NiFe2O4 Nanoparticles Synthesized by Dextrin from Corn-Mediated Sol-Gel Combustion Method and Its Polypropylene Nanocomposites Engineered with Reduced Graphene Oxide for the Reduction of Electromagnetic Pollution.

In this work, nickel ferrite (NiFe2O4) nanoparticles were synthesized by dextrin from corn-mediated sol-gel combustion method and were annealed at 600, 800, and 1000 °C. The structural and physical characteristics of prepared nanoparticles were studied in detail. The average crystallite size was 20.6, 34.5, and 68.6 nm for NiFe2O4 nanoparticles annealed at 600 °C (NFD@600), 800 °C (NFD@800), and 1000 °C (NFD@1000), respectively. The electromagnetic interference shielding performance of prepared nanocomposites of NiFe2O4 nanoparticles (NFD@600 or NFD@800 or NFD@1000) in polypropylene (PP) matrix engineered with reduced graphene oxide (rGO) have been investigated; the results indicated that the prepared nanocomposites consisted of smaller-sized nickel ferrite nanoparticles exhibited excellent electromagnetic interference (EMI) shielding characteristics. The total EMI shielding effectiveness (SET) for the prepared nanocomposites have been noticed to be 45.56, 36.43, and 35.71 dB for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposites, respectively, at the thickness of 2 mm in microwave X-band range (8.2-12.4 GHz). The evaluated values of specific EMI shielding effectiveness (SSE) were 38.81, 32.79, and 31.73 dB·cm3/g, and the absolute EMI shielding effectiveness (SSE/t) values were 388.1, 327.9, and 317.3 dB·cm2/g for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP, respectively. The prepared lightweight and flexible sheets can be considered useful nanocomposites against electromagnetic radiation pollution.

Introduction

Recently, the continuous development and the utilization of electronic devices in our daily life have raised the serious issue of electromagnetic interference pollution.[1,2] It has become an extensive concern owing to its potential influence on human health and electronic safety.[3,4] Therefore, substantial attempts have been carried out to develop high-performance electromagnetic interference shielding materials.[5,6] Traditional electromagnetic shielding materials associated with single dielectric loss or magnetic loss characteristics have provided an unsatisfactory electromagnetic shielding performance due to the issue of impedance mismatching.[7] One of the effective approaches to improve the performance of shielding material is to decorate spinel ferrite nanoparticles with reduced graphene oxide as impedance matching can be regulated with the synergistic impact of magnetic and dielectric loss.[8] An ideal electromagnetic interference shielding material should possess strong absorption characteristics, low density, light weight, design flexibility, thermal stability, etc.[9] Therefore, preparation of nanocomposite consisting of polymer-magnetic and dielectric inorganic material with high magnetic and dielectric losses could be a better choice. In addition, the physical properties of nanocomposites depend on their microstructures such as particle size, interfaces, density, etc.[10] Therefore, the physical characteristics of nanocomposites could be improved by the optimization of microstructure. It is of great interest to investigate the impact of microstructure on the electromagnetic interference shielding characteristics.[11,12] Spinel ferrite nanoparticles have received a great potential in the wide range of applications such as information storage, electronic devices to medical diagnostics, drug delivery, supercapacitors, anode materials for lithium-ion batteries, and microwave- and radar-absorbing material.[13,14] Nickel ferrite (NiFe2O4) is one of the important candidates among the spinel ferrite family because of its application in microwave, electronic, magnetic, and electrochemical devices.[15] It exhibits a low coercivity, eddy and hysteresis loss; therefore, it is beneficial for electronic devices such as telecommunications and high-frequency devices.[16] It has been also noticed that the electromagnetic wave absorption capability of spinel ferrite nanoparticles depends on its structural characteristics such as crystallinity, particle size, cation distribution, morphology, dopant ions, etc.[17,18] Accordingly, considerable efforts have been devoted to prepare spinel ferrite nanoparticles by various chemical synthesis techniques such as co-precipitation, hydrothermal/solvothermal method, microwave-assisted synthesis, microemulsion method, sonochemical synthesis, sol–gel combustion method, etc.[19−21] Among various chemical synthesis approaches, the sol–gel combustion method has various advantages such as a rapid synthesis approach, formation of high-purity product, homogeneous composition, stabilization of metastable phases, low energy consumption, and a simple, economic, and scalable synthesis method.[22,23]

