Sensors and Actuators B 135 (2009) 568–574 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Highly sensitive NO 2 sensors using lamellar-structured WO 3 particles prepared by an acidification method Tetsuya Kida a , Aya Nishiyama b , Masayoshi Yuasa a , Kengo Shimanoe a,∗ , Noboru Yamazoe a a Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, Japan b Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, Japan article info Article history: Received 13 May 2008 Received in revised form 24 September 2008 Accepted 28 September 2008 Available online 1 November 2008 Keywords: NO 2 WO 3 Lamella Acidification method abstract Tungsten trioxide (WO 3 ) was prepared by acidification of Na 2 WO 4 with acid solutions such as H 2 SO 4 , HCl, and HNO 3 (pH 0.5 to −0.8) and tested for its NO 2 sensing properties. Acidificationwithstrong acid solutions (pH −0.5, −0.8) was found to produce lamellar-structured WO 3 particles, which consisted of nano-sized crystalline plates that were 100–350 nm in lateral size and 20–50 nm in thickness, as observed by XRD and SEM analyses. The sizes of the primary and secondary particles were decreased by decreasing the pH of the acid solution used. This was accompanied by an increase in the specific surface area. The NO 2 responses of the prepared WO 3 lamellae were dependent on their morphology. The device using smaller WO 3 lamellae prepared with a H 2 SO 4 solution (pH −0.8) had the highest sensor response, exhibiting a high sensor response (S = 150–280), even to dilute NO 2 (50–1000 ppb) in air at 200 ◦ C. The use of smaller lamellae resulted in a decrease in the electrical resistance of the device, probably due to intimate contact between smaller lamellar particles, which allowed the detection of NO 2 in a rather wide concentration range. In addition, the developed device showed high NO 2 selectivity without substantial interference from NO. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The continuous detection and monitoring of NO 2 in the atmo- sphere have become highly important because of its toxic effects on both animals and plants. There has been a high demand for compact, cheap, and preferably portable devices able to detect low levels (ppb level) of NO 2 in the atmosphere, since available analyti- cal instruments based on Saltzman or chemiluminescence methods are large and expensive. Such a demand for high-performance NO 2 sensors is rapidly growing for other applications. It isenvisaged that in the next few years, an automatic damper (ventilation) system will be introduced in cars. This system needs a compact sensor that can monitor NO 2 inside and outside in a rather wide concentration range, from ppb to several ppm levels. Thus far, several solid- state NO 2 sensors, such as resistive [1–6], potentiometric [7–9], amperometric [10–13], capacitive [14,15], optic [16,17], and surface acoustic wave (SAW) types [18] have been developed. In particu- lar, resistive-type NO 2 sensors based on oxide-semiconductors are well-suited for the above applications due to their superior prop- erties and simple structure, and as such they have been intensively studied for about 20 years [1–6]. Through an extensive search for ∗ Corresponding author. Tel.: +81 92 583 7876; fax: +81 92 583 7538. E-mail address: simanoe@mm.kyushu-u.ac.jp (K. Shimanoe). NO 2 -sensitive materials, tungsten trioxide (WO 3 ) has been found to show very promising NO 2 sensing properties [4,5]. Notably, WO 3 -based sensors can detect dilute NO 2 in air without significant interference from CO 2 , methane, CO, or H 2 at low temperatures like 200 and 300 ◦ C [4]. It is important to note that the sensor response of WO 3 depends significantly on the preparation method. Many preparation routes for WO 3 sensors have been reported, including sputtering [19], vacuum evaporation [20], pulsed-laser deposition [21], sol–gel [22–25], pyrolysis [4,5,26], photochemical [27], and ion-exchange methods [28–31]. We have previously reported in a series of papers that thick and thin film devices using lamellar-structured WO 3 particles with nano-sized thickness, which were prepared by an ion-exchange method using a protonated cation-exchange resin and subsequent heat treatment, exhibited excellent NO 2 sensing properties at 200–300 ◦ C [28–31]. It was found that the sensor response was sig- nificantly increased with a decrease in the thickness of the WO 3 lamellae and was well-correlated with its thickness [30]. 