- 學位論文
利用微波溶劑熱法製備一維三氧化鎢奈 米棒及其光降解特性分析之研究
Studies on the Preparation of 1-Dimension WO 3 Nanorods using Microwave-Solvothermal and their Photodegradation Properties
摘要
奈米氧化過渡金屬近年來因為許多優秀的半導體特性而開始了廣泛的應用，像是磁性儲存材料、太陽能的轉換、觸媒等等。其中三氧化鎢因為其優秀的半導體特性，而具有廣泛的實用價值。本篇研究主要分為兩個部份來探討，第一部份首先是利用微波加熱溶劑熱法來製備出具有一維奈米棒結構的三氧化鎢；第二部份則為了降解水中汙染物目標而將三氧化鎢奈米棒當作光觸媒使用，進而研究光降解效率實驗。 在第一部份中，利用六氯化鎢當作鎢源溶解在乙醇中，並以聚乙二醇(PEG)當作分散劑以避免產物團聚，再使用微波加熱溶劑熱法製備三氧化鎢奈米棒，並且對三氧化鎢奈米棒進行XRD、SEM、EDX、HR-TEM、動態光散射儀(Dynamic Light Scattering)DLS以及TG/DTA等物性分析。 實驗過程中，分別控制不同的鎢源濃度、溶劑中乙醇的百分比、溶劑的種類、分散劑的種類以及添加量，以利得到形貌完整、粒徑分布均勻，且高純度的三氧化鎢奈米棒。研究結果發現，從XRD繞射圖分析及電子選區繞射圖(Selected Area Electron Diffraction, SAED)可以證明本研究成功合成出三氧化鎢，另外SEM以及HR-TEM則可以證明其形貌為一維的奈米棒，而其最佳實驗條件分別為0.005M WCl6、PEG(20k) 0.3g以及使用20%的乙醇當作溶劑來進行溶劑熱法，並與JCPDS資料庫88-0550之結果相符合，其長晶方向為[110]的單斜晶系；DLS所檢測出來的結果與SEM以及HR-TEM所檢測的結果相符合；TG/DTA則證明WO3在600℃下具有良好的熱穩定性，不會發生其他的氧化反應；另外溶劑濃度的變化會造成反應時間、結晶度以及顆粒形貌上的影響，反之則利用濃度變化來控制一維或是二維顆粒形貌；溶劑中有機物的含量關係到反應時間，當分散劑含量增加時，反應時間也會跟著增加；使用80%環己醇當作溶劑時，會因為壓力不夠以及對於WCl6相對低的溶解度，而造成結晶度不足的結果，且顆粒形貌上也容易形成方型結構；較長鏈的PEG(400k)當作分散劑時，會限制[110]方向的長晶進而幫助[002]方向的生長；鎢源濃度增加，則會產生粒徑分布較細長的一維WO3棒狀結構，其長度與寬度大約分別為1um及100nm，長徑比(aspect ratio)則為10左右。 在第二部份中，利用在第一部份成功合成出的WO3奈米棒作為光觸媒，在雙層一體型光反應器中照射波長254nm的紫外光來進行高濃度氮類型染料(dimethyl amino-and azo-groups)中的甲基橙(Methyl Orange, MO)的高反應物體積高效率光降解實驗。 實驗中嘗試使用不同WO3濃度以及一般常用的光觸媒TiO2濃度作為WO3光觸媒添加量、改變不同反應物溶液體積作光降解效率分析；另外再針對高濃度的染料尋找最佳的操作條件。實驗結果可以發現，當降解物為50 ppm之低濃度甲基橙(Methyl Orange, MO)時，250 ppm的WO3奈米棒為最佳操作條件，過少的光觸媒會不利於氫氧自由基的生成，而過多的光觸媒則會產生光穿透及光散射的問題；使用TiO2作為光觸媒時，會因為band gap大小、電洞生成效率以及酸性環境的原因，而導致效率不彰；溶劑體積的增加會提升光降解效率，當溶劑體積為800 mL時，其kapp為3.212，與100 mL有著45倍的差距，其主因應為體積的增加造成光輻射分佈更為平均，使得每個光觸媒不會因為遮蔽效應而造成電子躍遷的困難，進而大幅的增加光降解效率；溶劑體積在500 mL時，會因為光觸媒濃度太高導致光穿透降低及光散射，以及光觸媒濃度太低，會使得光生電洞的速率變成過慢等原因，而導致光降解效率降低；在高濃度的MO染料實驗狀況下，750 ppm的光觸媒濃度皆擁有較好的降解效率，但是並不會與其他濃度有太顯著的差距；在探討動力研究中可以發現，本降解反應符合Langmuir–Hinshelwood 1st order model。 因此可藉由微波加熱溶劑熱法快速的製備出符合經濟及時間效率的一維三氧化鎢奈米棒，並且期望可以應用在環境保護的光觸媒研究以及其他半導體材料相關的應用。
並列摘要
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.