Hierarchical structured tungsten oxide nanocrystals via hydrothermal route: microstructure, formation mechanism and humi
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Hierarchical structured tungsten oxide nanocrystals via hydrothermal route: microstructure, formation mechanism and humidity sensing Hua-Feng Pang · Zhi-Jie Li · Xia Xiang · Yong-Qing Fu · Frank Placido · Xiao-Tao Zu
Received: 29 July 2012 / Accepted: 26 November 2012 / Published online: 8 December 2012 © Springer-Verlag Berlin Heidelberg 2012
Abstract Hierarchical structured tungsten oxide nanocrystals were synthesized via the hydrothermal route assisted by a capping agent of ammonium benzoate (AB). The products were characterized using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The experimental results show that the crystal microstructures could be changed from flower-shape to star-shape by changing the mole ratio of ammonium benzoate to sodium tungstate (AB/ST). The crystal phases were changed from orthorhombic WO3 ·0.33H2 O to hexagonal WO3 with the increase in the concentration of AB. Based on the results from Fourier transform infrared spectroscopy and time-dependent growth analysis, a selfassembly growth mechanism has been proposed for the formation of flower, spherical, and star-netted microstructures at different mole ratios of the AB/ST. The star-netted WO3 nanocrystals were applied as a sensitive layer for humidity sensing performed using a Love-mode ZnO/36◦ Y-cut LiTaO3 surface acoustic wave device, and a stable and sensitive response to the change of relative humidity was obtained.
H.-F. Pang () · Z.-J. Li · X. Xiang · X.-T. Zu Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu, 610054, P.R. China e-mail: [email protected] X.-T. Zu () e-mail: [email protected] Y.-Q. Fu · F. Placido Thin Film Centre, Scottish Universities Physics Alliance (SUPA), University of the West of Scotland, Paisley, PA1 2BE, UK
1 Introduction Hierarchical structured oxides with the fine structure and large surface area have stimulated great interest in crystal engineering and development due to their wide applications in chemical sensors, photocatalysis, solar cell, and electrochemical device [1–5]. Size, specific shape, and crystal planes are the critical factors to determine the sensing, photocatalytic, and adsorption/desorption performance of the semiconductor materials with hierarchical microstructure [5, 6]. Therefore, shape-controlled synthesis of the hierarchical nanocrystals becomes a research focus in order to maximize their functional properties [7, 8]. Among these nanostructured oxides, the hierarchical tungsten oxide nanocrystals exhibit prominent sensitivity in chemical sensing, unique electrochromic, and photochromic properties for energy efficient window application [9–11]. Recently, hierarchical tungsten oxide (hydrates) nanocrystals have been synthesized using various reaction techniques including chemical vapor deposition, radio-frequency magnetron reactive sputtering deposition, the template directed method, and the thermal evaperation, and the hydrothermal method [12–17]. The hie
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