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 that were ca. 30 nm in thick- ness and 1 m in lateral dimension were used for the sensing film [31]. However, the electrical resistance of the sensing film was fairly high, exceeding 10 8 at 200 ◦ C in response to even low concen- 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.09.056 T. Kida et al. / Sensors and Actuators B 135 (2009) 568–574 569 trations of NO 2 , e.g., 500 ppb [30]. Such a high electrical resistance makes measuring the sensor signal with the simple electric circuits that are typically used for commercial resistive sensors difficult, which hinders the detection of NO 2 at higher concentrations (sub ppm level). In addition, a high electrical resistance can sometimes be a source of noise in the sensor signal. Although the resistance can be decreased by raising the operation temperature, this is coun- terbalanced by a decrease in the sensor response. In this study, in order to decrease the sensor resistance and to extend the detectable concentration range, we prepared smaller WO 3 lamellae by the acidification of WO 4 2− in a strong acid solution. We expected that nano-sized lamellae would contact more closely to each other than micro-sized lamellae, leading to a decrease in the sensor resistance. Acidification of W-polyanions is known to produce WO 3 ·2H 2 Ocrys- tals, a precursor of WO 3 [32–35]. The acidification method was found to be able to tune the size of WO 3 lamellae by changing the pH of the acid solutions used, as described below. 2. Experimental WO 3 particles were prepared by acidification of Na 2 WO 4 and subsequent calcination of the resulting precipitates. A Na 2 WO 4 solution was added drop by drop into an acidic solution under vig- orous stirring. The two solutions were mixed so as to set the molar ratio of Na + /H + at 1/10. The pH of the acidic solution was controlled with H 2 SO 4 , HNO 3 , and HCl to be between 0.5 and −0.8. The mix- ing quickly produced a yellow gel (crystalline WO 3 ·2H 2 O), which wasagedfor1dayat30 ◦ C. The gel was washed thoroughly with distilled water by centrifugation. The structure of the fabricated sensor device is shown in Fig. 1. 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 m; distance between lines: 90 m; sensing layer area: 64 mm 2 ). The Au electrodes were also fabricated by a screen-printing method using a commercial Au paste followed by calcination at 850 ◦ C. The paste deposited on the substrates was calcined at 300 ◦ C for 2 h in air to form a WO 3 sensing layer via the dehydration of the precursor, WO 3 ·2H 2 O. The surface morphology ofthe samples was analyzed with a field emission scanning electron microscope (FE-SEM). The thickness of the films was estimated to be 15–25 m by FE-SEM observa- tions. The crystal structure and specific surface area of the samples Fig. 1. Schematic structure of a NO 2 sensor device, in which a WO 3 thick film is deposited on an alumina substrate equipped with a pair of comb-type microelec- trodes. were measured using an X-ray diffractometer (XRD) with copper K␣ radiation 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 50–1000 ppb in air. Measurements were performed using a conventional gas flow apparatus equipped with an electric furnace at a gas flow rate of 100 cm 3 /min. The sensor response (S) was defined as the ratio of resistance in aircontaining NO 2 (R g ) to that in dry air (R a )(S = R g /R a ). 