| 摘要: | 摘要 Ⅰ.芳香基硫醇分子於鉑(111)電極上的吸附: (a)苯硫酚(Benzenethiol)的吸附結構 藉由掃描式電子穿隧顯微鏡（STM）觀察芳香基硫醇分子在鉑(111)電極上，於固定電位下苯硫酚分子吸附層的結構。於0.1 ~ 0.3 V電位區間形成規則之吸附結構，其排列方式和苯硫酚濃度有關，在10 ?M以下，苯硫酚分子形成覆蓋度為0.25的(2 × 2)吸附結構，當濃度提升到100 ?M時，覆蓋度增加至0.33，而結構轉變為(?3 × ?3)R30°。當電位調至0.6 V後，這些規則之吸附結構轉變為不規則的吸附，此一結構的轉變為不可逆。苯硫酚分子以其硫端與鉑(111)表面原子形成化學鍵結，在0.1 M KOH中，由於在-1 V時仍然未有還原電流出現，由此得知苯硫酚分子形成很強的表面化學鍵，由高解像STM圖判斷，兩相鄰苯硫酚分子間距為4.8 Å，以直立的位向吸附在鉑電極，硫鉑之共價鍵為主要的吸附力，苯基部份則傾斜於鉑表面。 (b)2-萘硫酚(2-Naphthalenethiol)的吸附結構 為研究分子結構對其吸附位向的影響，我們選擇萘環單硫分子(2-Naphthalenethiol)在鉑(111)上的吸附，高解像STM結果顯示，在低覆蓋度時2-萘硫酚以其雙環與硫端分子散亂地吸附，同時吸附於電極上，但當表面分子覆蓋度隨電位愈正而增加時，鉑(111)面上開始出現具規則排列之島狀特徵，這些特徵隨時間逐漸成長且數目明顯增加，持續觀察後，同時提高溶液中分子濃度，表面形成一高規則度之(?3 × ?3)R30°吸附結構，此一結構和苯硫酚分子的結果相同，因此二者應具相同的吸附方式，他們均以硫端與鉑電極表面鍵結，而苯及萘環部份則傾斜直立於鉑載體上。 Ⅱ.有機雙硫醇分子修飾於鉑(111)電極: 本論文研究中我們選用兩種烷基雙硫醇分子 : 1,6-己烷基雙硫醇、1,9-壬烷基雙硫醇，及兩種芳香基雙硫醇分子: 鄰苯雙硫醇、間苯雙硫醇，使用掃描式電子穿隧顯微鏡(Scanning Tunneling Microscopy，STM)及循環伏安儀(Cyclic Voltammetry，CV)研究單晶鉑(111)電極表面有機雙硫醇分子自然形成規則有序單層分子薄膜，即所謂自組裝單層膜(self- assembled monolayers, SAMs)。 當電位控制在0.1 ~ 0.3 V (vs.RHE)之間，雙硫醇分子均形成規則單層分子，電位控制決定電極表面分子吸附濃度，由STM成功觀察分子結構會由(2 × 2)轉變為較緊密的(?3 × ?3)R30°結構，覆蓋度分別是0.25及0.33，此結果是第一次由STM觀察出雙硫分子在鉑(111)電極規則吸附。由於單硫醇分子及雙硫醇分子均形成相同的結構，雙硫醇分子之末端硫原子和鉑載體之間的作用力主導分子吸附的最重要動力，在高解像STM影像判斷，分子空間結構吸附方式以硫端鍵結於鉑載體，呈現直立吸附排列。 Ⅲ.同分異構物之硝基酚分子在鉑(111)電極上研究: 選用鄰(Ortho-)、間(Meta-)、對(Para-)之硝基酚分子及硝基苯分子，探討在單晶電極表面有機分子的吸附結構及其氧化還原反應，由CV實驗觀察得知0.1 M過氯酸中，電位介於0.1至0.9 V之間，此等若干有機分子均能完整吸附於鉑(111)電極上。電壓較0.9 V正時，分子開始分解，判斷生成為未知的有機碎片，在0.08 V產生了一個還原波，此特徵峰電流隨著掃描圈數持續增加，得知此一特色應來自於分子本身的脫附及氫的吸附。STM結果顯示，選用的四種硝基酚有機分子在0.1 ~ 0.15 V均有規則結構吸附，對-硝酚分子在0.1V時同時有兩種結構生成，分別為: (5?3 × 4)，覆蓋度為0.3及(2?7 × 2?21)，覆蓋度為0.41。間-硝基苯分子結構為(?3 × 13)，覆蓋度為0.77。鄰-硝基苯分子結構為(6?3 × 2)，覆蓋度為0.208。硝基苯分子結構為(?3 × 7)，覆蓋度為0.214。分子結構不會隨電位改變而轉變，施加更正電位後，分子呈現不規則吸附，且為不可逆的分子吸附方式，上敘硝酚分子在鉑(111)表面均呈現特殊條紋狀結構吸附，則以對-硝酚為較穩定有機分子聚集。 Ⅳ.同分異構物之氟酚分子在鉑(111)電極上研究: 間(Meta-)、對(Para-)之氟酚分子，探討在單晶鉑電極表面有機分子的吸附結構及其氧化還原反應，可與同分異構物之硝基酚分子進行探討，在STM觀察得知，其分子吸附結構方式均呈現條狀規則分子吸附結構，與硝基酚分子呈現類似吸附方式，分子吸附方式為穩定吸附，分子並未有同一方向吸附，而是呈現60°、120°轉角，此旋轉區塊為相同吸附結構。 高解像STM結果顯示，在0.1 M過氯酸溶液中，定電位於0.1 V吸附氟酚分子，對-氟酚結構為(43 × 8)，覆蓋度為0.39，推測為分子垂直吸附，以氟離子與鉑鍵結，所以吸附結構主要是由載體與吸附物吸附之間π-stacking作用力形成，對-氟酚呈現特殊扭結(kinks)結構。間-氟酚為(33 × 7)結構，覆蓋度0.361，兩種分子吸附均呈現條紋狀吸附，在觀察諸此有機分子吸附結構均為條紋狀吸附，不同分子呈現不同結構吸附方式。 Abstract Ⅰ. Self-assembly of aromatic thiols molecules on a well-ordered Pt(111) (a)Benzenethiol on Pt(111) electrodes In situ scanning tunneling microscopy (STM) has been used to examine the spatial structures of arylthiols (benzenethiol, 2-naphthalenehtiol) on well-ordered Pt(111) electrodes in 0.1 M HClO4. The electrochemical potential dominated the coverages and the spatial structures of these organic adlayers. Ordered structures were observed only between 0.1 and 0.3 V. Depending on the dosage, two ordered structures were identified. Lower dosages (ca. 10 µM in concentration) of arylthiols resulted in a well ordered (2 × 2) structure with a coverage (θ) of 0.25, whereas