The Growth of Copper Clusters over ZnO: the Competition between Planar and Polyhedral Clusters

Abstract

Hybrid quantum and molecular mechanical calculations have been used to investigate the nucleation and growth of copper clusters on the (0001)-Zn polar surface of ZnO. Our method is based on the embedded molecular cluster approach developed to study point defects in polarizable ionic solids, where we make use of the chemically accurate exchange and correlation density functional, B97-1, for the treatment of interactions between the metal cluster and oxide support. Following the initial seeds of cluster growth at the anchor sites on the zinc terminated surface, we identify distinct families of structures for Cu4−Cu7 in the main oxidation states and show when the clusters move from 2- to 3-dimensional entities. Our calculations corroborate and rationalize the experimental evidence of the predominantly neutral nature of larger clusters, with the positive charge concentrating at the anchor site, which is a copper atom occupying a surface vacant zinc site. We unravel the mechanisms for the planar clusters stabilization around the anchor, with the upper copper atoms effectively wetting the ZnO surface, and for the formation of polyhedral configurations prompted by interaction with electron-rich oxide surface ions. From comparison of both binding and nucleation energies, we predict a dynamical equilibrium between planar and polyhedral cluster morphologies for small copper clusters, which can alternate upon cluster growth and chemical interactions. We conclude that the ability of anchor sites to stabilize copper clusters of different morphologies is the key factor that prevents sintering of supported metal clusters and maximizes the surface area of the catalyst.