Phase-controlled synthesis of iron phosphates via phosphation of β-FeOOH nanorods

Iron phosphates comprise an important class of materials with a wide range of applications. We have designed novel routes for the controlled synthesis of iron phosphate dihydrates of varying crystallographic phases (monoclinic and orthorhombic) and morphologies. Our approach comprises the phosphation of β-FeOOH nanorods with aqueous phosphoric acid solutions. Through a systematic parametric study coupled to an array of characterisation techniques, including XRD, TEM, HAADF-STEM, AAS, N2 sorption, TGA-MS, FTIR, UV-Vis, XPS, EXAFS, EPR, and 57Fe Mössbauer spectroscopy, we unravel the complex synthesis chemistry on both the macroscopic and microscopic levels. It is found that the formation of iron phosphates occurs exclusively under acidic conditions and in general involves the dissolution of β-FeOOH which, upon reaction with H3PO4, precipitates as FePO4·2H2O. The pH of the treatment solution determines the crystallographic phase of the resulting product by regulating the rate of β-FeOOH dissolution and of the precipitation of the iron phosphates, while the treatment time is decisive for the preservation of the morphology. The formation of the monoclinic phase entails a fast iron dissolution and subsequent precipitation in the solution. The generation of the orthorhombic analogue involves an interfacial reaction between H3PO4 and β-FeOOH, forming an amorphous layer of iron phosphate, which crystallises into a pure phase with increasing treatment time. The thermal transformation of hydrated to anhydrous iron phosphates is dependent on the phase and morphology of the precursors. The rod shape of iron-rich orthorhombic FePO4·2H2O can be preserved even after annealing at 923 K, with the formation of mesopores. These novel nanostructures may widen the applications of iron phosphates and the routes developed herein can be anticipated to guide the fabrication of other metal phosphates.