Sensors and Actuators B 94 (2003) 343–351

Nano-crystalline tungsten oxide NO2 sensor Shih-Han Wang a , Tse-Chuan Chou a,∗ , Chung-Chiun Liu b b

a Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan Electronics Design Center, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106-7200, USA

Received 1 July 2002; received in revised form 18 April 2003; accepted 24 April 2003

Abstract Sensitive porous tungsten oxide nano-crystalline based NO2 sensor was fabricated by thin film microfabrication technique. The sensitivity of this NO2 sensor was at parts per billion (ppb) level. The nano-crystalline porous tungsten oxide film was prepared from WCl6 by a sol–gel technique. The surface morphology and sensitivity to NO2 of the tungsten oxide films calcined at various temperatures were investigated. The NO2 adsorption behavior on the tungsten oxide surface was carried out by XPS measurement. Experimental results indicated that the tungsten oxide film calcined at 550 ◦ C for 1 h showed the best performance as a sensing material to NO2 , and the optimal operational temperature of the sensor was 300 ◦ C. The sensor showed high sensitivity to low NO2 concentration in the range from 50 to 550 ppb with relatively fast response time (∼3 min) and recovery time (∼1 min), respectively. © 2003 Elsevier B.V. All rights reserved. Keywords: Tungsten oxide; Nano-crystalline; Nitrogen dioxide sensor; Sol–gel technique

1. Introduction Nitrogen dioxide (NO2 ) is toxic with a pungent odor, and it is harmful to the environment as a major origin of acid rain and photochemical smog. NO2 is mainly produced from power plant, combustion engine and automobiles. The NO2 concentration has been regulated by the Environmental Protection Administration of the Republic of China, and the alarm threshold is set at 250 ppb [1]. Unfortunately, stable, sensitive, convenient and modest cost devices for ppb-level NO2 detection have not become reality. Metal oxide based sensors have been used extensively for sensing gases, which include toxic and pollution gases (NOx , H2 S, Cl2 , CO, SO2 , O3 , etc.) and combustible gases (H2 , CH4 and flammable organic vapors) [2–4]. Metal oxide materials, such as TiO2 , SnO2 , ZnO, WO3 , and others [2–4], have been used for gas sensing. Recently, many materials have been considered for NO2 detection, in particular YSZ [5], NASICON [6,7], In2 O3 [8], WO3 [9–15], and porous silicon [16]. However, few of these materials show the capability of detecting NO2 at very low concentration level, and only porous silicon [16] and NASICON [7] have been reported to detect ppb level NO2 in air. The preparation process or the structures of those sensors, however, ∗ Corresponding author. Tel.: +886-6-2757575x62639; fax: +886-6-2366836. E-mail address: [email protected] (T.-C. Chou).

0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00383-6

were rather complex. Among other metal oxides, tungsten oxide was considered to be a good candidate for low level NOx sensing. Tungsten oxide (WO3 ) film showed promising electrical and optical properties for various applications, such as efficient photolysis, electrochromic device, selective catalysts for oxidation and reduction reactions [17] and gas sensors [4]. The sensing properties of WO3 films strongly depended on the preparation methods and the growing conditions of the film itself. Tungsten oxide thin films have been prepared by various techniques, such as plasma-enhance chemical vapor deposition (PECVD) [9], thermal evaporation [10,11], sputtering [15], thick film [13,14], and sol–gel [12] technologies. In order to have a high productivity and low power consumption of a sensor, the current trend is to construct all sensing elements on a chip using silicon-based microfabrication technology. Sol–gel technology has the ability to control the film morphology and is compatible for the production of thin film sensor. Sol–gel method has been used for the preparation of tungsten oxide for electrochromic materials. However, this technique for application to sensor has not been fully explored. Semiconductor tungsten oxide (WO3 ) can be prepared from a tungsten alkoxide or an alcohol precursor, and this approach is applicable for many other metal oxides. Tungsten alkoxide, W(OR)6 , precursor is prepared by dissolving tungsten hexachloride, WCl6 , in alcohol. Tungsten oxide

