Procedia Chemistry www.elsevier.com/locate/procedia Proceedings of the Eurosensors XXIII conference Microstructure control of WO 3 film by adding nano-particles of SnO 2 for NO 2 detection in ppb level Kengo Shimanoe a *, Aya Nishiyama b , Masayoshi Yuasa a , Tetsuya Kida a , Noboru Yamazoe a a Faculty of Engineering Sciences,Kyushu University b Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan Abstract To fabricate more excellent NO 2 sensor with high sensor response and good linearity between the sensor response and NO 2 concentration, the microstructure of WO 3 lamellae was controlled by adding nano-particles of SnO 2 . It was found that the sintering of WO 3 lamellae was inhibited by adding nano-particles of SnO 2 . The device using WO 3 lamellae added a small amount of SnO 2 nano-particles had the highest sensor response, exhibiting a high sensor response (S = 60-540) even to dilute NO 2 (100- 1000 ppb) in air at 200°C. Keywords: Gas sensors, Microstructure control, Lamellar, WO 3 , NO 2 , SnO 2 1. Introduction It is well known that WO 3 is a semiconductor material to detect NO 2 gas and that the morphology and size of particles composing the sensing layers play an important role in determining the sensing properties. Previously we reported that NO 2 sensor using nano-sized WO 3 lamellae shows a high NO 2 sensitivity [1-5]. It was found that the sensor response was significantly increased with a decrease in the thickness of the WO 3 lamellae and was well- correlated with its thickness. Another important feature of the devices was the porous microstructure of the sensing layer packed with WO 3 lamellae with a high anisotropic shape. A sufficiently high sensor response was obtained, even to 10 ppb NO 2 in air, when WO 3 lamellae with ca. 30 nm in thickness and 1 μm in lateral dimension were used for the sensing film. In addition, the acidification of NaWO 4 with a strong acid solution produced lamellar- structured WO 3 particles with 100-350 nm in lateral size and 20-50 nm in thickness, resulting in excellent NO 2 sensing properties (S = 150 against 500 ppb NO 2 in air) at the low temperature of 200°C. On the other hand, however, the linearity between the sensor response and the NO 2 concentration was not well understood. It is sometimes observed that the sensor response showed a tendency to be saturated with increasing NO 2 concentration. Such saturation seems to be owing to that the lamellae particles agglomerated heavily by sintering were dispersed * Corresponding author. Tel.:+81-92-583-7876; fax:+81-92-583-7538. E -mail address:simanoe@mm.kyushu-u.ac.jp. 1876-6196/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.proche.2009.07.053 Procedia Chemistry 1 (2009) 212–215 Fig. 1 FE-SEM images of the surface for thick films of WO 3 (a), WO 3 -SnO 2 (1:0.01) (b) and WO 3 -SnO 2 (1:0.1) (c) calcined at 300°C. C. into the sensing film. In this study, in order to extend the detectable concentration range by improving the sensor resp onse at high NO 2 concentration, we investigated the microstructure control of WO 3 film by adding nano- particles of SnO 2 . 2. Experimental Sol of WO 3* 2H 2 O with lamellar-structure was prepared by the acidification of NaWO 4 with a strong acid solution (H 2 SO 4 at pH = -0.8) [5]. On the other hand, sol of crystalline SnO 2 with mean grain (crystallite) size of 7 nm was prepared by hydrothermal treatments [6]. Both sols were mixed together with W:Sn=1:0-0.1 in molar ratio and stirred for 24 h. The mixed sols were washed with distilled water by centrifugation. The obtained precipitates were mixed with water to form a paste. The resulting paste was screen-printed on an alumina substrate equipped with a pair of comb-type Au microelectrodes (line width: 180 μm; distance between lines: 90 μm; sensing layer area: 64 mm 2 ). The paste deposited on the substrates was calcined at 300-500°C for 2 h in air to form a sensing layer of SnO 2 -dispersed WO 3 via the dehydration of the precursor, WO 2H 3* 2 O. The surface morphology of the samples was analyzed with a field emission scanning e lectron microscope (FE- SEM). The thickness of the films was estimated to be 15-25 μm by FE-SEM observations. The crystal structure and specific surface area of the samples were measured using an X-ray diffractometer (XRD) with copper Kα radi ation and a BET surface area analyzer, respectively. The NO 2 sensing properties of the devices were examined at an operating temperature of 200°C in a concentration range of 50 to 1000 ppb in air. Measurements were performed using a conventional gas flow apparatus equipped with an electric furnace at a gas flow rate of 0.1 dm 3 / min. The sensor response (S) was defined as the ratio of resistance in air containing NO 2 (R g ) to that in dry air (R a ) (S = R g /R ). a 3. Results and Discursion Figure 1 shows FE-SEM images of the surface for thick films of WO 3 (a), WO 3 -SnO 2 (1:0.01) (b) and WO 3 -SnO 2 (1:0.1) (c) calcined at 300°C. The morphology of the lamellar particles seems to differ a little depending on amount of adding SnO 2 . In the case of only WO 3 , comparatively large agglomerated particles are seen. However by adding SnO 2 nano-particles, the particle size was still kept small although the thickness of lamellae was seen as it increased. Table 1 shows specific surface area for each sample. By addition of a small amount