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Synthesis and Microwave Absorption Properties of BiFeO3 Nanowire-RGO Nanocomposite and First-Principles Calculations for Insight of Electromagnetic Properties and Electronic Structures Debabrata Moitra, Samyak Dhole, Barun Kumar Ghosh, Madhurya Chandel, Raj Kumar Jani, Manoj Kumar Patra, Sampat Raj Vadera, and Narendra Nath Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02836 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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Synthesis and Microwave Absorption Properties of BiFeO3 Nanowire-RGO Nanocomposite and First-Principles

Calculations

for

Electromagnetic

Properties

and

Insight

of

Electronic

Structures Debabrata Moitra,a Samyak Dhole,a Barun Kumar Ghosh,a Madhurya Chandel,a Raj Kumar Jani,b Manoj Kumar Patra,b Sampat Raj Vadera,b and Narendra Nath Ghosh*a

a

Nano-materials Lab, Department of Chemistry, Birla Institute of Technology and Science,

Pilani K.K. Birla Goa Campus, Goa- 403726, India. b

Defence Lab, Jodhpur 342011, India.

1 ACS Paragon Plus Environment

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ABSTRACT Here, we report a facile hydrothermal synthesis method to prepare BiFeO3 nanowire-reduced graphene oxide (BFO-RGO) nanocomposites. The unique properties of 2-D reduced graphene oxide (RGO), and 1-D BiFeO3 nanowires (BFO) were exploited to design nanocomposites to obtain high performing microwave absorber materials. The composite with 97 wt % BFO and 3 wt % RGO exhibited minimum reflection loss value of -28.68 dB at 10.68 GHz along with the effective absorption bandwidth (≥ -10 dB) ranging from 9.6 GHz to 11.7 GHz when the absorber thickness was only 1.55 mm. First-principles calculations based on density functional theory (DFT) of BFO, graphene, and BFO-RGO nanocomposites were performed to obtain information about their electronic structures to interpret their complex permittivity and its derived properties. To the best of our knowledge, this is the first time investigations on microwave absorption properties of the BiFeO3 nanowire, and BFO-RGO nanocomposites have been reported, and this nanocomposite shows its potential to be used as a light weight, high performing microwave absorber in the X-band region. 1. Introduction Nowadays, with the rapid development of wireless communication and other electronic devices, electromagnetic interference (EMI) has become a concern for its potential adverse effects on living systems and human body.1-6 Over exposure to microwave radiation through excessive use of mobile phones, personal computers, local area network, television, radar systems, etc. can cause severe interruption in electronically controlled systems.7-9 Moreover, it is suspected that microwave exposure might be harmful to the biological systems because it may increase the possibility of cancer, weaken immune response, damage DNA in brain cells, increase heart rate,



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etc.7, 10-11 Therefore, to overcome these EMI related problems, there is currently focused effort by scientists to develop efficient microwave absorbing materials which can absorb unwanted electromagnetic signals. Microwave absorbers with high efficiency and broadband absorption in X-band region (8.2-12.4 GHz) find potential applications in military applications, particularly in stealth technology.9, 12-13 In defense, the use of Radar Absorbing Materials (RAMs), which are capable of absorbing microwave radiation in the X-band region, is very much required to stealth an object by reducing its radar cross section (RAS). At present, the next generation radar stealth equipment demands novel RAMs with advanced features, namely, light weight, thin layers, strong absorption and wide effective bandwidth. ‘Light, thin, strong and wide’ are the key features of new electromagnetic wave absorbing materials,14-15 which are in high demand. To design high performance microwave absorbing materials several strategies have been employed by the researchers, such as (i) making core/shell structures, in which magnetic materials occupy the core and the dielectric materials form the shell:16-17 Here, the enhancement of microwave absorption can be achieved by impedance matching between core and shell. However, complicated synthetic methodologies and scale up of products are the problems associated with this strategy. (ii) mixing magnetic materials with high dielectric property materials.18 The synthesis technique is quite simple here, but ensuring homogeneous mixing of the two materials plays a critical role in determining their microwave absorbing property. (iii) assembling magnetic materials onto 2D nanomaterials (such as graphene) to form a hybrid structure.16 Currently, graphene has attracted immense attention due to its unique twodimensional sheet structure where sp2 bonded carbon atoms form hexagonal network.19-22 It is known that dielectric loss, magnetic loss, and effective complementarities of these two loss mechanisms play important roles in determining the microwave absorption performance of


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an absorber.1, 16, 23-24 The microwave loss of magnetic materials (e.g. Fe3O4, Fe, Co) depends on their magnetic properties.1, 24 Hence, a magnetic material alone is not capable of producing high dielectric and magnetic loss simultaneously, thus limiting their applications.1, 23 To overcome this limitation, combining magnetic nanoparticles with graphene to form a composite material is an attractive strategy. Graphene offers several advantages (such as low density, superior mechanical property, and electronic properties)4, 8, 21, 25-27 but its loss mechanism is mainly based on dielectric loss.1, 8, 27 Hence its microwave absorption properties are insufficient for wide range application. Therefore, through the introduction of magnetic materials (e.g. ferrite nanoparticles) the microwave absorption property of graphene can be improved significantly. Recently, several graphene-ferrite nanocomposites have been explored to develop high performing microwave absorbers. Microwave absorption properties of some of the representative ferrite-graphene nanocomposites and their synthesis methods are listed in Table S1 (ESI†).1-4, 7, 11, 19-22, 24, 26, 28-34 Among various ferrites, perovskite type BiFeO3 is an interesting material due to its outstanding ferroelectric and antiferromagnetic properties. It offers some unexpected advantages such as coupling to spintronics and conduction at domain walls.12, 35-36 Combination of both dielectric loss and magnetic loss properties of BiFeO3 has opened new perspectives for application of BFO as a microwave absorber.12 Doping with La, Ho, Nd, etc. has increased microwave absorption properties of BiFeO3.12,

37-38

However, BiFeO3 based nanoceramic

materials exhibited their minimum reflection loss (RL) beyond X-band region, and in most of the cases, the absorber thickness was comparatively high.37-40 1-D magnetic nanowires, exhibited some fascinating properties, such as high field anisotropy due to high surface to volume and high aspect ratios.20, 32, 41-43 These properties can be exploited to develop high performing microwave absorber. As microwave absorbers in X-band region are



