| dc.description.abstract | 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. | en_US |