| 摘要: | 本論文利用掃描式電子穿隧顯微鏡 (scanning tunneling microscopy, STM) 與循環伏安法 (cyclic voltammetry, CV) 來探討陰離子及電位對苯胺及聚苯胺分子吸附在金(111)電極表面上之結構及構型變化的影響。在含有30 mM苯胺的硝酸溶液中，苯胺分子與硝酸根離子在電位0.5 ~ 0.8 V (相對標準氫電極)時會共吸附在電極表面上形成一整齊結構 (3 × 2√3)，當電位增加至0.85 V時，此結構會轉變為另一個新結構 (3 × 2√21)，在達到聚合電位0.92 V時，苯胺分子會以 (3 × 2√21) 結構為基板進行氧化聚合形成沿著載體方向成長的線性聚苯胺鏈，接下來聚苯胺分子會慢慢以一維成長的方式形成至少兩層具高規則度的分子膜在電極表面上。相反地，苯胺分子在鹽酸溶液中及電位0.8 V下並未有任何整齊結構吸附在電極表面上，雖然缺少整齊吸附結構，苯胺分子依然可以在電位大於0.9 V下進行分子接合而形成規則的線性聚苯胺鏈。另外STM也被用來探討在硝酸、硫酸及過氯酸電解液下，電位對聚苯胺分子構型的影響，當電位在0.8 V及0.6 V來回跳動時，聚苯胺分子的化學結構會隨著電位產生變化進而導致聚苯胺分子會在線性及彎曲的構型之間來回變動。這個由電位主導的構型變化在硝酸中是可逆且快速的，但在硫酸中卻是明顯的不可逆。
本論文也利用即時的STM掃描探討不同醇類分子對聚苯胺分子構型的影響，首先發現在0.5 M硫酸含有30mM苯胺及1.2 M醇類分子的溶液中(甲醇、乙醇、正丙醇)，聚苯胺分子皆會從線性轉變為彎曲構型，推測是由於聚苯胺分子與醇類分子之間會形成氫鍵進而擠壓原先的線性分子鏈使其變彎。然而根據Fick’s law所計算出來的醇類分子擴散通量比利用STM結果所計算出來的數值來的大，表示擴散至電極上的醇類分子並未全數與聚苯胺分子作用，推測醇類分子可能需要具備特定的位向才能與聚苯胺分子形成氫鍵。另一方面，聚苯胺分子也會與醇類反應產生水解產物，這個反應的快慢與醇類分子大小有關，也就是說正丙醇是三種醇類中反應性最低的。
除了苯胺之外，利用CV及STM也探討了苯胺衍生物3-甲基苯胺分子與3-磺酸基苯胺分子在金(111)電極上的吸附及聚合過程，在硫酸溶液中，3-甲基苯胺分子首先會在0.5 V形成一整齊吸附結構 (5 × 2√3)，覆蓋度為0.2，當電位調整至0.8 V時，此結構會轉變為排列較鬆散的 (5 × 2√3) 及 (3√3 × 2√3) 結構，覆蓋度分別為0.1及0.11。當電位大於 0.9 V時，3-甲基苯胺分子會開始進行氧化聚合，在初始聚合物不滿一層時，聚3-甲基苯胺分子主要是呈現線性構型，但是隨著電位增加聚3-甲基苯胺分子會逐漸轉變為以彎曲的形式成長。而3-磺酸基苯胺分子也會在0.5 及0.8 V時分別形成 (√19 × √31) 與 (2√7 × √31) 兩種整齊結構，然而當電位大於1.0 V時，整齊結構 (2√7 × 31) 會被亞硫酸根離子從電極表面上置換掉。除此之外，本論文也探討了3-磺酸基苯胺與苯胺分子共吸附在電極表面上時的吸附結構，在含有30 mM 苯胺及3 mM 3-磺酸基苯胺分子的硫酸溶液中，苯胺會與3-磺酸基苯胺分子在0.8 V形成一個整齊的共吸附結構 (4 × 2√3)，進而引導分子們沿著載體重排的方向進行氧化聚合形成線性的分子鏈，可惜的是根據X射線光電子能譜 (X-ray photoelectron spectroscopy, XPS) 結果來看，所形成的分子鏈並非是苯胺與3-磺酸基苯胺分子共聚合出來的產物而主要是來自於苯胺分子自身的聚合反應。
最後，利用表面增強紅外光譜 (surface-enhanced infrared absorption spectroscopy, SEIRAS) 來探討當苯胺分子吸附在修飾金膜的矽電極上時，其吸附位向隨著電位的變化。從SEIRAS紅外光譜圖得知苯胺分子的吸附位向會隨著電位越正而從平躺轉變為近乎垂直的吸附，而亞硫酸根離子也會因正負電荷相吸的原因而與苯胺分子共吸附在電極表面上。除此之外，聚苯胺分子的化學結構也會隨著電位而改變，根據聚苯胺主鏈中的benzoid及quinoid結構的吸收強度比例可以判別在電位小於0.3 V時，聚苯胺分子主要是以完全還原態吸附在電極表面上，電位在0.3 ~ 0.6 V時，聚苯胺分子會轉變為半氧化(emeraldine)，當電位大於0.6 V時，另一種半氧化態會接著出現(nigraniline)，而完全氧化態則會開始出現在電位大於0.8 V時且亞硫酸根離子在0.5 V時會開始摻雜入聚苯胺分子中形成具有導電性的半氧化態(emeraldine salt)。
;To unravel effects of anion on the molecular structure of polyaniline (PAN), the adsorption of aniline and its subsequent oxidative polymerization on Au(111) electrode were examined by cyclic voltammetry (CV), chronoamperometry and in situ scanning tunneling microscopy (STM) in 0.5 M nitric and hydrochloric acids, respectively. Aniline molecules were coadsorbed with nitrate ions in a highly ordered (3 × 2√3)rect structure between 0.5 and 0.8 V (vs. reversible hydrogen electrode). Raising the potential to 0.85 V forced rearrangement of the (3 × 2√3)rect structure into a hitherto unidentified (3 × 2√21) structure, yielding one-dimension PAN band aligned in the <110> directions of the Au(111) substrate at 0.92 V. PAN film then grew to form a uniform film up to two layers in nitric acid. Oppositely, no ordered molecular adlattice of aniline was noted at the onset potential (~0.8 V) for polymerization in hydrochloric acid. This lack of ordered aniline structure however did not affect coupling of aniline molecules into well-defined linear PAN molecules as the potential was raised to > 0.9 V. Furthermore, the effects of potential on the PAN’s conformation produced on Au(111) electrode in nitric, sulfuric and perchloric acids were also studied by in situ STM. As the potential was modulated between 0.8 and 0.6 V, PAN molecules changed their oxidation states, which manifested in dramatic changes in the molecular conformations between linear and winding conformations. This potential - driven process was fast and reversible in nitric acid, but was largely irreversible in sulfuric acid.
