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WO3 and WTiO thin-film gas sensors prepared by sol–gel dip-coating
Sensors and Actuators B 86 (2002) 75±80
WO3 and W±Ti±O thin-®lm gas sensors prepared by sol±gel dip-coating J. Shieha,*, H.M. Fenga, M.H. Hona, H.Y. Juangb a
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan b Cyntec Company Ltd., Hsin-Chu 300, Taiwan Received 16 July 2001; received in revised form 12 February 2002; accepted 5 March 2002
Abstract The WO3 and W±Ti±O thin-®lms were prepared by sol±gel dip-coating on alumina substrate using tungsten chloride, 2,4-pentanedione (PTN), ethanol and water as sources and calcined at 400 8C. The structure was characterized by XRD, Raman, SEM and TEM analyses. The effect of titanium doping on the structure was then discussed and related to the gas sensing properties. The results show that the sensitivities for detecting 10 ppm NO2 were 82 and 1768 by WO3 and W±Ti±O gas sensors at 200 8C, respectively. The huge increase in sensitivity by adding titanium ion arose from the grain size reduction of the ®lms and the doping effect on Debye length. The improved sensitivity, recovery, and reliability suggest that WO3 and W±Ti±O thin-®lms prepared by sol±gel dip-coating can be used for NO2 gas sensing. # 2002 Elsevier Science B.V. All rights reserved. Keywords: NO2 gas sensor; Tungsten oxide; Tungsten±titanium oxide; Sol±gel
1. Introduction Our environment has suffered damage as the economy expands. Chromatography, spectrology, electrochemistry and semiconductors are adopted to analyze kinds and concentration of the noxious gases. In order to improve the drawbacks of expensive, heavy, and time-consuming conventional gas analysis, many studies have focused on the thin-®lm gas sensors recently. Among the semiconductor thin-®lms for gas sensing, tungsten oxide (WO3) has the greatest sensitivity for detecting NO2 [1]. The processes to synthesize WO3 thin-®lms include physical vapor deposition [2,3], plasma enhanced chemical vapor deposition [4], sol± gel technology [5], and so on. In opposition to the drawback of expensive cost and compact structure in vapor deposition process, sol±gel process is a potential candidate for gas sensor fabrication. Moreover, with aqueous experimental conditions, self-assembly mechanism can be utilized to synthesize the nanomaterials (e.g. mesoporous materials) with the advantage of increasing speci®c surface areas to improve the sensitivity. The factors that in¯uence the sensitivity include grain size and the charge carrier concentration of the ®lms [6±8], which are in sequence determined by the type and concentration of doped impurities. For the WO3, titanium addition * Corresponding author. Tel.: 886-6-2380208; fax: 886-6-2380208. E-mail address: [email protected] (J. Shieh).
was found to have the advantages of inhibiting grain growth and improving sensitivity. Most of the studies on W±Ti±O thin-®lms were prepared by sputtering [9±12] and screenprinting technologies [13,14]. In this article, the WO3 and W±Ti±O thin-®lms prepared by dip-coating process were investigated for detecting NO2. The results show that both the WO3 and W±Ti±O thin-®lms prepared by sol±gel dipcoating had high NO2 sensitivity and recovery with fast and reproducible response. By means of titanium doping, the sensitivity was improved to the value of 1768 at 200 8C in the ¯owing NO2 gas of 10 ppm concentration, demonstrating that this process is an easy and promising way for fabricating NO2 gas sensor. 2. Experimental Sol±gel was prepared using a method similar to that of Nishide and Mizukami [15] as follows: 1 g WCl6 was ®rstly dissolved in 10 ml ethanol. After the solution turning blue, 1 ml of 2,4-pentanedione (PTN) was then added into the solution as an organic ligand. The sol was allowed to stand for 2 h, and 0.05 ml H2O was then added. The preparation of the W±Ti±O sols was the same as that of the WO3 sols except that additional TiCl4 was added into the solution. The Pt electrode with interdigital structure was deposited on alumina substrate by sputtering process, and the sensing ®lms were deposited on alumina substrate by dip-coatings
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and withdrew at a speed of 1 cm/min and dried at 70 8C. After four coats, the samples were calcined at 400 8C for 1 h. The surface morphology was characterized by Philip XL-40FEG ®eld emission scanning electron microscopy (FE-SEM) and the structure was analyzed by X-ray diffraction using a Rigaku D/MAX diffractometer with a Cu Ka source; the chemical characteristics were investigated by Raman spectroscopy (Coherent, mode Innova 90); the composition of coatings was determined by energy dispersive spectrometry (EDS) equipped in SEM. Additional structure characterization was performed by transmission electron microscopy (TEM) at 300 kV with a Hitachi model HF2000 ®eld emission transmission electron microscopy. The samples for TEM were prepared by dispersing particles scraped from the coatings onto carbon ®lm on a copper grid. For the sensing behavior determination, the sensor was placed in a glass chamber (85 ml) with the gas ¯ow rate of 200 ml/min. The NO2 gas and air were metered by mass ¯ow controllers, respectively. The data of the electrical response were acquired by a computer with an IEEE interface board (HP 34970A). The gas sensitivity S is de®ned as the ratio of electrical resistance in NO2 (Rg) and that in air (Ra), i.e. S Rg =Ra . 