Materials Research Bulletin 48 (2013) 4775–4779

Synthesis, electrical and dielectric characterization of cerium dopednano copper ferrites

Muhammad Aslam Malana *, Raheela Beenish Qureshi, Muhammad Naeem Ashiq,Zafar Iqbal Zafar

Department of Chemistry, Bahauddin Zakariya University, Multan 60800, Pakistan

A R T I C L E I N F O

Article history:

Received 29 July 2012

Received in revised form 29 July 2013

Accepted 16 August 2013

Available online 28 August 2013

Keywords:

A. Semiconductors

D. Dielectric properties

D. Electrical properties

A. Nanostructures

C. X-ray diffraction

A B S T R A C T

The nanosized CuFe2�xCexO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8) ferrites doped with cerium are synthesized by

chemical co-precipitation method. The synthesized materials are characterized by XRD, FTIR, TGA and

SEM. XRD analysis of cerium substituted copper ferrites confirms the cubic spinel structure. The average

crystallite size calculated by using Scherrer’s formula ranges from 37 to 53 nm. The values of cell

constant and cell volume vary with the dopant concentration. These variations can be explained in terms

of their ionic radii. The DC electrical resistivity, measured by two point probe method, increases with

increase in dopant concentration while it decreases with rise in temperature exhibiting semiconductor

behaviour. Energy of activation of these ferrites is calculated by using Arrhenius type resistivity plots.

Dielectric measurements of the synthesized compounds show exponential decrease in dielectric

constant and dielectric loss factor with increase in frequency. This indicates the normal dielectric

behaviour of ferrites.

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1. Introduction

For the last two decades nanomaterials (crystallite size<100 nm) have been considered the most popular emerging fieldof research. Many studies have been focused on the preparation ofnovel nanometal oxides because of their exclusive size basedproperties [1,2]. Similar features observed in a class of suchmaterials, spinel ferrites, MFe2O4 (where M is a divalent cation) hasrendered it of enormous technological and industrial importancedue to their improved electrical and magnetic properties [3].Ferrites when nanosized exhibit higher efficiency, low cost andproper dielectric loss and hence, find potential applications inmemory core and microwave devices [4–6] . Moreover, they havevast applications in the fabrication of magnetics, high frequencytransformers, analogue devices, antennae and radar devices [7–9].The electrical properties of spinel ferrites depend on the method ofpreparation, chemical composition, grain size, doping additives[3,10,11] and the concentration of ferrous and ferric ions as well astheir distribution among the tetrahedral and octahedral sites [12].

The substitution of different cations in ferrite materials inducesvariations in electrical properties [13,14]. The rare earth ion

* Corresponding author. Tel.: +92 61 9210092; fax: +92 61 9210085.

E-mail addresses: draslammalana@gmail.com, draslamalana@yahoo.com

(M.A. Malana).

0025-5408/$ – see front matter � 2013 Published by Elsevier Ltd.

http://dx.doi.org/10.1016/j.materresbull.2013.08.021

substitution in ferrites has been reported by many researchers toenhance their electrical and magnetic properties [15–20].

The conventional method for the synthesis of ferrites is solid statereaction method in which mixing of oxides or carbonates withintermittent grinding is followed by annealing at high temperature(1573 and 1973 K) [21]. This process of synthesis is simple yet it hasmany limitations such as larger particle size, less homogeneity,greater time consumption and high reaction temperature [22]. Onthe other hand, wet methods such as co-precipitation, sol-gel, citrategel, micro emulsion and spray pyrolysis, produce sub-micron sizedparticles with high degree of homogeneity and good control ofstoichiometry [3]. In the present studies, chemical co-precipitationmethod is selected due to its simplicity, lower cost and freedom ofthe synthesized material from contaminations [23].

We report the synthesis of cerium doped copper ferrites anddetailed studies of their electrical and dielectric characteristics inthis manuscript. Thermal, spectroscopic and microscopic char-acterizations of these nanomaterials are also discussed.

2. Experimental

Chemicals used in the synthesis of Ce substituted copperferrites were CuCI2�2H2O (99%; Merck), Fe(NO3)3�9H2O (97%;Riedel Dehaen Seelze, Germany), CeCl3�7H2O (98.5%; Merck) andNH3 solution (33%; Merck).

Ferrite nanoparticles CuFe2�xCexO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8)have been synthesized by chemical co-precipitation method [3].

