Amphiphilic and photocatalytic behaviors of TiO2 nanotube arrays on Ti prepared via electrochemical oxidation

Amphiphilic TiO2 nanotube arrays (TiO2 NTs) were fabricated through electrochemical oxidation of Ti in solution containing H3PO4 and NaF. Scanning electron microscopic analysis shows that the as-prepared TiO2 NTs have an average pore diameter of 100 nm and a wall thickness of 15 nm. The electrochemical oxidation of Ti can be divided into four stages. In the first stage, when the potential is very low, oxygen formation and Ti dissolution are the major reactions. The second stage corresponds to a slightly higher potential, but less than 2.5 V. In this stage, the formation of TiO2 film occurs. When the potential is increased to the even higher range from 2.5 V to 6 V, the TiO2 film dissolves and nanoporous surface structure is generated. This is the third stage. Further increase of the potential enters stage four. The high potentials cause the self-organization of the nanostructure and allow the formation of well-aligned TiO2 NTs. We also found that the change in surface condition of Ti by annealing heat treatment affects the film dissolution kinetics. As compared with TiO2 thin film, the TiO2 NTs show higher photocatalytic activity on decomposing Rhodamine B. The surface of the TiO2 NTs can be wetted by both water and oil. Such an amphiphilic property comes from the capillary effect of the nanochannel structure of the TiO2 NTs. Because of the amphiphilic property and the photocatalytic activity, we conclude that the TiO2 NTs have the capability of self-cleaning. (C) 2011 Elsevier B.V. All rights reserved.