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Preparation of Fe2O3(0.9)–SnO2(0.1) by hydrazine method: application as an alcohol sensor
Sensors and Actuators B 81 (2002) 170±175
Preparation of Fe2O3(0.9)±SnO2(0.1) by hydrazine method: application as an alcohol sensor C.V. Gopal Reddy*, W. Cao, O.K. Tan, W. Zhu Sensors & Actuators Laboratory, School of EEE, Nanyang Technological University, Singapore 639798, Singapore Received 11 December 2000; received in revised form 16 February 2001; accepted 30 August 2001
Abstract Nanocrystalline Fe2O3(0.9)±SnO2(0.1) powders have been prepared using a hydrazine method by adding hydrazine monohydrate to an aqueous solution of ferric nitrate nanohydrate, (Fe(NO3)39H2O) and tin tetra chloride (SnCl4), followed by washing and drying. This material has been characterized by different techniques such as gravimetric-differential thermal analysis (TGA/DTA), X-ray diffraction (XRD). Sensors made from this material have been proved to be highly sensitive and selective in the detection of ethanol. The sensitivity for ethanol has been compared with a 10 wt.% of ZrO2 and SnO2 loaded in Fe2O3. The ethanol sensitivity of pure and Pt doped Fe2O3(0.9)± SnO2(0.1) and Fe2O3(0.9)±ZrO2(0.1) has been investigated for its electrical resistance. The high sensitivity of the sensor to the ethanol could be explained on the basis of a SnO2, ZrO2 activity that invokes the acid±base properties of sensing materials towards the sensitive detection of ethanol vapor in air. Its cross-sensitivity to other gases like CH4, CO, and H2 has also been carried out. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Iron oxide; Thick-®lms; Ethanol sensor; Long-term stability; Selectivity
1. Introduction There exists an increasing demand for semiconducting gas sensors for several monitoring applications having high sensitivity, selectivity and reliability on a long-term scale. In recent years, gas sensors have become more popular in the detection of in¯ammable gases (LPG, H2, etc.) and toxic gases (CO, Cl2, etc.) in indoor as well as outdoor spaces [1± 3]. Fine particles of ferrites are of great interest due to their applications in the preparation of high-density ferrite cores, as suspension materials in ferromagnetic liquids, and also as catalysts. Among these, iron oxides in particular are technologically useful as pigments and also for their magnetic properties [4]. Recently, the study of ®ne particles has received an increased attention. Reducing the particle size to as low as in the nanometer range is accompanied by altered electrical, magnetic, electro-optical and chemical properties [5]. Semiconducting thick-®lms of Fe2O3 have earlier been studied as sensor for CH4, H2 and NH3 [6]. Cantalini et al. [7±9] have also reported a-Fe2O3-based gas sensors. Zeng et al. have studied high stability g-Fe2O3 as a * Corresponding author. Present address: Department of Materials Science & Engineering, The Ohio State University, Columbus, OH-43210, USA. E-mail address: [email protected] (C.V. Gopal Reddy).
possible gas sensors for detecting several reducing gases [10]. SnO2±Fe2O3 thin-®lm have also been reported as a effective humidity sensor [11]. Ethanol sensors are expected to ®nd a wide range of applications such as traf®c management, food ferment, wine making and medical processes. Zhang et al. [12] have studied on an alcohol sensor based on cadmium ferrite, but its operating temperature is high with less sensitivity. In the present work, we report on the preparation and characterization of Fe2O3(0.9)±SnO2(0.1) and Fe2O3(0.9)±ZrO2(0.1) for application as an ethanol sensor at a low operating temperature. These promising performances indicate that Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) could be used as a breath alcohol checker, because of its high sensitivity, selectivity and a rapid response to low concentration of ethanol is a prerequisite of this application. 2. Experimental 0.9 M ferric nitrate nanohydrate (Fe(NO3)39H2O), 0.1 M tin tetrachloride, (SnCl4, 99% pure), 0.1 M zirconium tetrachloride (ZrCl4) and hydrazine monohydrate ((NH2)2H2O) (TCI, 98%) were used as the starting materials. Ferric nitrate and tin tetrachloride solution were prepared by dissolving above amounts in deionized water (18 MO). This aqueous solution (pH 1:75) was poured in a beaker and was stirred
0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 9 4 8 - 0
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on a magnetic stirrer at 60 8C for 30 min. Hydrazine monohydrate 0.8 M was added slowly to this solution through a burette by maintaining a constant stirring until the resulting precipitate reached to pH 8 [13]. The suspension was separated by centrifugation, washed more than 15 times with DI water to remove adsorbed hydrazine and nitrate ions as tested by the brown ring test of copper ®lings with concentrated H2SO4, and then dried in open air. Similarly Fe2O3(0.9)±ZrO2(0.1) was also prepared by using the above procedure. Thoroughly washed and dried material was then subjected to thermal gravimetric-differential thermal analysis (TGA/ DTA) and X-ray diffraction (XRD). Perkin-Elemer TGA-7 and DTA-7 were used to measure the thermal properties of the materials. TGA/DTA was conducted in air at a heating rate of 10 8C/min in the temperature range from the room temperature (RT) to 1000 8C; a-Al2O3 was used as the reference. These materials were studied by using a Rigaku Ê ). X-ray diffractometer (Cu Ka radiation, l 1:5406 A Pastes of these materials were prepared using a commercial organic vehicle 400 (from ESL) and screen-printed onto Al2O3 substrates with inter-digital Au electrodes. The thickness of sensor ®lm is about 60 mm. the samples were then sintered at temperatures from 400 to 600 8C for 1 h in air. The gas sensing properties were measured by using a computer-controlled gas sensing characterization system. 3. Results and discussion Fig. 1(a) shows the TGA/DTA curves of Fe2O3(0.9)± SnO2(0.1) untreated material which was heated in a dry air ¯ow. A signi®cant loss of weight was observed from RT to 280 8C and reached a plateau at 350 8C with a total loss was about 15%. This value indicates that the hydrated form of Fe2O3H2O, SnO2H2O, which appear on the DTA curve as endothermic peak at 100 8C is the result of the loss of water molecules. The exothermic peak at 286 8C on the DTA is attributed to the changes of the phase from geothite to aFe2O3. Similarly, the TGA/DTA data of Fe2O3(0.9)±ZrO2(0.1) are shown in Fig. 1(b). An endothermic peak appears at 103 8C representing the loss of surface water. An exothermic peak appears at 273 8C that correspond to the change of phase from Fe2O3 (Maghemite-Q) to a-Fe2O3. Note that endothermic peak associated with the loss of hydroxyl group is missing because this method directly produces oxides. Another small exothermic peak is observed at 725 8C without any associated change in the weight loss. This may be related to possible phase transformation in ZrO2. Since the thermal decomposition shows that in the region of the DTA peak no weight loss has been recorded, therefore, an exothermic peak has been observed that could be attributed to the change from an amorphous to a crystalline phase. The XRD patterns were recorded on a Rigaku X-ray diffractometer. The XRD patterns of Fe2O3(0.9)±SnO2(0.1) powder calcined from RT to 500 8C for 2 h are shown in
Fig. 1. TGA/DTA curves of (a) Fe2O3(0.9)±SnO2(0.1) and (b) Fe2O3(0.9)± ZrO2(0.1).
