Please cite this article in press as: T.D. Senguttuvan, et al., Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipi- tation method, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.053 ARTICLE IN PRESS G Model SNB-12436; No. of Pages 5 Sensors and Actuators B xxx (2010) xxx–xxx Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipitation method T.D. Senguttuvan a,∗ , Vibha Srivastava a , Jai S. Tawal b , Monika Mishra a , Shubhda Srivastava a , Kiran Jain a a Electronic Materials Division, National Physical Laboratory, Dr. K.S. Kishnan Marg, New Delhi 110012, India b Materials Characterization Division, National Physical Laboratory, Dr. K.S. Kishnan Marg, New Delhi 110012, India article info Article history: Received 9 December 2009 Received in revised form 23 June 2010 Accepted 26 June 2010 Available online xxx Keywords: WO 3 Metal oxide Gas sensor Ammonia sensor Thick film Chemical synthesis abstract WO 3 ·2H 2 O samples were prepared by acidic precipitation of sodium tungstate solution. Nanocrystalline WO 3 powders were obtained after 350 and 600 ◦ C calcinations. XRD patterns of these samples showed a diffraction profile similar to that of monoclinic WO 3 . Calcinations at 600 ◦ C yield WO 3 powders with par- ticle sizes ranging from 60 to170 nm whereas that for 350 ◦ C calcinations are in the range of 30–150 nm. The gas sensing properties of these powders in the form of thick film were investigated with and without platinum doping. Gas response of thick films sintered at two different temperatures was measured at four different operating temperatures. We confirm that the sensor elements made from 600 ◦ C calcined powders doped with platinum sintered at 800 ◦ C are highly responsive and selective to ammonia vapour at 350 ◦ C operating temperature as compared to sensor elements made from commercial powders. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Gas sensors are used for many applications such as process controls in chemical industries, detection of toxic environmental pollutants, and for the prevention of hazardous gas leaks. Different oxide semiconductors such as SnO 2 ,WO 3 , ZnO, MoO 3 , TiO 2 ,In 2 O 3 and mixed oxides have been studied and showed promising appli- cations for detectingtarget gases suchas NO x ,O 3 ,NH 3 , CO, CO 2 ,H 2 S and So x [1–3] The working principle ofthese sensorsis based on the detection of a change in resistance on exposure to a gas. Due to the constraints of gas permeation only the surface layers are affected by such reactions. Among various oxide sensors, WO 3 is responsive to NO x ,H 2 S, and NH 3 [4–6] In order to achieveimprovements in the gas sensing properties such as enhancing their response, selectiv- ity to a given target species and reduce the operating temperature, small amounts of noble metals (Pt, Pd and Ag) are added to active metal oxide layers [7,8]. The sensor characteristics strongly depend on preparation techniques and the resulting material microstruc- ture of active metal oxide layer [9,10]. Cantalini et al. reported positive effects on the response cross-sensitivity by increases in the annealing time for the WO 3 thin films [11,12]. Tamaki et al. have shown that the response to NO 2 increases dramatically with decreasing grain size of the tungsten oxide for sintered block type ∗ Corresponding author. Tel.: +91 11 45609461; fax: +91 11 25726938. E-mail address: tdsen@mail.nplindia.ernet.in (T.D. Senguttuvan). WO 3 -based gas sensors [13]. This generated a wide interest in synthesizing nanosized powder and investigating gas sensor prop- erties. Nanosized powders of WO 3 have been prepared by sol–gel, spray-pyrolysis, radio frequency magnetron sputtering and aque- ous chemicalroute etc [9–12,14]. Comparedwith other techniques, aqueous chemical process is attractive because of its cost effective- ness and the ease of material preparation. This technique was well reported for NO 2 sensor fabrication [15]. However, it is not widely explored for ammonia gas sensors preparation. In the present work, we investigate ammonia gas sensing prop- erties of nanocrystalline tungsten oxide with and withoutplatinum doping under different processing conditions. 