Formation of hierarchical porosity in oxidation of Ag films by reactive sputtering deposition of metal oxides via the Kirkendall effect

In this work, the conversion of Ag films to silver oxide nanorod arrays with hierarchical porosity which was realized in an oxidation environment by reactive sputtering deposition of metal oxides was reported. The mechanisms from the oxidation of Ag films to the formation of hierarchical porosity were investigated by varying the deposition parameters with the determination of phase structure, hierarchical porosity, diffusion coefficient, and chemical states of elements as well as by a comparison with the oxidation of Ag films by direct exposure to plasma. The oxidation of Ag films can be ascribed to the atomic oxygen in the environment. The formation of silver oxide nanorods and the creation of hierarchical porosity were substantiated to be due to the Kirkendall effect occurring between the oxidized Ag films and the deposited metal oxides with the assistance of the shading effect in deposition. In addition, the sample temperature and the attributes of metal oxides were found to have significant effects on the formation of hierarchically structured porous silver oxide films. The principle revealed in this work is of significance for the synthesis of porous materials with hierarchical porosity, especially for fabrication of hierarchically porous silver oxide films with improved properties, which are needed in the research fields of solid-state thin film batteries, photocatalysts, bactericides, gas sensors, etc.