Fabrication of self-branched TiO2 nanotubes by anodization method, ordering and crystallinity

This paper reports on the self-branching of TiO2 nanotubes during anodization of Ti metal carried out at a fixed applied potential. These nanotubes are prepared by a two-step anodization carried out in an aqueous electrolyte containing ethylene glycol and ammonium fluoride at a range of anodization voltages of 30–60 V. The self-branching of nanotubes is more pronounced in the first-step anodization than during the second-step anodization at each voltage studied. The nanopits on the titanium surface, exposed after the dissolution of the oxide layer formed in the first-step anodization are observed to be highly regular and uniform. These nanopits provide nucleation sites producing less branched arrays during second step of anodization. Moreover, symmetric pits also result in straight tubes from top to bottom. Considering different applied voltages, the TiO2 nanotubes prepared at 50 V are comparatively more uniform and tend to grow straighter, whereas, numerous turns and pore branching are observed at other applied voltages. In this way, the best order in the tube arrays is found at anodization voltage of 50 V shown by the quantitative analysis as well. In addition, the nanotubes prepared by second-step anodization have shown higher crystallinity and with anatase as the major crystalline phase.