Electron Transfer in Dye‐Sensitised Semiconductors Modified with Molecular Cobalt Catalysts: Photoreduction of Aqueous Protons

Abstract

A visible-light driven H(2) evolution system comprising of a Ru(II) dye (RuP) and Co(III) proton reduction catalysts (CoP) immobilised on TiO(2) nanoparticles and mesoporous films is presented. The heterogeneous system evolves H(2) efficiently during visible-light irradiation in a pH-neutral aqueous solution at 25 °C in the presence of a hole scavenger. Photodegradation of the self-assembled system occurs at the ligand framework of CoP, which can be readily repaired by addition of fresh ligand, resulting in turnover numbers above 300 mol H(2) (mol CoP)(-1) and above 200,000 mol H(2) (mol TiO(2) nanoparticles)(-1) in water. Our studies support that a molecular Co species, rather than metallic Co or a Co-oxide precipitate, is responsible for H(2) formation on TiO(2). Electron transfer in this system was studied by transient absorption spectroscopy and time-correlated single photon counting techniques. Essentially quantitative electron injection takes place from RuP into TiO(2) in approximately 180 ps. Thereby, upon dye regeneration by the sacrificial electron donor, a long-lived TiO(2) conduction band electron is formed with a half-lifetime of approximately 0.8 s. Electron transfer from the TiO(2) conduction band to the CoP catalysts occurs quantitatively on a 10 μs timescale and is about a hundred times faster than charge-recombination with the oxidised RuP. This study provides a benchmark for future investigations in photocatalytic fuel generation with molecular catalysts integrated in semiconductors.

Efficient and Limiting Reactions in Aqueous Light-Induced Hydrogen Evolution Systems using Molecular Catalysts and Quantum Dots

“…22,23 Our results show that the catalytic activity of Co(III)-1 is superior to that of the cobaloxime-type catalyst Co(III)-2, that has also been used as proton reduction catalyst in water (Chart 1). 24 An aqueous equimolar mixture of ascorbic acid/sodium ascorbate (H 2 A/NaHA) was chosen as both buffer and sacrificial electron donor to trap the photogenerated holes in the CdTe QDs. A detailed study of the thermodynamics and kinetics of the system based on the components depicted in Chart 1, has allowed us to build a complete energetics-kinetics scheme of the proton reduction catalysis and identify what are the major advantages and limitations in this kind of photo-driven hydrogen evolution processes.…”

Enzymes as Exploratory Catalysts in Artificial Photosynthesis

“…The lifetime of the charge-separated state is also considerably long, with a value of ca. 1 ms [ 74 ]. Electron injection into an enzyme however has not been measured to date.…”

“…Electron injection into an enzyme however has not been measured to date. It is known that ET from TiO 2 to a synthetic cobaloxime H + reduction catalyst occurs with a rate constant of 10 5 s -1 [ 74 ]. This gives a ratio of forward ET/back ET of 100, which is more favourable than in the CdS:HydA system.…”

Dye-Sensitized Photocatalytic Water Splitting and Sacrificial Hydrogen Generation: Current Status and Future Prospects

“…Carboxyl [49][50][51][52] and phosphonate [53][54][55] are the two most important terminal groups of Ru-complexes that have been studied. Choi's team has studied the photocatalytic activity of semiconductors using Ru-complexes with various terminal groups.…”