Visible-light sensitive hydrogen evolution photocatalyst ZnRh2O4

We investigated the ability of the oxide ZnRh2O4 to serve as a solar H2-evolution photocatalyst due to the predicted potential of its conduction band bottom, which may allow thermodynamically favorable H2 evolution in spite of its small band-gap of 1.2 eV. ZnRh2O4 produced H2 in the presence of HCHO, but only scarcely in the presence of CH3OH, indicating that the potential of the valence band top of ZnRh2O4 lies at ∼0.1 V (vs. SHE). Thus, the conduction band bottom potential (∼−1.1 V) lies much more negative than the potential of H+/H2, allowing thermodynamically favorable H2 evolution. In addition, the irradiated-light-wavelength dependence of the quantum efficiency (QE) for H2 evolution was consistent with the solar spectrum, and the QE was quite high (∼27%), even at a wavelength of 770 ± 25 nm. Taken together, our findings indicate that ZnRh2O4 can utilize solar light effectively, not only the entire range of UV and visible light, but is also sensitive to infrared light.