Self-organized and uniform TiO2 nanotube arrays with optimized NH4F concentration in electrolyte by high voltage electrochemical anodization

Abstract

Large diameter and ordered TiO₂ nanotubes (NTs) were fast fabricated in an electrolyte containing lactic acid and ethylene glycol with different amount of NH₄F at various high anodization voltages up to 220 V. In this work, we could optimize F⁻ ionic concentration in the electrolyte at each anodization voltage and the uniform films without any oxide breakdown were successfully achieved. The optimum NH₄F concentration at which NTs can be formed homogeneously, decreases with the increment of anodization voltage. As a result, the fastest mean growth rate of 2.45 μm min⁻¹ was attained in 0.075 M NH₄F at 150 V. Growth rate of TiO₂ nanotubes is explained taking into account the role of F⁻ ions and their limited diffusion through TiO₂ nanotubes from bulk electrolyte. The interpore distance of the nanotubes is increased with enhanced anodization voltage.