一、 以植晶法水相合成立方體銀奈米晶體及其形貌變化 二、 製備系統性形狀演繹的金銀核殼結構及其光學性質探討

I.Seed-Mediated Growth of Silver Nanocubes and Their Morphological Transformation II.Synthesis of Au–Ag Core–Shell Heterostructures with Systematic Shape Evolution and Their Optical Properties

曹育琪

指導教授 ： 黃暄益

國立清華大學/理學院/化學系/碩士(2014年)

https://doi.org/10.6843/NTHU.2014.00263

摘要

一、 以植晶法水相合成立方體銀奈米晶體及其形貌變化

銀奈米粒子形狀合成通常是在高溫有機相中反應，若在水相中合成則可減少能源的消耗及達到以綠色化學為目的的製備方法，由文獻中得知，在水相中合成立方體銀奈米晶體仍需要較長的時間且高溫下反應，故本研究主要致力於能有效縮短反應時間及降低反應溫度的立方體銀奈米粒子合成方法，並且探討其反應速率對其形狀影響。
在本研究當中，我們利用植晶方法在水相系統中製備銀奈米晶體，以硝酸銀作為銀的來源、氯化十六烷基三甲基銨鹽作為保護劑及維生素C作為還原劑，反應溫度控制在60 ºC而約兩個小時可製備出成立方體銀奈米晶體。藉由粉末X–RAY 繞射鑑定、穿透式電子顯微鏡電子繞射鑑定與掃描式電子顯微鏡影像拍攝可證明為立方體銀奈米粒子且是由{100}的面所組成 。合成出的粒子大小其範圍在46至55奈米之間。另外，為了研究反應速率對其形狀演變我們加入氨水並發現在加速反應速率的情況下會促進{111}的面生成，反之減低反應速率會致使{100}的面生成。由不同時間點反應顏色的影像拍攝可佐證確實反應速率是會對形狀造成影響。

二、 製備系統性形狀演繹的金銀核殼結構及其光學性質探討

本研究中我們以菱形十二面體金奈米晶體作為板模使銀成長於其上並利用植晶法製備系統性形狀演繹銀包金核殼結構。立方體、截角立方體、截半立方體、截角八面體及八面體可利用硝酸銀、氯化十六烷基三甲基銨鹽、維生素C和氫氧化鈉這些試劑製得。形狀演變是藉由改變還原劑的濃度或是硝酸銀的量，此合成時間僅需50分鐘且在室溫下進行，故此方法可減少能量的消耗縮短反應時間。這些奈米粒子具有窄的大小分布情形所以其有很好的自組裝排列現象。藉由觀測不同反應時間的溶液顏色變化及調控溫度，得到還原反應速率快時會形成八面體而在反應速率較慢的情況下，立方體會產生。
立方體及八面體銀包金核殼結構大小是可調控的以及我們利用這些奈米粒子去探討其光學性質，利用紫外-可見光光譜可發現其光學吸收和金殼大小及外層銀的厚度是息息相關的，利用此一性質不僅可用調整粒子大小更可以利用調控金殼去改變其光學性質。

關鍵字

系統性形狀演繹 ； 金銀核殼結構 ； 光學性質探討

並列摘要

CHAPTER 1

Seed-Mediated Growth of Silver Nanocubes and Their Morphological Transformation

Silver nanoparticles are often synthesized in organic solvents with the use of high reaction temperatures. If nanoparticles can be synthesized in aqueous solution, the method would be energy-saving and environmentally friendly. In the literature, long reaction time and high temperatures are still need to synthesize silver nanocubes. Here we present a facile and low temperature approach to prepare silver nanocubes in aqueous solution and investigat how the reaction rate controls the final product morphology.
In this study, we have developed a seed-mediated growth method to synthesize Ag nanocrystals in aqueous solution. The method involves the addition of a small volume of a seed solution to an aqueous solution of silver nitrate (AgNO3), cetyltrimethylammonium chloriode (CTAC), and ascorbic acid (AA). We utilized AgNO3 as silver source, CTAC as surfactant, and AA as reducing agent. Silver nanocubes were generated in 2 hours at 60 ºC. Transmission electron microscopy (TEM), powder X–ray diffraction (PXRD) pattern, and scanning electron microscopy (SEM) have been employed to characterize the nanocubes enclosed by {100} facets. The edge length of cubes can also be tuned from 46 to 55 nm. Here, we also present the effects of NH3 solution on morphological transformation. The acceleration of the reaction rate by introducing NH3 solution promotes the formation of the {111} facets. The solution color at different time points during synthesis also proved that the reaction rate controlled the final particle morphology.

CHAPTER 2

Synthesis of Au–Ag Core–Shell Heterostructures with Systematic Shape Evolution and Their Optical Properties

In this study, we have utilized rhombic dodecahedral gold nanocrystals as the structure-directing cores for the growth of Ag shells in aqueous solution. Au–Ag core–shell heterostructures with different morphologies can be directly synthesized. The reagents we used are silver nitrate (AgNO3), cetyltrimethylammonium chloriode (CTAC), ascorbic acid (AA), and sodium hydroxide (NaOH). By simply varying the concentration of reducing agent or silver source, shape evolution from cubes, truncated cubes, cuboctahedra, truncated octahedra and octahedra were obtained. The reaction was finished within 50 minutes at 30 ºC. This is a time- and energy saving method. These monodisperse nanocrystals can readily form self-assembled structures. By monitoring the solution color at different time points during synthesis or changing the temperature, particle growth rates was found to be fastest for octahedra covered by {111} facets. On the other hand, a slower reaction rate favors the generation of cubes enclosed by {100} facets.
The nanocube and nanooctahedra size can also be tuned within a range. UV–vis spectra were used to investigate their unique optical property and suggested that their optical responses are closely related to silver shell thickness and gold core size. Both spectral blue-shifts and red-shifts of the Au–Ag nanocrytals compared to Au cores have been observed. With very thin shell thickness, spectral blue-shift was recorded. As particle size increases, red-shift occur.

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