Induced Mesocrystal-Formation, Hydrothermal Growth and Magnetic Properties of α-Fe₂O₃ Nanoparticles in Salt-Rich Aqueous Solutions

Iron oxide nanoparticles are widely prevalent in our aqueous environment (e. g., streams, seawater, hydrothermal vents). Their aggregation and crystal growth depend on their chemical surroundings, for instance just a change in pH or salt concentration can greatly affect this. In turn this influences their properties, mobility, fate, and environmental impact.

We studied the growth of α-Fe2O3 (hematite), starting from 8 nm hematite particles in weakly acidic (HNO3) aqueous suspension with different states of particle aggregation, using salt (NaCl and NaH2PO4) to control their initial aggregation state. The samples were then subject to hydrothermal treatment at 100-140 degrees C.

We followed the development in aggregation state and particle size by dynamic light scattering, X-ray diffraction, small angle neutron scattering and transmission electron microscopy, and the magnetic properties by Mossbauer spectroscopy. The addition of NaCl and NaH2PO4 both led to aggregation, but NaCl led to linear chains of hematite nanoparticles (oriented parallel to their hexagonal c-axis), such that the crystalline lattice planes of neighboring hematite particles were aligned.

However, despite this oriented alignment, the particles did not merge and coalesce. Rather they remained stable as mesocrystals until heat-treated. In turn, the addition of NaCl significantly increases the rate of growth during hydrothermal treatment, probably because the nanoparticles, due to the chain formation, are already aligned and in close proximity.

With hydrothermal treatment, the magnetic properties of the particles transform from those characteristic of small (aggregated) hematite nanoparticles to those of particles with more bulk-like properties such as Morin transition and suppression of superparamagnetic relaxation, in correspondence with the growth of particle size.