Role of aliphatic ligands and solvent composition in the solvothermal synthesis of iron oxide nanocrystals

Abstract

This paper describes a new method for controlling nanocrystal size and crystal phase in solvothermal syntheses of iron oxide nanocrystals by altering the carbon chain length of aliphatic iron(III) carboxylate precursors and reaction solvent composition, respectively. Hematite (α-Fe2O3) nanocrystals were synthesized from iron(III) carboxylate precursors in a 2:1 water:ethanol mixture in the presence of free aliphatic carboxylic acids or aliphatic primary amine ligands. In the presence of amine, increasing the length of the carbon chain of the iron(III) carboxylate precursor from two to eighteen carbons produced pseudocubic nanocrystals with size dispersities of ∼10% and diameters that decreased from 65 nm to 25 nm. Changing the chain length of the amine does not affect the size of the nanocrystals. In contrast, free carboxylic acid ligands produce polydisperse, polycrystalline hematite nanoparticles. The crystal structure of the iron oxide nanocrystals synthesized by this method depends on the mole-fraction of water (xwater) in the water/ethanol solvent mixture: xwater = 0 produces maghemite (γ-Fe2O3), xwater = 0.67 produces hematite, and xwater = 1 produces a mixture of goethite (α-FeOOH) and hematite. The dependence of the nanocrystal size on ligand chain length is attributed to decreasing solubility of partially hydrolyzed iron(III) carboxylate precursors in the reaction solvent mixture with increasing chain length. The dependence of nanocrystal phase on solvent composition is attributed to the different reactivities of water and ethanol toward hydrolysis of the iron(III) carboxylate precursors.