Nanostructured Spinel ZnFe2O4 for the Detection of ... - ResearchGate

Sensors & TransducersVolume 134 Issue 11November 2011www.sensorsportal.com ISSN 1726-5479Editors-in-Chief: professor Sergey Y. Yurish, tel.: +34 696067716, e-mail: editor@sensorsportal.comEditors for Western EuropeMeijer, Gerard C.M., Delft University of Technology, The NetherlandsFerrari, Vittorio, Universitá di Brescia, ItalyEditor for Eastern EuropeSachenko, Anatoly, Ternopil State Economic University, UkraineEditors for North AmericaDatskos, Panos G., Oak Ridge National Laboratory, USAFabien, J. Josse, Marquette University, USAKatz, Evgeny, Clarkson University, USAEditor South AmericaCosta-Felix, Rodrigo, Inmetro, BrazilEditor for AfricaMaki K.Habib, American University in Cairo, EgyptEditor for AsiaOhyama, Shinji, Tokyo Institute of Technology, JapanEditor for Asia-PacificMukhopadhyay, Subhas, Massey University, New ZealandEditorial Advisory BoardAbdul Rahim, Ruzairi, Universiti Teknologi, MalaysiaAhmad, Mohd Noor, Nothern University of Engineering, MalaysiaAnnamalai, Karthigeyan, National Institute of Advanced Industrial Scienceand Technology, JapanArcega, Francisco, University of Zaragoza, SpainArguel, Philippe, CNRS, FranceAhn, Jae-Pyoung, Korea Institute of Science and Technology, KoreaArndt, Michael, Robert Bosch GmbH, GermanyAscoli, Giorgio, George Mason University, USAAtalay, Selcuk, Inonu University, TurkeyAtghiaee, Ahmad, University of Tehran, IranAugutis, Vygantas, Kaunas University of Technology, LithuaniaAvachit, Patil Lalchand, North Maharashtra University, IndiaAyesh, Aladdin, De Montfort University, UKAzamimi, Azian binti Abdullah, Universiti Malaysia Perlis, MalaysiaBahreyni, Behraad, University of Manitoba, CanadaBaliga, Shankar, B., General Monitors Transnational, USABaoxian, Ye, Zhengzhou University, ChinaBarford, Lee, Agilent Laboratories, USABarlingay, Ravindra, RF Arrays Systems, IndiaBasu, Sukumar, Jadavpur University, IndiaBeck, Stephen, University of Sheffield, UKBen Bouzid, Sihem, Institut National de Recherche Scientifique, TunisiaBenachaiba, Chellali, Universitaire de Bechar, AlgeriaBinnie, T. 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Sensors & Transducers JournalContentsVolume 134Issue 11November 2011www.sensorsportal.com ISSN 1726-5479Research ArticlesNanomaterials and Chemical SensorsSukumar Basu and Palash Kumar Basu ............................................................................................ 1Fabrication of Phenyl-Hydrazine Chemical Sensor Based on Al- doped ZnO NanoparticlesMohammed M. Rahman, Sher Bahadar Khan, A. Jamal, M. Faisal, Abdullah M. Asiri ..................... 32Nanostructured Ferrite Based Electronic nose Sensitive to Ammonia at room temperatureU. B. Gawas, V. M. S. Verenkar, D. R. Patil....................................................................................... 45A Humidity Sensor Based on Nb-doped Nanoporous TiO 2 Thin FilmMansoor Anbia, S. E. Moosavi Fard................................................................................................... 56A Resistive Humidity Sensor Based on Nanostructured WO 3 -ZnO CompositesKarunesh Tiwari, Anupam Tripathi, N. K. Pandey.............................................................................. 65Highly Sensitive Cadmium Concentration Sensor Using Long Period GratingA. S. Lalasangi, J. F. Akki, K. G. Manohar, T. Srinivas, Prasad Raikar, Sanjay Kherand U. S. Raikar ................................................................................................................................. 76MEMS Based Ethanol Sensor Using ZnO Nanoblocks, Nanocombs and Nanoflakes asSensing LayerH. J. Pandya, Sudhir Chandra and A. L. Vyas ................................................................................... 85Simple Synthesis of ZnCo 2 O 4 Nanoparticles as Gas-sensing MaterialsS. V. Bangale,S. M. Khetre D. R. Patil and S. R. Bamane................................................................. 95Nanostructured Spinel ZnFe 2 O 4 for the Detection of Chlorine GasS. V. Bangale, D. R. Patil and S. R. Bamane..................................................................................... 107Fabrication of Polyaniline-ZnO Nanocomposite Gas SensorS. L. Patil, M. A. Chougule, S. G. Pawar, Shashwati Sen, A. V. Moholkar, J. H. Kim and V. B. Patil 120Synthesis and Characterization of Nano-Crystalline Cu and Pb 0.5 -Cu 0.5 - ferrites byMechanochemical Method and Their Electrical and Gas Sensing PropertiesV. B. Gaikwad ,S. S. Gaikwad, A. V. Borhade and R. D. Nikam........................................................ 132Surface Modification of MWCNTs: Preparation, Characterization and Electrical PercolationStudies of MWCNTs/PVP Composite Films for Realization of Ammonia Gas SensorOperable at Room TemperatureSakshi Sharma, K. Sengupta, S. S. Islam.......................................................................................... 143An Investigation of Structural and Electrical Properties of Nano Crystalline SnO2: Cu ThinFilms Deposited by Spray PyrolysisJ. Podder and S. S. Roy ..................................................................................................................... 155