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important for military applications, we thought of exploiting the interesting multiferroic properties of BiFeO3 and the advantages of combined properties of 1-D magnetic nanowires with the nanometer thin structure of reduced graphene oxide. Therefore, we have synthesized nanocomposites, consisting of BiFeO3 nanowires (BFO) and reduced graphene oxide (RGO) and investigated their microwave absorption property in X-band region. In this paper, we report a simple hydrothermal route to synthesize BiFeO3 nanowire-reduced graphene oxide (BFO-RGO) nanocomposites, where BiFeO3 nanowires are anchored on the surface of nanometer thin reduced graphene oxide sheets. Microwave absorption properties of the synthesized BFO-RGO nanocomposites were investigated in the X-band region. To better interpret the inner mechanisms of interaction between BFO and RGO, first-principles calculations based on density functional theory (DFT) were performed. DFT calculations provided the information about the electronic structures of BFO, graphene, and BFO-graphene nanocomposites which were analyzed to understand their complex permittivity and derived properties.44-45 2. Experimental Section 2.1. Materials Bismuth (III) nitrate pentahydrate (Bi(NO3)3.5H2O), iron (III) chloride hexahydrate (FeCl3.6H2O), acetone, ammonium hydroxide were purchased from Fischer Scientific. Sodium hydroxide, sodium nitrate, sulphuric acid, potassium permanganate, and 30% H2O2 solution were purchased from Merck, India, and graphite powder (mean particle size of < 20 mm) was purchased from Sigma Aldrich and used without further purification. Distilled water was used throughout the reactions described here.


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2.2. Synthesis of Graphene Oxide (GO) Graphene oxide was synthesized from graphite powder according to the method reported by Hummers and Offeman.46 In this synthesis process, 1 g graphite and 0.6 g NaNO3 were mixed with 35 ml H2SO4 (18 M) at 0⁰C. The mixture was stirred for 6 h. Then 3.8 g KMnO4 was slowly added to this suspension. The temperature of the solution was raised to 35⁰C and maintained for 8 h to complete the oxidation reaction. Then 60 ml distilled H2O was slowly added and the reaction temperature was increased to 98⁰C. This temperature was maintained for 1 h, after that 2 ml 30% H2O2 solution was added to the mixture followed by stirring for 0.5 h. The mixture was centrifuged and washed with 10% HCl solution and distilled H2O. The yellowish brown precipitate of graphene oxide was obtained and dried at 60⁰C. 2.3. Synthesis of BiFeO3 nanowires-reduced graphene oxide nanocomposite (BFO-RGO) Synthesis of BiFeO3 nanowires-reduced graphene oxide nanocomposites has been discussed in our previous paper.47 A hydrothermal method was used to prepare BFO-RGO nanocomposites. The preparation of BFO-RGO nanocomposites is illustrated in Scheme 1. Briefly, Bi(NO3)3.5H2O and FeCl3.6H2O were dissolved in acetone (99.8%) with stirring and ultrasonication for 30 min. Then an aqueous dispersion of GO (0.03189 g in 50 ml) was added to the mixture. Concentrated ammonia was then added with vigorous stirring until the pH of the solution was 10-11. The filtrate thus formed was centrifuged and washed with distilled water several times until pH became neutral. Then under vigorous stirring, 5 M NaOH aqueous solution was added to this suspension, and this solution was transferred to a stainless steel autoclave with a Teflon liner and heated at 140⁰C for 24 h. The final black powder thus formed was separated from the reaction mixture and washed with distilled water followed by drying at 60⁰C. BFO-RGO nanocomposites with 1, 2 and 3 wt % RGO were prepared (confirmed by TGA



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vide infra) and designated as 99BFO-1RGO, 98BFO-2RGO, and 97BFO-3RGO respectively. We have also prepared pure BFO using the same method but without adding GO.

Scheme 1 Formation of BFO-RGO nanocomposites by hydrothermal method. 2.4. Characterization The synthesized pure BFO and BFO-RGO nanocomposites were characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), Raman spectroscopy, Thermogravimetric analysis (TGA), Transmission Electron Microscopy (TEM), Energy Dispersive X-Ray analysis (EDX), and Vibrating Sample Magnetometer (VSM) (details of the techniques used for structural characterization are discussed in ESI†). 2.5. Microwave absorption measurement HP 8510 Vector Network Analyzer was used for the measurement of microwave absorption of the synthesized BFO-RGO nanocomposites in X-band (8.2-12.4 GHz) range. Reflection loss (RL) was calculated using the measured values of complex permittivity and permeability. (Details of sample preparation for this measurement are provided in supporting in ESI†). 7 ACS Paragon Plus Environment

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2.6. First-principles calculations We have used first-principles quantum mechanical calculations based on DFT to obtain the electronic structures of BFO, graphene, and BFO- RGO nanocomposites. The calculations were performed using Pwscf code of Quantum ESPRESSO, an open-source distribution electronic structure calculations and materials.48 The exchange-correlation functional was approximated with generalized gradient approximation (GGA)49 parameterized by Perdew, Burke, and Ernzerhof (PBE)50 for geometric optimization of all structures. Since GGA approach underestimates the band gap for both semiconductors and insulators, a final single point energy calculation using screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06) was used for calculating the final electronic properties of the structures, which were optimized using PBE. Since the weak interactions are not well described by the standard PBE functional, an empirical dispersion-corrected density functional theory (DFT-D2) approach, which has proposed by Grimme,51-53 was adopted. All calculations were performed considering spin polarization. Ultrasoft pseudopotentials, which were used for all systems (e.g. super lattices of BFO, graphene, and BFO-graphene nanocomposite), were constructed by using 15, 14, 6, and 4 electrons for Bi (5d106s26p3), Fe (3p63d64s2), oxygen (2s22p4), and carbon (2s22p2) respectively.54-55 The geometry optimization and energy calculations were well converged with an energy cutoff of 50 Ry (~680 eV), and the energy tolerance was 10-5 Ry (1.36 x 10-5 eV/atom), while the force tolerance was set to 0.05 eV/Å. Here, three different systems were chosen: (i) BiFeO3 with rhombohedral distorted perovskite type structure with space group R3c,56-59 (ii) graphene superlattices and (iii) BFO-graphene nanocomposite. The details of Monkhorst-Pack mesh of k-points,56, 60-61 which were used to calculate the density of state and Brillouin zone, for geometry optimization of BFO, graphene, and BFO-RGO nanocomposites are