3. Results and discussion 3.1. Crystal structure and microstructure Fig. 2 shows the XRD patterns of WO 3 particles prepared using the acidification method using H 2 SO 4 with different pH solutions (pH 0.5 to −0.8) and subsequent calcination at 300 ◦ C. All XRD peaks were assigned to monoclinic WO 3 (JCPDS 43–1035) for all samples. Other peaks are ascribable to Si, which was mixed with samples as an internal standard for the XRD measurements. It is noted that (0 0 1)-oriented WO 3 particles were formed when a H 2 SO 4 solution with pH 0.5 was used. On the other hand, such a preferential orientation in the (0 0 1) plane became weak, i.e., the intensity of the (0 0 2) peak decreased with a decrease in the pH of the solution while those of the (0 2 0) and (2 0 0) peaks increased. In a previous report, we found that (0 0 1)-oriented WO 3 particles were formed by dehydrating (0 1 0)-oriented WO 3 ·2H 2 O crystallites, which were prepared via the acidification of Na 2 WO 4 with a protonated cation-exchange resin and repeated washing- centrifugation treatments [28]. Quite strikingly, the preferential orientation in layered WO 3 ·2H 2 O was preserved even through the dehydration step leading to WO 3 . Thus, in the present case, the observed (0 0 1) orientation in WO 3 at pH 0.5 also reflects the pref- erential (0 1 0) orientation in the precursor WO 3 ·2H 2 O. On the other hand, the observed loss in the crystal orientation at lowe r pH can be interpreted as follows: since WO 3 ·2H 2 O is formed through the con- densation of W-polyanions by acidification, the condensation rate, which depends on the pH of the precursor solution, may affect the orientation of the WO 3 ·2H 2 O crystallites. It is thus suggested that with a significant decrease in pH of the solution from 0.5 to −0.8, Fig. 2. XRD patterns of WO 3 particles prepared using H 2 SO 4 solutions with different pH (a) 0.5, (b) 0, (c) −0.5, and (d) −0.8. 570 T. Kida et al. / Sensors and Actuators B 135 (2009) 568–574 Fig. 3. FE-SEM images of WO 3 particles prepared using H 2 SO 4 solutions with different pHs (a) 0.5, (b) 0, (c) −0.5, and (d) −0.8. the crystal growth rate in other directions such as (2 0 0) and (0 2 0) became dominant, as observed in the XRD patterns. However, it remains uncertain how such a difference in the direction of the crystal growth occurred at different pHs. Further studies are thus necessary to draw conclusions. Fig. 3 shows FE-SEM images of WO 3 particles prepared using H 2 SO 4 solutions with different pHs (pH 0.5 to −0.8) and subsequent calcination at 300 ◦ C. The morphology of the particles differed con- siderably depending on the pH of the precursor solution. Particles prepared at pH 0.5 were leaf shaped and were ca. 2.5 m in lat- eral size and 0.2–0.5 m in thickness, as estimated from the image (Fig. 3(a)). The size of the particles is in almost the same range as those prepared by the ion-exchange method [28]. Balazsi and Pfeifer also reported a similar morphology of WO 3 ·2H 2 O crystals prepared by an acidification method [33]. Conversely, particles pre- pared at lower pHs were of a rectangular platelet shape (lamellar) and the size of the particles decreased with a decrease in the pH of the precursor solution. In accordance with the decrease in the lateral size, the thickness also decreased, as observed in the images (Fig. 3(b–d)). The estimated lateral sizes and thicknesses of lamellar particles are summarized in Table 1, together with the specific sur- face areas and the crystalline sizes obtained from XRD peaks using Scherrer’s equation: D = 0.9 (B cos Â) (1) where D represents mean crystalline size, B stands for full width at half maximum of the peak, is the wavelength of the X-ray, and  is the center angle of the peak. In the estimation of crystalline size, XRD peaks around 22–26 ◦ are decomposed into three components resulting from the (0 0 2), (0 2 0), and (2 0 0) planes. Apparently, the surface area of lamellar particles was increased with a decrease in the lateral size and thickness. The crystal size was decreased with decreasing precursor solution pH. It appears that acidification of WO 4 − in a strong acid solution yields nano-sized lamellar WO 3 particles. The obtained results suggest that, in a highly acidic solu- tion, the rate of crystal nucleation may be faster than that of crystal growth, resulting in a decrease in the crystalline size. WO 3 particles were also prepared using different acid solutions including HNO 3 and HCl. Figs. 4 and 5 show the XRD patterns Table 1 Particle size, crystalline size, specific surface area, and resistance in air of WO 3 particles prepared by the acidification method. Acid solution Particle size (nm) Crystallite size (nm) Specific surface area (m 2 /g) Resistance in air () Name pH Diameter Thickness (2 00) (0 2 