Effect of Rh valence state and doping concentration on the structure and photocatalytic H ₂ evolution in (Nb,Rh) codoped TiO ₂ nanorods

The simultaneous realization of visible light response and high photocatalytic activity remains a challenging task for TiO2 despite extensive research. Herein, (Nb,Rh) codoping is adopted to extend the absorption band of anatase TiO2 into the visible-light region. Meanwhile, the dependence of the electronic structure, visible-light absorption, and photocatalytic performance on the dopant ratio as well as doping concentration is studied. Open shell t2g5 Rh(iv) and closed shell t2g6 Rh(iii) coexist in Rh-doped TiO2, and the codoped Nb promotes a change in valence state from Rh(iv) to Rh(iii). Rh(iii) is the main active species in charge of the excellent photocatalytic performance, while Rh(iv) doping introduces electron/hole recombination centres. However, surprisingly, a trace of Rh(iv)-doping contributes to a decrease in electron transfer resistance and an increase in donor density, which help to improve photocatalytic performance. By virtue of the controlled content of Rh(iii) and Rh(iv), Ti1-2xNbxRhxO2 exhibits a high hydrogen evolution rate of ∼9000 μmol g-1 h-1 in methanol solution, along with a remarkable photocurrent density of ∼9 μA cm-2 under visible-light irradiation, which are about 170 and 30 times higher than those of pristine TiO2 nanorods, respectively.