Prospects for the incorporation of cobalt into α-Fe2O3 nanorods during hydrothermal synthesis

A feasibility study on the incorporation of cobalt into α-Fe2O3 nanorods (NRs) during hydrothermal synthesis (HS) is presented as a function of FeCl3 and CoCl2 concentration, phosphate surfactant concentration and pH value, with samples assessed using X-ray diffractometry, transmission electron microscopy, selected area electron diffraction and energy dispersive X-ray analysis. No evidence was found for the incorporation of cobalt into α-Fe2O3 NRs at low pH, whilst synthesis at intermediate and high pH values favoured the formation of CoFe2O4 NPs. The critical role of pH value over the precipitation, size and phase purity of the nanostructured reaction products is emphasised. At pH ~2, large, well crystalline α-Fe2O3 nanoparticles (NPs) and NRs were grown from FeCl3 solution in the absence and presence of phosphate, respectively, whilst no evidence was found for Co precipitation or incorporation in α-Fe2O3 following HS in the presence of CoCl2. At pH ~8, smaller α-Fe2O3 NPs, as well as Co3O4 and CoFe2O4 NPs were synthesised from FeCl3, CoCl2, or a mixture thereof. HS at pH ~12 produced a mixture of larger CoFe2O4 NPs and α-Fe2O3 NPs depending on the Fe:Co molar ratio. The formation of intermediate metastable (oxy)hydroxide phases is considered pH dependent, providing for a variety of different reaction pathways. Further, inclusion of preformed Co3O4 and CoFe2O4 NPs to the FeCl3 solution at pH ~2 in the presence of phosphate surfactant resulted in the synthesis of α-Fe2O3 NRs with residual Co3O4 and CoFe2O4 NPs attached to their surfaces. The CoFe2O4 NPs encouraged local dissolution leading to the formation of α-Fe2O3 NR surface corrugations.