Iron carbide nanoparticles (ICNPs) are considered to have great potential in new energy conversion, nanomagnets and biomedical applications due to their intrinsically peculiar magnetic and catalytic properties. However, the synthetic routes were greatly limited in morphology and phase controlled synthesis. In this article, we present a versatile solution chemistry route towards colloidal ICNPs (Fe₂C-hexagonal and monoclinic syngony, Fe₅C₂-monoclinic syngony and Fe₃C-orthorhombic syngony) derived from body centered cubic Fe@Fe₃O₄ by introducing heteroatoms to restrain their phase transformation. We found that the phases of Fe₂C NPs could be controlled by direct phase transformation in the drastic thermally driven procedure (defined as thermodynamical manner). Meanwhile, the selective adsorption of Cl ions weakened the bonding between Fe and C atoms, thus interfering with the penetration of C atoms to form lower carbon content Fe₅C₂ and Fe₃C NPs. The kinetic mechanisms were evaluated using density functional theory (DFT) simulations focusing on the bonding energy between Fe–C and Fe–Cl atoms. All the obtained ICNPs exhibited typically soft ferromagnetic properties with the highest saturation magnetization value of 101.2 emu g⁻¹ and the highest Curie temperature of 497.8 K.

Developing efficient and economical noble-metal free catalysts for oxygen reduction reaction (ORR) is one of the essential factors for the industrialization of fuel cells. Recent studies on transition metal ORR catalysts have become the priority to practical low-temperature fuel cells. Herein, we proposed a novel in situ design, Fe₂N nanoparticles (NPs) in an N doped porous carbon matrix (Fe₂N@NPC) derived from metal organic frameworks (MOFs) as high-performance ORR catalysts. The decorated Fe₂N NPs increase the amount of Fe–N/C bonding as catalytic site. Meanwhile, the coated NPC facilitates the mass transport of the reaction and improves the conductivity simultaneously. The catalyst exhibits an onset potential of −0.038 V, a half-wave potential of −0.175 V in 0.1 M KOH, and superior stabilities and methanol tolerance performances to commercial Pt/C catalysts. More importantly, the proposed in situ ‘two-step’ annealing route provides potential possibilities for producing other non-precious metal catalysts for ORR.

A potential solid-state magnetic cooling candidate Gd₃BWO₉is proposed, which shows an exceptionally large magnetocaloric effect.