M.A. Malana et al. / Materials Research Bulletin 48 (2013) 4775–47794776

The aqueous solutions of CuCl2�2H2O, Fe(NO3)3�9H2O and CeCl3�7H2O in equal volumes with their desired concentrations weremixed in a 2000 ml beaker with constant stirring and heating up to333 K. After half an hour, 2.0 M ammonia solution was added dropwise (as precipitating agent) to the reaction mixture and the pH waskept between 10 and 11. The reaction temperature was kept at 333 Kfor 3 h. Precipitates thus obtained were washed with deionizedwater to remove the water soluble impurities. The precipitates werekept overnight in an electric oven at 373 K to remove water contents.The dried precursor was grinded to fine powder in a clean agatemortar and pestle and annealed in air at 1073 K for 8 h in an electricfurnace (PLF 160/7) programmed at 5 K/min.

The structural characterization of the prepared materials wascarried out by XRD analysis using Philips X’Pert PRO 3040/60diffractometer which uses CuKa as a radiation source. The FTIRspectra of CuFe2O4 were also recorded using FTIR-8400 (Shimdzu,Japan) for structural elucidation of the synthesized compound. Thethermal characterization of CuFe2O4 was carried out by TGA byusing Perkin Elmer TG/DTA Diamond instrument.

JEOL-JSM-6700F field emission scanning electron microscope(SEM) was used to obtain the SEM micrographs of the synthesizedcompounds. For this purpose, they were mounted on aluminiumstud using adhesive graphite tape and sputter coated gold beforeanalysis. Temperature dependence of dc electrical resistivity for Cedoped copper ferrites was measured by two point probe method.The pellets used for these measurements were 0.13 cm in diameterand 0.14-0.26 cm in thickness. Dielectric measurements of thesynthesized materials were carried out at room temperature in afrequency range of 100 Hz to 3.0 MHz by using the instrument(6440B, Wayne Kerr).

3. Results and discussion

3.1. Characterization

3.1.1. Thermal analysis

TGA/DTG curve of CuFe2O4 is shown in supplementary material.The 1st weight loss at 363 K in TGA curve shows the loss of free

Fig. 1. (a) FTIR spectrum of unannealed CuFe2O

water from the copper ferrite material and has a correspondingendothermic peak at the same temperature in DTG curve. The 2nddrop in weight at 483 K can be ascribed to the elimination ofhydrated water which is in agreement with the endothermic peakat about 473 K in DTG curve. A weight loss occurring in the rangefrom 523 K to 633 K may be attributed to the removal of ammoniaand nitrates adsorbed at the surface of metal hydroxides. Thematching peaks for this loss in DTG curve are observed at 623 and693 K. The conversion of metal hydroxide into oxide is indicated at703–863 K which is represented as endothermic peak in DTG curveat 873 K. Above 863 K, no change in weight loss is observed. Thisconstancy of the curve corresponds to the formation of pure spinelphase [24].

3.1.2. FTIR analysis

FTIR spectrum of unannealed CuFe2O4 is shown in Fig. 1a. Thepeaks at 449 cm�1, 516 cm�1 and 570 cm�1 correspond to metaloxygen bonds. The peaks at 829 cm�1 and 1375 cm�1 correspondto N–O bending and N–O stretching vibrations, respectively, whichshow the presence of some residual nitrates in the synthesizedcopper ferrite. The other significant bands in the spectrum appearat 1604 cm�1 and 3500–3000 cm�1. The former represents O–Hbending vibrations and the latter bands correspond to O–Hstretching of water present as moisture.

FTIR spectrum of annealed CuFe2O4 is illustrated in Fig. 1b.Disappearance of three peaks in the region 700–400 cm�1 andappearance of single peak at 599 cm�1 confirms the formation ofpure spinel phase after annealing. The peaks at 819 cm�1 and1375 cm�1 (corresponding to N–O bending and stretching vibra-tions) observed in FTIR spectrum of unannealed compound areabsent in the spectrum of annealed copper ferrite material. Thisshows complete removal of nitrates (adsorbed at surface of thesynthesized material) during annealing. The peak at 1604 cm�1 inunannealed material show less intensity in annealed nanomaterialat 1708 cm�1 depicting removed O–H bending vibrations. Simi-larly, peaks in 3500–3000 cm�1 region are also invisible showingthe conversion of hydroxide into oxide on annealing. Less intensepeaks in the 4000–3000 cm�1 region show O–H stretching

4. (b) FTIR spectrum of annealed CuFe2O4.

