glucose - mediated hydrothermal synthesis and gas sensing characteristics of wo3 hollow microspheres

Sensors and Actuators B 142 (2009) 236–242 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Glucose-mediated hydrothermal synthesis and gas sensing characteristics of WO 3 hollow microspheres Choong-Yong Lee, Sun-Jung Kim, In-Sung Hwang, Jong-Heun Lee ∗ Department of Materials Science and Engineering, Korea University, Anam-Dong, Sungbuk-Gu, Seoul 136-713, Republic of Korea article info Article history: Received 21 May 2009 Received in revised form 29 July 2009 Accepted 18 August 2009 Available online 21 August 2009 Keywords: WO 3 NO 2 sensor Hollow microspheres Carbon template abstract Tungsten-coated carbon microspheres were prepared by one-pot hydrothermal reaction of an aqueous solution containing glucose and sodium tungstate. The spheres were converted into WO 3 hollow micro- spheres by the decomposition of their core carbon. The [glucose]/[sodium tungstate] ratio of the stock solution determined not only the morphology of the precursors but also the phase of the powders after calcination. The WO 3 hollow microspheres showed a higher gas response and more selective detec- tion of 0.5–2.5 ppm NO 2 than WO 3 solid and nano-porous microspheres did. The enhanced NO 2 sensing characteristics are explained in relation to the surface area, pore volume, and hollow morphology. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Oxide semiconductor nanostructures with hollow morphology have a variety of functional applications such as photocatalysts, energy storage, piezoelectric materials, and gas sensors [1–3].In particular, when the shells of hollow spheres are thin and nano- porous, the gas can diffuse toward the entire sensing surface, which enhances the gas sensing behaviors [4]. In contrast, when the pri- mary particles are agglomerated into dense and large secondary particles, the gas sensing reaction involving the resistance change occurs only on the outer part of the agglomerates, which decreases the gas responses. The chemical routes used to prepare oxide hollow structures are divided into two groups according to the use of templates. The representative approaches to prepare hollow structures without template are the hydrothermal self-assembly reaction [5], ultra- sonic spray pyrolysis [6], Ostwald ripening of porous secondary particles [7], and outward oxidation of metal spheres by Kirkendall effect [8]. Although one-step reaction is simple and convenient, the precise control of wall thickness and/or hollow morphology remains a challenging issue. By contrast, the thinness and porosity of the wall and the size of the hollow spheres can be manipulated by optimizing the use of well-defined templates. The chemical routes include layer-by-layer assembly [9], heterocoagulation [10], and controlled hydrolysis [11]. ∗ Corresponding author. Tel.: +82 2 3290 3282; fax: +82 2 928 3584. E-mail address: jongheun@korea.ac.kr (J H. Lee). Sun and Li [12] suggested the synthetic route to prepare monodisperse carbon microsphereswith hydrophilic surface by the hydrothermal reaction of glucose and demonstrated the reactiv- ity of carbon spheres through the uniform coating of Ag. Titirichi [13] prepared various metal oxide (Fe 2 O 3 , NiO, Co 3 O 4 , MgO, CuO) hollow spheres by one-pot hydrothermal reaction of a solution con- taining metal ions and glucose and subsequent decomposition of core carbon. In addition, the hollow structures of SnO 2 and ZnO [14,15] have been prepared by similar chemical routes. However, to the best of our knowledge, the preparation of WO 3 hollow struc- tures using one-pot hydrothermal reaction of a solution containing the precursors of carbon templates and tungsten component has not been reported. In this contribution, WO 3 hollow microspheres are prepared by one-pot hydrothermal reaction and their gas sensing char- acteristics are investigated. The study focuses on the formation mechanism of tungsten-precursor-coated carbon spheres and hol- low WO 3 microspheres and the effect of particle morphology on the NO 2 sensing characteristics. 