Studies on the Preparation of 1-Dimension WO 3 Nanorods using Microwave-Solvothermal and their Photodegradation Properties

Autor: Chia-Ting Lin, 林嘉鼎
Rok vydání: 2012
Druh dokumentu: 學位論文 ; thesis
Popis: 101
In the recent years, nanoparticles of transition metal oxides found a wide range of applications due to their superior semiconducting properties as magnetic storage materials, solar energy conversion materials, and photocatalysts. Among these materials, tungsten trioxide (WO3) nanomaterials has been considered as the most valuable and practical one. This thesis consists two parts of studies related to WO3 nanorods. The first part is the study of the preparation of one-dimensional WO3 nanorods using microwave-assisted solvothermal method. The second part is the study of the photocatalytic activity of the above-described one-dimensional WO3 nanorods using photdegradation of methyl orange as a probe reaction. In the preparation of one-dimensional WO3 nanorods using microwave-assisted solvothermal method, tungsten hexachlorid dissolved in ethanol was used as tungsten source while polyethylene glycol was used as a dispersing agent. The physical properties of tungsten oxide nanorods were analyzed by XRD, SEM, EDX, HR-TEM, DLS and TG/DTA. In order to obtain tungsten trioxide nanorods with complete morphology, narrow size distribution, and high purity, the effects of the concentration of the tungsten source, the concentration of ethanol in the solvent, solvent type, the type and amount of dispersant were studied. The results showed that tungsten trioxide could be synthesized successfully and confirmed by the XRD diffraction pattern and SEAD analysis. The morphology of tungsten trioxide nanorods was demonstrated as one-dimensional via SEM and HR-TEM. The optimum synthesis conditions appeared to be the 0.005M WCl6 with 0.3g PEG (20k) using 20% ethanol as solvent. The direction of crystal growth of monoclinic was showed as [110] which was consistent with the results of the database of the JCPDS 88-0550. Moreover, the results of DLS analysis were in accordance with the SEM and HR-TEM results. The TG/DTA analysis proved that WO3 has good thermal stability and the oxidation reaction would not occur under 600 ℃. The reaction time, crystallinity and particle morphology would be affected by the changes of solvent concentration. The changes of solvent concentration could be applied to control the one-dimensional or two-dimensional particle morphology as well. It was found that the organic compound contents in the solvent were related to the reaction time. When an 80% cyclohexanol was used as solvent, a low reaction pressure and low WCl6 solubility would cause a shortage in crystallinity of the WO3 obtained. The particles of the WO3 obtained tended to form a square structure. Besides, the experimental results indicated that when PEG (400k) was used as a dispersing agent, the crystal growth along the [110] direction would be limited and the growth along the [002] direction would be increased. Furthermore, when the concentration of tungsten source was raised, a slender one-dimensional WO3 rod-like structure was formed. The slender one-dimensional WO3 rod-like structure was approximately 1 um long and 100nm wide with an aspect ratio of 10. On the other hand, the photocatalytic activity of the one-dimensional WO3 nanorods was studied using photdegradation of methyl orange as a probe reaction. The photocatalysis was carried out in a double-layer integrated reactor under 254nm UV irradiation to degradate the dimethyl amino- and azo-groups of the methyl orange. In photodegradation experiments, the effects of the concentrations of WO3 and TiO2 catalysts and the changes in different reactant solution volume on the photodegradation efficiency were investigated. The optimum conditions were targeted especially on systems with high concentrations of methyl orange. The experimental results showed that a 50 ppm methyl orange was photodegradated by 250 ppm WO3 nanorods with an efficiency higher than that by commercially available TiO2 (Degussa P-25) catalyst. The poor performance of TiO2 catalyst was explained by the band gap, the efficiency of electron hole generation, and the acidic environment. It was found that an increase in the volume of solvent would increase the photodegradation efficiency. When the solution volume was 800mL, a kapp of 3.212 was obtained. This high photodegradation efficiency was about 45 times of the kapp observed with a solution volume of 100mL. The increase in solution volume would make the light radiation distribution more even and thus substantially increased the photdegradation efficiency. Moreover, slightly higher degradation efficiency toward high methyl orange concentrations was observed when the concentration of the WO3 catalyst was increased from 100 ppm to 750 ppm. Based on the kinetics study results, it was found that the degradation reaction of methyl orange on the one-dimensional WO3 nanorods was consistent with Langmuir- Hinshelwood 1-st order model. The photocatalytic activity of the one-dimensional WO3 nanorods synthesized by microwave-assisted solvothemal method was observed; further application in environmental protection could be expected.
Databáze: Networked Digital Library of Theses & Dissertations
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