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Development of CoFe2O4 thin films for nitrogen dioxide sensing at moderate operating temperature
Accepted Manuscript Development of CoFe2O4 thin films for nitrogen dioxide sensing at moderate operating temperature A.A. Bagade, K.Y. Rajpure PII:
S0925-8388(15)31366-9
DOI:
10.1016/j.jallcom.2015.10.115
Reference:
JALCOM 35670
To appear in:
Journal of Alloys and Compounds
Received Date: 24 July 2015 Revised Date:
5 October 2015
Accepted Date: 13 October 2015
Please cite this article as: A.A. Bagade, K.Y. Rajpure, Development of CoFe2O4 thin films for nitrogen dioxide sensing at moderate operating temperature, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.10.115. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
A.A. Bagade, K.Y. Rajpure*
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Development of CoFe2O4 thin films for nitrogen dioxide sensing at moderate operating temperature
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Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur-416 004, India
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Development of CoFe2O4 thin films for nitrogen dioxide sensing at moderate operating temperature
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A.A. Bagade, K.Y. Rajpure* Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur-416 004, India Abstract
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Cobalt ferrite thin films were deposited onto quartz substrates by spray pyrolysis technique and their gas sensing properties were investigated. X-ray diffraction study confirms
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that films are polycrystalline in nature and exhibit single phase spinel cubic crystal structure. DC electrical resistivity measurement indicates the semiconducting nature of the films. The surface morphology study confirms the formation of grains structure with grain size about 200-400 nm. Room temperature variation of dielectric properties with frequency suggests the normal dielectric performance of the cobalt ferrite thin film. The influence of quantities of spraying solutions on the operating temperature, sensitivity and selectivity, gas concentrations, response
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and recovery characteristics of gas sensor are systematically tested. The maximum gas response 95 % was observed for film prepared at 50 mL quantity of solution for 80 ppm NO2. The films are extremely selective towards NO2 as compared to other target gases. Gas response attains
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nearly 90 % of its initial value after 90 days indicates good durability of the films.
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Keywords: Spray Pyrolysis; Cobalt ferrite; XRD; FESEM; Dielectric properties; Gas sensing.
*Corresponding author
Email- [email protected]
Phone: +91-231-2609435; Fax- +91-231-2691533
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1. Introduction Spinel structured ferrites are imperative scientific materials as they possess both semiconductor and magnetic properties [1]. Spinel ferrite materials have attracted the research interest due to their practical applications in many research fields [2-4]. The spinel ferrites
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having a chemical composition M2+Fe2O4 (M= Cu, Mn, Mg, Co, Zn, Ni, Cd, etc.) are widely used as magnetic materials.
Recently, some spinel ferrites thin films have been studied in the gas sensor application as they show high sensitivity and selectivity to certain gases present in the atmosphere [5]. The
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general gas sensing mechanism is based on the chemisorptions of oxygen on the surface of gas sensing material, followed by charge transfer during the reaction between oxygen with gas
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molecules. Therefore, its resistance changes during and after the expose of the gas on the surface of metal oxide semiconductor [6]. The spinel ferrite in which the divalent transition metal ion i.e. M2+ was incorporated into the lattice of the parent structure was established to be a promising material in detecting of the gases presence of the atmosphere [7]. From the gas sensor application point of view low density and high surface area is required because they show good sensitivity and stability for detection of various gases [8]. The gas response of the sensor depends on the
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morphology, annealing temperature, ferrite composition, concentration and which type of target gas to be used for detection [9]. Cobalt ferrite thin films have been used in industrial applications because they have strong anisotropy, good mechanical hardness and chemical stability [10, 11]. Lee et al. [12] synthesized the cobalt ferrite thin films by sol-gel technique and studied the
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structural and magnetic properties. They reported that the CoFe2O4 film prepared by sol-gel technique could be used for high-density recording media if the grain size is 10 nm while
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maintaining its magnetic property. The spinel cobalt ferrite thin films have extensively used in research because of their applications in frequency device [13, 14]. Pulsed laser deposition (PLD) [15], RF magnetron sputtering [16], Sol-gel method [17], spray pyrolysis [18], electron beam evaporation [19] etc. have been used to prepare ferrite thin films. Amongst the various techniques, spray pyrolysis is one of the best techniques as it is simple, cost-effective and useful for preparation of ferrite thin films [20]. Tudorache et al. [21] prepared NiCoFe2O4 thin films by spin coating technique for gas sensing application. The highest sensitivity for ethyl alcohol vapors was observed for low cobalt contents and higher response was observed for acetone at large cobalt contains thin films. Xiangfeng et al. [22] reported nanocrystalline CoFe2O4 by 2
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hydrothermal method for ethanol gas sensor and investigated their gas sensing properties. They found that the CoFe2O4 nano crystals are used for detecting very low concentration of triethylamine and ethanol gases. Sutka et al. [23] prepared n-type ZnFe2O4 thin films by spray pyrolysis technique for gas sensor application and they obtain fast response time for ethanol gas
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at 1 ppm concentration. The similar type of other spinel oxide semiconductor (MFe2O4) shows gas sensor response to NO2 gas at various experimental conditions. Zhuiykov et al. [24] investigated NO2 gas sensing properties over the temperature range 600-700 oC of the zinc ferrite. They observed maximum gas response at 436 ppm at 700 oC operating temperature. The
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gas response also increases with increase in NO2 concentration. Darshane and Mulla [25] reported the gas sensor performance towards NOx of magnesium ferrite nano-particles by using a
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simple molten salt method. They found that 52 % gas response for 1% Pd doping then decreases further increasing in Pd doping. Zhang et al. [26] prepared ZnCo2O4 nanotube by using a template assisted method and their gas sensing properties towards various gases was studied. The ZnCo2O4 nanotube shows maximum 1.72 gas response for 400 ppm NO2 at 300 oC operating temperature. Niu et al. [27] synthesized zinc ferrite by micro-emulsion method and studied their gas sensing properties. The maximum gas response S=2.4 was observed for ZnFe2O4 for 50 ppm
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NO2 at 270 oC operating temperature.
We have already reported [20] physicochemical properties of spray deposited CoFe2O4 thin films. The substrate temperature was optimized to be 400 oC. In continuation of this work, in this paper effect of the quantity of spraying solution on CoFe2O4 thin films is discussed. Further
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2. Experimental
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their NO2 gas sensing properties are studied.
2.1 Synthesis of CoFe2O4 thin films The cobalt ferrite thin films were deposited onto the preheated quartz substrates using
simple chemical spray pyrolysis technique [20]. The spraying solution was sprayed onto preheated quartz substrates at different quantities of spraying solutions an interval of 10 mL. The as-deposited cobalt ferrite thin films were annealed at the 900 oC for 4 h in ambient atmosphere at 2o/min constant heating rate. These annealed ferrite thin films were characterized for their physicochemical as well as gas sensing properties.
