Black TiO₂ Synthesis by Chemical Reduction Methods for Photocatalysis Applications

Applications of TiO₂ nanomaterials in photocatalysis, batteries, supercapacitors and solar cells, have seen widespread development in recent decades. Nowadays, black TiO₂ have won attention due to enhancing the solar light absorption by the formation of oxygen vacancies and Ti³⁺ defects, to promote the separation of photo-generated charge carriers leading to the improvement of the photocatalytic performance in H₂ production and pollutants degradation. The enhanced photocatalytic activity of black TiO₂ is also due to a lattice disorder on the surface and the presence of oxygen vacancies, Ti³⁺ ions, Ti-OH and Ti-H groups. Enhancing the optical absorption characteristics of TiO₂ and change of energy level and band-gap of materials have been successfully demonstrated to improve their photocatalytic activities, especially for black TiO₂ nanoparticles, which promote visible light absorption. The current review focuses on the investigation of the chemical reduction synthetic route for black TiO₂ nanomaterials, and their proposed association with green applications such as photodegradation of organic pollutants and photocatalytic water splitting. The synthesis methods of black TiO₂ involves the changes from Ti⁴⁺ to Ti³⁺ state, into different strategies: (1) The use of highly active hydrogen species such as H₂, H₂/Ar or H₂/N₂ gases, and metal hydrides (NaBH₄, CaH₂), (2) the reduction by active metals such as aluminum, magnesium and zinc, and (3) organic molecules such as imidazole and ascorbic acid.