Yttrium-Doped Cobalt Nanoferrites Prepared by Sol–Gel ...heeman/paper/Y-doped... · are named...

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RESEARCH ARTICLE Copyright © 2013 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 13, 3535–3538, 2013 Yttrium-Doped Cobalt Nanoferrites Prepared by Sol–Gel Combustion Method and Its Characterization M. K. Shobana 1 , Wonjong Nam 2 , and Heeman Choe 2 1 Center for Advanced Material Technology, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea 2 School of Advanced Materials Engineering, Kookmin University, Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea Ferrites are extremely important magnetic ceramics in the production of electronic components because they reduce the energy losses by the induced currents acting as electrical insulators. Sim- ilarly, the spinel-structured cobalt-based ferrites are promising materials for stress, torsion sensors and energy storage applications (anode materials in lithium batteries, fuel cells and solar cells). Therefore, many studies have focused on cobalt ferrites obtained using conventional techniques. Different sintering conditions, types and levels of substitution result in different microstructures and magnetostriction coefficients under a wide range of preparation conditions. Despite many attempts, there are no specific reports on the trivalent substitution of yttrium in cobalt ferrite to the best of our knowledge. In the present study, yttrium-doped cobalt ferrite was prepared with different con- centrations to identify the crystallite size with respect to the yttrium concentration, temperature and changes in the structural and electrical properties. In addition, the resistance of the nanostruc- tured yttrium-doped cobalt ferrites nanopowders was analyzed. The resistance was increased by the addition of yttrium to cobalt ferrites. Keywords: Nanostructured Ferrite, Sol–Gel Combustion Method, X-Ray Diffraction. 1. INTRODUCTION Mixed ferrite materials, particularly cobalt-based ferrite materials, have remarkable physical and technological applications. 1–3 More importantly, cobalt-based ferrites have importance in sensors, transformers and catalytic materials. 4–6 For this reason, engineers and scientists are passionately interested in determining their novel proper- ties. Ferrite plays a useful role in many magnetic applica- tions owing to its low electrical conductivity compared to other magnetic materials. Moreover, the electrical proper- ties of ferrite materials have been found to change depend- ing on the substitution of different valance cations and preparation conditions: sintering temperatures, sintering times, and the rate of heating and cooling. 7 8 On the other hand, the effect of the substitution of cations, such as Al 3+ , Dy 3+ and Cr 3+ , on the physical properties of ferrites has already been studied. 9–11 Generally, cobalt ferrite is a significant material for its magnetic and catalytic properties, which depend on the structural and morphological characteristics. Mostly, this Author to whom correspondence should be addressed. type of ferrite is in a spinel form but exhibits large coer- civity that is different from other spinel ferrites. Therefore, many authors 12–15 have examined the saturation magneti- zation and coercivity at room temperature as a function of the crystallite size. Moreover, the spinel ferrites have the resourceful magnetic and electrical properties among other ferrites. In particular, CoFe 2 O 4 has attracted considerable atten- tion because of its structural, magnetic and electrical con- ductivity under a range of conditions 16–19 Furthermore, CoFe 2 O 4 possesses a partially inverse structure with a degree of inversion that depends on the method of prepa- ration and heat treatment 20 21 CoFe 2 O 4 has a spinel crystal structure and crystallizes in a cubic close packed struc- ture of oxygen ions, in which tetrahedral [A] and octahe- dral [B] sites are occupied by cations. At the same time, the magnetization, coercivity, permittivity and conductiv- ity are greatly affected by the porosity, grain size and microstructure of the sample. Similarly, ferrites contain- ing cobalt exhibit interesting properties that make them suitable for a wide range of applications but there are no reports of yttrium doped with cobalt ferrite. Therefore, this study examined the properties of yttrium-doped cobalt J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 5 1533-4880/2013/13/3535/004 doi:10.1166/jnn.2013.7250 3535

Transcript of Yttrium-Doped Cobalt Nanoferrites Prepared by Sol–Gel ...heeman/paper/Y-doped... · are named...

