Water molecule switching heterogeneous proton-coupled electron transfer pathway

Figuring out the specific pathway of semiconductor-mediated proton-coupled electron transfer (PCET) driven by light is essential to solar energy conversion systems. In this work, we reveal that the amount of adsorbed water molecules determines the photo-induced PCET pathway on the TiO2 surface through systematic kinetic solvent isotope effect (KSIE) experiments. At low water content (< 1.7 wt%), the photoinduced single-proton/single-electron transfer on TiO2 nanoparticles follows a stepwise PT/ET pathway with the formation of high-energy H+/D+- O]C or H+/D+- O-C intermediates, resulting in an inverse KSIE (H/D) similar to 0.5 with (Bu3ArO)-Bu-t center dot and KSIE (H/D) similar to 1 with TEMPO in methanol-d(0)/d(4) systems. However, at high water content (> 2 wt%), the PCET reaction follows a concerted pathway with a lower energy barrier, leading to normal KSIEs (H/D) >= 2 with both reagents. In situ ATR-FTIR observation and DFT calculations suggest that water molecules' existence significantly lowers the proton/electron transfer energy barrier, which coincides with our experimental observations.