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nano-crystalline produced by tungsten alkoxide showed good sensing performance to O3 [18]. The sensitivity of porous WO3 or SnO2 increases with decreasing of grain size [19]. The surface-to-bulk ratio for a nano-crystalline material is greater than that of a material with large grain size. Tamaki et al. [19] showed that the sensitivity of the WO3 -based sensors to NO2 increased dramatically with decreasing of the tungsten oxide grain size. Consequently, the particle size distribution, the surface morphology and the defect structure of the tungsten oxide were important factors for nitrogen dioxide sensing. For potential ppb level NO2 detection, new type of WO3 based sensor is needed. In this study, a sol–gel technique was employed to produce WO3 from tungsten(VI) hexachloride dissolved into alcohol. The tungsten oxide film was formed on an alumina substrate by spin-coating. The microstructure of the tungsten oxide films, and the effect of calcining temperature on film structure, grain size, as well as on the sensor response to the ppb level of NO2 will be described. NO2 adsorption behavior on the tungsten oxide surface at various operating temperatures was also investigated.

2. Experimental 2.1. Tungsten oxide thin film preparation The sol–gel process to produce tungsten(VI) oxide film is shown in Fig. 1. The hydrolysis of tungsten alkoxide was carried out under ice cooling. Typically, 7 g of tungsten(VI) hexachloride (WCl6 , Aldrich Chemical, >99.9%) was mixed with 100 ml of ethanol and a lemon yellow color solution was obtained. Because W6+ was reduced by the alcohol, the solution became blue after 10 min. White tungsten(VI) oxide gel was obtained by dropwise adding 10 ml of 0.5 M ammonia hydroxide (NH4 OH, Fisher Scientist, reagent grade) solution. The chloride ion was carefully removed from the precipitation by de-ionized-water washing and centrifuging until no precipitation appeared when titrated with 0.1 M silver nitrate solution. The washed precipitate was peptized by ammonia hydroxide, and 50 ␮l of surfactant (Sigma, Triton X-100) was added into the solution. In this batch process, 100 ml of tungsten oxide contained sol was obtained. 2.2. The production of tungsten(VI) oxide films Tungsten(VI) oxide film was produced by spin-coating from the corresponding sol–gel solution. Approximately 0.1 ml of the sol solution was added onto the surface of highly polished alumina substrate area (15 mm × 15 mm) at 2000 rpm for 2 min, and the resulted film thickness was about 0.2 ␮m. Tungsten(VI) oxide sol–gel films formed were then calcined at temperatures of 350, 450, 550 and 650 ◦ C, for 1 h in air. The characteristics of the calcined films were then evaluated.

Fig. 1. The flow chart of tungsten oxide preparation by a sol–gel technique.

2.3. Surface characterization The surface morphology and the particle size of the tungsten oxide films were examined using scanning electron microscopy (SEM, XL-40 FEG, Philips). The specific surface area was evaluated from nitrogen adsorption data using BET method. The film crystalline structure was examined by X-ray diffraction measurement (XRD, Rint-2200, Rigaku, Cu K␣ radiation). X-ray photoelectron spectroscopy (XPS, Fison (VG) ESCA 210) with a monochromatic Al K␣ radiation source was employed to determine the elemental composition on the tungsten oxide surface. XPS was also used to examine the tungsten oxide surface wherein it reacted with nitrogen dioxide. Based on this adsorption behavior, the mechanism that reduced the sensitivity of WO3 to NO2 at high temperature could then be derived. The surface adsorption phenomenon of NO2 on the WO3 surface was accomplished by XPS measurement of N (1s) peak. The photoelectron spectra were collected from: (i) the fresh tungsten oxide was the reference blank; (ii) the fresh tungsten oxide was exposed in

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Fig. 2. The top-view of the micro-NO2 -sensor fabricated by thin film microfabrication technique.

