microstructure characterization and no2 sensing properties of tungsten oxide

Please cite this article in press as: Y. Qin, et al., Microstructure characterization and NO 2 -sensing properties of tungsten oxide nanostructures, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.063 ARTICLE IN PRESS G Model SNB-12446; No. of Pages 7 Sensors and Actuators B xxx (2010) xxx–xxx Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Microstructure characterization and NO 2 -sensing properties of tungsten oxide nanostructures Yuxiang Qin ∗ , Ming Hu, Jie Zhang School of Electronics and Information Engineering, Tianjin University, No 92, Weijjin Road, Nankai District, Tianjin 300072, PR China article info Article history: Received 16 November 2009 Received in revised form 28 June 2010 Accepted 29 June 2010 Available online xxx Keywords: Tungsten oxide Nanowires Nanosheets Solvothermal synthesis Gas sensors abstract Nanowires and nanosheets of tungsten oxide were synthesized by solvothermal method with differ- ent tungsten hexachloride (WCl 6 ) concentrations in 1-propanol solvent. The morphology and crystal structure of the tungsten oxide nanostructures were investigated by means of field emission scanning electron microscope, X-ray diffraction and transmission electron microscope. The specific surface area and pore size distribution were characterized by Brunauer–Emmett–Teller gas-sorption measurements. One-dimensional W 18 O 49 nanowire bundles were synthesized at the WCl 6 concentration of 0.01M. With the concentration increasing to 0.02 M, the structure of the pure two-dimensional WO 3 nanosheets was formed. The NO 2 gas sensing properties of W 18 O 49 nanowires and WO 3 nanosheets were investigated at 100 ◦ Cupto250 ◦ C over NO 2 concentration ranging from 1 to 20 ppm. Both nanowires and nanosheets exhibit reversible response to NO 2 gas at different concentrations. In comparison to WO 3 nanosheets, W 18 O 49 nanowire bundles showed a much higher response value and faster response–recovery charac- teristics to NO 2 gas, especially a much quicker response characteristic with response time of 19 s at the concentration of 5 ppm. © 2010 Published by Elsevier B.V. 1. Introduction With theindustrialdevelopment, airpollutionis becomingmore and moreserious. Especially, nitrogenoxide NO x (NO 2 or NO)which results fromcombustionand automotiveemissions is amain source of acid rain and photochemical smog [1]. So detection of toxic NO x gas is very important for the environmental protection and human health. Thus far, several kinds of solid-state NO 2 sensors, such asresistive[2], capacitive[3],and surfaceacoustic wave (SAW) [4] types have been developed. In particular, resistive-type sen- sors based on metal oxide semiconductors are well suited for NO 2 detection owing to their remarkable gas sensing performance and simple structures [2,5]. Among various metal oxide semiconduc- tors, tungsten oxide (WO 3−x ), which is a wide band-gap n-type semiconductor, has been found to be a promising material for detection of NO 2 gas [6,7]. However, most tungsten oxide sen- sors based on nanocrystalline powders or films have been studied widely and shown too slow response–recovery time; and their sensitivity still needs to make further improvement for practical application. In these years, novel nanostructures such as nanowires, nan- otubes, nanorods and nanobelts, have been evaluated as ideal candidates for gas sensing applications due to their larger spe- ∗ Corresponding author. Tel.: +86 22 27402372; fax: +86 22 27401233. E-mail address: qinyuxiang@tju.edu.cn (Y. Qin). cific surface area and smaller dimensions than the Debye length [8,9]. In fact, gas sensing materials such as SnO 2 [10,11], ZnO [12] and In 2 O 3 [13] with well-established nanostructure have exhib- ited higher sensitivity and quicker response to detect gases at low concentrations than the corresponding thin film materials [14,15]. Likely, tungsten oxide in nanostructures like