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Article Cite This: ACS Omega 2019, 4, 12632−12646

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Magnetic, Pseudocapacitive, and H2O2‑Electrosensing Properties of Self-Assembled Superparamagnetic Co0.3Zn0.7Fe2O4 with Enhanced Saturation Magnetization Rituparna Mondal,† Koyel Sarkar,† Subhrajyoti Dey,†,‡ Dipanwita Majumdar,§ Swapan Kumar Bhattacharya,∥ Pintu Sen,# and Sanjay Kumar*,† †

Department of Physics and ∥Department of Chemistry, Jadavpur University, Kolkata 700032, India Swami Vivekananda Institute of Science & Technology, Sonarpur, Kolkata 700145, India § Department of Chemistry, Chandernagore College, Chandannagar, West Bengal 712136, India # Variable Energy Cyclotron Centre, HBNI, 1/AF Bidhannagar, Kolkata 700064, India

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‡

S Supporting Information *

ABSTRACT: The present work explores the structural, microstructural, optical, magnetic, and hyperfine properties of Co0.3Zn0.7Fe2O4 microspheres, which have been synthesized by a novel template-free solvothermal method. Powder X-ray diffraction, electron microscopic, and Fourier transform infrared spectroscopic techniques were employed to thoroughly investigate the structural and microstructural properties of Co0.3Zn0.7Fe2O4 microspheres. The results revealed that the microspheres (average diameter ∼121 nm) have been formed by self-assembly of nanoparticles with an average particle size of ∼12 nm. UV−vis diffuse reflectance spectroscopic and photoluminescence studies have been performed to study the optical properties of the sample. The studies indicate that Co0.3Zn0.7Fe2O4 microspheres exhibit a lower band gap value and enhanced PL intensity compared to their nanoparticle counterpart. The outcomes of dc magnetic measurement and Mössbauer spectroscopic study confirm that the sample is ferrimagnetic in nature. The values of saturation magnetization are 76 and 116 emu g−1 at 300 and 5 K, respectively, which are substantially larger than its nanosized counterpart. The infield Mössbauer spectroscopic study and Rietveld analysis of the PXRD pattern reveal that Fe3+ ions have migrated from [B] to (A) sites resulting in the cation distribution: (Zn2+0.46Fe3+0.54)A[Zn2+0.24Co2+0.3Fe3+1.46]BO4. Comparison of electrochemical performance of the Co0.3Zn0.7Fe2O4 microspheres to that of the Co0.3Zn0.7Fe2O4 nanoparticles reveals that the former displays greater specific capacitance (149.13 F g−1) than the latter (80.06 F g−1) due to its self-assembled porous structure. Moreover, it was found that Co0.3Zn0.7Fe2O4 microspheres possess a better electrochemical response toward H2O2 sensing than Co0.3Zn0.7Fe2O4 nanoparticles in a wide linear range.

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particles to become superparamagnetic (SPM).8−10 Usually, ultrafine magnetic nanoparticles exhibit SPM behavior above their so-called blocking temperature (TB).8−12 The SPM nanoparticles possess moderate saturation magnetization, can be easily dispersed in a liquid medium, and respond to the change in external magnetic fields.11,13−16 Agglomeration resulting from strong magnetic interaction does not take place in SPM nanoparticles as interparticle interaction is very feeble for this system. These features of SPM nanoparticles make them suitable for different biomedical applications. On the other hand, the reduction of magnetization due to the finite size effect and spin canting is a common feature of SPM

INTRODUCTION

Over the past few years, superparamagnetic ferrite nanoparticles have drawn considerable attention because of the fundamental physics involved and their potential technological application in drug delivery, magnetic resonance imaging, medical diagnosis, ferrofluids, catalysis, water and air purification, photocatalytic degradation of toxic compounds, gas separation, and magnetic fluid hyperthermia treatment of cancer.1−7 If the size of a magnetic substance is reduced below a certain size limit, the thermal energy (kBT, where kB and T are Boltzmann constant and temperature, respectively) becomes comparable to the magnetic anisotropy energy (KV, where K and V are the anisotropy energy constant and volume of the particle, respectively) responsible for clutching the magnetization along a certain orientation, and in consequence, the magnetic moment flips randomly with time causing the © 2019 American Chemical Society

Received: May 11, 2019 Accepted: July 11, 2019 Published: July 24, 2019 12632

DOI: 10.1021/acsomega.9b01362 ACS Omega 2019, 4, 12632−12646

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nanoparticles.17−19 SPM nanoparticles cannot be efficiently manipulated using a moderate magnetic field because of their low magnetization, and this limits their applications severely.11,13 The magnetization usually increases with the increase of particle size. However, simply making particles larger cannot be a solution for overcoming the conflict between the requirement of high magnetization and the inherent character of the SPM nanoparticles since, with the increase of size, the particles lose their SPM character and become magnetically ordered resulting in strong aggregation, and thus, those particles do not remain rapidly dispersible in solution.11,13,15 Efforts have been made to prepare submicronsized SPM ferrites with high saturation magnetization through composite formation or by embedding the nanometric SPM particles in the polymer and silica matrix.14,20−24 However, uniform loading of the magnetic nanoparticles in these systems is very difficult as ferrites do not disperse evenly during the growth process of such synthesis techniques. An alternative strategy of forming hierarchical self-assembled and monodispersed microspheres of ferrite composed of SPM particles appears to be more attractive because of the possibility of enhancing the magnetization while retaining the SPM characteristic. Recently, we have performed a detailed study on structural, microstructural, magnetic, and hyperfine properties of the SPM Co0.3Zn0.7Fe2O4 nanoparticle system.25 In this background, it will be interesting to examine whether the submicron-sized self-assembled spheres composed of SPM Co0.3Zn0.7Fe2O4 nanoparticles can show enhancement of saturation magnetization compared to their nanosized counterpart while retaining the SPM character of their constituent nanoparticles. Further, we have chosen Co0.3Zn0.7Fe2O4 nanoparticles as the building blocks of self-assembled hierarchical ferrite microspheres as it will help us to study as well as compare the structural, magnetic, optical, hyperfine, and electrochemical properties of both Co0.3Zn0.7Fe2O4 nanoparticles and microspheres. It may be noted that supercapacitors are considered as among the most promising next-generation energy storage devices due to their high power density, fast rate of charging− discharging, excellent reversibility, stable life cycle, and safe mode of operation compared to other electrochemical energy storage devices.26−32 Supercapacitors are generally classified into two categories based on their fundamental energy storage mechanism.27 The first one is the electrochemical double layer capacitor (EDLC) where the energy is stored by the electrostatic accumulation of charges, and the other type is the pseudocapacitor, which stores electrical energy by a fast and reversible faradaic redox reaction.27,28 Even though EDLCs provide high power density and excellent life cycle, they can store a limited amount of energy due to finite electrical charge separation at the interface of electrode and electrolytes.27,28 In this context, it is relevant to mention that the pseudocapacitors have higher specific capacitance and energy density than EDLCs, which can be beneficially used in fabrication of high-quality energy storage devices.27,28 RuO2 is a distinctive example of a pseudocapacitor with high pseudocapacitance, high reliability, and excellent reversibility.27,32 Recently, attempts have been made to explore the pseudocapacitive properties of some other transition-metal oxides like Fe2O3, V2O5, NiO, MnO2, and Mn2O3 as an alternative to RuO2 since it is very much costly.33−38 However, most of these materials suffer from low capacitance and poor cycling stability, which produces insurmountable obstruction