Fig. 2(a). From the RT to 150 8C, a-FeO(OH) (geothite), Fe3O4 and a-Fe2O3 structures was observed. All lines are matching with the reported JCPDS data (Card nos. 29-0713, 26-1136 and 33-0664). For the geothite peaks are exactly matching with the reported data [14]. Berry et al. reported the XRD of tin doped a-FeO(OH), which was prepared by the co-precipitation method. After increasing the calcination temperature from 150 to 500 8C the geothite phase changed to and shows the a-Fe2O3 and g-Fe2O3. The DTA also show an exothermic peak at 286 8C, evidence for a change of the phase. At 500 8C we observed a-Fe2O3 peaks (Card no. 330664) and g-Fe2O3. We did not ®nd any peak for SnO2 and hence, it might be amorphous at these temperature or low quantity of SnO2. The particle size of the a-Fe2O3 calculated Ê . From the XRD, using Scherrer formula [15], is 82 A we observed the intensities of peaks did not change much with an increasing temperature that indicates the crystalline nature of particles at low temperature. Fig. 2(b) shows the XRD patterns of Fe2O3(0.9)±ZrO2(0.1) that are treated at different temperatures. Here, we did not observe the geothite structure. However, from RT to 300 8C, only a Fe2O3 with a tetragonal structure (Maghemite-Q, Card no. 25-1402) was
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Fig. 3. Sensitivity to ethanol (1000 ppm) as a function of the operating temperature of Fe2O3(0.9)±SnO2(0.1) annealed from 400 to 600 8C for 1 h.
Fig. 2. XRD patterns of (a) Fe2O3(0.9)±SnO2(0.1) and (b) Fe2O3(0.9)± ZrO2(0.1) calcined at different temperatures: (i) as-prepared; (ii) 150 8C, 12 h; (iii) 300 8C, 2 h; (iv) 400 8C, 2 h; (v) 500 8C, 2 h. Here, a, g, m, g and Q stand for a-Fe2O3, g-Fe2O3, magnetite (Fe3O4), geothite and Maghemite-Q, respectively.
observed. Soon after that with an increasing of the calcination temperature the a-Fe2O3 peaks appeared. The gas sensitivity (S) is de®ned as the ratio of the resistance of the sensor in air (Rair) to that in the test gas (Rgas). In regard of the gas sensing properties, we have ®rst examined the effect of different weight percentages SnO2 content on gas sensing properties of sensors. It was found that 10 wt.% SnO2 doped Fe2O3 sensor has a much better sensitivity to ethanol gas than a pure Fe2O3 based sensor. Experiments have been carried out an pure Fe2O3(0.9)± SnO2(0.1) to ethanol (1000 ppm). Fig. 3 shows the sensitivity to 1000 ppm ethanol vapor as a function of the operating temperature from 160 to 300 8C of Fe2O3(0.9)±SnO2(0.1) without noble metal doping. This sensor device is sintered at different temperatures from 400 to 600 8C for 1 h in air. It is seen that as sintering temperature increases from 400 to 600 8C, the gas sensitivity to ethanol is decreasing. The best sintering temperature for our devices is at 400 8C. For the sintering temperature to be lower than 400 8C, the organic constituents (binder) are not fully decomposed, and hence, would render the device unstable. The good sensitivity is observed for the sample
sintered at 400 8C for 1 h. The maximum sensitivity observed to ethanol at 220 8C is 1122. We impregnated different weight percentages of Pt on the above material. Aqueous solutions of corresponding Pt chloride (H2PtCl6) solution (99%, Aldrich) were impregnated in Fe2O3(0.9)± SnO2(0.1) and Fe2O3(0.9)±ZrO2(0.1). The resulting solution mixture was evaporated on a water bath and then dried at 120 8C for 15 h in an oven. The dried material was calcined at different temperatures in air to get rid of the chloride. The details of the procedure for impregnation method have been described earlier [16]. After annealing this device at different temperatures from 400 to 600 8C to verify the effect on ethanol sensitivity, but we got a maximum sensitivity at 400 8C for 1 h annealing element for ethanol (Fig. 4). Here, we observed the sensitivity is increased compared to Fe2O3(0.9)±SnO2(0.1). The maximum sensitivity value at an operating temperature of 220 8C is 1574 on Fe2O3(0.9)± SnO2 0:1 Pt (1 wt.%) annealed at 400 8C for 1 h. The effects of gas sensitivity on the changing of SnO2 with ZrO2 in Fe2O3 are shown in Figs. 5 and 6. These graphs show the sensitivity versus operating temperature for ethanol
Fig. 4. Sensitivity to ethanol (1000 ppm) as a function of the operating temperature of Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%), annealed from 400 to 600 8C for 1 h.
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Fig. 5. Sensitivity to ethanol (1000 ppm) as function of the operating temperature of Fe2O3(0.9)±SnO2(0.1) and Fe2O3(0.9)±ZrO2(0.1) annealed at 400 8C for 1 h.
