Halide-Controlled Polymorphism in the Synthesis of Nickel Nanocrystals

• Naomi Hobbs, Chemistry, University of Delaware

• Emil Hernandez-Pagan, Chemistry, University of Delaware

Intense research efforts have been dedicated to the synthesis of metal nanocrystals driven by both fundamental studies and applications. Nickel nanocrystals are of particular interest because of their catalytic, magnetic, and electronic properties. Like for many materials, these properties depend on the crystal structure. Nickel can form in two crystal structures (or polymorphs), face-centered cubic (FCC) and hexagonally close-packed (HCP). For example, it has been shown that HCP Ni is the more active polymorph for green energy-relevant electrocatalytic reactions, such as the Oxygen Evolution Reaction (OER) for water splitting.     Therefore, developing easily tunable synthesis methods to control polymorphism in this and other systems is crucial yet still remains a challenge in nanomaterials synthesis. Recent work from our research group demonstrated that varying the metal halide precursor can direct polymorph control in manganese and tin chalcogenide nanocrystals. Motivated by these findings, this work investigates the extension of halide-directed polymorph control in the synthesis of nickel nanocrystals. The nanocrystals were synthesized using a high-temperature colloidal method in which initially the nickel halide precursor was varied while all other reaction conditions were kept constant. The role of the solvent, as well as the reaction time and temperature were also investigated. Powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to determine crystal structure and particle morphology, respectively. This work establishes a correlation between halide identity and polymorph selectivity in nickel nanoparticles. Nickel iodide favors the FCC polymorph whereas nickel bromide and nickel chloride favor HCP, however, can produce either HCP or FCC depending on reaction conditions. Ultimately, this research contributes towards the molecular understanding of the synthetic handles that enable polymorph selectivity and whether halide-directed polymorph control can be extended to nickel nanocrystals.