Sensors and Actuators B 45 (1997) 141–146 Spin-coated thin films of SiO 2 –WO 3 composites for detection of sub-ppm NO 2 Xusheng Wang, Go Sakai, Kengo Shimanoe, Norio Miura, Noboru Yamazoe * Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu Uni6ersity, Kasuga-shi, Fukuoka 816 , Japan Received 21 May 1997; received in revised form 18 August 1997; accepted 20 August 1997 Abstract Thin films of SiO 2 –WO 3 composites with various SiO 2 contents (0–20 wt%) were prepared by means of sol-gel method. The grain size of WO 3 decreased with increasing SiO 2 content, resulting in an increase in NO 2 sensitivity. On the other hand, the response transient to NO 2 was brought to be the quickest at 5% SiO 2 . The thin film of SiO 2 (5%)–WO 3 was far more sensitive to NO 2 than the thin film or sintered block of pure WO 3 . It could detect dilute NO 2 (0.1–2 ppm) in air at 350°C sensitively and fairly selectively. The NO 2 sensing characteristics were stable over the whole test period of 10 days. © 1997 Elsevier Science S.A. Keywords : Sol-gel; Thin films; NO 2 sensor; WO 3 ; SiO 2 ; AFM 1. Introduction From an environmental concern, there has been an increasing demand for sensory detection of nitrogen oxides (NO x : NO and NO 2 ), which are typical air pollutants released from combustion facilities and auto- mobiles. Especially NO 2 is highly toxic to human nerves and respiratory organs so that high-sensitivity detection of it is desired for air quality monitoring. Various solid-state sensors to detect NO 2 and/or NO have been proposed by using semiconducting oxides [1– 4], solid electrolytes [1,5] and SAW devices [6]. However, the detection of NO 2 in the vicinity of envi- ronmental standard (40 –60 ppb in Japan) has remained to be a challenge. Recently, semiconductor sensors using WO 3 have been proved to be very sensitive to NO 2 [2,3,7– 10]. The NO 2 sensitivity has been shown to be improved by using fine particles of WO 3 [3] or thin films of WO 3 prepared by vacuum evaporation [8], RF sputtering [9], or sol-gel methods [10,12]. These reports suggest that further improvements in NO 2 sensitivity may be possible by controlling the microstructure of WO 3 . Compared with the other techniques, sol-gel methods allow one to control relatively easily the parti- cle size and dispersion of components in composite films, as illustrated for silica thin films [11]. In this paper we examined the NO 2 sensing properties of SiO 2 - mixed WO 3 composite thin films prepared by a sol-gel method. 2. Experimental Thin films were fabricated on an alumina substrate (unpolished, 9× 13 mm 2 ) with comb-shaped gold elec- trodes (area 9 ×9mm 2 ) by a spin-coating technique. Coating solutions were prepared as follows. Te- traethoxysilane was mixed with ethanol and water (1:4:4 in molar ratio), together with a trace amount of HCl catalyst, and stirred for 30 min. This solution (transparent) was further mixed with an aqueous solu- tion (2.5 wt%) of ammonium paratungstate ((NH 4 ) 10 W 12 O 41 ·5H 2 O) to designated proportions. A small amount of each coating solution thus obtained was dropped on the substrate, allowed to spread over by spinning (1500 rpm, 20 s) and dried at 350°C for 10 min. The same procedures were repeated 15 times to increase the film thickness up to about 0.8 mm, before firing was carried out at 550°C for 1 h. The SiO 2 content was set to 0, 5, 10 or 20 wt% to WO 3 and is denoted like SiO 2 (5%)– WO 3 hereafter. Thin films were characterized by means of XRD analysis (Riguku 4011) and AFM observation * Corresponding author. Tel.: +81 92 5837539 fax: +81 92 5752318. 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S09 2 5 -4 005(97)00 2 86 -4 X. Wang et al. / Sensors and Actuators B 45 (1997) 141 – 146 142 (Nanoscope IIIa; Digital Instruments Inc.). Sensing characteristics were measured in a conventional flow apparatus (gas flow rate 100 cm 3 min −1 ) in the temper- ature range of 250–450°C. Gas sensitivity (S) was defined as R g /R a , where R a and R g are the electrical resistances in air and in the sample gas, respectively. 