Page 1: Yttrium-Doped Cobalt Nanoferrites Prepared by Sol–Gel ...heeman/paper/Y-doped... · are named CYF1, CYF2, CYF3 and CYF4, respectively. 3. CHARACTERIZATION The prepared nanoferrites

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Copyright © 2013 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 13, 3535–3538, 2013

Yttrium-Doped Cobalt Nanoferrites Prepared by Sol–GelCombustion Method and Its Characterization

M. K. Shobana1�∗, Wonjong Nam2, and Heeman Choe21Center for Advanced Material Technology, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu,

Seoul 136-702, Republic of Korea2School of Advanced Materials Engineering, Kookmin University, Jeongneung-dong, Seongbuk-gu,

Seoul 136-702, Republic of Korea

Ferrites are extremely important magnetic ceramics in the production of electronic componentsbecause they reduce the energy losses by the induced currents acting as electrical insulators. Sim-ilarly, the spinel-structured cobalt-based ferrites are promising materials for stress, torsion sensorsand energy storage applications (anode materials in lithium batteries, fuel cells and solar cells).Therefore, many studies have focused on cobalt ferrites obtained using conventional techniques.Different sintering conditions, types and levels of substitution result in different microstructures andmagnetostriction coefficients under a wide range of preparation conditions. Despite many attempts,there are no specific reports on the trivalent substitution of yttrium in cobalt ferrite to the best ofour knowledge. In the present study, yttrium-doped cobalt ferrite was prepared with different con-centrations to identify the crystallite size with respect to the yttrium concentration, temperature andchanges in the structural and electrical properties. In addition, the resistance of the nanostruc-tured yttrium-doped cobalt ferrites nanopowders was analyzed. The resistance was increased bythe addition of yttrium to cobalt ferrites.

Keywords: Nanostructured Ferrite, Sol–Gel Combustion Method, X-Ray Diffraction.

1. INTRODUCTION

Mixed ferrite materials, particularly cobalt-based ferritematerials, have remarkable physical and technologicalapplications.1–3 More importantly, cobalt-based ferriteshave importance in sensors, transformers and catalyticmaterials.4–6 For this reason, engineers and scientists arepassionately interested in determining their novel proper-ties. Ferrite plays a useful role in many magnetic applica-tions owing to its low electrical conductivity compared toother magnetic materials. Moreover, the electrical proper-ties of ferrite materials have been found to change depend-ing on the substitution of different valance cations andpreparation conditions: sintering temperatures, sinteringtimes, and the rate of heating and cooling.7�8 On the otherhand, the effect of the substitution of cations, such as Al3+,Dy3+ and Cr3+, on the physical properties of ferrites hasalready been studied.9–11

Generally, cobalt ferrite is a significant material for itsmagnetic and catalytic properties, which depend on thestructural and morphological characteristics. Mostly, this

∗Author to whom correspondence should be addressed.

type of ferrite is in a spinel form but exhibits large coer-civity that is different from other spinel ferrites. Therefore,many authors12–15 have examined the saturation magneti-zation and coercivity at room temperature as a function ofthe crystallite size.Moreover, the spinel ferrites have the resourceful

magnetic and electrical properties among other ferrites.In particular, CoFe2O4 has attracted considerable atten-tion because of its structural, magnetic and electrical con-ductivity under a range of conditions16–19 Furthermore,CoFe2O4 possesses a partially inverse structure with adegree of inversion that depends on the method of prepa-ration and heat treatment20�21 CoFe2O4 has a spinel crystalstructure and crystallizes in a cubic close packed struc-ture of oxygen ions, in which tetrahedral [A] and octahe-dral [B] sites are occupied by cations. At the same time,the magnetization, coercivity, permittivity and conductiv-ity are greatly affected by the porosity, grain size andmicrostructure of the sample. Similarly, ferrites contain-ing cobalt exhibit interesting properties that make themsuitable for a wide range of applications but there are noreports of yttrium doped with cobalt ferrite. Therefore,this study examined the properties of yttrium-doped cobalt

J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 5 1533-4880/2013/13/3535/004 doi:10.1166/jnn.2013.7250 3535

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Yttrium-Doped Cobalt Nanoferrites Prepared by Sol–Gel Combustion Method and Its Characterization Shobana et al.

ferrite, particularly the structural and electrical properties,for novel applications.