2. Experimental 2.1. Preparation of sample d(+)-Glucose monohydrate (C 6 H 12 O 6 ·H 2 O, 99.5% Fluka Chemi- cal Co., Ltd.) and sodium tungstate (IV) (Na 2 WO 4 ·2H 2 O, 99% Kanto Chemical Co., Ltd.) were used as carbon precursor and source material, respectively. Table 1 shows the composition of the stock solution for hydrothermal reaction. Glucose (3.963 g, 0.02 mol) was 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.08.031 C Y. Lee et al. / Sensors and Actuators B 142 (2009) 236–242 237 Fig. 1. (a) Electrode configuration, (b) sensor structure, and (c) schematic diagram of testing system. dissolved in 20 ml of distilled water and a designated amount of Na 2 WO 4 ·2H 2 O was dissolved in 20 ml of distilled water. After mix- ing the two solutions and subsequent mild stirring, the solutions were transferred toa Teflon-lined, stainless steelautoclave (volume 100 cm 3 ), sealed, and then heated at 200 ◦ C for 24 h. After cooling, the reaction products were washed with water and ethanol and dried at 60 ◦ C for 24 h. The as-prepared precursors were converted into WO 3 by the decomposition of core carbon via heat treatment (HT) at 450 ◦ C. The samples were designated according to the molar ratio between glucose and Na 2 WO 4 ·2H 2 O (denoted as ‘R’) in the stock solution. For example, R30 precursor and R30 powders mean the precursors and WO 3 powders prepared from the stock solution with R = 30, respectively. In order to investigate the effect of HT on the morphology of the WO 3 microspheres, the heat-treatment temperature (HTT) and duration at HTT were varied. 2.2. Characterization of sample The morphologies of the precursors and powders were analyzed by scanning electron microscopy (SEM, Hitachi S-4300) and trans- mission electron microscopy (TEM, FEITecnai 20). The crystal phase was analyzed using X-ray diffraction (XRD, Rigaku D/MAX-2500 V/PC). The surface areas of the powders were investigated using Brunauer–Emmett–Teller (BET) method (Tristar 3000, Micromerit- ics Co., Ltd.). 2.3. Gas sensing characteristics The WO 3 spherical powders after HT were prepared in a paste form and applied to an alumina substrate having two Au electrodes (Fig. 1(a) and (b), substrate: 1.5 mm × 1.5 mm, spacing between two electrodes: 0.2 mm). The sensor element was heat treated at 400 ◦ C for 30 min to decompose the organic component of the paste. The films after HT were ∼15 ␮m thick. The sensor was installed in a quartz tube and the furnace temperature was stabilized at a constant sensing temperature (300 ◦ C) (Fig. 1(c)). The gas concen- tration was controlled by changing the mixing ratio of the parent gases (5ppm NO 2 , 200ppm C 2 H 5 OH, 200ppm CH 3 COCH 3 , 100ppm CO, 100 ppm C 3 H 8 , 1000 ppm CH 4 , all in air balance) and dry syn- thetic air. A flow-through technique with a constant flow rate (500 ml/min) was used. The gas response (S = R a /R g or R g /R a ) was measured at 400 ◦ C by comparing the resistance of the sensor in Table 1 Sample specification and composition of the stock solution for hydrothermal reaction. Specification [Glucose]/[Na 2 WO 4 ·2H 2 O] Glucose Na 2 WO 4 ·2H 2 O R30 30 0.5 M 0.0166 M R5 5 0.5 M 0.1 M R0.5 0.5 0.5 M 1 M high-purity air (R a ) with that in the target gases (R g ). The electri- cal resistance sensor was monitored using a Picoammeter/Voltage Source (Keithley 6487) interfaced with a computer. 