Facile synthesis of nickel based nanostructures from Ni[EMIM]Cl₂ ionic liquid precursor: Effects of thermal and chemical methods on the properties of nanoparticles

New N,N′-dialkylimidazolium salts of the type [RMIM]₂[MCl_n], where [RMIM+] = 1-alkyl-3-methylimidazolium and M = Ni(ii) have been prepared by refluxing the imidazolium with nickel chloride in a methanolic solution. The resulting salt has been characterized by FT-IR, ¹H, ¹³C NMR, and elemental analysis. The salt melts around 40 °C and TGA measurements show that it decomposes in air in several steps eventually forming the corresponding metal, the onset of decomposition being observed around 220 °C, clearly indicating an ionic liquid window from 40-220 °C. This ionic liquid was then used as a precursor for the synthesis of nickel nanoparticles using NaBH₄ as a chemical reductant and also by calcining the ionic liquid at 500 °C for four hours. The nanoparticles have been characterized by powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. These results indicate that spherical shaped nickel nanoparticles (Ni NPs) of smaller size were obtained in the case of chemical reduction of the [EMIM][NiCl₂] IL precursor, whereas, hexagonal shaped large size particles of NiO NPs were formed by the thermal decomposition method. Both IL and the calcination temperature play a critical role in defining the morphology and phase of NPs.