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Oxidation of polycrystalline tin by hyperthermal atomic oxygen: an investigation using electron energy-loss spectroscopy

2000; Elsevier BV; Volume: 106; Issue: 1 Linguagem: Inglês

10.1016/s0368-2048(99)00092-4

1873-2526

Jason F. Weaver, Timothy J. Campbell, Gar B. Hoflund, Ghaleb N. Salaita,

Electron and X-Ray Spectroscopy Techniques

Abstract The oxidation of polycrystalline Sn by hyperthermal atomic oxygen was systematically investigated using electron energy-loss spectroscopy (ELS) and X-ray photoelectron spectroscopy (XPS). Depth-sensitive information was also obtained after each gas exposure by varying the energy of the primary-electron beam used in the ELS measurements. These spectra have been interpreted based on features in the ELS spectra of well-characterized reference materials: metallic Sn, SnO and SnO. During the 50-L atomic oxygen (AO) exposure, SnO forms in the near-surface region and a small quantity of transitional oxide with a composition between that of SnO and SnO 2 forms beneath the SnO. Metallic Sn is still present in the very near surface region after the 50-L exposure, indicating that the SnO forms in clusters in the near-surface region. With increasing AO exposures to 980 L and 2840 L, the SnO penetrates more deeply into the solid and more transitional oxide and SnO 2 form beneath the SnO at the interface between the oxide region and the bulk metal. After these higher AO exposures, the metallic Sn detected with ELS resides mostly beneath the oxide film. This distribution of Sn oxides as a function of depth indicates that the diffusion of oxygen atoms in the solid is faster than oxide formation in the near-surface region. By comparing the XPS and ELS data obtained after different atomic and molecular oxygen exposures, it is found that hyperthermal atomic oxygen is greater than seven orders of magnitude more reactive than O 2 towards oxidizing polycrystalline Sn. Thus, the rate-limiting step in the oxidation of Sn by O 2 is dissociative adsorption.