Electrical and gas-sensing properties of WO 3 semiconductor material Wang Yu-De * , Chen Zhan-Xian, Li Yan-Feng, Zhou Zhen-Lai, Wu Xing-Hui Department of Materials Science and Engineering, Yunnan University, Kunming 650091, People's Republic of China Received 28 November 2000; received in revised form 22 January 2001; accepted 19 February 2001 Abstract In this paper, the electrical and gas-sensing properties of calcined tungsten trioxide semiconductor materials were investigated. X-ray diraction, scan electron microscopy and infrared were used to characterize structure and perfor- mance of WO 3 semiconductor material. The average grain size of WO 3 was 22 nm after calcination at less than 800°C and 24±26 nm at more than 900°C for 1 h. The sensors of indirect heating type were fabricated. The eects of calcining temperature and operating temperature on electrical resistance and sensitivity, and sensitivity-gas concentration properties of the WO 3 -based sensors were investigated. The sensor based on WO 3 exhibited high sensitivity and good response characteristics to ethanol gas. The electrical properties of WO 3 were analyzed and the sensitive mechanism was discussed. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: WO 3 ; Gas-sensing properties; Calcination temperature; Operating temperature; Sensitivity 1. Introduction Numerous metal oxide semiconductor materials were reported to be usable as semiconductor gas sensors, such as ZnO, SnO 2 ,WO 3 , TiO 2 , a-Fe 2 O 3 and so on. These candidates have non-stoichiometrics structures, so free electrons originating from oxygen vacancies contribute to electronic conductivity [1]. The demands for accurate and dedicated sensors to provide precise process control and automation in manufacturing process, and also to monitor and control environmental pollution, have ac- celerated the development of new sensing materials and sensors technology over the last decade [2,3]. Some new types of sensing materials are still being studied and exploited at present time. WO 3 is n-type semiconductor whose electron concentration is determined mainly by the concentration of stoichiometric defects such as oxy- gen vacancy like other metal oxide semiconductors [4]. WO 3 gas sensor was ®rst reported for detection of H 2 by Shaver [5], who showed that the conductivity of WO 3 ®lms changed greatly upon the exposure to the H 2 am- bient. Following this pioneering work, many works have been performed on the structural, electrical properties and sensing characteristics of WO 3 ®lms. It was dem- onstrated by dierent authors that WO 3 -based thin and thick ®lms were both sensitive to NO x gas [1,4,6±10]. It has been reported that WO 3 materials have good sensi- tivity for low concentration of NO x gas [6]. However, most reports focused on the NO x gas sensors, and the study of sensing to other gases was rare. In this paper, we have investigated the electrical and gas-sensing characteristics of WO 3 semiconductor material to other gases such as ethanol, petrol, butane and methane. The experimental results indicated that the sensor based on WO 3 exhibited high sensitivity and good response characteristics to ethanol gas. The electrical and gas- sensing mechanism were analyzed and discussed. Solid-State Electronics 45 (2001) 639±644 * Corresponding author. Present address: Department of Materials Science and Engineering, Tsinghua University, Beij- ing 100084, People's Republic of China. Tel.: +86-0871- 5033371; fax: +86-0871-5031410. E-mail address: wang_yude@263.net (W. Yu-De). 0038-1101/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 038-1 101 ( 0 1)0 0 1 26- 5 2. Experimental 2.1. Materials All the chemical reagents used in the experiments were purchased from commercial sources as guaranteed- grade reagents and used without further puri®cation. The purity of WO 3 powder was 99.99%. 2.2. Characterization of samples The crystal structures of WO 3 samples were analyzed using X-ray diraction (XRD, Rigaku D/MAX-3B powder diractometer). In order to obtain high resolu- tion and to minimize the signal to noise ratio, we have performed the measurements with ®xed slits. The mean crystallite sizes (D) were measured from XRD peaks that were obtained at a scan rate of 2° min À1 . D is based on the Scherrer's equation: D  k=DW cos h. Where k is the wavelength of X-ray (k  1:5418  A), h the Bragg's diraction angle, and DW the true half-peak width. The microstructure of powder was characterized by scan electron microscopy (SEM, CSM950). The conductance type of the WO 3 was measured with the hot probe. 2.3. Fabrication of sensor elements In order to prepare series of sensors, we have chosen the indirect heating type as the structure of sensor. The sensor were fabricated according to the literature [11]. WO 3 semiconductor materials with SiO 2 (4 wt.