Study On Hydrothermal Synthesis Of WO₃Nanostructures And Their Gas-sensing Properties

Tungsten trioxide （WO₃） is a typical n-type semiconductor functional materials, itis also one of the common metal oxide semiconductor （MOS）. In the past few decades,WO₃has attracted the affection of many researchers due to its numerous potentialapplications in gas sensors, electrochromism, chemical catalysis and electrochemistry,etc. The design and rational synthesis of metal oxide semiconductors with tailoredcharacteristics （morphology, crystal phase, components, crystallinity, etc） have attractedincreasing attention because they largely determine the various properties of thematerials. The introduction of modern nanometer material science and technology eithercan enhance some performance of materials to a certain extent or makes it show someunexpected properties.The hydrothermal solution method and water bath method, which are belong tosolution method, have the advantages of simple economy, low energy consumption, andmaking the nano material high crystallinity and purity, etc. However, the changing PHvalue, hydrothermal temperature and time, kinds and concentrations of auxiliary agentin the solution have important influence on the size, shape, structure and crystallinestructure of WO₃. At the same time, for the same kind of crystal phase, the influence ofdifferent size and morphology of WO₃on properties of its gas sensitivity remains to befurther improved. Therefore, hydrothermal preparation of nanometer WO₃, growthmechanism and gas sensitive properties researches have extremely importantsignificance on theory and practice. Main content of this article is as follows:Firstly, we studied the influence of different size and morphology of WO₃on theperformance of gas-sensing properties. We prepared cubic-like particle monoclinicphase nanometer WO₃, plate-like and layered flower-like WO₃·H₂O by hydrothermalmethod and water bath method, respectively, where sodium tungstate was using as rawmaterials, CTAB and oxalate as auxiliary additives. The WO₃·H₂O transformed tomonoclinic WO₃and keep the original plates and flower-like structures after sinteringprocessing. The best working temperature of the different size and morphology of WO₃is300℃and flower-like structure showed the highest sensitivity. The results showedthat the morphology and size monoclinic phase of WO₃have great influence ongas-sensing properties. We explained gas-sensing mechanism of the three samples byband depletion theory model. Then, we studied the influence of different measurement of auxiliary additives onthe morphology and crystal phase of WO₃in the hydrothermal process. We found thatmalic acid played a significant role in governing morphologies of precursors duringhydrothermal process. A possible formation mechanism was also proposed in detail.Experimental results showed that the optimized hydrothermal precursor could bedehydrated to mixed composition of hexagonal and monoclinic WO₃with the uniquesphere-like porous architecture after being annealed. Besides, gas-sensing measurementindicated that the well-defined3D assembled sphere-like architectures exhibited thehighest sensor response to ethanol gas at the optimal temperature of250℃among thesamples.Finally, we studied the influence of different kinds of auxiliary additives on themorphology and crystal phase of WO₃in the hydrothermal process. We prepared1D（nanorods and nanowires） hexagonal phase WO₃with three different auxiliary additives（Na₂C₂O₄、Na₂SO₄and H₂C₂O₄） by hydrothermal method under the180℃and24hcondition. Combined with the experimental results and related references, we focus onthe influence of common ions Na⁺, SO₄^2-, C₂O₄^2-and H₂C₂O₄-on the nucleation andgrowth role of WO₃in the process of preparation. Results showed that the threedifferent types of auxiliary additives have no effect on the crystal phase of the product.Subsequent gas-sensing measurement indicated that Na₂C₂O₄assisted synthesis of WO₃nanorods showed the highest sensitivity, which was mainly attributed to its highdispersion degree and crystallinity.