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Investigation of WO3 Micro/Nano Structure Synthesized by Hydrothermal Method on Sensing and Photoelectrical Properties

Author: ZuoAiHua
Tutor: XieChangSheng
School: Huazhong University of Science and Technology
Course: Materials Science
Keywords: WO3 Micro/nanostructure Gas-sensing properties Ion doping Phase transformation Photoelectric properties Light activated gas-sensing properties
CLC: TB383.1
Type: PhD thesis
Year: 2011
Downloads: 165
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Abstract


Metal oxide semiconductor has been widely used to detect gas, such as indoor organic volatile gases and all kinds of harmful gases in the atmosphere. However, there are still some problems about the sensitivity, selectivity and stability, which should be further investigated. Moreover, it is more important to degrade harmful gases than to detect them. As an important gas-sensing material, WO3 has attracted more attention in recent years. It should be noted that WO3 is also a good kind of catalytic materials because of its narrow forbidden band and effective adsorption for the light with long wavelength. In this work, WO3 was used as host material. The influence of morphology on gas-sensing properties has been analyzed in order to enhance sensitivity and to decrease detection limit, as well as operation temperature. Moreover, the morphology, crystal structure, energy band and gas-sensing properties after doping metal ions have also been investigated in detail in order to improve photoelectric catalytic properties. The thesis mainly includes six parts as follows:Firstly, crystal structure, important properties and preparation method of WO3 semiconductor were introduced. And then gas-sensing properties and photoelectric properties were discussed in detail, which illustrated the objective of the thesis.Secondly, experimental materials, apparatus, procedure and characterization of WO3 micro/nano-structure have been introduced, as well as gas-sensing theory, preparation of gas sensor, performance index, equipment and method about gas-sensing properties.Thirdly, WO3 materials with nanoparticle, nanobrick and nanosheet morphologies have successfully synthesized using surfactant as template in water solvent through changing surfactants, synthesized time and temperature. The results show that the products are nanobrick and nanoparticle using F127 surfactant and CTAB, respectively. While nanosheet could be gained using mixed solvents of F127 and CTAB. Sensing properties show that WO3 nanobrick has the best sensitivity towards NH3, benzene, acetone, methanol and formaldehyde. The samples synthesized using 10% CTAB as surfactant takes second place and big nanosheet using 20% CTAB as surfactant has the worst sensitivity. But big nanosheet has lowest resistance, which is beneficial to gas-sensing testing.Fourthly, three-dimensional WO3 nanowall microspheres have been successfully synthesized through changing the ratio between ethanol and water. The results show that only nanosheet could be gain after adding water solvent into ethanol solvent, indicating that ethanol play an important role in morphology control. Gas-sensing properties show that nanowall microsphere has better sensitivity toward benzene, acetone, methanol and formaldehyde than nanosheet. Moreover, the recovery rate of nanowall sample is quicker than nanosheet. Under the illumination of UV light,3D WO3 nanowall has good sensitivity and response rate compared to commercial WO3.Fifthly, WO3 nanowire netted microspheres and WO3 nanoneedle clusters have been successfully synthesized using glycol and ethanol as solvent. The results show that only WO3 nanowire netted microspheres could be gained only using glycol as solvent, while nanoneedle clusters would be present when adding ethanol solvent in glycol solvent. Moreover, synthesized time, temperature, tungsten amount and F127 amount could also influence the morphology. XRD results show that all the diffraction peaks of nanowire netted microsphere before annealing can be indexed as hexagonal W03(H20)o.333, while nanoneedle clusters can be indexed as tetragonal H0.23WO3. After annealing, all the samples convert monoclonal WO3. Gas-sensing properties show that nanoneedle cluster has better sensitivity towards NH3 and benzene than nanowall samples and nanowire microspheres. But nanowall sample has better sensitivity towards NO2 gas than nanowire microshpheres and nanoneedle cluster.Sixthly, Nb5+, Sn4+, In3+ and Zn2+ ions were used to improve the properties of WO3. The results show that all the four ions can be imbedded into WO3 crystal lattice and change the morphology and energy band of WO3. FESEM shows that nanowall is gradually evolved into homogeneous nanoparticle after doping. XRD show that crystalline phase of WO3 change from Y-WO3 toα-WO3 after doping few ions. Moreover, forbidden band width of WO3 is also changed, which could influence its UV adsorption and emitting properties. Photoelectric properties show that dark current and photocurrent increase firstly and then decrease with the increase of Nb5+ and Sn4+ concentration, which could be attributed to the hole defect. Gas-sensing properties under light illumination show that Sn-WO3 has best sensitivity towards formaldehyde compared to dimethylbenzene and methylbenzene.

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