Structural and micro structural studies of PbO-doped SnO2 sensor for detection of methanol,propanol

Available online at www.sciencedirect.com| Joumal ofScienceDirect7I Natural GasChemistryEL SEVIERJournal of Natural Gas Chemistry 20(201 1)179-183www.elsevier.com/locate/jngcStructural and micro structural studies of PbO-doped SnO2 sensorfor detection of methanol, propanol and acetoneJ. K. Srivastava*，Preeti Pandey, V. N. Mishra, R. DwivediCenter for Research in Microletronics, Department of Electronics Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India[ Manuscript reeived September 25, 2010; revised November 23, 2010 ]AbstractIn the present work the structural information of PbO doped SnO2 thick film sensor has been investigated with X -ray diffractometer (XRD)and scanning electron microscope (SEM). Initially, SnO2 powder was derived using sol-gel process and was subsequently doped with PbOand ground up to nanosized particles. A suitable gas sgnsor structure was fabricated on 1"x 1" alumina substrate using thick film technology.he necessary paste for screen printing was also dey oped. SEM rel sults showed sol-gel derived powder gets more agglomerated in the thickfilm form. The sensitivity of the sensor has been inv stigated at affetent tnt tempelures (150 °C- 350 °C) upon exposure to methanol, propanoland acetone, yielding a maximum at 250 °C for acetope yith 1while at 350 °C for propanol with 3wt% PbO-doping of thesensor. The reduction of particle size to nanometers (valid ated tHrd shAAt>Aca 's to a dramatic improvement in sensitivity of sensors for tHchosen organic vapors. The results also correlat wel vith the n c ostucuralpr perties oflthe material and the dopant.Key wordsgas sensor; nanosized; SnO2; Sol-gel; thick film1. Introductionan increase of sensing surface area of SnO2, and the nano-sized particles provide better and faster interaction betweenoxygen species and detected gases due to the availability ofAmong the semiconducting metal oxides used for gas sen-larger sensing surface area resulting in a lesser response timesors, SnO2 is the most widely used material due to its low cost, .[5,6]. Hence, overall performance of the sensor is improvedbetter endurance and superior reproducibility [1,2]. Tin oxidewhen nanosized particles are used. It is also reported that witsensors can detect both reducing and oxidizing gases, such asthe gas exposure, sensors having microsized particles, only tHeCH4, H2, C2HsOH, CO, C2H2, NO2, NO and H2S [3]. SnO2surface properties of the grain change, whereas sensors witis an n-type, wide band-gap semiconductor [4]. Its tetrago-nanosized particles, the total grain properties change, typinal unit cell is composed of two tin and four oxygen atoms, cally, when crystal dimension is comparable to the thicknesswith each tin atom octaedrically coordinated by six oxygenof the charge depletion layer. As bending of the energy bandatoms. It is reported that the microstructure of SnO2 controlsextends into the bulk of the grains, it results in drastic im-the sensitivity of gas sensors [5,6]. The microstructure alsoprovement in the overall performance of the nanostructureddepends on the temperature treatment, concentration of thesensor [7]. Botter et al. [8] studied the relationship betweendopant and the technological steps undertaken for its prepa-the microstructure and the sensitivity by investigating the re-ration. Sharp increase in sensitivity has also been reportedlation between coordination number of the sensor particle andfor crystallite sizes of about 6 nm, i.c. twice the thickness ofpercolation theory. Recently Wang et al. [9] and Williams etSchottky barrier penetration in tin oxide grains [5,6]. In theseal. [10] used computer simulation to study the effect of mi-nanosized crystals, depletion of electrons takes place through-crostructrual factors on the sensitivity of the gas sensors andout the crystal due to chemisorbed oxygen, producing a dra-found that the particle size and the grain growth affected thematic improvement in the