金屬/半導體核殼奈米晶體之光催化特性研究

Abstract

本研究論文之主要成果乃在於開發出一新穎製程，以獲得具有均勻殼層包覆之Au-CdS核殼奈米晶體，經由合成條件的改變，其殼層的厚度可被精確地調控。Au-CdS核殼晶體乃被證實具有光誘發載子分離之效果，此一現象源自於Au與CdS之相對能帶結構所致，經照光後於CdS殼層內被激發之電子，可藉由Au-CdS異質介面的存在而轉移至Au端，並且留下大量電洞於CdS殼層內。藉由載子傳輸動力學的量測，此發生於Au-CdS核殼奈米晶體內之載子分離現象，可有效被定量化描述與討論，量測結果顯示電子經由CdS傳輸至Au端之速率常數，隨著殼層厚度的增加而上升。由於此Au-CdS核殼奈米晶體在光照後具有良好的載子分離效果，故當其進一步被運用於光催化反應系統中時，也展現出相當優異之催化效能；其中，CdS殼層越厚之Au-CdS奈米晶體其光催化效果越佳，此現象與電子傳輸速率常數變化的趨勢相符。在可見光的照射下，此Au-CdS核殼奈米晶體的光催化效能也優於N-doped P-25 TiO2 和 CdS塊材粉末之商用品，使其相當有潛力成為具高效光催化能力之可見光觸媒。此外，為提昇其耐久性以利長時間之應用，吾人在製備過程中加入Zn，以形成Au-Cd1-xZnxS核殼奈米晶體。在重複使用於光催化的過程中，Au-Cd1-xZnxS核殼奈米晶體展現了更佳的穩定性。由本研究所開發之製程亦可延伸應用於以其它硫化物做為殼層的金-半導體核殼奈米晶體之合成，例如Au-ZnS，且可將其應用於光電化學電池中甲醇氧化反應之催化。

A facile and reproducible synthetic approach for preparation of core-shell Au-CdS nanocrystals with controllable shell thickness was reported. Due to the difference in band structures between Au and CdS, a pronounced photoinduced charge separation took place at the interface of Au and CdS, resulting in the electron-charged Au core and the hole-enriched CdS shell. The electron-charging of Au core in Au-CdS nanocrystals was revealed with the charge carrier dynamics measurement. An increase in the electron-transfer rate constant was observed for Au-CdS nanocrystals with increasing shell thickness, probably due to the less pronounced electron-hole interaction of thicker CdS that enabled a fuller extent of participation of photoexcited electrons in the charge separation process. On the other hand, the hole-enriched CdS shell of Au-CdS nanocrystals upon light illumination was characterized with a photocatalytic process. The photocatalytic activity of Au-CdS nanocrystals was found to increase with increasing shell thickness, attributable to the greater capability of light absorption achieved by the extensive growth of the CdS shell. The correlation of photocatalytic activity with the shell thickness of Au-CdS nanocrystals corresponded well with that of the electron-transfer rate constant. As compared to the relevant commercial products like N-doped P-25 TiO2 and CdS powders, the as-synthesized Au-CdS nanocrystals exhibited superior photocatalytic performance under visible light illumination, demonstrating their potential as an effective visible-light-driven photocatalyst. To further enhance the durability performance, Zn was introduced into the shell of Au-CdS, producing Au-Cd1-xZnxS nanocrystals that showed relatively high stability in photocatalysis. The present synthetic route can be readily extended to obtain other sulfide-semiconductor-coated Au nanocrystals, such as Au-ZnS, which may find potential use for methanol oxidation in the photoelectrochemical cell.