Structures of Aniline and Polyaniline Molecules Adsorbed on Au(111) Electrode: as Probed by in Situ STM, 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) were used to examine the electrified interface of Au(111) in 0.1 M H2SO4 containing 0.030 M aniline. In agreement with cyclic voltammogram (CV), which revealed two pairs of peaks at 0.48 and 0.62 V, in situ STM imaging yielded two highly ordered aniline adlattices, (root 19 x 5) at 0.55 V and (3 x 2 root 3)rect at 0.85 V [vs reversible hydrogen electrode, RHE]. According to XPS results obtained with Au(111) emersed at 0.85 V from 0.1 MH2SO4 + 0.030 M aniline, bisulfate anions were coadsorbed in an amount equal to that of aniline. The (3 x 2 root 3)rect-aniline structure was examined carefully by STM using different imaging conditions. Results revealed that imaging with a tunneling current of 10 nA at a -300 mV bias voltage allowed molecular resolution of both aniline admolecules and bisulfate anions. These species could form acid-base pairs and mingled uniformly on the Au(111) electrode. NEXAFS results were also obtained at 0.85 V, showing that the phenyl rings of aniline admolecules on average was tilted away from the Au(I 11) substrate by 47 degrees. At E > 0.95 V, aniline molecules were oxidized to cation radicals, which initiated intermolecular coupling between aniline molecules to form polyaniline (PAN). The as-formed PAN assuming the form of emeraldine salt exhibited distinct linear conformations, which is proposed to derive from a unique head-to-tail arrangement of aniline monomers in the (3 x 2 root 3)rect structure. The coadsorbed bisulfate anions played an important role in the production of surface-bound PAN emeraldine salts, whose high conductivities facilitated molecular resolution STM imaging up to a thickness of four PAN layers.