3. Results and discussion 3.1. X-ray diffraction analysis Fig. 2 shows the XRD patterns of the R30, R5, and R0.5 powders prepared by HT of precursors at 450 ◦ C for 1 h. The R30 powders Fig. 2. X-ray diffraction patterns of the powders prepared by heat treatment of (a) R30, (b) R5, and (c) R0.5 precursors at 450 ◦ C for 1 h. 238 C Y. Lee et al. / Sensors and Actuators B 142 (2009) 236–242 showed a pure monoclinic phase of WO 3 , while the R5 and R0.5 powders were identified as Na 2 W 2 O 7 and Na 2 WO 4 , respectively. The [Na]/[W] ratio in the Na 2 WO 4 powders prepared at R = 0.5 was 2, which was 2 times higher than the same ratio in the Na 2 W 2 O 7 powders prepared at R = 5. The Na content of the stock solution at R =0.5 was 10 times higher than that at R = 5. Accordingly, the high Na 2 WO 4 ·2H 2 O concentration of the stock solution was thought to have increased the Na content of the precursors and powders, despite the removal of a large portion of Na by washing. 3.2. Particle morphology The morphology of the precursors was closely dependent on the R values. At R = 0.5 and 5, irregular precipitate morphologies were prepared (Fig. 3(c) and (e)). In contrast, nearly mono-disperse, spherical precursors with clean surface were prepared (Fig. 3(a)). The morphology remained similar after HT at 450 ◦ C for 1 h (Fig. 3(b), (d) and (f)). Thus, the morphology of the powders could be designed in the stage of hydrothermal reaction. The WO 3 micro- spheres after HT (R30 powders) consisted of many small primary particles (10–20 nm) (inset in Fig. 3(b)), whereas the average pri- mary particle sizes of R5 and R0.5 powders were ∼400 nm and ∼1 ␮m (insets in Fig. 3(d) and (f)). These results indicated that the R values determined the phase, morphology and the primary particle sizes of the powders. The scale bar of Fig. 3(b) is 10 times smaller than that of Fig. 3(a). The sizes of more than 300 spheres were measured using the SEM observation of well-dispersed precursors and powders. The aver- age particle diameters of the R30 precursors and R30 powders were 1300 ± 249 nm and 226 ± 41 nm, respectively, indicating that the spherical precursors shrank to 17% of their precursor’s diameter during HT. The significant decrease of sphere sizes during HT in the present study can be attributed to the decomposition of the core carbon and subsequent sintering between primary particles. To confirm this, the R30 precursors were heat treated at a more rapid heating rate for a shorter duration at HTT (Fig. 4). The sphere diameters decreased with increasing thermal energy input dur- ing HT. At a high heating rate (20 ◦ C/min) and short duration at HTT (10 min), the powders retained their black color after HT, indi- cating the incomplete decomposition of core carbon (R30-RH-SD Fig. 3. Scanning electron micrographs of (a) R30 precursors, (b) R30 powders, (c) R5 precursors, (d) R5 powders, (e) R0.5 precursors, and (f) R0.5 powders. The powders (b), (d), and (f) were prepared by heat treatment of the precursors (a), (c), and (e) at 450 ◦ C for 1 h, respectively. C Y. Lee et al. / Sensors and Actuators B 142 (2009) 236–242 239 Fig. 4. The sphere diameters according to heat treatment schedules and transmission electron micrographs of (a) R30 precursors, (b) R30-RH powders heat treated at 450 ◦ C for 40 min (heating rate: 20 ◦ C/min) and (c) R30 powders heat treated at 450 ◦ C for 1h (heating rate: 5 ◦ C/min) (RH-SD: rapid heating and short duration at heat treatment temperature; RH: rapid heating). powders). However, whitish yellow powders could be prepared by increasing the duration at HTT from 10 min to 40 min (R30-RH powders). The TEM image of the R30 precursor showed a very clean sur- face (Fig. 4(a)). Even the small spheres showed a completely black contour, indicating that the precursors were not hollow but solid and consisted of carbon and W precursors. After HT with grad- ual heating rate and long duration at HTT (R30 powders), most of the powders showed a solid and nano-porous morphology but a few spheres showed a hollow morphology (arrow and inset in Fig. 4(c)). In contrast, most of the spheres showed a hollow mor- phology due to the rapid heating