SHORT RESEARCH COMMUNICATION

The effects of lithium and yttrium substitution on the opticaland structural properties of cobalt ferrites

M K Shobana1*, H-S Nam2 and H Choe2

1Department of Physics, School of Advanced Science, VIT University, Vellore, Tamil Nadu 632014, India

2School of Materials Science and Engineering, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul 02707, Republic of Korea

Received: 13 December 2017 / Accepted: 22 May 2018

Abstract: Known as a hard magnetic material, cobalt ferrite belongs to one of the most attractive classes of materials

owing to its diverse technical applicability such as in catalysis, sensors, batteries. In this study, we synthesized Li- and

Y-substituted cobalt ferrite nanoparticles using a sol–gel combustion method. Their mean crystallite sizes with spinel

structures were calculated as 15–19 and 2–5 nm and their lattice constant values were 7.8 ± 0.02 and 8.5 ± 0.3 A,

respectively. The results also revealed that the band gap energy values of the prepared Li- and Y-doped cobalt ferrites

decreased after doping compared with that of pure cobalt ferrite, and thus they appear to be promising for energy

generation and storage applications.

Keywords: Nanostructures; X-ray diffraction; Optical properties; Ferrite

PAC Nos.: 75.75.Lf; 73.63.-b; 75.50.Gg; 78.67.Bf; 61.46.Df

1. Introduction

Presently, spinel-structured nanometal oxides with one or

more substituted metal ions are considered to belong to one

of the most promising emerging classes of materials owing

to their remarkable properties such as low melting tem-

perature, high specific heating, large expansion coefficient,

low saturation magnetic moment, and low magnetic tran-

sition temperature [19, 32]. Based on these excellent

properties, spinel nanoferrites have numerous technical

applications in a number of different areas, such as catal-

ysis, sensors, and batteries [5, 7, 8, 10, 12, 15, 21]. More

specifically, numerous researchers have demonstrated the

characteristics of several substituted cobalt ferrites for

different applications [1, 2, 4, 6, 9, 18, 20, 23, 28]. How-

ever, little attention has been paid to the analysis of some

of the important properties of cobalt ferrites (CoFe2O4),

such as their structural and optical characteristics, espe-

cially since those containing Li or Y have exhibited such

interesting ones.

Given that there is a strong correlation between the

functional properties of substituted cobalt ferrites and their

microstructures, composition, and particle/crystal sizes, the

selection of the applied synthesis method plays a vital role

in obtaining excellent functional properties. For example,

substituted ions in ferrites are stabilized in octahedral sites,

thereby giving rise to a degenerate or near-degenerate

orbital ground state and thus different performances.

Therefore, in the present study, we synthesized and mod-

ified the structural and optical properties of cobalt ferrites

by doping them with Li or Y through a sol–gel auto-

combustion method. Additionally, we examined the struc-

tural and optical properties of the Li- and Y-doped cobalt

ferrites using X-ray diffraction (XRD), scanning electron

microscopy (SEM), and Fourier transform infrared (FTIR)

and ultraviolet (UV) spectroscopy studies. Their average

crystallite size, lattice constant values, and band gap energy

values were calculated based on the XRD and UV results

according to the varied amounts of Li or Y.

*Corresponding author, E-mail: mkshobana@gmail.com

Indian J Phys

https://doi.org/10.1007/s12648-018-1292-3

� 2018 IACS

2. Experimental details

All nanoferrite samples were synthesized using commer-

cially available constituent nitrates. Analytical-grade

cobalt nitrate (99%, Sigma-Aldrich, USA), yttrium nitrate

(99%, Sigma-Aldrich, USA), iron nitrate (99%, Sigma-

Aldrich, USA), and citric acid (99%, Sigma-Aldrich, USA)

were used as source materials for the preparation of the

ferrite nanoparticles. The Li- and Y-substituted cobalt

ferrites were prepared using a sol–gel combustion method

[26]. Nitrates of the metals, citric acid, and polyvinyl

alcohol (PVA, used as a chelating agent) were dissolved in

distilled water and stirred for 3 h without interruption;

additionally, the solution was prepared with proper heat

treatment, as reported previously [26, 27].

The prepared Li- and Y-doped cobalt ferrites were

characterized using powder XRD (Bruker D8 ADVANCE

X-ray diffractometer) using CuKa radiation (1.54056 A).

Additionally, the particle sizes of the samples were con-

firmed using SEM (ZEISS EUVA 8). FTIR spectroscopy

was carried out in KBr medium using an FTIR-8400

(Shimadzu) with the wave number ranging from 400 to

4000 cm-1 at a resolution of 4 cm-1. UV–Vis diffuse

reflectance spectroscopy was carried using a JASCO UV–

visible spectrophotometer (V-670 PC).

3. Results and discussion

Figures 1 and 2, respectively, show the Li- and Y-substi-

tuted cobalt ferrite nanoparticles with different composi-

tions synthesized using the sol–gel combustion method.

