Morphological Changes at a Silver Surface Resulting from Exposure to Hyperthermal Atomic Oxygen

Abstract

Bulk single crystals of Ag(100) and Ag(111) at a sample temperature of 220 °C were exposed to a beam containing hyperthermal atomic oxygen with a nominal translational energy of 5.2 eV. The resultant oxide scales and interface structures were characterized by cross-sectional (scanning) transmission electron microscopy ((S)TEM) and scanning electron microscopy (SEM). The oxide scales formed on Ag(100) and Ag(111) were more than 10 microns thick and contained numerous micropores, microchannels, grain boundaries, and defects (e.g., twins), which can provide pathways for the rapid transport of oxygen atoms through the thick oxide scales to the oxide/Ag interface. Energy dispersive X-ray spectroscopy (EDS) and nanoarea electron diffraction pattern (NA-EDP) investigations revealed that the oxide scales were predominately polycrystalline silver, containing only ∼3−5 atom % of oxygen, which is mainly in the form of Ag2O. Additional low-fluence exposures of thin foils of single-crystal Ag followed by analysis with TEM and NA-EDP identified crystalline grains of Ag, with dimensions of 500−800 nm, imbedded in a porous region of nanometer-sized Ag2O grains during the initial stages of surface transformation by hyperthermal atomic oxygen. A mechanism based on the rapid oxidation of Ag by atomic oxygen followed by thermal reduction of the Ag2O at 220 °C is proposed to explain the formation of the thick and unique polycrystalline silver “oxide” scales that were observed.