Preparation of WO 3 nanoparticles and application to NO 2 sensor Dan Meng, Toshinari Yamazaki * , Yanbai Shen, Zhifu Liu, Toshio Kikuta Faculty of Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan 1. Introduction Nitrogen dioxide, NO 2 , is toxic and deteriorates the environ- ment through acid rain, photochemical smog, and the production of ozone [1]. Therefore, the development of an NO 2 gas sensor for environmental monitoring has been expected. Other than to show a high sensitivity, the NO 2 gas sensor is desired to operate at a low temperature, if possible, at room temperature. This low operating temperature is important for low electric power consumption and long-term stability. Numerous attempts have beenmade to use various metal-oxide semiconductors as gas sensors [1–14]. Among these semiconduc- tors, tungsten trioxide (WO 3 ), an n-type semiconductor, is known as a promising material for sensing NO 2 [1,5–13]. The NO 2 sensing is due to the change in the resistance caused by the adsorption of NO 2 on WO 3 surface and electron exchange between WO 3 and adsorbed species. As the gas adsorption occurs at the surface, a large surface-to-volume ratio of the WO 3 sensor is expected to lead to a high sensitivity. Thus, many researchers have focused their attention on a gas sensor made of WO 3 nanostructures accom- panied with a large surface-to-volume ratio [7–11]. Various methods for preparing WO 3 for a sensing material, such as magnetron sputtering [12], sol–gel [13], and gas evaporation [14,15], have been extensively explored. Of these methods, the gas evaporation is a method extensively studied by Kimoto et al. [16]; they evaporated various metals using resistive heating under inert gases at a low pressure. Kaito et al. [17] applied this method to the formation of metal-oxide nanoparticles, such as WO 3 , MoO 3 , etc.; they evaporated various metals in Ar and O 2 mixture. The first application of the gas evaporation to the formation of metal-oxide nanoparticles for a gas sensor was attempted by Ogawa et al. [18]. They applied SnO 2 nanoparticles formed by plasma-assisted gas evaporation to a gas sensor and found that this sensor was highly sensitive to combustible gases. It was Solis et al. [19] and Reyes et al. [20] who first applied WO 3 nanoparticles formed by gas evaporation adopting induction heating to a gas sensor; they applied their sensor to H 2 S, NO 2 , and CO sensing. Mahan et al. [21] and Pal and Jacob [22] have recently shown that WO 3 nanoparticles were easily formed by gas evaporation; they heated a tungsten filament by introducing electric current through the filament. In the present study, similarlyto them, we adopted this easy gas evaporation to the formation of WO 3 nanoparticles and firstly studied the sensing properties of gas sensors made of these nanoparticles. The WO 3 nanoparticles deposited under various oxygen pressures were annealed at different temperatures, and the structure of the particles were investigated. The particle size depended on both the annealing temperature and the pressure during deposition. We made the gas sensors using the particles with different sizes and investigated the sensing property to NO 2 gas. Thus, the relation between the sensitivity and the particle size was obtained and discussed. 2. Experimental WO 3 nanoparticles were deposited on oxidized silicon sub- strates by resistive heating of a tungsten filament under an oxygen atmosphere at a low pressure. A schematic diagram of the apparatus used for the formation of WO 3 nanoparticles is shown in Fig. 1. A spiral W filament was 0.5 mm in diameter, 150 mm in length, and 99.9% in purity. After the chamber was evacuated to Applied Surface Science xxx (2009) xxx–xxx ARTICLE INFO Article history: Available online xxx Keywords: WO 3 Gas evaporation Nanoparticle Gas sensor NO 2 ABSTRACT WO 3 nanoparticles were prepared by evaporating tungsten filament under a low pressure of oxygen gas, namely, by a gas evaporation method. The crystal structure, morphology, and NO 2 gas sensing properties of WO 3 nanoparticles deposited under various oxygen pressures and annealed at different temperatures were investigated. The particles obtained were identified as monoclinic WO 3 . The particle size increased with increasing oxygen pressure and with increasing annealing temperature. The sensitivity increased with decreasing particle size, irrespective of the oxygen pressure during deposition and annealing temperature. The highest sensitivity of 4700 to NO 2 at 1 ppm observed in this study was measured at a relatively low operating temperature of 50 8C; this sensitivity was observed for a sensor made of particles as small as 36 nm. