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, X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy, and inductively coupled plasma optical emission spectrometry, the formation mechanisms of alpha-Fe2O3 nanotubes and nanorings were revealed. At early stages, the precursor molecules polymerized and aggregated into large disordered particles, from which beta-FeOOH nanorods grew up. When the NaH2PO4 concentration was low (e.g., 1 mM in a solution of 23. mM FeCl3), the beta-FeOOH nanorods were relatively stable and underwent side-by-side aggregation into spindle-like particles. Phase transformation into self-orientated alpha-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 alpha-Fe2O3 nanotubes. When the NaH2PO4 concentration was high (e.g., 4 mM), the individual beta-FeOOH nanorods decomposed into alpha-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 the photocurrent response of the nanotubes was three times higher than that of the nanorings. The newly established nonclassical formation mechanisms of these crystals may help us to understand the development of many other novel morphologies of metal oxides via a hydrolysis process.