to HTT (R30-RH powders and Fig. 4(b)), although solid and nano-porous spheres were also found. The hollow shell was ∼30 nm thin (inset in Fig. 4(b)). Sun and Li [12] suggested that monodisperse carbon micro- spheres with hydrophilic surface can be prepared by the hydrothermal reaction of glucose. The polymerization of glucose into aromatic compounds and oligosaccharides and their subse- quent carbonization into spheres were suggested as the formation mechanism. The OH and CHO groups bonded to the surfaces of the carbon spheres are advantageous for the reaction with metal cations. Titirichi [13] prepared metal-ion-coated carbon spheres by one-pot hydrothermalreaction of a solution containing glucose and metal salts and transformed these spheres into metal oxide hollow spheres by the decomposition of the core carbon. In the present study, the hollow morphologies were found at the R30-RH powders. This indicates that the carbon spheres with hydrophilic surfaces are formed by the cross linking between glu- cose in the beginning stage of reaction and W ions are coated on the negatively charged surface of carbon spheres in the later stage of reaction. Thus, the precursors in Fig. 4(a) can be regarded as carbon spheres loosely coated with W-precursors. The precursor spheres were transformed to hollow WO 3 spheres by the decomposition of the core carbon (Fig. 4(b)) and then further transformed into solid and nano-porous spheres by the sintering between the WO 3 primary particles within the spheres (Fig. 4(c)). 3.3. Surface area and porosity The nitrogen adsorption–desorption isotherms plots and corre- sponding pore-size distribution plots of R30-RH, R30, R5, and R0.5 powders are given in Fig. 5. The volumes of pores in the size range of 2–70 nm for the R30-RH and R30 powders were markedly larger that those for the R5 and R0.5 powders. The surface area of the R30- RH, R30, R5, and R0.5 powders were 19.0, 12.7, 7.22, and 1.27 m 2 /g, respectively, indicating that the spherical WO 3 powders prepared at R = 30 (R30 and R30-RH powders) were advantageous to achieve high gas responses. Indeed, the gas responses of the R5 and R0.5 powders to NO 2 ,C 2 H 5 OH, CH 3 COCH 3 , CO, C 3 H 8 , and CH 4 were very low (not shown), while the R30 and R30-RH powders showed very high gas responses (as presented below). The large pore volume of the R30 powders (Fig. 5), despite their apparently solid interior structure (Fig. 4(c)), was attributed to their evolution from hollow spheres. The further increase of pore volumes in the size range of 5–20 nm by the employment of rapid heating indicates that the hollow morphology of R30-RH powders is advantageous to achieve both of high surface area and large pore volume. 3.4. Gas sensing characteristics As stated above, the gas responses of the R5 and R0.5 powders were negligible. Thus, the gas responses of the R30-RH and R30 240 C Y. Lee et al. / Sensors and Actuators B 142 (2009) 236–242 Fig. 5. Nitrogen adsorption–desorptionisotherm plots and corresponding pore-size distribution plots of R30-RH, R30, R5, and R0.5 powders. powders at 300 ◦ C were measured and the results are shown in Fig. 6. Note that the gas response to NO 2 is the R g /R a value and those to other gases are the R a /R g values. The increase of resis- tance upon exposure to NO 2 in WO 3 can be explained by the adsorption of the more negatively charged oxygen on the surface (NO 2 (g)+e − → NO(g)+ O − (surf)). The gas response (R g /R a )ofR30 powders to 1 ppm NO 2 was 20.1, while the responses (R a /R g )to 100 ppm CH 3 COCH 3 , CO, C 3 H 8 ,CH 4 , and C 2 H 5 OH were very low (1.1–3.3) (Fig. 6(b)). The concentration of NO 2 was 1/100 of those of the other gases. These results demonstrated the high gas response and selective detection of WO 3 nano-porous microspheres to NO 2 . The selective detection and gas responses were enhanced further by the use