to design a practical energy storage device.27 A possible way out to overcome these disadvantages is to develop nanostructured oxide materials as they exhibit higher capacitive performance due to their high surface area and short ion transfer pathway.27 However, these oxide nanomaterials have been found to be of little convenient use because of their poor intrinsic conductivity and unstable life cycle.27 In this background, recently, some research groups have proposed MFe2O4 (where M is a divalent metal ion)-type spinel ferrite heterostructures as very good candidates for electrochemical energy storage application as they exhibit better electrical conductivity, rich redox chemistry, and higher electrochemical activity than oxides containing one type of metal atom.27 There are several reports available regarding the synthesis and electrochemical performance of ZnFe2O4, Fe3O4, CuFe2O4, CoFe2O4, MnFe2O4, and other ferrite-based self-assembled heterostructures.27,39−44 In this work, we have presented a comparative study on the electrochemical properties and pseudocapacitive behavior of self-assembled Co0.3Zn0.7Fe2O4 microspheres and Co0.3Zn0.7Fe2O4 nanoparticles. Besides the issues related to magnetic and electrochemical properties, iron-based biocompatible, nontoxic, chemically stable, and low-cost oxide materials have been successfully used in electrochemical sensing of gas, humidity, ethanol, H2O2, hydrazine, and DNA.45−49 Hydrogen peroxide (H2O2) is an efficient liquid oxidant and is widely used in the biomedical, biopharmaceutical, environmental, clinical, and industrial fields.48,50 It is one of the most common reactive oxygen species and when in contact with a living organism without proper precaution can damage cellular proteins, nucleic acids, and lipid molecules and thereby may cause diabetes, cancer, cardiovascular, and neurodegenerative disorders.50,51 Hence, sensitive and precise detection of H2O2 is of prime challenge in the chemical, pharmaceutical, and biomedical research fields. Until now, many techniques like calorimetry, electrochemistry, chemiluminescence, and fluorescence have been proposed to detect H2O2 qualitatively and quantitatively.50 Among all the proposed methods, nonenzymatic amperometric sensing has attracted prime attention because this method is very simple, sensitive, and does not require high-cost complex instrumentation.48,50 As a result, a great deal of effort has been paid for fabrication of nonprecious electrochemical probes for rapid and sensitive detection of H2O2. Nanostructured spinel ferrites have appeared as potential candidates for electrochemical probing of H2O2 due to their large surface area, excellent catalytic activity, and cost-effective synthesis process. 15,52 It has been reported that the substitution of iron ions of the spinel ferrite system by Mn2+, Co2+, Ni2+, and Zn2+ ions leads to the improvement of the physicochemical and electrical conductivity of the system due to the formation of new donor−acceptor chemisorption sites and fast electron transfer mechanism between the cations.52 Recently, a comparative study has revealed that the order of reactivity of electrospun MFe2O4 nanofibers toward peroxide determination/reduction is as follows: CoFe2O4 > CuFe2O4 > NiFe2O4 > MnFe2O4 > Fe2O3 systems.53 Highly porous, hierarchical morphology of mixed ferrites promotes fast mass transfer kinetics across their channels besides increasing the density as well as reactivity of exposed electrocatalytic active sites.54,55 The above explorations reveal that further improved and prominent electrochemical behavior would be expected in porous microsphere morphology compared to densely packed 12633


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0.0199° and 3 s/step, respectively. The surface morphology of the sample was probed by field-emission scanning electron microscopy (FESEM, FEI INSPECT F50). The highresolution transmission electron microscopy (HRTEM) was performed by using a JEOL JEM 2100 instrument to determine particle size distribution. The crystalline character of the sample was further investigated by recording the selected area electron diffraction (SAED) pattern using a JEOL JEM 2100 instrument. The presence of constituent elements in CZMS was checked by energy-dispersive X-ray spectroscopy (EDS). The EDS spectrum of the sample was recorded by a Bruker EDS system attached with HRTEM equipment. Fourier transform infrared spectroscopy was performed by using a PerkinElmer spectrometer (Spectrum Two) equipped with an attenuated total reflectance (ATR) attachment. The diffuse reflectance spectroscopy (DRS) spectra of the samples were recorded by a PerkinElmer UV−vis spectrometer (Lambda 35) with a solid-state measurement attachment. The UV−vis and photoluminescence (PL) spectra of the samples were recorded by JASCO V-630 and JASCO FP-6700 spectrophotometers, respectively. The magnetic property of CZMS was investigated by a Cryogenic vibrating sample magnetometer (VSM). The standard zero-field cooled (ZFC) and field cooled (FC) magnetization protocol measurement was performed at an external magnetic field of 500 Oe in the temperature range of 5−300 K. The isothermal variation of magnetization (M) with the change of the external magnetic field (H) was recorded at 300 and 5 K in the field range of ±5 T. The 57Fe Mössbauer spectroscopic measurement of the sample was carried out in transmission mode at 300 and 10 K. The 300 K Mössbauer spectrum was recorded by using a constant acceleration drive (CMTE-250) equipped with a 10 mCi 57Co source embedded in the Rh matrix. The 10 K Mössbauer spectrum was recorded with the help of a JANIS SVT-400 MOSS cryostat system. The Mössbauer spectrum at 10 K in the presence of a 5 T external field applied parallel to the γ-ray direction was recorded by a superconducting magnet (JANIS SuperOptiMag) with a 40 mCi 57Co source. The velocity calibration was performed by using a natural iron foil, and the values of isomer shifts were estimated with reference to standard α-Fe at 300 K. Windows-based Recoil software was used for the qualitative analysis of the Mössbauer spectra.62 Fabrication of Working Electrode and Electrochemical Characterization. The working electrode was fabricated by adopting the following procedure. At first, a graphiticcarbon rod with a working electrode area of 0.13 cm2 was washed thoroughly in distilled water by ultrasonication. Then the washed rod was wrapped by a Teflon sheet keeping both ends open. The sample was mixed in distilled water and Nafion (5 wt %) by ultrasonication to form an aqueous solution of concentration 1.6 mg mL−1. 10 μL of the resulting solution was dropped on the rounded flat surface of the graphite rod and dried at ambient temperature overnight. All electrochemical measurements for supercapacitive studies and electrochemical sensing of H2O2 were performed at 25 °C using a conventional three-electrode system with the drop-coated sample on the graphitic-carbon rod acting as the working electrode, a saturated calomel electrode as the reference electrode, and a Pt wire as the counter electrode. Cyclic voltammetric and chronoamperometric measurements were performed in 1 and 0.1 M NaOH aqueous solutions, respectively, as electrolytes using a potentiostat instrument

