Lamellar Co3O4 nanoparticles recycled from synthetic cobalt carbonate: Core/shell morphology and magnetic properties

In a world where the ubiquity of Co-Li batteries in smartphones, laptops and electric vehicles is giving rise to an increase of cobalt concentration in mining soils and landfill leachates, the absorption of Co2+ ions by calcite provides a potential method to remove metal pollutants from aqueous media. Here, we show that the cobalt carbonates resulting from this process can be further recycled to produce lamellar Co3O4 nanoparticles with sizes around 30 nm. The cores of the nanoparticles possess a Fd (3) over barm spinel crystal structure and order antiferromagnetically at a Neel temperature of T-N similar to 35 K. On the other hand, the loss of crystallinity in a 1-2 nm-thick surface shell leads to the appearance of magnetically disordered spins. Finally, the magnetic exchange coupling between the core and shell spins gives rise to a moderate exchange bias effect (similar to 500 Oe) at T = 5 K.