Abstract
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 CoIII to CoII 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 CoII to CoI, 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 CoII. Dye sensitisation enables visible light photoactivity, although this is partly offset by slower, and less efficient, hole scavenging.
Highlights
The development of photocatalytic systems for the hydrogen generation from water splitting is receiving renewed attention as a renewable fuel synthesis strategy.[1,2] The photochemicalPaper kinetics that are critical in determining the efficiency of the system
Rapid progress is currently being made in the development of synthetic and bio-inspired molecular catalysts for proton reduction, which can be integrated in heterogeneous photocatalytic systems through the functionalisation of semiconductors such as TiO2
We focus on the electron transfer dynamics in a series of model systems based upon mesoporous, nanocrystalline TiO2 lms functionalised with a cobalt based H+ reduction catalyst, and thereby elucidate how these charge carrier dynamics impact the proton reduction efficiency
Summary
The development of photocatalytic systems for the hydrogen generation from water splitting is receiving renewed attention as a renewable fuel synthesis strategy.[1,2] The photochemicalPaper kinetics that are critical in determining the efficiency of the system. Electron transfer dynamics between molecular photosensitisers and TiO2 has been widely studied in the context of dye sensitised solar cells.[14,15,16] Such devices are based on the electron injection from the photosensitiser into the TiO2 semiconductor. H+ reduction by molecular catalysts immobilised on TiO2 requires the reverse electron transfer reaction – from the TiO2 conduction band to the molecular catalyst. This reverse electron transfer reaction is an undesired recombination pathway in dye sensitised solar cells. The parameters required to optimise the efficiency of this reaction have received relatively little attention to date, and are the subject of this paper
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