The complex permittivity and permeability of composites were calculated using scattering parameters (S11 and S21) based on the theoretical calculations given by Nicholson–Ross and Weir.[23] The real part and the imaginary part of permittivity are mainly associated with the amount of polarization occurring in the material and to the dissipation of energy, respectively. Further, material dielectric performance is determined by its ionic, electronic, orientational (arising due to the presence of bound charges), and space charge polarizations (originating from the heterogeneity in the system). A heterogeneous system can possess different dielectric constants and conductivities in the measured frequency range.[44]Figure a,b shows variation of real and imaginary permittivity of RGO/Fe3O4, Fe3O4/PANI, and RGO/Fe3O4/PANI in the frequency range of 2–8 GHz, respectively. It is noted that following range of magnitude is observed corresponding to frequency range under investigation: ε′: RGO/Fe3O4/PANI (165–42) > RGO/Fe3O4 (100–25) > Fe3O4/PANI (80–21); ε″: RGO/Fe3O4/PANI (173–44) > RGO/Fe3O4 (91–23) > Fe3O4/PANI (55–17). In all probability, the presence of comparatively higher conductivity of RGO compared to PANI accounts for higher ε′ and ε″ values in RGO/Fe3O4 than Fe3O4/PANI.[62,70] It can also be ascribed to the higher space charge polarization in RGO/Fe3O4 compared to Fe3O4/PANI.[44]Figure a,b also shows variations of real and imaginary permittivity of all the composites, which decrease exponentially with increasing frequency. This is due to the interfacial polarization (Maxwell–Wagner polarization) and reduction of space charge polarization on account of conducting RGO and PANI components present in the composites.[44] However, real and imaginary parts of permittivity showed dramatic enhancement in the RGO/Fe3O4/PANI ternary composite in comparison to the other binary composites. This is in all probability due to the higher number of interfaces and space charge polarization due to the presence of conducting RGO as well as PANI. The complex permeability (μ) of a material is related to real (μ′) and imagnary (μ″) parts as μ = μ′ – jμ″, where μ′ and μ″ represent storage and loss capabilities of the magnetic energy, respectively. Figure c,d shows variation of μ′ and μ″ with frequency of RGO/Fe3O4, Fe3O4/PANI, and RGO/Fe3O4/PANI composites. These findings indicate the following trend on magnitude corresponding to the lowest (2 GHz) and highest frequencies (8 GHz):

Figure 10

Plot of (a) frequency vs ε′, (b) frequency vs ε″, (c) frequency vs μ′, and (d) frequency vs μ″ of RGO/Fe3O4, Fe3O4/PANI, and RGO/Fe3O4/PANI.

μ′: RGO/Fe3O4/PANI (0.93–0.23) > RGO/Fe3O4 (0.59–0.14) ≈ Fe3O4/PANI (0.58–0.13)

μ″: RGO/Fe3O4/PANI (0.97–0.24) > RGO/Fe3O4 (0.53 to 0.13) > Fe3O4/PANI (0.4 to 0.10)

It is inferred that μ′ of RGO/Fe3O4 and Fe3O4/PANI remain more or less close. However, it increased significantly in RGO/Fe3O4/PANI possibly due to the presence of an additional interface facilitating thereby the flow of electrons beneficial to enhance dipole polarization, magnetic loss, and dielectric loss.[76] In contrast, a higher magnitude of μ″ (2–8 GHz) indicated greater magnetic loss in RGO/Fe3O4/PANI compared to RGO/Fe3O4 and Fe3O4/PANI. In totality, higher dielectric and magnetic losses over the whole frequency range in RGO/Fe3O4/PANI could account for enhanced microwave absorption properties originating due to the cooperative effect of PANI, RGO, and Fe3O4.[44]

EMI Shielding

The ratio between the intensity of incoming EM wave and its intensity after shielding is termed as electromagnetic interference shielding effectiveness (EMI SE) or total SE (SET) and used to express the shielding performance of a material.[79,80] The attenuation of EM waves by a shielding material is accompanied by three different types of mechanisms, namely, reflection (R), absorption (A), and multiple internal reflections (M).[9,23,81] Therefore, EMI SE (SET) of a material can be expressed (dB) aswhere SER, SEA, and SEM correspond to contributions due to reflection, absorption, and multiple internal reflections, respectively. If the distance between the reflecting surfaces exceeds the skin depth, shielding due to multiple reflections, SEM, can be ignored and the equation can be written asWhen EM radiation penetrates on the shield material, the summation of power coefficient of reflectivity (R), absorptivity (A), and transmissivity (T) must be equal to 1, i.e., R + A + T = 1.[9,23] The resultant complex scattering parameters (S) of the sample measured in a network analyzer are related according to their direction of propagation to correlate with the reflectance (R) and transmittance (T) as below:[81]where S11, S12, S21, and S22 correspond to the forward reflection coefficient, forward transmission coefficient, backward transmission coefficient, and reverse reflection coefficient, respectively. SE, SE, and SEA data of the samples can be calculated based on the parameters as follows:[81]

