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Repeated oxidation and reduction of a platinum electrode, which can be compared to the switching ON and OFF of a device (e.g. fuel cell, electrolyzer,…), leads to the roughening of the surface, caused by the nucleation and growth of nano-islands, which first grow only laterally (2D) and then exclusively in height (3D). Surprisingly, this particular nucleation and growth combines several remarkable aspects that all deviate from standard text book- and literature-knowledge. The basic mechanism is rooted in the larger lattice constant of platinum-oxide such that the growth is promoted by the creation of adatoms and vacancies, implying that not only growth processes but also etching processes have to be considered. It is remarkable that the nano-islands arrange in an almost perfect, hexagonal pattern, which clearly deviates from standard random growth. Surprisingly, many other chemical surface systems develop highly ordered nano-islands during cycles of surface reaction and restoration. In the case of platinum the nano-islands growth eventually enhances the dissolution, which makes electrochemical energy applications, like in fuel cells and electrolysers, economically less feasible, as it is scarce and expensive. Preventing nucleation would be most effective for preventing roughening. However, little is known about the atomic details regarding the nucleation; a process almost impossible to observe. I will show that platinum exhibits four different (pre-)oxidation stages that are responsible for the mysterious observations, all leading to very peculiar atomic changes and surface structures, before real bulk oxidation occurs. First the adatoms of the surface-background adatom gas are oxidized, but the equilibrium adatom pressure shoots up simultaneously such that nucleation does not occur during the oxidation, where the atoms are created and pushed on top of the surface. Surprisingly, they nucleate during the reduction. Secondly, analysing the island distance-distribution, we derive a rarely observed non-random nucleation, which can only occur if there is a repulsive interaction between the depositing atoms or the existence of preferential nucleation sites. Fundamentally interesting, as never observed before, we do show that special, preferential nucleation sites that a priori do not exist, develop initially via a precursor and eventually form a semi-ordered Pt-oxide structure: the famous spoke wheels, which are assembled from Pt atoms that exchanged their places with oxygen atoms going subsurface. At low coverages the system is fully reversible and restores upon reduction. We can model the reversible place exchange coverage with a Frumkin isotherm combined with an Arrhenius diffusion term. At high coverages irreversible place exchange atoms are created that are pushed onto the Pt(111) surface, leading eventually to the ordered nano-islands. Interestingly, the pre-oxidation stages are expected to occur also on many other surfaces during oxidation and nano-islands growth has been reported before also on Cu, Ag, and Au. We expect similar precursors and pre-stages to form also during nitride and sulfide formation.

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Dr. Marcel Rost, Universität Leiden, NL
Seminar Room of the Institute of Physics II
Contact: Thomas Michely, Matteo Cacco