Fig. 2. (a) SEM micrograph of CuFe2O4. (b) SEM micrograph of CuFe1.6Ce0.4O4.

20 30 40 50 60 702-Theta (degree )

a

d

e

220

Inte

nsity

(au

) c

b

311222 400

511331

422 440 531

Fig. 3. Powder XRD patterns for CuFe2�xCexO4 nanomaterials (a) x = 0.0, (b) x = 0.2,

(c) x = 0.4, (d) x = 0.6 and (e) x = 0.8.

Table 1Structural parameters of the synthesized CuFe2�xCexO4 (x = 0.0–0.8).

Compound D (nm) a (A) V (A3) rx (g/cm3)

CuFe2O4 37.352 8.211 553.719 5.728

CuFe1.8Ce0.2O4 52.880 8.338 579.736 5.858

CuFe1.6Ce0.4O4 41.496 8.375 587.494 6.162

CuFe1.4Ce0.6O4 43.320 8.373 587.216 6.547

CuFe1.2Ce0.8O4 53.806 8.320 575.939 7.064

M.A. Malana et al. / Materials Research Bulletin 48 (2013) 4775–4779 4777

vibrations which may be interpreted in terms of hygroscopicnature of the synthesized material [25].

3.1.3. SEM analysis

The SEM images of CuFe2O4 and CuFe1.6Ce0.4O4 are shown inFig. 2a and b, respectively. The particle size calculated from SEMimage of CuFe2O4 is in the range of 20–40 nm. The surface of thecopper ferrite material appears to be rough and nanoparticles lookspherical in shape. The particles have no clear boundaries as theyare aggregated. Roughness of the surface may be due toagglomeration of individual nanoparticles forming aggregates[24]. When cerium is doped in CuFe2O4 at the place of iron thenanoparticles configure edged shape with particle size in the rangeof 20–50 nm.

3.2. X-ray diffraction analysis

Fig. 3a–e shows the XRD patterns of the prepared nanoma-terials. All the peaks match with the standard pattern (ICSD 01-077-0010) confirming the presence of cubic spinel phase. Theaverage crystallite size of the synthesized compounds calculatedby using Scherrer’s formula (Eq. (1)) [26] is in the range of 37–53 nm.

D ¼ klb cos u

(1)

where k is Scherrer’s constant, l is the wavelength of X-rayused, b is the full width at half maxima (FWHM) and u is Bragg’sangle.

The lattice constant (a), cell volume (V) and X-ray density (rx)are calculated by using Eqs. (2)–(4) [27,28] and the results aregiven in Table 1.

a ¼ ½d2ðh2 þ k2 þ l2Þ�1=2

(2)

V ¼ a3 (3)

rx ¼ZM

NAV(4)

where Z is the number of formula unit in a unit cell, M is themolecular weight of the compound and NA is the Avogadro’snumber.

The lattice constant increases with dopant (cerium) concen-tration, the change being interpreted in terms of ionic radii. Theionic radius of Fe3+ is 0.645 A [29] and that of Ce3+ is 1.143 A [30].When cerium is doped at the place of iron, lattice constantincreases. However, when concentration of cerium exceeds(x = 0.5) then lattice constant decreases. This behaviour can beinterpreted in terms of lower solubility of cerium as compared tothat of iron. The peaks shifting has also been observed in the XRDpatterns with the substituent content which also indicate thevariation in lattice parameters.

Cell volume is found to vary in a similar manner to that of thelattice constant (Table 1). X-ray density for the synthesizednanomaterials increases as the concentration of cerium increases.This can be attributed to greater molar mass of the dopant (cerium)as compared to that of iron.

050001000015000200002500030000350004000045000

350 400 450 500

ρ *

106

(ohm

cm

)

T (K)

x=0.0x=0.2x=0.4x=0.6x=0.8

Fig. 4. Graph of DC electrical resistivity of CuFe2�xCexO4 (x = 0.0–0.8) vs.

temperature.

Table 2Activation energies calculated from Arrhenius type plots.

Compound Activation energy (eV)

CuFe2O4 0.219

CuFe1.8Ce0.2O4 0.600

CuFe1.6Ce0.4O4 0.436

CuFe1.4Ce0.6O4 0.256

CuFe1.2Ce0.8O4 0.254

0

100

200

300

400

500

600

700

800

900

0 5 10 15 20

K

lnf

x=0.0

x-0.2

x=0.4

x=0.6

Fig. 6. Graph of dielectric constant (K) of CuFe2�xCexO4 (x = 0.0–0.6) vs. ln f.