Figure a–d shows the plots of EMI of SEA, SER, SET, and reflection loss versus frequency of Fe3O4/PANI, RGO/Fe3O4, and RGO/Fe3O4/PANI in the range of 2–8 GHz, and the corresponding findings are expressed as below:

Figure 11

Variation of (a) SEA, (b) SER, (c) SET, and (d) reflection loss vs frequency of Fe3O4/PANI, RGO/Fe3O4, and RGO/Fe3O4/PANI.

SEA: Fe3O4/PANI (5–11 dB), RGO/Fe3O4 (12.5–17.5 dB), RGO/Fe3O4/PANI (27–28 dB)

SER: Fe3O4/PANI (17.5–13 dB), RGO/Fe3O4 (13–8 dB), RGO/Fe3O4/PANI (0–1.5 dB)

SET: Fe3O4/PANI (22-23 dB), RGO/Fe3O4 (∼26 dB), RGO/Fe3O4/PANI (28–29 dB)

It is noted that the SET values of Fe3O4/PANI and RGO/Fe3O4 in the entire frequency range are around 22 and 26 dB, respectively. However, the SEA of Fe3O4/PANI increases from 5 to 10 dB and the SER of Fe3O4/PANI reduces from 17 to 12 dB as the frequency progresses from 2 to 8 GHz. Similarly, the SEA of RGO/Fe3O4 continuously increases from 13 to 18 dB and the SER reduces continuously from 13 to 8 dB with the frequency progression from 2 to 8 GHz. The above results clearly indicate that SEA increases with the increase in frequency and SER decreases with the increase in frequency in the entire frequency range for both Fe3O4/PANI and RGO/Fe3O4 composites. Figure a,b also demonstrates that SER is dominated in Fe3O4/PANI in contrast to RGO/Fe3O4 where SEA is dominated. The SEA domination of RGO/Fe3O4 is attributed to the high dielectric loss and magnetic loss of the composite compare to Fe3O4/PANI.[44,67] This high SEA leads to the higher SET for RGO/Fe3O4 (26 dB) compared to PANI/Fe3O4 (22 dB). Moreover, the low impedance (high conductivity) of coated PANI on the Fe3O4 surface (supported by FESEM), which is an intrinsically conducting material leading to the higher SER. On the contrary, in the RGO/Fe3O4 composite, most of the surface of RGO is covered by the Fe3O4 nanoparticle (supported by FESEM), which have high impedance due to its non-conducting nature of Fe3O4. Interestingly, RGO/Fe3O4/PANI showed a remarkably high SET (∼28 dB) through an absorption-dominant mechanism. This is possibly due to the greater extent of interfacial polarization existing in the multiple interfaces in the RGO/Fe3O4/PANI composite.[54] However, a huge contribution of SEA in SET (28 dB) is attributed to the significantly high dielectric loss and high magnetic loss in the ternary composite.[54]

The microwave absorbing properties of absorbing materials can also be studied considering reflection loss (RL) curves. According to the classical transmission line theory, RL can be calculated according to the following equation:[82]where f is the EM wave frequency, d is the thickness of the sample, and c is the velocity of EM wave. Accordingly, variation of the RL calculated values of Fe3O4/PANI (thickness: 0.82 mm), RGO/Fe3O4 (thickness: 0.73 mm), and RGO/Fe3O4/PANI (thickness:0.45 mm) as a function of frequency (2–8 GHz) are displayed in Figure d. It is noted that the maximum RL values of Fe3O4/PANI, RGO/Fe3O4, and RGO/Fe3O4/PANI in the range of 2–6 GHz correspond to −25 to −35, −40 to −46, and approximately −64 dB, respectively. All these findings clearly indicated that the addition of PANI in RGO/Fe3O4 significantly enhanced the EM wave absorption of RGO/Fe3O4/PANI. This is in all probability due to the good impedance matching between permittivity/permeability and dielectric/magnetic losses. Alternatively, the minimum RL value of RGO/Fe3O4/PANI suggested the possibility of more interfacial polarization, relaxations, and multiple reflections playing a significant role in its microwave absorption properties. Table records microwave absorption performances in terms of RL of earlier reported RGO/Fe3O4/PANI composites fabricated by different approaches. It is inferred that RGO/Fe3O4/PANI exhibits excellent microwave absorption properties with relatively much thinner thickness. This feature makes RGO/Fe3O4/PANI an excellent candidate as a microwave absorber.

