| 摘要: | 利用掃描式電子顯微鏡(in situ scanning tunneling microscopy, STM)及循環伏安法(cyclic voltammetry, CV)，探討在三種不同磺酸溶液中，在未達聚合電位前，苯胺分子在單晶電極金(111)上，所發生的電化學過程與吸附層的空間結構,由STM圖可證實，分子排列結構隨電位而改變,這些過程具可逆性。 苯胺於甲基磺酸、苯磺酸、樟腦磺酸溶液中，電位在0.9 V(相對於可逆氫電極)形成規則吸附層，結構分別為(3 × 2√3)， (4 × 2√3)， 及(4 × 2√3)，苯胺與酸根共吸附的構形會因為酸根大小而改變，三者中最小的甲基磺酸根與苯胺共吸附時形成(3 × 2√3)結構，而較大的苯磺酸和樟腦磺酸根，與苯胺共吸附時，分子間距增加導致不同的共吸附結構。 不同的陰離子也會對聚苯胺鏈的構形有影響，在硫酸和甲基磺酸中，苯胺單體在金(111)上聚合前有相同的吸附結構- (3 × 2√3)，聚苯胺在金(111)電極上以二維線性的方式沿著原子密排的方向成長。在苯磺酸和樟腦磺酸中，苯胺單體在金(111)上形成相同的吸附結構 - (4 × 2√3)，達聚合電位時，聚苯胺於苯磺酸中以線性或三維方式成長，在樟腦磺酸中則必定以三維的方式成長。 造成在苯磺酸中苯胺分子以三維方式成長的原因可能和苯胺分子所在位置有關，尤其，吸附於金電極表面的苯胺分子可能較溶液中的苯胺分子難以氧化，因此苯胺分子優先於溶液中形成，爾後再沉積於電極上，如此產生的聚苯胺分子通常不具明確的分子構形。如降低苯胺濃度至0.3mM可降低溶液中的反應速率，，並提高表面苯胺分子的聚合趨勢，因而此時可在苯磺酸中得到線性的聚苯胺，幾乎所有的聚苯胺分子沿著√3方向成長，與硫酸中結果不同。 藉由接觸式原子力顯微鏡(atomic force microscopy, AFM)測量以電化學所製備的聚苯胺高分子的導電度，發現導電度和電解質的種類有關，其導電度依序為甲基磺酸≧苯磺酸>樟腦磺酸，此結果符合我們的預期，由STM得知在甲基磺酸及苯磺酸中可得到線 II 性的聚苯胺鏈，線性聚苯胺鏈具較佳的共軛程度，而有利於電子的傳遞，，因此可得較好的導電度，在樟腦磺酸中，聚苯胺不具清楚的構形，因此相較之下導電度較差。 In situ scanning tunneling microscopy (STM) was used to obtain molecular insights of the oxidative polymerization of aniline on Au(111) single crystal electrode. In situ STM yielded highly ordered structures of (4 × 2√3), (3 × 2√3), and (4 × 2√3) at 0.9 V (the onset potentials for polymerization of aniline) [vs. reversible hydrogen electrode,RHE] in 30 mM aniline-containing 0.1 M benzenesulfonic acid (BSA), methylsulfonic acid (MSA), and (CSA) solutions, respectively. These adlayer structures are thought to be made of aniline monomers and anions of BSA, MSA, and CSA, respectively. The physical size of these anions could be an important factor in determining the arrangements of these co-adsorbed adlayers, which explains the most compact structure of (3 × 2√3) observed with the smallest anions of MSA. The arrangements of aniline monomers played key roles in guiding the intermolecular coupling between aniline admolecules on the Au(111) electrode. More specifically, aniline admolecules in these ordered arrays could be adsorbed in a manner where two neighboring entities were oriented in a head-to-tail configuration, which facilitated intermolecular coupling upon oxidation. Indeed, in situ STM imaging indicates that electropolymerization of aniline at E > 1 .0 V in MSA and BSA proceeded predominantly at the electrode surface, producing mostly linear polyaniline (PAN) molecules aligned preferentially in the <121> directions. These results are comparable to those reported for PAN in sulfuric acid, except PAN chains were aligned differently. In stark contrast, PAN produced in HCSA were mostly crooked, regardless of the concentration of aniline used in the polymerization reaction. The reason for the production of poorly defined PAN in CSA could be attributed to the weaker interaction between PAN molecules and counter anions of CSA, as compared to BSA. This weaker electrostatic interaction could allow coupling reactions to IV proceed in the middle of the PAN chains, rather than at the ends of PAN chains, as observed with MSA and BSA. However, it is possible that polymerization reaction in CSA could proceed faster in the solution, as compared to that on electrode surface. In other words, in situ STM could image precipitated polymer molecules formed in the solution phase. It appears that these solution-formed PAN molecules could have poorly defined molecular conformations. By using conductive atomic force microscope (AFM), I - V relationships were obtained with PAN doped with the anions of BSA, MSA, and CSA and the conductivity of each PAN film was extrapolated from these measurements. The conductivity of PAN chains followed the order of MSA≧ BSA >CSA. If the conductivity of PAN films produced in these media were mainly affected by the degree of conjugation in the polymeric chains, we can explain these results by noting that well-defined linear PAN chains were produced in MSA and BSA but not in CSA. |
