Impact of sonochemical synthesis condition on the structural and physical properties of MnFe2O4 spinel ferrite nanoparticles

Contents lists available at ScienceDirect Ultrasonics - Sonochemistry journal homepage: www.elsevier.com/locate/ultson Impact of sonochemical synthesis condition on the structural and physical properties of MnFe 2 O 4 spinel ferrite nanoparticles Raghvendra Singh Yadav a,⁎ , Ivo Kuřitka a , Jarmila Vilcakova a , Thaiskang Jamatia a , Michal Machovsky a , David Skoda a , Pavel Urbánek a , Milan Masař a , Michal Urbánek a , Lukas Kalina b , Jaromir Havlica b a Centre of Polymer Systems, University Institute, Tomas Bata University in Zlín, Trida Tomase Bati 5678, 760 01 Zlín, Czech Republic b Materials Research Centre, Brno University of Technology, Purkyňova 464/118, 61200 Brno, Czech Republic ARTICLE INFO Keywords: Spinel ferrite Nanoparticles Sonochemical synthesis Magnetic property Electrical property ABSTRACT Herein, we report sonochemical synthesis of MnFe 2 O 4 spinel ferrite nanoparticles using UZ SONOPULS HD 2070 Ultrasonic homogenizer (frequency: 20 kHz and power: 70 W). The sonication time and percentage amplitude of ultrasonic power input cause appreciable changes in the structural, cation distribution and physical properties of MnFe 2 O 4 nanoparticles. The average crystallite size of synthesized MnFe 2 O 4 nanoparticles was increased with increase of sonication time and percentage amplitude of ultrasonic power input. The occupational formula by Xray photoelectron spectroscopy for prepared spinel ferrite nanoparticles was (Mn 0.29 Fe 0.42 )[Mn 0.71 Fe 1.58 ]O 4 and (Mn 0.28 Fe 0.54 ) [Mn 0.72 Fe 1.46 ]O 4 at sonication time 20 min and 80 min, respectively. The value of the saturation magnetization was increased from 1.9 emu/g to 52.5 emu/g with increase of sonication time 20 min to 80 min at constant 50% amplitude of ultrasonic power input, whereas, it was increased from 30.2 emu/g to 59.4 emu/g with increase of the percentage amplitude of ultrasonic power input at constant sonication time 60 min. The highest value of dielectric constant (ε′) was 499 at 1 kHz for nanoparticles at sonication time 20 min, whereas, ac conductivity was 368 × 10 −9 S/cm at 1 kHz for spinel ferrite nanoparticles at sonication time 20 min. The demonstrated controllable physical characteristics over sonication time and percentage amplitude of ultrasonic power input are a key step to design spinel ferrite material of desired properties for specific application. The investigation of microwave operating frequency suggest that these prepared spinel ferrite nanoparticles are potential candidate for fabrication of devices at high frequency applications. 1. Introduction Recently, nanoparticles of spinel ferrite have received a considerable attention among researchers due to its widespread technological applications such as magnetic resonance imaging (MRI) contrast agent, drug-delivery, magnetically recoverable efficient photo-catalyst, ferrofluids, gas sensor, hyperthermia cancer treatment, magnetic refrigeration (MR), anode material for Li-ion battery, magnetic recording media with higher storage density, microwave devices, spintronic devices, super-capacitors, paint industry, and water splitting for hydrogen production, etc. [1–5]. In addition, the magnetic and dielectric characteristics of spinel ferrite nanoparticles make them very attractive functional material. In spinel ferrite with general formula AFe 2 O 4 (A = Mn 2+ ,Ni 2+ ,Co 2+ , etc.), there is a cubic close-packed oxygen lattice with metal ions inhabiting 1/8 of the tetrahedral and 1/2 of the octahedral voids. Therefore, the number of tetrahedrally coordinated ionic types in the system (8 per unit cell) is half of the octahedrally coordinated (16 per unit cell). The system in which A 2+ and Fe 3+ ions fully sited on the tetrahedral and octahedral site, respectively, is normal spinel system, whereas, the A 2+ ions occupied the octahedral sites with half of the Fe 3+ ions located on tetrahedral sites is inverse spinel. In partial spinel structure, a fraction of A 2+ ions, known as inversion degree, located on the octahedral site. In the bulk, MnFe 2 O 4 is partially inverse, NiFe 2 O 4 and CoFe 2 O 4 are inverse, and ZnFe 2 O 4 is a normal spinel [6]. However, the spinel ferrite nanoparticles exhibit high level of cation inversion [7]. B. Nandan et al. [8] reported the cation inversion in Co 2+ doped NiFe 2 O 4 nanoparticles. M. Fantauzzi et al. [9] noticed the cation inversion in CoFe 2 O 4 spinel ferrite nanoparticles. S. K. Gore et al. [10] observed the cation inversion in Bi 3+ doped CoFe 2 O 4 nanoparticles. For technological applications, the performance of spinel ferrite can be regulated by particle size, microstructure, cation distribution at