Sensitivity properties of a novel NO2 gas sensor based on mesoporous WO3 thin film

Sensors and Actuators B 96 (2003) 219–225

Sensitivity properties of a novel NO2 gas sensor based on mesoporous WO3 thin film L.G. Teoh a , Y.M. Hon a , J. Shieh b , W.H. Lai a , M.H. Hon a,∗ a

Department of Materials Science and Engineering, National Cheng Kung University, 1 Ta-Hsueh Road, Tainan 70101, Taiwan, ROC b National Nano Device Laboratories, 1001-1 Ta-Hsueh Road, Hsinchu 30050, Taiwan, ROC Received 19 December 2002; received in revised form 20 May 2003; accepted 27 May 2003

Abstract Mesoporous WO3 thin films micro-gas sensor was fabricated and the NO2 gas-sensing as well as electrical properties have been investigated. The film had nano-sized grains, porous structure with a relative surface area of 143 m2 /g as calcined at 250 ◦ C. Upon exposure to NO2 , the electrical resistance of a semiconducting mesoporous WO3 thin films is found to dramatically increase. The sensitivity of mesoporous WO3 thin film sensors is substantially higher than that from other reports. In addition, the mesoporous WO3 thin film sensor calcined at 250 ◦ C and operated at 35 ◦ C shows an excellent sensitivity of 23, as we know it is unique NO2 gas sensor which has the sensitivity at such a low temperature. © 2003 Elsevier B.V. All rights reserved. Keywords: Mesoporous WO3 ; NO2 gas sensor; Sol–gel process

1. Introduction The detection of NO2 is important for monitoring environmental pollution resulting from combustion or automotive emissions [1]. Existing gas sensor materials include semiconducting metal oxides [1], silicon [2,3] and organic materials [4,5]. Semiconducting metal oxides such as WO3 and SnO2 had been widely used for NO2 detection [1,6]. These sensors have to operate at 200–500 ◦ C in order to improve the sensitivity by enhancing the chemical reaction between gas and the sensor material [7,8]. Obviously, it would be desirable for many applications if the sensor could operate at temperatures <100 ◦ C or even at room temperature, especially for battery-operated devices. Recently it also has been reported that ZrO2 –SnO2 [9] and ZnO [10] materials can be used as H2 S and NH3 gas sensors at room temperature, respectively, but their sensitivity was low. WO3 was reported to exhibit promising electrical and optical properties for various applications like efficient photolysis, electrochromic devices, selective catalysts and gas sensors [6]. WO3 is an n-type semiconductor whose electron concentration is determined mainly by the concentration of stoichiometric defects such as oxygen vacancy like other metal oxide semiconductors. The first work on the feasibility of ∗ Corresponding author. E-mail address: [email protected] (M.H. Hon).

0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00528-8

WO3 thin films as a gas sensor was reported by Shaver [11] who observed that the conductivity of WO3 thin films changed greatly upon the exposure to the H2 ambient. Following this pioneering work, many works have been performed on the structural and electrical properties and sensing characteristics of WO3 thin films. These sensors have been reported to have good selectivity for low concentration NOx gas [12]. In this study, we developed a novel NO2 gas sensors based on mesoporous WO3 thin film to detect small concentration of NO2 at low operating temperatures. Li and Kawi [13] have shown that a linear relationship was found between the surface areas of SnO2 sensors and their sensitivities to 500 ppm of H2 . Accordingly, mesoporous WO3 with a higher surface area provides more surface adsorption sites for the reaction of NO2 gas, which is beneficial to the operating temperature and sensitivity of the sensor. Mesoporous materials are generally prepared by amphiphilic self-assembling surfactants as templates [14,15]. In this study, the mesoporous WO3 thin film sensors synthesized by sol–gel process using triblock copolymer as the template were reported. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) and conductivity measurements were used to characterize the microstructure and electrical properties of mesoporous WO3 gas-sensing films that were deposited by dipping on Al2 O3 substrate.