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provided as computational details in supporting information (ESI†). The initial structures of BFO, graphene, and BFO-graphene superlattices are shown in Figure S1 (ESI†), and sizes of the unit cells of the systems simulated are listed in Table S2 (ESI†). Details of the sample input files for geometric optimization of BFO, graphene, and BFO-graphene superlattices are provided in the supplementary, (ESI†). 3. Result and Discussion 3.1. Characterization of the Materials Room temperature wide angle powder XRD patterns of GO, pure BFO, BFO-RGO nanocomposites are presented in Figure 1. XRD pattern of pure BFO sample shows diffraction peaks at 2θ = 22.50⁰, 31.69⁰, 32.14⁰, 38.88⁰, 39.53⁰, 45.73⁰, 51.35⁰, 51.72⁰, 56.44⁰, 57.01⁰, 66.30⁰, 67.03⁰, 71.25⁰ and 76.16⁰ corresponding to (101), (012), (110), (003), (021), (012), (113), (211), (104), (122), (024), (220), (303) and (214) planes respectively of BiFeO3 (JCPDS card No 20-0169), which indicates its rhombohedral distorted perovskite structure with space group R3c and lattice parameters of a =b =c = 5.62043 Å and α =β =γ = 59.35381⁰. These values are consistent with what has been reported in the literature.47, 62 In case of GO, the XRD pattern shows a strong diffraction peak at 2θ = 9.76⁰ and a small peak at 2θ = 42.14⁰ corresponding to (001) and (101) planes respectively of GO.11,

47, 63

In the XRD patterns of BFO-RGO

nanocomposites all the diffraction peaks corresponding to BFO are observed. However, the absence of diffraction peaks for GO implies that during preparation of BFO-RGO nanocomposite, GO flakes were converted to RGO and RGO sheets were exfoliated.26 However, it was observed that the formation of pure BiFeO3 was affected by the presence of RGO. When the content of RGO was increase to 3 wt % in the BFO-RGO nanocomposite, very small peaks of Bi2O2CO3 and graphite were observed. This might be due to the limited migration


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of Bi3+ and Fe3+ ions in the presence of 3 wt % RGO.64 Therefore we have not prepared BFORGO nanocomposite with more than 3 wt % RGO content.

Figure 1 Room temperature wide angle powder XRD pattern of (a) pure BFO, (b) 99BFO1RGO, (c) 98BFO-2RGO, (d) 97BFO-3RGO and (e) pure GO The transformation of GO to RGO in BFO-RGO nanocomposite was further confirmed from the results obtained from FT-IR (Figure S2 (ESI†)) and Raman spectra (Figure S3 (ESI†)). From TGA thermogram (Figure S4 (ESI†)) it was observed that BFO-RGO nanocomposites are fairly stable up to 300ºC and the amounts of RGO present in the composites are consistent with the desired composition of RGO and BFO. (Detailed analysis of FT-IR, Raman spectra and TGA are provided in ESI†). Representative TEM micrographs of as synthesized pure BFO are presented in Figure 2(A) and (B). These images clearly show that the BFO sample consists of uniform cylindrical structures with diameters between 40 to 200 nm and lengths varied from hundred nanometers to several microns. As these BFO samples have an aspect ratio (major axis /minor axis) in the range of 50-100, we have considered this synthesized BiFeO3 as BFO nanowires.65



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Figure 2 TEM micrographs of synthesized (A) BFO, (B) individual BFO, (C) TEM micrograph and (D) HRTEM micrograph of a typical portion of GO, (E) and (F) TEM micrograph and FESEM micrograph of 97BFO-3RGO nanocomposite.


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The High Resolution Transmission Electron Microscopy (HRTEM) image (Figure S5(A) (ESI†)) and selected area electron diffraction (SAED) pattern (Figure S5(B) (ESI†)) of BFO show the planes with interplaner spacing of 0.257 and 0.45 nm representing (211) and (001) crystal face of BiFeO3 nanowires respectively.62 Figure 2(C) shows nanometer thin sheet of pure GO. The SAED pattern for GO (Figure S5(C) (ESI†)) shows the unresolved diffraction dots, indicating the GO sheets are amorphous in nature.19 A HRTEM image of GO (Figure 2(D)) shows the lattice fringe of 0.76 nm, which agrees well with the XRD pattern of GO.19 Figure 2(E) and (F) show BiFeO3 nanowire on the surface of nanometer thin RGO sheet.

Figure 3 (A) FESEM image, (B)-(F) Elemental mapping and (G) EDX spectra of the synthesized 97BFO-3RGO nanocomposite.



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FESEM micrograph (Figure 3(A)) of BFO-RGO nanocomposites reveals that BFO nanowires are dispersed on the surface of RGO sheet. Elemental mapping of BFO-RGO nanocomposites are shown in Figure 3((B)-(F)), which indicates the presence of Bi, Fe, O, and C in these composites. EDX analysis of BFO-RGO nanocomposite (Figure 3(G)) also indicates the presence of Bi, Fe, O, and C in these composites. The results obtained from XRD, HRTEM, FT-IR, Raman spectroscopy, and TGA clearly indicated the formation of BiFeO3 nanowires and BiFeO3 nanowire-RGO composite. Formation of the BiFeO3 nanowire can be presented by the following reactions47:

The growth of BFO crystals in nanowire form proceeds via Oswald ripening process.65-66 Before hydrothermal treatment, the precipitate contains nanoparticles of Fe(OH)3 and (Bi2O2)(OH)Cl (as identified by XRD) (Figure S6 (ESI†)). During hydrothermal treatment, these nanoparticles undergo aggregation and rupturing (as a result of Rayleigh instability),65,

67

followed by

dissolution due to their high free energy. Then the crystal growth starts from the aggregates of smaller particles and nanowires are formed.65 Room temperature magnetic property measurements of synthesized BFO and BFO-RGO nanocomposites by vibrating sample magnetometer (VSM) show that saturation magnetization (Ms) values of the composites decrease with increasing RGO content in the composites (Figure 4). Ms and coercivity (Hc) values of BFO and BFO-RGO composites are listed in Table 1. As in these composites, where BFO nanowires are anchored on the surface of nanometer thin sheets of RGO, several factors play important roles in determining the Ms value of these composites. Some of these factors that influence the magnetic properties are particle surface spin, disordered 13 ACS Paragon Plus Environment

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surface spin structure, dipolar inter particle interactions, etc.7, 19, 68-69 The magnetic properties of these composites can be tailored by simply manipulating the weight ratio of RGO and BFO in the composite.