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2.2 Characterization techniques To analyze the crystalline quality and phase identification of annealed CoFe2O4 thin films, XRD studies were carried out using Bruker D2-Phaser X-ray powder diffractometer with Cu-Kα (λ=1.5406 Å) radiation recorded in ranges of 20-80o. The thickness of the annealed films
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was measured by using AMBIOS, USA XP-I surface profiler and scanning electron microscopy. The DC electrical resistivity of ferrite thin films was measured by using the two probe method with a high sensitivity Keithley electrometer model-6514. The surface morphology of the films was investigated using a Mira-3, Tescan, Brno-Czech Republic, Field emission scanning electron
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microscope (FE-SEM). The dielectric measurements of the cobalt ferrite thin films were carried out using a high precision LCR meter bridge (HP-6284A) in the frequency range 20 Hz to 1
2.3 Gas sensing Measurements
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MHz at room temperature.
For gas sensing measurements the silver contacts were made into the CoFe2O4 thin film, which were placed in a specially designed stainless steel chamber having diameter 7 cm and height 15 cm. The operating temperature was varied, ranging from 150 oC to 250 oC. The NO2
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gas was injected into the glass chamber with a calibrated syringe. The Cromel–Alumel thermocouple was used to measure the operating temperature and connected to the heating system. The gas response of the films in both air and in presence of the gas was measured at various gas concentrations at constant operating temperature 150 oC. Gas sensing measurements
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were carried out at locally fabricated gas sensing unit equipped with gas flow meters and change in resistance was measured on the Keithley electrometer (6514). The gas response was calculated
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by using the relation [28].
Gas response (%) =
R air − R gas R air
×100 (1)
where, Rair is the resistance in the air and Rgas is the resistance in the presence of the gas. Response and recovery times mean the time for the sensor to reach 90% of the final resistance value during gas exposure and at the end of gas exposure, respectively.
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3. Results and discussion 3.1 X-ray diffraction studies Figure 1 shows X-ray diffraction (XRD) patterns of the annealed CoFe2O4 thin films deposited at various quantities of the spraying solution. A matching of inter planar spacing
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values (d) of CoFe2O4 using JCPDS card No-02-1045 confirms the spinel cubic crystal structure. It is observed that, as the quantity of the spraying solution increases the crystallinity of films increases up to 50 ml and then decreases. The lower intensity peaks are observed in CoFe2O4 films due to the lower film thickness. The crystallite size is calculated by using the Scherrer’s
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formula [29]. The crystallite size of the films varies with varying quantities of the spraying solution. In the present study, the thickness of the films increases with increase in quantities of
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the spraying solutions. Crystallite size becomes larger in the thinnest films and is directly correlated to the film thickness. Similar variation of the crystallite size as a function of film thickness is reported by Bouderbala et al. [30]. The exact value of the lattice parameter has been calculated by plotting the graph of Nelson Riley function (NRF) versus the lattice parameter for every plane. The NRF is a function of the Bragg angle θ calculated using the well known equation [31].
ଵ ௦మ ఏ
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ܴܰ = ܨቀ + ଶ ௦ఏ
௦మ ఏ ቁ ఏ
(2)
where, θ is the Bragg angle. Then by using a linear fit technique exact value of lattice constant was determined. Now the linear fitted straight line cuts the y-axis is the point of actual lattice parameter of the material. It is seen that the values of lattice constant increases with increase in
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quantities of the spraying solutions (Table 1). A similar relation between the lattice constant and film thickness was also observed by Purohit et al. [32]. They reported that lattice constant
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increases with increase in the film thickness due to the change in density as well as the nature of native imperfection of the thin films.
3.2 DC electrical resistivity DC electrical resistivity is an important property of the ferrite materials. The cobalt ferrite
possesses high resistive spinel ferrite material with high activation energy. The preparation method, substitution of ions at tetrahedral (A) and octahedral (B) site, particle dimension and sintering temperature influences the electrical properties of spinel ferrites [33]. The thickness of the thin film is a very important parameter for thin film technology; it affects the electrical 5
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properties of the material. In the present investigation the thickness of the films was measured by AMBIOS XP-I surface profiler. From Fig. 2 it is seen that the thickness of films increased with quantity of solution, up to 50 ml quantity of spraying solution and then saturates for higher
film deposited at 50 ml quantity of spraying solution.
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quantities of spraying solutions. The inset of Fig.2 shows the cross section area of the typical
Figure 3 shows the DC electrical resistivity of annealed cobalt ferrite thin films deposited at various quantities of spraying solutions. The resistivity decreases with increase in temperature showing the semiconducting behavior of the films. In ferrites material the resistivity was depends
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on the electron hopping mechanism between the trivalent metal ions present in the sample, ionic substitution, method of preparation and crystal structure of the material [34]. The activation
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energy in the studied temperature region was calculated by using the Arrhenius Equation [35]. The slight variation of resistivity is observed due to the effect of thickness. From Table 1 it is seen that the activation energy slightly varies with changing solution quantities as all the thin film samples were annealed at a fixed temperature (900 oC) and stoichiometry of the samples does not change with change in solution quantity. The similar values of activation energy of
3.3 Morphological studies
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cobalt ferrite prepared by the sol-gel route are reported by Gul and Maqsood [33].
Figure 4 shows the FESEM images of cobalt ferrite thin films prepared at different quantities of spraying solutions. This morphology of the films includes grains whose average
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size increases with the increase in quantity of spraying solutions. In the present study, grains have an incredibly large size from 200 to 400 nm compared to that obtained by XRD results.
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From these observations, it is seen that the grains appear on the film surface are clusters of crystallites [36]. The average grain size increases with the increase in quantity of spraying solution, this variation of grain size is in good agreement with the XRD results. This increases the surface area of film for the NO2 adsorption and hence enhances the gas sensor response. It is seen that the surface morphology with grain size within the nanometer range have covered the surface, suggesting that spray pyrolysis make possible to produce nanocrystalline growth of CoFe2O4 ferrite thin films. These formations of the grain structure of ferrite are useful for adsorption of gas molecules to the film surface and get better gas sensor response [37].
