PVD Ti-films for plasma-facing first walls, Part I: Characterization of surface chemistry during H2 and O2 exposure

A surface analysis study was carried out to investigate the gettering behavior of PVD Ti films in ambiences pertinent to plasma research experiments. By controlling the type and amount of gas flux, the effect of wall history on the Ti chemical pumping of H2 and O2 was studied using X-ray photoelectron spectroscopy (XPS). The Ti-2p XPS spectra at various gas exposure stages were interpreted using reference spectra obtained at selected conditions. Our findings are in accord with some of the previously reported notions regarding Ti pumping of hydrogen and oxygen; (a) hydrogen is pumped by Ti until all available sites are filled, and (b) oxygen exposure on Ti results in a passive surface oxide layer which inhibits further oxidation as well as hydride formation. However, our observations conflict the understanding that oxygen pumping of Ti is not affected by previous hydrogen absorption. In contrast, when exposed to O2, Ti films that were previously exposed to H2 yielded considerably lower oxidation compared to unexposed Ti films.