of SnO 2 nano-particles, it is found that the sintering of WO 3 lamellae was controlled and the porosity was kept as that result. It can be considered that SnO 2 nano-particles were inserted between the thin WO 3 lamellae and they played a part in inhibiting grain growth of WO 3 . (a) (b) (c) Table 1 Specific surface area of WO 3 -SnO 2 based samples calcined at 300° K. Shimanoe et al. / Procedia Chemistry 1 (2009) 212–215 213 Figure 2 shows the sensor response as a function of NO 2 concentration at 200°C. In the figure, the properties of sensor prepared through an ion-exchange method also indicated for comparison. These devices also responded to dilute NO 2 and showed a sufficient ability to detect ppb level NO 2 in the atmosphere. Especially the device using WO 3 lamellae added SnO 2 nano-particles indicates excellent sensor response. However, the sensor response of the devices differed depending on amount of adding SnO 2 . The sensor fabricated with WO 3 -SnO 2 (1:0.01) showed the best NO 2 response, but the device could not measure high concentration because the electric resistance was as high as exceeding a measurement limit. Such high sensor response can be explained from the viewpoint of the specific surface area as shown in Table 1. The sensor fabricated with WO 3 -SnO 2 (1:0.01) has more porous microstructure, as compared with other devices. It is because the agglomeration of lamellae by sintering was inhibited by adding nano-particles of SnO 2 . However the amount of addition of SnO 2 nano-particles seems to have the most suitable value. The sensor response, when the amount of addition increased, lowered like a case of (b) in Fig. 2, although it was more sensitive than the device without adding SnO 2 nano-particles. In addition, the excessive amount of addition seems to make linearity between the sensor response and the NO 2 concentration poor. In order to confirm the linearity between th e sensor response and the NO 2 concentration for the sensor fabricated with WO 3 -SnO 2 (1:0.01), the calcination temperature was elevated. Figure 3 shows the sensor response as a function of NO 2 concentration at 300°C for the devices calcined at 400 and 500°C. The sensing properties were measured at 300°C to restrain electric resistance in less than a measurement limit. It was found that the sensor response decreased with increasing the calcination temperature. It can think about such a tendency that WO 3 particles grow due to the rise in calcination temperature. However though the calcination was made in high temperature, the linearity between the sensor response and the NO 2 concentration was clearly observed for each device. This result means that the sensor fabricated with WO 3 -SnO 2 (1:0.01) holds porous structure still and fully. If Au electrodes for measurement can be optimized by using MEMS technology to reduce the electric resistance, more excellent sensor, which can detect NO 2 of the wide concentration range, would be obtained at operating temperature of 200°C. Fig. 2 Sensor response as a function of NO concentration at 200°C for the devices using 2 -SnO (1:0.01), (b) WO -SnO (1:0.1), (c) WO by acidification method, and (a) WO 3 2 3 2 3 by ion-exchange method. These devices were calcined at 300°C. (d) WO 3 K. Shimanoe et al. / Procedia Chemistry 1 (2009) 212–215 214 Fig. 3 Sensor response as a function of NO concentration at 300°C for the devices 2 -SnO (1:0.01)) calcined at (a) 400 and (b) 500°C (WO 3 2 4. Conclusions To extend the detectable concentration range by improving the sensor response at high NO 2 concentration, the microstructure control of WO 3 film was investigated by adding nano-particles of SnO 2 . It was found that the developed devices can detect NO 2 high-sensitively in a wide concentration range of 50-1000 ppb. Acknowledgements This work was financially supported in part by NISSAN SCIENCE FOUNDATION. References 1. Y G. Choi, G. Sakai, K. Shimanoe, N. Miura, N. Yamazoe, Preparation of aqueous sols of tungsten oxide dehydrate from sodium tungstate by an ion-exchange method, Sens. Actuators B, 87 (2002) 63-72. Choi, G. Sakai, K. Shimanoe, Y. Teraoka, N. Miura, N. Yamazoe, 2. Y G . Preparation of size and habit-controlled nano crystallites of tungsten oxide, Sens. Actuators B, 93 (2003) 486-494. Choi, G. Sakai, K. Shimanoe, N. Miura, N. Yamazoe, Wet process-prepared thick films of WO for NO sensing, 3. Y G. 3 2 Sens. Actuators B, 95 (2003) 258-265. Choi, G. Sakai, K. Shimanoe, N. Yamazoe, Wet process-based fabrication of WO t hin film for NO detection, 4. Y G. 3 2 Sens. Actuators B, 101 (2004) 107-111. 5. T .Kida, A.Nishiyama, M.Yuasa, K.Shimanoe, N.Yamazoe, Highly sensitive NO 2 sensors using lamellar-structured WO3 particles prepared by an acidification method, Sens. Actuators B, 135 (2009) 568-574. 6. D. D. Vuong, G. Sakai, K. Shimanoe, N. Yamazoe, Preparation of grain size-controlled tin oxide sols by hydrothermal treatment for thin film sensor application, Sens. Actuators B, 103 (2004) 386-391. K. Shimanoe et al. / Procedia Chemistry 1 (2009) 212–215 215 . Proceedings of the Eurosensors XXIII conference Microstructure control of WO 3 film by adding nano-particles of SnO 2 for NO 2 detection in ppb level. the sintering of WO 3 lamellae was inhibited by adding nano-particles of SnO 2 . The device using WO 3 lamellae added a small amount of SnO 2 nano-particles