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +81 764456882; fax: +81 764456882. E-mail address: yamazaki@eng.toyama-u.ac.jp (T. Yamazaki). G Model APSUSC-18761; No of Pages 4 Please cite this article in press as: D. Meng, et al., Preparation of WO 3 nanoparticles and application to NO 2 sensor, Appl. Surf. Sci. (2009), doi:10.1016/j.apsusc.2009.05.075 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.05.075 2 Â 10 À6 kPa, oxygen gas was introduced into the chamber to obtain a prescribed pressure from 1 to 10 kPa. Then, an electric current of about 100 A was introduced through the filament to make the filament redheated. Half an hour later, WO 3 particles were deposited on the substrate placed at a position 180 mm over the W filament. After the deposition, the WO 3 particles on the substrate were annealed at various temperatures from 400 to 800 8C in air for 1 h. The structure of the particles on the substrate was investigated using an X-ray diffractometer (XRD) (Shimadzu XRD-6100) with Cu K a radiation and a field-emission scanning electron microscope (FE-SEM, JEOL JSM-6700F). The structure was also observed using a transmission electron microscope (TEM, JEOL EM002B). The particles collected from the substrates were dispersed in ethanol, and a drop of the ethanol suspended with particles was transferred onto a mesh for the TEM observation. Gas sensors were prepared by pouring a few drops of ethanol suspended with the annealed particles onto oxidized silicon substrates equipped with a pair of interdigitated Pt electrodes with a gap length of 0.035 mm. These sensors were annealed at 350 8C for 30 min in air before the evaluation. In the measurement of sensing properties, the sensor samples were placed in a quartz tube that was inserted into a tubular electric furnace. Dry synthetic air was first introduced in the quartz tube as a reference gas, and then dry synthetic air including NO 2 at a concentration of 200 ppm was added to the reference gas to obtain NO 2 at a concentration of 1 ppm. The gas-flow rate was fixed at 200 ml/min using mass flow Fig. 1. Schematic diagram of the apparatus used for the formation of WO 3 nanoparticles. Fig. 3. (a)–(c) FE-SEM images of WO 3 nanoparticles deposited at 1, 3, and 10 kPa, respectively, and annealed at 600 8C. (d)–(f) FE-SEM images of WO 3 nanoparticles deposited at 1 kPa and annealed at 400, 600, and 800 8C, respectively. Fig. 2. XRD patterns of WO 3 nanoparticles prepared at different conditions. (a) WO 3 nanoparticles deposited at 1–10 kPa and annealed at 600 8C for 1 h in air. (b) WO 3 nanoparticles deposited at 1 kPa and annealed at a temperature of 400–800 8C for 1 h in air. D. Meng et al. / Applied Surface Science xxx (2009) xxx–xxx 2 G Model APSUSC-18761; No of Pages 4 Please cite this article in press as: D. Meng, et al., Preparation of WO 3 nanoparticles and application to NO 2 sensor, Appl. Surf. Sci. (2009), doi:10.1016/j.apsusc.2009.05.075 controllers. The sensing properties were measured at an operating temperature from room temperature (RT = 25 8C) to 300 8C. The electrical resistance of the gas sensors was determined by measuring the electric current flowing through the sensor sample at a constant voltage of 10 V between interdigitated Pt electrodes. The gas sensitivity was defined as (R g À R a )/R a , where R a and R g were the electrical resistances in air and in air including NO 2 , respectively. 