A Correlated Series of Au/Ag Nanoclusters Revealing the Evolutionary Patterns of Asymmetric Ag Doping

Doping of metal nanoclusters is an effective strategy for tailoring their functionalities for specific applications. To gain fundamental insight into the doping mechanism, it is of critical importance to have access to a series of correlated bimetal nanoclusters with different doping levels and further reveal the successive transformations. Herein, we report asymmetric doping of Ag into an Au₂₁ nanocluster to form a series of new Au/Ag bimetal nanoclusters and the effects of doping on the evolution of size, structure, and properties based upon X-ray crystallography and optical spectroscopy analyses. The asymmetric doping discovered in the series reveals two important rules. First, the heteroatom doping-induced kernel transformation mechanism is revealed, explaining the successive conversions from Au₂₁(S-Adm)₁₅ with an incomplete cuboctahedral kernel to Au₂₀Ag₁(S-Adm)₁₅ with a complete cuboctahedral Au₁₂Ag₁ kernel and then to Au₁₉Ag₄(S-Adm)₁₅ with an icosahedral Au₁₀Ag₃ kernel. The electron density accumulated on the central Au atom(s) is rationalized to force an expansion of radial metal-metal bond angles, which triggers the cuboctahedral-to-icosahedral kernel conversion. This mechanism is generalized by elucidating several other cases. Second, through comparison of a series of seven nanoclusters (all protected by adamantanethiolate), we find that the unit cell symmetry of their crystals is correlated with the symmetry of the cluster's kernel. Specifically, we observe a sequential change from triclinic to monoclinic to trigonal unit cell in the series with increasing kernel symmetry. The kernel structure-dependent optical properties are also discussed.