Mechanistic Model of Nanotube Array Formation

To help understand the process of nanotube formation, FESEM images of the surface of the samples anodized at 20 V for different durations were taken and analyzed. At the start the anodization the initial oxide layer [111], formed due to interaction of the surface Ti ions with oxygen ions (0 ) in the electrolyte, can be seen uniformly spread across the surface. The overall reactions for anodic oxidation of titanium can be represented as 

In the initial stages of the anodization process field-assisted dissolution dominates chemical dissolution due to the relatively 

In formamide electrolyte containing fluoride ion, the starting anodization current does not drop instantly as observed in aqueous bath. The gas evolution which is indicative of electronic conduction was observed at the anode. The anodization current drops steeply thereafter due to the initial formation of an insulating oxide layer, see Fig. 5.10. In this region, electronic conduction decreases due to the blocking action of the formed oxide, and ionic conduction increases. Once the oxide layer is completely formed over the entire exposed surface of the anode, electronic conduction becomes negligible and ionic conduction dominates the mechanistic behavior. Nanotube formation reduces the surface area available for anodization with a correlated decrease in current density, while deepening of the pore occurs. 

The conductivity of the electrolytes also plays a role in controlling nanotube array growth. Ethylene glycol containing 2% water and 0.35 % NH4F have a conductivity of 460 pS/cm which is much lower than the conductivity of the formamide based electrolytes ( 2000 pS/cm) [27]. The total applied anodization voltage is the sum of the potential difference at the metal-oxide interface, the potential drop across the oxide, the potential difference at the oxide-electrolyte interface, and the potential drop across the 

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