Recently, polymer nanocomposites-based advanced electromagnetic interference shielding materials have received an interest among researchers and academicians due to their advantages of light weight, easy preparation, tunable conductivity, corrosion resistance, etc. in comparison to traditional metal-based shielding material.[24] Carbon material-based fillers in the polymer matrix have shown promising electromagnetic interference shielding characteristics.[25] This type of polymer composite cannot offer effective magnetic loss effect; therefore, it has limitation in electromagnetic waves absorption. To achieve the presence of both electric and magnetic dipoles for excellent electromagnetic interference shielding nanocomposites, various magnetic nanoparticles with carbon-based materials have been used as fillers in the polymer matrix.[26,27] The natural resonance, eddy current loss, and hysteresis losses generated by NiFe2O4 spinel ferrite can provide efficient magnetic loss in shielding material to efficiently absorb the electromagnetic waves.[28] Reduced graphene oxide exhibits a high dielectric loss; therefore, it provides attenuation characteristics to electromagnetic waves because of its high conductivity.[29] The combination of conducting reduced graphene oxide and spinel ferrite can provide high dielectric and magnetic loss in the polymer matrix, which can generate efficient electromagnetic interference shielding composite material.[30] Polypropylene is one of the most common thermoplastics, and it is chosen as the polymer matrix due to its extremely low cost and excellent characteristics such as chemical inertness, facile processing, good mechanical properties, etc.[31,32] In our previous work, our research group reported the effect of graphite, graphene oxide, and reduced graphene oxide as a filler with NiFe2O4 nanoparticles in polypropylene matrix on its electromagnetic interference shielding properties.[33] In that work, NiFe2O4 nanoparticles were synthesized by the honey-mediated sol–gel autocombustion method. Further, our research group investigated the electromagnetic interference shielding characteristics of NiFe2O4 nanoparticles in the presence of in situ thermally reduced graphene oxide (rGO) in polypropylene matrix with the variation of reduced graphene oxide content.[34] In that article, NiFe2O4 spinel ferrite nanoparticles were synthesized by the starch-mediated sol–gel combustion synthesis approach.

In this present work, the major objective is to prepare NiFe2O4 spinel ferrite nanoparticles via a simple, cost-effective, and greener approach and to further perform a detailed investigation on the impact of size of NiFe2O4 spinel ferrite nanoparticle as a filler with reduced graphene oxide in polypropylene on its electromagnetic interference shielding characteristics. Therefore, NiFe2O4 nanoparticles were prepared by dextrin from the corn-mediated sol–gel combustion method. To the best of the authors’ knowledge, this is the first report on the synthesis of NiFe2O4 nanoparticles by dextrin from the corn-mediated sol–gel combustion method. Further, the correlation among particle size, physical properties of NiFe2O4 nanoparticles, and electromagnetic interference shielding characteristics (permittivity, permeability, dielectric and magnetic loss) with reduced graphene oxide in the polypropylene matrix is investigated in detail. In addition, the obtained results indicate that the NiFe2O4–polypropylene nanocomposite engineered with reduced graphene oxide has provided high-performance electromagnetic interference shielding characteristics. A lightweight and flexible nanocomposite with excellent electromagnetic interference characteristics can be achieved by appropriate-sized NiFe2O4 nanoparticles with reduced graphene oxide in the polypropylene matrix. This preparation strategy can be also utilized to prepare other ferrite nanoparticles and its nanocomposites for its considerable potential applications.

Results and Discussion

X-ray Diffraction Study

Figure a presents the X-ray diffraction pattern of prepared NiFe2O4 nanoparticles with annealing at 600, 800, and 1000 °C. All of the XRD patterns of the prepared NiFe2O4 nanoparticles exhibited diffraction peaks of the (111), (220), (311), (222), (400), (422), (511), (440), and (533) crystal planes, corresponding to the spinel ferrite structure of the NiFe2O4.[35] The absence of impurity peak in the XRD patterns indicate high purity of the synthesized NiFe2O4 nanoparticles. It can be also noticed that the intensity of diffraction peaks increased with increase of annealing temperature, which signified the higher crystallinity of the NiFe2O4 nanoparticles prepared at a higher annealing temperature. The average crystallite size of the prepared spinel ferrite nanoparticles was evaluated through the Debye–Scherrer equation[36]with k = 0.9, λ is the X-ray wavelength (Cu Kα, 1.5418 Å), β is the full width at half-maximum (FWHM) of the (311) diffraction peak, and θ is the diffraction angle. The evaluated crystallite size was 20.6, 34.5, and 68.6 nm for NiFe2O4 nanoparticles annealed at 600, 800, and 1000 °C, respectively, as mentioned in Table S1. In addition, the Williamson–Hall method (Figure S1) and structural parameter investigation of the prepared nanoparticles are mentioned in detailed Supporting Information. Further, the crystal-phase information of the prepared reduced graphene oxide (rGO) and graphene oxide (GO) was acquired by XRD studies, as shown in Figure b. The graphene oxide prepared by Hummer’s method possesses a strong X-ray diffraction peak at 11.2° associated with the (001) reflection plane. In addition, the X-ray diffraction pattern of reduced graphene oxide (rGO) exhibits a diffraction peak at 24.81° corresponding to the (002) plane and another peak at 43.4° corresponds to the (100) crystal plane of graphene. Furthermore, the crystal phases of prepared nanocomposites were acquired by XRD studies, as shown in Figure c. It can be noticed that the XRD pattern of prepared nanocomposite is similar to nanoparticles, and no diffraction peak corresponding to rGO was noticed due to low X-ray diffraction intensity of rGO in the prepared nanocomposites.[37] In other words, the presence of high diffraction peak intensities of NiFe2O4 nanoparticles with disappeared peak of rGO and PP indicates high crystallinity and weight percentage (wt %) of NiFe2O4 nanoparticles in the prepared nanocomposite.

