large - scale synthesis and gas sensing application of vertically

Sensors and Actuators B 143 (2009) 325–332 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Large-scale synthesis and gas sensing application of vertically aligned and double-sided tungsten oxide nanorod arrays Xiaoping Shen a,b , Guoxiu Wang a,∗ , David Wexler a a Institute for Superconducting and Electronics Materials, School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, Wollongong, New South Wales 2522, Australia b School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212000, China article info Article history: Received 25 June 2009 Received in revised form 17 August 2009 Accepted 6 September 2009 Available online 17 September 2009 Keywords: Tungsten oxide Nanorod arrays Hydrothermal synthesis Gas sensing Sensor abstract Large-scale vertically aligned and double-sided Co-doped hexagonal tungsten oxide nanorod arrays have been successfully synthesized by a facile hydrothermal method without using any template, catalyst, or substrate. Scanning electron microscopy and transmission electron microscopy analyses reveal an interesting three-order hierarchical nanostructure from small, single-crystalline nanorods via nanorod bundles to double-sided nanorod arrays. The optical absorption properties of the Co-doped WO 3 sam- ples were investigated by ultraviolet–visible spectroscopy, and the results indicate that the Co-doped WO 3 nanostructures are semiconducting with direct band gaps of 2.26eV and 2.77 eV. The gas sens- ing performance of the as-prepared Co-doped WO 3 double-sided nanorod arrays was tested towards a series of typical organic solvents and fuels. The sample shows excellent gas sensing performance towards 1-butanol vapor, with rapid response and high sensitivity. We propose that the double-sided nanorod arrays are formed from urchin-like microspheres via a self-assembly and fusion process. This new syn- thesis strategy could be extended to prepare other well-aligned nanorod arrays for many functional applications. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Enormous efforts have been devoted to the development of syn- thetic strategies for highly organized hierarchical nanostructures consisting of nanoscale building blocks [1–4], because assembling the synthesized nanoscale building blocks into advanced struc- tures is a necessary approach for applications in integrated devices. Highly ordered arrays of nanorods or nanowires in particular have aroused continuous interest due to their diverse properties and potential applications in data storage, catalysis, sensing, field elec- tron emission, and optoelectronic devices [5–8]. Two versatile strategies basedon templates and patterned catalysts, respectively, have been developed to produce nanorods in the form of large- area arrays [9–11]. However, although the template-based method can provide good control over the uniformity and dimensions of nanorods, removal of the template through a post-synthesis process may cause damage to the nanorod arrays. In addition, most nanorods synthesized using the template-based method are polycrystalline instructure, whichmay limit their use in device fab- rication and fundamental studies. The method involving the use of a patterned catalyst is able to generate nanorods with controllable ∗ Corresponding author. Fax: +61 2 42215731. E-mail address: gwang@uow.edu.au (G. Wang). sizes and highly crystalline structures. However, the catalyst may cause contamination of the resultant nanorod arrays,which is often a great disadvantage to their application. As a template-free and catalyst-free method, epitaxial growth of free-standing nanorod arrays has recently been attained, in which a substrate with an excellent lattice match to the overlying materials is vital to guide the assembly of one-dimensional (1D) nanoarrays [12,13].Asa result, this method has largely been limited to particular materials, notably zinc oxide. Therefore, the development of novel and more effective strategies for preparing highly ordered nanorod arrays is of great significance for their practical applications in nanotechnol- ogy. Tungsten oxide (WO 3 ), as an important n-type semiconductor, has received wide attention owing