Non-dispersive second-order charge recombination at mesoporous TiO₂/adsorbed molecules/electrolyte interfaces

The large surface areas of mesoporous metal oxide electrodes have been widely exploited for various applications including photocatalysis, electrosynthesis cells, dye-sensitized solar cells, and perovskite solar cells. Understanding interfacial electron transfer kinetics is essential for these applications. Charge recombination between electrons in the electrode and surface-adsorbed molecules has frequently been reported to exhibit highly dispersive kinetics. For more than two decades, these results have been interpreted primarily with electron trapping and de-trapping within TiO₂. Here, we show that charge recombination at the molecular-anchored mesoporous TiO₂/electrolyte interface follows a non-dispersive second-order rate law and exhibit little temperature dependence under appropriate conditions. Furthermore, we show that the same TiO₂ displays dispersive recombination depending on the structure of adsorbed molecules and electrolyte cations, demonstrating that the dispersive recombination is not an intrinsic property of mesoporous TiO₂ and cannot be attributed solely to intra-band charge traps. Instead, the results suggest that electronic coupling disorder of the molecules on the TiO₂ surface is a major origin of the dispersion.