https://doi.org/10.1016/j.ultsonch.2019.104839 Received 14 August 2019; Received in revised form 18 October 2019; Accepted 21 October 2019 ⁎ Corresponding author. E-mail address: [email protected] (R. Singh Yadav). Ultrasonics - Sonochemistry 61 (2020) 104839 Available online 23 October 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved. T octahedral and tetrahedral sites, etc [11]. A research group, E. Ranjith Kumar et al. [12] investigated particle size dependent physical properties of spinel ferrite nanoparticles. Another research group, L. Zhang et al. [13] noticed the dependence of physical characteristics on the microstructure of spinel ferrite nanoparticles. M. Abbas et al. [14] observed the size-controlled physical properties of cobalt ferrite nanostructures. Further, L. I. Granone et al. [15] noticed the influence of degree of inversion in ZnFe 2 O 4 spinel ferrite on the electrical conductivity. V. G. Harrisa et al. [16] reported the effect of cation inversion in spinel ferrite nanoparticles on the physical properties. R. Lamouri, et al. [17] investigated the impact of cation inversion on the magnetic properties of spinel ferrite. Furthermore, L. I. Granone et al. [18] reported the impact of the degree of inversion in spinel ferrite on optical characteristics. In addition, L. I. Granone et al. [19] observed the influence of the cation redistribution on the photoelectrochemical activity. A controllable nucleation and growth of nanoparticles during chemical synthesis can provide desired physical properties for specific application. Spinel ferrite nanoparticles have been synthesized by several chemical synthesis approach such as hydrothermal method, coprecipitation method, microemulsion method, sol-gel method, sonochemical method, solution combustion method, etc. [20,21]. A research group, P. Dolcet et al. [7] noticed the various level of inversion in spinel ferrite nanoparticles by wet chemical synthesis. In addition, S.I. El-Dek et al. [22] observed the influence of different chemical synthesis method on the physical properties of spinel ferrite. Among chemical synthesis approaches, the sonochemical synthesis method is one of the chemical synthesis approach for the preparation of spinel ferrite nanoparticles with controllable particle size, morphology and its physical characteristics [23,24]. In sonochemical synthesis method, the chemical impact of ultrasonic irradiation arises from acoustic cavitation, i.e., the rapid formation, growth and implosive collapse of consecutive generated bubbles in liquid medium [25,26]. Further, the collapse of bubble provides the creation of an instantaneously high thermal energy and pressure pulse within a very few time. These conditions provide formation of noble nanostructures [27,28]. The significant advantage of this sonochemical synthesis approach is the cost-effectiveness, high reaction rate, controllable synthesis condition, high purity, narrow size distribution, environmentally friendly nature and scalability, etc. [29,30]. These advantages make this synthesis technique as potential alternative than other chemical synthesis approaches [31,32]. In our previous report, we investigated the impact of Gd 3+ substitution in CoFe 2 O 4 nanoparticles synthesized by sonochemical method on its physical properties [33]. Other research group, P. Balasubramanian et al. [34] reported the sonochemical synthesis of calcium ferrite and its application as electrochemical sensor. Further, N. Lenin et al. [35] observed the impact of neodymium ion in NiFe 2 O 4 spinel ferrite nanoparticles prepared by sonochemical method on its structural, dielectric and magnetic properties. Furthermore, S. R. Yousefi et al. [36] investigated the impact of sonication time and power in sonochemical preparation of ZnFe 2 O 4 nanostructures and further studied its photocatalytic activity. In addition, M.A. Almessiere et al. [37,38] reported the impact of Gd 3+ and Tm 3+ ion in cobalt ferrite nanoparticles synthesized by sonochemical method on its structural, optical and magnetic properties. In the present work, we report the preparation of MnFe 2 O 4 spinel ferrite nanoparticles by sonochemical synthesis approach via. controllable sonochemical synthesis time and percentage amplitude of ultrasonic power input. The structural evolution of MnFe 2 O 4 nanoparticles over sonochemical synthesis conditions have been investigated in details. In addition, physical characteristics such as magnetic, optical, dielectric, electrical, impedance, modulus spectroscopy, and operating microwave frequency characteristics have been investigated in-depth. 2. Experimental section 2.1. Materials The reagents Iron Nitrate [Fe(NO 3 ) 3 ·9H 2 O], Manganese nitrate [Mn (NO 3 ) 2 ·4H 2 O,] and sodium hydroxide (NaOH) were purchased from Alfa Aesar GmbH & Co KG, Germany. 