3. Results and discussion The perfect crystal structure of WO3 is ReO3 type with the corner-sharing packing of [WO6]6 octahedra, which is distorted as the fabrication temperature is low as well as titanium ion is doped. Although the doped elements usually occupy the interstitial sites of WO3 to form perovskite crystal structure [16], titanium atoms prefer to replace tungsten atoms in ReO3 structure [10,17]. Fig. 1 presents the XRD patterns for the WO3 and W±Ti±O coatings annealed at 400 8C, which were identi®ed with JCPDS number of 830951 for the monoclinic indicating. It can be observed that WO3 ®lm is strongly (2 0 0) orientation, and W±Ti±O ®lm is strongly (0 2 0) orientation. Cantalini et al. stated that the (2 0 0) preferred orientation might originate from the epitaxial growth of WO3 ®lm on Al2O3 substrate because the distance between oxygen atoms in the Al2O3 (1 1 6) plane and the WO3 (2 0 0) plane is close [18]. For W±Ti±O ®lm, Sangaletti et al. have reported that the preferred orientation derived from the disordered cubic phase [9]. The relative intensity of (0 0 2), (0 2 0), and (2 0 0) peaks different from the monoclinic phase with a more symmetry pro®le indicates that the monoclinic lattice was distorted. Depero et al. [10] demonstrated that this cubic-like X-ray diffraction pattern might result from a static disorder introduction as edge-sharing octahedra randomly distributed in the lattice by oxygen de®ciency since the diffraction peak of (1 1 0) plane of WO2.9 is located at 23.7718 that is near the {2 0 0} diffraction peaks of WO3. Raman spectroscopy, which is sensitive to the changes in translational symmetry, was then employed to con®rm the
Fig. 1. X-ray diffraction patterns for: (a) W±Ti±O (Ti/(W Ti) 19.38 at.%), and (b) WO3 thin-films annealed at 400 8C with alumina substrate and JCPDS files of WO3, and WO2.9 as reference.
chemical state, as shown in Fig. 2. The bands appeared at 250, 700, and 780 cm 1 corresponding to the WO3 monoclinic phase [19], and the broad pro®le is indicative of the defects that is abundant in n-type semiconductor [20]. In addition, because oxygen was de®cient as Ti with the lower oxidation state substituted for W, Ti cations also contributed to the disorder structure through the transformation of corner-sharing octahedra to edge-sharing octahedra.
Fig. 2. Raman spectra of WO3 and W±Ti±O Ti= W Ti 19:38 at.%) thin-films annealed at 400 8C.
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Fig. 3. SEM surface morphology of: (a) WO3, and (b) W±Ti±O Ti= W Ti 19:38 at.%) thin-films annealed at 400 8C.
Surface morphology characterization by SEM observation indicates that the coatings consisted of nanograins with average sizes of 76.7 nm and 56.4 nm for WO3 and W±Ti±O sensors, respectively, as shown in Fig. 3. The atomic ratio of Ti/(Ti W) in the W±Ti±O ®lm analyzed by EDS was 19.38 at.%. However, the grain size of WO3 and W±Ti±O thin-®lms determined by Scherrer's equation [21] from XRD patterns were 33.8 and 17.9 nm, respectively. The different results might be attributed to the agglomeration of grains in the SEM observation. Fig. 4 shows the TEM bright ®eld images of WO3 and W±Ti±O particles that agreed with the results of grain size determination obtained from XRD analyses, indicating that the grain growth was suppressed by impurity effect during crystallization. The gas sensing mechanism is that as the solid adsorbs more electronegative gas, the solid loses electron to the gas, causing a space charge layer (or depletion layer) formed near the surface where the uncompensated donor ions survive. Schottky barrier is then built up which the electron should overcome to cross this region. Since NO2 has a stronger electron af®nity than O2, the resistance is increased as NO2 is adsorbed. This resistance variation is used to detect NO2 in air. Fig. 5 presents the effects of temperature on the response and recovery transients of WO3 for 10 ppm NO2, showing that the slower desorption rate of NO2 at a lower temperature led to the longer recovery time. Fig. 6 shows the sensitivity as a function of temperature for WO3 sensor in 10 ppm NO2 ambient. The sensitivity reaching a peak value of 82 at 200 8C derived from the competition between gas adsorption and desorption mechanisms. The sensitivity increased as temperature increased at a low temperature since
Fig. 4. TEM bright field images of: (a) WO3, and (b) W±Ti±O Ti= W Ti 19:38 at.%) films annealed at 400 8C.