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3.4 Dielectric properties The effect of solution quantity on the dielectric behavior of cobalt ferrite thin films has been studied for all the films at room temperature. Figure 5 shows the room temperature variation of real part dielectric constant (ε’) of cobalt ferrite thin films deposited at various
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quantities of the spraying solution. The dielectric constant of cobalt ferrite thin films is measured over a frequency range from 20 Hz to 1 MHz [38]. From Fig.5 it is clear that at lower frequencies the dielectric constant slowly decreases and at higher frequencies it remains constant. This type of behavior is almost similar to all the films prepared at various quantities of
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the spraying solution. The change in the dielectric constant is due to the electron hopping conduction mechanism between Fe3+-Fe2+ and Co3+-Co2+ pairs of ions. In ferrites the dielectric
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polarization mechanism is similar to the electrical conduction mechanism [39]. The variation of dielectric constant can be related to the combined activities of electronic charge carriers. The polarization occurs in ferrite material is through a mechanism similar to the conduction process. Due to the presence of Fe3+ and Fe2+ ions ferrite materials referred as a dipolar. The rotational displacement of dipoles creates orientation of polarization which may be visualized as the exchange of electrons between the ions so that, the dipoles align in response to the changes of
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frequency of the electric field [40]. The polarization at lower frequencies results from the conduction mechanism or electron hopping between Fe3+ and Fe2+ ions in the ferrite lattice. The decrease in polarization with increase in frequency can be explained by Maxwell-Wagner polarization based on Koop’s phenomenon hypothesis. According to this hypothesis the well
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conducting grains at high frequencies are separated by the poorly conducting grain boundaries at low frequencies [41]. At lower frequencies the dielectric constant is higher due to the space
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charge polarization and at higher frequencies it remains constant this type of phenomenon can be attributed to the electronic polarizability of ions [42]. In ferrite the conductivity and dielectric dispersion are possible due to the exchange of charge carriers situated on two sub-lattice sites in the spinel structure. The dielectric constant is inversely proportional to the electrical resistivity and similar behavior is observed in the present study. The room temperature variations in loss tangent (tanδ) with frequency of CoFe2O4 thin films for various solution quantities are shown in Fig.6. From this figure, it has been seen that at lower frequencies higher loss tangent is observed and then decreases for higher frequencies. The imaginary part of dielectric constant ε” was calculated using the relation, ε”=ε’tanδ where, tanδ is the dielectric loss tangent and ε’ is the real 7
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part of the dielectric constant. Figure 7 shows the variation of the imaginary part of the dielectric constant (ε") with frequency at room temperature. The imaginary part of the dielectric constant of the samples occurs due to the polarization behind the alternating electric field and is caused by the crystal lattice impurities which depend on stoichiometry, composition and structural
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homogeneity and sintering temperature [43]. In present study there is a slight variation of ε’ and ε" are observed due to the stoichiometry of the samples cannot change and all samples are annealed at constant temperature.
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3.5 Gas sensing properties
The gas sensing properties of CoFe2O4 thin films as a function of operating temperature
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and gas concentration have been carried out. Gas response is measured by heating the films for one hour at the desired operating temperature and the stabilized resistance is taken as the reference resistance for gas sensing measurements. The gas response was measured at different operating temperatures viz., 100, 150, 200 and 250 oC shown in Fig.8. From this graph it seen that at 150 oC operating temperature the response is higher. The maximum gas response is observed at 150 oC operating temperature, it may be due to at this temperature NO2 highly react
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with the pre-adsorbed oxygen species O2− of the film surface [44]. At high operating temperatures (200-250 oC) the pre-adsorbed oxygen species on the film surface was reduced and gas molecule can less accessible to respond the NO2 gas therefore, the short response was observed at higher operating temperatures. Therefore, optimized operating temperature of 150 oC
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was selected for all measurements of NO2 gas sensing. It is seen that the response is increased up to film deposited at the 50 mL quantity of solutions and then decreases because the availability
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of the adsorption sites in the NO2 molecules is higher than other thin film samples [45]. Figure 8a shows that variation of gas response of typical cobalt ferrite thin film prepared at the 50 mL quantity of solution for different gas concentrations (20-80 ppm) at 150
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C operating
temperature. From figure it is seen that NO2 gas response increases with increase in gas concentration. The gas response increases from 69% to 95% with NO2 concentrations. Figure 8b shows the change in gas response of cobalt ferrite thin film prepared in the various quantities of the solution, under exposure of different NO2 gas concentrations at 150
o
C operating
temperature. It is seen that the film deposited at 50 ml solution quantity the gas response is higher than other thin film samples. It can be seen that films show lower response at 20 ppm NO2 8
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concentration and then increases with gas concentration due to enough availability of the adsorption sites at lower concentration. The selectivity toward various target gases viz. Sulfur dioxide (SO2), Ammonia (NH3), Hydrogen sulfide (H2S), Nitrogen dioxide (NO2) are shown in Fig.8c. It was seen that the higher
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sensing response observed toward NO2 compared to other gases at an operating temperature of 150 oC. The other gases are exposed to film surface the gas sensor response was found to be decreased. From Fig.8c it is seen that, the response to SO2, NH3 and H2S, was just 3%, 6%, and 7% respectively, which was less than the response towards NO2 (95%) gas at the concentration
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of 80 ppm, it means that the response to NO2 was nearly 13 times higher than that of other test gases. The other type of spinel ferrites such as, NiFe2O4, ZnFe2O4, MgFe2O4, ZnAl2O4, CoAl2O4
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and MgAl2O4 shows the gas response 10-20 % for H2S and 1 to 10 % for NH3 gas at 300 oC operating temperature, which is a comparably similar for our reported values [46]. This result suggested that the CoFe2O4 thin film based gas sensor are well selective to NO2 in presence of various oxidizing and reducing gases. In order to check the reproducibility of the gas sensor the film was tested after every 10 days shown in Fig.8d. It is found that gas response of the film decreases initially and remains nearly constant after 90 days. The CoFe2O4 thin film based gas
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sensor shows a fast response (5 s) and recovery (117 s) time with NO2 gas. The recovery time of metal oxide semiconductor gas sensor is also an important parameter. The resistance of the sensor increases due to adsorption and resistance decreases due to desorption, this desorption time is termed as recovery time of the sensor. The recovery time depends on the bonding force
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between NO2 gas molecule with the surface of the film and also flowing rate of NO2 [44]. The gas response, response time and recovery time of the gas sensor are illustrated in Table 2. All gas
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sensing results indicates that the CoFe2O4 based thin film gas sensor is very potential material for the development of high performance NO2 gas sensor.
4. Conclusions
The cobalt ferrite thin films onto the quartz substrate were successfully deposited using
the spray pyrolysis technique. X-ray diffraction study confirms that films are polycrystalline nature and exhibit spinel cubic crystal structure. The DC resistivity of the films decreases with increase in temperature indicates the semiconducting nature of the films. Surface morphology studies showed the grain structure with an average grain size about 200-400 nm. Dielectric 9
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measurements for all the thin film samples show a normal dielectric dispersion due to space charge polarization. The gas sensing measurements reveals that at 150 oC operating temperature films shows the maximum gas sensor response 95 % for 80 ppm NO2. The response of the gas sensor increases with increase in NO2 gas concentrations. The selectivity study reveals that the
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cobalt ferrite thin film based gas sensor shows higher gas response to NO2 than other test gases. After 90 days the thin film achieved a better response indicates that superior durability of the film.