The observed XRD peaks indicated the presence of pure

spinel ferrite structures represented by reflections at (220),

(311), (400), and (511), which is comparable with previ-

ously reported standard data [3, 26]. The average crystallite

sizes of the Li- and Y-substituted cobalt ferrite nanoparti-

cles at 300 �C lay in the range of 13–19 and 2–5 nm,

respectively. The lattice constant values were 7.8 ± 0.02

(AU

)

(degrees)

(a)

(b)

(c)

Fig. 1 XRD patterns of the Li-substituted cobalt ferrite: (a) x = 0.2,

(b) x = 0.8, and (c) x = 1

0 20 40 60 80 100

0

500

1000

1500

2000

2500

3000

3500

4000

4500

(e)

(d)

(c)

(b)

(a)

(440

)(5

11)

(222

)

(220

)

(400

)

(311

)

Inte

nsity

(AU

)

2 theta (degrees)

Fig. 2 XRD patterns of the Y-substituted cobalt ferrite: (a) x = 0.2,

(b) x = 0.4, (c) x = 0.6, (d) x = 0.8, and (e) x = 1

Fig. 3 SEM images of the Li-substituted cobalt ferrite: (a) x = 0.2, (b) x = 0.8, and (c) x = 1

M K Shobana et al.

Fig. 4 SEM images of the Y-substituted cobalt ferrite: (a) x = 0.2, (b) x = 0.4, (c) x = 0.6, (d) x = 0.8, and (e) x = 1

0

1

2

3

4

5

6

7

(h

)2 (eV

/m)2

Photon energy (eV)

0

5

10

15

20

25

% (?

(h

)2 (eV

/m)2 Y

)

Photon energy (eV)0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

0

5

10

15

20

25

(h

)2 (eV

/m)2

Photon energy (eV)

(a) (b)

(c)

Fig. 5 UV spectra of the Li-substituted cobalt ferrites: (a) x = 0.2, (b) x = 0.8, and (c) x = 1

The effects of lithium and yttrium substitution

and 8.5 ± 0.3 A for the Li- and Y-substituted samples,

respectively. The variation of the lattice constant is due to

ionic radius of the lithium and yttrium ion constituents

[29, 33]. The prepared sample shows the noisy and amor-

phous phase due to low temperature treatment, and it can

be reduced by increasing temperature.

Figures 3, 4, 5, and 6 illustrate the morphological features

of Li- and Y-substituted cobalt ferrite nanoparticles with

different compositions; the micrographs indicate the cubic

morphology of the nanoparticles. Figures 5 and 6 show UV–

visible spectroscopy absorption in the UV region for the Li-

and Y-substituted cobalt ferrites, respectively. With this

method, molecules comprising P-electrons or non-bonding

n-electrons could absorb energy in the form of ultraviolet or

visible light to excite the electrons to higher or anti-bonding

molecular orbits [30]. The band gap energy is calculated as

0

5

10

15

20

(h

)2 (e

V/m

)2

Photon energy (eV)

(d)

0

2

4

6

8(

h)2

(eV

/m)2

Photon energy (eV)

(a)

0

2

4

6

8

(h

)2 (eV

/m)2

Photon energy (eV)

(b)

0

2

4

6

8

(h

)2 (e

V/m

)2

Photon energy (eV)

(c)

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

0

5

10

15

20

(h

)2 (e

V/m

)2

Photon energy (eV)

(e)

Fig. 6 UV spectra of the Y-substituted cobalt ferrites: (a) x = 0.2, (b) x = 0.4, (c) x = 0.6, (d) x = 0.8, and (e) x = 1

1000 800 600 400 2000.0

0.2

0.4

0.6

0.8

1.0

1.2

(c)

(b)

Wavelength (nm)

Abs

orpt

ion

(AU

)

(a)

Fig. 7 UV absorption spectra of (a) Li-substituted cobalt ferrites:

(a) x = 0.2, (b) x = 0.8, and (c) x = 1

1000 800 600 400 2000.2

0.4

0.6

0.8

1.0

1.2

(c)

(e)

(b)

(a)

Wavelength (nm)

Abs

orpt

ion

(AU

)

(d)

Fig. 8 UV absorption spectra of the Y-substituted cobalt ferrites:

(a) x = 0.2, (b) x = 0.4, (c) x = 0.6, (d) x = 0.8, and (e) x = 1

M K Shobana et al.

ahm ¼ A hm� Eg

� �n; ð1Þ

where a is the absorption coefficient, A is a constant, Eg is

the optical energy band gap, and n is an integer equal to 0.5

or 2 for a direct or an indirect band gap, respectively.

A plot of (ahm)2 versus energy (eV) was used to estimate

the value of the indirect band gap energy of the cobalt

ferrite by extrapolating the curve to the zero absorption.

The slope of (aht)2 represents phonon involvement in an

optical process [11, 22]. Figures 5 and 6 show plots of

(ahm)2 with photon energy (hm) to determine the optical

energy gap of CoFe2O4 in the Li- and Y-doped samples,

respectively. On the other hand, Figs. 7 and 8 show the

respective UV absorption bands in the range of

330–450 nm, which originated primarily from the absorp-

tion and scattering of UV radiation by the magnetic

nanoparticles and is in accordance with Koutzarova et al.

