Review on Modification Methods of TiO2 Nanotubes for Photocatalytic Degradation of Dyes

Annually, insoluble biodegradable dyes in textile, leather, paper and food industries, photovoltaic cells are evacuated to natural water flows. The highly ordered TiO2 nanotube arrays made with titanium anodizing, due to its large surface area, strong oxidizing ability and excellent charge transfer performance, have been widely used as photocatalyst for decomposition of dyes in recent years. Nevertheless, the disadvantages of TiO2 are the vast energy gap (3.0-3.2 eV) and the high recombination of electron-hole derived from light that limits the use of TiO2 nanotubes in the use of visible light to decompose organic pollutants. For this purpose, in recent years, many attempts have been made to modify TiO2 nanotubes to increase light absorption to visible light, limit the band gap, increase surface area, and prevent the recombination of charge carriers. To overcome these limitations and increase photocatalytic activity of nanotubes, various methods have been investigated: decoration with noble metal particles, doping with metal elements, modification with semiconductor and doping with non-metallic elements.