Sensors and Actuators B 140 (2009) 514–519 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Facile synthesis and NO 2 gas sensing of tungsten oxide nanorods assembled microspheres Zhifu Liu a,∗ , Masashio Miyauchi a,∗ , Toshinari Yamazaki b , Yanbai Shen b a Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan b School of Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan article info Article history: Received 14 January 2009 Received in revised form 31 March 2009 Accepted 25 April 2009 Available online 7 May 2009 Keywords: Tungsten oxide Microsphere Nanorod NO 2 Gas sensor abstract Tungsten oxide nanorods assembled microspheres were synthesized by a facile hydrothermal process at 180 ◦ C using ammonium metatungstate and oxalic acid as starting materials. The morphology and structural properties were investigated using scanning electron microscopy, powder X-ray diffraction, and transmission electron microscopy. The as-synthesized microspheres are composed of orthorhombic WO 3 ·xH 2 O nanorods with diameter less than 100 nm. These microspheres lose water gradually during annealing and transfer to monoclinic WO 3 when annealed at 550 ◦ C. The gas sensing properties of the microspheres annealed at different temperatures were studied by exposing the gas sensors made from microspheres to NO 2 gas. The results indicated that the crystalline phase of the microspheres has no obvious effect on the gas sensing performance. The microspheres annealed at 350 ◦ C showed fast and the highest response to NO 2 gas due to the three-dimensional network based on the nanorods and the high effective surface area. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Nanostructured materials are considered as good candidates for gas sensing applications due to their large surface area-to-volume ratio and the size effect. Since the report of enhanced gas sensing performance of tin oxide nano-crystallites in 1990s [1], nanomate- rials based gas sensors attracted more and more attentions [2,3]. Nanostructures of well-established gas sensing materials like tin oxide [4–6], zinc oxide [7], tungsten oxide [8–10], titanium oxide [11,12], and indium oxide [13,14] have shown higher sensitivity, faster response, lower operating temperature, and/or enhanced capability to detect low concentration gases compared withthe thin film counterparts. Tungsten oxides are a class of versatile materials that offer manifold technological applications including gas sensors [15,16], opto-electrochromic and optical modulation devices [17,18], pho- tocatalysis [19], hydrophilic surface design [20], etc. Gas sensors based on tungsten oxide are sensitive to a variety of gases such as NO 2 ,O 3 ,H 2 ,H 2 S, andNH 3 [21]. In particular, tungsten oxide showed superior sensitivity and selectivity in detecting NO 2 gas [22,23].On the other hand, nanostructural tungsten oxide such as nanorods [24], nanowires [25], nanotubes [26], nanoflakes [27], nanodisks ∗ Corresponding authors. E-mail addresses: zhifu liu@yahoo.com (Z. Liu), m-miyauchi@aist.go.jp (M. Miyauchi). [28], and nanotrees [29] have been synthesized using high tem- perature evaporation, precipitation, hydrothermal reaction, and electrochemical or template assisted methods. These nanostruc- tures provide good blocks for developing high performance gas sensors. Herein, we report the synthesisof tungsten oxide nanorods assembled microspheres by a facile hydrothermal method. To our knowledge, there isno report on thegas sensing of the microsphere- like tungsten oxide nanostructures. We expect that this kind of microsphere with nanorod substructure will benefit to the gas sens- ing performance of their based gas sensors. 