Figure 1

XRD patterns of (a) NiFe2O4 nanoparticles, (b) reduced graphene oxide and graphene oxide, and (c) prepared nanocomposites.

Figure a depicts the TEM image of prepared NFD@600 nanoparticles, which indicates the formation of spherical nanoparticles of size 10–25 nm. Figure b presents the HRTEM image of this nanoparticle, which suggest a lattice distance of 0.25 nm corresponding to the (311) lattice plane of nickel ferrite. It confirms the crystalline nature of prepared nanoparticles. The TEM image of the prepared reduced graphene oxide is shown in Figure c. It indicates that the sample exhibits aggregated and crumpled few layers of rGO enclosed together. Furthermore, the HRTEM image (Figure d) of prepared reduced graphene oxide has a lattice spacing of 0.33 nm.[38]

Figure 2

(a) TEM image and (b) HRTEM image of NFD@600; (c) TEM image and (d) HRTEM image of reduced graphene oxide.

Figure a–c depicts the FE-SEM image of the cross section of prepared nanocomposites NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP. The FE-SEM image indicates the presence of nickel ferrite and reduced graphene oxide fillers in the polypropylene matrix. It can be also noticed from Figure a–c that smaller nanosized NiFe2O4 nanoparticles have better dispersion with reduced graphene oxide in the polypropylene matrix. The dispersion of nanoparticles in polymer depends on the interfacial area and interfacial interaction.[39] In addition, the poor dispersion of graphene in nonpolar polymer such as polypropylene is associated with the large polarity difference and low interaction energy.[40] Further detailed microstructural and structural characterization of NiFe2O4 spinel ferrite nanoparticles and its polypropylene nanocomposites engineered with reduced graphene oxide characterized by field emission-scanning electron microscopy (FE-SEM) with energy-dispersive X-ray spectroscopy (EDX) (Figure S2), Raman spectroscopy (Figure S3), Fourier transform infrared spectroscopy (Figure S4), and X-ray photoelectron spectroscopy (Figures S5 and S6) is given in the Supporting Information.

Figure 3

FE-SEM image of cross section of prepared nanocomposite: (a) NFD@600-rGO-PP, (b) NFD@800-rGO-PP, and (c) NFD@1000-rGO-PP.

Magnetic Property of Nanoparticles and Nanocomposites

Figure depicts magnetic hysteresis curves of the prepared nanoparticles and nanocomposites at room temperature. It is clear from Figure a that all of the samples exhibited ferromagnetic behavior. The saturation magnetization (Ms), coercivity (Hc), and remanent magnetization (Mr) for all of the prepared nanoparticles powder samples are tabulated in Table S2. It is clear from Table S2 that the saturation magnetization is increased with increase of the crystallite size, while coercivity and remanent magnetization vary randomly. The values of saturation magnetization were 32.37, 37.74, and 43.15 emu/g for NFD@600, NFD@800, and NFD@1000 samples, respectively. Further, the values of coercivity were 63.65, 93.66, and 9.94 Oe for NFD@600, NFD@800, and NFD@1000 samples, respectively.

Figure 4

Magnetic hysteresis curves of (a) nanoparticles and (b) nanocomposites. The inset is its enlarged view.

