Photocatalyst-Mediator Interface Modification by Surface-Metal Cations of a Dye-Sensitized H₂ Evolution Photocatalyst

To develop highly active H₂ evolving dye-sensitized photocatalysts (DSPs) applicable for Z-scheme water splitting, we synthesized a series of Ru(II)-dye-double-layered DSPs, X'-RuCP-Zr-RuP@Pt-TiO₂ (X'-DSP) with different surface-bound metal cations (X' = Fe²⁺, Y³⁺, Zr⁴⁺, Hf⁴⁺, and Bi³⁺). In 0.5 M KI aqueous solution, the photocatalytic H₂ evolution activity under blue light irradiation (λ = 460 ± 15 nm) increased in the following order: nonmetal-modified DSP, H-DSP (turn over number for 6 h irradiation = 35.2) < Fe-DSP (54.9) ≈ Bi-DSP (55.2) < Hf-DSP (65.5) ≈ Zr-DSP (68.3) ≈ Y-DSP (71.5), suggesting that the redox-inactive and highly charged metal cations tend to improve the electron donation from the iodide electron mediator. On the other hand, DSPs having heavy metal cations, Hf-DSP (18.4) and Bi-DSP (16.6), exhibited better activity under green light irradiation (λ = 530 ± 15 nm) than Zr-DSP (15.7) and H-DSP (7.80), implying the contribution of a heavy atom effect of the surface-bound metal cation to partially allow the spin-forbidden metal-to-ligand charge-transfer excitation.