Controllable Preparation And Phase Transition Of Tungsten Oxide Nanowires With Different Diameters

The tungsten oxides is a typical semiconducting material with a wide band gap and has a wide variety of unique properties,such as photochronmic,thermochromic and electrochromic,indicating that one-dimensional nanostructure tungsten oxides have a broad application prospect in constructing "smart windows" and display devices.Nowadays,it is of great significance and value to study the heat-induced phase transition of one-dimensional nanostructure of tungsten oxide.Tungsten trioxide is a three dimensional network of octahedral crystal structures.The order of the crystal structure of tungsten trioxide with the temperature increasing as the following:monoclinic(ε phase)→triclinic(δphase)→monoclinic(γ phase)→orthorhombic(β phase)→tetragonal(a phase).In the process of thermochromic phase transition,the step change of resistance inside the material will be caused,which will lead to the change of optical property and storage property of resistance.In recent years,there have been more studies on bulk material phase and less on nanomaterials on account of its higher surface energy than bulk materials.Therefore,it is of great significance to study the phase change of tungsten oxide nanowires with different diameters.Simple operation and low cast chemical vapor deposition method was used to prepare monoclinic tungsten trioxide nanowires with good dispersion,uniform diameter and high purity.By controlling the pressure of the reaction chamber,the distance between the source and the substrate,oxygen flow rate,the controlled preparation of tungsten oxide nanowires was realized successfully.The synthesize samples contain orthorhombic and monoclinic phase corresponding to W32O84 and WO3 nanowires.As the pressure and oxygen flow of the reaction chamber increased,the diameter of the nanowires increased.The diameter decreases when the substrate is far away from the source materials.The band gap of the nanowires related to the diameter.According to the absorption,the smaller diameter nanowires has increased bandgap.By using in situ variable temperature Raman,we find that the WO3 nanowires with diameter of 600-800 nm is monoclinic phase at 17 ℃.When the temperature increasing to 220 ℃,its structure changed from monoclinic y phase to orthorhombic β phase.Furthermore,its phase change from orthorhombic β phase to tetragonal α,phase at the temperature of 5 ℃.These phase transition temperature from monoclicnic phase to orthorhombic phase and orthorhombic phase to tetragonal phase is lower than that of bulk materials.The diameter of 200-330 nm is orthorhombic W32O84 nanowires at 17 ℃,its phase change from orthorhomic to tetragonal at 500 ℃.The phase transition of orthorhombic W32O84 nanowires with diameter of 50-150 nm can’t be detected due to the surface effect.