2. Experimental Tungsten trioxide powder was prepared by acidic precipitation route. 5 g Sodium Tungstate was dissolved in 200 ml of de-ionised water to obtain atransparent solution. To this solution5% dilute HCl is added drop-wise to obtain yellow tungstic acid precipitate. It was then washed with water to remove sodium and chlorine ions till no chlorine ion was detected on AgNO 3 test. At this stage the precipi- tate was filtered and dried at 100 ◦ C. These powders were calcined at 350 and 600 ◦ C to obtain powder A and powder B respectively. For Pt doping, calcined powders A and B were mixed with 0.4 wt% chloroplatinic acid solution in water and dried to obtain pow- ders C and D. X-ray powder diffraction (XRD) analysis on powder A and B were carried out using Bruker Analytical X-ray diffrac- 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.06.053 Please cite this article in press as: T.D. Senguttuvan, et al., Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipi- tation method, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.053 ARTICLE IN PRESS G Model SNB-12436; No. of Pages 5 2 T.D. Senguttuvan et al. / Sensors and Actuators B xxx (2010) xxx–xxx tometer equippedwith graphite monochromatized CuK␣ radiation ( = 1.5418 Å). The nano-scale characterization was carried out by a transmission electron microscope (TEM, model JEOL JFM 200x). The morphologies of thick films were observed by using scanning electron microscope (LEO 440 SEM). Pure powders A and B, platinium doped powders C and D, micron sized commercial powder (Sigma–Aldrich) and platinum doped commercial powders were used tomake thickfilm paste. Thick film paste was prepared in butyl carbitol medium containing a small amount of ethyl cellulose and terpineol oil. These pastes were then screen printed on alumina substrates that had gold finger contacts. Sensor films were coated overthe gold finger contacts, and sintered at 600 ◦ C and 800 ◦ C for 10 min. Thus we have obtained eleven dif- ferent sensor elements. They are designated as AP 600 (for sensor prepared from pure powder A sintered at 600 ◦ C), BP 600 (for sensor prepared from pure powder B sintered at 600 ◦ C), AP 800 (for sensor prepared from pure powder A sintered at 800 ◦ C), BP 800 (for sensor prepared from pure powder B sintered at 800 ◦ C), CPt 600 (for sen- sor prepared from Pt doped powder C sintered at 600 ◦ C), DPt 600 (for sensor prepared from Pt doped powder D sintered at 600 ◦ C), CPt 800 (for sensor prepared from Pt doped powder C sintered at 800 ◦ C), DPt 800 (for sensorprepared fromPt doped powderD csin- tered at 800 ◦ C), Comm Powder (sensor prepared from commercial powder Sintered at 800 ◦ C, Comm-600 (sensor prepared from Pt doped commercial C sintered at 600 ◦ C) and Comm-800 (sensor prepared from Pt doped commercial C sintered at 800 ◦ C). Resistiv- ity and gas response for CNG, NO 2 ,NH 3 , CO, LPG and Ethanol gas was measured using Keithley 2000 multimeter in a static system. The sensor element was placed onto an externally heated sample holder and the working temperature of the thick films was deter- mined with a thermocouple attached near the sensor element. The gas or vapour of required amount was injected into the chamber using a 1 ml syringe. The sample’s resistance was measured in air and after exposure to targeted gas or vapour. After completing the measurement, the gas was leaked out. To get uniform temperature distribution throughout the sensor elements, they were heated to targeted temperature in dry air for 1 h before measurements were carried out. Gas sensing properties of the thick films were carried out at operating temperatures ranging from 250 to 450 ◦ C. The gas response (S) was definedas the ratioR a /R g or R g /R a , for reducing and oxidizing gases, respectively; where R a is the resistance in pure