%) were fabricated on an alumina tube with Au electrodes and Platinum wires. The SiO 2 was used to add intensity of sensitive material of WO 3 , but it did not in¯uence the gas-sensing properties of WO 3 -based sensor. A Ni±Cd alloy crossing alumina tube was used as a resistor. This resistor ensured both substrate heating and temperature control. Each element was sintered at dierent temper- ature (350±800°C) for 1 h in air. Thickness of the sen- sitive bodies after sintering was approximately 0.6±0.8 mm. 2.4. Measurement of sensing properties The gas-sensing properties were examined in a chamber through which air or a sample gas (petrol, methane, butane and ethanol diluted with air) was al- lowed to ¯ow at a rate of 160 cm 3 /min. The sensor's resistance was measured by using a conventional circuit in which the element was connected with an external resistor in series at a circuit voltage of 10 V. The elec- trical response of the sensors was measured with an automatic test system, controlled by a personal com- puter. In order to improve their stability and repeat- ability, the gas sensors were aged at 250°C for 150 h in air. The gas sensitivity (b) was de®ned as the ratio of the electrical resistance in air (R a ) to that in gas (R g ). 3. Results 3.1. Structure of samples The average grain size of WO 3 was 22 nm after cal- cination at less than 800°C and 24±26 nm at more than 900°C for 1 h. The X-ray powder diraction patterns of the WO 3 powder calcined at 500°C are indexed as monoclinic and triclinic WO 3 compounds, which have shown high degree of crystallinity (Fig. 1). Their particle sizes based on Scherrer's equation are 21 nm. The av- erage particle sizes of SEM show consistency with the results of XRD. With the increasing of calcining tem- perature, WO 3 powder was crystallized with sharp peaks, the size of grains and macro pores gradually in- creased. According to the examined results by the hot probe, WO 3 is n type semiconductive material. This re- sult is in good accordance with the literature [4]. 3.2. Resistance±temperature characteristics Fig. 2 shows resistance±calcination temperature curves for the WO 3 -based sensor in the air. The resis- tance of sensor appeared the largest value for low calc- ining temperature and the smallest value for high calcining temperature at the operating temperature of 200°C and 250° C, respectively. As shown in Fig. 3, the resistance±operating temperature properties of the sen- sor shows the characteristic of a typical surface-con- trolled model [12±14]. Electron concentration of WO 3 semiconductor is determined mainly by the concentra- tion of stoichiometric defects such as oxygen vacancy like other metal oxide semiconductors. From 100°Cto Fig. 1. X-ray powder diraction pattern of WO 3 calcined at 500°C for 0.5 h (a) monoclinic WO 3 ( b) triclinic WO 3 . 640 W. Yu-De et al. / Solid-State Electronics 45 (2001) 639±644 300°C, the shape of their resistance±operating temper- ature curves is attributed to the change in charge state of the chemisorbed oxygen-related species, such as O À 2ads , O À ads ,OH À ads and O 2À ads . However, above 300°C their in- trinsic defects, such as oxygen vacancies are responsible for the conductance of the sensor. Generally, a higher sintering temperature is needed during the fabrication of gas-sensing elements, and gas sensors have to operate in the temperature range from 200°C to 400°C for a long time [15]. So it is important for gas sensor to have good thermal stability. In Fig. 3, it can be seen that the eect of operating temperature from 175°C to 225°C on re- sistance of WO 3 sensor is smaller than that of the other temperatures. So that sensor's resistance changed little in this temperature range, and the sensor based on WO 3 has good thermal stability when their operating tem- peratures are in this range. This thermal stability is of signi®cance to apply the sensors to certain control and monitoring. 3.3. Gas-sensing properties In this study, we ®rst examined the eect of calcining temperature on gas-sensing properties of sensor. It was found that the calcining temperature has great in¯uence on the sensitivity of the sensor to the sample gases, such as ethanol, petrol, and butane. Fig. 4 shows the gas sensitivity to the sample gases changes as a function of calcination temperature for WO 3 -based sensors at 200°C operating temperature. It can be clearly realized that there are an increase in sensitivity for ethanol (100 ppm) gas as the calcining temperature increasing from 350°C to 500°C, and the sensitivity decreased above 550°C. It is seen from the above results that as the calcination temperature increase, the crystallite size increases, thereby decreasing the surface area, which in turn af- fected the gas sensitivity. In order to maintain the crystallite size, WO 3 sensor calcined at 500°C has been chosen, since this is the temperature at which the max- imum sensitivity is obtained for ethanol gas. It is clear from Fig. 5 that the operating temperature has an obvious in¯uence on the sensitivity of sensor to Fig. 2. The resistance±calcination temperature behavior of the WO 3 -based sensor in air at operating temperature (a) 200°C and (b) 250°C. Fig. 3. The relation between the resistance and operating temperature in ambient humidity air. Fig. 4. The in¯uence of calcinations temperature on the sensi- tivity gas concentration  100 ppm). Fig. 5. The in¯uence of operating temperature on the sensi- tivity of the sensor for ethanol and petrol 100 ppm, and for butane and methane 1000 ppm. W. Yu-De et al. / Solid-State Electronics 45 (2001) 639±644 641 sample gases. The maximum sensitivity to 100 ppm ethanol gas occur at 80°C is about 40. However, sensi- tivities to ethanol is reduced with the increasing of op- erating temperature in the range of 80±200°C. On the other hand, the sensitivity of sensor to other gases such as petrol, butane and methane is very low, though gas concentration is 1000 ppm. Fig. 6 shows the relationship between sensitivity and sample gases concentration for sensor operating at 200°C (because sensor has good thermal stability at this temperature). When the sensor is operated at 200°C, the sensitivity exhibits a good dependence on ethanol gas concentration. Long time stability of the WO 3 sensor in the whole investigated time rang is shown in Fig. 7. 