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Brunauer–Emmett–Teller (BET) surface areas were estimated over a relative pressure (P/P0 ) range from 0 to 1.0. Pore size distribution was obtained from the analysis of the adsorption branch of the isotherms using the Barrett– Joyner–Halenda (BJH) model. The pore volume was taken at the P/P0 = 0.983 signal point. The resistance of the films was obtained by measuring the current through the film at a constant voltage of 1 V and recorded by a multimeter (HP 3458 A). The samples under test were placed in a quartz chamber (85 cm3 ) and exposed to 3 ppm NO2 gas and 4000 ppm H2 , respectively. Gas-sensing properties of the films were studied at various operating temperatures Tg in the range of 35 ◦ C < Tg < 100 ◦ C. The sensitivity is defined as Rg /Ra , where Rg and Ra are the electric resistance in test gas and air, respectively.

2. Experimental Poly(alkylene oxide) block copolymer (BASF Pluronic EO100 PO64 EO100 , F127) was used as a template material. About 0.5 g of F127 copolymer was dissolved in 10 g of ethanol. Then 0.01 mole of the anhydrous tungsten chloride precursor, WCl6 (Aldrich 99.9%), was added into the F127 ethanol solution with vigorously stirring for 1 h. The resulting sol solution was gelled in an open Petri dish at 60 ◦ C in air. Alternatively, the sol solution can be used to prepare thin films on Al2 O3 substrate that was coated with Pt electrode by dip coating. The thin films can be dried within several hours at 60 ◦ C. The as-made bulk samples or thin films were calcined at 250 ◦ C for 5 h and then washed by ethanol to remove the residual block copolymer. X-ray powder diffraction (XRD) patterns were obtained on a Rigaku D/max-X-diffractometer using Cu K␣ radiation with Ni filter. Transmission electron microscopy (TEM) studies were carried out on a Hitachi Model HF-2000 electron microscope operating at 200 keV. The samples for TEM were prepared by directly dispersing the fine powders of the product onto 200 mesh Cu grids. The morphology of mesoporous WO3 films was observed by scanning electron microscope (SEM, Philips XL-40 FEG). The nitrogen adsorption and desorption isotherms at 77 K were measured using a Micromeritics ASAP 2010 system after the samples were vacuum-dried at 150 ◦ C for 10 h in N2 atmosphere.

3. Results and discussion 3.1. Microstructure characterizations Fig. 1 shows the XRD pattern of mesoporous WO3 thin films calcined at 250 ◦ C for 5 h indicating that this crystallographic nucleation actually occurs during the calcination, but is limited to formation of nanocrystallite domains. The diffraction peaks of the WO3 thin films are assigned based on monoclinic structure (JCPDS card no. 05-0363).

∆ WO 3 ∗ Substrate

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Intensity (arb. units)

∆

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∗

∆

∗

∆ ∗ ∆

∆ ∆

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2 θ (deg.) Fig. 1. X-ray diffraction pattern for a mesoporous WO3 thin film calcined at 250 ◦ C for 5 h.

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Fig. 2. SEM micrograph of mesoporous WO3 thin film calcined at 250 ◦ C for 5 h.

After employing Scherrer’s formula, the calculated grain size of WO3 is approximately 3.8 nm. These grains contact contributes to the gas-sensing properties of the mesoporous WO3 films (the smaller grain size increases gas sensitivity since the diameter is comparable with or less than the space charge region of the grain). Fig. 2 shows the SEM micrographs of the WO3 thin films calcined at 250 ◦ C for 5 h. The sample exhibits porous structure with a spherical powder of approximately 1.5 ␮m. It means that such a structure of film is likely to facilitate the adsorption process of NO2 molecules because of the capillary pore and large surface area. This implies the conclusion that this type of film will offer a good sensitivity to NO2 gas. The morphology of the mesoporous WO3 thin film was characterized by TEM. Fig. 3a shows a bright field TEM image, in which the pores with a mean size of ∼5 nm can be clearly observed. The size of the mesopores estimated by TEM is in agreement with the values determined from the adsorption data (BET). Selected-area electron diffraction patterns recorded on mesoporous WO3 that is characteristic of diffuse electron diffraction rings demonstrate that the walls of our material are made up of nanocrystallite. This is also supported by the dark field TEM image (Fig. 3b), which reveals that the framework consists of nanocrystals (the bright spots in the image correspond to WO3 nanocrystals, ∼3 nm) and agree with the result of grain size determination obtained from XRD analyses. The results lead to conclude that the crystallized WO3 is essential for obtaining high sensitivity or expected to afford higher sensitivity toward gas-sensing reactions. The pore size and the wall thickness can be estimated from TEM in 5.4 and 1.8 nm, respectively. Nitrogen adsorption–desorption isotherms exhibiting a type IV curve is shown in Fig. 4, which is characteristic of mesoporous WO3 [16]. Barrett–Joyner–Halenda (BJH) analyses show that the calcined mesoporous WO3 exhibits mean pore size of 5 nm (Fig. 4 inset). From the absolute adsorption, we can calculate a specific surface of 143 m2 /g. This underlines that most pores are really accessible from

