Parameters affecting electron transfer dynamics from semiconductors to molecular catalysts for the photochemical reduction of protons

The aim of this work is to use transient absorption spectroscopy to study the parameters affecting the kinetics and efficiency of electron transfer in a photocatalytic system for water reduction based on a cobalt proton reduction catalyst (CoP) adsorbed on a nanocrystalline TiO₂ film. In the first approach, water is used as the proton and electron source and H₂ is generated after band gap excitation of TiO₂ functionalised with CoP. The second system involves the use of a sacrificial electron donor to regenerate the TiO₂/CoP system in water at neutral pH. The third system consists of CoP/TiO₂ films co-sensitised with a ruthenium-based dye (RuP). In particular, we focus on the study of different parameters that affect the kinetics of electron transfer from the semiconductor to the molecular catalyst by monitoring the lifetime of charge carriers in TiO₂. We observe that low catalyst loadings onto the surface of TiO₂, high excitation light intensities and small driving forces strongly slow down the kinetics and/or reduce the efficiency of the electron transfer at the interface. We conclude that the first reduction of the catalyst from Co^III to Co^II can proceed efficiently even in the absence of an added hole scavenger at sufficiently high catalyst coverages and low excitation densities. In contrast, the second reduction from Co^II to Co^I, which is required for hydrogen evolution, appears to be at least 10⁵ slower, suggesting it requires efficient hole scavenging and almost complete reduction of all the adsorbed CoP to Co^II. Dye sensitisation enables visible light photoactivity, although this is partly offset by slower, and less efficient, hole scavenging.