Halogen element modified titanium dioxide for visible light photocatalysis

Halogen elements, i.e. fluorine, chlorine, bromine, and iodine, have attracted intensive interests in modification of TiO₂ for photocatalytic oxidation of organic pollutants under visible light irradiation. Compared to other metal and non-metal elements, halogens show 3-fold advantages, such as improvement of UV activity, various cations or anions for substitution of Ti⁴⁺ and/or O^2- in the TiO₂ matrix, narrowing the band gap of TiO₂ and tuning the band position. In this paper synthesis, physicochemical properties, mechanism of visible response, and photocatalytic activities of halogen modified TiO₂ are reviewed. It is found that introduction of a halogen element into TiO₂ crystals could lead to the enhancement of surface acidity, formation of surface hydroxyl radicals, more active sites, creation of oxygen vacancies or Ti³⁺, narrowing the band gap and tuning the valence band position. As a consequence, halogen modified TiO₂ exhibited high activity for organics oxidation under visible light radiation in aqueous and gas phases.