Synthesis and structural characterization of iron-cementite nanoparticles encapsulated in carbon matrix

Carbon-encapsulated iron-cementite (Fe-Fe₃C) nanoparticles, promising nanomaterials for medicine due to their valuable magnetic properties, were synthesized by a single-step solid-state pyrolysis of iron phthalocyanine. To obtain required magnetic characteristics of such nanoparticles by governing of the pyrolysis conditions one needs reliable structural information of the atomic architecture of the obtained nanoparticles of composition (Fe-Fe₃C), in which Fe atoms have different types of the local surrounding. The latter complicates the structural characterization of samples, which was performed using the complementary methods of TEM, SAXS, XRD, XANES, and EXAFS and the results of simulations by the method of reactive force field molecular dynamics (ReaxFF MD). The size of the particles is on the order of 10 nm with cementite concentration of about 60–70 wt%. The simulations enabled to reveal that the most plausible combinations of the local structures of Fe atoms in (Fe-Fe₃C) nanoparticle result in the difference of corresponding atomic pair radial distribution functions relatively to iron (RDF), which can be further filtered through the comparison with experimentally obtained RDF for iron atoms in the studied sample. Such RDF was derived from experimental Fe K-edge EXAFS in the sample by Fourier transform multi-shell processing within harmonic approximation and using the results of the analysis of SAXS, XRD, and XANES. The used approach, based on the filtering of ReaxFF MD-calculated RDFs via comparison with the EXAFS derived RDF, revealed that for particles of composition (Fe-Fe₃C) with XRD derived iron:cementite ratio of $\sim 40$:60 wt% and sizes bigger than 4 nm, the architecture with iron in core region of particle and cementite in its shell (Fe@Fe₃C) is the most probable for the mean nanoparticle comparing with architectures of the inverted core-shell (Fe₃C@Fe) or the mixture of iron and cementite phases (Fe+Fe₃C).