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Materials Chemistry and Physics

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The influence of lithium on the optical and electrical properties of CoLixFe1-xO4 nanoferritesM.K. Shobanaa,∗, Hyeji Parkb, Heeman Choeba Department of Physics, School of Advanced Sciences, VIT University, Vellore, Tamil Nadu, 632014, Indiab School of Materials Science and Engineering, Kookmin University, Seoul, 02707, Republic of Korea

H I G H L I G H T S

• The crystalline nature of the synthesized nanoparticles decreased with increasing Li content.• SEM analysis reveals cubic structure.• The calculated bandgap energies tended to decrease with increasing Li concentration.• The discharge (charge) capacity and coulombic efficiency shows 1258 mAhg−1 (1010 mAh g−1) and 80%.

A R T I C L E I N F O

Keywords:Nanostructured materialsElectronic propertiesOptical properties

A B S T R A C T

Current research on the application of nanoferrites as an electrode material for Li-ion batteries (LIBs) hasreached a tremendously high level. The structural, electrical, and optical properties of nanoferrite materials forLIBs have been characterized in several studies. The intention in the present study was to introduce new ferritematerials for improving the cyclic property and efficiency of Li batteries and to provide further characterizationof the optical and electrical properties of these CoLixFe1-xO4 nanoferrites synthesized using a sol-gel combustiontechnique. Moreover, the synthesized CoLixFe1-xO4 ferrite samples were characterized by identifying theirstructural, electrical, and optical properties. A UV analysis illustrated a decrease in the bandgap energy valueswith increasing Li concentration, while a Fourier transform-infrared spectroscopy study confirmed a spinelferrite structure. Electrochemical performance of the synthesized CoLixFe1-xO4 nanoferrites showed an initialcoulombic efficiency of 80% and an initial discharge capacity of 1258 mAh g−1.

1. Introduction

Ferrites are a great class of oxides with exceptional magneticproperties, which have been examined and applied over the past 50years [1,2]. Their usage covers an extraordinary range of applicationsranging from integrated circuitry with millimeter wave to simple per-manent magnets, power handling, and magnetic recording [3–5]. Forsome applications, ferrites cannot be exchanged by ferromagnetic me-tals; nevertheless, they frequently compete with metals due to com-mercial reasons, among others [2]. The possibility of producing nano-sized ferrites has opened a new, innovative research field withrevolutionary capabilities that are applicable not only in electronics butalso in the field of biotechnology [6].

Research on ceramic compounds mixed with iron oxide and one ormore metals has fascinated the relevant scientific community owing totheir novel properties and technological applications, especially when

the particle size approaches the nanometer scale, as more novel elec-trical and magnetic behavior has been observed in comparison withtheir bulk counterparts [7,8]. In general, the transport properties ofnanomaterials are primarily controlled by the grain boundaries ratherthan the grains [9]. Based on this finding, these magnetic materialshave been explored for use in an extensive range of practical applica-tions and thus are currently replacing conventional materials [10].

It is evident that the phases, crystallite size, and morphology offerrites have a significant impact on their magnetic and electricalproperties, and potential applications; thus, the meticulous fabricationof nanostructured materials with novel morphologies has recentlydrawn considerable attention [11–13]. Since iron is a conventionalmagnetic material, nanoferrites have been used in various fields be-cause of their unique electrical and magnetic properties [14,15]. Nu-merous innovative and effective approaches have been developed toyield nanoferrites with various shapes such as nanowires, nanorods,

https://doi.org/10.1016/j.matchemphys.2019.01.063Received 11 July 2018; Received in revised form 13 January 2019; Accepted 25 January 2019

∗ Corresponding author.E-mail address: mkshobana@gmail.com (M.K. Shobana).

Materials Chemistry and Physics 227 (2019) 1–4

Available online 25 January 20190254-0584/ © 2019 Elsevier B.V. All rights reserved.

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nanotubes, etc. [16–19]. Nanoferrites have suitable properties and haveshown excellent efficiency in the fields of lithium-ion batteries (LIBs),wastewater treatment, microwave devices, solar cells, and drug de-livery [20–25].

In this study, Li-substituted cobalt ferrites were synthesized using asol-gel combustion method with various concentrations of lithium.Subsequently, the synthesized samples were characterized usingpowder X-ray diffraction (XRD) and scanning electron microscopy(SEM) to elucidate their structures, and using UV spectroscopy andFourier transform-infrared (FTIR) spectroscopy to ascertain their op-tical properties. In addition, their band-gap energy values were calcu-lated and electrochemical performance examined by conducting coin-cell testing.

