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    题名: 掃描式電子穿隧顯微鏡對乙基取代苯胺分子在金(111)電極上的吸附及聚合研究;Electropolymerization of ethylaniline molecular on Au(111) probed by in situ scanning tunneling microscopy
    作者: 黃忠正;Chung-Cheng Huang
    贡献者: 化學研究所
    关键词: 電化學聚合;乙基苯胺;ethylaniline;electropolymerization
    日期: 2009-07-07
    上传时间: 2009-09-22 10:21:02 (UTC+8)
    出版者: 國立中央大學圖書館
    摘要: 摘要 首先利用循環伏安法(cyclic voltammetry,CV)及掃描式電子穿隧顯微鏡(scanning tunneling microscopy,STM)來探討單晶金(111)電極上,鄰、間、對-乙基苯胺分子的吸附層結構。C V結果顯示在30 mM濃度下,這三個分子都導致三對特徵峰,由於特徵峰所在的電位反映分子與電極的作用力強弱,由此得知鄰、對-乙基苯胺分子較間-乙基苯胺分子吸附力強,此一結果反映出位於鄰及對位的乙基可增加苯胺官能機上的電子密度,因而使兩者有較強的吸附力。 由STM觀察其吸附結構隨著電位往正移動覆蓋度增加,其中在聚合前的結構對於聚合反應尤為重要。由CV及STM結果皆看到對-乙基苯胺並不會聚合,而是生成水解產物,間接應證要以頭對尾方是才利於聚合;在聚合電位之前的鄰-乙基苯胺是(4√3 × 4√3)聚集結構,聚合以彎曲散亂狀沉積在金(111)上;間-乙基苯胺則是(4 × 2√3),沿√3方向直線沉積在金(111)上。在XPS及NEXAFS的結果:聚間-乙基苯胺的氮/硫比為2/1、質子化的氮/未質子化的氮比為1/1,得知所得到的聚間-乙基苯胺為鹽式中間態,苯環與金表面平均傾角約47度。 第二部分探討乙基苯胺與苯胺混合狀況下,其吸附結構及聚合方式,在苯胺與鄰-乙基苯胺各30mM混合的情況下,在電位0.6V時,由STM觀察到共吸附結構(3√3 × √43)、(3√3 × √57)、和(3√7 × √21),往正電位到0.8V之後便轉為苯胺的(3 × 2√3)結構,對應CV的特徵峰,其變化來自於苯胺的置換,聚合的部分也和苯胺相同,沿載體密排方向直線成長;在苯胺與間-乙基苯胺各30mM混合的情況下,從 0.8 V之後其結構均為(4 × 2√3),聚合的部分類似間-乙基苯胺,沿√3方向直線成長。 Abstract Cyclic voltammetry (CV) and scanning tunneling microscopy (STM) were used to examine the adsorption of o-, m-, and p-ethylaniline on single crystal Au(111) electrode in sulfuric acid. Each molecule resulted in three pairs of peaks in the CV profiles obtained in 0.5 M sulfuric acid containing 30 mM o-, m-, and p-ethylaniline, respectively. The features found at the most negative potentials were associated with the adsorption-desorption of these molecules. These peaks observed with o- and p-ethylaniline appeared at potentials more negative than that of m-ethylaniline, suggesting that the former were more strongly bonded to the Au(111) electrode, mostly likely via the amino groups.This is consistent with the fact that ethyl substituents on the benzene are electron-donating groups, which increase the electronic density at the amine group of these molecules. Molecular resolution STM images were acquired to reveal the adlayer structures of these molecules on the Au(111) electrode. Generally, more and more ethylaniline molecules became adsorbed with more positive potential. Those structures observed at onset potentials of oxidation (~0.95 V) were most relevant to the polymerization of these molecules. In situ STM imaging conducted under potential control revealed fast oxidation and polymerization of o- and m-ethylaniline, but not p-ethylaniline. These results supported that these molecules could follow the head-to-tail coupling mechanism established for electrolymerization of anilines. The structures of o- and m-ethylaniline observed at 0.85 V (onset of polymerization) were (4√3 × 4√3) (4 × 2√3),respectively. Raising the potential to >0,95 V, these molecules were oxidized and polymerized with rather different molecular conformations, as revealed by the in situ STM. While o-ethylaniline produced ill-defined polymers, 3-ethylaniline resulted in linear chains of polymers preferentially aligned in the <112> directions of the Au(111) electrode surface. According to XPS and NEXAFS results, the nitrogen/sulfur elemental ratio in the poly3-ethylaniline film is 2/1, and the ratio of protonated /unprotonated nitrogen was 1/1, which is consistent with the expected value for the emeraldine salt of poly3-ethylaniline. The tilt angle defined by the benzene ring and the gold surface is approximately 47?. I also explore the possibility of forming two-component polymers, such as polyethylaniline-aniline. In situ STM imaging at a fixed potential of 0.6 V showed that aniline and o-ethylaniline could be coadsorbed on Au(111) in 0.5 M sulfuric acid containing 30 mM of aniline and 30 mM o-ethylaniline. The ordered structures were identified as (3√3 × √43), (3√3 × √57), and (3√7 × √21), which were transformed into (3 × 2√3) as the potential was increased to 0.8 V. This structure was previously observed with aniline, suggesting that aniline, being more strongly adsorbed on Au(111) than o-ethylaniline, could displace foreigner admolecules and prevail. The conformation of the produced polymers at E > 0.9 V was the same as that of aniline, where linear chains were produced in the main axis of the Au(111) substrate. In the mixed system of aniline and 3-ethylaniline, in situ STM revealed a (4 × 2√3) structure at 0.8 V. Polymerization at E > 0.9 V resembles that of 3-ethylaniline, where straight chains were found in the <112> directions.
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