sensitivity of gas sensors. It is alsosensitivity and response time of the sensor.an established fact that the sensitivity increases linearly withThus, getting motivated to achieve better sensor perform-中国煤化工" Corresponding author. Tel: +91-8800264795; E-mail: jksjitendra@ rdiffmail.comMYHCNM HGCopyrightO201 1, Dalian Institute of Chemical F, ChineAcademy of Sciences.doi:10.1016/S 10039953(10)60168-5180J. K. Srivastava et al./ Journal of Natural Gas Chemistry Vol. 20 No.22011microstructure and sensor performance in present study, sol-gel derived nanosized PbO-doped (1 wt%，2 wt%, 3 wt%，4 wt% and 5 wt%) SnO2 powders were used to fabricate fiveindependent thick film sensors. The fabricated sensors weretested for different concentration of methanol, propanol andMultimeteracetone. It is found that sensor with particle size aroundD.U.T17.9 nm shows better sensitivity than the sensors with largerparticle sizes.Closed chamber withgas injection facility2. Fabrication of sensor deviceFigure 2. Schematic diagram of measurement setupFive SnO2 sensors doped with PbO (1 wt%, 2 wt%,3 wt%, 4 wt%, 5 wt%) was fabricated using thick film screen3. Performance characteristics of the sensorprinting technique. It consists of gas sensing layer (PbO-doped SnO2), a pair of electrodes underneath the gas sensingThe variation of sensitity (% change in resistance i.e.layer to serve as a contact pad of the sensor, and a heater ele-S=(Ra - Rg)x 100/Ra, where Ra and Rg are the resistancesment on the backside of the substrate to generate the desiredof sensor in clean air and in presence of the gas, respec-temperature necessary for gas sensing. Alumina (96%) hastively) with different concentrations of organic vapors, wasbeen used as the substrate for sensor fabrication and schematicstudied for all five fabricated sensors at different fixed tem-diagram of the fabricated sensors is shown in Figure 1. Theperatures (150°C- -350 °C). It is observed that sensor with入SnO2 nanoparticles were synthesized using sol-gel method.1 w1% PbO doping exhibits the highest sensitivity at 250 °CHere, pure SnO2 powder was prepared by slow reaction ofwhile the sensors with 2 wt%, 3 wt%, 4 wt% and 5 wt%SnCl4 :5H2O with ammonia water (NH4OH). After some time,PbO doping show the highest sensitivity at 350 °C for all thetin hydroxide, in the white precipitated form, was obtained. chosen vapors. Figure 3 to Figure 5 show the scnsitities ofThe precipitate was then washed with distilled water so as to1 wt%, 2 wt%, 3 wt%, 4 wt% and 5wt% PbO- doped sensors toremove excess ammonium chloride. The precipitate was thenmethanol, propanol and acetone, respectively. It is clear fromfiltered and dried in an oven at about 150 °C. The powder so口obtained is the tin hydroxide which, when calcined at 400 °C sensitivity towards acetone than methanol and propanol, whilesfor 4 h, yields the desired tin oxide for sensor development.at 350 °C, 2 wt%, 4 wt% and 5 wt% PbO-doped sensors showDoped SnO2 powder was obtained by mechanically mixingbetter sensitivity towards propanol than methanol and acetonethe appropriate amounts of PbO. This mixture was ball milledand the 3 w1% PbO-doped sensor shows maximum sensitiv-for 15 h to get homogeneous powder and was then sintered atity towards propanol. The influence of PbO concentration on600 °C for 2h in a furnace. To get a proper paste, PbO-doped the particle size and the sensitivity of the sensors for chosenSnO2 powder was mixed with lead glass powder and the or- organic vapors (ie. methanol, propanol and acetone) is pre-ganic binder followed by ball milling for 1 h, then Q-terpineolsented in Table 1.and diethyl glycol monobuty1 ether were added to the mix-ture and kept at 80°C for 24 h. The phase identification of0Fthe powder was carried out by X-ray diffractometer (XRD).The fabricated sensor was then exposed to varying concentra-50tions of acetone, methanol and propanol in a locally devel-oped test chamber of volume 2047cm3 [1]. The change inresistance of sensor is measured using KEITHELY 195A mul-timeter. The schematic