以電化學法製備金奈米結構及其應用之研究

Fabrication and Characterization of Nanostructured Au by Electrochemical deposition and Thier Applications

林苔瑄(Tai-Hsuan Lin)

指導教授 ： 洪偉修

國立臺灣師範大學/理學院/化學系/博士(2009年)

摘要

摘要

奈米材料於近幾十年備受矚目並引起研究熱潮，成為當紅的尖端科技，最主要的原因在於隨著尺寸的縮小及構形的改變，奈米級結構會呈現出有別於塊材之獨特的物理與化學性質。就我們所知，金為高化學惰性的貴重金屬，但當金的尺吋微小至數奈米時，其物理和化學性質會隨粒徑奈米化而改變，隨著粒徑縮小，比表面積大幅增加、粒子邊緣及裸露角增加、與載體之接觸表面積增加，金的表面形成凹凸不平的原子台階，而呈現獨特催化活性，如果再搭配可還原性金屬氧化物載體即可成為高活性的奈米金觸媒。除此之外，金奈米結構所具有之優異獨特的光學、化學以及電子學性質，使之可更廣泛地應用在電子、化學以及生醫等領域（如：奈米電子元件、化學感測器、燃料電池、觸媒催化、生物感測器…等）。在此論文中，我們利用電化學脈衝電鍍法製備所需之金奈米粒子以及具有樹枝狀分支的金奈米結構。
金奈米粒子以脈衝電鍍沉積於表面改質或未改質的玻璃碳電極基板上；苯磺酸分子以電氧化方式進行嫁接達到改質修飾表面的目的，藉由X光光電子能譜儀、循環伏安法以及交流阻抗來分析探討嫁接苯磺酸分子後，玻璃碳電極表面性質的變化。接著，經由調變電鍍液的濃度和電鍍時間，可控制奈米粒子的尺寸大小以及於基版上的分散性，完整製備後的電極於富含氧氣的磷酸緩衝溶液裡進行電催化活性的量測。金奈米粒子生成於苯磺酸分子修飾後之電極，其無論粒子的分散性或比表面積都有顯著的增加，相較於未經修飾的玻璃碳電極與多晶的金電極更表現出較佳的電催化活性。
以相同的電化學沉積法，在玻璃碳電極上製備出具有良好結晶性並呈現三軸對稱的支狀金結構，此乃由於電鍍溶液裡添加了含有硫醇官能基之氨基酸的緣故。由於金的不同晶面 (facet) 對於硫原子具有不同的鍵結強度，因此利用氨基酸在各個晶面形成硫金鍵 (S－Au bond)，佐以提供適當的電鍍電位來進行金奈米結構的控制生長實驗。金樹狀結構主體呈現三軸對稱，由皆沿著 (111) 晶面方向成長的主幹、分支以及奈米葉片共三個部分組成，我們利用x光光電子能譜儀、掃瞄及穿透式電子顯微鏡、x光繞射儀來確定鑑定其結構成分。再者，對於甲醇氧化以及氧氣還原的電催化活性，金樹狀結構的表現優於多晶的金電極。

關鍵字

電化學電鍍 ； 金奈米粒子 ； 表面改質 ； 金奈米樹 ； 電位控制

並列摘要

Abstract

Nanostructured Au has been the focus of intense researches due to their fascinating chemical, optical, electronic properties and potential applications in chemical sensing, biosensor, electronic devices and catalysis. In this thesis, the Au nanostructures are fabricated on the glassy carbon electrode by electrochemical deposition under various deposition conditions. Subsequently, these as-resulted electrodes are characterized with surface analysis and electrochemical measurements.
A glassy carbon (GC) electrode was covalently grafted with a layer of sulfanilic acid (SAA) via the formation of amine cation radicals under electro-oxidation. The surface compositions of the resulting electrode were characterized with X-ray photoelectron spectra. The electron transfer of Fe(CN)63/4 to the SAA-modified GC (SAA/GC) electrode in solutions at varied pH was studied with cyclic voltammetry and analysis of electrochemical impedance. The electrodeposition of gold nanoparticles (AuNP) on the SAA/GC electrode exhibited a dependence on the solution pH, which is attributed to a variation of the terminal charge state of the grafted SAA. The modification of the GC surface with grafted SAA resulted in an enhanced electrodeposition of AuNP. The catalytic activities of AuNP/GC and AuNP/SAA/GC electrodes for reduction of oxygen are compared in phosphate buffer solution with cyclic voltammetry. Two distinct reduction features are observed and attributed to a two-step, four-electron reduction of O2 to H2O through intermediate H2O2. The reduction signals of O2 on the AuNP/SAA/GC electrode exhibit a shift toward a positive potential, relative to those observed on the AuNP/GC electrode. Hence, AuNP on the SAA/GC electrode exhibits an increased catalytic activity for reduction of oxygen.
The synthesis of the three-fold symmetrical and crystalline Au dendrites formed on the GC electrode is performed in a solution of HAuCl4 and H2SO4 with the presence of cysteine. The electrodeposited Au dendrites are characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), and cyclovoltammetry (CV). The deposited Au dendrite shows a three-fold symmetric hierarchical architecture and is composed of trunks, branches and nanorod leaves, which grow all along the <111> direction. The formation of Au dendrite results from the preferential desorption of the cysteine from the Au(111) facet. The morphology and shape of the Au dendrite vary with the deposition potential and the concentration of cysteine. Compared with the polycrystalline Au electrode, the electrodeposited Au dendrites exhibits significantly better electrocatalytic activity for the oxidation of methanol and reduction of oxygen.

並列關鍵字

electrochemical deposition ； gold nanoparticle ； surface modification ； sulfanilic acid ； Au dendrite ； cysteine ； potential control

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