nanowires, nanosheets and nanorods were investigated [16–18], and they revealed good sensing properties while detecting toxic and haz- ardous gases. For example, very good results for H 2 S gas sensor based on tungsten oxide nanowires and nanosheets have been reported. Chen co-workers [19] found that the single-crystalline potassium-doped tungsten oxide nanosheets could exhibit high sensitivity, fast response time and good stability to H 2 S, acetone and Cl 2 . The investigation of Rao co-workers [20] indicated that the WO 2.72 nanowires had much higher response value to H 2 S than the nanoparticles or nanoplatelets of WO 3 . Very recently, Ger- litz et al. [21] reported that the sensor based on tungsten oxide nanotubes can detect dilute NO 2 as low as 200 ppb at 200 ◦ C and exhibit response two to three orders-of-magnitude higher than the one based on WO 3 thin film. These results clearly demonstrate the potential of tungsten oxide nanostructures in toxic gas detection. In this work, we synthesized one-dimensional bundled nanowires and two-dimensional nanosheets of tungsten oxide by the sim- ple solvothermal method and evaluated their sensing properties towards NO 2 gas ranging from 1 to 20 ppm at operating temper- ature of 100–250 ◦ C. Our study indicates that both nanowires and nanosheets of tungsten oxide have high sensitivities to NO 2 gas; 0925-4005/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.snb.2010.06.063 Please cite this article in press as: Y. Qin, et al., Microstructure characterization and NO 2 -sensing properties of tungsten oxide nanostructures, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.063 ARTICLE IN PRESS G Model SNB-12446; No. of Pages 7 2 Y. Qin et al. / Sensors and Actuators B xxx (2010) xxx–xxx Furthermore, the nanowires exhibit a much quick response char- acteristic with response time of 19 s to 5 ppm NO 2 . 2. Experimental Nanowires and nanosheets of tungsten oxide were synthesized by solvothermal method with tungsten hexachloride (WCl 6 )as precursor and 1-propanol as solvent. First, a certain amount of WCl 6 was dissolved in a little ethanol to form a solution in a beaker. The ratio of WCl 6 mass to ethanol volume is 0.1 g/ml. Then, 1-propanol was added to the solution which was subsequently transferred to and sealed in a 100 ml Teflon-lined stainless steel autoclave. The volume of 1-propanol is80 mland the concentration of WCl 6 in 1-propanol varied from 0.01 to 0.02M in our experi- ments. The solvothermal reaction was conducted at 200 ◦ C for 9 h in an electric oven. After that, the autoclave was cooled naturally to room temperature.Thefinal productswerecentrifuged andwashed sequentially by deionized water and ethanol several times, and the obtained powder was dried at 70 ◦ C for 6h in air. The morphology,crystal structure,and phasecomposition ofthe tungsten oxides were characterized using a field emission scanning electron microscope (FESEM, FEI Nanosem 430), a X-ray diffrac- tometer (XRD, RIGAKU D/MAX 2500V/PC, Cu K␣ radiation) and a field emission transmission electron microscope (FETEM, TEC- NAI G 2 F-20). In order to evaluate the specific surface area and pore size distribution of the products, Brunauer–Emmett–Teller (BET) gas-sorption measurements were carried out using Quan- tachrome NOVA automated gas-sorption system after the samples were vacuum-dried at 200 ◦ C for 10 h. The specific surface area was estimated by nitrogen gas isotherms at a relative pressure (P/P 0 ) ranging from 0.005 to 0.1. Pore size distribution was obtained from the analysis of the desorption branch of nitrogen gas isotherms using the Barrett–Joyner–Halenda (BJH) model, and total pore volume was determined by the amount of nitrogen adsorbed at P/P 0 = 0.99. The gas sensors were fabricated by spin coating the slurry of synthesized tungsten oxide nanostructureson the cleaned alumina substrates which were attached with a pair of interdigitated Pt electrodes with a thickness of 100 nm. Fig. 1(a and