Accelerated Recombination Reaction through Interfacial <i>Fe^IV=O</i> Accumulation on Photoanode Surfaces

The observation of third-order water oxidation kinetics by <i>Fe</i>^<i>IV</i><i>=O</i> accumulation at the semiconductor-electrolyte interface is a milestone for understanding the four-electron transfer reaction mechanism. However, the consequences of such <i>Fe</i>^<i>IV</i><i>=O</i> 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 <i>Fe</i>^<i>IV</i><i>=O</i> 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 <i>Fe</i>^<i>IV</i><i>=O</i>. We refer to this phenomenon as "accumulation-accelerated recombination (AAR)" and highlight the adverse role of <i>Fe</i>^<i>IV</i><i>=O</i> accumulation at the interface. Further, we demonstrate that this fast second-order <i>AAR</i> 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 <i>Fe</i>^<i>IV</i><i>=O</i> with less active <i>Co</i>^<i>IV</i><i>=O</i> 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.