mixed-ferrite nanoparticles. The observation that selfassembled ferrite microstructures often exhibit striking electrical, optical, magnetic, and catalytic properties compared to their nanoparticle counterparts,52 thus, has inspired us to investigate the performance of Co0.3Zn0.7Fe2O4 self-assembled microspheres as nonenzymatic H2O2 sensors. It is well known that nanoparticles, self-assembled heterostructures of nanoparticles, and core−shell nano/microstructures exhibit outstanding optical properties like linear absorption, photoluminescence emission, and nonlinear optical properties. In this context, the study of optical properties of different hierarchically developed nano/microstructures will surely help to probe their potential toward different optoelectronic and sensor devices and photocatalytic degradation of toxic dyes.40,56−59 Spinel ferrites generally exhibit lower band gap values, which are very crucial in their optical applications.60,61 In this context, it would be worthy to study the optical properties of the Co0.3Zn0.7Fe2O4 microspheres and nanoparticles to gather more information on their band gap values. In this work, self-assembled microspheres of Co0.3Zn0.7Fe2O4 have been successfully synthesized by a facile, one-pot, lowtemperature, low-cost solvothermal method. We have thoroughly characterized the sample by powder X-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, photoluminescence, diffuse reflectance spectroscopy, Mössbauer spectroscopy, and dc magnetic measurement. We have performed a detailed study on electrochemical properties of the sample and its nanosized counterpart to compare the pseudocapacitive electrical storage capacity and sensing capability of these samples toward detection of H2O2.

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EXPERIMENTAL METHODS Synthesis Procedure of Co0.3Zn0.7Fe2O4 Microspheres (CZMS). Analytical-grade transition-metal chlorides CoCl2· 6H2O, ZnCl2·4H2O, and FeCl3·6H2O of extremely high purity (∼99.99%) were purchased from Sigma-Aldrich and used without further purification. Sodium acetate (NaAc, CH3COONa·3H2O), ethylene glycol (EG, HOCH2CH2OH), and polyethylene glycol (PEG, MW = 4000) of analytical purity were procured from Merck India. Co0.3Zn0.7Fe2O4 microspheres (CZMS) were synthesized by a simple, template-free, low-temperature, one-pot solvothermal method. At first, 10 mM FeCl3·6H2O, 1.5 mM CoCl2·6H2O, and 3.5 mM ZnSO4·6H2O were dissolved in 80 mL of EG through vigorous magnetic stirring. Afterward, 7.2 g of NaAc along with 2 g of surfactant PEG was added to the salt solution, and the mixture was stirred vigorously at room temperature for 2 h to obtain a homogeneous brown solution. Then the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 20 h in an oven. The autoclave was cooled down to room temperature naturally and taken out from the oven. After that, the precipitate was collected and washed several times with deionized water and ethanol by centrifugation to remove organic and inorganic residues. Finally, the precipitate was dried in vacuum at 65 °C for 8 h to obtain a black fine powder of CZMS. Characterization Techniques. We have recorded the powder X-ray diffraction (PXRD) pattern of CZMS at 21 °C over the 2θ range of 10−80° using a Bruker D8 Advance diffractometer with Cu Kα (λ = 1.54184 Å) radiation. The PXRD data was collected with a step size and counting time of 12634


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refinement curve is shown in Figure 1, which shows that the experimental and the simulated data match with acceptable agreement. The refinement parameters, metal-oxide bond lengths and bond angles, and fractional coordinates along with occupancies of different ions are listed in Tables 1 and 2.

(Digi-ivy, model No. DY2300). The galvanostatic charging− discharging performance was studied in 1 M NaOH aqueous solution with the same three-electrode setup using an Autolab PGSTAT30 instrument. All the CV scans for electrochemical peroxide sensing by its reduction were recorded in the potential range between −0.2 to −0.6 V at a scan rate of 10 mV/s with different concentrations of H2O2 (0, 0.058, 1.18, and 1.47 mM) in a 0.1 M aqueous NaOH electrolyte, while chronoamperometric investigations of H2O2 reduction were carried out in 0.1 M aqueous NaOH solution with different molar amounts of H2O2 at potentials of −0.42 and −0.45 V, respectively, for 120 s.

Table 1. Structural and Microstructural Parameters along with Metal−Oxygen (M−O) Bond Lengths and Bond Angles of the Sample Acquired from Rietveld Analysis of PXRD Pattern of CZMS by GSAS Program

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RESULTS AND DISCUSSION Structural, Microstructural, and Morphological Study. The PXRD pattern of CZMS is depicted in Figure 1,

parameters

values

formula weight crystal system space group lattice parameter (Å) volume (Å3) density (g cm−3) metal−oxygen bond lengths (Å) metal−oxygen bond angles

239.13 cubic Fd-3m 8.407(7) 594.249(15) 5.346 1.913(4) (A site), 2.050(5) (B site) 109.47°(9) (A site), 93.03°(4) (B site)

Table 2. Fractional Coordinates and Occupancies of Different Ions Obtained from the Rietveld Refinement by GSAS Program ions

x

y

z

occupancy (±0.003)

Zn (A) Fe (A) Zn (B) Co (B) Fe (B) O

0.125 0.125 0.5 0.5 0.5 0.2544

0.125 0.125 0.5 0.5 0.5 0.2544

0.125 0.125 0.5 0.5 0.5 0.2544

0.46 0.54 0.12 0.15 0.73 1.00

The unit cell of CZMS together with the tetrahedral and octahedral lattice sites is illustrated in Figure 2. The Rietveld

