Regular Arrays of Pt Clusters on Alumina: A New Superstructure on Al₂O₃/Ni₃Al(111)

Alumina ultrathin films obtained by high-temperature oxidation of a Ni₃Al(111) surface are a good template to grow regular arrays of metal clusters. Up to now, two hexagonal organizations called “dot” and “network” structures have been observed with distances between clusters of 4.1 and 2.4 nm, respectively. In the present article, we report on an investigation by in situ grazing incidence small-angle X-ray scattering, showing that Pt deposited at room temperature and for a low coverage forms a new hexagonal structure with a distance between clusters being 1.38 nm. For the first time, an assembly of tiny Pt clusters (1–6 atoms) with a very high density (5.85 × 10¹³ cm^–2) and presenting a good organization on an alumina surface is obtained. This system could be used to investigate by surface science techniques the new emerging field of single-atom catalysis. By deposition at 573 K, small Pt clusters are organized on the network structure. By deposition of Pt at 573 K on preformed Pd seeds, large Pt (Pd) clusters containing about a hundred atoms are organized on the dot structure and they remain organized up to 733 K. We show that the three structures are interrelated. The different organizations of the Pt clusters on the alumina surface are explained by the presence of three types of sites corresponding to different adsorption energies for Pt atoms.