Fig. 5. Resistance response of WO3 gas sensor for 10 ppm NO2 in dry air at different temperatures between 120 and 250 8C.
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Fig. 6. Relation between sensitivity and operating temperature for WO3 gas sensor.
temperature provided the active energy for chemisorption. At a higher temperature, on the other hand, the surface recovery was reduced by a progressive gas desorption to decrease the sensitivity [20]. Fig. 7 shows the response to NO2 at 200 8C for WO3 sensor calcined at 400 8C. The response and recovery time (time for 90% of resistance change) for NO2/air mixture in the 10±25 ppm range was about 4 min with a good reproducibility. The calibration curve shown in Fig. 8 indicates that the relation between sensitivity and concentration is linear, which is a bene®t for an actuator to respond to detect different concentrations of noxious gas. Fig. 9 shows the electrical responses of WO3 and W±Ti±O sensors at 200 8C in ¯owing NO2 gas of 10 ppm. The sensitivities for WO3 and W±Ti±O were 82 and 1768, respectively, as shown in Figs. 8 and 10. The pronounced sensitivity increased by Ti doping is explained as follows. The relation between the thickness of space charge layer (L) and Debye length (LD) is r 2eVs L LD (1) kT
Fig. 7. Response transients of WO3 gas sensor to different NO2 concentration between 10 and 25 ppm at 200 8C.
Fig. 8. Sensitivity variation as a function of NO2 concentration for WO3 gas sensor operated at 200 8C.
where eVs is the height of surface barrier that depends on the kind of adsorbed gases, and kT corresponds to the thermal energy [6,20]. As the grain size is larger than the dimension of 2L, the sensitivity is independent of grain size since the resistance arises from the grain boundary where the environment gases adsorb (grain boundary control model). On the other hand, as the grain size is smaller than the dimension of 2L, the sensitivity is determined by grain itself since the carrier is depleted in the whole grain (grain control model) [22]. Using these grain size models of sensitivity, Tamaki et al. [7] have discussed the grain size effects on the sensitivity of WO3. They controlled the grain size by changing the sintering temperature and demonstrated that steeply increased sensitivity arose at a critical grain size in which the
Fig. 9. Response transients of WO3 and W±Ti±O Ti= W Ti 19:38 at.%) gas sensors to 10 ppm NO2 at 200 8C.
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4. Conclusion Sol±gel dip-coated WO3 and W±Ti±O gas sensors with nanocrystallite characteristic and disorder-cubic structure were obtained as identi®ed by XRD, Raman, SEM and TEM analyses. The WO3 ®lms annealed at 400 8C exhibit an optimal working temperature of 200 8C for 10 ppm NO2. The sensitivity was increased from 82 to 1768 as titanium was added into the WO3 gas sensor. The reduced grain size and increased space charge layer arose from Ti doping effect and corresponded to this sensitivity improvement. The high sensitivity with good recovery and the linear relation between sensitivity and gas concentration indicate that WO3 and W±Ti±O gas sensors prepared by sol±gel dip-coating process are suitable for detecting the noxious NO2 gas. Fig. 10. Sensitivity variation as a function of NO2 concentration for W±Ti±O Ti= W Ti 19:38 at.%) gas sensor operated at 200 8C.