higher dosages (ca. 100 µM) resulted in a (?3 × ?3)R30° adlattice, θ = 0.33. The degree of ordering of this benzenethiol adlayer deteriorated substantially once the potential was raised above 0.3 V. This structural change, irreversible with potential modulation, was due to further deposition of organic ad-molecules, preferentially at domain boundaries of ordered (?3 × ?3)R30° arrays. All molecular adlayers were completely disordered by 0.6 V. Since all organosulfur ad-molecules were arranged similarly to those of sulfur adatoms, it is likely that they were bonded to Pt(111) mainly through their sulfur headgroups. They were adsorbed so strongly on Pt(111) electrodes that none of them was reductively removed at a potential as negative as –1.0 V in 1 M KOH. (b)2-Naphthalenethiol on Pt(111) To see how the molecular structure of ad-molecules affects their adsorption configuration, we examined the adsorption of 2-naphthalene molecules on Pt(111). The dosage of this molecule dominated its coverage and spatial structures. Lower dosages of 2-Naphthalenethiol resulted in a disorder adlayer. High quality STM imaging reveals the naphthalene and sulfur headgroup of 2-naphthalenethiol molecules, suggesting that 2-naphthalenethiol was adsorbed parallel to the Pt(111) surface at the initial stage of adsorption. Patches of hexagonal arrays, identified as (?3 × ?3)R30°, was observed at more positive potentials, where more 2-naphthalene molecules were adsorbed. The thus-formed patches gradually grew with time to displace the disordered domains, and finally a well-ordered adlayer was produced. In other words, the adsorption of 2-naphthalenethiol proceeded in a disorder-to-order phase transition at higher dosage. 2-Naphthalenethiol ad-molecules could be adsorbed in a tilted configuration with their sulfur headgroup bound to the Pt(111) surface, as identified for benzenethiol on Pt(111). Since the intermolecular spacing of 4.8 Å is less than the van der Waals diameter of 2-nanphthalenethiol, it is likely that this molecule was adsorbed also tilted on Pt(111). Ⅱ. Organodithiols monolayers on Pt(111) electrodes We have employed in site scanning tunneling microscopy (STM) and cyclic voltammetry (CV) to study the structures of alkanedithiols (1,6-hexanethiol and 1,9-nonadithiol) aryldithiols (benzene-1,2-dithiol and benzene-1,3-dithiol) on well-ordered Pt(111) electrodes in 0.1 M HClO4. Self-assembled monolayers (SAMs) of organodithiols were adsorbed in ordered structures between 0.1 and 0.3 V. A well-ordered (2 ×2) structure predominated at 0.15 V , which rearranged into an ordered (?3 × ?3)R30° adlattice, as a result of a slight increase of coverage at more positive potentials. Only between 0.1 and 0.3 V did in situ STM reveal long range ordered adlattices of (2 ×2) and (?3 ×?3)R30°. Stepping potential positively to more than 0.5 