It is well known that the magnetic parameters such as saturation magnetization, coercivity, and remanent magnetization depend on the preparation method, microstructure, chemical composition, particle size, morphology, and cation distribution.[41] The increase in saturation magnetization with increase of annealing temperature is related to the larger particle size and high degree of crystallinity, resulting in negligible surface spin canting.[42] The coercivity of the ferrite nanoparticles depends on the magnetocrystalline anisotropy, strain, interparticle interaction, particle size, and morphology.[43] The value of anisotropy constant can be evaluated by utilizing the value of coercivity and saturation magnetization in the following relation[44]The evaluated value of anisotropy constant is listed in Table S2. The magnetic moment (ηB) observed per unit formula in the Bohr magneton (μB) is evaluated by the following equation[45]where M is the molecular weight and Ms is the saturation magnetization. The observed magnetic moments are 1.35, 1.58, and 1.81 μB for NFD@600, NFD@800, and NFD@1000 samples, respectively. Figure b presents the magnetic hysteresis curves of polypropylene (PP) and its prepared nanocomposites with nickel ferrite nanoparticles and reduced graphene oxide. It can be observed that PP symbolizes nonmagnetic behavior and nanocomposites ferromagnetic characteristics. The evaluated values of saturation magnetization (Ms), coercivity (Hc), and remanent magnetization (Mr) from magnetic hysteresis curves for nanocomposites samples are tabulated in Table S3. The saturation magnetization values of the nanocomposites were 19.26, 22.95, and 25.29 emu/g for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposite samples, respectively. The saturation magnetization values of the nanocomposites are smaller than those of nickel ferrite nanoparticles because of the existence of nonmagnetic rGO and PP in nanocomposites. Nevertheless, the value of coercivity of nanocomposites increases as listed in Table S3. The high value of coercivity can enhance the absorption performance at GHz due to increased natural resonance frequency of spinel ferrite.[46] In addition, the detailed frequency dependence of dielectric constant (permittivity), ac conductivity, and modulus spectroscopy characteristics of the prepared nanoparticles in the frequency range of 1–107 Hz is given in the Supporting Information (Figure S7).

Electromagnetic Interference Shielding Effectiveness and Electromagnetic Properties of Nanocomposites

The electromagnetic interference shielding effectiveness (EMI SE) is defined as the ratio of the incident power (PI) and transmitted power (PT) of electromagnetic wave and generally expressed in the unit of decibel (dB). The electromagnetic interference shielding effectiveness of the prepared nanocomposites over the frequency range of 8.2–12.4 GHz with a vector network analyzer has been investigated using the waveguide method. In addition, the theoretical details of electromagnetic interference shielding are given in the Supporting Information. Further, Figure a–c presents the frequency-dependent total EMI shielding effectiveness (SET) and its reflection part (SER) and the absorption loss part (SEA) of prepared nanocomposites. For reflection of incident EM waves, the shielding material should exhibit mobile charge carriers. It means that the shielding material should possess good conductivity. For absorption of incident EM waves, the shielding material must possess electric and magnetic dipoles. In shielding material, the electric dipole can be obtained from the material having a high value of dielectric constant and further the magnetic dipole can be achieved from the material possessing a high value of magnetic permeability. It can be noticed from Figure that the values of SET were 45.56, 36.43, and 35.71 dB for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposites, respectively. Further, the values of SEA were 28.14, 16.53, and 18.84 dB for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP, respectively. In addition, the values of SER were 18.05, 16.34, and 17.28 dB for the prepared nanocomposites NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP, respectively.

Figure 5

Frequency-dependent electromagnetic interference: (a) total shielding effectiveness (SET), (b) reflection loss (SER), and (c) absorption loss (SEA) for nanocomposites.

The performance of the EMI shielding composite could be described in more reality, in terms of specific EMI shielding effectiveness (SSE, EMI shielding effectiveness divided by density) and absolute EMI shielding effectiveness (SSE/t, SSE divided by thickness of material) for its space and defense application where weight and thickness are important parameter. The evaluated values of SSE were 38.81, 32.79, and 31.73 dB·cm3/g for the prepared nanocomposites NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP, respectively. Further, the calculated values of SSE/t was 388.1, 327.9, and 317.3 dB·cm2/g for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP, respectively. Recently, Jiantong Li et al.[47] have noticed the SSE value of 21.3 dB·cm3/g for microcellular epoxy/multiwall carbon nanotube nanocomposite foam. Another research team, Hao-Bin Zhang et al.[48] observed the SSE value of 25 dB·cm3/g for graphene–polymer microcellular foams. Further, Haijun Liu et al.[49] reported the SSE value of 37.03 dB·cm3/g for porous graphene nanoplatelets/Fe3O4/epoxy nanocomposites. Furthermore, Hongming Zhang et al.[50] noticed the SSE value of 50 dB·cm3/g for microcellular PMMA/Fe3O4@MWCNTs nanocomposite foam.

