- 學位論文
Highly facet-dependent photocatalytic properties of Cu2O-ZnO heterostructures
氧化亞銅與氧化鋅的異質結構在光催化活性上的晶面效應
摘要
我們利用水相合成法簡易合成出具氧化亞銅與氧化鋅異質結構的材料，並進行甲基橙的光降解研究探討。在先前我們實驗室已發展出多種形貌演變的氧化亞銅奈米粒子，並且藉由染料光降解實驗證明具單一晶面的奈米氧化亞銅立方體、八面體及菱形十二面體其光催化活性順序。而我們所提出的氧化亞銅立方體無光催化活性挑戰了過往普遍對奈米材料的認知，因此我們在此利用氧化鋅修飾上氧化亞銅立方體、八面體及菱形十二面體，進行對甲基橙的光催化降解實驗想再次印證我們的觀點。理論上這種半導體與半導體間的界面能夠幫助光催化過程中產生的電子電洞對良好分離，降低它們的再結合率，進而提升光降解效果，也是我們最初決定試驗本篇研究的想法，但是實驗結果並不完全如預期發展。雖然對於氧化亞銅立方體晶面性質的部分有得到進一步的驗證，但是氧化亞銅八面體的表現卻遠出乎於意料，從原本能在75分鐘左右的時間將甲基橙的濃度消耗至近乎零，變成在經過兩小時的光激發下僅僅減少微量的有機染料濃度。透過HRTEM的鑑定，我們統整出在這裡的氧化鋅材料幾乎以{101}晶面與氧化亞銅的{111}晶面相連接，而對於氧化亞銅的{100}及{110}晶面上則無特定的生長面。此結果讓我們認為除了氧化亞銅的晶面效果，氧化鋅一側的表面也會有很大的晶面效應。人們認為能帶符合電子與電洞分離的半導體異質結構能提升光催化效果，結果顯示在不同的晶面相連接之下，可能造成完全不同的結果。
並列摘要
We have synthesized Cu2O-ZnO heterostructured nanocrystals by using a simple method in aqueous solution, and performed a series of photodegradation experiments. Our lab has previously demonstrated strongly facet-dependent photocatalytic properties of Cu2O cubes, octahedra and rhombic dodecahedra. We found Cu2O cubes have no activity to photodegrade methyl orange (MO) dye molecules. This result violates our general understanding of photocatalysis mechanism, so we want to extend our investigation to semiconductor–semiconductor heterojunctions by growing ZnO on Cu2O crystals. Because formation of semiconductor heterostructures with proper band alignment should improve the photogenerated electron–hole pair separation, we expect the photocatalytic activity of Cu2O cubes should remain inactive or become somewhat active with contribution from the ZnO side, while Cu2O octahedra and rhombic dodecahedra decorated with ZnO should show enhanced photocatalytic activities. Surprisingly, while Cu2O cube-ZnO composite structures remain inactive, ZnO-decorated Cu2O octahedra also become inactive. ZnO-decorated Cu2O rhombic dodecahedra show good but a slightly decreased activity possibly due to Cu2O surface coverage. The Cu2O-ZnO heterostructured nanocrystals have been characterized by various techniques, showing maintenance of their composition before and after photocatalysis, although some CuO has been formed. Through extensive analysis of interfacial HRTEM images, we found that the {101} facets of ZnO preferentially grew epitaxially on the {111} faces of Cu2O, but multiple ZnO lattice planes can grow on Cu2O cubes and rhombic dodecahedra. Assuming the (101) planes develop a large band bending preventing photoexcited electrons from migrating to the ZnO side, this unfavorable situation explains the lack of photocatalytic activity of ZnO-Cu2O octahedra. This finding has profound implication, showing that formation of unfavorable heterojunctions can lead to sharp decline in charge carrier transport ability even when band alignment of the semiconductors forming the heterojunctions predicts catalytic enhancement. This is another dramatic demonstration of semiconductor facet effects.
參考文獻
延伸閱讀
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