Hydrothermal evolution, optical and electrochemical properties of hierarchical porous hematite nanoarchitectures
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 ß-FeOOH phase was observed to precipitate alongside small primary α-Fe₂O₃ nanoparticles. Dissolution of the ß-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 PO₄³⁻ 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 PO₄³⁻ 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. Monodentate PO₄³⁻ absorption is considered weaker and hence easily displaced during growth, as compared to absorbed PO₄³⁻ 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.