4. Discussion 4.1. Electrical properties The oxygen adsorbed on the surface of the material in¯uences the conductance of the WO 3 -based sensor. The oxygen adsorbed depends on the particle size, large speci®c area of the material, and the operating temper- ature of the sensor. With increasing temperature in air, the state of oxygen adsorbed on the surface of WO 3 undergoes the following reactions: O 2gas 6 O 2ads O 2ads  e À 6 O À 2ads O À 2ads  e À 6 2O À ads O À ads  e À 6 O 2À ads The oxygen species capture electrons from the ma- terial, leading to increasing of the hole concentration and decreasing of the electron concentration. WO 3 is a kind of the acidic oxide and can react with the alkali. Besides the state of oxygen adsorbed on the surface of WO 3 , there is OH À that comes from water. The exis- tence form of chemisorbed water on WO 3 is more complicated. The reaction can be summarized as W lat  H 2 O 6W lat À OH À H  ads where W lat is Lewis acid site, which can form covalent bond with OH À ,H  ads is the adsorbed hydrogen ion, which is Bronsted acid site that is can be removed easily in catalytic reaction. OH À ,O À 2ads and O À ads are dominat- ing oxygen-related species on materials surface at low temperature [16,17]. As shown in Fig. 3, from room temperature to 175°C, the resistance decreases with in- creasing of the operating temperature because the ther- mal energy causes electrons to emit from low energy levels (such as donor levels or valence band) to con- duction band. In the range of 175±225°C, the change in resistance is very small. It is because the electrons of donor level are ionized completely, and the electronic concentration of intrinsic exciting is less than the con- centration of donor in this temperature region. How- ever, when the temperature is higher than 250°C, chemisorbed water desorbs and transfers to O À ads and O 2À ads . The resistance starts to go up in the temperature ranging from 250°C to 300°C, which may be attributed to such electron depletive type mechanisms [15]. 4.2. Gas-sensing mechanism The gas-sensing mechanism is based on the changes in the conductance of WO 3 . The reducing gas reacted with oxygen adsorbed on the surface of the sensor and the possibility of the reaction between reducing gas and lattice oxygen was very small. The reducing gas acting on the WO 3 sensor surface can be explained as [18]: R  O À ads 6 RO  e À To maintain neutrality, the electrons release back WO 3 material, resulting in the increase of the electron con- Fig. 6. The eects of gas concentration on the sensitivity of sensor at 200°C. Fig. 7. Long time stability of the sensitivity for ethanol gas concentration  100 ppm). 642 W. Yu-De et al. / Solid-State Electronics 45 (2001) 639±644 centration and the decrease of the resistance. This change of the electrical resistance determined sensitivity of the WO 3 based sensor to reducing gases. The reacting products of C 2 H 5 OH on the surface of WO 3 were studied by infrared (IR, Mattson ALPHA CENTAURT FT-IR) spectrum. As shown in Fig. 8, it is found that C 2 H 5 OH is decomposed to ethyl ether at 250°C but peaks of aether vanished at 400°C. Ethylene is produced at 350°C and the amount of it increases with tempera- ture. Ethanol is adsorbed and reacted with Lewis acid site of surface (that is W ion) to produce O±CH 2 ±CH 3 . The possible process of the reaction can be explained as follows: where O±CH 2 ±CH 3 is negatively charged and H  ads is activated. 2H  ads  O À ads 3 H 2 O gas H  ads  OH À ads 3 H 2 O gas H  ads  O 2À lat 3 O lat ±H À  V O where V O is oxygen vacancy. At high temperature 350°C, O±CH 2 ±CH 3 and OH group form ethylene through dehydration and the reaction product of eth- ylene increases with temperature. The reaction can be written as If temperature is low and oxygen is de®cient, two O±CH 2 ±CH 3 can interact to produce ethyl ether: Fig. 8. IR spectrum of ethanol after reacting with WO 3 material at (a) 145°C, (b)200°C, (c) 250°C, (d) 295°C, (e) 350°C, (f) 407°C, (g) 455°C, (h) 500°C. W. Yu-De et al. / Solid-State Electronics 45 (2001) 639±644 643 All these reactions release electrons into the WO 3 ma- terial, leading to the increase of the electron concentra- tion, and the decrease of the resistance of WO 3 -based sensor. This result is in good accordance with the above analysis. 5. Conclusions Monoclinic and triclinic WO 3 compounds can be used as a gas-sensing material for ethanol gas. The analysis of the electrical properties and gas-sensing mechanism of WO 3 -based sensors revealed that the calcining and operating temperature of sensor obviously in¯uence on the resistance change and gas sensitive characteristics of the WO 3 sensor. 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