2. Experimental 2.1. Synthesis A facile hydrothermal process was employed to synthesize the samples. Ammonium metatungstate and oxalic acid (99.9%, Wako Pure Chemicals Co.) were used as starting materials. In a typical experiment, 0.53 g ammonium metatungstate and 0.72 g oxalic acid (OA) (the mole ratio of OA/W is 4:1) were dissolved in 50 ml deion- ized water. Clear solution was obtained after stirring for 30 min. Then, the mixture solution was transferred into a 100 ml Teflon- lined stainless autoclave. The autoclave was sealed and maintained at 180 ◦ C for 8 h. After the reaction completed, the resulting prod- uct was centrifuged and washed with deionized water for three times, and then dried at 60 ◦ C overnight. Part of the product thus treated was annealed at 350, 450, and 550 ◦ C for 5 h, respectively, 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.04.059 Z. Liu et al. / Sensors and Actuators B 140 (2009) 514–519 515 for investigating the crystal structure, morphology change, and the gas sensing properties. For comparison, samples were also synthe- sized at OA/W ratios of 2:1, 3:1, and 5:1 with a fixed tungsten ion concentration by the same synthesis process. 2.2. Structural characterization X-ray diffraction (XRD) measurements were performed on an X-ray diffractometer (Rigaku, Ultrax 18SF) with an imaging plate detector using Cu Ka radiation. A Hitachi S-4800 field emission scanning electron microscope (FESEM) was used to investigate the morphology of the samples. Transmission electron microscopy (TEM) characterization was carried out on a Hitachi S-9000 transmission electron microscope. The effective surface area was measured using physical adsorption/desorption of Kr on a Quan- tachrome AUTOSORB-1-MP facility. 2.3. Gas sensing measurements The gas sensors were made by drop-casting method. Briefly, desired amount of the synthesized powder was dispersed in methanol with the assist of ultrasonic. Then, the suspension was dispensed dropwise onto the oxidized Si substrate with a pair of interdigitated Pt electrodes. The gas sensors were ready for char- acterization after dried and then aged at 350 ◦ C for 2 h. The gas sensing properties were measured in a tube system with a coil resistance heater. The carrier gas (dry synthetic air) mixed with a desired concentration of NO 2 gas was flowed at 200 ml/minthrough the quartz tube (45 mm in diameter and 400 mm in length) kept at a set temperature. The electrical measurement was performed by a voltamperometric method at a constant bias of 10 V, and a multimeter (Agilent 34970A) was used to monitor the change of electrical resistance upon turning the target gas on and off. The sensor response is defined as (R a − R o )/R o , where R o is the resis- tance in air and R a is the maximum resistance after the NO 2 gas was introduced. 3. Results and discussion 3.1. Structure and morphology All the as-synthesized products are powders with white blue color. The XRD patterns of the samples dried at 60 ◦ C are shown in Fig. 1. The results indicate that the products synthesized with OA/W ratio of 2:1, 3:1, 4:1, and 5:1 are all crystallized and have the same crystalline structure. The peaks of the XRD patterns can match well with the documented diffraction pattern of orthorhombic WO 3 ·0.33H 2 O (JCPDS card no. 35-0270). Considering the possibility of the variation of water in the structure during drying and anneal- ing, we assign the formula of WO 3 ·xH 2 O to the samples containing water in our experiments. Despite of the same phase composition, the morphology of the products synthesized with different OA/W ratio is very different. Fig. 2 presents the FESEM images of the samples synthesized with OA/W ratio of 2:1, 3:1, 4:1, and 5:1. The sample synthesized with OA/W ratio of 2:1 shows sphere-like aggregate with nanoplatelet substructure. The nanoplatelet substructure can still be observed when the ratio of OA to