Nanocrystalline Cobalt-doped SnO2 Thin Film: A Sensitive Cigarette Smoke SensorPatil Shriram B., More Mahendra A., Patil Arun V.............................................................................. 163Effect of Annealing on the Structural and Optical Properties of Nano Fiber ZnO FilmsDeposited by Spray PyrolysisM. R. Islam, J. Podder, S. F. U. Farhad and D. K. Saha.................................................................... 170Amperometric Acetylcholinesterase Biosensor Based on Multilayer Multiwall CarbonNanotubes-chitosan CompositeXia Sun, Chen Zhai, Xiangyou Wang................................................................................................. 177Fabrication of Biosensors Based on Nanostructured Conducting Polyaniline (NSPANI)Deepshikha Saini, Ruchika Chauhan and Tinku Basu....................................................................... 187Authors are encouraged to submit article in MS Word (doc) and Acrobat (pdf) formats by e-mail: editor@sensorsportal.comPlease visit journal’s webpage with preparation instructions: http://www.sensorsportal.com/HTML/DIGEST/Submition.htmInternational Frequency Sensor Association (IFSA).

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-119Sensors & TransducersISSN 1726-5479© 2011 by IFSAhttp://www.sensorsportal.comNanostructured Spinel ZnFe 2 O 4 for the Detectionof Chlorine Gas1S. V. Bangale, 2 D. R. Patil and 1 S. R. Bamane1Metal Oxide Research, Laboratory, Department of Chemistry, Dr. Patangrao KadamMahavidyalaya, Sangli 416416 (M.S.) India2Bulk and Nano Materials Research Lab, Dept. of Physics, Rani Laxmibai CollegeParola, Dist – Jalgaon, 425111Tel.: 0233-2535993, fax: No.0233-2535993E-mail: bangale_sv@rediffmail.co, sambhajibamane@yahoo.comReceived: 1 July 2011 /Accepted: 21 November 2011 /Published: 29 November 2011Abstract: Semiconductive nanometer-size material ZnFe 2 O 4 was synthesized by a solutioncombustion reaction of inorganic reagents of Zn(NO 3 ) 3 . 6H 2 O, Fe(NO 3 ) 3 .9H 2 O and Urea as a fuel. Theprocess was a convenient, environment friendly, inexpensive and efficient preparation method for theZnFe 2 O 4 nanomaterial. Effects of the calcining temperature on the phase constituents characterized byTG-DTA, X-ray diffraction (XRD) was used to confirm the material structure, The as-preparedsamples were further characterized by scanning electron microscopy (SEM) equipped with energydispersiveX-ray spectroscopy (EDX), transmission electron microscopy (TEM). To depict thecrystallite microstructure. Conductance responses of the nanocrystalline ZnFe 2 O 4 thick film weremeasured by exposing the film to reducing gases like Acetone, Ethanol, Ammonia (NH 3 ), Hydrogen(H 2 ), Hydrogen sulphide (H 2 S), Chlorine (Cl 2 ) and Liquefied petroleum gas (LPG). It was found thatthe sensors exhibited various sensing responses to these gases at different operating temperature.Furthermore, the sensor exhibited a fast response and a good recovery. The results demonstrated thatZnFe 2 O 4 can be used as a new type of gas-sensing material which has a high sensitivity and goodselectivity to Chlorine (CL 2 ). Copyright © 2011 IFSA.Keyword: Solution combustion reaction, Synthesis, ZnFe 2 O 4 nanoparticles, Gas sensor.1. IntroductionThe need for a novel gas sensor material capable of providing reliable operation in harsh environmentsis now greater than ever. Such sensors find a range of applications, including the monitoring of traffic107