0) (0 0 2) HNO 3 −0.5 100–300 30–50 24.4 20.0 14.4 10.8 1.16E+06 HCl −0.5 100–250 30–50 25.2 20.0 14.2 11.0 1.34E+06 H 2 SO 4 0.5 ca. 2500 150–300 28.9 22.3 21.7 7.9 2.63E+05 0.0 200–450 35–60 26.1 21.7 14.4 6.9 1.18E+06 −0.5 100–350 30–50 19.5 20.5 9.1 9.0 1.11E+06 −0.8 100–350 20–50 17.0 19.5 10.7 18.8 6.74E+05 T. Kida et al. / Sensors and Actuators B 135 (2009) 568–574 571 Fig. 4. XRD patterns of WO 3 particles prepared using different acid solutions (pH −0.5). (a) H 2 SO 4 , (b) HNO 3 , and (c) HCl. and FE-SEM images, respectively, of WO 3 particles prepared by the acidification method using HCl and HNO 3 solutions (pH −0.5) and subsequent calcination at 300 ◦ C. Their specific surface area and estimated cr ystal size are also summarized in Table 1. No significant changes in the physical properties such as morphology, crystalline size, and surface area were observed when WO 3 was prepared using different acid solutions with the same pH. It can be concluded that the presence of anions such as SO 4 − ,Cl − , and NO 3 − has no drastic influence on the physical properties of the WO 3 particles. 3.2. Electrical properties The value of electrical resistance is one of the important param- eters for resistive-type semiconductor gas sensors. If the electrical resistance is too high, reliable gas detection becomes difficult due to the generation of noise in sensor devices based on bridge cir- cuits. Such a problem becomes more serious for the detection of oxidizing gases, such as NO 2 and O 3 , using n-type semiconductors, in which target gases are detected by a sharp increase in resistance. This is in contrast to the detection of reducing gases such as H 2 and CO, in which target gases are detected by a sharp decrease in resistance. Thus, it is difficult to evaluate higher NO 2 concentra- tions with a conventional WO 3 -based sensor with a high electrical resistance. Fig. 6 shows the dependence of the electrical resistance on NO 2 concentration at 200 ◦ C for various devices using WO 3 particles pre- pared with different acid solutions (pH −0.5), together with that for the device using WO 3 particles prepared by the ion-exchange method. The electrical resistances in air of the WO 3 particles used are listed in Table 1. Obviously, the resistances were successfully decreased by almost one order of magnitude when smaller WO 3 lamellae, prepared by the acidification method, were used. The decreases in the resistance were observed for all devices using WO 3 lamellae in the same size range. The observed decrease in the resistance is suggested to originate from the formation of intimate contacts among lamellae by decreasing their lateral size. Fig. 5. FE-SEM images of WO 3 particles prepared using different acid solutions (pH −0.5). (a) H 2 SO 4 , (b) HNO 3 , and (c) HCl. 572 T. Kida et al. / Sensors and Actuators B 135 (2009) 568–574 Fig. 6. Dependence of the electrical resistance on NO 2 concentration at 200 ◦ Cfor the devices using WO 3 lamellae prepared by the acidification method using different acid solutions (pH −0.5). (a: solid triangle) H 2 SO 4 , (b: solid circle) HNO 3 , and (c: mark) HCl.Fo r comparison, the electrical resistance of thedevice using WO 3 lamellae prepared by the ion-exchange method is also shown. 3.3. NO 2 sensing properties The NO 2 sensing properties of the sensor devices were exam- ined at 200 ◦ C. Fig. 7 shows the sensor response as a function of NO 2 concentration for the devices fabricated with a H 2 SO 4 solu- tion at different pH values. For comparison, the sensor response of the device fabricated by the ion-exchange method is also plotted. It was found that a large NO 2 response was obtained using nano- sized WO 3 lamellae, as compared to the case using micro-sized lamellae prepared via the ion-exchange method. The device using the smallest WO 3 lamellae prepared with a H 2 SO 4 solution at pH −0.8 showed the highest sensor response (Fig. 7(d)). It has been Fig. 7. Sensor response (S = R g /R a ) as a function of NO 2 concentration at 200 ◦ Cfor the devices using WO 3 lamellae prepared using H 2 SO 4 solutions with different pH values. (a: mark) pH 0.5, (b: open triangle) 0, (c: solid circle) −0.5, and (d: open circle) −0.8. For comparison, the sensor