Effect of Rh valence state and doping concentration on the structure and photocatalytic H2 evolution in (Nb,Rh) codoped TiO2 nanorods

Abstract

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 t_2g⁵ Rh(IV) and closed shell t_2g⁶ 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), Ti_1−2xNb_xRh_xO₂ 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.