3. Results and discussion 3.1. Microstructure Fig. 1 shows the XRD patterns of a series of SiO 2 – WO 3 films prepared, together with that of the uncoated substrate. The XRD peaks other than those of Al 2 O 3 (substrate) and Au (electrode) were all assigned to a monoclinic phase of WO 3 (JCPDS 43-1035), the inten- sity pattern being very similar to that of the WO 3 powder obtained from ammonium paratungstate by pyrolysis. It was also obvious that the SiO 2 component included was amorphous. The XRD peaks of WO 3 tended to become broader with increasing SiO 2 content. The average grain size of WO 3 was estimated from the full width at half maximum of the strongest peak (200) at 2q =24.2° based on Sherrer’s equation. As shown in Fig. 2, the grain size decreased with an increase in SiO 2 content, indicating that the growth of WO 3 grains was suppressed by SiO 2 . Fig. 2. Average grain size of WO 3 evaluated from XRD as a function of SiO 2 content. Fig. 3 shows AFM images of the thin films. The bright spots, corresponding to the grains of WO 3 , were 50– 100 and 30– 50 nm in diameter for WO 3 (a) and SiO 2 (20%)– WO 3 (b), respectively. These values are in fair agreement with those obtained from the XRD peak above, if one considers the tip size (15 nm in diameter) of AFM probe. 3.2. NO 2 sensing characteristics The electrical resistance of each film in air (R a ) and in 0.4 ppm NO 2 -containing air (R g ) as well as the resulting sensitivity to 0.4 ppm NO 2 are shown as a function of temperature in Fig. 4. As already reported [7 –10], R g was always larger than R a , indicating anionic adsorp- tion of NO 2 on the surface of WO 3 (n-type oxide). At a fixed temperature, both R a and R g tended to increase with an increase in SiO 2 content except that SiO 2 (5%)– WO 3 had larger R a and R g values than WO 3 . However, R g was more dependent on SiO 2 content than R a , giving Fig. 1. XRD patterns of WO 3 ,WO 3 +5wt% SiO 2 ,WO 3 + 10wt% SiO 2 and WO 3 +20wt% SiO 2 thin films deposited on alumina substrates with Au electrodes. Fig. 3. AFM images of thin films (a): WO 3 , (b): SiO 2 (20%)–WO 3 . X. Wang et al . / Sensors and Actuators B 45 (1997) 141 – 146 143 Fig. 4. NO 2 sensing properties of SiO 2 –WO 3 thin films as a function of temperature. SiO 2 content: 0% (1), 5% (2), 10% (3), 20% (4). (a): Resistance in air (R a ) or 0.4 ppm NO 2 containing air (R g ); (b): sensitivity to 0.4 ppm NO 2 in air. seen to consist of two steps, although such steps be- came less visible at 0.2 and 0.4 ppm NO 2 and totally disappeared at 0.8 ppm and above. The same two-step behavior at low NO 2 concentrations was also observed with the other films, and is considered to appear from the coexistence of strong and weak adsorption of NO 2 as described shortly. The correlations between normalized resistances (R a / R g ) and NO 2 concentrations for the films of pure WO 3 and SiO 2 (5%)– WO 3 at 350°C are shown in Fig. 6. R g /R a was almost linear to the NO 2 concentration and the slope of SiO 2 (5%)– WO 3 was almost twice as steep as that of WO 3 , indicating the higher sensitivity of the former film. Since R g /R a should be unity at zero con- centration of NO 2 , the correlations should have a sharp inflection in the low concentration range as schemati- cally illustrated by a dotted line. Qualitatively speaking, such an inflection can be accounted for by assuming two types of NO 2 adsorption, i.e. one type which is strong in bonding but small in capacity and the other which is weak but large in capacity. It is considered that the saturation of the first type adsorption at a low concentration