Table 2

EMI Shielding Data of Fe3O4/RGO/PANI vis-à-vis Binary and Ternary Composites Reported in the Literature

material	thickness	shielding data	ref
Fe3O4/C/PANI	1 mm	RLmin: −33 at (2–8 GHz)	(41)
RGO/Fe3O4/PANI	3.34 mm	SEA (max) = 28 dB at 18 GHz (12–18 GHz)	(44)
Fe3O4/RGO/PANI	3 mm	RLmax.: −43.7 dB at 10.7 GHz (2–18 GHz)	(45)
N-doped graphene/PANI/Fe3O4	2.7 mm	RLmax.: −40.8 at 14.8 GHz (2–18 GHz)	(47)
RGO/porous Fe3O4/PANI	1 mm	RLmin.: −29.51 at 14.96 GHz (2–18 GHz)	(50)
RGO/PPy/Fe3O4	3 mm	RLmax.: −49.2 at 11.8 GHz (2–18 GHz)	(51)
PANI/GO/Fe3O4	3.91 mm	RLmax.: −53.5 dB at 7.5 GHz (2–18 GHz)	(69)
RGO/PANI/FeNi3	4.8 mm	RLmax.: −43.17 at 6.2 (2–18 GHz)	(23)
Fe3O4/SiO2/PPy	1 mm	SET (max) = 32 dB at 2 GHz (2–8 GHz)	(83)
RGO/Co-doped ZnNi ferrite/PANI	1.7 mm	RLmax.: −24.2 at 17 GHz (2–18 GHz)	(84)
NiFe2O4/RGO/PANI	2.5 mm	RLmax.: −50.5 at 12.5 GHz (2–18 GHz)	(85)
Ni/PANI/RGO	3.5 mm	RLmax.: −51.3 at 4.9GHz (2–18 GHz)	(86)
FeNi/C/PANI	1.3 mm	RLmin.: −49.2 at 16.6 GHz (2–18 GHz)	(87)
RGO/Fe3O4/PANI	0.45 mm	RLmax.: approximately −64 dB (2–8 GHz) SET; 28–29 dB (2–8 GHz)	present work

The mechanism of EMI shielding and structure of RGO/Fe3O4/PANI are demonstrated in Figure . The SET in RGO/Fe3O4/PANI is mainly contributed by the absorption phenomena. The higher dielectric/magnetic loss in RGO/Fe3O4/PANI as evident from the complex permittivity and complex permeability measurements enhanced the microwave absorption properties. Alternatively, enhancements in electronic polarization, interfacial polarization, and magnetic loss could increase the EM absorption properties of RGO/Fe3O4/PANI due to improved impedance matching.[46,51,52] The incident EM waves are also likely to undergo multiple reflections and scattering due to the presence of interfaces between RGO/Fe3O4, Fe3O4/PANI, and RGO/PANI. RGO sheets of high specific area could also result in the absorption of EM waves and account its absorption following the dissipation of their energy as heat.[45,88] Further, the role of enhanced electron tunneling and electronic clouds in RGO/Fe3O4/PANI accounting for the conversion from EM wave to thermal energy also cannot be ruled out.[89,90]

Figure 12

Schematic illustration of the EMI shielding mechanism of RGO/Fe3O4/PANI.

Conclusions

RGO/Fe3O4 binary composites have been initially synthesized at room temperature following the growth of the Fe3O4 nanoparticle on the RGO surface from the respective precursors. Subsequently, the RGO/Fe3O4/PANI ternary composite was fabricated by subjecting RGO/Fe3O4 to low temperature in situ aniline polymerization following its characterization and performance in electromagnetic interference shielding. It was inferred that RGO/Fe3O4/PANI showed remarkably high SET (∼28 dB) compared to RGO/Fe3O4 (26 dB) and Fe3O4/PANI (22 dB). The dominant mechanism in RGO/Fe3O4/PANI arises due to the contribution of SEA originating from a significantly high dielectric loss and high permeability loss in the ternary composite and interfacial polarization. The excellent microwave absorption properties was also evidenced by the maximum reflection loss (RLmax) in RGO/Fe3O4/PANI (approximately −64 dB: 2–8 GHz) even at its relatively much thinner thickness (0.45 mm).