Figure 4 Room temperature magnetic hysteresis loops of Pure BFO, 99BFO-1RGO, 98BFO2RGO, and 99BFO-3RGO

Table 1: Magnetic properties of BFO and BFO-RGO nanocomposites

Saturation magnetization (Ms) (emu/g)

Coercivity (Hc)

BFO

5.42

4.20

99BFO-1RGO

5.35

4.30

98BFO-2RGO

5.30

4.15

97BFO-3RGO

5.22

4.14

Material

(Oe)



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3.2. First-principles calculations of electronic structure The superlattice structures of BFO, and BFO-graphene nanocomposites after full relaxation are shown in Figure S1 (ESI†). It was observed that the relaxed superlattice structures of graphene and BFO are in close agreement with reported theoretical and experimental results56-59, 70 as shown in Table S3 (ESI†).

Figure 5 (A) Electronic total charge density and (B) difference charge density plots of the BFOgraphene composite interface.


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In BFO-graphene superlattice, lattice distortion after relaxation was observed. BFO slab in BFOgraphene superlattice was expanded in the z-direction by 3.4% when compared to the relaxed geometry of BFO slab, indicating a strong interaction between BFO and graphene. Binding energy was calculated from the difference between the total energy of the BFO-graphene superlattice and the sum of each system alone, and the obtained value was -5.4eV. The equilibrium interlayer distance between graphene and BFO slab was 2.79 Å. The electronic total charge density contour plot for the interface between graphene and BFO slab (Figure 5(A)) shows the overlap of electron orbital overlap between graphene and BFO in the interface and indicates the C-O and C-Fe interactions. Figure 5(B) displays the difference charge density plot, i.e., charge density of the composite interface minus superposition of isolated atomic densities. The red color represents charge accumulation, while the blue color represents charge depletion. On the graphene sheet, it is found that charges depleted on the C atoms that are close to O atoms, while accumulated on the C atoms that are close to Fe atoms. To understand the electronic properties of the nanocomposite and the details of the interaction between BiFeO3 slab and graphene super lattices, the band structure and projected density of states (PDOS) were calculated by projecting the electron wave functions onto spherical harmonics centered on each type of atom,70-71 and shown in Figure 6 and Figure 7, respectively. The Fermi level was referenced at zero energy. Before investigating the electronic properties of the nanocomposite BFO-graphene superlattices, the electronic properties of pure graphene and BiFeO3 slab were studied. Figure 6(A) shows the band structures of graphene superlattices. The conduction and the valence bands are either separated by a gap or overlap with each other intersecting in two inequivalent points, which are known as Dirac points in the first Brillouin zone.70 16 ACS Paragon Plus Environment


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Figure 6 Electronic band structures of (A) graphene superlattices, (B) BiFeO3 superlattices for spin up, (C) BiFeO3 superlattices for spin down, (D) BiFeO3-Graphene superlattices for spin up and (E) BiFeO3-Graphene superlattices for spin down.

It is a well-established fact that GGA approach underestimates the band gap for both semiconductors and insulators due to self-interaction error of electrons.56, 72 Hence, the electronic structures of BFO, graphene, and BFO-RGO have been calculated by performing a single point energy calculation using a hybrid density functional (HSE) on the respective relaxed geometries obtained with GGA-PBE. Graphene was found to have zero-band gap (Figure 6(A)). For pure BiFeO3 (Figure 6(B) and (C)) a majority spin band gap of 2.45 eV and minority spin band gap of 1.6 eV were obtained and the value of band gap, obtained with majority spin, is in close agreement with the experimentally obtained optical band gap of 2.5 eV and 2.74 eV as reported


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by several researchers.73-77 Though according to Li et al.64, the experimentally obtained band gap for BFO-RGO is 1.7 eV, but in the present study, no band gap was observed for BFO-graphene interface. This result is consistent with the observations from the reported work on interfaces of graphene with semiconductors composite (such as ZnO-graphene and BaTiO3- graphene) by Geng et al71 and Luo et al70 and is due to the fact that the DFT calculation considers only a 2-D interface between BiFeO3 and graphene. Also, the defects which are present in real RGO have not been considered in this calculation. The defects influence the band gap value of graphene based composites. However, from the band structure of BFO-graphene superlattices (Figure 6(D) and (E)) it was observed that some new bands appear near the Fermi level compared to BiFeO3 superlattice, which is a clear evidence for the effects of graphene on the electronic properties of BFO-graphene superlattices. These new bands originate from C 2p states of graphene. To understand the effect of graphene and formation of new bands on BFO-graphene superlattices, DOS analysis offers a further insight which helps to elucidate the nature of contact between BFO and graphene and hence the electronic structure. Therefore, before calculating the DOS for BFO-graphene superlattices, we have individually calculated the DOS of BFO and graphene superlattices. Figure 7(A) shows the DOS calculation of graphene superlattices. It was observed that graphene has zero-band gap and its Fermi energy coincides with the Dirac point, which agrees with calculated results of other researchers.44,

78

All contributions in graphene

originate from C 2s and C 2p states. Figure 7(B) shows the density of states of pure BFO superlattice. It was observed that the peaks at 18 eV, 11 eV and 4 eV below the Fermi level correspond to O 2s, Bi 6s and O 2p states, respectively. The contributions of different orbitals are clearly observed near the Fermi level (0 eV). The major contributions in BFO superlattices originate from Bi 6p, O 2p, and Fe 3d states. To be specific, the valence band of BFO is mainly



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constituted of O 2p and Fe-3d states, where as Bi 6p states dominate the conduction band. The overlap of Fe 3d and O 2p states is associated with the hybridization between these states. Figure 7(C) shows DOS of BFO-graphene superlattices. Here, the predominant localization of electronic charge around O 2p and Fe 3d states are major contributors to the valence band. C 2p of graphene also partially contributes to the valence band maximum. The electronic structure of graphene layer in the BFO- graphene composite changes significantly due to the orbital overlap, which was also observed in the electronic total charge density plot of the BFO-graphene composite interface (Figure 5(E)). The conduction band is largely comprised of Bi 6p and partly from C 2p states. The inclusion of graphene introduces several new states just above the Fermi level. These new states generally originate from C 2p states of graphene. Strong hybrid interactions between O 2p and Fe 3d orbitals are also observed. This hybridization shortens the distance between O and Fe atoms, causing a distortion which in turn produces the imbalance of charges for the whole structure. This strong hybrid interaction between O 2p and Fe 3d orbitals are the origin of ferroelectricity of BFO-graphene superlattices.56 In the BFO-graphene composite, the hybridization between C 2p states of graphene and O 2p and Fe 3d states of BFO increases the conductance of the BFO-graphene superlattices. The displacement of outer electrons of Bi in BFO-graphene composite might cause the electronic polarization. These factors are responsible for the increase in the permittivity value of the composite44-45, 70 which in turn enhance the dielectric loss properties of the nanocomposites. It has already been reported by several researchers that, dielectric loss plays an important role in the microwave absorption properties of an absorber.1, 3, 16 Therefore, based on this theoretical analysis we can predict that the dielectric loss, as well as microwave absorption properties of the BFO-RGO composites, will


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be improved due to the inclusion of RGO and this prediction agrees well with our experimental results which are discussed in section 3.3.

