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Journal of Science: Advanced Materials and Devices (2018) 145e150 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Fabrication of lithium substituted copper ferrite (Li-CuFe2O4) thin film as an efficient gas sensor at room temperature V Manikandan a, *, Monika Singh b, B.C Yadav b, Juliano C Denardin c a Department of Physics, RVS Technical Campus, Coimbatore, Tamil Nadu 641402, India 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 b a r t i c l e i n f o a b s t r a c t Article history: Received 19 February 2018 Received in revised form 17 March 2018 Accepted 27 March 2018 Available online April 2018 A Li-CuFe2O4 thin film has a well-defined nanocrystalline structure with a crystallite size of ~17 nm The TEM and SEM images of the Li-CuFe2O4 thin film 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-CuFe2O4 thin film has a semiconducting nature, and the band gap of the thin film is determined to be 1.15 eV The gas sensor analysis demonstrates repeatability of the sensing behavior of the Li-CuFe2O4 thin film 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/) Keywords: Gas sensor Thin film Surface growth Ferrites 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 respiratory systems There are different types of air pollutants such as LPG, CO, CO2, H2S and Cl2 and 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 oxygen and gas molecules A number of researchers have reported the gas sensing at high temperature which is inconvenient for * Corresponding author E-mail address: manikandan570@gmail.com (V Manikandan) Peer review under responsibility of Vietnam National University, Hanoi domestic and industrial purposes So that, development of highly efficient room temperature gas sensors is needed Spinel ferrites [MFe2O4; where M ¼ 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 particular, 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-CuFe2O4 thin film sensor was fabricated Until now, there is no report available on use of the Li-CuFe2O4 thin film sensors at room temperature The crystallite size of the prepared 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 (http://creativecommons.org/licenses/by/4.0/) 146 V Manikandan et al / Journal of Science: Advanced Materials and Devices (2018) 145e150 thin film has ~17 nm and it shows the high sensitivity and percentage of the sensor response Further reduction of crystallite size has potential to improve the high sensitivity and sensor response Experimental 2.1 Synthesis of nanomaterial Lithium chloride (LiCl), copper chloride (CuCl2) and ferric chloride (FeCl2) were used as starting materials Firstly, these materials were dissolved in de-ionized (DI) water and the mixtures of solutions were kept in vigorous stirring for h Subsequently, sodium hydroxide (NaOH) solution was added until the pH of solution reached to 11 Upon reaching to pH ¼ 11, the mixture solution was changed into dark brown precipitate The brown precipitate was washed with DI water to remove the chlorine and other impurities Then, the samples were dried overnight in oven at 100 C Finally, the dried samples were put into mortar and grinded manually for h to obtain fine powders Fig XRD pattern of the lithium substituted copper ferrite thin film 2.2 Fabrication of the sensing film The thin film was deposited on borosilicate substrate by using spin coating technique The substrate has the dimension of 1.5 Â 1.5 cm2 and then washed in ultrasonic cleaner by immersing in de-ionized water followed by iso propyl alcohol and then acetone for 15 each Then the substrate was dried on a hot air oven at 150 C for 10 One layered thin film of Li-CuFe2O4 was deposited on the substrate using a photoresist spinner at a speed of 3000 rpm and then dried at 70 C on a hot plate The prepared thin film was sintered at 900 C for h in muffle furnace Finally, the sintered thin film was used as sensing element towards LPG sensing 2.3 Characterization 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 LiCuFe2O4 thin film Thus, the particles are irregular polyhedron form This type of formation would lead to abrupt change in gas sensing Fig (b) shows the EDX spectrum of the Li-CuFe2O4 thin film and reveals the presence of Li, Cu, Fe, and O Fig shows 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 homogeneous Also, mixed size of nanoparticles are present in prepared thin film The nano ferrite film 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) Crystalline nature was recorded through Transmission Electron Microscope with Selected Area Electron Diffraction Optical absorption spectrum was obtained by UV-Vis spectrophotometer (Evolution 201) The prepared thin film was subjected into FTIR analysis and Fig shows a spectrum of the Li-CuFe2O4 thin film The spectrum emits only two peaks The peak at 1483 cmÀ1 is assigned to eCeH bending Also, the peak at 468 cmÀ1 is attributed to the general behavior of ferrites [22] Results and discussion 3.4 Optical analysis 3.1 Structural analysis The optical band-gap of the Li-CuFe2O4 thin film was calculated from Tauc's plot drawn with the absorbance and wavelength data obtained by a spectrophotometer Fig (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: The observed