Initiation and Growth of Self-Organized TiO[sub 2] Nanotubes Anodically Formed in NH[sub 4]F∕(NH[sub 4])[sub 2]SO[sub 4] Electrolytes

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Abstract
The anodic formation of self-organized porous TiO2TiO2 on titanium was investigated in 1 M (NH4)2SO4(NH4)2SO4 electrolytes containing 0.5 wt % NH4FNH4F by potential sweeps to 20VSCE20VSCE . 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 TiO2TiO2 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.