Rare Metal Materials and Engineering Volume 39, Issue 5, May 2010 Online English edition of the Chinese language journal

Cite this article as: Rare Metal Materials and Engineering, 2010, 39(5): 0753−0755.

Received date: May 17, 2009 Foundation item: National “973” Project (2007CB936601); National Natural Science Foundation (50872064) of China; Knowledge Innovation Project of Chinese Academy of Sciences (KGCX2-YW-341); China Postdoctoral Science Foundation (20090450581); K. C. Wong Education Foundation , Hong Kong Corresponding author: Xu Yuxing, Ph. D., Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China, Tel: 0086-10-62772623, E-mail: xuyx04@mails.tsinghua.edu.cn

Copyright © 2010, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

ARTICLE

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Sol-Gel Derived Cubic-Phase WO3 Nanowires on Nano- Porous Alumina Template Xu Yuxing1,2, Tan Qiangqiang1, Tang Zilong2, Zhang Zhongtai2, Yuan Zhangfu1 1Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China; 2State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China

Abstract: WO3 nanowires with various morphologies were prepared on nanoporous alumina template by a simple sol-gel method. X-ray diffraction analysis reveals that the obtained materials are cubic WO3. Scanning electron microscopy shows that the products contain chrysanthemum-shaped WO3 nanowires. The diameter of WO3 nanowires is about 10-80 nm and their length is up to several micrometers. Argon gas atmosphere is more favorable for WCl6-triblock copolymer sol to form WO3 nanowires with different morphologies on nanoporous alumina template than air atmosphere.

Key words: sol-gel; nanoporous alumina template; WO3 nanowires

In the last few years, one-dimensional nano-structures of nanowires, nanobelts and nanorods have attracted extensive attention because of their fascinating physicochemical properties and their promising applications in fabricating nanoscale devices and nanosensors. It is expected that the nanowires-based one-dimensional materials will be the focus of nanomaterials research in the next decade[1]. Among the numerous transition metal oxides, tungsten oxides are of intense interests and have been investigated extensively for their distinctive properties[2]. Various WO3-based materials have been characterized as promising gas sensing materials for the detection of NOx, NH3, CO and so on[3-5]. Since the adsorption is a surface effect, one of the most important parameters to evaluate the sensitivity of the sensor materials is the surface area. Some results indicate that increase in the surface area can increase the sensitivity of the sensor[6]. For this purpose, much effort has been devoted to the synthesis of one-dimensional WO3 gas sensing materials[7,8]. However, these one-dimensional WO3 nanomaterials must be coated on certain substrate and then sintered before they are measured for gas sensing. Therefore, the surface-to-volume ratio of the materials will be decreased greatly and the gas sensing properties will be influenced largely.

In this work, WO3 nanowires with various morphologies have been prepared on nanoporous alumina template by a simple sol-gel method. Thin nanoporous alumina template with enlarged surface area has been employed as support material for fabricating WO3 nanowires. The structure characteristics of WO3 nanowires have been discussed in detail.

1 Experimental

Poly (ethylene glycol) - poly (propylene glycol) - poly (ethylene glycol) triblock copolymer (P123, Mn=5800, Aldrich), tungsten hexachloride (WCl6, purity: 99.9%, Aldrich) and ethanol (purity: 99.98%) were used as raw materials. The solution was prepared by a simple sol-gel process, with a colloidal solution of WCl6 stabilized by the addition of P123 as the structure-directing agent to form one-dimensional tungsten oxide. Firstly, a solution with 2.5 g of P123 dissolved in 50.0 g ethanol was prepared and stirred rapidly for 0.5 h at room temperature. Then, 5.0 g WCl6 was slowly added to the above solution and stirred vigorously for about 15 min at room temperature. Thus, WCl6-triblock copolymer sol was prepared.

Commercial nanoporous alumina template from Whatman

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(Anodisc 25) with a pore diameter of ~200 nm and thickness of ~60 μm was used as support material. The details of the formation process are as follows[9]. A piece of nanoporous alumina template was placed into a sand funnel, and then the WCl6-triblock copolymer sol was poured onto the template. The as-prepared WCl6 sol was extruded into the pores of the template under the mechanical pump pressure of 0.1 MPa. Finally, the sample surface was rinsed quickly with ethanol and distilled water in turn several times and then dried at 65 °C for 6 h. The dried samples were sintered at 500 °C for 5 h at the rate of 2 °C/min in argon gas atmosphere and air atmosphere, respectively, to remove the residual triblock copolymer. The flow rate of the gas was kept at 0.35 L/min throughout the sintering process.

