Unveiling the photocatalytic degradation mechanism of methylene blue on the octahedral Cu2O(111) facets.

Visible-light photocatalysis provides a promising route for removing persistent organic pollutants from water. However, the molecular-level relationship between exposed crystal facets, pollutant adsorption, and radical-induced bond cleavage remains poorly understood. Here, octahedral Cu2O with predominantly exposed (111) facets was synthesized at room temperature by a wet-chemical method. The photocatalytic degradation of methylene blue (MB) was investigated experimentally and analyzed using density functional theory (DFT). The calculations indicated that MB preferentially adopts a parallel adsorption configuration through Cu--S and Cu--C interactions on the Cu2O(111) surface. As a hydroxyl radical (˙OH) approaches a carbon center in MB, the resulting redistribution of electron density weakens the adjacent covalent bonds. A single ˙OH radical induces local bond cleavage followed by partial structural recovery; five simultaneous ˙OH radicals cause irreversible ring opening; and six radicals lead to extensive fragmentation of the three-ring molecular framework into smaller species. These findings connect facet-selective adsorption with cooperative radical-induced molecular decomposition, providing an atomistic framework for understanding and designing facet-engineered photocatalysts for organic-pollutant degradation.