Coalescence of Atomically Precise Clusters on Graphenic Surfaces

The interaction of ultrasmall metal clusters with surfaces of graphene is important for developing promising future applications of graphenic materials. In the experiment, chemically synthesized reduced graphene oxide (RGO) in water was mixed with Au₂₅SR₁₈ (where SR, SCH₂CH₂Ph, is a ligand protecting the cluster core) in tetrahydrofuran, and a completely new cluster, larger in mass, was formed at the liquid–liquid interface. Matrix assisted laser desorption ionization mass spectrometry of the product attached to RGO show that the peak due to Au₂₅SR₁₈ disappears gradually upon reaction and a single sharp peak referred to here as “135 ± 1 kDa cluster” appears. The composition of the new cluster is very close to the well-known magic cluster, Au₁₄₄SR₆₀ while the peak maximum is at Au₁₃₅SR₅₇. The formation of 35 ± 1 kDa cluster from the parent Au₂₅ is proposed to be governed by the trapping of smaller clusters in a deep potential well generated at the graphene surface. We theoretically model the active role of the surface in stabilizing the large clusters. Our studies indicate a general mechanism of stabilization of clusters of precise size via the competition between the interfacial fluctuations and the energy scales of interaction of the clusters with the surface. The chemical transformation occurs at deformable surfaces at reduced particle densities which is in good agreement with the theoretical model. Transformations of this kind are important in controlled tuning of particles at graphenic surfaces