air and R g is the sensor resistance in the presence of a species diluted in air 3. Results and discussion Fig. 1 shows the XRD pattern of powders A and B. The charac- teristic peaks observed at 2Â values 23.2, 28.88 and 34.17 confirms that these powders are monoclinic WO 3 [JCPDS 43-1035]. The sharp diffraction peaks imply good crystallinityof WO 3 powders. Absence of characteristicpeaks corresponding to other impurities suchas W or W(OH) 6 indicates the phase purity of WO 3 . The tungsten triox- ide can exists in several polymorphic forms such as monoclinic, hexagonal and pyrochlore around room temperature. Choi et al. have shown in the case of WO 3 derived from sol prepared by ion exchange method both hexagonal and pyrochlore crystals switch completely to monoclinic phase if it is heated to more than 500 ◦ C and cooled downto room temperature[16,17]. However inour case both the powders A and B are calcined at 350 and 600 ◦ C show only monoclinic WO 3 Fig. 2 shows the TEM microstructure of the powders A and B respectively. It is evident from the micrographs that the pow- der A has particles in the size range of 30–150 nm and powder B has particles in the range of 60–170 nm. This increase in parti- cle size is understandable since higher calcination temperatures Fig. 1. XRD pattern of powders calcined at 350 and 600 ◦ C. lead to increased diffusion rate that in turn coalesce the adjacent grains. 3.1. Gas Response—pure WO 3 Response to ammonia gas was measured for AP 600, BP 600, AP 800 and BP 800 sensor elements. The sensor response was eval- uated asa function of operating temperature. Wecould not observe Fig. 2. TEM micrograph of powders calcined at 350 and 600 ◦ C. Please cite this article in press as: T.D. Senguttuvan, et al., Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipi- tation method, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.053 ARTICLE IN PRESS G Model SNB-12436; No. of Pages 5 T.D. Senguttuvan et al. / Sensors and Actuators B xxx (2010) xxx–xxx 3 Fig. 3. Response of AP 800 (sensor prepared from pure powder A), BP 800 (sensor prepared from pure powder B) and Comm Powder (sensor prepared from commer- cial powder) all sintered at 800–4000 ppm NH3 gas. noticeable sensor response for AP 600 andBP 600 sensor elements. Fig. 3 shows the sensor response for AP 800 and BP 800 sensor elements to 4000 ppm NH 3 gas. For both the sensors elements maximum response was observed at 400 ◦ C operating tempera- ture, where as for the sensor elements prepared from micron sized commercial highest response was observed at 400 ◦ C operating temperature. WO 3 powders calcined at 600 ◦ C better response as compared to powders calcined at 350 ◦ C. The TEM results have con- firmed lower particle size for powder A than for powder B. As per linear extrapolation one would expect lower grain size for AP 800 sensor element and hence a higher response which is contrary to our results. The reason for this discrepancy can be understood by seeing the SEM micrographs of AP 800 sensor element (Fig. 4) and BP 800, sensor element (Fig. 5). AP 800 sensor element showed polycrystalline grain morphology consisting of spherical grains of grain size 100nm together with plate like grains of grain size in the range 300–500 nm. The average grain size of this sensor element was calculated by linear intercept method (EN 623-3) andit is found Fig. 4. SEM micrograph of AP 800 (sensor prepared from pure powder A sintered at 800 ◦ C). Fig. 5. SEM micrograph of BP 800 (sensor prepared from pure powder B sintered at 800 ◦ C). to be 680 nm. BP 800, sensor element showed polycrystalline grain morphology consisting of mostly of spherical grains with grainsizes ranging from 130 to 600 nm. The average grain size of this sen- sor element is 300 nm. We observe better a response in smaller grain size sensor element. These results are similar to increase in response of tungsten oxide sensor with decrease in grain size as reported by Tamaki et al. [13]. The sensing properties of WO 3 film are also controlled by the surface defects [18]. The higher response to NH 3 for BP 800 sensor element can also be because of its large irregular voids. Further,more oxygen vacanciesare expected onthe surface of tungsten oxide since it is calcined at higher temperature in accordance with the results obtained by Liu et al. [19]. 