Structural transitions in 2,2′-bipyridine adlayers on Au(111) -: an in-situ STM study

The adsorption and phase formation of 2,2'-bipyridine on the Au(lll) \ aqueous electrolyte interface has been studied using ac-voltammetry and in-situ scanning tunneling microscopy. At positive charge densities 2,2'-bipyridine forms in neutral and acidic solutions an ordered monolayer of 'molecular chains' (phase I) determined by directional substrate-adsorbate coordination and lateral pi-stacking. The distance between adjacent parallel rows is 9.6 +/- 0.5 Angstrom. Molecules are tilted from the normal to the chain axis by an angle of 28 +/- 2 degrees. The rotation angle gamma = 30 degrees between the stacking rows and the [11 (2) over bar]-direction of the former Au(lll)-(p x root 3) reconstruction lines was obtained, and a commensurate (4 x 2 root 3) unit cell is proposed. A second ordered 'stacking phase' of 2,2'-bipyridine, which is incommensurate with the underlying substrate surface, was found at negative charge densities in acidic solutions (phase V). Nearly defect-free molecular rows decorate the reconstruction lines of Au(lll)-(p x root 3) at an angle of 86 degrees; each molecule is tilted from the chain axis by either + 23 or - 16 degrees. Identical tilt angles repeat every other row. The dynamics of film formation and dissolution as well as the substrate stability were investigated by in-situ STM as a function of electrode potential and temperature. It was found that both formation and dissolution of phase I are governed by nucleation and growth processes. Several metastable adlayer structures were found during these transitions. Their appearance and characteristic properties depend strongly on the applied substrate potential perturbation, accompanied by structural changes of the electrode surface. The 2,2'-bipyridine phase I stabilizes the Au(lll)-(p x root 3) reconstruction only in neutral solutions, but inhibits, in general, the onset of gold oxidation at more positive potentials. The 'break-off' of the adlayer starts with a broadening of domain boundaries, the formation of channels, holes and smaller adlayer patches until the organic phase can no longer be resolved at E>1.25 V. Remarkably, the entire process is fairly reversible. (C) 1998 Elsevier Science S.A. All rights reserved.