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-119pollutants or food quality in specially designed electronic noses [1- 2]. Semiconductor metal oxidesensors are alternatives for non expensive and robust detection system. Spinel-type oxidesemiconductors with formula MFe 2 O 4 have been reported to be sensitive materials to both oxidizingand reducing gases [3]. Liu et al [4] reported the high sensitivity of CdFe 2 O 4 to ethanol vapour; Reddyet al [5] investigated NiFe 2 O 4 as sensor to detect Cl 2 in air. A few Cl 2 gas sensors have already beendeveloped, Transparent conducting oxide (TCO) thin film sensors operate at higher temperature(~300 0 C). However it is inconvenient to operate. It is, therefore, essential to develop a sensor, whichcould detect CL 2 gas at Room temperature. Zirconia tubes, MgO-In 2 O 3 , Zn 2 O 5 -MgIn 2 O 4 (TCO),ZnO-CaO etc., are materials utilized in Cl 2 sensing applications. [6-10]. Chen et al [11] revealed thatMgFe 2 O 4 and CdFe 2 O 4 are sensitive and selective to LPG and C 2 H 2 . Other earlier report about theapplication study of zinc ferrite as a gas sensing material include ZnFe 2 O 4 particles to H 2 S [12], adirectly deposited film to Co [13], ultrafine powder to Cl 2 [14] CdO-doped ZnFe 2 O 4 to ethanol [15], aZnO/ZnFe 2 O 4 thick film to propanol [16], ZnFe 2 O 4 tubes synthesis and application to gas sensors [17].Our latest work about MCo 2 O 4 (M = Ni, Cu, Zn) nanotubes, which possess combined characteristics ofhallow-inside and 1D structure, has displayed excellent selectivity and high sensitivity [18]. In theseregards, zinc ferrite tubes, which not only hold the above mentioned 1D tubular structure but also haveadvantages of low cost and environmental friendliness, are expected to be a good candidate as sensormaterials.Herein, we prepared ZnFe 2 O 4 nanopowder by this simple solution combustion reaction. One of ouraims is to develop a general synthesis method and explore the gas sensing properties of the ZnFe 2 O 4nanopowder obtained. We found that the process is a convenient, environment friendly, inexpensiveand efficient for preparation of ZnFe 2 O 4 nanomaterial with the grain size of about 15-35 nm.Furthermore, the ZnFe 2 O 4 obtained possesses excellent gas-sensing responses to reducing gas. In thepresent paper we report the development of thick film zinc iron oxide Cl 2 sensors.2. Experimental2.1. Sample Preparation and CharacterizationIn this study polycrystalline ZnFe 2 O 4 powder was prepared using urea as fuel. By combustion Rout.The materials used as precursors were Zinc nitrate hexahydrate Zn (NO 3 ) 2 6H 2 O, Fe (NO 3 ) 2 6H 2 O Ironnitrate hexahydrate (all these were procured from A. I. Grade of Qualligen) and (3) urea (Nuclearband). Urea possesses a high heat of combustion. It is an organic fuel and provides a platform forredox reactions during the course of combustion. Initially the Zinc nitrates, Iron nitrates and urea aretaken in the 1:1 :4 stoichiometric amount and dissolved in a 250 ml beaker then slowly string by usingglass rod then clear solution was obtained. Solution formed was evaporated on hot plate in temperaturerange 70 0 C to 80 0 C gives thick gel. The gel was kept on a hot plate for auto combustion and heated inthe temperature range 170 0 C to 180 0 C. The nanocrystalline ZnFe 2 O 4 powder was formed within40-50 minute. And then sintered at about 300, 500, 800 and 1000 0 C for about 4 hours then we getbrown colour shining powder of nanocrystalline ZnFe 2 O 4.Zinc iron oxide powder was ground in an agate pastel-moter to ensure sufficiently fine particle size.The fine powder was calcined at 800 0 C for 24 h in air and re-ground. The thixotropic paste [19-20]was formulated by mixing the resulting ZnFe 2 O 4 fine powder with a soluction of ethyl cellulose (atemporary binder) in a mixture of organic solvents such as butyl carbitol acetate and turpineol. Theratio of inorganic and organic path was kept as 75:25 in formulating the paste. The paste was then usedto prepare thick films. The thixotropic paste was screen printed on a glass substrate in desired patterns.The films prepared were fired at 500 0 C for 24 h.108