200 ppb NO2 at 300 ◦ C, and followed by the air purge; (iii) the fresh samples exposed in 200 ppb NO2 at 350 ◦ C for 20 min; and (iv) the sample in (iii) then was purged by air. 2.4. Sensing elements and manufacture of the NO2 sensor Sensing properties of the tungsten oxide film to NO2 were assessed using a WO3 based sensor. The sensor was produced by thin film microfabrication technique. The sensor consisted of a temperature detector, a heater, an interdigitated sensing element and a calcined WO3 film. A highly polished Al2 O3 was used as substrate, and the top-view of the sensor structure is shown in Fig. 2. Eleven pairs of platinum interdigitated electrodes with 100 ␮m line and gap width and 2.3 mm in length were formed by ion bean sputtering metallization and photoresist pattering techniques. The tungsten oxide film was spin-coated over the interdigitated electrodes. Platinum resistance type heater and temperature detector were formed by sputtering in the vicinity of the electrodes, as shown in Fig. 2. The resistance of the heater and temperature detector was 1.5 and 2.8 k, respectively, at ambient temperature. A constant dc voltage was applied to the platinum interdigitated electrodes, and the sensor current was measured and displayed through a potentiostat (EG & G M 263A). The NO2 sensitivity, defined by the ratio RNO2 /Rair , where RNO2 was the sensor resistance in a NO2 -presenting environment and Rair was the resistance in dry air. The sensor was placed inside a glass chamber of 30 ml volume which was kept under continuous flowing of testing gas mixtures or air at a constant flow rate of 120 ml min−1 . A gas blander was used to adjust the flow rate of dry air and nitrogen dioxide gas (20 ppm blended with air) through mass

flow controllers. The gas was premixed at a pre-determined NO2 concentration ranging from 0 to 550 ppb before entering the glass chamber. The sensors were stabilized for at least 4 h at the operating temperature in air before measuring NO2 gas in the chamber.

3. Results and discussion 3.1. Characteristics of tungsten oxide film The crystalline structure and the surface morphology of the produced tungsten oxide were characterized by XRD and SEM. The XRD measurements on the tungsten oxides calcined at various temperatures in the range from 350 to 650 ◦ C for 1 h were shown in Fig. 3. The XRD patterns indicated that the tungsten oxide crystal was well formed. When the calcination temperature was above 450 ◦ C, the crystal structure was monoclinic. The crystallite size of the prepared tungsten oxide was evaluated from the diffraction line of (221) based on Scherrer equation [20]. Dhkl =

0.9λ βhkl cos(θhkl )

(1)

where λ is the wavelength of the incident radiation, βhkl the full width at half-maximum (FWHM) of the peak in the radians, and θ hkl the Bragg angle. The average crystallite sizes of various films were estimated at approximately Scherrer formula, as shown in Table 1. Tungsten oxide films coated on a highly polished alumina surface were calcined at 350, 450, 550 and 650 ◦ C for 1 h. The surface morphologies of these films are shown in Fig. 4a–d, respectively. The particle size of the sol-derived

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Fig. 3. XRD patterns of prepared tungsten oxide calcined at various temperatures. The Miller indices were determined from the monoclinic structure (JCPDS, 5-0363).

Fig. 4. Surface morphologies of tungsten oxide thin films (on the highly polished alumina substrate) at various calcination temperatures: (a) 350 ◦ C; (b) 450 ◦ C; (c) 550 ◦ C; and (d) 650 ◦ C.

