Enhancement of visible-light-induced photodegradation over hierarchical porous TiO2 by nonmetal doping and water-mediated dye sensitization

Dye sensitization and nonmetal doping on TiO2 are important for visible-light utilization in many fields. In this work, N, N&S and N&F doped hierarchical macro-/mesoporous TiO2 was prepared using hydrothermal method. Most of the nonmetals exist as surface impurities before calcination, without any visible-light response. Thermal calcination makes N effectively implanted into TiO2 lattice and causes red shift in optical absorption, but S and F are mainly on the surface. The activity of prepared samples for the photodegradation of rhodamine B under visible light was evaluated. The reaction over doped samples without calcination proceeds solely via self-sensitization, and calcination effectively enhances the photodegradation due to nonmetal doping. After being stored for ca. half a year, the activity of all samples are promoted significantly due to the water-mediated adsorption switch from covalent to electrostatic adsorption, caused by pre-bonding of water to surface bridging hydroxyls. The activity of doped samples is further enhanced with water treatment, attributed to the formation of more water-mediated electrostatic modes. The combination of nonmetal doping and water-mediated adsorption switch greatly enhances the visible-light activity of TiO2 (e.g., water-treated N&F-codoped sample shows 6.8-fold higher activity than pure TiO2).