Figure 7 Projected density of states of (A) graphene, (B) BiFeO3, and (C) BFO-graphene superlattices. The Fermi level is referenced to zero energy, as indicated by the dotted line. 3.3. Microwave absorbing properties To evaluate microwave absorption properties of pure BFO and BFO-RGO nanocomposites, we have measured the complex permittivity (ε' and ε'' stand for real and imaginary permittivity) and permeability (µ' and µ'' stand for real and imaginary permeability) of the samples. Figure 8 shows the complex permittivity and permeability for BFO and BFO-RGO nanocomposites with varying RGO content from (1 to 3 wt. %) in the X-band frequency range (8.2-12.4 GHz). Generally, real parts (ε' and µ') are measures of the storage capability of dielectric and magnetic energy, whereas the imaginary parts (ε'' and µ‫ )׳׳‬characterize the dissipation of dielectric and magnetic energy of the samples.3, 79



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Figure 8 Frequency dependence (A) real part and (B) imaginary part of relative complex permittivity, (C) Dielectric loss, (D) real part and (E) imaginary part of relative complex permeability and (F) magnetic loss values of pure BFO and BFO-RGO nanocomposites with different RGO content From Figure 8(A) and (B) it was observed that initially values of ε' and ε'' increased with increasing RGO content in the BFO-RGO nanocomposites. ε' values were in the range of 19.1220.55 (pure BFO), 19.79-20.58 (99BFO-1RGO), 20.29-19.97 (98BFO-2RGO), and 21.08-20.08 (97BFO-3RGO). Similarly, ε'' values (Figure 8(B)) fall in the range of 1.69-2.17 (pure BFO), 3.45-2.53 (99BFO-1RGO), 3.82-4.14 (98BFO-2RGO) and 4.04-4.20 (97BFO-3RGO). The fluctuation of values of ε' and ε'' became more and more pronounced with increasing RGO content in the composite. In ε'' versus frequency plots (Figure 8(B)) resonance peaks appeared in the range of 10.5-11.0 GHz for all RGO dominated composites. In the case of 97BFO-3RGO, a


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strong resonance peak was observed at 10.75 GHz. Therefore it is clear that resonance peaks can be adjusted by varying the BFO and RGO ratio in the composite. The appearance of resonance peaks might be due to the displacement current lag caused by the interfaces between BFO and RGO in BFO-RGO nanocomposites.1, 80-81 Dielectric tangent loss (tan δε = ε″/ ε‫ )׳‬with changing frequency for pure BFO and BFO-RGO nanocomposites was also calculated and is shown in Figure 8(C). BFO-RGO nanocomposites possess higher dielectric loss than that of pure BFO. 97BFO-3RGO showed a strong resonance peak at 10.75 GHz. A resonance peak in the same region was also observed in ε″ versus frequency curve of 97BFO-3RGO (Figure 8(B)). These results suggest that BFO-RGO nanocomposites possess improved dielectric properties. Two important factors (i) conductivity loss and (ii) combined loss of the dipole polarization and interfacial polarizations contribute to the dielectric loss of the absorber.1, 3, 9 The contribution of conductivity loss, originating from the RGO content in the BFO-RGO nanocomposite, can be explained with the help of free electron loss theory. Although polarization plays a role in the imaginary part, free electrons also have significant effect on it, due to the good electrical conductivity of RGO.82-85 According to the free electron theory,1, 3, 82-85 ε‫ ׳׳‬could, therefore, be obtained as ε″ ≈ σ(T)/2πε0f

(3)

where, σ(T) is temperature-dependent electrical conductivity, f is frequency and ε0 is the dielectric constant in vacuum. Equation (3) shows that σ(T) plays an important role in ε″. As with the increasing RGO content in BFO-RGO nanocomposites the resistivity of the nanocomposites decreases because of the high conductivity of RGO, it is expected that the composites having more RGO will exhibit higher conductance loss.3,

25

Present studies also

revealed that 97BFO-3RGO exhibited highest ε″ value.



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To explain the contribution of combined loss of the dipole polarization and interfacial polarizations towards dielectric loss of nanocomposites Debye theory3 has been employed. The defects present in RGO are probably responsible for dipole polarization, whereas the interfacial polarization originates from the interfaces between BFO nanowire and RGO. In BFO-RGO nanocomposites the presence of large numbers of inner and outer surface interfaces causes the interfacial polarizations, which are associated with relaxation and contribute significantly to the dielectric loss.1, 9, 25 Debye dipolar relaxation greatly influences the permittivity behaviors of microwave absorbers. According to Debye dipolar relaxation,1, 3, 25, 86-87 the relative complex permittivity εr can be expressed by the following equation:

∞ = + ′′= ∞ +

(4)

Here, f is frequency, τ0 is polarization relaxation time, εs and ε∞ are the stationary permittivity and optical dielectric constant at high-frequency limit respectively. Equations (5) and (6) can be deduced from equation (4)

′ = ∞ + (

=

( ) ( )

(5)

)

()

+

(6)

From equation (5) and (6), the relationship between ε′ and ε″ can be expressed as ( ′ −

) +

( ) = (

)

(7)

Debye dipolar relaxation of microwave absorbers is generally determined from Cole-Cole semicircle plots of ε′ versus ε″. Each semicircle represents one Debye relaxation process. The Cole-Cole plots of pure BFO and BFO-RGO nanocomposites are shown in Figure 9. The presence of more than one semicircle for each sample suggests the existence of multiple Debye


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dipolar relaxation process.3 Under alternating electromagnetic radiation, the lags of induced charges originating from BFO-BFO, BFO-RGO, and RGO-RGO interfaces, which meet the externally applied field, lead to the relaxation and transform the electromagnetic energy to thermal energy.2-3 Furthermore, the lattice defects and functional groups in RGO also lead to self-doping.2-3 This induces additional carriers between the RGO and BFO interface and is also beneficial to Debye relaxation.

