Synthesis of NiF₂ and NiF₂·4H₂O Nanoparticles by Microemulsion and Their Self-Assembly

Superstructures or self-assembled nanoparticles open the development of new materials with improved and/or novel properties. Here, we present nickel fluoride (NiF₂) self-assemblies by successive preparatory methods. Originally, the self-assemblies were obtained by exploiting the water-in-oil microemulsion technique as a result of auto-organization of hydrated NiF₂ (NiF₂·4H₂O) nanoparticles. The nanostructuration of NiF₂·4H₂O nanoparticles was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) data. The size and shape of NiF₂·4H₂O nanoparticles and their subsequent self-assemblies varied slightly as a function of water-to-surfactant and water-to-oil ratios. Scanning electron microscopy (SEM) and TEM characterizations revealed that the nanoparticles are organized into a succession of self-assemblies: from individual nanoparticles assembled into layers to truncated bipyramids, which further auto-organized themselves into almond-shaped superstructures. Anhydrous NiF₂ was achieved by heating NiF₂·4H₂O self-assemblies under the dynamic flow of molecular fluorine (F₂) at a moderate temperature (350 °C). Preservation of self-assemblies during the transformation from NiF₂·4H₂O to NiF₂ is successfully achieved. The obtained materials have a specific surface area (SSA) of about 30 m²/g, more than 60% of that of bulk NiF₂. The lithium-ion (Li⁺) storage capacities and the mechanism of the nanostructured samples were tested and compared with the bulk material by galvanostatic cycling and X-ray absorption spectroscopy (XAS). The nanostructured samples show higher capacities (∼650 mAh/g) than the theoretical (554 mAh/g) first discharge capacity due to the concomitant redox conversion mechanism of NiF₂ and solid-electrolyte interphase (SEI) formation. The nanostructuration by self-assembly appears to positively influence the lithium diffusion in comparison to the bulk material. Finally, the magnetic properties of nanostructured NiF₂·xH₂O (x = 0 or 4) have been measured and appear to be very similar to those of the corresponding bulk materials, without any visible size reduction effect. The hydrated samples NiF₂·4H₂O show an antiferromagnetic ordering at T_N = 3.8 K, whereas the dehydrated ones (NiF₂) present a canted antiferromagnetic ordering at T_N = 74 K.