Ordered Intermetallic Pt–Sn Nanoparticles: Exploring Ordering Behavior across the Bulk Phase Diagram

Abstract

Because the bulk phase diagram of Pt–Sn contains five different compounds (Pt₃Sn, PtSn, Pt₂Sn₃, PtSn₂, and PtSn₄) with very different congruent or incongruent melting points, we chose this system to investigate factors that influence the homogeneity and structure of intermetallic nanoparticles with different compositions prepared by the same solution phase co-reduction reaction. Pt and Sn chloride precursors were reduced by alkali metal borohydrides in tetrahydrofuran. Of the five compositions, only PtSn forms ordered intermetallic nanoparticles (average domain size of 4.3 nm) at room temperature. As-prepared Pt₃Sn nanoparticles adopt an alloy FCC structure and ordered into the cubic Cu₃Au structure at the unexpectedly low annealing temperature of 200 °C. Despite their low bulk incongruent melting temperatures, the tin-rich compositions (PtSn_<i>x</i>, where <i>x</i> = ³/₂, 2, or 4), by contrast, form highly disordered products at room temperature and require annealing at ≥200 °C for crystallization and ordering to be observed. Phases prepared from tin­(II) chloride exhibit co-reduction behavior different from that of tin­(IV) chloride, the latter requiring overall higher temperatures to produce similarly ordered particles. Spectroscopic studies indicate that this behavior may be due to the formation of Pt–Sn complexes in solution prior to the reduction when tin­(II) is used as a precursor, but not for tin­(IV), resulting in more consistent nucleation of the target stoichiometric phases. The process of crystallization by annealing is discussed