Oxide formation on Fe and Ti thin films and on Fe thin films modified with ultrathin layers of Ti

Abstract

Abstract Reactions of O 2 and H 2 O to from thin oxides on Fe, Ti and Ti‐modified Fe thin films have been studied by XPS, following O 2 and H 2 O exposure in the range 0‐600 L, to form oxides with thicknesses of < 40 Å. XPS Iineshape analysis is used, utilizing a special combination of reflection electron energy‐loss spectroscopy (REELS) and nonlinear least‐squares fitting routines to model the intrinsic and extrinsic energy losses that accompany potoemission for the Ti and Fe 2p lines. During the formation of the thinnest oxides, this approach yields a unique picture of the composition of the oxide, while for the thicker layers, there is little significant difference between this fitting approach and (1) fitting approaches using an integral background approach or (2) direct deconvolution methods using the REELS data. Both Fe and Ti are quite reactive to O 2 , yielding a surface oxide that is apparently an FeO‐dominated FeO/Fe 3 O 4 bilayer on Fe surface oxide that is predominantly TiO 2 on the Ti surface. Pure Fe and Ti surfaces are unreactive to H 2 O. Predosing of thewse surfaces with low levels of O 2 (5 L) does not increase the reactivity appreciably toward H 2 O. Ti dispersed on the Fe surface as an adatom layer, with an equivalent thickness of 3 Å, greatly suppresses the reactivity of Fe toward O 2 , while the Ti is oxidized primarily to TiO 2 , In contrast to the pure Ti layers, these Ti adatom layers are quite reactive toward H 2 O, yielding oxides (no evidence for hydroxide) in a uniform distribution of oxidation states (Ti +2 , Ti +3 and Ti +4 ).