of NO 2 gives rise to the inflection men- tioned above. It follows that these two types differ in adsorption kinetics, giving rise to the two-step response transients at lower NO 2 concentrations. Correspond- ingly the replots of the same data in a log-log scale (Fig. 7) gave the correlations, the slopes of which were close to unity in the higher NO 2 concentration range and decreased with decreasing NO 2 concentration. For comparison, the data of the sintered block type element of WO 3 measured at 300°C are also indicated in the same figure. Roughly speaking, the sensitivity decreases to about 1/2 with a rise of temperature by 50°C. Taking this into account, the present thin film of SiO 2 (5%)– WO 3 is estimated to have higher sensitivity than the sintered block of WO 3 by more than one order of magnitude. 3.3. Selecti 6ity and stability A practical NO 2 sensor should be resistant enough to interferences by coexistence gases. Thus the sensing properties of SiO 2 (5%)– WO 3 to 0.4 ppm NO 2 in air were examined under the coexistence of 100 ppm CO, 100 ppm CO 2 , or 5000 ppm H 2 O at 350°C. Fig. 8 shows the influence of these gases on R a and R g . These gases tended to reduce R a slightly or significantly (H 2 O), while such tendencies were less pronounced for R g values. As seen from Fig. 8, the NO 2 sensing properties were fairly stable to the coexistence of CO and CO 2 . However, the interference by water vapor was signifi- cant, needing further investigations to overcome it. To examine the stability of NO 2 sensing perfor- mance, the SiO 2 (5%)– WO 3 film was exposed repeat- edly to 0.4 ppm NO 2 at a frequency of three times a rise to an increase in sensitivity (S) at a fixed tempera- ture. With lowering temperature, R g increased more steeply than R a , leading to a rather sharp increase in S. As seen from Fig. 4, S tails off at about 500°C when temperature is increased. On the other hand, lowering temperature resulted in a sharp loss in the rates of response and recovery. As a trade-off between the sensitivity and the response kinetics, an operating tem- perature of 350°C was selected in the subsequent study. Table 1 summarizes some sensing characteristics of the composite films to 0.4 ppm NO 2 in air at 350°C. The sensitivity (S) tended to increase gradually with increasing SiO 2 content. The times of 90% response and 90% recovery were rather large in all cases, being 5– 7 and 7–15 min, respectively. Nevertheless SiO 2 (5%)– WO 3 showed the shortest recovery time of 7 min. From this feature as well as the rather high sensitivity, this film was selected for further investigations of NO 2 sensing properties. Fig. 5 shows the response and recovery transients of SiO 2 (5%)– WO 3 film on switching on and off various concentrations of NO 2 in air between 0.1 and 2.0 ppm at 350°C. The response transient to 0.1 ppm NO 2 is X. Wang et al. / Sensors and Actuators B 45 (1997) 141 – 146 144 Table 1 Sensing characteristics of SiO 2 –WO 3 thin films at 350°C 90% Recovery time (min)Resistance (R a , V) 90% Response time (min)Sensitivity a (R a /R g )Film 4.7 6WO 3 128.6×10 5 57SiO 2 (5%)–WO 3 3.2×10 5 8.1 8.2 7SiO 2 (10%)–WO 3 1.7×10 6 15 11.2 1151.9×10 6 SiO 2 (20%)–WO 3 a To 0.4 ppm NO 2 in air. day for 10 days. As shown in Fig. 9, both R g and R a were fairly stable over the whole test period. This confirms that the film is stable enough at least for a short term. 3.4. Imprecation of SiO 2 –WO 3 composite films In the present preparation conditions of SiO 2 –WO 3 composite films, SiO 2 was deposited through a sol-gel process [3]. On the other hand, it was confirmed by XRD analysis that WO 3 took form through the crystal- lization of WO 3 · 0.33H 2 O from the solution followed by its dehydration. It is considered that the dispersion of fine particles of SiO 2 hinders the grain growth of WO 3 · 0.33H 