Synthesis of Cubic Ni₃Sn Bimetallic Nanoparticles as a New Polymorph by Sequential Exsolution.

Abstract

The non-equilibrium phase among the polymorphs of the materials provides a new solution to harness the unique properties for various applications. However, advanced synthetic routes need to be developed to experimentally realize the novel phases to overcome the thermodynamic stability of their equilibrium counterparts. Here, we synthesized cubic Ni₃Sn bimetallic nanoparticles (NPs), known as a non-equilibrium polymorph, by the sequential exsolution of Ni and Sn metallic cation species from the perovskite stannate lattice framework. Following the prediction from the Ellingham diagram, Sn species decompose at a higher onset temperature than Ni species, enabling sequential formation of Ni (600°C) and Ni-Sn alloy (800°C) exsolved NPs by increasing the annealing temperature. Strikingly, due to the kinetic guidance of pre-formed cubic Ni NPs, subsequently exsolved Ni₃Sn alloy NPs exhibited the unconventional cubic Fm <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mover><mn>3</mn> <mo>¯</mo></mover> <annotation>$\bar{3}$</annotation></semantics> </math> m space group, which has a higher formation energy compared to the thermodynamically stable hexagonal Ni₃Sn with P6₃/mmc. Interestingly, the cubic Ni₃Sn NPs show unique catalytic pathways in CO^* dissociation, which leads to CO-free methanol dissociation. This discovery offers a fresh perspective on the synthetic routes of non-equilibrium bimetallic polymorphs to harness distinct catalytic pathways.