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Hematite nanotubes and nanorings

There is a long tradition in the synthesis of iron oxide particles with tailored chemical composition, shape and size. In order to modify the shape of iron oxide particles a lot of different surfactants, organic molecules, cations and anions have been used. In particular, phosphate anions have often been used to this end. For instance, Ozaki et al. (J. Colloid Interface Sci. 102 (1984) 146) synthesized uniform spindle-shaped colloidal hematite (α-Fe2O3) particles of narrow size distribution by using the forced hydrolysis of ferric chloride solutions at an elevated temperature. It has been shown that the concentration of phosphate ions has a significant impact on the axial ratio of hematite particles. However, in spite of a long tradition of the synthetic iron oxide chemistry a convenient and reproducible one-pot procedure for a large-scale synthesis of iron oxide nanotubes has been recently developed by Jia et al. (Angew. Chem. Int. Ed. 44 (2005) 4328). These authors have shown that phosphate anions can induce the preferential dissolution of the hematite spindle precursor to form single-crystalline iron oxide nanotubes. The same authors (J. Am. Chem. Soc. 130 (2008) 16968) introduced sulphate anions into the system in order to achieve a better control of its morphology and this double anion mediation resulted in the synthesis of iron oxide nanorings. Gotić et al. (J. Mol. Struct. 993 (2011) 167) have exploited the benefits of introducing the divalent metal cations M2+ (M = Mn, Cu, Zn, Ni) into the system. The addition of the divalent metal cations gradually modified the nanotube into the nanoring particle morphologies. The modification by the divalent metal cations could be used as a general approach to the control of hematite nanoparticle morphology using forced hydrolysis in highly acidic aqueous solutions without bringing about any changes in the chemical and phase composition of the product.

hematite, nanotubes, nanorings

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