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

Abstract

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 μm-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 chromatograph–mass 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.

Introduction

Many attractive properties of TiO₂ such as a high refractive index, biocompatibility, and photocatalytic effect [[1], [2], [3]] have been utilized in water splitting [1,2,4,5], solar cells [[6], [7], [8], [9], [10], [11], [12], [13]], capacitors [14,15], sensors [16], sun cream etc. Especially, the nanoscale structures of TiO₂ have been investigated a lot because its various nanostructures generate new useful electronic and optical properties. Various attempts have been made to produce a good quality of nanostructures after the discovery of TiO₂ nanotubes by Zwilling et al. [17]. Previously, the highly ordered regular arrays of TiO₂ nanotubes were achieved by electropolishing and two step anodization method [18]. The reduced surface roughness and mirror-finish by electropolishing and the regular arrays of curvatures produced by the first anodization on which the electric field is concentrated, give rise to the uniformly well-organized nanostructures. Under the similar principle just mentioned above, ion beam guided anodization [19] and the anodization of pretextured Ti by nanoimprinting [20] were also reported for the synthesis of ordered TiO₂ nanotubes. The morphology and the uniformity of the TiO₂ nanotubes vary over a wide range according to different process parameters such as viscosity [[21], [22], [23]], electrolyte composition and concentration [9,24], temperature [24,25], pH [9,26], and voltage [9,23]. The uniform and regular arrays of metal oxide nanotubes are produced in the result of delicate balance between the growing and etching rates of metal oxides. Although they were highly ordered, the process took much time. For practical and economic applications, the fast and hard anodization with a wide range of pore diameters, interpore distances, and lengths, is desired. Controlling the medium viscosity, the long [9,[21], [22], [23]] and smooth [27] nanotubes were synthesized. Under the high concentration of fluoride ions and water, the growth and etching rates of the nanotubes are enhanced, however, the top of the grown nanotubes is excessively etched away and becomes nanograss [[28], [29], [30]] within a shorter time. The neutral pH condition of electrolyte solution obtained by using NH₄F instead of HF mitigates the dissolution of TiO₂ nanotube wall and maintains the shapes. The viscosity of the medium affects the diffusion of ions. The pore size of the TiO₂ nanotubes is influenced by the viscosity of the electrolyte solution. Anodized nanostructured TiO₂ growth is electrochemically activated by temperature and the rate-determining step is the transport of oxygen species across the TiO₂ layer at the pore bottom. The oxide thickness, pore diameter and porosity of grown TiO₂ layers increase gradually with increasing temperature [25]. In order to achieve the rapid growth of regular and uniform TiO₂ nanotubes, the fast and hard anodization process requires high current densities. As for the applied voltage, if it exceeds the dielectric breakdown limit of the oxide, the oxide will be no longer resistive and lead to sparking. The long TiO₂ nanotubes with high anodization voltage are attributed to the enhanced driving force for ionic transport through the barrier layer at the bottom of the pore. In 2012, ‘burning’ or breakdown was observed during the hard anodization under the strong electric field for ultrafast growth of highly ordered TiO₂ nanotubes in lactic acid [31]. The cause of ‘burning’ or breakdown was not well understood, but the brief description was given at that time. In order to achieve the fast growth of metal oxide nanotubes at high current density and voltage, we should prevent the burning occurring at high voltages. The less dissolution of the metal oxides, in fluoride-containing solution with organic electrolyte and less water content, aids the formation of longer nanotubes.

The formamide of a high static dielectric constant has been used as an organic solvent for electrostatic self-assembly of polymer film [32] and nanocrystals [33] and a drying-control chemical additive [34,35]. The formamide was a key molecule to generate fatty acids, amino acids and organic acids through its decomposition under catalytic conditions with TiO₂ [36], ZrO₂ [37], phosphate [38], borate [39] and meteorite [40]. In this report, utilizing high dielectric formamide to prevent the breakdown by relaxing probable excessive current at high anodization voltage through its electrochemical reactions on TiO₂, we investigate the practical and economic growth of uniformly regular arrays of TiO₂ nanotubes at high voltage and current in a short time in terms of the fast and hard anodization in formamide. Since the electrochemical reactions of formamide on TiO₂ generated various organic acids which could relax an excessive current, formamide was selected to study ‘burning’ or breakdown in the fast and hard anodization. The organic fatty acids originating from formamide rapidly stabilized anodization current at high voltages and thus, the highly ordered 17 μm-long TiO₂ nanotube arrays were obtained just in 5 min anodization without any burning. Formamide can relax an excessive current to produce organic fatty acids which in turn, are overcoated or are adsorbed on the TiO₂. The fatty acids attached to TiO₂ nanotubes prevent the burning or breakdown at high voltages. We report the prevention of the burning or breakdown during the fast hard anodization in formamide for the ultrafast growth of highly ordered arrays of TiO₂ nanotubes and the generation of fatty acids from formamide.