Electronic Structures and Photocatalysis Properties under Visible Irradiation of F-doped TiO2 Nanotube Arrays

Nanosized TiO2 has excellent photocatalytic activity to degrade most of pollutants. The most important focus has been on the improvement of photocatalytic activity and the efficiency of utilizing solar energy. Self-organized F-doped TiO2 nanotube arrays were fabricated by a one-step electrochemical anodization process on a Ti sheet. The prepared samples were annealed in air and detected by SEM, XRD and XPS. SEM images show that the well ordered TiO2 nanotube arrays with the average tube diameter of 40 nm and the average tube length of 700 nm are successfully fabricated. The substitutional F atoms that occupied oxygen sites in the TiO2 crystal lattice cause the appearance of active spices Ti3+, as evidenced from XPS. The sample annealed at 673 K exhibits the highest photocurrent intensities under visible light irradiation, which may be attributed to the Ti3+. The removal efficiency reaches 37.2% of AO7 at 180 min and the photoelectrocatalytic synergetic factor is 1.33. DFT calculations suggests that the valence and conduction band of F-TiO2 consist of the electrons of O2p and Ti3d, while the small amount of electrons of F2p do not change the band gap of TiO2.