Effect of annealing on microstructure and NO₂-sensing properties of tungsten oxide nanowires synthesized by solvothermal method

Abstract

Quasi-orienting W₁₈O₄₉ nanowire bundles were synthesized by solvothermal method. The microstructures and the NO₂-sensing properties of the as-synthesized nanowires annealed at different temperatures were studied. To characterize the morphology and crystalline structure of the annealed tungsten oxide, field emission scanning electron microscope, X-ray diffraction and transmission electron microscope were employed. It was found that with annealing temperature increasing, the nanowire bundles became straighter and slightly thicker, and eventually a nonobelt-like structure was formed via the nanowires coalescence at 450 °C. Meanwhile, the monoclinic W₁₈O₄₉ was transformed to monoclinic WO₃ when annealing temperature rising from 350 to 450 °C. In comparison to the W₁₈O₄₉ nanowire bundles-based sensor, the sensor based on the 450 °C annealing-induced WO₃ nanobelt-like structure exhibited markedly higher response value and gas selectivity, as well as much better response-recovery characteristics to NO₂ gas due to its favorable microstructure feature to gas-sensing.

Introduction

Detection of toxic gases such as NO_x, O₃, NH₃, CO, H₂S, and SO_x is very important for environmental protection and human health. At present, various metal oxide semiconductors including SnO₂, WO_3−x, ZnO, MoO₃, and TiO₂, have been widely used for sensing these toxic gases [1], [2], [3], [4], [5]. Their sensing mechanism lies in changes in the electrical conductivity of metal oxides in the presence of toxic gases and oxygen due to catalytic reduction/oxidation reactions occurring at the oxide surface. Recently, it was reported that one-dimensional nanostructured materials such as nanowires, nanotubes, nanorods and nanobelts, showed high sensitivity, quick response, and enhanced ability to detect gases at low concentrations because of their large specific surface area, single crystalline structure and dimensions comparable to Debye length [6], [7], [8], [9], [10]. Among various metal oxide semiconductors, tungsten oxide (WO_3−x), which is a wide band-gap n-type semiconductor, has been considered as a promising sensing material for detection of toxic gases [11], [12]. Recent studies about one-dimensional tungsten oxide nanostructures like W₁₈O₄₉ nanowires and WO₃ nanotubes have clearly demonstrated the good prospects of tungsten oxide nanostructures in toxic and hazardous gas detection [13], [14], [15].

For metal oxide-based gas sensors, annealing treatment during the device fabrication is necessary to stabilize sensing film microstructure and then improve the reliability and stability of the sensors. However, thermal annealing at elevated temperature may lead to obvious changes of crystallinity, grain size and shape, porosity, active surface area, and even the phase composition, which are known to be the main factors determining the gas-sensing properties of oxide semiconductor-based sensors [16]. It is expected that nanostructured materials may undergo similar microstructure changes as those in their bulk form at high temperature. Therefore, it is significant and necessary to comprehensively understand the relationship between the annealing-reduced microstructure change and the gas-sensing properties of the promising one-dimensional tungsten oxide. In our previous work, we studied the NO₂-sensing properties of the W₁₈O₄₉ nanowires synthesized by solvothermal method [17]. In this paper, we further investigate the effect of annealing on the microstructure and the NO₂-sensing properties of the solvothermally synthesized W₁₈O₄₉ nanowires. Some interesting results were obtained.