Characterization and magnetism of Co-modified γ-Fe_2O_3 core-shell nanoparticles by enhancement using NaOH

摘要

During synthesis of γ-Fe_2O_3 nanoparticles by chemically-induced transition in FeCl_2 solution, Co-modification of the particles has been attempted by adding Co(NO_3)_2 (sample (1)) or Co(NO_3)_2/NaOH (samples (2) and (3)). Using VSM, TEM, XRD, EDS, and XPS, the specific magnetization, morphology, crystal structure, and bulk and surface chemical compositions have been characterized. Particles in sample (1) are composed of γ-Fe_2O_3 and CoCl_2 · 6H_2O, and particles in samples (2) and (3) have CoFe_2O_4 in addition to γ-Fe_2O_3 and CoCl_2 • 6H_2O. Detailed analysis shows that the Co-modification can be enhanced by additional NaOH to form γ-Fe_2O_3/CoFe_2O_4 core-shell nanoparticles coated by CoCl_2 · 6H_2O. The molar, mass, and volume ratios of each phase have been estimated for each sample, and the average density and saturation magnetization are derived. Accordingly, it is revealed that for these nanoparticle systems, the magnetization and coercivity depend on the grain size of the spinel structure crystallites based on γ-Fe_2O_3 or γ-Fe_2O_3/CoFe_2O_4, but the anisotropy constant remains at 1.48 × 10~(-1) J/cm~3.