Acknowledgements conducting mechanism changed from grain boundary control to grain control as WO3 was exposed to air and NO2, respectively. In the present study, as Ti was added to WO3, the carrier concentration was decreased as follows: WO3
Ti ! Ti00W 2h
(2)
The creation of hole eliminated some electrons and then increased the Debye length since WO3 is an n-type semiconductor in which electron is a major charge carrier. Under the same gas type and temperature, L depends on LD that increases as the carrier concentration decreases, according to the relation [23] LD
EkT q2 N
1=2
(3)
where E is the dielectric constant, k the Boltzmann's constant, T an absolute temperature, q the electrical charge of the carrier, and N the carrier concentration. The enlarged Debye length has the similar effects as grain size reduction on the sensitivity when the grain size is about twice of space charge layer. Both of them are conducive to the model transition from grain boundary control to grain control. Therefore, the sensitivity improvement of W±Ti±O sensor in Fig. 9 indicates that the value of 2L was smaller than grain size as the sensor gas was exposed to air, but after adsorbing NO2, the value of 2L was larger than the grain size. The identical electric resistance of the WO3 sensor and the W±Ti±O sensor as they were exposed to air indicates that the value of 2L in air was smaller than 17.9 nm, and the steeply increasing electrical resistance of the W±Ti±O sensor indicates that the value of 2L in 10 ppm NO2 was between 17.9 nm and 33.8 nm. Similar calibration curve for W±Ti±O ®lms is presented in Fig. 10, in which a linear relation between concentration and sensitivity is also favorable for actuator to control the gas sensor.
This study was supported by the National Science Council of Taiwan, the Republic of China (NSC 89-2218-E006-017), and Cyntec Company, which are greatly appreciated. References [1] N. Yamazoe, N. Miura, Environmental gas sensing, Sens. Actuators B 20 (1994) 95±102. [2] S. Santucci, L. Lozzi, M. Passacantando, S.D. Nardo, A.R. Phani, C. Cantalini, M. Pelino, Study of the surface morphology and gas sensing properties of WO3 thin films deposited by vacuum thermal evaporation, J. Vac. Sci. Technol. A 17 (1999) 644±649. [3] T.S. Kim, Y.B. Kim, K.S. Yoo, G.S. Sung, H.J. Jung, Sensing characteristics of dc reactive sputtered WO3 thin films as an NOx gas sensor, Sens. Actuators B 62 (2000) 102±108. [4] C.E. Tracy, D.K. Benson, Preparation of amorphous electrochromic tungsten oxide and molybdenum oxide by plasma enhanced chemical vapor deposition, J. Vac. Sci. Technol. A 4 (1986) 2377±2383. [5] C. Cantalini, M.Z. Atashbar, Y. Li, M.K. Ghantasala, S. Santucci, W. Wlodarski, M. Passacantando, Characterization of sol±gel prepared WO3 thin films as a gas sensor, J. Vac. Sci. Technol. A 17 (1999) 1873±1879. [6] N. Yamazoe, N. Miura, in: S. Yamauchi (Ed.), Chemical Sensor Technology, Vol. 4, Elsevier, Amsterdam, 1992, pp. 19±42. [7] 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. [8] X. Wang, S.S. Yee, W.P. Carey, Transition between neck-controlled and grain-boundary-controlled sensitivity of metal-oxide gas sensors, Sens. Actuators B 24/25 (1995) 454±457. [9] L. Sangaletti, E. Bontempi, L.E. Depero, R. Salari, M. Zocchi, P. Nelli, G. Sberveglieri, P. Galinetto, M. Ferroni, V. Guidi, G. Martinelli, W± Ti±O layers for gas-sensing applications: structure, morphology, and electrical properties, J. Mater. Res. 13 (1998) 1568±1575. [10] L.E. Depero, S. Groppelli, I.N. Sora, L. Sangaletti, G. Sberveglieri, E. Tondello, Structure studies of tungsten±tianium oxide thin films, J. Solid State Chem. 121 (1996) 379±387. [11] L.E. Depero, M. Ferroni, V. Guidi, G. Marca, G. Martinelli, P. Nelli, L. Sangaletti, G. Sberveglieri, Preparation and micro-structural characterization of nanosized thin film of TiO2±WO3 as a novel material with high sensitivity towards NO2, Sens. Actuators B 35/36 (1996) 381±383.