S.-H. Wang et al. / Sensors and Actuators B 94 (2003) 343–351 Table 1 The average particle sizes, crystallite sizes and surface area of the prepared tungsten oxide films calcined at various temperatures for 1 h Calcination temperature (◦ C)

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Particle size (nm) Crystallite size (nm) Surface area (m2 g−1 )

18.1 ± 3.6 23.2 ± 4.7 25.7 ± 2.6 69.9 ± 10.7 9.3 16.4 17.1 30 58.4

450

36.8

550

32.3

650

10.7

thin film appeared to be controlled by the calcination temperature. The particle size distribution and the average particle sizes were obtained by measuring 100 particles in the neighborhood area [21], and the results were shown in Fig. 5, and Table 1, respectively. The calculated average particle sizes were 18.1, 23.2, 25.7 and 69.9 nm for the films calcined at 350, 450, 550 and 650 ◦ C for 1 h, respectively. As shown in Figs. 4 and 5, the temperature effect on the particle size is evident. Tungsten oxide film calcined at a higher temperature yielded large size particles. Also, the particle size distribution became broader for the film calcined at 650 ◦ C than those calcined at lower temperatures. This observation was consistent with that reported by Jin et al. on sol–gel formed nano-crystalline tin oxide film [21]. Namely, at a rel-

Fig. 5. Particle size distributions of the tungsten oxide films calcined at various temperatures for 1 h. The temperatures in the figure represent the calcination temperature.

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atively high calcined temperature the particle size itself and its distribution in the film appeared to become larger. The SEM data were also consistent with the information from the X-ray diffraction. 3.2. Sensitivity of the WO3 sensor to NO2 3.2.1. Effect of the calcinations temperature The sensitivity of the tungsten oxide films calcined at various temperatures was evaluated at a fixed NO2 level of 100 ppb. Fig. 6 shows the sensor response to NO2 in dry air as a function of operating temperature in the range from 30 to 350 ◦ C. The best sensitivity of the NO2 sensor was obtained using tungsten oxide film calcined at 550 ◦ C for 1 h. Tamaki et al. [19] reported that the sensitivity of the sensor increased as the grain-size of tungsten oxide decreased. As shown in Table 1, the smallest crystallite size, 9.3 nm, was obtained by calcination at 350 ◦ C, and the grain sizes of the tungsten oxide films calcined at 450 and 550 ◦ C were similar, 16.4 and 17.1 nm, respectively. However, our experimental results indicated that the highest sensitivity occurred for the film calcined at 550 ◦ C. The particle size of this film was not the smallest one among these films. Therefore, grain-size theory was insufficient to explain the sensor performance to our sensor using nano-crystalline porous tungsten oxide. Similar phenomena appeared in SnO2 nano-crystalline to CO sensing which was reported by Jin et al. [21] Semiconductor sensors sensed gas using its non-stoichiometric structure and the free electrons originated from the oxygen vacancy. The sensing properties of WO3 film were controlled by the surface defect and structure rather than the native properties according to Bringans et al. [22]. There were more oxygen vacancies on the surface of tungsten oxide calcined at higher temperature [23]. Also, the total surface area of tungsten oxide film decreased as the calcination temperature increased. These factors suggested that the sensitivity of the tungsten oxide film to NO2 was not controlled by the grain size of the tungsten oxide alone. Rather, the sensitivity of the film was also affected by the surface structure and the geometrical heterogeneity of the film. These properties of the film were strongly influenced by the calcination temperature of the film. 3.2.2. Effect of the operating temperature Sensitivity of the sensors increased with the operating temperature, over the range of 30 to 300 ◦ C, as shown in Fig. 6. The potential theory suggested that a potential barrier was formed between the surface of the semiconductor film and the ambient environment [24]. In our case, NO2 was adsorbed onto WO3 surface, and the oxygen of NO2 served as the accepter, extracting electrons from the conduction band of WO3 . Therefore, the barrier of the WO3 was increased and the resistance of the WO3 film was also increased whenever NO2 was adsorbed on WO3 surface. The density of electrons at the tungsten oxide surface increased with the operating temperature. Hence, the electron in the conduction band of