tungsten is increased to 3:1. However, the products change to nanorod-like morphology when the ratio of OA to tungsten is 4:1. These nanorods assemble to microspheres with average diameter of around 3 m. For the sample synthesized at an OA/W ratio of 5:1, radial nanorod aggregates are obtained. Since no other templates or assistant agents were used in our experiments, the formation of various microstructures at different OA/W ratios should be ascribed to the interaction between tungsten ions and OA. It is known that OA can stabilize the hydrolyzed tungsten oxide Fig. 1. XRD patterns of the products synthesized with oxalic acid/tungsten mole ratios of (a) 2:1; (b) 3:1; (c) 4:1; and (d) 5:1. nanocrystals in aqueous solution by forming coordination com- plex [30]. The OA ligand would af fect the growth direction of the nanocrystals by binding to specific surface of the nanocrystals. The WO 3 ·xH 2 O nanocrystals can grow in the platelet habit in the pres- ence of a small amount of OA. However, in the presence of large amount of OA, for example when the OA/W ratio is 4, the crystal- lization habit is changed by the surrounding OA ligands, leading to the formation of rods like morphology. On the other hand, the intermolecular force among the OA molecules may contribute to the formation of the microsphere morphology [31]. The NH 4 + ions in the solution may also affect the microstructure formation [32]. More detailed work should be done to clarify the self-assembly mechanism of the microspheres. Fig. 3 shows a typical TEM image of the nanorods obtained with OA/W ratio of 4:1. These nanorods have an average diameter less than 100 nm and length in micrometer level. A correspond- ing diffraction pattern of the nanorods is also presented in Fig. 3. Diffraction rings can be clearly seen. The diffraction pattern, which can be indexed to orthorhombic phase, is consistent with the XRD results. 3.2. Effect of annealing on structural properties Here we choose the microspheres synthesized with OA/W ratio of 4:1 to investigate the gas sensing properties. Since gas sensor requires a material to work continuously at high temper- ature condition, the microspheres were annealed to stabilize the microstructure. Fig. 4 represents the XRD patterns of the micro- spheres annealed at 350, 450, and 550 ◦ C, respectively. The sample lost the crystalline water after annealed at 350 ◦ C and transferred to hexagonal WO 3 (JCPDS card no.33-1387). With the increase of the annealing temperature, the diffraction peaks at 23–25 ◦ and the peaks at 26–30 ◦ separate gradually, indicating that the phase changed after annealing at higher temperature. The sample com- pletely transferred to monoclinic WO 3 (JCPDS card no. 43-1035) after annealed at 550 ◦ C for 5 h. The FESEM images of the samples annealed at different temper- atures are shown in Fig. 5. For the samples annealed at 350 and 450 ◦ C, the nanorods on the microsphere surface were damaged to some extent. However, the nanorods inside the microspheres can still be clearly seen. The nanorod substructure was totally destroyed for the sample annealed at 550 ◦ C and these nanorods changed to nanoparticles. However, it was noticed that the microsphere morphology still exists for the samples annealed at all conditions. 516 Z. Liu et al. / Sensors and Actuators B 140 (2009) 514–519 Fig. 2. SEM images of the products synthesized with oxalic acid/tungsten mole ratios of (a) 2:1; (b) 3:1; (c) 4:1; and (d) 5:1. These microspheres are very helpful for forming porous sensing layer. The effective surface areas of the samples were evaluated by isothermal Kr gas physical adsorption/desorption measurements and are shown in Fig. 6. The sample dried at 60 ◦ C has an effec- tive surface area of 36 m 2 /g. The effective surface area decreases after annealing and is 20, 13, and 6 m 2 /g for the samples annealed at 350, 450, and 550 ◦ C, respectively. Crystal growth and the par- tial collapse of the substructure of the microspheres should be the main reason of the decrease of effective surface area of the annealed samples. Fig. 3. TEM image of the WO 3 ·xH 2 O nanorods synthesized with an oxalic acid/tungsten ratio of 4:1. Inset is the corresponding diffraction pattern. 