太陽能與海水是地球上十分充沛的資源，利用半導體能將太陽能轉化為化學電勢，以加工海水做為電解液，進行光電化學反應之能源轉換，生成氫氣如此潔淨之綠色能源，氫能在燃燒後僅會產生水；此外，能源轉換能夠降解溫室氣體二氧化碳，生成工業用甲酸，反應過程中的反應物及產物均非汙染物，後續能源應用相較於石化燃料而言，氫能是極低汙染的燃料。這種能源生成是極具潛力的，期待未來能夠成為永續且優異的綠色能源轉換方法。
本篇論文主要探討試圖利用氧化鈷材料保護層加強氮化鎵在光電化學系統中的耐久性，加入氧化鈷這樣的催化劑可以在其與半導體之間的界面形成內建電場增加電荷分離效率。此外，有研究顯示氧化反應的催化劑可以透過從光陽極中截取光生電洞來改善光陽極的穩定性，進而抑制光腐蝕現象。首先我們討論磁控濺鍍不同厚度的四氧化三鈷於氮化鎵基板上的光電化學表現，發現緻密濺鍍氧化鈷薄膜的阻抗高，較不利於載子傳輸，易被內部缺陷複合，導致整體光電流密度表現較差。為了改善緻密濺鍍氧化鈷薄膜不利於載子傳輸的問題，我們利用鹼性電解液使鈷離子相轉變至利於光陽極氧化反應的操作位點，造成的表面破孔也能使光陽極與電解液接觸面積增加，能有效提升反應的飽和光電流密度。
另外利用熱蒸鍍鈷金屬退火的製程手法形成部分覆蓋的氧化鈷薄膜在氮化鎵基板上。此方法一樣可以改善緻密濺鍍氧化鈷薄膜的缺點，鈷離子扮演良好的氧化反應催化劑，相較於n-GaN標準試片，除了過電位的降低，也在飽和電流有提升的趨勢。證實利用鈷離子這樣的觸媒可以有較將光陽極操作在較佳的位點去進行光電化學產氫。

This thesis mainly discusses the attempt to enhance the durability of gallium nitride in photoelectrochemical systems by using a protective layer of cobalt oxide. The addition of a catalyst such as cobalt oxide can form a built-in electric field at the interface between the semiconductor and cobalt oxide to increase the charge separation efficiency. Firstly, we discuss the photoelectrochemical performance of sputtering different thicknesses of cobalt oxide on a gallium nitride substrate. It is found that the densely sputtered cobalt oxide film has high impedance, is not conducive to carrier transport, also easily recombined by internal defects, resulting in overall photocurrent density performance is poor. In order to improve the problem of densely sputtered cobalt oxide film, we use an alkaline electrolyte to convert the cobalt ion phase to an operating site that facilitates photoanodizing, and the surface cracking that also increases the contact area between photoanode and electrolyte, which can effectively increase the saturated photocurrent density of the PEC reaction.
In addition, a partially covered cobalt oxide film is formed on the gallium nitride substrate by thermal evaporation of cobalt metal annealing. This method can improve the shortcomings of the densely sputtered cobalt oxide film. The cobalt ion is a good oxidation reaction catalyst. This method can increase the saturation current. It has been confirmed that a catalyst such as cobalt ion can perform photoelectrochemical hydrogen production by operating the photoanode at a preferred phase.

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