Fig. 3. TEM images of mesoporous WO3 thin film calcined at 250 ◦ C for 5 h: (a) bright field TEM image; (b) dark field TEM image obtained on the same area of (a); (a) inset: selected-area electron diffraction pattern recorded on the sample.

the outside and the pore system is fully interconnected (from adsorption and desorption lines of N2 ). 3.2. Gas-sensing properties In order to check the sensitivity of WO3 sensors for the concentration of 3 ppm NO2 , WO3 sensors were maintained at various temperatures from 35 to 100 ◦ C. Fig. 5 shows that WO3 sensors respond to turning-on and turning-off NO2 flow by the reversible changes of electrical resistance. The resistance values in air decrease with a rise in operating temperature, which is a typical characteristic of ceramics. When the NO2 was introduced into the test chamber, the resistance of sensor increased and soon afterwards it became saturated. When the gas was turning-off, the resistance of

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80

1.6

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Pore volume (cc/g)

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0.8

0.4

0 10

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Pore diameter,(Å) 20 0

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Relativep ressure (P/P0) Fig. 4. Nitrogen adsorption(+)–desorption(䊊) isotherms and BJH pore size distribution curves for mesoporous WO3 calcined at 250 ◦ C for 5 h.

the sensor decreased. The time response of WO3 sensors, which shows good sensitivity to NO2 , is shown in Fig. 6. After initial resistance was stabilized, NO2 was injected into the closed chamber in the batch system and vented the gas after being maintained for 5 min. The sensors with operating temperatures >70 ◦ C and operating temperatures <50 ◦ C have a 90% response time of 1–2 min and above 10 min, respectively. Fig. 7 illustrates NO2 gas sensitivities of WO3 sensors to 3 ppm NO2 from 35 to 100 ◦ C. It is evident that the films are able to detect 3 ppm of NO2 in air at low temperature. The results show the systematic changes of WO3 conductivity with decreasing operating temperatures. More importantly, the results clearly show that a higher surface area WO3 sensor has a much better sensitivity response to a low concentration gas. For comparison, it was reported that a metal oxide sensor (thick film type WO3 by screen printing) operated at 100 ◦ C for detecting 100 ppm of NO2 with a sensitivity of ∼200 [17] and a high-performance metal oxide sensor (Cd-doped SnO2 ) operated at 250 ◦ C for detecting 100 ppm of NO2 with a sensitivity of ∼300 [4,18]. Thus, the mesoporous WO3 sensors have the advantage of 100 ◦ C temperature operation for detecting 3 ppm with sensitivity up to 226 over these materials. The experiment on the selectivity of the mesoporous gas sensor was carried out by monitoring the electrical resistance change in H2 atmosphere, as shown in Fig. 5. It can

be seen that the sensor operating at 100 ◦ C and 4000 ppm of H2 exhibits a sensitivity of ∼3. Although the sensitivity is much smaller than that of NO2 , the opposite response in resistance (NO2 increases the resistance, while H2 decreases the resistance) demonstrates that the mesoporous gas sensor has the selectivity to distinguish oxidizing and reducing gases. The stability of the sensing characteristics was examined several times in a week. It was found that the initial resistance was approximately maintained, but with a slightly increased resistance at saturation. For example, the sensitivity change was about 10% over this period. This indicates that the long-term stability properties should be improved. The most important factors that influence the WO3 sensor characteristics are probably microstructure and surface area. The films exhibiting a porous structure have a large fraction of atoms residing at surfaces and interfaces between the pores, which suggests that the microstructure of the films is suitable for gas-sensing purposes. In the other words, it can be said that the high sensitivity of a mesoporous sensor can be attributed to the full exposure of surface adsorption sites to chemical environments. As for the microstructure, maintaining smaller crystal sizes can improve device performance. The mesoporous WO3 thin film contains crystallites ∼3.8 nm in diameter embedded in an amorphous matrix. Semiconductor gas sensor

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1.2E+009

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↑H2 off 0.0E+000 0

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Time (sec) Fig. 5. Dependence of electrical resistance of WO3 thin films upon operating temperatures of (a) 35, 50, 70 and 100 ◦ C for 3 ppm NO2 gas and (b) 100 ◦ C for 4000 ppm H2 gas. 250

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Temperature (oC) Fig. 6. Time response of WO3 thin film sensor.