250

M.A. Malana et al. / Materials Research Bulletin 48 (2013) 4775–47794778

3.3. Electrical resistivity measurements

The electrical resistivity of the Ce doped copper ferrites wasmeasured by two point probe method [31] as a function oftemperature. The resistivity values exhibit a decreasing behaviourwith increasing temperature (Fig. 4) showing the semiconductornature of these ferrites. This loss in resistivity with temperature isattributed to the thermally activated mobility of the chargecarriers [32].

Conduction mechanism in ferrites, however, is different fromthat in semiconductor. In ferrites, temperature affects the mobilityof charge carriers which influences the conductivity while carriersconcentration remains unaffected with temperature variation. Thecharge carriers in ferrites are localized at magnetic atoms while insemiconductors charge carriers occupy states in wide energybands. According to hopping mechanism, the change in themobility of charge carriers with temperature leads to theconduction of current by hoping from one iron atom to the next[33]. Hopping between A (tetrahedral) sites does not occur due tothe reason that there are only Fe3+ ions at A sites. Fe2+ ions occupy B(octahedral) sites only. So conduction in ferrites is attributed tohopping of electrons between Fe2+ and Fe3+ ions at octahedral sites[34]. When cerium is doped at the place of iron, resistivity ofsample increases. It can be explained on the basis of greaterresistivity of cerium (75 mV cm) as compared to that of iron(9.71 mV cm) [35].

The energy of activation (Ea) given in Eq. (5) [3] is calculatedfrom the slope of the line obtained by plotting a graph between ln rand reciprocal of temperature (Fig. 5) and its values are listed inTable 2.

ln r ¼ ln r0 þ Ea

kbT(5)

The energy of activation of the synthesized ferrites increasesfrom 0.21 to 0.60 eV up to dopant concentration of x = 0.0–0.2 andthen decreases at higher dopant content.

1516171819202122232425

2 2.2 2. 4 2.6 2. 8

ln ρ

1000/T (K-1)

x-0.0x=0.2x=0.4x=0.6x=0.8

Fig. 5. Graph of ln r vs. reciprocal of temperature for CuFe2�xCexO4 (x = 0.0–0.8).

3.4. Dielectric properties

It can be seen from Figs. 6 and 7 that the value of dielectricconstant (K) and dielectric loss factor (D) decreases continuouslywith increasing frequency at room temperature showing normaldielectric behaviour of ferrites. This dielectric behaviour of ferriteswas also observed by many investigators [36–38].

It can be noticed from Figs. 6 and 7 that pure sample (CuFe2O4)has greater value of dielectric constant, dielectric loss factor andconductivity as compared to any cerium substituted copperferrites. This high value can be explained on the basis that it hasthe maximum number of Fe2+, which exchange electrons betweenFe2+ and Fe3+ and that they are responsible for the maximumpolarization [36].

By doping cerium at the place of iron, the value of dielectricconstant, dielectric loss factor and conductivity decreases. Thismay be due to the decrease in Fe–Fe interactions [39]. Murthy andSobhanadri [40] reported a strong correlation between thedielectric behaviour and conduction mechanism of ferrite.Assuming that the mechanism of the conduction process in ferriteto be similar to that of the polarization process, they concludedthat electronic exchange between Fe2+ and Fe3+ causes localdisplacements which determine the polarization of the ferrites.

0

50

100

150

200

0 5 10 15 20

D

lnf

x=0.0

x=0.2

x=0.4

x=0.6

Fig. 7. Graph of dielectric loss (D) of CuFe2�xCexO4 (x = 0.0–0.6) vs. ln f.

M.A. Malana et al. / Materials Research Bulletin 48 (2013) 4775–4779 4779

4. Conclusion

For synthesis of cerium doped copper ferrites CuFe2�xCexO4

(x = 0.0, 0.2, 0.4, 0.6, 0.8), co-precipitation method was used whichis simple and economical. The crystallite size of the synthesizedferrites is in the range of 37–53 nm i.e. sufficiently small to be usedin high density recording media. The lattice constant increases upto x = 0.4 due to larger ionic size of cerium and then decreases dueto lower solubility of cerium as compared to that of iron. The DCelectrical resistivity increases up to x = 0.2 and after that electricalresistivity decreases. The dielectric constant and dielectric lossfactor decrease exponentially with increase in frequency reflectingnormal dielectric behaviour of ferrites.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in

the online version, at http://dx.doi.org/10.1016/j.materresbull.2013.

08.021.

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