Figure 9 Typical Cole-Cole semicircles for (A) BFO, (B) 99BFO-1RGO, (C) 98BFO-2RGO, and (D) 97BFO-3RGO in the frequency range 8-12 GHz.

These factors are responsible for multiple relaxation processes of BFO-RGO nanocomposites. Thus, during this process, the EM wave is attenuated to a certain degree. Moreover, besides the Debye relaxation effect, the Maxwell-Wagner relaxation due to the accumulation of bound charges at the heterogeneous interfaces also causes to absorb EM waves.1-3, 25



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The magnetic loss is also an important factor that contributes to the EM wave attenuation in BFO-RGO nanocomposites. Figure 8(D) shows the changes of µ' values with varying RGO content in BFO-RGO composites. µ' values of pure BFO and BFO-RGO nanocomposites fall in the range of 1.059-0.990 (pure BFO), 1.050-1.007 (99BFO-1RGO), 1.030-1.002 (98BFO2RGO) and 1.030-1.02 (97BFO-3RGO). Similarly, µ'' values (Figure 8(E)) fall in the range of 0.044-0.0399 (pure BFO), 0.041-0.030 (99BFO-1RGO), 0.026-0.003 (98BFO-2RGO) and 0.038-0.025 (97BFO-3RGO). 97BFO-3RGO composite shows a broad resonance peak at 8.711.5 GHz with a maximum value at 10.75 GHz. Figure 8(F) shows the change of magnetic tangent loss (tan δµ = µ″/ µ‫ )׳‬with changing frequency for all the samples. tanδµ for pure BFO first declines in the frequency range 8.2-8.78 GHz, then remains constant with small fluctuations in the frequency range 8.78-11.21 GHz and finally increases in the frequency range of 11.2112.4 GHz. But, in the case of 97BFO-3RGO, tan δµ first declines following a similar trend as pure BFO in the frequency range 8.2-8.78 GHz, followed by a broad resonance peak at 8.78-11.4 GHz with a maximum value at 10.68 GHz, and then remains almost constant in the frequency range 11.4-12.4 GHz. Theoretically, the magnetic loss of magnetic materials is related to hysteresis loss, domain wall resonance, exchange resonance, eddy current loss, and natural resonance.3,

88

However,

hysteresis loss can be neglected when a weak field is applied.3 As the domain wall contribution towards magnetic loss usually occurs in the megahertz frequency range its contribution can also be neglected in the present case (8-12 GHz).3 Exchange resonance also makes very little contribution to magnetic loss in the high frequency range 8-12 GHz.16 Therefore, the contributing factors which should be considered for magnetic loss in the present case are eddy


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current loss and natural resonance in the microwave frequency band.89 The effect of eddy current loss can be estimated using the following equation:1-3, 33, 87 µ″≈ 2π µ0 (µ‫ )׳‬2 σ d 2f / 3

(8)

C0 = µ″ (µ‫)׳‬-2f -1= 2π µ0 d 2σ

(9)

where C0 is eddy current coefficient, µ0 is permeability in vacuum, d is the thickness of the absorber and σ is electrical conductivity of the composite. When the value of µ″ (µ‫)׳‬-2f -1does not change with increasing frequency then it can be assumed that the eddy current loss is contributing towards magnetic loss.1,

3

Another important factor which contributes to the

magnetic loss is the natural resonance and can be described by natural resonance equation (equation 10):1, 3, 26, 33 2 = !"

(10)

!" = 4|% |/3() *+

(11)

% = () *+ !, /2

(12)

where Ha is anisotropy energy, γ is gyromagnetic ratio, |% | is anisotropy coefficient, µ0 stands for permeability in free space (4π × 10-7 Hm-1), Hc is coercivity, Ms is saturation magnetization and fr is the resonance frequency. Figure 10 shows the change in eddy current coefficient (C0) with increasing frequency for all the samples. In the case of pure BFO, as the eddy current coefficient (C0) changes with increasing frequency, it can be assumed that the magnetic loss is not caused by eddy current effect. In case of BFO-RGO samples, it was observed that C0 remains almost constant in the frequency range of 11.5 to 12.4 GHz and this might originate from eddy current loss. Additionally, C0 values of the nanocomposites change in the frequency range from 8.2 to 11.5 GHz, and the resonance peaks are located at 10.90 GHz and 10.63 GHz for 98BFO-2RGO and



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97BFO-3RGO respectively. This might be the contribution from natural resonance phenomena.1, 3

Therefore, we can conclude that magnetic loss in case of BFO-RGO nanocomposite comes up with a mixed mechanism comprised of both eddy current loss and natural resonance phenomenon. Furthermore, from equation 10, it is also clear that natural resonance depends upon anisotropic energy (Ha) and higher anisotropic energy favors the microwave absorption properties at high frequency. But this enhancement in Ha can be achieved by lowering Ms of the absorber (equation 11).1,

3, 26

In the present case, the Ms value of BFO-RGO nanocomposite

decreases with the increasing RGO content in the composite and is lowest for 97BFO-3RGO (Figure 4). Thus, the anisotropic energy for 97BFO-3RGO nanocomposite is highest among all BFO-RGO nanocomposites. So it is expected that due to high anisotropic energy 97BFO-3RGO can show enhanced microwave absorption properties compare to pure BFO.

Figure 10 Plots of µ″ (µ')-2f-1 vs. frequency for the sample BFO, 99BFO-1RGO, 98BFO-2RGO, and 97BFO-3RGO in the frequency range 8-12 GHz.


