Crystallite Size Variations of Nanosized Fe2O3 Powders during γ- to α-Phase Transformation

Crystallite size variations of nanosized iron oxide particles during γ- to α-phase transformation were examined using DTA/TG, FTIR, XRD, TEM, and BET-N2 techniques. Synthetic γ-FeOOH and Fe3O4, prepared as precursors, both converted to γ-Fe2O3 and then completed the phase transformation under thermal treatment. Starting with γ-FeOOH, the γ-Fe2O3 obtained grew from the original size of 5 nm to a size about 25 nm or larger, then transformed to α-Fe2O3. The α-crystallites obtained showed sizes similar to that of γ-crystallite. With continuous heating, the α-Fe2O3 coarsened to a size of ∼55 nm. Identical phenomena were observed when α-Fe2O3 was prepared from Fe3O4 via γ-Fe2O3. The study also found that α-Fe2O3 crystallites obtained by calcinations of amorphous Fe(OH)3 will show sizes smaller than 5 nm. The decomposition temperature can be lowered to 200 °C.