H.B. Sharma, N.G. Boinis Singh, S. Bobby Singh and Th. Dhanabati Devi

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Synthesis and Characterization of Cobalt Ferrite (Cofe2O4)Nanoparticles by Sol-Gel Autocombustion Method

H.B. SHARMA, N.G. BOINIS SINGH, S. BOBBY SINGH* and TH. DHANABATI DEVIDepartment of Physics, Manipur University, Canchipur-795 003

*E-mail: sbsoram@gmail.com

AbstractCobalt ferrite nanoparticles were synthesized by the sol-gel autocombustion method. To modify the features ofthe particles according to different requirements, the molar ratio of the metal nitrates to citric acid were varied.The final products obtained with different starting conditions were characterized through X-ray diffraction,Scanning Electron Microscopy and Energy Dispersive X-ray. Dielectric and magnetic measurements werefinally carried out and the connection between structural and magnetic properties will be discussed.

Key words : Nanoparticle, ferrite, sol-gel, dielectric, magnetic.

1. Introduction

In recent years, nanocrystalline materials arebecoming a subject of intense research because oftheir unique properties, both from the fundamentaland application point of view [1]. Among the ferrites,the most popular type is the cubic spinel structure,which has tetrahedral [A] site and octahedral [B]site in the AB2O4 crystal structure [2]. Due to thelarge electronegativity of oxygen, the ionic type ofbonds prevails in almost all oxide spinels [3]. Softspinel ferrite (MFe2O4, Ni, Zn, Mn, etc.)nanoparticles have been intensively investigated dueto their remarkable magnetic and electricalproperties and wide practical applications inferrofluids, magnetic drug delivery, magnetic high-density information storage [4-6]. Hard spinelferrites, CoFe2O4 is an interesting magnetic materialbecause of its high coercivity, moderate saturationmagnetization, excellent chemical stability andmechanical hardness. The properties of ferritenanoparticles are influenced by the composition andmicrostructure, which are sensitive to thepreparation methodology used in their synthesis.There are several different synthesis methods usedto generate ferrites such as sol-gel [7], co-

Invertis Journal of Science and Technology, Vol. 7, No. 2, 2014. ; pp. 78-84

precipitation [8], hydrothermal [9], etc. Recently, asol-gel auto-combustion method has attracted muchattention due to its inherent advantages of lowprocessing temperature, homogeneous reactantdistribution, the products obtained by this methodexhibit high crystalline quality, narrow sizedistribution and uniform shape [10]. It has beenreported that in chemical solution method pH hasstrong effect on the properties of ferritenanoparticles. Herein, we reported a simple sol gelauto-combustion method to synthesizenanocrystalline cobalt ferrite. The structural,dielectric and magnetic properties of nanocrystallinecobalt ferrites in relation to the metal nitrate to citricacid ratios were investigated.

2. Experimental

Nanocrystalline cobalt ferrite were prepared bysol gel auto-combustion method, using theanalytically pure grade [Fe(NO3)3.9H2O],[Co(NO3)26H2O], and citric acid (C6H8O7). Citricacid is used as a fuel. A stoichiometric amount ofmetal nitrates were dissolved in a certain amount ofdeionized water by stirring on a hotplate at 50oC.Citric acid was then added to the mixture solution

Synthesis and Characterization of Cobalt Ferrite (Cofe2O4) Nanoparticles by Sol-Gel Autocombustion Method

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to chelate metal ions. Four samples of differentmolar ratios of metal nitrate to citric acid such as3:1, 3:2, 3:3 and 3:4 were prepared. These foursamples will be named as CFO1, CFO2, CFO3 andCFO4 respectively hereafter. The pH of thecorresponding solutions thus obtained resulted <1.The dark brown solutions were allowed to evaporateon a hot plate maintaining the solution temperatureat 80-90°C. As a result of the increasingconcentration, a gel started to form. A suddenincrease of the temperature (T>200°C) allowed toconvert directly the gels into the correspondingnanocrystalline CoFe2O4 powders. The as-preparedpowders were then calcined at 400oC for 4 hrs infree atmosphere. X-ray diffraction (XRD) patternsof the samples were collected on a Philips X'pertPro X-ray diffractometer. SEM images and EDAXwere carried out on a FEI QUANTA 250 scanningelectron microscope. Magnetic measurements werecarried out at room temperature using a vibratingsample magnetometer (VSM).

