二氧化鈦奈米管氫能源製備系統之設計

Abstract

二氧化鈦(TiO2)具有良好的化學穩定性及光催化活性，因此常使用於光觸媒領域，但由於本身為高能隙的半導體材料並且只能吸收紫外光波段的能量，使得應用性受到侷限。本研究利用陽極氧化法(anodization)製備具高比表面積的TiO2奈米管結構，探討比表面積的不同對TiO2奈米管的光反應性與光電解水的表現，並且採用還原氣氛熱處理(reduced atmosphere heat treatment)增加TiO2奈米管中的缺陷，以期可提升TiO2光分解水製氫的能力及降低其能隙進而可吸收可見光。 鈦的陽極氧化處理使用三種不同電解液：0.5wt%氫氟酸(HF)、1M磷酸(H3PO4)+0.3wt%氫氟酸及1M硫酸鈉(Na2SO4)+0.5wt%氟化鈉(NaF)。實驗結果顯示，於室溫下，操作電壓皆為20伏特，分別進行不同時間的陽極處理，則使用中性緩衝溶液1MNa2SO4+0.5wt%NaF所製得的TiO2奈米管具有最高的比表面積，且奈米管長度最長可達3微米(μm)。於三種電解液中陽極處理完畢的非晶質TiO2奈米管在空氣(Air)下進行400℃熱處理並持溫2小時，由XRD可知當TiO2奈米管的長度愈長，銳鈦礦(anatase)的特徵峰強度愈強；由UV/Vis光譜分析與光電化學分析的結果可知TiO2奈米管的長度愈長，則對紫外光的反應更明顯；在氫氣收集的結果顯示，TiO2奈米管的長度愈長，則TiO2奈米管的光電解水製氫的表現愈佳，因此TiO2奈米管的長度愈長，即比表面積愈高，則對光的反應性愈顯著。 在氣氛熱處理方面選用兩種成分的還原氣氛：99%N2+1%H2(O2：12ppm)與97%N2+3%H2(O2：0.6ppm)，由實驗結果發現，在99%N2+1%H2下進行400℃至800℃的退火，TiO2的能隙值下降，最低降至3.01eV，但其能隙還不足以在可見光下進行反應，推測造成此能隙的下降是因為發生相轉變，由銳鈦礦相(anatase，3.2eV)變成金紅石相(rutile，3.0eV)；在97%N2+3%H2下進行400℃至700℃退火，TiO2奈米管的能隙值增加，能隙值的增加可能是因為97%N2+3%H2造成TiO2變為TiO2-x而導致結晶度下降。在800℃時，rutile相的生成則主導能隙值下降，因此會使臨界吸收波長往長波長處移動。 實驗結果發現，同樣在400℃退火且持溫2hr，在空氣中熱處理比起還原氣氛99%N2 + 1%H2及97%N2+3%H2對TiO2奈米管在光電流密度與光電解水製氫的表現較佳，即在同一溫度400℃時，氣氛的含氧量降低會使光分解水製氫的能力變差。

Titanium oxide (TiO2) exhibits outstanding resistance to corrosion and photocorrosion in aqueous environments. Due to its high photosensitivity, TiO2 is often used in the applications of photocatalysis. The wide band gap of TiO2 causes it to absorb only UV radiation, and consequently, limits its photocatalytic applications. In this study, the photosensitivities of anodized TiO2 nanotubes of different aspect ratios and prepared under different annealing conditions are investigated. The performances of photoelectrolysis of various nanotube TiO2 systems are also examined. The experimental results indicate that a high aspect ratio for the TiO2 nanotubes can be achieved by adopting the buffer electrolyte 1MNa2SO4+0.5wt%NaF during the anodization procedure. Characterizations using X-Ray Diffractometer Spectrometer (XRD), UV-Vis-NIR Spectrometer (UV/Vis), electrochemical analysis and hydrogen production by water-splitting support the high photosensitivity of high-aspect-ratio TiO2 nanotubes. In this study, three different atmospheres: Air, 99%N2+1%H2 (O2:12ppm) and 97%N2+3%H2 (O2:0.6ppm) are chosen for the annealing procedure. The results indicate that the band gap of the TiO2 nanotubes decreases to a minimum value about 3.01eV by annealing in between 400°C to 800°C in 99%N2+1%H2(O2：12ppm) for 2 h. This is due to the formation of rutile phase at high temperatures. However, this band gap value is still not low enough to absorb visible light radiation. In contrast, when annealing in between 400°C to 800°C in 97%N2+3%H2(O2：0.6ppm) for 2 h, the band gap first increases modestly and than decreases sharply at 800°C. The strong reduction power of annealing atmosphere proceduces crystallgraphic defects (e.g., TiO2→TiO2-x) which increase the band gap value of the TiO2 nanotubes. However at 800°C, the formation of rutile phase becomes the dominate factor, and consequently, lowers the band gap. It is found that the stronger the reduction power of the annealing atmosphere, the poorer the water splitting ability of the TiO2 nanotubes.