<i>In Situ</i> X‑ray Scattering Guides the Synthesis of Uniform PtSn Nanocrystals

Compared to monometallic nanocrystals (NCs), bimetallic ones often exhibit superior properties due to their wide tunability in structure and composition. A detailed understanding of their synthesis at the atomic scale provides crucial knowledge for their rational design. Here, exploring the Pt–Sn bimetallic system as an example, we study in detail the synthesis of PtSn NCs using <i>in situ</i> synchrotron X-ray scattering. We show that when Pt­(II) and Sn­(IV) precursors are used, in contrast to a typical simultaneous reduction mechanism, the PtSn NCs are formed through an initial reduction of Pt­(II) to form Pt NCs, followed by the chemical transformation from Pt to PtSn. The kinetics derived from the <i>in situ</i> measurements shows fast diffusion of Sn into the Pt lattice accompanied by reordering of these atoms into intermetallic PtSn structure within 300 s at the reaction temperature (∼280 °C). This crucial mechanistic understanding enables the synthesis of well-defined PtSn NCs with controlled structure and composition via a seed-mediated approach. This type of <i>in situ</i> characterization can be extended to other multicomponent nanostructures to advance their rational synthesis for practical applications