Initiation and growth of self-organized TiO2 nanotubes anodically formed in NH4F/(NH4)(2)SO4 electrolytes

The anodic formation of self-organized porous TiO2 on titanium was investigated in 1 M (NH4)(2)SO4 electrolytes containing 0.5 wt % NH4F by potential sweeps to 20 V-SCE. By a combination of electrochemical, morphological, and compositional information we show that the sweep rate has a significant influence on the initiation and growth of the porous structures. In the first phase of the anodization process, a precursor barrier type of oxide film is formed; underneath this film pores then start growing first randomly and then self-organize. High-aspect-ratio TiO2 nanostructures can be obtained under optimized electrochemical conditions. These nanotubular oxide layers have single-pore diameter ranging from 90 to 110 nm, average spacing of 150 nm, and porosity in the order of 37 - 42%. The current work indicates that the nature of the initial barrier-type layer has a strong influence on establishing optimized pore growth conditions. (c) 2005 The Electrochemical Society.