半導體奈米異質結構之製備與光催化及類過氧化酶催化之應用

Abstract

本論文著重在研究兩種不同類型之半導體異質結構，分別為半導體-石墨烯的複合結構以及半導體-金屬蛋黃-蛋殼奈米晶體。由於材料能帶結構之差異，在光照射下半導體上被激發的電子會自發性的傳遞至具有較低功函數的石墨烯，使載子可以有效分離而提升其光電轉換效率。對半導體-金屬蛋黃-蛋殼奈米晶體而言，殼層提供一保護層可使核心的金屬保有長期穩定的催化活性，故可利用在各式化學催化反應。 在第一部份，我們發展出一個簡單且對環境無害的反溶劑製程來製備石墨烯(reduced graphene oxide, RGO)與ZnO的奈米複合材料，本研究提出的合成方法為先將ZnO粉末溶於深共熔離子液體(deep eutectic solvent, DES)，將其注入對ZnO不具溶解能力的大量反溶劑中後，ZnO因溶解度下降會析出與成長成奈米顆粒，若在此反溶劑程序中事先將具單層結構之RGO添加於反溶劑內，則ZnO奈米顆粒會成長於RGO表面，進而形成RGO-ZnO之奈米複合結構。由於RGO具有極高的表面積以及優異的導電性，在與半導體奈米材料相接合時，可有效的將半導體材料上照光被激發的電子導走使其與電洞分離，以降低整體材料電子電洞之複合機率，進而提升其光電轉換效率。本研究欲藉由合成不同比例之ZnO與RGO之複合材料，研究相對比例成份對於RGO/ZnO之光電轉換效率的影響，以期能將石墨烯/半導體奈米複合材料應用於相關光電轉換用途，例如太陽能電池與光催化分解水產氫等方面。 第二部份是合成蛋黃－蛋殼之Au@Cu7S4結構，發現其具有極高的類過氧化酶之催化作用，藉著先合成Au@Cu2O核殼結構作為模板，接著透過硫化反應以及Kirkendall 效應來形成Au@Cu7S4蛋黃－蛋殼之奈米結構，接著將此些樣品置於過氧化氫之環境中，其類過氧化酶之催化性質可將過氧化氫分解產生氫氧自由基，此氫氧自由基可進一步與受質分子結合以完成酵素反應。此主題意在觀察比較兩種異質結構之類過氧化酶催化特性，此二種結構之殼層皆具有保護Au 以及防止聚集之作用，同時亦能與過氧化氫反應以產生氫氧自由基，故兩者皆具有良好之類過氧化酶之催化效率。蛋黃－蛋殼相較於核殼結構具有更優越之催化活性，由於殼內之中空結構可有效侷限反應物分子於內做均勻相反應，此外，蛋黃－蛋殼結構內可移動的Au核，乃有助分子在其表面的吸脫附，故蛋黃－蛋殼奈米結構展現較優異之催化活性。此研究進一步探討蛋黃－蛋殼結構之中空尺寸以及殼層厚度對於其類過氧化酶催化性質的影響，。結果顯示隨著中空尺寸的增大，蛋黃－蛋殼結構之催化活性相應增加，而殼層厚度較厚的樣品亦展現較佳的催化活性。

In this study, we investigated two kinds of semiconductor heterostructures including semiconductor-graphene composites and metal-semiconductor yolk-shell nanocrystals. Due to the difference in band structure, the photoexcited electrons of semiconductor would preferentially transfer to graphene, which promotes charge carrier separation to enhance the photoconversion efficiency. For yolk-shell nanocrystals, the surrounding shell serves as a protection layer for the inner core, which assists in maintaining the long-term catalytic activity for metal. The yolk-shell nanocrystals therefore are appealing for relevant catalytic applications. In the first part, we have developed a facile, environmentally benign antisolvent approach to prepare reduced graphene oxide (RGO)-ZnO nanocomposites. The antisolvent process involved the dissolution of ZnO powders in a deep eutectic solvent (DES), followed by the precipitation and growth of ZnO from DES upon injection into a bad solvent. When ZnO-containing DES solution was injected into a bad solvent (e.g. water) showing no solvation ability toward ZnO, ZnO would be precipitated from the solution owing to the dramatic decrease in its solubility. With the addition of RGO sheets in the bad solvent, growth of ZnO in antisolvent process was accompanied by RGO decoration, resulting in the formation of RGO-ZnO nanocomposites. Because of the high electrical conductivity of RGO, the photoexcited electrons of ZnO would preferentially transfer to RGO, leaving positively charged holes in ZnO to achieve charge carrier separation. The significant charge separation may further enhance the performance of RGO-ZnO nanocomposites when applied in photoconversion processes such as dye degradation and water splitting. For the second part, we have synthesized Au@Cu7S4 yolk-shell nanocrystals using Au@Cu2O core-shell nanocrystals as the growth template. The formation of yolk-shell structures involved the sulfurization treatment and Kirkendall effect. Both Au@Cu7S4 and Au@Cu2O were found to show peroxidase-like catalytic properties, which may catalyze H2O2 to generate OH radicals for further carrying out enzyme reactions. This study aimed to compare the catalytic efficiency between Au@Cu7S4 yolk-shell and Au@Cu2O core-shell nanocrystals. The two samples both had outer shells to protect the inner Au cores from aggregation, therefore showing noticeable peroxidase-like catalytic activity. As compared to core-shell sample, Au@Cu7S4 yolk-shell nanocrystals displayed higher catalytic activity, ascribable to the existence of void space that confined the reactants to make the reaction environment more homogeneous.. In addition, the movable metal core of Au@Cu7S4 also benefited the catalysis since it possessed larger exposed surface for promoting the molecular adsorption and desorption. This work further studied the influence of void size and shell thickness on the catalytic efficiency of Au@Cu7S4 nanocrystals. The results showed that large void space and thicker shell thickness both led to higher catalytic activity.