b) shows the schematic diagrams of the interdigitated electrodes and the sensor respectively. The electrodes were deposited using RF magnetron sputtering method. The coating slurry was prepared by ultrasoni- cally dispersing tungsten oxide powders in mixed organic solvents of terpineol and ethanol with 2:1 volume ratio for 2 h. A physical mask is used during spin coating to avoid the presence of slurry at the end of the substrate. The coated sensing films were dried in air for 30 min subsequently annealed at 300 ◦ C for 1 h at ambient atmosphere in order to burn out the organic solvent used in prepa- ration of coating paste and to enhance the adherence of the sensing film to the sensor substrates. Temperature was raised from ambi- ent to 300 ◦ C using a slow ramp of 2.5 ◦ C/min in order to avoid the occurrence of cracks in the films. The NO 2 gas sensing measurements were carried out in a home-built computer-controlled static gas sensing characteriza- tion system consisting of a glass test chamber, a flat heating plate, a professional digital multimeter and a data acquisition system. The schematic diagram of the gas sensing test system is shown in Fig. 1(c). The sensors were placed on the heating plate fixed in test chamber. The pure NO 2 gas was injected into the chamber directly to get the desired concentration, and the sensor was recovered by opening thetop coverof thetest chamberand settingup the electric blower fixed at the bottom of the chamber. An UNI-T UT70D pro- fessional digital multimeter with the function of measuring range automatic adjustment was used for continuously monitoring the resistance change of the sensors during the whole measurement process. The electrically connection between the Pt electrodes and Fig. 1. Schematic diagrams of the interdigitated Pt electrodes (a), the sensor (b) and the gas sensing test system (c). the digital multimeter was realized by a pair of elastic Au-coated copper probes. The acquired resistance data were stored in a PC for further analysis. The sampling interval was set to 1 s. The operating temperature of the sensing films was changed from 100 to 250 ◦ C by adjusting the temperature controller of heat plate. The sensor response (S) was defined as S =(R g − R 0 )/R 0 , where R g and R 0 are the resistance of the sensor in the measuring gas and that in clean air, respectively. The response time is defined as the time required for the resistance rising to 90% of the equilibrium value since the test gas is injected. Conversely, therecovery time is the time for the resistance in equilibrium to go down to 10% of the original value in air since the test gas is released. 3. Results and discussion 3.1. Structural characterization The morphologies of tungsten oxide nanostructures synthe- sized at different WCl 6 concentrations after annealing at 300 ◦ C for 1 h were shown in Fig. 2(a–d). It can be seen from Fig. 2(a), Please cite this article in press as: Y. Qin, et al., Microstructure characterization and NO 2 -sensing properties of tungsten oxide nanostructures, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.063 ARTICLE IN PRESS G Model SNB-12446; No. of Pages 7 Y. Qin et al. / Sensors and Actuators B xxx (2010) xxx–xxx 3 Fig. 2. (a, c, d) SEM images of tungsten oxide nanostructures synthesized at WCl 6 concentration of 0.01M, 0.015M and 0.02 M, respectively, after annealing at 300 ◦ C for 1h. (b) TEM image of the annealed tungsten oxide synthesized at 0.01M. The insets in (a) and (d) are the SEM images of the corresponding product before annealing. the product synthesized at the WCl 6 concentration of 0.01 M after thermal annealing exhibited one-dimensional nanowire bundles features with diameters in 70–90 nm and lengths in 500–1000 nm. Further TEM examination can identify their bundled feature, giv- ing evidence that many nanowires with diameters of about 10 nm assembled alongtheir maingrowth direction andformed abundled structure, asshown in the Fig. 2(b). Comparedwith