二氧化鈦奈米管陣列與複合結構之形貌與結構特性及其光催化性質之研究

Abstract

本研究針對使用陽極處理法製備二氧化鈦奈米管陣列(TiO2 nanotubes arrays, TNAs)與其奈米線複合結構(TNWs/TNAs)之製程參數與後熱處理方法進行一連串的顯微結構分析與形貌演進之探討。首先，以兩種不同氟化物:氫氟酸(Hydrogen fluoride, HF)與氟化銨(Ammonium fluoride, NH4F) 製備TNAs後，進行不同溫度之後熱處理，再以X-光粉末繞射儀(XRD)、掃瞄式電子顯微鏡(SEM)和X光近緣結構(XANES)進行分析討論。其結果顯示以HF製備出TNAs含90%非晶相(amorphous)與10% Ti2+ (TiO) 與 Ti3+ (Ti2O3)之低氧量鈦化物。而經過400oC熱處理之後，其結構轉變成93%銳鈦礦相(anatase)、6% 非晶相與1% 低氧鈦化物。相反地，以NH4F製備出TNAs則是含較低之非晶相TiO2 (82%)與較高的低氧鈦化物(18%)，其原因乃是由於電解質中只有少量的1 wt% 水添加量，使得溶液中氧離子供應量不足而產生較多低氧鈦化物。經過400oC熱處理之後，其二氧化鈦奈米管之結晶度只增加至86%銳鈦礦相。此低結晶度可推論是因為NH4解離之NH4+與TiF62- 反應形成(NH4)2TiF6化合物所致。 另一方面，本研究利用準分子雷射以垂直式(parallel mode)與旋轉式(tilted mode)兩種方法來進行在二氧化鈦後熱處理。其研究結果發現垂直式照射試片表面之後熱處理只能達到相較400oC 1h後熱處理之50%結晶度。因為垂直式後熱處理使得一維二氧化鈦奈米管的熱傳遞方向只能從表面開始向下產生相變化，再加上過薄的滲透深度與太短的脈衝持續時間(25 ns)，而限制了材料之結晶度。然而，若使用旋轉式，當角度旋轉至與雷射源呈85o¬¬¬時，其結晶度可達相較400oC 1h後熱處理90%之結晶度。推測因為旋轉式增加了雷射照射的面積以及較佳的雙向熱傳導方式為其主要原因。 此外，本研究亦順利以乙二醇(ethylene glycol)與NH4F之含水電解質在無攪拌環境中以一階段方式製備出二氧化鈦奈米線直接連接奈米管之複合結構(TNWs/TNAs)，實驗中利用改變電壓與時間所得之結構形貌來推論其複合結構形成機制包含以下四個步驟: (1) 在未攪拌系統中產生管壁厚度不均勻的TNAs，其管口部分受到蝕刻使得管壁漸漸變薄，(2) 較薄的管壁開始被蝕刻出小洞，且小洞開始連結，(3) 連結的小洞將TNAs漸漸分開成奈米線，最後(4) 奈米線的尺寸也受到蝕刻而隨著時間越來越細。除此之外，在光催化性質之分析結果發現: 由於較高之比表面積與電子傳輸特性，二氧化鈦奈米線/奈米管之結構相較二氧化鈦奈米管對具有較佳之光催化特性，而且 20 nm TNWs/40 nm TNAs可達到與二氧化鈦奈米粉末相接近光觸媒特性。

In this study, the evolution of morphology and microstructure of anodized TiO2 films by changing the anodizing parameters and post annealing process were investigated and compared. First, TiO2 nanotube arrays fabricated with HF and NH4F electrolytes as a function of annealing temperature up to 400 oC was investigated and compared using x-ray diffraction (XRD), scanning electron microscopy (SEM), and x-ray absorption near-edge structure spectroscopy (XANES). Results showed that TiO2 nanotube arrays grown in HF electrolyte contained 90% amorphous TiO2 and 10% lower oxidation states of titanium from Ti2+ (TiO) and Ti3+ (Ti2O3) cations. After annealing at 400oC, TiO2 nanotube arrays underwent charge transfer and phase transformation to 93% anatase phase, 6% amorphous TiO2, and 1% suboxides. In contrast, as-grown TiO2 nanotube arrays using NH4F electrolyte possessed less amorphous TiO2 (82%) but more suboxides (18%) due to lower oxygen ion formation from scanty 1wt% H2O addition. Moreover, when annealed to 400oC, the crystallinity of TiO2 nanotube arrays increased only to 86% for the anatase phase. The lower anatase phase could be attributed to the formation of (NH4)2TiF6 type compounds presumably formed by the reaction of TiF62- and NH4+ ions dissociated from NH4F. On the other hand, for TNA post annealing technology, the excimer laser annealing (ELA) were investigated as a function of the laser fluence using parallel and tilted modes. Results showed that the crystallinity of the ELA-treated TNAs reached only about 50% relative to that of TNAs treated by furnace anneal at 400oC for 1 hr. The phase transformation starts from the top surface of the TNAs with surface damage resulting from short penetration depth and limited one-dimensional heat transport from the surface to the bottom under extremely short pulse duration (25 ns) of the excimer laser. When a tilted mode was used, the crystallinity of TNAs treated by ELA at 85o was increased to 90% relative to that by the furnace anneal. This can be attributed to the increased area of the laser energy interaction zone and better heat conduction to both ends of the TNAs. Furthermore, TiO2 nanowires connected directly with TiO2 nanotubes arrays (TNWs/TNAs) were successfully fabricated with a mixture of ethylene glycol and water that contained NH4F electrolyte via a one-step method without mechanical stirring. The morphology of the TNWs/TNAs structure was investigated by changing the anodizing voltage and processing time to elucidate its formation mechanism. Well-developed anodic oxide nanowires are only observed under specific anodizing voltage and processing time conditions. The evolution of TNWs follows four stages: (1) thinning of the tube wall thickness with high roughness near the TNA mouths, (2) forming strings of through holes in the upper section of the TNAs, (3) splitting into nanowires, and (4) collapsing and further thinning of nanowires. For photocatalytic application, TNWs/TNAs film demonstrated a better photocatalytic performance than regular TNAs due to higher surface area and improved charge transport. Moreover, TNWs/TNAs film (20 nm wire/40 nm pore diameter) achieved a performance comparable to that of the film made from TiO2 nanoparticles.