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 after immersion freshly flame annealed Au(100) crystals are completely covered by a well-ordered ''hex'' reconstruction, comparable to that observed in UHV. The dependence of this initial surface topography on the sample pretreatment is demonstrated. Domains of parallel running modulation rows, which are the main feature of the reconstructed phase, can have spatial extensions of several micrometers. The potential-induced lifting of the reconstruction starts at the termination of these rows at step edges and domain boundaries. It proceeds by a quasi-one-dimensional growth along the rows, the speed of which is a function of potential. Due to the approximately 25% higher density of the reconstructed surface layer, Au atoms are expelled to the surface during the transition. They form monoatomic islands of isotropic shape, which grow by a ripening process. The reverse (1 x 1) --> ''hex'' transition proceeds 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 kinetic behavior of the phase transition.