Fig. 1. Schematic illustration of the reaction scheme. (i) A solution of red Co(NO3)2·6H2O in OLA is heated in a round-bottomed flask. (ii) Injection of a base (typically aqueous NaOH) induces the formation of green α-Co(OH)2. (iii) Further heating leads to the precipitation of hydroxide precursor NPs. In contact with air, the mixture adopts a brown-green color presumably due to the presence of an oxyhydroxide oxidation product. (iv) The precursor particles are pseudomorphically converted into black Co3O4 NPs.
