金奈米柱-不均勻二氧化矽核殼粒子之光致熔化現象與界面結構之關係

The Effect of Interfacial Structure on The Photo-induced Melting of Gold Nanorod in Gold Nanorod@non-uniform Silica Core-Shell Nanosystem

吳致廷(CHIH-TING Wu)

指導教授 ： 王崇人

國立中正大學/理學院/化學暨生物化學系/碩士(2016年)

摘要

本研究主要在探討金奈米柱與鍍層間之界面結構對光致熔化現象的關係。我們採用不均勻二氧化矽包覆的金奈米柱核殼粒子（silica coated gold nanorod with non-uniform thickness in major and minor axes; AuNR-nu-SiO2）做為基本的奈米系統探討金奈米柱的光致熔化現象，其中兩端厚度（end-thickness, ET）極薄約為1 nm左右，金奈米柱於兩端接近裸露；而其側邊厚度（side-thickness, ST）約為12 nm左右。至於此一奈米系統，我們設計兩種金奈米柱與鍍層間之界面結構：其一為化學鍵結，另一則為物理吸附；前者標示為AuNR-SH-nu-SiO2而後者則標示為AuNR-CTAB-nu-SiO2。前者的鍍層工作是以（3-硫基丙基）三甲氧基矽烷（(3-mercaptopropyl)trimethoxysilane; MPS）作為水解縮合步驟的前驅物，而後者則以四乙氧基矽烷（tetraethyl orthosilicate; TEOS）為之。在AuNR-CTAB-nu-SiO2奈米結構中，溴化十六烷基三甲銨（hexadecyltrimethylammonium bromide; C16TAB）為金奈米柱原本的穩定劑，在殼層生長過程中並未被取代，並擔任了轉接層的角色。我們這項設計主要想證實界面結構的不同將會導致在側邊熱傳導的差異，進而造成金奈米柱內的中間與兩端的溫差。我們期待金奈米柱的光致熔化現象將提供我們有效的機制藉以觀察界面結構的這項效應。
我們也瞭解金奈米柱在吸收其表面電漿特徵波長的光照後，將極有效率地轉變為熱能，而此熱釋放在殼層的熱傳導速率會受殼層的材質與緻密度影響。在我們設計的奈米系統中，殼層的材質同為空洞型的二氧化矽，為了凸顯界面結構對於異向性熱釋放之效應，我們首先必須確認兩種不同界面的殼層緻密度是相同的。這一項鑑定工作是透過吸收光譜中金奈米柱的長軸表面電漿共振特徵吸收波帶的偏移量，以及孔洞表面積的量測作為證據。針對具有兩種不同界面結構的奈米粒子，光致熔化實驗的結果顯示，界面結構為化學鍵的奈米粒子，AuNR-SH-nu-SiO2，其中的金奈米柱則產生高達約70%的縮熔（melting; rod-to-sphere and its intermediates），同時未發現有任何裂熔（split-melting）產物的生成。然而在界面結構無化學鍵的AuNR-CTAB-nu-SiO2粒子中，金奈米柱僅約20%的粒子會產生縮熔，同時我們觀察到有相當程度比例的裂熔產物生成，其比例約有40%。追究這兩個奈米結構的光致熔化現象有如此明顯差異的緣由，我們歸因於界面作用力大小不同所致。界面作用力的大小會影響熱傳導速率，作用力越大者導熱越快，所以在殼層材質與緻密度相同的條件下，AuNR-CTAB-nu-SiO2因為界面結構的作用力相對來說較小（約9 kcal/mol），因此導熱較慢，造成金奈米柱內的中間與兩端溫度差相對來說較大，而此較大的位差足以產生約40%的裂熔。反觀AuNR-SH-nu-SiO2因界面結構為化學鍵，其作用力相對較大（約45 kcal/mol），因此導熱較快而使該溫差較不明顯，造成絕大部分進行縮熔，而無裂熔形變的現象。我們也嘗試將AuNR-CTAB-nu-SiO2粒子的殼層厚度增加，觀察到在界面結構與殼層對熱釋放的加成效果下，金奈米柱的的裂熔比例隨殼層厚度增加而降低至30%，縮熔比例則是上升達45%。認為是因為殼層增厚將金奈米柱全包覆，降低了熱釋放的方向性差異，但由於界面結構對熱釋放的局限，所以仍有裂熔形變的產生。同時也觀察到金奈米柱裂熔形變的情況隨殼層厚度增加而從原先裂成兩顆球狀變成裂熔為兩短柱狀粒子。
關鍵詞：金奈米柱、光致熔化

關鍵字

金奈米柱 ； 光致熔化

並列摘要

This thesis focuses on the effect of the interfacial structure between gold nanorod（AuNR）and silica in the photo-induced melting of the AuNR core. We synthesized silica coated gold nanorod with non-uniform thickness in major and minor axes（AuNR-nu-SiO2）as a basic nanosystem. The particle side-thickness（ST）and end-thickness（ET） are well controlled with the typical values of ca. 12 nm for ST and ca. 1 nm for ET. There are two kinds of interfacial structures between AuNR and silica we proposed herein, one is chemical bonding and the other one is physical adsorption. The former is Au-S covalent bonding, symbolized as AuNR-SH-nu-SiO2, and the latter is physical adsorption of CTAB on the AuNR surface, symbolized as AuNR-CTAB-nu-SiO2. The chemical bonding of the interfacial structure was introduced into the nanosystem by choosing the precursor of the sol-gel process as (3-mercaptopropyl)trimethoxysilane (MPS) for the silica coating The physisorption of the interfacial structure in the other nanosystem was accomplished by the use of tetraethyl orthosilicate (TEOS) as the precursor of the sol-gel process while the CTAB remain intact to the AuNR surface. Two nanosystems with different interfacial structures were designed to demonstrate a clear difference for the heat conductivity along the AuNR side to the silica and were expected that we should be able to observe different photo-induced melting products.
It is well-known that AuNR will efficiently transform photon energy by light absorption to heat at its surface plasma resonance (SPR). Also, the transportation rate of heat flux is influenced by the porosity of the coated silica. In order to extract a clear evidence regarding the interfacial structure effect on the photo-induced melting process, we need to confirm that the porosity of the coated silica in both nanosystems are similar to begin with. The porosities were confirmed by examining the extent of the SPR spectral shift and also data collected from the surface area and porosimetry analyzer. The results of our photo-induced melting measurements clearly indicate that the melting process in AuNR-SH-nu-SiO2 system follows the conventional melting after absorbing single pulsed photon energy, AuNR melts to give sphere or shorter rod. A high yield of ca. 70% for such melting products was observed without any indication for the spilt-melting products. However, in AuNR-CTAB-nu-SiO2 nanosystem, after laser irradiation the split-melting products was clearly observed to give ca. 40% yield while the yield of the melting products is about 20%. We rationalized the split-melting result compared to the conventional melting process by the only reason that the temperature difference between the central region of AuNR and its ends is greatly enhanced in the AuNR-CTAB-nu-SiO2 nanosystem. The enhanced temperature gradient are attributed to the poorer thermal conductivity through its interface with weaker interaction. This less efficient thermal conductivity then results in higher temperature retained in the central region of the AuNR. Additionally, we also increased the both directions of side and end thickness of AuNR-CTAB-nu-SiO2. In those cases, we observed that increased percentage of the AuNRs melting particles via conventional pathway as we increased the thickness. It can be contributed by that the heat flux becomes more and more isotropic.
Keywords：Gold nanorod, photo-induced

並列關鍵字

Gold nanorod ； Photo-induced

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