In Situ PDF Study of the Nucleation and Growth of Intermetallic PtPb Nanocrystals

The mechanism of Pt and PtPb nanocrystal formation under supercritical ethanol conditions has been investigated by means of in situ X-ray total scattering and pair distribution function (PDF) analysis. The metal complex structures of two different platinum precursor solutions, chloroplatinic acid and Pt(acac)$_2$ (acac=acetylacetonate) provide atomic-scale detail about the nucleation mechanisms after initiation of the reaction with Pb(acac)$_2$ by heating. The stronger Pt−O chemical bonding in the Pt(acac)$_2$ precursor complex compared with the Pt−Cl bonding in the chloroplatinic acid precursor complex leads to a much slower reduction of the Pt center, and this allows more optimal co-reduction conditions providing a pathway for formation of phase-pure intermetallic PtPb product. The matching chemistry of the Pt(acac)$_2$ and Pb(acac)$_2$ precursors allow development of a facile continuous flow supercritical ethanol process for obtaining phase-pure hexagonal PtPb nanocrystals. The study thus highlights the importance of in situ studies in revealing atomic-scale information about nucleation mechanisms, which can be used in design of specific synthesis pathways, and the new continuous-flow process to obtain PtPb nanocrystals holds potential for large-scale production