3.3. Gas sensing properties The gas sensing properties of the annealed microspheres were evaluated by exposing the microspheres based gas sensors to NO 2 gas. Fig. 7 shows the typical resistance change profiles of the microsphere based gas sensors upon exposed to 1 ppm NO 2 gas at different operating temperatures. The sensor responses quickly to NO 2 gas at all operating temperatures. The response times (the time for the resistance increase to 90% of the maximum) are less than 3 min in all cases, which are much quicker than that of sput- tered WO 3 thin film sensors measured using the same system [33]. However, we also notice that the sensor cannot recover to initial resistance at low operating temperatures of 100 and 150 ◦ C after the Fig. 4. XRD patterns of the WO 3 ·xH 2 O microspheres (a) dried at 60 ◦ C and annealed at (b) 350 ◦ C; (c) 450 ◦ C; and (d) 550 ◦ C. Z. Liu et al. / Sensors and Actuators B 140 (2009) 514–519 517 Fig. 5. SEM images of the WO 3 ·xH 2 O microspheres annealed at (a) 350 ◦ C; (b) 450 ◦ C; and (c) 550 ◦ C. NO 2 gas was turned off. The sensor can recover to initial resistance only at temperatures above 200 ◦ C. Fig. 8 represents the responses of the sensors based on 350, 450, and 550 ◦ C annealed microspheres as a function of operat- ing temperatures. These sensors exhibit very high response at low operating temperature. For example, the sensor based on micro- spheres annealed at 350 ◦ C showed a sensor response up to 3000 when operated at 100 ◦ C. It is more than 10 times larger than the sensor response of the thin film counterpart [33]. For all the three kinds of materials, the sensor response decreases with the increase of operating temperature. However, it can be noticed that the sen- Fig. 6. Effective surface area of the WO 3 ·xH 2 O microspheres (a) dried at 60 ◦ C and annealed at (b) 350 ◦ C; (c) 450 ◦ C; and (d) 550 ◦ C. Fig. 7. The resistance change profile of the microspheres (annealed at 350 ◦ C) based gas sensor to 1 ppm NO 2 gas at different operating temperatures. Fig. 8. The sensor response of the microspheres based gas sensor to 1 ppm NO 2 gas as a function of operating temperatures. 518 Z. Liu et al. / Sensors and Actuators B 140 (2009) 514–519 Fig. 9. The dynamic response of the gas sensors based on microspheres annealed at (a) 350 ◦ C, (b) 450 ◦ C, and (c) 550 ◦ C to 1, 3, 5, 10, and 20 ppm NO 2 gas pulses. The sensor response as a function of gas concentration is shown in (d). sor based on microspheres annealed at 350 ◦ C showed a relatively higher response than the others. The quick and high response of the sensors should be ascribed to the distinctive microsphere structure with nanorod substructure. It is accepted that, upon exposure to NO 2 gas, the NO 2 gas molecules are directly absorbed on the active sites on tungsten oxide surface. Charge transfer is likely to occur from WO 3 to absorbed NO 2 because of the strong electron-withdrawing power of the NO 2 molecules, which leads to the increase of thickness of the depletion layer [34]. The nanorod substructure in the microspheres can be fully depleted by exposing to NO 2 gas. As a result, the barrier heights at the bound- aries between the nanorods increase significantly, resulting in the large increase in electrical resistance, i.e., the high sensor response. On the other hand, for a thin film and thick film gas sensor, the