Electron transfer in dye-sensitised semiconductors modified with molecular cobalt catalysts: Photoreduction of aqueous protons

A visible-light driven H₂ evolution system comprising of a Ru^II dye (RuP) and Co^III proton reduction catalysts (CoP) immobilised on TiO₂ nanoparticles and mesoporous films is presented. The heterogeneous system evolves H₂ efficiently during visible-light irradiation in a pH-neutral aqueous solution at 25 C in the presence of a hole scavenger. Photodegradation of the self-assembled system occurs at the ligand framework of CoP, which can be readily repaired by addition of fresh ligand, resulting in turnover numbers above 300 mol H₂ (mol CoP)^-1 and above 200,000 mol H₂ (mol TiO₂ nanoparticles) ^-1 in water. Our studies support that a molecular Co species, rather than metallic Co or a Co-oxide precipitate, is responsible for H₂ formation on TiO₂. Electron transfer in this system was studied by transient absorption spectroscopy and time-correlated single photon counting techniques. Essentially quantitative electron injection takes place from RuP into TiO₂ in approximately 180 ps. Thereby, upon dye regeneration by the sacrificial electron donor, a long-lived TiO₂ conduction band electron is formed with a half-lifetime of approximately 0.8 s. Electron transfer from the TiO₂ conduction band to the CoP catalysts occurs quantitatively on a 10 μs timescale and is about a hundred times faster than charge-recombination with the oxidised RuP. This study provides a benchmark for future investigations in photocatalytic fuel generation with molecular catalysts integrated in semiconductors. Long-lived charge separation! Photoexcitation of a ruthenium dye (RuP) on TiO₂ results in ultrafast electron injection into the semiconductor, and the resulting long-lived conduction-band electron has sufficient time to reach co-attached cobalt catalysts (CoP) for H ₂ evolution in a pH-neutral solution and at room temperature (see figure).