Experimental Section

Materials

Ferric chloride hexahydrate (FeCl3.6H2O), ferrous sulfate heptahydrate (FeSO4·7H2O), ammonium persulfate (NH4)2S2O8, aniline, and sodium dodecyl benzene sulfonate (SDBS) were all procured from Merck, India. Ethanol (C2H5OH) and graphite Micro-850 were obtained from SRL Pvt. Mumbai and Asbury Graphite Mills, Inc., Asbury, NJ, respectively.

Synthesis of SDBS-Wrapped Water-Soluble Reduced Graphene Oxide (RGO/SDBS)

A total of 200 mg graphene oxide (GO) synthesized by using the modified Hummer’s method[91] was placed inside a 250 cc round bottom flask comprising 100 mL distilled H2O and sonicated for 30 min. Thereafter, 200 mg SDBS and 2 mL hydrazine hydrate were added to this solution and subjected to stirring for 30 min. Subsequently, the round bottom flask was placed on an oil bath at 90 °C for 24 h to form SDBS-wrapped water-soluble reduced graphene oxide (RGO/SDBS).

Synthesis of the RGO/Fe3O4 Composite

A total of 1.8 g FeSO4·7H2O (0.0064 mol)/3.501 g FeCl3·6H2O (0.0129 mol) were dissolved in 50 mL H2O in a 250 mL round bottom flask comprising earlier-prepared 50 mL of RGO/SDBS (2 mg/mL) dispersed in distilled H2O. Subsequently, ammonia (25%) was added dropwise to this solution at room temperature under stirring continued for 1 h. The product obtained in this manner was filtered and washed several times with distilled water to remove SDBS and dried at 60 °C in a vacuum oven.

Synthesis of RGO/Fe3O4/PANI and Fe3O4/PANI

A total of 750 mg of earlier-prepared RGO/Fe3O4 was added to a 50 mL 1 (N) HCl/1 mL aniline mixture taken earlier in a round bottom flask of 100 mL capacity and then placed in ice bath. Subsequently, aniline was in situ polymerized by adding 2.624 g of APS to earlier contents according to the method reported earlier at about 0 °C for 24 h. Finally, the product was filtered and washed several times with distilled water and ethanol and kept in a vacuum oven at 60 °C for 24 h for drying. The synthesis of Fe3O4/PANI was also carried out under identical conditions by taking 700 mg Fe3O4 instead of RGO/Fe3O4.

Synthesis of Fe3O4 and Reduced Graphene Oxide (RGO)

Fe3O4 was synthesized by the same procedure in the absence of water-soluble reduced graphene oxide, and RGO was produced from graphite oxide in the absence of SDBS.

Characterization Techniques

X-ray diffraction (XRD) analysis of samples were performed on a Bruker XRD instrument with Cu Kα radiation (λ = 1.54 Å) at a scan rate of 10° min–1. Fourier transform infrared spectroscopy (FTIR) was carried out in the frequency range of 400 to 4000 cm–1 on a PerkinElmer RXI FTIR spectrometer, USA. Field emission scanning electron microscopy (FESEM) was used to analyze the morphological aspect of samples by Zeiss MERLIN. The samples for this purpose were prepared by drop casting on Al foil following its extensive sonication in ethanol. High-resolution transmission electron microscopy (HRTEM) was performed on a JEOL2100 microscope at a voltage of 200 keV. The samples used in this analysis were subjected to extensive dispersion in the presence of acetone followed by drop casting on a carbon-coated Cu grid. Raman analysis was carried out on a MODEL T64000 (Make Jobin Yvon Horiba, France) spectrometer using an exciting source of argon–krypton mixed ion gas laser. The magnetic measurement of samples were performed on Lake shore VSM of model no. 7410 at room temperature in the applied field ranging between −10,000 and 10,000 Oe. The electromagnetic shielding efficiency measurements were carried out on an Agilent E5071C Vector Network Analyzer over the frequency range of 2 to 8 GHz on the Fe3O4/PANI, RGO/Fe3O4 and RGO/Fe3O4/PANI samples, which are compressed molded in the form of a pellet of diameter 1.4 cm and thickness of ∼0.5 mm.[41]