The electromagnetic interference shielding effectiveness characteristic is highly dependent on the complex permittivity (εr = ε′ + ε″) and complex permeability (μr = μ′ + μ″) of the shielding material. The real part of the permittivity (ε′) and permeability (μ′) signifies the storage ability, and the imaginary part of the permittivity (ε″) and permeability (μ″) signifies the loss ability of electric and magnetic energy[51]Figure a represents the variation of the real part of permittivity (ε′) of the prepared nanocomposites in the frequency range of 8.2–12.4 GHz. The values of the real part of permittivity (ε′) were 5.70–5.86, 5.75–5.97, and 5.67–5.85 for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposite samples, respectively. The variation of the imaginary part of permittivity (ε″) over the frequency range of 8.2–12.4 GHz is shown in Figure b. The observed values of the imaginary part of permittivity (ε″) were in the ranges of 0.21–0.38, 0.20–0.37, and 0.15–0.36 for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposite samples, respectively. The results show the variation of complex permittivity of nanocomposites with variation of particle size of nickel ferrite nanoparticles in the presence of reduced graphene oxide in the polypropylene matrix. It can be noticed that the nanocomposite which is consisted of smaller-size nickel ferrite nanoparticles have improved complex permittivity, which is associated with the improvement of the degree of polarization in combination of reduced graphene oxide with smaller-sized nickel ferrite nanoparticles in the polypropylene matrix.[52] Generally, the values of the imaginary part of permittivity of EMI shielding material are related with the electrical conductivity and further the high conductivity is beneficial for the high value of complex permittivity.[53] The relation between electrical conductivity and the imaginary part of the permittivity (ε″) can be expressed as[54]where σAC is the electrical conductivity, εo is the dielectric constant of the free space, ε″ is the imaginary part of the permittivity, and f is the frequency. Figure c depicts the variation of electrical conductivity of prepared nanocomposites over the frequency range of 8.2–12.4 GHz. The values of electrical conductivity were in the ranges of (0.80–1.87) × 10–3, (0.74–1.82) × 10–3, and (0.58–1.77) × 10–3 S/cm for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposite samples, respectively. The magnetic filler NiFe2O4 nanoparticles provide magnetic characteristics to the prepared nanocomposites, which further improves the EMI shielding characteristics.

Figure 6

(a) Frequency-dependent real part of permittivity, (b) frequency-dependent imaginary part of permittivity, (c) ac conductivity, and (d) Cole–Cole plots, for nanocomposites.

In general, the dielectric loss is generated from polarization, namely, Debye dipolar relaxation, interface polarization, and electron polarization. In addition, the most important mechanism for an electromagnetic wave-absorbing material is the dielectric relaxation. The polarization and their related relaxation processes enhance the performance of shielding material. Cole–Cole plots have been utilized to understand polarization and their related relaxation process in nanocomposite shielding material. In view of the Debye theory, the relationship between ε′ and ε″ can be expressed as[55]The plot between ε′ and ε″ should be a single semicircle, corresponding to one Debye relaxation process, which is assigned as the Cole–Cole semicircle.

Figure d depicts the Cole–Cole plots for prepared nanocomposites. Several multiple semicircles can be noticed from the Cole–Cole plots of the nanocomposites, as shown in Figure d, which represents diverse relaxation mechanisms such as Debye dipolar polarization and interface polarization at the heterogeneous junction among nickel ferrite nanoparticles, reduced graphene oxide, and polypropylene.[56,57] The presence of interfaces in the heterogeneous nanocomposites provides the interfacial polarizations, and it is easy to happen in the reduced graphene oxide with relatively high conductivity, consequently accumulation of charges at interfaces and the creation of large dipoles on NiFe2O4 nanoparticles.[58] Therefore, the interfacial polarizations and associated relaxations contribute to the EMI shielding characteristics.

The real part of permeability (μ′) of the prepared nanocomposites is shown in Figure a. The values of the real part of permeability are in the ranges of 1.19–2.08, 0.96–1.03, and 0.99–1.91 for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposite samples, respectively. Figure b presents the imaginary part of the permeability (μ″) of the nanocomposites. The values of the imaginary part of permeability are in the ranges of 0.06–0.14, 0.01–0.05, and 0.01–0.09 for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP samples, respectively. The dielectric loss tangent (tan δε = ε″/ε′) and magnetic loss tangent (tan δμ = μ″/μ′) are two effective electromagnetic parameters to determine the contribution of dielectric loss and magnetic loss to the performance of EMI shielding nanocomposites.

