| dc.description.abstract | Abstract
In situ scanning tunneling microscopy (STM) has been used to examine the adsorption of sulfur-oxygen compounds (ex. sodium sulfite (Na2SO3) and sodium dithionite (Na2S2O4)) on well-ordered Au(111), Au(100) and Pt(111) electrodes in 0.1 M HClO4 under potential control. High-quality STM atomic resolution results in three adlattices on Au(111) immersed in acidic Na2SO3. These structures are presumably due to SO2, which represents the most prominent chemical species in the mixed solutions of HClO4 and Na2SO3. A well-packed (5 ? 2?39)R16.1? predominates at 0.2 V, which rearranges into a (?13 ? ?21)R46.1? adlattice as a result of a slight increase of coverage at 0.4 V. Finally, at 0.7 V clusters of sulfur atoms coadsorbed with water molecules to form a highly ordered adlattice of (5 ? 3?7)R19.1?.
In situ STM was also used to probe the structures of SO2 on Au(100) in 0.1 M HClO4. With a concentration of 1 mM or higher, SO2 are adsorbed in (?2 ? ?2)R45?, ? ? 0.25, whereas at potential negative of
0.6 V lower concentration results in (?13 ? 5?2)R45?, ? ? 0.16 and (2?5 ? 2?5)R26.6?, ? ? 0.22, depending on the potential of Au(100). At more positive potential the coverage of SO2 ad-molecules increases to produce a more compact (2 ? 2), ? ? 0.25 structure. In the case of Pt(111), SO2 are spontaneously reduced to sulfur adatoms arranging in (?3 ? ?3)R30? at 0.2 V.
The adsorption of Na2S2O4 on Au(111) and Pt(111) were also investigated by in situ STM and cyclic voltammetry (CV). In strong contrast to the formation of multiple adlattices for SO2, only one structure, (?3 ? ?3)R30?, is identified at Au(111) and Pt(111) by STM. Tentatively, according to the results from cyclic voltammetry, this structure, at least in the case of Pt(111), is due to the adsorption of sulfur adatoms, produced from the irreversible reduction of SO2 upon its contact with Pt electrodes. However, in the case of Au(111), no reduction feature is noted at potential negative of 0.3 V, indicating that S2O42- anions are adsorbed intact. Due to the close spacing (5.1 Å) of two neighboring S2O42- anions in the structure, they are likely to adsorbed through a sulfur atom.
We employed in situ STM and CV to study the adsorption of poly-3-alkyithiophene derivatives (3TOESNa and P3TOESH) onto a well-ordered Pt(111) electrode in 0.1 M HClO4. The STM shows the atomic image of Pt(111) - (2 ? 2)-poly-3-alkyithiophene derivatives structure. Two nearest protrusions are separated by 0.55 nm. The atomic rows are parallel to the close-packed directions of Pt(111).
In situ STM was also applied to study the electropolymerization of aniline on well-defined Pt(111) electrodes in 0.1 M HClO4. The two reversible peaks emerging at 0.5 and 0.7 V indicate the success of electropolymerization of aniline in the acidic media. High-resolution STM imaging of polyaniline on Pt(111) reveals the formation of electronically conducting organic phases with an average corrugation height of 6 nm. Stable STM imaging was possible for the first hour at 0.7 V in 1 mM aniline, whereas tip-and-substrate contact becomes so severe that it was impossible to have a clear view of the samples as the films get thicker. | en_US |