| dc.description.abstract | Two separate subjects are addressed in this thesis. Part I describes cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) results obtained with cysteine betaine adsorbed on a Au(111) electrode as a function of potential, anion and pH. In 0.1M perchloric acid, cysteine betaine was adsorbed mainly in stripe structures designated as(√21×√37) and((8×8) with a coverage (Ɵ) of 0.093 between 0.4 to 0.7V (vs. reversible hydrogen electrode). At 0.75V, these stripe structure transformed into a honeycomb-like structure characterized as (6×6), Ɵ = 0.083. Cysteine betaine molecule was oxidized at E > 0.8V. In 0.1M sulfuric acid, a structure(4×5), Ɵ = 0.2 consisted of betaine between 0.3 to 0.5V, but bisulfate anions competed with cysteine for adsorption on the Au(111) electrode, resulting in a pronounced order-to-disorder phase transition at 0.7V. This transition was quasi-reversible, as the stripe structure appeared again, once the potential was made to 0.3V. The main stripe structure became(√7×7) with a coverage of 0.19 at 0.3V, whose coverage is 5% lower than that of (4×5). At 0.6V, the stripe structure turned into two vertical structures(4√3×√21)and(√43×√21)with a coverage of 0.055 and 0.059 among a mainly disordered adlayer. To both perchloric acid and sulfuric acid, adding cysteine betaine 0.3μM resulted in stripe structures, (4×5) and (√7×7), between 0.3 to 0.5V. (8×5√3) and (4×3√3) structures (Ɵ = 0.0375 and 0.167) were found in sulfuric acid, which contrast (√21×12) and (4×4√3), (Ɵ = 0.133 and 0.125) in perchloric acid. Being a zwitterion ion, betaine is either positively charged or neutral when pH is lower or greater than 4.36. In pH3, 7, 10.9, and 12.67 media, in situ STM revealed (4×5),(√19×3√3)and(√7×7) and structures under proper potential control.
Part II addresses the decomposition of polyaniline (PANI) at positive potential and the conductivity of PANI using STM. Regardless of monolayer or multilayer PANI, CV results show decomposition between 0.9 to 1.4V by CV. PANI chains shortened at 0.92V and vastly degraded at 0.98V. The reversibility of PANI’s conformation between straight and crooked forms in sulfuric acid were examined in nitric acid, sulfuric acid, and perchloric acid. The conversion from crooked to straight form occurred at 0.88V in sulfuric acid, as opposed to 0.8V seen in nitric acid or perchloric acid. The conductivity of different conformation PANI measured by STM. Results show that poor correlation between conductivity and anions and variation with potential, in particular, conductivity decreased with more positive potential.
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