Initial stages of oxide formation on titanium surfaces during oxygen bombardment at room temperature: An XPS study

In this work, we investigated the oxidation of metal titanium (Ti) induced by ion bombardment using X-ray photoelectron spectroscopy (XPS). The UNIFIT program was used for data processing and analysis, which enables the precise numerical deconvolution of XPS spectra. The photoemission spectra were numerically deconvoluted using a combination of Gaussian and Lorentzian functions to determine the different oxidation states of the elements and to monitor the early stages of the oxidation process. The analysis of the photoemission spectra revealed significant transformations of the titanium electronic structure during the oxidation process. Before exposing the sample to oxygen ions, the spectrum around the Ti 2p atomic level showed only the metallic phase of titanium. However, with an increase in the bombardment time, Ti 2p photoemission spectra reveal the presence of different titanium oxides (TiO, and TiO₂), indicating the progressive coverage of the titanium surface with oxide layers. After 180 minutes of bombardment with ions, the TiO₂ phase becomes dominant, although metallic Ti and lower oxides are still present on the sample's surface. These conclusions were further confirmed by the analysis of XPS spectra around the valence band, which, with the increasing oxygen irradiation time, showed a decrease in the intensity of the peak characteristic for metallic titanium and an increase in the intensities of the features associated with TiO₂. Our analysis shows that an increase in the thickness of the TiO₂ surface layer follows, consistent with Wagner's theory. In addition, our study suggests that the mechanism of titanium oxidation is primarily influenced by the diffusion transport of Ti cations through singly charged cation vacancies, formed during the bombardment of the material.