Figure 7

Frequency dependence of (a) the real part of permeability, (b) the imaginary part of permeability, (c) dielectric tangent loss values, and (d) magnetic tangent loss values for nanocomposites.

The frequency dependence dielectric loss of the nanocomposites is shown in Figure c. It can be observed that the dielectric tangent loss values are in the ranges of 0.030–0.065, 0.027–0.064, and 0.022–0.062 for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP samples, respectively. It is well known that the dielectric loss is associated with mainly electric dipole and interfacial polarization. Specially, interfacial polarization occurs when neighboring fillers have different complex permittivities and conductivities[59] In prepared nanocomposites, the interface is mainly created between nickel ferrite nanoparticles and reduced graphene oxide in the polypropylene matrix. The presence of reduced graphene oxide may create electric dipole due to the presence of defects, chemical species on the surface, etc.[60,61]Figure d depicts the frequency dependence variation of magnetic loss of the prepared nanocomposites. The magnetic tangent loss values are noticed in the ranges of 0.047–0.110, 0.008–0.047, and 0.010–0.087 for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP samples, respectively. Normally, the EMI shielding characteristics of nanocomposites are results of combined contribution of dielectric and magnetic loss, and consequently, a higher value supports the efficient performance of the EMI shielding nanocomposites. As shown in Figure c,d, the magnetic tangent loss value is higher than the dielectric loss value, suggesting that the magnetic loss is the main contributor to the high performance of the NFD@600-rGO-PP nanocomposite. In general, the magnetic loss of classified ferrites is associated with eddy current loss, hysteresis loss, natural resonance, and domain wall resonance.[62] For NiFe2O4-rGO-PP nanocomposites, magnetic loss is due to the time lag of the magnetization vector M, behind the magnetic field vector H. In addition, a reversible rotational magnetization process occurs under an applied weak magnetic field to NiFe2O4-rGO nanocomposites. The permeability at a high frequency is caused by the reversible rotation of the magnetization vector.[63] Under weak magnetic field, the hysteresis loss is negligible. The domain wall resonance takes place at a very low frequency range in multidomain materials. The eddy current is an important parameter for electromagnetic wave absorption. It is associated with the electrical conductivity (σ) and thickness (d) of the samples by the following relation[64]where μo and σ are the permeability in the vacuum and electrical conductivity of the material, respectively. If with the variation of frequency, Co is constant, then the magnetic loss is caused by eddy current loss.[65] It can be noticed from Figure a that the value of Co decreases with serious fluctuations in the whole frequency range, which signifies that the eddy current effect has no significant contribution to the electromagnetic wave absorption. In addition, it implies that magnetic loss is caused by natural resonance instead of the eddy current effect. Further, the natural resonance loss can be expressed by the following expression[66]where |K| is the anisotropic coefficient, r is the gyromagnetic ratio (2.8 GHz·kOe–1), Ha is the anisotropic energy, and Ms is the saturation magnetization. On the one hand, a smaller-size nanoparticle has higher anisotropy energy due to the surface anisotropic field by the small-size effect.[67] On the other hand, it is observed in Figure b and Table S3 that the Ms value of the NFD@600-rGO-PP nanocomposite is lower than that of the NFD@800-rGO-PP and NFD@1000-rGO-PP nanocomposites. Therefore, the anisotropy energy of the NFD@600-rGO-PP nanocomposite is higher. The higher anisotropy energy is helpful in the improvement of electromagnetic wave absorption characteristics, particularly at high frequency.[68]

Figure 8

(a) Eddy current loss, (b) skin depth, (c) attenuation constant, and (d) impedance matching coefficient (η) for nanocomposites.

To elucidate the capability of prepared nanocomposites to shield the electromagnetic waves, the skin depth, attenuation constant, and impedance matching coefficient were studied. The high-frequency electromagnetic wave penetrates only near-surface within the material. This is known as the skin-depth effect. Skin depth is defined as the depth at which field drops up to 1/e or 37% times of its original value.[69] It is expressed by the following relation[70]where f is the frequency, μ is the magnetic permeability of the material, and σ is the electrical conductivity. Therefore, skin depth has dependence on frequency, electrical conductivity, and permeability. Figure b displays the skin depth of prepared nanocomposites as a function of frequency. It can be noticed that the prepared nanocomposites exhibited skin depth in the range of 0.9–2.6 μm. Generally, a low skin depth is found in high conductive metals.[71] Hence, the prepared nanocomposites exhibited shielding characteristics as metals, while in metals, the primarily shielding is due to reflection, but in our prepared nanocomposites, the primary shielding is due to absorption.