2. EXPERIMENTAL DETAILS

The prepared ferrites samples were synthesized usingcommercially available constituent metal nitrates. Ana-lytical grade cobalt ferrite (99% Sigma Aldrich, Korea),yttrium nitrate (99% Sigma Aldrich, Korea), iron nitrate(99% Sigma Aldrich, Korea) and citric acid (99% SigmaAldrich, Korea) were used as a source material for thepreparation of proposed ferrite nanoparticles. The ferritenanoparticles were synthesized using a sol–gel combus-tion technique.22 Metal nitrates tals, citric acid (agglomer-ation reducing agent) and polyvinyl alcohol (PVA, burningagent) were dissolved in distilled water and stirred continu-ously for 3 h. The resulting mixture was heat treated underthe appropriate conditions, as reported previously.22 Theprepared powders were calcined at the required tempera-tures for 1 h. The prepared samples are named as follows:yttrium with cobalt ferrite (x = 0�2, ACYF), yttrium withcobalt ferrite (x= 0�4, BCYF) and yttrium with cobalt fer-rite (x= 0�6, CCYF). The above 3 samples were treated at200 �C. The samples calcined at 300,600, 900 and 1200 �Care named CYF1, CYF2, CYF3 and CYF4, respectively.

3. CHARACTERIZATION

The prepared nanoferrites were characterized by pow-der X-ray diffraction (XRD, D-MAX-2200) using CuK�1radiation (1.54056 Å). The structure of the samplewas confirmed using high resolution scanning electronmicroscopy (HR-SEM, NOVA NANO-200). Double-sidedtape was used as the coating surface for the HR-SEMstudies. The sample was sputter-coated with Pt–Pd for100–120 s. Fourier transform infrared spectroscopy (FTIR,Shimadzu, FTIR-8400) was carried out in a KBr mediumover the wave number range, 400–4000 cm−1 with a reso-lution of 4 cm−1. The electrical conductivity of the samplewas analyzed by measuring the sheet resistance using afour point probe (Chang Min Co. Ltd., Korea).

4. RESULTS AND DISCUSSION

Figure 1 shows XRD patterns of the YxCoFe2−xO4

(x = 0�2, 0.4 and 0.6) system; x= 0�2 and 0.4 had a cubicspinel structure and x = 0�6 was biphasic. The peak cor-responding to 33.11� 2� (indicated by ∗ in Fig. 1) wasassigned to a secondary phase at the grain boundaries andwas identified as FeYO3 (iron yttrium oxide) according tothe ICDDPDF #39-1489. In addition, the secondary phaseon the grain boundaries appeared due to the high reactivityof Fe3+ ions with Y3+ ions.23

The lattice constant (a) increased slightly for all com-positions (x = 0�2 to 0.6), such as 8.27, 8.33 and 8.39,

(a)

(b)

Fig. 1. (a) XRD pattern of yttrium doped cobalt ferrite (x = 0�2 to0.6). (b) XRD pattern of yttrium doped cobalt ferrite with respect totemperature.

respectively. This variation can be explained based onthe ionic radii of the substituted ions. The transfer ofsmaller Fe3+ ions (0.64 Å) with larger Y3+ ions (0.95 Å)causes a dilation of the host spinel lattice, which resultsin an increase in lattice constant. This study is consistentwith results reported elsewhere, in that the substitution ofyttrium ions increases the lattice constant.25�26