of hollow R30-RH powders. The R g /R a value to 1 ppm NO 2 was increased to 53.9 whereas those to the other gases decreased to 1.1–1.9 (Fig. 6(a)). Fig. 6. Gas responses to 1 ppm NO 2 and 100 ppm CH 3 COCH 3 ,CO,C 3 H 8 ,CH 4 , and C 2 H 5 OH at 300 ◦ C in air: (a) R30-RH powders heat treated at 450 ◦ C for 40 min (heat- ing rate: 20 ◦ C/min) and (b) R30 powders heat treated at 450 ◦ C for 1 h (heating rate: 5 ◦ C/min). The gas response to NO 2 is the R g /R a value and those to other gases are the R a /R g values. The sensing transients to 0.5–2.5 ppm NO 2 of the R30-RH and R30 powders at 300 ◦ C are shown in Fig. 7(a) and (b), respectively. The sensor resistance in air (R a ) was rather high (40–50 M). When exposed to NO 2 , the sensor resistance greatly increased up to a few G level. The gas responses of the R30-RH sensor to 0.5–2.5 ppm NO 2 were 38.6–81.5, which were 2.2–2.9 times higher than those of the R30 sensor (13.4–36.5) (Fig. 7(c)). Table 2 summarizesthe NO 2 responses of theundoped WO 3 sen- sors estimated from the literature data [16–27]. Most of the WO 3 sensors were operated at 100–300 ◦ C and the response to NO 2 var- ied greatly according to the morphology and preparation method of the sensor materials. Our result of R g /R a = 53.9 to 1 ppm NO 2 is compares very favorably to the values reported in the literature [16–27], although a few studies [19,20,22,25] have reported higher gas responses. The higher gas response of the R30 microspheres than those of the R5 and R05 powders was attributed to the higher surface area for gas sensing reaction and the large pore volume for the effective gas diffusion (Fig. 5). The gas response was improved fur- Fig. 7. Dynamic sensing transients of (a) R30-RH and (b) R30 sensors and (c) gas responses (R g /R a ) of R30-RH and R30 sensors to 0.5–2.5 ppm NO 2 . C Y. Lee et al. / Sensors and Actuators B 142 (2009) 236–242 241 Table 2 Gas responses to NO 2 in the present study and those reported in the literature [16–27]. WO 3 sensing materials (preparation) [NO 2 ] R g /R a Sensing temperature ( ◦ C) Reference Hollow microspheres 1 ppm 53.9 300 Present study Thin film (thermal evaporation) 0.5 ppm ∼22 100 [16] Thin film (rf sputtering) 1 ppm ∼5 370 [17] Thin film (vacuum thermal deposition) 1 ppm ∼4 200 [18] Thin film (Aerosol-assisted CVD) 0.2 ppm ∼80 150 [19] Thin film (reactive magnetron sputtering) 1 ppm ∼450 200 [20] Nanocrystalline powders 5 ppm ∼75 150 [21] Lamellar structures (acidification) 1 ppm ∼280 200 [22] Nanostructures (thermal evaporation) 5 ppm ∼3 250 [23] WO 3 nanopetals (dealloying of W–Al alloy and thermal oxidation) 5 ppm ∼4 250 [24] NWs network (thermal evaporation) 1 ppm ∼150 300 [25] NWs array (thermal evaporation) 1 ppm ∼10 180 [26] Mesoporous nanostructures 1 ppm ∼10 230 [27] ther by the employment of a more hollow morphology through the rapid decomposition of the core carbon (R30-RH powders). Although the R30 powders are relatively porous, the diffusion of NO 2 to the centerof the WO 3 secondary spheres canbe hampered in part by thetortuousconfiguration of the nanopores,which becomes more significant when the secondary spheres become larger. This decreases the gas response because only the surface region of the microspheres changes the resistance by the gas sensing reaction. In contrast, a hollow morphology with a thin shell configuration and large pore volume facilitates the in-diffusion of the sensing gas and counter-diffusion of the reactant gas. With effective gas diffu- sion, the resistance of all the primary particles within the secondary hollow spheres is affected upon NO 2 exposure, which increases the gas response. Thus, the WO 3 hollow microspheres examined in the present study are promising sensing materials to detect NO 2 with a high sensitivity and selectivity. 