Further, electromagnetic wave attenuation by EMI shielding material is an important factor which plays an important role in the performance of shielding characteristics. The attenuation constant (α) signifies the attenuation ability of the shielding material, which can be expressed by the following relation[72]where f and c are the frequency of the electromagnetic wave and velocity of the light, respectively. The evaluated value of attenuation constant (α) of the prepared nanocomposites is displayed in Figure c. It can be noticed that the NFD@600-rGO-PP nanocomposite has a larger attenuation constant (α) in comparison to other nanocomposites with variation of frequency, which implies better shielding performance of this nanocomposite.

Furthermore, the impedance matching coefficient (η) can be evaluated by using the following expression[73]where Zo is the impedance in free space, Z is the impedance of the shielding nanocomposite material, εr and μr are the relative complex permittivity and permeability of the shielding nanocomposite material, respectively, εo and μo are the permittivity and permeability of the vacuum, respectively. The higher impedance matching coefficient signifies the better impedance matching possessed by the shielding material.[74] In addition, impedance matching decides how much of electromagnetic wave to propagate into the shielding material.[75]Figure d depicts the frequency dependence impedance matching coefficient for prepared nanocomposites. It can be noticed from Figure d that the NFD@600-rGO-PP nanocomposite provides a high impedance matching coefficient, resulting in a superior electromagnetic wave absorber. The prepared NFD@600-rGO-PP nanocomposite exhibited a high value of attenuation constant as well as impedance matching coefficient; consequently, it has a superior electromagnetic interference shielding performance. A detailed schematic demonstration of the electromagnetic interference shielding mechanism, as discussed above, is shown in Figure . The above results demonstrate that the prepared nanocomposites could be utilized as a high-performance electromagnetic interference shielding material.

Figure 9

Schematic illustration of the mechanism of electromagnetic interference shielding for prepared nanocomposites of nickel ferrite nanoparticles with reduced graphene oxide in polypropylene matrix.

Mechanical Properties of Nanocomposites

The mechanical properties of nanocomposites strongly depend on size, morphology, and interfacial adhesion between the components.[76,77]Figure displays a typical stress–strain behavior of the prepared nanocomposites. The tensile properties of the prepared nanocomposites are shown in Table S4. The values of evaluated Young’s modulus are 33.34, 28.19, and 24.55 MPa for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposites, respectively. Further, the values of tensile strength are 1.89, 2.21, and 2.36 MPa for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposites, respectively. In addition, the values of elongation at break are 24.01, 77.38, and 259.34% for NFD@600-rGO-PP, NFD@800-rGO-PP, and NFD@1000-rGO-PP nanocomposites, respectively. The above results indicate that the size of nickel ferrite nanoparticle evidently also affects the mechanical characteristics of prepared nanocomposites.[78]

Figure 10

Typical stress–strain curves of prepared nanocomposites.

Conclusions

In this work, various sizes of NiFe2O4 nanoparticles were synthesized by dextrin-mediated sol–gel combustion method, followed by annealing at 600, 800, and 1000 °C. The annealing temperature played an important role in tuning particle size and physical characteristics of NiFe2O4 nanoparticles. In addition, the particle size played an important role in controlling the electromagnetic interference shielding performance and the electromagnetic properties of nanocomposites based on nickel ferrite nanoparticles with reduced graphene oxide as a nanofiller in the polypropylene matrix. For nanocomposites with a smaller-sized NiFe2O4 nanoparticle as a filler with reduced graphene oxide, the maximum total shielding effectiveness reaches 45.56 dB at thickness 2 mm. The main electromagnetic interference shielding mechanism is magnetic loss, dielectric loss, synergetic effect, high value of attenuation constant, and good impedance matching. The prepared nanocomposites are a promising material for electromagnetic interference shielding application.

Experimental Section

Materials

Dextrin from corn Type I powder, graphite flakes, and potassium permanganate (KMnO4) were the products of Sigma-Aldrich, Germany. Sodium nitrate (NaNO3) was purchased from Lach-Ner, the Czech Republic. Nickel nitrate and iron nitrate were products of Alfa Aesar GmbH & Co. KG (Germany). Polypropylene (Vistamaxx 6202) was a product of Exxon Mobil (Machelen, Belgium). Vitamin C (Livsane) was a product of Dr. Kleine Pharma GmbH, Germany.