Fig. 6. Sensitivity to ethanol (1000 ppm) as function of the operating temperature of Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) and Fe2O3(0.9)± ZrO2 0:1 Pt (1 wt.%) annealed at 400 8C for 1 h.
gas on Fe2O3, Fe2O3(0.9)±SnO2(0.1), Fe2O3(0.9)±ZrO2(0.1), Fe2O3 Pt (1 wt.%), Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%), and Fe2O3(0.9)±ZrO2 0:1 Pt (1 wt.%). The maximum sensitivity observed for Fe2O3(0.9)±SnO2(0.1), Fe2O3(0.9)±ZrO2(0.1) and Fe2O3 are 1122, 600 and 344 for 1000 ppm ethanol at 220, 230 and 240 8C, respectively. From this graph, we have observed Fe2O3, Fe2O3(0.9)±ZrO2(0.1) sensitivity values are lower and the operating temperature is higher compared to Fe2O3(0.9)±SnO2(0.1). Fig. 6 presents the sensitivity versus operating temperature of Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%), Fe2O3(0.9)±ZrO2 0:1 Pt (1 wt.%) and Fe2O3 Pt (1 wt.%) for 1000 ppm ethanol. This graph shows the maximum sensitivity value for ethanol is 1574 on Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%), compared to Fe2O3(0.9)±ZrO2 0:1 Pt (1 wt.%) is 807 and Fe2O3 Pt (1 wt.%) [13] sensitivity value is 462. From these graphs, we observed more sensitivity on Fe2O3 doped with SnO2, Pt than Fe2O3, Pt doped Fe2O3 and Fe2O3(0.9)±ZrO2(0.1). Related investigations have revealed that the ethanol sensitivity tends to be promoted with basic oxides [17,18] like SnO2, while it decreases with the acidic oxides [17,18] like ZrO2. The Fe2O3(0.9)±SnO2(0.1) with 1 wt.% Pt showed the maximum sensitivity for 1000 ppm ethanol vapor at 220 8C.
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Fig. 7. Sensitivity vs. different concentration of ethanol on Fe2O3(0.9)± SnO2 0:1 Pt (1 wt.%) in air at 220 8C.
Hence, for further studies, we selected the 1 wt.% Pt sample. We have studied further with different concentration of ethanol, cross-sensitivity for other interfering gases, like CH4, CO and H2 and response time for ethanol. Fig. 7 shows the relationship between sensitivity and ethanol gas concentration for Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) at 220 8C. The sensitivity exhibits a good dependence on ethanol gas concentration in the range below 600 ppm. Since the sensitivity to 1 ppm vapor is as high as 18.8, it is able to detect signi®cantly low concentration even less than 1 ppm of ethanol in air at this temperature. Fig. 8 shows the current versus time in seconds for different concentration of ethanol in air on Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) an operating temperature at 220 8C. It is seen that by increasing the concentration of the ethanol, the rise time is faster, but the time taken for the sensor to come back to the ground state is also longer. Fig. 9 shows the sensitivity versus resistance of the sensor for 1000 ppm ethanol cycle at an 220 8C on Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%). It clearly shows the resistance of the sensor is coming to original value within 1 h. Fig. 10 shows the sensitivity of Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) to various test gases as a function of the operating temperature. It is seen that the sensor exhibits a very good selectivity by sensing only ethanol vapor at 220 8C compared
Fig. 8. Current vs. time in seconds for different concentrations of ethanol in air on Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%).
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earlier clari®ed that when the test gas has a complex molecular structure or a reactive functional group, the surface reactions could differ depending on the acid±base properties. Iron oxide is a basic oxide, it shows the similar gas sensing properties like SnO2. When SnO2 is added to Fe2O3 the basicity of the material increases compared pure Fe2O3. The sensor surface is Fe2O3(0.9)±SnO2(0.1), which is predominantly a basic oxide, ethanol (C2H5OH) vapor decomposes to CH3CHO and ®nally CO2 and H2O as follows: C2 H5 OH
Fe2 O3 0:9 SnO2 0:1
!
CH3 CHO H2
CH3 CHO 3Oads ! 2CO2 2H2 O 3e Fig. 9. The response time of the Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) sensor to 1000 ppm ethanol in air at 220 8C.
to other interfering gases like CH4, CO and H2 for 1000 ppm concentration in air. The sensitivities to these interfering gases at 220 8C are 12, 19 and 22.5, respectively. These sensitivity values are very insigni®cant compared to ethanol. This could be effectively used as a breath sensor. The possible reasons for this selective sensitive to ethanol could be explained as follows. 3.1. Sensing mechanism The mechanism underlying this sensing process could be explained from the chemical reactions that are possible on the sensor surface. As we are aware from the catalytic chemistry that the acid±base properties of an oxide surface could be advantageously utilized to favor a particular reaction on the surface. With the prior knowledge of the type of a reducing gas, the sensor surface could accordingly be modi®ed in order to preferentially sense the test gas over the other interfering gases. This could probably be one of the reasons why the Fe2O3-based sensors have such high sensitivity to alcohol. Yamazoe and Miura [19] have
Fig. 10. Sensitivity of the Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) sensor to C2H5OH, CH4, CO and H2 gases (1000 ppm) as a function of the operating temperature.
(1) (2)
where Fe2O3(0.9)±ZrO2(0.1) is more acidic than Fe2O3(0.9)± SnO2(0.1). Since ZrO2 is a weak acid, so it decreases the basicity of Fe2O3. Ethanol (C2H5OH) vapor dehydrogenation route goes through a C2H4 intermediate. C2 H5 OH
Fe2 O3 0:9 ZrO2 0:1
!
C2 H4 2H2 O
(3)
It is also known that basic oxides generally tend to increase the ethanol sensitivity when added to a base semiconducting oxide, while the acidic ones decrease the sensitivity [18,20]. It appears that a comparatively high sensitivity to ethanol gas could be achieved with the oxidation route via CH3CHO given above. 4. Conclusions It can be concluded that hydrazine reduction of iron salts directly form g-Fe2O3 at about 130 8C without any further calcination. But when we add different doping oxides, we got different structures of Fe2O3. From these studies, the following speci®c conclusions can be drawn. 1. From the XRD analysis, it is observed that different structures exist in the final material at different calcination temperatures. In Fe2O3(0.9)±SnO2(0.1) observed at the RT and 150 8C, one is FeO(OH) geothite, Magnetite (Fe3O4) and another is a-Fe2O3 respectively. Whereas in Fe2O3(0.9)±ZrO2(0.1) from RT to 300 8C, only Fe2O3 (Maghemite-Q) phase exists the a-Fe2O3 is only formed after 300 8C. 2. The ethanol vapor sensitivity could be increased tremendously with an addition of a basic metal oxide such as SnO2 to Fe2O3. Due to the change of the structure of the material, the sensitivity for ethanol is found to be different. The doping of Pt causes a remarkable improvement in the sensitivity of ethanol. However, the 1 wt.% Pt incorporated material shows a good sensitivity even at 1 ppm with a sufficient sensitivity. The sensor is highly selective and sensitive to ethanol vapor than the other interfering gases like CH4, CO and H2. It is also known that the basic oxides generally tend to increase the
C.V. Gopal Reddy et al. / Sensors and Actuators B 81 (2002) 170±175
ethanol sensitivity when added to a base semiconducting oxide, while acidic ones decrease the sensitivity. 3. A maximum sensitivity is observed with the Pt (1 wt.%) doped Fe2O3(0.9)±SnO2(0.1) device annealed at 400 8C for 1 h and operating temperature at 220 8C for 1000 ppm ethanol vapor at 1574 compare to Fe2O3(0.9)±ZrO2(0.1), Fe2O3 devices which is 807, 462 at 230 8C.
