Determining and Controlling Cu-Substitution Sites in Thiolate-Protected Gold-Based 25-Atom Alloy Nanoclusters

Alloying has been studied actively for thiolate (SR)-protected metal nanoclusters (NCs) because heteroatom mixing enables the creation of novel electronic structures, physical/chemical properties, and functions. Among them, SR-protected gold (Au)-based 25-atom alloy NCs (Au25-xMx(SR)(18); M = heteroatom) are the most studied NCs. However, a geometrical structure of copper (Cu)-substituted [Au25-xCux(SR)(18)](-) has not been determined by single-crystal X-ray diffraction (SC-XRD) analysis to date. In this study, we discovered suitable crystallization conditions for [Au25-xCux(PET)(18)](-) (PET = phenylethanethiolate) and revealed by SC-XRD that Cu substitutes Au preferentially in the staple regardless of synthesis methods. Cu substitution also occurred at the surface of metal core when the number of Cu substitutions increased. The main Cu-substitution site can be changed from the staple to the surface of metal core when the central Au atom of the NCs is substituted by platinum. Experimental and theoretical results on the electronic structure indicate that the effect of Cu substitution on the electronic structure of Au-based 25-atom alloy NCs is similar to that of silver substitution. These findings are expected to lead to clear design guidelines to create NCs with new physical/chemical properties and functions.