On the mechanism of inter-cluster alloying reactions: two-stage metal exchange of [Au25(PET)18]− and [Ag25(DMBT)18]− clusters†
Alloying is an effective way to tune the physical and chemical properties of metal nanoparticles (NPs) and nanoclusters (NCs). This study revealed the two-stage metal exchange mechanism of the alloying reaction between atomically precise noble metal clusters. It was found that the inter-cluster alloying reaction of [Au25(PET)18]− and [Ag25(DMBT)18]− clusters proceeded through the formation of the dianionic adduct [Au25Ag25(PET)18(DMBT)18]2−. Once the dianionic adduct was formed, its structure evolved gradually by breaking and recombining the ligand shells. During this process, the ligand shell metal exchange took place first (stage I). Thereafter, the heterometal atom in the ligand-shell was swapped with the metal atom in the icosahedral M13-kernel (stage II). The mechanisms of two kinds of metal exchange processes were determined. For the ligand-shell/ligand-shell metal exchange, it proceeded via the formation of a key intermediate containing two doubly bridged metal clusters. In the case of the ligand-shell/metal-kernel metal exchange, a metal cluster 'catalyzes' heterometal atom diffusion mechanism was unraveled. Through comprehensively studying the metal exchange pathways, our studies further revealed that the [Au25(PET)18]− cluster had a higher activity to form the kernel-doped alloy cluster than the [Ag25(DMBT)18]− cluster.