thebundle mor- phology before annealing shown in the inset in Fig. 2(a), it can be seen that the thermal annealing results in the nanowire bundles becoming thicker, shorter and straighter, indicating that a possi- ble agglomeration had occurred. Increasing WCl 6 concentration to 0.015 M, apparent evolution of the morphology can be observed. The SEM image shown in Fig. 2(c) exhibited the annealed prod- uct was a mixture structure of nanowires and nanosheets. Up to a high WCl 6 concentration of 0.02 M, a pure nanosheets structure with thicknesses of 10–30 nm was formed (inset in Fig. 2(d)), and showed unobvious change after annealing treatment (Fig. 2(d)). From above results, it can be speculated that the WCl 6 concentra- tion has a great effect on the specific morphologies of tungsten oxide nanostructures synthesized by solvothermal method. This is in good agreement with the previous report [22]. Lower solution concentration contributedto the lower supersaturation oftungsten source, promoting the growth of tungsten oxide nanowires [23]. At higher concentration, the highly saturated WCl 6 could prohibit the growth of tungsten oxide nanowires along the main growth direction. XRD analysis was carried out to identify the crystalline struc- ture of the tungsten oxide before and after annealing at 300 ◦ C for 1h. Fig. 3(a, c, e) and (b, d, f) respectively shows the XRD pat- terns of the as-synthesized and annealed samples. As shown in Fig. 3(a), the main diffraction peaks of the bundled nanowires syn- thesized at WCl 6 concentration of 0.01 M can be well indexed as the monoclinic cell of W 18 O 49 with cell parameters of a =18.32 Å, b =3.79 Å, c = 14.04 Å and ˇ = 115.04 ◦ (JCPDS No. 65-1291). The strongest peak intensity of (0 10) plane indicates that the crys- Fig. 3. XRD patterns of tungsten oxide nanostructures: (a, c, e) nanowires, mixture and nanosheets synthesized at WCl 6 concentrations of 0.01 M, 0.015 M and 0.02 M, respectively, before annealing, (b, d, f) nanowires, mixture and nanosheets after annealing. Please cite this article in press as: Y. Qin, et al., Microstructure characterization and NO 2 -sensing properties of tungsten oxide nanostructures, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.063 ARTICLE IN PRESS G Model SNB-12446; No. of Pages 7 4 Y. Qin et al. / Sensors and Actuators B xxx (2010) xxx–xxx Table 1 Measured resistances and sensor responses in the presence of 5ppm of NO 2 for tungsten oxide nanowires and nanosheets as a function of temperature. Operating temperature ( ◦ C) W 18 O 49 nanowires WO 3 nanosheets Baseline resistance (M) Equilibrium resistance (M) Sensor response Baseline resistance (M) Equilibrium resistance (M) Sensor response 100 43.9 ± 0.8 631.7 ± 9.7 13.4 ± 0.5 44.2 ± 0.7 535.2 ± 31.0 11.1 ± 0.9 125 9.6 ± 0.4 1020.7 ± 39.1 105.1 ± 0.5 10.1 ± 0.7 665.6 ± 40.6 65.2 ± 0.5 150 3.1 ± 0.1 468.8 ± 12.7 151.2 ± 0.5 3.3 ± 0.4 354.1 ± 43.1 107.3 ± 0.3 175 2.1 ± 0.1 307.1 ± 9.8 144.6 ± 0.4 1.3 ± 0.1 121.3 ± 15.5 95.2 ± 0.4 200 1.8 ± 0.04 226.7 ± 4.6 123.6 ± 0.3 0.6 ± 0.1 42.5 ± 9.2 73.5 ± 0.4 225 1.4 ± 0.1 68.6 ± 2.9 48.0 ± 0.2 0.6 ± 0.1 13.7 ± 1.7 21.1 ± 0.2 250 1.1 ± 0.1 2.4 ± 0.1 1.2 ± 0.1 0.7 ± 0.1 8.0 ± 0.7 10.2 ± 0.1 Fig. 4. BET plots of N 2 adsorption isothermsfor tungstenoxide nanostructures after annealing at 300 ◦ C for 1 h. tal grows preferentially along the b-axis, i.e. the [0 10] direction. The XRD pattern of the nanosheets obtained at WCl 6 concentration of 0.02 M corresponds to the monoclinic structure of WO 3 with lattice of a= 7.297Å, b = 7.539 Å, c =7.688 Å and ˇ =90.91 ◦ (JCPDS No. 43-1035), seen in Fig. 3(e). From this XRD pattern, the two strongest diffraction peaks appear at 2Â = 23.58 ◦ and 2Â = 24.34 ◦ corresponding to (0 20) and (2 00) facets and the peak intensity of the (0 02) reflection is muchweaker, which implies the nanosheets grow along the [0 10] and [1 00] crystallographic direction and is enclosed by ±(0 01) facets. The sample synthesized at WCl 6 concentrations of 0.015 M is a mixture of monoclinic W 18 O 49 and monoclinic WO 3 according to the XRD analysis in Fig. 3(c), which is in accordance with the SEM characterization result shown in Fig. 2(c). Fig. 3 shows the comparison between the XRD patterns of the samples before and after thermal annealing. It is obvious that the crystal structures of the W 18 O 49 nanowires and WO 3 nanosheets remained unchanged by the annealing treatment at 300 ◦ C. However, the much sharper peaks observed in the XRD pat- terns of the annealed tungsten oxides indicated an increase degree of crystallinity. 