4. Conclusion WO 3 hollow microspheres were prepared by the glucose- mediated, hydrothermal synthesis of W-coated carbon spheres and their calcination at 450 ◦ C. With the input of increasing thermal energy during calcination, the hollow WO 3 microspheres were gradually transformed into solid microspheres. Both hollow and solid WO 3 microspheres showed high response and selective detec- tion to 0.5–2.5 ppm NO 2 . In particular, the responses to NO 2 were increased 2.2–2.9 times by using hollow morphology, which was attributed to the high surface area for gas sensing and the effec- tive diffusion of NO 2 toward all the primary particles through the nano-porous and thin shell layers. Acknowledgements This work was supported by KOSEF NRL program grant funded by the Korean Government (MEST) (No. R0A-2008-000-20032-0) and a grant from the Fundamental R&D program for Core Tech- nology of Materials (M2008010013) funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] F. Caruso, Nanoengineering of particle surfaces, Adv. Mater. 13 (2001) 11–22. [2] R. Meyer Jr., H. Weitzing, Q. Xu, Q. Zhang, R.E. Newnham, Lead zirconate titanate hollow-sphere transducers, J. Am. Ceram. Soc. 77 (1994) 1669. [3] S. Han, B. Jang, T. Kim, S.M. Oh, T. Hyeon, Simple synthesis of hollow tin dioxide microspheres and their applications to lithium-ion battery anodes, Adv. Funct. Mater. 15 (2005) 1845–1850. [4] J H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B. 140 (2009) 313–336. [5] Q. Zhao, Y. Gao, X. Bai,C.Wu,Y. Xie,FacilesynthesisofSnO 2 hollow nanospheres and applications in gas sensors and electrocatalysts, Eur. J. Inorg. Chem. (2008) 1643–1648. [6] V. Jokanovi ´ c, A.M. Spasi ´ c, D. Uskokovi ´ c, Designing of nanostructured hollow TiO 2 spheres obtained by ultrasonic spray pyrolysis, J. Colloid Interface Sci. 278 (2004) 342–352. [7] X.W. Lou, Y. Wang, C. Yuan, J.Y. Lee, L.A. Archer, Template-free synthesis of SnO 2 hollow nanostructures with high lithium capacity, Adv. Mater. 18 (2006) 2325–2329. [8] Y. Yin, R.M. Rioux, C.K. Erdonmez, S. Hughes, G.A. Somorjai, A.P. Alivisatos, For- mation of hollow nanocrystals through the nanoscale Kirkendall effect, Science 30 (2004) 711–714. [9] F. Caruso, X. Shi, R.A. Caruso, A. Susha, Hollow titania spheres from layered precursor deposition onsacrificial colloidal core particles, Adv.Mater. 13 (2001) 740–744. [10] N. Kawahashi, E. Matijevi ´ c, Preparation and properties of uniform coated col- loidal particles, J. Colloid Interface Sci. 138 (1990) 534–542. [11] J Y. Lee, J H. Lee, S H. Hong, Y.K. Lee, J Y. Choi, Coating of BaTiO 3 nano-layer on spherical Ni powders for MLCC, Adv. Mater. 15 (2003) 1655–1658. [12] X. Sun, Y. Li, Colloidal spheres and their core/shell structures with noble-metal nanoparticles, Angew. Chem. 116 (2004) 607–611. [13] M M. Titirici, M. Antonietti, A. Thomas, A generalized synthesis of metal oxide hollow spheres using a hydrothermal approach, Chem. Mater. 18 (2006) 3808–3812. [14] H.X. Yang, J.F. Qian, Z.X. Chen, X.P. Ai, Y.L. Cao, Multilayered nanocrystalline SnO 2 hollow microspheres synthesized by chemically induced self-assembly in the hydrothermal environment, J. Phys. Chem. C 111 (2007) 14067– 14071. [15] J. Yu, X. Yu, Hydrothermal synthesis and photocatalytic activity of zinc oxide hollow spheres, Environ. Sci. Technol. 42 (2008) 4902–4907. [16] A. Ponzoni, E. Comini, M. Ferroni, G. Sberveglieri, Nanostructured WO 3 deposited by modified thermal evaporation for gas-sensing applications, Sens. Actuators B 490 (2005) 81–85. [17] M. Stankova, X. Vilanova, E. Llobet, J. Calderer, C. Bittencourt, J.J. Pireaux, X. Correig, Influence of the annealing and operating temperatures on the gas- sensing properties of rf sputtered WO 3 thin-film sensors, Sens. Actuators B 105 (2005) 271–277. [18] G. Xie, J. Yu, X. Chen, Y. Jiang, Gas sensing characteristics of WO 3 vacuum deposited thin films, Sens. Actuators B 123 (2007) 909–914. [19] S. Ashraf, C.S. Blackman, R.G. Palgrave, S.C. Naisbitt, I.P. Parkin, Aerosol