Temperature controlled shape evolution of iron oxide nanostructures in HMTA media

This work reported an improved approach to the synthesis of iron oxide nanostructures using iron(III) chloride as the precursor and hexamethylenetetramine (HMTA) as the key auxiliary. A range of iron oxide (alkoxide) nanostructures including nanosheets, hierarchical flowers (assembled by thin nanosheets), mesoporous hollow nanospheres and solid nanospheres were obtained only by altering the reaction temperature from 180 °C to 240 °C in a single synthetic protocol. Supplementary experiments driven by reaction time were designed in order to further clarify the morphological evolution behaviors of these nanostructures, which discovered that the spherical morphology with the size of about 150–200 nm formed from the inside of micro-scaled flower-like clusters gradually by the condensing and weaving of curled nanosheets, suggesting that the hollow nanospheres were obtained consequently by the further condensation of incompact nanospheres with the assistance of the rearrangement of surfactant micelles, followed by the oriented attachment assembly and Ostwald ripening.