2 OorWO 3 , leading to a decrease in WO 3 grain size with increasing SiO 2 content (Fig. 2). It is rather striking that the decreasing behavior of grain size is exactly reflected by the sensitivity behavior of the films (Table 1). This means that the sensitivity to NO 2 is primarily determined by the grain size of WO 3 in agreement with literature [3]. The higher sensitivity of the SiO 2 –WO 3 films than that of the WO 3 sintered block previously reported [2] by about one order of magnitude or more may also be ascribable primarily to a difference in grain size. Apart from the grain size of WO 3 , the dispersion of SiO 2 would also change the microstructure of the film, e.g. porosity and pore size distribution which affects the rate of response. In this respect, the SiO 2 (5%)– WO 3 film showing the shortest response and recovery times appears to have the mi- crostructure most porous for NO 2 molecules to diffuse inside and outside. However, the response kinetics were not quick enough even with this film. The microstruc- ture would depend not only on the SiO 2 content but also on many other factors, e.g. SiO 2 particle size, agglomeration state and film thickness, which are all related with the methods and conditions of film prepa- ration. Further studies to optimize the microstructure from these viewpoints would be necessary. As mentioned above, incorporation of SiO 2 in the WO 3 sensing film to form SiO 2 –WO 3 composite was fairly effective in suppressing the grain growth of WO 3 and in enhancing the porous structure. These effects have lead to higher sensitivity and quicker response, Fig. 5. Response transients of SiO 2 (5%)–WO 3 thin film on turning on and off various concentrations of NO 2 diluted in air. Fig. 6. Normalized resistance of thin films under exposure to various concentrations of NO 2 in air (350°C). X. Wang et al. / Sensors and Actuators B 45 (1997) 141 – 146 145 Fig. 7. Normalized resistance vs. concentration correlations in a log–log scale for WO 3 -based thin films (350°C) and sintered block (300°C). Fig. 9. Short-term stability of electrical resistance of SiO 2 (5%)–WO 3 film sensor in air and in 0.4 ppm NO 2 at 350°C. Acknowledgements This work was partially supported by a Grant-in- Aid for Scientific Research from The Ministry of Ed- ucation, Science, Sports and Culture of Japan, and a grant from Steel Industry Foundation for the Ad- vancement of Environmental Protection Technology. References [1] N. Yamazoe, N. Miura, Environmental gas sensing, Sensors and Actuators B 20 (1994) 95–102. [2] M. Akiyama, J. Tamaki, N. Miura, N. Yamazoe, Tungsten oxide-based semiconductor sensor highly sensitive to NO and NO 2 , Chem. Lett. 1 (1991) 1611–1614. [3] J. Tamaki, Z. Zhang, K. Fujimori, M. Akiyama, T. Harada, N. Miura, N. Yamazoe, Grain-size effects in tungsten oxide-based sensor for nitrogen oxides, J. Electrochem. Soc. 141 (1994) 2207–2210. [4] G. Sberveglieri, G. Faglia, S. Groppelli, P. Nelli, Methods for the preparation of NO, NO 2 and H 2 sensors based on tin oxide thin films, grown by means of the r.f. magnetron sputtering technique, Sensors and Actuators B 8 (1992) 79–88. [5] N. Miura, S. Yao, Y. Shimizu, N. Yamazoe, New auxiliary sensing materials for solid electrolyte NO 2 sensors, Solid State Ionics 70/71 (1994) 572–577. [6] M.S. Nieuwenhuizen, A.J. Nederlof, Preliminary results with a silicon-based surface acoustic wave chemical sensor for NO 2 , Sensors and Actuators 19 (1989) 385–392. [7] T. Inoue, K. Ohtsuka, Y. Yoshida, Y. Matsuura, Y. Kajiyama, Metal oxide semiconductor NO 2 sensor, Sensors and Actuators B24/25 (1995) 388–391. [8] C. Cantalini, H.T. Sun, M. Faccio, M. Pelino, S. Santucci, L. Lozzi, M. Passacantando, NO 2 sensitivity of WO 3 thin film obtained by high vacuum thermal evaporation, Sensors and Actuators B 31 (1996) 