assisted chemical vapour deposition of WO 3 thin films from tungsten hexacarbonyl and their gas sensing properties, J. Mater. Chem. 17 (2007) 3708–3713. [20] Y. Shen, T. Yamazaki, Z. Liu,D. Meng, T. Kikuta, N. Nakatani, Influence of effective surface area on gas sensing properties of WO 3 sputtered thin films, Thin Solid Films 517 (2009) 2069–2072. [21] H. Xia, Y. Wang, F. Kong, S. Wang, B. Zhu, Z. Guo, J. Zhang, Y. Wang, S. Wu, Au-doped WO 3 -based sensor for NO 2 detection at low operating temperature, Sens. Actuators B 134 (2008) 133–139. [22] T. Kida, A. Nishiyama, M. Yuasa, K. Shimanoe, N. Yamazoe, Highly sensitive NO 2 sensors using lamellar-structured WO 3 particles prepared by an acidification method, Sens. Actuators B 135 (2009) 568–574. [23] T. Sicilliano, A. Tepore, G. Micocci, S.D. Manno, E. Filippo, WO 3 gas sensors prepared by thermal oxidation of tungsten, Sens. Actuators B 133 (2008) 321– 326. [24] Z. Liu, T. Yamazaki, Y. Shen, D. Mang, T. Kikuta, N. Nakatani, T. Kawabata, Deal- loying derived synthesis of W nanopetal films and their transformation into WO 3 , J. Phys. Chem. C 112 (2008) 1391–1395. [25] A. Ponzoni, E. Comini, G. Sberveglieri, Z. Zhou, Z. Deng, N.S. Xu, Y. Ding, Z.L. Wang, Ultrasensitive and highly selective gas sensors using three-dimensional tungsten oxide nanowires networks, Appl. Phys. Lett. 88 (2006) 203101. [26] B. Cao, J. Chen, X. Tang, W. Zhou, Growth of monoclinic WO 3 nanowire array for highly selective NO 2 detection, J. Mater. Chem. 19 (2009) 2323–2327. 242 C Y. Lee et al. / Sensors and Actuators B 142 (2009) 236–242 [27] Rossinyol, A. Prim, E. Pellicer, J. Arbiol, F. Hernández-Ramírez, F. Peiró, A. Cornet, J.R. Morante, L.A. Solovyov,B. Tian, T. Bo, D. Zhao,Synthesis and characterization of chromium-doped mesoporous tungsten oxide for gas sensing applications, Adv. Funct. Mater. 17 (2007) 1801–1806. Biographies Choong-Yong Lee studied materials science and engineering and received his BS degree from Korea University in 2007. He is currently a master course student at Korea University. His research involves the preparation of oxide nanostructures for gas sensor applications. Sun-Jung Kim studied materials science and engineering and received his BS and MS degrees in 2006 and 2008, respectively, at Korea University in Korea. He is cur- rently studying for PhD degree at Korea University. His research interests are oxide nanostructures for chemical sensor applications and the combinatorial design of gas sensing materials. In-Sung Hwang studied materials science and engineering and received his BS from Kumoh National University, Korea, in 2004. In 2006, he received his MS degree from Korea University. He is currently studying for a PhD at Korea University. His research interest is oxide nanostructure-based electronic devices. Jong-Heun Lee has been a Professor at Korea University since 2008. He received his BS, MS, and PhD degrees from Seoul National University in 1987, 1989, and 1993, respectively. Between 1993 and 1999, he developed automotive air–fuel- ratio sensors at the Samsung Advanced Institute of Technology. He was a Science and Technology Agency of Japan (STA) fellow at the National Institute for Research in Inorganic Materials (currently NIMS, Tsukuba, Japan) from 1999 to 2000, a research professor at Seoul National University from 2000 to 2003, and an asso- ciate professor at Korea University from 2003 to 2008. His current research interests include chemical sensors, functional nanostructures, and solid oxide electrolytes. . homepage: www.elsevier.com/locate/snb Glucose-mediated hydrothermal synthesis and gas sensing characteristics of WO 3 hollow microspheres Choong-Yong Lee, Sun-Jung. higher gas responses. The higher gas response of the R30 microspheres than those of the R5 and R05 powders was attributed to the higher surface area for gas sensing