Application of Analytical Electron Microscopy to the Physical and Chemical Characterisation of Ferrihydrites

Ferrihydrite is a poorly crystalline iron (oxy)hydroxide which exists exclusively in a disordered nanocrystalline form. Lacking long range three-dimensional structural order, possessing a high density of defects and displaying variable composition depending on origin has meant that to date a clear consensus regarding a structural model has yet to be reached. A departure from traditional means of crystallographic investigation is therefore required prompting the exploration of hitherto unexplored and potentially novel methods in refining this elusive structure. Recent years have witnessed significant interest in the investigation of ferrihydrites formed in the presence of compounds such as phosphate and citrate. Novel forms of ferrihydrite have been reported, produced by the hydrothermal treatment of these modified ferrihydrites, which have the potential to increase understanding of ferrihydrite in general. This work demonstrates the application of the latest generation of aberration corrected electron microscopes to the structural and chemical characterisation of ferrihydrite produced both in vitro and within the protein cage of ferritin. Two-line ferrihydrite (2LFh), coprecipitated in the presence of varying concentrations of phosphorus (P-doped) and subject to hydrothermal and dry air annealing heat treatments, has also been investigated. By way of a thorough characterisation additional physicochemical characterisation techniques have been applied to confirm the phase purity of specimens and benchmark them against those reported in the literature. The effects of prolonged exposure to the 200 keV electron beam of the transmission electron microscope (TEM) has been investigated by the simultaneous acquisition of Fe-L and O-K edge electron energy-loss (EELS), EELS and energy dispersive X-ray (EDX) compositional and selected-area electron diffraction (SAED) data. A safe fluence level of 108 electrons nm-2 has been established, below which both 2LFh and P-doped 2LFh can be observed in their pristine state. Beyond the safe fluence limit alteration of both pristine 2LFh and the P-doped 2LFh proceeds by preferential loss of oxygen, reduction of iron and concomitant phase transformation to a material with characteristics similar to that of magnetite. A novel low-electron fluence method of scanning TEM EELS acquisition (SmartAcquisition) has been developed and its applicability to the characterisation of ferrihydrite nanoparticles proven. Results here are consistent with independently published data which confirm ferrihydrite to be a predominantly Fe(III) bearing iron-oxide material in which the iron atoms reside in six-fold coordination with oxygens and/or hydroxyls. Fe-L EELS edge analysis provides support for a new and controversial model for the structure of ferrihydrite in which a significant fraction of iron is in four fold coordination. High-angle aberration corrected scanning electron microscopy is used to investigate the morphology of ferritin mineral cores. Atomically resolved sub-unit structure is observed with individual cores comprising several crystalline units which appear to have nucleated independently of one another. The principle aim of this work it to explore the applicability of the latest generation of analytical electron microscopes for the elucidation of chemical and structural properties of challenging nanoparticulate systems.