81–87. [9] P. Nelli, L.E. Depero, M. Ferroni, S. Groppelli, V. Guidi, F. Ronconi, L. Sangaletti, G. Sberveglieri, Sub-ppm NO 2 sensors based on nanosized thin films of titanium-tungsten oxides, Sen- sors and Actuators B 31 (1996) 89–92. [10] N. Yamazoe, N. Miura, Some basic aspects of semiconductor gas sensors, in: S. Yamauchi (Ed.), Chemical Sensor Technology, vol. 4, Kodansha, Tokyo, 1992, pp. 19–42. respectively. Although the NO 2 sensing performance achieved here are not good enough, still it is expected that environmental monitoring of NO 2 (standard 40 ppb) would be possible by elaborating such composite films further. 4. Conclusions The preparation of SiO 2 –WO 3 thin films by a sol- gel method was useful for enhancing the NO 2 sensing characteristics. The grain growth of WO 3 was sup- pressed more intensively with increasing SiO 2 content, whereas the film microstructure was brought to be most easily accessible by NO 2 molecules at 5wt% SiO 2 . The SiO 2 (5%)– WO 3 film showed fairly good sensing performances to NO 2 in air in the range of 0.1– 2 ppm. Fig. 8. Influences of coexistent CO 2 ,COorH 2 O on the resistance of SiO 2 (5%)–WO 3 film in air (R a ) and in 0.4 ppm NO 2 -containing air (R g ) at 300°C. X. Wang et al. / Sensors and Actuators B 45 (1997) 141 – 146 146 [11] X. Yao, L. Zhang, S. Wang, Pore size and pore-size distribution control of porous silica, Sensors and Actuators B 24/25 (1995) 347–352. [12] L. Armelao, R. bertoncello, G. Granozzi, G. Depaoli, E. Ton- dello, G. Battaglin, High purity WO 3 sol-gel coatings: synthesis and characterization, J. Mater. Chem. 4 (1994) 407–411. Biographies Xusheng Wang has been an associate professor at Xidian University of China since 1993. He received the MSc degree in 1985 from the university. He has been with Xian Jiaotong University since 1994. Now he is with Kyushu University as a visiting researcher. His current research interests the ferroelectric and semicon- ducting functional materials and devices. Go Sakai has been a research associate at Kyushu University since 1996. He received the BEng degree in Applied Chemistry in 1991 and a DEng degree in 1996 from Kyushu University. His current research works is focused on the development of chemical sensor based on semiconductive oxides. Kengo Shimanoe has been a research associate at Kyushu University since 1995. He received the BEng degree in Applied Chemistry in 1983 and MEng degree in 1985 from Kagoshima University and Kyushu Uni- versity, respectively. He joined the advanced material and technology laboratory of Nippon Steel Corp. and has studied the electronic characterization on semicon- ductor surface and the electrochemical reaction on ma- terials. He received a DEng degree in 1993 from Kyushu University. His current research interests in- clude the development of chemical sensors and ECD as well as the analysis of solid surface. Norio Miura has been an Associate Professor at Kyushu University since 1982. He received the BEng degree in Applied Chemistry in 1973, MEng degree in 1975 from Hiroshima University and the DEng degree in 1980 from Kyushu University. His current research concentrates on development of new chemical sensors as well as other electrochemical functional devices such as ECD and secondary batteries. Noboru Yamazoe has been a professor at Kyushu University since 1981. He received the BEng degree in Applied Chemistry in 1963 and a DEng degree in 1969 from Kyushu University. His current research interests include the development and application of functional inorganic materials. . . Sensors and Actuators B 45 (1997) 141–146 Spin-coated thin films of SiO 2 –WO 3 composites for detection of sub-ppm NO 2 Xusheng Wang, Go Sakai, Kengo. transients of SiO 2 (5%)–WO 3 thin film on turning on and off various concentrations of NO 2 diluted in air. Fig. 6. Normalized resistance of thin films under