A study of the growth mechanism of large-diameter double-wall TiO2 nanotube arrays fabricated by high voltage anodization.

Abstract

Research on the growth mechanism of titanium dioxide (TiO2) nanotube arrays fabricated by anodic oxidation is essential to achieve artificial control of the microstructure and to expand their applications. In our previous work, we reported the preparation of highly ordered large-diameter double-wall TiO2 nanotube arrays prepared by high voltage anodization. In this paper, we observed and analyzed the initial growth process of large-diameter double-wall TiO2 nanotube arrays anodized at 120 V in ethylene glycol electrolyte containing aluminum fluoride (NH4F) and water (H2O), such as the evolution of surface and cross-sectional morphologies, the influence of current density on growth rate, the transition process from nanoholes to nanotubes, and the evolution of dimples on the remaining substrate. On the basis of our observations and inspirations from the existing viewpoints, we established growth models of large-diameter double-wall TiO2 nanotube arrays corresponding to different growth stages to explain the growth process. The growth rate of anodic oxide film changes accordingly with the current density. The compact anodic oxide film formed initially actually contains outer layer and inner layer, with no obvious interface between them. Then, the bottom even levels of the inner layer and outer layer bulge towards the substrate and become individual hemisphere-like structures. The inner layer becomes the outer wall, and the outer layer becomes inner wall. Eventually, V-shaped large-diameter and double-wall TiO2 nanotube arrays form. The results presented in this work are significant and provide a better understanding of the growth mechanism of large-diameter double-wall TiO2 nanotube arrays anodized by high voltage.