3.2. Physical adsorption–desorption measurements To examine the porous structure of the W 18 O 49 nanowire bun- dles and WO 3 nanosheets, the specific surface area and pore size distribution of the samples annealed at 300 ◦ C for 1 h are deter- mined by the physical adsorption–desorption measurements in N 2 gas. BET plots of N 2 gas adsorption isotherms for tungsten oxide nanostructures are shown in Fig. 4. Here, W is the weight of N 2 gas adsorbed at a relative pressure P/P 0 . P/P 0 is the pressure of N 2 gas divided by its saturation vapor pressure. It can be seen that the data points are on a straight line for every sample, suggest- Fig. 5. Pore size distributions per unit mass of tungsten oxide nanowires and nanosheets after annealing at 300 ◦ C for 1 h. ing that the specific surface area determined by BET analysis is reliable. The pore size distributions per unit mass of the annealed sam- ples determined by adsorption–desorption isotherms of N 2 gas using the BJH model are represented in Fig. 5. Y-axis represents the pore surface area when the pore size is in a certain range. The pore surface area and the pore size distribution above 8 nm appear sim- ilar between W 18 O 49 nanowires and WO 3 nanosheets. But when the pore size is less than 8 nm, some discrepancy occurs. W 18 O 49 nanowires show larger pore surface areas than WO 3 nanosheets. Meanwhile, it can be seen that for W 18 O 49 nanowires, the pore size distribution has a peak at about 2.1 nm, while the pore size distri- bution peak shifts to a larger size of 2.3 nm for WO 3 nanosheets. The measured specific surface area and pore volume of the annealed bundled W 18 O 49 nanowires are 72.03 m 2 /g and 0.13 cc/g, whilst for the WO 3 nanosheets they are 41.85 m 2 /g and 0.12 cc/g, respectively. These values are slight lower than those obtained from the samples before annealing. For the as- synthesized W 18 O 49 nanowires and WO 3 nanosheets, the specific surface area and pore volume are respectively 89.89 m 2 /g/0.14 cc/g and 46.67 m 2 /g/0.12 cc/g. The relative higher specific surface area of W 18 O 49 nanowires than that of WO 3 nanosheets is related to the bundles feature. The individual nanowires comprising the bundles were observed in thin and long one-dimensional nanostructured materials from the SEM and TEM images shown in Fig. 2(a and b). It’s obvious that the high specific surface area of the bundled nanowires is in part ascribed to a combination of the ultra-thin feature of individual nanowires and the unique packing character- istic of the bundles themselves [24]. Also, it is associated with the sizes and distributions of the pores [25], which can be proved from Fig. 5. Theformation ofnanosheets structurewill consequently lead to the decrease of pore surface area, which in turn resulted in the decreased pore volume and specific surface areas. Please cite this article in press as: Y. Qin, et al., Microstructure characterization and NO 2 -sensing properties of tungsten oxide nanostructures, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.063 ARTICLE IN PRESS G Model SNB-12446; No. of Pages 7 Y. Qin et al. / Sensors and Actuators B xxx (2010) xxx–xxx 5 3.3. NO 2 -sensing properties The gas sensing properties of the sensors based on tungsten oxide nanostructures towards 5 ppm NO 2 were tested at operating temperatures ranging from 100 to 250 ◦ C. Table 1 shows the mea- sured