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[12] V. Guidi, D. Boscarino, E. Comini, G. Faglia, M. Ferroni, C. Malagu, G. Martinelli, V. Rigato, G. Sberveglieri, Preparation and characterization of titanium±tungsten sensors, Sens. Actuators B 65 (2000) 264±266. [13] D.S. Lee, S.D. Han, J.S. Huh, D.D. Lee, Nitrogen oxides-sensing characteristics of WO3-based nanocrystalline thick film gas sensor, Sens. Actuators B 60 (1999) 57±63. [14] D.S. Lee, S.D. Han, S.M. Lee, J.S. Huh, D.D. Lee, The TiO2-adding effects in WO3-based NO2 sensors prepared by coprecipitation and precipitation method, Sens. Actuators B 65 (2000) 331±335. [15] T. Nishide, F. Mizukami, Crystal structures and optical properties of tungsten oxide films prepared by a complexing-agent-assisted sol±gel process, Thin Solid Films 259 (1995) 212±217. [16] A. Hjelm, C.G. Granqvist, J.M. Wills, Electronic structure and optical properties of WO3, LiWO3, NaWO3, and HWO3, Phys. Rev. B 54 (1996) 2436±2445. [17] L. Sangaletti, L.E. Depero, G. Sberveglieri, B. Allieri, E. Bontempi, S. Groppelli, Growth of WO3 crystals from W±Ti±O thin films, J. Cryst. Growth 198/199 (1999) 1240±1244. [18] C. Cantalini, M. Pelino, H.T. Sun, M. Faccio, S. Santucci, L. Lozzi, M. Passacantando, Stability of NO2 sensors from WO3 thin film, Sens. Actuators B35/36 (1996) 112±118. [19] B. Pecquenard, H. Lecacheux, J. Livage, C. Julien, Orthorhombic WO3 formed via a Ti-stabilized WO31/3H2O phase, J. Solid State Chem. 135 (1998) 159±168. [20] M.J. Madou, S.R. Morrison, Solid-state background, in: Chemical Sensing with Solid State Devices, Academic Press, Boston, 1989, pp. 13±65. [21] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd Edition, Wiley, New York, 1974, pp. 618±708 (Chapter 9).
[22] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Grain size effects on gas sensitivity of porous SnO2-based elements, Sens. Actuators B 3 (1991) 147±155. [23] H. Ogawa, M. Nishikawa, A. Abe, Hall measurement studies and an electrical conduction model of tin oxide ultrafine particle films, J. Appl. Phys. 53 (1982) 4448±4455.
Biographies Jiann Shieh received his BS and MS degrees of Materials Science and Engineering from National Cheng Kung University, Tainan, Taiwan in 1995 and 1997, respectively. He has been a PhD candidate at National Cheng Kung University, Tainan, Taiwan, 1997. His major research has related to PECVD Ti±Al±C±N system nanocomposite thin-films, semiconductor gas sensor, and mesoporous materials. Hui Ming Feng received his BS and MS degrees of Materials Science and Engineering from National Cheng Kung University, Tainan, Taiwan in 1999 and 2001, respectively. 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, and nanomaterials. Horng Yih Juang received his PhD degree in Materials Science and Engineering from Nation Cheng Kung University in 1996. He is now a manager at Cyntec Company in Hsin-Chu, Taiwan.
WO3 and W±Ti±O thin-®lm gas sensors prepared by sol±gel dip-coating J. Shieha,*, H.M. Fenga, M.H. Hona, H.Y. Juangb a
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan b Cyntec Company Ltd., Hsin-Chu 300, Taiwan Received 16 July 2001; received in revised form 12 February 2002; accepted 5 March 2002
Abstract The WO3 and W±Ti±O thin-®lms were prepared by sol±gel dip-coating on alumina substrate using tungsten chloride, 2,4-pentanedione (PTN), ethanol and water as sources and calcined at 400 8C. The structure was characterized by XRD, Raman, SEM and TEM analyses. The effect of titanium doping on the structure was then discussed and related to the gas sensing properties. The results show that the sensitivities for detecting 10 ppm NO2 were 82 and 1768 by WO3 and W±Ti±O gas sensors at 200 8C, respectively. The huge increase in sensitivity by adding titanium ion arose from the grain size reduction of the ®lms and the doping effect on Debye length. The improved sensitivity, recovery, and reliability suggest that WO3 and W±Ti±O thin-®lms prepared by sol±gel dip-coating can be used for NO2 gas sensing. # 2002 Elsevier Science B.V. All rights reserved. Keywords: NO2 gas sensor; Tungsten oxide; Tungsten±titanium oxide; Sol±gel
1. Introduction Our environment has suffered damage as the economy expands. Chromatography, spectrology, electrochemistry and semiconductors are adopted to analyze kinds and concentration of the noxious gases. In order to improve the drawbacks of expensive, heavy, and time-consuming conventional gas analysis, many studies have focused on the thin-®lm gas sensors recently. Among the semiconductor thin-®lms for gas sensing, tungsten oxide (WO3) has the greatest sensitivity for detecting NO2 [1]. The processes to synthesize WO3 thin-®lms include physical vapor deposition [2,3], plasma enhanced chemical vapor deposition [4], sol± gel technology [5], and so on. In opposition to the drawback of expensive cost and compact structure in vapor deposition process, sol±gel process is a potential candidate for gas sensor fabrication. Moreover, with aqueous experimental conditions, self-assembly mechanism can be utilized to synthesize the nanomaterials (e.g. mesoporous materials) with the advantage of increasing speci®c surface areas to improve the sensitivity. The factors that in¯uence the sensitivity include grain size and the charge carrier concentration of the ®lms [6±8], which are in sequence determined by the type and concentration of doped impurities. For the WO3, titanium addition * Corresponding author. Tel.: 886-6-2380208; fax: 886-6-2380208. E-mail address: [email protected] (J. Shieh).
