Formation of Nanosized Metallic Ag Grains by Oxidation of Ag Single Crystals with Hyperthermal Atomic Oxygen

摘要

Silver single crystals (Ag(100), Ag(111)) were exposed to 5eV hyperthermal atomic oxygen, created by a laser detonation of molecular oxygen at a substrate temperature of 220℃ for 7 hours. Oxide scales of more than ten microns formed on both Ag (100) and Ag (111) substrates. The microstructural investigation of the oxide layers by high resolution transmission electron microscopy (HRTEM) revealed that the "oxide" scales are predominately composed of nanosized polycrystalline silver grains (5-100nm) as well as a small amount of nanosized silver oxides. The results were remarkably different than the O_2 oxidation. The HRTEM investigation suggested that the grains of polycrystalline silver were first carved off from the substrate into "oxide" scale by lattice expansion and decohesion, which are driven by atomic oxygen diffusion in Ag lattice, occupation of oxygen atoms at the interstitial sites of Ag lattice, and partially internal oxidation. The grains in the scale were also subject to continuing oxidations with the atomic oxygen--secondary poly-crystallization, and changed to smaller grains. The preferred oxidation fronts in silver lattice is along the {111} planes.