to its promising application in gas sensors, heterogeneous catalysts, chromogenic devices, solar- energy devices, and field electron emission [14–16]. The syntheses of various WO 3 nanostructures have been reported, including nanorods, nanowires, nanotubes, nanobelts, urchin-like super- structures, and nanostructured thin films [17–19]. Moreover, it has been found that metal-doped tungsten oxide may exhibit improved optical and electrical properties. For example, Na-doped tungsten oxide has proved to be a high temperature supercon- ductor [20], and Cr-doped WO 3 nanocrystals showed excellent gas sensing performance towards acetone [21]. Herein, we report a novel strategy for preparing large-area Co-doped WO 3 double- 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.09.015 326 X. Shen et al. / Sensors and Actuators B 143 (2009) 325–332 sided nanorod arrays without using any template, catalyst, or substrate. The resulting products were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), Raman spectroscopy, and ultraviolet–visible (UV–vis) spectroscopy. A mechanism is proposed to explain the growth of the double-sided nanorod arrays. Moreover, the gas sensing performance of the as-synthesised Co-doped WO 3 nanos- tructures was investigated towards a series of flammable and toxic organic solvents. We found that the as-synthesised Co-doped WO 3 nanorod arrays have a fast, highly sensitive, and fairly selective response to 1-butanol gas. 2. Experimental details All chemicals are ACS reagent and were used directly as purchased from Sigma–Aldrich. In a typical experiment for synthe- sizing Co-doped hexagonal tungsten oxide double-sided nanorod arrays, 0.815 g of Na 2 WO 4 ·2H 2 O was dissolved in 10mL of distilled water. The solution was acidified to a pH range of 1–1.2 using HCl solution (3mol L −1 ). Then, 0.63 g of H 2 C 2 O 4 was added to the mix- ture and the mixture was diluted to 25 mL. After that, a stable WO 3 sol was formed. 16 mL of the WO 3 sol was transferred into a 20 mL Teflon-lined autoclave, and then 0.80 g Na 2 SO 4 and 5.0mg Co(NO 3 ) 2 ·6H 2 O were added to the solution. The autoclave was sealed and maintained at 180 ◦ C for 10 h. After the autoclave was naturally cooled to room temperature, the resulting solid products were separated by centrifugation, washed three times with dis- tilled water and ethanol, respectively, and dried at 60 ◦ C overnight. To further improve the crystallinity and phase purity, the products were annealed at 400 ◦ C for 4h. The as-prepared samples were characterized by X-ray diffrac- tion (XRD, Cu K␣ radiation, Philips 1730), scanning electron microscopy (SEM, JSM-6460 and JSM-6700F), transmission elec- tron microscopy (TEM), and high-resolution TEM (HRTEM, JEOL 2011). Samples for TEM were prepared by dropping the products on a carbon-coated copper grid after strong ultrasonic dispersion in absolute ethanol. X-ray photoelectron spectroscopy (XPS) mea- surements were carried out with an ESCALab220i-XL spectrometer by using a twin-anode Al K␣ (1486.6 eV) X-ray source. All the spectra were calibrated according to the binding energy of the adventitious C1s peak at 284.8eV. The band gap of the Co-doped WO 3 was determined by UV–vis spectroscopy (Shimadzu 1700). Raman spectroscopy (HR 800) was performedat room temperature with an excitation wavelength of 632.8 nm. The gas sensing properties were measured using a WS-30A gas sensor measurement system. The gas sensor was fabricated as fol- lows: the Co-doped WO 3 sample was mixed with polyvinyl acetate (PVA) binder (1 wt.%) to form a slurry, and then pasted onto a ceramic tube (2 mm in diameter) by a doctor blade to form a thin film between two Au electrodes,which had been previously printed on the ceramic tube and were connected with four platinum wires. As a comparison, gas sensing properties of commercial WO 3 pow- der (Fluka) were also measured. The commercial WO 3 powder is a well crystallinepowder witha crystalsize inthe rangeof afew hun- dreds nanometers. Fig. S-1 shows the FE-SEM image of commercial WO 3 powders (Supplementary data). Given amounts of test gases were injected into the testing chamber by a microsyringe. Gas sens- ing measurements were carried out at a working