Quasi-reversible chloride adsorption/desorption through a polycationic organic film on Cu(100)

Combined cyclic voltammetry and in situ scanning tunneling microscopy studies were employed to gain information about the interfacial structure of a chloride modified Cu(100) electrode surface exposed to an acidic electrolyte solution that contained redox-active dibenzylviologens (DBV, 1,1′-dibenzyl-4,4′-bipyridinium molecules). A particular focus of this contribution lies in the structural characterization of the electrode surface under nonequilibrium reactive conditions, for example, during the occurrence of an electron-transfer reaction. Typically, two pairs of clearly distinguishable current waves denoted as Pl/Pl′ and P2/P2′ appear in the cyclic voltammogram of Cu(100) in a mixture of 10 mM HCl and 0.1 mM DBVCl₂, provided the cathodic potential limit remains restricted to values of E_work > -425 mV vs reversible hydrogen electrode. Systematic variations of the DBV solution concentration and the nature of the counterion strongly suggest that Pl has to be assigned to the first electron-transfer reaction reducing the dicationic DBV²⁺ to the radical monocationic DBV⁺ species while Pl′ represents the corresponding oxidation process. Not only solution but also preadsorbed viologen species are involved in this charge-transfer reaction. Triggered by the electron transfer, the more open DBV_ads²⁺ "cavitand" structure formed on top of the preadsorbed c(2 × 2)-Cl layer prior to the electron transfer transforms into a more compact polymeric (DBV_ads⁺)_n stacking phase upon reaching P1 Both the reactants and products of the electron-transfer reaction form condensed and laterally ordered 2D phases. In particular, the quite stable (DBV_ads⁺)_n stacking phase maintains its structural integrity during the ongoing electron-transfer reaction involving solution species. Passing P2 in the cyclic voltammogram, however, initiates an order-disorder transition within the organic film with defect lines or point defects in the (DBV_ads⁺)_n stacking phase acting as active sites for this structural transition. The driving force for this further phase transition is the starting chloride desorption through the (DBV_ads⁺)_n film. In the presence of the covering viologen film, the chloride desorption occurs at a potential that is ΔE_desorp ≈ 100 mV lower than that in the pure supporting electrolyte pointing to a significant additional activation barrier for that process. Reduced monomeric and oligomeric viologen species reveal a significantly lower lateral mobility on the. metallic substrate than that on the chloride lattice. In the reverse potential sweep, chloride anions are forced to readsorb on the metallic copper substrate through the disordered viologen film resulting in a full restoration of the c(2 × 2)-Cl lattice in direct contact to the metallic copper and, in addition, in the full restoration of the ordered (DBV_ads⁺)_n stacking phase on top of the chloride lattice.