A review on TiO2-based nanotubes synthesized via hydrothermal method: Formation mechanism, structure modification, and photocatalytic applications

TiO2 nanotubes or titanate nanotubes (TNTs), which are produced by hydrothermal synthesis, have received increasing attention for photocatalytic applications due to their unique physicochemical properties, such as nanotubular structure with layered walls, high surface area, ion-exchange ability, and photocatalytic activity. The hydrothermal method that has been widely applied for preparation of catalysts in industry like zeolites, is a simple and mature synthesis technique, in which the properties of products depend on the formation mechanism and hydrothermal conditions. Focusing on the structure of TNTs including crystallography and morphology from the extensive literature, this paper discusses the formation mechanisms of TNTs during hydrothermal synthesis. The effects of synthesis conditions (TiO2 precursor, hydrothermal temperature and duration), auxiliary methods (ultrasonication and microwave-assistance), and post treatment (acid washing and calcination) on the formation of titanate nanotubes are reviewed. Furthermore, structural modifications and current progress toward photocatalytic activity of TNTs as photocatalysts are also summarized in this review.