Fabrication of lithium substituted copper ferrite li cufe2o4 thin film as an efficient gas sensor at room temperature

The TEM and SEM images of the Li-CuFe2O4thinfilm show a polyhedron shape of nanoparticles with uneven sizes, resulting in a significant change in its gas sensing characteristics such as se

Original Article

V Manikandana,*, Monika Singhb, B.C Yadavb, Juliano C Denardinc

a Department of Physics, RVS Technical Campus, Coimbatore, Tamil Nadu 641402, India

b Nanomaterials and Sensors Research Laboratory, Department of Applied Physics, Babasaheb Bhimrao University, Lucknow, UP 226025, India

c Department of Physics, University of Santiago, CEDENNA, Santiago, Chile

a r t i c l e i n f o

Article history:

Received 19 February 2018

Received in revised form

17 March 2018

Accepted 27 March 2018

Available online 3 April 2018

Keywords:

Gas sensor

Thin film

Surface growth

Ferrites

a b s t r a c t

A Li-CuFe2O4thinfilm has a well-defined nanocrystalline structure with a crystallite size of ~17 nm The TEM and SEM images of the Li-CuFe2O4thinfilm show a polyhedron shape of nanoparticles with uneven sizes, resulting in a significant change in its gas sensing characteristics such as sensitivity and sensor response An optical analysis shows that the Li-CuFe2O4thinfilm has a semiconducting nature, and the band gap of the thinfilm is determined to be 1.15 eV The gas sensor analysis demonstrates repeatability

of the sensing behavior of the Li-CuFe2O4thinfilm and this ensures a reliable and efficient gas sensor at room temperature

© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Fast globalization, different types of machineries and increase of

vehicles are solo in charge of natural and environmental disaster

Owing to the globalization, nature is polluted and it is directly

affecting human lifespan Presently, air pollution is one of the

essential problems in the world which is related to human

respi-ratory systems There are different types of air pollutants such as

LPG, CO, CO2, H2S and Cl2and it can cause diseases like weakening

function of lung, mesothelioma, pneumonia, and leukemia Among

these pollutants, liquid petroleum gas (LPG) has several advantages

and on the other hand, it has certain disadvantages as well[1e4]

Mostly, metal oxide based gas sensors are utilized to monitor the

harmful pollutants and they offer considerable sensitivity and

stability[5,6] Among the oxide materials, ferrites were promising

sensing materials for reducing and oxidizing gases[7e9] From the

literature survey, the gas sensing behavior of a material relies on

microstructure, particle-size and also the method of synthesis used

Also, gas sensing characteristics depend on numerous factors such

as dopants, grain size, surface states, and amount of adsorbed

ox-ygen and gas molecules A number of researchers have reported

the gas sensing at high temperature which is inconvenient for

domestic and industrial purposes So that, development of highly

efficient room temperature gas sensors is needed

Spinel ferrites [MFe2O4; where M¼ divalent metallic ions such

as Cu, Zn, Ca, Mg, Co etc] have specific properties such as low magnetic transition temperature, melting point and high electrical conductivity[7,10] Also, the spinel ferrites are used in most of the devices such as gas sensors, photo catalytic, semiconductors and micro-wave deices Thus, the spinel ferrites are considered advanced functional nanomaterials In the past few years, much attention has been paid to fabricate spinel ferrites because of their controlled phase, size and surface morphology[11e13] In partic-ular, substitution of metal cations in copper ferrites has shown novel features for gas sensing since conduction in these spinel ferrites occurs via electron or hole transfer between equal cation s located in octahedral sites that's highly sensitive to chemical composition [14] Copper ferrite has a high electrical resistance Substitution of lithium can increase its conductivity, which is an advantage for gas sensing

Singh et al has reported the gas sensing behavior of the copper ferrite[15]and it has a low sensitivity (1.5) with the percentage of the sensor response (57%) Vakil et al has investigated zinc doped copper ferrite for gas sensing The sensing analysis shows the 55.55% sensor response[16] In order to increase the sensitivity and sensor response, a Li-CuFe2O4thinfilm sensor was fabricated Until now, there is no report available on use of the Li-CuFe2O4thinfilm sensors at room temperature The crystallite size of the prepared

* Corresponding author.