3. Results and discussion Fig. 2(a) shows XRD patterns for WO 3 nanoparticles deposited at 1–10 kPa and annealed at 600 8C for 1 h while Fig. 2(b) shows XRD patterns for WO 3 nanoparticles deposited at 1 kPa and annealed at a temperature of 400–800 8C for 1 h. All XRD patterns showed the formation of monoclinic WO 3 indicated in the Joint Committee of Powder Diffraction Standards (JCPDS) card No. 43- 1035. The diffraction peaks tended to be sharper with increasing oxygen pressure and increasing annealing temperature, showing the increase in the particle size. FE-SEM images of WO 3 particles deposited at various condi- tions are shown in Fig. 3. At an annealing temperature of 600 8C, the particle size increased with increasing oxygen pressure (Fig. 3(a)–(c)). The average particle size was relatively small and was about 60 nm at a low oxygen pressure of 1 kPa. It was about 250 nm at a high oxygen pressure of 10 kPa. At a fixed pressure of 1 kPa, the particle size increased with increasing annealing temperature (Fig. 3(d)–(f)). The average size of the particles annealed at 400 8C was approximately 36 nm and that of the particles annealed at 800 8C was approximately 150 nm. For confirmation, the particle size was also measured by TEM, and it was found that the size determined by TEM was consistent with that determined by FE-SEM. It was also found that the nanoparticles were single-crystal monoclinic WO 3 in all condi- tions. The temporal responses of a sensor made of WO 3 nanoparticles deposited at 1 kPa and annealed at 600 8C upon exposure to 1 ppm NO 2 at different operating temperatures are represented in Fig. 4. The resistance quickly increased when the sensor was exposed to NO 2 . The resistance recovered to the initial value in 10 min after removal of NO 2 at a high operating temperature of 250 8C. However, it took a long time for the resistance to recover to the initial value at a low operating temperature. Fig. 5 illustrates the relation between the sensitivity to 1 ppm NO 2 and operating temperature for the sensors made of WO 3 nanoparticles formed at different conditions. Fig. 5(a) shows the results for the sensors made of WO 3 nanoparticles deposited at 1, 3, and 10 kPa and annealed at 600 8C while Fig. 5(b) shows the results for the sensors made of WO 3 nanoparticles deposited at 1 kPa and annealed at 400 and 600 8C. All sensors showed a peak operating temperature at which the sensitivity indicated a maximum. In Fig. 5(a), the peak temperature decreased from 150 to 90 8C with decreasing pressure during deposition, that is, with deceasing particle size from 250 to 60 nm. Because the average sizes of the particles annealed at 400 and 600 8CinFig. 5(b) were as small as 36 and 60 nm, the peak temperatures of these sensors were as low as 60 and 90 8C, respectively. The sensitivities were lower than 50 at an operating temperature above 200 8C for every sensor. The highest sensitivity obtained in this study was 4700, which was observed at 50 8C for the sensor made of WO 3 nanoparticles deposited at a pressure of 1 kPa and annealed at 400 8C. The average particle size of this sensor was as small as 36 nm. The sensitivity at a temperature below 150 8C largely depended on the condition of the formation of the particles or the particle size. Thus, the sensitivities at 100 8C, around which most sensors showed their maximum sensitivities, are summarized as a function of particle size in Fig. 6. It is noticeable that the relation between the sensitivity and particle size is expressed by a unique curve, irrespective of the oxygen pressure during deposition and annealing temperature. The sensitivity increased with decreasing particle size, which is consistent with the result reported by Tamaki et al. [11]. To clarify the influence of the particle size on the sensitivity, the temperature dependences of resistance in air and in air including NO 2 are shown in Fig. 7. Fig. 7(a) and (b) show the result for the sensor made of small particles with an average particle size of 36 nm deposited at 1 kPa and annealed at 400 8C and that for the sensor made of large particles with an average particle size of Fig. 4. Temporal responses to 1 ppm NO 2 measured for the sensors made of WO 3 nanoparticles deposited at 1 kPa and annealed at 600 8C. The results for various operating temperatures are shown. Fig. 5. Relation between the sensitivity to 1 ppm NO 2 and operating temperature. (a) WO 3 nanoparticles deposited at 1, 3, and 10 kPa and annealed at 600 8C. (b) WO 3 nanoparticles