Controlling Ag-doping in [Ag_xAu_25-x(SC₆H₁₁)₁₈]^- nanoclusters: cryogenic optical, electronic and electrocatalytic...

Doping metal nanoclusters with a second type of metal is a powerful method for tuning the physicochemical properties of nanoclusters at the atomic level and it also provides opportunities for a fundamental understanding of alloying rules as well as new applications. Herein, we have devised a new, one-phase strategy for achieving heavy Ag-doping in Au₂₅(SR)₁₈ nanoclusters. This strategy overcomes the light doping of silver by previous methods. X-ray crystallography together with ESI-MS determined the composition of the product to be [Ag_xAu_25-x(SC₆H₁₁)₁₈]^- with x ∼ 21. Cryogenic optical spectroscopy (80-300 K) revealed fine features in optical absorption peaks. Interestingly, the heavy doping of silver does not significantly change the electron-phonon coupling strength and the surface phonon frequency. DFT simulations reproduced the experimentally observed trend of electronic structure evolution with Ag doping. We further investigated the electrocatalytic performance of such heavily Ag-doped nanoclusters for oxygen reduction in alkaline solutions. The mass activity of ligand-off [Ag_xAu_25-x(SC₆H₁₁)₁₈]^- nanoclusters (217.4 A g^-1_metal) was determined to be higher than that of ligand-on nanoclusters (29.6 A g^-1_metal) at a potential of -0.3 V (vs. Ag/AgCl). The rotating disk electrode (RDE) studies revealed the tunable kinetic features of the nanoclusters by ligand removal.