Sensors and Actuators B 96 (2003) 219–225 Sensitivity properties of a novel NO 2 gas sensor based on mesoporous WO 3 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 WO 3 thin films micro-gas sensor was fabricated and the NO 2 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 m 2 /g as calcined at 250 ◦ C. Upon exposure to NO 2 , the electrical resistance of a semiconducting mesoporous WO 3 thin films is found to dramatically increase. The sensitivity of mesoporous WO 3 thin film sensors is substantially higher than that from other reports. In addition, the mesoporous WO 3 thin film sensor calcined at 250 ◦ C and operated at 35 ◦ C shows an excellent sensitivity of 23, as we know it is unique NO 2 gas sensor which has the sensitivity at such a low temperature. © 2003 Elsevier B.V. All rights reserved. Keywords: Mesoporous WO 3 ;NO 2 gas sensor; Sol–gel process 1. Introduction The detection of NO 2 is important for monitoring environ- mental pollution resulting from combustion or automotive emissions [1]. Existing gas sensor materials include semi- conducting metal oxides [1], silicon [2,3] and organic mate- rials [4,5]. Semiconducting metal oxides such as WO 3 and SnO 2 had been widely used for NO 2 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, es- pecially for battery-operated devices. Recently it also has been reported that ZrO 2 –SnO 2 [9] and ZnO [10] materials can be used as H 2 S and NH 3 gas sensors at room temper- ature, respectively, but their sensitivity was low. WO 3 was reported to exhibit promising electrical and optical proper- ties for various applications like efficient photolysis, elec- trochromic devices, selective catalysts and gas sensors [6]. WO 3 is an n-type semiconductor whose electron concen- tration is determined mainly by the concentration of stoi- chiometric defects such as oxygen vacancy like other metal oxide semiconductors. The first work on the feasibility of ∗ Corresponding author. E-mail address: mhhon@mail.ncku.edu.tw (M.H. Hon). WO 3 thin films as a gas sensor was reported by Shaver [11] who observed that the conductivity of WO 3 thin films changed greatly upon the exposure to the H 2 ambient. Following this pioneering work, many works have been per- formed on the structural and electrical properties and sens- ing characteristics of WO 3 thin films. These sensors have been reported to have good selectivity for low concentration NO x gas [12]. In this study, we developed a novel NO 2 gas sensors based on mesoporous WO 3 thin film to detect small concentration of NO 2 at low operating temperatures. Li and Kawi [13] have shown that a linear relationship was found between the surface areas of SnO 2 sensors and their sensitivities to 500 ppm of H 2 . Accordingly, meso- porous WO 3 with a higher surface area provides more surface adsorption sites for the reaction of NO 2 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 WO 3 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 WO 3 gas-sensing films that were deposited by dipping on Al 2 O 3 substrate. 0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00528-8 220 L.G. Teoh et al./ Sensors and Actuators B 96 (2003) 219–225 2. Experimental Poly(alkylene oxide) block copolymer (BASF Pluronic EO 100 PO 64 EO 100 , 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, WCl 6 (Aldrich 99.9%), was added into the F127 ethanol solution with vigorously stirring for 1 h. The result- ing 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 Al 2 O 3 substrate that was coated with Pt elec- trode by dip coating. The thin films can be dried within sev- eral 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 elec- tron 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 WO 3 films was observed by scanning electron microscope (SEM, Philips XL-40 FEG). The nitrogen ad- sorption 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 N 2 atmosphere. 20 30 40 50 60 2 θ (deg.) Intensity (arb.units) ∆ ∆ ∆ ∆ ∆ ∗ ∗ ∗ ∗ ∗ ∆ WO 3 ∗ Substrate ∆ ∗ Fig. 1. X-ray diffraction pattern for a mesoporous WO 3 thin film calcined at 250 ◦ C for 5 h. Brunauer–Emmett–Teller (BET) surface areas were esti- mated over a relative pressure (P/P 0 ) 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/P 0 = 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 cm 3 ) and exposed to 3 ppm NO 2 gas and 4000ppm H 2 , respectively. Gas-sensing properties of the films were studied at various operating temperatures T g in the range of 35 ◦ C <T g < 100 ◦ C. The sensitivity is defined as R g /R a , where R g and R a are the electric resistance in test gas and air, respectively. 