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

The oxidation of polycrystalline Sn by hyperthermal atomic oxygen was syste matically 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-elect ron beam used in the ELS measurements. These spectra have been interpreted based on features in the ELS spectra of well-characterized reference materi als: 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, 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 penetrate s more deeply into the solid and more transitional oxide and SnO, form bene ath the SnO at the interface between the oxide region and the bulk metal. A fter these higher AO exposures, the metallic Sn detected with ELS resides m ostly 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 faste r than oxide formation in the near-surface region. By comparing the XPS and ELS data obtained after different atomic and molecular oxygen exposures, i t 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 adso rption. (C) 2000 Elsevier Science BN. All rights reserved.