Quantitative analysis of the volume expansion of nanotubes during constant voltage anodization

Abstract The formation process of anodized nanotubes and the reason of volume expansion are still the focus of attention. So far, the traditional field-assisted dissolution and ejection mechanisms have only qualitatively discussed the phenomenon that the growth efficiency of the porous oxide film is about 60% of that of the barrier film, and the corresponding volume expansion factor is about 1.44 (2.4 × 60%). Here, the nanotubes prepared by applying different constant voltages in five kinds of NH4F concentration electrolyte were studied in detail. For the first time, the charges transferred in the process of constant voltage anodization was calculated by integral, thus the volume expansion factors under different conditions were calculated. We found that when the number of charges transferred by the reaction was similar, the total volume hardly changed, and it is independent of the NH4F concentration. The volume expansion factors (about 2.0) did not change with the increase of NH4F content, which is contrary to the expectations of the field-assisted dissolution and ejection. This study will help to promote the qualitative to quantitative study of the volume expansion of anodized TiO2 nanotubes under constant voltage. It is helpful to understand the growth kinetics of porous oxides.