Magnetic properties of transition metal nanoparticles enclosed in carbon nanocages

We have demonstrated a simple, scalable, and low-cost method of producing superparamagnetic nanoparticles (SPNs) encapsulated in carbon nanocages (CNCs). These materials show a very high saturation magnetization and have the potential to find applications in various technologies, including a direct drug delivery, supercapacitors, catalysis, etc. Using a facile method, it was shown that via a simple annealing process of precursors based on a mixture of transition metal (TM) (TM: Ni, Co or Fe) salt (acetate) and citric acid, TM nanoparticles encapsulated in CNCs (TM@CNC) can be readily produced. Our study indicates, that Ni@CNC nanoparticles synthesized at temperatures 500 °C-600 °C are in the 3-4 nm size range and show superparamagnetic behavior while the annealing at 700 °C and higher temperatures leads to the ferromagnetic behavior due to a nanoparticle agglomeration through the Ostwald ripening mechanism. Further, the superparamagnetic Co@CNC nanoparticles synthesized at 600 °C show slightly larger, 4-5 nm, sizes but have much higher saturation magnetization compared to Ni@CNC nanoparticles synthesized at the same temperature making Co@CNC more sensitive to external magnetic field and more useful for direct drug delivery. Finally, for Fe-based precursor, the agglomeration occurs at much lower temperatures. The Fe@CNC nanoparticles synthesized at 600 °C are much larger, in the 10-100 nm size range, show only ferromagnetic behavior, and may be used for magnetic nanoparticle hyperthermia in cancer treatment. The magnetic properties of samples are found to correspond to the theoretical estimates of the critical size of single-domain particles in these TM systems.