Is platinum-loaded titania the best material for dye-sensitized hydrogen evolution under visible light?

A dye-sensitized photocatalyst combining Pt-loaded TiO₂ and Ru(II) tris-diimine sensitizer (RuP) was constructed and its activity for photochemical hydrogen evolution was compared with that of Pt-intercalated HCa₂Nb₃O₁₀ nanosheets. When the sacrificial donor ethylenediaminetetraacetic acid (EDTA) disodium salt dihydrate was used, RuP/Pt/TiO₂ showed higher activity than RuP/Pt/HCa₂Nb₃O₁₀. In contrast, when NaI (a reversible electron donor) was used, RuP/Pt/TiO₂ showed little activity due to back electron transfer to the electron acceptor (I₃^–), which was generated as the oxidation product of I^–. By modification with anionic polymers (sodium poly(styrenesulfonate) or sodium polymethacrylate) that could inhibit the scavenging of conduction band electrons by I₃^–, the H₂ production activity from aqueous NaI was improved, but it did not exceed that of RuP/Pt/HCa₂Nb₃O₁₀. Transient absorption measurements showed that the rate of semiconductor-to-dye back electron transfer was slower in the case of TiO₂ than HCa₂Nb₃O₁₀, but the electron transfer reaction to I₃^– was much faster. These results indicate that Pt/TiO₂ is useful for reactions with sacrificial reductants (e.g., EDTA), where the back electron transfer reaction to the more reducible product can be neglected. However, more careful design of the catalyst will be necessary when a reversible electron donor is employed.