Recent Advances of Spontaneous Ageing Effect on TiO2 Nanotubes Anodized from Pristine TA6V Alloy

Nanotubes TiO2 prepared by electrochemical anodization of titanium foil are promising due to their great physical/chemical properties and large surface area for applications in electro-energy synthesis and storage devices as an active material in ion batteries and supercapacitors, solar cells, and photocatalysis. In the present work, nanotubes TiO2 were manufactured by anodizing Ti–6Al–4V alloy at constant applied voltage of 60 V for 3 h, in a fluoride ethylene glycol electrolyte and water content of 20 wt %. The latter sample was left unattended and exposed to environmental conditions (i.e., humidity and ambient temperature) for more than four years (from 2018 to 2022) before being evaluated for any significant alteration/changes and ageing effects on the surface morphology, surface composition, and microelectronic structure. Surface morphology and surface chemical composition of nanotubes TiO2 were recorded using an environment scanning electron microscope, energy dispersive X-ray spectroscopy, X-rays fluorescence. A change in the crystallite size was investigated by X-ray diffraction. To our knowledge, this has not been yet reported for nanotubes TiO2. The micro-electronic structure alteration was depicted using electrochemical impedance spectroscopy measurements. Comparing the scanning electron microscope pictures of the freshly synthesized and the aged samples revealed a noticeable alteration in the morphology and a disparity in the nanotubes TiO2 external diameter. The aged sample showed darker areas, which may correspond to a loss of material or a presence of an oxide layer. Energy dispersive X-ray spectroscopy analysis depicts a large amount of Titan and a lower amount of oxygen in the aged samples. The result affirms a strong presence of corrosion on the aged TiO2 NTs surfaces due to the storage period. Moreover, the disappearance of the dual morphology structure (α, β) (generally detected in freshly synthesized samples) and the detection of the α phase on the aged sample makes the surface less stable (degraded). The Nyquist diagrams on the fresh and aged samples confirm a distinguished behaviour modification between the two samples, which indicates a clear surface alteration. The excellent properties depicted on the freshly synthesized became a huge drawback for the aged sample, where the surface degradation and the microelectronic structure alteration were noticeable due to exposition to normal environmental conditions, ambient temperature, pressure and humidity, let alone being exposed to an aggressive environment of a battery when used as an active material. This opens up a new insight into the optimisation of storage and disposal conditions.