Characterization of tin oxides by X-ray-photoemission spectroscopy

Using well-defined samples of SnO and SnO2, we have focused our attention on the way the two tin oxides could be distinguished using x-ray-photoemission spectroscopy (XPS). Polycrystalline SnO, oxidized in air to give SnO2, sputtered by argon-ion bombardment to give SnO and single-crystalline SnO2 have been examined using XPS in order to study the formal valencies of tin in these partly ionic compounds. On the basis of a tin 3d-level line-shape analysis, we show that a sizable chemical shift of 0.7±0.05 eV exists between (formal) Sn4+ and Sn2+. Using a least-squares fitting routine, we are able to follow the evolution of both ionic species upon argon-ion bombardment. This evolution shows up more strongly in the valence-band region, where SnO is characterized by an additional structure attributed to Sn 5s–derived levels. Our experimental results are interpreted using calculated tight-binding bulk densities of states. Finally, we propose a procedure for the quantitative evaluation, by XPS, of the relative concentration of the two oxides.