Effect of increasing etchant concentration to accelerate synthesis of 9-μm-long TiO_2 nanotubes

摘要

During electro-deposition of a TiO_2 film on titanium foil, an etching agent supplies ionic species that create the desired morphology by chemical dissolution of the oxide layer. The etchant ions attack both titanium ions and the oxide film to form hexafluorotitanate complexes, water and hydroxyl species. In this study, an attempt was made to reduce the duration of a TiO_2 film electro-deposition process by increasing the chemical dissolution rate. Two 127 μm-thick titanium foil substrates were anodised for different durations in an anhydrous polar organic solvent containing different amounts of an etchant. This yielded 9 μm-long nanotubes. Anodisation current density measurements for a two hour process showed slow chemical dissolution when the amount of etchant was low. In an attempt to speed-up the electro-deposition process, the amount of etchant was increased and anodisation carried out for only an hour. This yielded films (consisting of nanotube arrays) that are equal in thickness to that obtained in the two hour process. However, scanning electron microscopy revealed un-etched regions of the film between the tubes when the etchant concentration is high. These fragments are dimensional irregularities in an otherwise uniform tubular structure and hence are defect sites that could reduce charge transport efficiency. The study shows that the nanotube array quality is reduced when the duration of the anodisation process is shortened by an increased etch rate, without further optimisation of the anodisation parameters.