Sensitization of WO3 and SnO2 photoanodes with molecular dyes for water splitting

Dyes endowed with strongly oxidizing ground state oxidation potentials are exploited in dye sensitized photoelectrochemical cells for water splitting with the aim of characterizing their interfacial separation dynamics. In the first example [(N,N-Bis(2-(trimethylammonium)-ethylene) perylene 3,4,9,10-tetracarboxylic acid bisimide)-(PF6)2] [1] was observed to spontaneously adsorb onnanocrystalline WO3 surfaces via aggregation/hydrophobic forces. Under visible irradiation (λ > 435 nm), the excited state of the peylene underwent oxidative quenching by electron injection (kinj > 108 s-1) to WO3, leaving a strongly positive hole (Eox ≈ 1.7 V vs SCE), which allows to drive demanding photo-oxidation reactions in photoelectrochemical cells (PECs). The casting of IrO2 nanoparticles (NPs), acting as water oxidation catalysts (WOCs) on the sensitized electrodes, led to a 4-fold enhancement in photoanodic current, consistent with hole transfer from oxidized dye to IrO2 occurring on the microsecond time scale. In a second case study the charge transfer dynamics involving a new Ru(II) polypyridine complex developed to generate strongly oxidizing photoholes for water oxidation, was studied by electrochemical, photo electrochemical and spectroscopic means. Interestingly this species, loaded on TiO2, underwent a change in the injection mechanism in the presence of ascorbic acid, consistent with the reductive quenching of the MLCT excited state and injection from the photo generated reduced state. On the other hand the usual oxidative quenching was observed on SnO2 photo anodes, where the activation of IrO2 by the oxidized state of the sensitizer was observed. Once the interaction with suitable WOCs is optimized, these molecular designs may hold potentialities for the straightforward building of molecular level devices for solar fuel production.