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

The hydrothermal growth mechanism of α-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 α-Fe₂O₃ nanorods. An intermediate β-FeOOH phase was observed to precipitate alongside small primary α-Fe₂O₃ nanoparticles. Dissolution of the β-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 α-Fe₂O₃ nanoparticles, which in turn coalesced to form lenticular α-Fe₂O₃ nanorods. The critical role of the PO₄^3- surfactant on mediating the lenticular shape of the α-Fe₂O₃ nanorods is emphasised. Strong phosphate anion absorption on α-Fe₂O₃ crystal surfaces stabilised the primary α-Fe₂O₃ nanoparticle size to < 10 nm. FT-IR investigation of the quenched reaction products provided evidence for PO₄^3- absorption on the α-Fe₂O₃ nanoparticles in the form of mono or bi-dentate (bridging) surface complexes on surfaces normal and parallel to the crystallographic α-Fe₂O₃ c-axis, respectively. Mono-dentate PO₄^3- absorption is considered weaker and hence easily displaced during growth, as compared to absorbed PO₄^3- bi-dentate species, which implies the α-Fe₂O₃ c-planes are favoured for the oriented attachment of primary α-Fe₂O₃ nanoparticles, resulting in the development of filamentary features which act as the basis of growth, defining the shape of the lenticular α-Fe₂O₃ nanorods.