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Acknowledgments
Authors are very much thankful to the Council of Scientific and Industrial Research
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(CSIR), New Delhi, India for the financial support through its project No. 03(1284/13/EMR-II).
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[36] H. Arabi, N.K. Moghadam, Nanostructure and magnetic properties of magnesium ferrite thin films deposited on glass substrate by spray pyrolysis, J. Magn. Magn. Mater. 335 (2013) 144-148.
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[37] Y.F. Sun, S.B. Liu, F.L. Meng, J.Y. Liu, Z. Jin, L.T. Kong, J.H. Liu, Metal oxide nanostructures and their gas sensing properties: a review, Sensors. 12 (2012) 2610-2631. [38] A.R. Babar, S.S. Shinde, A.V. Moholkar, K.Y. Rajpure, Magnetic and electrical properties of In doped cobalt ferrite nanoparticles, J. Alloys. Compd. 505 (2010) 743-749. [39] R.S. Devan, D.R. Dhakras, T.G. Vichare, A.S. Joshi, S.R. Jigajeni, Y.R. Ma, B.K. Chougule, Li0.5Co0.75Fe2O4+BaTiO3 particulate composites with coupled magnetic-electric properties, J. Phys. D: Appl. Phys. 41 (2008) 105010. [40] J. Parashar, V.K. Saxena, Jyoti, D. Bhatnagar, K.B. Sharma, Dielectric behaviour of Zn substituted Cu nano-ferrites, J. Magn. Magn. Mater. 394 (2015) 105-110. 13
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[41] C.G. Koops, On the Dispersion of Resistivity and Dielectric Constant of Some Semiconductors at Audio frequencies, Phys. Rev. 83 (1951) 121-124. [42] H.V. Keer, Principles Solid State, New Age International Publishers, New Delhi, (2000) 302.
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[43] S.S. Bellad, B.K. Chougule, Composition and frequency dependent dielectric properties of Li-Mg-Ti ferrites, Mater. Chem. Phys. 66 (2000) 58-63.
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[44] V.V. Ganbavle, S.K. Patil, S.I. Inamdar, S.S. Shinde, K.Y. Rajpure, Effect of Co doping on structural, morphological and LPG sensing properties of nanocrystalline ZnO thin films, Sens. Actuators, A. 216 (2014) 328-334. [45] A. Gurlo, N. Barsan, M. Ivanovskaya, U. Weimar, W. Gopel, In2O3 and MoO3-In2O3 thin film semiconductor sensors: interaction with NO2 and O3, Sens. Actuators, B. 47 (1998) 92-99.
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[46] V.D. Kapse, Preparation of Nanocrystalline Spinel-type oxide Materials for Gas sensing applications, Res. J. Chem. Sci., 5 (2015) 7-12.
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Figure Captions
Figure 1 X-ray diffraction patterns of annealed CoFe2O4 thin films deposited at various
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quantities of spraying solutions
Figure 2 Variation in thickness of annealed CoFe2O4 thin films and inset shows the cross section image of the film
Figure 3 Variation in DC electrical resistivity of annealed CoFe2O4 thin films with
SC
temperature
Figure 4 FESEM images of annealed CoFe2O4 thin films with various quantities of spraying solutions (a) 30 mL, (b) 40 mL, (c) 50 mL (d) 60 mL, (e) 70 mL
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Figure 5 Variation in room temperature dielectric constant with frequency of CoFe2O4 thin films deposited at various quantities of spraying solutions
Figure 6 Variation in loss tangent with a frequency of CoFe2O4 thin films deposited at various quantities of spraying solutions
Figure 7 Variation in dielectric loss (ε”) with frequency of CoFe2O4 thin films deposited at
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various quantities of spraying solutions
Figure 8 A variation in gas response of CoFe2O4 thin films for 80 ppm NO2 concentrations at different operating temperatures 8a) Variation in gas response of typical CoFe2O4 thin film at 50 mL solution quantity (SQ) with different NO2 concentrations, 8b) Change in
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gas response of films prepared at the various quantities of solution under exposure of different NO2 concentrations at constant 150 oC operating temperature, 8c) Selectivity of the typical CoFe2O4 film towards various gases at constant 150 oC operating temperature,
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8d) Reproducibility of the film exposed to 80 ppm NO2 gas at 150 oC operating temperature with a number of days
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Table captions Table 1
Structural and electric parameters of CoFe2O4 thin films for different quantities
Table 2
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of solutions
Gas response, response time and recovery time of the typical 50 mL film for
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different NO2 concentrations
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Tables
Lattice constant (Å) 8.2669
Crystallite size D (nm) 30
40
8.3033
33
50
8.3659
37
60
8.3658
37
70
8.3746
38
0.69 0.61 0.55
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0.49
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Table 1
Activation Energy (eV) 0.71
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Solution quantity (mL) 30
Gas response (%) 69
Response time (s) 3
Recovery time (s) 84
40
79
6
127
60
85
5
117
95
5
114
80
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Gas concentration (ppm) 20
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Table 2
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Figures
20
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30
40
EP
50 mL
40 mL
30 mL
50
2θ (degree)
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60 mL
(440)
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(511)
(400)
(311)
(220)
Intensity (A.U.)
