The generation of renewable H2 through an efficient photochemical route requires photoinduced electron transfer (ET) from a light harvester to an efficient electrocatalyst in water. Here, we report on a molecular H2 evolution catalyst (NiP) with a DuBois-type [Ni(P2R′N2R″)2]2+ core (P2R′N2R″ = bis(1,5-R′-diphospha-3,7-R″-diazacyclooctane), which contains an outer coordination sphere with phosphonic acid groups. The latter functionality allows for good solubility in water and immobilization on metal oxide semiconductors. Electrochemical studies confirm that NiP is a highly active electrocatalyst in aqueous electrolyte solution (overpotential of approximately 200 mV at pH 4.5 with a Faradaic yield of 85 ± 4%). Photocatalytic experiments and investigations on the ET kinetics were carried out in combination with a phosphonated Ru(II) tris(bipyridine) dye (RuP) in homogeneous and heterogeneous environments. Time-resolved luminescence and transient absorption spectroscopy studies confirmed that directed ET from RuP to NiP occurs efficiently in all systems on the nano- to microsecond time scale, through three distinct routes: reductive quenching of RuP in solution or on the surface of ZrO2 (“on particle” system) or oxidative quenching of RuP when the compounds were immobilized on TiO2 (“through particle” system). Our studies show that NiP can be used in a purely aqueous solution and on a semiconductor surface with a high degree of versatility. A high TOF of 460 ± 60 h–1 with a TON of 723 ± 171 for photocatalytic H2 generation with a molecular Ni catalyst in water and a photon-to-H2 quantum yield of approximately 10% were achieved for the homogeneous system.

Solar-light-driven H2 production in water with a [NiFeSe]-hydrogenase (H2ase) and a bioinspired synthetic nickel catalyst (NiP) in combination with a heptazine carbon nitride polymer, melon (CNx), is reported. The semibiological and purely synthetic systems show catalytic activity during solar light irradiation with turnover numbers (TONs) of more than 50 000 mol H2 (mol H2ase)−1 and approximately 155 mol H2 (mol NiP)−1 in redox-mediator-free aqueous solution at pH 6 and 4.5, respectively. Both systems maintained a reduced photoactivity under UV-free solar light irradiation (λ>420 nm).

Layer-by-layer assembly of a Ru dye and Ni catalyst on a p-type NiO photocathode enables photoelectrochemical H2 generation in water.

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 TiO 2 film. In the first approach, water is used as the proton and electron source and H 2 is generated after band gap excitation of TiO 2 functionalised with CoP. The second system involves the use of a sacrificial electron donor to regenerate the TiO 2 /CoP system in water at neutral pH. The third system consists of CoP/TiO 2 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 TiO 2. We observe that low catalyst loadings onto the surface of TiO 2 , 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 10 5 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. Broader context The photochemical reduction of water into H 2 offers the possibility of harvesting sunlight and storing this energy in the form of chemical bonds. Rapid progress is currently being made in the development of synthetic and bio-inspired molecular catalysts for proton reduction, which can also be integrated in heterogeneous photocatalytic systems through the functionalisation of semiconductors such as TiO 2. The development of efficient hybrid photocatalytic H 2 evolution systems involves not only the study of its catalytic activity, but also the analysis of the electron transfer kinetics and the control of the main recombination pathways. Molecular catalysts for H + reduction require the accumulation of two electrons in one single catalyst, which is potentially a key limiting factor of their efficiency. In this paper, we study three different photochemical systems based on TiO 2 functionalised with a molecular cobalt catalyst and a ruthenium photosensitiser. In particular, we address the kinetics of the two-electron transfer reactions needed for the H 2 production by monitoring the charge carriers (electrons and holes) of the semiconductor by transient absorption spectroscopy.

Splitting water into hydrogen and oxygen with 3d transition metal molecular catalysts and light has been accomplished.