diagram of the measurement setup isshown in Figure 2.40 E% 30SnO2 sensing AluminaHeaterlayerubstrate20+ 1 w%PbO)- 2 wt%PbO←3 wt%PbO4 w%PbO-5 w%PbOSilverclectrode中国煤化工0005000二 ]|CNMHGFigure 1. Schematic diagram of fabricated sensorsSnOsonenrspoeronretomatanalwt%,2wt%,3wt%,4wt%and5wt%)SnO2 sensors on exposure to methanolConcent rat i on( ppn),Journal of Natural Gas Chemistry Vol. 20 No.2 2011Table 1. Comparison of doping concentration with sensitivity andStructural and microstructural analysis was carried outparticle size for methanol, propanol and acetonewith powder X-ray diffractometer (Seifert, Germany, model-Maximum sestivityParticleID-3000) and SEM (QUANTA 200F). The XRD patterns ofSamplemethanolpropanolacetonesize (nm)SnO2 (1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt% PbO-doped)1 wt%PbO60.425.292.620.5powders, calcined for 2 h at 600 °C, are shown in Figure 6.2 wt%PbO30.246.422.33 wt%PbO698681.817.9The X-ray diffraction patterns of all the samples are almost4 wt%PbO5342.929.6.similar. The observed peaks in each sample are indexed and5 wt%PbO47.036.332.2correspond to SnO2 cassiterite structure (JCPDS, Joint Com-mittee on Powder Diffraction Standards, standard card no. 411445) with lattice constant a= 6.0294A, c= 5.0285A andno peak corresponding to PbO phase is observed, suggest-90ing that the concentration of PbO phase in the sample maybe small. The particle size was calculated from XRD results80using Scherrer equation [12].70D=k入β cos60where, D is the average particle size;入is the X-ray wave-50- 1 wt%PbOlength (1 .54056A); k is the Scherrer constant (0.9) and 6)- 2 wt%PbOis angular width of the diffracted peak at the half maximumh 40WODOO(FWHM) for the diffraction angle 20. The average crystallite-◆-5wt%PbOsizefor1wt%,2wt%,3wt%,4wt%and5wt%PbOdopedpowders is 20.5 nm, 22.3 nm, 17.9 nm, 29.6 nm and 32.2 nm,respectively. The lowest grain size of about 17.9 nm is ob->20tained for 3 wt% PbO-doped powder. The results indicatethat an increased PbO concentration favors the production of410larger crystallites, except for 3 wt% PbO, for which the parti-口cle size is 17.9 nm. The SEM images (Figure 7) after printingGand firing of PbO-doped SnO2 thick film sensors show some2000300040005000non-uniformity in the shape and size of the particles but theConcentration (ppm)maximum particle size in all cases is less than 80 nm. An in-Figure 4. Response of PbO doped (1 wt%, 2 wl%, 3 w1%, 4 wt% and 5 wt%)crease in the grain size (as observed in SEM image) in printedSnO2 sensors on exposure to propanolthick film of PbO-doped SnO2 is seen, which is due to thefiring and melting of lead borosilicate glass at 850 °C, result-00 ring in particle binding and agglomeration [13]. SEM analysis .also reveals the fact that as the concentration of PbO doping+ ! wt%PbO-2 wt%PbOis increased, the agglomeration of the particles also increases.一3 wt%PbOThe basic principle regarding the sensing mechanism for8C-◆5w%PbOSnO2 sensor assumes that electrons are depleted due to ad-sorption of oxygen on the surface of SnO2. The oxygenchemisorption causes the formation of depletion layer aroundthe surface of crystallite (the thickness of this depletion layeris generally termed as Debye length L) causing the materialconductivity to decrease. On exposure of reducing gas on theSnO2 surface, the interaction between the gas molecules and40the chemisorbed oxygen takes place, resulting in the reduction .of depletion layer width (due to inverse charge transfer) andthe increase in conductivity of the material [14,15].20-The concept of Debye length L [16] can be taken into ac-count to explain the correlation between the grain size D andthe sensitivity of SnO2 based thick film sensor. Depending onthe relation between grain size and width of depletion layer L,1000two cases can aris中国煤化工size, i.e. whenD>2L, formationYHCN MH Ger is restrictedFigure 5. Response of PbO-doped(1 wt%, 2 wt%, 3 Wt%, 4 wt% and 5 wt%)only at the surfac1, i. uap udrges are foundSnO2 sensors on exposure to acetone :only at the boundary of the grain