利用掃描式電子穿隧顯微鏡研究羧基硫醇分子於銅(111)的吸附行為及結構

note

陳宇映(Yu-ying Chen)

指導教授 ： 姚學麟

國立中央大學/理學院/化學系/碩士(2014年)

摘要

本研究利用循環伏安法(CV)和掃描式電子穿隧顯微鏡(EC-STM)探討羧基硫醇分子於銅(111)上之吸附行為及平面結構。實驗著重於三種不同分子Mercaptoacetic acid (MAA)、3-Mercaptopropionic acid (MPA)、4-mercaptobutyric acid (MBA)進行兩種濃度(100 µM及1 µM)及兩種 pH值(1、4)下分子吸附的結構。
　　第一部份研究在電解液為pH1硫酸系統下吸附羧基硫醇分子， MAA於銅(111)電極上的自組裝吸附及結構。在高濃度100 µM條件下，在0 ~ -0.3 V電位區間時，吸附分子形成規則的花紋狀(moiré pattern)結構，其覆蓋度為0.4。MAA分子可能以其SH官能基在脫氫後與銅形成離子鍵，分子直立於銅(111)載體上。而低濃度1µM條件下，MAA分子在-0.25 ~ -0.35 V間以(√3 × √3) R30°吸附結構，其覆蓋度為0.33;當將電位移至-0.35 V(分子脫附前)，MAA分子轉換為條紋結構。而MPA及MBA分子在高濃度100µM條件下，多層的吸附分子形成薄膜，高解像掃描顯示分子無整齊有序的結構。而在低濃度1µM條件下，MPA及MBA分子在電位區間-0.25~ -0.35 V以(√3 × √3)R30°吸附結構，其覆蓋度為0.33; 於較負電位-0.35 V(分子脫附前)，MPA及MBA分子同樣形成條紋的吸附結構。這些結構的變化皆為可逆的過程。
　　在含氯的條件下，氯會和MAA分子競爭吸附於銅電極上，電位是控制吸附的重要參數，STM結果顯示在正電位下，吸附物形成整齊排列的moiré pattern結構，此為MAA的吸附結構;負電位下，moiré pattern 排列明顯變亂，而銅平台上出現整齊的條紋狀結排列，其特徵和氯的結果一致。
　　第二部份研究在了解pH對MAA及MPA分子在銅電極上吸附的影響，在此將硫酸電解液的pH值提高到4，對MAA而言，從0到-0.3 V間，分子密集排列形成moiré pattern結構，在較負電位(-0.3 ~ -0.5 V) 分子由高覆蓋度的moiré pattern結構轉變為較低覆蓋度的(√7 × √7)R19.1°，其覆蓋度為0.29，之後在-0.5到-0.7 V間，分子結構繼續轉變成(2√7 × 2√7)R19.1°，其覆蓋度為0.11。在MPA分子部份，0 V到-0.3 V電位區間為密集排列形成整齊結構，將電位調整到-0.3 V到-0.5 V，MPA分子由亮點結構轉變為(√7×√7)R19.1°，更負電位觀察，-0.55 V到-0.8 V，MPA分子結構吸附由(√7×√7)R19.1°轉變成(2√7 ×2√7)R19.1°。這些結構隨電位的變化和單純硫原子的行為相同，因此推測硫醇分子的吸附主要決定再於硫端與銅載體間的作用力，短碳鏈及羧酸官能基並不影響其吸附結構。

關鍵字

銅 ； 自組裝吸附 ； 硫醇分子

並列摘要

In situ scanning tunneling microscope (EC-STM) and cyclic voltammetry (CV) were used to examine the adsorption behavior and structure of carboxyl thiol molecules on Cu(111). In particular, three molecules with different chain length, mercaptoacetic acid (MAA), 3-Mercaptopropionic acid (MPA), 4-mercaptobutyric acid (MBA), were deposited onto Cu(111) electrode from aqueous solutions containing 100 and 1 μM of these three molecules. In situ STM provided molecular – resolution views of the spatial structures of these molecular adlayers as a function of potential.
The structures of carboxyl thiol molecules adsorbed on Cu(111) electrode were examined in pH1 sulfuric acid containing 100 μM MAA, MPA and MBA, respectively. These admolecules were adsorbed in compact, long - range ordered moiré structures at coverage () of 0.41 (number of admolecules/number of Cu atoms). MAA molecules were presumed to adsorb via their SH end, forming ionic bond with the Cu substrate. By contrast, MPA and MBA molecules tended to be adsorbed in multilayer in the presence of 100 μM molecules. High resolution STM imaging revealed largely disordered structures in these multilayer thin films. In electrolyte containing 1 μM of these molecules, results were nearly the same. They all formed ordered (√3 × √3)R30° ( = 0.33) between -0.25 ~ -0.35 V (vs. Ag/AgCl). Shifting potential negatively resulted in restructuring into a stripe phase before admolecules continued to desorb at E < -0.35 V. These phase transition events were reversible. Given the nearly same results observed with these molecules, chain length of thiol molecules had an insignificant effect on molecular structures.
The presence of chloride in the electrolyte appeared to complicate the adsorption of these molecules. Aided by molecular – resolution STM imaging, it is thought that MAA arranging in ordered moiré structure was adsorbed more strongly than chloride at positive potentials. However, shifting potential negatively resulted in displacement of the moire pattern with a striped phase attributable to adsorbed chloride.
　　The adsorption of MAA and MPA in pH 4 sulfate media was examined with in situ STM. MAA molecules were adsorbed in the compact moiré structure between 0 to -0.3 V, which was converted the (√ 7 × √ 7) R19.1 °( = 0.29) structure when the potential was adjusted to -0.3 V, followed by another phase transition to (2√7 × 2√7) R19.1°( = 0.11) at -0.5 V. These results are essentially the same as those observed with sulfur adatoms on Cu(111) electrode, which suggests that S-Cu interaction was of prime importance in guiding the adsorption of thiol molecules on Cu(111). Chain length seemed to be secondary under the present conditions.

並列關鍵字

STM ； SAM

參考文獻

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