Growth mechanism and morphology control of double-layer and bamboo-type TiO2 nanotube arrays by anodic oxidation

We have synthesized multilayer and bamboo-type TiO2 nanotube arrays via alternating-voltage anodization steps in hydrous ethylene glycol (EG) containing NH4F and investigated their growth mechanisms using experimental and theoretical approaches. Current transients are recorded to study real-time morphological evolution of anodic TiO2 films during anodization at high and low voltages (V-high, V-low). Current changes after each voltage ramp to V-high are observed along with sequential origination of pits, pores and tubes in a compact barrier layer at the base of oxide film. Two anodization steps at V-high separated by one step at V-low with equal holding time yield double-layer smooth-walled TiO2 nanotubes. However, repetition of this sequence does not produce nanotubes of more layers, but makes lower-layer nanotubes longer and induces ridges on their walls to form bamboo-type tubes. Formation mechanisms of double-layer TiO2 consisting of smooth-walled or bamboo-type nanotubes are explored. A proper holding time of low-voltage anodization is required for ridge formation, but ridge spacing is determined by high-voltage anodization time. The ridge spacing increases linearly with the high-voltage anodization time, and can be theoretically calculated for lower-layer bamboo-type nanotubes formed in EG electrolytes with 5 vol% H2O. Less water (2 vol%) in electrolyte results in larger ridge spacing, while more water (10-15 vol%) not only reduces the ridge spacing, but also causes instability during growth of multilayer TiO2 nanotube arrays and eventually leads to formation of a disordered porous TiO2 structure. (C) 2012 Elsevier Ltd. All rights reserved.