Essential distinction between one-step anodization and two-step anodization of Ti

[Display omitted] •One-step anodization (OSA) and two-step anodization (TSA) of Ti are compared.•Constant current anodizations were designed to control charge transfer.•Essential distinction between the OSA and the TSA was analyzed for the first time.•The distinction was clarified by oxygen bubble mould and plastic flow model.•The concaves contribute to the growth of highly-ordered ATNTs during TSA. One-step anodization (OSA) and two-step anodization (TSA) have been widely used to fabricate anodic TiO2 nanotubes (ATNTs). It has been reported that TSA can fabricated more ordered nanotubes, as compared to OSA. However, the essential distinction between OSA and TSA of Ti has never been clarified. Here, constant current anodizations were designed to study the effects of TSA and OSA on the morphology of ATNTs. The significant differences between nanotubular architecture (length, diameter, porosity) of ATNTs fabricated by OSA and TSA were observed and analyzed for the first time. For TSA, the relationship between the nanotube length (y) and anodizing current (x) corresponds to the liner equation y=0.259x-2.3, and the relationship between nanotube outside diameter (Y) and anodizing current (x) corresponds to the linear equation Y=3.352x+65.73. However, no similar linear relationships were found for OSA. The essential distinction between OSA and TSA was clarified by oxygen bubble mould combined with plastic flow model.