gas diffusion is one of the key factors that determines the sensor response and response time [35]. In the present work, the sensing layer made by microspheres is highly porous. The gas can reach the deep layer of the microspheres based thick film quickly through the pore network. So the effects of gas diffusion can be ignored and the surface phenomena such as adsorption/desorption of NO 2 molecules should be the dominating factor of the sensor perfor- mance. This is supported by the fast response of the sensors to NO 2 gas. In order to investigate the relation between the sensor response and gas concentration, the sensors were exposed to NO 2 gas with concentrations of 1, 3, 5, 10, and 20 ppm. Fig. 9a–c indicates the dynamic response of the sensors based on the microspheres annealed at 350, 450, and 550 ◦ C at an operating tempera- tureof200 ◦ C. All the three kinds of sensors exhibit good response/recovery cycle to the NO 2 gas pulses and the sensor responses increase with the increase of gas concentrations. Fig. 9d shows the profiles of the sensor responses as a function of NO 2 gas concentrations. The sensor responses increase nearly linearly with the increase of NO 2 gas concentration. It can also be noticed that the concentration coefficient (the slop of the lines, assigned as S A350 ,S A450 , and S A550 , respectively) of the sensor response depends on the annealing temperature and follow the trend: S A350 >S A450 >S A550 . The higher effective surface area should ben- efit to the higher response of the sensor based on 350 ◦ C annealed microspheres. In addition, as shown previously, the phase compositions of the annealed microspheres are different and the crystal phase changes from hexagonal to monoclinic structure gradually when the anneal- ing temperature increases from 350 to 550 ◦ C. The gas sensing of monoclinic tungsten oxide has been extensively studied. There are also reports on the gas sensing of hexagonal tungsten oxide [36,37]. In our present work, there is no obvious difference in the gas sens- ing performance among the hexagonal, monoclinic, and the mixed phase tungsten oxide. This implies that it is possible to obtain higher sensor response by using the materials annealed at lower tempera- ture such as 350 and 450 ◦ C under which the nanorod substructure can be well kept. 4. Conclusions In conclusion, the microspheres composed of WO 3 ·xH 2 O nanorod were synthesized by a facile hydrothermal process. The microsphere morphology and the nanorod substructure can be reserved when annealed at temperatures lower than 450 ◦ C. Gas sensing properties of the microspheres annealed at different tem- peratures were investigated by exposing the microspheres based gas sensors to NO 2 gas. The gas sensor based on 350 ◦ C annealed Z. Liu et al. / Sensors and Actuators B 140 (2009) 514–519 519 microspheres showed relative higher response to NO 2 gas than oth- ers. Phase composition of the microspheres had no obvious effect on the gas sensing performance. This kind of microspheres with nanorod substructure provides a new block for developing high performance gas sensors. Acknowledgements This work is supported by the New Energy and Industrial Tech- nology Development Organization (NEDO) in Japan and was partly conducted using the AIST Nano-Processing Facility, which is sup- ported by the “Nanotechnology Support Project” of the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] J. Tamaki, C. Xu, N. Miura, N. Yamazoe, Grain size effects on gas sensitivity of porous SnO 2 -based elements, Sens. Actuators B 3 (2) (1991) 147–155. [2] E. Comini, Metal oxide nano-crystals for gas sensing, Anal. Chim. Acta 568 (1-2) (2006) 28–40. [3] A. Kolmakov, M. Moskovits, Chemical sensing and catalysis by one-dimensional metal-oxide nanostructures, Ann. Rev. Mater. Res. 34 (2004) 151–180. [4] A. Kolmakov, Y.X. Zhang, G.S. Cheng, M. Moskovits, Detection of CO and O 2 using tin oxide nanowire sensors, Adv. Mater. 