Carbonaceous materials that include metallic nanoparticles (NPs) have attracted extensive interest during the last decades, especially those of metals like Ni and NiO in core/ shell morphologies. However, in order to improve NiO properties for those applications, a correlated analysis of its microstructure and magnetic properties, should be done.
In particular, Nickel oxide is widely studied due to its importance in technological applications (i.e., catalysis, batteries, ceramics, etc). To achieve this purpose, we have prepared five samples of 2-methylimidazole Nickel (NIOF) nanoparticles with carbonization temperatures between 400°C and 600°C; characterized their crystal structure and microstructure by X-Ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Additionally, their magnetic properties were studied by SQUID magnetometer through ZFC-FC and M(H) curves.
The samples exhibit two crystallographic phases of Ni: face centered cubic-FCC and hexagonal compact phase-HCP. Additionally, at the lowest carbonization temperature Ni3C was also detected. XRD peaks become narrower and symmetrical as the carbonization temperature raises, suggesting that the Ni-NPs mean diameter increases. Inter planar distances were measured by analysing into detail HRTEM images. These studies corroborate XRD results and the existence of Ni3C phase on samples synthesized at the lowest carbonization temperature.
The analysis of M(H) curves recorded at room temperature, reveal that the saturation magnetization is low on samples that contain antiferromagnetic Ni3C or NiO phases. Besides, the saturation magnetization values (Ms) and mean blocking temperature values (TB) increases as the carbonization temperature rises because larger NPs are synthesized in those conditions.
From the magnetic analysis, we suggest that each NPs can be described as consisting of a metallic Ni core, surrounded by very thin shell of NiO.