temperature of 200 ◦ C with 30–40% relative humidity. Fig. S-2 shows a schematic diagram of the sensor system (Supplementary data). The gas sensor was fabricated according to the method described in Section 2. The working temperature of the sensor can be controlled by adjusting the heating voltage (V heating ) across a resistor inside the ceramic tube. A reference resistor is put in series with the sensor to form a complete measurement circuit. In the test process, a working voltage (V working ) was applied. By monitoring the voltage (V output ) across the reference resistor, the response of the sensor in air or in a test gas could be measured. The gas sensing response is defined as the ratio of the stationary electrical resistance of the sensor in air (R air ) to the resistance in the test gas (R gas ), i.e., R = R air /R gas . 3. Results and discussion The phase of the obtained products was determined by XRD. As shown in Fig. 1, all the diffraction peaks of the annealed product can be readilyindexed to hexagonalstructure WO 3 with lattice con- stants a = 7.3223 Å and c =7.6574 Å, which are slightly smaller than the standard values for bulk hexagonal WO 3 (JCPDS No. 85-2460, a =7.3242 Å and c = 7.6624Å). Before annealing, the sample showed several impurity peaks, suggesting that the annealing is necessary to obtain pure phase WO 3 . In addition, the relative strength of the (0 02) peak was significantly increased by the annealing treatment, suggesting that annealing also improves the crystallinity of the product. It is well known that hexagonal (h) WO 3 is a metastable phase and can transform into monoclinic (m) WO 3 at high tem- perature. Recently, Szilágyi et al. pointed out that the structure of hexagonal WO 3 cannot be maintained without some stabilizing ions or molecules in the hexagonal channels, and thus the exis- tence ofstrictly stoichiometric hexagonal WO 3 is questionable[22]. X-ray photoelectron spectroscopy (XPS) (Fig. 2) was employed to determine the chemical composition and oxidation states of the elements in the present sample. It was revealed that the product contains the elements tungsten, oxygen, sodium and cobalt with a W:O:Na:Co molar ratio of about 1:2.92:0.40:0.02. The presence of such a high Na + content of the product strongly supports Szilá- gyi’s viewpoint. This may explain why alkaline (Na + ,K + ,Cs + , etc.) or NH 4 + ions were needed to prepare h-WO 3 . Alkaline (Na + ,K + ,Cs + , etc.) or NH 4 + ions can stabilize the hexagonal structure in such a way that they are located in the hexagonal channels of crystallites and block the thermodynamically favored hexagonal–monoclinic transformation. As shown in Fig. 2, the O1s peak is located at 530.7 eV, which is ascribed to the W–O peak. The W4f peaks located at 36.4eV and 38.4 eV can be attributed to W4f 7/2 and W4f 5/2 , respectively, which are in good agreement with the reported values [23]. These two peaks are well separated without any shoulder, which indicates that almost all W atoms are in the +6 oxidization state [24]. The Fig. 1. X-ray diffraction patterns of Co-doped WO 3 nanorod arrays: (a) before annealing and (b) after annealing. X. Shen et al. / Sensors and Actuators B 143 (2009) 325–332 327 Fig. 2. XPS spectra of Co-doped WO 3 nanorod arrays. Na1s peak at 1070.9 eV is consistent with the +1 oxidation state of sodium [25]. A Co2p signal located at 780.3 eV was detected, showing that Co exists in the +2 oxidation state in the product [26]. However, the Co2p signal is weak due to the low Co content in the product. Therefore, the above results confirmed the successful preparation of the Co-doped hexagonal tungsten oxide. Fig. 3 shows FE-SEM images of the Co-doped WO 3 sample. Fig. 3(a) is a general view of a double-sided nanorod array. It can be seen that the double-sided nanorod array has a uniform thickness of about 10 ␮m, with the nanorods growing from the center in two opposite directions. Fig. 3(b) and (c) are top views of the nanorod array at different magnifications. It can be seen that the nanorods, with a diameter of about 200 nm, are densely arranged in a well ordered way to form a large-area ordered array. It was found that the area of a single ordered array can reach 1.0 × 