STRUCTURAL TRANSITIONS IN URACIL ADLAYERS ON GOLD SINGLE-CRYSTAL ELECTRODES

The formation of uracil adlayers on Au(111), Au(100) and Au(110) in aqueous solutions has been studied by current-potential, capacitance-potential and current-time measurements. Besides adsorption of uracil on gold at low coverages and negative potentials, the formation of two distinctly different organized uracil layers was found. The first one at medium coverages has been observed only for the densely-packed surfaces of Au(111) and reconstructed Au(100) and it corresponds to a two-dimensional condensed physisorbed film. The second type of adlayer is formed at very positive potentials on all gold surfaces, irrespective of their crystallographic orientation. This layer, which has been imaged by in-situ STM, has a hcp structure with a next-neighbour distance of 0.4 nm and is assigned to chemisorbed uracil. The kinetics of the structural transitions within the uracil adlayer has been studied by current transients and analyzed by standard nucleation-and-growth models which were modified to account for the influence of surface defects.