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.