Accelerated Recombination Reaction through Interfacial Fe^IV=O Accumulation on Photoanode Surfaces

The observation of third-order water oxidation kinetics by Fe^IV=O accumulation at the semiconductor-electrolyte interface is a milestone for understanding the four-electron transfer reaction mechanism. However, the consequences of such Fe^IV=O accumulation for the associated recombination reaction kinetics at the interface have not been fully explored so far. Here, we observe fast second-order recombination reaction kinetics for Fe^IV=O as the result of its accumulation at the model hematite-electrolyte interface, compared to the first-order recombination reaction kinetics for a lesser amount of available Fe^IV=O. We refer to this phenomenon as "accumulation-accelerated recombination (AAR)" and highlight the adverse role of Fe^IV=O accumulation at the interface. Further, we demonstrate that this fast second-order AAR could be slowed down to first-order kinetics by (i) deprotonation of the metal oxide surface; (ii) evacuating the conduction band electrons; and (iii) partial substitution of Fe^IV=O with less active Co^IV=O species. Such an insight is vital not only for understanding the efficiency loss mechanisms at the semiconductor-electrolyte interface but also for interpreting the interfacial behavior of photovoltaic systems and photocatalysts.