異質接面和氫摻雜對氧化鋅-氧化鈷奈米複合材料光電性質之影響

Abstract

氧化鋅材料因為擁有許多有價值的物理和化學特性，被應用於不同的領域之中。在光電化學水分解系統中，氧化鋅作為光電材料也十分常見。強化光電材料的光催化能力非常重要，強化的方式包括提升可見光波段的吸收，或是改善能帶結構來增加載子分離的效果。本研究嘗試以氧化鋅奈米柱陣列結構作為基底，發展氧化鋅及氧化鈷材料的複合異質結構，來提高光電化學水分解反應之效果。另一方面，嘗試引入陰極充氫處理，改善氧化鋅的電性，如增加載子濃度和降低電阻率，來提高光催化能力。 本研究以ITO導電玻璃作為基板，經過旋轉塗佈法及原子層沉積技術鍍覆晶種層後，透過水熱法沉積氧化鋅奈米柱陣列，並且使用簡易的電鍍法在氧化鋅奈米柱陣列上沉積氧化鈷奈米顆粒，形成擁有Type-II半導體能帶結構的異質結構，並應用於光電化學水分解系統中。因為此兩種材料的能帶結構有相對差異，光電極照光後被激發的電子會由氧化鈷傳遞至氧化鋅，光激發的電洞則以反方向由氧化鋅傳遞至氧化鈷，從而降低電子電洞對的複合，並提高載子分離的效果。氧化鋅的能隙大小位在紫外光波段，所以使用擁有較小能隙的氧化鈷進行改質，可以促進可見光波段的吸收，能有效率地利用太陽光所涵蓋的光譜波段，促使氧化鋅及氧化鈷的異質結構產生較佳的光電化學水分解反應。 本文的另一項研究主題為，在氧化鋅不同奈米結構下進行陰極充氫處理，摻雜氫氣的行為不同，造成光催化能力變化之比較。以氧化鋅晶種層進行陰極充氫處理，可以促使氫氣吸附於材料表面，進一步擴散至材料內部，轉換成可以傳遞電荷的載子。此載子的擴散能力比氧空缺更好，藉此提升載子濃度並降低電阻率，有助於材料獲得較佳的的光催化能力。在陰極充氫處理之後，製備氧化鋅及氧化鈷的異質結構可以大幅提升光催化能力。氧化鋅晶柱表面以非極性晶面為主，晶體成長的環境中晶面上的氧原子易與氫原子鍵結。若以氧化鋅奈米柱陣列進行陰極充氫處理，過程中的還原環境促使晶柱表面的氫原子還原，產生的氫氣由表面脫附，因而無法達到摻雜氫氣的效果，造成光催化能力下降。

Because of its valuable physical and chemical properties, the semiconductor material, Zinc Oxide (ZnO) is widely used in various fields including the photoelectrochemical system. There are some ideas to improve the catalytic activity of ZnO, such as increasing the absorption of visible light and enhancing the separation of carriers. In this study, a heterogeneous composite nanostructure was formed by ZnO nanowire arrays as a substrate and cobalt oxide (CoO) deposited on it. This process could enhance the effect of the photoelectrochemical water splitting reaction in the visible light region. Besides, the introduction of a cathodic hydrogen charging treatment might increase the carrier concentration and decrease the resistivity of ZnO to get the better catalytic ability. In this study, after plating the seed layer by spin coating and atomic layer deposition techniques on the conductive glass (Indium Tin Oxide, ITO), ZnO nanowire arrays were grown by the hydrothermal method, and the photoanode was made from the electrodeposition method that we coated CoO nanoparticles on the ZnO nanowire arrays to get a heterostructure with type-II band structure. Since the difference of the energy band levels between these two materials, the photo-excited electrons would be transferred from the conduction band of CoO to that of ZnO, and the holes would be transferred from the valence band of CoO to that of ZnO. This could reduce the recombination of electron and hole pairs and enhance the separation of carriers. The bandgap of ZnO corresponds to the energy of UV light, and CoO has narrower bandgap. Thus, the heterosturcture enhances the absorption of visible light, effectively utilize the whole range of sun light and improve the reaction of photoelectrochemical water splitting. Another subject of this paper is the comparison of the influence of cathodic hydrogen charging (CHC treatment) applied to different nanostructures of ZnO. CHC treatment can promote hydrogen adsorbing on the surface of ZnO seed layer, further diffusing into the material and converting into a carrier whose diffusion ability is better than oxygen vacancies, then increase the carrier concentration and lower the resistivity of ZnO. This helps ZnO to obtain better photocatalytic ability. After CHC treatment, the heterostructure of ZnO and CoO enhances the photocatalytic activity greatly. However, CHC treatment applied to ZnO nanowire arrays causes hydrogen desorption from the non-polar surface where the bonding of hydrogen with oxygen has generated during the process of crystal growth. There is no hydrogen doping in ZnO nanowire arrays, so its photocatalytic activity decreases.