2.2. Preparation In the sonochemical synthesis of MnFe 2 O 4 spinel ferrite nanoparticles, the sonication time was varied as 20 min, 40 min, 60 min and 80 min. For preparation, 2.6 g of Mn(NO 3 ) 2 ·4H 2 O and 7.54 g of Fe (NO 3 ) 2 ·9H 2 O was added in a beaker containing 60 ml of deionised water, and stirred on a magnetic stirrer at room temperature for 15 min. A base solution was prepared in another beaker by adding 4 g of NaOH in 60 ml deionised water. This solution was then poured into the beaker containing the metal precursors. The stirring was continued at room temperature and then thick precipitates was observed immediately after the addition of base solution. The reaction mixture was then exposed to high-intensity ultrasonic irradiation (frequency: 20 kHz and power: 70 W) (Ultrasonic homogenizer UZ SONOPULS HD 2070) for 20 min. After the completion of the sonication for 20 min, the precipitate was centrifuged, and washed with deionised water three-four times and dried in oven at 40 °C. Following above procedures, other samples were prepared for 40 min, 60 min, and 80 min of sonication time. These prepared nanoparticles were named as MF20, MF40, MF60, and MF80 corresponding to sonication time 20 min, 40 min, 60 min, and 80 min, respectively. The above samples were synthesized by using 50% amplitude of the ultrasonic power input. Further, MnFe 2 O 4 spinel ferrite nanoparticles were synthesized by this sonochemical method at different percentage amplitude of ultrasonic power input. The percentage (%) amplitude of ultrasonic power input was 75% and 100% for a constant time 60 min and these synthesized samples were named as MFP75 and MF-P100, respectively. During sonochemical synthesis, water decomposes to provide hydrogen and hydroxyl radicals [39]. These created hydrogen radicals may combine to produce hydrogen and also the hydroxyl radicals may combine to provide hydrogen peroxide. Further, iron nitrate and manganese nitrate hydrolyse to provide their corresponding hydroxides [40]. Furthermore, there is formation of oxides by initiation of oxidation of their corresponding hydroxides under ultrasonic cavitation. In addition, all formed oxides Fe 3 O 4 and Mn 3 O 4 due to ultrasound cavitation oxidizes to form MnFe 2 O 4 spinel ferrite [41]. 2.3. Characterization The crystal phase formation of MnFe 2 O 4 nanoparticles at different sonication time was investigated by using Rigaku MiniFlex 600 X-ray Spectrometer. The Raman modes related to vibration of metal ionoxygen ion at octahedral site and tetrahedral site in MnFe 2 O 4 at different sonication time were investigated by Raman microscope Nicolet DXR. The absorption band associated to vibration of metal ions at octahedral and tetrahedral sites in MnFe 2 O 4 nanoparticles at different sonication time were studied by using FTIR spectrometer Nicolet 6700 (Thermo Scientific). The morphology of the particles of MnFe 2 O 4 spinel ferrite at different sonication time was investigated by the field emission scanning electron microscopy (FE-SEM) and elemental analysis was done by energy dispersive X-ray spectroscopy (EDX) by scanning electron microscope Nova NanoSEM450 (FEI company). In addition, the morphology and lattice fringes of MnFe 2 O 4 nanoparticles at different sonication time was examined by the high-resolution transmission electron microscope (JEOL JEM 2100). The oxidation state and cation distribution of prepared MnFe 2 O 4 nanoparticles at different sonication time was studied by X-ray photoelectron spectroscopy (Kratos Analytical Axis Ultra DLD). The magnetic hysteresis curves of R. Singh Yadav, et al. Ultrasonics - Sonochemistry 61 (2020) 104839 2 synthesized MnFe 2 O 4 spinel ferrite nanoparticles were measured by using a vibrating sample magnetometer (VSM 7407, Lake Shore). The dielectric and electrical characteristics of prepared MnFe 2 O 4 nanoparticles were examined by using a Broadband Dielectric Impedance Analyzer Concept 40 (Novocontrol, Germany). In addition, the complex impedance characteristics of these prepared MnFe 2 O 4 nanoparticles were studied by a standard sample cell BDS 1200 employing RC model. The specific surface area was calculated based upon the multipoint Brunauer-Emmet-Teller analysis of nitrogen adsorption/desorption isotherms at 77 K recorded by Belsorp-mini II (BEL Japan, Inc.). Samples were outgassed for 3 h at 80 °C prior to the measurements. Optical property was investigated by using diffuse reflectance spectroscopy (Lambda 1050 UV/VIS/NIR spectrometer from Perkin Elmer). 