Green Synthesis and Catalytic Application of Gold Nanomaterials

本論文利用四氯金酸鈉水溶液和茶萃取物在常溫及常壓的條件下，製備多分支狀金奈米材料(Branched Gold Nanomaterials)。我們更進一步發現，調控四氯金化鈉水溶液和茶萃取物的濃度比例可以操控金奈米材料的大小及形狀，如將茶萃取物的濃度降低可獲得球型的金奈米材料。茶萃取物亦可將其他貴金屬離子如銀、鉑、鈀下還原形成不同形狀的奈米粒子。藉由如穿透式電子顯微鏡、吸收光譜儀、X-ray能量散佈分析儀、X-ray粉末繞射儀來鑑定金奈米材料的特性，發現多分支狀金奈米材料(大小為50 nm)是由金奈米棒（寬8 nm，長12 nm）所自組裝而成。經由實驗証實，茶萃取物中之多酚類扮演金離子還原形成多分支狀奈米結構及穩定奈米粒子的重要角色。由對硫醇苯甲酸(4-mercaptobenzoic acid)拉曼的實驗，我們發現此多分支狀金奈米材料具備表面增強拉曼散射的效果。我們更利用銀奈米殼層(Ag nanoshells)的沉積來使其訊號增加約為十倍，此金銀奈米複合材料可提供穩定的拉曼訊號及乾淨的表面以利修飾的潛力。在光催化部分，首先藉由多酚類與鈦金屬間之作用力(ligand to metal charge transfer)，將此多分支狀金奈米材料修飾於二氧化鈦奈米材料表面。此複合材料可進一步增加催化甲基藍(methylene blue)之分解速率，在TiO2(P25)相較TiO2(P210)光催化效率分別提升2.05倍及2.50倍。本研究證實多分支狀金奈米材料及其和TiO2的複合物分別在表面增強拉曼散射基材及光催化具有發展潛力。In this work, three-dimensional branched Au nanomaterials were produced at high yield by reacting an aqueous solution of sodium tetrachloroaurate with a tea extract at ambient temperature and pressure. By varying the tea concentrations at a constant amount of sodium tetrachloroaurate, different size and shape of Au nanomaterials were separately prepared. We also synthesized other noble metal nanoparticles such as Ag, Pt and Pd by tea extract. UV-vis absorption, energy dispersive X-ray, X-ray diffraction, and transmission electron microscopy measurements were conducted to characterize the as-prepared Au nanomaterials. The results revealed that the branched Au nanomaterials (50 nm) were formed through the self-assembly of short nanorods (8 nm in width and 12 nm in length). According to the experiments, we find out that the polyphenols in tea extract play an important role of reducing Au3+ ion to Au and stabling the branched Au nanomaterials. By conducting Raman measurements, we found that the branched Au nanomaterials was useful on enhanced signal further than 10 times through surface-enhanced Raman scattering (SERS) effect and Ag nanoshells when adopting 4-mercaptobenzoic acid as report. We believe the Au/Ag nanomaterials have the potential to be good SERS substrates because of the stable signal and clean surface. Through the interaction of polyphenol and Titanium metal (ligand to metal charge transfer), the branched Au nanomaterials were easily deposited on the surface of TiO2 nanomaterials. On the photocatalysis, we have found the photodegradation of the methylene blue is further enhanced in TiO2(P25) about 2.50 folds and TiO2(P210) about 2.05 folds through the deposition of the branched Au nanomaterials. Consequently, we believe the branched Au materials have the potential to be good SERS substrates and photocatalysis enhancers because of the stable signal and clean surface