10 4 ␮m 2 . Furthermore, the high-magnification FE-SEM image (Fig. 3(d)) reveals that the nanorods are actually composed of a number of smaller nanorods with a diameter of about 20 nm. Therefore, they will be referred to as nanorod bundles hereafter. We also performed FE-SEM observation on a cross-section of the double-sided nanorod array (as shown in Fig. S-3, supplemen- tary data). Elemental energy dispersive X-ray (EDX) mapping of Co was obtained on the same area. We found that the distribu- tion of the dopant element Co is uniform in the cross-section. The above results unambiguously demonstrate that a novel Co-doped WO 3 nanostructure with a three-order hierarchical architecture from small nanorods via nanorod bundles to double-sided nanorod arrays has been achieved. A further investigation of the Co-doped WO 3 nanorod arrays was conductedby TEM and HRTEM analysis.The low-magnification TEM image (Fig. 4(a)) shows some straight nanorod bundles with a diameter of about 200 nm and a length up to about 5 ␮m. A high-magnification TEM image (Fig. 4(b)) reveals that the nanorod bundles consist of smaller nanorods with a diameter of about 20 nm. These results are consistent with the FE-SEM observations. Selected area electron diffraction (SAED) pattern (inset in Fig. 4(b)) taken from this nanorod bundle shows regular diffraction spots, which can be indexed as hexagonal WO 3 single crystal recorded along the [1 1 0]zone axis anddemonstrates that theWO 3 nanorods grow along the [0 01] direction. The SAED pattern also reveals that the nanorods in the bundles consist of a perfectly oriented assembly, which is further confirmed by the HRTEM analysis. We have performed extensive TEM observationson different individual bundles and found that all of them consist of small nanorods and grow along [0 01] crystal direction (Fig. S-4, supplementary data). As shown in Fig. 4(c) and (d), the parallel lattice fringes among the different primary nanorods and grain boundaries clearly show the oriented aggregation and high crystallinity of the primary nanorods. The degree of fusion of the primary nanorods in the bundles shows a gradual increase from the top to the base of the bundles. Fig. 4(e) shows a HRTEM image of the middle section of a primary nanorod, from which the (1 0 0) lattice planes with a d- spacing of 0.64 nm can be clearly distinguished. It was found that preferential growth occurred parallel to the (1 00) lattice plane, which is in consistent with the [0 01] growth direction determined by SAED. Fig. 4(f) shows a lattice resolved HRTEM image of the tip of a primary nanorod. Thecorresponding SAED pattern is presented as the inset in Fig. 4(f), confirmed the single-crystalline nature of the primary nanorod. Based on the experimental observations, we propose that the formation process of the Co-doped WO 3 double-sided nanorod arrays can be divided into three steps. In the initial stage, the nanoparticles are quickly grown and spontaneously aggregate into large spheres to minimize their surface area. In the second step, the as-formed microspheres serve as substrates for epitaxial growth of c-axis oriented WO 3 nanorods with the assistance of SO 4 2− ions. At the same time, because of their high surface energy, the adja- cent WO 3 nanorods tend towards oriented attachment and then 328 X. Shen et al. / Sensors and Actuators B 143 (2009) 325–332 Fig. 3. FE-SEM images of Co-doped WO 3 nanorod arrays: (a) a general side view of double-sided nanorod arrays, (b) and (c) top views of the nanorod arrays at different magnifications, (d) high-magnification FEG-SEM image of the Co-doped WO 3 nanorod arrays, with the inset showing a high-resolution view of a single nanorod bundle. partly fuse to form bunch-like structures [27–29]. As observed by TEM analysis, the bunch-like structures become more smooth and regular from the tip to the base of the bundles. We believe that Ostwald-ripening works simultaneously with the oriented attachment to remedy the defects, leading to a smooth and regu- lar surface of the base parts. Thus, urchin-like architectures with WO 3 nanorod bundles on the surfaces of the microspheres are formed. This is supported by the evidence that several hemispher- ical regions (Fig. 5(a)) and sphere-like cores with nanorod bunches on their surfaces (Fig. 5(b)) have been found in the Co-doped WO 3 products. This process is similar to what happens in the synthesis of WO 3 urchin-like microspheres. Finally, the as-formed urchin- like microspheres further self-assemble and fuse into well-aligned double-sided nanorod arrays with the help of dopant Co 2+ and an accompanying Ostwald-ripening process. To the best of our knowl- edge, thisis the first time that the observation of large-area uniform double-sided nanorodarrays formedby self-assemblyand fusion of urchin-like nanostructures has been reported. Scheme 1 shows the schematics of the formation process of the Co-doped WO 3 double- sided nanorod arrays, elucidating the growth mechanism of the nanorod arrays. Raman scattering is very sensitive to the microstructure of nanocrystalline materials, so it was employed to determine the nanostructure of the Co-doped WO 3 nanorod arrays. As shown in Fig. 6(a), the Raman spectrum of the Co-doped WO 3 nanostruc- tures shows five obvious Raman peaks located at around 270 cm −1 , 327 cm −1 , 713cm −1 , 808cm −1 , and 927 cm −1 . The Raman shifts are consistent with the fundamental modes of crystalline h-WO 3 . The bands at 713 cm −1 and 808cm −1 can be assigned to the O–W–O stretching modes, while the bands at 270cm −1 and 327 cm −1 cor- respond to the O–W–O bending modes of the bridging oxygen. The weak Raman peak at 927 cm −1 may be attributed to a stretching mode of the terminal W O. Although this latter band is charac- teristic of tungsten oxide hydrates, it can appear in WO 3 via the adsorption of water molecules [30]. The Raman spectrum provides clear evidence for the highstructural qualityand phase-pure nature of Co-doped WO 3 nanorod arrays. Theoptical absorptionproperties of the as-prepared Co-doped WO 3 nanostructures were investi- gated at room temperature by UV–visible spectroscopy. As shown in Fig. 6(b), the spectrum shows one absorption peak at about 281 nm. WO 3 is an n-type semiconductor [31], and its optical band gap can be estimated using the following formula: (˛h) n = B(h − E g ) (1) where ˛ is the absorption coefficient, h is the photon energy, B is a constant relative to the material, E g is the band gap, and n is either 1/2 for an indirect transition or 2 for a direct transition. The (˛h) 2 versus h curve for the product is shown in the inset in Fig. 4(b). The value of h extrapolated to ˛ = 0 gives the absorp- tion band gap energy. Two regions with a linear relationship are observed in the ranges of 3.5–4.2eV and 5.0–6.0 eV, respectively, giving two E g values of 2.77 eV and 2.26eV. The band gap of 2.77 eV can be attributed to the transition between the 2p valence band formed by oxygen and the 5d conduction band of tungsten, while the 2.26eV bandgap may be associated with the O −II → Co II charge transfer process (with the Co II level located below the conduction band). However, the band gap of 2.77 eV is much lower than the reported directband gapsof WO 3 . Theband edgeposition for amor- phous WO 3 in contact with an aqueous electrolyte at a pH of ∼1is about 3.2 eV [32]. Two distinct direct interband transition energies of 3.52eV and 3.74 eV for WO 3 were also observed by Koffyberg et al. [33]. The lower band gap of the Co-doped WO 3 may reflect doping effects. It is well known that in doped compound semi- conductors, in contrast with un-doped ones, the impurity states play a special role in the electronic energy structures and transi- tion probabilities. In addition, it is found that the best fit of Eq. (1) to the absorption spectrum of the product gives n = 2, which sug- gests that the as-obtained Co-doped WO 3 is semiconducting with direct transitions at these energies. Chemical sensors play an important role in the areas of emis- sions control, environmental protection, public safety, and human health. Much more public concern over serious environmental issues is further promoting the development of sensors with both high sensitivity and rapid response. It has been well documented that the ultra-high surface-to-volume ratios of nanostructured materials make their electrical conductivities extremely sensitive to surface-adsorbed