3. Result and discussion 3.1. X-ray diffraction study X-ray diffraction technique is utilized to investigate the structural formation and evolution of sonochemically prepared MnFe 2 O 4 nanoparticles with increase of sonication time and percentage amplitude of ultrasonic power input. Fig. 1 depicts the X-ray Diffraction pattern of prepared MnFe 2 O 4 nanoparticles by sonochemical synthesis method. The prepared MnFe 2 O 4 nanoparticles exhibited a single phase cubic spinel structure. The observed diffraction peak in prepared MnFe 2 O 4 nanoparticles can be index to (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) corresponding to plane of cubic unit cell of the cubic spinel structure [42,43]. The observed broadening in the Xray diffraction peak of prepared MnFe 2 O 4 nanoparticles at lower sonication time indicate the nanocrystalline nature of the prepared MnFe 2 O 4 nanoparticles by sonochemical method. The diffraction peak intensity was increased with increase of sonication time and percentage amplitude of ultrasonic power input, which indicate the increase of crystallinity and crystallite size. The observed lower diffraction intensity with broad diffraction peak of amorphous like/nano-crystalline nature of prepared spinel ferrite at low sonication time attribute that there was less nucleus activation energy to form highly crystalline spinel ferrite nanoparticles at lower sonication time [44]. The average crystallite size of prepared MnFe 2 O 4 spinel ferrite nanoparticles was evaluated by using the Scherrer formula: = d kλ βcosθ where k is 0.9, λis the X-ray wavelength, βis the full width at half maximum of the (3 1 1) diffraction peak of MnFe 2 O 4 spinel ferrite nanoparticles, and θis the diffraction angle. The average crystallite sizes of prepared nanoparticles were noticed to increase from 1.8 nm to 22.1 nm, with increase of sonication time 20 min to 80 min at constant 50% amplitude of ultrasonic power input, as tabulated in Table 1. However, the crystallite size was increased from 19.3 nm to 25.5 nm with increase of percentage of amplitude of ultrasonic power input at constant sonication time 60 min. Further, the lattice parameter of sonochemically synthesized MnFe 2 O 4 spinel ferrite nanoparticles can be determined by using following relation: =++ aλh k l Sinθ () 2 2221/2 The calculated values of lattice parameter for prepared MnFe 2 O 4 nanoparticles are tabulated in Table 1. It can be noticed that the lattice parameter was increased with increase of sonication time and percentage amplitude of ultrasonic power input. The lattice parameter of sonochemically prepared MnFe 2 O 4 nanoparticles was observed to increase from 8.2685 Å to 8.4831 Å with increase of sonication time 20 min to 80 min at constant 50% amplitude of ultrasonic power input. However, the lattice parameter was increased from 8.4551 Å to 8.4972 Å with increase of percentage of amplitude of ultrasonic power input at constant sonication time 60 min. The increase in lattice constant with sonication time and percentage amplitude of power input obeys Vegard’s law [45]. In spinel ferrite structure, the lattice constant has correlation with microstructure, surface defects, thermal triggered ordering/reordering of cations, valence states and chemical bonds in this crystal structure [46]. In this work, the increase in lattice constant is associated with variation of microstructure, surface defects and ultrasonic activated ordering/reordering of cations in spinel ferrite crystal. R.D. Raland et al. [47] noticed the increase of lattice parameter from 8.422 Å to 8.459 Å with increase of crystallite size from 19 nm to 32 nm of MnFe 2 O 4 nanoparticles by co-precipitation method. The X-ray density (ρ X-ray ) of synthesized MnFe 2 O 4 spinel ferrite nanoparticles at different sonication time can be determined by utilizing the following relation: = − ρ ZM Na Xray A3 where, Z is the number of molecules per unit cell, M is the molecular weight of MnFe 2 O 4 spinel ferrite, N A is the Avogadro’s number and ‘a’is the lattice parameter of MnFe 2 O 4 spinel ferrite nanoparticles prepared at different sonication time. The evaluated values of X-ray density of the prepared MnFe 2 O 4 spinel ferrite nanoparticles at different sonication time are tabulated in Table 1. It was noticed that the X-ray density of sonochemically prepared MnFe 2 O 4 nanoparticles was decreased with increase of sonication time and percentage amplitude of ultrasonic Fig. 1. XRD pattern of MnFe 2 O 4 spinel ferrite nanoparticles by sonochemical method. R. Singh Yadav, et al. Ultrasonics - Sonochemistry 61 (2020) 104839 3 power input. X-ray density was decreased from 5.419 g/cm 3 to 5.018 g/ cm 3 with increase of sonication time from 20 min to 80 min at constant 50% amplitude of ultrasonic power input. However, the X-ray density was decreased from 5.068 g/cm 3 to 4.993 g/cm 3 with increase of percentage of amplitude of ultrasonic power input at constant sonication time 60 min. Kurnia et al. [48] observed the decrease of X-ray density from 5.26 g/cm 3 to 