Figure 1. Indexed PXRD pattern (black dots) of CZMS and illustrative Rietveld refinement plot (red line) of that pattern fitted by GSAS program. The green line indicates the respective residue.

which reflects that all the characteristic X-ray diffraction peaks ascribing to the cubic spinel ferrite system are present in the diffractogram.25 The indexing of the PXRD pattern was performed by DICVOL06 and TREOR90 of the FullProf 2000 package and NTREOR of the EXPO2009 package adopting the methodology described in our previous works.12,17−19,63−65 The space group of CZMS was determined by FINDSPACE of the EXPO2009 package, which suggests that the sample crystallizes in the Fd-3m space group.66 The crystallographic information (Miller indices, space group) obtained from the preliminary analysis of the PXRD data is in excellent agreement with the JCPDS database of ZnFe2O4 and CoFe2O4 systems (JCPDS No. 89-1012 and 22-1086 for ZnFe2O4 and CoFe2O4, respectively).67 In order to get detailed information regarding the structural (lattice parameter, atomic coordinates) and microstructural (crystallite size and lattice strain) parameters of the sample, we have performed Rietveld refinement of the PXRD pattern by using the MAUD2.33 software package, and the method of refinement is described elsewhere.20,68 The fitted PXRD pattern is shown in Figure S1, and the final refinement parameters are listed in Table S1. For a deeper understanding of the crystal structure and to estimate the bond lengths and bond angles, we have also performed the Rietveld refinement of the PXRD pattern of CZMS by the GSAS program with the EXPGUI interface.69,70 The PXRD data along with the

Figure 2. (a) Unit cell of CZMS, bond angles and bond lengths of (b) tetrahedral (A) and (c) octahedral (B) sites.

refinement of the PXRD data by MAUD2.33 and GSAS software programs suggests that CZMS is single-phase cubic spinel ferrite, which crystallizes in the Fd-3m space group. The FESEM micrograph (Figure 3a) clearly indicates the formation of three-dimensional hierarchical self-assembled microspheres of Co0.3Zn0.7Fe2O4 nanoparticles. The average diameter of the microsphere is observed to be ∼124 nm (Figure S2). The FESEM micrographs also suggest that the microspheres are formed by aggregation of a large number of Co0.3Zn0.7Fe2O4 nanoparticles, which leads to an uneven outer surface of the microspheres. 12635


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the solvothermal reaction technique, only the variation of the reaction temperature by constraining other reaction parameters (concentration of the reactants, choice of the solvent, presence of the surfactant, pH of the reaction medium, reaction time, etc.) may lead to the formation of different kinds of morphological forms of self-assembled heterostructures.83−87 By performing PXRD, SEM, and TEM studies on the MFe2O4 (M = Mn, Co, Ni) microspheres with average diameters of 100−300 nm synthesized by the solvothermal technique at different temperatures, Reddy and Mohamed have shown that the optimal temperature for synthesizing such ferrite microspheres with good crystallinity and narrow size distribution is 180 °C.87 So, to fulfill the target of synthesizing monodispersed, highly crystalline microspheres, we have chosen the solvothermal method as the synthesis technique and 180 °C as the reaction temperature. From the structural and microstructural characterization of CZMS, it is clear that this target has been achieved. FTIR Study. The FTIR spectra of CZMS and CZNP recorded in the wavenumber range of 400 to 2000 cm−1 are displayed in Figure 4 and Figure S6, respectively. In the FTIR

Figure 3. (a) FESEM image of CZMS, HRTEM images showing (b) size distribution, (c) single microsphere, and (d) lattice fringes of CZMS.

For the verification of the outcomes obtained by the PXRD and FESEM studies and to acquire more knowledge about the structural and microstructural properties of the sample, we have recorded the HRTEM images of the sample. The TEM images at high and low magnification along with the SAED pattern are shown in Figure 3b,c, respectively. The highresolution TEM study suggests that the average diameter of the Co0.3Zn0.7Fe2O4 microsphere is ∼121 nm. Further assessment of the HRTEM images indicates that the microspheres consist of self-assembled Co0.3Zn0.7Fe2O4 nanoparticles of ∼12 nm size (Figure 3c), which is in good agreement with the value of crystallite size (∼14 nm) obtained from the Rietveld refinement of the PXRD data (the slight mismatch in the crystallite size and the particle size is due to the difficulty in accurately distinguishing the individual nanoparticles in the microspheres). The good crystallinity of the sample has been reflected by the presence of clear lattice fringes in the HRTEM micrograph (Figure 3d) and bright Debye−Scherrer ring in the selected area diffraction (SAED) pattern (Figure S3). The crystallographic d values obtained from the radius of the rings corresponding to different lattice spacings of the SAED pattern corroborate with those acquired from the PXRD study. The distinct peaks originating from the constituent elements (Co, Zn, Fe, and O) were clearly observed in the EDS spectra of both CZMS (Figure S4) and CZNP (Figure S5). The ratios of atomic percentages of Co, Zn, and Fe in CZMS and CZNP obtained from the EDS spectrum are 0.92:2.23:5.37 and 5.24:11.95:33.66, respectively, which confirm that the samples are in proper stoichiometry. The morphology (shape and size), phase, and degree of purity of nanostructured materials synthesized by sol−gel,71,72 sonochemical,73−75 thermal decomposition,76,77 hydrothermal,78,79 and Schiff base ligand mediated methods80 can be controlled by properly tuning the reaction parameters like reaction temperature, pressure, pH, time, reactant concentration, and choice of the precursor and surfactant.59,71−80 Among all these synthesis techniques, the hydrothermal method is the most favorable procedure for developing hierarchical self-assembled heterostructures due to its capability of inducing preferred orientational morphological growth.78,79,81,82 Further, the solvothermal/hydrothermal technique is a facile, energy-efficient, low-cost, and green synthesis technique.78,79,81−86 It is relevant to mention that, in

Figure 4. FTIR spectrum of CZMS.

