Large scale free-standing open-ended TiO2 nanotube arrays: stress-induced self-detachment and in situ pore opening

In this study, the free-standing and open-ended TiO2 nanotube arrays with large area are synthesized under high field anodization. The membrane is firstly self-detached from Ti foil during anodization followed by a pore opening process of in situ field-assisted chemical dissolution. A model of stress accumulation is proposed to explain the self-detachment of the as-formed TiO2 nanotube membrane from the underlying Ti substrate. It is believed that the stress accumulation in the membrane is the collective effects of the stress generation during tube growth and the stress release due to chemical dissolution. When the stress accumulates to a critical value, the TiO2 nanotube membrane would be self-detached from the Ti substrate. It is found that the magnitude of stress is associated with the thickness of a stressed layer rather than the thickness of the as-formed TiO2 membrane. By changing the anodization parameters, the stress accumulation rate can be facilely controlled, which finally determines the detaching time and thickness of the detached membrane.