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-119The as –prepared samples were characterized by TG/DTA thermal analyzer (SDT Q600 V 20.9 Build20), XRD Philips Analytic X-ray B.V. (PW-3710 Based Model diffraction analysis using Cu-K αradiation), scanning electron microscope (SEM, JEOL JED 2300) coupled with an energy dispersivespectrometer (EDS JEOL 6360 LA), A JEOL JEM–200 CX transmission electron microscopeoperating at 200 kV analysis.2.2. Fabrication and Analysis of Gas SensorsThe sensing performance of the sensors was examined using a “static gas-sensing system. There wereelectrical feeds through the base plate. The heat was fixed on the base plate to heat the sample undertest up to the required operating temperatures. The current passing through the heating element wasmonitored using a relay with adjustable ON and OFF time intervals. A Cr-Al thermocouple was usedto sense the operating temperature of the sensors. The output of the thermocouple was connected todigital temperature indicators. A gas inlet valve was fitted at one port of the base plate. The requiredgas concentration inside the static system was achieved by injecting a known volume of test gas usinga gas-injecting syringe. A constant voltage was applied to the sensors, and current was measured by adigital Pico-ammeter. Air was allowed to pass into the glass dome after every Gases exposure cycle.3. Result and Discussion3.1. Spinel Structure and Formation Analysis2C 2 NH 3 O 2 + (9/2) O 2 → N 2 + CO 2 + 5H 2 OZn (NO 3 ) 3 + Fe (NO 3 ) 3 + 4C 2 H 5 NO 2 → ZnFe 2 O 4 + 8H 2 O + 10H 2 O + 5N 2TG-DTA analysis was performed at a heating rate of 10 K min -1 to investigate the thermal propertiesof. ZnFe 2 O 4. The TG spectrum and its 1 st derivative in Fig. 1 show the thermal decomposition ofZnFe 2 O 4 is the curve indicates that the slight weight loss in ZnFe 2 O 4 powder due to little loss about14.66 at temperature up to 200 0 C in ZnFe 2 O 4 of moisture, carbon dioxide and nitrogen gas. The DTAcurve of ZnFe 2 O 4 recorded in static air and in shown in Fig. 1. The curve shown that ZnFe 2 O 4 did notdecompose, but weight loss was due to dehydrogenation, decarboxylation and denitration. Furtherweight loss of about 16 % between the temperature range 400 0 C and continuous loss in weight about33.33 % up to 550 0 C is attributed to loss of organic materials and yield final product at 600 0 C. Thisweight loss and weight gained was very negligible. This weight change was in the range of 800 0 Cthese indicating that the synthesized powder was almost stable from the begging. The formationtemperature in the present work is found to be comparatively similar than that reported forcorresponding solid state reaction route.The X-ray diffraction pattern of ZnFe 2 O 4 powder is shown in Fig. 2 The observed d values comparedwith standard d values and are in good agreement with standard d value JCPDS data card number82-1042. The structure possesses the cubic may be attributed to the different preparation method whichmay yield different structural defects. The crystalline size was determined from full width of halfmaximum (FWHM) of the most intense peak obtained by shown scanning of X-ray diffraction pattern.The grain size was calculated by using Scherrer’s formula.d = 0.9λ/ βcosθ109

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-119Fig. 1. TG-DTA curve of mixed precursor.The crystalline size can be calculated by using Scherrer equation. [21, 22]. Where, d is the crystallinesize, λ is the X-ray wavelength of the Cu K α source (λ=1.54056 A 0 ), β is the FWHM of the mostpredominant peak at 100 % intensity, θ is the Braggs angle at which peak is recorded. In order toobtain pure nanocrystalline ZnFe 2 O 4 particles and understand the thermal characterizations, the asprepared ZnFe 2 O 4 powder is further calcined at 180, 300, 500, 800 and 1000 0 C respectively (thecalcined temperature assigned as T C ). Fig.2 present XRD patterns for ZnFe 2 O 4 oxide nanoparticles.The effects of the calcinations temperature on the crystallite size of ZnFe 2 O 4 particles can bedemonstrated. Traces of ZnFe 2 O 4 crystallites phases (111), (311), (400), (422) and (511) are detectedin the XRD pattern for all calcined temperatures and then their intensities increase abruptly when theT C above 1000 0 C .In general, the sharpness of the XRD peak (i.e. high crystallinity) is increased as theT C increases. According to the (311) diffraction pattern of ZnFe 2 O 4 crystalline, the particle size ofZnFe 2 O 4 can be calculated from the full width at haif-maximum using the Scherrer equation.Obviously, the particle size of ZnFe 2 O 4 changes as the Tc controlled fewer than 180, 300, 500, 800 and1000 0 C, the order is 22.15, 22.90, 25.58, 32.21 and 32.34 nm, respectively. These indicate that thecrystallinity of ZnFe 2 O 4 is accelerated as the Tc above 500 0 C. It illustrates the relationship betweenthe annealing temperature and the average crystal size of the ZnFe 2 O 4 nanoparticles. It is obvious thatthe ZnFe 2 O 4 nanoparticle grows slowly at 300-5000 C and800-1000 0 C, respectively, the nanoparticle grow rapidly at 800 0 C.3.2. Partical Size AnalyzerParticle size distribution studies have been carried out by using dynamic light scattering techniques(DLS) Via Laser input energy of 632nm). It was observed that Zinc Chromium oxide nanoparticleshave narrow size distribution within the range of about 25-30 nm. Fig. 3. which well matches withcalculated from Debye-Scherrer equation.3.3. Morphology and MicrostructureThe microstructure of the sintered samples can be visualized from scanning electron microscope(SEM) tool. Fig. 4 Shown the particle morphology of high resolution, the particles are most irregularin shape with a Nanosize range.110

