Surface modified titanium dioxide using transition metals: nickel as a winning transition metal for solar light photocatalysis

Titanium dioxide has been widely used as an antimicrobial agent, UV-filter and catalyst for pollution abatement. Herein, surface modifications with selected transition metals (Me) over colloidal TiO2 nanopartides and immobilization with a colloidal SiO2 binder as composite films (MeTiO2/SiO2) on a glass carrier were used to enhance solar-light photoactivity. Colloidal TiO2 nanopartides were modified by Loading selected transition metals (Me = Mn, Fe, Co, Ni, Cu, and Zn) in the form of chlorides on their surface. They were present primarily as oxo-nanoclusters and a portion as metal oxides. The structural characteristics of bare TiO2 were preserved up to an optimal metal Loading of 0.5 wt%. We have shown in situ that metaloxo-nanodusters with a redox potential dose to that of O-2/O-2(center dot-) were able to function as co-catalysts on the TiO2 surface which was excited by solar-light irradiation. The materials were tested for photocatalytic activity by two opposite methods; one detecting O-2(center dot-) (reduction, Rz ink test) while the other detecting (OH)-O-center dot (oxidation, terephthaEic acid test). It was shown that the enhancement of the solar-light activity of TiO2 by the deposition of transition metal oxo-nanoclusters on the surface depends strongly on the combination of the reduction potential of such species and appropriate band positions of their oxides. The latter prevented excessive self-recombination of the photogenerated charge carriers by the nanoclusters in Ni and Zn modification, which was probably the case in other metal modifications. Overall, only Ni modification had a positive effect on solar photoactivity in both oxidation and reduction reactions.