Size and Size Distribution Control of γ-Fe2O3 Nanocrystallites: An in Situ Study

The formation and growth of maghemite (γ-Fe 2 O 3 ) nanocrystals during the hydrothermal synthesis from aqueous solutions of ammonium iron(III) citrate (C 6 H 8 O 7 · x Fe(III)· y NH 3 ) have been studied by in situ powder X-ray diffraction (PXRD). Data analysis by Rietveld refinement and whole powder pattern modeling (WPPM) reveals that the crystallite size and size distribution can be precisely tuned through simple adjustments of the reaction temperature and time. Increasing the reaction temperature causes faster growth and results in larger crystallites while the size distribution broaden as reaction times increase, regardless of temperature. The crystallization kinetics were investigated by fitting the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetic model to the growth curves. The activation energy was found to be 67(15) kJ/mol, and the limiting mechanisms of crystallite formation were determined. The growth was studied with various precursor compositions of ammonium iron(III) citrate and Fe(NO 3 ) 3 ·9H 2 O or FeCl 3 ·6H 2 O. Increasing the fraction of Fe(NO 3 ) 3 ·9H 2 O in the precursor results in larger maghemite nanocrystallites. The addition of Fe(NO 3 ) 3 ·9H 2 O also results in the formation of the thermodynamically more stable hematite (α-Fe 2 O 3 ) phase as a byproduct.