3.2. Gas response—Pt doped WO 3 The highest magnitude of response to 4000 ppm NH 3 observed for BP 800 sensor element was 9. We have tried to improve the response by doping with platinum as explained earlier. Response to ammonia gas was measured for sensor elements CPt 600, DPt 600, Fig. 6. CPt 600 (sensor prepared from Pt doped powder C sintered at 600 ◦ C), CPt 800 (sensor prepared from Pt doped powder C sintered at 800 ◦ C) sensor, Comm-600 (sensor prepared from Pt doped commercial C sintered at 600 ◦ C) and Comm-800 (sensor prepared from Pt doped commercial C sintered at 800 ◦ C) response for to 4000 ppm NH 3 gas. Please cite this article in press as: T.D. Senguttuvan, et al., Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipi- tation method, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.053 ARTICLE IN PRESS G Model SNB-12436; No. of Pages 5 4 T.D. Senguttuvan et al. / Sensors and Actuators B xxx (2010) xxx–xxx Fig. 7. DPt 600 (sensor prepared from Pt doped powder D sintered at 600 ◦ C) DPt 800(sensor prepared from Pt doped powder D sintered at 800 ◦ C) Comm-600 (sensor prepared from Pt doped commercial C sintered at 600 ◦ C) and Comm-800 (sensor prepared from Pt doped commercial C sintered at 800 ◦ C) response for to 4000 ppm NH 3 gas. Fig. 8. The response of DPt 800(sensor prepared from Pt doped powder D sintered at 800 ◦ C) to NH 3 gas operating at 350 ◦ C. CPt 800 and DPt 800. Fig. 6 shows the sensor response for CPt 600, CPt 800, Comm-600 and Comm-800 sensor element to 4000 ppm NH 3 gas. The maximum response was observed at 350 ◦ C for all sensor elements except for Comm-600. However, the response magnitude was better for CPt 800 sensor element and it was found to be 82 as compared to all other elements. Fig. 7 shows the sensor response for DPt 600 sensor element and DPt 800 sensor element along with Comm-600 and Comm-800 sensor elements to 4000 ppm NH 3 gas. The maximum response wasobserved at 350 ◦ C, and was highest for sensor annealed at 800 ◦ C. Above 400 ◦ C oper- ating temperature both sensor elements (DPt 600 and DPt 800) show same response to 4000 ppm NH 3 gas. The response magni- tude of 125 was observed for DPt 800 sensor element as compared to a magnitude of 9 for BP 800 sensor element. Fig. 8 shows the linear response of DPt 800 sensor element to NH 3 gas within the concentration range 100–4000 ppm operating at 350 ◦ C. The response time represents the time required by the response factor to undergo 90% variation with respect to its equilibrium value following a step increase in the test gas concentration. The recovery time represents the time required by the sensitivity factor to return to 10% below its equilibrium value in air following the zeroing of Fig. 9. Resistance vs time graph for DPt 800 (sensor prepared from Pt doped powder D sintered at 800 ◦ C) at different operating temperatures. Fig. 10. The response of DPt 800 (sensor prepared from Pt doped powder D sintered at 800 ◦ C) to 800 ppm of different gases (NH 3 , LPG, CNG, CO ethanol and NO 2 .) the test gas Fig. 9 shows response time graph for DPt 800 sensor element at different operating temperatures. It took less than 20 s at an operating temperature of 400 ◦ C and 60s for 200 ◦ C operating temperature. It should also be noted that tungsten oxide nanopar- ticles prepared by gas deposition yield films that can be used even at room temperature with similar and greater sensitivities for H 2 S gas [20]. Our results indicate the limitations posed by fluid-based chemistry that was used to fabricate the nanoparticle films. The Pt–WO 3 (DPt 800 sensor element) sensor was highly selec- tive towards ammonia at an operating temperature of 350 ◦ C. Fig. 10 shows the response to NH 3 , LPG, CNG, CO ethanol and NO 2 were recorded. 