spectrum of ferrites, two broad bands appear at ∼590 and 405 cm−1 due to the stretching vibration of (A) and [B] site metal−oxygen (M−O) bonds, respectively.88 Both CZMS and CZNP displays absorption bands, each at 580 and 417 cm−1 and 541 and 450 cm−1, respectively, in their respective FTIR spectrum. This indicates that both samples are cubic spinel ferrites. Study of Optical Properties. The optical properties of the samples have been investigated by UV−vis diffuse reflectance spectroscopic (DRS) and photoluminescence (PL) studies to determine the band gaps of the samples. It is well known that the band gap is dependent on various factors like crystallite size, dopant concentration, and structural parameters.56,60,61,89−91 The DRS spectra of CZMS and CZNP are shown in Figure 5a. It can be ascertained from the DRS spectrum of CZMS that it displays an absorption edge at around 775 nm. On the other hand, CZNP exhibits a broad absorption band with no such detectable absorption edge, and thus, it is not possible to calculate the band gap value of CZNP from this absorption band. Hence, the band gaps of the samples have been estimated from the intercept of the (αhν)2 versus hν plot (Kubelka−Munk plot), where α is the absorption coefficient.60 The Kubelka−Munk plots for CZMS and CZNP are presented in Figure 5b. The estimated band gaps of CZMS and CZNP are 1.6 and 2.45 eV, 12636


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Figure 5. (a) DRS spectra and (b) (αhν)2 versus hν plots of CZMS and CZNP.

ordering, cation distribution, and spin-canting effect of ironcontaining oxide materials.17,93,94 The room-temperature Mössbauer spectrum (Figure 7a) of CZMS was fitted by the

respectively. The band gap of pure ZnFe2O4 is 2.1 eV, and it generally decreases with the increase in cobalt doping concentration.60,61 In the present case, CZMS follows this trend, but there is an increase in the band gap of CZNP, which may be attributed to the quantum confinement effect related to the small size of the nanoparticles.61 The UV−vis spectra of both CZMS and CZNP (Figure S7) exhibit a broad absorption band around ∼420 nm. Thus, both samples have been excited at a frequency of ∼420 nm to record the PL spectra of CZMS and CZNP (Figure 6). CZMS

Figure 7. Mössbauer spectra of CZMS at (a) 300 K, (b) 5 K zerofield, and (c) 5 K in the presence of the 5 T external magnetic field. Solid black circles represent the experimental data points, and solid lines represent the simulated spectrum.

“Lorentzian site analysis” method of the Recoil program, and the fitted parameters are listed in Table 3. The spectrum exhibits a diffused sextet resulting from the collective behavior of magnetic nanoparticles assembled in the form of microspheres. We have fitted the room-temperature Mössbauer spectrum with two hyperfine split sextets. The values of the isomer shift (IS) suggest that only the high-spin Fe3+ ions are present in the sample and no signature of the Fe2+ ion was detected.17 Thus, the room-temperature Mössbauer spectroscopic study indicates that CZMS is in the magnetically ordered state at room temperature. It is noteworthy that CZMS has been formed by the aggregation of large numbers of individual nanoparticles and the aggregation process has masked the SPM relaxation in the system as the individual anisotropy energy barrier is modified due to the onset of strong interparticle interactions. As a distinct anisotropy energy barrier cannot be defined for an individual particle, CZMS displays the collective magnetic state, which gives rise to a diffused sextet in its room-temperature Mössbauer spectrum.18,95 For quantitative estimation of the relative distribution of Fe3+ ions among tetrahedral (A) and octahedral [B] sites and to probe the spin-canting effect in CZMS, we have recorded the Mössbauer spectrum of the sample at 5 K in the presence of a 5 T external magnetic field. The infield Mössbauer spectrum exhibits two clearly resolved subspectra, which

Figure 6. Photoluminescence spectra of CZMS and CZNP.

and CZNP display emission peaks at ∼632 and 630 nm. The intensity of the PL peak of CZNP is lower than that of CZMS. It may be noted that CZMS has been prepared by the surfactant-assisted solvothermal technique. These surfactants generally reduce surface defects and decrease the number of trap sites in the system.40 So, in such a system, a majority of electrons relax through direct transitions between conduction and valence bands. On the other hand, CZNP was synthesized by the high-energy ball milling method, which introduces lattice strain, causes contraction of lattice parameter, and gives rise to defects and vacancies in the system. All these together can initiate indirect transitions in the system, thus reducing the PL intensity. The particle size and morphological features of a sample strongly influences its band gap energy, which is further important in determining its electrochemical activity.92 So, it is expected that the electrochemical performances of CZMS and CZNP will be different, which has indeed been observed in the electrochemical study. Mössbauer Spectroscopic Study. 57Fe Mössbauer spectroscopy is a widely used tool for verifying the magnetic 12637


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Table 3. Hyperfine Parameters Obtained by Fitting the 300 K Mössbauer Spectra of the Sample sample

spectra

IS (±0.03) (mm s−1)

QS (±0.03) (mm s−1)

HMF (±0.4) (T)

width (±0.03) (mm s−1)

area (±0.2) (%)

CZMS

sextet1 sextet2

0.28 0.36

0.00 0.00

49.31 42.00

0.50 0.70

27.46 72.54

Table 4. Values of Zero-Field and Infield Mössbauer Parameters of the Sample at 5 K Determined by Lorentzian Profile Fitting temperature/ field 5 K/0 T 5 K/5 T

site

width (mm s−1) (±0.03)

IS (mm s−1) (±0.03)

2ε (mm s−1) (±0.03)

[Fe3+A] [Fe3+B] [Fe3+A] [Fe3+B]

0.30 0.37 0.30 0.37

0.39 0.47 0.39 0.47

0.00 0.00 0.00 0.00

Beffa (T) (±0.14)

Bhf (T) (±0.14)

55.78 47.35

50.78 52.12 50.78c 52.12c

A23 (degree) (±0.1)

0.0007 0.21

θb (%) (±0.02)

area (mm s−1) (±0.2)

1.07 18.41

27 73 27 73

a Observed HMF (BHF) is the vector sum of the internal HMF and the external applied magnetic field. bThe average canting angle estimated from the ratio of the intensities of lines 2 and 3 from each subspectra I2/I3 (A23) according to θ = arccos[(4− I2/I3)/(4 + I2/I3)]1/2, where I2/I3 = A23. c Estimated according to the relationship of Beff, Bhf, and applied field.