70 mL
Figure 1
60
70
80
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550
500
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Film thickness (nm)
600
450
400
40
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50
60
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Solution quantity (mL) Figure 2
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16 15 14
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30 mL 40 mL 50 mL 60 mL 70 mL
13
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11
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ln ρ
12
10 9 8 2.0
2.2
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1.8
2.4
2.6 -1
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1000/T (K )
Figure 3
2.8
3.0
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40 mL
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30 mL
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50 mL
70 mL
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60 mL
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Figure 4
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118
30 mL 40 mL 50 mL 60 mL 70 mL
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114 112
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110 108
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Dielectric constant (ε')
116
106 104 102 100 100
1000
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10
10000
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Frequency (Hz) Figure 5
100000
1000000
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30 mL 40 mL 50 mL 60 mL 70 mL
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0.35 0.30 0.25
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0.15
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Tan δ
0.20
0.10 0.05 0.00 100
1000
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10
10000
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Frequency (Hz) Figure 6
100000
1000000
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40
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30 mL 40 mL 50 mL 60 mL 70 mL
30
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25 20 15
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Dielectric loss (ε")
35
10 5 0 100
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10000
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Frequency (Hz) Figure 7
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120
30 mL 40 mL 50 mL 60 mL 70 mL
80 ppm NO2
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80
60
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Gas response (%)
100
40
20
0 125
150
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100
175
200
225 o
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Operating temperature ( C) Figure 8
250
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280 o
50 mL SQ
Operating temperature 150 C
60 ppm
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180
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200
40 ppm
220 20 ppm
160
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Resistance (MΩ)
240
80 ppm
260
140 120 100 400
600
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200
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0
800
1000 1200 1400 1600 1800
Time (s)
Figure 8a
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140
100
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80 60
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Gas response (%)
120
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20 ppm 40 ppm 60 ppm 80 ppm
40 20 0
40
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30
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60
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Solution quantity (mL) Figure 8b
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100 Gas concentration 80 ppm
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90
70
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60 50 40
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Gas response (%)
80
30 20 10 0
NH3
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SO2
H2S
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Test gas
Figure 8c
NO2
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140
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120 100
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80 60
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Gas response (%)
50 mL
Gas concentration 80 ppm o Operating temperature 150 C
40 20 0 10
20
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0
50
60
70
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Number of days Figure 8d
80
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Research highlights Polycrystalline CoFe2O4 thin films were developed onto quartz substrate and their
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physicochemical as well as gas sensing properties was studied. XRD, FESEM for structural and morphological and electrical properties were studied. The gas sensing parameters towards various conditions and various oxidizing gases are systematically discussed.
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The films show higher gas response to NO2 gas as compared to other target gases at moderate operating temperature.
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We observed the maximum gas sensor response 95 % to NO2 gas at 150 oC operating
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temperature.
S0925-8388(15)31366-9
DOI:
10.1016/j.jallcom.2015.10.115
Reference:
JALCOM 35670
To appear in:
Journal of Alloys and Compounds
Received Date: 24 July 2015 Revised Date:
5 October 2015
Accepted Date: 13 October 2015
Please cite this article as: A.A. Bagade, K.Y. Rajpure, Development of CoFe2O4 thin films for nitrogen dioxide sensing at moderate operating temperature, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.10.115. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
A.A. Bagade, K.Y. Rajpure*
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Development of CoFe2O4 thin films for nitrogen dioxide sensing at moderate operating temperature
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Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur-416 004, India
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Development of CoFe2O4 thin films for nitrogen dioxide sensing at moderate operating temperature
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A.A. Bagade, K.Y. Rajpure* Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur-416 004, India Abstract
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Cobalt ferrite thin films were deposited onto quartz substrates by spray pyrolysis technique and their gas sensing properties were investigated. X-ray diffraction study confirms
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that films are polycrystalline in nature and exhibit single phase spinel cubic crystal structure. DC electrical resistivity measurement indicates the semiconducting nature of the films. The surface morphology study confirms the formation of grains structure with grain size about 200-400 nm. Room temperature variation of dielectric properties with frequency suggests the normal dielectric performance of the cobalt ferrite thin film. The influence of quantities of spraying solutions on the operating temperature, sensitivity and selectivity, gas concentrations, response
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and recovery characteristics of gas sensor are systematically tested. The maximum gas response 95 % was observed for film prepared at 50 mL quantity of solution for 80 ppm NO2. The films are extremely selective towards NO2 as compared to other target gases. Gas response attains
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nearly 90 % of its initial value after 90 days indicates good durability of the films.
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Keywords: Spray Pyrolysis; Cobalt ferrite; XRD; FESEM; Dielectric properties; Gas sensing.
*Corresponding author
Email- [email protected]
Phone: +91-231-2609435; Fax- +91-231-2691533
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1. Introduction Spinel structured ferrites are imperative scientific materials as they possess both semiconductor and magnetic properties [1]. Spinel ferrite materials have attracted the research interest due to their practical applications in many research fields [2-4]. The spinel ferrites
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having a chemical composition M2+Fe2O4 (M= Cu, Mn, Mg, Co, Zn, Ni, Cd, etc.) are widely used as magnetic materials.
Recently, some spinel ferrites thin films have been studied in the gas sensor application as they show high sensitivity and selectivity to certain gases present in the atmosphere [5]. The
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general gas sensing mechanism is based on the chemisorptions of oxygen on the surface of gas sensing material, followed by charge transfer during the reaction between oxygen with gas
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molecules. Therefore, its resistance changes during and after the expose of the gas on the surface of metal oxide semiconductor [6]. The spinel ferrite in which the divalent transition metal ion i.e. M2+ was incorporated into the lattice of the parent structure was established to be a promising material in detecting of the gases presence of the atmosphere [7]. From the gas sensor application point of view low density and high surface area is required because they show good sensitivity and stability for detection of various gases [8]. The gas response of the sensor depends on the
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morphology, annealing temperature, ferrite composition, concentration and which type of target gas to be used for detection [9]. Cobalt ferrite thin films have been used in industrial applications because they have strong anisotropy, good mechanical hardness and chemical stability [10, 11]. Lee et al. [12] synthesized the cobalt ferrite thin films by sol-gel technique and studied the
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structural and magnetic properties. They reported that the CoFe2O4 film prepared by sol-gel technique could be used for high-density recording media if the grain size is 10 nm while
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maintaining its magnetic property. The spinel cobalt ferrite thin films have extensively used in research because of their applications in frequency device [13, 14]. Pulsed laser deposition (PLD) [15], RF magnetron sputtering [16], Sol-gel method [17], spray pyrolysis [18], electron beam evaporation [19] etc. have been used to prepare ferrite thin films. Amongst the various techniques, spray pyrolysis is one of the best techniques as it is simple, cost-effective and useful for preparation of ferrite thin films [20]. Tudorache et al. [21] prepared NiCoFe2O4 thin films by spin coating technique for gas sensing application. The highest sensitivity for ethyl alcohol vapors was observed for low cobalt contents and higher response was observed for acetone at large cobalt contains thin films. Xiangfeng et al. [22] reported nanocrystalline CoFe2O4 by 2
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hydrothermal method for ethanol gas sensor and investigated their gas sensing properties. They found that the CoFe2O4 nano crystals are used for detecting very low concentration of triethylamine and ethanol gases. Sutka et al. [23] prepared n-type ZnFe2O4 thin films by spray pyrolysis technique for gas sensor application and they obtain fast response time for ethanol gas
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at 1 ppm concentration. The similar type of other spinel oxide semiconductor (MFe2O4) shows gas sensor response to NO2 gas at various experimental conditions. Zhuiykov et al. [24] investigated NO2 gas sensing properties over the temperature range 600-700 oC of the zinc ferrite. They observed maximum gas response at 436 ppm at 700 oC operating temperature. The
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gas response also increases with increase in NO2 concentration. Darshane and Mulla [25] reported the gas sensor performance towards NOx of magnesium ferrite nano-particles by using a
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simple molten salt method. They found that 52 % gas response for 1% Pd doping then decreases further increasing in Pd doping. Zhang et al. [26] prepared ZnCo2O4 nanotube by using a template assisted method and their gas sensing properties towards various gases was studied. The ZnCo2O4 nanotube shows maximum 1.72 gas response for 400 ppm NO2 at 300 oC operating temperature. Niu et al. [27] synthesized zinc ferrite by micro-emulsion method and studied their gas sensing properties. The maximum gas response S=2.4 was observed for ZnFe2O4 for 50 ppm
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NO2 at 270 oC operating temperature.