Synthesis of Nanoparticles

Nanofiller spinel ferrite NiFe2O4 was prepared by the dextrin-mediated sol–gel combustion method. Analytical-grade nickel nitrate (Ni(NO3)2·6H2O), iron nitrate (Fe(NO3)3·9H2O), and dextrin from corn, (C6H10O5), were utilized as the starting materials and were dissolved in deionized water. The mixed solution was stirred and heated at 110 °C. After 3 h of continuous stirring and heating, the mixed solution was converted into a viscous gel. This formed gel was heated at 310 °C until it is automatically converted into a fluffy spinel ferrite powder by self-combustion. Then, the as-prepared powder was further annealed at 600, 800, and 1000 °C to achieve a set of spinel ferrite nanoparticles of varied particle size. A schematic illustration of the synthesis of NiFe2O4 nanoparticles by the dextrin-mediated sol–gel combustion method is shown in Figure . The resultant NiFe2O4 nanoparticles were termed as NFD@600, NFD@800, and NFD@1000 corresponding to annealing at 600, 800, and 1000 °C. The formation of NiFe2O4 nanoparticles by the dextrin-mediated sol–gel combustion method can be expressed by the following equation

Figure 11

Schematic illustration of the synthesis of NiFe2O4 nanoparticles by the dextrin-mediated sol–gel combustion method.

Synthesis of Graphene Oxide (GO) and Reduced Graphene Oxide (rGO)

Graphene oxide (GO) was synthesized by the modified Hummer’s method.[79] The detailed preparation procedure of graphene oxide (GO) is shown in the Supporting Information. For the preparation of reduced graphene oxide (rGO), vitamin C was utilized as a reducing agent. The prepared graphene oxide was mixed in deionized water with ultrasonication for 15 min. Then, vitamin C was added slowly to the solution, and the suspension was stirred for 3 h at 90 °C. The obtained product was centrifuged and washed with deionized water. Finally, it was annealed at 60 °C for 15 h to get reduced graphene powder (rGO) powder.

Preparation of Nanocomposites

Nanocomposites of polypropylene (40 wt %) with nanofiller (55 wt % NiFe2O4 + 5 wt % rGO) were prepared. Nanofillers and polypropylene were premixed and then compounded by Micro-compounder Xplore MC15 (DSM Xplore Instruments BV, Sittard, the Netherlands). Melt treatment was conducted at 200 °C for 5 min with a speed of 50 rpm. The sheets of nanocomposites were prepared by the hot-press method. These sheets of nanocomposites were utilized for structural and physical characterization of nanocomposites. Three nanocomposite samples, namely, (i) NFD@600-rGO-PP, (ii) NFD@800-rGO-PP, and (iii) NFD@1000-rGO-PP, were prepared. A digital photograph of the prepared NFD@600-rGO-PP nanocomposite is shown in Figure .

Figure 12

(a–c) Digital photograph of lightweight, flexible, and efficient electromagnetic interference shielding nanocomposite.

Characterization

The crystal structure and phase purity of nanoparticles and nanocomposites were examined by X-ray powder diffraction (Rigaku Corporation, Tokyo, Japan). Raman spectroscopy measurements of nanoparticles and nanocomposites were examined by a Raman spectrometer (Thermo Fisher Scientific, Waltham, MA) at an excitation wavelength of 532 nm. The morphology of the nanoparticles was studied with a scanning electron microscope (FEI NanoSEM450). The cross section of nanocomposites was prepared by freeze-fracturing in liquid nitrogen and then utilized for the field emission scanning electron microscope. Further, the morphology of nanoparticles was characterized by a high-resolution transmission electron microscope Jeol JEM 2100. XPS images of nanoparticles were measured with an X-ray photoelectron spectroscope (Kratos Analytical Ltd.). The magnetic hysteresis curves of nanoparticles and nanocomposites were measured by using a vibrating sample magnetometer (VSM 7407, Lake Shore). The electromagnetic interference (EMI) shielding effectiveness (SE) of the prepared nanocomposite was investigated using vector network analyzer (Agilent N5230A) in the frequency range of 8.2–12.4 GHz (X band). The EMI SE of the nanocomposite was measured using a 23.4 × 10.2 × 2.0 mm3 sheet of prepared nanocomposites, which was fit into a waveguide sample holder. The EMI SE characteristics of the prepared nanocomposites were evaluated from the scattering (S) parameters, which was utilized to estimate total shielding effectiveness (SET), reflection loss (SER), and absorption loss (SEA). A tensile test of the prepared nanocomposites was carried out on a Testometric universal testing machine of type M 350-5CT (Testometric Co. Ltd., Rochdale, UK), equipped with a load cell of 300 kN.