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-119311Intensity1114001000 O C800 O C422511500 O C300 O C180 O C20 40 60 80 1002Fig. 2. XRD pattern of calcinied mixed precursor at 300 0 C, 500 0 C. 800 0 C, 1000 0 C in air for 4 h.1.21PL Intensity0.80.60.40.20261012.3515.2418.8123.2328.6735.4Wavelength in nmFig. 3. Particle Size Distribution of ZnFe 2 O 4 oxide Nanoparticale.(a)(b)Fig. 4. SEM images of mixed precursor at 800 0 C in air for 4 h (a) low resolution, and (b) high resolution.111

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-119Some particles are found as agglomerations containing very fine particles the particles shapes are notdefined porous nature and small and large core approximately (5µm), spongy pores are seen in themicrograph. The energy dispersive X-ray microanalysis was carried out to know the presence of zinc,cobalt and oxygen peaks, confirms in Fig. 5. The TEM image of the mixed precursor calcined at800 0 C for 4 h are shown in Fig. 6 (a). It indicates the presence of ZnFe 2 O 4 nanoparticles with size 30-40 nm which form beed type of oriental aggregation throughout the region. Which are correlated wellwith the XRD result. The selected area electron diffraction (SAED) pattern Fig. 6 (b) shows the spottype pattern which is indicative of the presence of single crystalline particles. No evidence was foundfor more than one pattern, suggesting the single phage nature of the material.Fig. 5. EDX pattern of mixed precursor at 800 0 C in air for 4 h.(a)(b)Fig. 6. TEM (a) images of nanostructured ZnFe 2 O 4 (b) SAED pattern.4. Electrical Properties4.1. I-V CharacteristicsFig. 7 depicts I-V characteristics of ZnFe 2 O 4 films. It is clear from the symmetrical I-V characteristicsthat the silver contacts on the films were ohmic in nature.112

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-119Fig. 7. I-V characteristics of the sensor.4.2. The Conductance of ZnFe 2 O 4 SensorsFig. 8 shows the conductance-temperature behavior of ZnFe 2 O 4 based sensors in air a conductancepeak occurring in the C-T curve at about 400 0 C indicates the characteristic of a typical surfacecontrolledsensor model [23]. In air, from 30 to 400 0 C, the conductance of sensor increases with theincreasing of temperature, that is the intrinsic characteristic of semiconductor, but the oxygenchemically adsorbed on the surface of ZnFe 2 O 4 undergoes the following reactions with the temperatureincreasing,O 2(ad) ↔ O - 2(ad) ↔ 2O - (ad) ↔ 2O 2- (ad) (1)When temperature increases, the equilibrium moves to the right, O 2(ad) captures the electrons from theconduction band of sensor, result in the decrease of conductance. When temperature is higher than400 0 C, the effect of equilibrium (1) on the conductance play a major role that leads to the conductancedecreasing of sensor. The gas sensing mechanism is based on the changes of the surface conductanceof ZnFe 2 O 4 . The reducing gas R acting on the surface of ZnFe 2 O 4 can be described as:2e - + O 2 → 2O - (2)O - + R → RO e - ,where e- represents a conduction band electron, In the absence of reducing gas R, electron are removedfrom the ZnFe 2 O 4 conduction band by the reduction of O 2 , resulting in the formation of O - species andconsequently the conductance of ZnFe 2 O 4 decrease. When R is introduced, it react with surfaceabsorbedoxygen species O - to from RO, and electron enter into the conduction band of ZnFe2O4,leading to increase in conductance.5. Sensing Performance of the Sensor5.1. Measurement of Gas Response, Selectivity, Response and Recovery TimeGas response (S) is defined as the ratio of the change in conductance of the sensor on exposure to thetarget gas to the original conductance in air. The relation for S is as: S = G g -G a /G a ,where G a and G gare the conductance of sensor in air and in a target gas medium, respectively.113