E-mail address: manikandan570@gmail.com (V Manikandan).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.03.008

2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license

to remove the chlorine and other impurities Then, the samples were

dried overnight in oven at 100C Finally, the dried samples were put

into mortar and grinded manually for 1 h to obtainfine powders

2.2 Fabrication of the sensingfilm

The thinfilm was deposited on borosilicate substrate by using

spin coating technique The substrate has the dimension of

1.5 1.5 cm2and then washed in ultrasonic cleaner by immersing in

de-ionized water followed by iso propyl alcohol and then acetone

for 15 min each Then the substrate was dried on a hot air oven at

150C for 10 min One layered thinfilm of Li-CuFe2O4was deposited

on the substrate using a photoresist spinner at a speed of 3000 rpm

and then dried at 70C on a hot plate The prepared thinfilm was

sintered at 900C for 4 h in muffle furnace Finally, the sintered thin

film was used as sensing element towards LPG sensing

2.3 Characterization

The nano ferritefilm was kept in Rigaku X-ray diffraction (Model

ULTIMA III) to interpret the structural information Surface

morphology was visualized using Scanning Electron Microscope

(SEM JEOL 5600V) Functional group analysis has done through

Fourier Transform Infrared Spectroscopy (Model-MAGNA 550)

Crys-talline nature was recorded through Transmission Electron

Micro-scope with Selected Area Electron Diffraction Optical absorption

spectrum was obtained by UV-Vis spectrophotometer (Evolution 201)

3 Results and discussion

3.1 Structural analysis

The observed XRD pattern of the Li-CuFe2O4thinfilm is shown in

Fig 1 The XRD pattern having the reflection peaks such as (111), (220),

(311), (400), (511), (440) and found to be in good agreement with

JCPDS card 40-1120 The Li-CuFe2O4thinfilm reveals cubic structure

From the analyses, it is found that thinfilm has some amount of

hematite phase This hematite phase arises from the loss of divalent

element in the prepared thinfilm materials[17] By increasing the

sintering temperature, the formation of hematite phase is removed

[18,19] The average crystallite size and lattice constant of the thinfilm

was ~17 nm and 8.303Å The crystallite size has an important role in

gas sensing because of its small size The lattice constant and

crys-tallite size of thefilm have been calculated from the most prominent

peak (311) by using Scherrer formula[20]

3.2 Surface morphology

The obtained SEM image of the Li-CuFe2O4thinfilm is shown in

Fig 2(a) From the SEM image, nanoparticles are well dispersed and

seemed to be nearly spheroidal with irregular polyhedron The formation of polyhedron nanocrystalline may be caused as a result

of the solid state reaction and takes place on interfaces of reactants [21] Additionally, some nano rod structure was found in the Li-CuFe2O4 thin film Thus, the particles are irregular polyhedron form This type of formation would lead to abrupt change in gas sensing.Fig 2(b) shows the EDX spectrum of the Li-CuFe2O4thin film and reveals the presence of Li, Cu, Fe, and O.Fig 3shows the TEM image of the Li-CuFe2O4 thin film The image exposes the irregular size of particles and nanoparticle sizes are within the range of 100 nm It is clearly shown that particles are not homo-geneous Also, mixed size of nanoparticles are present in prepared thinfilm

3.3 FTIR spectroscopy The prepared thinfilm was subjected into FTIR analysis and Fig 4shows a spectrum of the Li-CuFe2O4thinfilm The spectrum emits only two peaks The peak at 1483 cm1is assigned toeCeH bending Also, the peak at 468 cm1is attributed to the general behavior of ferrites[22]

3.4 Optical analysis The optical band-gap of the Li-CuFe2O4thinfilm was calculated from Tauc's plot drawn with the absorbance and wavelength data obtained by a spectrophotometer.Fig 5(a) depicts the variation of absorbance with wavelengths for the Li-CuFe2O4 thin film The maximum absorbance was observed in a visible region at ~600 nm The optical band gap of the material has been calculated by using the following formula:

a¼k hw  Eg
m

wherea is the absorption coefficient, k is a constant, hw is the photon energy, Egis the optical band gap of the thinfilm ‘‘m’’ is a number which characterizes the mechanism of a transition process

m¼ 1/2, 3/2, for direct transitions and 1, 2, 3, for indirect transi-tions The Tauc's plot was used for calculating the value of the direct optical energy band gap by extrapolating curve to zero absorption and shown inFig 5(b) The calculated value of the band gap was 1.15 eV The thinfilm has very low band gap as compared to other pristine (copper ferrite) materials This reduction may be caused as Fig 1 XRD pattern of the lithium substituted copper ferrite thin film.

results of additional sub-bandgap energy levels are induced with

aid of abundant surface and interface defects within the

nano-particle formation[23e25] Thus, the preparedfilm has a

semi-conducting nature

3.5 Gas sensing

The gas sensing setup is shown in Fig 6 It consists of gas

chamber wherein the sensing element isfixed using silver pastes

Variations in electrical resistance with respect to different

con-centrations of LPG of the sensing element have been monitored by

Keithley Electrometer

The gas sensing characteristics of the Li-CuFe2O4thinfilm have

been investigated and results of which are shown in Fig 7 (a)

Sensing parameters like sensitivity, percentage of sensor response,

response& recovery time have been calculated from the sensing

behavior of the Li-CuFe2O4thinfilm Sensitivity is calculated by the

account of the sensing curve and it is defined as[26];