References [1] H. Yamaura, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, Selective CO detection by using indium oxide-based semiconductor gas sensor, J. Electrochem. Soc. 143 (1996) L36±37. [2] C.V. Gopal Reddy, S.V. Manorama, Room temperature hydrogen sensor based on La2O3:SnO2, J. Electrochem. Soc. 147 (2000) 390±393. [3] T. Miyata, T. Minami, Stability and sensing mechanism of high sensitivity chlorine gas sensors using transparent conducting oxide thin-films, Thin Solid Films 355/356 (1999) 35±40. [4] R.M. Cornell, U. Schwertmann, The Iron Oxides, VCH, New York, 1996, p. 115. [5] M. Gotic, S. Popovic, N. Ljubesic, S. Music, Structural properties of precipitates formed by hydrolysis of Fe3 ion in aqueous solutions containing NO3 and Cl ions, J. Mater. Sci. 29 (1994) 2474±2480. [6] V.V. Malyshev, A.V. Eryshkin, E.A. Koltypin, A.E. Varfolomeev, A.A. Vasiliev, Gas sensitivity of semiconductor Fe2O3-based thickfilm sensors to CH4, H2 and NH3, Sens. Actuators B 19 (1994) 434±436. [7] C. Cantalini, M. Faccio, G. Ferri, M. Pelino, Micro structure and electrical properties of Si-doped a-Fe2O3 humidity sensor, Sens. Actuators B 16 (1993) 293±298. [8] C. Cantalini, M. Faccio, G. Ferri, M. Pelino, The influence of water vapour on carbon monoxide sensitivity of a-Fe2O3 micorporous ceramic sensors, Sens. Actuators B 18/19 (1994) 437±442. [9] C. Cantalini, H.T. Sun, K.S. Patel, M. Faccio, M. Pelino, S. Santucci, L. Lozzi, M. Passacantando, NO2 sensing properties in the Fe2O3± WO3 system, in: Proceedings of the East Asia Conference on Chemical Sensors, Vol. 2, 1995, pp. 77±80. [10] H. Zeng, H. Luo, T. Zhang, R. Zhang, Synthesis of high stability gFe2O3 and its gas sensing characteristics, Gongneng Cailia 26 (1995) 305±308. [11] T.M. Racheva, O. Stamb, I.T. Lova, T. Donchev, Humidity sensitive characteristics of SnO2±Fe2O3 thin-films prepared by spray pyrolysis, J. Mater. Sci. 29 (1994) 281±284. [12] T. Zhang, P. Hing, J. Zhang, L.B. Kong, Ethanol sensing characteristics of cadmium ferrite prepared by chemical co-precipitation, Mater. Chem. Phys. 61 (1999) 192±198. [13] C.V. Gopal Reddy, K. Kalyana Seela, S.V. Manorama, Preparation of g-Fe2O3 by the hydrazine method application as an alcohol sensor, Int. J. Inorg. Mater. 2 (2000) 301±307. [14] F.J. Berry, O. Helgason, A. Bohorquez, J.F. Marco, J. Mc Manus, E.A. Moor, S. Morup, P.G. Wynn, Preparation and characterization of tin-doped a-FeO(OH), J. Mater. Chem. 10 (2000) 1643±1648.
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[15] H.P. Klug, L.E. Alexande, X-ray Diffraction Procedures, Wiley, New York, 1962, p. 125. [16] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, Preparation and characterization of barium stannate: application as a liquefied petroleum gas sensor, J. Mater. Sci. Mater. Electr. 12 (2001) 137± 142. [17] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sens. Actuators B 5 (1991) 7±19. [18] T. Jinkawa, G. Sakai, J. Tamaki, N. Miura, N. Yamazoe, Relationship between ethanol gas sensitivity and surface catalytic property of tin oxide sensors modified with acidic or basic oxides, J. Mol. Catal. A: Chem. 155 (2000) 193±200. [19] N. Yamazoe, N. Miura, in: Shigeru Yamauchi (Ed.), Chemical Sensor Technology, Vol. 4, Elsevier, Amsterdam, 1992, pp. 39±40. [20] S. Matsushima, T. Maekawa, J. Tamaki, N. Miura, Y. Yamazoe, Role of additives on alcohol sensing by semiconductor gas sensor, Chem. Lett. (1989) 845±848.
Biographies C.V. Gopal Reddy received his PhD from Osmania University, India. His research interest is mainly direct towards the development of gas sensors based on semiconducting oxide thick-films with different structures like perovskites, spinels, mixed oxides and synthesis of nanostructured metal oxides for gas sensor applications. Dr. Reddy joined in the School of EEE in December 1999 as a research fellow. Wenqing Cao received his MSc and BSc in engineering from Xi'dian University, China. He is currently doing his PhD on semiconductor oxidebased gas sensors with the Sensors and Actuators Group, Microelectronics Centre, Nanyang Technological University, Singapore. His research interests are oxide semiconductor thick-film gas sensors, microelectronics, and hybrid microcircuit process. Ooi Kiang Tan received his BSc in engineering from the Nanyang Technological University, Singapore, MSc in engineering from the University of Edinburgh, UK, and PhD from Nanyang Technological University, Singapore. He is currently an associate professor at Nanyang Technological University, Singapore. His areas of research interest include silicon IC designs, thick- and thin-films, especially semiconductor and ferroelectric films for gas sensor applications and integration on silicon. Dr. Tan is a member of American Ceramic Society and IEEE. Weiguang Zhu received is BSc and MSc from Shanghai Jiaotong University, China, and PhD from Purdue University, USA. He was a post-doctoral research associate at Purdue University, research fellow at Nanyang Technological University, Singapore, and currently an associate professor at Nanyang Technological University, Singapore. Dr. Zhu is a member of American Physical Society, American Ceramic Society, IEEE, Materials Research Society, and the New York Academy of Sciences. He has published widely in electronics materials, thin-films, ferroelectric materials and memory applications, gas sensors, intelligent/smart materials integrated with silicon devices, diamond films and applications, etc.
