Shaoyu Zhang

Facile Method to Enhance the Adhesion of TiO2 Nanotube Arrays to Ti Substrate

The weak adhesion of anodic TiO2 nanotube arrays (TNTAs) to the underlying Ti substrate compromises many promising applications. In this work, a compact oxide layer between TNTAs and Ti substrate is introduced by employing an additional anodization in a fluoride-free electrolyte. The additional anodization results in an about 200 nm thick compact layer near the nanotube bottoms. Scratch test demonstrates that the critical load of TNTAs with the compact oxide layer is a more than threefold increase in comparison with those without the compact layer. Moreover, this facile method can also improve the photoactivity and supercapacitor performances of TNTAs markedly.

Simulation of anodizing current-time curves and morphology evolution of TiO2 nanotube arrays

Through introducing appropriate additives into the electrolytes, the morphology and growth efficiency of TiO2 nanotube arrays (TNTAs) have been greatly influenced. The anodizing current transients and the corresponding morphology of TNTAs were investigated and compared in detail by SEM. To further understand the mechanism, the measured current-time curves obtained during the anodization of titanium in the electrolytes with different additives are simulated. Notably, in the total anodizing current, the ionic current is separated from the electronic current according to the present model, and that the electronic current and ionic current make different contributions to the growth of TNTAs. It is found that the initiation of nanopores may be caused by the rupture of the oxygen bubbles occluded in the growing oxide, and the opening of nanotubes is thought to be close related to the disturbance effect of the rising bubbles (caused by electronic current). The present results would be helpful for understanding the formation mechanism of TNTAs from the perspective of ionic and electronic current. And practically, the nanotube length can be predicted and deduced quantitatively via simulating and comparing electronic and ionic current.

Formation mechanism of multilayer TiO2 nanotubes in HBF4 electrolyte

Multilayer anodic TiO2 nanotubes are first fabricated in HBF4-containing electrolyte by a one-step galvanostatic anodization. These nanotubes demonstrate unique A-shaped sidewalls, which are different from the traditional V-shaped nanotubes formed in NH4F-containing electrolyte. Further, the formation mechanism of the multilayer TiO2 nanotubes is proposed. During the anodizing process, the total anodizing current could be separated into ionic current and electronic current. The oxygen bubbles, induced by the electronic current, play a significant role in shaping the nanotube architectures. The bottoms of TiO2 nanotubes could be broadened under the pressure of oxygen bubbles. Thus the wall at the bottom of the nanotube becomes thinner. When the pressure of oxygen bubbles reaches a certain value, it will break the sidewalls of the nanotubes, resulting in the formation of the A-shaped sidewalls of TiO2 nanotubes. However, in NH4F-containing electrolyte, the oxygen bubbles escape from the top of nanotubes, and then the V-shaped sidewall thickness profiles of TiO2 nanotubes are formed. Owing to the inflating effect of oxygen bubbles on the nanotube walls, the whole TiO2 nanotube layer is finally divided into nanotube multilayers in HBF4-containing electrolyte.