Reduction under hydrogen of ferrite MFe_2O_4 (M: Fe, Co, Ni) nanoparticles obtained by hydrolysis in polyol medium: A novel route to elaborate CoFe_2, Fe and Ni_3Fe nanoparticles

摘要

A novel method to process metal and various alloy particles of nanometric size is described. The first step consists in the elaboration of MFe_2O_4 (M: Fe, Ni or Co) spinel nanoparticles in polyol medium via hydrolysis and the second one in gently reducing these latter under hydrogen at 300 °C. X-ray diffraction analysis shows that pure Fe and CoFe_2 alloy are well obtained by reducing Fe_3O_4 and CoFe_2O_4, respectively. This is not the case when we try to reduce NiFe_2O_4. A mixture of Fe and Ni_3Fe is observed. TEM analysis reveals that the size of metal particles stays within the range of a few tenths of nm up to 150 nm, while the precursors (MFe_2O_4) never exceed 5 nm. Our results show that the formation of metal particles occurs via two main steps: (i) reduction of the spinel oxide nanoparticles into metal ones and (ii) aggregation of the latter, leading to larger metal nanoparticles. Magnetic measurements indicate that the as-obtained metallic materials have good magnetic properties mainly affected by the sizes of the nanoparticles and the purity of the reduced phases.