Morphology and chemical states of size-selected Ptn clusters on an aluminium oxide film on NiAl(110)

Abstract

The adsorption states of size-selected Pt_n clusters (7 ≤ n ≤ 20) soft-landed on an Al₂O₃/NiAl(110) substrate were investigated using scanning tunneling microscopy, infrared reflection absorption spectroscopy, and temperature programmed desorption. Pt_n clusters lay flat on the surface with a planar structure (n ≤ 18), and three-dimensional two-layer clusters start to appear at n ≥ 19. By considering the Pt–Pt and Pt–oxide bonds in the cluster, the morphological transition could be reasonably explained. Using CO probe molecules, the chemical states of the Pt atoms inside the clusters were investigated. Two ontop CO species were observed inside the clusters, and were assigned as adsorbed CO on neutral and slightly cationic Pt atoms. Despite the first layer Pt atoms, the Pt_n clusters are composed of two kinds of Pt atoms. The observed size dependence of the Pt atoms inside the clusters may contribute to the size-dependent chemical reactivity of Pt_n clusters on the Al₂O₃ surface.