was found to have the advantages of inhibiting grain growth and improving sensitivity. Most of the studies on W±Ti±O thin-®lms were prepared by sputtering [9±12] and screenprinting technologies [13,14]. In this article, the WO3 and W±Ti±O thin-®lms prepared by dip-coating process were investigated for detecting NO2. The results show that both the WO3 and W±Ti±O thin-®lms prepared by sol±gel dipcoating had high NO2 sensitivity and recovery with fast and reproducible response. By means of titanium doping, the sensitivity was improved to the value of 1768 at 200 8C in the ¯owing NO2 gas of 10 ppm concentration, demonstrating that this process is an easy and promising way for fabricating NO2 gas sensor. 2. Experimental Sol±gel was prepared using a method similar to that of Nishide and Mizukami [15] as follows: 1 g WCl6 was ®rstly dissolved in 10 ml ethanol. After the solution turning blue, 1 ml of 2,4-pentanedione (PTN) was then added into the solution as an organic ligand. The sol was allowed to stand for 2 h, and 0.05 ml H2O was then added. The preparation of the W±Ti±O sols was the same as that of the WO3 sols except that additional TiCl4 was added into the solution. The Pt electrode with interdigital structure was deposited on alumina substrate by sputtering process, and the sensing ®lms were deposited on alumina substrate by dip-coatings
0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 1 5 0 - 8
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and withdrew at a speed of 1 cm/min and dried at 70 8C. After four coats, the samples were calcined at 400 8C for 1 h. The surface morphology was characterized by Philip XL-40FEG ®eld emission scanning electron microscopy (FE-SEM) and the structure was analyzed by X-ray diffraction using a Rigaku D/MAX diffractometer with a Cu Ka source; the chemical characteristics were investigated by Raman spectroscopy (Coherent, mode Innova 90); the composition of coatings was determined by energy dispersive spectrometry (EDS) equipped in SEM. Additional structure characterization was performed by transmission electron microscopy (TEM) at 300 kV with a Hitachi model HF2000 ®eld emission transmission electron microscopy. The samples for TEM were prepared by dispersing particles scraped from the coatings onto carbon ®lm on a copper grid. For the sensing behavior determination, the sensor was placed in a glass chamber (85 ml) with the gas ¯ow rate of 200 ml/min. The NO2 gas and air were metered by mass ¯ow controllers, respectively. The data of the electrical response were acquired by a computer with an IEEE interface board (HP 34970A). The gas sensitivity S is de®ned as the ratio of electrical resistance in NO2 (Rg) and that in air (Ra), i.e. S Rg =Ra . 3. Results and discussion The perfect crystal structure of WO3 is ReO3 type with the corner-sharing packing of [WO6]6 octahedra, which is distorted as the fabrication temperature is low as well as titanium ion is doped. Although the doped elements usually occupy the interstitial sites of WO3 to form perovskite crystal structure [16], titanium atoms prefer to replace tungsten atoms in ReO3 structure [10,17]. Fig. 1 presents the XRD patterns for the WO3 and W±Ti±O coatings annealed at 400 8C, which were identi®ed with JCPDS number of 830951 for the monoclinic indicating. It can be observed that WO3 ®lm is strongly (2 0 0) orientation, and W±Ti±O ®lm is strongly (0 2 0) orientation. Cantalini et al. stated that the (2 0 0) preferred orientation might originate from the epitaxial growth of WO3 ®lm on Al2O3 substrate because the distance between oxygen atoms in the Al2O3 (1 1 6) plane and the WO3 (2 0 0) plane is close [18]. For W±Ti±O ®lm, Sangaletti et al. have reported that the preferred orientation derived from the disordered cubic phase [9]. The relative intensity of (0 0 2), (0 2 0), and (2 0 0) peaks different from the monoclinic phase with a more symmetry pro®le indicates that the monoclinic lattice was distorted. Depero et al. [10] demonstrated that this cubic-like X-ray diffraction pattern might result from a static disorder introduction as edge-sharing octahedra randomly distributed in the lattice by oxygen de®ciency since the diffraction peak of (1 1 0) plane of WO2.9 is located at 23.7718 that is near the {2 0 0} diffraction peaks of WO3. Raman spectroscopy, which is sensitive to the changes in translational symmetry, was then employed to con®rm the
Fig. 1. X-ray diffraction patterns for: (a) W±Ti±O (Ti/(W Ti) 19.38 at.%), and (b) WO3 thin-films annealed at 400 8C with alumina substrate and JCPDS files of WO3, and WO2.9 as reference.