15 (12) (2003) 997–1000. [5] E. Comini, G. Faglia, G. Sberveglieri, D. Calestani, L. Zanotti, M. Zha, Tin oxide nanobelts electrical and sensing properties, Sens. Actuators B 111 (2005) 2–6. [6] Y.B. Shen, T. Yamazaki, Z.F. Liu, D. Meng, T. Kikuta, N. Nakatani, M. Saito, M. Mori, Microstructure and H 2 gas sensing properties undope d and Pd-doped SnO 2 nanowires, Sens. Actuators B 135 (2009) 524–529. [7] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors, Appl. Phys. Lett. 84 (18) (2004) 3654–3656. [8] A. Ponzoni, E. Comini, G. Sberveglieri, J. Zhou, S.Z. Deng, N.S. Xu, Y. Ding, Z.L. Wang, Ultrasensitive and highly selective gas sensors using three-dimensional tungsten oxide nanowire networks, Appl. Phys. Lett. 88 (20) (2006) 203101. [9] K.M. Sawicka, A.K. Prasad, P.I. Gouma, Metal oxide nanowires for use in chemical sensing applications, Sens. Lett. 3 (1) (2005) 31–35. [10] C.S. Rout, A. Govindaraj, C.N.R. Rao, High-sensitivity hydrocarbon sensors based on tungsten oxide nanowires, J. Mater. Chem. 16 (40) (2006) 3936–3941. [11] O.K. Varghese, C.A. Grimes, Metal oxide nanoarchitectures for environmental sensing, J. Nanosci. Nanotechnol. 3 (4) (2003) 277–293. [12] A.S. Zuruzi, A. Kolmakov, N.C. Macdonald, M. Moskovits, Highly sensitive gas sensor based on integrated titania nanosponge arrays, Appl. Phys. Lett. 88 (10) (2006) 102904. [13] C. Li, D.H. Zhang, X.L. Liu, S. Han, T. Tang, J. Han, C.W. Zhou, In 2 O 3 nanowires as chemical sensors, Appl. Phys. Lett. 82 (10) (2003) 1613–1615. [14] C. Li, D.J. Zhang, X.L. Liu, S. Han, T. Tang, C.W. Zhou, Doping dependent NH 3 sensing of indium oxide nanowires, Appl. Phys. Lett. 83 (9) (2003) 1845–1847. [15] H.T. Sun, C. Cantalini, L. Lozzi, M. Passacantando, S. Santucci, M. Pelino, Microstructural effect on NO 2 sensitivity of WO 3 thin film gas sensors. Part 1. Thin film devices, sensors and actuators, Thin Solid Films 287 (1996) 258–265. [16] C. Cantalini, W. Wlodarski, Y. Li, M. Passacantando, S. Santucci, E. Comini, G. Faglia, G. Sberveglieri, Investigation on the O 3 sensitivity properties of WO 3 thin films prepared by sol–gel, thermal evaporation and r.f. sputtering techniques, Sens. Actuators B 64 (1-3) (2000) 182–188. [17] T. Oi, Electrochromic materials, Ann. Rev. Mater. Sci. 16 (1986) 185–201. [18] S.K. Deb, Opportunities and challenges in science and technology of WO 3 for electrochromic and related applications, Solar Energy Mater. Solar Cells 92 (2008) 245–258. [19] R. Abe, H. Takami, N. Murakami, B. Ohtani, Pristine simple oxides as visible light driven photocatalysts: highly efficient decomposition of organic compounds over platinum-loaded tungsten oxide, J. Am. Chem. Soc. 130 (2008) 7780–7781. [20] M. Miyauchi, A. Nakajima, T. Watanabe, K. Hashimoto, Photocatalysis and pho- toinduced hydrophilicity of various metal oxide thin films, Chem. Mater. 14 (2002) 2812–2816. [21] G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Oxide materials for development of integrated gas sensors—a comprehensive review, Crit. Rev. Solid State Mater. Sci. 29 (2004) 111–188. [22] M. Akiyama, J. Tamaki, N. Miura, N. Yamanoe, Tungsten oxide based semi- conductor sensor highly sensitive to NO and NO 2 , Chem. Lett. 20 (1991) 1611–1614. [23] M. Akiyama, Z. Zhang, J. Tamaki, N. Miura, N. Yamanoe, Tungsten oxide based semiconductor sensor for detection of nitrogen oxides in combustion exhaust, Sens. Actuators B 14 (1993) 619–620. [24] Y.S. Kim, S.C. Ha, K. Kim, H. Yang, S.Y. Choi, Y.T. Kim, Room-temperature semi- conductor gas sensor based on nonstoichiometric tungsten oxide nanorod film, Appl. Phys. Lett. 86 (2005) 