At present, one of the most studied molecular device for the conversion of sunlight into electricity is the dye‐sensitised solar cell (DSSC). To date, ruthenium polypyridyl complexes have shown the ...highest light‐to‐energy conversion efficiencies because of their photophysical, photochemical and electrochemical properties. In addition, the overall efficiency achieved by DSSCs is strongly dependent on the interfacial charge‐transfer reactions that take place between the different components of the solar cell: the injection of electrons into the conduction band of the semiconductor by the dye, the transport of electrons through the semiconductor towards the working electrode contact, dye regeneration by the redox pair present in the electrolyte and the recombination reaction between the photoinjected electrons in the semiconductor and the oxidised species of the dye and the electrolyte. This microreview comprises: (i) the operational principles of complete functional photovoltaic devices and (ii) several synthetic methods, properties and main applications of most relevant homoleptic and heteroleptic ruthenium complexes reported in the literature.
The review describes dye solar cells and the use of ruthenium complexes as sensitisers in this molecular device.
Abstract
Sunlight plays a critical role in the development of emerging sustainable energy conversion and storage technologies. Light-induced CO
2
reduction by artificial photosynthesis is one of the ...cornerstones to produce renewable fuels and environmentally friendly chemicals. Interface interactions between plasmonic metal nanoparticles and semiconductors exhibit improved photoactivities under a wide range of the solar spectrum. However, the photo-induced charge transfer processes and their influence on photocatalysis with these materials are still under debate, mainly due to the complexity of the involved routes occurring at different timescales. Here, we use a combination of advanced in situ and time-resolved spectroscopies covering different timescales, combined with theoretical calculations, to unravel the overall mechanism of photocatalytic CO
2
reduction by Ag/TiO
2
catalysts. Our findings provide evidence of the key factors determining the enhancement of photoactivity under ultraviolet and visible irradiation, which have important implications for the design of solar energy conversion materials.
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 sub(2) film. In the first approach, water is used as the proton and electron source and H sub(2) is generated after band gap excitation of TiO sub(2) functionalised with CoP. The second system involves the use of a sacrificial electron donor to regenerate the TiO sub(2)/CoP system in water at neutral pH. The third system consists of CoP/TiO sub(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 sub(2). We observe that low catalyst loadings onto the surface of TiO sub(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 super(III) to Co super(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 super(II) to Co super(I), which is required for hydrogen evolution, appears to be at least 10 super(5) slower, suggesting it requires efficient hole scavenging and almost complete reduction of all the adsorbed CoP to Co super(II). Dye sensitisation enables visible light photoactivity, although this is partly offset by slower, and less efficient, hole scavenging.
A series of heteroleptic ruthenium(II) polypyridyl complexes containing phenanthroline ligands have been designed, synthesized, and characterized. The spectroscopic and electrochemical properties of ...the complexes have been studied in solution and adsorbed onto semiconductor nanocrystalline metal oxide particles. The results show that for two of the ruthenium complexes, bearing electron-donating (−NH2) or electron-withdrawing (−NO2) groups, the presence of the redox-active I−/I3 − electrolyte produces important changes in the interfacial charge transfer processes that limit the device performance. For example, those dyes enhanced the electron recombination reaction between the photoinjected electrons at TiO2 and the oxidized redox electrolyte. In an effort to understand the details of such striking observations, we have monitored the charge transfer reactions taking place at the different interfaces of the devices using time-resolved single photon counting, laser transient spectroscopy, and light-induced photovoltage measurements.
The development of synthetic systems for the conversion of solar energy into chemical fuels is a research goal that continues to attract growing interest owing to its potential to provide renewable ...and storable energy in the form of a ‘solar fuel’. Dye-sensitised photocatalysis (DSP) with molecular catalysts is a relatively new approach to convert sunlight into a fuel such as H 2 and is based on the self-assembly of a molecular dye and electrocatalyst on a semiconductor nanoparticle. DSP systems combine advantages of both homogenous and heterogeneous photocatalysis, with the molecular components providing an excellent platform for tuning activity and understanding performance at defined catalytic sites, whereas the semiconductor bridge ensures favourable multi-electron transfer kinetics between the dye and the fuel-forming electrocatalyst. In this tutorial review , strategies and challenges for the assembly of functional molecular DSP systems and experimental techniques for their evaluation are explained. Current understanding of the factors governing electron transfer across inorganic-molecular interfaces is described and future directions and challenges for this field are outlined.
The development of synthetic systems for the conversion of solar energy into chemical fuels is a research goal that continues to attract growing interest owing to its potential to provide renewable ...and storable energy in the form of a 'solar fuel'. Dye-sensitised photocatalysis (DSP) with molecular catalysts is a relatively new approach to convert sunlight into a fuel such as H
2
and is based on the self-assembly of a molecular dye and electrocatalyst on a semiconductor nanoparticle. DSP systems combine advantages of both homogenous and heterogeneous photocatalysis, with the molecular components providing an excellent platform for tuning activity and understanding performance at defined catalytic sites, whereas the semiconductor bridge ensures favourable multi-electron transfer kinetics between the dye and the fuel-forming electrocatalyst. In this
tutorial review
, strategies and challenges for the assembly of functional molecular DSP systems and experimental techniques for their evaluation are explained. Current understanding of the factors governing electron transfer across inorganic-molecular interfaces is described and future directions and challenges for this field are outlined.
Recent progress and basic understanding in the emerging field of dye-sensitised photocatalysis for light-driven hydrogen production is reviewed.
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(P2 R′N2 R″)22+ core (P2 R′N2 R″ = 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.
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