IN SITU SCANNING TUNNELING MICROSCOPY TOPOGRAPHY CHANGES OF GOLD (111) IN AQUEOUS SULFURIC ACID PRODUCED BY ELECTROCHEMICAL SURFACE OXIDATION AND REDUCTION AND RELAXATION PHENOMENA

Abstract

The electrochemical formation and reduction of O -layers on gold (111) films in 1 m sulfuric acid under different potentiodynamic routines are investigated utilizing in situ scanning tunneling microscopy. The surface dynamics is interpreted considering the anodic and cathodic reaction pathways recently proposed complemented with concurrent relaxation phenomena occurring after gold (111) lattice mild disruption (one gold atom deep) and moderate disruption (several atoms deep). The dynamics of both oxidized and reduced gold topographies depends on the potentiodynamic routine utilized to form OH / O surface species. The topography resulting from a mild oxidative disruption is dominated by quasi-2D holes and hillocks of the order of 5 nm, involving about 500–600 gold atoms each, and their coalescence. A cooperative turnover process at the O -layer, in which the anion ad-layer and interfacial water play a key role, determines the oxidized surface topography. The reduction of these O -layers results in gold clusters, their features depending on the applied potential routine. A moderate oxidative disruption produces a surface topography of hillocks and holes several gold atoms high and deep, respectively. The subsequent reduction leads to a spinodal gold pattern. Concurrent coalescence appears to be the result of an Ostwald ripening that involves the surface diffusion of both gold atoms and clusters. These processes produce an increase in surface roughness and an incipient gold faceting. The dynamics of different topographies can be qualitatively explained employing the arguments from colloidal science theory. For 1.1 V ≤ E ≅ E pzc weak electrostatic repulsions favor gold atom/cluster coalescence, whereas for E < E pzc the attenuated electrostatic repulsions among gold surfaces stabilize small clusters over the substrate producing string-like patterns.