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The frequency dependence of reflection loss (RL) values of pure BFO and BFO-RGO nanocomposites were estimated from the complex permittivity (εr= ε'- jε'') and permeability (µr= µ'- jµ'') using a single layered plane wave absorber model, proposed by Naito and Suetake.11, 90 -. = -) (( / ) / tan ℎ 34 5

ƒ6 ,

7 (( . ) / 9

A A

RL = 20 log @ABC A @ BC

(13) (14)

Where f is the microwave frequency, d is the absorber thickness, Z0 is the impedance of free space and Zin is the input impedance of absorber. The contribution of complex permittivity (εr= ε'- jε'') and permeability (µr= µ'- jµ'') in the light weight absorber can be expressed by quarter-wavelength matching model equation 16:3, 16, 24, 87

tm= nλ/4 = nc/4fm (εrµr)1/2

n = 1, 2, 3…

(15)

where λ, fm and tm are the wavelength of the materials, the frequency of complex permeability and complex permittivity and the absorber matching thickness at maximum microwave absorption. According to equation 15, absorbers with low thickness (tm) possess a high value of εr. As εr is dependent on ε' and ε'', and the values of ε' and ε'' are influenced by the composition of the composites, the thickness of the absorber can be adjusted by judiciously selecting the composition of the absorber. It is clear from equation 13 and 14 that, in addition to good permittivity and permeability values, excellent impedance matching is another key factor for improving microwave absorption.24, 86 The modulus of the normalized characteristic impedance Z = |Zin/Z0| has been calculated according to equations 13 and 14 (Figure 11(A)-(C)). It represents the ability of microwave to enter into the absorber and be converted to thermal energy or dissipated through interference. When the value of Z is equal or close to 1, it is beneficial for improving microwave 28 ACS Paragon Plus Environment


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absorption of the absorber.24, 87 As tm can be calculated from equation 15, a fitting curve was constructed by plotting tm

fit

(calculated from equation 15) versus frequency (Figure 11(A)).

Matching thickness can also be obtained directly from RL curves (Figure 11(C)), which can be denoted as tm exp. When tm exp values (as black dots) were placed in tm fit curve (red line), it was observed that tm fit and tm exp are in good agreement. In the case of 97BFO-3RGO fm was 10.68 GHz when tm was 1.55 mm. Here Z was found to be very close to 1. For pure BFO and BFORGO nanocomposites having various compositions, reflection loss (RL) values were calculated and shown in Figure 11(D) and (E). The important points are: (i) Pure BFO exhibited minimum RL of -45 dB (i.e. ~99.99%) at 10.78 GHz when the thickness was 4.5 mm (Figure 11(D)). Microwave absorption of the synthesized BFO nanowire in X-band region was found to be better than that of polycrystalline BiFeO3 nanoparticles,12, 37-40 which might be due to the 1-D nanowire like the structure of synthesized BFO. However, a

thickness of 4.5 mm is quite large as a light weight absorbing materials. Moreover, the effective band width was ~1 GHz which is also very low. When the thickness was lowered, the minimum RL values of BFO were decreased. When the thickness was 1.55 mm, minimum RL was -6.7 dB (~79 %) at 10.62 GHz (Figure 11(E)). (ii) With increasing RGO content in the composite minimum RL value was increased. (iii) Effective band width (i.e. RL < -10 dB) was found to be increased with increasing RGO content in the composite. (iv) The composite is having 97 wt% BFO and 3 wt% RGO (97BFO-3RGO) exhibited superior microwave absorption properties compare to pure BFO and other compositions of BFO-RGO nanocomposite. 97BFO-3RGO possessed highest value of minimum RL of -28.68 dB (i.e. 99.75% absorption) at 10.68 GHz when the thickness was 1.55 mm with effective band width in the 9.6-11.7 GHz range. To demonstrate the influence of thickness on microwave absorption properties, a 3D image map and


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a contour map of reflection loss in the frequency range 8-12 GHz for 97BFO-3RGO were plotted and are shown in Figure 11(F) and (G). It is important to note that 97BFO-3RGO exhibited minimum 99% microwave absorption (> -20.0 dB) for the thickness ranging from 1 to 2 mm at various frequencies covering the whole X-band region (8.2-12.4 GHz).

Figure 11 (A) Comparison of the calculated matching thickness (tm fit) under n=1 to the tm exp obtained from RL values of 97BFO-3RGO, (B) the modulus of the normalized impedance Z and (C) RL values of 97BFO-3RGO with different thickness. Frequency dependence minimum reflection loss values for synthesized (D) BFO with a variation of thickness (3-5 mm) and (E) minimum reflection loss values of BFO and BFO-RGO nanocomposites with varying RGO content. (F) Three dimensional and (G) Two-dimensional contour representations of frequency dependence of reflection loss values of 97BFO-3RGO nanocomposites with the variation of thickness (1-2 mm). Several factors are responsible for the enhanced microwave absorption property of BFORGO nanocomposites. Some of the important factors are (i) the existence of the residual defects 30 ACS Paragon Plus Environment


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and functional groups in RGO favor electromagnetic energy absorption, (ii) the high aspect ratio and high conductivity of RGO sheets provide better microwave absorption properties, (iii) the presence of interfaces between BFO-BFO, BFO-RGO, and RGO-RGO causes interfacial polarization and associated Debye relaxation which contribute to the dielectric loss of BFO-RGO nanocomposites. At the same time, eddy current loss and natural resonance loss also contribute to the magnetic loss of the BFO-RGO nanocomposites. These factors play important roles in making BFO-RGO nanocomposites excellent microwave absorbing materials. To provide a visual demonstration of microwave absorbing mechanism of BFO-RGO, a schematic diagram is presented in Scheme 2.

Scheme 2 Possible microwave absorbing mechanisms of the BFO-RGO nanocomposites.


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To the best of our knowledge, this is the first time microwave absorption properties of 1-D BiFeO3 nanowire-RGO nanocomposites have been reported and the composite having 97 wt% BFO and 3 wt% RGO exhibited excellent RL in X-band region. The method of preparation of various RGO-ferrite nanocomposites and their microwave absorption properties1-4, 7, 11, 19-22, 24, 26, 28-34

has been listed in Table S1 (ESI†). From Table S1 it is clear that the microwave absorption

properties of 97BFO-3RGO in X-band region are comparable and even superior to many RGOferrite nanocomposites. 4. Conclusion Here, preparation of BiFeO3 nanowire-Reduced graphene oxide (BFO-RGO) nanocomposites and their microwave absorption properties in X-band region have been described. To the best of our knowledge, this is the first time microwave absorption property of BiFeO3 nanowireReduced graphene oxide has been reported. The microwave absorption property of these nanocomposites has been improved with the increasing RGO content (up to 3 wt %) in the composites. The synergy of dielectric loss and magnetic loss along with multiple interfaces between RGO and BiFeO3 nanowires are mainly responsible for the enhanced microwave absorption property of BFO-RGO nanocomposites. First-Principles Calculations also verified the hybridization among C 2p states of graphene and O 2p, Fe 3d states of BiFeO3 in BFO-graphene nanocomposites. This hybridization enhanced the interfacial polarization in the composite, which improves the permittivity value of the nanocomposite. The microwave absorption properties of these composites have been explained in details considering various factors which influence the dielectric loss and magnetic loss of these materials. The 1-D nanowire structure of BiFeO3 greatly influences the microwave absorption properties of BFO-RGO nanocomposites. In the case of the nanocomposite containing 97 wt % BFO and 3 wt % RGO (97BFO-3RGO), proper