3. Results and discussion 3.1. Microstructure characterizations Fig. 1 shows the XRD pattern of mesoporous WO 3 thin films calcined at 250 ◦ C for 5 h indicating that this crys- tallographic nucleation actually occurs during the calcina- tion, but is limited to formation of nanocrystallite domains. The diffraction peaks of the WO 3 thin films are assigned based on monoclinic structure (JCPDS card no. 05-0363). L.G. Teoh et al./ Sensors and Actuators B 96 (2003) 219–225 221 Fig. 2. SEM micrograph of mesoporous WO 3 thin film calcined at 250 ◦ C for 5 h. After employing Scherrer’s formula, the calculated grain size of WO 3 is approximately 3.8 nm. These grains contact contributes to the gas-sensing properties of the mesoporous WO 3 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 WO 3 thin films calcined at 250 ◦ C for 5 h. The sample exhibits porous struc- ture 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 NO 2 molecules because of the cap- illary pore and large surface area. This implies the conclu- sion that this type of film will offer a good sensitivity to NO 2 gas. The morphology of the mesoporous WO 3 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 WO 3 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 WO 3 nanocrystals, ∼3 nm) and agree with the result of grain size determination obtained from XRD analyses. The results lead to conclude that the crystallized WO 3 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 1.8nm, respec- tively. Nitrogen adsorption–desorption isotherms exhibiting a type IV curve is shown in Fig. 4, which is characteristic of mesoporous WO 3 [16]. Barrett–Joyner–Halenda (BJH) analyses show that the calcined mesoporous WO 3 exhibits mean pore size of 5 nm (Fig. 4 inset). From the absolute adsorption, we can calculate a specific surface of 143m 2 /g. This underlines that most pores are really accessible from Fig. 3. TEM images of mesoporous WO 3 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 N 2 ). 3.2. Gas-sensing properties In order to check the sensitivity of WO 3 sensors for the concentration of 3 ppm NO 2 ,WO 3 sensors were maintained at various temperatures from 35 to 100 ◦ C. Fig. 5 shows that WO 3 sensors respond to turning-on and turning-off NO 2 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 NO 2 was introduced into the test chamber, the re- sistance of sensor increased and soon afterwards it became saturated. When the gas was turning-off, the resistance of 222 L.G. Teoh et al./ Sensors and Actuators B 96 (2003) 219–225 Pore diameter,(Å) 20 40 60 80 100 0 0.2 0.4 0.6 0.8 1 Relativep ressure (P/P 0 ) Volume adsorbed (cc/g, STP) 10 100 1000 0 0.4 0.8 1.2 1.6 Pore volume (cc/g) Fig. 4. Nitrogen adsorption(+)–desorption(᭺ ) isotherms and BJH pore size distribution curves for mesoporous WO 3 calcined at 250 ◦ C for 5 h. the sensor decreased. The time response of WO 3 sensors, which shows good sensitivity to NO 2 , is shown in Fig. 6. After initial resistance was stabilized, NO 2 was injected into the closed chamber in the batch system and vented the gas after being maintained for 5min. The sensors with operat- ing temperatures >70 ◦ C and operating temperatures <50 ◦ C have a 90% response time of 1–2 min and above 10 min, re- spectively. Fig. 7 illustrates NO 2 gas sensitivities of WO 3 sensors to 3ppm NO 2 from 35 to 100 ◦ C. It is evident that the films are able to detect 3 ppm of NO 2 in air at low tem- perature. The results show the systematic changes of WO 3 conductivity with decreasing operating temperatures. More importantly, the results clearly show that a higher surface area WO 3 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 WO 3 by screen print- ing) operated at 100 ◦ C for detecting 100 ppm of NO 2 with a sensitivity of ∼200 [17] and a high-performance metal oxide sensor (Cd-doped SnO 2 ) operated at 250 ◦ C for de- tecting 100ppm of NO 2 with a sensitivity of ∼300 [4,18]. Thus, the mesoporous WO 3 sensors have the advantage of 100 ◦ C temperature operation for detecting 3 ppm with sen- sitivity up to 226 over these materials. The experiment on the selectivity of the mesoporous gas sensor was carried out by monitoring the electrical resis- tance change in H 2 atmosphere, as shown in Fig. 5.Itcan be seen that the sensor operating at 100 ◦ C and 4000 ppm of H 2 exhibits a sensitivity of ∼3. Although the sensitivity is much smaller than that of NO 2 , the opposite response in re- sistance (NO 2 increases the resistance, while H 2 decreases the resistance) demonstrates