以硫化鎘奈米線為主體之異質結構:載子動力學與其光能轉換應用

Abstract

利用各種半導體能帶結構的不同，透過適當的選擇與搭配所形成的type-II奈米異質結構，可獲得或改善原單一半導體所無法得到的光轉換效率與光電特性。本論文之重點在於研究由CdSnO3及Cu2O奈米晶體所分別修飾的CdS奈米線異質結構之特性，對CdS-CdSnO3而言，其能帶結構的組合可使光激發電子傳遞至CdSnO3奈米晶體，電洞留在CdS奈米線上，進而得到載子分離的效果，藉由時間解析螢光光譜的量測可量化其界面載子的傳輸效率，結果顯示CdSnO3接枝含量達到2.5 at% 時，有最顯著的載子分離效率，當CdSnO3 的含量超越此最佳值時，橫跨界面的載子再複合程序會使得整體載子分離效果下降，進而導致載子傳輸速率減低，此結果與其應用於光催化分解汙染物時的分解速率趨勢一致，因此可知橫跨界面的載子再複合行為為造成CdSnO3含量超過最佳值時，其光催化速率下降的主要因素。另一方面，利用Cu2O奈米晶體的修飾，使得CdS奈米線的光激發電洞可導出至Cu2O，避免CdS累積過多的電洞，進而改善其光腐蝕現象，實驗結果顯示，透過Cu2O的修飾不僅可提升CdS的光電轉換效率，亦可增強其穩定性並能長時間的重複使用之。

Semiconductor nanoheterostructures with type-II band offset have exhibited unique optoelectronic properties that are beneficial to photoconversion applications. Debatable arguments however exist in the literature for interpreting the most enhanced photocatalytic performance of type-II semiconductor nanoheterostructures when an optimal content of the constituents is employed. In this work, time-resolved photoluminescence is used to investigate the interfacial charge carrier dynamics for CdSnO3-decorated CdS nanowires (NWs), a prototype type-II nanoheterostructures system, with varying CdSnO3 contents. Our results show that the CdSnO3 content of 2.5 at% rendered CdS-CdSnO3 NWs the most significant charge carrier separation, above which electron-hole recombination across CdS/CdSnO3 interface mediated carrier transfer to compromise the overall charge separation efficiency. The carrier dynamics results are in good accordance with that of photoconversion performance evaluation in dye photodegradation, which assists in resolving the very critical but still controversial issue as to the factors causing the depressed photocatalytic efficiency of type-II nanoheterostructures when the constituent content exceeds the optimal value. On the other hand, in order to improve the long-term stability and recyclability of CdS NWs, Cu2O nanoparticles were used to decorate CdS . Because of the higher valance band potential of Cu2O nanoparticles than that of CdS, the holes would transfer to Cu2O upon light irradiation, alleviating the possible photocorrosion issue and increasing the long-term stability of CdS NWs during their use as photocatalysts.