3. Results and Discussion

3.1 X-ray Diffraction Analysis

Figure 1 shows the X-ray diffraction patternsof the four CFO samples calcined at 400oC for 4hrsin free atmosphere. These patterns confirm theformation of cubic spinel type lattice of CoFe2O4,which matches well with the standard XRD pattern(ICDD card No: 22-1086). All peaks correspondto a cubic spinel type lattice (space group Fd3m). Itis observed from Fig. 1, that with increase in molaramount of the citric acid the corresponding XRDpeaks shifts towards the lower angle suggesting anincrease in lattice parameter. The lattice parametera=b=c of the unit cell were evaluated from theXRD pattern and found to increase from 8.33 to8.36 Å approaching to that of standard value (8.39Å)of bulk CFO sample [11], with the increase in citricacid amount the crystallite size increases. Theaverage crystallite size of the CFO samples were

Inte

nsity

(arb

.uni

ts)

2θ (Degrees)

(220

)

CF01

CF04

CF02

CF03

20 30 40 50 60 70

(440

)

(511

)(4

42)

(222

)

(440

)

(311

)

Fig. 1. X-ray diffraction pattern of CoFe2O4 samples calcined at 400°C for 4 hrs

Table 1Structural and dielectric properties of CoFe2O4 nanoparticles

CoFe2O4 a Crystallite size Grain size Dielectric DissipationSample (nm) (nm) from (nm) from constant (ε′) factor (D)

XRD SEM at 100 kHz at 100 kHz

CFO1 8.33 27 42 104 0.05CFO2 8.33 41 64 218 0.05CFO3 8.35 63 82 313 0.05CFO4 8.36 72 95 354 0.06

H.B. Sharma, N.G. Boinis Singh, S. Bobby Singh and Th. Dhanabati Devi

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determined from the full width at half maximum(FWHM) of the major peak, in Fig. 1 using theScherrer's formula and are reported in Table 1. Thecrystallite sizes of the samples were found toincrease from 27 nm to 72 nm with the increase inmolar amount of citric acid.

3.2 Morphological Behavior of CoFe2O4

The structural morphology of nanoparticles wasinvestigated through SEM. Figure 2 shows the SEMimages of CoFe2O4 nanoparticles. The SEM images

Fig. 2. SEM photograph of four different CoFe2O4 nanoparticles

CF01 CF02

CF03 CF04

show that the CoFe2O4 nanoparticles prepared bysol-gel auto-combustion method have uniformspherical structure morphology with a narrow sizedistribution of particles. Some moderatelyagglomerated particles as well as separated particles

are also present in the images. The average grainsize was found to increase from 42 nm to 95 nmwith increase in mole amount of citric acid present.From the morphology it is interesting to note thatwhen citric acid amount content increases the shapedoes not change but affects only the radius of thenanoparticles.

3.2.1 EDAX spectrum

Figure 3 shows the EDAX spectra of the CFO1sample and confirmed the ratio of the transition

metal atoms in each material according to the nominalstoichiometry. The atomic ratio of Fe:Co for theentire sample (Fig. 3) is maintained at ~ 2:1. Theseresults revealed the sustained spinel structure of theCoFe2O4 crystal at low calcinations temperature.