2. Experimental methods

2.1. Sample preparation

The nanoferrite samples were synthesized using commerciallyavailable nitrates as source materials: analytical-grade chromium ni-trate (99% purity, Sigma-Aldrich Co., USA), nickel nitrate (99% purity,Sigma-Aldrich Co., USA), iron nitrate (99% purity, Sigma-Aldrich Co.,USA), and citric acid (99% purity, Sigma-Aldrich Co., USA). The Li-doped cobalt ferrites were synthesized using a sol-gel combustionmethod. Metal nitrates, citric acid, and polyvinyl alcohol (PVA) weredissolved in distilled water and stirred continuously for 3 h (the PVAwas used as a chelating agent). Additionally, they were subjected toheat treatment, as previously reported [26].

2.2. Characterization

The synthesized Li-doped cobalt nanoferrites were characterizedusing XRD with a Bruker D8 Advance X-ray diffractometer and CuKαradiation (1.54056Å). The particle sizes of the samples were confirmedusing SEM (Zeiss Euva 8). An FTIR spectroscopic study was carried outin a KBr medium using a Shimadzu FTIR-8400 in the wave numberranging from 400 to 4000 cm−1 at a resolution of 4 cm−1 and a UV-diffusion reflectance spectra study was performed using a JascoUV–Visible Spectrophotometer (V-670 PC). The electrochemical per-formance was evaluated using a standard coin-cell test with the syn-thesized nanoferrites as the working electrode and Li metal as both thecounter and reference electrodes. The electrolyte was 1.0M LiPF6 in asolution of ethylene carbonate (EC) and diethylene carbonate (DEC)mixed at a volume ratio of 3:7. Galvanostatic test (CTS-Lab, BaSyTec,Germany) was conducted at a constant current of 100 mA g−1 in thevoltage window between 3.0 V and 0.01 V vs. Li+/Li at 25 °C.

3. Results and discussion

The XRD patterns of the CoLixFe1-xO4 (x= 0.2, 0.4, 0.6, 0.8, and 1)nanoferrites are displayed and compared in Fig. 1; they exhibit peakpatterns similar to those of pure cobalt ferrite [27]. The (220), (311),(400), (422), (511), and (440) planes in the XRD patterns reveal thecubic structures of the samples. At the same time, it is seen that thecrystalline nature decreases with increasing Li concentration. More-over, this decrease in intensity is also observed in the XRD patterns withincreasing Li concentration [28]. This is associated with an increase instructural disorder as the crystalline nature decreased with increasing Liconcentration, which led to different inter-atomic distances betweenthe nanoparticles’ surface layers relative to those at their cores and asmaller mass of substituent ions [29,30]. The average particle sizeranged from 20 to 78 nm. The observed pattern indicates a single-phasestructure since there are no additional peaks of any impurities.

Based on the SEM analysis in Fig. 2, the morphological features ofthe CoLixFe1-xO4 nanoferrites with different amounts of Li are com-pared, showing that they all possess cubic morphology. The observed

morphology is in good agreement with TEM analysis reported in pre-vious literatures [31–33]. In addition, the optical properties of theCoLixFe1-xO4 nanoferrites are analyzed and compared in Fig. 3. The

Fig. 1. XRD patterns of the synthesized CoLixFe1-xO4 nanoferrites (x= 0.2, 0.4,0.6, 0.8, and 1).

Fig. 2. SEM images of the synthesized CoLixFe1-xO4 nanoferrites: (a) x=0.2,(b) x= 0.4, (c) x= 0.6, (d) x= 0.8, and (e) x=1.

Fig. 3. Tauc plots to measure the optical bandgaps of the synthesized CoLixFe1-xO4 nanoferrites: (a) x= 0.2, (b) x=0.4, (c) x= 0.6, (d) x= 0.8, and (e)x=1.

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bandgap-energy values of the Li-substituted Co ferrites according to theconcentration of Li are observed. Tauc plots of (αhν)2 vs. photon energy(eV) are used to calculate the indirect bandgap energy values of the Li-substituted Co ferrite by extrapolating the curve to zero absorption. Theslope of the (αhυ)2 values denotes the phonon involvement in an opticalprocess [34,35]. The results show a decrease in bandgap energy withincreasing Li concentration as Eg=0.5 eV, 0.4 eV, 0.3 eV, 0.2 eV, and0.1 eV for x=0.2, 0.4, 0.6, 0.8, and 1, respectively. Based on the ob-served results, the conductivity of the Li-substituted Co ferrites steadilyimproves with increasing Li concentration. Therefore, the results fromthe UV studies suggest that the synthesized CoLixFe1-xO4 nanoferritematerials are applicable in LIBs.

