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 H(2)SO(4) 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 MH(2)SO(4) + 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

3-Mercapto-1-propanesulfonic acid (MPS) and bis(3-sulfopropyl) disulfide (SPS) adsorbed on a Au(111) electrode were studied by using in situ scanning tunneling microscopy (STM). Although the adsorptions of MPS and SPS are known to be oxidative and reductive, respectively, on an Au(111) electrode, these two admolecules behave similarly in terms of phase evolution, surface coverage, potential for stripping, and characteristics of cyclic voltammetry. However, different adsorption mechanisms of these molecules result in different structures. Raising electrode potential causes more MPS and SPS molecules to adsorb, yielding ordered adlattices between 0.67 and 0.8 V (vs reversible hydrogen electrode). The ordered adlattices of MPS and SPS appear as striped and netlike structures with molecules adsorbed parallel to the Au(111) surface. Switching potential to 0.9 V or more positive still does not result in upright molecular orientation, possibly inhibited by electrostatic interaction between the end group of -SO(3)(-) and the Au(111) electrode. Lowering the potential to 0.4 V disrupted the ordered adlayer. Stripping voltammetric experiments show that MPS and SPS admolecules are desorbed from Au(111) at the same potential, suggesting that these molecules are both adsorbed via their sulfur headgroups. The S-S bond in SPS is likely broken upon its adsorption on Au(111).