二氧化鈦奈米管之管徑與銀擔載量對於其在0.5M硫酸中氧還原催化活性之影響

Effect of Ag Loading and Diameter of Selforganized TiO2 Nanotubes on their Catalytic Activity for Oxygen Reduction in 0.5 M H2SO4

本研究在室溫下含氟電解液中，採用兩極式電化學氧化法來製備二氧化鈦奈米管，藉由工作偏壓、電解液氟離子濃度、pH、添加劑和電解液溫度等實驗參數的改變，來探討其對奈米氧化鈦管徑之影響。結果顯示：二氧化鈦奈米管之管徑，會隨工作偏壓（由5 V增高至20 V）而由30 nm增至100 nm、電解液中氟離子濃度（由0.1 M增濃至0.7 M）及電解液溫度提高（20℃to 80℃），而由70 nm增至134 nm；反之，管徑會隨電解液之pH值由1增加到7，而由100 nm降至45 nm，浴中甘油添加濃度由0 M增加至5 M而由100 nm降至87 nm。電化學氧化法得到非晶質結構之二氧化鈦奈米管，經在480℃爐中加溫3小時後轉變成銳鈦礦晶體。
經加溫處理之二氧化鈦奈米管，在擔載不同含量之銀與鈀後，浸泡於含飽和氧氣之0.5M硫酸溶液中，測試其對氧還原的催化活性。結果顯示：二氧化鈦奈米管之催化活性，隨奈米管之管徑減小而增大，隨銀擔載量增加而增強，其中尤以65 nm管徑之二氧化鈦奈米管經照射1小時紫外光（λ=254 nm）擔載銀觸媒的催化反應電流密度最高（在242μA/cm2）。

Self-organized titanium dioxide nanotubes were fabricated by electrochemical anodization in fluoride solution at room temperature. The influence of experimental parameters such as applied voltage, fluoride concentration, pH, additives, and bath temperature on the diameter of the fabricated titanium dioxide nanotubes was explored. The results revealed that the diameter of the nanotubes increased from 30 nm to 100 nm with increasing applied voltage from 5 V to 20 V and increased from 70 nm to 134 nm with increasing fluoride concentration from 0.1 M to 0.7 M at bath temperatures ranging from 20 ℃ to 80 ℃. However, the diameter decreased from 100 nm to 45 nm with increasing pH from 1 to 7 and decreased from 100 nm to 87 nm with increasing glycerol concentration from 0 M to 5 M in the bath. All nanotubes fabricated by electrochemical anodization were in amorphous form, and they turned into anatase crystal after being annealed at 480 ℃ for 3 h.
The annealed self-organized anatase nanotubes were treated with silver and palladium to increase their catalytic activity for electrochemical reduction of oxygen in a 0.5 M sulfuric acid solution saturated with oxygen at room temperature. It was found that the catalytic activity increased while the diameter of the nanotubes decreased. However, it increased with an increase in silver loading. The anatase nanotubes with a diameter of 65 nm treated with silver by immersion in 0.2 M silver nitrate and by illumination with ultra-violet light (λ= 254 nm) for 1 h exhibited the highest catalytic activity (with highest current density 242 μA/cm2).