X-ray diffraction pattern (XRD) of the as-prepared product was obtained with a D8 Advance diffractometer using Cu Kα radiation (λ=0.154178 nm). The 2θ range used in the measurement of WO3 nanowires was from 20° to 70° in step of 5° with a count time of one minute. Scanning electron microscopy (SEM) measurements of WO3 nanowires without etching off nanoporous alumina template were performed on a high-resolution field-emission scanning electron microscopy (FESEM: LEO 1530). X-ray photoelectron spectroscopy (XPS) studies were carried out by a PHI Quantera SXM spectrometer with a monochromatized Al Kα X-ray source. The working pressure inside in the analysis chamber was 6.7×10-8 Pa. The overall instrumental resolution was about 0.5 eV.

2 Results and Discussion Fig.1 shows the XRD pattern of the as-prepared WO3

nanowires on nanoporous alumina template. It is indicated that the phase of the nanowires is cubic WO3 (JCPDS card 46-1096), whose indexed peaks are corresponding to (200), (220), (222), (400), (420) and (422) lattice planes, respectively. The space group is similar to that of cubic ReO3 (Pm3m). The peaks marked with triangle are the impurities coming from the nanoporous alumina template as shown in Fig.1.

Fig.2 shows the SEM images of the as-prepared WO3 nano- wires on nanoporous alumina template with different morpho- logies. As shown in it, the diameter of the as-prepared WO3 nanowires is about 10-80 nm and their length is up to several micrometers. WO3 nanowires sintered in argon gas at 500 °C for

5 h with chrysanthemum-shaped nanowires are shown in Fig.2b. WO3 nanoparticles sintered at 500 °C for 5 h in air are shown in Fig.3. It can be seen from it that only nanoparticles were produced when the sample was heat treated in air atmosphere. For comparison, the same experiment was carried out in argon atmosphere except that nanoporous alumina template was substituted by Al2O3 ceramic plate. The results show that no nanowires were produced. Therefore, it can be proposed that the sintering atmosphere has displayed an important factor. Argon gas atmosphere is more favorable for WCl6-triblock copolymer sol to form WO3 nanowires with different morphologies than air atmosphere. Since the dimensionality and the size of the materials are regarded as critical factors that may bring some fantastic or unexpected properties, WO3 nanowires prepared on nanoporous alumina template will have promising applications especially in gas sensor.

The XPS results for W4f and O1s of the as-prepared WO3 nanowires on nanoporous alumina template are shown in Fig.4a and Fig.4b, respectively. Fig.4a reveals the W4f core level spectra comprising the well resolved spin orbit split doublet peaks pertaining to W4f7/2 and W4f5/2 states. The binding energies of the peaks are 35.72 and 37.72 eV corresponding to W4f7/2 and W4f5/2, respectively. Fig.4b shows the XPS core-level spectrum of O1s. The binding energy of O1s is 530.72 eV and it is assigned to the oxygen atoms that form the strong W=O bonds in the oxide. The XPS results for W4f and O1s of the as-prepared WO3 nanowires are

Fig.1 XRD pattern of the as-prepared WO3 nanowires on nanoporous alumina template

Fig.2 SEM images of WO3 nanowires sintered in argon gas at 500 °C for 5 h with different morphologies: (a,c) nanowires and (b) chrysanthemum-shaped nanowires

a b c

200 nm 200 nm 200 nm

2θ/(°)

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Fig.3 SEM image of WO3 nanoparticles sintered at 500 °C for 5 h in air

Fig.4 XPS spectra of WO3 nanowires: (a) W4f core level spectrum and (b) O1s core level spectrum

in good agreement with those of tungsten (VI) trioxide[10], which also agrees well with the XRD analysis.

3 Conclusions

1) The diameter of WO3 nanowires is about 10-80 nm, with the length up to several micrometers.

2) The presence of six-valent tungsten (W6+) in the nanowires has been confirmed from W4f core level binding energies, which agrees well with the XRD result.

3) Argon gas atmosphere is more favorable for WCl6- triblock copolymer sol to form WO3 nanowires with different morphologies than air atmosphere. This novel synthesis process provides an alternative way for high-quality chrysanthemum- shaped WO3 nanowires synthesis on nanoporous alumina template.

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200 nm

Binding Energy/eV