Preparation of Fe2O3(0.9)±SnO2(0.1) by hydrazine method: application as an alcohol sensor C.V. Gopal Reddy*, W. Cao, O.K. Tan, W. Zhu Sensors & Actuators Laboratory, School of EEE, Nanyang Technological University, Singapore 639798, Singapore Received 11 December 2000; received in revised form 16 February 2001; accepted 30 August 2001
Abstract Nanocrystalline Fe2O3(0.9)±SnO2(0.1) powders have been prepared using a hydrazine method by adding hydrazine monohydrate to an aqueous solution of ferric nitrate nanohydrate, (Fe(NO3)39H2O) and tin tetra chloride (SnCl4), followed by washing and drying. This material has been characterized by different techniques such as gravimetric-differential thermal analysis (TGA/DTA), X-ray diffraction (XRD). Sensors made from this material have been proved to be highly sensitive and selective in the detection of ethanol. The sensitivity for ethanol has been compared with a 10 wt.% of ZrO2 and SnO2 loaded in Fe2O3. The ethanol sensitivity of pure and Pt doped Fe2O3(0.9)± SnO2(0.1) and Fe2O3(0.9)±ZrO2(0.1) has been investigated for its electrical resistance. The high sensitivity of the sensor to the ethanol could be explained on the basis of a SnO2, ZrO2 activity that invokes the acid±base properties of sensing materials towards the sensitive detection of ethanol vapor in air. Its cross-sensitivity to other gases like CH4, CO, and H2 has also been carried out. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Iron oxide; Thick-®lms; Ethanol sensor; Long-term stability; Selectivity
1. Introduction There exists an increasing demand for semiconducting gas sensors for several monitoring applications having high sensitivity, selectivity and reliability on a long-term scale. In recent years, gas sensors have become more popular in the detection of in¯ammable gases (LPG, H2, etc.) and toxic gases (CO, Cl2, etc.) in indoor as well as outdoor spaces [1± 3]. Fine particles of ferrites are of great interest due to their applications in the preparation of high-density ferrite cores, as suspension materials in ferromagnetic liquids, and also as catalysts. Among these, iron oxides in particular are technologically useful as pigments and also for their magnetic properties [4]. Recently, the study of ®ne particles has received an increased attention. Reducing the particle size to as low as in the nanometer range is accompanied by altered electrical, magnetic, electro-optical and chemical properties [5]. Semiconducting thick-®lms of Fe2O3 have earlier been studied as sensor for CH4, H2 and NH3 [6]. Cantalini et al. [7±9] have also reported a-Fe2O3-based gas sensors. Zeng et al. have studied high stability g-Fe2O3 as a * Corresponding author. Present address: Department of Materials Science & Engineering, The Ohio State University, Columbus, OH-43210, USA. E-mail address: [email protected] (C.V. Gopal Reddy).
possible gas sensors for detecting several reducing gases [10]. SnO2±Fe2O3 thin-®lm have also been reported as a effective humidity sensor [11]. Ethanol sensors are expected to ®nd a wide range of applications such as traf®c management, food ferment, wine making and medical processes. Zhang et al. [12] have studied on an alcohol sensor based on cadmium ferrite, but its operating temperature is high with less sensitivity. In the present work, we report on the preparation and characterization of Fe2O3(0.9)±SnO2(0.1) and Fe2O3(0.9)±ZrO2(0.1) for application as an ethanol sensor at a low operating temperature. These promising performances indicate that Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) could be used as a breath alcohol checker, because of its high sensitivity, selectivity and a rapid response to low concentration of ethanol is a prerequisite of this application. 2. Experimental 0.9 M ferric nitrate nanohydrate (Fe(NO3)39H2O), 0.1 M tin tetrachloride, (SnCl4, 99% pure), 0.1 M zirconium tetrachloride (ZrCl4) and hydrazine monohydrate ((NH2)2H2O) (TCI, 98%) were used as the starting materials. Ferric nitrate and tin tetrachloride solution were prepared by dissolving above amounts in deionized water (18 MO). This aqueous solution (pH 1:75) was poured in a beaker and was stirred
0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 9 4 8 - 0
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on a magnetic stirrer at 60 8C for 30 min. Hydrazine monohydrate 0.8 M was added slowly to this solution through a burette by maintaining a constant stirring until the resulting precipitate reached to pH 8 [13]. The suspension was separated by centrifugation, washed more than 15 times with DI water to remove adsorbed hydrazine and nitrate ions as tested by the brown ring test of copper ®lings with concentrated H2SO4, and then dried in open air. Similarly Fe2O3(0.9)±ZrO2(0.1) was also prepared by using the above procedure. Thoroughly washed and dried material was then subjected to thermal gravimetric-differential thermal analysis (TGA/ DTA) and X-ray diffraction (XRD). Perkin-Elemer TGA-7 and DTA-7 were used to measure the thermal properties of the materials. TGA/DTA was conducted in air at a heating rate of 10 8C/min in the temperature range from the room temperature (RT) to 1000 8C; a-Al2O3 was used as the reference. These materials were studied by using a Rigaku Ê ). X-ray diffractometer (Cu Ka radiation, l 1:5406 A Pastes of these materials were prepared using a commercial organic vehicle 400 (from ESL) and screen-printed onto Al2O3 substrates with inter-digital Au electrodes. The thickness of sensor ®lm is about 60 mm. the samples were then sintered at temperatures from 400 to 600 8C for 1 h in air. The gas sensing properties were measured by using a computer-controlled gas sensing characterization system. 3. Results and discussion Fig. 1(a) shows the TGA/DTA curves of Fe2O3(0.9)± SnO2(0.1) untreated material which was heated in a dry air ¯ow. A signi®cant loss of weight was observed from RT to 280 8C and reached a plateau at 350 8C with a total loss was about 15%. This value indicates that the hydrated form of Fe2O3H2O, SnO2H2O, which appear on the DTA curve as endothermic peak at 100 8C is the result of the loss of water molecules. The exothermic peak at 286 8C on the DTA is attributed to the changes of the phase from geothite to aFe2O3. Similarly, the TGA/DTA data of Fe2O3(0.9)±ZrO2(0.1) are shown in Fig. 1(b). An endothermic peak appears at 103 8C representing the loss of surface water. An exothermic peak appears at 273 8C that correspond to the change of phase from Fe2O3 (Maghemite-Q) to a-Fe2O3. Note that endothermic peak associated with the loss of hydroxyl group is missing because this method directly produces oxides. Another small exothermic peak is observed at 725 8C without any associated change in the weight loss. This may be related to possible phase transformation in ZrO2. Since the thermal decomposition shows that in the region of the DTA peak no weight loss has been recorded, therefore, an exothermic peak has been observed that could be attributed to the change from an amorphous to a crystalline phase. The XRD patterns were recorded on a Rigaku X-ray diffractometer. The XRD patterns of Fe2O3(0.9)±SnO2(0.1) powder calcined from RT to 500 8C for 2 h are shown in
Fig. 1. TGA/DTA curves of (a) Fe2O3(0.9)±SnO2(0.1) and (b) Fe2O3(0.9)± ZrO2(0.1).