that the mesoporous gas sen- sor has the selectivity to distinguish oxidizing and reduc- ing 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 WO 3 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 WO 3 thin film contains crystallites ∼3.8nm in diameter em- bedded in an amorphous matrix. Semiconductor gas sensor L.G. Teoh et al./ Sensors and Actuators B 96 (2003) 219–225 223 Time (sec) 0.0E+000 4.0E+008 8.0E+008 1.2E+009 Resistance( Ω ) 0 1000 2000 3000 0 1000 2000 3000 ↓ On ← Off 100 o C 70 o C 50 o C 35 o C NO 2 NO 2 (a) Time (sec) 0.0E+000 2.0E+005 4.0E+005 6.0E+005 Resistance ( Ω) H 2 on ↓ H 2 off ↑ 100 o C (b) Fig. 5. Dependence of electrical resistance of WO 3 thin films upon operating temperatures of (a) 35, 50, 70 and 100 ◦ C for 3 ppm NO 2 gas and (b) 100 ◦ C for 4000 ppm H 2 gas. 20 40 60 80 100 Temperature ( o C) 0 10 20 Time (min) Fig. 6. Time response of WO 3 thin film sensor. 20 40 60 80 100 120 Operating temperature ( 0 C) 0 50 100 150 200 250 Sensitivity (R g /R a ) Fig. 7. Sensitivity of mesoporous WO 3 thin film upon operating temper- atures from 35 to 100 ◦ C. 224 L.G. Teoh et al./ Sensors and Actuators B 96 (2003) 219–225 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 WO 3 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 NO 2 -sensing mechanism of the present WO 3 is depicted as the following. The molecular NO 2 has an unpaired electron and is known as a strong oxidizer. Upon NO 2 adsorption, charge transfer is likely to occur from mesoporous WO 3 to NO 2 because of the electron-withdrawing powerof the NO 2 molecules. The NO 2 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 NO 2 might originate from the superior number of NO + absorbates than NO − adsorbates, even at room temper- ature. Consequently, the electron transfer to surface species in connection with NO 2 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 WO 3 with the adsorbed NO 2 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 WO 3 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 WO 3 film that has the sensitivity to NO 2 at such a low temperature. Therefore, it can be assumed that the mesoporous WO 3 film exhibits a high sensitivity at a low temperature to 3 ppm NO 2 . 4. Conclusions This study has shown that mesoporous WO 3 thin films with unique microstructure lead to excellent sensing prop- erties upon exposure to low concentration of NO 2 in air at low temperatures and enabled the selective detection of NO 2 and H 2 gases. A high surface area and small crystal- lites present in the mesoporous WO 3 films are the factors contributing to this behavior. Apart from small grain sizes, the main feature of mesoporous WO 3 thin film sensors is that they operate at low temperatures and low concentration of NO 2 with sensitivity as high as 226. Thus, mesoporous WO 3 thin film should be promising for advanced miniatur- ized chemical sensors. Acknowledgements This work was financially supported by the National Sci- ence Council of Taiwan, ROC, grant No. NSC 90-2216- E-006-064, which is gratefully acknowledged. References [1] Y. Shimizu, M. Egashira, Basic aspects and challenges of semicon- ductor gas sensors, MRS Bull. 24 (1999) 18–24. [2] H.M. 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Teoh et al./ Sensors and Actuators B 96 (2003) 219–225 225 [21] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Grain-size effects on gas sensitivity of porous SnO 2 -based elements, Sens. Actuat. B 3 (1991) 147–155. [22] X. Wang, S.S. Yee, W.P. Carey, Transition between neck-controlled and grain-boundary-controlled sensitivity of metal-oxide gas sensors, Sens. Actuat. B 24 (1995) 454–457. [23] H. Arai, H. Tominaga, An infrared study of nitric oxide adsorbed on rhodium–alumina catalyst, J. Catal. 43 (1976) 131–142. 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 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, semi- conductor gas sensor, mesoporous materials, nanocrystal, and nanowire materials. Wei Hao Lai received his BS and MS degrees of Materials Science and En- gineering 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 Sci- ence and Engineering in National Cheng Kung University, Tainan, Tai- wan. His research interest includes thin film deposition, battery materials, gas sensor, lead-free solder, biomaterials, nanomaterials, and mesoporous materials. . candidate at National Cheng Kung University, Tainan, Taiwan, 1999. Her major research has related to mesoporous materials, semiconductor gas sensor, and. 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