Synthesis of carbon nano-onions filled with γ-Fe and Gd/GdCl₃: A candidate multifunctional system

Stabilization of high-spin high-volume ferromagnetic γ-Fe nanocrystals inside carbon nano-onions (CNOs) has recently attracted an important attention for possible applications in magnetic hyperthermia cancer therapy. Here we report an advanced investigation of the chemical vapour synthesis (CVS) fabrication methodology of the filled CNOs, with focus on 1) the stabilization of the γ-Fe nanocrystals in presence of larger concentration of Cl radicals and 2) the structural functionalization, through simultaneous encapsulation of both ferromagnetic γ-Fe and paramagnetic GdCl₃ phases. By employing HRTEM we demonstrate an anomalous structural configuration of the CNOs, involving the coexistence of defect-rich concentrical-like and radially-oriented graphitic layers. XPS and Raman spectroscopy evidence the presence of C–S bond-formation and defect nucleation as key parameters for the growth of the radially-oriented graphitic layers. This unusual structural arrangement of the CNO is found to stabilize the γ-Fe phase at room temperature. We highlight the absence of FCC to BCC structural relaxation within timescales of 8 months from sample-fabrication. XRD, Rietveld refinements, together with magnetometry confirm the presence of a ferromagnetic transition arising below T ∼ 70 K within γ-Fe. This interpretation is further corroborated through extended characterization with ESR spectroscopy. Structural manipulation and functionalization of the CNOs is demonstrated by including tris(tetramethyl-cyclopentadienyl)gadolinium(III) as a precursor in the pyrolysis experiments. The functionalized CNO (filled with ferromagnetic γ-Fe/Fe₃C and paramagnetic Gd/GdCl₃) can be visualized as a multifunctional-container and a candidate for magnetic-hyperthermia-cancer-therapy and MRI contrast applications.