Fig. 2(a). From the RT to 150 8C, a-FeO(OH) (geothite), Fe3O4 and a-Fe2O3 structures was observed. All lines are matching with the reported JCPDS data (Card nos. 29-0713, 26-1136 and 33-0664). For the geothite peaks are exactly matching with the reported data [14]. Berry et al. reported the XRD of tin doped a-FeO(OH), which was prepared by the co-precipitation method. After increasing the calcination temperature from 150 to 500 8C the geothite phase changed to and shows the a-Fe2O3 and g-Fe2O3. The DTA also show an exothermic peak at 286 8C, evidence for a change of the phase. At 500 8C we observed a-Fe2O3 peaks (Card no. 330664) and g-Fe2O3. We did not ®nd any peak for SnO2 and hence, it might be amorphous at these temperature or low quantity of SnO2. The particle size of the a-Fe2O3 calculated Ê . From the XRD, using Scherrer formula [15], is 82 A we observed the intensities of peaks did not change much with an increasing temperature that indicates the crystalline nature of particles at low temperature. Fig. 2(b) shows the XRD patterns of Fe2O3(0.9)±ZrO2(0.1) that are treated at different temperatures. Here, we did not observe the geothite structure. However, from RT to 300 8C, only a Fe2O3 with a tetragonal structure (Maghemite-Q, Card no. 25-1402) was
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Fig. 3. Sensitivity to ethanol (1000 ppm) as a function of the operating temperature of Fe2O3(0.9)±SnO2(0.1) annealed from 400 to 600 8C for 1 h.
Fig. 2. XRD patterns of (a) Fe2O3(0.9)±SnO2(0.1) and (b) Fe2O3(0.9)± ZrO2(0.1) calcined at different temperatures: (i) as-prepared; (ii) 150 8C, 12 h; (iii) 300 8C, 2 h; (iv) 400 8C, 2 h; (v) 500 8C, 2 h. Here, a, g, m, g and Q stand for a-Fe2O3, g-Fe2O3, magnetite (Fe3O4), geothite and Maghemite-Q, respectively.
observed. Soon after that with an increasing of the calcination temperature the a-Fe2O3 peaks appeared. The gas sensitivity (S) is de®ned as the ratio of the resistance of the sensor in air (Rair) to that in the test gas (Rgas). In regard of the gas sensing properties, we have ®rst examined the effect of different weight percentages SnO2 content on gas sensing properties of sensors. It was found that 10 wt.% SnO2 doped Fe2O3 sensor has a much better sensitivity to ethanol gas than a pure Fe2O3 based sensor. Experiments have been carried out an pure Fe2O3(0.9)± SnO2(0.1) to ethanol (1000 ppm). Fig. 3 shows the sensitivity to 1000 ppm ethanol vapor as a function of the operating temperature from 160 to 300 8C of Fe2O3(0.9)±SnO2(0.1) without noble metal doping. This sensor device is sintered at different temperatures from 400 to 600 8C for 1 h in air. It is seen that as sintering temperature increases from 400 to 600 8C, the gas sensitivity to ethanol is decreasing. The best sintering temperature for our devices is at 400 8C. For the sintering temperature to be lower than 400 8C, the organic constituents (binder) are not fully decomposed, and hence, would render the device unstable. The good sensitivity is observed for the sample
sintered at 400 8C for 1 h. The maximum sensitivity observed to ethanol at 220 8C is 1122. We impregnated different weight percentages of Pt on the above material. Aqueous solutions of corresponding Pt chloride (H2PtCl6) solution (99%, Aldrich) were impregnated in Fe2O3(0.9)± SnO2(0.1) and Fe2O3(0.9)±ZrO2(0.1). The resulting solution mixture was evaporated on a water bath and then dried at 120 8C for 15 h in an oven. The dried material was calcined at different temperatures in air to get rid of the chloride. The details of the procedure for impregnation method have been described earlier [16]. After annealing this device at different temperatures from 400 to 600 8C to verify the effect on ethanol sensitivity, but we got a maximum sensitivity at 400 8C for 1 h annealing element for ethanol (Fig. 4). Here, we observed the sensitivity is increased compared to Fe2O3(0.9)±SnO2(0.1). The maximum sensitivity value at an operating temperature of 220 8C is 1574 on Fe2O3(0.9)± SnO2 0:1 Pt (1 wt.%) annealed at 400 8C for 1 h. The effects of gas sensitivity on the changing of SnO2 with ZrO2 in Fe2O3 are shown in Figs. 5 and 6. These graphs show the sensitivity versus operating temperature for ethanol
Fig. 4. Sensitivity to ethanol (1000 ppm) as a function of the operating temperature of Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%), annealed from 400 to 600 8C for 1 h.
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Fig. 5. Sensitivity to ethanol (1000 ppm) as function of the operating temperature of Fe2O3(0.9)±SnO2(0.1) and Fe2O3(0.9)±ZrO2(0.1) annealed at 400 8C for 1 h.
