Challenges in the Surface Analytical Characterisation of Anodic TiO₂ Films – a Review

Abstract

Surface analytical findings of thick anodic passive layers on titanium are discussed and analysed in order to give a critical overview about possible artefacts for the system Ti/TiO₂. In line with previous studies, the pretreatment of the surface can have an influence on the morphology of the passive film. On polycrystalline surfaces, the oxide film shows a different composition and morphology on differently oriented grains, but although the crystallographic orientation of the substrate influences the thickness and the morphology of the passive layer the defect density does not seem to depend so strongly on the substrate condition. Ion sputtering strongly alters the TiO₂ surface morphology and composition which can be directly seen in SEM images of the surface and indirectly from the change in XPS spectra throughout a sputter depth profile. The material removal is not homogeneous and sputtering leads to reduction of the surface oxide. In AES experiments the passive layer on titanium undergoes changes induced by the primary electron beam that is used for the analysis. All these effects make the Ti/Ti-oxide a challenging system particularly when employing surface analytical methods for its investigation. Recent developments like the growth of nanotube structures on titanium give the system a renewed importance.