Mechanistic insight into conflicting valence fluctuation of Cu(I) photocorrosion pathways: The role of charge behaviors.

Photocorrosion remains a major challenge that hinders the development of photoelectrochemical (PEC) solar fuel production. Although Cu2O is a promising photocathode material due to its suitable band structure and visible light absorption, both oxidative and reductive photocorrosion pathways have been reported, with no consensus established in the research community in the PEC system. In this work, we fabricate three Cu2O photocathodes with distinct charge transport properties to investigate the correlation between charge accumulation and photocorrosion behavior. Both oxidative and reductive photocorrosion were observed depending on the type and location of charge accumulation. Notably, we propose a refined oxidative photocorrosion mechanism in which photogenerated holes induce surface Cu2O dissolution to Cu2+, followed by coordination with adsorbed *OH to form Cu(OH)2/CuO under illumination, which is contrary to the widely assumed direct oxidation from Cu2O to CuO. This work not only provides a unified understanding of the dual photocorrosion pathways in Cu2O, but also offers a generalizable framework for understanding photo-induced degradation in metal oxide photocathodes and for guiding the design of more robust PEC systems through charge transport engineering.