H2O-steered size/phase evolution and magnetic properties of large-scale, monodisperse FexOy nanomaterials

An easy mixed solvent solvothermal/hydrothermal method was developed for the one-step synthesis of monodisperse, single-crystal Fe3O4 and alpha-Fe2O3 nanomaterials. The morphologies can be varied from spherical, to octahedral, to rice like, and even to fusiform; the size can be continuously tuned to a range within 30-290 nm. The morphology-, dimension-, and phase-controlled growth of FexOy nanocrystals can be achieved by tuning kinetic factors, such as the H2O volume fraction (gamma), Fe3+ concentration, reaction temperature, and the ratio of alkali/Fe3+. The threshold value gamma (about 25%) for the H2O-steered size and phase evolutions was theoretically and experimentally inferred. The size-and phase-dependent saturation magnetization (M-s) and coercivity (H-c) were systematically investigated. High Ms was observed in single-crystal Fe3O4 nanomaterials because of high crystallinity; significantly enhanced Hc was exhibited by the as-obtained Fe3O4-alpha-Fe2O3 hybrid nanocrystals because of the unique unidirectional anisotropy and exchange bias. This study provided insights into the size and phase evolution mechanisms of nanocrystals in the EG-H2O system and served as efficient guidance for the tunable synthesis of monodisperse nanomaterials in a mixed solvent system. The uniform single-crystalline Fe3O4 and alpha-Fe2O3 nanomaterials can provide better platforms for studying their size-and phase-dependent optical, electric, and magnetic performances.