V resulted in disordering of the adlayer, this phase transition was irreversible to the modulation of potential. All organodisulfur ad-molecules exhibited the same adsorption behavior. It is illustrated that the electrochemical potential played a decisive role in guiding the coverages and spatial structures of SAMs on Pt(111). Ⅲ. Isomers of Nitrophenol adsorbed on Pt(111) Organic monolayers of three isomers of nitrophenol and nitrobenzene (Extraction have been studies for the ortho-, meta- and para-Nitrophenol monolayers.) on well-ordered Pt(111) electrodes were examined with electrochemistry, in-situ STM and CV in 0.1 M HClO4. The chemical and physical characteristics of the interface are significantly and differently affected by the isomers comprising the SAMs. A pronounced reductive feature peaking at 0.08 V, attributed to reductive adsorption of nitrophenol and proton reduction, is observed. High-resolution STM images reveal that p-nitrophenol are arranged in well-ordered (5?3 × 4) and (2?7 × 2?21) adlattices at coverages (θ) of 0.3 and 0.41. For m-nitrophenol and o-nitrophenol, (?3 × 13), (θ = 0.77), and (6?3 × 2) structures (θ = 0.208) were identified. in comparison, nitrobenzene molecule was adsorbed in an order (?3 × 7) structure (θ = 0.214). It is evident that the real-space structures varied greatly with the molecular structures of ad-molecules. Since the intermolecular spacings between two neighboring molecules for all cases amounts to 4 or so, it is likely that all molecules were adsorbed in stand-up orientations. The decrease in the degree of freedom imposed by the surface controls the extent of interaction of the nitro group with the solution as well as with the adjacent molecules. It is illustrate ad-molecules were arranged similarly to those of nitro adatoms, the phenyl ring is tilted the surface normal in the monolayer. Ⅳ. Isomers of Fluorophenol adsorbed on Pt(111) To further explore the effect of molecular structure on the spatial arrangements of organic ad-molecules on Pt(111), we examined the spatial structures and binding configurations of the geometric isomers (m- and p-) of fluorophenol.While p-fluororphenol arranges in well-ordered (4?3 × 8) with a coverage (θ) of 0.39, m-fluorophenol was adsorbed in an ordered (3Ö3 × 7) structure (θ = 0.361). These two structures appeared as striped patterns with intermolecular spacings of 3.5 ~ 4 Å. To our knowledge, there has no report of fluoro group compounds adsorbed on Pt crystal electrodes in ambient. While similar nitrophenol adsorbed situation of SAMs at Pt surface. The structure of the adlayer is determined by substrate-adsorbate coordination and lateral π-stacking. The most characteristic difference to the p-fluorophenol stacking phase on Pt(111). We found that these kink positions propagate among parallel stacking rows of one domain. |
