Efficient Photocatalytic Hydrogen Production over Rh-Doped Inverse Spinel Zn₂TiO₄

Introducing foreign elements is an appealing means to endow wide band gap semiconductors with visible-light responses. Here, we investigated a series of Rh-doped Zn₂TiO₄ as water reduction photocatalysts. Our results show that Rh doping is effective to improve the visible-light response as well as surface hydrophilicity of Zn₂TiO₄. Efficient photocatalytic hydrogen evolution was observed after Rh doping and was found to be compositional dependent. The best performance was recorded for sample Zn₂Ti_0.96Rh_0.04O₄, with an apparent quantum efficiency of ≈0.74 % under full-range irradiation (λ≥250 nm) and ≈0.25 % under visible-light irradiation (λ≥420 nm). The light absorption and surface hydrophilicity are likely the controlling factors for the better activity. Theoretical calculations suggest that Rh doping is responsible for a band gap reduction and is due to a newly formed valence band composed mainly of Rh 4d orbitals at the top of the original valence band. The visible-light activity can be attributed to the electron excitations from this new valence band to the conduction band.