Synthesis,Characterization and Properties of Tungsten Oxide Nano-/Micro-Structures with Different Stoichiometric Ratios

Author:Shen Zhen Guang

Supervisor:peng zhi jian wu jing

Database:Doctor

Degree Year:2019

Download:8

Pages:120

Size:9060K

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As an important strategic nonferrous metal,the highly efficient utilization and high value-added processing of tungsten resources are of great significance.Among all kinds of tungsten materials,tungsten oxides(WO3-x,x≤3)have received considerable attention due to their earth-abundance,highly tunable composition,high chemical stability at appropriate pH value,and excellent electrical conductivity.In addition,tungsten oxides have the following attractive characteristics.(i)Tungsten oxides can absorb about 12%of the solar spectrum and have a good electron conduction rate of about 40 times that of titanium dioxide,which makes tungsten oxides one of the most attractive candidate products for photocatalysis and photoelectric catalysis.(ii)Because of the good charge storage/transfer between the W atoms in tungsten oxides,tungsten oxides have become a kind of excellent electrode materials for electrochromic devices and supercapacitors.(iii)Tungsten oxides have high sensitivity to various gases,which makes them one of the most potential gas sensor materials.In this disseration,tungsten oxide crystalline nano-/micro-rods with different stoichiometric ratios were prepared by a simple thermal evaporation method.Their photocatalytic,electrical,humidity-sensitive and electrochemical properties were investigated.The effect of oxygen vacancies on the properties of WO3-x-x nanomaterials was explored.(1)Tungsten oxide(WO3-x)nanomaterials with controlled morphology and composition were fabricated by thermal evaporation of WO3 and S powders at different temperatures(800-1300℃)in a vacuum tube furnace.Their photocatalytic degradation on organic contaminants was evaluated by decomposing methylene blue(MB)in aqueous phase under sunlight,in which WO3 particles presented higher photocatalytic activity than its oxygen-deficient counterparts,WO2.90 and W18O49.But the W18O499 nanorods had higher adsorption ability to MB in all the samples.This result indicates that the presence of oxygen vacancies enhances the adsorption of WO3-x to MB in the photocatalytic process.(2)WO3-x crystalline nano-/micro-rods with identical morphology but different stoichiometric ratios were prepared by thermally evaporating fixed amount of WO3powder in reductive atmosphere from different amounts of S power at 1150oC in a vacuum tube furnace.With increasing content of oxygen vacancies,the devices fabricated with WO3-x crystals present a negative to positive resistance response to relative humidity.Under dry atmosphere,for the devices with increasing x,the strong response to light changed from short to long wavelength;under light irradiation,the conducting ability of the devices was enhanced,due to the more efficient separation and transportation of the photogenerated carriers.These results indicate that WO3-x-x crystalline nano-/micro-rods have the potential to fabricate various humidity sensors and optoelectronic devices.The electrochemical properties of WO3-x crystalline nano-/micro-rods with the same morphology but different oxygen vacancy contents were examined.It was revealed that the oxygen vacancies can substantially improve the conductivity and capacitance of WO3-x crystals.The obtained WO3-x crystals exhibit pseudocapacitive properties.WO3-x nanocrystals with oxygen vacancy exhibits enhanced specific capacitance,which might have broad prospects for application in energy storage devices.(3)With a series of crystalline tungsten oxide nano-/micro-rods with different stoichiometric ratios but identical morphology feature,various nanoscaled electrical devices were fabricated by micro-fabrication through a focused ion beam technique.By studying the relationship between the nonlinearity of WO3-x device and the oxygen vacancy concentration,it was revealed that the I-V characteristic curve of WO3-x-x device with oxygen vacancy is non-linear,while the I-V characteristic curve of WO3crystal is linear.Such nanoscaled WO3-x-x varistors may be a promising low-voltage electrical/electronic device.(4)W18O49@Cu2O nano-/micron-structures were successfully prepared by loading Cu2O on the surface of W18O49 nano-/micron-rods.Methylene blue aqueous solution was selected as the target degradation substance to simulate their photocatalytic degradation performance of waste water.The results showed that with a longer time for the W18O49 nano-micron rods were immersion in the saturated copper acetate solution,more Cu2O was load on the catalyst after heat treatment.The photocatalytic efficiency of the prepared W18O49@Cu2O nano-/micron-structures materials has been greatly improved compared with that of the untreated W18O49nano-/micron-rods.