Distance dependent charge separation and recombination in semiconductor/molecular catalyst systems for water splitting

The immobilisation of molecular catalysts on semiconductors for solar fuel production is an attractive strategy to exploit electrocatalysts in a heterogeneous photocatalytic environment. Efficient H2 production in such hybrid systems requires effective electronic coupling between the light harvesting unit and the electrocatalyst. Current examples based on non-precious metal complexes include Co, Ni and Fe electrocatalysts attached to narrow band-gap or dye-loaded wide band-gap semiconductors that allow for visible light absorption. In order to reduce protons to H2 through amononuclear heterolytic route, the semiconductors have to transfer two electrons to one molecular catalyst. Previously, we reported that recombination of the reduced catalyst with valence band holes in the semiconductor limits the efficiency in these photocatalytic systems. Thus, achieving essentially uni-directional (vectorial) electron transfer from the semiconductor to the catalyst is crucial for enhancing long-lived charge separation and allowing the slow catalytic reactions to take place before electron–hole recombination. Understanding and controlling the influence of the molecular structure on interfacial electron transfer dynamics has been a key requirement to enhance the efficiency of dye sensitised solar cells (DSSCs). Analogously to DSSCs, one might expect that changes in the molecular structure of the catalyst in such hybrid systems for solar fuels will also affect the kinetics of charge separation and recombination (in reverse direction of charge separation compared to DSSCs, see Fig. 1). However, systematic studies addressing the effect of molecular structure of the catalyst on charge transfer dynamics are scarce. In this study, we compare the kinetics of charge separation and recombination when a semiconductor (TiO2) is functionalised with three related cobalt electrocatalysts, whose molecular structure varies the physical separation between the catalytic core and the semiconductor surface (Fig. 1). In this hybrid system, the semiconductor acts as light harvester and the H2 evolution is driven by the anchored molecular catalyst. The molecular catalysts and nanocrystalline anatase TiO2 films employed herein were synthesised as reported elsewere. Functionalisation of the TiO2 films with a monolayer of molecular catalyst (ca. 900 molecules of Co1, 1000 of Co2 and 1050 of Co3 per TiO2 particle, see ESI† for detailed calculations) was achieved by dipping the films into 10 4 M catalyst aqueous solutions for 12 h at rt in the dark. The kinetics of charge separation were studied by monitoring the photogenerated charge carriers (electrons and holes) in the Fig. 1 (a) Electron transfer processes in TiO2 functionalised with a molecular catalyst for H2 production after UV-light excitation. The solid black and dashed red arrows indicate charge separation and recombination, respectively. Molecular structures of the catalysts for H reduction are shown in (b) for Co1, (c) for Co2 and (d) for Co3 (charges omitted for clarity). The blue arrows indicate the distance between the anchoring groups and the catalyst metal centre (r, Å), as determined by energy minimised DFT calculations (Fig. S1, ESI†).‡