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Fig. 6. Sensitivity comparisons of tungsten oxide films calcined at various temperatures and at various operating temperatures. The temperatures in the figure were the corresponding calcination temperature.

tungsten oxide more easily tunneled to through the barrier and reacted with the adsorbed NO2 molecular. Therefore, the sensitivity of tungsten oxide to NO2 gas increased with the operating temperature in the range below 300 ◦ C. Consequently, the resistance of the WO3 film became lower while exposed in air, and the sensitivity increased with temperature [24]. The adsorption of NO2 to the WO3 film surface was a reversible chemisorption reaction NO2 + e− ↔ NO2 −

(2)

Also, the reaction rate increased with the operating temperature. However, when the operating temperature was above 300 ◦ C, the sensitivity of the tungsten oxide film

to NO2 decreased, and the highest sensitivity occurred at 300 ◦ C. In general, NO2 was adsorbed by nitrio type (ONO− ), and dissociated into nitrosyl forms (NO− , NO+ ) along with the increasing of temperature [14,25,26]. Consequently, the reversible sensitivity of the tungsten oxide film to NO2 gas decreased dramatically at operating temperature above 300 ◦ C. The response time of the sensor also depended on the operating temperature, as shown in Fig. 7. Base on the net contribution of reaction, adsorption and desorption rate, the shortest response time was obtained at an operating temperature of 300 ◦ C. Fig. 8 shows the XPS spectra of N (1s) for fresh unused tungsten oxide film (a), exposed to NO2 at the operating

Fig. 7. The response time of the tungsten oxide calcined at 550 ◦ C to 100 ppb NO2 at various temperatures.

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Fig. 8. N 1s XPS spectra of tungsten oxide after 550 ◦ C calcination and treated by the following steps: (a) fresh unused WO3 ; (b) exposed in 200 ppb NO2 atmosphere at 300 ◦ C for 20 min then following by dry air purge; (c) exposed in 200 ppb NO2 atmosphere at 350 ◦ C for 20 min; (d) exposed in 200 ppb NO2 atmosphere at 350 ◦ C for 20 min then following by dry air purge.

temperature of 300 ◦ C (b), and 350 ◦ C (c), and the spectra of the tungsten oxide films after purging by dry air (d), respectively. In principal, the binding energy of nitrosyl (NO+ , NO− ) shifted to the lower binding energy than the nitrio type functional group in the N 1s spectra. There was no peak observed in the spectra of fresh WO3 film calcined at 550 ◦ C as shown in Fig. 8a, and no significant peak in the spectra was found when the sensor was exposed in NO2 at 300 ◦ C then purged by dry air as shown in Fig. 8b. In Fig. 8c, there was a peak observed in N (1s) when the sensor operated at 350 ◦ C. The peak comprised two peaks, which located at 402 and 403 eV. After the film was purged

by dry air, a single peak remained at 402 eV as shown in Fig. 8d. In principle, the binding energy of NO was located at 402 eV and ONO was at 403 eV [26,27]. Our results suggested that most of NO2 desorpted in high vacuum condition. Our results also indicated that most of the residual was nitrosyl type compounds. After the tungsten oxide film in Fig. 8c was purged by dry air, NO+ or NO− stayed in the tungsten oxide as shown in Fig. 8d. The residuals, NO+ and NO− , occupied the active sites on tungsten oxide film surface for NO2 adsorption, consequently, the sensitivity of tungsten oxide to NO2 operated at 350 ◦ C decreased. These results implied that an irreversible phenomenon occurred

Fig. 9. The sensing response of the tungsten oxide film calcined at 550 ◦ C for 1 h in air and in NO2 /air mixture at 300 ◦ C.

