Conformations of Polyaniline Molecules Adsorbed on Au(111) Probed by in Situ STM and ex Situ XPS and NEXAFS

In situ scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) have been used to examine the conformation of a monolayer of polyaniline (PAN) molecules produced on a Au(1 11) single-crystal electrode by anodization at 1.0 V [vs reversible hydrogen electrode (RHE)] in 0.10 M H2SO4 containing 0.030 M aniline. The as-produced PAN molecules took on a well-defined linear conformation stretching for 500 angstrom or more, as shown by in situ and ex situ STM. The XPS and NEXAFS results indicated that the linear PAN seen at 1.0 V assumed the form of an emeraldine salt made of PAN chains and (bi)sulfate anions. Shifting the potential from 1.0 to 0.7 V altered the shape of the PAN molecules from straight to crooked, which was ascribed to restructuring of the Au(111) electrified interface on the basis of voltammetric and XPS results. In situ STM showed that further decreasing the potential to 0.5 V transformed the crooked PAN threads into a mostly linear form again, with preferential alignment and formation of some locally ordered structures. PAN molecules could be reduced from emeraldine to leucoemeraldine as the potential was decreased to 0.2 V or less. In situ STM showed that the fully reduced PAN molecules were straight but mysteriously shortened to similar to 50 angstrom in length. The conformation of PAN did not recuperate when the potential was shifted positively to 1.0 V.