Process Map for the Hydrothermal Synthesis of α-Fe₂O₃ Nanorods

Abstract

A “process map” for the hydrothermal synthesis (HS) of single crystalline α-Fe2O3 nanorods from aqueous FeCl3 is presented, as a function of temperature, time, and phosphate concentration, as assessed using the combined techniques of X-ray diffractometry, transmission electron microscopy, selected area electron diffraction, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy. The process map provides insight into the nature of intermediate β-FeOOH nanorod precipitation, dissolution and subsequent α-Fe2O3 growth, along with the effect of PO43− anion concentration on the α-Fe2O3 particle shape. Increasing the processing temperature in the absence of a surfactant promoted the dissolution of initially formed β-FeOOH nanorods and the nucleation and growth of equiaxed α-Fe2O3 nanoparticles with rhombohedral morphology. Increasing additions of phosphate surfactant resulted in a shape change of the α-Fe2O3 nanoparticles into lenticular α-Fe2O3 nanorods with increasing aspect ratio but with progressive inhibition of α-Fe2O3 phase formation. Increasing the synthesis temperature in the presence of PO43− anions was associated with the recovery of well-defined single crystal, lenticular nanorods. Increasing the time of synthesis in the presence of PO43− anions was similarly associated with the progressive formation and dissolution of β-FeOOH and the growth of well-defined lenticular α-Fe2O3 nanorods. An HS processing temperature of 200 °C and an Fe3+−PO43− molar ratio of 31.5 yielded optimized crystalline lenticular α-Fe2O3 nanorods with an aspect ratio of ∼7. Chemical analysis indicated that some P was retained within the bulk of the developed α-Fe2O3 nanorods.