species and make them excellent candidates for gas sensing applications [34,35]. The gas sensing performance X. Shen et al. / Sensors and Actuators B 143 (2009) 325–332 329 Fig. 4. TEM and HRTEM images of Co-doped WO 3 nanorod arrays: (a) low TEM images of separated nanorod bundles, (b) high-magnification TEM images showing the bundle-like nanostructures, (c) and (d) HRTEM images showing the lattice structures of the nanorod bundles, (e) and (f) lattice resolved HRTEM images of the primary nanorods. The inset in (b) and (f) are the corresponding SAED patterns. of the Co-doped WO 3 nanorod arrays was investigated for sev- eral ordinary organic solvents and fuels, including acetone, ethanol, propanol, butanol,toluene, heptane, acetic acid, and gasoline. Some of these chemicals are very important industrial raw materials, and the others are arousing more and more attention because of the possibility of their use as automotive fuels or gasoline compo- nents. Therefore, highly sensitive gas sensors are important to the practical applications of these flammable gases. The real-time sensing responses towards 1-butanol of sensors based on the Co-doped WO 3 nanostructures and on commercial WO 3 powder are displayed in Fig. 7(a). It can be seen that the sensing response (R air /R gas ) of the sensor increased abruptly on the injection of 1-butanol, then decreased rapidly, and recovered to its initial value after the test gas was purged. The magnitude of the response of the sensor based on the Co-doped WO 3 nanorod arrays improved dramatically with increasing concentration of the Fig. 5. (a) FE-SEM image shows a hemispherical region (enclosed by the circle) in a nanorod array. (b) Cross-sectional FE-SEM image showing the sphere-like cores (as indicated by the circles) in the Co-doped WO 3 double-sides nanorod arrays. 330 X. Shen et al. / Sensors and Actuators B 143 (2009) 325–332 Scheme 1. Schematic diagram of the proposed growth mechanism of the Co-doped WO 3 double-sided nanorod arrays. test gas and was much higher than that of the commercial pow- der. This means that the Co-doped WO 3 nanorod arrays are much more sensitive to 1-butanol than the commercial powder. After many cycles between the test gas and fresh air, the resistance of the sensor was still able to recover its initial state, which indi- cates that the sensor has an excellent reversibility. The response time and recovery time (defined as the time required to reach 90% Fig. 6. (a) Raman spectrum of the Co-doped WO 3 double-sided nanorod arrays. (b) UV–vis spectrum of the Co-doped WO 3 double-sided nanorod arrays. The inset is (˛h) 2 vs. h curve showing the band gap energies. of the final equilibrium value) of the sensor were only 1–2 s and 2–4 s, respectively. The response characteristic curves of the sen- sors towards other gases are similar to that for 1-butanol and are not shown here. The major charge carriers are electrons for n-type semiconductors. Upon exposure to a reducing gas, the density of n-type charge carriers (electrons) would increase due to surface Fig. 7. (a) Real-time sensing responses towards 1-butanol of the sensor made from the Co-doped WO 3 double-sided nanorod arrays and commercial WO 3 powders. (b) Sensing responses vs. gas concentrations of various gases, including 1-butanol, acetone, toluene, 2-propanol, acetic acid, ethanol, gasoline, and heptane. X. Shen et al. / Sensors and Actuators B 143 (2009) 325–332 331 adsorption and chemical reaction between the gas and the oxy- gen adsorbates (electron acceptors), resulting in a decrease in the sensor resistance. The sensing responses as a function of vapor concentration from 5 ppm to 1000 ppm are shown in Fig. 7(b). It can be seen that the responses took on an exponential rate of increase at first (below 200 ppm), which then changed to a linear increase in the range of 200–1000 ppm. As a whole, the sensing responses decreased in the sequence of 1-butanol, acetone, toluene, 2-propanol, acetic acid, ethanol,gasoline, andheptane. TheCo-doped WO 3 nanostruc- tures exhibited a high sensing response to 1-butanol vapor, and the R air /R gas value was 8.5 at the very low concentration of 5 ppm, but reached 232 at 1000 ppm. The sensing responses to 