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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/6380

    Title: 利用掃描式電子穿隧顯微鏡研究苯胺分子在金(111)和金(100)電極上的吸附結構及聚合機制;Direct Visualization of an Aniline Admolecule and Its Electropolymerization on Au(111) and Au(100) with in Situ Scanning Tunneling Microscope
    Authors: 張今仁;Chin-Zen Chang
    Contributors: 化學研究所
    Keywords: 聚苯胺;polyaniline STM Au(111)
    Date: 2008-07-03
    Issue Date: 2009-09-22 10:18:32 (UTC+8)
    Publisher: 國立中央大學圖書館
    Abstract: 本論文分為兩部分,第一部分主要利用掃描式電子穿隧顯微鏡(in situ scanning tunneling microscopy, STM )及循環伏安法(cyclic voltammetry , CV)探討單結晶金(111),在含苯胺的硫酸溶液中,在未達聚合電位前,分子在金(111)所發生的電化學與吸附層的空間結構。在30mM濃度的苯胺溶液中,在0.55V和0.7V分別形成兩吸附層,結構為(5 × 2√3)、(3 × 2√3),覆蓋度為0.1及0.16,當電位愈正分子覆蓋度愈高。從XPS跟NEXAFS結果顯示苯胺分子在(3 × 2√3)結構中,單位晶格中有一苯胺陽離子與硫酸根或亞硫酸根共吸附,另一苯胺則是以胺基跟金鍵結,且苯環與金表面的傾角約為47度。在達聚合電位時,苯胺單體沿載體方向聚合成線性的聚苯胺鏈,高解像的聚苯胺STM圖證實苯胺以頭對尾的方式聚合形成聚苯胺鏈,兩苯環距離0.46nm當將電位從0.8往0.7V移動,聚苯胺鏈從線性的變為彎曲狀的聚苯胺鏈,在此同時第二層聚苯胺鏈從第一層聚苯胺鏈水解脫附,從XPS和NEXAFS的結果指向這種改變是來自於物理性的變化而不牽涉到氧化還原反應,將電位從0.3往0.15V移動則可以觀察到彎曲的聚苯胺分子鏈變為線性的分子鏈,但此時分子鏈斷裂至5~10nm,當將電位從0.35往0.5V移動,則觀察到分子鏈再度變為原本彎曲狀,電位再往0.7V移動則觀察到一吸附結構(8 × 2√3),推測為苯胺的二聚物如4-hydroxydifenylamine((C6H5)2(NH)(OH) )。不同的陰離子對聚合出的聚苯胺鏈也會有構形上的不同,在硫酸和過氯酸中,苯胺單體在金(111)上聚合前有相同的吸附結構,聚苯胺在金(111)電極上以2D線性的方式進行成長,而在硝酸溶液中,在聚合電位前與硫酸過氯酸同樣是(3 × 2√3),但在達聚合電位時,聚苯胺並不是在金表面以線性的方式形成,而是以3D的方式成長,所形成的分子鏈為纖維狀。 第二部分為利用in situ STM 探討單結晶金(100),在含苯胺的過氯酸溶液中,在未達聚合電位前,分子在金(100)所發生的電化學與吸附層的空間結構。在30mM濃度的苯胺溶液中,在0.7、0.8和0.95V分別形成三吸附層,結構為(7√2 × 2√2)、(4√2 × 2√2)及(√10× √10),覆蓋度為0.143、0.125及0.1,當電位愈正分子覆蓋度愈減少,覆蓋度的減少可能與苯環跟金的作用力增加,使得分子躺得更平。在達聚合電位時,聚苯胺鏈沿a、b、c三個方向生長,三個方向彼此夾60度,方向a跟方向b出現的比例遠大於方向c的聚苯胺鏈,方向a與方向b與Au(100)載體密排方向夾了18度,a與b是沿著?10的方向,c則是?2的方向,a b方向的聚苯胺鏈,分子鏈呈鉅齒狀的線性,c方向的聚苯胺鏈則是線性的但比例較少。由1往0.7V移動,第二層的苯胺從第一層苯胺上水解脫落,第一層的聚苯胺分子鏈,我們可以發現它從原來沿?10跟?2的方向轉變到沿載體密排方向,分子鏈跟分子鏈有著90度或180度的彎曲。當將電位從0.3調至0.15V可觀察到聚苯胺鏈構形從彎曲狀變為線性的分子鏈,但此時分子鏈斷裂至5~10nm,分子鏈沿載體方向吸附。 This thesis is divided into two parts. First, in situ scanning tunneling microscopy was used to unveil the structures of aniline monomer and the product of anodization - polymerization on Au(111). Within a solution of 30 mM aniline + 0.1 M H2SO4, aniline molecules were adsorbed in (5 × 2√3) and (3 × 2√3) at 0.55 and 0.7 V (vs. reversible hydrogen electrode RHE), respectively. Their coverage were is 0.1 and 0.16. The results of XPS and NEXAFS indicated that the unit cell of (3 × 2√3) has two aniline adspecies with one exiting as an aniline cation co-adsorbed with a (bi)sulfate anion, and the other aniline as an aniline molecule bonded to the gold electrode. In average NEXAFS results suggest a tilt angle of ~ 47°from the Au(111) surface. At potentials positive of 0.9 V (vs. reversible hydrogen electrode), aniline admolecules were oxidized and formed polyaniline. in situ STM revealed that this polymerization reaction was highly anisotropic, leading to polyaniline molecules aligned preferentially along the close-packed atomic directions of the Au(111) substrate. Molecular-resolution STM images showed that two benzene rings were separated by 0.46nm from each other, suggesting that polyaniline molecules were likely formed by coupling between two neighboring molecules in a head-to-tail fashion. Switching potential from 0.8 to 0.7 V modified the molecular conformation of polyaniline, as the polyaniline chain changed from linear to crooked form. Meanwhile, the upper layer of polyaniline was desorbed. Making the potential from 0.3 to 0.15 V resulted in breakdown of polyaniline as the conformation turned from crooked to short linear form with an average length of 7 nm. These potential-dependent changes of molecular conformation were reversible. Meanwhile, in situ STM revealed an hitherto unseen adlattice structure of (8 × 2√3). because it was found after substantial decomposition of polyaniline, this local adlattice is believed to be associated with products such as 4-hydroxydifenylamine from the decomposition reaction. The conformation of polyaniline chains varied with the chemical nature of the supporting electrolyte. While linear polyaniline chain grew individually and laterally in sulfuric acid and perchloric acid, polyaniline molecules tended to agglomerate and grew in 3D-like manner in nitric acid. Second , In situ STM was used to unveil the structure of aniline monomer and its oxidative polymerization on Au(100) in perchloric acid. Three different structures (7√2 × 2√2)、(2√2 × 4√2)、(√10 × √10 ) of aniline monomers were identified at 0.7、0.8、and 0.95 V, respectively. The coverage was consecutively decreased from 0.143 to 0.125 and then to 0.1. This result could be associated with relatively stronger adsorption of benzyl portion of the aniline molecule at potentials much more positive (> 0.5 V) than the point of zero charge (about 0.3 V vs. RHE). In situ STM was also used to study the molecular conformations of polyaniline adsorbed on Au(100). Results show that potential caused changes of polyaniline molecule structure similar to those found on Au(111).
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