STRUCTURAL TRANSITIONS IN URACIL ADLAYERS ON GOLD SINGLE-CRYSTAL ELECTRODES

The formation of uracil adlayers on Au(111), Au(100) and Au(110) in aq ueous solutions has been studied by current-potential, capacitance-pot ential and current-time measurements. Besides adsorption of uracil on gold at low coverages and negative potentials, the formation of two di stinctly different organized uracil layers was found. The first one at medium coverages has been observed only for the densely-packed surfac es of Au(111) and reconstructed Au(100) and it corresponds to a two-di mensional condensed physisorbed film. The second type of adlayer is fo rmed 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 str uctural transitions within the uracil adlayer has been studied by curr ent transients and analyzed by standard nucleation-and-growth models w hich were modified to account for the influence of surface defects.