Figure 8. (a) ZFC-FC magnetization curves of CZMS at 500 Oe. The FC and ZFC magnetization are represented by pink and green lines, respectively. (b) M−H (hysteresis) loop of CZMS at 300 and 5 K, between ±5 T. The M−H loops at 300 and 5 K between ±0.18 and ± 1, respectively, are shown in the inset for clarity.

pattern of the sample. We have also fitted the 5 K without field Mössbauer spectrum by constraining the values of the Fe3+A/ Fe3+B ratio as obtained from the fitting of the infield pattern. It is evident that the 5 K without field spectrum (Figure 7b) has been well fitted with the experimental data and the fitted values of the hyperfine parameters (Table 4) corroborate with those obtained from the fitting of the 5 K infield spectrum. This validates the observations of the infield study. Thus, the Mössbauer study indicates that CZMS is ferrimagnetically ordered at low temperature; there is substantial migration of iron ions from [B] to (A) site, and the surface spins are canted. dc Magnetic Study. The ZFC-FC magnetization measurement as a function of temperature is a very useful tool to study the particle size distribution, magnetic anisotropy energy, and magnetic ordering of iron-based oxide nanoparticles.19,96 The ZFC-FC magnetization curves in the temperature range between 5 and 300 K and at a 500 Oe external magnetic field are shown in Figure 8a. The FC magnetization remains almost constant throughout the measurement temperature range, and the ZFC magnetization falls sharply below 300 K. From the Mössbauer spectroscopic study, it has been observed that the sample exhibits a prominent sextet at and below 300 K. Thus, the fall in ZFC magnetization below 300 K can be ascribed to the spin-glass-like freezing of the magnetically coupled spins in the sample.8 The isothermal variations of magnetization (M) as a function of the external magnetic field (H) at 300 and 5 K

suggest that CZMS is ferrimagnetically ordered at 5 K. We have fitted the infield spectrum using the Lorentzian site analysis method of the Recoil program by taking a pair of sextet assigned to the (A) and [B] site iron ions. The fitted spectrum is shown in Figure 7c, and the hyperfine parameters are listed in Table 4. The non-vanishing intensities of the 2nd and 5th lines of the six-finger pattern suggest the presence of spin canting in the sample.17 The average values of (A) and [B] site canting angles (1.07 and 18.41° for (A) and [B] site moments, respectively) have been estimated by taking into account the ratio of the line intensities of the 2nd and 3rd lines of the sextets ascribed to the (A) and [B] sites, respectively, using the standard formula.17,93 The relative ratio of Fe3+ ions among (A) and [B] sites (Fe3+A/Fe3+B) has been determined from the areal intensity of the sextets corresponding to the (A) and [B] sites. For the infield spectrum, the value of Fe3+A/ Fe3+B was found to be 0.37. Hence, it is clear from the outcome of the infield Mössbauer study that Fe3+ ions have migrated from [B] to (A) sites. By keeping in mind that Co2+ has a strong tendency to occupy the [B] site due to its favorable atomic radius and charge distribution in the octahedral crystal field, we have estimated the structural formula of the CZMS to be (Zn2+0.46Fe3+0.54)A[Zn2+0.24Co2+0.3Fe3+1.46]BO4, which is different from the equilibrium cation configuration (Zn2+0.7Fe3+0.3)A[Co2+0.3Fe3+1.7]BO4. The structural formula obtained from the infield Mössbauer spectrum corroborates with the one obtained from Rietveld refinement of the PXRD 12638


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Figure 9. CV plots showing current density versus voltage at different scan rates for (a) CZMS and (b) CZNP.

in the field range of ±5 T are shown in Figure 8b. The saturation magnetizations (MSAT) of the sample have been calculated from the M versus 1/H plot using the law of approach to saturation.17 The values of MSAT at 300 and 5 K are 76 and 116 emu g−1, respectively, which are considerably higher than that of the Co0.3Zn0.7Fe2O4 nanoparticles reported in our previous work.25 The values of coercivity (HC) at 300 and 5 K are 99 and 1826 Oe, respectively. We have also determined the values of anisotropy constant (K) by fitting the M−H curve using the modified Langevin function with the help of MATLAB software.97−99 The value of anisotropy constant of the sample determined from the fitting of the virgin curve of the M−H loop (Figure S8) is 10.63 × 106 J m−3 at 5 K. Electrochemical Performance. To study the electrochemical properties of Co0.3Zn0.7Fe2O4 microspheres (CZMS) and nanoparticles (CZNP), we have prepared the electrodes coated with CZMS and CZNP using 1 M NaOH aqueous solution as the electrolyte and performed the cyclic voltammetry (CV) and galvanostatic charging−discharging (GCD) measurements. The cyclic voltammograms of CZMS and CZNP electrodes at scan rates of 10, 20, 50, and 100 mV s−1 in the potential range of ±0.4 V with reference to the saturated calomel electrode are illustrated in Figure 9a,b. From the cyclic voltammograms of CZMS and CZNP, it is clear that they do not exhibit ideal rectangular current−potential voltammograms of the electrical double layer capacitor.100,101 Moreover, no redox peaks have been observed in the cathodic or anodic cycles of the CV curves of CZMS and CZNP. The deviation from the rectangular shape of the CV curves can be ascribed to the pseudocapacitive behavior of CZMS and CZNP originated from the transfer of charges between electrode and electrolyte.102 The charge transfer mechanism can be accomplished by oxidation−reduction reaction, electrosorption, and intercalation processes.103 In the case of CZMS and CZNP, the pseudocapacitance has been instigated due to charge transfer between the electrodes and the electrolyte through electrosorption and intercalation processes, which can be understood from the following reaction

indicates that the pseudocapacitive behavior of this material takes place through interconversion of Co2+/Co3+ states coupled with simultaneous insertion/deintercalation of OH− ions during the charging−discharging process. The values of specific capacitance (CS) of the working electrodes containing CZMS and CZNP were calculated from CV graphs taken at scan rates of 10, 20, 50, and 100 mV s−1 using the equation: CS = i , where m is the mass of the active mv

electrode material, v is the potential sweep rate, and i is the current response that can be obtained by integrating the area of the curve i =

∫

Va

Vc

i(V )dV , Vc − Va

where Va and Vc represent the anodic

and cathodic voltages, respectively.104,105 The values of specific capacitance at scan rates of 10, 20, 50, and 100 mV s−1 are 149.13, 97.47, 57.76, and 44.78 F g−1, respectively, for CZMS and 80.06, 58.16, 36.85, and 24.97 F g−1, respectively, for CZNP. It has been observed that the specific capacitance of CZMS is much higher than that of CZNP at all scan rates, and for both samples, the values of specific capacitance decrease with the increase in the scan rate. At higher scan rates, ions get attached to the atoms of the electrode and thereby increase the ionic resistance. The increase of effective ionic resistance promotes obstruction in the charge transfer process, which leads to the drop of capacitance with the increase of the scan rate.106 Moreover, the area of the CV curves increases with the sweep rate, keeping the shape of the curves unaltered, which also reflects the good electrochemical response of the samples.39 Galvanostatic charging−discharging (GCD) is another tool to characterize the capacitive performance of the electrode materials. The specific capacitances of CZMS and CZNP can be evaluated from the GCD curve according to the equation I Δt CS = mΔV , where I is the current density, Δt and m denote the discharge time and mass of the active electrode material, respectively, and ΔV is the potential drop during discharge.105 The charging−discharging curves of CZMS and CZNP at a current density of 5.3 A g−1 within the potential window between ±0.4 V are displayed in Figure 10a,b. The value of the specific capacitance of CZNP (29.39 F g−1) was found to be lower than that of CZMS (31.45 F g−1), which is in good agreement with the findings of CV measurement at a scan rate of 100 mV s−1. The deviation from linearity in the shape of the GCD curve indicates that the samples are pseudocapacitive in