response of the device using WO 3 lamellae prepared by the ion-exchange method is also shown. proposed that the adsorption of NO 2 on the n-type semiconductor WO 3 induces electron-depleted space-charge layers inside the WO 3 surfaces [36]. For nano-sized thin WO 3 lamellae, a whole region of the lamellae can be occupied by the space-charge layer upon NO 2 adsorption (full depletion). This significantly increases the dou- ble Schottky barrier heights at the boundaries between lamellae, resulting in a large increase in the electrical resistance. Such an effect can explain why thinner nano-sizedlamellae exhibit a greater response, i.e., a large resistance change upon NO 2 adsorption. It is noteworthy that the developed devices with lower electri- cal resistances can detect NO 2 in a wide concentration range of 50–1000 ppb. This feature is very important when the sensor is applied to environmental monitoring or air-quality control. How- ever, the sensor response was gradually saturated at higher NO 2 concentrations. This may be explained in terms of the full depletion of small WO 3 lamellar crystals induced by formation of space- charge layers in the whole region of the crystals upon higher NO 2 adsorption. It is thought that the ratio of the number of fully depleted crystals to that of non-fully depleted cr ystals increases rapidly with decreasing crystalline size. Consequently, the proba- bility of finding pairs of neighboring non-fully depleted crystals, and of connecting a conductive path to the next, would decrease sharply with decreasing crystalline size. In this case, although some small WO 3 lamellae have the capability of NO 2 adsorption, most conductive paths are interrupted by fully depleted crystals. This situation may bring about the observed saturation of the sensor response (resistance change) at higher NO 2 concentrations. How- ever, the sensor response of the device (c) showed such saturation at higher NO 2 concentration than 500 ppb. The possible explana- tion seems that the NO 2 adsorption sites on WO 3 lamellae were decreased by the agglomeration of WO 3 lamellae. As can be seen in Table 1, the surface area of the device (c) was about halves of the device (d), although the crystallite size of the device (c) was the almost same as that of the device (d). In short, it is expected that the surface of agglomerated lamellae would not supply more sites for NO 2 adsorption, meaning that further extension of space charge layer was difficult. Fig. 8 shows the sensor response of devices fabricated with three different acid solutions at pH −0.5 as a function of NO 2 concen- tration. These devices also responded to dilute NO 2 and showed a Fig. 8. Sensor response (S = R g /R a ) as a function of NO 2 concentration at 200 ◦ Cfor the devices using WO 3 lamellae prepared with different acid solutions (pH −0.5). (a: solid circle) H 2 SO 4 , (b: solid triangle) HNO 3 , and (c: mark) HCl. T. Kida et al. / Sensors and Actuators B 135 (2009) 568–574 573 Fig. 9. Response transients to NO 2 at 200 ◦ C of the device using WO 3 lamellae pre- pared with a H 2 SO 4 solution (pH −0.8). sufficient ability to detect ppb level NO 2 in the atmosphere. How- ever, the sensor response of the devices differed depending on the acid solutions used for preparing the WO 3 particles. The sensor fab- ricated with HNO 3 showed the best NO 2 response, but the device made using HCl showed a lower response. Since the physical prop- erties of the sensing layers, such as particle size, crystalline size, and surface area, are not ver y different among the three devices, it is difficult to account for the observed difference on the basis of these properties. As possible reasons, differences in surface proper- ties such as composition, porosity, and remaining impurities such as Cl − can be considered. However, further experimental evidence is required to elucidate the reasons for this. As noted above, the preparation using a strong acid solution (H 2 SO 4 ,pH−0.8) produced the most NO 2 -sensitive WO 3 lamellae, which were 30–50 nm in thickness. To explore the possibilities of using this material, other NO 2 sensing properties, such as response speed and selectivity, were examined. Response transients of the device using WO 3 lamellae prepared with a H 2 SO 4 solution at pH −0.8 are shown in Fig. 9. The device responded reversibly to NO 2 with changes in the resistance at 200 ◦ C. However, the response was rather sluggish, suggesting slow adsorption and desorption ratesofNO 2 . Times for 90% response and recovery were 4 and Fig. 10. Sensor response (S = R g /R a ) as a function of NO x ((a) NO 2 and (b) NO) concen- tration at 200 ◦ C for the device using WO 3 lamellae prepared with a H 2 SO 4 solution (pH −0.8). 