Fig. 6. Sensitivity to ethanol (1000 ppm) as function of the operating temperature of Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) and Fe2O3(0.9)± ZrO2 0:1 Pt (1 wt.%) annealed at 400 8C for 1 h.
gas on Fe2O3, Fe2O3(0.9)±SnO2(0.1), Fe2O3(0.9)±ZrO2(0.1), Fe2O3 Pt (1 wt.%), Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%), and Fe2O3(0.9)±ZrO2 0:1 Pt (1 wt.%). The maximum sensitivity observed for Fe2O3(0.9)±SnO2(0.1), Fe2O3(0.9)±ZrO2(0.1) and Fe2O3 are 1122, 600 and 344 for 1000 ppm ethanol at 220, 230 and 240 8C, respectively. From this graph, we have observed Fe2O3, Fe2O3(0.9)±ZrO2(0.1) sensitivity values are lower and the operating temperature is higher compared to Fe2O3(0.9)±SnO2(0.1). Fig. 6 presents the sensitivity versus operating temperature of Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%), Fe2O3(0.9)±ZrO2 0:1 Pt (1 wt.%) and Fe2O3 Pt (1 wt.%) for 1000 ppm ethanol. This graph shows the maximum sensitivity value for ethanol is 1574 on Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%), compared to Fe2O3(0.9)±ZrO2 0:1 Pt (1 wt.%) is 807 and Fe2O3 Pt (1 wt.%) [13] sensitivity value is 462. From these graphs, we observed more sensitivity on Fe2O3 doped with SnO2, Pt than Fe2O3, Pt doped Fe2O3 and Fe2O3(0.9)±ZrO2(0.1). Related investigations have revealed that the ethanol sensitivity tends to be promoted with basic oxides [17,18] like SnO2, while it decreases with the acidic oxides [17,18] like ZrO2. The Fe2O3(0.9)±SnO2(0.1) with 1 wt.% Pt showed the maximum sensitivity for 1000 ppm ethanol vapor at 220 8C.
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Fig. 7. Sensitivity vs. different concentration of ethanol on Fe2O3(0.9)± SnO2 0:1 Pt (1 wt.%) in air at 220 8C.
Hence, for further studies, we selected the 1 wt.% Pt sample. We have studied further with different concentration of ethanol, cross-sensitivity for other interfering gases, like CH4, CO and H2 and response time for ethanol. Fig. 7 shows the relationship between sensitivity and ethanol gas concentration for Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) at 220 8C. The sensitivity exhibits a good dependence on ethanol gas concentration in the range below 600 ppm. Since the sensitivity to 1 ppm vapor is as high as 18.8, it is able to detect signi®cantly low concentration even less than 1 ppm of ethanol in air at this temperature. Fig. 8 shows the current versus time in seconds for different concentration of ethanol in air on Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) an operating temperature at 220 8C. It is seen that by increasing the concentration of the ethanol, the rise time is faster, but the time taken for the sensor to come back to the ground state is also longer. Fig. 9 shows the sensitivity versus resistance of the sensor for 1000 ppm ethanol cycle at an 220 8C on Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%). It clearly shows the resistance of the sensor is coming to original value within 1 h. Fig. 10 shows the sensitivity of Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) to various test gases as a function of the operating temperature. It is seen that the sensor exhibits a very good selectivity by sensing only ethanol vapor at 220 8C compared
Fig. 8. Current vs. time in seconds for different concentrations of ethanol in air on Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%).
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earlier clari®ed that when the test gas has a complex molecular structure or a reactive functional group, the surface reactions could differ depending on the acid±base properties. Iron oxide is a basic oxide, it shows the similar gas sensing properties like SnO2. When SnO2 is added to Fe2O3 the basicity of the material increases compared pure Fe2O3. The sensor surface is Fe2O3(0.9)±SnO2(0.1), which is predominantly a basic oxide, ethanol (C2H5OH) vapor decomposes to CH3CHO and ®nally CO2 and H2O as follows: C2 H5 OH
Fe2 O3 0:9 SnO2 0:1
!
CH3 CHO H2
CH3 CHO 3Oads ! 2CO2 2H2 O 3e Fig. 9. The response time of the Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) sensor to 1000 ppm ethanol in air at 220 8C.
to other interfering gases like CH4, CO and H2 for 1000 ppm concentration in air. The sensitivities to these interfering gases at 220 8C are 12, 19 and 22.5, respectively. These sensitivity values are very insigni®cant compared to ethanol. This could be effectively used as a breath sensor. The possible reasons for this selective sensitive to ethanol could be explained as follows. 3.1. Sensing mechanism The mechanism underlying this sensing process could be explained from the chemical reactions that are possible on the sensor surface. As we are aware from the catalytic chemistry that the acid±base properties of an oxide surface could be advantageously utilized to favor a particular reaction on the surface. With the prior knowledge of the type of a reducing gas, the sensor surface could accordingly be modi®ed in order to preferentially sense the test gas over the other interfering gases. This could probably be one of the reasons why the Fe2O3-based sensors have such high sensitivity to alcohol. Yamazoe and Miura [19] have
Fig. 10. Sensitivity of the Fe2O3(0.9)±SnO2 0:1 Pt (1 wt.%) sensor to C2H5OH, CH4, CO and H2 gases (1000 ppm) as a function of the operating temperature.
(1) (2)
where Fe2O3(0.9)±ZrO2(0.1) is more acidic than Fe2O3(0.9)± SnO2(0.1). Since ZrO2 is a weak acid, so it decreases the basicity of Fe2O3. Ethanol (C2H5OH) vapor dehydrogenation route goes through a C2H4 intermediate. C2 H5 OH
Fe2 O3 0:9 ZrO2 0:1
!
C2 H4 2H2 O
(3)
It is also known that basic oxides generally tend to increase the ethanol sensitivity when added to a base semiconducting oxide, while the acidic ones decrease the sensitivity [18,20]. It appears that a comparatively high sensitivity to ethanol gas could be achieved with the oxidation route via CH3CHO given above. 4. Conclusions It can be concluded that hydrazine reduction of iron salts directly form g-Fe2O3 at about 130 8C without any further calcination. But when we add different doping oxides, we got different structures of Fe2O3. From these studies, the following speci®c conclusions can be drawn. 1. From the XRD analysis, it is observed that different structures exist in the final material at different calcination temperatures. In Fe2O3(0.9)±SnO2(0.1) observed at the RT and 150 8C, one is FeO(OH) geothite, Magnetite (Fe3O4) and another is a-Fe2O3 respectively. Whereas in Fe2O3(0.9)±ZrO2(0.1) from RT to 300 8C, only Fe2O3 (Maghemite-Q) phase exists the a-Fe2O3 is only formed after 300 8C. 2. The ethanol vapor sensitivity could be increased tremendously with an addition of a basic metal oxide such as SnO2 to Fe2O3. Due to the change of the structure of the material, the sensitivity for ethanol is found to be different. The doping of Pt causes a remarkable improvement in the sensitivity of ethanol. However, the 1 wt.% Pt incorporated material shows a good sensitivity even at 1 ppm with a sufficient sensitivity. The sensor is highly selective and sensitive to ethanol vapor than the other interfering gases like CH4, CO and H2. It is also known that the basic oxides generally tend to increase the
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ethanol sensitivity when added to a base semiconducting oxide, while acidic ones decrease the sensitivity. 3. A maximum sensitivity is observed with the Pt (1 wt.%) doped Fe2O3(0.9)±SnO2(0.1) device annealed at 400 8C for 1 h and operating temperature at 220 8C for 1000 ppm ethanol vapor at 1574 compare to Fe2O3(0.9)±ZrO2(0.1), Fe2O3 devices which is 807, 462 at 230 8C.
