Understanding the Effects of Iron Precursor Ligation and Oxidation State Leads to Improved Synthetic Control for...

Iron oxide nanocrystals have the potential for use in a wide variety of applications if we can finely control and tune the diverse structural attributes that lead to specific, desired properties. At the high temperatures utilized for thermal decomposition based syntheses, commonly used Fe(III) alkylcarboxylate precursors are inadvertently reduced and produce wüstite (FeO), which is paramagnetic, as opposed to the desired ferrimagnetic spinel phases of magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃). To circumvent this issue, we carried out syntheses at lower temperatures (∼230 °C) using an esterification-mediated approach. Under these conditions, formation of the FeO phase can be avoided. However, we found that the precursor oxidation state and ligation had a surprisingly strong influence on the morphologies of the resulting nanocrystals. To investigate the cause of these morphological effects, we carried out analogous nanocrystal syntheses with a series of precursors. The use of Fe(III) oleate precursors yielded highly crystalline, largely twin-free nanocrystals; however, small amounts of acetylacetonate ligation yielded nanocrystals with morphologies characteristic of twin defects. During synthesis at 230 °C, the Fe(III) oleate precursor is partially reduced, providing sufficient quantities of Fe(II) that are needed to grow the Fe₃O₄ nanocrystals (wherein one-third of the iron atoms are in the Fe(II) state) without twinning. Our investigations suggest that the acetylacetonate ligands prevent reduction of Fe(III) to Fe(II), leading to twinned structures during synthesis. Harnessing this insight, we identified conditions to predictably and continuously grow octahedral, spinel nanocrystals as well as conditions to synthesize highly twinned nanocrystals. These findings also help explain observations in the thermal decomposition synthesis literature which suggest that iron oxide nanocrystals produced from Fe(acac)₃ are less prone to FeO contamination in comparison to those produced from Fe(III) alkylcarboxylates.