Alda Khairunnisa, Weichen Liao, Shuehlin Yau
Despite many efforts studying polypyrrole (PPy) with scanning probe techniques in the past several decades, its molecular structure and conformation have been elusive. This study presents the first atomic and molecular-resolution scanning tunneling microscopy (STM) images of pyrrole (Py) and PPy electrochemically produced on bare and iodine-modified Au(100) electrodes in pH 1 and 4 sulfate media. First, STM reveals a highly ordered Au(100) - (42 x 42) R45° - Py between 0.15 and 0.5 V (vs saturated calomel electrode) in 0.1 M H2SO4 containing 30 mM Py. By contrast, a disordered Py adlayer is seen in 0.1 M HClO4 medium, indicating the coadsorption of Py and an acid. This anion effect is manifested in voltammetric results obtained with Au(100), showing a pair of sharp peaks at 0.12 V in 0.1 M H2SO4 + 30 mM Py and a featureless profile seen with Au(111) or Au(100) in HClO4. Py and bisulfate (and perchloric acid) attract each other and are coadsorbed on Au(100) electrodes. The favorable adsorption of bisulfate anions on Au(100) benefits the adsorption of Py. Raising the potential to 0.6 V causes irreversible oxidation of Py, yielding a dark deposit on the Au(100) electrodes. The IR results indicate that this deposit is PPy. The as-prepared PPy film yields a pair of reversible redox peak at -0.4 V in pH4 K2SO4. Molecular-resolution STM imaging at 0.4 V in 0.1 M H2SO4 reveals the oxidized PPy molecules, which assume loop, circle, and wigging conformations. The main molecular features observed by STM can be explained by structural models, based on α-α coupling and syn-conformation of Py rings; however, the α-β coupling of Py is also possible. Linear segments 2-3 nm long have anti-conformation of Py. The PPy molecules are not stable at E < -0.1 V and >0.7 V, possibly due to degradation and overoxidation. (Figure Presented). © 2016 American Chemical Society.
Department of Chemistry, National Central University, 300 Jhongda Road, Jhongli District, Taoyuan City, 320, Taiwan; Department of Chemistry, Faculty of Mathematics and Science, Brawijaya University, East Java, 65145, Indonesia
