合成小尺寸具系統性形狀演繹的金銀核殼結構及其光學性質和應用探討

Synthesis of Ultrasmall Au@Ag Nanocrystals with Shape Evolution and Their Optical Properties and Applications

姜傑(Chiang, Chieh)

指導教授 ： 黃暄益

國立清華大學/理學院/化學系/碩士(2015年)

https://doi.org/10.6843/NTHU.2015.00085

摘要

此實驗利用小顆八面體金奈米粒子作為模板並使用種晶植法使銀成長於其上合成具形狀演繹的銀包金核殼結構，藉由使用氯化十六烷基三甲基銨鹽為界面活性劑、加入八面體金種、硝酸銀當銀殼的前驅物、使用維生素C當還原劑、NaOH控制還原速率可得到立方體、截半立方體、八面體的形狀，系列的形狀演變最主要由加入金種跟硝酸銀的比例和還原速度調控，在30度C下反應一小時而得，比起有機合成法是更具省時、省能、環保的方法。這些可小於50奈米的粒子具有一致性的形狀和大小分佈所以其有很好的自組裝排列現象。藉由觀測不同還原速率時溶液隨時間的顏色變化，我們可發現還原速率快時，易形成八面體，相反的還原速率變慢時，易得到立方體的形狀。
我們可由同樣大小的金核合成出不同大小的立方體及八面體銀包金核殼結構，這代表銀殼的厚度是可調控的，利用這些不同大小的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外層銀的厚度及形狀是息息相關的，利用此一性質不僅可用調整粒子大小更可以利用調控銀殼去改變其光學性質。
這些金屬奈米粒子除了特殊光譜現象，在吸收光後，其容易以放熱的方式來釋放能量，我們進一步以雷射去照射含有奈米粒子的液體，觀察液體溫度的改變。除此之外因為小尺寸和一致的形狀會展現出很好的自組裝排列現象，我們可以利用此性質，在不同溫度下，使濃液滴在充滿水氣的瓶子裡慢慢乾燥，奈米粒子會自組裝成不同形狀的超級晶體。

關鍵字

銀包金 ； 金銀核殼結構 ； 形狀演繹 ； 小尺寸 ； 光學性質 ； 應用 ； 核殼結構 ； 超級晶體 ； 光熱效應 ； 奈米粒子

並列摘要

In this study, we have utilized octahedral gold nanocrystals as the structure-directing cores to grow Ag shells in aqueous solution. Au–Ag core–shell heterostructures with different morphologies can be directly synthesized by using cetyltrimethylammonium chloride (CTAC) as capping agent, octahedral gold nanocrystals as seed, silver nitrate (AgNO3) as precursor of Ag shell, ascorbic acid (AA) as reducing agent and sodium hydroxide (NaOH) as controller of reducing rate. By simply varying the ratio of gold seed to AgNO3 and the concentration of reducing agent, shape evolution from cubes to cuboctahedra and octahedra can be achieved. The reaction was finished within 1 hour at 30 ºC. This is a time- and energy-saving method. These monodispersed nanocrystals can have sizes less than 50 nm and readily form self-assembled structures. By tuning the reduction rate and monitoring the solution color at different time points during synthesis, octahedra covered by {111} facets were found to grow at a faster rate. On the other hand, a slower reaction rate favors the generation of cubes enclosed by {100} facets.
We can use the same gold cores to synthesize different sized Au‒Ag core‒shell cubes and octahedral with tunable shell thickness. UV–vis spectra were used to investigate their optical properties and suggested that their optical responses are closely related to silver shell thickness and gold core size. With very thin shell thicknesses, a spectral blue-shift was recorded. As particle size increases, red-shift appears.
After absorbing light, these metal nanoparticles release the energy through heat generation. Upon laser illumination, rapid and significant solution temperature increase was recorded. In addition, the small polyhedral nanocrystals can form different shapes of supercrystals in a saturated moist atmosphere by using a simple drop-casting method at different temperatures.

並列關鍵字

Au@Ag ； Au-Ag core-shell nanocrystals ； shape evolution ； ultrasmall ； optical properties ； applications ； core-shell ； supercrystals ； photothermal effect ； nanocrystals

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