5.04 g/cm 3 with increase of crystallite size 13.1 nm to 18.4 nm for MnFe 2 O 4 nanoparticles synthesized by co-precipitation method. Further, the structural parameters such as the tetrahedral and octahedral site radii (r A and r B ), hopping length in tetrahedral and octahedral site (d A and d B ), tetrahedral and octahedral bond lengths (d Ax and d Bx ), tetrahedral edge, shared and unshared octahedral edges (d AxE , d BxE and d BxEU ) for sonochemically prepared MnFe 2 O 4 nanoparticles can be determined by using following equation [49]: =⎛ ⎝−⎞ ⎠−ru a R 1 43 Ao =⎛ ⎝−⎞ ⎠−ruaR 5 8 B o = d a 1 43 A = d a 1 42 B =⎛ ⎝− ⎞ ⎠ d au31 4 Ax =⎡ ⎣−⎛ ⎝⎞ ⎠+⎛ ⎝⎞ ⎠⎤ ⎦ d au u311 4 43 64 Bx 2 1/2 =⎛ ⎝− ⎞ ⎠ d au22 1 2 AxE =− d au2(1 2 ) BxE =⎡ ⎣−+ ⎛ ⎝⎞ ⎠⎤ ⎦ d au u43 11 16 BxEU 2 1/2 where, a is the lattice parameter of sonochemically prepared MnFe 2 O 4 nanoparticles, u is the oxygen positional parameter (u = 0.384 Å), and R o (=1.32 Å) is the radius of the oxygen. The calculated values of structural parameters for MnFe 2 O 4 spinel ferrite nanoparticles are tabulated in Tables 1 and 2. The increase of hopping length with increase of sonication time and percentage amplitude of ultrasonic power input was observed. It implies that the distance between the magnetic ions in sonochemically prepared MnFe 2 O 4 nanoparticles increases with increase of sonication time and percentage of amplitude of ultrasonic power input. It is related with increase of crystallite size and cation redistribution in MnFe 2 O 4 nanoparticles with increase of sonication time and power. The observed change in structural characteristics triggered by sonication time and power can lead the change in physical characteristics of sonochemically prepared MnFe 2 O 4 spinel ferrite nanoparticles. Table 1 Crystallite size, lattice parameter, X-ray density and ionic radii for MnFe 2 O 4 spinel ferrite nanoparticles synthesized by sonochemical method. Sample Sonication time (min) %Amplitude of Power Input Crystallite size D (nm) Lattice Parameter a (Å) X-ray density dx (g/cm 3 ) Ionic Radii r A (Å) r B (Å) MF20 20 50 1.8 8.2685 5.419 0.5990 0.6727 MF40 40 50 2.0 8.3494 5.263 0.6177 0.6923 MF60 60 50 19.3 8.4551 5.068 0.6423 0.7176 MF80 80 50 22.1 8.4831 5.018 0.6488 0.7144 MF-P75 60 75 22.8 8.4878 5.010 0.6499 0.7255 MF-P100 60 100 25.5 8.4972 4.993 0.6521 0.7278 Table 2 Hopping length, tetrahedral bond, octahedral bond, tetrahedral edge, octahedral edge (shared and unshared) for MnFe 2 O 4 spinel ferrite nanoparticles synthesized by sonochemical method. Sample Hopping Length Tet. Bond d Ax (Å) Oct. Bond d Bx (Å) Tet. Edge d AxE (Å) Oct. Edge (Shared & Unshared) d A (Å) d B (Å) d BxE (Å) d BxEU (Å) MF20 3.5803 2.9229 1.9190 1.9954 3.1334 2.7125 2.9271 MF40 3.6153 2.9515 1.9378 2.0149 3.1640 2.7390 2.9557 MF60 3.6611 2.9888 1.9623 2.0404 3.2041 2.7737 2.9931 MF80 3.6732 2.9987 1.9688 2.0472 3.2147 2.7829 3.0031 MF-P75 3.6752 3.0004 1.9699 2.0483 3.2164 2.7844 3.0047 MF-P100 3.6793 3.0037 1.9721 2.0506 3.2200 2.7874 3.0081 Fig. 2. (a) TEM image of MF20, (b) TEM image of MF40, (c) TEM image of MF60, (d) HRTEM of MF60, (e) TEM image of MF80, and (f) HRTEM of MF80. R. Singh Yadav, et al. Ultrasonics - Sonochemistry 61 (2020) 104839 4 3.2. TEM and HRTEM study Fig. 2 (a & b) shows the TEM image of MF 20 and MF40 sample, which attribute smaller sized spherical nanoparticles of 2–4 nm. Fig. 2(c) depicts TEM image of MF60 sample, which indicate that sample consist of 2–5 nm sized particles with spherical morphology. Fig. 2(d) shows the HRTEM image of MF60 sample, which indicate lattice fringes with inter-planer distance 0.30 nm and 0.25 nm corresponding to plane (2 2 0) and (3 1 1), respectively, for MnFe 2 O 4 spinel ferrite crystal structure. Fig. 2(e) illustrates the TEM image of MF80 sample, indicating that the sample has spherical nanoparticles of size 50–80 nm, which is also assembly of nanoparticles of 5–10 nm. Fig. 2(f) represents typical HRTEM image of MF80 sample, which is consisted of well resolved two-dimensional lattice fringes with spacing of 0.48 nm, 0.25 nm, and 0.30 nm corresponding to lattice plane (1 1 1), (3 1 1) and (2 2 0), respectively for MnFe 2 O 4 spinel ferrite system. 3.3. FE-SEM and EDX study Fig. 3 (a & b) depict the FE-SEM image of MF20 and MF40 samples, which indicates nanocrystalline spinel ferrite of size 2–4 nm. Fig. 3(c) depicts the FE-SEM image of MF60 sample, indicating spherical nanoparticles of 2–5 nm, which is consistent with the TEM image. Further, Fig. 3(d) illustrates the FE-SEM image of MF80 sample, it exhibited spherical nanoparticles of 50–80 nm, which is assembly of nanoparticles of 5–10 nm. Therefore, there was agglomeration of smaller size nanoparticles with increase of sonication time, and further bigger nanoparticles was formed. Fig. 3 (e & f) depict the FE-SEM image of MF-P75 and MF-P100 sample, which indicate increase of size of spherical nanoparticles with increase of the percentage amplitude of ultrasonic power input. The inset in Fig. 3(c-d) is EDX pattern, indicating elemental presence of Mn, Fe and Oxygen in prepared spinel ferrite nanoparticles. 