baseline resistances, equilibrium resistances and calculated sensor responses for the W 18 O 49 nanowires and WO 3 nanosheets, respectively, as a function of operating temperature. It should be noted that three continual tests were preformed to the same sen- sor sample at every operating temperature in order to ensure the reliability of the testing data. The baseline resistances and the equi- librium resistances in three tests were found to be similar, and every data shown in Table 1 is the average value of three data obtained from three tests respectively. The standard deviations for each valuewere also shownin the table. From Table1, it canbe seen that the baseline resistances of tungsten oxide decrease along with increasing temperature, which is consistent with typical semicon- ductor materials. It is well known that the response of the sensor is much dependent on the operating temperatures. Such relation is illustrated in Table 1. The response tests’ results show maxi- mum values of 151.2 and 107.3 at 150 ◦ C for the W 18 O 49 nanowires and WO 3 nanosheets, respectively. When operating temperature is above 200 ◦ C, the nanowires and nanosheets lose their response ability quickly. Especially, for W 18 O 49 nanowires, the response value at 250 ◦ C only was 1.2, which is less than 1% percent of that at 150 ◦ C. The above results can be understood as following: As been reported, tungsten oxide is a typical n-type semiconductor, and its gas sensing mechanism belongs to the surface-controlled type, and the change of conductivity is dependent on the species and the amount of chemisorbed oxygen on the surface [26].Atlow temperature, oxygen species on the film surface are not active, so that a low interaction happens between adsorbed oxygen species and detected NO 2 gas. Thus, the response of the tungsten oxide film is low. Conversely, some of the adsorbed oxygen species may be desorbed from the film surface at high temperature, which also leads to low response value. As a result, there should be an optimal operating temperature to balance the above two effects in order to achieve the maximum gas response. It is clear from Table 1 that the measurement carried out at temperatures ranging from 150 to 200 ◦ C can obtain relatively high NO 2 response, and the highest response is achieved at 150 ◦ C. From Table 1,W 18 O 49 nanowires exhibited much higher NO 2 response than WO 3 nanosheets at dif- ferent operating temperature ranging from 100 to 200 ◦ C. While it is noteworthy that, at 250 ◦ C, the NO 2 response of WO 3 nanosheets sensor is almost 10 times higher than that of W 18 O 49 nanowires sensor. The response and recovery time of the W 18 O 49 nanowires and WO 3 nanosheets to 5 ppm NO 2 at various operating temperatures are shownin Fig.6. Fromthis figure, the response timeand recovery time of the two samples are both decrease quickly with increasing operating temperature. When the operating temperature rise to 200 ◦ C or above, both samples show the much fast response and recovery characteristics, despite of their low response shown in Table 1. It is also clear from this figure that the W 18 O 49 nanowires show fasterresponse–recoverythan theWO 3 nanosheets atvarious operating temperatures. Fig. 7 shows the dynamic responses of tungsten oxide nanos- tructures to NO 2 gas in varying concentration. The operating temperature is 200 ◦ C. Fig. 7(a and b) shows the results of the W 18 O 49 nanowires and WO 3 nanosheets synthesized at WCl 6 con- centrations of 0.01 M and 0.02 M, respectively. As shown in this figure, the measured resistances increased upon exposure to NO 2 gas. This result is expected because the oxidizing analyte NO 2 withdraws electrons from the n-type tungsten oxide surface and induces the formation of electron-depleted space-charge layers [27]. Notably, the resistances could almost recover to its initial Fig. 6. Response time and recovery time of tungsten oxide nanowires and nanosheets to 5 ppm NO 2 as a