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Fig. 10. Variation in sensitivity of the tungsten oxide film calcined at 550 ◦ C as correlated with NO2 concentration.

when exposing the sensor in NO2 atmosphere and operated at 350 ◦ C. 3.2.3. Characteristics of tungsten oxide sensor calcined at 550 ◦ C The best performance of the NO2 sensor was obtained using a tungsten oxide film calcined at 550 ◦ C and at an operating temperature of 300 ◦ C. Fig. 9 shows the sensor response to NO2 . The response and the recovery times of the sensor (time for 90% of total conductance change) for NO2 /dry air mixture in the range from 50 to 500 ppb were about 3 and 1.5 min, respectively. The sensitivity was in the range of 1.5–4 in the NO2 concentration range from 50 to 550 ppb. This sensitivity for low NO2 concentration is attractive for practical sensor development. The surface element analysis in this study was carried out by XPS analysis. According to the XPS results, there was no N (1s) (402 eV) appeared before and after long-term exposed in NO2 atmosphere following by dry air purge. The results indicated that the NO2 adsorption on the surface of WO3 film calcined at 550 ◦ C was reversible at this operating condition. The sensor showed relatively fast response and baseline recovery for NO2 detection. The surface area of our tungsten oxide was larger and the particle size was smaller than those reported in previous studies [9–15]. Thus, the minimum detectable NO2 level of our sensor was lower than those reported in the literatures [9–15]. A typical calibration curve of the sensor was shown in Fig. 10. The linear detection range from 50 to 550 ppb was obtained for the sensor with tungsten oxide film calcined at 550 ◦ C for 1 h and operated at 300 ◦ C. Thus, it is feasible to apply this method to develop a practical sensor to detect nitrogen dioxide at low ppb range, such as 50–550 ppb.

4. Conclusions A WO3 micro-NO2 -sensor was produced using thin film microfabrication technology. Nano-crystallite tungsten oxide was produced by a sol–gel method. Experimental results indicated that the porous tungsten oxide films had high degree of sensitivity to low NO2 concentration in the range from 50 to 550 ppb with relatively fast response and recovery time. The sensitivity of the prepared tungsten oxide film depended on the surface structure, grain size, and the geometrical heterogeneity of the films which were controlled by the calcination temperature. The sensitivity of WO3 to NO2 also depended on the NO2 adsorped form on the surface which was affected by the operating temperature. The optimal sensing condition was using a tungsten oxide film calcined at 550 ◦ C for 1 h and operating the sensor at 300 ◦ C. Acknowledgements The support by the National Science Council of the Republic of China (NSC 90-2214-E006-001), Ministry of Education of the Republic of China (EX-91-E-FA09-5-4), National Cheng Kung University, Electronics Design Center, Case Western Reserve University and The Li Foundation, New York is gratefully appreciated. We would like to thank the help of Laurie Dudik and Qinghai Wu for the design and fabrication of the thin film sensor. References [1] Air Pollution Control Act, Environmental Protection Administration of the Republic of China on Taiwan, 1986.

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Biographies Shih-Han Wang received her BS degree from National Cheng Kung University, Taiwan. Since 1998, she has been working towards a PhD degree in the Department of Chemical Engineering at National Cheng Kung University, Tainan, Taiwan. Her research interests include the development of ion-selective electrodes and gas sensor by microfabrication technology. Tse-Chuan Chou is the Professor of Chemical Engineering at National Cheng Kung University. He is also the Director of the Engineering and Technology Promotion Center, National Science Council, Taiwan. His research interests include chemical, and gas sensors, biosensors, photo-catalysts, catalysts and photoelectrochemistry, organic electrochemistry and microfabrication. Chung-Chiun Liu is the Wallace R. Persons Professor of Sensor Technology and Control and Professor of Chemical Engineering at Case Western Reserve University. He is also the Director of the Electronic Design Center, a multidisciplinary education and research center focusing on microfabrication and micromachining processes at the University. His research interests consist of development of microfabricated chemical, gaseous and biosensors, functional nano-structure materials and microfabrication and micromachining of non-silicon materials.