Influence of Content and Type of Lanthanide on the Structure of Ln2O3-Covered Carbon Nanoflakes: The EPR and XPS Study

Synthesized Ln₂O₃ (Ln = La, Nd or Gd) nanoparticles with sizes of 1-3 nm, 5-6 nm and 10-15 nm were stabilized by carbon nanoflakes (CNFs). The weight content of Ln₂O₃ in the Ln₂O₃/CNF composites was 20-50 wt. %, which makes these composites potentially suitable for practical use as computed tomography and magnetic resonance imaging contrast agents. The structure of CNFs and Ln₂O₃/CNF composites was investigated by X-ray diffraction data, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The EPR spectra of raw CNFs were silent. The oxidation of the CNF surface resulted in the appearance of paramagnetic centers associated with two types of unpaired electrons in the carbon support. After impregnation of the CNFs with the Ln³⁺ ion solution, the number of unpaired electrons was reduced, presumably due to the formation of C-O-Ln bonds. All Ln³⁺ ions changed the composites' EPR spectra by reducing the number of unpaired electrons in the CNF structure.