金屬/半導體奈米異質界面與其光誘發載子分離、光催化產氫與電子儲存特性研究

Abstract

由金屬與半導體材料所組成之奈米異質界面(heterostructures)，因其所具有之高效 率載子分離(charge separation)特性，可廣泛應用於光觸媒分解有機分子、光催化產氫、 太陽能光電電池以及光催化燃料電池等，於近年來吸引了諸多研究熱潮。其中，金屬奈 米粒子在此複合結構中乃扮演著電子捕捉者的角色，當奈米半導體材料經照光而激發出 電子電洞對時，由於能帶結構的相對關係，電子會從半導體端轉移至金屬端，此時半導 體端成為電子施體而帶有大量的電洞，而金屬端則扮演電子陷阱角色而帶有大量自由電 子。此正負載子的分離經導出利用後，可應用於光催化產氫與光誘發轉移之電子儲存 上。為了要使電子轉移後半導體電子施體能一直維持正電(富含電洞)狀態，亦即被誘發 轉移之電子能完全侷限於金屬端，以提高電子電洞分離效率，因此金屬與半導體之相對 能階位置的設計與材料的選取是非常重要的。我們欲將半導體材料結構設計為單晶奈米 線，因一維奈米線先天乃具有單一方向通道(徑向)，可利載子於其內傳輸，搭配單晶結 構特性來減少載子被捕捉於晶體缺陷中(如晶粒邊界, grain boundary)之機率；經表面接 枝上金屬奈米顆粒後，以便來提高此金屬/半導體異質界面之光誘發載子分離率，進而 增進其應用於光催化產氫之效能。此外，藉由金屬-半導體核殼奈米粒子的製備，我們 嘗試將光誘發載子分離所轉移的自由電子，儲存於金屬奈米粒子內，進而達到光誘發電 子儲存的目的。

Efforts to employ metal-semiconductor nanoheterostructures have been extensively explored in recent years to facilitate and improve charge separation efficiency in the semiconductor nanostructures. Of particular interest is the use of such heterostructures for photocatalytic decomposition of organics, photocatalytic hydrogen production, photochemical solar cells and photocatalytic fuel cells. Metal nanoparticles, which act as a reservoir for photogenerated electrons, promote an interfacial charge-transfer process as contacted to semiconductors. Free electrons generated from semiconductors will transfer to the domain of metals due to the discrepancy between their band structures, leaving positively holes in semiconductors. To maintain the positively charging condition in semiconductors after the charge separation, in which the induced transferred electrons can be totally confined in the metal nanoparticles, it is quite important to design these materials with appropriate band structures discrepancy. The separated electrons and holes, once depleted and utilized, bring the promising application in photocatalytic hydrogen production and photoinduced transferred electron storage. This project aims to prepare composite nanomaterials with metal/semiconductor heterostructures and employ them as the active catalysts in the photocatalytic hydrogen production and photogenerated electron storage. We intend to obtain metal nanoparticles-modified single-crystalline semiconductor nanowires and expect a remarkable efficiency in water splitting to produce hydrogen. The unidirectional conduction path for carriers in nanowires and their single crystalline nature ensure the rapid collection of carriers generated throughout the system, leading to the enhanced charge separation efficiency and their better performance in hydrogen production. Furthermore, the metal-semiconductor core-shell nanoparticles, in which the photogenerated free electrons can be transferred and stored within the metal, will be prepared to investigate the possibility of collecting these photoinduced transferred electrons.