Structural transitions in 4,4′-bipyridine adlayers on Au(111)—an electrochemical and in-situ STM-study

The adsorption and phase formation of 4,4′-bipyridine (4,4′-BP) on the Au(111)–(1×1) ∣ aqueous electrolyte interface has been studied using capacitance measurements, cyclic voltammetry (CV), surface enhanced infrared reflection/absorption spectroscopy (SEIRAS) and in-situ scanning tunneling microscopy (STM). The organic molecules form, dependent on the electrode potential applied and in the absence of Faradaic reactions, three highly ordered monolayers. The high coverage adlayer I is composed of 4,4′-BP co-ordinated with one nitrogen atom to the underlying, positively charged electrode surface, arranged in interdigitated, densely packed molecular stacks and stabilized by co-adsorbed interfacial water molecules. The corresponding unit cell (a=1.15 nm, b=1.3 nm, α=74°) contains six molecules, which gives rise to a coverage Γ=6.9×10−10 mol cm−2. Excursions of the electrode potential towards more negative values triggers two first-order phase transitions, characterized by the loss of parallel stacking rows, accompanied by an orientational change of 4,4′-BP from perpendicular towards an inclined position and a rearrangement of co-adsorbed interfacial water molecules. The adlayer coverage decreases from 6.8×10−10 mol cm−2 (phase I) via 5.7×10−10 mol cm−2 (phase II, striped structure) to 3.4×10−10 mol cm−2 (phase III, rhombohedral structure). The vectors which determine the unit cells of the adlayers I and III are basically identical. II and III exhibit strict registry with the substrate surface geometry, the substrate–adsorbate corrugation potential in phase I appears to be somewhat smaller. The unperturbed imaging of the steady-state 4,4′-BP adlayers II and III was possible only with tunneling currents lower then 30 pA, often down to 6 pA. The STM contrast patterns of all three steady-state structures depend significantly on the applied tunneling conditions indicating that the electron transfer between tip and substrate is most probably mediated by the conjugated π-system of the molecule, e.g. ‘through bond’ tunneling occurs. On the other hand, current distance curves yielded barrier heights of 1.00±0.05 eV, rather independent of the type of adlayer. Chronoamperometric potential step- and dynamic in-situ STM—experiments revealed that the first order phase transitions I→II, II→III and III→II proceed preferentially by two-dimensional (hole) nucleation with surface-diffusion controlled growth processes. These phase transitions are quasi-reversible provided that all perturbations are limited to the potential range of an ideal polarizable organic adlayer.