Atomic-Level Composition Engineering in M ₂₅ Nanoclusters for Photocatalytic Degradation of Methylene Blue: From Homometallic Au ₂₅ , Ag ₂₅ , and Cu ₂₅ to Heterometallic Au ₁ Cu ₂₄

Atomically precise metal nanoclusters provide an ideal platform for elucidating composition–activity relationships in photocatalysis. Herein, four structurally defined M 25 nanoclusters, Au 25, Ag 25, Cu 25, and Au 1 Cu 24, were supported on g-C 3 N 4 to establish an atomic-level composition-engineering model for photocatalytic methylene blue (MB) degradation. Among the homometallic clusters, Au 25 exhibited the highest activity, followed by Ag 25, whereas Cu 25 showed the lowest performance. Remarkably, substitution of a single Au atom into Cu 25 to form Au 1 Cu 24 markedly enhanced the photocatalytic activity to a level comparable to that of Au 25 . Au 1 Cu 24 /g-C 3 N 4 achieved over 95% MB degradation within 120 min, a TOC removal efficiency of 71.6%, and retained above 88% degradation efficiency after five cycles. Photoelectrochemical measurements indicate that the improved performance of Au 1 Cu 24 is associated with more efficient interfacial charge transfer and suppressed charge-carrier recombination relative to the Cu 25 -based system. These results demonstrate that single-atom heterometal incorporation can effectively regulate photocatalytic function and provide a practical strategy for designing efficient and economical nanocluster-based photocatalysts.