2-propanol, ethanol and gasoline are 71, 50, and 37, respectively, at the concen- tration of 1000 ppm. As shown in Fig. 5(b), the sensing responses of the Co-doped WO 3 nanorod arrays towards acetone, toluene, acetic acid, and heptane are 122, 95, 66, and 31, respectively. These results indicate that the Co-doped WO 3 nanostructure-based sen- sor is highly sensitive to these organic gases. It should be noted that the relativelylow operation temperature helps to decrease thecon- sumption of energy and can improve the suitability of the sensor in some particular situations. It is well known that WO 3 ,asann- type semiconductor, is a good candidate for detecting the inorganic gases O 3 ,NO x , and H 2 S [36], but is less sensitive to hydrocarbons. Nevertheless, our investigations illustrate that doping, and control of the morphology and size of the nanorod arrays has endowed WO 3 with better sensing performance towards hydrocarbon gases at the relatively low operation temperature of 200 ◦ C. 4. Conclusions In summary,well-aligned Co-doped WO 3 double-sided nanorod arrays have been synthesized by a facile hydrothermal method without using any template, catalyst, or substrate. An interest- ing three-order hierarchical nanostructure, from small nanorods via nanorod bundles to double-sided nanorod arrays, has been observed by FE-SEM and TEM. It was discovered that the double- sided nanorod arrays are formed from urchin-like microspheres via a self-assembly and fusion process. This may provide a new strategy for large-scale preparation of well-aligned nanorod arrays, with the advantages of simplicity, low cost, and no introduced alien species. The Co-doped WO 3 nanorod arrays are semiconducting, with direct band gaps of 2.26eV and 2.77 eV, and show good sens- ing performance towards 1-butanol vapor, with rapid response and high sensitivity. The results highlight the potential applications of the Co-doped WO 3 double-sided nanorod arrays in monitoring flammable and toxic organic gases. Acknowledgements This work was financially supported by the Australian Research Council (ARC) through an ARC Discovery project (DP0559891). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2009.09.015. References [1] F.X. Redl, K.S. Cho, C.B. Murray, S. O’Brien, Three-dimensional binary superlat- tices of magnetic nanocrystals and semiconductor quantum dots, Nature 423 (2003) 968–971. 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Chem. 18 (2008) 965–969. 332 X. Shen et al. / Sensors and Actuators B 143 (2009) 325–332 [36] C.S. Rout, M. Hegde, C.N.R. Rao, H 2 S sensors based on tungsten oxide nanos- tructures, Sens. Actuators B 128 (2008) 488–493. Biographies X.P. Shen received his PhD degree in inorganic chemistry in 2005 from Nanjing University, China. He is a professor at School of Chemistry and Chemical Engineer- ing, Jiangsu University since 2006. He is currently working as a visiting Professor at School of Mechanical, Materials and Mechatronic Engineering, University of Wol- longong, Australia. His major research interests include nanostructured materials and molecule-based magnetic materials. G.X. Wang received his PhD degree in Materials Science and Engineering in 2001 from University of Wollongong, Australia. He currently is working as an Associate Professor at School of Mechanical, Materials and Mechatronic Engineering, Univer- sity of Wollongong. His major research interests include nanostructured functional materials, materials chemistry in energy storage and conversion, and development of chemical and biological sensors. D. Wexler received his PhD degree in Materials Science and Engineering in 1991 from Monash University, Australia. He currently is working as a senior research fellow at School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong. His major research interests include nanomaterials synthesis and TEM and HRTEM characterization of materials. . Chemical journal homepage: www.elsevier.com/locate/snb Large-scale synthesis and gas sensing application of vertically aligned and double-sided tungsten oxide nanorod. surface-adsorbed species and make them excellent candidates for gas sensing applications [34,35]. The gas sensing performance X. Shen et al. / Sensors and Actuators