We have already reported [20] physicochemical properties of spray deposited CoFe2O4 thin films. The substrate temperature was optimized to be 400 oC. In continuation of this work, in this paper effect of the quantity of spraying solution on CoFe2O4 thin films is discussed. Further
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2. Experimental
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their NO2 gas sensing properties are studied.
2.1 Synthesis of CoFe2O4 thin films The cobalt ferrite thin films were deposited onto the preheated quartz substrates using
simple chemical spray pyrolysis technique [20]. The spraying solution was sprayed onto preheated quartz substrates at different quantities of spraying solutions an interval of 10 mL. The as-deposited cobalt ferrite thin films were annealed at the 900 oC for 4 h in ambient atmosphere at 2o/min constant heating rate. These annealed ferrite thin films were characterized for their physicochemical as well as gas sensing properties.
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2.2 Characterization techniques To analyze the crystalline quality and phase identification of annealed CoFe2O4 thin films, XRD studies were carried out using Bruker D2-Phaser X-ray powder diffractometer with Cu-Kα (λ=1.5406 Å) radiation recorded in ranges of 20-80o. The thickness of the annealed films
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was measured by using AMBIOS, USA XP-I surface profiler and scanning electron microscopy. The DC electrical resistivity of ferrite thin films was measured by using the two probe method with a high sensitivity Keithley electrometer model-6514. The surface morphology of the films was investigated using a Mira-3, Tescan, Brno-Czech Republic, Field emission scanning electron
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microscope (FE-SEM). The dielectric measurements of the cobalt ferrite thin films were carried out using a high precision LCR meter bridge (HP-6284A) in the frequency range 20 Hz to 1
2.3 Gas sensing Measurements
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MHz at room temperature.
For gas sensing measurements the silver contacts were made into the CoFe2O4 thin film, which were placed in a specially designed stainless steel chamber having diameter 7 cm and height 15 cm. The operating temperature was varied, ranging from 150 oC to 250 oC. The NO2
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gas was injected into the glass chamber with a calibrated syringe. The Cromel–Alumel thermocouple was used to measure the operating temperature and connected to the heating system. The gas response of the films in both air and in presence of the gas was measured at various gas concentrations at constant operating temperature 150 oC. Gas sensing measurements
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were carried out at locally fabricated gas sensing unit equipped with gas flow meters and change in resistance was measured on the Keithley electrometer (6514). The gas response was calculated
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by using the relation [28].
Gas response (%) =
R air − R gas R air
×100 (1)
where, Rair is the resistance in the air and Rgas is the resistance in the presence of the gas. Response and recovery times mean the time for the sensor to reach 90% of the final resistance value during gas exposure and at the end of gas exposure, respectively.
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3. Results and discussion 3.1 X-ray diffraction studies Figure 1 shows X-ray diffraction (XRD) patterns of the annealed CoFe2O4 thin films deposited at various quantities of the spraying solution. A matching of inter planar spacing
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values (d) of CoFe2O4 using JCPDS card No-02-1045 confirms the spinel cubic crystal structure. It is observed that, as the quantity of the spraying solution increases the crystallinity of films increases up to 50 ml and then decreases. The lower intensity peaks are observed in CoFe2O4 films due to the lower film thickness. The crystallite size is calculated by using the Scherrer’s
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formula [29]. The crystallite size of the films varies with varying quantities of the spraying solution. In the present study, the thickness of the films increases with increase in quantities of
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the spraying solutions. Crystallite size becomes larger in the thinnest films and is directly correlated to the film thickness. Similar variation of the crystallite size as a function of film thickness is reported by Bouderbala et al. [30]. The exact value of the lattice parameter has been calculated by plotting the graph of Nelson Riley function (NRF) versus the lattice parameter for every plane. The NRF is a function of the Bragg angle θ calculated using the well known equation [31].
ଵ ௦మ ఏ
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ܴܰ = ܨቀ + ଶ ௦ఏ
௦మ ఏ ቁ ఏ
(2)
where, θ is the Bragg angle. Then by using a linear fit technique exact value of lattice constant was determined. Now the linear fitted straight line cuts the y-axis is the point of actual lattice parameter of the material. It is seen that the values of lattice constant increases with increase in
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quantities of the spraying solutions (Table 1). A similar relation between the lattice constant and film thickness was also observed by Purohit et al. [32]. They reported that lattice constant
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increases with increase in the film thickness due to the change in density as well as the nature of native imperfection of the thin films.
3.2 DC electrical resistivity DC electrical resistivity is an important property of the ferrite materials. The cobalt ferrite
possesses high resistive spinel ferrite material with high activation energy. The preparation method, substitution of ions at tetrahedral (A) and octahedral (B) site, particle dimension and sintering temperature influences the electrical properties of spinel ferrites [33]. The thickness of the thin film is a very important parameter for thin film technology; it affects the electrical 5
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properties of the material. In the present investigation the thickness of the films was measured by AMBIOS XP-I surface profiler. From Fig. 2 it is seen that the thickness of films increased with quantity of solution, up to 50 ml quantity of spraying solution and then saturates for higher
film deposited at 50 ml quantity of spraying solution.
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quantities of spraying solutions. The inset of Fig.2 shows the cross section area of the typical
Figure 3 shows the DC electrical resistivity of annealed cobalt ferrite thin films deposited at various quantities of spraying solutions. The resistivity decreases with increase in temperature showing the semiconducting behavior of the films. In ferrites material the resistivity was depends
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on the electron hopping mechanism between the trivalent metal ions present in the sample, ionic substitution, method of preparation and crystal structure of the material [34]. The activation
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energy in the studied temperature region was calculated by using the Arrhenius Equation [35]. The slight variation of resistivity is observed due to the effect of thickness. From Table 1 it is seen that the activation energy slightly varies with changing solution quantities as all the thin film samples were annealed at a fixed temperature (900 oC) and stoichiometry of the samples does not change with change in solution quantity. The similar values of activation energy of
3.3 Morphological studies
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cobalt ferrite prepared by the sol-gel route are reported by Gul and Maqsood [33].
