Molecular ordering at electrified interfaces: Template and potential effects

A combination of cyclic voltammetry and in situ scanning tunneling microscopy was employed to examine the adsorption and phase transition of 1,1'-dibenzyl-4,4'-bipyridinium molecules (abbreviated as DBV(2+)) on a chloride-modified Cu(111) electrode surface. The cyclic voltammogram (CV) of the Cu(111) electrode exposed to a mixture of 10 mM HCl and 0.1 mM DBVCl2 shows three distinguishable pairs of current waves P1/P'1, P2/P'2, and P3/P'3 which are assigned to two reversible electron transfer steps, representing the reduction of the dicationic DBV(2+) to the corresponding radical monocationic DBV(+•) (P1/P'1) and then to the uncharged DBV(0) (P3/P'3) species, respectively, as well as the chloride desorption/readsorption processes (P2/P'2). At positive potentials (i.e., above P1) the DBV(2+) molecules spontaneously adsorb and form a highly ordered phase on the c(p × √3)-precovered Cl/Cu(111) electrode surface. A key element of this DBV(2+) adlayer is an assembly of two individual DBV(2+) species which, lined up, forms a so-called "herring-bone" structure. Upon lowering the electrode potential the first electron transfer step (at P1) causes a phase transition from the DBV(2+)-related herring-bone phase to the so-called "alternating stripe" pattern built up by the DBV(+•) species following a nucleation and growth mechanism. Comparison of both observed structures with those found earlier at different electrode potentials on a c(2 × 2)Cl-precovered Cu(100) electrode surface enables a clear assessment of the relative importance of adsorbate-substrate and adsorbate-adsorbate interactions, i.e., template vs self-assembly effects, in the structure formation process of DBV cations on these modified Cu electrode surfaces.