4. Conclusions Nanocrystalline tungsten oxide powder was produced by an acid precipitation of sodium tungstate suitable for ammonia gas sensors. These powders were monoclinic WO 3 . The particles with size distribution of 30–150 and 60–170 nm resulted from 350 and 600 ◦ C calcinations respectively. Sensor element obtained from 350 ◦ C calcined powders showed a polycrystalline grain morphol- ogy consisting ofspherical grains ofsize 100nm together withplate like grains of size 300–500 nm. Whereas 600 ◦ C calcined powder resulted in a sensor elements with polycrystalline grain morphol- ogy consisting only of spherical grains of size ranging from 130 to 600 nm. Sensor elements made with pure WO 3 powders calcined at 600 ◦ C are better than those calcined at 350 ◦ C. Platinum doping resulted in better response irrespective of calcination temperature. The maximum response to NH 3 was achieved at an operating tem- perature of 350 ◦ C for all the platinum dopedsensor elements and at 400 ◦ C for undoped sensors. The best response for NH 3 is observed Please cite this article in press as: T.D. Senguttuvan, et al., Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipi- tation method, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.053 ARTICLE IN PRESS G Model SNB-12436; No. of Pages 5 T.D. Senguttuvan et al. / Sensors and Actuators B xxx (2010) xxx–xxx 5 in the sensor element annealed at 800 after it was green formed from Pt doped powder calcined at 600 ◦ C. The response magni- tude of 125 observed for the best Pt doped WO 3 sensor element was significantly than a magnitude 9 for the best pure WO 3 sensor element. Acknowledgements We are thankful to Mr Jain, CEERI Pilani for providing the sensor substrates. We are also thankful to Dr. S.K. Halder for measurement of XRD. One of the authors (VS) thanks Council of scientific and industrial research for the award of Research Associateship. References [1] W. Göpel, Ultimate limits in the miniaturization of chemical sensors, Sensors and Actuators A: Physical 56 (1996) 83–102. [2] K. Dieter, Function and applications of gas sensors, Journal of Physics D: Applied Physics (2001) R125. [3] G. Korotcenkov, Metal oxides for solid-state gas sensors: what determines our choice? Materials Science and Engineering: B 139 (2007) 1–23. [4] W. Yu-De, C. Zhan-Xian, L. Yan-Feng, Z. Zhen-Lai, W. Xing-Hui, Electrical and gas-sensing properties of WO 3 semiconductor material, Solid-State Electronics 45 (2001) 639–644. [5] B.T. Marquis, J.F. Vetelino, A semiconducting metal oxide sensor array for the detection of NO x and NH 3 , Sensors and Actuators B: Chemical 77 (2001) 100–110. [6] V. Srivastava, K. Jain, Highly sensitive NH3 sensor using Pt catalyzed silica coating over WO 3 thick films, Sensors and Actuators B: Chemical 133 (2008) 46–52. [7] M. Penza, C. Martucci, G. Cassano, NO x gas sensing characteristics of WO 3 thin films activated by noble metals (Pd, Pt, Au) layers, Sensors and Actuators B: Chemical 50 (1998) 52–59. [8] P. Ivanov, E. Llobet, F. Blanco, A. Vergara, X. Vilanova, I. Gracia, C. Cané, X. Correig, On the effects of the materials and the noble metal additives to NO 2 detection, Sensors and Actuators B: Chemical 118 (2006) 311–317. [9] S.C. Moulzolf, S a. Ding, R.J. Lad, Stoichiometry and microstructure effects on tungsten oxide chemiresistive films, Sensors and Actuators B: Chemical 77 (2001) 375–382. [10] V. Guidi, M. Blo, M.A. Butturi, M.C. Carotta, S. Galliera, A. Giberti, C. Malagù, G. Martinelli, M. Piga, M. Sacerdoti, B. Vendemiati, Aqueous and alcoholic synthe- ses of tungsten trioxide powders for NO 2 detection, Sensors and Actuators B: Chemical 100 (2004) 277–282. [11] C. Cantalini, M.Z. Atashbar, Y. Li, M.K. Ghantasala, S. Santucci, W. Wlo- darski, M. Passacantando, Characterization of sol-gel prepared WO 3 thin films as a gas sensor, Journal of Vacuum Science & Technology A 17 (1999) 