Real-time STM imaging also reveals that linear PAN became crooked upon exposure to methanol, ethanol and propanol, which resulted in formation of hydrogen bonds and twists of PAN molecules. Since diffusion fluxes of these molecules calculated from Fick’s law exceeded that determined from STM results, orientation of alcohol molecules impinging on the electrode could be important in making alcohol-PAN adducts. PAN molecules could decompose by interacting with alcohol molecules. The bulky n-propanol was the least reactive molecule among the alcohols studied here.
In addition to aniline, CV and in situ STM were also used to examine the adsorption and electropolymerization of 3-methylaniline (3-MA) and metanilic acid (MA) on the Au(111) electrode in 0.5 M H2SO4, respectively. 3-MA admolecules were adsorbed in a (5 × 2√3)rect structure (θ = 0.2) at 0.5 V, but rearranged into two less compact adlattices, (5 × 2√3)rect (θ = 0.10) and (3√3 × 2√3)rect structures (θ = 0.11) at 0.8 V. Raising the potential to 0.9 V resulted in oxidation and polymerization of 3-MA. The poly(3-MA) molecules produced in the early stage assumed linear conformation, but became predominantly crooked upon the increase of overpotential. MA molecules also were adsorbed in highly ordered (√19 × √31)rect and (2√7 × √31)rect structures at 0.5 and 0.8 V. These adlattices however were displaced by bisulfate anions at E > 1.0 V. Furthermore, MA and aniline molecules could be coadsorbed in a highly ordered (4 × 2√3)rect structure at 0.8 V in 0.5 M H2SO4+ 30 mM aniline + 3 mM MA, which led to anisotropic oxidative polymerization in the <121> directions of the Au(111) electrode. However, x-ray photoelectron spectroscopy (XPS) results show that the as-produced linear polymeric chains were mainly PAN, rather than copolymer of aniline and MA.
Finally, molecular structures of PAN as a function of potential were investigated by surface-enhanced infrared absorption spectroscopy (SEIRAS). Results obtained show that aniline molecules were adsorbed in flat and upright orientations at negative and positive potentials, respectively. Bisulfate ions were coadsorbed with aniline molecules on the gold electrode as a result of the need to compensate for the charge. SEIRAS shows that PAN molecules were fully reduced at < 0.3 V, and oxidized to emeraldine (0.3 ~ 0.6 V) and nigraniline at E > 0.6 V. These species were characterized by comparing intensity of IR bands due to benzoid and quinoid species in the backbone of PAN molecules. The fully oxidized PAN or pernigraniline was produced first at 0.8 V. IR bands due to ring structures in doped PAN molecules and bisulfate ions were most pronounced at 0.5 V, supporting the chemical form of PAN as emeraldine salt. |