11 min, respectively, for 1000 ppb NO 2 at 200 ◦ C. Such a sluggish response seems to be due to close packing of the particles. As can be seen in Fig. 3(d), there are few meso- and macro-pores use- ful for gas diffusion in the film for WO 3 lamellae prepared at pH −0.8. To improve the response and recovery response behaviors, the sizes and distribution of the pores and length of gas diffusion path (thickness or radius of secondary particles of grains) need to be optimized. Fig. 10 shows the dependence of the sensor response on NO x (NO and NO 2 ) concentration at 200 ◦ C for the device using WO 3 lamellae prepared with aH 2 SO 4 solution (pH −0.8). The device showed a much lower sensor response to NO (S < 2.3) than NO 2 in the 200–400 0 ppb concentration range. As highlighted in the liter- ature [5], NO is detected through the conversion of NO to NO 2 and subsequent NO 2 adsorption onto WO 3 . Thus, the obtained results suggest that WO 3 lamellae prepared by the acidification method have little catalytic activity for converting NO to NO 2 at 200 ◦ C. It is also possible that the improved response to NO 2 successfully diminished the interference from NO. 4. Conclusion The acidification of NaWO 4 with a strong acid solution (pH −0.5 or −0.8) produced lamellar-structured WO 3 particles that were 100–350 nm in lateral size and 20–50 nm in thickness. The electri- cal resistance of the device was successfully reduced by one order of magnitude by using the nano-sized WO 3 lamellae, probably due to improved contact among lamellae. 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Int. 33 (2007) 931–936. [36] J. Tamaki, Z. Zhang, K. Fujimori, M. Akiyama, T. Harada, N. Miura, N. Yama- zoe, Grain-size effects in tungsten oxide-based sensor for nitrogen oxides, J. Elechem. Soc. 141 (1994) 2207–2210. Biographies Tetsuya Kida has been an Associate Professor at Kyushu University since 2006. He received his M. Eng. Degree in materials science in 1996 and his Dr. Eng. Degree in 2001 from Kyushu University. His current research interests include the devel- opment of chemical sensors, nano-particle synthesis methods, and self-assemble d inorganic–organic hybrid materials. Aya Nishiyama received her B. Eng. Degree in materials science in 2007 from Kyushu University. She is currently a Masters course student at the Department of Molecular and Material Sciences in Kyushu University. Masayoshi Yuasa has been an Assistant Professor at Kyushu University since 2005. He received his M. Eng. Degree in materials science in 2003. His current research interests include the development of chemical sensors and active electrocatalysts for oxygen reduction. Kengo Shimanoe has been a Professor at Kyushu University since 2005. He received a BE degree in Applied Chemistry in 1983 and a M. Eng. Degree in 1985 from Kagoshima University and Kyushu University, respectively. He joined Nippon Steel Corp. in 1985, and received a Dr. Eng. Degree in 1993 from Kyushu University. His cur- rent research interests include the development of gas sensors and other functional devices. Noboru Yamazoe had been a professor at Kyushu University since 1981 until he retired in 2004. He received his M. Eng. Degree in Applied Chemistry in 1963 and Dr. Eng. Degree in 1969 from Kyushu University. His research interests were directed mostly to the development and application of functional inorganic materials. . B: Chemical journal homepage: www.elsevier.com/locate/snb Highly sensitive NO 2 sensors using lamellar-structured WO 3 particles prepared by an acidification. thick and thin film devices using lamellar-structured WO 3 particles with nano-sized thickness, which were prepared by an ion-exchange method using a protonated