在含氟甘油電解液中陽極氧化鈦箔製備二氧化鈦奈米管

Abstract

奈米管狀結構相較於粉末結構有著更大的表面積，加上管狀排列能減少質傳阻礙，增進光電轉換的效率。以陽極處理法來製作奈米鈦管，具有低成本、高效率、製程相對簡單、結構排列有序等優點。陽極處理的電解液選擇多元，可大致分為水溶液以及非水溶液系統。文獻指出，在相同的操作條件下，以非水溶液系統進行陽極處理製備奈米鈦管時，能成長出較佳的外觀形貌，在後續應用上亦具有較良好的效率。 本研究採用陽極處理法在純鈦箔片上成長奈米鈦管，電解液則採用非水溶液系統，以甘油（glycerol）為主體，加入0.5wt%NH4F 配置成電解液，改變實驗參數：陽極處理電壓(20、40、60、80V)、電解液溫度(5、15、25、40、50℃)、陽極處理時間(3、6、9、12、24h)、電解液含水量(0、5、10、90%)，藉此釐清在甘油系統中，實驗參數對氧化膜形貌的影響，並對氧化膜生長機制做討論。結果顯示，鈦管管徑寬與操作電壓成正相關，陽極氧化時間、電解液的溫度及含水量若提高，將增加自由氟離子攻擊氧化膜表面的機會，造成管徑寬的分布有較大的範圍，呈現較不均勻的形貌。氧化膜厚度在陽極氧化時間12小時內，與陽極氧化時間成正相關，在50 ℃以內，膜厚也與溫度有同步趨勢。實驗過程中，甘油扮演調整溶液黏度的角色，不參與反應；鈦在陽極處理中需經過三階段的氧化，即鈦的氧化數變化順序為＋2、＋3、＋4，亦即在氧足量存在時，鈦以高氧化價態存在；當氧的存在量不足時，鈦則以高低氧化價態混和存在。 針對氧化膜表面完整度進行改善，在原本電解液配置中，再加入NH4Cl作為輔助電解質，進行陽極處理後形成一緻密的含氯氧化層，其中並無氟的存在，氧化膜伴隨些微蝕孔以及穿孔存在，無法形成預期的管狀結構，顯示此方法並不能達到期望效果。

Compared to titanium dioxide (TiO2) nanoparticles, self-organized TiO2 nanotubes display larger specific areas and mass transportation rates, which, in turn, enhance the photoelectric conversion efficiency. Over the past few years, self-organized TiO2 nanotube layers prepared by electrochemical methods have received ever-increasing interest. Among the electrochemical methods, the anodization method has several advantages such as low cost, high efficiency, and comparatively simple in operation. Self-organized TiO2 nanotubes can be made on pure Ti foils anodized in aqueous or non-aqueous electrolytes. TiO2 nanotubes formed in non-aqueous electrolytes show better morphology and performance than those formed in aqueous electrolytes. In the present work, TiO2 nanotubes have been prepared on pure Ti foils anodized in non-aqueous glycerol solution containing 0.5wt% NH4F. Several anodizing parameters were studied to gain better understanding about the formation and growth mechanism of TiO2 nanotubes, including anodization voltage (20, 40, 60, 80V), electrolyte temperature (5, 15, 25, 40, 50℃), anodization time (3, 6, 9, 12, 24h), and electrolyte water concentration (0, 5, 10, 90 vol%). The results show that the average diameter of the tubes increased with increasing anodization voltage. After prolonged anodization in the solution containing more water at higher temperatures, the tubes exhibited a broader distribution in tube diameter due to severer attack by free fluorine ions. Longer anodization times up to 12 h and higher electrolyte temperatures resulted in thicker anodic film composed of TiO2 nanotubes. Glycerol in the electrolyte influenced the solution viscosity, but did not involve in the formation of TiO2 nanotubes as Ti was sequentially oxidized to +2, +3, and finally +4 valence. That is, Ti was oxidized to 4 valence in the presence of abundant oxygen source, while was in a mixture of various valence states in the deficit of oxygen. To improve the surface integrity of the nanotubes, NH4Cl was added to the electrolyte. However, the anodic film formed in the electrolyte containing Cl- and F- was a rather compact oxide layer in which Cl species was detected, but F species was absent. Moreover, nanotubes were not observed in this compact oxide film. Consequently, the presence of Cl- in the electrolyte suppressed the formation of TiO2 nanotubes on Ti anodized in glycerol solution containing F-.