Figure 4 shows the FESEM images of cobalt ferrite thin films prepared at different quantities of spraying solutions. This morphology of the films includes grains whose average
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size increases with the increase in quantity of spraying solutions. In the present study, grains have an incredibly large size from 200 to 400 nm compared to that obtained by XRD results.
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From these observations, it is seen that the grains appear on the film surface are clusters of crystallites [36]. The average grain size increases with the increase in quantity of spraying solution, this variation of grain size is in good agreement with the XRD results. This increases the surface area of film for the NO2 adsorption and hence enhances the gas sensor response. It is seen that the surface morphology with grain size within the nanometer range have covered the surface, suggesting that spray pyrolysis make possible to produce nanocrystalline growth of CoFe2O4 ferrite thin films. These formations of the grain structure of ferrite are useful for adsorption of gas molecules to the film surface and get better gas sensor response [37].
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3.4 Dielectric properties The effect of solution quantity on the dielectric behavior of cobalt ferrite thin films has been studied for all the films at room temperature. Figure 5 shows the room temperature variation of real part dielectric constant (ε’) of cobalt ferrite thin films deposited at various
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quantities of the spraying solution. The dielectric constant of cobalt ferrite thin films is measured over a frequency range from 20 Hz to 1 MHz [38]. From Fig.5 it is clear that at lower frequencies the dielectric constant slowly decreases and at higher frequencies it remains constant. This type of behavior is almost similar to all the films prepared at various quantities of
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the spraying solution. The change in the dielectric constant is due to the electron hopping conduction mechanism between Fe3+-Fe2+ and Co3+-Co2+ pairs of ions. In ferrites the dielectric
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polarization mechanism is similar to the electrical conduction mechanism [39]. The variation of dielectric constant can be related to the combined activities of electronic charge carriers. The polarization occurs in ferrite material is through a mechanism similar to the conduction process. Due to the presence of Fe3+ and Fe2+ ions ferrite materials referred as a dipolar. The rotational displacement of dipoles creates orientation of polarization which may be visualized as the exchange of electrons between the ions so that, the dipoles align in response to the changes of
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frequency of the electric field [40]. The polarization at lower frequencies results from the conduction mechanism or electron hopping between Fe3+ and Fe2+ ions in the ferrite lattice. The decrease in polarization with increase in frequency can be explained by Maxwell-Wagner polarization based on Koop’s phenomenon hypothesis. According to this hypothesis the well
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conducting grains at high frequencies are separated by the poorly conducting grain boundaries at low frequencies [41]. At lower frequencies the dielectric constant is higher due to the space
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charge polarization and at higher frequencies it remains constant this type of phenomenon can be attributed to the electronic polarizability of ions [42]. In ferrite the conductivity and dielectric dispersion are possible due to the exchange of charge carriers situated on two sub-lattice sites in the spinel structure. The dielectric constant is inversely proportional to the electrical resistivity and similar behavior is observed in the present study. The room temperature variations in loss tangent (tanδ) with frequency of CoFe2O4 thin films for various solution quantities are shown in Fig.6. From this figure, it has been seen that at lower frequencies higher loss tangent is observed and then decreases for higher frequencies. The imaginary part of dielectric constant ε” was calculated using the relation, ε”=ε’tanδ where, tanδ is the dielectric loss tangent and ε’ is the real 7
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part of the dielectric constant. Figure 7 shows the variation of the imaginary part of the dielectric constant (ε") with frequency at room temperature. The imaginary part of the dielectric constant of the samples occurs due to the polarization behind the alternating electric field and is caused by the crystal lattice impurities which depend on stoichiometry, composition and structural
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homogeneity and sintering temperature [43]. In present study there is a slight variation of ε’ and ε" are observed due to the stoichiometry of the samples cannot change and all samples are annealed at constant temperature.
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3.5 Gas sensing properties
The gas sensing properties of CoFe2O4 thin films as a function of operating temperature
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and gas concentration have been carried out. Gas response is measured by heating the films for one hour at the desired operating temperature and the stabilized resistance is taken as the reference resistance for gas sensing measurements. The gas response was measured at different operating temperatures viz., 100, 150, 200 and 250 oC shown in Fig.8. From this graph it seen that at 150 oC operating temperature the response is higher. The maximum gas response is observed at 150 oC operating temperature, it may be due to at this temperature NO2 highly react
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with the pre-adsorbed oxygen species O2− of the film surface [44]. At high operating temperatures (200-250 oC) the pre-adsorbed oxygen species on the film surface was reduced and gas molecule can less accessible to respond the NO2 gas therefore, the short response was observed at higher operating temperatures. Therefore, optimized operating temperature of 150 oC
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was selected for all measurements of NO2 gas sensing. It is seen that the response is increased up to film deposited at the 50 mL quantity of solutions and then decreases because the availability
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of the adsorption sites in the NO2 molecules is higher than other thin film samples [45]. Figure 8a shows that variation of gas response of typical cobalt ferrite thin film prepared at the 50 mL quantity of solution for different gas concentrations (20-80 ppm) at 150
o
C operating
temperature. From figure it is seen that NO2 gas response increases with increase in gas concentration. The gas response increases from 69% to 95% with NO2 concentrations. Figure 8b shows the change in gas response of cobalt ferrite thin film prepared in the various quantities of the solution, under exposure of different NO2 gas concentrations at 150
o
C operating
temperature. It is seen that the film deposited at 50 ml solution quantity the gas response is higher than other thin film samples. It can be seen that films show lower response at 20 ppm NO2 8
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concentration and then increases with gas concentration due to enough availability of the adsorption sites at lower concentration. The selectivity toward various target gases viz. Sulfur dioxide (SO2), Ammonia (NH3), Hydrogen sulfide (H2S), Nitrogen dioxide (NO2) are shown in Fig.8c. It was seen that the higher
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sensing response observed toward NO2 compared to other gases at an operating temperature of 150 oC. The other gases are exposed to film surface the gas sensor response was found to be decreased. From Fig.8c it is seen that, the response to SO2, NH3 and H2S, was just 3%, 6%, and 7% respectively, which was less than the response towards NO2 (95%) gas at the concentration
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of 80 ppm, it means that the response to NO2 was nearly 13 times higher than that of other test gases. The other type of spinel ferrites such as, NiFe2O4, ZnFe2O4, MgFe2O4, ZnAl2O4, CoAl2O4
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and MgAl2O4 shows the gas response 10-20 % for H2S and 1 to 10 % for NH3 gas at 300 oC operating temperature, which is a comparably similar for our reported values [46]. This result suggested that the CoFe2O4 thin film based gas sensor are well selective to NO2 in presence of various oxidizing and reducing gases. In order to check the reproducibility of the gas sensor the film was tested after every 10 days shown in Fig.8d. It is found that gas response of the film decreases initially and remains nearly constant after 90 days. The CoFe2O4 thin film based gas
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sensor shows a fast response (5 s) and recovery (117 s) time with NO2 gas. The recovery time of metal oxide semiconductor gas sensor is also an important parameter. The resistance of the sensor increases due to adsorption and resistance decreases due to desorption, this desorption time is termed as recovery time of the sensor. The recovery time depends on the bonding force
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between NO2 gas molecule with the surface of the film and also flowing rate of NO2 [44]. The gas response, response time and recovery time of the gas sensor are illustrated in Table 2. All gas
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sensing results indicates that the CoFe2O4 based thin film gas sensor is very potential material for the development of high performance NO2 gas sensor.
