The organization of pyridazine adsorbed on Au(1 1 1) electrode in sulfuric and perchloric acids – As probed by in situ STM

The adsorption of pyridazine (PD) on a gold electrode served as a model to study the orientation and arrangement of an organic adsorbate bearing a heterocyclic molecular structure at an electrified interface. Because the two N-ends of PD could interact with a Au electrode, the acid-base equilibrium of PD at an electrified interface can be different from that in a solution phase. Unprotonated and protonated PD interacted with the Au electrode differently, leading to dissimilar spatial structures on the Au(1 1 1) electrode. Moreover, having a large dipole moment of 4.22 D, PD adopted different adsorption configurations, as the Au potential was modulated. The indispensable anion present in the supporting electrolyte can compete with PD for surface sites on the Au electrode. Since the potential of a charged conductor is established by the physical contact with an electrolyte, the study of the adsorption of PD on a Au(1 1 1) electrode calls for the use of an in situ tool, such as a scanning tunneling microscope (STM), which provides sub-nanometer information of the interface in real-time and real-space. Different phases of adsorbed PD molecules on an ordered Au(1 1 1) electrode were revealed as a function of potential, pH and anion. The most notable event for PD adsorbed on Au(1 1 1) is attributed to the sharp transition from a disordered state to highly organized structures in 0.1 M H₂SO₄ and HClO₄. The obtained STM images enabled a direct measurement of the spacing between PD admolecules, from which the molecular reorientations on the Au electrode were inferred. PD admolecules flipped from the horizontal to the upright configuration, tethered to Au substrate via the two N-ends, at positive potentials. Different ordered PD structures seen in H₂SO₄ and HClO₄ imply anions were coadsorbed with PD molecule on the Au electrode. The acid-base equilibrium of PD at the Au electrode was examined with STM in pH 1 and 3 media.