Tuning the shape, size, phase composition and stoichiometry of iron oxide nanoparticles: The role of phosphate anions

Highlights: • Microwave synthesis of iron oxide hollow nanotubes, solid nanorods and nanodisks. • Influence of phosphate anions in impacting size, shape, phase composition and stoichiometry. • Shape-dependent magnetic properties and existence of magnetic vortex in nanodiscs. • Mechanistic description of the chemical processes and core@shell models. -- Abstract: This work describes a microwave synthetic approach for the controlled assembly of α-Fe₂O₃ nanosystems with defined morphologies, such as hollow nanotubes (NTs), solid nanorods (NRs) and nanodisks (NDs). The morphological control is aided during the crystallization processes by using phosphate anions as key surfactants in solution. Furthermore, the thermal reduction under H₂ atmosphere of these NTs, NRs and NDs α-Fe₂O₃ systems to the correspondent Fe₃O₄ nanomaterials preserved their initial morphologies. It was observed that the concentration of phosphate anions and volume of solvent had significant impact not only on controlling the shapes and sizes, but also phase composition and stoichiometry of the NTs, NRs and NDs nanoparticles. X-ray Rietveld refinement analysis of the NTs, NRs and NDs systems, after reduction in H₂, revealed the presence of zero-valent iron (Fe⁰) in the final materials, with Fe⁰ fractions that decreased gradually in % from NTs (∼16%), NRs (∼11%) to NDs (∼0%) upon increasing amount of phosphate anions. Bulk magnetic susceptibility measurements showed clear alterations of the Verwey transition temperatures (T