From Single Pt Atoms to Pt Nanocrystals: Photoreduction of Pt²⁺ Inside of a PAMAM Dendrimer

The dynamics of structural transformations of Pt aggregates in the "quantum size" range where their molecular structure transforms into crystalline nanoparticles with metallic properties is an important issue in nanoscience. Here, we show high-resolution transmission electron microscopy (HRTEM) and spectroscopic observations of the polyamidoamine (PAMAM) dendrimer-mediated system, Pt–PAMAM, after UV irradiation that reveal the formation of small Ptδ+n clusters (n = 2–8) with linear chains of −Pt–Pt–, which are the building blocks of stable nanocrystals. Dynamic imaging in an aberration corrected TEM at atomic resolution shows intermediate molecular and crystalline states and coalescence of Pt clusters into stable nanocrystals via an oriented attachment assembly process. We propose that the structural transformation from Pt aggregates to nanocrystals occurs between 1.5 and 2 nm and that a phase transition of the type "disordered-to-crystalline" exists depending on the number of atoms in the cluster.