A Comprehensive Study of Adsorption and Polymerization of Aniline and its Derivatives on Au(111) by In Situ Electrochemical STM

• Main Authors: Sih-zih Chen, 陳思孜
• Other Authors: Shueh-lin Yau
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/83649693223070608295

博士 === 國立中央大學 === 化學學系 === 102 === To unravel effects of anion on the molecular structure of polyaniline (PAN), the adsorption of aniline and its subsequent oxidative polymerization on Au(111) electrode were examined by cyclic voltammetry (CV), chronoamperometry and in situ scanning tunneling microscopy (STM) in 0.5 M nitric and hydrochloric acids, respectively. Aniline molecules were coadsorbed with nitrate ions in a highly ordered (3 × 2√3)rect structure between 0.5 and 0.8 V (vs. reversible hydrogen electrode). Raising the potential to 0.85 V forced rearrangement of the (3 × 2√3)rect structure into a hitherto unidentified (3 × 2√21) structure, yielding one-dimension PAN band aligned in the <110> directions of the Au(111) substrate at 0.92 V. PAN film then grew to form a uniform film up to two layers in nitric acid. Oppositely, no ordered molecular adlattice of aniline was noted at the onset potential (~0.8 V) for polymerization in hydrochloric acid. This lack of ordered aniline structure however did not affect coupling of aniline molecules into well-defined linear PAN molecules as the potential was raised to > 0.9 V. Furthermore, the effects of potential on the PAN’s conformation produced on Au(111) electrode in nitric, sulfuric and perchloric acids were also studied by in situ STM. As the potential was modulated between 0.8 and 0.6 V, PAN molecules changed their oxidation states, which manifested in dramatic changes in the molecular conformations between linear and winding conformations. This potential - driven process was fast and reversible in nitric acid, but was largely irreversible in sulfuric acid. Real-time STM imaging also reveals that linear PAN became crooked upon exposure to methanol, ethanol and propanol, which resulted in formation of hydrogen bonds and twists of PAN molecules. Since diffusion fluxes of these molecules calculated from Fick’s law exceeded that determined from STM results, orientation of alcohol molecules impinging on the electrode could be important in making alcohol-PAN adducts. PAN molecules could decompose by interacting with alcohol molecules. The bulky n-propanol was the least reactive molecule among the alcohols studied here. In addition to aniline, CV and in situ STM were also used to examine the adsorption and electropolymerization of 3-methylaniline (3-MA) and metanilic acid (MA) on the Au(111) electrode in 0.5 M H2SO4, respectively. 3-MA admolecules were adsorbed in a (5 × 2√3)rect structure (θ = 0.2) at 0.5 V, but rearranged into two less compact adlattices, (5 × 2√3)rect (θ = 0.10) and (3√3 × 2√3)rect structures (θ = 0.11) at 0.8 V. Raising the potential to 0.9 V resulted in oxidation and polymerization of 3-MA. The poly(3-MA) molecules produced in the early stage assumed linear conformation, but became predominantly crooked upon the increase of overpotential. MA molecules also were adsorbed in highly ordered (√19 × √31)rect and (2√7 × √31)rect structures at 0.5 and 0.8 V. These adlattices however were displaced by bisulfate anions at E > 1.0 V. Furthermore, MA and aniline molecules could be coadsorbed in a highly ordered (4 × 2√3)rect structure at 0.8 V in 0.5 M H2SO4+ 30 mM aniline + 3 mM MA, which led to anisotropic oxidative polymerization in the <121> directions of the Au(111) electrode. However, x-ray photoelectron spectroscopy (XPS) results show that the as-produced linear polymeric chains were mainly PAN, rather than copolymer of aniline and MA. Finally, molecular structures of PAN as a function of potential were investigated by surface-enhanced infrared absorption spectroscopy (SEIRAS). Results obtained show that aniline molecules were adsorbed in flat and upright orientations at negative and positive potentials, respectively. Bisulfate ions were coadsorbed with aniline molecules on the gold electrode as a result of the need to compensate for the charge. SEIRAS shows that PAN molecules were fully reduced at < 0.3 V, and oxidized to emeraldine (0.3 ~ 0.6 V) and nigraniline at E > 0.6 V. These species were characterized by comparing intensity of IR bands due to benzoid and quinoid species in the backbone of PAN molecules. The fully oxidized PAN or pernigraniline was produced first at 0.8 V. IR bands due to ring structures in doped PAN molecules and bisulfate ions were most pronounced at 0.5 V, supporting the chemical form of PAN as emeraldine salt.