Hydrothermal synthesis and characterisation of α-Fe2O3 nanorods

Abstract

The hydrothermal synthesis (HS) of α-Fe2O3 nanorods (NRs) is investigated using a combination of complementary analytical techniques. The construction of an HS ‘process map’ as a function of temperature, time and phosphate (PO43-) concentration provides insight into the nature of intermediate β-FeOOH NR precipitation, dissolution and subsequent α-Fe2O3 growth, along with the effect of PO43- anion concentration on the development of α-Fe2O3 particle shape. An HS processing temperature of 200˚C and an Fe3+ : PO43- molar ratio of 31.5 yielded crystalline acicular α-Fe2O3 NRs with an aspect ratio of ~ 7 (~ 420 nm long, ~ 60 nm wide). The additional effects of FeCl3 concentration, pH, stage of phosphate addition and α-Fe2O3 seed content on the growth of α-Fe2O3 NRs is investigated. The development of a novel valve-assisted pressure autoclave is described, facilitating the rapid quenching of hydrothermal suspensions into liquid nitrogen, providing ‘snapshots’ closely representative of the in situ physical state of the synthesis reaction products. Examination of the samples acquired as a function of reaction time and known reaction temperature provides fundamental insight into the anisotropic crystal growth mechanism of the acicular α-Fe2O3 NRs. It is considered that the release of Fe3+ ions back into solution through intermediate β-FeOOH dissolution supplies the nucleation and growth of primary α-Fe2O3 nanoparticles (NPs) (< 10 nm) which subsequently coalescence through a mechanism of oriented attachment (OA) with increasing temperature, into larger, acicular α-Fe2O3 NRs. Fourier transform infra-red spectroscopy investigation of the quenched reaction products provides evidence for PO43- absorption on the α-Fe2O3 NPs, in the form of mono or bi-dentate (bridging) surface complexes, on surfaces normal and parallel to the crystallographic α-Fe2O3 c-axis, respectively. The balance between bi-dentate and mono-dentate phosphate absorption is considered to be critical in mediating the acicular shape of the α-Fe2O3 NRs. A feasibility study on the incorporation of ferromagnetic cobalt, Co3O¬4 NPs or CoFe2O4 NPs into α-Fe2O3 NRs during HS is presented. In all cases, there is no evidence for the incorporation of cobalt within the α-Fe2O3 NRs or the formation of hetero-nanostructures with the Co3O4 or CoFe2O4 NPs. The overall growth mechanism of single crystalline acicular α-Fe2O3 NRs involves the anisotropic growth and dissolution of intermediate β-FeOOH NRs, governed by its crystallographic structure, and the OA of primary α-Fe2O3 NPs, mediated by the preferential absorption of phosphate surfactant