Element-specific anisotropic growth of shaped platinum alloy nanocrystals

Abstract

Morphological shape in chemistry and biology owes its existence to anisotropic growth and is closely coupled to distinct functionality. Although much is known about the principal growth mechanisms of monometallic shaped nanocrystals, the anisotropic growth of shaped alloy nanocrystals is still poorly understood. Using aberration-corrected scanning transmission electron microscopy, we reveal an element-specific anisotropic growth mechanism of platinum (Pt) bimetallic nano-octahedra where compositional anisotropy couples to geometric anisotropy. A Pt-rich phase evolves into precursor nanohexapods, followed by a slower step-induced deposition of an M-rich (M = Ni, Co, etc.) phase at the concave hexapod surface forming the octahedral facets. Our finding explains earlier reports on unusual compositional segregations and chemical degradation pathways of bimetallic polyhedral catalysts and may aid rational synthesis of shaped alloy catalysts with desired compositional patterns and properties.

Octahedral PtNi nanoparticles with controlled surface structure and composition for oxygen reduction reaction

“…From the HRTEM images of the individual nanoparticles shown in Fig. 1d, the obvious concave structure is observed, similar to the observation in previous studies [8,19]. The appearance of the concaveness indicates the site-specific composition segregation occurs during the synthesis of bimetallic PtNi nanoparticles [8,20].…”

Elemental Anisotropic Growth and Atomic-Scale Structure of Shape-Controlled Octahedral Pt–Ni–Co Alloy Nanocatalysts

“…The amount of metal precursors and/or the reaction stage in which they are added controls the size, shape and the intraparticle composition. To the 5 best of our knowledge, there are no previous reports addressing the evolution of elemental anisotropy in shaped ternary alloy nanoparticles.Trimetallic PtNiCo shape-controlled nanoparticles were prepared using a surfactant-free, low temperature solvothermal synthesis 21,25,26,43 . In one synthesis, henceforth referred to as "one pot approach", Pt(acac) 2 , Ni(acac) 2 and Co(acac) 2 were simultaneous reduced in N,Ndimethylformamide (DMF) at 130 ºC for 42 hours.…”

“…Since then, many synthetic routes to obtain PtNi nanooctahedra, which ideally exhibit 8 facets with (111) atomic arrangement, have been developed, in which the co-reduction of the metal precursors takes place in the solution-phase [20][21][22][23][24] . A recent study 25 has shed light onto the growth mechanism of binary PtNi octahedral nanoparticles, revealing an anisotropic growth where a Pt-rich phase first develops into hexapod-like concave NCs in a ligand-controlled kinetic process, followed by a layer-by-layer deposition of a Ni-rich phase at the concave {111} facets. The degradation pathway of these nanocatalysts in acidic media 25,26 showed the selective etching of the Ni-rich {111} facets, which first results in the formation of concave nanooctahedra and finally in Pt-rich skeleton frameworks.…”

“…Trimetallic PtNiCo shape-controlled nanoparticles were prepared using a surfactant-free, low temperature solvothermal synthesis 21,25,26,43 . In one synthesis, henceforth referred to as "one pot approach", Pt(acac) 2 , Ni(acac) 2 and Co(acac) 2 Table S1).…”

“…Nevertheless, it has been reported that obtaining PtCo nanoparticles with octahedral shape is possible 25 . In addition, our results from the "one-step" synthesis point out that the Co reduction is inhibited by the presence of Ni.…”