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Operating temperature ( 0C) Fig. 7. Sensitivity of mesoporous WO3 thin film upon operating temperatures from 35 to 100 ◦ C.

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typically utilizes the gas-induced variations in potential barrier height at grain boundaries (i.e. changes in thickness of the space charge layer), and it is well known that the gas sensitivity increases with decreasing the particle size [19,20]. It is also noted that earlier work by Xu et al. [21] stated that the gas sensitivity is controlled by a grain size effect for WO3 crystallites smaller than 33 nm; this result is consistent with a theoretical model by Wang et al. [22]. Based on these fundamental aspects, a possible NO2 -sensing mechanism of the present WO3 is depicted as the following. The molecular NO2 has an unpaired electron and is known as a strong oxidizer. Upon NO2 adsorption, charge transfer is likely to occur from mesoporous WO3 to NO2 because of the electron-withdrawing power of the NO2 molecules. The NO2 ions adsorbed at low temperatures on oxide semiconductor surfaces are thought to be ONO− (nitrito type adsorbates) and dissociate into nitrosyl type adsorbates (NO+ , NO− ) [23]. This enables to conclude that the normal response of sensor for NO2 might originate from the superior number of NO+ absorbates than NO− adsorbates, even at room temperature. Consequently, the electron transfer to surface species in connection with NO2 chemisorption creates Schottky energy barrier at the surface yielding a large resistance of the film. It is clear that the response is related to a catalytic reaction of WO3 with the adsorbed NO2 ions. A release of electrons from the surface species increases the height of the surface barrier, thereby resulting in an increase of the film resistance. In a model of Wang et al. [22], a small grain size, such as in the deposited nanocrystalline WO3 films after sintering at 480 ◦ C, improves the gas sensitivity. In summary, for polycrystalline conductors, grain boundaries contribute most of the resistance. The surface resistivity of an oxide crystal depends on the electron concentration near the surface, which in turn is affected by the nature of the chemisorbed species. Theoretically, the smaller the crystal size, the greater the sensitivity of overall resistance to the surrounding atmosphere. In addition, the films also show a sensitivity of 23 at 35 ◦ C. As we know it is unique for the WO3 film that has the sensitivity to NO2 at such a low temperature. Therefore, it can be assumed that the mesoporous WO3 film exhibits a high sensitivity at a low temperature to 3 ppm NO2 .

4. Conclusions This study has shown that mesoporous WO3 thin films with unique microstructure lead to excellent sensing properties upon exposure to low concentration of NO2 in air at low temperatures and enabled the selective detection of NO2 and H2 gases. A high surface area and small crystallites present in the mesoporous WO3 films are the factors contributing to this behavior. Apart from small grain sizes, the main feature of mesoporous WO3 thin film sensors is that they operate at low temperatures and low concentration of NO2 with sensitivity as high as 226. Thus, mesoporous

WO3 thin film should be promising for advanced miniaturized chemical sensors.