4. Conclusions
The cobalt ferrite thin films onto the quartz substrate were successfully deposited using
the spray pyrolysis technique. X-ray diffraction study confirms that films are polycrystalline nature and exhibit spinel cubic crystal structure. The DC resistivity of the films decreases with increase in temperature indicates the semiconducting nature of the films. Surface morphology studies showed the grain structure with an average grain size about 200-400 nm. Dielectric 9
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measurements for all the thin film samples show a normal dielectric dispersion due to space charge polarization. The gas sensing measurements reveals that at 150 oC operating temperature films shows the maximum gas sensor response 95 % for 80 ppm NO2. The response of the gas sensor increases with increase in NO2 gas concentrations. The selectivity study reveals that the
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cobalt ferrite thin film based gas sensor shows higher gas response to NO2 than other test gases. After 90 days the thin film achieved a better response indicates that superior durability of the film.
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Acknowledgments
Authors are very much thankful to the Council of Scientific and Industrial Research
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(CSIR), New Delhi, India for the financial support through its project No. 03(1284/13/EMR-II).
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Figure Captions
Figure 1 X-ray diffraction patterns of annealed CoFe2O4 thin films deposited at various
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quantities of spraying solutions
Figure 2 Variation in thickness of annealed CoFe2O4 thin films and inset shows the cross section image of the film
Figure 3 Variation in DC electrical resistivity of annealed CoFe2O4 thin films with
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temperature
Figure 4 FESEM images of annealed CoFe2O4 thin films with various quantities of spraying solutions (a) 30 mL, (b) 40 mL, (c) 50 mL (d) 60 mL, (e) 70 mL
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Figure 5 Variation in room temperature dielectric constant with frequency of CoFe2O4 thin films deposited at various quantities of spraying solutions
Figure 6 Variation in loss tangent with a frequency of CoFe2O4 thin films deposited at various quantities of spraying solutions
Figure 7 Variation in dielectric loss (ε”) with frequency of CoFe2O4 thin films deposited at
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various quantities of spraying solutions
Figure 8 A variation in gas response of CoFe2O4 thin films for 80 ppm NO2 concentrations at different operating temperatures 8a) Variation in gas response of typical CoFe2O4 thin film at 50 mL solution quantity (SQ) with different NO2 concentrations, 8b) Change in
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gas response of films prepared at the various quantities of solution under exposure of different NO2 concentrations at constant 150 oC operating temperature, 8c) Selectivity of the typical CoFe2O4 film towards various gases at constant 150 oC operating temperature,
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8d) Reproducibility of the film exposed to 80 ppm NO2 gas at 150 oC operating temperature with a number of days
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Table captions Table 1
Structural and electric parameters of CoFe2O4 thin films for different quantities
Table 2
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of solutions
Gas response, response time and recovery time of the typical 50 mL film for
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different NO2 concentrations
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Tables
Lattice constant (Å) 8.2669
Crystallite size D (nm) 30
40
8.3033
33
50
8.3659
37
60
8.3658
37
70
8.3746
38
0.69 0.61 0.55
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0.49
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Table 1
Activation Energy (eV) 0.71
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Solution quantity (mL) 30
Gas response (%) 69
Response time (s) 3
Recovery time (s) 84
40
79
6
127
60
85
5
117
95
5
114
80
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Gas concentration (ppm) 20
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Table 2
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Figures
20
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30
40
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50 mL
40 mL
30 mL
50
2θ (degree)
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60 mL
(440)
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(511)
(400)
(311)
(220)
Intensity (A.U.)
70 mL
Figure 1
60
70
80
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550
500
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Film thickness (nm)
600
450
400
40
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30
50
60
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Solution quantity (mL) Figure 2
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16 15 14
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30 mL 40 mL 50 mL 60 mL 70 mL
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ln ρ
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10 9 8 2.0
2.2
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1.8
2.4
2.6 -1
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1000/T (K )
Figure 3
2.8
3.0
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40 mL
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30 mL
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50 mL
70 mL
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60 mL
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Figure 4
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118
30 mL 40 mL 50 mL 60 mL 70 mL
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114 112
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110 108
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Dielectric constant (ε')
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106 104 102 100 100
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Frequency (Hz) Figure 5
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30 mL 40 mL 50 mL 60 mL 70 mL
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0.35 0.30 0.25
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0.15
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Tan δ
0.20
0.10 0.05 0.00 100
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Frequency (Hz) Figure 6
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40
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30 mL 40 mL 50 mL 60 mL 70 mL
30
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Dielectric loss (ε")
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10 5 0 100
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Frequency (Hz) Figure 7
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120
30 mL 40 mL 50 mL 60 mL 70 mL
80 ppm NO2
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80
60
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Gas response (%)
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40
20
0 125
150
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175
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225 o
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Operating temperature ( C) Figure 8
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280 o
50 mL SQ
Operating temperature 150 C
60 ppm
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180
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40 ppm
220 20 ppm
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Resistance (MΩ)
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80 ppm
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Time (s)
Figure 8a
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Gas response (%)
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20 ppm 40 ppm 60 ppm 80 ppm
40 20 0
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Solution quantity (mL) Figure 8b
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100 Gas concentration 80 ppm
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Gas response (%)
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30 20 10 0
NH3
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SO2
H2S
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Test gas
Figure 8c
NO2
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140
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Gas response (%)
50 mL
Gas concentration 80 ppm o Operating temperature 150 C
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Number of days Figure 8d
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Research highlights Polycrystalline CoFe2O4 thin films were developed onto quartz substrate and their
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physicochemical as well as gas sensing properties was studied. XRD, FESEM for structural and morphological and electrical properties were studied. The gas sensing parameters towards various conditions and various oxidizing gases are systematically discussed.
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The films show higher gas response to NO2 gas as compared to other target gases at moderate operating temperature.
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We observed the maximum gas sensor response 95 % to NO2 gas at 150 oC operating
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temperature.