function of operating temperature. value after NO 2 removal, indicating a good reversibility of these nanostructure materials. From this figure, it also can be seen that the increase in the resistance of the W 18 O 49 nanowires upon expo- sure to NO 2 is much larger than that of the WO 3 nanosheets, which indicates that the W 18 O 49 nanowires have higher NO 2 response. The response values of the W 18 O 49 nanowires upon exposure to 1, 5, 10 and 20 ppm NO 2 are 13.4, 123.6, 203.4 and 332.3, while those of the WO 3 nanosheets are 13.3,73.5, 144.2 and279.8, respectively. As has been reported, the gas sensing mechanism of tung- sten oxide belongs to the surface-controlled type in which the surface states and oxygen adsorption play an important role [26,28]. Atmospheric oxygen adsorbs electrons from the conduc- tion band of the sensing metal oxide and occurs on the surface Fig. 7. Dynamic response of (a) bundled W 18 O 49 nanowires, (b) WO 3 nanosheets to varying NO 2 concentration at an operating temperature of 200 ◦ C. Please cite this article in press as: Y. Qin, et al., Microstructure characterization and NO 2 -sensing properties of tungsten oxide nanostructures, Sens. Actuators B: Chem. (2010), doi:10.1016/j.snb.2010.06.063 ARTICLE IN PRESS G Model SNB-12446; No. of Pages 7 6 Y. Qin et al. / Sensors and Actuators B xxx (2010) xxx–xxx Fig. 8. Response time and recovery time curves of W 18 O 49 nanowires and WO 3 nanosheets to different NO 2 concentration at an operating temperature of 200 ◦ C. in the form of O − ,O 2− and O 2 − . The reaction between NO 2 and the surface adsorbed species (O 2 − ,O − and O 2− , etc.) induces the formation of electron-depleted space-charge layers inside the tungsten oxide surfaces and thus the increase in the resistance [29]. According to the BET measurements, the W 18 O 49 nanowires showed much larger specific surface area (72.03 m 2 /g) than the WO 3 nanosheets (41.85 m 2 /g). The larger surface area can provide more adsorption–desorption sites and a larger amount of surface adsorbed oxygen species interacting with detected gas molecules. Thus, W 18 O 49 nanowires with higher specific surface area show much larger change in resistance upon exposure to NO 2 than the WO 3 nanosheets with lower specific surface area. Besides, non- stoichiometric crystal structure of W 18 O 49 is another important factor for highresponse [30]. There existmuch more oxygenvacan- cies inthecrystal structureof non-stoichiometricW 18 O 49 than fully oxidized WO 3 [31], and the large amounts of oxygen vacancies can serve as adsorption sites of gas molecular and effect on the elec- tron density in oxide, which is beneficial to achieving much higher gas response [32,33]. Above analysis can explain why the W 18 O 49 nanowires exhibithigher responsethan theWO 3 nanosheets. How- ever, as shown in Table 1, the NO 2 response of WO 3 nanosheets is higher than that of W 18 O 49 nanowires for almost 10 times at the operating temperature of 250 ◦ C. This result implies that some other factor dominates the gassensing performance ofthe one- and two-dimension tungstenoxidenanostructure. Itis foundfrom Fig.2 that thermal treatment resulted in a more evident change in the microstructure of W 18 O 49 nanowires than that of WO 3 nanosheets, indicating that the microstructure of W 18 O 49 nanowires is much more sensitive to temperature than WO 3 nanosheets. Therefore, it is possible that the microstructure change of W 18 O 49 nanowires (e.g. further agglomeration) at high operating temperature affects the gas diffusion and then induces the much low response. Accord- ing to this analysis, WO 3 nanosheets might show much better stability in the gas response due to much better thermal stability in the microstructure in comparison withW 18 O 49 nanowires when operating at high temperature. Fig. 8 shows the response and recovery time curves of the W 18 O 49 nanowires and WO 3 nanosheets to different concentra- tion of NO 2 at 200 ◦ C. It can be seen that, for the tungsten oxide nanowires and nanosheets, the response times decrease whereas the recovery times increase with rising concentration of NO 2 . Worth of mention is the faster response–recovery of the W 18 O 49 nanowires compared to that of the WO 3 nanosheets when both expose to the same NO 2 concentration. Especially, the W 18 O 49 nanowires exhibit very fast responsecharacteristic to NO 2 gas with the response times of42, 19, 16 and 14 s to 1, 5, 10 and 20ppm NO 2 , respectively. 