Co0.3Zn 0.7Fe2O4 + 1.3OH− + H 2O → 0.3CoOOH + 2FeOOH + 0.7[Zn II(OH)]+ + 2e−

The previous characterization studies inform that iron is present in the +3 oxidation state while cobalt and zinc are present in the +2 oxidation state. The above equation thus 12639


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to 1000 cycles. Figure 11b shows the variation of specific capacitance of Co0.3Zn0.7Fe2O4 nanoparticles with respect to the cycle number. The specific capacitance of CZNP increases slightly up to 100 cycles, and afterward, it is remarkably stable over the increase in the number of charging and discharging cycles. The specific capacitances (CS) of CZMS and CZNP vary almost similarly with the increase of the cycle number, and the nature of the CS versus cycle number curves can be explained considering how the electroactive sites get exposed and utilized with the increase of the cycle number. At the initial cycles of the charging/discharging processes, the electrochemical active materials remain partially wetted by the electrolyte. It is thus only the surface sites that are available for charge storage, and hence, the efficiency of charge transfer is low. With increasing GCD cycles, through continuous ion intercalations and deintercalations, more and more inner electroactive sites get recognized and utilized, and as a result, the capacitance rises until it attains a constant value where most of the active sites get exposed and exploited. Thus, this so-called “activated state” is the cause of the above capacitance rise, which is greatly dependent on the nature and morphology of the electrode material.107 We have recorded the FESEM micrograph of the sample before and after the performance of 1000 cycles of charging and discharging, and the micrographs are depicted in Figure S9. It is observed from the FESEM micrographs that some of the hierarchical spherical microstructures have broken into an irregular agglomeration of nanoparticles with the increase of the number of cycles. This phenomenon leads to the increase in porosity of the sample, and these pores adsorb the electrolytes during the charging and discharging process, which in turn increases the specific capacitance of the sample.108 Electrochemical Sensing of H2O2. To probe the potentials of CZMS and CZNP toward the electrochemical reduction of H2O2, the cyclic voltammograms (CVs) of the samples were studied in the potential window of −0.2 to −0.6 V at a fixed potential scan rate of 10 mV s−1 with varying H2O2 concentrations (0, 0.058, 1.18, and 1.47 mM) in a 0.1 M aqueous NaOH electrolyte, and the results are displayed in Figure 12. The result suggests that the reduction peaks at about −0.42 and −0.45 V for CZMS and CZNP, respectively, get intensified with the increase of the molar concentration of H2O2, signifying electrochemical reduction of H2O2 on the electrode surface. It has also been observed that, with the

Figure 10. Galvanostatic charging−discharging behavior of (a) CZMS and (b) CZNP at a current density of 5.3 A g−1.

nature.39 The symmetric GCD curves indicate that the samples exhibit reversible performance during faradaic reactions.39 To investigate the cyclic stability of the samples, we have performed the charging−discharging measurement up to 1000 cycles at a current density of 5.3 A g−1. The cyclic performance of CZMS is shown in Figure 11a, which indicates that the value

Figure 11. The variations in the values of specific capacitance (CS) with the cycle number of (a) CZMS and (b) CZMP are shown for 1000 cycles measured at a current density of 5.3 A g−1.

of specific capacitance of CZMS increases from 31.45 to 48.33 F g−1 up to 100 cycles, and thereafter, it is almost constant up

Figure 12. CVs of (a) CZMS and (b) CZNP with increasing concentrations of H2O2 in 0.1 M NaOH at a scan rate of 10 mV s−1. 12640


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Figure 13. Chronoamperometric profiles for (a) CZMS and (b) CZNP catalysts for H2O2 in 0.1 M aqueous NaOH solution at potentials of −0.42 and −0.45 V, respectively, up to 120 s.

Figure 14. Calibration plots of H2O2 concentration versus current of (a) CZMS and (b) CZNP.

Table 5. Comparison of the Analytical Performance of Various Micro- and Nanostructured Material Coated Electrodes toward the Detection of H2O2 detection limit electrode material α-Fe2O3 microsnowflake MnO2 nanosheet/carbon foam Pt−TiO2/RGO Mn3O4-MnCo2O4 Co3O4−rGO Fe3O4−Fe2O3 nanocomposites Fe3O4 nanoparticle Fe3O4/rGO MnFe2O4/rGO CoFe2O4 nanoparticles CoFe2O4−chitosan nanocomposite NiFe2O4 nanoparticles NiFe2O4−chitosan nanocomposite CoFe2O4 hollow nanostructure ZnFe2O4/rGO pristine CuHCF Cu2+-rich CuHCF Co0.3Zn0.7Fe2O4 microspheres Co0.3Zn0.7Fe2O4 nanoparticles

sensitivity (μA μM−1 cm−2) 7.16 54 × 10−3 40

linear range (μM) 100−5500 2.5−2055 100−22500 0.1−1274.3 15−675 200−1800 1−2500 1−20000 0.1−1 2−1500 30−8000 2−1500 300−1200 10−1200 25−10200 2−10 0.5−35 58−4120 29−11760

1.14 × 103 20.325 5.7 × 10−3 387.6 13.55 0.99 × 10−3 3.3 × 10−6 0.74 × 10−3 79.29 × 10−3 17 × 10−3 621.64 1.81 × 10−2 3.17 × 10−2 22.73 5.42

lower (μM) 10 0.12 0.020 2.4 200 7.3 0.17 50 × 10−3 14.0 2.0 9.2 14.0 2.5 2.12 2 0.5 58.8 29.4

ref 48 110 111 112 113 114 115 116 117 115 118 115 119 52 120 121 121 this work this work

The nonenzymatic H2O2 sensing properties of CZMS and CZNP were further investigated by the amperometric method. The amperometric current−time response graphs (Figure 13a,b) of CZMS and CZNP were recorded by varying the molar concentration of H2O2 in the 0.1 M aqueous NaOH

increase of the molar concentration of H2O2, the peak current increases gradually.15 It is assumed that Co2+ ions in the electrode material reduce H2O2 to H2O and get oxidized to Co3+ ions plausibly. Thus, these results indicate that CZNP and CZMS can act as nonenzymatic biosensors of H2O2. 12641