chemical state, as shown in Fig. 2. The bands appeared at 250, 700, and 780 cm 1 corresponding to the WO3 monoclinic phase [19], and the broad pro®le is indicative of the defects that is abundant in n-type semiconductor [20]. In addition, because oxygen was de®cient as Ti with the lower oxidation state substituted for W, Ti cations also contributed to the disorder structure through the transformation of corner-sharing octahedra to edge-sharing octahedra.
Fig. 2. Raman spectra of WO3 and W±Ti±O Ti= W Ti 19:38 at.%) thin-films annealed at 400 8C.
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Fig. 3. SEM surface morphology of: (a) WO3, and (b) W±Ti±O Ti= W Ti 19:38 at.%) thin-films annealed at 400 8C.
Surface morphology characterization by SEM observation indicates that the coatings consisted of nanograins with average sizes of 76.7 nm and 56.4 nm for WO3 and W±Ti±O sensors, respectively, as shown in Fig. 3. The atomic ratio of Ti/(Ti W) in the W±Ti±O ®lm analyzed by EDS was 19.38 at.%. However, the grain size of WO3 and W±Ti±O thin-®lms determined by Scherrer's equation [21] from XRD patterns were 33.8 and 17.9 nm, respectively. The different results might be attributed to the agglomeration of grains in the SEM observation. Fig. 4 shows the TEM bright ®eld images of WO3 and W±Ti±O particles that agreed with the results of grain size determination obtained from XRD analyses, indicating that the grain growth was suppressed by impurity effect during crystallization. The gas sensing mechanism is that as the solid adsorbs more electronegative gas, the solid loses electron to the gas, causing a space charge layer (or depletion layer) formed near the surface where the uncompensated donor ions survive. Schottky barrier is then built up which the electron should overcome to cross this region. Since NO2 has a stronger electron af®nity than O2, the resistance is increased as NO2 is adsorbed. This resistance variation is used to detect NO2 in air. Fig. 5 presents the effects of temperature on the response and recovery transients of WO3 for 10 ppm NO2, showing that the slower desorption rate of NO2 at a lower temperature led to the longer recovery time. Fig. 6 shows the sensitivity as a function of temperature for WO3 sensor in 10 ppm NO2 ambient. The sensitivity reaching a peak value of 82 at 200 8C derived from the competition between gas adsorption and desorption mechanisms. The sensitivity increased as temperature increased at a low temperature since
Fig. 4. TEM bright field images of: (a) WO3, and (b) W±Ti±O Ti= W Ti 19:38 at.%) films annealed at 400 8C.
Fig. 5. Resistance response of WO3 gas sensor for 10 ppm NO2 in dry air at different temperatures between 120 and 250 8C.
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Fig. 6. Relation between sensitivity and operating temperature for WO3 gas sensor.
temperature provided the active energy for chemisorption. At a higher temperature, on the other hand, the surface recovery was reduced by a progressive gas desorption to decrease the sensitivity [20]. Fig. 7 shows the response to NO2 at 200 8C for WO3 sensor calcined at 400 8C. The response and recovery time (time for 90% of resistance change) for NO2/air mixture in the 10±25 ppm range was about 4 min with a good reproducibility. The calibration curve shown in Fig. 8 indicates that the relation between sensitivity and concentration is linear, which is a bene®t for an actuator to respond to detect different concentrations of noxious gas. Fig. 9 shows the electrical responses of WO3 and W±Ti±O sensors at 200 8C in ¯owing NO2 gas of 10 ppm. The sensitivities for WO3 and W±Ti±O were 82 and 1768, respectively, as shown in Figs. 8 and 10. The pronounced sensitivity increased by Ti doping is explained as follows. The relation between the thickness of space charge layer (L) and Debye length (LD) is r 2eVs L LD (1) kT
Fig. 7. Response transients of WO3 gas sensor to different NO2 concentration between 10 and 25 ppm at 200 8C.