On the other hand, Fig. 4 shows the absorption bands for different Liconcentrations as a function of wavelength, which originated primarilyfrom the absorption and scattering of UV radiation by the magneticnanoparticles in agreement with the previously reported results in theliterature [36]. Thus, it is evident that the Co ferrites showed a sig-nificant blue shift in the absorption peak relative to the bulk absorption,which might have been associated with the quantum size effect thatarose due to the very small size of the nanoparticles [37].

Fig. 5 shows the FTIR spectroscopy analysis recorded in the rage of4000–400 cm−1 for the Li-substituted Co ferrites with varying compo-sition. The peaks around 500 cm−1 show the common characteristics ofspinel ferrites [38]. The frequency-absorption band lies in the range of500–600 cm−1, which is fixed for the vibration of the tetrahedral metalcomplex consisting of a bond between the oxide ion and the tetrahedralsite metal ion (O-M). The existence of a higher frequency band isconfirmed by the stretching vibration within the bonds between mo-lecules/ions, whereas the lower frequency band is originated from thebending vibration. These band locations are in good matching with thetypical infrared absorption bands of Co ferrites [39]. In addition, thebands near 1650 and 3500 cm−1 are due to the C-H stretching vibrationband and O-H stretching interaction through the H band [40,41].

Fig. 6a and b illustrates the voltage profiles of the CoLixFe1-xO4nanoferrites and the cyclic performance of the CoLixFe1-xO4 at a con-stant current density of 100mA g−1 with a voltage between 3.0 and0.01 V. Li can be inserted and reacted with ferrite in the presence of MOand reduced to Fe and Li2O as follows:

M(O) Fe O 6Li 2Fe 3Li O M(O)2 3 2+ + + (1)

Based on Eq. (1), the approximate theoretical capacity of CoLixFe1-xO4 nanoferrites is predicted to be 714 mAh g−1 because the predictedcapacity is calculated based on the number of transferred electrons in

the electrochemical reaction [42].The initial discharge capatity (and the initial charge capacity) and

the initial coulombic efficiency of the synthesized nanoferrites aremeasured as 1258 mAh g−1 (1010 mAh g−1) and 80%, respectively,and exhibit superior capacity and coulombic efficiency as comparedwith its predicted theoretical capacity (714 mAh g−1) and previousreports [43,44]. Although a further systematic study is required to

Fig. 4. UV absorbance spectra of the synthesized CoLixFe1-xO4 nanoferrites: (a)x=0.2, (b) x= 0.4, (c) x=0.6, (d) x= 0.8, and (e) x= 1.

Fig. 5. FTIR spectroscopy of the synthesized CoLixFe1-xO4 nanoferrites: (a)x=0.2, (b) x=0.4 (c) x= 0.6, (d) x= 0.8, and (e) x=1.

Fig. 6. (a) The voltage profile and (b) cyclic performance of the synthesizedCoLixFe1-xO4 nanoferrites at a constant current of 100 mAh g−1 in the voltagewindow between 3.0 and 0.01 V.

M.K. Shobana, et al. Materials Chemistry and Physics 227 (2019) 1–4

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define its salient mechanism and explore its optimized conditions, wedemonstrate for the first time that the CoLixFe1-xO4 nanoferrites syn-thesized in this study can be a promising anode material of LIBs.

4. Conclusions

In this paper, we report the synthesis of Li-substituted Co ferritesusing a sol-gel combustion method with various concentrations of Li.The following conclusions can be drawn:

i. The crystalline nature of the synthesized nanoparticles decreasedwith increasing Li concentration.

ii. The SEM analysis revealed that they all had cubic crystallinestructure.

iii. The calculated and plotted bandgap energies tended to decreasewith increasing Li concentration.

iv. From the FTIR spectroscopy of the Li-substituted Co ferrites withvarying composition, the band positions were in good agreementwith the characteristic infrared absorption bands of Co ferrites.

v. The initial discharge (charge) capacity and initial coulombic effi-ciency were measured as 1258 mAh g−1 (1010 mAh g−1) and 80%,respectively. There is still plenty of room for further developmentdespite the successful demonstration of its potential use as a anodematerial of LIBs.

Acknowledgments

This research was supported by Korea Electric Power Corporation(Grant No.: R18XA06-41). The authors also thank VIT University forproviding ‘VIT SEED GRANT’ for carrying out this research work.

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The influence of lithium on the optical and electrical properties of CoLixFe1-xO4 nanoferritesIntroductionExperimental methodsSample preparationCharacterization

Results and discussionConclusionsAcknowledgmentsReferences