3.4. Raman spectroscopy Fig. 4 depicts the Raman spectra of sonochemically prepared MnFe 2 O 4 nanoparticles at different sonication time 20 min, 40 min, 60 min, and 80 min. Theoretical group analysis predicts the five Raman active mode, A 1g +E g +3T 2g for spinel ferrite crystal structure [50].It can be noticed from Fig. 4 that the Raman spectrum of prepared spinel ferrite nanoparticles exhibited all the characteristics Raman bands, i.e., E g mode (263 cm −1 ), T 2g mode (216 cm −1 , 368 cm −1 , 580 cm −1 ) and A 1g mode (610 cm −1 , 666 cm −1 ), which confirm spinel ferrite structure formation of MnFe 2 O 4 by sonochemical synthesis [51]. In addition, the Raman intensity increases with sonication time, indicating improvement in the crystallinity and crystallite size with increase of sonication time. Fig. 3. FE-SEM image of (a) MF20, (b) MF40, (c) MF60, (d) MF80, (e) MF-P75, and (f) MF-P100. EDX spectrum is shown as inset. R. Singh Yadav, et al. Ultrasonics - Sonochemistry 61 (2020) 104839 5 3.5. FTIR spectroscopy Fig. 5 depicts the FTIR spectra of MnFe 2 O 4 nanoparticles synthesized at different sonication time 20 min, 40 min, 60 min and 80 min. It can be seen in Fig. 5 that the FTIR spectra consist of two absorption bands, first at high wavenumber side 500–600 cm −1 and second at low wavenumber side 200–350 cm −1 . The existence of these two absorption bands confirm the spinel ferrite crystal structure formation of MnFe 2 O 4 nanoparticles by sonochemical synthesis method at different sonication time. The absorption band at high wavenumber side 500–600 cm −1 corresponds to intrinsic stretching vibration of metals at tetrahedral sites, however the absorption band at low wavenumber side 200–350 cm −1 is associated with octahedral-metal stretching. The observed two absorption band positions in tetrahedral and octahedral complexes of MnFe 2 O 4 nanoparticles is associated with the different distances between Fe 3+ –O 2− at the octahedral and tetrahedral sites [52]. Further, the shift in vibrational band position is associated with the increase of crystallite size and cation redistribution with increase of sonication time [53]. 3.6. X-ray photoelectron spectroscopy Fig. 6(a) depicts the XPS survey spectrum of MF80 sample, which represent the existence of Fe 2p, Mn 2p and O 1 s peaks at binding energy 710.9 eV, 641.6 eV and 528.5 eV, respectively. The deconvoluted high resolution XPS spectra of Fe 2p for MF20, MF40, MF60, and MF80 sample is shown in Fig. 6(b), (c), (d) & (e), respectively. Fig. 6(be) displays the presence of Fe 2p 3/2 and Fe 2p 1/2 , with satellite peak for Fe 2p 3/2 , confirming the Fe 3+ oxidation state in MnFe 2 O 4 nanoparticles synthesized by sonochemical method at different sonication time [54]. Fig. 7 (a-d) illustrates the deconvoluted high resolution XPS spectra of Mn 2p and indicate existence of Mn 2+ oxidation state in prepared spinel ferrite system [55]. The Fe region as shown in Fig. 6 (b-e) and Mn region as displayed in Fig. 7(a-d) were further fitted and analyzed for cation distribution at octahedral and tetrahedral site in MnFe 2 O 4 nanoparticles prepared at sonication time 20 min, 40 min, 60 min, and 80 min. Table 3 displays evaluated cation distribution of Fe and Mn at octahedral and tetrahedral site in MnFe 2 O 4 nanoparticles synthesized by sonochemical synthesis technique at different sonication time 20 min, 40 min, 60 min, and 80 min. It can be noticed that the Fe cation at tetrahedral site was increased, whereas at octahedral site it was decreased with the increase of sonication time. In case of Mn cation, it was altered randomly at octahedral and tetrahedral site. The occupational formula by using X-ray photoelectron spectroscopy for prepared spinel ferrite nanoparticles was (Mn 0.29 Fe 0.42 ) [Mn 0.71 Fe 1.58 ]O 4 , (Mn 0.34 Fe 0.48 ) [Mn 0.68 Fe 1.48 ]O 4 , (Mn 0.32 Fe 0.52 ) [Mn 0.68 Fe 1.48 ]O 4 and (Mn 0.28 Fe 0.54 ) [Mn 0.72 Fe 1.46 ]O 4 at sonication time 20 min, 40 min, 60 min and 80 min, respectively. 