In situ TEM investigation of the oxide/metal interface during the annealing of anodically formed titanium dioxide nanotubes

Titanium dioxide nanotubes (NTs) anodically grown on a titanium metal substrate are of great interest to many fields due to their unique physiochemical properties and electrochemical behavior. For many applications, a heat treatment is required to impart the NTs with a useful crystalline structure. The exact processes which occur in NTs during this high-temperature transformation, including the anatase-to-rutile transition (ART), were not well understood. Previous studies conducted to understand this phenomenon largely made use of conventional materials characterization techniques on annealed NTs attached to the titanium substrate. Therefore, it was challenging to determine if and/or how the substrate influenced the ART. In this study, in situ transmission electron microscopy was used to observe the morphological and phase changes occurring at the oxide/metal interface of anodically formed NTs during annealing. Samples were prepared both with and without the substrate to understand the effects of the substrate on phase changes within the NTs. These investigations revealed that when the substrate is present, the oxide/metal interface is a bilayer structure in which the ART is initiated from the upper layer of the interface while hydride and nitride precipitation occurs in the bottom layer. At elevated temperatures (> 500°C), the substrate undergoes spalling and loses its ordered morphology and crystallinity. When the NTs are not attached to the substrate, there is no evidence of ART or precipitation. These findings highlight the crucial role that the titanium substrate plays in the morphological and phase transformation of NTs during annealing. Graphical Abstract In situ transmission electron microscopy (enabled observation of the changes occurring at the oxide/metal interface during annealing. In situ TEM identified the oxide/metal interface as the epicenter of the morphological and crystal phase transformation. The in situ TEM investigation revealed the influence of substrate on anatase to rutile phase transformation.