References [1] H. Yamaura, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, Selective CO detection by using indium oxide-based semiconductor gas sensor, J. Electrochem. Soc. 143 (1996) L36±37. [2] C.V. Gopal Reddy, S.V. Manorama, Room temperature hydrogen sensor based on La2O3:SnO2, J. Electrochem. Soc. 147 (2000) 390±393. [3] T. Miyata, T. Minami, Stability and sensing mechanism of high sensitivity chlorine gas sensors using transparent conducting oxide thin-films, Thin Solid Films 355/356 (1999) 35±40. [4] R.M. Cornell, U. Schwertmann, The Iron Oxides, VCH, New York, 1996, p. 115. [5] M. Gotic, S. Popovic, N. Ljubesic, S. Music, Structural properties of precipitates formed by hydrolysis of Fe3 ion in aqueous solutions containing NO3 and Cl ions, J. Mater. Sci. 29 (1994) 2474±2480. [6] V.V. Malyshev, A.V. Eryshkin, E.A. Koltypin, A.E. Varfolomeev, A.A. Vasiliev, Gas sensitivity of semiconductor Fe2O3-based thickfilm sensors to CH4, H2 and NH3, Sens. Actuators B 19 (1994) 434±436. [7] C. Cantalini, M. Faccio, G. Ferri, M. Pelino, Micro structure and electrical properties of Si-doped a-Fe2O3 humidity sensor, Sens. Actuators B 16 (1993) 293±298. [8] C. Cantalini, M. Faccio, G. Ferri, M. Pelino, The influence of water vapour on carbon monoxide sensitivity of a-Fe2O3 micorporous ceramic sensors, Sens. Actuators B 18/19 (1994) 437±442. [9] C. Cantalini, H.T. Sun, K.S. Patel, M. Faccio, M. Pelino, S. Santucci, L. Lozzi, M. Passacantando, NO2 sensing properties in the Fe2O3± WO3 system, in: Proceedings of the East Asia Conference on Chemical Sensors, Vol. 2, 1995, pp. 77±80. [10] H. Zeng, H. Luo, T. Zhang, R. Zhang, Synthesis of high stability gFe2O3 and its gas sensing characteristics, Gongneng Cailia 26 (1995) 305±308. [11] T.M. Racheva, O. Stamb, I.T. Lova, T. Donchev, Humidity sensitive characteristics of SnO2±Fe2O3 thin-films prepared by spray pyrolysis, J. Mater. Sci. 29 (1994) 281±284. [12] T. Zhang, P. Hing, J. Zhang, L.B. Kong, Ethanol sensing characteristics of cadmium ferrite prepared by chemical co-precipitation, Mater. Chem. Phys. 61 (1999) 192±198. [13] C.V. Gopal Reddy, K. Kalyana Seela, S.V. Manorama, Preparation of g-Fe2O3 by the hydrazine method application as an alcohol sensor, Int. J. Inorg. Mater. 2 (2000) 301±307. [14] F.J. Berry, O. Helgason, A. Bohorquez, J.F. Marco, J. Mc Manus, E.A. Moor, S. Morup, P.G. Wynn, Preparation and characterization of tin-doped a-FeO(OH), J. Mater. Chem. 10 (2000) 1643±1648.
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[15] H.P. Klug, L.E. Alexande, X-ray Diffraction Procedures, Wiley, New York, 1962, p. 125. [16] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, Preparation and characterization of barium stannate: application as a liquefied petroleum gas sensor, J. Mater. Sci. Mater. Electr. 12 (2001) 137± 142. [17] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sens. Actuators B 5 (1991) 7±19. [18] T. Jinkawa, G. Sakai, J. Tamaki, N. Miura, N. Yamazoe, Relationship between ethanol gas sensitivity and surface catalytic property of tin oxide sensors modified with acidic or basic oxides, J. Mol. Catal. A: Chem. 155 (2000) 193±200. [19] N. Yamazoe, N. Miura, in: Shigeru Yamauchi (Ed.), Chemical Sensor Technology, Vol. 4, Elsevier, Amsterdam, 1992, pp. 39±40. [20] S. Matsushima, T. Maekawa, J. Tamaki, N. Miura, Y. Yamazoe, Role of additives on alcohol sensing by semiconductor gas sensor, Chem. Lett. (1989) 845±848.
Biographies C.V. Gopal Reddy received his PhD from Osmania University, India. His research interest is mainly direct towards the development of gas sensors based on semiconducting oxide thick-films with different structures like perovskites, spinels, mixed oxides and synthesis of nanostructured metal oxides for gas sensor applications. Dr. Reddy joined in the School of EEE in December 1999 as a research fellow. Wenqing Cao received his MSc and BSc in engineering from Xi'dian University, China. He is currently doing his PhD on semiconductor oxidebased gas sensors with the Sensors and Actuators Group, Microelectronics Centre, Nanyang Technological University, Singapore. His research interests are oxide semiconductor thick-film gas sensors, microelectronics, and hybrid microcircuit process. Ooi Kiang Tan received his BSc in engineering from the Nanyang Technological University, Singapore, MSc in engineering from the University of Edinburgh, UK, and PhD from Nanyang Technological University, Singapore. He is currently an associate professor at Nanyang Technological University, Singapore. His areas of research interest include silicon IC designs, thick- and thin-films, especially semiconductor and ferroelectric films for gas sensor applications and integration on silicon. Dr. Tan is a member of American Ceramic Society and IEEE. Weiguang Zhu received is BSc and MSc from Shanghai Jiaotong University, China, and PhD from Purdue University, USA. He was a post-doctoral research associate at Purdue University, research fellow at Nanyang Technological University, Singapore, and currently an associate professor at Nanyang Technological University, Singapore. Dr. Zhu is a member of American Physical Society, American Ceramic Society, IEEE, Materials Research Society, and the New York Academy of Sciences. He has published widely in electronics materials, thin-films, ferroelectric materials and memory applications, gas sensors, intelligent/smart materials integrated with silicon devices, diamond films and applications, etc.