Chemistry of hydrolysis of FeCl₃ in the presence of phosphate to form hematite nanotubes and nanorings

Through investigation of the intermediate specimens during the hydrolysis of FeCl3 in the presence of phosphate using Mass spectroscopy, XRD, EDX, SEM, HRTEM and ICP-OES Spectrometry, the formation mechanisms of α-Fe₂O₃ nanotubes and nanorings were revealed. At early stages, the precursor molecules polymerized and aggregated into large disordered particles, from which β-FeOOH nanorods grew up. When the NaH₂PO₄ concentration was low (e.g. 1 mM in a solution of 23 mM FeCl₃), the β-FeOOH nanorods were relatively stable and underwent side-by-side aggregation into spindle-like particles. Phase transformation into self-orientated α-Fe₂O₃ nanocrystallites then took place on the surface of these spindle particles, followed by Ostwald ripening, to form a single-crystalline shell. The ends and the core of the spindle particles were dissolved, forming α-Fe₂O₃ nanotubes. When the NaH₂PO₄ concentration was high (e.g. 4 mM), the individual β-FeOOH nanorods decomposed into α-Fe₂O₃ nanocrystallites, which underwent self-orientated aggregation into polycrystalline disks. Surface Ostwald ripening and dissolution of the central area turned these disks into nanorings. The exposed surface in the nanotubes is mainly (hk0), while it is (001) in the nanorings. Photoelectrochemical measurement indicated that photocurrent response of the nanotubes was three times higher than the nanorings. This newly established non-classical formation mechanisms of these crystals may help us to understand the development of many other novel morphologies of metal oxides via a hydrolysis process.