[16]. Thus, it is evident that the cobalt ferrite showed a

significant blue shift in the absorption peak relative to the

bulk absorption, which might have been related to the

quantum size effect that arose due to the very small size of

the nanoparticles. This makes them useful for gas sensing

applications [14].

For the Li-substituted cobalt ferrites, the Tauc optical

gaps associated with the samples were estimated as 0.509,

0.775, and 0.892 eV for x = 0.6, 0.8, and 1, respectively,

while those for the Y-doped cobalt ferrites were estimated

as 0.684, 0.693, 0.701, 0.716, and 0.721 eV for x = 0.2,

0.4, 0.6, 0.8, and 1, respectively. Hence, there was a slight

increase in band gap energy with increasing concentrations

of Li or Y in the cobalt ferrites, which might have been due

to an increase in their charge carrier mobility values. The

possible reason for the increasing resistance can be attrib-

uted to a change in the lattice constant since the band gap

energy values decreased compared with that of pure cobalt

ferrite (0.92 eV), suggesting that the doped ferrites might

be useful for energy applications. Note that the indirect

band gap value increased with an increase in Li or Y

content. Moreover, it can be affected by various factors

such as crystallite size, the structural parameter, and the

Fig. 9 FTIR spectroscopy of the Li-substituted cobalt ferrites: (a) x = 0.2, (b) x = 0.8, and (c) x = 1

The effects of lithium and yttrium substitution

presence of impurities [13]. Hence, the increase in the

indirect band gap for our synthesized samples might be

attributable to the further increase in the structural

parameter (lattice constant) with an increase in Li or Y

concentration [31].

Figures 9 and 10, respectively, exhibit the FTIR spectra

of the Li- and Y-substituted cobalt ferrites. The bending

peaks between 550 and 460 cm-1 are characteristic of

cobalt ferrites [24]. The stretching and bending peaks of

the Li-substituted cobalt ferrites were observed at 3354.21,

1608.63, and 530 cm-1 for x = 0.6; 3385.07, 1597.06, and

545.85 cm-1 for x = 0.8; and 3369.64, 1608.48, and

547.44 cm-1 for x = 1. On the other hand, those of the

Y-substituted cobalt ferrites were observed at 3547, 1618,

and 530.42 cm-1 for x = 0.2; 3547, 1531, and 540 cm-1

for x = 0.4; 3537, 1386, and 559 cm-1 for x = 0.6; and

3520, 1386, and 543 cm-1 for x = 1. The bands observed

at 420, 432, and 442 cm-1 were assigned to octahedral

group complexes [17], while the band at 549 cm-1 was

attributed to tetrahedral group complexes. The broad peak

near 3385.07–3354.21 cm-1 was due to the stretching of

the hydrogen and oxygen bond in water molecules [25].

The frequency between 550 and 460 cm-1 represents the

presence of a metal oxide stretching vibration in the pre-

pared samples.

4. Conclusions

Li- and Y-substituted cobalt ferrites with different com-

positions were successfully synthesized using a sol–gel

combustion method. Their average crystallite sizes were

calculated as 15–19 and 2–5 nm and their lattice constant

values were 7.8 ± 0.02 and 8.5 ± 0.3 A, respectively.

Morphological studies revealed the cubic structure of the

synthesized samples with various amounts of Li or Y.

Additionally, optical studies of the Li- and Y-substituted

cobalt ferrites confirmed a decrease in band gap values

compared with that of the baseline pure cobalt ferrite. The

conductivity of the synthesized samples was higher than

that of the pure cobalt ferrite owing to an increase in the

charge carrier mobility. Simultaneously, characteristic

50010001500200025003000350040000

20

40

60

80

100%T

3547

.09

3313

.71

1618

.28

1406

.11

840.

9671

9.45

530.

42 455.

20

50010001500200025003000350040000

20

40

60

80

100

%T

3547

.09

3334

.92

1531

.48

1402

.25

1259

.52

1093

.64

1018

.41

839.

0379

8.53

540.

07

5001000150020002500300035004000

1/cm

0

20

40

60

80

100

%T

1386

.82

543.

9346

0.99

416

62

5001000150020002500300035004000

1/cm

0

20

40

60

80

100

%T

3537

.45

3288

.63

1525

.69

1386

.82

835.

18

559.

36

(a) (b)

(c) (d)

Fig. 10 FTIR spectroscopy of the Y-substituted cobalt ferrites: (a) x = 0.2, (b) x = 0.4 (c) x = 0.8, and (d) x = 1

M K Shobana et al.

peaks around 400–600 cm-1 confirmed the presence of

spinel ferrite structures in the synthesized samples. Hence,

they can be considered suitable for energy generation

application.

Acknowledgements This study was supported by the Priority

Research Centers Program through the National Research Foundation

of Korea (NRF) funded by the Ministry of Education, Science and

Technology (2010-0028287; 2017R1A2B4012871).

Compliance with ethical standards

Conflict of interest The authors declare that they have no conflicts of

interest.

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The effects of lithium and yttrium substitution