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-119Fig. 8. Variation of log (conductivity) with reciprocal operating temperature (K -1 ).Selectivity or specificity is defined as the ability of a sensor to respond to a certain gas in the presenceof other gases.The time taken for the sensors to attain 90 % of the original conductance is the recovery time.5.2. Sensing Performance of ZnFe 2 O 4 Thick Films5.2.1. Gas Response and Operating TemperatureFig. 9 depicts the variation of 300 ppm gas responses with operating temperature. The ZnFe 2 O 4 sensorwas observed to be a temperature dependent gas sensor. From Fig. 8 it is observed that the sensorbetter responds to Cl 2 gas at 300 0 C, to H2S at 300 0 C, to NH 3 150 0 C, to Acetone 300 0 C, to LPG200 0 C, to C 2 H 5 OH 300 0 C, and to CO 2 300 0 C, and to H 2 at 350 0 C. The sensitivity towards each gasdepends on temperature. The same sensor could be used to identify Cl 2 , H 2 S, NH 3 , Acetone, LPG,Ethanol, CO 2 and H 2 just by varying the operating temperature. This is the most desirablecharacteristic of this sensor.Fig. 9. Variation of gas response of ZnFe 2 O 4 with operating temperature.114

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-1195.2.2. Active Region of the SensorThe film was exposed to varying concentrations of CL 2 , H 2 S, Acetone, NH 3 .For the ZnFe 2 O 4 samples,the response values were observed to increase continuously with increasing gas concentrations up to300 ppm, in Fig. 10. The rate of increase in response was relatively larger up to 300 ppm and sloweddown beyond 300 ppm. Thus, the active region of the sensor would be up to 300 ppm. At lower gasconcentration, the unimolecular layer of gas molecules would be formed on the surface of the sensor,which could interact more actively giving larger response. The multilayer of gas molecules, on thesensors surface, at higher gas concentration would result in saturation in response beyond 300 ppmgas.Fig. 10. Variation of gas response of ZnFe 2 O 4 with gas concentration.5.2.3. SelectivityFig. 10 depicts the, selectivity of the ZnFe 2 O 4 film for 300 ppm of various gases at varioustemperatures. Fig. 11 that in contrast to ZnFe 2 O 4 , the sample showed not only enhanced response butalso very high selectivity.Ethanol Acetone CO 2 NH 3 CL 2 H 2 S H 2 LPG2.8685 6.9166 2.7692 5.3633 18.023 9.421 2.12 3.300Fig. 11. Selectivity of ZnFe 2 O 4 thick film among various gases.115

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-1195.2.4. Response and Recovery TimeThe response of ZnFe 2 O 4 sensor was found to be quick (~ 7 s) to 300 ppm Cl 2 , while the recovery wasfast (~ 25 s). The fast response may be due to faster oxidation of the gas. The negligible quantity of thesurface reaction product and its high volatility explains its fast response and quick recovery to itsinitial chemical status.6. DiscussionWhen reducing gases such as ammonia, ethanol, LPG, chlorine, hydrogen, acetone, H 2 S react withdifferent oxygen species, complex series of reactions take place, ultimately oxidizing the ammonia,ethanol, LPG, chlorine, hydrogen, acetone, and H 2 S as:C 4 H 10 + 13O 2- + 300 0 C → 5H 2 O + 4CO 2 + 26e - (3)Cl 2 + 4O 2- + 300 0 C → 2Cl - + 2O 2 + 6e - (4)C 2 H 5 OH + 6O - + 300 0 C → 3H 2 O + 2CO 2 +6e - (5)4NH 3 + 3O 2 - + 150 0 C → 6H 2 O + 2N 2 + 3e - (6)4H 2 S + 4O 2- + 300 0 C → SO 2 + 4H 2 O + 8e - (7)2H + + 2O 2 - → H 2 O 2 + O 2 (8)There are four adsorption behaviors of chlorine on the oxide surface [24- 25]Cl 2 + O 2(ad) 2− → 2Cl (ad) − +O 2 +2e − (9)Cl 2 +2O ox→ 2Cl o − +O 2 +2e − (10)Cl 2 +2e → 2Cl (ad) −Cl 2 +2V o +2e − → 2Cl o − ,where subscripts ad and o mean the species adsorbed on the surface and the species occupying thelattice oxygen site, respectively. Vo is the oxygen vacancy. Pure ZnFe 2 O 4 showed response to chlorineat high temperature.This shows n-type conduction mechanism. Thus on oxidation, a single molecule of gas liberatesplurality of electrons in the conduction band, resulting in an increase in conductivity of the sensor. Wefound that the gas responses increase with operating temperature, reach to their responses increasewith operating temperature, reach to their respective maxima at a particular temperature and thendecrease with a further increase in operating temperature. An increase in operating temperature causesoxidation of a large number of gas molecules, producing a very large number of electrons. Therefore,the conductivity increases this is the reason why the gas response increases with operatingtemperature. The temperature at which the gas response is maximum is the actual temperature neededto activate the material for progressing the reaction. However, the response decreases at higheroperating temperature, as the oxygen adsorbates are desorbed from the surface of the sensor. Also, athigher temperature, the carrier concentration increases due to intrinsic thermal excitation and the116

