Photocatalytic activity of neodymium ion doped TiO2 for 2-mercaptobenzothiazole degradation under visible light irradiation

A series of neodymium ion-doped titanium dioxide (Nd3+ -TiO2) catalysts were prepared by means of a sol-gel method. The physical and chemical properties of the catalysts were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) N-2 sorbtion method, UV-visible diffusive reflective spectroscopy (DRS), and photoluminescence (PL) analyses. The adsorption behaviour and photocatalytic activity of Nd3+ -TiO2 under visible light irradiation were evaluated for aqueous 2-mercaptobenzothiazole (MBT) solution. The analytical results of XRD and BET demonstrate that the neodymium ion doping could reduce the crystallite size and increase the specific surface area of TiO2 catalysts. The analytical results of DRS show that Nd3+ doping did not shift the main absorption band edge significantly, but some new absorption peaks attributable to 4f internal electron transition existed in the visible region. It was further confirmed that significant PL emission occurred in the visible range of 350-700 nm, attributable to the electron transfer between Nd3+ and TiO2 owing to introduction of a Nd 4f level. The experimental results of adsorption isotherm tests demonstrate that both the saturated adsorption amount (Gamma(max)) and adsorption equilibrium constant (K-a) of Nd3+ -TiO2 catalysts increased significantly with the increased Nd3+ dosage. Furthermore, the Nd3+ -TiO2 catalysts demonstrated significant activity towards photocatalytic degradation of MBT in aqueous solution under visible light irradiation, whereas the TiO2 catalyst did not. An optimal dosage of Nd3+ doping was found to be 0.7%. We propose that the introduction of the Nd 4f level plays a crucial role in visible photosensitization and enhancement of the electron-hole separation.