Fig. 8. Sensitivity variation as a function of NO2 concentration for WO3 gas sensor operated at 200 8C.
where eVs is the height of surface barrier that depends on the kind of adsorbed gases, and kT corresponds to the thermal energy [6,20]. As the grain size is larger than the dimension of 2L, the sensitivity is independent of grain size since the resistance arises from the grain boundary where the environment gases adsorb (grain boundary control model). On the other hand, as the grain size is smaller than the dimension of 2L, the sensitivity is determined by grain itself since the carrier is depleted in the whole grain (grain control model) [22]. Using these grain size models of sensitivity, Tamaki et al. [7] have discussed the grain size effects on the sensitivity of WO3. They controlled the grain size by changing the sintering temperature and demonstrated that steeply increased sensitivity arose at a critical grain size in which the
Fig. 9. Response transients of WO3 and W±Ti±O Ti= W Ti 19:38 at.%) gas sensors to 10 ppm NO2 at 200 8C.
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4. Conclusion Sol±gel dip-coated WO3 and W±Ti±O gas sensors with nanocrystallite characteristic and disorder-cubic structure were obtained as identi®ed by XRD, Raman, SEM and TEM analyses. The WO3 ®lms annealed at 400 8C exhibit an optimal working temperature of 200 8C for 10 ppm NO2. The sensitivity was increased from 82 to 1768 as titanium was added into the WO3 gas sensor. The reduced grain size and increased space charge layer arose from Ti doping effect and corresponded to this sensitivity improvement. The high sensitivity with good recovery and the linear relation between sensitivity and gas concentration indicate that WO3 and W±Ti±O gas sensors prepared by sol±gel dip-coating process are suitable for detecting the noxious NO2 gas. Fig. 10. Sensitivity variation as a function of NO2 concentration for W±Ti±O Ti= W Ti 19:38 at.%) gas sensor operated at 200 8C.
Acknowledgements conducting mechanism changed from grain boundary control to grain control as WO3 was exposed to air and NO2, respectively. In the present study, as Ti was added to WO3, the carrier concentration was decreased as follows: WO3
Ti ! Ti00W 2h
(2)
The creation of hole eliminated some electrons and then increased the Debye length since WO3 is an n-type semiconductor in which electron is a major charge carrier. Under the same gas type and temperature, L depends on LD that increases as the carrier concentration decreases, according to the relation [23] LD
EkT q2 N
1=2
(3)
where E is the dielectric constant, k the Boltzmann's constant, T an absolute temperature, q the electrical charge of the carrier, and N the carrier concentration. The enlarged Debye length has the similar effects as grain size reduction on the sensitivity when the grain size is about twice of space charge layer. Both of them are conducive to the model transition from grain boundary control to grain control. Therefore, the sensitivity improvement of W±Ti±O sensor in Fig. 9 indicates that the value of 2L was smaller than grain size as the sensor gas was exposed to air, but after adsorbing NO2, the value of 2L was larger than the grain size. The identical electric resistance of the WO3 sensor and the W±Ti±O sensor as they were exposed to air indicates that the value of 2L in air was smaller than 17.9 nm, and the steeply increasing electrical resistance of the W±Ti±O sensor indicates that the value of 2L in 10 ppm NO2 was between 17.9 nm and 33.8 nm. Similar calibration curve for W±Ti±O ®lms is presented in Fig. 10, in which a linear relation between concentration and sensitivity is also favorable for actuator to control the gas sensor.
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Biographies Jiann Shieh received his BS and MS degrees of Materials Science and Engineering from National Cheng Kung University, Tainan, Taiwan in 1995 and 1997, respectively. He has been a PhD candidate at National Cheng Kung University, Tainan, Taiwan, 1997. His major research has related to PECVD Ti±Al±C±N system nanocomposite thin-films, semiconductor gas sensor, and mesoporous materials. Hui Ming Feng received his BS and MS degrees of Materials Science and Engineering from National Cheng Kung University, Tainan, Taiwan in 1999 and 2001, respectively. 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, and nanomaterials. Horng Yih Juang received his PhD degree in Materials Science and Engineering from Nation Cheng Kung University in 1996. He is now a manager at Cyntec Company in Hsin-Chu, Taiwan.