1873–1879. [12] C. Cantalini, W. Wlodarski, Y. Li, M. Passacantando, S. Santucci, E. Comini, G. Faglia, G. Sberveglieri, Investigation on the O-3 sensitivity properties of WO 3 thin films prepared by sol–gel, thermal evaporation and r.f. sputtering tech- niques, in: 10th International Conference on Solid-State Sensors and Actuators, Elsevier Science Sa, Sendai, Japan, 1999, pp. 182–188. [13] J Tamaki, Z. Zhang, K. Fujimori, M. Akiyama, T. Harada, N. Miura, N. Yamazoe, Grain size effect in tungsten oxide based sensor for nitrogen oxide, Journal of the Electrochemical Society 141 (1994) 2207–2210. [14] M. Sun, N. Xu, Y.W. Cao, J.N. Yao, E.G. Wang, Nanocrystalline tungsten oxide thin film: preparation, microstructure, and photochromic behavior, Journal of Materials Research 15 (2000) 927–933. [15] Y G Choi, G. Sakai, K. Shimanoe, N. Miura, N. Yamazoe, Wet process-prepared thick films of WO 3 for NO 2 sensing, Sensors and Actuators B: Chemical 95 (2003) 258–265. [16] Y G. Choi, G. Sakai, K. Shimanoe, Y. Teraoka, N. Miura, N. Yamazoe, Preparation of size and habit-controlled nano crystallites of tungsten oxide, Sensors and Actuators B: Chemical 93 (2003) 486–494. [17] Y G. Choi, G. Sakai, K. Shimanoe, N. Miura, N. Yamazoe, Preparation of aqueous sols of tungsten oxide dihydrate from sodium tungstate by an ion-exchange method, Sensors and Actuators B: Chemical 87 (2002) 63– 72. [18] R.D. Bringans, H. Höchst, H.R. Shanks, Defect states in WO 3 stud- ied with photoelectron spectroscopy, Physical Review B 24 (1981) 3481. [19] Z.F. Liu, T. Yamazaki, Y. Shen, T. Kikuta, N. Nakatani, Influence of annealing on microstructure and NO 2 -sensing properties of sputtered WO3 thin films, Sensors and Actuators B: Chemical 128 (2007) 173–178. [20] J.L. Solis, S. Saukko, L.B. Kish, C.G. Granqvist, V. Lantto, Nanocrystalline tungsten oxide thick films with high sensitivity to H 2 S at room temperature, Sensors and Actuators B: Chemical 77 (2001) 316–321. Biographies T.D. Sengutavan obtained his M.E. from REC Trichy in the field of Materials Science with specialization in Ceramics and Ph.D. in Sol–gel processing from IIT Delhi. He is working as scientist in National Physical Laboratory, New Delhi, for the past 13 years. His research interest includes ceramic structures, powder processing, microwave sintering and metal oxide gas sensors. Vibha Srivastava obtained her Ph.D. in 2004 from Gorakhpur University in materials science. Presently she is working as research associate at National Physical Labo- ratory, New Delhi. Her current research interests are nanoscience, nanostructure mesoporous materials and gas sensors. Jai S.Tawala obtained his M.Sc. in 2006 from NagpurUniversity in Physics. Presently he is working as technical assistant at National Physical Laboratory, New Delhi. His current research interests are nanostructure materials and characterization. Monika Mishra obtained her M.Sc. in 2006 from Bundelkhand University in Physics. Presently she is working as research intern at National Physical Laboratory, New Delhi. Her current research interests are nanostructure materials and gas sensors. Shubhda Srivastava obtained her M.Sc. in 2005 from Avadh University in Electron- ics. Presently she is doing her M.Tech. Project at National Physical Laboratory, New delhi. Her current research interests are nanostructure material and gas sensors. Kiran Jain did her Ph.D. in the field of high Tc superconductivity from Delhi Uni- versity. She is working as scientist in National Physical Laboratory, New Delhi, for the past 25 years on the diverse area of research material science such as high Tc superconductors, ceramics, nanocrystalline semiconductors, thin film photovoltaics and metal oxide gas sensors etc. . NH 3 gas. Please cite this article in press as: T.D. Senguttuvan, et al., Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid. Chemical journal homepage: www.elsevier.com/locate/snb Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipitation method T.D. Senguttuvan a,∗ ,