AN IN-SITU SCANNING-TUNNELING-MICROSCOPY STUDY OF ELECTROCHEMICALLY INDUCED HEX -][-(1X1) TRANSITIONS ON AU(100) ELECTRODES

The ''hex'' <-- --> (1 x 1) transitions on Au(100) electrodes immersed into sulfuric acid solutions were investigated from an atomic up to a micrometer scale by in-situ scanning tunneling microscopy. Directly a fter immersion freshly flame annealed Au(100) crystals are completely covered by a well-ordered ''hex'' reconstruction, comparable to that o bserved in UHV. The dependence of this initial surface topography on t he sample pretreatment is demonstrated. Domains of parallel running mo dulation rows, which are the main feature of the reconstructed phase, can have spatial extensions of several micrometers. The potential-indu ced lifting of the reconstruction starts at the termination of these r ows at step edges and domain boundaries. It proceeds by a quasi-one-di mensional growth along the rows, the speed of which is a function of p otential. Due to the approximately 25% higher density of the reconstru cted surface layer, Au atoms are expelled to the surface during the tr ansition. They form monoatomic islands of isotropic shape, which grow by a ripening process. The reverse (1 x 1) --> ''hex'' transition proc eeds by nucleation and growth of reconstruction domains originating at step edges. A detailed analysis of the mechanisms and interactions of both transitions reveals the importance of surface defects in the kin etic behavior of the phase transition.