Synthesis and Characterization of Cobalt Ferrite (Cofe2O4) Nanoparticles by Sol-Gel Autocombustion Method

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C:/edax32\genesis\genspc.spc

Label A: CFOSC

0K

CoK

FeK

CoK

FeK

FeL

FeL

1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 KeV

Fig. 3. EDAX line scanning spectrum of sample CFO1 nanopowder

3.3 Dielectric Properties

Figure 4 (a) and (b) displays the variation ofdielectric constant and dissipation factor for fourCFO samples as a function of frequency at roomtemperature from 1k Hz-1MHz. It has beenreported that the materials having high d.c. electricalresistivity acquires low values of dielectric constantand vice versa [12]. It is also reported that thepolarization in ferrites is mainly contributed by thespace charge polarization, which is governed by thenumber of space charge carriers and theconductivity in materials [13] and hopping exchangeof the charges between two localized statesgoverned by density of the localized state andresultant displacement of charges w.r.t. the externalfield. So, the decrease in dielectric constant withincreasing frequency may be attributed to the twofactors that (i) the electron exchange between Fe2+

and Fe3+ ions cannot follow the change of theexternal applied field beyond certain frequency [14]and (ii) the dipoles due to the space-chargepolarization or Maxwell-Wagner type interfacialpolarization do not respond at higher frequencies.

It is observed from the figure that the roomtemperature dielectric constant increases from 104to 354 with increase in citric acid amount content.The increase in dielectric constant in our samplemight me be due to the increase in grain size assuggested by XRD and SEM.

Figure 4(b) shows the variation of dissipationfactor of CFO samples as a function of frequency atroom temperature from 1 kHz-1MHz. The dielectricloss factor is considered to be the most importantpart of the total core loss in ferrites [15, 16]. All thesamples show low dielectric loss of ~0.05 at 100kHz. The dielectric loss originates from twomechanisms: resistive loss and relaxation loss.

3.4 Magnetic Properties of the Products

The magnetic properties of the CoFe2O4nanoparticles was measured through vibratingsample magnetometer (VSM) at a maximum appliedfield of 22,000 Oe at room temperature and isshown in Fig. 5. The saturation magnetization (Ms),remanent magnetization (Mr) and coercive field (Hc)of the sample were found to be 38 emu/g, 20.59

Energy (KeV)

H.B. Sharma, N.G. Boinis Singh, S. Bobby Singh and Th. Dhanabati Devi

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10 100 1000

(b)

Frequency

500

450

400

350

300

250

200

150

100

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

10 100 1000Frequency (kHz)

(Fig. a)

(Fig. b)

Fig. 4. Variation of room temperature (a) dielectric constant and (b) loss tangent as a function of frequencyfor CoFe2O4 nanoparticles

CF01CF02CF03CF04

CF01CF02CF03CF04

Synthesis and Characterization of Cobalt Ferrite (Cofe2O4) Nanoparticles by Sol-Gel Autocombustion Method

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CFO140

20

0

-20

-40

-18.0K -12.0K -6.0K 0 0 6.0K 12.0K 18.0K

Mag

netiz

atio

n (e

mu/

gm)

Applied magnetic field (Oe)

Fig. 5. Room temperature magnetic hysteresis loop of CoFe2O4 nanoparticles

emu/g and 1500 Oe, respectively. This value ofsaturation magnetization is much lower than the bulkvalue of 80 emu/g [23]. The high coercivity value inour sample indicates that the particles formed areindeed single domain with no nonmagnetic impurities.It is very well reported in literature that the magneticproperties of nanosize particles depend on thepreparation method and the particle size. The lowervalue of saturation magnetization and higher valueof coercivety in our sample might be due to smallergrain size.

4. Conclusion

Spinel CoFe2O4 ferrite system was successfullyprepared by Sol-gel autocombustion technique withnano-size dimension. The XRD revealed that theprepared samples possess single phase cubic spinelstructure. The nanocrystalline nature of theprepared samples was also confirmed from SEM.The EDAX confirmed the ratio of the transition metalatoms in each material according to the nominalstoichiometry. It is also observed that the dielectricconstant increases with increase in the molar ratioof metal nitrate to citric acid amount. The saturation

magnetization (Ms), remanent magnetization (Mr) andcoercive field (Hc) of the sample were found to be38 emu/g, 20.59 emu/g and 1500 Oe, respectively.

Acknowledgement

One of the authors, Th. Dhanabati Devi isthankful to the UGC, New Delhi, for providingfinancial assistance as project fellow.

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