SURFACE CHARACTERIZATION STUDY OF THE THERMAL-DECOMPOSITION OF AG2O

In this study the thermal decomposition of Ag2O was examined by collecting XPS data from a pressed-powder Ag2O sample before and after annealing at 100, 200, 300, and 490 degrees C. Comparative data were also collected from a polycrystalline Ag foil that had been cleaned by cycles of annealing at 500 degrees C and sputtering with 2-keV Ar+ ions for an extended period. A mixture of subsurface atomic oxygen and hydroxyl groups, characterized by a broad peak in the O 1s spectrum centered at a binding energy (BE) of 531.0 eV, remains on this foil after cleaning. The compositional and chemical-state changes that occur at the Ag2O surface during an anneal are characterized by the relative peak intensities, line-shape changes and BE shifts in the XPS spectra. The results show that the BE shift of the Ag 3d peaks obtained from Ag2O are -0.3 eV relative to the metallic state. Several different types of contamination are present on the Ag2O sample including a mixture of carbonate and bicarbonate species and hydrocarbon and oxygen-containing organic species. H2O may also be present in the near-surface region of the sample. During the 100 degrees C anneal, a small amount of AgO and Ag metal form by disproportionation of Ag2O. The resulting mixture yields broadened valence-band and Ag MNN features and a 0.15-eV broadening in the Ag 3(5/2) peak. Removal of about 20% of the carbon contamination also occurs during this anneal. Annealing at 200 degrees C causes most of the carbonate to decompose, and a small portion of the AgO decomposes to Ag2O. Dissociation of the AgO at 300 degrees C produces a surface that is predominantly Ag2O. However, a very small amount of Ag metal is present. Annealing at 490 degrees C for 60 min causes nearly all of the Ag2O to decompose to Ag metal. Also, impurities including Si, S, K, and Cd contained in the bulk of the sample migrate to the surface forming a contamination overlayer. This overlayer was removed by ion sputtering, resulting in a surface that exhibits spectral features characteristic of Ag metal. The oxygen species remaining on this surface after sputtering consist of a mixture of dissolved O and OH.