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-119Debye length decreases. This may be one of the reasons for the decreased gas response at highertemperature [26].The ZnFe 2 O 4 film gives responses to Cl 2 gas at 300 0 C, to H 2 S at 300 0 C, to NH 3 150 0 C, to Acetone300 0 C, to LPG 200 0 C, to C 2 H 5 OH 300 0 C, and to CO 2 300 0 C, and to H 2 at 350 0 C. The gas responsecan be tuned with operating temperature. The selective gas responses of the sensor to various gases atvarious temperatures may be attributed to oxidation of different gases by different oxygen species onthe surface (Fig. 8).The working principal of thick film semiconducting gas sensors is based on the change of theelectronic conductivity of the semiconducting material upon exposure of target gas. The interaction oftarget gas molecule with the surface of the thick film causes the transfer of electrons between thesemiconducting surface and the adsorbates. The atmospheric oxygen molecules O 2 are adsorbed on thesurface of the thick film. They capture the electrons from the conduction band of the thick filmmaterial as:O 2(air) + 4e - → 2O 2- (film surface)The ZnFe 2 O 4 showed response to chlorine at high temperature. This would be attributed to thereplacement of adsorbed oxygen by chlorine Fig. 12. Donating electrons to the base material reaction(9) when chlorine gas was exposed to ZnFe 2 O 4 film, it substitutes lattice oxygen on the film surface.The reaction (10) is responsible for sensing of chlorine in which chlorine substituted for lattice oxygento form Cl (ad) - inducing electron donation into the oxide.ZnFe 2 O 4 forms p–n heterojunctions, giving very high resistance even at room temperature. WhenZnFe 2 O 4 comes in contact with chlorine gas, lattice oxygen would be replaced by chlorine rupturingthe heterojunctions and the resistance drops down suddenly. In addition to this, the material gainselectrons.Fig. 12. Gas sensing mechanism of ZnFe 2 O 4 at 300 0 C.7. SummaryFrom the results, following statements can be made for the sensing performance of the presentZnFe 2 O 4 sensors.117