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impedance matching (Z close to 1) was observed. This composite exhibited minimum reflection loss of -28.68 dB (i.e. 99.75 % absorption) at 10.68 GHz with effective bandwidth 9.6-11.7 GHz with an absorber thickness of only 1.55 mm. Moreover, it also exhibited microwave absorption >-20 dB (99 % absorption) for the entire range of X-band (8.2-12.4 GHz) when the absorber thickness varied between 1 to 2 mm. BFO-RGO nanocomposites exhibit its potential as an efficient light weight microwave absorber in the X-band region, which is important for radar absorption applications. ASSOCIATED CONTENT Supporting Information Figure S1 Initial structure of (A) BFO unit cell, (B) Graphene superlattices and (C) BFO-RGO superlattices. The optimized structure of (D) BiFeO3 slab and (E) BFO-graphene superlattice after complete relaxation, Figure S2 FT-IR spectra of (A) GO, (B) RGO, (C) Pure BFO and (D) 97BFO-3RGO nanocomposite, Figure S3 Raman spectra of (A) GO, (B) Pure BFO, (C) pure RGO and (D) BFO–RGO nanocomposite. (E) Enlarged Raman spectra of BFO showing all the 13 Raman active phonon modes of pure BFO, Figure S4 TGA curve of (A) pure BFO, (B) BFORGO nanocomposite and (C) GO, Figure S5 (A) HRTEM image of BFO nanowire. SAED patterns of (B) BFO nanowire and (C) pure GO respectively, Figure S6 Room temperature wide angle power XRD pattern of the precipitate containing Fe(OH)3 and (Bi2O2)(OH)Cl phases. Table S1: Microwave absorption properties of various ferrites and ferrite based composites, Table S2: The sizes of the unit cells of simulated the systems, Table S3: Comparison of the optimized structural parameter of BiFeO3 unit cell and graphene superlattices with previously reported work. Details of the sample input files for geometric optimization of BFO unit cell,


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graphene, and BFO-RGO superlattices. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Corresponding author. Tel. /fax: +91 832 2580318/2557033. *E-mail address: [email protected] (N. N. Ghosh)

ACKNOWLEDGEMENTS Dr. N. N. Ghosh gratefully acknowledges financial support from DRDO, New Delhi, India (ERIP/ER/1305004/M/01/1523). Dr. Ghosh is thankful to Prof C.E. Branes and Dr. S Roy of Department of Chemistry, University of Tennessee Knoxville USA for their valuable suggestions during manuscript preparation. Dr. Ghosh is also thankful to Central Research Facility of BITS Pilani KK Birla Goa campus for providing FESEM facility. Reference (1) Jian, X.; Wu, B.; Wei, Y.; Dou, S. X.; Wang, X.; He, W.; Mahmood, N., Facile Synthesis of Fe3O4/GCs Composites and Their Enhanced Microwave Absorption Properties. ACS Appl. Mater. Interfaces 2016, 8, 6101-6109. (2) He, J. Z.; Wang, X. X.; Zhang, Y. L.; Cao, M. S., Small Magnetic Nanoparticles Decorating Reduced Graphene Oxides to Tune the Electromagnetic Attenuation Capacity. J. Mater. Chem. C 2016, 4, 7130-7140. (3) Liu, P.; Yao, Z.; Zhou, J.; Yang, Z.; Kong, L. B., Small Magnetic Co-Doped NiZn Ferrite/Graphene Nanocomposites and Their Dual-Region Microwave Absorption Performance. J. Mater. Chem. C 2016, 4, 9738-9749.



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(4) Yang, Z.; Wan, Y.; Xiong, G.; Li, D.; Li, Q.; Ma, C.; Guo, R.; Luo, H., Facile Synthesis of ZnFe2O4/Reduced Graphene Oxide Nanohybrids for Enhanced Microwave Absorption Properties. Mater. Res. Bull. 2015, 61, 292-297. (5) Organization, W. H., Establishing a Dialogue on Risks from Electromagnetic Fields; Geneva: World Health Organization, 2002. (6) Lin, J. C., Human Exposure to Rf, Microwave, and Millimeter-Wave Electromagnetic Radiation [Health Effects]. IEEE Microwave Magazine 2016, 17, 32-36. (7) Moitra, D.; Chandel, M.; Ghosh, B. K.; Jani, R. K.; Patra, M. K.; Vadera, S. R.; Ghosh, N. N., A Simple ‘In Situ’ Co-Precipitation Method for the Preparation of Multifunctional CoFe2O4Reduced Graphene Oxide Nanocomposites: Excellent Microwave Absorber and Highly Efficient Magnetically Separable Recyclable Photocatalyst for Dye Degradation. RSC Adv. 2016, 6, 76759-76772. (8) Wang, L.; Huang, Y.; Sun, X.; Huang, H.; Liu, P.; Zong, M.; Wang, Y., Synthesis and Microwave Absorption Enhancement of Graphene@Fe3O4@SiO2@NiO Nanosheet Hierarchical Structures. Nanoscale 2014, 6, 3157-3164. (9) Cao, J.; Fu, W.; Yang, H.; Yu, Q.; Zhang, Y.; Liu, S.; Sun, P.; Zhou, X.; Leng, Y.; Wang, S., Large-Scale Synthesis and Microwave Absorption Enhancement of Actinomorphic Tubular ZnO/CoFe2O4 Nanocomposites. J. Phys. Chem. B 2009, 113, 4642-4647. (10) Zhu, W.; Wang, L.; Zhao, R.; Ren, J.; Lu, G.; Wang, Y., Electromagnetic and MicrowaveAbsorbing Properties of Magnetic Nickel Ferrite Nanocrystals. Nanoscale 2011, 3, 2862-2864. (11) Moitra, D.; Ghosh, B.; Chandel, M.; Jani, R.; Patra, M.; Vadera, S.; Ghosh, N., Synthesis of a Ni0.8Zn0.2Fe2O4-RGO Nanocomposite: An Excellent Magnetically Separable Catalyst for Dye Degradation and Microwave Absorber. RSC Adv. 2016, 6, 14090-14096.


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