The prevention of 'burning' during the hard anodization in formamide for ultrafast growth of highly ordered arrays of TiO2 nanotubes

The growth rate of TiO2 nanotubes depends on temperature, etchant concentration, and the strength of electric field. Under the typical fast hard anodization condition such as the strong electric field at 120 V, the flow of current is concentrated through the thin layer of TiO2, resulting in the bent or collapsed TiO2 nanotubes or a break-down, called 'burning'. To prevent the adverse effects, top etching and 'burning', we introduced formamide of a high dielectric constant as an additive in the electrolyte. The organic acids were electrochemically generated from the decomposition of formamide on TiO2. The organic acids rapidly stabilized anodization current and thus, the highly ordered 17 mu m-long TiO2 nanotube arrays were obtained just in 5 min anodization. During the anodization with pure formamide mixed with 1.3 vol % water under the strong electric field, cyanides, ammonium ions, and fatty acids, originated from the decomposition of formamide adsorbed on the TiO2, were found by ion chromatograph and gas chro-matographemass spectrometer (GC-MS) equipped with a pyrolyzer. The major roles of fatty acids such as oleic acids etc. generated from formamide are the current stabilization, the prevention of burning, and the delicate balancing of speed of etching with oxide layer growth. (C) 2018 Published by Elsevier Ltd.