3.7. BET specific area analysis Fig. 8(a) depicts N 2 adsorption-desorption isotherm curves of synthesized MnFe 2 O 4 nanoparticles measured at 77 K. The specific surface area of these nanoparticles were evaluated by Brunauer-Emmett-Teller (BET) method [56]. The BET specific surface area were 253.6 m 2 /g, 307.4 m 2 /g, 162.3 m 2 /g, 66.5 m 2 /g, and their pore volume were 0.21 cm 3 /g, 0.28 cm 3 /g, 0.19 cm 3 /g and 0.24 cm 3 /g for MF20, MF40, MF60 and MF80 sample, respectively. Further, the pore size distribution of these synthesized nanoparticles were evaluated by BarrettJoyner-Halenda (BJH) method [57], as shown in Fig. 8(b). These porous material is associated with the agglomerates of nanoparticles with approximately uniform spheres. This uniformity of synthesized MnFe 2 O 4 nanoparticles was decreased at higher sonication time. Furthermore, the average size (d) can be evaluated by using following relation: =∗ d nm S ρ()(6/ )10 BET 3 , where S BET stands for specific surface area, ρis the density (4.98 g/cm 3 for MnFe 2 O 4 ). The evaluated average particle size by BET specific surface area was 4.75 nm, 3.91 nm, 7.42 nm, and 18.11 nm for MF20, MF40, MF60, and MF80, respectively. 3.8. Magnetic property Fig. 9 displays the magnetic hysteresis curves of MnFe 2 O 4 nanoparticles synthesized by sonochemical method. It is noticeable from Fig. 9 that sonochemically synthesized MnFe 2 O 4 nanoparticles at different sonication time exhibited ferromagnetic characteristics. The saturation magnetization (M s ), remanent magnetization (M r ) and coercivity (H c ) of MnFe 2 O 4 nanoparticles were evaluated from the magnetic hysteresis curves and mentioned in Table 4. The value of saturation magnetization was increased from 1.9 emu/g to 52.5 emu/g with increase of sonication time 20 min to 80 min at constant 50% amplitude of ultrasonic power input. However, the value of the saturation magnetization was increased from 30.2 emu/g to 59.4 emu/g with increase of the percentage amplitude of ultrasonic power input at constant sonication time 60 min. The increase in saturation magnetization with increase of sonication time and percentage amplitude of ultrasonic Fig. 4. Raman spectrum of sonochemically synthesized MnFe 2 O 4 nanoparticles at sonication time 20 min, 40 min, 60 min, and 80 min. Fig. 5. FTIR spectrum of sonochemically synthesized MnFe 2 O 4 nanoparticles at sonication time 20 min, 40 min, 60 min, and 80 min. R. Singh Yadav, et al. Ultrasonics - Sonochemistry 61 (2020) 104839 6 power input is associated with the larger particle size and high degree of crystallinity resulting in negligible surface spin canting [58]. The highest value of coercivity (H c ) was observed 45.0 Oe for MF20 sample and it was 24.8 Oe for MF-P100 sample. The coercivity of sonochemically synthesized MnFe 2 O 4 nanoparticles was decreased with increase of sonication time and percentage amplitude of ultrasonic power input. In general, the coercivity of the spinel ferrite nanoparticles is governed by on the magneto-crystalline anisotropy, strain, inter-particle interaction, grain size, and morphology, etc. [59]. Further, the coercivity is associated with anisotropy constant K by following equation [60]: =∗ HK M 0.96 c S where K is the anisotropy constant and M s is the saturation magnetization. The evaluated value of anisotropic constant for MnFe 2 O 4 spinel ferrite nanoparticles is mentioned in Table 4. Furthermore, the magnetic moment (η B ) observed per unit formula in the Bohr Magneton (μ B ) for sonochemically prepared MnFe 2 O 4 spinel ferrite nanoparticles can be determined by using following relation: =∗ η MM 5585 B s where M is the molecular weight and M s is the saturation magnetization of sonochemically prepared MnFe 2 O 4 nanoparticles. The evaluated value of magnetic moment of prepared MnFe 2 O 4 nanoparticles is mentioned in Table 4. The highest value of magnetic moment was 2.45 μ B for MF-P100 sample. In addition, a comparative property of MnFe 2 O 4 nanoparticles with same material by other researchers is provided in Table 5, in order to understand the advantage of sonochemical synthesis method. Fig. 6. (a) Survey XPS spectrum of MF80 sample, and high resolution XPS spectrum of Fe 2p of (b) MF20, (c) MF40, (d) MF60, (e) MF80. R. Singh Yadav, et al. Ultrasonics - Sonochemistry 61 (2020) 104839 7 3.9. Optical property The optical property of prepared MnFe 2 O 4 spinel ferrite nanoparticles were analysed by diffuse reflectance spectroscopy. The Kubelka-Munk Theory was utilized to evaluate band gap of prepared nanoparticles. The Kubelka-Munk equation can be expressed by the following relation [65]: ==− FR α S R R () (1 ) 2 2 where F(R) is the Kubelka-Munk function, αis the absorption coefficient, S is the scattering factor, and R is the reflectance. Kubelka-Munk function is utilized to get equivalent absorption coefficient from the diffuse reflectance. Therefore, the modified Tauc’s relation cab be expressed as [66,67]: =−FRhν Ahν E() ( ) gn where, E g is the energy gap, A is the proportionality constant, n is the transition coefficient which can be considered as ½ for direct allowed transition. Fig. 10 depicts the modified Tauc’s plots [F(R)E] 2 versus E (=hν), and the band gap can be evaluated by extrapolating the slop, where [F(R) E] 2 →0. The determined value of band gap for MnFe 2 O 4 nanoparticles was found 1.65 eV, 1.62 eV, 1.60 eV and 1.53 eV for MF20, MF40, MF60, MF80 sample, respectively. Several authors reported the band gap analysis by diffuse reflectance spectroscopy. S. Manzoor et.al. [68] reported the calculation of band gap of LaFeO 3 by Kubelka-Munk function. A. Baykal et al. [69] reported the band gap study by diffuse reflection spectroscopy and noticed the band gap value 1.2 eV –1.8 eV of Cr 3+ ion substituted copper ferrite nanoparticles. S. Yuvaraj et al. [70] also studied band gap of Mn 0.55 Cu 0.45 Fe 2 O 4 nanoparticles by diffuse reflection spectroscopy and noticed the band gap variation from 1.46 eV to 1.88 eV at different higher temperature annealing of nanoparticles. 3.10. Dielectric and electrical property Fig. 11 (a-b) displays the frequency dependent real and imaginary part of dielectric constant of MnFe 2 O 4 nanoparticles prepared at sonication time 20 min, 40 min, 60 min, and 80 min. It can be observed that the dielectric constant (ε′) of prepared MnFe 2 O 4 nanoparticles at different sonication time decreases with increase of frequency. It is associated with that at low frequency, the influence of grain boundary is more commanding over that of grains and consequently dielectric dispersion and further at high frequency, the space charge polarization is decreased as the charge carriers are inadequate to follow the applied electric field and results in the decrease of dielectric constant with increase of frequency in prepared MnFe 2 O 4 nanoparticles at different sonication time [71,72]. Further, the value of dielectric constant was decreased with increase of sonication time. It was decreased from 499 to 8 at 1 kHz frequency for MnFe 2 O 4 nanoparticles synthesized at Fig. 7. High resolution XPS spectrum of Mn 2p of (a) MF20, (b) MF40, (c) MF60, (d) MF80. Table 3 Cation distribution in sonochemically synthesized MnFe 2 O 4 nanoparticles evaluated by XPS analysis. Sample Cation Distribution Tetrahedral (A) site Octahedral (B) site MF20 Mn 0.29 Fe 0.42 Mn 0.71 Fe 1.58 MF40 Mn 0.34 Fe 0.48 Mn 0.66 Fe 1.52 MF60 Mn 0.32 Fe 0.52 Mn 0.68 Fe 1.48 MF80 Mn 0.28 Fe 0.54 Mn 0.72 Fe 1.46 R. Singh Yadav, et al. Ultrasonics - Sonochemistry 61 (2020) 104839 8 sonication time from 20 min to 80 min, as mentioned in Table 6. However, it was decreased from 1843 to 16 at 100 Hz for MnFe 2 O 4 nanoparticles with increase of sonication time 20 min to 80 min. In general, the dielectric constant of MnFe 2 O 4 spinel ferrite depends on particle size, morphology, cation inversion at octahedral and tetrahedral sites, etc. [73]. Fig. 11(c) depicts the frequency dependent dielectric loss (tanδ)of MnFe 2 O 4 nanoparticles synthesized at sonication time 20 min, 40 min, 60 min, and 80 min. The observed dielectric loss of MnFe 2 O 4 nanoparticles at different sonication time decreases with increase of the frequency and further it becomes constant at higher frequency. It is due to the polarization lag behind the applied electric field and it depends on the grain boundaries, impurities and imperfections in the MnFe 2 O 4 spinel ferrite nanoparticles synthesized at different sonication time [74]. The value of dielectric loss decreases from 2.49 to 0.89 at 1 kHz for MnFe 2 O 4 nanoparticles with increase of sonication time 60 min to 80 min. In addition, it decreases from 5.92 to 1.67 at 100 Hz from sonication time 60 to 80 min, as mentioned in Table 6. Further, Fig. 11(d) illustrates the frequency dependent ac conductivity of prepared MnFe 2 O 4 nanoparticles at sonication time 20 min, 40 min, 60 min, and 80 min. In MnFe 2 O 4 spinel ferrite nanoparticles, the conduction is due to hopping of electrons between the ions of the same element by the following relation [75]: +↔ + ++ + + M nFe MnFe 23 32 where Fe 3+ and Mn 2+ ions occupy the octahedral sites. The cation distribution at octahedral site can influence the ac conductivity of MnFe 2 O 4 nanoparticles. From the XPS analysis, it is found that the Fe 3+ ions at octahedral site decreases with increase of sonication time. Consequently, there is decrease in ac conductivity of prepared MnFe 2 O 4 nanoparticles with increase of sonication time. The value of ac conductivity decreases from 368 × 10 −9 S/cm to 3.1 × 10 −9 S/cm at 1 kHz for MnFe 2 O 4 nanoparticles prepared at sonication time from 20 min to 80 min. In addition, it decreases from 178 × 10 −9 S/cm to 1.6 × 10 −9 S/cm at 100 Hz for MnFe 2 O 4 nanoparticles prepared with increase of sonication time from 20 min to 80 min, as mentioned in Fig. 8. (a) The N 2 adsorption–desorption isotherms, and (b) pore size distributions of MnFe 2 O 4 nanoparticles synthesized by sonochemical method. R. Singh Yadav, et al. Ultrasonics - Sonochemistry 61 (2020) 104839 9