213105. [25] J. Polleux, A. Gurlo, N. Barsan, U. Weimar, M. Antonietti, M. Niederberger, Template-free synthesis and assembly of single-crystalline tungsten oxide nanowires and their gas-sensing properties, Angew. Chem. Int. Ed. 118 (2005) 267–271. [26] Z.G. Zhao, M. Miyauchi, Nanoporous-walled tungsten oxide nanotubes as highly active visible-light-driven photocatalysts, Angew. Chem. Int. Ed. 47 (2008) 7051–7055. [27] Z.F. Liu, T. Yamazaki, Y.B. Shen, D. Meng, T. Kikuta, N. Nakatani, Fabrication of WO 3 nanoflakes by a dealloying-based approach, Chem. Lett. 37 (2008) 296–297. [28] Y.G. Choi, G. Sakai, K. Shimanoe, Y. Teraoka, N. Miura, N. Yamazoe, Preparation of size and habit-controlled nano cr ystallites of tungsten oxide, Sens. Actuators B 93 (2003) 486–494. [29] M. Shibuya, M. Miyauchi, Site-selective deposition of metal nanoparticles on aligned WO 3 nano-trees for super-hydrophilic thin film, Adv. Mater. 21 (2009) 1373–1376. [30] E. Lassner, W. Schubert, Tungsten: Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds, Kluwer Academic, New York, 1999. [31] Z. Gu, T. Zhai, B. Gao, X. Sheng, Y. Wang, H. Fu, Y. Ma, J. Yao, Controllable assembly of WO 3 nanorods/nanowires into hierarchical nanostructures, J. Phys. Chem. B 110 (2006) 23829–23836. [32] J.H. Ha, P. Muralidharan, D.K. Kim, Hydrothermal synthesis and characteriza- tion of self-assembled h-WO 3 nanowires/nanorods using EDTA salts, J. Alloys Compd. 475 (2009) 446–451. [33] Z.F. Liu, T. Yamazaki, Y.B. Shen, T. Kikuta, N. Nakatani, Influence of annealing on microstructure and NO 2 -sensing properties of sputtered WO 3 thin films, Sens. Actuators B 128 (2008) 173–178. [34] J. Tamaki, Z. Zhang, K. Fujimori, M. Akiyama, T. Harada, N. Miura, N. Yama- noe, Grain-size effects in tungsten oxide-based sensor for nitrogen oxides, J. Electrochem. Soc. 141 (1994) 2207–2210. [35] N. Yamazoe, Toward innovation of gas sensor technology, Sens. Actuators B 108 (2005) 2–14. [36] C. Balázsi, L. Wang, E.O. Zayim, I.M. Szilágyi, K. Sedlacková, J. Pfeifer, A.L. Tóth, P.I. Gouma, Nanosize hexagonal tungsten oxide for gas sensing applications, J. Eur. Ceram. Soc. 28 (2008) 913–917. [37] L. Wang, J. Pfeifer, C. Balázsi, P.I. Gouma, Synthesis and sensing properties to NH 3 of hexagonal WO 3 metasatble nanopowders, Mater. Manufact. Proc. 22 (6) (2007) 773–776. Biographies Zhifu Liu is a researcher at National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan. He received his Ph.D. degree from Shanghai Insti- tute of Ceramics, Chinese Academy of Sciences, in 2004. His current research interest is nanostructured semiconductor materials and their based devices for environmen- tal and clean energy applications. Masahiro Miyauchi is a senior research scientist at National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan. He received his Ph.D. degree from the University of Tokyo in 2002. His research interests are in the areas of nanostructured materials for energy and environmental issues. Toshinari Yamazaki received his Ph.D. degree from Nagoya University, Nagoya, Japan, in 1983. He is currently an associate professor at University of Toyama. His research interests are in the areas of semiconducting oxide gas sensors and the deposition process of sputtered films. Yanbai Shen is a Ph.D. student at University of Toyama, Japan. He received his MS degree at Northeastern University, China, in 2004. His current research is focused on the microstructural, electrical, and gas sensing properties of oxide semiconductor thin films. . homepage: www.elsevier.com/locate/snb Facile synthesis and NO 2 gas sensing of tungsten oxide nanorods assembled microspheres Zhifu Liu a,∗ , Masashio. 2009 Keywords: Tungsten oxide Microsphere Nanorod NO 2 Gas sensor abstract Tungsten oxide nanorods assembled microspheres were synthesized by a facile hydrothermal