and the conductivity of theConcentr ati on( ppm)182J K. Srivastava et al./ Journal of Natural Gas Chemistry Vol. 20No. 2 2011sensor is due to charge transfer from one grain to other grain[16] that for D<2L, the depletion layer extends throughoutvia grain boundary (GB). Secondly, it has been already reported the grain and does not restrict just at the surface. It is alsostated [17] that the smaller grain size (nanosized) provideslarger surface area facilitating faster chemisorption process.So a smaller particle size increases the active surface area of01the sensor, resulting higher adsorption of oxygen and reducing211gases which in turn increases sensitivity of sensor [17]. Thisstudy also shows that the sensor with the smallest particle size(17.9 nm) i.e. 3 wt% PbO-doped sensor has maximum sensi-tivity for propanol (Figure 7). But the 1 wt% PbO-doped sen-sor shows the maximum sensitivity for acetone whose particle4 w!%PbOsize is greater than the 3 wt% PbO-doped sensor but smallerthan other (2 wt%, 4 wt% and 5 wt%) doped sensors. The ex-ceptional behavior of 3 wt% PbO doping (in terms of particlesize as well as sensitivity) is probably due to the optimized3 wt%PbOcomposition. Besides this, increased concentration of PbOalso increases the particle size as evidenced from the XRDand SEM analysis (Figure 6 and Figure 7).2 wt%PbOSensitivity of the sensors is decreased with the increase ofPbO concentration except 3 wt% PbO- doped sensor remainsan exceptional case. The reason for this behavior may be dueto the different radii of Pb2+ (1.06 A) and Sn4+ (0.69 A)[7]. Itu w%PbOjis rarely probable that Pb2+ will acquire a substitution positionin the tin oxide lattice and hence lead ions should occupy the1020304060 70 80interstitial sites in the lattice of SnO2. This interstitial position201(0 )of lead ions should result in a decrease in the concentration ofFigure 6. XRD patterns of SnO2 powders calcined at 600 °。C with differentoxygen vacancies and a decrease of depleted charge carriersPbO doping concentrationswhich ultimately causes the sensors to be less sensitive.d(b)」(c)c)中国煤化工YHCNMHGFigure 7. SEM images of SnO2 thick flms. (a) 1 wt% PbO-doped, (b) 2 wt% PbO-doped, (C) 3 wt% PbO-doped, (d) 4 wt% PbO- doped, (e) 5 wt% PbO-doped2030405060708020/(。)Journal of Natural Gas Chemistry Vol. 20 No.2 20111834. Conclusions[3] Niranjan R S, Mulla I S. Mater Sci Eng B, 2003, 103: 103It is thus concluded that the grain size of the fabricated[4] RobertsonJ. J Phys C, 1979, 12: 4767sensor has significant influence on the performance of PbO-[5] Moseley P T Sens Actuators B, 1992, 6: 149[6] Hauptmann P. Sensors: Principles and Applications, Salisbury:doped SnO2 gas sensor. It is also found that 3 wt% PbO-Prentice-Hall, 1991. 115doped sensor showed the maximum sensitivity for propanol[7] Zhang G, Liu M L. Sens Actuators B, 2000, 69: 144at 350°C. The grain size of the 3 wt% PbO-doped sensor,[8] Botter R, Aste T, Beruto D. Sens Actuators B, 1994, 22: 27estimated by the XRD and SEM analysis, is the smallest[9] Wang Y z, Liu Y H, Ciobanu C, Patton B R. J Am Ceram Soc,(17.9 nm) among the tested samples, whereas the grain sizes .were 20.5 nm, 22.3 nm, 29.6 nm and 32.2 nm for the 1 wt%，[10] Williams D E, PrattK FE. Sens Actuators B, 2000, 70: 2142wt%, 4 wt% and 5 wt% PbO-doped sensors, respectively. [11] Nayak M S, Dwivedi R, Srivastava S K. Microelectron J, 1994,This establishes the fact that the smaller the grain size the bet-25: 17ter the sensitivity is. It is also observed that increasing doping [12] Taylor A. X-ay Matllography. New York: John Wiley & Sons,concentration increases grain size and results in lower sensi-1961. 678[13] Ansari Z A, Ansari S G, Ko T, OhJ H. Sens Actuators B, 2002,tivity of the gas sensors.87::105[14] Watson J, Ihokura K, Coles G s V. Meas Sci Technol, 1993, 4:References[15] Gopel W; Schierbaum K D. Sens Actuators B, 1995, 26: 1[1] Maekawa T, Suzuki K, Takada T, Kobayashi T, Egashira M. Sens [16] Rahmani B, Yasouka K, Ishi S. 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