4. Conclusions One- and two-dimensional tungsten oxidenanostructures were synthesized at 200 ◦ C by solvothermal method with tungsten hex- achloride (WCl 6 ) as precursor and 1-propanol as solvent. The synthesis processes were preformed at different WCl 6 concen- trations (0.01, 0.015 and 0.02 M, respectively). One-dimensional W 18 O 49 nanowire bundles were obtained at a WCl 6 concen- tration of 0.01 M, while the structure of pure two-dimensional WO 3 nanosheets was formed at concentration of 0.02 M. Thermal annealing at 300 ◦ C could not change the crystal structure of the nanowires and nanosheets.BET measurements showedthe specific surface areas and pore volumes were 72.03 m 2 /g and 0.13 cc/g for annealed W 18 O 49 nanowire bundles and 41.85 m 2 /g and 0.12 cc/g for annealed WO 3 nanosheets, respectively. The gas sensing prop- erties measurements indicated that both W 18 O 49 nanowires and WO 3 nanosheets exhibit reversible response to different concen- trations of NO 2 . Compared to WO 3 nanosheets, W 18 O 49 nanowires showed quicker response–recovery and higher response value to different concentration of NO 2 gas due to the high specific sur- face area and the non-stoichiometric crystal structure, and their response time, recovery time andresponse value are 19 s, 112 s and 123.6 to 5 ppm NO 2 at 200 ◦ C, respectively. These results indicate the one-dimensional W 18 O 49 nanowire is a promising gas sensing material for high performance NO 2 gas sensor. Acknowledgments This work was financially supported by the National Natural Science Foundation (No. 60801018, 60771019), Tianjin Natural Science Foundation (No. 09JCYBJC01100) and the New Teacher Foundation of Ministry of Education (No. 200800561109) of China. References [1] G. Eranna,B.C.Joshi, D.P. Runthala,R.P. Gupta, Oxidematerialsfor development of integrated gas sensors-a comprehensive review, Crit. Rev. Solid State Mater. Sci. 29 (2004) 111–188. [2] A. Ruiz, A. Cornet, G. Sakai, K. 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Jiménez, M.A. Centeno, R. Scotti, F. Morazzoni, A. Cornet, J.R. Morante, NH 3 interaction with catalytically modified nano-WO 3 powders for gas sensing applications, J. Electrochem. Soc. 150 (2003) 72–80. Biographies Yuxiang Qin received a Ph.D. in microelectronics and solid-state electronics from Tianjin University in 2007. She is currently an associate professor in Department of Electronics Science and Technology in Tianjin University. Her research interest is in the areas of oxide semiconductor gas sensor, field emission materials and devices. Ming Hu received a M.S. in microelectronics and solid-stateelectronics from Tianjin University in 1991. She is now a professor in Department of Electronics Science and Technology in Tianjin University. Her research interests include MEMS, gas sensor, functional film devices. Jie Zhang received her Bachelor degree in microelectronics and solid-state elec- tronics from Tianjin University in 2008. She is now a graduate student at Tianjin University. Her current research is focused on the tungsten oxide based gas sensor and material adsorption properties simulation. . www.elsevier.com/locate/snb Microstructure characterization and NO 2 -sensing properties of tungsten oxide nanostructures Yuxiang Qin ∗ , Ming Hu, Jie Zhang School of Electronics. temperature of 200 ◦ C. Please cite this article in press as: Y. Qin, et al., Microstructure characterization and NO 2 -sensing properties of tungsten oxide