An XPS study of Si as it occurs in adsorbents, catalysts, and thin films

This paper is focused upon XPS studies of silicon in a number of diverse environments. First, the XPS analysis of Si in its “clean” zero valent form is reported. Novel observations of the corresponding XPS loss spectra are presented. Second, the changes in these XPS spectra during oxidation of Si0 to SiO2 are described. In addition to loss spectra, charge shift and valence band analyses are reported. Third, the changes experienced by silica are documented when metal aluminates are injected to create such common adsorbents as clays and zeolites. Contrary to previous suppositions these systems do exhibit “chemical shifts”, but these shifts are apparently motivated by group, rather than elemental, changes. As a fourth example, unique XPS analyses are reported of the changes experienced when silica is employed as a support for a metal oxide catalyst during on-line use situations. Finally, the study returns to elemental silicon by describing thin ( ∼ 500 Å) films of SiO2 on Si0. Contrary to previous contentions, it is shown that by utilizing loss spectra and charge shift analyses, a description of the SiO2 film, the resulting interface, and the Si0 underlayer may be achieved employing common ion-sputter etching. These studies demonstrate the interconnection of XPS techniques and results over fields that often seem totally divorced from one another.