STM observation of the mobility of Ag(100) surfaces and polycrystalline Pd surfaces: Atmospheric and aqueous conditions

Abstract

The mobility of chemically polished Ag(100) surfaces and polycrystalline Pd surfaces has been investigated by means of scanning tunnelling microscopy in the constant current mode under atmospheric conditions, as well as when the surfaces are immersed in nanopure H 2O. We have found experimental conditions for which tip—sample interactions are at a sufficiently low level that the intrinsic surface mobility can be studied. Under atmospheric circumstances the Pd surface was immobile, whereas the Ag surface was mobile. In nanopure H 2O both surfaces were mobile. We conclude that the local interface structure plays an important role in determining the observed mobility. Our observations lead us to the following, general lines of interpretation. Atomic motion along the crystal surface (which we shall refer to as surface diffusion) seems to be the most important process causing a significant surface mobility under vacuum conditions, both for Ag and Pd. Under atmospheric conditions this mobility may be reduced by chemisorption. We expect that both Ag and Pd surfaces are covered by, eg oxide layers and possibly some physisorbed H 2O. Since PdO is more rigid than Ag 2O we expect that the atmospheric Pd surface is less mobile than the atmospheric Ag surface. Immersed in nanopure water, exchange processes become important. We suggest that the electrochemical conditions are such that adsorption and desorption of the surface oxides via local redox reactions makes the surface oxide layer mobile or even removes it to a large extent. Then a dynamic equilibrium between the Ag (Pd) sample and Ag + (Pd 2+) ions in the electrolyte may be approached. The fluctuations around this equilibrium and the approach to it seem to be the most important processes causing the observed surface mobility and global flattening in nanopure H 2O.