Direct Observation of Interfacial Charge Transfer in Water Reduction at a Cu ₂ O Photocathode by In Situ Near‐Field Heterodyne Transient Grating Spectroscopy

We investigated the interfacial charge‐carrier dynamics of a Cu ₂ O photocathode during water reduction using near‐field heterodyne transient grating (NF‐HD‐TG) spectroscopy. Systematic measurements under various applied potentials, electrolyte conditions, and scavenger environments revealed multiple charge transfer and recombination processes, providing direct insight into reduction‐related electron dynamics. We were able to clearly observe signals arising from the interfacial electron transfer involved in the reduction process with a time constant of 1.33 × 10 ^{− 6}  s using NF‐HD‐TG spectroscopy at 0  V _RHE , where the water reduction reaction occurs, indicating that the electron transfer governing the reduction kinetics is relatively faster than the hole transfer during water oxidation. To elucidate the origin of the relatively fast electron transfer, we obtained temperature‐dependent rate constants using NF‐HD‐TG spectroscopy, from which the activation energy was extracted. The results revealed that the rapid electron transfer originates from the relatively low activation energy of the electron‐transfer process.