Acknowledgements This work was financially supported by the National Science Council of Taiwan, ROC, grant No. NSC 90-2216E-006-064, which is gratefully acknowledged. References [1] Y. Shimizu, M. Egashira, Basic aspects and challenges of semiconductor gas sensors, MRS Bull. 24 (1999) 18–24. [2] H.M. McConnell, The cytosensor microphysiometer: biological applications of silicon technology, Science 257 (1992) 1906–1912. [3] C. Christofides, Physics, Chemistry and Technology of Solid State Gas Sensor Devices, Wiley, New York, 1993. [4] J. Miasik, A. Hooper, B. Tofield, Conducting polymer gas sensors, J. Chem. Soc., Faraday Trans. 1 82 (1986) 1117–1126. [5] S. Capone, S. Mongelli, R. Rella, P. Siciliano, Gas sensitivity measurements on NO2 sensors based on copper (II) tetrakis (nbutylaminocarbonyl) phthalocyanine LB films, Langmuir 15 (1999) 1748–1753. [6] H.T. Sun, C. Cantalini, Microstructural effect on NO2 sensitivity of WO3 thin film gas sensors. Part 1. Thin film devices, sensors and actuators, Thin Solid Films 287 (1996) 258–265. [7] V. Demarne, A. Grisel, An integrated low-power thin film CO gas sensors on silicon, Sens. Actuat. B 13 (1988) 301–313. [8] N. Yamazoe, N. Miura, Environmental gas sensing, Sens. Actuat. B 20 (1994) 95–102. [9] G. Fang, Z. Liu, Z. Zhang, K.L. Yao, Preparation of ZrO2 –SnO2 thin films by sol–gel technique and their gas sensitivity, Phys. Status Solid A 156 (1996) 81–85. [10] G.S.T. Rao, D.T. Rao, Gas sensitivity of ZnO based thick film sensor to NH3 at room temperature, Sens. Actuat. B 55 (1999) 166–169. [11] P.J. Shaver, Activated tungsten oxide gas detectors, Appl. Phys. Lett. 11 (1967) 255–257. [12] M. Akiyama, J. Tamaki, N. Miura, N. Yamazoe, Tungsten oxidebased semiconductor sensor highly sensitive to NO and NO2 , Chem. Lett. 6 (1991) 1611–1614. [13] G.J. Li, S. Kawi, High-surface-area SnO2 : a novel semiconductoroxide gas sensor, Mater. Lett. 34 (1998) 99–102. [14] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism, Nature 359 (1992) 710–712. [15] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, Nonionic triblock and star diblock copolymer and oligometric surfactant syntheses of highly ordered, hydrothermally stable, mesoporous silica structures, J. Am. Chem. Soc. 120 (1998) 6024–6036. [16] Gregg, S.J. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. [17] Y.K. Chung, M.H. Kim, W.S. Um, Gas sensing properties of WO3 thick film for NO2 gas dependent on process condition, Sens. Actuat. B 60 (1999) 49–56. [18] G. Sberveglieri, S. Groppelli, P. Nelli, Highly sensitive and selective NOx and NO2 sensor based on Cd-doped, Sens. Actuat. B 4 (1991) 457–461. [19] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Correlation between gas sensitivity and crystallite size in porous SnO2 -based sensors, Chem. Lett. 2 (1990) 441–444. [20] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Relationship between gas sensitivity and microstructure of porous SnO2 , Denki Kagaku (Electrochemistry) 58 (1990) 1143–1148.

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at National Cheng Kung University, Tainan, Taiwan. His major research has related to mesoporous materials, and battery materials. Jiann Shieh received his BS, MS and PhD degrees of Materials Science and Engineering from National Cheng Kung University, Tainan, Taiwan in 1995, 1997 and 2002, respectively. He is now an associate researcher at National Nano Device laboratories Hsinchu, Taiwan. His major research has related to PECVD Ti-Al-C-N system nanocomposite thin films, semiconductor gas sensor, mesoporous materials, nanocrystal, and nanowire materials.

Biographies Lay Gaik Teoh received her BS and MS degrees of Materials Science and Engineering from National Cheng Kung University, Tainan, Taiwan in 1997 and 1999, respectively. She has been a PhD candidate at National Cheng Kung University, Tainan, Taiwan, 1999. Her major research has related to mesoporous materials, semiconductor gas sensor, and PVD Ba-Ti-Sn-O system thin films. Yi Ming Hon received his BS, MS and PhD degrees of Materials Science and Engineering from National Cheng Kung University, Tainan, Taiwan in 1995, 1996 and 2001, respectively. He is now a postdoctoral fellow

Wei Hao Lai received his BS and MS degrees of Materials Science and Engineering from National Cheng Kung University, Tainan, Taiwan in 2000 and 2001, respectively. He has been a PhD candidate at National Cheng Kung University, Tainan, Taiwan, 2001. His major research has related to mesoporous materials, semiconductor gas sensor, and nanomaterials. Min Hsiung Hon is a professor in the Department of Materials Science and Engineering in National Cheng Kung University, Tainan, Taiwan. His research interest includes thin film deposition, battery materials, gas sensor, lead-free solder, biomaterials, nanomaterials, and mesoporous materials.