The lattice parameter for the biphasic samples stillincreased with increasing yttrium content, which suggeststhat the spinel lattice is not compressed by the secondaryphase. Such observations in the lattice constant of rare-earth substituted ferrites were reported by Hemeda et al.26

in the case of Gd substituted in NiFe2O4. Figure 1(b)shows Y0�2CoFe1�8O4 nanoparticles calcined at differenttemperatures; the mean crystallite size was calculatedusing the Scherrer formula. As expected, the crystallitesize increased with increasing heat treatment temperature

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400 nm 400 nm 400 nm

(a) (b) (c)

Fig. 2. (a), (b), and (c) HR-SEM images of yttrium-doped cobalt ferrites (x = 0�2, 0.4, and 0.6).

(300, 600, 900 and 1200 �C) for all samples examined,ranging from ∼ 16 to 80 nm. The increasing crystallitesize with increasing calcination temperature agrees wellwith previous reports.18�22 XRD showed that an increasein yttrium concentration results in an increase in the lat-tice constant, and the crystallite size increases graduallywith increasing calcination temperature. Figure 2 showsthe morphology of the prepared ferrites samples confirm-ing the cubic structure.Figure 3 presents the FTIR spectra of the samples

prepared at 200 �C. The absorption bands were in theexpected range, confirming the formation of a spinel struc-ture. In spinel ferrites, the absorption band at approxi-mately 600 cm−1��1) was assigned to stretching vibrationsof tetrahedral complexes.27 The higher frequency band(�1) was almost constant at all compositions investigated.The bands observed at approximately 3400 and 1500 cm−1

were assigned to the stretching modes and H O H bendingvibration of the free or absorbed water molecules, respec-tively. The peak at approximately 1400 cm−1 was assignedto O H bending. The observed results concurred withthose of a previous report.28 From the FTIR spectra, thefunctional groups were analyzed to confirm the structure.The room temperature sheet resistance of the prepared

Fig. 3. FTIR spectra of yttrium-doped cobalt ferrite (x = 0�2 to 0.6).

sample was measured using a four point probe. Figure 4shows the conductivity as a function of the yttrium concen-tration. The resistance increased with increasing yttriumconcentration, and the conductivity was 0.0285, 0.0134and 0.0092 S/cm, respectively. Yttrium ions occupy theoctahedral sites29 owing to their large ionic radius. Theconcentration of Fe3+ ions decreased gradually at theB-sites when yttrium was substituted in the place of iron.The hopping rate of electron transfer will decrease withdecreasing Fe3+ ions. Therefore, the resistance increasedwith increasing yttrium concentration and thus the activa-tion energies should also increase. The higher resistance athigher yttrium concentrations highlights the strong block-ing of the conduction mechanism ferrous and ferric ionsdue to the presence of yttrium ions at B-sites.30 One pos-sible reason for the increasing resistance might be theincrease in lattice constant. In ferrites, the inter-ionic dis-tances increase with increasing lattice constant. This grad-ual increase in inter-ionic distance enhances the barrierheight encountered by charge carriers, which increases theresistance. According to the observed results, the con-ductivity decreased with increasing yttrium concentration,which might be a useful candidate for high frequencyapplications.

Fig. 4. Conductivity of the yttrium doped cobalt ferrite as a function ofyttrium concentration.

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5. CONCLUSIONS

Yttrium-doped cobalt ferrite (YxCoFe2−xO4� was preparedusing a sol–gel combustion technique. XRD suggested thatthe lattice constant increases with increasing yttrium con-centration and the crystallite size increases with increasingcalcination temperature. FTIR and HR-SEM confirmed thecubic structure of the prepared sample. The resistivity ofthe prepared samples increased with increasing yttrium, sothe conductivity should decrease with increasing yttriumaddition. Overall, these materials might be useful in highfrequency applications.

Acknowledgment: This study was supported by Pri-ority Research Centers Program through the NationalResearch Foundation of Korea (NRF) funded by theMinistry of Education, Science and Technology (2010-0028287).

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Received: 4 November 2011. Accepted: 3 April 2012.

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