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-119(I) Nanocrystalline ZnFe 2 O 4 has been synthesized by self combustion route. This synthesis route maybe used for the synthesis of other metal oxide.(II) Among all other additives tested, ZnFe 2 O 4 is outstanding in promoting the ethanol gas sensingmechanism.(III) ZnFe 2 O 4 to be optimum and showed highest response to Chlorine vapours at 300 0 C.(IV)The sensor showed very rapid response and recovery to Chlorine vapours.(VI) Sensing mechanism of ZnFe 2 O 4 was the substitution of lattice oxygen by chlorine gas. Materialgains electrons in this substitution.(VII) The sensor has good selectivity to chlorine against acetone, NH 3 , LPG, ethanol, CO 2 , H 2 .References[1]. K. Arshak, E. Moore, G. Lyons, J. Harris, S. Clifford, Sens. Rev., 24, 2004, pp. 181-191.[2]. L. Pirlola, H. Parviainen, T. Hussein, A. Valli, K. Hameri, P. Aaalta, A. Virtanen, J. Keskinen,T. A. Pakkanen, T. Makela, R. E. Hillamo, Atomos. Enviorn., 38, 2004, pp. 3625-3635.[3]. R. Nicolae, R. Elena, T. Florin, P. D. Popa, Sensors & Transducers, Vol. 78 Issue 4, 2007,pp. 1134-1142.[4]. X. Q. Liu, Z. L. Xu, Y. S. Shen, A new type ethanol material on CdFe 2 O 4 semiconductor, J. Yunnan Univ.,19, 1997, pp. 147-149.[5]. C. V. G. Reddy, S. V. Manorama and V. I. Rao, semiconducting gas sensor for chlorine based on inversespinel nickel ferrite, Sensors and Actuators B, 55, 1999, pp. 90-95.[6]. D. R. Patil, L. A, Patil, Room temperature Chlorine gas sensing using surface modified ZnO thick filmresistors, Sensors and Actuators B, 123, 2007, pp. 546-553.[7]. D. R. Patil, L. A. Patil P. P. Patil, Cr 2 O 3 -activated ZnO thick film resistors for ammonia gas sensingoperable at room temperature, Sens. Actuators B, 126, 2007, pp. 368-374.[8]. D. R. Patil, L. A. Patil, Al 2 O 3 - modified ZnO based thick film resistors for H 2 gas sensing, Sens.Transducers 81, 2007, pp. 1354-1363.[9]. D. R. Patil L. A. Patil, preparation and study of NH 3 gas sensing behavior of Fe 2 O 3 doped ZnO thick filmresistors, Sens. & Transducers, 70, 2006, pp. 661-670.[10]. G. H. Jain, L. A. Patil M. S. Wagh, S. A. Patil, D. R. Patil, D. P. Amalnerkar, Surface modified BaTiO 3thick film resistors as H 2 S gas sensor, Sens. Actuators B, 117, 2006, pp. 159-165.[11]. N. S. Chen, X. J. Yang, E. S. Liu and J. L. Huang, reducing gas-sensing properties of ferrite compoundsMFe 2 O 4 (M = Cu, Zn, Cd, Mg), Sensors and Actuators B, 66, 2000, pp. 178-180.[12]. C. V. Gopal Reddy, S. V. Manorama, V. J. Rao, preparation and characterization of ferrites as gas sensormaterials, J. Mater. Sci. Lett., 19, 2000, pp. 775-778.[13]. Z. Jiao, M. H. Wu, J. Z. Gu, Z. Qin, Preparation and gas sensing characteristics of nanocrystalline spinelzinc ferrite thin films, IEEE Sens. J., 3, 2003, pp. 435-438.[14]. X. S. Niu, W. P. Du, W. M. Du, Preparation and gas sensing properties of ZnM 2 O 4 (M = Fe, Co, Cr), Sens.and Actuators B, 99, 2004, pp. 405-409.[15]. X. F. Chu, X. Q. Liu, G. Y. Meng, Effect of CdO dopant on the gas sensitivity properties of ZnFe 2 O 4semiconductors, Sens. and Actuators B, 65, 2000, pp. 64-67.[16]. K. Arshak, I. Gaidan, Development of a novel gas sensor based on oxide thick film, Mater. Sci. Eng. B,118, 2005, pp. 44-49.[17] G. Zhang, C. Li, F. Cheng, J. Chen, ZnFe 2 O 4 tubes: Synthesis and application to gas sensors, Sensors andActuators B, 120, 2007, pp. 403-410.[18]. G. Y. Zhang, B. Guo, J. Chen, MCo 2 O 4 (M = Ni, Cu, Zn), nanotubes, Sens. and Actuators B, 114, 2006,pp. 402-409.[19]. M. S. Wagh, G. H. Jain, D. R. Patil, S. A. Patil and L. A. Patil, Modified zinc oxide thick film resistors asNH 3 gas sensors, Sens. Actuators B, 115, 2006, pp. 128-133.118

Sensors & Transducers Journal, Vol. 134, Issue 11, November 2011, pp. 107-119[20]. L. A. Patil and D. R. Patil, Heterocontact type CuO-modified SnO 2 sensor for the detection of a ppm levelH 2 S gas at room temperature, Sens. Actuators B, 120, 2006, pp. 316-323.[21]. P. Crambelli et al Tureo, Appl. Catal B, 24, 2000, pp. 243.[22]. B. Cullity, Elements of X-ray diffraction, Addison Weley, London, 1956, pp. 99.[23]. C. Xiangfeng, L. Xinggin, Sensors and Actuators B, 55, 1999, pp. 19-22.[24]. D. H. Dawson, D. E. Williams, Gas sensitive resistors: surface interaction of chlorine with semiconductingoxide, J. Mater. Chem., 6, 1996, pp. 406-414.[25]. X. F. Chu, High sensitivity chlorine gas sensors using CdIn 2 O 4 thick film prepared by Co-precipitationmethod, Mater. Res. Bull., 38, 2003, pp. 1705-1711.[26]. J. Mizsei, How can sensitive and selective semiconductor gas sensors be made?, Sens. Actuators B, 23,1995, pp. 173-176.___________________2011 Copyright ©, International Frequency Sensor Association (IFSA). All rights reserved.(http://www.sensorsportal.com)119

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