deposited at 1 kPa and annealed at 400 and 600 8C. D. Meng et al. / Applied Surface Science xxx (2009) xxx–xxx 3 G Model APSUSC-18761; No of Pages 4 Please cite this article in press as: D. Meng, et al., Preparation of WO 3 nanoparticles and application to NO 2 sensor, Appl. Surf. Sci. (2009), doi:10.1016/j.apsusc.2009.05.075 250 nm deposited at 10 kPa and annealed at 600 8C, respectively. In air, in both small particles and large particles, an electron deletion layer is formed only at the surface, and the electrons flow through the non-depleted bulk region. Thus, the temperature dependence of the resistance is small, reflecting the character of a fully ionized bulk semiconductor. On the other hand, in air including NO 2 , small particles are depleted in the whole region, indicating a very high resistance, while large particles are still depleted only at the surface. In small particles, as the temperature increases, the electrons trapped by surface states are thermally excited to conduction band, which results in the abrupt decrease in the resistance with increasing temperature. As a result of such temperature dependence, the difference between the resistance in air and that in air including NO 2 , namely, the sensitivity shows a large value at a low temperature. However, in large particles, even in air including NO 2 , the electrons flow through the non-depleted bulk region. Therefore, the temperature dependence of resistance is still small. Thus, both the sensitivity and its temperature dependence are small. The influence of the particle size on the resistance mentioned above causes the dependence of the sensitivity on the particle size shown in Fig. 6. 4. Conclusions WO 3 nanoparticles were deposited on oxidized silicon sub- strates by a gas evaporation method using resistive heating of tungsten filament under an oxygen atmosphere. The structure of the particles deposited at various oxygen pressures and annealed at different temperatures was systematically investigated using XRD, FE-SEM, and TEM. In addition, NO 2 sensing properties of the sensors made of WO 3 nanoparticles were evaluated. The results are summarized below. The obtained nanoparticles are monoclinic WO 3 . The average particle size increases with increasing oxygen pressure and with increasing annealing temperature. The sensitivity of the NO 2 sensors made of the WO 3 nanoparticles increases with decreasing particle size. The relation between the sensitivity and particle size is expressed by a unique curve, irrespective of the oxygen pressure during deposition and annealing temperature. Gas sensors made of WO 3 particles smaller than 100 nm show a high sensitivity to 1 ppm NO 2 at a low operating temperature. The highest sensitivity obtained in this study is 4700 at 50 8C measured for a sensor made of WO 3 nanoparticles deposited at an oxygen pressure of 1 kPa and annealed at 400 8C. These results demonstrate that WO 3 nano- particles made by the gas evaporation method shown in this study are a promising material for detecting NO 2 gas at a low operating temperature. Acknowledgement The authors wish to thank Dr. T. Kawabata for his cooperation in TEM measurements. References [1] J. Tamaki, A. Hayashi, Y. Yamamoto, M. Matsuoka, Sens. Actuators B 95 (2003) 111. [2] A.M. Ruiz, G. Sakai, A. Cornet, K. Shimanoe, J.R. Morante, N. Yamazoe, Sens. Actuators B 93 (2003) 509. [3] C. Cantalini, W. Wlodarski, H.T. Sun, M.Z. Atashbar, M. Passacantando, S. Santucci, Sens. Actuators B 65 (2000) 101. [4] A. Karthigeyan, R.P. Gupta, M. Burgmair, S.K. Sharma, I. Eisele, Sens. Actuators B 87 (2002) 321. [5] D.S. Lee, K.H. Nam, D.D. Lee, Thin Solid Films 375 (2000) 142. [6] X.L. He, J.P. Li, X.G. Gao, L. 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Meng, et al., Preparation of WO 3 nanoparticles and application to NO 2 sensor, Appl. Surf. Sci. (2009), doi:10.1016/j.apsusc.2009.05.075 . Preparation of WO 3 nanoparticles and application to NO 2 sensor Dan Meng, Toshinari Yamazaki * , Yanbai Shen, Zhifu Liu, Toshio Kikuta Faculty of Engineering,. similarlyto them, we adopted this easy gas evaporation to the formation of WO 3 nanoparticles and firstly studied the sensing properties of gas sensors made of