TiO2 nanotube arrays with a volume expansion factor greater than 2.0: Evidence against the field-assisted ejection theory

The mechanism of formation of anodic TiO2 nanotubes and the cause of volume expansion are still controversial. It is well known that fluoride ions participate in the field-assisted dissolution reaction as well as in field-assisted ejection to reduce the current efficiency and the volume expansion factor. Here, the anodizing of titanium in an electrolyte with three different concentrations using four current densities is discussed in detail. We found that the total volume of the nanotube array remained about the same for a given current, being independent of the concentration of NH4F (0.1, 0.3 and 0.5 wt%), and depending only on the anodizing current. Interestingly, regardless of the anodizing conditions, the volume expansion factor of the nanotubes was about 2.073 and the ratio of the total volume of the nanotube array to the volume of consumed titanium was about 2.285. These data are completely contrary to what would be expected on the basis of the field-assisted ejection and dissolution theories.