Sensitivity¼Rg

FromFig 7(a), the resistance of the sensingfilm increased with

respect to time upon the exposure of LPG Next, the sensing element

reached a constant value (resistance) and then decreased due to the

removal of LPG (recovery characteristics) from the gas chamber Also, the sensing curves of the sensor are very broadening due to the in-crease of LPG concentration It is found that the sensitivity of the sensor increases as the concentration of LPG increases At low con-centration, only few gas molecules would adsorb and it covers small area However, on increasing the concentration, more number of gas molecules and oxygen species would adsorb at the surface of the sensing element, it covers large area of the material Therefore, the sensitivity of the sensor was increased The maximum sensitivity of the sensor was found to be 1.82 for 4 vol% of LPG and it is a significant achievement at room temperature The variations of the sensitivity versus LPG concentration and percentage of sensor response versus LPG concentration (0.5e4 vol%) are shown inFig 7(b) and (c) The percentage of the sensor is defined as[26];

Percentage of sensor response¼ Ra Rg

FromFig 7(b)& (c), the sensitivity and percentage of the sensor response increased gradually with respect to the concentration of LPG The maximum percentage of the sensor was found to be 83.82 for 4 vol% of LPG which is the highest response at room tempera-ture Moreover, the sensing curve has repeatability Because, ambient temperature and environment condition remain constant Repeatability is defined as the sensing element has ability to produce the same response for successive measurement and it is related to precision Also, the repeatability reveals that the material enables an efficient and reliable LPG sensor

Response and recovery time are more important parameters for gas sensing which are defined as time to reach 90% of the resistance value while exposed to LPG The response and recovery times of the sensor were 2.7 and 19.36 min, respectively A comparative analysis between the response and recovery time has shown that the re-covery time of the sensor is quite long However, the LPG gas fast adsorbed and diffused inside the sensing element In the recovery sense, the gas desorbed gradually at room temperature and it took long time to recover The significant finding is the robust detection

of LPG, high sensitivity and percentage of the sensor response The sensing mechanism is based on the adsorption and desorption process at the surface of the sensingfilm[27,28] The environmental oxygen species adsorb the surface of the Li-CuFe2O4 thin film Then, takes out electron from the conduction band to form Ospecies that increases the resistance of thefilm The re-action can be explained by the following rere-action;

Fig 2 (a) SEM image, (b) EDX spectrum of the lithium substituted copper ferrite thin film.

Fig 3 TEM image of the lithium substituted copper ferrite thin film.

O2ðadsÞþe/O2 (5)

The chemisorbed oxygen reacts with LPG molecules The above

reaction would remove the adsorbed oxygen and then form

gaseous species and water vapor Also, the resistance of thefilm

was changed The following reaction has occurred between

hy-drocarbon and chemisorbed oxygen;

2CnH2nþ2þ 2O2/2CnH2nOþ 2H2Oþ 2e (6)

where CnH2n þ2 represents hydrocarbon While LPG reacts with

surface oxygen, the ignitable items exist, for example, water and

potential barrier to charge transport ought to be created The

development of the potential barrier is expected to decrease the

concentration of charge carriers (conduction) This thus increases

the resistance of thefilm with time In this way, the gas atoms were ceased and afterward oxygen in air would adsorb the surface of the film (catch of electron) This thus decreases the resistance of the sensingfilm

The LPG sensing properties of the Li-CuFe2O4thinfilm sensor at room temperature has not been reported so far in the literature Table 1shows a recent literature survey on LPG sensing perfor-mances of pure and doped copper ferrites From this table, the prepared thinfilm sensor shows the high sensitivity and response

4 Conclusion The prepared Li-CuFe2O4thinfilm has a cubic phase The SEM and TEM images show the polyhedron shape of nanoparticles with irregular sizes Also, the spinel ferrite has an usual behavior and it is Fig 4 FTIR spectrum of the lithium substituted copper ferrite thin film.

Fig 5 (a) Absorption vs wavelength spectrum, (b) Tauc's plot of the lithium substituted copper ferrite sensing film showing the optical energy band-gap of 1.15 eV.

Fig 6 Experimental set-up for Keithley Electrometer based electrical gas sensing.

Fig 7 (a) Gas sensing behavior of the lithium substituted copper ferrite thin film, (b) Sensitivity vs LPG concentration, (c) % Sensor response vs LPG concentration.

Acknowledgements

Monika Singh is thankful to DAE-BRNS, Govt of India for

financial support in the form of project, grant vide sanction number

2013/34/27/BRNS/2693

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nanocrystalline nickel ferrite thin films for development of a gas sensor at< /small>

room temperature, J Mater... the lithium substituted copper ferrite thin film.

Fig TEM image of the lithium substituted copper ferrite thin film.