Surface Impacts Of Nanoscale Oscillations

Oscillating reactions operating at the nanometer scale are guided by irregular surface structural features rather than by diffusion, as found for solution systems such as the classic Belousov-Zhabotinsky oscillating reaction, reports Jean-Sabin McEwen, Pierre Gaspard, Thierry Visart de Bocarmé, and Norbert Kruse of the Free University of Brussels (ULB), in Belgium (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0811941106). The researchers used field ion microscopy and theoretical modeling to examine the reaction of hydrogen and oxygen to produce water at a rhodium field-emitter tip. They found that the oscillation cycle starts when O₂ dissociatively adsorbs onto rhodium, first populating both the surface and subsurface area at the apex of the tip. The oxide layers then expand into a crosslike pattern that depends on the crystalline nanostructures of the rhodium surface. As subsurface oxygen builds up, it hinders further adsorption of O₂ on the surface, creating thermodynamic instability. After about 30 seconds, the instability leads to the sudden reduction of the oxide from the outer edges of the tip back toward the tip's apex, concurrent with release of the water product. The cycle then starts anew with oxygen adsorption.