XRD pattern of the Li-CuFe2O4 thin film is shown in Fig 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-CuFe2O4 thin film reveals cubic structure From the analyses, it is found that thin film has some amount of hematite phase This hematite phase arises from the loss of divalent element in the prepared thin film 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 thin film was ~17 nm and 8.303 Å The crystallite size has an important role in gas sensing because of its small size The lattice constant and crystallite size of the film 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-CuFe2O4 thin film is shown in Fig (a) From the SEM image, nanoparticles are well dispersed and 3.3 FTIR spectroscopy À a¼ k hw À Eg hw Ám (1) where a is the absorption coefficient, k is a constant, hw is the photon energy, Eg is the optical band gap of the thin film ‘‘m’’ is a number which characterizes the mechanism of a transition process m ¼ 1/2, 3/2, for direct transitions and 1, 2, 3, for indirect transitions 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 in Fig (b) The calculated value of the band gap was 1.15 eV The thin film has very low band gap as compared to other pristine (copper ferrite) materials This reduction may be caused as V Manikandan et al / Journal of Science: Advanced Materials and Devices (2018) 145e150 147 Fig (a) SEM image, (b) EDX spectrum of the lithium substituted copper ferrite thin film removal of LPG (recovery characteristics) from the gas chamber Also, the sensing curves of the sensor are very broadening due to the increase of LPG concentration It is found that the sensitivity of the sensor increases as the concentration of LPG increases At low concentration, 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 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 in Fig (b) and (c) The percentage of the sensor is defined as [26]; Percentage of sensor response ¼ Fig TEM image 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 nanoparticle formation [23e25] Thus, the prepared film has a semiconducting nature 3.5 Gas sensing The gas sensing setup is shown in Fig It consists of gas chamber wherein the sensing element is fixed using silver pastes Variations in electrical resistance with respect to different concentrations of LPG of the sensing element have been monitored by Keithley Electrometer The gas sensing characteristics of the Li-CuFe2O4 thin film have been investigated and results of which are shown in Fig (a) Sensing parameters like sensitivity, percentage of sensor response, response & recovery time have been calculated from the sensing behavior of the Li-CuFe2O4 thin film Sensitivity is calculated by the account of the sensing curve and it is defined as [26]; Sensitivity ¼ Rg Ra (2) From Fig (a), the resistance of the sensing film 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 R a À R g Ra * 100 (3) From Fig (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 vol% of LPG which is the highest response at room temperature 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 recovery 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 sensing film [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 OÀ species that increases the resistance of the film The reaction can be explained by the following reaction; O2ðgasÞ 4O2ðadsÞ (4) 148 V Manikandan et al / Journal of Science: Advanced Materials and Devices (2018) 145e150 Fig FTIR spectrum of the lithium substituted copper ferrite thin film Fig (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 O2adsị ỵe /O (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 the film was changed The following reaction has occurred between hydrocarbon and chemisorbed oxygen; 2Cn H2nỵ2 þ 2OÀ /2Cn H2n O þ 2H2 O þ 2e (6) where CnH2nỵ2 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 the film 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 sensing film The LPG sensing properties of the Li-CuFe2O4 thin film sensor at room temperature has not been reported so far in the literature Table shows a recent literature survey on LPG sensing performances of pure and doped copper ferrites From this table, the prepared thin film sensor shows the high sensitivity and response Conclusion The prepared Li-CuFe2O4 thin film 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 V Manikandan et al / Journal of Science: Advanced Materials and Devices (2018) 145e150 Fig Experimental set-up for Keithley Electrometer based electrical gas sensing Fig (a) Gas sensing behavior of the lithium substituted copper ferrite thin film, (b) Sensitivity vs LPG concentration, (c) % Sensor response vs LPG concentration 149 150 V Manikandan et al / Journal of Science: Advanced Materials and Devices (2018) 145e150 Table Sensing properties of previously reported pure and copper ferrite and other ferrites S No Sensing materials Target gas Sensitivity % sensor response References Copper ferrite Copper ferrite nanospheres Copper ferrite Zn-copper ferrite Ce-copper ferrite 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other ferrites S No Sensing materials Target gas Sensitivity % sensor response References Copper ferrite Copper ferrite nanospheres Copper ferrite Zn -copper ferrite
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