The Evolution of Surface Oxides during TiFe0.9M0.1 (M = Ni, Mn) Activation: An In Situ XPS Investigation

The nature of TiFe-based surface oxides and their evolution during conventional activation heat treatment were investigated in this study. The as-prepared TiFe alloy was found to possess an initial composite Ti and Fe amorphous surface oxide layer of about 6 nm. Depth profiling has shown that oxides steadily vanish with increasing depth, whereas metallic Fe contribution and mixed Ti oxides arise, before eventually being found in their metallic state at 7.5 nm. In situ XPS measurements, carried out to directly observe the evolution of oxides during the activation procedure, have indicated that the initial ternary oxide begins to transform to metallic Fe and mixed Ti oxides at a temperature as low as 200 °C. Consistent with the literature, the reduction of Ti oxides took a major turn at around 400 °C. Toward the end of the in situ measurements, oxygen was partially dissolved due to the limited measurement duration: TiO and metallic Fe remained beyond 400 °C. A similar overall reduction behavior was observed for the Ni- and Mn-substituted alloys, with a few subtle exceptions: Ni existed in its metallic state from 200 °C whereas Mn was reduced from Mn3+ to the Mn2+ state only beyond 400 °C due to a pronounced difference in the oxidation driving force of these two substitution metals.