Pt/Fe₃O₄ Core/Shell Triangular Nanoprisms by Heteroepitaxy: Facet Selectivity at the Pt–Fe₃O₄ Interface and the Fe₃O₄ Outer Surface

Pt/Fe3O4 core/shell triangular nanoprisms were synthesized using seed-mediated heteroepitaxy. Their well-defined shape, facets, and ordered-assembly allowed detailed analysis of mechanism of the heteroepitaxy. At the Pt-Fe3O4 interface, existence of both lattice and chemical mismatch resulted in facet-selective epitaxy along ⟨111⟩ directions of two lattices. X-ray absorption fine structure measurements demonstrated that the Pt seed nanocrystals were composed of an iron-rich Pt-Fe metallic thin layer sandwiched between the Pt core and a Fe-O outer-surface. The Fe-O outer-surface of the seed nanocrystals presumably offered epitaxial sites for the following deposition of the Fe3O4 shell. Each tip and side of a triangular nanoprism respectively possessed a groove and a ridge, and a (111) plane parallel to the basal planes linked all grooves and ridges. This interesting (111) plane approximately bisected the triangle nanoprisms and located near the Pt-seed. The outer surface of the hybrid nanocrystals was also found to be facet-selective, that is, solely {111} facets of Fe3O4 lattice. These polar {111} facets allowed the surface to be only occupied with high-density iron ions, and thus offered best surface coordination for the electron donating ligands in the solution.