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electrolyte at applied reduction potentials of −0.42 and − 0.45 V for CZMS and CZNP, respectively, with reference to the saturated calomel electrode. From the current−time response profiles of the samples, it is clear that the current decreases more rapidly in the case of CZNP compared to CZMS with the elapse of time. It is well known that the decrease in current density can be ascribed to the accumulation of adsorbed reaction intermediates on the surface of the active sites in the electrodes.109 As the surface to volume ratio for a nanomaterial is substantially higher than that for its microstructural counterpart, it is obvious that more adsorption of the reaction product over the electrode of CZNP will take place in comparison to the electrode of CZMS. Thus, the rapid decrease of current density in the case of CZNP compared to CZMS is expected. From Figure13, it is evident that both samples swiftly respond to H2O2 and the average times required to reach a steady state current are 42 and 8 s for CZNP and CZMS, respectively. CZMS and CZNP respond linearly to H2O2 in the concentration range from 58 μM to 4.12 mM and 29 μM to 11.76 mM, respectively, with values of correlation coefficient (R2) of 0.99 and 0.98 for CZMS and CZNP, respectively (Figure 14a,b). From the slope of the fitted straight line curves of current density versus molar concentration of H2O2, we have estimated the sensitivity of the samples as the H2O2 sensor. The values of sensitivity for CZMS and CZNP are 22.73 and 5.42 μA mM−1 cm−2, respectively, and the values of the lower detection limit are 58.8 and 29.4 μM, respectively, toward H2O2 detection. Thus, CZMS is electrochemically more active than CZNP, and it exhibits high sensitivity and rapid response toward the detection of H2O2. It can be concluded that the self-assembly of nanoparticles in microspherical morphology plays an important role in determining the electrochemical activity of these samples. It may further be noted that the band gap energy of CZMS is lower than that of CZNP. It may therefore be inferred that CZMS exhibits better electrochemical activity than CZNP due to its lower band gap energy. The values of sensitivity, linear range, and lower detection limit have been compared with those of several oxides, spinel nanostructures, nanocomposites, and microstructures, and the outcome is summarized in Table 5. It has been observed that the Co0.3Zn0.7Fe2O4 microsphere based electrode has moderate sensitivity toward H2O2 sensing.

in CZMS has been considerably masked by the strong interparticle interaction originated from aggregate formation. The values of MS for CZMS are considerably higher than those for its nanoparticle counterparts. The Lorentzian fitting of the infield Mössbauer spectrum suggests that the cation configuration of the system is (Zn2+0.46Fe3+0.54)A[Zn2+0.24Co2+0.3Fe3+1.46]BO4, which is different from the equilibrium cation distribution. The electrochemical properties of CZMS and CZNP were investigated by performing cyclic voltammetry (CV) and galvanostatic charging−discharging (GCD) measurements. Both the CV and GCD measurements indicate that the values of specific capacitance of CZMS are higher than those of CZNP. We have also recorded the CV of the samples in different concentrations of H2O2 to probe the potentials of CZMS and CZNP toward the electrochemical reduction of H2O2. The results suggest that both CZMS and CZNP can detect H2O2 and CZMS has better sensitivity toward the detection of H2O2 than CZNP. It has been found that CZMS exhibits better electrochemical activity than CZNP due to its self-assembled spherical morphology and lower band gap energy than CZNP. Thus, it can be inferred that CZMS-based sensors are more electrochemically active than CZNP-based H2O2 sensors.

CONCLUSIONS Three-dimensional self-assembled hierarchical microspheres of Co0.3Zn0.7Fe2O4 have been successfully synthesized by a facile, one-pot, and low-temperature solvothermal method. The structural, microstructural, optical, magnetic, and hyperfine properties of the sample have been thoroughly investigated by PXRD, FESEM, HRTEM, FTIR, DRS, PL, dc magnetic, and Mössbauer spectroscopic studies. The Rietveld refinement of the PXRD pattern suggests that the sample under investigation is single-phase cubic spinel ferrite, which crystallizes in the Fd3m space group. The PXRD and HRTEM studies suggest that CZMS with an average diameter of ∼121 nm is composed of individual Co0.3Zn0.7Fe2O4 ferrite nanoparticles of ∼10 nm size. The dc magnetic and Mössbauer spectroscopic studies indicate that CZMS is ferrimagnetic in nature at low temperature and at room temperature; it exhibits the collective magnetic state due to strong interparticle interaction. Although the size of the individual particles of CZMS is well below the superparamagnetic size limit, the superparamagnetic relaxation

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b01362.

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Powder X-ray diffraction pattern of Co0.3Zn0.7Fe2O4 microsphere (CZMS) fitted by MAUD2.33. Histogram showing the distribution of the diameter of the Co0.3Zn0.7Fe2O4 microspheres (CZMS). Selected area electron diffraction (SAED) pattern of CZMS. EDS spectrum of Co0.3Zn0.7Fe2O4 microspheres (CZMS). EDS spectrum of Co 0.3 Zn 0.7 Fe 2 O 4 nanoparticles (CZNP). FTIR spectrum of Co0.3Zn0.7Fe2O4 nanoparticles (CZNP). UV−vis spectra of CZMS and CZNP. Fitted the first quadrant virgin curve of M−H loop of the sample at 5 K using MATLAB. FESEM micrograph of the sample (a) before and (b) after the performance of 1000 cycles of charging and discharging. Structural, microstructural, and refinement parameters of the sample obtained from MAUD (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Swapan Kumar Bhattacharya: 0000-0002-1218-1860 Sanjay Kumar: 0000-0002-0584-0901 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.M. (IVR No. 201400001001) gratefully acknowledges the Department of Science and Technology (DST), Govt. of India, for providing the INSPIRE fellowship. K.S. (Ref. No: 20/12/ 2015(ii)EU-V) acknowledges UGC, New Delhi, for a senior research fellowship. We gratefully acknowledge the support received from the UGC-DAE CSR, Indore Center, in 12642


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connection to infield Mössbauer measurement. We also acknowledge Prof. V. R. Reddy of UGC-DAE CSR, Indore Center, for his immense help regarding the infield Mössbauer spectroscopic study. The UPE program of UGC and the PURSE program of DST, Govt. of India, are also acknowledged. D.M. acknowledges Chandernagore College, Hooghly, WB, India, for permitting honorary research work.

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