Hydrothermal growth mechanism of alpha-Fe2O3 nanorods derived by near in situ analysis

The hydrothermal growth mechanism of alpha-Fe2O3 nanorods has been investigated using a novel valve-assisted pressure autoclave. This approach has facilitated the rapid quenching of hydrothermal suspensions into liquid nitrogen, providing 'snapshots' representative of the near in situ physical state of the synthesis reaction products as a function of known temperature. Examination of the acquired samples using complementary characterisation techniques of transmission electron microscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy (FT-IR) has provided fundamental insight into the anisotropic crystal growth mechanism of the lenticular alpha-Fe2O3 nanorods. An intermediate beta-FeOOH phase was observed to precipitate alongside small primary alpha-Fe2O3 nanoparticles. Dissolution of the beta-FeOOH phase with increasing temperature, in accordance with Ostwald's rule of stages, led to the release of Fe3+ anions back into solution to supply the growth of alpha-Fe2O3 nanoparticles, which in turn coalesced to form lenticular alpha-Fe2O3 nanorods. The critical role of the PO43- surfactant on mediating the lenticular shape of the alpha-Fe2O3 nanorods is emphasised. Strong phosphate anion absorption on alpha-Fe2O3 crystal surfaces stabilised the primary alpha-Fe2O3 nanoparticle size to < 10 nm. FT-IR investigation of the quenched reaction products provided evidence for PO43- absorption on the alpha-Fe2O3 nanoparticles in the form of mono or bi-dentate (bridging) surface complexes on surfaces normal and parallel to the crystallographic alpha-Fe2O3 c-axis, respectively. Monodentate PO43- absorption is considered weaker and hence easily displaced during growth, as compared to absorbed PO43- bi-dentate species, which implies the alpha-Fe2O3 c-planes are favoured for the oriented attachment of primary alpha-Fe2O3 nanoparticles, resulting in the development of filamentary features which act as the basis of growth, defining the shape of the lenticular alpha-Fe2O3 nanorods.