Effect of phosphate ions on the formation of iron oxide/hydroxide as a stabilizer

Hematite (alpha-Fe2O3) nanoparticles were synthesized using a facile precipitation method under mild conditions by adding phosphate anions (PO43-). The solid-state phase transformation of alpha-Fe2O3 nanoparticles follows dehydration and internal rearrangement of ferrihydrite initially precipitated at a high pH, while PO43- ions acted as a stabilizer for preventing the dissolution of ferrihydrite. This phase transformation resulted in tiny spherical alpha-Fe2O3 nanoparticles with a size of approximately 5 nm, which is similar to that of the initial ferrihydrite. Conversely, acicular goethite (alpha-FeOOH) nanoparticles were formed without PO43- ions by dissolution and recrystallization of ferrihydrite. Furthermore, anions such as sulfate (SO42-) and chloride (Cl-) had no significant effect on the alpha-Fe2O3 formation due to their low Lewis basicity, resulting in an acicular alpha-FeOOH structure. These results extend the possibility to synthesize alpha-Fe2O3 nanoparticles, which could have various practical applications.